Chapter 9

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1996, BICSI LAN Design Manual - CD-ROM, Issue 11

Overview

Introduction

In earlier chapters, LAN designs were illustrated for a single-floor implementation usingEthernet and Token-ring technologies. As the demand for LAN access grows, it becomesnecessary to expand the network to accommodate new users. At this point, there are threeoptions available for expansion. They are as follows:

The Repeater Option.

With this option, the number of stations attached to the existing network can beincreased, up to the maximum number of connections allowed. This solution usesdevices called repeaters as needed to overcome excessive signal loss, whichoccurs as the length of the network cabling increases.

The Bridge Option.

With this option, multiple networks can be linked to each other using devices calledbridges. This solution overcomes the maximum number of connections rule. Itoffers better performance by linking several small networks together, rather thancreating one large LAN.

Introduction, continued

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Chapter 9 - Ethernet and Token-ring Expansion 1996, BICSI LAN Design Manual - CD-ROM, Issue 12

The Backbone Option.

With this option, multiple stations or networks can be linked to a common backbonenetwork, using either repeaters or bridges. With this optionand using bridgesperformance improves further and an even greater overall number of stations can

be connected together.In the following pages, each of these implementations will be described for both Ethernet andToken-ring. Such expansions are usually needed for LANs servicing multiple floors of abuilding. Linkages between buildings in a campus environment would be similar in designthe distances, however, would usually be greater.

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LAN expansion - The repeater option

Introduction

A repeater operates at the Physical layer of the OSI model. This layer deals solely withlinkages to the physical medium. The repeaters role is to link two cable segments byregenerating the incoming signal from one segment before rebroadcasting it onto the othersegment.

Because of its relatively simple function, a repeater is incapable of examining an incoming

message and making decisions based on its content.

A repeater can, however, be used to extend the physical reach of a LAN by sending theoutgoing signals over a different transmission medium, such as optical fiber cabling.

A repeater is also able to detect a signal-related malfunction on either of its attachedsegments. If necessary, it can isolate the faulty segment, thus preventing the failure fromdisabling the entire network.

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Ethernet repeaters

Copper Ethernet repeaters

Copper repeaters are commonly employed in Ethernet networks when the distances to be

covered are limited.

Coaxial cable basedEthernet

The original Ethernetspecifications, using thickcoaxial cable trunksegments, called for amaximum of five suchsegments between any twonodes. Each segment couldbe a maximum of 500 m

(1640 ft) in length and thefive segments wereconnected to each otherusing four repeaters.

FIGURE 9.1:

ETHERNET USINGCOPPER REPEATERS

Ethernet repeaters, continued

Repeater 1

Repeater 2

Repeater 3

Repeater 4

Trunk segment 1 - Network devices attached



Trunk segment 2 - No network devices attached

Trunk segment 4 - No network devices attached

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Ethernet repeaters are used to extend the overall length of the trunk cable. This may benecessary when the floor area to be covered is very large. Repeaters may also be requiredwhen more than 100 transceivers need to be connected to the network.

A repeater is attached to a transceiver on each of the two trunk cables to be connected

with an AUI (Attachment Unit Interface) cable. It is considered to be a network device and,therefore, becomes one of the maximum of 100 devices on a trunk cable.

Some considerations when installing repeaters are as follows:

Up to five trunk segments may be joined using four repeaters.

Stations may be connected to only three trunk segments. Other trunk segments areused for distance extension.

The maximum overall length of connected trunk segments is 2500 m (8200 ft).

The total number of devices on all trunk segments which have been combined withrepeaters cannot exceed 1024.


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With Thinnet (10Base-2) Ethernet, the layout when using repeaters is similar. It has thefollowing characteristics:

A maximum of five trunk segments can be connected using four repeaters.

Stations can be attached to only three of the segments. The remaining segments

are used for distance extension.

The maximum segment length is 185 m (607 ft).

The maximum overall length of the trunks is 925 m (3035 ft).

There can be a maximum of 30 attachments (including repeaters) per segment.

The total number of devices on all trunk segments which have been combined withrepeaters cannot exceed 1024.


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Multiport repeaters

In both the Thicknet and Thinnet environments, multiport repeaters can be used to attachmore than one station per attachment to the trunk cable. This type of configuration isshown below.

FIGURE 9.2:ETHERNET USINGMULTIPORT REPEATERS

Without such devices, a

Thicknet trunk cablewould support nomore than 100stations, while aThinnet trunkcable would belimited to 30

stations.

Multi-port Repeater

Transceiver

AUI cable

Stations and servers

Coaxial trunk cable


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10Base-T networks

In 10Base-T networks, where all devices are connected to a central hub, the repeater isphysically a part of the hub. Such a hub may be connected to a thick or thin coaxial trunkcable or it may be connected to another 10Base-T hub. Both are illustrated below.

FIGURE 9.3:10BASE-T HUBATTACHED

TO ACOAXIALTRUNK CABLE

10Base-T hub

Transceiver

AUI cable to A UI port on the hub

Stations and server

Coaxial trunk cable

UTP cable


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FIGURE 9.4: DAISY-CHAINED 10BASE-T HUBS

It should be noted that the ANSI/TIA/EIA-568-A standard accepts

the connection of devices in different telecommunications closets forthe purpose of maintaining a bus or ring topology.

10Base-T hubs Stationsand server

UTP cable

10Base-T hubs Stationsand server

UTP cable

UTP or opticalfiber repeater

cable


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Optical fiber Ethernet repeaters

By using optical fiber, the total length of the network can be extended considerably. Theillustration below shows how optical fiber repeaters may be included in an Ethernetnetwork.

FIGURE 9.5: ETHERNET USING OPTICAL FIBER REPEATERS

Repeater

Segment 1

Repeater

Segment 1

Repeater

Segment 2

Repeater

Optical fiber segment Optical fiber segment


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There are three specifications which address the role of optical fiber in such applications.

FOIRL - Fiber Optic Inter-Repeater Link

This is the original specification for linking Ethernet segments using optical fiber. FOIRL

follows the four-repeater limit and specifies a maximum length of 1000 m (3280 ft)between repeaters. This provides for approximately 2500 m (8200 ft) between twosegments located at opposite ends of a network using coaxial trunk cabling.

10Base-FL

The Fiber Link specification approved in 1993 replaces FOIRL. It allows for 2 kilometers

(6560 ft) between repeaters or between an optical fiber NIC and its corresponding hubport, in keeping with structured cabling standards recommendations. As a result, the totaldistance between two stations at opposite ends of the network is extended to4500 m (14760 ft).

For hub-based networks, there can be a total of five repeatersrather than the fourspecified in FOIRL. This allows five repeater-equipped hubs to be connected to each

other.


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10Base-FB

Whereas the 10Base-FL specification can be used for repeater or station-to-hub links, theFiber Backbone specification, also approved in 1993, defines specifications for repeaterconnections only. The repeaters can be up to 2 kilometers (6560 ft) apart, and multiple

repeaters can be connected to each other sequentially.

The FB specification calls for synchronous signaling between unitsunlike FOIRL and10Base-FL, which use asynchronous signaling. Such signaling improves the timing ofsignals, permitting a greater number of connections between stations located at oppositeends of a network.

SummaryIn all cases where the repeater option is used to expand the size of a single Ethernet, thelimiting factor is the amount of traffic generated on the resulting network. Although it ispossible to connect hundreds of devices using multiport repeaters, usually this will result ina highly congested network with many collisions and subsequent delays in processing.Such a situation can be avoided by using bridges, discussed later in this chapter.

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Token-ring repeaters

Copper Token-ring repeaters

In Token-ring networks, external repeaters are used to increase the overall size of the ring

connecting one passive MAU to another. The active hubs, or CAUs, use internalrepeaters. For both types of hubs, copper and optical fiber repeaters are available.

If the ring to be expanded operates at 4 Mbps, two types of copper repeaters are available.These are the following:

An external unit used with MAUs.

An internal unit included with CAUs.

For a 16 Mbps ring, only the second choiceusing CAUsis available.

CAU base units operate with LAMsthe CAU acts as the repeater, while the LAMsconnect to station NICs. Each CAU counts as three stations for the purpose of limiting theoverall number of devices on a ring, which cannot exceed a 250 device limit, as specifiedby IEEE.

Token-ring repeaters, continued

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FIGURE 9.6: CAU-BASED TOKEN-RING

When using Type 1 STP to connect the ports of the CAUs, a distance of 400 m (1312 ft)can exist between two CAUs operating at 4 Mbps and 200 m (656 ft) when operating

at 16 Mbps.

CAU

CAU

CAU

CAU

CAU

Token-ring repeaters, continued

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Optical fiber Token-ring repeaters

Optical fiber repeaters are recommended for extended distance links between CAUs orMAUs. This will often be the case where a ring spans multiple buildings on a campus.

FIGURE 9.7:TOKEN-RING USING

OPTICAL FIBERREPEATERS

When using62.5/125 mmultimodeoptical fiberto connecteither CAUs

or MAUs, adistance of

2000 m(6560 ft)can existbetween the

repeaters, at either 4 or 16 Mbps.This is consistent with structured

cabling standards recommendations for

such applications.

Optical fiber cabling

CAU/MAU

CAU/MAU

CAU/MAU

Opticalfiber

repeater

Optical

fiber

repeater

CAU/MAU

CAU/MAU

CAU/MAU

Opticalfiber

repeater

Optical

fiberrepeater

CAU/MAU

CAU/MAU

CAU/MAU

Optical

fiberrepeater

Optical

fiberrepeater

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LAN expansion - The bridge option

Introduction

Each new station functioning on a LANEthernet, Token-ring or any other typeincreasesthe load on the network. Repeaters, which make it possible to connect distant stations to thenetwork, contribute to this congestion. As newer, more complex software is adopted byusers, there is a need to limit the number of stations on the network to maintain acceptableperformance.

A bridge operates at the Data Link layer of the OSI model. It is used to link two or morenetworks to each other to permit message exchange between stations. Each segment or ringjoined this way stays distinctunlike with repeaters, where one large segment or ring resultsfrom the link.

Bridges are a better solution for LAN expansions than repeaters. They allow for a largenumber of stations to communicate with each other while maintaining excellent network

response and performance.There are two broad categories of bridgeslocal and remote. A local bridge linking two

networks, connects to the cabling system of each. A remote bridge linking two networksconnects to one using its cabling system and to the other using a telecommunications circuit.

Remote bridging is examined in greater detail in a later chapter. The following sectionsdescribe network expansion using local bridges.

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Ethernet bridges

Overview

Since a bridge operates at the Data Link layer of the OSI model, it has no knowledge or

understanding of the topologies or communications protocols used by the networks itconnects. An Ethernet bridge connecting two or more networks acts as an arbitrator for themessages generated by the devices on each network

FIGURE 9. 8:ETHERNET BRIDGES

Bridge B

LAN B

LAN A

OR,

Ethernet bridges, continued

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Upon connection to the networks to belinked, the bridge proceeds to examinethe source and destination addressesfound in all frames it sees. It thenrapidly builds an internal database

identifying the network associated witheach device. This activity is performedin seconds and is referred to as thelearning function of a bridge.

From this point onward, the bridgeforwards all frames whose destination

devices are on another network andfiltersor discardsall frames whosedestination is another device on thesame network.

Frames intended for all stations or anunknown station on the linked networks

are broadcast by the bridgesent to allattached networks. This process isreferred to as flooding.

Bridge module

Bridge module

LAN A

LAN B


T b id i

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Transparent bridging

As devices are activated on the connected networks, the bridge updates its databasewithout requiring intervention by the network administrator. For this reason, such bridgesare referred to as Transparent bridges.

When transparent bridging was introduced, there lacked a means of providing multiplepaths from one network to another. This would ensure a greater likelihood of networkavailability in the event of a bridge failure. The problem is illustrated below.

FIGURE 9.9: TRANSPARENT BRIDGING

LAN A

LAN C

LAN B

B1

B2

B3

OR,


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In such a design, bridge B3is a secondary path fromLAN A to LAN C. If active, itwould forward frames from

LAN A to LAN C. Bridges B1and B2 would do the samething, creating needlessframe duplication and aninefficient use of thechannels linking the

networks.

Spanning Tree Algorithm

To permit efficient redundant links using transparent bridging, the IEEE approved theSpanning Tree Algorithm as the IEEE 802.1D standard. Spanning Tree allows for loops tobe created between networks with Transparent bridges, with the following guidelines:

One bridge in the group of installed bridges is classified as the Root bridge.

Each path between bridges is assigned a cost.

Each bridge then calculates the least cost route to the Root bridge, referred to asthe primary path.

All redundant paths are classified as Standby or Backup paths. Bridges in suchpaths will not forward frames unless the Primary path develops a failure.

LAN A

LAN B

LAN C

B1

B2

B3


S mmar

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Summary

Bridges linking several Ethernet networks can coexist with repeaters linking severalEthernet segments on a single Ethernet network. This possibility, combined with stationcounts, multiple media, bridge performance and vendor enhancements, makes it difficult toprovide a definite limit on the total number of Ethernet networks that can be linked usingbridges. Rules-of-thumb suggesting a maximum of seven or eight bridges can be used.Ultimately, the deciding factor is network performance, itself a function of traffic load,timing and signal propagation delay.

Token ring bridges

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Token-ring bridges

Overview

Linked Ethernet networks rely on their bridges to decide on the forwarding or filtering of

frames. In the Token-ring environment, the stations generating the frames are responsiblefor specifying the path to the destination.

Token-ring bridges do not have internal databasestables are kept by each station andupdated as needed. In this environment, the bridges are referred to as Source Routingbridges. A message can pass through a maximum of seven bridges before reaching itsdestination.

Token-ring bridges, continued

Source routing

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Source routing

When a station has a message to send, it broadcasts a route discovery packet to thedestination station. Each bridge which receives this packet adds its own address andforwards it. Eventually, the destination station receives one or more of these packets,depending on the arrangement of the bridges.

The destination station picks the packet which took the shortest path to arrive andbroadcasts this information to the sending station, which uses this path for all subsequentmessages to that station.

FIGURE 9.10: SOURCE ROUTING

Because of this route-discovery method, Source-Routing bridges can be used to constructmultiple paths between networks. There is no requirementfor unique paths as with Transparent bridging.

In this example, if LAN B sends a message to LAN D itcan do so using four different routes. These are asfollows:

Via bridges B2 and B3.

Via bridges B1 and B4.

Via bridges B1, B5 and B3.

Via bridges B2, B5 and B4.

The route proving to take the shortest time to transmit the message is the one selected forsubsequent transmissions.

LAN A

LAN C

B1

B2

LAN DB3

B4

B5

LAN B

Linking Ethernet and Token-ring

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Linking Ethernet and Token-ring

Source routing transparent bridging

Token-ring and Ethernet networks differ in many ways, including frame size, frame format,

frame content and bridging architecture.In order to permit linking Ethernet and Token-ring networks using bridges, IBM proposed

the Source Routing Transparent bridging method to the IEEE 802.1 High-Level Interfacecommittee in March 1990.

As the name implies, such a device allows both the Ethernet and Token-ring bridgingmechanisms described previously to coexist on an internetwork. However, if the message

is passing from Ethernet to a Token-ring networkor vice versaframe translation is alsorequired. This may be performed by the bridge or software running on the networkstations.

Linking Ethernet and Token-ring,continued

FIGURE 9.11: SOURCE ROUTING TRANSPARENT BRIDGING

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FIGURE 9.11: SOURCE ROUTING TRANSPARENT BRIDGING

Transparent Bridge B

Ethernet LAN B

Ethernet LAN A

Token-ring LAN A Token-ring LAN B

B

Source Routing Bridge

B

Source Routing

TransparentBridge

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LAN expansion - The backbone option

Introduction

In most cases, the messages sent by a station are destined for a station or server on thesame network segment or ring. There is a rule referred to as the 80/20 rule80% of thenetwork traffic is local and 20% of the traffic is to another network. This rule focuses thedesign on the individual networks, with links to other networks being dealt with as growthoccurs.

For large-scale networking, where hundreds or thousands of stations are to be able tocommunicate with each other, the design effort must begin with the links between theeventual networks. The individual networks will be configured at a later time.

This approach is called backbone networking. It uses one or more segments or rings to actas a pathway for inter-network messaging. The individual networks are connected to thebackbone via either repeaters or, more commonly, bridges. Such a design also eliminates, forall practical purposes, the number of stations which can be linked to each other.

The following sections describe backbone networking in the Ethernet and Token-ring

environments using both repeaters and bridges. It is not necessary, however, for thebackbone network to use the same technology as the networks it connects. Two high-speed

technologies, FDDI (Fiber Distributed Data Interface) and ATM (Asynchronous TransferMode) can also be used to construct backbone networks. These technologies are discussedin later chapters.

Ethernet backbones

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Ethernet backbones

Using repeaters

Coaxial cable backbonesWith coaxial cable trunk Ethernet, individual segments are connected to a commonbackbone segment using repeaters.

A message sent by a station onone segment would bebroadcast over the

backbone to all of theother attached stations. In thismanner, only two repeaters arebetween any two stations wishing tocommunicate over the extendednetwork.

The maximum number of devicesover the extended network remains1024, since the combined segmentsrepresent only one Ethernet.

FIGURE 9.12:

COAXIAL CABLEETHERNET BACKBONE

Repeater

Repeater

Repeater

Repeater

Repeater

Backbone

Ethernet backbones, continued

10Base-T and 10Base-FL backbones

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Ethernet backbones can also be created using hubs connected to one another, with eachhub acting as a repeater. If 10Base-FL is used to create the backbone segment, a total offive repeaters can exist between any two stations, as shown below.

FIGURE 9.13: 10BASE-FL BACKBONE

Station

Server

Repeater 1

Repeater 2

Repeater 3

Repeater 4

Repeater 5

Optical Fiber


Collapsed backbone

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It is also possible to implement a backbone in a single hub. Such an arrangement isreferred to as a Collapsed Backbone.

One hub acts as the backbone hub, with all other hubs connected to its ports. With this

configuration, a maximum ofthree hubs or repeaters isbetween any two stationsneeding to communicate.

FIGURE 9.14:

COLLAPSED BACKBONE

Backbone hub

Station

Station

Server


In the most centralized model, the collapsed backbone as well as its connected segments

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are all found in the same hub. Such a device has several backplanes, or paths, to linkvarious hardware modules. Some of these modules connect to stations and servers, whileothers are used for bridging and network management.

With the ability to connect hundreds of network devices, these hubs are often referred to

as Enterprise hubsthey have the ability to link together every station in the organizationthrough one chassis.

FIGURE 9.15: THE ENTERPRISE HUB

Primary PowerSupply Mo dule

Secondary (redundant)Power Supply M odule

Managem entModule

Ethernetsegment

#1

Ethernetsegment

#2

Ethernetsegment

#3

BackboneEthernet segment


Using bridges

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Bridge-based Ethernet backbones are implemented in the same manner as repeater-basedones, in both coaxial trunk and hub configurations.

Since bridging permits each connected segment to remain a distinct network, the

maximum-number-of-repeaters rule and the 1024 device limit apply separately to each ofthe segments. This makes large scale networking possible.

FIGURE 9.16:COAXIAL CABLE BRIDGING

Backbone

B

B

B B B

B B


FIGURE 9.17: BRIDGING MODULES IN 10BASE-T ETHERNET

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Station

Station

Server

Bridging module

Token-ring backbones

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Introduction

Token-ring backbones are created using bridges to link discrete rings to a common

backbone ring. This is illustrated below.

FIGURE 9.18:TOKEN-RINGBACKBONE RING

RI RO

RI RO

RI RO

RI RO

B

RI

RO

B

B

B

Token-ring backbones, continued

Each ring, including the backbone, can operate at either 4 or 16 Mbps. When extendeddistances are involved as is the case in some multi building situations the backbone ring

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distances are involved, as is the case in some multi-building situations, the backbone ringcan be made up of MAUs or CAUs located centrally in each building. Optical fiberrepeaters are then used to connect these MAU/CAU clusters together to form thebackbone ring.

FIGURE 9.19: CAMPUS BACKBONE RING

Since there is a limitof seven source-routing bridges

between any tworings, the backbonering can be used tointernetwork morethan eight rings.Rather than passingfrom ring to ring, amessage goes fromits source ring to thebackbone ring andthen to thedestination ring,avoiding all other

rings which mayexist.

B

B

B

B

B

B

B

B

B

B

B

B

B

B

B

Building 1 Building 2 Building 3

Campus backbone ring

Repeater Repeater

Token-ring backbones, continued

Dual backbone rings

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For added protection against backbone failure due to a software, equipment or cable-related fault, a duplicate backbone system can be readily implemented in a Token-ringenvironment. In such systems, two bridges are used on each ring, with each bridgeconnecting to a different backbone ring. Since both backbones are available at all times,

the failure of one causes no disruption in network availabilitythe second handles allinternetwork traffic until repairs are made.

FIGURE 9.20:DUALBACKBONE

RINGS B

B

B

B

B

B

B

B

B

B

Primary Backbone ring Secondary (backup) Backbone ring

LAN expansion - The switch option

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LAN expansion - The switch option

Overview

Traditional LAN environments such as Ethernet and Token-ring have relied on using ashared transmission channel. In the case of Ethernet, the shared channel traditionallyprovided a total bandwidth of 10 Mbps, with 100 Mbps Ethernet currently making anappearance. Token-rings shared either 4 or 16 Mbps total bandwidth.

As network traffic makes increasing demands on the shared bandwidth, alternatives arebeing considered. Newer applications such as multimedia presentations, video-conferencing, imaging and other graphics and data-intensive software are causing networkcongestion problems. These problems may be indicated by low network throughput, slowedresponse times and in the case of Ethernet, high rates of collisions.

Possible solutions to network congestion problems include the following:

Using traditional LAN segmentation by using bridges and/or routers.

Using higher-speed technologies such as FDDI or 100 Mbps Ethernet.

Using LAN segmentation, but through switching hubs.

The first two solutions may improve performance. However, bridge and router-basedenvironments can become complex to administer and can potentially require costlyinvestments. Also, traditional higher-speed technologies still rely on the use of shared

media.

Overview, continued

The use of switching hubs may provide a solution to congested networks. They are capableof providing dedicated links to each attached device giving each device the bandwidth it

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of providing dedicated links to each attached device, giving each device the bandwidth itrequires.

Switching hub technology is also referred to as port switching. It allows LANs to be divided

into multiple, smaller independent segmentsmicrosegmentationand then interconnects

the segments at full network speeds as required.

The number of stations assigned to a single port on the hub can be as few as one. Or, fordevices producing lighter traffic loads, some of the switching hubs permit multiple devices toaccess a single port. In all cases, switches allow the separation of heavy network trafficproducers from those producing less network traffic.

FIGURE 9.21:MIXED SWITCHENVIRONMENT Switching hub

Shared LAN segment

Dedicated LANsegments

Overview, continued

FIGURE 9.22: DEDICATED (PRIVATE) SWITCH ENVIRONMENT

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Many port switching devices also allow stations to be reassigned to different logical LANsegments. This ability to create virtual LANs allows LAN administrators to define logicalworkgroups regardless of the physical LAN to which they are connected.

Switching hub

Each device has a dedicated or private link tothe hub. Each link has full network bandwidth.

Overview, continued

Switching environments

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Currently most of the switching hubs available are for use in Ethernet environments,although there are some units available for use in Token-ring and FDDI environments. Aswell, there are switching hubs that allow multiple environments to be connected to thesame hub.

One of the primary advantages associated with switches is the ability to install themtransparently. That is, they do not require any additional changes to be made to thenetwork environment. For example, a 10Base-T Ethernet network administrator whowishes to switch to dedicated 10 Mbps links for all stations needs only to purchase theappropriate switching hubs. The Network Interface Cards and cabling already in place canbe used to provide the dedicated links.

Switching hubs are generally categorized according to the physical capabilities of thehubthe number of ports and the technology supportedas well as to the style ofimplementation.

Three categories of hubs are broadly defined. They are as follows:

Workgroup hubs.

These are the smallest of the switching hubs. They usually have between 8 and 12ports per hub used to connect stations and the servers these stations need toaccess. If selecting this type of hub, it is important to ensure that there is room forgrowth. That is, that the hub can be connected to other hubs at a later time.

Departmental hubs.

These are hubs that work at the same level as workgroup hubsconnectingstations and serverbut have more ports available.

Overview, continued

Enterprise hubs.

These are the larger switching hubs used to connect multiple network segments

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These are the larger switching hubs used to connect multiple network segments,including workgroup and department hubs and switches, and common resourcessuch as backup devices and database servers. These hubs form the basis for the

collapsed backbone environment.

Switch operations

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Introduction

Most switches operate essentially as MAC-layer (or device address) bridges and are even

referred to by their vendors as multiport bridges.A switch needs to know the MAC-layer address of the destination device to be able to

forward the data packets. Switching hubs may provide dedicated port switching with asingle MAC address per port or shared port switching where multiple MAC addresses areacceptable per port. The switch learns the MAC addresses associated with a specific portas network packets appear at the port. The addresses are stored in an address database.Therefore, little or no manual administration is required.

When a data packet arrives at a port, the switch examines the MAC destination address.Depending on what the destination address is, the switch does one of three things:

If the destination address is local to the incoming port, the packet is filteredit isignored by the switch and not forwarded.

If the destination address is associated with another port, the packet is forwardedto the other port.

If the destination address is unknown, the packet is broadcastit is sent to everyport other than the incoming port.

In an environment where devices are assigned to dedicated ports, communications withother devices is done via the switchs backplane. It allows the devices to communicate at

network speeds.

Switch operations, continued

An advantage that switches have over traditional bridges is a high-performance backplanethat supports very high throughput. The total throughput of the backplane can be as high

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as the number of paths through the switch multiplied by the throughput of each individuallink.

EXAMPLE 9.1: SWITCH THROUGHPUT

A workgroup consisting of 12 users will be installing an Ethernet switch providingdedicated 10 Mbps links to each user. Users want to be able to communicate with eachother at maximum possible network speed. What would the minimum acceptablebackplane throughput be for the switch?

A 12-port hub could provide for a maximum of 6 links at one time. Each of the links could

transmit at a maximum of 10 Mbps. Therefore, the maximum backplane throughput wouldbe as follows:

Backplane throughput = 6 links x 10 Mbps per Link

Backplane throughput = 60 Mbps

With a backplane throughput of 60 Mbps, 6 links could each support 10 Mbps throughput.

Large switches, such as those found in Enterprise hubs, can have backplane throughputmeasured in Gbpsespecially those switches providing ports operating at 100 Mbps.

Well-designed, switch-based networks can benefit from aggregated total throughput.Additional switches can add to total network throughput and improve overall performance.


Switching methods

When packets arrive at an incoming port on the switch they must be directed to the

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When packets arrive at an incoming port on the switch, they must be directed to theappropriate outgoing port leading to the destination device. The traditional mechanismused is known as store-and-forward while a newer mechanism used by some switchinghubs is referred to as cut-through.

Store-and-forward

The store-and-forward method is a technology used in high-speed bridges. Switchesbased on this technology wait for the entire data packet to arrive before processing canbegin. Before sending the packet to its destination, error checking is performed on thepacket using a Cyclic Redundancy Check (CRC). If the packet is determined to be error-

free, it is forwarded to its destination.

Store-and-forward type switches may have additional features. Some have the ability toperform packet filteringthey can be programmed to ignore certain packets received fromcertain device addresses. Also, some of these switches provide for low-level routing. Thisallows networks to be logically segmented from a single location. It should be noted thateach added feature can slow network performance, which can be problematic in large

networks.


Cut-through

The cut-through method of switching represents a newer technology It is based on the

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The cut through method of switching represents a newer technology. It is based on thepremise that there is no need for the switch to wait for the arrival of the complete packet.The switch waits only long enough to read the destination address on the packet before it

begins forwarding the packet to its destination.

Network performance may or may not be significantly improved with such a technology,depending on the packet size used and the protocol used. It has been found that networkprotocols requiring an acknowledgment of every packet sent benefit from this method. Inaddition, networks configured to use packet sizes greater than 1024 bytes do not see asgreat an increase in performance as those using smaller packet sizes.

Also of concern is the lack of error checking done on the packets. Malformed packets and

corrupt packets are also passed on by the switch, resulting in a propagation of errors. Thiscan adversely affect network performance by requiring retransmission of these badpackets.

Full-duplex Ethernet

Associated with dedicated port switching is the introduction of full duplex Ethernet This

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Associated with dedicated port switching is the introduction of full-duplex Ethernet. Thistechnology allows for signals to be transmitted and received at full network speeds at thesame time. That is, full-duplex 10 Mbps Ethernet permits 10 Mbps transmission to happen inboth directions at the same time, resulting in a total throughput of 20 Mbps.

Full-duplex Ethernet is possible with a dedicated connection since there is no need fordevices to listen for collisions. Without the danger of a collision, connections can operate in

both directions at the same time.

Most station applications receive more network traffic than they transmit, making full-duplextransmission seem to be unnecessary. However, having an additional transmission channelallows acknowledgments and other housekeeping traffic to be transmitted while the station isstill receiving data.

The greatest benefit of full-duplex Ethernet is found in those environments where the trafficflow is balanced in the two directions. This is seen mostly in server-to-switch connectionsand in video-conferencing situations.

It should be noted that full-duplex Ethernet requires full-duplex Network Interface Cards and

special full-duplex switch ports.

Overview .................................................................................. 1

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Overview .................................................................................. 1

Introduction ....................................................................................... 1

LAN expansion - The repeater option ................................. 3

Introduction ....................................................................................... 3

Ethernet repeaters ........................................................................... 4Copper Ethernet repeaters................................................................ 4

Coaxial cable based Ethernet ......................................................... 4Multiport repeaters .......................................................................... 710Base-T networks ......................................................................... 8

Optical fiber Ethernet repeaters ...................................................... 10FOIRL - Fiber Optic Inter-Repeater Link .......................................1110Base-FL ...................................................................................... 1110Base-FB .................................................................................... 12

Summary ......................................................................................... 12

Token-ring repeaters ..................................................................... 13

Copper Token-ring repeaters........................................................... 13Optical fiber Token-ring repeaters................................................... 15

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LAN expansion - The bridge option .................................. 16

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Introduction ..................................................................................... 16

Ethernet bridges............................................................................. 17Overview ......................................................................................... 17

Transparent bridging ....................................................................... 19Spanning Tree Algorithm ................................................................ 20Summary ......................................................................................... 21

Token-ring bridges ......................................................................... 22Overview ......................................................................................... 22Source routing ................................................................................. 23

Linking Ethernet and Token-ring ................................................ 24Source routing transparent bridging ................................................ 24

LAN expansion - The backbone option ............................ 26

Introduction ..................................................................................... 26

Ethernet backbones ...................................................................... 27

Using repeaters ............................................................................... 27Coaxial cable backbones .............................................................. 2710Base-T and 10Base-FL backbones........................................... 28Collapsed backbone ...................................................................... 29

Using bridges .................................................................................. 31

Token-ring backbones .................................................................. 33Introduction...................................................................................... 33

Dual backbone rings ........................................................................ 35

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LAN expansion The switch option ..................................36

Overview .......................................................................................... 36Switching environments .................................................................. 39

Switch operations .......................................................................... 41Introduction...................................................................................... 41Switching methods .......................................................................... 43

Store-and-forward ......................................................................... 43Cut-through ................................................................................... 44

Full-duplex Ethernet ...................................................................... 45

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Example 9.1: Switch throughput .............................................. 42APTER

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Figure 9.1: Ethernet using copper repeaters ........................... 4

Figure 9 2: Ethernet using multiport repeaters 7

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Figure 9.2: Ethernet using multiport repeaters ........................ 7

Figure 9.3: 10Base-T hub attached to a coaxial trunk cable .. 8

Figure 9.4: Daisy-chained 10Base-T hubs .............................. 9

Figure 9.5: Ethernet using optical fiber repeaters ................. 10

Figure 9.6: CAU-based Token-ring ......................................... 14

Figure 9.7: Token-ring using optical fiber repeaters .............. 15

Figure 9.8: Ethernet bridges .................................................... 17

Figure 9.9: Transparent bridging ............................................. 19

Figure 9.10: Source Routing ..................................................... 23

Figure 9.11: Source routing transparent bridging ................... 25

Figure 9.12: Coaxial cable Ethernet backbone ...................... 27

Figure 9.13: 10Base-FL backbone .......................................... 28

Figure 9.14: Collapsed backbone ........................................... 29

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Figure 9.15: The Enterprise Hub ............................................ 30

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Figure 9.16: Coaxial cable bridging ........................................ 31

Figure 9.17: Bridging modules in 10Base-T Ethernet .......... 32Figure 9.18: Token-ring backbone ring .................................. 33

Figure 9.19: Campus backbone ring ...................................... 34

Figure 9.20: Dual backbone rings ........................................... 35

Figure 9.21: Mixed switch environment .................................. 37

Figure 9.22: Dedicated (private) switch environment ........... 38

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Chapter 9

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Transcript of Chapter 9