Telecom Transmission Made Simple

Test Equipment used in Optical fiber maintenance

noreply@blogger.com (hi) — Sun, 09 Nov 2014 04:22:00 +0000

Optical Power Meter (OPM)

•OPM measure the average optical power out of an optical fiber.
•Power meters are calibrated at the typical wavelengths used in fiber optics, 850, 1300 and 1550 nm.
•Some meter uses Auto wavelengths Recognition feature.

Optical Light Source(OLS)

•OLS along with Optical Power Meter is used to make measurements of optical loss or attenuation in fibers and connectors.
•It should be compatible with the type of fiber in use (singlemode or multimode with the proper core diameter) and the wavelength.
•Sources may be either LED's or LASERS.
•Sometimes optical power meters are combined with an Optical Light Source (OLS) or Visual Fault Locator (VFL).

OTDR

•Optical Time Domain Reflectometer (OTDR) is used for estimating the fiber’s length and overall attenuation, including splice and mated-connector losses and to locate faults, such as breaks.
•OTDR functions by injecting a series of optical pulses into the fiber under test, using LASER.
•It requires access to only one end of the fiber.

Fiber Trace at OTDR

Optical Spectrum Analyzer (OSA)

•Optical signal monitoring on live channels.

•Able to analyze

–Optical Power

–Optical wavelength

–OSNR of each wavelength

–Spectral width of optical source

Visual Fault Locator

•Visual Fault Locator is used to inject the visible light to visually trace fiber.

•It covers the range where OTDRs are not useful.

•This is used for buffered fiber and even jacketed single fiber cable if the jacket is not opaque to the visible light.

Dispersion in Optical Fiber / Notes on Dispersion

noreply@blogger.com (hi) — Mon, 03 Nov 2014 08:19:00 +0000

Dispersion in Optical Fiber

Various wavelengths of the signal have different propagation velocities. Dispersion is the spreading out of a light pulse in time as it propagates down the fiber. Dispersion limits the information carrying capacity of a fiber. It is measured as ps/nm.km

Types of Dispersion

–Intermodal dispersion
–Material dispersion
–Waveguide dispersion.
( Also read about losses in optical fiber )

Intermodal Dispersion:

•Each mode enters the fiber at a different angle, take different path, travel different distance and arrive at different time at the fiber output.
•Multimode fibers have many different light modes since they have much larger core size.
•The light pulse spreads out in time which can cause signal overlapping.

Material Dispersion

•Dependence of refractive index of the material on wavelength.
•The velocity of light through a fiber depends on its wavelength.
•Different wavelengths travel at different velocity due to refractive index, hence the different propagation causes material dispersion.
•Material dispersion at 1300nm for silica is zero.

Waveguide Dispersion

•Distribution of light between core and cladding is a function of the wavelength travelling in waveguide.
•Light ray that travels in the cladding travels faster than that in the core causes waveguide dispersion.
•Single mode fiber suffers from waveguide dispersion as it has small core diameter

Chromatic Dispersion

•Combined effect of Material and Waveguide Dispersion

Polarization Mode Dispersion (PMD)

•Asymmetry, due to Mechanical and thermal stresses during fiber manufacturing introduces small index of refraction differences for the two polarization states.
•Polarization Mode Dispersion (PMD) a form of modal dispersion, causes broadening of the input pulse due to a phase delay between input polarization states.

Effect of Dispersion

•Spreading of pulses which can cause Inter Symbol Interference ‘ISI.
•Detection of individual pulse is not easy at receiver.
•Poor BER performance.
•Limit the communication distance.
•Limit the transmission rate.

Losses in Optical Fiber

noreply@blogger.com (hi) — Wed, 29 Oct 2014 11:48:00 +0000

Signal Degradation in Optical Fiber :

Attenuation and dispersion determine the maximum distance an optical signal can be transmitted before the receiver is unable to detect it.
The attenuation and dispersion of a fiber are wavelength dependant.

Transmission Losses in Fibers:

Transmission loss or attenuation of the signal in an optical fiber is measured in dB/km.

Types of Losses

–Material Absorption

–Rayleigh scattering

–Waveguide imperfections

–Radiative loss

Material Absorption

•Intrinsic Absorption:

–Intrinsic absorption losses correspond to absorption by fused silica.

–intrinsic material absorption for silica in the wavelength range 0.8~1.6um is below 0.1dB/km.

•Extrinsic Absorption:

–Extrinsic absorption is related to losses caused by impurities within silica.

–The main source of extrinsic absorption silica fibers is the presence of water vapors.

Rayleigh Scattering

•Silica molecules move randomly in the molten state and freeze in place during fiber fabrication.

•Density fluctuation lead to random fluctuations of the refractive index.

•Light scattering in such a medium is known as Rayleigh scattering.

Waveguide Imperfections

•Imperfections at the core-cladding interface, such as random core-radius variations, can lead to losses.

•This has been taken good care of in optical fiber manufacturing and the core radius is made sure not to vary significantly along the fiber length.

Radiative Losses

•Radiative losses occur whenever an optical fiber undergoes a bend of finite radius of curvature.

•Fibers can be subjected to two types of bends.

–Macroscopic bending:

Bends having diameter smaller than the specified by manufacturer e.g. when a fiber cable turns a corner.

–Microscopic bending:

Random microscopic bends of the fiber axis that can arise when the fibers are incorporated into cables, e.g. deformation of axis.

Gap- Loss :

•Gap-loss happens when there is a space, breakage, between fiber connection.

•Light can cross this gap, but spreads out and is weakened and diffused

Types of Fiber

noreply@blogger.com (hi) — Mon, 20 Oct 2014 11:30:00 +0000

Optical fiber is classified based on material used, mode & refractive index.

Step Index Optical Fiber:

In the step index fiber, the refractive index of the core is uniform throughout.
Different light modes in a step-index multimode fiber follow different lengths along the fiber in zigzag way
The arrival of different modes of the light at different times causes Inter Modal Dispersion.
50 mm core for step index.

Graded Index Optical Fiber:

Graded-index fiber’s refractive index decreases gradually away from its center.
Light rays follow sinusoidal paths.
Reduces inter modal dispersion.
62.5 mm core for graded index.

Single Mode Fiber:

In a single mode fiber only one mode can propagate through the fiber.
Core of SM fiber is smaller (8.3 to 10 microns). Common size is 9um.
Laser is used as light source for single mode fiber.
Used in long distance communication upto100 Gbit/s data in DWDM.
1310 or 1550 nm wavelength is used in SM fiber with higher data rate.

Multimode Fiber:

Multimode fibers allow a large number of modes.
A multimode optical fiber has larger core (50 to 100um). Most common size is 50um or 62.5um.
DWDM is not normally used on multi-mode fiber.
850 or 1300 nm wavelength is used in multimode fiber.
LED is used as a light source in multimode fibers .

Practically used Optical Fibers:

MM Step index: 62.5/125 µm
MM Graded index: 50/125 µm
SM Step index: 9/125 µm
Single mode graded index is not practically used

Fiber Design

noreply@blogger.com (hi) — Fri, 17 Oct 2014 11:45:00 +0000

Construction of Fiber

An optical fiber consists of a core, cladding and buffer (a protective outer coating). The cladding guides the light along the core by using the method of total internal reflection. In the glass fiber core and cladding are made of high quality silica glass.

Principle of Optical Communication

Optical fiber communication works on the principle of Total Internal Reflection.

Total Internal Reflection

– For all angles of incidence greater than the critical angle, the incident ray will get reflected back into the denser medium itself. This phenomenon is called total internal reflection.

Wavelengths in Optical Fiber

Normally we use light wavelengths around 850, 1310 and 1550 nm for fiber optics with glass fibers. These wavelengths are falling in the infrared region and having less attenuation.

Optical Fiber as Telecommunication Medium

noreply@blogger.com (hi) — Tue, 14 Oct 2014 11:13:00 +0000

Physical path between transmitter and receiver is called transmission medium, it may be wireline or wireless.

Wireless (unguided) media

–Microwave radio.

–Communication satellites

•Wireline (guided) medium

–Copper wires

–Fiber-optic cables.

Fiber Medium Vs Microwave medium

Optical Fiber Communication

Transmitting information from one place to another by sending pulses of light (wavelength) through an optical fiber. Modulation used: Light Intensity Modulation

Advantages of Optical Fibers

Very high information carrying capacity
Less attenuation (order of 0.2 db/km)
Small in diameter and size & light weight
Greater safety and immune to EMI & RFI, moisture & corrosion
It is dielectric in nature so can be laid in electrically sensitive surroundings
Difficult to tap fibers, so secure
No cross talk and disturbances

Disadvantages of Optical Fibers

The terminating equipment is still costly as compared to copper equipment.
It is delicate so has to be handled carefully
High Cost
Last mile is still not totally fiberised due to costly subscriber premises equipment
Optical fiber splicing is a specialized technique and needs expertly trained manpower.
The splicing and testing equipments are very expensive as compared to copper equipments.

Applications of Optical Fiber

Long distance communication: Backbones
Broadband services
Computer data communication (LAN, WAN etc..)
Medical Industry
Military application

Network Management Basics

noreply@blogger.com (hi) — Fri, 12 Apr 2013 03:54:00 +0000

Operational Tasks:

Following basic operational tasks are performed by network management system:

Protection :

Protection switching takes place within milliseconds ( sub 50 ms) & hence Circuit recovery in milliseconds ( failure should not be detected by voice customers)

Restoration:

By doing manual configuration, circuit recovery achieved in seconds or

Provisioning:

Allocation of capacity to preferred routes (according to certain time schedules)

Consolidation:

Moving traffic from unfilled bearers onto fewer bearers to reduce waste trunk capacity

Grooming:

Sorting of different traffic types from mixed payloads into separate destinations for each type of traffic.

OAM Functions and Layers

Level 1 - Regenerator Section: Loss of synchronization, signal quality degradation

Level 2 - Multiplex Section : Loss of frame synchronization, degraded error performance

Level 3 – Path : Assembly and disassembly, cell delineation control.

Data Communication Channel (DCC)

DCC is a in-band channel to facilitate communication between all Network Elements (NE) in a network. This facilitates remote login, alarms reporting, software download, provisioning

Understanding Synchronization Protection Basics in Transmission network

noreply@blogger.com (hi) — Mon, 08 Apr 2013 09:44:00 +0000

Let us consider a ring network. Normal synchronization works around a ring. In this case, Nodes B-F are line timed. Node A is timed to an external reference. When a sync source is failed, new time source should be selected in a reasonable amount of time

If synchronization is not restored , BER will be increasing through time.

SPS Timing Loops (SPS = Synchronization Protection Switching):

During a ring failure, simple reference switching would result in timing loops as shown below.

Operations – Normal Flow :

Let us read about the operation. Following diagram shows the normal flow of operation. Synchronization messaging in normal operation.

S1 = Stratum 1 Traceable

DU = Don’t Use

HO = Holdover

Operations – Fiber Cut:

In the ring, if fiber cuts between B & C, Node C goes into short term holdover as shown below.

Then, Node F switches to timing from Node A as shown below.

Finally ring is reconfigured and all nodes are again synchronized to BITS as shown below.

Please let me know if any clarification required :)

Reference Clocks & alternative clock source in SDH network.

noreply@blogger.com (hi) — Sat, 06 Apr 2013 13:45:00 +0000

Reference Clocks:

Let us read about reference clocks. Precision of internal clock is classified into so called “Stratum” levels. Accuracy of reference clock is defined as the ratio of bit slip happening (causing a bit error)

Stratum 1 => 1 x 10^-11 (synchronization to atomic clock)

Stratum 2 => 1.6 x 10^-9

Stratum 3E => 1 x 10^-6

Stratum 3 => 4.6 x 10^-6

Stratum 4 => 32 x 10^-6 (typical for IP routers)

When we are distributing the clock in the network, accuracy level might decrease at each hop in clock distribution. Originally providing Stratum 1 clocks for each network element was far from being economical, even providing this service at multiple locations was too much demanding. So clock distribution methods were developed to minimize the number of high accuracy clocks needed in the network.

Global Positioning System (GPS) includes Stratum 1 atomic clocks on the satellites. Cheap GPS receivers are available in the market and they make it possible to have a Stratum 1 time source at almost any place. This reduces the need for time synchronization network (might even go away in the future…).

Clock Distribution Methods :

Various clock distribution methods are as described below.

When all equipment is at the same location, External clock input might be used. This is usually BITS = Building Integrated Timing Signal. It uses an empty T1 or E1 framing to embed clock signal. Might be provided as a dedicated bus reaching into each rack in a CO environment. BITS should be generated from a Stratum 1 clock. Typically it will be deployed with a hot spare alternative source for fail-over.

Network elements not close to a BITS source should recover clock from the line. While distributing the clock, Clock distribution network should not have loops, so a tree distribution topology should be configured. Usually carrier network element will have Stratum 3 accuracy when running free. By synchronization to the reference clock, this clock is running at the same rate as the reference clock (that is Stratum 1). Minimum requirement for any network element is 20 ppm (that is between Stratum 3 and Stratum 4).

Alternative Clock Sources:

If the trail to the reference clock source is lost, the network element still continues normal operation. However, alarm might be generated. After some time the clock might drift away so much, that bit errors would occur. Some time is left for switching over to an alternative clock source. Then the network element gets into a holdover state. Requirement is to have less than 255 errors in 24 hours.

A hierarchy of potential clock sources should be configured at each network element to achieve a high-availability operation. Typically a maximum 3 alternative time reference sources might be configured. This is meaningful only if there are different paths to the alternative time reference sources. If only one natural path exists to a single time reference source, then the path must be protected by automatic protection switching. This requires some extra signaling to do it properly, called SPS = Synchronization Protection Switching.

Synchronization requirements & modes of timing in SDH transmission networks

noreply@blogger.com (hi) — Thu, 04 Apr 2013 13:12:00 +0000

For synchronization of a transmission network, Frequency variation of bits transmitted should be inside the limits determined by the next hop’s ability to transmit these bits further. Stuffing allows for some limited tolerance. In order to guarantee a low level of BER Frequencies should be synchronized all over the network. Usually Synchronization is done by recovering the embedded clock signal from the input signal . Synchronization source should have a very precise clock (reference clock). Reference clock might be reached only by multiple hops, but number of hops should be minimized.

Synchronization modes for transmission networks:

In a transmission network, Each network element has to be configured for time synchronization. Time reference distribution should minimize delay.

Various timing alternatives available are:

– External

– Line

– Loop

– Through.

Let us see the details .

External timing:

In this mode, all signals transmitted from a node are synchronized to an external source received by that node; i.e. BITS timing source.

Line Timing :

In this mode, All transmitted signals from a node are synchronized to one received signal.

Loop timing:

In this mode, the transmit signal in a optical link, east or west, is synchronized to the received signal from the same optical link.

Through timing :

In this mode, the transmit signal in one direction of transmission around the ring is synchronized to the received signal from that same direction of transmission.

Detailed operation of BLSR & Squelching.

noreply@blogger.com (hi) — Wed, 03 Apr 2013 13:37:00 +0000

Operation – Traffic flow :

Bi-directional traffic between two nodes is transported over a subset of the "ring sections" or "spans". In this configuration, Minimum capacity equals line rate. Capacity is in general expressed as number of AU4, or bandwidth. The bandwidth is provided by an integer number of AU4 payload.

Maximum bandwidth capacity :

Here, each span has, in each direction, a capacity of up to half the number of AU4 in the STM-N (i.e. 8 AU4 for an STM-16 section). All traffic from a node goes to adjacent nodes.

Max. capacity = 0.5 (line rate) x number of nodes.

Note: This Max is achieved only of the working traffic is transported only between two adjacent nodes.

Extra Traffic:

We can utilize shared protection bandwidth for Extra traffic. This extra traffic is not protected & it could be lost when a failure of working traffic occurs

Operations – Fiber Cut :

Let us consider a scenario, where fiber cuts between A&B. We have a working traffic from A-C and C-A. This failure interrupts A-C and C-A traffic . Now Node A and Node B detect failure

Now node A and node B will switch the traffic to protection path. No dedicated protection bandwidth - only used when protection required. Only nodes next to the failure know about the protection switch. No traffic lost.

Operations – Node Failure:

Let us consider that we have live traffic from D-F and F-D. If node B fails, Failure interrupts D-F and F-D traffic. Node A and C detect failure

Now Both node A & C switch the traffic to protection channels. Only nodes next to the failure know about the protection switch. In this scenario, only Traffic to/from failed node lost.

Squelching Problem :

When a node fails, traffic terminating on those nodes cut off by failures could be misconnected to other nodes on the ring in case of using a local fail-over decision .

Consider a scenario, where we have active traffic from Node F-B , B-F and E-B, B-E. If Node B fails,

Squelching misconnection occur : Node F now talking to Node E instead of Node B

This can be avoided by path AIS Insertion. STM Path AIS is inserted instead of the looped STM-1#7. No mis-connections

Squelching Summary :

Squelching is in general used when extra traffic is used, it is used when normal traffic is switched to the protection entity and replaces the extra traffic. Squelching prevents that in case protection switch is active the normal traffic is output instead of the original extra traffic by outputting AU-AIS. You can also read clause 7.2.3.2 of ITU-T G.841

Squelching is required to assure that misconnections are not made. It is required for bidirectional line switched rings only, since it is the only ring to provide a reuse capability of STM-1s around the ring. This is only required when nodes are cut off from the ring. Also this is only required for traffic terminating on the cut off nodes.

A ring map that includes all STM and VC Paths on the ring is available at every node on the ring. Squelching is also required for extra traffic since the extra traffic may be dropped when a protection switch is required.

Scrambling SDH signal, why scrambler is used in SDH?

noreply@blogger.com (hi) — Sat, 30 Mar 2013 14:51:00 +0000

In SDH/SONET system, receivers recover clock based on incoming signal. Insufficient number of 0-1 transitions causes degradation of clock performance. In order to avoid this problem and to guarantee sufficient transitions, SONET/SDH employ a scrambler.

All data except first row of section overhead is scrambled . Scrambler is 7 bit self-synchronizing X⁷ + X⁶ + 1 . Scrambler is initialized with ones

This type of short scrambler is sufficient for voice data. But this is not sufficient for data which may contain long stretches of zeros. So, while sending data an additional payload scrambler is used.

This modern standards use 43 bit X⁴³ + 1. It run continuously on ATM payload bytes (suspended for 5 bytes of cell tax) . Run continuously on HDLC payloads

Scrambler :

Types of switching in SDH Rings.

noreply@blogger.com (hi) — Thu, 28 Mar 2013 13:40:00 +0000

SPAN SWITCHING & RING SWITCHING

Span switching :

This type of switching uses only protection fibers on the span where fault is detected.

Ring switching :

In this type of switching, traffic is switched away from filed span to adjacent node via the protection fibers on the long path.

REVERTIVE & NON-REVERTIVE SWITCHING

We can implement two modes of protection switching in SDH networks, revertive or non-revertive.

In revertive switching, once the fault condition has cleared, the network enters a "wait to restore state. One the configured WTR time is elapsed, traffic will be switched back to main path. This will be useful, wjen main path is much shorter than protection path.

In non-revertive mode, even after clearance of fault condition, traffic will not switch back to main path automatically.

How Protection Switching is implemented in SDH?

noreply@blogger.com (hi) — Wed, 27 Mar 2013 14:33:00 +0000

For protection switching, mainly K1, K2 bytes and B2 bytes in Multiplex Section Overhead of SDH frame are used. Normally K bytes carried in protection fiber are used to carry APS protocol. B2 bytes contain bit interleaved parity check of the previously transmitted MSOH plus the VC-n payload.

K1/K2 Byte strycture:

K1/K2 byte structure is as shown in above diagram. Upto maximum 16 nodes can be supported in a SDH ring with protection. This is because, only 4 bytes are used for source and destination ID. In 4F-rings, APS protocol is only active on the protection fibers. APS protocol is optimised for AU level of operation. Each node in the ring should be configured with a ring map. This ring map contains information about the channels that node handles. Also, Each node in the ring is given a unique Id number within the range 0 to 15.

At any point of time, each node will be knowing the current status of the ring ( normal or protected). When the protection switches are not active, each node sends K-bytes in each direction indicating "no bridge request". At the time of failure in the ring between two adjacent nodes, two paths may exist for communication. Short path is the one, which directly connects both the nodes. Longer path connects these two nodes via all other nodes on the ring. When a node receives a non-idle K-byte message containing a destination ID of another node on the ring, that node will change to pass through mode.

Let us read about types of ring protection in next post

BLSR,Bi-directional Line Switched ring

noreply@blogger.com (hi) — Tue, 26 Mar 2013 14:30:00 +0000

There are two types of BLSR deployed in various networks.
i. 2-fiber BLSR
ii. 4-fiber BLSR

2-fiber BLSR:

This system is also known as two fiber multiplex-section shared protection ring. Here, service traffic flows bi-directionally. Both the fibers carries service and protection channels.

When the protection channels are not required, they can be used to carry extra traffic, but at the time of protection switching, this extra traffic is dropped. Only ring switching is supported by this architecture. At the time of ring switching, those channels carrying service traffic are switched to the channels that carry the protection traffic in the opposite direction.

4-Fiber BLSR:

This system is also known as four-fiber multiplex-section shared protection ring. This is the most robust ring architecture. This is most expensive to implementbecause of the extra optical hardware required.

In this system, bi-directional pairs of fibers are used to connect each span in the ring. One bi-directional pair carries the working channels, while the other pair carries protection channels. 4F-BLSR supports both span switching and ring switching. ( but both not at the same time). Multiple span switches can coexist on the ring. This is because, only the protection channels along one span are used for each span switch.

What triggers a protection?

Protection switching is triggered in following cases.

1. Signal Fail , detected as Loss of Signal (LOS) at receiver input. This may be due to faulty hardware in the upstream network equipment or due to broken fiber.

2.Signal degrade, this is monitored by monitoring B2 bytes.

In next post let us read about : How is protection switching implemented in SDH ( K1 & K2 Bytes)

Ring networks - G.841 - Interview notes for UPSR

noreply@blogger.com (hi) — Mon, 25 Mar 2013 17:32:00 +0000

Further to Linear protection, let us read about ring protection. ITU-T recommendation covers several types of ring network architectures. Ring protection switching can be implemented at path level or at line level. Rings can be uni-directional or bi-directional. & they may utilise 2-fiber or 4-fibers.

UPSR : Uni-directional Path Switched Ring :

In a uni-directional ring, service traffic flows in one direction. ( clockwise in below diagram). Protection traffic flows in opposite direction (counter clockwise)

In this example, traffic from C to B travels in clockwise direction via A. Traffic from B to C travels directly in clockwise direction. This configuration is also known as multiplex section dedicated protection ring. This is because, one fiber carries service traffic, while the other is dedicated to protect the main path.All traffic is added in both directions. Decision as to which to use at drop point (no signaling). Normally non-revertive, so effectively two diversity paths

Main advantage of this configurations are :

single ended switching, simple to implement and does not require any protocol. Single ended switching is always faster while compared to dual ended switching. Chances of restoring traffic under multiple fail conditions is high. Also, implementation of this architecture is least expensive.

However this arcitecture is Inefficient for core networks. There is no spatial reuse. Node needs to continuously monitor every tributary to be dropped.

In next post let us read about BLSR - Bi-directional line switched ring

Traffic Protection on SDH Optical Networks, Interview notes for SDH protection

noreply@blogger.com (hi) — Sun, 24 Mar 2013 07:07:00 +0000

Service survivability has become more important than ever. This is because telecommunication is used increasingly for vital transactions such as electronic fund transfer, order processing, inventory control & many other business activities ( e.g : e-mail, internet access). Users are willing to pay more to get guaranteed service.

In SDH transmission system, Automatic Protection Switching ( APS) algorithms and performance/alarm monitoring are built in. This system allows the construction of linear point-to-point networks and synchronous ring topology networks which are self- healing in the event of failure. Also, to minimize the disruption of traffic, the protection switching must be completed within the specified time limit ( sub 50ms) recommended by ITU-T G.783 (linear networks) and ITU-T G.841 (ring networks).

Upon detection of a failure (dLOS, dLOF, high BER), the network must reroute traffic (protection switching) from working channel to protection channel. The Network Element that detects the failure (tail-end NE) initiates the protection switching. The head-end NE must change forwarding or to send duplicate traffic. Protection switching may be revertive (automatically revert to working channel)

Key ITU-T recommendations :

ITU-T recommendations define methods of protecting service traffic in SDH networks. Two important recommendations are :

1.Recommendation G.783 covers linear point to point networks.

2.Recommendation G.841 covers various configurations of multiplex section rings.

Linear ( point to point) protection :

In a linear network, protection is achieved through an extra protection fibre. It can protect the network from fiber or NE card failure. Different variants of linear protection are 1+1, 1:1 and 1:N.

How it works ?

Head-end and tail-end NEs have bridges (muxes). Head-end and tail-end NEs maintain bidirectional signaling channel. Signaling is contained in K1 and K2 bytes of protection channel. K1 – tail-end status and requests. K2 – head-end status .

Linear 1+1 protection :

This is simplest form of protection. Can be at OC-n level (different physical fibers) or at STM/VC level (called SubNetwork Connection Protection) or end-to-end path (called trail protection) Head-end bridge always sends data on both channels. Tail-end chooses channel to use based on BER, dLOS, etc. No need for signaling. For non-revertive cases, there is no distinction between. working and protection channels. BW utilization is 50%.

Linear 1:1 protection :

In this case, Head-end bridge usually sends data on working channel. When tail-end detects failure it signals (using K1) to head-end. Head-end then starts sending data over protection channel. When not in use, protection channel can be used for (discounted) extra traffic (pre-emptible unprotected traffic).

Linear 1:N protection:

This is verymuch similar to 1:1 protection with a small difference. Here, in order to save BW we allocate 1 protection channel for every N working channels. Here, N limited to 14.

Let us read about ring networks in next post.

Tributary Unit (TU) Frames, Interview notes on Tributary Unit frames in SDH

noreply@blogger.com (hi) — Sat, 23 Mar 2013 08:33:00 +0000

Different sizes of Tributary Unit frames are used in SDH & we have seen basic SDH multiplexing structur e in earlier post.

Different TU-Sizes provided in SDH are TU-11, TU-12. TU-2 and TU-3 .

1. TU-11 : A TU-11 frame consists of 27 bytes, structured as 3 columns of 9 bytes. These 27 bytes provide a transport capacity of 1.728 Mbps at a frame rate of 8000 Hz. It will accommodate the mapping of 1.544 Mbps DS1 signal. 84 TU-11's can be multiplexed into a STM-1 frame. Structure as as shown below.

2.TU-12 : A TU-12 frame consists of 36 bytes, structured as 4 columns by 9 bytes.These 36 bytes provide a transport capacity of 2.304 Mbps at a frame rate of 8000 Hz. It will accommodate the mapping of 2.048 Mbps E1 signal. 63 TU-12's can be multiplexed into a STM-1 frame. Structure as as shown below.

3.TU-2 : A TU-2 frame consists of 108 bytes, structured as 12 columns by 9 bytes.These 108 bytes provide a transport capacity of 6.912 Mbps at a frame rate of 8000 Hz. It will accommodate the mapping of DS2 signal. 21 TU-2's can be multiplexed into a STM-1 frame. Structure as as shown below.

4.TU-3 : A TU-3 frame consists of 774 bytes, structured as 86 columns by 9 bytes.These 36 bytes provide a transport capacity of 49.54 Mbps at a frame rate of 8000 Hz. It will accommodate the mapping of 34 Mbps E3 signal and North American DS3 signal. 3 TU-3's can be multiplexed into a STM-1 frame. Structure as as shown below.

Maintenance signals in SDH & abbreviations

noreply@blogger.com (hi) — Fri, 22 Mar 2013 04:48:00 +0000

Let us read about maintenance signals in SDH.

LOS Drop of incomming optical power level causes BER of 10-3 or worse

OOF A1, A2 incorrect for more than 625 us

LOF If OOF persists of 3ms

B1 Error Mismatch of the recovered and computed BIP-8

MS-AIS K2 (bits 6,7,8) =111 for 3 or more frames

B2 Error Mismatch of the recovered and computed BIP-24

MS-RDI If MS-AIS or excessive errors are detected, K2(bits 6,7,8)=110

MS-REI M1: Binary coded count of incorrect interleavedbit blocks

AU-AIS All "1" in the entire AU including AU pointer

AU-LOP 8 to 10 NDF enable or 8 to 10 invalid pointers

HP-UNEQ C2="0" for 5 or more frames

HP-TIM J1: Trace identifier mismatch

HP-SLM C2: Signal label mismatch

HP-LOM H4 values (2 to 10 times) unequal to multiframesequence

B3 Error Mismatch of the recovered and computed BIP-8

HP-RDI G1 (bit 5)=1, if an invalid signal is received in VC-4/VC-3

HP-REI G1 (bits 1,2,3,4) = binary coded B3 errors

TU-AIS All "1" in the entire TU incl. TU pointer

TU-LOP 8 to 10 NDF enable or 8 to 10 invalid pointers

LP-UNEQ VC-3: C2 = all "0" for >=frames;

VC-12: V5 (bits 5,6,7) = 000 for >=5 frames

LP-TIM VC-3: J1 mismatch; VC-12: J2 mismatch

LP-SLM VC-3: C2 mismatch; VC-12: V5 (bits 5,6,7) mismatch

BIP-2 Err Mismatch of the recovered and computed BIP-2 (V5)

LP-RDI V5 (bit 8) = 1, if TU-2 path AIS or signal failure received

LP-REI V5 (bit 3) = 1, if >=1 errors were detected by BIP-2

LP-RFI V5 (bit 4) = 1, if a failure is declared

Abbreviations :

AU Administration unit

HP High path

LOF Loss of frame

LOM Loss of miltiframe

LOP Loss of pointer

LOS Loss of signal

LP Low path

OOF Out of frame

REI Remote error indication (FEBE)

RDI Remote defect indication (FERF)

RFI Remote failure indication

SLM Signal label mismatch

TIM Trace identifier

TU Tributary unit

UNEQ Unequipped

VC Virtual Container

C container

Detailed study of multiplexing process in SDH - Interview notes - Part III

noreply@blogger.com (hi) — Thu, 21 Mar 2013 04:00:00 +0000

Let us continue from previous post, where we studied about multiplexing of VC-12 into VC-4. In this post, you will read about VC-4 Path overhead and Mapping of VC-4 into STM-1 frame.

VC-4 Path Overhead:

The VC-4 Path Overhead forms the start of the VC-4 payload area and consists of one whole column of nine bytes as shown below. The POH contains control and status messages (similar to the V5 byte) at the higher order.

J1 - Higher Order Path trace. This byte is used to provide a fixed length user configurable string, which can be used to verify the connectivity of 140 Mbit/s connections.

B3 - Bit Interleaved Parity Check (BIP-8). This byte provides an error monitoring function for the VC-4 payload.

G1 - Higher Order Path Status. This byte is used to transmit back to the distant end, the results of the BIP-8 check in the B3 byte

K3 -Automatic protection Switching (APS). K3 provides for automatic protection switching control with VC-4 payloads. Similar to the K4 bits in the 2 Mbit/s overheads

Mapping of a VC-4 into an STM-1 frame.

An AU pointer is added to the VC-4 to form an AU-4 or Administrative Unit -4.

The AU pointers are in a fixed position within the STM-1 frame and are used to show the location of the first byte of the VC-4 POH.

The AU-4 is then mapped directly into an AUG or Administrative Unit Group, which then has the Section Overheads or SOH, added to it. These section overheads provide STM-1 framing, section performance monitoring and other maintenance functions pertaining to the section path.

The VC-4 payload, plus AU pointers and Section Overheads, together form the complete STM-1 transport frame.

If you have any question , please write to me

Detailed study of multiplexing process in SDH - Interview notes - Part II

noreply@blogger.com (hi) — Wed, 20 Mar 2013 05:02:00 +0000

Further to previous post, let us read about mapping VC-12 into TU-12, TU-12 to TUG-2, TUG-2 to TUG-3 & TUG-3 to VC4.

Mapping of a VC-12 into a TU-12 signal.

In order to detect the start of the 2 Mbit/s signal and thereby the start of the customers data, The V5 byte must be seen be the distant end. This is achieved by adding four overhead bytes to the multiframe, which together form a calculated byte count to the start of V5. This is called a pointer value and is known as the TU Pointer.

There are four pointer bytes called V1, V2, V3 and V4, which are used to calculate the location of V5.

Multiplexing of TU-12 into a TUG-2:

Each VC-12 consists of 36 bytes of information and these 36 bytes fill up exactly 4 columns of the STM-1 frame.

3 separate TU-12's are directly mapped together to form a TUG-2.

The 3 TU-12's will fit exactly into 12 columns of the STM-1 frame .

Mapping of a TUG-2 into a TUG-3 signal:

The mapping of TUG-2's into TUG-3's uses fixed column interleaving and is shown below.

Mapping of a TUG-3 into a VC-4 signal:

The three TUG-3's are column interleaved to form the VC-4 payload. At this point two columns of fixed stuffing are added and the 'VC-4 Path Overhead' is added to the start.

These are the steps followed in multiplexing VC-12 into VC-4. In next post, we will read about VC-4 Path over head

If you have any questions, you can add them in comments section. I will provide you the answer.

Detailed study of multiplexing process in SDH - Interview notes

noreply@blogger.com (hi) — Tue, 19 Mar 2013 05:13:00 +0000

Let us study the overview of the process followed by a 2 Mbit/s PDH input signal until it becomes part of an STM-1 frame. You can compare the details of each individual stage to SDH multiplexing structure provided in previous post.

Mapping of a 2 Mbit/s PDH signal into a C-12:

The 2 Mbit/s PDH input signal is mapped into a Container 12 (C-12). The input frame consists of 32 bytes of information and this fits directly into the C-12 as shown

Mapping of a C-12 into a VC-12:

At the time of mapping a C-12 into a VC-12, we need to add four bytes of overhead control information. But we can add only one byte per frame of customers' data So, this process takes place over 4 consecutive frames & described below: -

Frame number One has two bytes of fixed stuffing added to it. One byte is added at the start and one byte at the end. One byte of overhead control information added to the start. This byte of over head is called the V5 byte and is known as the VC-12 Path OverHead (POH).

Two more important features of the V5 byte are:

BIP-2 is Bit Interleaved Parity Check-2. This looks at the data in the C-12. It counts all of the binary one's that it sees in the odd bit positions (i.e. bits 1,3,5,7 etc) and then it counts all of the binary one's that it sees in the even bit positions (i.e. bits 2,4,6,8 etc). This BIP-2 is then recalculated at the distant end. If the count is different, then some bit corruption has occurred.

FEBE is Far End Bit errors. This bit is set correspondingly to the result of the BIP-2 check. If errors are received at the distant end then there needs to be a mechanism for informing the sender of the problem.

Frame number Two has two bytes of fixed stuffing added to it. One byte is added at the start and one byte at the end. It then has one byte of overhead control information added to the start. This control byte in frame 2 is the Lower Order Path Trace or J2 byte. J2 is used to check continuity of a 2 Mbit/s path.

Frame number Three has two bytes of fixed stuffing added to it. One byte is added at the start and one byte at the end. One byte of overhead control information added to the start. This control byte N2, in frame 3 is called the Network Operator or Tandem Control byte. N2 is used to transmit performance-monitoring information where the circuit spans differing vendors networks.

Frame number Four has one byte of fixed stuffing added to the end. It also has one byte of variable stuffing added to the start. One byte of overhead control information added to the start. Control byte in frame 4 is called K4 and it is used for 2 Mbit/s Automatic Protection Switching or APS. APS is used to automatically switch a single 2 Mbit/s circuit to its alternate path if a fault condition occurs.

In the next post we will learn about :

Mapping of a VC-12 into a TU-12 signal.

SDH Concatenation, Interview notes on Contiguous concatenation

noreply@blogger.com (hi) — Mon, 18 Mar 2013 04:30:00 +0000

There are two types of concatenation in SDH. They are Contiguous concatenation and Virtual concatenation. In this article, let us learn about contiguous concatenation.

Contiguous Concatenation :

The SDH frame can be thought of as transport lorry. The data to be transported is placed in the VC-4 'Container'. This is then hitched to the SOH 'Cab unit' that 'drives' the data to its destination.The maximum carrying capacity of the vehicle is determined by the size of the 'container'. Therefore although the SDH signal is 155 Mbit/s in size, the largest single circuit that can be transmitted at any one time by the customer is limited to the size of the VC-4 i.e. 140 Mbit/s.

When using higher rates of SDH (STM-4, STM-16 etc), multiple 'containers' and 'cabs' are added one after another, to form a bigger vehicle. The customer is still limited to a single circuit size of 140 Mbit/s however, because each individual 'container' is still the same size (140 Mbit/s). They can however transmit multiple 140 Mbit/s circuits simultaneously.

Standard STM-4 structure is given below

The limitation of 140 Mbit/s per individual circuit is not a efficient way of managing bandwidth. In order to overcome this limitation, a method of combining 'containers' together has been developed which is called 'Concatenation'.

STM-4 concatenated structure (VC-4-4C) is as shown below

Concatenated paths are commonly defined as VC-4-xC circuits (where x is size of the concatenation), as shown below:

STM-4 concatenation (written as VC-4-4c), provides a single circuit with a bit rate of approximately 600M (actually 599.04 Mbit/s). STM-16 concatenation (written as VC-4-16c), provides a single circuit with a bit rate of approximately 2.2G (actually 2.2396160 Gbit/s).

STM-1 Frame Structure & Section Over head

noreply@blogger.com (hi) — Sun, 17 Mar 2013 07:26:00 +0000

STM-1 Frame Structure

STM-1 frame contains 2430 bytes of information. Each byte contains 8 data bits (i.e. a 64kbit/s channel). Duration of STM-1 transport frame is 125ms. The number of frames per second is 1 second / 125ms = 8000 Frames per second.

So, rate of STM-1 frame is calculated as follows: -

8 bits x 2430 bytes x 8000 per second = 155,520,000 bits/s or 155 Mbit/s.

STM-1 frame chopped up into 9 segments, stacked on top of each other as shown in the diagram below. The bits start at the top left with byte number one and are read from left to right and top to bottom. They are arranged as 270 columns across and 9 rows down.

STM-1 Section Overheads

The STM-1 Section Overhead (SOH) consists of nine columns by nine rows as shown below. It forms the start of the STM-1 frame.The SOH contains control and status messages at the optical fibre level.

First three rows are RSOH ( Regenerator Section Overhead), Fourth row is AU-4 pointer. Fifth to Ninth row are MSOH ( Multiplexer section Overhead).

A1 & A2 - STM-1 Frame Alignment. These 6 bytes are used for STM-1 frame alignment. They are the first bytes transmitted. Frame alignment takes place over three STM-1 frames.

J0 - STM-1 Section Path Trace. This byte is used to provide a fixed length user configurable string, which can be used to verify network topology connections.

B1 - Byte Interleaved Parity Check 8 (BIP-8). This byte provides an error monitoring function for the entire STM-1 frame after encoding.

B2 - Byte Interleaved Parity Check 24 (BIP-24). These 3 bytes provide an error monitoring function for the STM-1 frame before encoding.A comparison between the BIP-8 and BIP-24 checks reveal if there were any encoding errors.

D1 to D12 - Data Communications Channel (DCC). These bytes provide a data channel for the use of network management systems.

K1 - Automatic protection Switching (APS). This byte is used to perform automatic protection switching of the optical fibre.

X - Reserved. These bytes are reserved for national use.

All unmarked bytes are reserved for future international standardisation.

SDH Principles and Interview questions on SDH Multiplexing structure

noreply@blogger.com (hi) — Sat, 16 Mar 2013 06:22:00 +0000

Overview

The SDH standard defines a number of 'Containers' each corresponding to an existing PDH input rate. Information from the incoming PDH signal is placed into the relevant container.Each container then has some control information known as the 'Path Overhead' (POH) and stuffing bits added to it. The path overhead bytes allow the system operator to achieve end to end monitoring of areas such as error indication, alarm indication and performance monitoring data. The container and the path overhead together form a 'Virtual Container' (VC).

Due to clock phase differences, the start of the customers' PDH data may not coincide with the start of the SDH frame. Identification of the start of the PDH data is achieved by adding a 'Pointer'. The VC and its relevant pointer together form a 'Tributary Unit' (TU).

Tributary units are then multiplexed together in stages (Tributary User Group 2 (TUG-2) - Tributary User Group 3 (TUG-3) - Virtual Container 4 (VC-4)), to form an Administrative Unit 4 (AU-4). Additional stuffing, pointers and overheads are added during this procedure.This AU-4 in effect contains 63 x 2 Mbit/s channels and all the control information that is required.

Finally, Section Overheads (SOH) are added to the AU-4.These SOH's contain the control bytes for the STM-1 section comprising of framing, section performance monitoring, maintenance and operational control information.An AU-4 plus its SOH's together form an STM-1 transport frame.