Tel3pedia

Planning for Frequency Hopping

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1. Frequency Plan:

Frequency Hopping plan differs from the conventional fixed frequency plan. The plan depends upon the type of Frequency Hopping system used. In case of SFH including BCCH frequency in hopping sequence is not a practical option, as it results in loss of traffic channels on BCCH carrier. A separate frequency plan is prepared for the BCCH carriers. This planning is very much similar to the conventional fixed frequency plan with lesser number of frequencies. This plan needs to be done very carefully as the system monitors cells based on the BCCH frequency only. Since BCCH carrier radiates continuously without downlink power control, frequencies used for BCCH on one cell should not be used as hopping frequencies on other cell. The reason is to avoid continuous interference from BCCH carriers. The benefits of hopping increase if more frequencies are available for hopping. Generally the frequency band is divided into two parts, one used for BCCH frequency plan and other for hopping frequencies. The division of frequency band for allocation of BCCH and hopping carriers should be done to maintain reasonable C/I for BCCH carriers as well as to have enough frequencies for hopping.

Example:

Consider a network with 31 frequencies, using 12 frequencies for BCCH and using 18 for hopping with 1 frequency as guard, is the ideal option. But it may not be practically possible to plan BCCHs with 12 frequencies (4/12 reuse). Using 15 for BCCH plan and 15 for hopping frequencies is more practical. There always exists a trade-off between BCCH and hopping plans. Using very less frequencies for BCCH plan might result in poor quality on BCCH carrier and the advantages of having quality improvement on hopping carriers may be lost. The ratio between hopping and BCCH frequencies should be decided based on the ratio of number of BCCH and NBCCH carriers in the network.

In case of BBH, generally BCCH carrier is included in the hopping sequence. The benefits of BBH can be obtained only when most of the sites in the network are having more than one NBCCH carriers. Benefits of BBH comparable to SFH can only be obtained by equipping additional hardware in order to include more frequencies in hopping sequence. However BBH without additional hardware will result in quality improvements and provide scope of additional capacity as compared to fixed frequency plan though the benefits may not be as significant as seen in SFH.

2. Planning of HSN:

HSN allocation to the cells is done in random fashion. Various scenarios are explained below:

a. MA list is same for all the sector of one site – In this case HSN is kept same for all the cells of the site. MAIO is used on per carrier basis to provide offset for starting frequency in hopping sequence and avoid hits among carriers of the site. Practically it is possible to achieve 0% hit rate within the site, as all the cells of the same site are synchronized.

b. MA list is same for ALL sector of different sites – In this case HSN should be different for all such cells. MAIO can be same or different in this case as HSN is different.

c. MA list is different for the cells – In this case HSN planning is not important, as there can not be any hits between these cells.

d. HSN is set to 0 – This is the case of cyclic hopping. The sequence for hopping remains same and is repeated continuously. This is not recommended in the urban environment where frequency reuse is more. This is because the network is not synchronized so if there is any one hit it will result in continuous sequence of hits. Cyclic hopping is preferred in rural environment as it provides the maximum benefits of frequency diversity.

3. Planning of MAIO:

The benefits of MAIO planning can be best achieved only in case when sectors having same MA list are synchronized. For non-synchronized sectors MAIO can be the same. In the previous version (GSR2), Motorola did not provide manual MAIO setting. It was set automatically by the system. However from GSR3 onwards it is be possible to set MAIO manually. It has to be changed on a case to case basis. In cases where there are large numbers of hits, MAIO change can be effective as it adds the offset in the hopping sequence and hit-rate can be reduced.

4. Planning of FHI:

This parameter is not specified in GSM. FHI is the Motorola defined hopping system. It actually means an independent hopping system consisting of MA and HSN. Total of 4 such hopping systems can be set in a cell.

FHI can be defined on a timeslot basis.

e.g. consider a cell with 3 carriers i.e. 2 carriers are hopping. It is then possible to define 4 different FHIs for 16 timeslots. That means timeslot 0 to 3 of 1 carrier can have one FHI and so on.

Benefits and Drawbacks of FHI

· Separate FHI can be defined even for each carrier with separate MA list.

· For a fully utilized cell, FHI can be used to control increase in hitrate during peak hours. This can be done by defining different MA list associated with a FHI for one of the carriers.

· Main benefits of FHI can be obtained in BBH. Consider a cell with 2 carriers using BBH with BCCH included in the hopping sequence. Timeslot 0 of BCCH will not hop. A separate FHI (with MA list without BCCH frequency) has to be defined for timeslot 0 of NBCCH.

· Different FHIs in the same cell is not used extensively in Motorola networks with SFH, where BCCH frequency is not included in hopping sequence.

· One drawback of using FHI on timeslot basis is that it adds more complexity to the database.

5. Reuse pattern for hopping carriers:

Conventionally there are 3 main reuse patterns followed for hopping frequencies.

1 X 1: It means all the cells in the network use the same frequencies for hopping.

e.g. If 15 frequencies are to be used for hopping, then every cell will have all 15 frequencies in the MA list. This type of reuse is useful in urban areas, where capacity requirement is large. However there is very less planning involved and so less control over quality problems.

3 X 9: Three hopping groups are used in 3 sites, one per site. In this case all the sites should be considered as omni sites for planning frequency reuse. The advantage of this scheme is it provides better isolation between sites using same hopping frequencies. The problem with this method is that, addition of new site may require frequency replan for the area.

1 X 3: This scheme is very commonly used in Motorola networks. Hopping frequencies are divided in 3 groups. Each Sector on a site uses one group and it is repeated on all sites. e.g. consider a network with standard orientation, all V1 sectors will use the same group and so on. It is very easy to add a site in the network. This reuse scheme is suitable for homogeneous network with minimum overlapping areas. The problem with this scheme is in peak hours there may be more hits.

Loading Factor: This parameter is a deciding factor for number of carriers that can be equipped on a sector or a site. Number of carriers equipped on a site or sector should not be greater than 50% of the number of frequencies in the MA list of the sector or site. This factor is a major distinguishing factor between 1 x 1 and 1 x 3.

Loading Factor = (BCCH / NBCCH) * 100

In Case Of Reliance Telecom Loading Factor =12/18 *100 = 66 %

6. Tools for simulation and drive test:

Motorola uses a tool “Handsem” which can simulate SFH plan (different reuse patterns and HSN plan). Latest versions of plaNET and Golf are supposed to support Frequency Hopping simulation. Drive test tools that display decoded layer 3 information are used for monitoring frequency hopping networks. TEMS is one of the drive test tools that can be used for the purpose.

Types of Frequency Hopping

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There are two ways of implementing Frequency Hopping in a Base Station System, one referred as Base Band Frequency Hopping (BBH) and another as Synthesizer Frequency Hopping (SFH). Their operation differs in the way they establish the Base to Mobile Station link (downlink), however there is not difference at all between Mobile Station to Base Station link in both types of hopping. Motorola does not allow BBH and SFH to be used together on the same site

1. Base Band Frequency Hopping

This is accomplished by routing the traffic channel data through fixed frequency DRCUs via the TDM highway on a timeslot basis. In this case, the DRCU would have fixed tuned transmitters combined either in low loss tuned combiners or hybrid combiners.

· TRX always transmits fixed frequency.

· The information for every call is moved among the available TRX on a per burst basis. (Burst of 577 µs)

· Call hops between same timeslots of all TRX.

· Processing (coding and interleaving) is done by digital part associated with TRX on which call was initially assigned.

· For uplink – call is always processed by TRX on which the call was initially assigned.

· Number of TRX needed is equal to the number of frequencies in the hopping sequence.

· BCCH frequency can be included in the hopping sequence.

· Power control does not apply to BCCH or bursts transmitting BCCH frequency.

· BCCH, timeslot 0 will never hop.

· Any timeslot with CCCH will never hop.

· Timeslot carrying all SDCCHs can hop.

If a network running with fixed frequency plan is switched over to BBH (BCCH included in MA list) without any frequency changes, significant quality improvement can be observed in the network. As a result drop call rate reduces in the network. Alternatively, for the existing network quality additional capacity can be provided. FHI can be used effectively in BBH. Further details regarding FHI planning are discussed later in the document.

2. Synthesizer Frequency Hopping

This is accomplished by high speed switching of transmit and receive frequency synthesizers of the individual DRCUs. As a result of dynamic nature of the transmit frequency, broadband (hybrid) combining of the transmitters is necessary.

· TRX changes transmitting frequency every burst.

· Call stays on the same DRCU where it started.

· Remote tune combiners (RTC) are not allowed.

· Number of TRX is not related to number of frequencies in hopping sequence.

· BCCH can be included in the hopping sequence:

If BCCH is included in the hopping sequence, timeslots 1 to 7 can not be used to carry traffic. They transmit dummy burst when BCCH frequency is not in the burst. Whenever BCCH frequency is being transmitted in a burst by DRCU, it will be transmitted at full power.
BCCH DRCU will never hop. It either carries traffic in timeslots 1 to 7 or it transmits dummy bursts.

· Transmission and reception is done on the same timeslot and same DRCU.

· Motorola allows to have NBCCH on fixed frequency hopping on the same sector.

Frequency Hopping Parameters

GSM defines the following set of parameters:

Mobile Allocation (MA): Set of frequencies the mobile is allowed to hop over. Maximum of 63 frequencies can be defined in the MA list.

Hopping Sequence Number (HSN): Determines the hopping order used in the cell. It is possible to assign 64 different HSNs. Setting HSN = 0 provides cyclic hopping sequence and HSN = 1 to 63 provide various pseudo-random hopping sequences.

Mobile Allocation Index Offset (MAIO): Determines inside the hopping sequence, which frequency the mobile starts do transmit on. The value of MAIO ranges between 0 to (N-1) where N is the number of frequencies defined in the MA list. Presently MAIO is set on per carrier basis.

Motorola has defined an additional parameter, FHI.

Frequency Hopping Indicator (FHI): Defines a hopping system, made up by an associated set of frequencies (MA) to hop over and sequence of hopping (HSN). The value of FHI varies between 0 to 3. It is possible to define all 4 FHIs in a single cell.

Motorola system allows to define the hopping system on a per timeslot basis. So different hopping configurations are allowed for different timeslots. This is very useful for interference averaging and to randomize the distribution of errors.

GSM algorithm

GSM has defined an algorithm for deciding hopping sequence. The algorithm is used to generate Mobile Allocation Index (MAI) for a given set of parameters.

ARFCN: absolute radio frequency channel number

MA: mobile allocation frequencies.

MAIO: Mobile allocation offset (0 to N-1), where N is the number of frequencies defined in MA.

HSN: Hopping sequence number (0-63)

T1: Super frame number (0-2047)

T2: TCH multiframe number (0-25)

T3: Signaling multiframe number (0-50)

This algorithm generates a pseudo-random sequence of MAIs. MAI along with MAIO and MA will decide the actual ARFCN to be used for the burst.

Frequency Hopping

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Frequency Hopping is an old technique introduced firstly in military transmission system to ensure the secrecy of communications and combat jamming. Frequency Hopping is mechanism in which the system changes the frequency (uplink and downlink) during transmission at regular intervals. It allows the RF channel used for signaling channel (SDCCH) timeslot or traffic channel (TCH) timeslots, to change frequency every TDMA frame (4.615 ms). The frequency is changed on a per burst basis, which means that all the bits in a burst are transmitted in the same frequency.

Advantages of Frequency Hopping

1. Frequency Diversity

In cellular urban environment, multi-path propagation exists in most cases. Due to Rayleigh fading, short-term variations in received level are frequently observed. This mainly affects stationary or quasi-stationary mobiles. For a fast moving mobile, the fading situation can be avoided from one burst to another because it also depends on the position of the mobile so the problem is not so serious. Frequency Hopping is able to take the advantage due to frequency selective nature of fading to decrease the number of errors and at the same time they are temporally spread. As a result, the decoding and de-interleaving processes can more effectively remove bit errors caused by bursts received whilst on fading frequencies (errors will be randomly distributed instead of having long bursts of errors). This increase in effectiveness leads to a transmission quality improvement of the same proportion.

· Frame Erasure Rate reduces due to 6 dB to 8 dB gain.

· Number of reports with rxqual 6 and 7 reduce.

· Reported values of rxlev are more concentrated around mean.

2. Interference Averaging

Interference Averaging means spreading raw bit errors (BER caused by the interference) in order to have random distribution of errors instead of having burst of errors, and therefore, enhance the effectiveness of decoding and de-interleaving process to cope with the BER and lead to better value of FER.

With hopping, the set of interfering calls will be continually changing and the effect is that all the calls experience average quality rather than extreme situations of either good or bad quality. All the calls suffer from controlled interference but only for short and distant periods of time, not for all the duration of the call.

· For the same capacity, Frequency Hopping improves quality and for a given average quality Frequency Hopping makes possible increase in capacity.

· When more than 3 % of the reports have rxqual of 6 or 7 then voice quality disturbances start to appear.

· Gains (reduction in the C/I value needed to satisfy the quality requirements involved in the criterion) from hopping relative to fixed frequency operation can be achieved.

1/3 interference: 1 dB gain

i.e. if 1 out of 3 frequencies are experiencing a continuous interference a gain of 1 dB in C/I requirement is obtained.

Similarly,

1/4 interference: 4 dB gain

1/5 interference: 6 dB gain

2/4 interference: 0 dB gain

2/5 interference: 4 dB gain

The effective gain obtained with Frequency Hopping is due to the fact that the interference effect is minimized and it is easier to keep it under control.

Parameter

SQIà +18 above is good (19 is ok)

C/Ià +9 above good in case of hopping(particular channel 200khz/total interference)

C/Ià +12 above good in case of Non hopping(particular channel 200khz/total interference) this is case of co-channel interference

C/Aà -9 above good (e.g -8,-7,-6) this carrier upon adjacent channel interference

Co-channel interference;-

(1) Level is high but call is drop due to handoff problem and this is due to interference on same channel in which call is latch.

(2) problem in access.

Adjacent interference;- check by quality , Rx level is good

Path Balance:- path balance gives the difference between uplink reception and down link reception

Path balance = path Loss +110

Path Loss = Up path Loss – DL path Loss

Where

Uplink Path Loss = Actual Ms Tx power --- Rx Lev_UL

Downlink Path Loss = Actual BTS Tx power --- Rx Lev_dL

+- 10 db is acceptable in link budget

negative value means your BTS Rx is bad

Positive value means your BTS Tx is bad

Effect of Frequency Hopping

1. Handovers:

When SFH is implemented, BCCH plan is done using lesser number of frequencies as compared to fixed frequency plan. This may result in quality degradation. However quality of hopping carriers improves than before. Also, quality threshold for handovers on hopping carrier should be increased as compared to fixed frequency plan. In the present version (GSR3), different quality threshold settings are set BCCH and NBCCH. By setting lower quality thresholds for BCCH as compared to NBCCH, number of dropped calls can be controlled. Handover Success Rate may go down because of the BCCH replan (less frequencies). This reduction may get compensated due to improvement in quality of hopping carriers (improvement in TCH assignment success rate).

2. Call setup:

In call setup, SDCCH hopping is also possible. There are no separate settings required for SDCCH hopping. Since GSR3 allows control over SDCCH configuration (location of SDCCH on timeslot basis), SDCCH hopping depends on the location of SDCCH. In case of SFH (with BCCH not included in MA list), if SDCCHs are on BCCH carrier they will not hop whereas SDCCHs on NBCCH carriers may hop. Generally it is preferred to keep SDCCHs on BCCH carrier as SDCCH timeslot is used continuously and it will increase interference on hopping carriers. Call success rate will depend on the cleanliness of BCCH carriers. Call Success Rate may reduce after BCCH replan. This reduction may fet compensated due to improvement in quality of hopping carriers (improvement in TCH assignment success rate).

3. Frame Erasure Rate (FER):

FER indicates the number of TDMA frames that could not be decoded by the mobile due to interference. This parameter gives the indication of hit-rate. FER improves (gain of 6 to 8 dB) after implementation of frequency hopping. FER is represented in percentage terms. FER less than 10% is considered to be good. But this is a subjective issue and good value should be decided by doing multiple drives. In future Motorola is planning to include FER as a statistics in the OMCR.

SHORT MESSAGE SERVICE TRAFFIC CASES

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The Short Message Service (SMS) provides a means of sending text messages containing up to 160 alpha numerical characters to and from MS’s. SMS makes use of a SMS Center (SMS-C), which acts as a store and forward center for short messages. SMS consists of two basic services:

• Mobile terminated SMS: from an SMS–C to an MS

• Mobile originated SMS: from an MS to an SMS–C In the two cases described below, the MS is in idle mode. If the MS is in active mode, short message is transmitted on the SACCH. No paging, call set-up, authentication, etc. needs to be performed in that case.

MOBILE ORIGINATED SMS

Mobile originated SMS transfers a short message submitted by the MS to an SMS-C. It also provides information about the delivery of the short message, either by a delivery report or failure report.

Mobile originating short message

1. An MS establishes a connection to the network, as in the case of a normal call set-up. This step is not performed if the MS is in active mode, since the connection already exists.

2. If authentication is successful, the MS sends the short message using SDCCH to the SMS–C via the MSC/VLR. The SMS–C in turn forwards the short message to its destination. This could be an MS or a terminal in the fixed network, such as a PC.

MOBILE TERMINATED SMS

Mobile terminated SMS has the capability to transfer a short message from the SMS–C to an MS.

Mobile terminating short message

1. A user sends a message to a SMS–C.

2. The SMS–C sends the message to the SMS–GMSC.

3. The SMS–GMSC queries the HLR for routing information.

4. The HLR returns routing information to the SMS-GMSC.

5. The SMS-GMSC re-routes the message to the MSC/VLR.

6. The MS is paged and a connection is set up between the MS and the network, as in the normal call set-up case.

7. If authentication is successful, the MSC/VLR delivers the message to the MS. Short messages are transmitted on the allocated signaling channel, SDCCH.

8. If the delivery was successful, a report is sent from the MSC/VLR to the SMS–C, if not, the HLR is informed by the MSC/VLR, and a failure report is sent to SMS–C.

In the case of an unsuccessful delivery, the SMS-C informs the HLR and VLR that there is a message waiting to be delivered to the MS. The HLR then informs the SMS–C when the MS becomes available. Mobile terminated SMS can be input to the SMS–C via a variety of sources, e.g. speech, telex, facsimile or internet.

INTERNATIONAL TRAFFIC CASES

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One of the primary features of GSM is the ability to perform international roaming and to handle international call cases. In order for a mobile subscriber to be able to make calls while roaming in a different GSM network, there must be an agreement between the subscriber’s home network operator and the visited network operator. This also applies to international roaming. Although there are less significant affects on other traffic cases, the two traffic cases that are affected most are outlined here.

IMSI ATTACH

When an MS is roaming internationally, the following occurs:

1. The MS is switched on and scans all GSM frequencies within one frequency band (e.g. GSM 900). It is searching for a BCCH carrier. The MS tunes to the BCCH carrier that has the strongest signal strength and reads its system information. This includes the identity of the network operator.

2. The MS compares this network identity with the list of forbidden PLMN’s in the SIM memory. This list contains all network identities, which the subscriber’s home operator does not have an international roaming agreement with. If the network, which the MS has tuned to, is a forbidden network, the MS continues to scan for a permitted network.

3. If the MS does not find a permitted network, but has identified a forbidden network, it displays the message “Emergency Calls Only”. If the MS finds a permitted network, it tunes to it and sends an IMSI attach message.

4. The remainder of this traffic case is identical to that of the normal IMSI attach case, with the only difference being that the subscriber’s HLR is located in another country.

CALL TO AN MS

When an MS is roaming internationally and a call is made to it, the procedure used is identical to when the MS is in their home network. The only major difference is that the GMSC and HLR used are in the home network, while the MSC/VLR is in a network in another country.

DROPBACK FUNCTIONALITY

The following traffic case demonstrates the advantages of using dropback functionality. It involves two subscribers:

• Subscriber A is from France and is located in France under the control of the MSC/VLR-A.

• Subscriber B is from Sweden but is currently roaming internationally in France under the control of the same MSC/VLR-A.

1. Subscriber A calls subscriber B. The call is routed internationally from France to Sweden.

2. The Swedish network then identifies that B is in under the control of MSC/VLR-A in France and routes the call back to France. The subscribers are connected to each other and continue their call.

• Without dropback: the speech on the call goes through the GMSC in Sweden.

• With dropback: the speech on the call is switched within MSC/VLR-A, thus saving on processing and transmission costs.

Traffic case showing the advantage of using dropback

Handover

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The process of changing cells during a call is called handover in GSM terminology. To choose the best target cell, the MS and the RBS perform measurements. Because the MS contributes to the handover decision, this type of handover is often called Mobile Assisted HandOver (MAHO).

Locating

An MS continuously measures signal strength and quality on it’s own cell and signal strength on the BCCH carriers of the neighboring cells. The measurements are carried out on the downlink while MS is in active mode. The measurement results are sent to the RBS on SACCH at regular intervals. The serving RBS measures signal strength and quality on the uplink.

Measurements sent to BSC

The measurements from the RBS and MS are sent to the BSC in the form of measurement reports. Based on these reports, the BSC decides if a handover is necessary and to which cell. This is called locating. As soon as a neighboring cell is considered to be better than the serving cell, a handover is attempted. Another reason for attempting a handover, apart from signal strength and quality, is when the Timing Advance (TA) used by MS exceeds a threshold value set by the operator. This usually happens when the MS is moving over the cell border to another cell. When the MS has changed cells, the new RBS informs the MS about the new neighboring BCCH carriers so measurements can be taken again. If the MS has also switched to a new LA, a location updating type normal takes place after the call has finished. Handover can be used for load balancing between cells. During a call setup in a congested cell, the MS can be transferred to a cell with less traffic if an acceptable connection quality is likely to be obtained. Another area where forced handover is a useful tool is maintenance. Channels can be released from traffic if necessary, e.g. for RBS maintenance reasons. There are several types of handover, including:

• Intra-cell handover

• Handover between cells controlled by the same BSC

• Handover between cells controlled by different BSC’s, but the same MSC/VLR

• Handover between cells controlled by different MSC/VLRs

Each of these traffic cases is described in greater detail below. In each case, the decision to perform a handover has already been made and a target cell has been identified.

Intra-Cell Handover

A special type of handover is the intra-cell handover. It is performed when the BSC considers the quality of the connection too low, but receives no indication from the measurements that another cell would be better. In that case the BSC identifies another channel1 in the same cell which may offer a better quality, and the MS is ordered to retune to it.

Handover between Cells Controlled by the Same BSC

When performing a handover between two cells controlled by the same BSC, the MSC/VLR is not involved. However, the MSC/VLR will be informed when a handover has taken place. If the handover involves different LA’s, location updating is performed once the call is finished.

Handover: cells controlled by the same BSC

1. The BSC orders the new RBS to activate a TCH.

2. The BSC sends a message to the MS, via the old RBS, containing information about the frequency and time slot to change to and also the output power to use. This information is sent to the MS using FACCH.

3. The MS tunes to the new frequency, and transmits handover access bursts in the correct time slot. Since the MS has no information yet on TA, the handover bursts are very short (only 8 bits of information).

4. When the new RBS detects the handover bursts, it sends information about TA. This is also sent via FACCH.

5. The MS sends a Handover Complete message to the BSC via the new RBS.

6. The BSC tells the old RBS to release the old TCH.

Handover between Cells Controlled by Different BSC’s but the Same MSC/VLR

When another BSC is involved in a handover, the MSC/VLR must also be involved to establish the connection between the two BSC’s.

Handover: different BSC’s but the same MSC/VLR

1. The serving (old) BSC sends a Handover Required message to the MSC containing the identity of the target cell.

2. The MSC knows which BSC controls this cell and sends a Handover Request to this BSC.

3. The new BSC orders the target RBS to activate a TCH.

4. The new BSC sends a message to the MS via the MSC and the old RBS.

5. MS tunes to the new frequency and transmits handover access bursts in the correct time slot.

6. When the new RBS sends information about TA.

7. MS sends a Handover Complete message to MSC via the new BSC.

8. MSC sends the old BSC an order to release the old TCH.

9. The old BSC tells the old RBS to release the TCH.

Handover between Cells Controlled by Different MSC/VLRs

Handover between cells controlled by different MSC/VLRs can only be performed within one PLMN and not between two PLMN’s. Cells controlled by different MSC/VLRs also means that they are controlled by different BSC’s.

Handover: cells controlled by different MSC’s

1. The serving (old) BSC sends a Handover Required message to the serving MSC (MSC–A), with the identity of the target cell.

2. MSC–A identifies that this cell belongs to another MSC, (MSC–B), and requests help.

3. MSC–B allocates a handover number to reroute the call. A Handover Request is then sent to the new BSC.

4. The new BSC orders the target RBS to activate a TCH.

5. MSC–B receives the information, and passes it on to MSC– A together with the handover number.

6. A link is set up to MSC–B, possibly via PSTN.

7. MSC–A sends a handover command to the MS, via the old BSC.

8. The MS tunes to the new frequency and transmits handover access bursts in the correct time slot.

9. When the new RBS detects the handover bursts it sends information about TA.

10. The MS sends Handover Complete message to the old MSC via the new BSC and the new MSC/VLR.

11. A new path in the group switch in MSC–A is established, and the call is switched through.

12. The old TCH is deactivated by the old BSC (not shown in the picture).

The old MSC, MSC–A, retains main control of the call until the call is cleared. This is because it contains the information about the subscriber and call details such as charging. After call release, the MS must perform location updating because a LA never belongs to more than one MSC/VLR service area. The HLR is updated by the VLR–B, and will in turn tell VLR–A to delete all information about the mobile subscriber.

TRAFFIC CASES: MS IN ACTIVE MODE

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An MS is in active mode when there is a call (speech, fax or data), or a call set up procedure taking place.

Cases which activate an MS and cases when MS is in active mode

CALL FROM AN MS

This section describes what happens when a mobile subscriber wants to set up a voice call to a subscriber in the PSTN. Data and text message calls are described separately.

Call set-up MS to PSTN

1. The MS uses RACH to ask for a signaling channel.

2. The BSC/TRC allocates a signaling channel, using AGCH.

3. The MS sends a call set-up request via SDCCH to the MSC/VLR. Over SDCCH all signaling preceding a call takes place. This includes:

• Marking the MS as “active” in the VLR

• The authentication procedure

• Start ciphering

• Equipment identification

• Sending the B-subscriber’s number to the network

• Checking if the subscriber has the service “Barring of outgoing calls” activated

4. The MSC/VLR instructs the BSC/TRC to allocate an idle TCH. The RBS and MS are told to tune to the TCH.

5. The MSC/VLR forwards the B–number to an exchange in the PSTN, which establishes a connection to the subscriber.

6. If the B-subscriber answers, the connection is established.

CALL TO AN MS

The major difference between a call to an MS and a call from an MS is that in a call to an MS the exact location of the mobile subscriber is unknown. Therefore, the MS must be located using paging before a connection can be established. Below is the description of the call set-up procedure for a call from a PSTN subscriber to a mobile subscriber. A call from an MS to a mobile subscriber operates according to the same process, the only difference being that the GMSC is contacted by another MSC/VLR instead of by a PSTN node.

Call to MS from PSTN

1. The PSTN subscriber keys in the MS’s telephone number (MSISDN). The MSISDN is analyzed in the PSTN, which identifies that this is a call to a mobile network subscriber. Aconnection is established to the MS’s home GMSC.

2. The GMSC analyzes the MSISDN to find out which HLR the MS is registered in, and queries the HLR for information about how to route the call to the serving MSC/VLR.

3. The HLR translates MSISDN into IMSI, and determines which MSC/VLR is currently serving the MS. The HLR also checks if the service, “Call forwarding to C–number” is activated, if so, the call is rerouted by the GMSC to that number.

4. The HLR requests an MSRN from the serving MSC/VLR.

5. The MSC/VLR returns an MSRN via HLR to the GMSC.

6. The GMSC analyses the MSRN and routes the call to the MSC/VLR.

7. The MSC/VLR knows which LA the MS is located in. A paging message is sent to the BSC’s controlling the LA.

8. The BSC’s distribute the paging message to the RBS's in the desired LA. The RBS’s transmit the message over the air interface using PCH. To page the MS, the network uses an IMSI or TMSI valid only in the current MSC/VLR service area.

9. When the MS detects the paging message, it sends a request on RACH for a SDCCH.

10. The BSC provides a SDCCH, using AGCH.

11. SDCCH is used for the call set-up procedures. Over SDCCH all signaling preceding a call takes place. This includes:

• Marking the MS as “active” in the VLR

• The authentication procedure

• Start ciphering

• Equipment identification

12. The MSC/VLR instructs the BSC/TRC to allocate an idle TCH. The RBS and MS are told to tune to the TCH. The mobile phone rings. If the subscriber answers, the connection is established.

TRAFFIC CASES: MS IN IDLE MODE

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Traffic cases when MS is in idle mode

ATTACHING TO THE NETWORK

IMSI attach

When an MS is switched on, the IMSI attach procedure is executed. This involves the following steps:

1. The MS sends an IMSI attach message to the network indicating that it has changed state to idle.

2. The VLR determines whether there is a record for the subscriber already present. If not, the VLR contacts the subscriber’s HLR for a copy of the subscription information.

3. The VLR updates the MS status to idle.

4. Acknowledgement is sent to the MS.

IMSI attach

Location Updating, type IMSI Attach

If the MS has changed LA while powered off, the IMSI attach procedure may lead to an update to the location of the MS. During IMSI attach, the VLR may determine that the current LAI of the MS is different from the LAI stored in the MS’s subscription information. If so, the VLR updates the LAI of the MS.

ROAMING IN THE NETWORK

Changing Cells within an LA

MS’s are constantly moving around in the cellular network. The MS location information stored in the VLR is the LA. If an MS changes cells within a LA, the network is not updated. The MS knows that the new cell belongs to the same LA by listening to the BCCH in the new cell. The BCCH broadcasts the cell’s LAI. The MS compares the last LAI received with the new LAI. If they are the same, it means that the MS has not changed LA's and does not need to inform the network.

Location Updating, Same MSC/VLR

If an MS detects a change in LAI on the BCCH, it informs the network. When the MS sends the Location Updating message, the MSC/VLR determines whether it is an MS, which is already registered, or if it is an MS visiting from another MSC/VLR.

Location updating, same MSC/VLR

1. The MS listens to BCCH in the new cell to determine the LAI. The received LAI information is compared to the old one. If they differ, a location update is necessary.

2. The MS establishes a connection with the network via SDCCH. Authentication is performed.

3. If authentication is successful, the MS sends a Location Updating Request to the system.

4. The system acknowledges Location Updating and requests RBS and MS to release the signaling channel.

Location Updating, New MSC/VLR

When an MS roams into a new LA, location updating is performed. However, unknown to the MS, the LA may belong to a new MSC/VLR. When the Location Update Request is received by the new VLR, it executes the procedure below.

Location updating, new MSC/VLR

1. Authentication is performed. If authentication is successful, the VLR checks its database to determine whether or not it has a record for this MS-subscription.

2. When the VLR finds no record for the MS, it sends a request to the subscriber’s HLR for a copy of the MS-subscription.

3. The HLR passes the information to the VLR and updates its location information for the subscriber. The HLR instructs the old VLR to delete the information it has about the MSsubscription.

4. The VLR stores its subscription information for the MS including the latest location and status (idle). The VLR sends acknowledgement to the MS.

Location Updating, type Periodic Registration

Periodic registration is a feature which forces MS’s to send a registration message to the network at predefined intervals. If an MS should miss such a registration, the network will mark the MS as detached. This may occur if an MS is out of the area of coverage and ensures that needless paging is not performed. If the network uses periodic registration, the MS will be informed, on the BCCH of how often periodic registration must be performed. Periodic registration has an acknowledgment message. The MS tries to register until it receives this message.

DETACHING FROM THE NETWORK

IMSI Detach

IMSI detach enables the MS to indicate to the network that it is switched off. At power off, the MS sends an IMSI detach message to the network. On reception, the VLR marks the corresponding IMSI as detached. The HLR is not informed. No acknowledgement is sent to the MS.

Implicit Detach

If the MS sends an IMSI detach message to the system and the radio link quality is poor, the system might not be able to decode the information. Because no acknowledgment is sent to the MS, no further attempt is made. In this case, the system still regards the MS as attached. If periodic registration is in use, the system will soon determine that the MS is detached. The VLR then performs an implicit detach, marking the MS as detached.

MS Purging

MS purging is used to inform the HLR that the VLR is about to remove a subscriber record from the VLR. The HLR then sets the MS purged flag and treats the subscriber as unreachable. This saves unnecessary network signaling and database lookup. For example, an UK MS travels to Australia and performs a location update in an MSC/VLR in Australia. Later, the subscriber travels back to the UK, which takes some time. During this period, the subscriber is not active. If MS purging is not used, when a caller makes a call to the MS, the HLR identifies the MS as registered in the Australian MSC/VLR and routes the call to it. The MSC/VLR then informs the HLR that the subscriber is unreachable. If MS purging is used; the UK subscriber’s record will have been purged from the Australian MSC/VLR. When a call is made to the subscriber, the HLR identifies the MS as unreachable and does not contact the Australian MSC/VLR.

GSM NETWORK IDENTITIES

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Network identities are numbers that a GSM network uses to locate a mobile subscriber when it is establishing a call to that subscriber. As the network relies on these identities to route calls to subscribers, it is important that each identity is unique and correct. Numbering plans are used to identify different networks as specified by the International Telecommunications Union - Telecommunications (ITU-T). For a telephone number in the PSTN/ISDN network, ITU-T's numbering plan E.164 is used.

SUBSCRIBER-RELATED IDENTITIES

Mobile Station ISDN number (MSISDN)

The Mobile Station ISDN number (MSISDN) uniquely identifies a mobile telephone subscription in the PSTN numbering plan. This is the number dialed when calling a mobile subscriber. As the MSISDN is the actual telephone number of the mobile subscriber, it is the only network identity that subscribers are aware of. All other network identities discussed in this chapter are for internal network use and subscribers do not need to be aware of them.

CME 20 MSISDN

In CME 20, the MSISDN consists of the following:

CME 20 MSISDN

CC Country Code

NDC National Destination Code

SN Subscriber Number

An NDC is allocated to each PLMN. For example, in Ireland the NDC's 086 and 087 indicate the PLMN's of two different network operators. In some countries, more than one NDC may be required for each PLMN. The international MSISDN number may be of variable length. The maximum length is 15 digits, prefixes not included. A German subscriber calling an Irish GSM subscriber would dial the following number:

CME 20 MSISDN

CMS 40 MSISDN

In CMS 40, the MSISDN consists of the following:

CMS 40 MSISDN

CC Country Code

NPA Number Planning Area

SN Subscriber Number

The NPA is allocated to each GSM 1900 PLMN. The length of MSISDN is determined by the structure and operating plan for each operator. The maximum length is 15 digits, prefixes notincluded. A Swedish subscriber calling a Canadian GSM 1900 subscriber would dial the following number:

CMS 40 MSISDN

International Mobile Subscriber Identity (IMSI)

The International Mobile Subscriber Identity (IMSI) is a unique identity allocated to each subscriber that facilitates correct subscriber identification over the radio path and through the network. It is used for all signaling in the PLMN. All network related subscriber information is connected to an IMSI. TheIMSI is stored in the SIM, the HLR and in the serving VLR. The IMSI consists of three different parts:

IMSI

MCC Mobile Country Code

MNC Mobile Network Code

MSIN Mobile Station Identification Number

According to the GSM specifications, the IMSI has a maximum length of 15 digits.

Temporary Mobile Subscriber Identity (TMSI)

The Temporary Mobile Subscriber Identity (TMSI) is a temporary IMSI number made known to an MS at registration. It is used to protect the subscriber's identity on the air interface. The TMSI has local significance only (that is, within the MSC/VLR area) and is changed at time intervals or when certain events occur such as location updating. Every operator can chose TMSI structure, but should not consist of more than 8 digits.

EQUIPMENT-RELATED IDENTITIES

International Mobile Equipment Identity (IMEI)

The International Mobile Equipment Identity (IMEI) is used to uniquely identify MS equipment to the network. The IMEI is used for security procedures such as identifying stolen equipment and preventing unauthorized access to the network. According to the GSM specifications, IMEI has a total length of 15 digits, and consists of the following:

IMEI

TAC Type Approval Code, determined by a central GSM body FAC Final Assembly Code, identifies the manufacturer SNR Serial Number, an individual serial number of six digits uniquely identifies all equipment within each TAC and FAC spare A spare digit for future use. When transmitted by the MS this digit should always be zero

International Mobile Equipment Identity and Software Version number

(IMEISV)

The International Mobile Equipment Identity and Software Version number (IMEISV) provides a unique identity for every MS and also refers to the version of software which is installed in the MS. The version of software is important as it may affect the services offered by the MS or its speech coding capabilities. For example, mobile networks need to know the MS speech coding capabilities when a call is being made (i.e. half rate/full rate, etc). This will be indicated by the IMEISV. The IMEISV consists of the following:

IMEISV

SVN Software Version Number allows the mobile equipment manufacturer to identify different software versions of a given type approved mobile. SVN value 99 is reserved for future use – 182 – EN/LZT 123 3321 R3B

International Mobile Equipment Identity and Software Version number

(IMEISV)

IMEISV

SVN Software Version Number allows the mobile equipment manufacturer to identify different software versions of a given type approved mobile. SVN value 99 is reserved for future use

LOCATION-RELATED IDENTITIES

Mobile Station Roaming Number (MSRN)

The Mobile Station Roaming Number (MSRN) is a temporary network identity which is assigned during the establishment of a call to a roaming subscriber. More information about the use of MSRN can be found in the "Traffic Cases" section later in the book. The MSRN consists of three parts:

MSRN

Note: In this case, SN is the address to servicing MSC/VLR.

SN= Servicing Node

Location Area Identity (LAI)

The Location Area Identity (LAI) is a temporary network identity, which is also required for routing. The two main purposes of the LAI are:

1. Paging, which is used to inform the MSC of the LA in which the MS is currently situated

2. Location updating of mobile subscribers The LAI contains the following:

LAI

LAC Location Area Code, the maximum length of LAC is 16 bits, enabling 65,536 different location areas to be defined in one PLMN

Cell Global Identity (CGI)

The Cell Global Identity (CGI) is used for identifying individual cells within a LA. Cell identification is achieved by adding a Cell Identity (CI) to the LAI components. The CI has a maximum length of 16 bits. The CGI consists of:

CGI

Base Station Identity Code (BSIC)

The Base Station Identity Code (BSIC) enables MS's to distinguish between different base stations sending on the same frequency. The BSIC consists of:

BSIC

NCC Network Color Code (3 bits) identifies the PLMN. Note that it does not uniquely identify the operator. NCC is primarily used to distinguish between operators on each side of a border BCC Base Station Color Code (3 bits) identifies the Base Station to help distinguish between RBS using the same control frequencies

Location Number (LN)

The Location Number (LN) is a number related to a certain geographical area, which the network operator specifies by “tying” the location numbers to cells, location areas, or MSC/VLR service areas. The LN is used to implement features like regional/local subscription and geographical differentiated charging. The LN consists of the following:

LSP Locally Significant Part

Regional Subscription Zone Identity (RSZI)

For each regional subscription, zones/regions need to be defined. This is achieved by using the Regional Subscription Zone Identity (RSZI). The RSZI consist of the following:

RSZI

ZC The length of the Zone Code, is two octets

SUBSCRIBER IDENTITY CONFIDENTIALITY

Subscriber identity confidentiality means that the IMSI is not disclosed to unauthorized individuals, entities or processes. This function protects a subscriber’s identity when the subscriber is using PLMN resources. It also prevents tracing the mobile subscriber’s location by listening to the signaling exchanges on the radio path.

Subscriber Identity Confidentiality Procedure

Each time a mobile station requests a system procedure (e.g. location updating, call attempt or service activation), the MSC/VLR can allocate a new TMSI to an IMSI. The MSC/VLR transmits the TMSI to MS that stores it on the SIM card. Signaling between MSC/VLR and MS utilizes only the TMSI from this point on. Thus, the real subscriber identity, IMSI, is not transmitted over the radio path again. TMSI is half the length of IMSI; thus allowing twice as many MS's to be paged in the same paging message.IMSI is only used in cases when location updating fails or when the MS has no allocated TMSI.

MOBILE STATION FEATURES

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An MS feature is defined as a piece of equipment or a function which relates directly to the operation of the MS.

TYPES OF MS FEATURES

MS features are defined as mandatory or optional. Mandatory features must be implemented as long as they pertain to the MS type. The choice of implementing optional features is left up to the manufacturers. Manufacturers are responsible for ensuring that the MS features neither conflict with the air interface nor interfere with the network, any other MS, or the MS itself. Standardization of a minimum set of features is desirable to make a simple and uniform set of MS features independent of the MS manufacturer and type. This minimum set includes all the mandatory features.

There are three categories of MS features:

• Basic:

Basic MS features are directly related to the operation of basic telecommunication services. Each feature is classed as being mandatory or optional.

• Supplementary:

A supplementary MS feature is directly related to the operation of the supplementary service (e.g. display of calling line number). All supplementary MS features are optional.

• Additional:

Additional features are neither basic nor supplementary. All additional MS features are optional.

BASIC MOBILE STATION FEATURES

Mandatory

Display of called number:

This feature enables the caller to check that the selected number is correct before call set-up.

Dual Tone Multi Frequency function (DTMF):

The MS must be able to send DTMF tones. There is special function in MSC to generate DTMF tones. MS just sends a digital signal to this function in MSC.

Indication of call progress signals:

Indications are given such as tones, recorded messages or a visual display based on signaling information returned from the PLMN. On data calls, this information may be signaled to the Data Terminating Equipment (DTE).

Country/PLMN indication:

The country/PLMN indicator shows in which GSM/PLMN the MS is currently registered. This indicator is necessary so that the user knows when roaming is taking place and that the choice of PLMN is correct. Both the country and PLMN are indicated. When more than one GSM/PLMN is available in a given area this information is indicated.

Country/PLMN selection:

If more than one GSM/PLMN is available, the user must have the ability to select their preferred choice.

Service indicator:

The user is informed that there is adequate signal strength (as far as can be judged from the received signal) to allow a call to be made and that the MS has successfully registered on the selected PLMN. This can be combined with the Country/PLMN Selection.

Subscription identity management:

The IMSI is part of the SIM card and is physically secured and standardized in the GSM system. If the user can remove the SIM, its removal detaches the MS from the network causing a call in progress to be terminated, and preventing the initiation of further calls (except emergency calls).

International Mobile station Equipment Identity (IMEI):

Each MS must have a unique identity and must transmit this if requested by the PLMN. The IMEI is incorporated into a module which is built into the MS and is physically secured. The of each individual module is to be carried out by the manufacturer.

Support of A5/1 and A5/2:

Provisions are made for support of up to seven algorithms as well as the support of ‘no encryption’. It is mandatory for A5/1 and A5/2 and non encrypted mode to be implemented, but other algorithms are optional.

Short message indication and acknowledgment:

This feature allows delivery of short messages to a MS from a service center. Such messages are submitted to the service center by a telecommunications network user who can also request information on the message status from the service center. The service center then transmits the message to the active MS user. The MS must therefore provide an indication to the user that a message has been received from the service center and must also send an acknowledgment signal to the PLMN, to show that this indication has been activated. The PLMN then returns this acknowledgment to the service center.

Short message overflow indication:

An MS user using the short message service will be informed when an incoming message cannot be received due to insufficient memory.

Emergency call capabilities:

It must be possible to make an emergency call even without a valid SIM.

Optional

On/Off switch:

The MS can be equipped with the means of switching its power supply on and off. Switch-off is generally “soft”, so that the MS completes housekeeping functions, such as deregistration, before actually switching off.

Keypad:

A physical means of entering numbers, generally in accordance with the layout below:

Keypad

DTE/DCE interface:

This is a standard connector for attaching Data Terminating Equipment (DTE)/Data Communication Equipment (DCE) to an MS and is used with data services.

ISDN ’S’ terminal interface:

A standard connector for attaching equipment to ISDN.

International access function ( ‘+’ key):

This enables a direct, standard method of gaining international access. For this purpose, the MS may have a key whose primary or secondary function is marked “+”. This has the effect of generating the International Access Code (IAC) in the network. This is useful because the IAC varies from country to country and may be unknown by a subscriber who is roaming internationally.

Short Message Service Cell Broadcast (SMSCB) screening:

This feature enables the user of the MS to receive only those broadcast messages that are of interest to the user and in this way save on consumption of battery power. This may be useful in the future to ignore advertisements sent as short messages.

SUPPLEMENTARY MOBILE STATION FEATURES

Charge indication:

This feature enables the display of charging information provided by the PLMN on a per call basis.

Control of Supplementary Services:

It is mandatory that Supplementary Services can be controlled from the MS.

ADDITIONAL MOBILE STATION FEATURES

Abbreviated dialing:

The feature stores a full directory number or part of a directory number in the MS with the abbreviated address. After retrieval, the directory number may appear on the display. An incomplete directory number must be supplemented by means of the keypad function or a second stored number. The full directory number is then transmitted on the radio path.

Fixed number dialing:

Using this feature in conjunction with an electronic lock makes it possible to place a bar on calling any numbers other than those pre-programmed into the MS. There are two sub-categories to this service:

• All calls are made to only one predetermined number.

• Calls can be made to several predetermined numbers. The required number is selected by means of an abbreviated address code.

Sub-addresses may be added to the predetermined number. In both cases, the actual directory number is transmitted on the radio path.

Barring of outgoing calls:

This feature allows outgoing calls to be blocked with the exception of emergency calls. The barring condition may be activated/deactivated by using a key word. The barring may be selective, that is, applied to individual services, individual call types (e.g. long distance, international calls) or supplementary services. No network signaling is involved.

DTMF control digits separator:

This enables subscribers to enter DTMF digits with a telephone number. When the called party answers, the ME automatically sends the DTMF digits to the network after a delay of three seconds. This may be useful for accessing a voice mailbox, when the subscriber knows the actions which they wish to perform. For example, the sequence 087 5551234#31 may dial the mailbox (087 5551234) access menu 3 and delete message 1.

Call charge units meter:

The MS may incorporate a call charge units indicator. This call charge indicator gives information about the actual call charge units consumed during the last conversation or information about accumulated call charge units for each PLMN.

Selection of directory number in short messages:

The short message (point-to-point mobile-terminated or mobile-originated, or cell broadcast) can be used to convey a directory number which the receiver wishes to call. This can be indicated by enclosing the directory number in a pair of inverted commas (“ “). If the displayed message contains these characters enclosing a directory number then a call can be set up. The message can contain more than one directory number, in which case the receiver selects the desired one. This service is useful for giving someone a telephone number via SMS.

Last numbers dialed (LND):

The MS can store the last ‘N’ numbers dialed in the SIM and/or the ME. ‘N’ can take the value up to 10 in the SIM but there is a greater number storage capacity in the ME.

SUBSCRIBER DATA STORED IN THE MOBILE EQUIPMENT

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All subscriber related information transferred to the ME during operation must be deleted after the removal of the SIM or deactivation of the MS. Examples of such information are the PIN and the PUK codes.

PIN MANAGEMENT

A SIM is required to have a PIN function even if it is deactivated by a user. The PIN consists of 4 to 8 digits. An initial PIN is loaded by the network operator at subscription time. Afterwards the PIN, including the length, can be changed by the user. In addition, the user can decide whether to use the PIN function or not by activating an appropriate SIM-ME function called the PIN disabling function. The PIN is disabled until the user changes the status of the function. This PIN disabling function can be blocked at subscription time by a person authorized to do so. If an incorrect PIN is entered, the user is informed. After three consecutive incorrect entries the SIM is blocked, even if between attempts the SIM has been removed or the MS has been switched off.

BLOCKING/UNBLOCKING OF SIM

When a SIM is blocked GSM network operations are forbidden. To unblock the SIM, the user must enter the PIN Unblocking Key, PUK. The PUK is an 8 digit numerical code. If the PUK is entered incorrectly the user is informed. The user can make 10 attempts to enter the PUK before the system blocks entry, in which case the subscriber must contact their network operator.

SUBSCRIBER IDENTITY MODULE (SIM)

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A key feature of the GSM standard is the Subscriber Identity Module (SIM) card. A SIM card contains information about the subscriber and must be plugged into the ME to enable the subscriber to use the network. With the exception of emergency calls, MSs can only be operated if a valid SIM is present. The SIM stores three types of subscriber related information:

• Fixed data stored before the subscription is sold: e.g. IMSI, authentication key and security algorithms

• Temporary network data: e.g. the location area of the subscriber and forbidden PLMNs

• Service data: e.g. language preference, advice of charge

GSM phase 1 SIMs contain all necessary network control information, while phase 2 SIMs include a large number of extra features such as a language identifier and a preferred language option.

TYPES OF SIM CARD

Two physical types of SIMs are specified. These are the ”ID-1 SIM” and the ”Plug-in SIM”. The logical and electrical interfaces are identical for both types of SIM.

ID-1 SIM

The format and layout of the ID-1 SIM comply with ISO standards for Integrated Circuit (IC) cards (i.e. credit card size).

ID-1 SIM card

Plug-in SIM

The plug-in SIM is smaller than the ID-1 SIM. It is intended for semi-permanent installation in the ME.

Plug-in SIM card

SECURITY FEATURES

GSM defines a number of security features to be supported by SIMs. These are:

• Authentication algorithm, A3

• Subscriber authentication key, Ki

• Ciphering key generation algorithm, A8

• Ciphering key, Kc

• Control of access to data stored and functions performed in SIM

SIM INFORMATION STORAGE REQUIREMENTS

A SIM contains information for GSM network operations. This information can be related to the mobile subscriber, GSM services or PLMN. The data storage requirements of a SIM are divided into two categories: mandatory and optional.

Mandatory storage

A SIM must provide storage capability for the following:

• Administrative information: describes the SIM’s mode of operation, e.g. normal or type approval

• IC card identification:

Unique information identifying the SIM and the card issuer

• SIM service table:

Indicates which optional services are provided by the SIM (e.g. last numbers dialled, call length indication, PLMN selection, etc.)

• International Mobile Subscriber Identity (IMSI):

An identitynumber used by the network to identify the subscription

• Location information:

Comprising LAI, current value of periodic location updating timer and location update status

• Ciphering key (Kc) and ciphering key sequence number

• List of carrier frequencies to be used for cell selection

• Forbidden PLMNs

• Language preference:

Subscriber's preferred languages

The location information, Kc and the Kc sequence number may be updated at each call termination. In addition the SIM must be able to manage and provide storage in accordance with the security requirements:

• Personal Identification Number (PIN)

• PIN enabled / disabled indicator

• PIN error counter

• PIN Unlock Key (PUK)

• PUK error counter

• Subscriber authentication key (Ki)

MOBILE STATION CLASSES

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Different types of MSs have different output power capabilities and therefore different ranges. Hand-held phones generally have a lower output power and consequently a shorter range than a vehicle-mounted phone. The output power varies according to the distance from the BTS. The further away from the source, the weaker the signal will be. According to GSM specifications, MSs are categorized into five classes according to MS output power. These classes are listed in the following table:

MS Power Classes

The location of the MS also affects the received power of the transmitted signal. An MS located at the top of a high building has a greater range than one that is located at or below ground level.

Mobile Stations

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INTRODUCTION

The MS is the equipment used to access the network. The MS consists of two independent parts:

• Subscriber Identity Module (SIM) card

• Mobile Equipment (ME)

A SIM card is an electronic smart card which stores information about the subscription. The ME is the actual telephone terminal.

Mobile station

MOBILE STATION FUNCTIONS

TRANSMISSION AND RECEIPT

As described previously, the transmission and receipt process in an MS includes the steps shown in Figure.

Transmission and receipt in an MS

MEASUREMENTS MADE BY THE MOBILE STATION

The measurements made by an MS are used in making decisions about signal strength and handovers. Measurements are taken in both idle and active mode.

Idle Mode

Cell selection is made at “power on” of the mobile:

1. The mobile scans all radio frequency channels in the GSM system and calculates average levels for each. The mobile tunes to the strongest carrier and determines if it is a BCCH carrier. If so, the mobile reads the BCCH information to find out if the cell is locked (e.g. chosen PLMN, barred cell, etc.). Otherwise the mobile tunes to the second strongest carrier, and so on until a valid BCCH carrier is found.

2. The mobile may optionally include a BCCH carrier memory of valid BCCH carriers in the home PLMN. In this case it only needs to search these carriers. If this ends unsuccessfully, the mobile performs as in 1.

3. If no valid BCCH carrier is found, but a BCCH carrier belonging to another GSM network operator is found, the mobile will display the message “Emergency calls only”. Every emergency call is permitted onto a GSM network, even if the subscriber has not subscribed to the network the MS chooses.

Alternatively, if no valid BCCH carrier is found, and no other network can be found, the MS will display the message “No network”. In this case no calls (including emergency calls) can be made. Once it has tuned to a valid BCCH carrier, the mobile is informed which BCCH carriers it is to monitor for cell re– selection purposes. These are the BCCH carriers in neighboring cells. A list of the strongest carriers is updated regularly by the MS as a result of the measurements.

Active Mode

During a call, the mobile continuously reports (via SACCH) to the system how strong the received signal strength is from BTSs. Both signal strength and quality are measured on the MS’s “own” BTS. These measurements are used by the BSC to make fast decisions about target cells when a handover is required.

SS= Signal strength

BSIC= Base Station Identity code

Measurements on neighboring cells during a call takes place when the mobile is idle, i.e. when it is not transmitting or receiving.

POWER SAVING FUNCTIONS

Discontinuous Transmission (DTX)

Discontinuous Transmission (DTX) is a method of saving battery power for the MS. An MS with the DTX function detects the input "voice" and turns the transmitter ON only while "voice" is present. When there is no voice input, the transmitter is turned OFF. When the MS detects that speech is absent during the conversation, it sends out a signal called “Post” to report a transmission output state OFF for the TCH. Conversely, when the MS detects that speech is present again, it sends out a signal called “Pre” to report the transmission output state ON for the TCH. The Post signal incorporates background noise information from the MS, which enables the TRAU to generate background noise. This ensures that the other subscriber on the call hears something and does not think that the mobile subscriber has ended the call. The MS transmits the Post signal periodically during a speech pause, to enable the BTS to update the background noise.

jhu

Another method used to conserve power at the MS is Discontinuous Reception (DRX). The paging channel, used by the BTS to signal an incoming call, is structured into subchannels. Each MS is assigned one of these sub-channels and needs to listen only to its own sub-channel. In the time between successive paging sub-channels, the mobile can go into “sleep mode”, when almost no power is used.

RADIO BASE STATION (RBS)

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RBS INTRODUCTION

An RBS includes all radio and transmission interface equipment needed on site to provide radio transmission for one or several cells. The RBS 2000 family is Ericsson's second generation of RBS offering products with a low total lifetime cost1. This is achieved by functions including long Mean Time Between Failure (MTBF) performance and short Mean Time To Repair (MTTR). In addition, this product line is quick and easy to install thus giving the possibility to achieve a rapid network roll out. RBS 2000 provides products for both indoor and outdoor installations and is available for GSM 900, GSM 1800 and GSM 1900.

Examples of the RBS 2000 series

RBS FUNCTIONS

RBS functionality can be divided into the following areas:

• Radio resources

• Signal processing

• Signaling link management

• Synchronization

• Local maintenance handling

• Functional supervision and testing

Radio Resources

An RBS's main function is to provide connection with the MSs over the air interface. This includes the following tasks:

• Configuration and system start:

Site configuration involves loading of software from the BSC and setting parameters prior to system startup, including:

− Transmitter and receiver frequencies

− Maximum output power

− Base Station Identity Code (BSIC)

• Radio transmission:

To transmit several frequencies using the same antenna, a combiner or a set of combiners are needed. Transmission power is controlled from the BSC.

• Radio reception:

In addition to reception of traffic on the physical channels, a primary RBS function the detection of channel requests from MSs (e.g. when a call is being made).

The RBS listening for channel requests and

measuring the uplink on an established connection

Signal Processing

An RBS is responsible for the processing of signals before transmission and after reception. This includes:

• Ciphering using the ciphering key

• Channel coding and interleaving

• Adaptive equalization

• Realization of diversity

• Demodulation

Signaling Link Management

An RBS manages the signaling link between the BSC and MS, applying the appropriate protocols to the information being sent.

Synchronization

Timing information is extracted from the PCM-links from the BSC and is sent to a timing module within the RBS. That enable the RBS to synchronize with the correct frequency reference and TDMA frame number.

Local Maintenance Handling

An RBS enables operation and maintenance functions to be carried out locally at the RBS site, without BSC connection. In this way, field technicians can maintain RBS equipment and software on site.

Functional Supervision and Testing

Supervision and testing of RBS functions is supported, using either built-in tests during normal operation or tests executed by command.

RBS 2000 IMPLEMENTATION

All types of RBS within the RBS 2000 series have the following characteristics:

• Support for user flexibility by providing modular hardware and software designs.

• Transceiver oriented design, which stresses using as little common equipment as possible ensuring dependable performance.

• Design and use are aimed at keeping system life cycle costs low.

The RBS 2000 series is based on standardized hardware units called Replaceable Units (RU). The major RU’s are:

• Distribution switch Unit (DXU)

• TRansceiver Unit (TRU)

• Combining and Distribution Unit (CDU)

• Power Supply Unit (PSU)

• Energy Control Unit (ECU)

Replaceable units in RBS 2000

Distribution switch Unit (DXU)

The DXU performs the following tasks:

• Provides an interface to the BSC

• Manages the link resources and connects the traffic time slots from the BSC link to the TRU’s

• Controls signaling to the BSC and performs concentration

• Extracts synchronization information from the link and generates a timing reference for the RBS In addition, the DXU has a database which stores information about installed hardware.

TRansceiver Unit (TRU)

One TRU includes all functionality needed for handling one radio carrier (i.e. the 8 time slots in one TDMA frame). It is responsible for radio transmitting, radio receiving, power amplification and signal processing. The TRU contains a radio frequency test loop between the transmitter and the receiver. This facilitates TRU testing by generating signals and looping them back. TRU’s are connected by a bus to enable frequency hopping. Some RBS products can contain up to 6 TRUs.

Combining and Distribution Unit (CDU)

The CDU is the interface between the TRUs and the 2-way antenna system. The task of the CDU is to combine signals to be transmitted from various transceivers and to distribute received signals to the receivers. All signals are filtered before transmission and after reception using bandpass filters. A range of CDU types have been developed to support different configurations within the RBS 2000 family. They consist of different types of CDUs, including:

• Without combiners

• With hybrid combiners

• With filter combiners to support large configurations

CDUs with duplex filters make it possible to transmit and receive using the same antenna.

Example of a CDU type C

Power Supply Unit (PSU)

The PSU rectifies the power supply voltage to the +24 VDC necessary for RBS operation.

Energy Control Unit (ECU)

The ECU controls and supervises the power equipment and regulates the environmental conditions inside the cabinet. The RBS 2000 is pre-assembled at the factory including program load and parameter settings making a quick startup possible. Assembly can also be carried out on site. The RBS software isdownloaded from the BSC and stored in a non-volatile (flash memory) program store. In a working RBS, this flash memory keeps cell down time low because traffic does not need to be interrupted. Power failure recovery can also be done quickly.

RBS 2000 IN A NETWORK

The Transmission Drop and Insert (TDI) function makes it possible to connect RBS’s together. This is an important cost saving feature of Ericsson’s RBS’s, as an RBS need not be connected to the BSC directly via a dedicated link. Instead it may be more economic to connect that RBS to another RBS in the region, thus saving on expensive transmission costs. The following network topologies are supported:

• Star:

This is the traditional architecture, where each RBS is connected directly to a BSC

• Cascade:

A cascade architecture includes RBS’s connected to each other without a loop, thus using transmission resources efficiently

• Loop:

This architecture includes RBSs connected to each other with a loop, ensuring that even if one link fails, another path is available

Ericsson’s RBS 2000 Network Configurations

RBS 2000 SERIES DESCRIPTIONS

RBS 2000 Series

RBS 2101

RBS 2102

RBS 2103

RBS 2202

RBS 2301. RBS 2302 is almost identical

BSC/TRC

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It is possible to combine the functions of the TRC and BSC in one AXE-based node. The subsystems in a BSC/TRC are the same as those used in a stand-alone BSC.

BSC/TRC hardware configuration

The hardware that is specific to the BSC/TRC is:

• RPG/TRH

• TRAU

BASE STATION CONTROLLER (BSC)

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BSC FUNCTIONS

The BSC controls a major part of the radio network. Its most important task is to ensure the highest possible utilization of the radio resources. The main functional areas of the BSC are:

• Radio Network Management

• RBS Management

• TRC Handling

• Transmission Network Management

• Internal BSC Operation and Maintenance

• Handling of MS connections

Radio Network Management

Radio network management includes the following tasks:

• Administration of radio network data including:

− Cell Description Data (e.g. cell identity, BCCH channel number, maximum and minimum output powers in the cell, RBS type, etc.)

− System information data (e.g. information about whether or not the cell is barred from access, maximum output power allowed in the cell, BCCH channel identities in neighboring cells)

− Locating data (e.g. cell rank used in HCS and high traffic load situations)

− Cell load sharing data, i.e. parameters for forcing early handovers from congested cells

• Traffic and event measurements: (e.g. number of call

attempts, congestion, traffic levels for a cell, traffic levels for an MS, number of handovers, number of dropped connections, etc.).

• Idle channel measurement:

The RBS collects statistics from the MS’s about signal strength and quality. These statistics are then used during the channel allocation process, so that a channel with low interference is allocated for a call.

RBS Management

Ericsson's RBS implementation is transceiver-orientated, ensuring good redundancy features. This means that as little as possible of the equipment is common to several transceivers. This philosophy inevitably leads to a master slave relationship between the BSC and the transceivers in the RBS. A logical model of the RBS is built up within the BSC and RBS equipment can be logically defined, connected and disconnected. The main tasks of RBS management are:

• RBS configuration:

This involves the allocation of frequencies to channel combinations and power levels for each cell according to available equipment. If equipment becomes faulty causing the loss of important channels,n reconfiguration of the remaining equipment is activated, sacrificing less important channels.

• RBS software handling:

This involves the control of program loads.

• RBS equipment maintenance:

RBS faults and disturbances are recorded and logged continuously.

TRC Handling

Although TRAUs are located in a TRC, the BSC, as controller of the radio resources of a GSM network, actually co-ordinates the sourcing of a TRAU for a call. During call set-up, the BSC instructs the TRC to allocate a TRA device to the call. If one is available, the TRC confirms the allocation of a TRA device. The TRA device is considered to be under the control of the BSC for the duration of the call.

Transmission Network Management

The transmission network for a BSC includes the links to and from MSC/VLRs and RBS’s. This involves the following tasks:

• Transmission interface handling:

This provides functions for administration, supervision, test and fault localization of the links to RBS’s. The BSC configures, allocates and supervises the 64 kbits/s circuits of the PCM links to the RBS. It also directly controls a remote switch in the RBS that enables efficient utilization of the 64 kbits/s circuits.

Internal BSC Operation and Maintenance

Operation and maintenance tasks can be performed locally in the BSC or remotely from the OSS. Internal BSC operation and maintenance involves the following features:

• TRH maintenance:

Administration, supervision and test of the TRansceiver Handler (TRH) is carried out in the BSC. The TRH consists of both hardware and software. A TRH is located on a Regional Processor for the Group switch (RPG). One RPG thus serves several transceivers. There can be several RPGs in the BSC.

• Processor load control in the BSC:

This function ensures that during processor overload situations, a large number of calls can still be handled by the BSC. If too many calls are accepted, real time requirements such as call set-up times can not be fulfilled. To prevent this, some calls need to be rejected in situations of high load. Calls already accepted by the system are given full service and are not affected by the overload situation.

Handling of MS Connections

Call Set Up

Call set up involves the following processes:

• Paging:

The BSC sends paging messages to the RBS’s defined within the desired LA. The load situation in the BSC is checked before the paging command is sent to the RBS.

• Signaling set-up:

During call set-up, the MS connection is transferred to an SDCCH allocated by the BSC. If the MS initiated the connection, the BSC checks its processor load before the request is further processed.

• Assignment of traffic channel:

After SDCCH assignment, the call set-up procedure continues with the assignment of a TCH by the BSC. As this takes place, the radio channel supervision functions in the BSC are informed that the MS has been ordered to change channels. If all TCHs in the cell are occupied an attempt can be made to utilize a TCH in a neighboring cell.

During a Call

The main BSC functions during a call are:

• Dynamic power control in MS and RBS:

The BSC calculates adequate MS and BTS output power based on the received measurements of the uplink and downlink. This is sent to the BTS and the MS every 480 ms to maintain good connection quality.

• Locating:

This function continuously evaluates the radio connection to the MS, and, if necessary, suggests a handover to another cell. This suggestion includes a list of handover candidate cells. The decision is based on measurement results from the MS and BTS. The locating process is beingb executed in the BSC.

• Handover:

If the locating function proposes that a handover take place, the BSC then decides which cell to handover to and begins the handover process. If the cell belongs to another BSC, the MSC/VLR must be involved in the handover. However, in a handover, the MSC/VLR is controlled by the BSC. No decision making is performed in the MSC because it has no real time information about the connection.

• Frequency Hopping:

Two types of hopping are supported by the BSC:

− Baseband hopping: this involves hopping between frequencies on different transceivers in a cell

− Synthesizer hopping: this involves hopping from frequency to frequency on the same transceiver in a cell

BSC IMPLEMENTATION

The BSC is implemented on a non-AM-based AXE platform consisting of standard APZ and APT subsystems and the following APT subsystems:

BSC Subsystems

BSC hardware configuration

• Regional Processor for the Group switch (RPG)/TRansceiver Handler (TRH)

TRANSCODER CONTROLLER (TRC)

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TRC FUNCTIONS

The primary functions of a TRC are to perform transcoding and to perform rate adaptation.

Transcoding

As previously explained, the function of converting from the PCM coder information (following A/D conversion) to the GSM speech coder information is called transcoding. This function is present in both the MS and the BSS.

Rate Adaptation

Rate adaptation involves the conversion of information arriving from the MSC/VLR at a rate of 64 kbits/s to a rate of 16 kbits/s, or transmission to a BSC (for a full rate call). These 16 kbits/s contains 13 kbits/s of traffic and 3 kbits/s of inband signaling information. This is an important function. Without rate adaptation the links to BSC’ would require four times the data rate capabilities. Such transmission capabilities form an expensive part of the network. By reducing the rate to 16 kbits/s, it is possible to use one quarter of the transmission links and equipment. In some GSM systems, the TRC contains units, which perform transcoding and rate adaptation. These hardware units are called Transcoder and Rate Adaptation Units (TRAUs). All TRAUs are pooled, meaning that any BSC connected to the TRC can request the use of one of the TRAUs for a particular call. The TRC also supports discontinuous transmission. If pauses in speech are detected, comfort noise is generated by the TRAU in the direction of the MSC/VLR.

TRC IMPLEMENTATION

The TRC is implemented on the AXE platform consisting of standard APZ and APT subsystems and the following APT subsystems:

BSC Subsystems

TRC hardware configuration

The hardware that is specific to the TRC is:

• Transcoder and Rate Adapter Unit (TRAU)

Base Station System (BSS)

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INTRODUCTION

The Base Station System (BSS) is responsible for all the radiorelated functions in the system, such as:

• Radio communication with the mobile units

• Handover of calls in progress between cells

• Management of all radio network resources and cell configuration data.

BSS consists of three components:

• Base Station Controller (BSC):

-The BSC is the central node within a BSS and co-ordinates the actions of TRC’s and RBS’s.

• Transcoder Controller (TRC):

-The TRC provides the BSS with rate adaptation capabilities. This is necessary because the rate used over the air interface and that used by MSC/VLRs are different - 33.8 kbits/s and 64 kbits/s respectively. A device, which performs rate adaptation is called a transcoder.

• Radio Base Station (RBS):

An RBS acts as the interface between MS’s and the network, by providing radio coverage functions from their antennae.

BSS in GSM systems

BASE STATION CONTROLLER AND TRANSCODER CONTROLLER

There two main options available for implementing the TRC and BSC in Ericsson’s BSS:

• BSC/TRC:

A combined BSC and TRC on the same AXE. This is suitable for medium and high capacity applications, e.g. urban and suburban area networks. The node can handle up to 1,020 transceivers (TRXs). 15 remote BSC’s can be supported from one BSC/TRC.

• Stand-alone BSC and stand-alone TRC:

The stand-alone BSC (without transcoders) is optimized for low and medium capacity applications and is a complement to the BSC/TRC, especially in rural and suburban areas. It caters for up to 300 TRX’s. The stand-alone TRC is located at the MSC/VLR to increase transmission efficiency. A stand-alone TRC can support 16 remote BSCs.

TRC utilization and transmission rates in BSS

RADIO BASE STATIONS

Radio Base Station (RBS) 2000 series of base stations implements the GSM-defined BTS. This includes the following RBS’s:

• RBS 2101

• RBS 2102

• RBS 2103

• RBS 2202

• RBS 2301

• RBS 2302

• RBS 2302 MAXITE

• RBS 2401

OTHER ACCESS NETWORK EQUIPMENT

Many mobile networks include additional equipment to provide improved coverage. Examples of such equipment includes:

• Repeaters:

These are placed in locations throughout the access network to repeat the digital signal from the MS and BTS. This helps to reduce the BER and thus provide better quality calls to subscribers. A typical location for a repeater may be on top of a building

• Leaky cable:

This is simply a cable carrying the electromagnetic energy,which has “holes” in it to leak out this radio signal at regular intervals. This may be suitable in areas,which are difficult to cover using traditional base station equipment. For example, a leaky cable could be used to provide coverage within an underground train system

Switching System

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INTRODUCTION

The Switching System in GSM systems contains the following components:

Switching System

Switching System components

MSC FUNCTIONS

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The primary node in a GSM network is the MSC. It is the node, which controls calls both to MS’s and from MS’s. The primary functions of an MSC include the following:

• Switching and call routing:

A MSC controls call set-up, supervision and release and may interact with other nodes to successfully establish a call. This includes routing of calls from MS’s to other networks such as a PSTN.

• Charging:

An MSC contains functions for charging mobile calls and information about the particular charge rates to apply to a call at any given time or for a given destination. During a call it records this information and stores it after the call, e.g. for output to a billing center.

• Service provisioning:

Supplementary services are provided and managed by a MSC. In addition, the SMS service is handled by MSC’s.

• Communication with HLR:

The primary occasion on which an MSC and HLR communicate is during the set-up of a call to an MS, when the HLR requests some routing information from the MSC1.

• Communication with the VLR:

Associated with each MSC is a VLR, with which it communicates for subscription information, especially during call set-up and release.

• Communication with other MSC’s:

It may be necessary for two MSC’s to communicate with each other during call setup or handovers between cells belonging to different MSC’s.

• Control of connected BSC’s:

As the BSS acts as the interface between the MS’s and the SS, the MSC has the function of controlling the primary BSS node: the BSC. Each MSC may control many BSC’s, depending on the volume of traffic in a particular MSC service area. An MSC may communicate with its BSC’s during, for example, call set-up and handovers between two BSC’s.

• Direct access to Internet services:

Traditionally, an MSC accessed the Internet nodes of an Internet Service Provider (ISP) via existing networks such as the PSTN. However, this function enables an MSC to communicate directly with Internet nodes, thus reducing call set-up time. Direct access can be provided by using an access server called Tigris (from Advanced Computer Communications). This may be integrated in an MSC or stand-alone connected to an MSC.

Internet access via GSM/PSTN (traditional method)

Direct access to Internet

• ISDN Primary Rate Access (PRA):

This function enables an MSC to provide PRA services to subscribers. One network operator can offer PABX connection services, through the PLMN. In this way the operator can compete directly with PSTN operators for ISDN business subscribers.

BSC/TRC

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It is possible to combine the functions of the TRC and BSC in one AXE-based node. The subsystems in a BSC/TRC are the same as those used in a stand-alone BSC.

BSC/TRC hardware configuration

The hardware that is specific to the BSC/TRC is:

• RPG/TRH

• TRAU

SERVICE SWITCHING FUNCTION (SSF), SERVICE CONTROL FUNCTION (SCF) AND SERVICE DATA POINT (SDP)

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This section gives a brief introduction to the Mobile Intelligent Network (MIN) handling capabilities of GSM systems. Mobile Intelligent Network (MIN) nodes can be added to a basic GSM network to provide value-added services such as Freephone and Personal Number to subscribers. GSM MIN nodes include:

• Service Switching Function (SSF):

An SSP acts as an interface between the call control functions of the mobile network and the service control functions of a Service Control Point (SCP). Ericsson's SSP is an AXE-based AM (SSFAM) and may be integrated within an MSC/VLR (recommended) or stand-alone.

• Service Control Function (SCF):

A SCP contains the intelligence of a MIN service or services. This intelligence is realized in software programs and data. SCP is also an AXE-based AM (SCFAM) and the recommended configuration is as a stand-alone node, accessible by all MSC/SSPs.

• Service Data Point (SDP):

An SDP manages the data which is used by a MIN service.SDP is a stand-alone node based on Unix.

MESSAGE CENTER (MC)

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MC FUNCTIONS

An MC may be added to a GSM network to provide one or more of the following messaging services:

• Voice mail

• Fax mail

• Short Message Service (SMS) text messages

• SMS Cell Broadcast (SMSCB) text messages

These services can generate considerable revenue for a network operator, as they are becoming increasingly popular.

Voice Mail

Voice mail ensures that all calls to a person can be completed, even when a person does not answer calls. A calling party can record a voice message for the subscriber they are calling. A subscriber can use their MS to select diversion to voice mail based on a particular event or status (e.g. busy, unreachable). The subscriber is informed that they have voice messages in their mailbox by means of either a short text message or phone call from the network at regular intervals. If their MS is detached, this indication is sent when the subscriber next attaches to the network. The subscriber can then retrieve their voice mail messages at a later stage. Functions for storing voice messages over a long period also exist.

Fax Mail

Fax mail operates similarly to voice mail. For MS’s that support fax, a subscriber can set diversion for all or some fax calls to a fax mailbox. When the MS is next attached to the network, the network will deliver the fax message to a fax machine identified by the MS.

SMS

A short text message consists of up 160 alphanumeric characters, entered at a Short Message Entity (SME) such as an MS (using the keypad) or computer terminal. A short message always originates or terminates in a GSM network, meaning that a short message can not be sent between two SMEs residing outside a GSM network. The short message originator knows if the message delivery is successful or unsuccessful via notification. When a message is submitted, the deferred delivery option can be requested. This option makes it possible to specify the time the message is to be delivered. An MC, which handles SMS messages is often referred to as an SMS Center (SMS-C). When a message is to be forwarded to an MS, the system must first determine where the MS is situated. As in ordinary voice traffic, a gateway requests the routing information. The gateway is called the SMS GMSC. Each short message is time stamped by the SMS-Center when it is submitted. A message is deleted once the delivery is successful or once the time specified in deferred delivery expires. When a message is buffered, the SMS-C regularly attempts to deliver the message, at intervals defined by the operator.

SMSCB

The SMSCB service enables a message of up to 93 alphanumeric characters to be delivered to all attached MS’s in one cell. This may be useful for identifying key phone numbers in the cell’s area such as that of a hospital or police station. Alternatively, it may be used for advertising services within the cell (e.g. “Superfood Restaurant in this area at the junction of M8 and I33”).

MC IMPLEMENTATION

One MC node may handle one or more messaging service. For example, depending on the amount of SMS traffic, it may be more efficient to have one MC acting as an SMS-C only, with other messaging services handled by another separate MC. It is also possible to integrate SMS-C functions on an MSC, leading to the term SMS InterWorking MSC (SMS-IWMSC). Additionally, the SMS GMSC functions may reside in the same node as the GMSC functions used for voice calls. In some GSM systems the MC is implemented by MXE product. The most important component of MXE is the message kernel. The message kernel is the central message store and forward nucleus responsible for safe storage of messages, routing and retry attempts.

SMS-C on an MXE

DATA TRANSMISSION INTERFACE (DTI)

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This section gives a brief introduction to the data handling capabilities of GSM systems. For a more detailed survey of such functions, please refer to the appendix titled “Data Services”.

DTI FUNCTIONS

The DTI implements the GSM Inter-Working Function (IWF). It performs data handling functions such as data rate conversion and provides the functions necessary for data interworking between GSM networks and other networks, including:

• Data Traffic to/from PSTN:

This involves modem and fax calls. For connections to the PSTN a modem is selected by the DTI to perform the necessary rate and format conversions.

• Data Traffic to/from ISDN:

The whole set of data communications towards ISDN is available, since the MSC/DTI is capable of signaling and mapping basic service information between the ISDN and the GSM network.

• Data Traffic to/from PDNs:

The DTI handles data traffic to and from Public Data Networks (PDNs) such as the Packet Switched PDN (PSPDN) and Circuit Switched PDN (CSPDN).

• Data Traffic between mobiles:

The data traffic inside the PLMN must pass through the DTI to handle the protocol used for rate adaptation in the radio path.

• HSCSD:

This version of High Speed Circuit Switched Data (HSCSD) allows the connection of 2, 3, or 4 time slots on one radio channel each carrying 9.6 kbits/s. The DTI handles rate conversion to PSTN or ISDN as appropriate.

DTI IMPLEMENTATION

The DTI is integrated within an MSC/VLR. The DTI is managed by the Data Transmission Subsystem (DTS). The DTI sub-rack contains eight plug-in units, each one supporting four data channels. Therefore, each DTI sub-rack can support a total of 32 simultaneous data calls.