CHAPTER 3books.mhprofessional.com/downloads/products/0072193123/...network services (such as packet switching, Frame Relay (FR), or Asynchronous Transfer Mode, or ATM), some form of

CHAPTER 3

Introduction toTransmissionTechnologies

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94 Data Network Design, Third Edit ion


This chapter provides an overview of common network topologies, circuits, connec-tions, and transmission types, along with the fundamentals of bridging, switching,and routing. Five major network topologies are defined in this chapter and form the

basis for most network designs. The presentation then covers the types of connectionsand circuits that interconnect these topologies. Each circuit type has characteristics, suchas the direction of data flow, bit- or byte-oriented transmission, and physical characteris-tics. Finally, the major data communications hardware types that form the buildingblocks of today’s data networks are discussed, including bridges, routers, hubs, switches,and gateways.

GENERAL NETWORK TOPOLOGIESThe five most commonly used network topologies for computer and data communica-tions networks are point-to-point, multipoint (or common bus), star, ring (or loop), andmesh. The term node will be used to designate a network data communications elementsuch as a router, switch, or multiplexer. The term link will be used to designate a circuitconnection between nodes. A link may be either logical, such as a Permanent Virtual Cir-cuit (PVC), or physical, as with a dedicated private line. Illustrated examples are pro-vided for each network topology.

Point-to-PointPoint-to-point connectivity is the simplest topology, providing a single link between twonodes. This link can be composed of multiple physical and/or logical circuits. Figure 3-1shows three examples of point-to-point links. The first example (A.) shows a single linkbetween node A and node B with a single physical and logical (or virtual) circuit. Noticethat communications can flow in both directions simultaneously over the same physicaland logical circuit. The second example (B.) depicts a single link between node A andnode B with multiple logical circuits riding over a single physical link. The third example(C.) depicts a single path between node A and node B with multiple (four to be exact)physical circuits, each one carrying multiple logical circuits.

Point-to-point configurations are the most common method of connectivity. Manydata communications networks and the applications that ride them use point-to-point to-pologies in metropolitan area network (MAN) and wide area network (WAN) configura-tions. For example, almost every time a user accesses the many types of MAN or WANnetwork services (such as packet switching, Frame Relay (FR), or Asynchronous TransferMode, or ATM), some form of point-to-point topology is used. An example of thepoint-to-point topology is the private-line or dedicated circuit.

Common Bus (Multipoint)A widely used function of multipoint is the common bus topology, where all nodes arephysically connected to a common bus structure. Figure 3-2 shows the multipoint com-mon bus topology, where nodes A through F communicate via a common physical and



Chapter 3: Introduction to Transmission Technologies 95


Figure 3-1. Point-to-point topology examples

Figure 3-2. Multipoint common bus topology example



logical bus. The IEEE 802.3 Ethernet protocol uses a common bus topology. The commonbus is also called a shared medium topology.

For many years, a multidrop analog line was commonly used for the Systems Net-work Architecture (SNA) Synchronous Data Link Control (SDLC) local loop access. Inthis example, an analog signal was broadcast from a primary station to all secondary sta-tions. In the return direction, all secondary signals were added and returned to the pri-mary. This was a very cost-effective mode of communication during the late twentiethcentury. The SNA SDLC protocol, which can use a multipoint circuit for access, was de-fined in great detail in Chapter 2.

Many companies are now using protocols such as the Internet Protocol (IP) and FR totransport their SNA traffic. Multidrop lines are predominately used in industries thatneed a small amount of bandwidth to many locations, such as banking (connections toautomated teller machines, the other ATM) and gaming (connections to slot machines).

Other conceptual examples of the multipoint topology are illustrated in Figure 3-3.Another commonly used multipoint topology is that of broadcast, or point-to-multipoint,which is defined in the Broadband Integrated Services Digital Network (B-ISDN) standards



Figure 3-3. Conceptual illustration of multipoint topologies



as the case in which one sender’s data is received by many other nodes. Yet another exam-ple is that of incast, or multipoint-to-point, where multiple senders’ signals are received atone destination, such as in the secondary-to-primary direction. In this conceptual illus-tration, note that the multipoint-to-multipoint topology (that is, shared medium ormulticast) is effectively the combination of a point-to-multipoint and a multipoint-to-point topology, as the name implies. The point-to-point topology is also illustrated forcomparison purposes.

StarThe star topology was developed during the era when most computer communicationswere centrally controlled by a mainframe. It also has its analogy in the voice world whereone central switch is connected to multiple remote switching nodes, each serving hun-dreds to even thousands of telephones. This network radiates in a star-like fashion fromthe central switch through the remote switches to the telephones on people’s desks. Alldevices in the network are connected to the central (often headquarters site) node, whichusually performs the processing and switching. Nodes communicate with each otherthrough point-to-point or multidrop links radiating from the central node. The differencebetween this topology and that of the multipoint topology is that the central node onlyprovides point-to-point connections between any edge node on either a physical or a log-ically switched basis.

Figure 3-4 shows a star topology, where node A serves as the center of the star andnodes B through E communicate with each other via connections switched through node A.Another example of a legacy star topology is many remote terminal locations accessinga centralized host processor. These terminals are often called dumb terminals since theprocessing power and intelligence are resident in the host.

The physical star topology is widely used to connect devices to a central hub. The cen-tral hub may logically organize the physical star as a logical bus or ring, as is commonlydone in local area network (LAN) wiring hubs and switches. An example of this is havingmultiple personal computers connected to an Ethernet switch. The physical connectionsare organized as a physical star, but logically the devices communicate via a logical bus(Ethernet).

In some WAN designs, the central hub site is typically a headquarters location wherethe main applications reside and to which all remote sites are connected. This topology isalso called “hub-and-spoke.” Network designs based on the original SNA architectureresemble a star topology, where remote and local terminals directly communicate withcommunications controllers, which would then pass the information in a star fashion tofront-end processors (FEPs) that pass the information to the host. Communications backto the remote users must follow the same hierarchy. Modern Application Service Pro-vider (ASP) configurations also use a star design, as many remote locations access a cen-tral server where the application resides, but instead of this server residing at a separateheadquarters location, it resides within the network typically on a service provider’shosted server.







RingThe loop, or ring, topology is used for networks in which communications data flow isunidirectional according to a particular protocol. A physical and/or logical ring is estab-lished, and each device attached to the ring passes information in the direction of thering’s traffic flow. In the example of the Institute of Electrical and Electronics Engineers(IEEE) 802.5 Token Ring protocol, each station has an opportunity to seize the token, passdata, and then release the token. The destination station [based on the Media Access Con-trol (MAC) address] identifies the data as destined for it, strips the information off thering, and then releases the token to pass more data. This method is referred to as a band-width reservation scheme, as opposed to the collision scheme used in Ethernet. The To-ken Ring protocol operates similar to a subway system where each car can be loaded withpeople, unloaded at the destination, and reused to carry more people.

Figure 3-5 shows a ring network where node A passes information (frame 1) to node Cvia the ring and through node D (steps 1 and 2). Node C then returns a confirmation

Figure 3-4. Star topology example





(frame 2) to node A via node B (step 3), at which point node A removes this data from thering (step 4). There is a reuse of capacity in this ring example because the destination re-moves the information from the ring to make better use of capacity. Examples of the ringtopology are the IEEE 802.5 Token Ring and the Fiber Distributed Data Interface (FDDI).

Note that the IEEE 802.6 physical topology is often drawn as a ring; however, its oper-ation is logically a bus. Synchronous Optical Network (SONET) technology also uses aphysical point-to-point architecture where the SONET-intelligent devices form logical

Figure 3-5. Ring or loop topology example



ring architecture. SONET protection rings use a ring topology and are distinguished froma mesh topology by the difference in nodal switching action from that of a mesh of circuitswitches.

MeshMost switched networks employ some form of mesh architecture. Mesh networks havemany nodes that are connected by multiple links. Figures 3-6 and 3-7 show two types ofmesh networks. Figure 3-6 shows a partial-mesh network where nodes B, C, D, E, F, and Ghave a high degree of connectivity by virtue of having at least three links to any other node,while nodes A and H has only two links to other nodes. Often, the number of links con-nected to a node is called its degree (of connectivity).

Figure 3-7 shows a full-mesh network where each node has a link to every other node.Almost every major computer and data communications network uses a mesh topologyto give alternate routes for backup and traffic loads, but few networks use a full-mesh to-pology primarily because of high cost factors associated with having a large number oflinks. This is because a full-mesh n-node network has n(n –1)/2 links. For networks with ngreater than 4 to 8 nodes, partial-mesh networks are usually employed.



Figure 3-6. Partial-mesh network topology example





CONNECTION AND CIRCUIT TYPES AND SERVICESThis section takes a detailed look at the characteristics of the three major types of connec-tions that define the data flows used in network topologies: simplex, half-duplex, and du-plex. We next look at multidrop and private-line circuits, which form the fundamentalcomponents of connectivity for most types of data communications architectures. Addi-tionally, most service providers, defined in legacy terms of Local Exchange Carriers (LECs),independent telephone and bypass companies, and inter-exchange carriers (IXCs), offer pri-vate lines and multidrop circuits as a tariff service. Finally, we look at a more recent type ofhigh-speed local loop that has become very popular: the Digital Subscriber Line (DSL). Ca-ble access will be covered later.

Connection Types: Simplex, Half-Duplex, and DuplexData terminal equipment (DTE) to data communications equipment (DCE) connectionsprovide a local, limited-distance physical connection between DTEs or terminal equip-ment (TE), such as a computer, and DCEs, such as a modem or channel service unit/dataservice unit (CSU/DSU). The physical medium can be two-wire, four-wire, coaxial, fiberoptic, or a variety of other interfaces.

Figure 3-7. Full-mesh network topology example



The following illustration depicts a connection between a DTE and a DCE runningsimplex communications, which means that transmission is possible only in a singledirection.

The following illustration displays a DTE/DCE connection using half-duplex com-munications, which means that a transmission can occur in either direction (as illustratedby the two-headed arrow), but only one direction is allowed at a time. The change oftransmission direction is accomplished via the control leads between the DTE and DCE atthe physical layer.

The next illustration depicts full-duplex communication, which means that transmis-sion not only can occur in both directions, but can occur in both directions simultaneously.

A separate ground lead is shown for each data signal, indicating a balanced interfacethat supports higher transmission speeds over longer distances on DTE-to-DCE connec-tions. An unbalanced interface shares a ground lead between multiple signal leads andoperates only over shorter distances. All of these examples demonstrate a point-to-pointtopology.





Multidrop CircuitsWhen one user, typically the originator of information, needs to communicate with mul-tiple users over a shared facility, a multidrop circuit can be used. Figure 3-8 shows atwo-wire multidrop circuit, and Figure 3-9 shows a four-wire multidrop circuit. In SNAmultidrop networks, many remote users (B, C, and D) share a single, low-cost multidropaccess circuit to a central site (A). Only one user (B, C, or D) may send data to the main legof the circuit (A) at any one time. When using multidrop circuits, there is a primary-sec-ondary relationship between the primary device, A, and the secondary devices, B, C, andD. A typical application is where the primary A is a cluster controller and B through D aredumb terminals connected to a host (either local or remote) to provide cost-effective ac-cess to a centralized host.

Note that when dealing with an SDLC loop operating on a two-wire multidrop cir-cuit, only a half-duplex connection protocol can be used, as indicated by the two-headedarrow, while a full-duplex operation can be accommodated on a four-wire circuit, as indi-cated by the single-headed arrow on the path in each direction. In a half-duplex opera-tion, the primary sends out data to the secondary devices and polls them for anyresponse. The secondary device sets a “final” bit in its response, indicating the last frameto be returned. A full-duplex operation is similar, except now the primary can send con-tinuously and uses this channel to poll the secondary devices.



Figure 3-8. Two-wire multidrop connection





Private Lines and Local LoopsA private, or leased, line is a dedicated circuit leased from a service provider for a prede-termined period of time, usually in increments of months or years. A private line may beupgraded by paying extra for a defined quality of service, such that conditioning is per-formed to ensure that a lower error rate is achieved, which makes a tremendous differ-ence in data communications. When service providers installed all-fiber networks, digitalprivate lines with much lower error rates are replacing the old voice-grade analog datacircuits at the same or even lower cost. Although analog voice-grade lines are still avail-able, most communications are carried on digital optical facilities. In the United States,there are very few private lines left that use microwave or analog circuits. This is espe-cially true in major metropolitan areas. Service level agreements (SLAs) on private linestypically guarantee minimum availability, delay, throughput, and loss compliance.

When leased lines are used to access other services, they are called access lines or localloops. A local loop typically connects customer premises equipment (CPE) to the service pro-vider’s central office (CO). Leased access lines can be purchased through LECs, competitiveaccess providers (CAPs), IXCs, or user-owned access arrangements. Access from these alter-nate sources is generally less expensive than the local telephone company prices. But, ofcourse, the alternative access service provider usually “cream-skims” the lucrative traffic(metropolitan area services) and leaves the “skimmed milk” of the smaller, more remote, oroccasional users for the LEC to serve. This is typically true for most high-speed Internet ac-cess Digital Subscriber Line (xDSL) services.

Figure 3-9. Four-wire multidrop connection





Private lines in Europe and Pacific rim countries are still very expensive, as is transo-ceanic fiber access. A service provider also must make an agreement with the party at theother side of a fiber to offer the transoceanic private-line service. A significant investmentis required for a small amount of bandwidth. The high cost of international private linesmay justify the cost of sophisticated, statistical multiplexers or statistical multiplexingservices like FR to utilize the expensive bandwidth as efficiently as possible. New fi-ber-optic technologies like erbium-doping techniques are enabling long transoceanic fi-ber runs that do not require repeaters, thus significantly driving down the cost oftranscontinental communications. The erbium-doping technique uses lasers to activateerbium ions in fiber to boost the optical signals transmitted through the fiber. Techniqueslike this create savings that eventually reach the consumer as lower private-line prices.

Digital Subscriber Line (DSL)Another form of DSL rate signal, operating over four wires, is the HDSL. HDSLs elimi-nate the cost of repeaters every 2,000 feet, as in a standard T1 repeater system, and are notaffected by bridge taps (splices). Users need to be within 12,000 feet of the serving CO,which covers over 80 percent of the DSL customers in the United States. ADSLs are alsoavailable and offer higher speeds (up to 640 Kbps) as well as better performance. The goalof ADSL technology is to deliver a video signal and telephone service over a majority ofthe existing copper twisted pairs currently connected to homes. Fiber-To-The-Curb(FTTC) and Fiber-To-The-Home (FTTH) have yet to see a wide-scale deployment. Both ofthese fiber implementations will enable services like ATM and higher-rate SDSL-liketechnologies to proliferate to the residential consumer. We will discuss more DSL optionsas well as other access technologies such as cable and satellite later in the book.

PRIVATE LEASED LINES VERSUS SWITCHED NETWORKSIn keeping with the discussion matter of this chapter, it is important to review the benefitsand risks of private-line networks versus switched networks. There are three general op-tions to data-transport networks today: private-line or dedicated leased-line networks,switched networks (including circuit and packet switched varieties), and hybrid designsincorporating a mix of both. Dedicated lines, also called private or leased lines, are dedi-cated circuits between two or more user devices. This type of circuit represents a dedi-cated private portion of bandwidth between two or more ports on the network, hence theterm private line. A private line is dedicated to one customer; the opposite is a publicservice shared among multiple customers. This private circuit is available to a customer 24hours a day, 7 days a week for a set fee (usually an initial nonrecurring fee and a monthlyrecurring fee). High volumes of traffic with frequent use justify this type of circuit. Theonly users of the circuit are the ports at both ends of the circuit. The bandwidth resourcesare in no way shared within the network. Although dedicated to a particular customer,these circuits are leased from the service provider, hence the term leased lines.





The second alternative is a switched network transport circuit. This can range fromsimple circuit switching, in which users dynamically select from a pool of multiple publicservice lines with fixed bandwidths, to intelligent, ubiquitous switched access networkswhere bandwidth is only allocated and used when needed, such as packet, frame, andcell relay networks.

Corporate communications usually comprise a hybrid network employing both so-lutions. The circuits requiring dedicated bandwidth to accommodate predictable vol-umes of constant-bandwidth traffic use dedicated circuits, while users requiringone-to-many connectivity, bandwidth-on-demand, and flexible or more dynamic accessuse switched network access. These decisions are also influenced by other factors such asthe burstiness of data (burstiness is defined as the peak-to-average traffic ratio, wherevideo traffic is less bursty and LAN-to-LAN TCP/IP traffic is typically very bursty), traf-fic patterns, bandwidth maximums, and minimum delay, to name a few. The three typesof network services that match the previous access types include private-line servicesdedicated to one customer, virtual private services that look like private services but rideon a public or shared network platform, and public network switched services.

Private (Leased) Line NetworksDedicated or private-line circuits are the simplest form of point-to-point communication.Circuits leased from a service provider (leased lines) are a form of private line. Private linesprovide a dedicated circuit of fixed bandwidth between two points. Figure 3-10 shows threeuser devices connected via three private lines. User A has a dedicated 56-Kbps circuit to userB as well as a dedicated 1.544-Mbps circuit to user C. Users B and C have a dedicated1.544-Mbps circuit between them.

Private-line bandwidths will vary, but typically follow standard electrical speed conven-tions of 56/64 Kbps (DS0), NxDS0 (56/64-Kbps increments) 1.544 Mbps, NxDS1 (1.5 Mbpsincrements), and optical speed conventions of OC-N where N is in increments of 51.83 Mbps.The digital and optical hierarchy (DS0, DS1, DS3, OC-N, and so on) will be explained in moredetail in Chapters 4 and 5.

Users will generally lease a private line when the entire bandwidth will always be avail-able between two points of choice whenever it is needed. They do not want to share thisbandwidth with anyone else, nor do they want to contend with other users to establish thecircuit (or wait during the delay time to establish it) to receive their required bandwidth. Thisbandwidth also affords the highest level of security and performance predictability.

Leased lines come in many grades and speeds. The most basic traditional serviceavailable consists of analog and digital leased lines having DS0, fractional DS1, DS1, frac-tional DS3, and DS3 speeds. These lines require a modem for digital-to-analog conver-sion and transmission over analog lines, or a CSU/DSU for line conditioning as well asproper framing and formatting for transmission over a digital line. This type of serviceranges from economic analog circuits to higher-grade Digital Data Service (DDS) trans-missions offered by the major IXCs and LECs, both inter-local access and transport area(LATA) and intra-LATA, respectively. Alternate access providers also offer DDS pri-vate-line connectivity. DDS is a private line digital service for data transmission and isgenerally more expensive and more reliable than analog leased lines.

The options for higher bandwidth access include SubRate Data Multiplexing(SRDM), fractional T1 (FT1), and dedicated T1, T2, and T3. SRDM offers the same access





speeds as DDS, but enables the aggregation of many low-speed channels into a singleDS0 for cost savings. FT1 and FT3 offer the same type of service, but at a DS1 and DS3level, respectively. Dedicated DS1 and DS3 offer just what they state: a single, dedicatedhigh-bandwidth circuit to access the service provider. If a user wants additional func-tionality, reliability, and availability, switched services are the alternative. In later chap-ters, the emergence of SONET, WDM, and GbE, will show the technological quantumleap that is now moving cost-effective user access speeds into the gigabits.

Although dedicated private line circuits provide the benefit of guaranteed availablebandwidth, they are typically nonredundant. At the network level, the digital cross-con-nects (DXCs) carrying private lines have network-layer disaster recovery, so the risk is atthe access level, unless LEC/CAP access redundancy is ordered and routed. If the privateline of any of its associated transmission equipment should fail or be taken out of com-mission (such as a fiber cut), the users on each end cannot communicate (unless they havesome method of reconnecting or dialing around the failure). Thus, the user must decidewhich level of availability is needed for communications between facilities. A decisionfor switched services backup is predicated on these trade-offs. Most large service provid-ers have SONET backbones that automatically reconfigure the user’s private line to alter-nate facilities during a backbone circuit or node failure, but the local loop will not berestored if cut (unless alternate LEC facilities are provisioned). Many options are avail-

Figure 3-10. Leased-line network




able for ensuring high availability in private lines. Some methods of routing trafficaround physical network failures and using wireless access alternatives are discussed inlater chapters.

Switched NetworksSwitched networks can range from simple circuit switching to advanced packet, frame,and cell switching, and they can include technologies such as IP, FR, and ATM. The maincharacteristics of switched networks include

▼ Addressing capability

■ Multiple protocol and interface support

■ One-to-many, many-to-one, and many-to-many connectivity

▲ Network intelligence above the physical Open Systems InterconnectionReference Model (OSIRM) layer

Circuit and packet (including packet, frame, and cell) switching are the most commonswitched network techniques. Each of these techniques will be defined in detail later. Theultimate method of achieving cost-effective high availability and throughput is by usinga switched network. Three examples of switched service offerings are FR, ATM, and IPVPNs. Examples of switched networks will be shown in later chapters.

Hybrid NetworksHybrid networks consist of some measure or mixture of private line and switched net-work access services, or even traffic that transits a private line before entering or after ex-iting a switched network. Either way, the important considerations are hardware,software, and protocol compatibility (or transparency). Network management and sup-port become more difficult when multiple network elements are crossed. Since switchednetworks offer significant cost savings and concentration benefits over dedicated lines,the driving factor should be to move from dedicated lines to switched network access assoon as possible where it makes sense. In some cases, a high-utilization, dedicated circuitbandwidth is required and a private line must be used. But in many cases, multiple trafficflows can be multiplexed together into switched access to achieve significant cost savings.

Today, the big choice in switched data is between switched services (includingSwitched 56/T1/T3), FR, and ATM. Routed services like IP will be covered in later chap-ters, along with combination switched and routed services like IP-FR and MPLS. Ubiqui-tous network access, quickly changing technologies with enhanced reliability, and theapparent economics of scale are driving many network designs to a switched network so-lution. Network support and management are also driving factors. Each of these deci-sions should be accompanied by a cost comparison of separate networks to a singlenetwork, or a dedicated line to a switched solution.





TRANSMISSION BASICSDigital Data transmission methods are often characterized as being either asynchronous orsynchronous. The terms asynchronous and synchronous are used in different contexts andhave entirely different meanings. The most common use of these terms is in the compari-son of asynchronous versus synchronous character or message transmissions. Since thistext presents a study of ATM technology, it is important to review another use of these twoterms, as in Synchronous versus Asynchronous Transfer Mode (STM and ATM). These twoentirely different meanings of the same term can be confusing, which is why this sectionpresents them together so that the reader can appreciate and understand the differences.

Asynchronous and Synchronous Data TransmissionAsynchronous character transmission has no clock either in or associated with the trans-mitted digital data stream. Characters are transmitted as a series of bits, with each characteridentified separately by start and stop bits, as illustrated in the example of ASCII charactersin Figure 3-11. There will typically be a variable amount of time between each charactertransmission. Analog modem communication employs this method extensively. The baudrate defines a nominal clock rate, which is the maximum asynchronous bit rate. The stop bitcan be greater than a baud interval in duration. Since at least 10-baud intervals are requiredto represent each character, the usable bit rate is no more than 80 percent of the baud rate (agood example of the overhead found even at Layer 1 of the OSIRM).

Asynchronous character transmission usually operates at low speeds (typically9600 bps). Asynchronous interfaces include RS232-C and D, as well as X.21. Modern-dayV.90 modems operate at an average of 56Kbps-capable throughput, but in actuality havelower throughputs.


Figure 3-11. Asynchronous modem character transmission




Synchronous data transmission clocks the bits at a regular rate by a clocking signal ei-ther associated with or derived from the transmitted digital data stream. Therefore,sender and receiver must have a means to derive a common clock within a certain fre-quency tolerance. On a parallel interface there is often a separate clock lead. Data flows incharacter streams are called message-framed data. Figure 3-12 shows a typical synchro-nous data stream. The message begins with two synchronization (SYNCH) charactersand a Start of Message (SOM) character. The Control (C) character(s) denotes the type ofuser data or message following. The user data follows next. The Cyclic RedundancyCheck (CRC) character checks the data for errors, and the End of Message (EOM) charac-ter signals the end of the transmission stream. The equipment then looks for another twoSYNC characters for the next piece of information.

Synchronous data transmission usually operates at speeds of 1,200 bps and higher. Syn-chronous data interfaces include V.35, RS449/RS-442 balanced, RS232-C and -D, and X.21.

Asynchronous Versus Synchronous Transfer ModeSince this text pays particular attention to ATM, or asynchronous time divisionmultiplexing, it is useful to compare this method to the commonly used STM, or synchro-nous time division multiplexing. Both of these methods have significant differences.

Figure 3-13 shows an example of STM and ATM. Figure 3-13 (A.) illustrates an STMstream where each time slot represents a reserved piece of bandwidth dedicated to a sin-gle channel, such as a DS0 in a DS1. Each frame contains n dedicated time slots per frame;for example, n is 192 in a DS1. Overhead fields identify STM frames that often contain op-eration information as well, such as the 193rd bit in a DS1 frame. Thus, if a channel is nottransmitting data, the time slot remains reserved and is still transmitted, without any use-ful payload. In this case, if the other channels have more data to transmit, they have towait until their reserved, assigned time slot occurs again. Frequent empty time slots re-sult in low line utilization.

ATM uses a completely different approach. A header field prefixes each fixed-lengthpayload, as shown in Figure 3-13 (B.). The header identifies the virtual channel. There-fore, the time slots are available to any user who has data ready to transmit. If no users are


Figure 3-12. Synchronous framed data message





ready to transmit, then an empty, or idle, cell is sent. ATM as compared with STM usuallycarries traffic patterns that are not continuous much more efficiently. The current ap-proach is to carry ATM cells over very high-speed STM transmission networks, such asSONET/Synchronous Digital Hierarchy (SDH). The match between high transmissionspeeds of SONET and SDH and the flexibility of ATM is a good one. We discuss the de-tails of SONET/SDH in Chapter 5.

HARDWARE SELECTION IN THE DESIGN PROCESSData-networking circuit and packet topologies form the roads by which data can travel.We will now explore the wide variety of LAN and WAN devices that utilize these roads.The various types of equipment available now or under development for use in the local,metropolitan, and wide areas are reviewed in this section. The principal networking equip-ment categories in place today include hubs, switches, and routers. There are also a numberof other (usually lower-level functions) that can be used in a building block manner, suchas multiplexers, concentrators, CSU/DSUs, and bridging devices, that are summarizedhere. Multiplexers and switch access devices will be considered in Chapter 4.

Here is a critical item: Before bridges, LAN switches, and routers were available, thefunctions that these devices now serve were once performed in mainframes and FEPs. Infact, the FEP was the first true router. The migration was from cluster controllers and

Figure 3-13. STM and ATM multiplexing





FEPs to simple routing devices and then to router-based networks. This was a major par-adigm shift in the last three decades of the twentieth century. As personal computing ar-rived on the scene, and as Millions of Instructions per Second (MIPS) moved to thedesktop during decentralization, the bridging and routing functions traditionally accom-plished at the FEP/host complex migrated toward the LAN and desktop. The advent ofpersonal computing, along with local and wide area networking, made routing andswitching outside the mainframe environment a necessity.

Perhaps the greatest driving factor was the multiple MAC and network-layer proto-cols operating within and between LANs as they moved into the WAN environment.Due to these diverse markets, technologies, and protocol suites, there evolved a need tomake diverse LAN, WAN, and operating-system protocols speak one language (or atleast provide a translation between similar languages on similar types of LANs). Whenbridges (predecessors to L2 switches) and routers (predecessors to L3 switches) first camealong, they were designed to deal with lower-speed LANs. Now the functions of bridg-ing, routing, and switching have merged in many cases into a single device. And with in-creased processor speeds, more advanced technologies, and concomitant reduced costs,these devices now support an extension of diverse LAN speeds over the WAN with ac-cess speeds from the Kbps up into the Gbps speeds.

Each hardware type mentioned in this chapter provides a different set of functional-ity, which can either be provided separately or together in one piece of equipment. Eachprovides protocol support for certain levels of the OSIRM as well as other proprietary ar-chitectures. For simplicity of discussion, and since the OSIRM seems to be the commonarchitectural point of reference (along with the IEEE 802.X protocol model), the protocolsupport for each hardware device will be given in reference to the seven-layer OSIRMreference model. Repeaters and legacy bridges have come to play a reduced role, yield-ing to remote-access switches and routers and Small Office Home Office (SOHO) de-vices that require only a subset of their larger counterparts. Intelligent LAN hubs andswitches that provide LAN connectivity and concentration, bridging, routing, andswitching will also be covered in detail. Each of these new hardware technologies offersspecific advantages and disadvantages depending upon user applications, protocols,addressing, and data transport needs. The network designer must understand each ofthese technologies to ensure successful LAN-MAN-WAN connectivity, interoperability,and integration. Starting with the simplest devices will allow us to work toward themost complex.

RepeatersRepeaters are inexpensive distance-extension devices, providing physical distance exten-sion through signal regeneration for point-to-point circuits. This enables a network to ex-tend the distance between network devices, similar to an extension cord for electricity,while providing electrical isolation during problem conditions. Thus, repeaters offer thecapability to extend an existing LAN or WAN segment at the L1 protocol interface. Re-peaters possess very little intelligence. They are commonly used as signal regenerators,protecting against signal attenuation while improving signal quality. Due to this lack ofintelligence, repeaters add value by maintaining the integrity of all data being passed, but





they are completely transparent to all data content. Drawbacks to using repeaters includepossible network congestion caused by the overhead they may add due to repeating andjitter imposed by signal delay. The effects of excessive jitter will be covered in later chap-ters. Repeaters form the core component of hubs and use only the physical layer of theOSIRM. Figure 3-14 portrays user A and user B communicating via a repeater in relationto the OSIRM. Note that while a repeater uses only the physical layer, it may operate elec-trically or optically.

Modems, Line Drivers/Limited-Distance Modems (LDMs)A modem modulates outgoing digital signals from a computer or other digital device toanalog signals and demodulates the incoming analog signal and converts it to a digitalsignal for the digital device. Signals are transmitted and received over unshieldedtwisted pair phone lines. Most modems today are 56 Kbps using the V.90 standard, whichwas derived from the x2 technology of 3Com (US Robotics) and Rockwell’s K56flex tech-nology. DSL services are accessed via the same medium, but use special DSL modems.DSL modems use various modulation techniques including Discrete Multitone Technol-ogy (DMT), Carrierless Amplitude Modulation (CAP), and Multiple Virtual Line (MVL).Broadband cable data services are also available that offer even higher bandwidth alter-natives than traditional analog and DSL modems. Cable modems connect to both thecomputer or LAN and the Cable TV and offer bandwidth to both. Cable modems have aconnection to the cable wall outlet and one to the PC (NIC card), router, switch, or set-topbox. Cable modems modulate between analog and digital signals, and attach to the coax-ial cable that communicates with a Cable Modem Termination System (CMTS) at the lo-cal cable TV company office. All cable modems can receive from and send signals only to

Application

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repeateror

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Figure 3-14. A repeater, LDM, CSU/DSU, and the OSIRM





the CMTS, but not to other cable modems on the line. Some services have the upstreamsignals returned by telephone rather than cable, in which case the cable modem is knownas a telco-return cable modem. Bandwidth can approach 27Mbps downstream and2.5Mbps upstream, although current services offer much less bandwidth.

Line drivers, also called Limited-Distance Modems (LDMs), are used to extend thedistance of a physical circuit. Basic telecommunications courses teach that modems pro-vide modulation/demodulation between analog and digital data. LDMs provide thesame functionality, but in the form of a repeater. Figure 3-14 shows the OSIRM layer usedby line drivers and LDMs as Layer 1.

CSU and DSUThe terms channel service unit (CSU) and data service unit (DSU) are often incorrectly usedinterchangeably. Originally, the CSU was developed to protect CPE from voltage surgesin the access line. Additional remote testing and monitoring capabilities such as bipolarviolation and loop-back testing were later added. The DSU was typically the lower-speeddevice, providing signal format and protocol translation, timing recovery, and synchro-nous sampling, as well as acting as the termination point for DDS 56 Kbps and below. TheCSU was a higher-speed device, used at DS1 rates, which could also be used at lowerspeeds. The CSU terminates digital circuits with the same features as a DSU, but it alsoprovides many feature functions not provided by the DSU, such as filtering, line equal-ization, line conditioning, signal regeneration and amplification, circuit loop-back testingcapabilities, and error control protocol conversion (that is, B8ZS) peculiar to DS1 service.

Today, a device is available that has merged CSU and DSU functionality. CSUs/DSUshave the capability for Extended Super Frame (ESF) monitoring and testing as well as ad-vanced Simple Network Management Protocol (SNMP) monitoring functionality [withtheir own management information bases (MIBs)]. Some even have the capability to multi-plex traffic from multiple input ports into a single point-to-point or multidrop circuit.

DSUs come in many speeds and with many different functions. There are six majorcategories of DSUs. First, fixed-rate DSUs operate at speeds of 19.2 Kbps and below(subrate) or at the fixed speed of 56 Kbps. Second, multirate DSUs can be purchased thatoperate at variable speeds at or below 56 Kbps. Third, these two types of DSUs can also beobtained with a secondary channel for network management. The fourth type of DSU isthe switched 56-Kbps DSU, which operates with 56 switched digital services. T1 dedi-cated and switched DSUs are also available. Standard CSUs provide a T1 circuit interfaceand can have properties similar to the DSUs mentioned previously. Finally, CSUs andDSUs are capable of multiplexing multiple T1, V.35, and RS-232 user ports into a singledata stream for “integrated access” into a LEC or IXC switch, where they can bedemultiplexed and passed to the appropriate service. Some CDU/DSUs may even havemanagement and reporting for higher-layer protocols that pass through their interfaces,or even provide part of the protocol functionality (such as FR CSU/DSU and ATMCSU/DSUs, respectively), as we will see in a minute.

Many CSU/DSU vendors also market DS-3/E-3 products, which provide theHigh-Speed Serial Interface (HSSI) for direct DS-3 connectivity. A few of the vendors,such as Quick Eagle Networks (formerly known as Digital Link), Visual Networks, andKentrox offer SMDS and ATM support. But let the buyer beware, caveat emptor, for many





of these SMDS and ATM support functions are proprietary between the CSU/DSU ven-dors and a particular hardware vendor.

With the emergence of broadband services such as SMDS and ATM Data ExchangeInterfaces (DXIs), the CSU takes on an entirely new function apart from its normal func-tionality. Some SMDS and ATM CSUs actually perform some of the required protocolconversion and cell segmentation, going far beyond their original function as a powerprotector interface. For example, with SMDS DXI, special SMDS CSUs actually take thehigh-level L3_PDU frame and segment it into L2_PDU cells, performing part of theSMDS protocol function within the CSU. The CSU then interfaces to the SMDS networkthrough a SMDS Interface Protocol (SIP). SIP will be reviewed in detail in Chapter 11.

Also note that many times a specific CSU or DSU setting or feature works best de-pending on the Layer 2 (L2) protocol being passed, such as with FR, where B8ZS is the op-tion of choice for DS1 speeds using ESF.

DSU/CSU standard interfaces include 56-Kbps, FT1, and DS1 using EIA-232-C, V.35,and HSSI (on DS-3 models). CSU and DSU operation is similar to that shown in Figure 3-17with repeaters, and the OSIRM layer used by CSUs and DSUs is shown in this same figure.Note that only physical Layer 1 is used by a CSU or DSU (except with the SMDS, ATM DSU,and some FR CSU/DSUs).

Hubs and LAN SwitchesLAN hubs, also called concentrator hubs, are devices that connect multiple LAN seg-ments (such as Ethernet and Token Ring) or single workstations (or servers) and combinethem on a shared backplane. Thus, a hub can be used to combine multiple workstationsor servers onto a single LAN segment, or multiple LAN segments onto a single LAN seg-ment. The former case is the hub acting as a repeater for multiple LAN network interfacecards (NICs); the workstations interface to the LAN media (that is, 10BASE-T). Largerhubs typically have multiple repeater cards and serve many workstations. In either case,these devices work as Layer 1 devices. Switching hubs, hubs combined with Layer 2switching intelligence, can perform both the segment-joining functions as well as switch-ing directly between workstations and servers or between LAN segments.

LAN hubs have been classified into four generations. Figure 3-15 illustrates each gen-eration of LAN hub along with its LAN protocol support at each stage. Hub configura-tions tend to model the star topology, with the hub as the center and each LAN device (formultiple LAN) attached directly to the hub. The first-generation LAN hub appeared in1984 and acted as a repeater for a single type of LAN connectivity (that is, an Ethernet ho-mogeneous environment). These hubs provide the functionality of a LAN concentrationpoint, supporting a single bus to provide physical connectivity for multiple ports on oneor more LAN operating on the same architecture (later, different LAN types could bemultiplexed, such as Ethernet and Token Ring). This function is similar to that of a combi-nation patch panel and repeater, as shown in Figure 3-15 (A.). One example is IBM’sMultistation Access Unit (MAU) wiring concentrator used for Token Ring coaxialhubbing. This architecture can now be purchased in stores in the form of a $30 four-portdumb hub and can be quite useful for small home area networks (HANs) sharing a singlecable modem (the authors advocate a combination hub or switch with an integratedrouter/firewall to provide maximum security over a cable modem connection).





Second-generation LAN hubs provide the same bus architectures, but accommodatedifferent LAN architectures over multiple ports, such as Ethernet and Token Ring. Addi-tional features such as local and remote network management and configuration capabil-ities have also been added, as shown in Figure 3-15 (B.).

Third-generation LAN hubs/switches provide multiple buses for connectivity simi-lar to the second generation, but also add L2 bridging [and occasionally Layer 3 (L3) rout-ing] functions, now called a LAN or L2 switch. Vendors support a wide range of mediaand often multiple multimegabit or gigabit buses in the hub architecture. Third-genera-tion LAN hubs/switches also have additional network management features and aresometimes called smart hubs. This generation also saw the appearance of ATM interfaces.These hubs are shown in Figure 3-15 (C.). Support for some form of network manage-ment protocol, like the SNMP, is a must.

Ethernet

Ethernet Ethernet

Ethernet

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D.C.

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LANhub

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coax

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Multiple buses Multiple busesATM ATM

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intelligence

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coax

coax coax

NM

10/100/1000Mbps

10/100/1000Mbps

10/100 Mbps10/100 Mbps

10/100 Mbps

10/100 Mbps

Figure 3-15. Four generations of wiring hubs





Fourth-generation switching hubs appeared in the latter part of the 1990s. Switchinghubs offer all the capabilities mentioned previously along with the addition of MAC-layer switching, transparent bridging, and standard wide area trunk interfaces. At times,these fourth-generation switching hubs offer simple routing and maybe even elementaryfirewall functions. These switching hubs are shown in Figure 3-15 (D.).

Figure 3-16 shows a building using a LAN switch to connect distributed LAN hubs ofthe same type or different types (Ethernet to Ethernet, Ethernet to Token Ring, TokenRing to Token Ring) from multiple floors. The following sections on bridges and routerswill explain these techniques from the protocol and OSIRM point of view. Figure 3-17shows the OSIRM layers used by LAN hubs and switches. Notice that hubs use the physi-cal layer, while LAN bridges and switches use the data-link layer (DLL).

Hub and LAN switch designs are typically built in a hierarchical nature, as shown inFigure 3-18. Here we see the wiring collection, segmentation, and network managementfunctions performed by hubs and LAN switches. Usually, many Ethernet or Token Ringtwisted pair lines, FDDI, and ATM over twisted pair to individual workstations are runto a central hub or LAN switch often located in the wiring closet. Hubs enable adminis-trators to assign individual users to a resource (such as an Ethernet segment), shown asan ellipse in the figure, via network management commands. Lower-level hubs are oftenconnected in a hierarchy to higher-level hubs or LAN switches, sometimes viahigher-speed protocols such as Fast Ethernet, Gigabit Ethernet, FDDI, or ATM over opti-cal fiber and high-grade twisted pairs. These devices are often employed in a hierarchicalmanner to concentrate access for many individual users to a shared resource, such as aserver or router, as shown in Figure 3-18. The highest-level hubs and LAN switches arecandidates for collapsed backbone architecture based on protocols like Ethernet, FDDI,and ATM, which also support high-speed access to shared resources such as routers andother LAN switches. Stackable modular implementations are also available and providean easy method of upgrade as the number of users increases.

BridgesBridges typically provide connectivity between LANs of similar architecture, such asEthernet-to-Ethernet or Token Ring-to-Token Ring connectivity, forming one of the sim-plest of LAN and WAN connections. The exception to this rule is the translation bridgethat translates from one media format to another. Many years ago, bridges werestand-alone boxes. Today, bridging can be performed in intelligent hubs, LAN switches,L3 switches or routers, or any device that runs bridging software. For the purposes of thisdiscussion, we will refer to any device that performs bridging as a bridge.

A bridge uses a minimal amount of processing and thus is a less expensive way to linkLANs having the same physical- and link-layer protocols. A hallway or an entire city canseparate these LANs. Bridges can also connect devices using physical- and link-layer pro-tocols to devices using the higher-level IEEE 802.X protocol suite (including FDDI). Sincebridges are protocol transparent above Layer 2, they do not provide flow control or rec-ognize higher-level protocols. They use only the physical and link layers of the OSIRMand support both the logical link control (LLC) and the MAC layers of LAN transmission.Modern-day LAN switches are in reality very fast multiport bridges.





Figure 3-16. LAN hub building plan

Figure 3-17. Hub and LAN switch communications via the OSIRM





Figure 3-19 portrays user A and user B communicating via a bridge over the physicaland link layers of the OSIRM. Bridges operate at the MAC layer of the OSI DLL. Both usersare implementing the same protocol stack for Layers 1 and 2, and the bridge does not mod-ify the information flow in any way (except a required MAC-layer conversion in translationbridging, depending on whether a header is included as part of the “information flow”). Thebridge supports linking at the physical and link levels, but provides no addressing orswitching functionality. Thus, the user provides all addressing and protocol translation.Bridges simply pass traffic from one network segment to another based on the destinationMAC address of the packet being passed. If the destination address of the frame received bythe bridge is not local to the bridge, the frame is obviously destined for another LAN andthus the bridge simply forwards the frame on to the next network device.

Bridges will store and forward packets between bridges as packet switches would, butthe bridge cannot act as a switch. The exception to this rule is the LAN switch, which alsoperforms bridging, in most cases. Bridges send each packet to a remote user based upon adestination address. Bridges can recognize either a fixed routing table scheme or, for moreexpensive bridges, a dynamic learning routing scheme. Bridges can “learn” the networkthrough the use of intelligent bridging and routing protocols, and some bridges are able todynamically update their forwarding tables. Bridging protocols will be discussed later.

Figure 3-18. Hub and LAN switch interface, functions, and architecture




Another key function of bridges is their capability to filter data. But a major drawbackto bridges is that they cannot forward data if operating at their maximum filtering rate.Bridges deployed in a network do not have advance knowledge of the network to whichthey are attached. They are blind to devices other than those that attach to their logicalpath structure. The flexibility of bridge connectivity will be discussed further when wecover bridge protocols in Chapter 7.

There are four major types of bridging: transparent, translating, encapsulating, andsource route bridging. Each provides different functionality for the various LAN archi-tectures.

When operating in transparent mode, bridges at both ends of a transmission supportthe same physical media and link-layer (MAC-level) protocols from the IEEE 802.X suite(or possibly FDDI), but transmission rates may vary. From the point of view of the net-work node, transparent bridges take no part in the route discovery or selection process.The higher-level protocols (OSI L3 and higher) need to be the same or compatible for allconnected applications, because bridges are transparent to protocols at, or above, the net-work layer.

Figure 3-20 shows examples of transparent bridging between two local EthernetLANs and two local Token Ring LANs. Encapsulation bridging between two TokenRing LANs is also shown, requiring CSUs or DSUs. In encapsulation bridging, the LANframe is placed in a serial encapsulation [High-level Data Link Control (HDLC),Point-to-Point Protocol (PPP), or proprietary] over the point-to-point circuit and is thende-encapsulated without modification at the other end.


Figure 3-19. Bridge communications via the OSIRM





Sometimes the bridge needs to send data between dissimilar LANs, such as from anEthernet LAN to a Token Ring LAN. When operating in translation mode, bridges at bothends of the transmission can use different physical media and link (MAC-level) proto-cols. Translating bridges thus translate from one media format to another, manipulatingthe MAC-layer frame structure associated with each media type. Protocols in the net-work layer and higher must still be compatible.

The following illustration shows an example of translation bridging between dissimi-lar local Ethernet and Token Ring LANs.

Figure 3-20. Transparent bridging





Translation bridges do not provide segmentation services, so the frame sizes of eachLAN host must be configured for the same supportable length (usually a maximum of thelargest allowable frame size of the limiting LAN technology).

When operating in encapsulation mode, bridges at both ends of the transmission mustuse the same physical and link-layer (MAC-level) LAN protocols, but the transmissionnetwork between the bridges can provide a similar or different physical media andMAC-level protocol. Encapsulating bridges provide a network interconnection or exten-sion by placing received frames within a media-specific “envelope” and forwarding theencapsulated frame to another bridge for delivery to the destination. This is commonwhen a Token Ring or FDDI backbone serves multiple Ethernet segments. The backboneprotocol then serves as the WAN protocol.

Figure 3-21 shows two examples of encapsulation bridging. The first example illus-trates two remote 10-Mbps Ethernet LANs being bridged via a metropolitan 100-MbpsFDDI network. The second example shows the same two Ethernet LANs, this timebridged over a 4- or 16-Mbps Token Ring network. Pay heed when encapsulating largeMAC frames into smaller frame sizes. Remember that the maximum frame size for Ether-net is 1,500 bytes, whereas the Token Ring frame size can be up to 17,972 bytes.

The fourth type of bridging is through source route bridging. Figure 3-22 shows asource route bridging scheme between two remote Token Ring LANs and three sourceroute bridges. The third Token Ring LAN is used only for transit. Source route bridgingwill be discussed in detail in Chapter 7.

Bridges are best used in small, geographically concentrated networks, which do notrequire a large customer address base and are needed to connect a fairly static networkdesign. Bridging speeds vary, supporting subrate DS0 through T1 to T3, and even FDDIand Ethernet 100-Mbps and higher (gigabit Ethernet) bridging speeds. These highspeeds are needed to support the high-speed LANs connected to the bridge, such as10/100/1000-Mbps Ethernet and 4/16-Mbps Token Ring. Bridges provide local, remote,or both local and remote configuration support.

Careful future planning is required when deploying a bridged/switched network so-lution. The manager or engineer who employs a bridge solution may find that very soonhis or her bridge solution will resemble the wood and stone bridge, built in 1850 and de-signed to accommodate a horse and carriage. Soon, there will be a need to drive not only acar, but also trucks over the bridge, but one year later rather than 150 years later. Thus,bridges can be good solutions for networks utilizing only one protocol and one architec-ture with no plans to change, or for very static network designs with multiple protocolsand architectures that have close local control.

Some major disadvantages are associated with bridging. For example, bridges aresusceptible to multicast or broadcast storms. For a packet with an unknown MAC ad-dress (one not associated with a port), a bridge floods that packet out all ports except theone on which it was received. Bridges in and of themselves do not create broadcaststorms. Such storms occur when there is a bridging loop in the topology. Bridging prob-lems increase with the size of the network and the number of users attached.

To minimize the problem, smart bridging/L2 switching techniques can provide somelevel of traffic isolation. Some bridges cope with broadcast storms by segmenting thebridged network into domains that restrict broadcast storms to a limited area. This con-





tainment method coupled with a multicast traffic ceiling effectively controls broadcaststorms. Bridges are also limited in both address retention and memory. They are de-signed to retain a limited amount of information and can handle only limited networkchanges. The more changes occurring in the network, the greater is the traffic passing be-tween routers to update routing tables; thus, an unstable network could occur.

Due to these disadvantages and limited capabilities, bridging should not be used innetwork designs calling for multiple protocol support, dynamic networks requiring fre-quent changes, or large networks of greater than 50 nodes. For networks with these re-quirements, more intelligent and robust devices will provide much of the bridgingfunctionality and additional routing intelligence as well as eliminate the disadvantagesof bridging. Enter the switch and router.

Figure 3-21. Encapsulation bridging

Figure 3-22. Source route bridging





SwitchesThere are four general classes of switches. Workgroup or local switches switch trafficwithin a workgroup, such as between workstations or local LANs. Enterprise switchesconnect multiple departments or workgroups. Both workgroup and enterprise switchesare typically 100/1000-Mbps Ethernet or ATM. Edge switches serve as access or entryswitches to a public data service and can be packet (X.25 or IP), frame (FR, Ethernet, orFDDI) or cell (ATM), or optical SONET/WDM switches. Service provider backbone (CO)switches (typically packet, frame, or cell switches) act as high-speed interconnects foredge switches. Figure 3-23 shows these four types of switches.

Figure 3-23. Router communications via the OSIRM





Usually, a private switched network is connected to one or more public switched net-works. L2 switches are connection-oriented devices. The CPE interfaces to the serviceprovider’s switch and communicates the connection request information via a user-to-network interface (UNI) signaling protocol. An interswitch protocol may be used be-tween switches. Networks are interconnected via a more complex network-to-networkinterface (NNI) signaling protocol. Signaling functions may be emulated by networkmanagement protocols where individual cross-connects are made. These switching fun-damentals are illustrated in Figure 3-24.

There is much confusion as to which layers of the OSIRM are used when switching.There are basically two types of switches. L2 switches are commonly referred to as LANswitches, and as switch frames typically at the MAC layer (examples include Ethernet,Token Ring, FDDI, and ATM). Hybrid L2 and L3 switches are used when some form ofpacket, frame, or cell switching is being used, such as when routing IP or accessing FR orATM services. Sometimes the function of the switch and other devices like routing willmerge. For example, there are many backbone sites that place routing within ATMswitches, as they can provide a better quality of service through protocol features specific

Figure 3-24. Router functions





to ATM. But nothing is free and there is a trade-off: The virtual circuits (VCs) used inATM are expensive network resources. With routing that uses quality of service, eachrouter must maintain the state of every other switch in the network. The same is true forATM switches that include routing and maintain quality of service. More on ATMswitching will be covered in later chapters, and switching methods are covered in detailin Chapter 9. As you can see, once you have mastered the basic principles of networking,they will appear again and again—in technologies not yet invented.

RoutersRouters provide interconnectivity between like and unlike devices on LANs and WANsas well as extend the LAN into the metropolitan and wide area networking arena.Routers are protocol sensitive and can either bridge or route a large suite of net-work-layer and higher-layer protocols. Thus, they support various LAN devices that canemploy a variety of networking protocols and addressing schemes. Routers understandthe entire network, not just locally connected devices, and will route based on many fac-tors to determine the best path. Routers have formed the core of the next generation ofcomputer internetworking devices.

As mentioned previously, routers have subsumed many of the legacy functions ofcluster-controllers-to-FEP-to-mainframe communications. Almost all Internet traffic tra-verses multiple routers. Routers emerged into the marketplace over the last two decadesas the hottest thing since multiplexers [starting with the first-generation routers that ap-peared at the Massachusetts Institute of Technology (MIT), Stanford University, and Car-negie Mellon University (CMU) in 1983 and ARPANET predecessors three years earlier],with much more intelligence than bridges or multiplexers. Dozens of multiplexer ven-dors that did not anticipate nor adapt to the routing wave were quickly swept awaywithin a few short years. Routing was a prime example of a technology that dramaticallychanged the vendor landscape, as evidenced by the rapid rise to fame and fortune ofCisco and Wellfleet (acquired by Bay Networks and later by NorTel Networks) based onrouting technology.

Routers use the physical, data-link, and network layers of the OSIRM to provide ad-dressing and switching functionality. Figure 3-25 shows the relation of the router to theOSIRM. Both users may exercise the same protocol stack up to L3. A router’s main func-tionality resides in the data-link- and network-layer protocols, but it also uses the physi-cal layer. Applications at both ends of the transmission do not need to support the sameLAN protocol from the IEEE 802.X suite, or protocols up to OSI level 3 within the same ar-chitecture, but they do need to have the same protocol from the fourth through seventhlayers of the OSIRM (or at least the intelligence at the user end to provide the gatewayfunctionality, if needed).

Routers use their own internetworking protocol suite. Through the use of routing ta-bles, as shown in Figure 3-26, and routing protocols, such as Open Shortest Path First(OSPF) and the Routing Information Protocol (RIP), routers constantly pass informationto each other that helps them build topology tables of the network. This artificial intelli-gence, called dynamic knowledge, allows them to be globally aware of the entire network.They then interface to L1, L2, or L3 WAN services like PL, FR, ATM, or IP. These routing





protocols can discover network topology changes and provide rerouting based upon dy-namically populated routing tables. For example, in OSPF each router keeps a map of theentire network. When changes in the network happen, only the change is propagated toother routers, versus RIP where the entire routing table is propagated.

Figure 3-25. Routing interfaces, functions, and architecture

Figure 3-26. Switching interfaces, functions, and architectures





Routers can limit the number of hop counts by their intelligent routing protocols. Hopcount limits are determined by the routing protocols and the general protocol itself. For ex-ample, Internetwork Packet Exchange (IPX) RIP has a hop limit of 16, as defined in the IPXstandard. IP has a hop limit of 255, as determined by the max value of the Time to Live (TTL)field. With the TCP/IP suite, different protocols have different values. RIPv1 is 15 hops, asdefined in the standard. (E)IGRP is by default 100, but can be increased to 255. Again, the IPTTL limits itself. OSPF has no hop count limit per se, but an IP packet cannot “live” beyond255 hops, as defined in RFC 1812, and thus in the TCP/IP suite itself.

Routers employ large addressing schemes, typically up to 4 bytes worth of addressesin a logical network [or more with Novell IPX or the OSI Connectionless Network LayerProtocol (CLNP)]. Routers also support large packet sizes in the many thousands ofbytes. Internal bus speeds are also much higher, typically in excess of a gigabit per sec-ond. The other major advantage of routers is their capability to perform these functionsprimarily through the use of software, which makes future revisions and support for up-grades much easier.

Routers use routing protocols to route packets from node to node based on thepacket-defined protocol information used by the routing protocols. Routing protocols arenot the only means of placing routes into a routing table, or forwarding information base,as some call it. Routing protocols in and of themselves do not route. They are but onemeans of “path determination” (other means include static routes, directly connected in-terfaces, and the redistribution of routes between routing protocols). Routers route (or, assome say, forward) packets based on the routing/forwarding table. But routing protocolsthemselves are not part of this forwarding process. This information typically includesleast-cost routing, minimum delay, minimum distance, and least-congestion conditions,depending upon the protocol and its support for features. Type of service (TOS) hasnever been implemented in TCP/IP OSPF, for example, because the Internet EngineeringTask Force (IETF) could never figure out what it was they wanted to accomplish. Hence,the TOS bit is seldom used. Now with RSVP and QoS, particularly in conjunction withVoice over IP (VoIP), the TOS bits can and are manipulated and used.

Least-cost routing is an interesting term, which is usually seen used in conjunctionwith Private Branch Exchange (PBX) functionality for voice calls. How does this termwork in a data network where the transmission cost is fixed? Multiprotocol routers canprovide support for multiple protocols simultaneously.

Figure 3-27 illustrates the range of interfaces and the scope of routing. Physical or vir-tual circuits often connect routers. Routers have very sophisticated software, and are nowbeing delivered with special-purpose firmware and hardware to increase packet-routingthroughput. Current routers can forward in excess of 40,000,000 IP packets per second.

Routers automatically discover the addresses of devices connected to each router inan internetwork. They use an interior routing protocol (within their internetwork) andconnect from their internetwork to outside networks using an exterior routing protocol.Static routing where the router is manually configured is also possible and not uncom-mon. Indeed, even the naming of networks as subnetworks of a larger network hasproven to be a very scalable concept. Notice that the term network can be anything from apiece of cable that forms a LAN segment to many devices internetworked across a largegeographic area that are managed by a single system and share a common addressing





scheme. Packets are routed based upon the destination address, sometimes using thesource address as well, or even an end-to-end route specification. Routers connect dis-similar protocols by way of routing and data protocol conversion. Routers can also han-dle both connection-oriented and connectionless network services, and can interconnectdissimilar media via media conversion.

Routing protocols continually monitor the state of the links that interconnect routersin an internetwork or the links with other networks through a variety of routing proto-cols. These protocols include RIP, OSPF, and Cisco’s proprietary Enhanced Interior Gate-way Routing Protocol (EIGRP) within their internetwork, as well as the Border GatewayProtocol (BGP) routing protocol, which is used to connect to other networks or the publicInternet. Each of these routing protocols is covered in detail later. Through these proto-cols, routers can discover network topology changes and provide dynamic reroutingaround link and node failures.

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R

Figure 3-27. Switching fundamentals





As access devices, routers accept multiple protocols and either route them to anotherlocal port on the router or convert them to a WAN protocol, such as IP, for transfer over aWAN link. When doing so, the router encapsulates, for example, IP traffic into FR frames,or ATM cells across a Layer 1 channel (such as a DS1) for transmission to or across a pub-lic network service. Routers can also switch WAN transport protocols between in-terfaceports, such as switching frames or cells. When routers send data over a WAN using FR,SMDS, or ATM, they may need to look inside a sequence of frames, packets, or cells in or-der to perform the routing function.

Three prime examples of routers with these capabilities are the Cisco 7600 seriesInternet router, Nortel Network’s Backbone Concentrator Node (BCN) router, and Juni-per Networks’ M-series Internet backbone routers. Many routers today support ATM in-terface and trunk cards, along with some form of ATM switching and LAN emulation.Routers typically are manageable through network management protocols such as IPand the Common Management Information Services/Common Management Informa-tion Protocol (CMIS/CMIP).

Routing Compared to Bridging and SwitchingA great sage once wrote, “Route when you can, bridge when you must.” Although routingis much more complex, it is also more feature-rich and has many advantages over bridging.Routers provide a level of congestion control not present in bridges, thus enabling the routerto dynamically reroute traffic over, for example, the least cost path (where the cost metric canbe the shortest path, the least cost ($) path, the lowest delay route, or a variety of other vari-ables). Routers reduce broadcast storm danger by providing a segmentation capabilitywithin the network; that is, they do not forward broadcasts across segments, other than incertain specifically designated circumstances [such as Dynamic Host Configuration Proto-col (DHCP) requests].

Thus, the network designer can build a hierarchical addressing scheme and designsmart routing tables, which operate somewhat similarly to the filtering capabilities ofbridges, but with the additional flexibility to define virtual network subsets within a largernetwork definition. Routers differ from bridges in that they provide protocol translation be-tween users at the link level, while bridges just pass information in a store-and-forwardmode between devices of similar protocol structures. Additionally, routers are required toaccess the public Internet, unless you are using a DSL L2 (usually ATM) access servicewhere the service providers have the router that accesses the Internet, versus having therouter on your customer’s premise. Greater detail will be forthcoming when we discussrouter access network design.

Routers that utilize IP routing schemes can solve packet-fragmentation problemscaused by technologies such as X.25 and FDDI. Packet fragmentation is necessary when-ever two media types with different sized packets are used. Routers have the capability totranslate between MAC layers. Unlike bridges, routers can be isolated and routed aroundwhen network problems exist. Routers contain a level of investment protection over lessintelligent (yet faster) switching and bridging devices.

However, there are also a few disadvantages to routers. Routing algorithms, dis-cussed in great detail later, typically require more system memory resources than





bridges and L2 switches and can be more complex to design and manage. Modern rout-ing algorithms and implementations (IS-IS, OSPF, EIGRP, and BGP) are comparable tobridging in the amount of bandwidth overhead. This is because of the additional intelli-gence needed in the routing protocols and the various congestion-control techniquesimplemented.

Many router vendors have implemented multiple processors as well as faster plat-forms and processors [such as Reduced Instruction Set Computer (RISC) processor tech-nology] to eliminate throughput problems caused by increased traffic loads of routingprotocols. Table 3-1 shows a comparison of bridge and router uses and capabilities. Notethat we use the term bridging and L2 switching interchangeably.

It is a good idea to bridge/switch when you need less overhead, have the same LAN me-dia type across the entire network, have a small centralized LAN with a simple topology, orneed to transport protocols that cannot be routed, such as the Network Basic Input OutputSystem (NetBIOS) and the Compaq (formerly Digital Equipment Corporation) Local AreaTransport (LAT). Spanning trees enable responses to topology changes, but they are slowerthan modern routing protocols.

Routing should be performed when you want to route traffic based upon network pa-rameters like the least-cost route, have multiple MAC protocol environments, have large,dynamic networks with complex topologies, want dynamic routing around failed linksover paths that run in parallel, or have network and subnetwork addressing requirements.

At present, Cisco still holds the largest share (over half) of the router market. In fact,most of the routers on the Internet backbone are Cisco. Every major router vendor sup-ports bridging and routing capabilities. The prices of routers have dropped significantly,

Functionality Bridging/Switching Routing

Data sources One source and destination Multiple sources anddestinations

Network addressing No (MAC addressing is L2) Yes (IP addressing)

Packet handling Pass packet transparently Interpret packet

Forward packets Out all ports (except porton which packet received)

Out specific port todestination

Global networkintelligence

Local only Knows status ofall devices

Priority schemes Some (L2) Yes

Security Poor, based on isolatingLAN segments orMAC filtering

Good, based on routingprotocol combinedwith filtering or IPSecurity (IPSec)

Table 3-1. Bridging to Routing Comparison





and many providers offer low-end remote access products that have a subset of protocoland memory resources from their larger counterparts. In fact, low-end SOHO routers canbe purchased for a few hundred dollars or less today (and many at that cost with built-infirewalls and multi-port switching).

Many bridging and routing functions have been built into workstations, PCs, andservers. Although this offers the cost and management advantages of using only a singledevice, users should be aware of product support, scale, upgrade, and manageability lim-itations. Choose the right device that will grow with your network. Purchase a device thatcan support both routing and switching—you may not need routing today, but as yournetwork grows you can upgrade without having to replace the device. It may be less ex-pensive in the long run to purchase a router with a port expansion, rather than taking thenetwork down and installing a larger or more feature-rich router later. Port sizing andtraffic growth patterns will typically dictate the size of the switch and router.

BroutersThe term brouter is a conflated word formed by bridge and router. Brouters perform thefunctions of both bridges and routers; that is, they have the capability to route some pro-tocols and bridge others. Some protocols need to be bridged (such as LAT and NetBIOS)rather than routed. Brouters were developed from the need to expand single-port bridgesto multiple ports to support IBM’s source route bridging and source route transparentbridging algorithms. The routing done by brouters is transparent to both the net-work-layer protocols and end stations, and is accomplished in the MAC address. Thus,brouters do not look at the network-level address. Rather, they route based on the MACheader. Router logical functionality is similar to that of bridges and routers, as shown inprevious figures. The term brouter is rarely used anymore, and the more common form isin remote access routers that only require a subset of bridging and routing.

GatewaysGateways provide all of the interconnectivity provided by routers and bridges, but in ad-dition they furnish connectivity and conversion between the seven layers of the OSIRMas well as other proprietary protocols. Gateways can be performed in hardware, soft-ware, or both. Some applications use priority schemes not consistent between the OSIlayers and proprietary protocol structures. Gateways are often application specific and,because of their complex protocol conversions, are often slower than bridges, switches,and routers.

One example of a gateway function is interfacing a device using SNA with a device us-ing the OSI protocol stack. The gateway will convert from SNA to an OSI protocol structure(as well as the reverse conversion from OSI to SNA). Thus, the gateway’s main functionalityresides in its role of protocol translator for architectures such as SNA, IPX, TCP/IP, and OSI.It can also translate between IEEE 802.X architectures such as Ethernet to Token Ring LANsand vice versa. If protocol functionality is needed in excess of that found in routers, then thegateway is the device of choice. Gateways can reside within workstations, servers, mini-computers, or mainframes, and are considerably more expensive than routers. Some routershave limited built-in gateway functionality. Figure 3-28 portrays the same two users as the





last example, this time connected via a gateway. This figure also shows the relationship ofgateways to the OSIRM. Both users may have different protocol stacks in any of the sevenlevels with both OSI and non-OSI protocol architectures.

There are three major disadvantages of gateways: low throughput during peak trafficconditions, user-to-gateway priority handling, and store-and-forward characteristics. Dur-ing periods of peak traffic, a gateway may become the network’s main congestion point,having to spend the majority of its time translating between many protocol suites. Gate-ways are often store-and-forward devices, forwarding only the information requested bythe destination node. In spite of these drawbacks, and the high expense of gateways, thereis a growing need for their functionality. Gateways will fill an important niche for manyyears to come in uniting disparate protocols. One key example is a voice gateway that actsas the intermediary node connecting a voice call between a packetized voice user (from anIP network) and a circuit-switched voice user [from the Public Switched Telephone Net-work (PSTN) network].

From Bridges to Routers to HubsSince the last edition of this book, routers and intelligent L2/L3 switches have predomi-nated most major data internetworks. Bridge networks, on all but large SNA shops, havevanished in lieu of homogeneous router- and switch-based networks served by100/1000-Mbps Ethernet switches. Switches are playing a key role in this shift because

Figure 3-28. Gateway communications via the OSIRM





they aggregate many LAN segments much more cost-effectively and they providegreater flexibility. This change is pushing the routing function further toward the WAN.This is an important trend to understand. We discuss this trend in more detail in Chapter 7.

Private Branch eXchange (PBX)For years, the PBX has been the key device that has separated the voice and data worlds.With the advent of ATM and IP technology, users, vendors, and service providers areadopting both ATM and IP switching approaches to PBXs. PBXs provide an automaticsetup of circuits between telephone sets today, which most current ATM and IP switchesdo not plan to do. Traditional PBX call processing and call control are slowly being builtinto ATM and IP switching architectures. The PBX vendors do not see this happening; infact, they see the opposite: ATM and IP-ready PBX products replacing ATM and IP LANswitches. One likely scenario will be a coexistence of ATM and IP-ready PBXs and ATMand IP switches with call-processing and control capabilities in both the campus andwide area. ATM and IP interfaces are now available for PBXs. One example is the SphereCommunications’ Sphericall 3.x ATM Telephony System and 3Com’s SuperStack 3 NBXNetworked Telephony solution. The more probable scenario is that high-end ATM andIP network modules with front-end PBXs to the WAN as well as the feature-rich,call-processing software in the PBX will continue to handle the traditional voice trafficrequirements.

REVIEWIn data communications, various topologies, circuit types, transmissions, and hardwaretypes have evolved. The main topologies include point-to-point, multipoint, star, ring,and mesh. The three types of signal transfer are simplex, half-duplex, and duplex, as wellas asynchronous and synchronous data transfers. The chapter concluded with a detaileddiscussion of each major type of network communications hardware and some L2 and L3switching devices, such as bridges, switches, routers, and gateways, leaving a detaileddiscussion of multiplexers and switches to the next chapter.



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CHAPTER 3books.mhprofessional.com/downloads/products/0072193123/...network services (such as packet switching, Frame Relay (FR), or Asynchronous Transfer Mode, or ATM), some form of