Knowledge Guru

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Network+ Guide 1 2 3

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Fiber-Optic Cable

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In almost every way, fiber-optic cable is the ideal cable for data transmission. Not only does this type of cable accommodate extremely high bandwidths, but it also presents no problems with EMI and supports durable cables and cable runs as long as several kilometers. The two disadvantages of fiber-optic cable, however, are cost and

installation dificulty. Despite these disadvantages, fiber-optic cable is now often installed into buildings by telephone companies as the cable of choice. The center conductor of a ?ber-optic cable is a ?ber that consists of

highly refined glass or plastic designed to transmit light signals with little loss. A glass core supports a longer cabling distance, but a plastic core is typically easier to work with. The ?ber is coated with a cladding or a gel that re?ects signals back into the fiber to reduce signal loss. A plastic sheath protects the fiber (see Figure. A fiber-optic network cable consists of two strands separately enclosed in plastic sheaths. One strand sends and the other receives.

Two types of cable con?gurations are available: loose and tight configurations. Loose con?gurations incorporate a space between the fiber sheath and the outer plastic encasement; this space is filled with a gel or other material. Tight configurations contain strength wires between the conductor and the outer plastic encasement. In both cases, the plastic encasement must supply the strength of the cable, while the gel layer or strength wires protect the delicate fiber from mechanical damage. Optical fiber cables don't transmit electrical signals. Instead, the data signals must be converted into light signals. Light sources include lasers and light-emitting diodes (LEDs). LEDs are inexpensive but produce a fairly poor quality of light suitable for only less-stringent applications. The end of the cable that receives the light signal must convert the signal back to an electrical form. Several types of solid-state components can perform this service. One of the signi?cant dificulties of installing ?ber-optic cable arises when two cables must be joined. The small cores of the two cables

(some are as small as 8.3 microns) must be lined up with extreme precision to prevent excessive signal loss.

RJ 45 CONNECTOR

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Category 5 UTP CABLES

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Category 5. This data-grade cable, which also consists of four twisted-pairs, is suitable for data rates up to 100Mbps. Most new cabling systems for 100Mbps data rates are designed around Category 5 cable.

Category 4 UTP CABLES

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Category 4. This data-grade cable, which consists of four twisted-pairs, is suitable for data rates up to 16Mbps.

Category 3 UTP CABLES

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Category 3. As the lowest data-grade cable, this type of cable generally is suited for data rates up to 10Mbps. Some innovative schemes utilizing new standards and technologies, however, enable the cable to support data rates up to 100Mbps. Category 3, which uses four twisted pairs with three twists per foot, is now the standard cable used for most telephone installations.

Categories 1 and 2. UTP CABLES

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Categories 1 and 2. These voice-grade cables are suitable only for voice and for low data rates (below 4Mbps). Category 1 was once the standard voice-grade cable for telephone systems. The growing need for data-ready cabling systems, however, has caused Categories 1 and 2 cable to be supplanted by Category 3 for new installations.

Unshielded Twisted-Pair (UTP) Cable

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Unshielded twisted-pair cable doesn't incorporate a braided shield into its structure. However, the characteristics of UTP are similar in many ways to STP, differing primarily in attenuation and EMI. As shown in Figure , several twisted pairs can be bundled together in a single cable. These pairs are typically color-coded to distinguish them.

Telephone systems commonly use UTP cabling. Network engineers can sometimes use existing UTP telephone cabling (if it is new enough and of a high enough quality to support network communications) for network cabling. UTP cable is a latecomer to high-performance LANs because engineers only recently solved the problems of managing radiated noise and susceptibility to EMI. Now, however, a clear trend toward UTP is in operation, and all new copper-based cabling schemes are based on UTP.

Connectors for STP

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AppleTalk and Token Ring networks can be cabled using UTP cable and RJ-45 connectors (described later in this chapter), but both net- works originated as STP cabling systems. For STP cable, AppleTalk also employs a DIN-type connector. Figure 3.15 shows an IBM connector connected to a network card having a DIN (DB-9) connectorusing a STP cable. The IBM Data Connector is unusual because it doesn't come in two gender configurations. Instead, any IBM Data Connector can be snapped to any other IBM Data Connector.

Shielded Twisted-Pair (STP) Cable

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Shielded twisted-pair cabling consists of one or more twisted pairs ofcables enclosed in a foil wrap and woven copper shielding. Figure shows IBM Type 1 cabling, the ?rst cable type used with IBMToken Ring. Early LAN designers used shielded twisted-pair cable because the shield performed double duty, reducing the tendency of the cable to radiate EMI and reducing the cable's sensitivity to outside interference. Coaxial and STP cables use shields for the same purpose. The shield is connected to the ground portion of the electronic device to which the cable is connected. A ground is a portion of the device that serves as an electrical reference point, and usually, it is literally connected to a metal stake driven into the ground. A properly grounded shield prevents signals from getting into or out of the cable. The picture in Figure is an example of IBM Type 1 cable, an STP cable, and includes two twisted pairs of wire within a single

shield. Various types of STP cable exist, some that shield each pair individually and others that shield several pairs. The engineers who design a network's cabling system choose the exact configuration. IBM designates several twisted-pair cable types to use with their Token Ring network design, and each cable type is appropriate for a given kind of installation. A completely different type of STP is the standard cable for Apple's AppleTalk network.Because so many different types of STP cable exist, describing pre- cise characteristics is dificult. The following sections, however, offer

some general guidelines.

Twisted-Pair Cable

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Twisted-pair cable has become the dominant cable type for all new network designs that employ copper cable. Among the several reasons for the popularity of twisted-pair cable, the most significant is its low cost. Twisted-pair cable is inexpensive to install and offers the lowest cost per foot of any cable type. Your telephone cable is an

example of a twisted-pair type cable. A basic twisted-pair cable consists of two strands of copper wire

twisted together (see Figure). The twisting reduces the sensitivi- ty of the cable to EMI and also reduces the tendency of the cable to radiate radio frequency noise that interferes with nearby cables and electronic components, because the radiated signals from the twisted wires tend to cancel each other out. (Antennas, which are purposely designed to radiate radio frequency signals, consist of parallel, not twisted, wires).

Twisting of the wires also controls the tendency of the wires in thepair to cause EMI in each other. As noted previously, whenever twowires are in close proximity, the signals in each wire tend to producecrosstalk in the other. Twisting the wires in the pair reduces crosstalkin much the same way that twisting reduces the tendency of thewires to radiate EMI.A twisted-pair cable is used in most cases to connect a PC to either a HUB or a MAU. "Connectivity Devices and Transfer Mechanisms." Two types oftwisted-pair cable are used in LANs: shielded and unshielded,

Coaxial Cable

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Coaxial cables were the ?rst cable types used in LANs. As shown in Figure coaxial cable gets its name because two conductors share a common axis; the cable is most frequently referred to as a "coax." A type of coaxial cable that you may be familiar with is your television cable.

The components of a coaxial cable are as follows:

A center conductor, although usually solid copper wire, is sometimes made of stranded wire. An outer conductor forms a tube surrounding the center conductor. This conductor can consist of braided wires, metallicfoil, or both. The outer conductor, frequently called the shield,serves as a ground and also protects the inner conductor fromEMI.
An insulation layer keeps the outer conductor spaced evenlY from the inner conductor. A plastic encasement (jacket) protects the cable from damage.

CABLE MEDIA

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three types of network cabling media, as follows:

Coaxial cable

Twisted-pair cable

Fiber-optic cable

Electromagnetic Interference

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Electromagnetic interference (EMI) consists of outside electromagnetic noise that distorts the signal in a medium. When you listen to an AM radio, for example, you often hear EMI in the form of noise caused by nearby motors or lightning. Some network media are more susceptible to EMI than others. Crosstalk is a special kind of interference caused by adjacent wires. Crosstalk occurs when the signal from one wire is picked up by another wire. You may have experienced this when talking on a telephone and hearing another conversation going on in the background. Crosstalk is a particularly signi?cant problem with computer networks because large numbers of cables often are locatedclose together, with minimal attention to exact placement.

Attenuation

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Attenuation is a measure of how much a signal weakens as it travels through a medium, Attenuation is a contributing factor to why cable designs must specify limits in the lengths of cable runs. When signal strength falls below certain limits, the electronic equipment that receives the signal can experience difIculty isolating the original signal from the noise present in all electronic transmissions. The effect is exactly like trying to tune in distant radio signals. Even if you can lock on to the signal on your radio, the sound generally still contains more noise than the sound for a local radio station. As mentioned in the previous chapters, repeaters are used to regenerate signals; hence one solution to deal with attenuation is to add a repeater.

Time-Division Multiplexing

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Time-division multiplexing (TDM) divides a channel into time slots that are allocated to the data streams to be transmitted, as illustrated in Figure If the sender and receiver agree on the time-slot assignments, the receiver can easily recover and reconstruct the original data streams.

TDM transmits the multiplexed signal in baseband mode. Interest- ingly, this process makes it possible to multiplex a TDM signal as one of the data channels on an FDM system. Conventional TDM equipment utilizes Fixed time divisions and allocates time to a channel, regardless of that channel's level of activity.If a channel isn't busy, its time slot isn't being fully utilized. Because The time divisions are programmed into the conFigurations of the multiplexers, this technique is often referred to as synchronous TDM. If using the capacity of the data medium more efIciently is important, a more sophisticated technique, statistical time-division multiplexing (StatTDM), can be used. A stat-mux uses the time-slot

technique but allocates time slots based on the trafic demand on theindividual channels, as illustrated in Figure
Notice that Channel B is allocated more time slots than Channel A, and that Channel C is allocated the fewest time slots. Channel D is idle, so no slots are allocated to it. To make this procedure work, the data transmitted for each time slot includes a control field thatidenti?es the channel to which the data in the time slot should be assigned.

Frequency-Division Multiplexing

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Figure illustrates frequency-division multiplexing (FDM). This technique works by converting all data channels to analog form. Each analog signal can be modulated by a separate frequency (called a "carrier frequency") that makes it possible to recover that signal during the demultiplexing process. At the receiving end, the demultiplexer can select the desired carrier signal and use it to extract the data signal for that channel. FDM can be used in broadband LANs. (A standard for Ethernet also exists.) One advantage of FDM is that it supports bidirectional signaling on the same cable. That is, a frequency can originate from both ends of the transmission media at once.

TRANSMISSION MEDIA

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Each type of transmission media has special characteristics that make it suitable for a specific type of service. You should be familiar with these characteristics for each type of media:

Cost

Installation requirements

Bandwidth

Band usage (baseband or broadband)

Attenuation

Immunity from electromagnetic interference

These characteristics are all important. When you design a network for a company, all these factors play a role in the decision concerning what type of transmission media should be used.

Cost

Installation Requirements

Installation requirements typically involve two factors. One is that some transmission media require skilled labor to install. Bringing in a skilled outside technician to make changes to or replace resources on the network can bring about undue delays and costs. The second has to do with the actual physical layout of the network. Some types of transmission media install more easily over areas where people are spread out, whereas other transmission media are easier to bring to
clusters of people or a roaming user.

Bandwidth
In computer networking, the term bandwidth refers to the measure of the capacity of a medium to transmit data. A medium that has a high capacity, for example, has a high bandwidth, whereas a medium that has limited capacity has a low bandwidth Bandwidth can be best explained by using water hoses as an analogy. If a half-inch garden hose can carry water FLow from a trickle up to two gallons per minute, then that hose can be said to have a band width of two gallons per minute. A four-inch ?re hose, however, might have a bandwidth that exceeds 100 gallons per minute. Data transmission rates are frequently stated in terms of the bits that can be transmitted per second. An Ethernet LAN theoretically can

Band Usage (Baseband or Broadband)

The two ways to allocate the capacity of transmission media are with baseband and broadband transmissions. Baseband devotes the entire capacity of the medium to one communication channel. Broadband enables two or more communication channels to share the band width of the communications medium. Baseband is the most common mode of operation. Most LANs function in baseband mode, for example. Baseband signaling can be accomplished with both analog and digital signals. Although you might not realize it, you have a great deal of experience with broadband transmissions. Consider, for example, that the TV cable coming into your house from an antenna or a cable provider is a broadband medium. Many television signals can share the bandwidth of the cable because each signal is modulated using a separately assigned frequency. You can use the television tuner to select the frequency of the channel you want to watch. This technique of dividing bandwidth into frequency bands is called frequency-division multiplexing (FDM) and works only with analog signals. Another technique, called time-division multiplexing (TDM), supports digital signals. Both of these types of multiplexing contrasts the difference between baseband and broadband modes of operation.

Multiplexing
Multiplexing is a technique that enables broadband media to support multiple data channels. Multiplexing makes sense under a number of circumstances: When media bandwidth is costly. A high-speed leased line, such as a T1 or T3, is expensive to lease. If the leased line has sufIcient bandwidth, multiplexing can enable the same line to carry mainframe, LAN, voice, video conferencing, and various other data types. When bandwidth is idle. Many organizations have installed
FIber-optic cable that is used to only partial capacity. With the proper equipment, a single ?ber can support hundreds of
megabits-or even a gigabit or more-of data per second. When large amounts of data must be transmitted through low-
capacity channels. Multiplexing techniques can divide the original data stream into several lower-bandwidth channels, each of which can be transmitted through a lower-capacity medium. The signals then can be recombined at the receiving end.

Multiplexing refers to combining multiple data channels for transmission on a common medium. Demultiplexing refers to recovering the original separate channels from a multiplexed signal. Multiplexing and demultiplexing are performed by a multiplexer (also called a mux), which usually has both capabilities.

transmit 10 million bits per second and has a bandwidth of 10 megabits per second (Mbps). The bandwidth that a cable can accommodate is determined in part by the cable's length. A short cable generally can accommodate

greater bandwidth than a long cable, which is one reason all cable designs specify maximum lengths for cable runs. Beyond those limits, the highest-frequency signals can deteriorate, and errors begin to occur in data signals. You can see this by taking a garden hose and snapping it up and down. You can see the waves traveling down the hose get smaller as they get farther from your hand. This loss of the wave's amplitude represents attenuation, or signal degradation.

One main factor in the purchase decision of any networking component is the cost. Often the fastest and most robust transmission media is desired, but a network designer must often settle for some thing that is slower and less robust, because it more than suf?ces for the business solution at hand. The major deciding factor is almost always price. It is a rare occasion in the ?eld that the sky is the limit for installing a network. As with nearly everything else in the computer field, the fastest technology is the newest, and the newest is the most expensive. Over time, economies of scale bring the price down, but by then, a newer technology comes along.

TRANSMISSION FREQUENCIES

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Transmission media make possible the transmission of the electronic signals from one computer to another. These electronic signals express data values in the form of binary (on/off ) impulses, which are the basis for all computer information (represented as 1s and 0s). These signals are transmitted between the devices on the network, using some form of transmission media (such as cables or radio) until they reach the desired destination computer. All signals transmitted between computers consist of some form of electromagnetic (EM) waveform, ranging from radio frequencies through microwaves and infrared light. Different media are used to transmit the signals, depending on the frequency of the EM wave form. Figure illustrates the range of electromagnetic waveforms (known as the electromagnetic spectrum) and their associated fre-quencies.

The electromagnetic spectrum consists of several categories of wave forms, including radio frequency waves, microwave transmissions, and infrared light. The frequency of a wave is dependent upon the number of waves or oscillations that occur during a period of time. An example that all people can relate to is the difference between a high-pitched sound, such as a whistle, and a low-pitch sound such as a fog horn. A high- pitched sound has a very high frequency; in other words, numerous cycles of oscillation (or waves) occur each second. Whereas, a low frequency sound, such as the fog horn, is based on relatively few cycles or waves per second (see Figure ). Although sound is not an example of electromagnetic energy (it's mechanical energy), the principles are similar.

Radio frequency waves are often used for LAN signaling. Radio fre- quencies can be transmitted across electrical cables(twisted-pair or coaxial) or by radio broadcast. Microwave transmissions can be used for tightly focused transmis- sions between two points. Microwaves are used to communicate between earth stations and satellites, for example, and they are also used for line-of-sight transmissions on the earth's surface. In addition, microwaves can be used in low-power forms to broadcast signals from a transmitter to many receivers. Cellular phone networks are examples of systems that use low-power microwave signals to broadcast signals. Infrared light is ideal for many types of network communications.

Infrared light can be transmitted across relatively short distances and can be either beamed between two points or broadcast from one point to many receivers. Infrared and higher frequencies of light also can be transmitted through ?ber-optic cables. A typical television remote control uses infrared transmission. The next sections examine the major factors you should consider when evaluating what type of transmission media should be implemented.

Connection-Oriented and Connectionless

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Compare the implications of using connection-oriented communications with connectionless communications.
The OSI Network layer determines the route a packet will take as it passes through a series of different LANs from the source PC to the destination PC. The complexity and versatility of Network layer addressing gives rise to two different communication modes for passing messages across the network, both of which are recognized under OSI:

Connection-oriented mode. Error correction and ?ow control

are provided at internal nodes along the message path.

Connectionless mode. Internal nodes along the message path do

not participate in error correction and flow control. To understand the distinction between connection-oriented and connectionless communications, you must consider an important distinction between the OSI model's Data Link and Network layers. In theory, the Data Link layer facilitates the transmission of data across a single link between two nodes. The Network layer describes the process of routing a packet through a series of nodes to a destination on another link on the network. An example of this latter scenario is a message passing from a PC on one LAN segment through a series of routers to a PC on a distant part of the network. The internal nodes forwarding the packet also forward other packets between other end nodes. In connection-oriented mode, the chain of links between the source and destination nodes forms a kind of logical pathway connection. The nodes forwarding the data packet can track which packet is part of which connection. This enables the internal nodes to provide flow control as the data moves along the path. For example, if an internal node determines that a link is malfunctioning, the node can send a notification message backward, through the path to the source computer. Furthermore, because the internal node distinguishes among individual, concurrent connections in which it participates, this node can transmit (or forward) a "stop sending" message for one of its
connections without stopping all communications through the node. Another feature of connection-oriented communication is that internal nodes provide error correction at each link in the chain. Therefore, if a node detects an error, it asks the preceding node to retransmit. Connectionless mode does not provide these elaborate internal control mechanisms; instead, connectionless mode relegates all error correcting and retransmitting processes to the source and destination nodes. The end nodes acknowledge the receipt of packets and retransmit if necessary, but internal nodes do not participate in flow control and error correction (other than simply forwarding messages between the end nodes).
The advantage of connectionless mode is that connectionless communications can be processed more quickly and more simply because the internal nodes only forward data and thus don't have to track connections or provide retransmission or flow control. The differences between connection-oriented and connectionless modes of communication may be easier to understand by analogy Imagine talking to someone and then having her reafirm that she understood what you have told her after each sentence. Connectionless mode is like having a conversation with someone, but the speaker just carries on and assumes that the listener under- stands. Connection-oriented is slower, yet more reliable. Connectionless is faster, but has less capability to correct errors (misunderstandings in the conversation example) as they occur. Connectionless mode does have its share of disadvantages, however, including the following:

Messages sometimes get lost due to an over?owing buffer or a

failed link along the pathway.

If a message gets lost, the sender doesn't receive noti?cation.

Retransmission for error correction takes longer because a

faulty transmission can't be corrected across an internal link.

It is important to remember that the OSI model is not a set of rules for communication; the OSI model is a framework in which models of communication are explained. As such, individual imple-mentations of connectionless protocols can attenuate some of the preceding disadvantages. It is also important to remember that connection-oriented mode, although it places much more emphasis on monitoring errors and controlling traf?c, doesn't always work either. Ultimately, the choice of connection-oriented or connectionless communications mode depends on interoperability with other systems, the premium for speed, and the cost of components.

How Peer OSI Layers Communicate

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Communication between OSI layers is both vertical within the OSI layers, and also horizontal between peer layers in another computer (see Figure). This is important to understand, because it affects how data is passed within a computer, as well as between two computers. When information is passed within the OSI model on a computer, each protocol layer adds its own information to the message being sent. This information takes the form of a header added to the beginning of the original message. The sending of a message always goes down the OSI stack, and hence headers are added from the top to the bottom (see Figure). When the message is received by the destination computer, eachlayer removes the header from its peer layer. Thus at each layer headers are removed (stripped ) by the receiving computer after the information in the header has been utilized. Stripped headers are removed in the reverse order in which they were added. That is, the last header added by the sending computer, is the ?rst one stripped off and read by the receiving computer. In summary, the information between the layers is passed along vertically. The information between computers is essentially horizontal, though, because each layer in one computer talks to its respective layer in the other computer.

THE OSI REFERENCE MODEL

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Having a model in mind helps you understand how the pieces of the networking puzzle together. The most commonly used model is the Open Systems Interconnection (OSI) reference model. The OSI model, first released in 1984 by the International Standards Organization (ISO), provides a useful structure for de?ning and describing the various processes underlying networking communications. The OSI model is a blueprint for vendors to follow when developing protocol implementations. The OSI model organizes communication protocols into seven levels. Each level addresses a narrow portion of the communication process. Figure 2.1 illustrates the levels of the OSI model.

Standards Organizations and the ISO

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The development and implementation of de jure standards is regulated by standards organizations. For example, the CCITT (this is a French acronym that translates to the International Consultative Committee for Telegraphy and Telephony) and the Institute of Electrical and Electronic Engineers (IEEE), among other organizations, are responsible for several prominent network standards that support the International Standards Organization's objective of network interoperability. The International Standards Organization (ISO)-whose name is derived from the Greek pre?x iso, meaning "same"-is located in Geneva, Switzerland. ISO develops and publishes standards and coordinates the activities of all national standardization bodies. In 1977, the ISO initiated efforts to design a communication standard based on the open systems architecture theory from which computer networks would be designed. This model came to be known as the Open Systems Interconnection (OSI) model. This model has become an accepted framework for analyzing and developing networking components and functionality

STANDARDS

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The network industry uses two types of standards: de facto standards and de jure standards. To understand the concept of open systems architecture, you must be familiar with the concepts of de facto and de jure standards. De facto standards arise through widespread commercial and educational use. These standards often are proprietary and usually remain unpublished and unavailable to outside vendors. Unpublished and unavailable standards are known as closed system standards. Published and accessible standards, on the other hand, are known as open system standards. Through the growing acceptance of the concept of interoperability, many closed, proprietary systems (such as IBM's Systems Network Architecture) have started to migrate toward open system standards. Certainly, de facto standards are not always closed system standards. Some examples of proprietary open system standards include Novell's NetWare network operating system and Microsoft's Windows. The second type of standards, de jure standards, are nonproprietary, which means that no single company creates them or owns the rights to them. De jure standards are developed with the intent of enhancing connectivity and interoperability by making speci?cations public so that independent manufacturers can build to such specifications.TCP/IP, "Transport Protocols," is an example of a nonproprietary de jure standard. Several permanent committees comprised of industry representatives develop de jure standards. Some examples of these committees are

the IEEE (Institute of Electrical and Electronic Engineers) and the IRTF (Internet Engineering Task Force). Although these committees are supported by manufacturer subscriptions, and in some cases government representatives, they are intended to represent the interests of the entire community and thus remain independent of any one

manufacturer's interests. Subscribing to de jure standards reduces the risk and cost of developing hardware and software for manufacturers. After a standard has been ?nalized, a component manufacturer sub- scribing to it can develop products with some confidence that the products will operate with components from other companies that also subscribe to the same standards.

An example of a de jure standard is the set of rules that guide how web pages are transferred between computers or how ?les are trans- ferred between systems. These de jure standards are created by the IRTF to facilitate communication between different systems. One problem of de jure standards, though, is the possibility of a vendor choosing to follow only part of a given standard. The frequent result is a product that claims to conform to the standard, but that in reality fails to operate with other products in the way one might believe or expect.