Chapter 8LAN Architectures

Part II: Understanding Internet Access TechnologiesPart II: Understanding Internet Access Technologies

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Topics Addressed in Chapter 8 What is meant by LAN architecture?What is meant by LAN architecture? LAN topologiesLAN topologies Data link and media access control protocolsData link and media access control protocols Physical layer data encoding in LANsPhysical layer data encoding in LANs Ethernet architecturesEthernet architectures Token ring LANsToken ring LANs FDDI and 100VG-AnyLANFDDI and 100VG-AnyLAN ATM LANsATM LANs Virtual LANsVirtual LANs Wireless LAN architecturesWireless LAN architectures LAN architecture selection criteriaLAN architecture selection criteria IEEE LAN standardsIEEE LAN standards

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What is Meant By LAN Architecture? LAN architecture is the overall design of a LAN. It LAN architecture is the overall design of a LAN. It

includes:includes: LAN hardwareLAN hardware LAN softwareLAN software LAN topologyLAN topology Media access control (MAC) protocolMedia access control (MAC) protocol

The LAN’s network operating system is sometimes The LAN’s network operating system is sometimes also considered to be part of LAN architecture also considered to be part of LAN architecture

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LAN Topologies There are two types of LAN topologies: physical and There are two types of LAN topologies: physical and

logicallogical Physical LAN topology refers to the physical layout of Physical LAN topology refers to the physical layout of

the networkthe network The way in which the communication is configured The way in which the communication is configured

and how nodes attach to the networkand how nodes attach to the network Because the focus is on physical connections among Because the focus is on physical connections among

hardware component, physical topologies correspond hardware component, physical topologies correspond to the physical layer of the OSI reference modelto the physical layer of the OSI reference model

Logical topology is concerned with how messages are Logical topology is concerned with how messages are passed from node to node within the networkpassed from node to node within the network

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Physical Topologies

LAN’s have three basic physical topologies:LAN’s have three basic physical topologies: Bus: all nodes attach to a common communication pathway Bus: all nodes attach to a common communication pathway

or channel or channel Ring: the medium forms a loop to which all nodes are Ring: the medium forms a loop to which all nodes are

attachedattached Star: uses a central station (hub or switch) to which all other Star: uses a central station (hub or switch) to which all other

nodes have point-to-point connections; all communication nodes have point-to-point connections; all communication among nodes occurs through this central stationamong nodes occurs through this central station

These are illustrated in Figure 8-1These are illustrated in Figure 8-1 Physical star topologies are most common in today’s LANsPhysical star topologies are most common in today’s LANs

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Figure 8-1

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Bus Topologies

In a classic bus topology, the medium consists of a single wire or In a classic bus topology, the medium consists of a single wire or cable to which other nodes are attached via connectors or transceiverscable to which other nodes are attached via connectors or transceivers Variations include a primary bus with spurs (see Figure 8-2)Variations include a primary bus with spurs (see Figure 8-2) Disadvantages include the potential for loose connections or Disadvantages include the potential for loose connections or

breaks in the bus to disrupt the entire networkbreaks in the bus to disrupt the entire network Early Ethernet LAN implementations were typically physical bus Early Ethernet LAN implementations were typically physical bus

architectures; today, most Ethernet implementations are physical starsarchitectures; today, most Ethernet implementations are physical stars However, an Ethernet shared media hub is sometimes called a However, an Ethernet shared media hub is sometimes called a

“bus in a box”“bus in a box” Both IEEE 802.3 standard and IEEE 802.4 standards and their Both IEEE 802.3 standard and IEEE 802.4 standards and their

protocols address communication over LANs with bus topologiesprotocols address communication over LANs with bus topologies

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Ring Topologies

In a physical ring topology, the communication medium In a physical ring topology, the communication medium forms a closed loop (ring) and all stations are connected to forms a closed loop (ring) and all stations are connected to the loopthe loop Data is transmitted node-to-node in one direction on the Data is transmitted node-to-node in one direction on the

ring (see Figure 8-3)ring (see Figure 8-3) Similar to a linear bus, the entire network could be Similar to a linear bus, the entire network could be

disrupted if one of the connectors or links in the ring disrupted if one of the connectors or links in the ring should failshould fail

Physical ring topologies are less common than bus or star Physical ring topologies are less common than bus or star topologiestopologies

Token ring and FDDI LANs have physical ring topologiesToken ring and FDDI LANs have physical ring topologies

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Figure 8-3

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IEEE 802.5 and 802.6 LANs The most widely used microcomputer ring network is the token The most widely used microcomputer ring network is the token

passing ring. It conforms to the IEEE 802.5 standardpassing ring. It conforms to the IEEE 802.5 standard Token ring networks physically look like a star topology, but Token ring networks physically look like a star topology, but

technically they are physical ringstechnically they are physical rings Token ring nodes attach to multistation access units (MAUs) – Token ring nodes attach to multistation access units (MAUs) –

see Figure 8-4see Figure 8-4 MAUs can be described as “a ring in a box”, because nodes MAUs can be described as “a ring in a box”, because nodes

attach to the physical ring by connecting to the MAU (see attach to the physical ring by connecting to the MAU (see Figure 8-5)Figure 8-5)

MAUs can be interconnected to form larger rings (see Figure MAUs can be interconnected to form larger rings (see Figure 8-6)8-6)

IEEE 802.6 addresses dual-ring metropolitan area network IEEE 802.6 addresses dual-ring metropolitan area network (MAN) architectures (see Figures 8-24 and 8-25)(MAN) architectures (see Figures 8-24 and 8-25)

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Figure 8-4

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Figure 8-5

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Figure 8-6

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Star Topologies

In LANs with star topologies, all nodes are connected to some kind of In LANs with star topologies, all nodes are connected to some kind of wiring center such as a hub or switch (see Figure 8-7)wiring center such as a hub or switch (see Figure 8-7) Today, most LAN implementations physically resemble star Today, most LAN implementations physically resemble star

topologiestopologies Each node is isolated on its own network segment in a physical star Each node is isolated on its own network segment in a physical star

topology which minimizes the possibility of total network disruption by topology which minimizes the possibility of total network disruption by a malfunctioning connector, NIC, or linka malfunctioning connector, NIC, or link However, the network is vulnerable to wiring center failureHowever, the network is vulnerable to wiring center failure

The use of central connection points also facilitates network traffic The use of central connection points also facilitates network traffic monitoring and network management, including network security monitoring and network management, including network security managementmanagement

ARCnet was one of the first LAN architectures with a star topology (see ARCnet was one of the first LAN architectures with a star topology (see Figure 8-8b)Figure 8-8b)

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Figure 8-7

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Logical Topologies Every LAN has both a physical and logical topologyEvery LAN has both a physical and logical topology A LAN’s A LAN’s logical topologylogical topology specifies how messages are passed from specifies how messages are passed from

node to node within the networknode to node within the network It corresponds to the media access control (MAC) protocol used It corresponds to the media access control (MAC) protocol used

in the LANin the LAN Two logical LAN topologies exist:Two logical LAN topologies exist:

SequentialSequential (or logical ring): data is passed from one node to (or logical ring): data is passed from one node to another in a ring-like sequenceanother in a ring-like sequence

Token passing in token ring and FDDI LANs are examplesToken passing in token ring and FDDI LANs are examples BroadcastBroadcast: nodes transmit frames/packets to all other nodes in the : nodes transmit frames/packets to all other nodes in the

network; only the intended recipient processes the entire network; only the intended recipient processes the entire frame/packetframe/packet

Ethernet LANs use a logical broadcast topologyEthernet LANs use a logical broadcast topology

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Data Link Protocols Data link protocols, including those used in LANs, are Data link protocols, including those used in LANs, are

responsible for establishing the rules by which nodes gain responsible for establishing the rules by which nodes gain access to a network’s communication medium and exchange access to a network’s communication medium and exchange messages. Such protocols describe several important aspects of messages. Such protocols describe several important aspects of the message exchange process including:the message exchange process including: Delineation of data Delineation of data Error controlError control AddressingAddressing TransparencyTransparency Code independenceCode independence Media access--this is governed by media access control Media access--this is governed by media access control

(MAC) protocols(MAC) protocols

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LLC and MAC Sublayers

LANs employ two primary data link protocols: LANs employ two primary data link protocols: contention and token passingcontention and token passing

In IEEE 802 standards, the data link layer is divided In IEEE 802 standards, the data link layer is divided into two sublayers LLC and MAC (see Figure 8-10)into two sublayers LLC and MAC (see Figure 8-10) LLC (logical link control) is responsible for flow LLC (logical link control) is responsible for flow

control, message sequencing, message control, message sequencing, message acknowledgement, and error checkingacknowledgement, and error checking

MAC (media access control) enables network MAC (media access control) enables network nodes to access the communication medium via nodes to access the communication medium via contention or token passingcontention or token passing

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Figure 8-10

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CSMA/CD

CSMA/CD (Carrier Sense Multiple Access and Collision DetectionCSMA/CD (Carrier Sense Multiple Access and Collision Detection) is ) is most widely used contention-based MAC used in LANsmost widely used contention-based MAC used in LANs It is the MAC protocol used in Ethernet LANsIt is the MAC protocol used in Ethernet LANs

In a true contention MAC (like CSMA/CD), each node has equal In a true contention MAC (like CSMA/CD), each node has equal access to the mediumaccess to the medium

As noted in Table 8-1, each node monitors the medium for data traffic As noted in Table 8-1, each node monitors the medium for data traffic and if none is detected, it begins transmitting dataand if none is detected, it begins transmitting data

A A collisioncollision occurs when two or more nodes begin to transmit at the occurs when two or more nodes begin to transmit at the same timesame time

To avoid collision recurrence, each node waits a random time interval To avoid collision recurrence, each node waits a random time interval (hardwired in its NIC) before attempting to retransmit(hardwired in its NIC) before attempting to retransmit

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Table 8-1

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CSMA/CA

CSMA/CA (Carrier Sense Multiple Access and CSMA/CA (Carrier Sense Multiple Access and Collision AvoidanceCollision Avoidance) is a variation of CSMA/CD ) is a variation of CSMA/CD used in wireless LANs because it is difficult to used in wireless LANs because it is difficult to detect collisions in such networksdetect collisions in such networks

When CSMA/CA is used, each node must wait a When CSMA/CA is used, each node must wait a random time interval (hardwired in the wireless random time interval (hardwired in the wireless NIC) after detecting a clear medium before NIC) after detecting a clear medium before transmittingtransmitting

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Token Passing

Token passing is the other major MAC protocol found in LANsToken passing is the other major MAC protocol found in LANs It is used in token ring and FDDI LANs and other networks with It is used in token ring and FDDI LANs and other networks with

logical ring topologieslogical ring topologies The token is a pre-defined bit pattern that is passed among network The token is a pre-defined bit pattern that is passed among network

attached computers until one of them wants to use the medium to attached computers until one of them wants to use the medium to transmit datatransmit data

Token passing is summarized in Table 8-2Token passing is summarized in Table 8-2 In token ring networks that resemble physical star topologies, token In token ring networks that resemble physical star topologies, token

passing takes place within MAUs (see Figure 8-12)passing takes place within MAUs (see Figure 8-12) Token passing can be used in bus topologies as well as in physical ring Token passing can be used in bus topologies as well as in physical ring

topologies (see Figure 8-11)topologies (see Figure 8-11) Table 8-3 compares token passing and CSMA/CDTable 8-3 compares token passing and CSMA/CD

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Table 8-2

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Figure 8-12

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Table 8-3

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Physical Layer Data Encoding

Baseband transmission is common in LANsBaseband transmission is common in LANs When LAN nodes use the communication medium, their NICs When LAN nodes use the communication medium, their NICs

transmit digital signals to represent the bits in data link layer transmit digital signals to represent the bits in data link layer protocols frames directly onto the mediumprotocols frames directly onto the medium

Some of the encoding schemes used in LANs are illustrated in Some of the encoding schemes used in LANs are illustrated in Figure 8-13. These include:Figure 8-13. These include: Manchester encoding (used in 10 mbps Ethernet)Manchester encoding (used in 10 mbps Ethernet) 4B5B (used in 100 mbps Ethernet)4B5B (used in 100 mbps Ethernet) Differential Manchester encoding (used in token ring LANs)Differential Manchester encoding (used in token ring LANs) NRZI (used in FDDI LANs)NRZI (used in FDDI LANs)

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Figure 8-13

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Ethernet LAN Architectures

IEEE 802.3-compliant LANs are better known as Ethernet IEEE 802.3-compliant LANs are better known as Ethernet LANsLANs

There are a variety of IEEE 802.3-compliant LANs (see There are a variety of IEEE 802.3-compliant LANs (see Table 8-4)Table 8-4)

Today, most Ethernet LANs have physical star topologies; Today, most Ethernet LANs have physical star topologies; some have physical bus topologiessome have physical bus topologies

All Ethernet LANs have broadcast logical topologies and All Ethernet LANs have broadcast logical topologies and use CSMA/CD as the MAC protocoluse CSMA/CD as the MAC protocol

Figure 8-9 illustrates widely used Ethernet frame formatsFigure 8-9 illustrates widely used Ethernet frame formats

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Table 8-4

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Figure 8-9

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Some Key Ethernet Implementations

Particularly important Ethernet implementations include:Particularly important Ethernet implementations include: Fast EthernetFast Ethernet (e.g. 100BaseT and 100BaseFX) (e.g. 100BaseT and 100BaseFX)

The IEEE 802.3u specification covers Fast EthernetThe IEEE 802.3u specification covers Fast Ethernet Gigabit EthernetGigabit Ethernet (e.g. 1000BaseT, 1000BaseSX, and (e.g. 1000BaseT, 1000BaseSX, and

1000BaseLX)1000BaseLX)The IEEE 802.3z specification addresses Gigabit The IEEE 802.3z specification addresses Gigabit

EthernetEthernet Iso-EthernetIso-Ethernet enables Ethernet LANs at different enables Ethernet LANs at different

geographic locations to be connected over ISDN (see geographic locations to be connected over ISDN (see Figure 8-14)Figure 8-14)IEEE 802.9a addresses Isochronous EthernetIEEE 802.9a addresses Isochronous Ethernet

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Figure 8-14

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Token Ring Architectures

Token ring networks are addressed in the IEEE Token ring networks are addressed in the IEEE 802.5 specification802.5 specification

Physically, token ring LANs resemble star Physically, token ring LANs resemble star topologies, but technically they are ringstopologies, but technically they are rings Ring is physically implemented in MAUsRing is physically implemented in MAUs UTP is the most common cablingUTP is the most common cabling

Speed is typically 16 mbps, however, 4 mbps and Speed is typically 16 mbps, however, 4 mbps and 100 mbps token ring networks exist100 mbps token ring networks exist

IEEE 802.5 frame formats are illustrated in Figure IEEE 802.5 frame formats are illustrated in Figure 8-158-15

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Figure 8-15

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FDDI LANs

Fiber Distributed Data Interface (FDDI) was first recognized in Fiber Distributed Data Interface (FDDI) was first recognized in ANSI’s X3T9.5 specificationANSI’s X3T9.5 specification

Physically, it has a dual ring topologyPhysically, it has a dual ring topology It has a sequential/ring logical topology and uses a variation of It has a sequential/ring logical topology and uses a variation of

token passing as the MAC protocoltoken passing as the MAC protocol Key FDDI technologies are identified in Figure 8-16Key FDDI technologies are identified in Figure 8-16

These include These include single attached stations (SAS),single attached stations (SAS), dual attached dual attached stations (DAS),stations (DAS), FDDI concentrators, and FDDI/Ethernet FDDI concentrators, and FDDI/Ethernet bridgesbridges

FDDI is often used as a backbone network architecture (see FDDI is often used as a backbone network architecture (see Figure 8-18)Figure 8-18)

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Figure 8-16

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100VG-AnyLAN 100VG-AnyLAN (aka 100BaseVG) is capable of 100VG-AnyLAN (aka 100BaseVG) is capable of

transporting both IEEE 802.3 and IEEE 802.5 framestransporting both IEEE 802.3 and IEEE 802.5 frames It provides a mechanism for interconnecting 100 mbps It provides a mechanism for interconnecting 100 mbps

token ring and 100BaseT Ethernet LANs via token ring and 100BaseT Ethernet LANs via specialized hubs and routers (see Figure 8-20)specialized hubs and routers (see Figure 8-20) 100VG-AnyLAN-compliant adapters are also 100VG-AnyLAN-compliant adapters are also

neededneeded It uses demand priority access (DPA) rather than It uses demand priority access (DPA) rather than

CSMA/CD as the MAC protocol in order to enable CSMA/CD as the MAC protocol in order to enable real-time voice and video frames to be given priority real-time voice and video frames to be given priority over other data framesover other data frames

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Figure 8-20

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ATM LANs

ATM (asynchronous transfer mode) is a switched network ATM (asynchronous transfer mode) is a switched network architecture that employs 53-octet cells to transmit dataarchitecture that employs 53-octet cells to transmit data

Two data link layers are defined:Two data link layers are defined: ATM adaptation layer (AAL)ATM adaptation layer (AAL) ATMATM

ATM physical topologies are starsATM physical topologies are stars ATM NICs with 25 speeds of 25, 100, or 155 mbps are ATM NICs with 25 speeds of 25, 100, or 155 mbps are

available for workstationsavailable for workstations Ethernet and token ring LANs can interface with an Ethernet and token ring LANs can interface with an

organization’s ATM backbone network via ATM organization’s ATM backbone network via ATM gateway/access switches (see Figure 8-21)gateway/access switches (see Figure 8-21)

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Figure 8-21

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Wireless LAN Architectures IEEE 802.11x standards are the most important wireless LAN IEEE 802.11x standards are the most important wireless LAN

(WLAN) specifications that exist today (see Table 8-6)(WLAN) specifications that exist today (see Table 8-6) WLANs are typically implemented as physical starsWLANs are typically implemented as physical stars

Nodes connect to wireless hubs called access pointsNodes connect to wireless hubs called access points CSMA/CA is the MAC protocol for IEEE 802.11-compliant LANsCSMA/CA is the MAC protocol for IEEE 802.11-compliant LANs IEEE 802.11 addresses FHSS (frequency hopping spread spectrum), IEEE 802.11 addresses FHSS (frequency hopping spread spectrum),

DSSS (direct sequence spread spectrum), and diffuse infrared DSSS (direct sequence spread spectrum), and diffuse infrared transmissiontransmission User “roaming” capabilities are also addressedUser “roaming” capabilities are also addressed

WiFi (Wireless Fidelity) certification has been developed to promote WiFi (Wireless Fidelity) certification has been developed to promote interoperability among WLAN productsinteroperability among WLAN products

The WISPR (Wireless ISP Roaming) standard is designed to enable The WISPR (Wireless ISP Roaming) standard is designed to enable users to roam from one publicly accessible WLAN to anotherusers to roam from one publicly accessible WLAN to another

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Table 8-6

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Choosing Among LAN Architectures A number of factors should be considered when selecting among LAN A number of factors should be considered when selecting among LAN

architecturesarchitectures Some of the major factors are described in Table 8-7; others are described in Table Some of the major factors are described in Table 8-7; others are described in Table

8-10)8-10) Especially important factors to consider include:Especially important factors to consider include:

Immediate and recurring LAN costs (see Table 8-8)Immediate and recurring LAN costs (see Table 8-8) Total cost of ownership (TCO)Total cost of ownership (TCO) Number of concurrent users that can be supportedNumber of concurrent users that can be supported Transmission speed and data throughputTransmission speed and data throughput Vendor supportVendor support ManageabilityManageability Scalability/expandabilityScalability/expandability SecuritySecurity Adherence to widely accepted standardsAdherence to widely accepted standards

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Table 8-7

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IEEE LAN Standards & Committees

802.1 High-Level 802.1 High-Level InterfaceInterface

802.2 Logical Link 802.2 Logical Link ControlControl

802.3 CSMA/CD802.3 CSMA/CD 802.4 Token Bus802.4 Token Bus 802.5 Token Ring802.5 Token Ring 802.6 Metropolitan Area 802.6 Metropolitan Area

Networks (MANs)Networks (MANs)

802.7 Broadband 802.7 Broadband Technical Advisory GroupTechnical Advisory Group

802.8 Fiber Optic 802.8 Fiber Optic Technical Advisory GroupTechnical Advisory Group

802.9 Integrated Data and 802.9 Integrated Data and Voice NetworksVoice Networks

802.10 LAN Security802.10 LAN Security 802.11 Wireless LANs802.11 Wireless LANs 802.12 Demand Priority 802.12 Demand Priority

Access MethodAccess Method

Chapter 8LAN Architectures

Part II: Understanding Internet Access TechnologiesPart II: Understanding Internet Access Technologies