Local Area Networks

ContentChapter 14: Advanced Review (Part I)

Switched Ethernet

Ethernet Evolution

Shared vs. Switched LANs

Transparent Learning

Spanning Tree Protocol

Tutorial Questions

Additional Notes

Ethernet Evolution

Developed in the mid-1980s as a shared bus LAN– Operated over coaxial cable– Used CSMA/CD channel access algorithm

Repeater and hubs (Layer 1 relays) extend distance– Number restricted to four between any

two nodes on 10Mbps Ethernet Bridges (Layer 2 relays) overcome restrictions

on number of repeaters– Spanning Tree Protocol (IEEE 802.1D)

addresses bridge resilience issues Twisted pair cabling introduced in late 1980s

– Reduced network diameter– More resilient than physical bus– Ethernet hub replaced repeater

Repeateror bridge

Bridge

Hub Hub

CSMA/CD = carrier sense multiple access with collision detection

Ethernet Evolution(continued)

Fast Ethernet standardized by mid-1990s– Supported on legacy UTP-3 and upcoming UTP-5 cabling– Reduced network diameter compared to 10Base-T

Auto-negotiating 10/100 interfaces self-configure speed and duplex mode– Flow control prevents overrun on 10 Mbps interfaces

Data rate

Cable types and distance limitations (meters)

Coaxial cable(10Base5 and

10Base2)

UTP-3 4-pair or UTP-5

(10Base-T and 100Base-TX)

MMF(10Base-F and 100Base-FX)

10 Mbps

500 (no repeaters)

2500 (max. 4 repeaters)

100 (no repeaters)500 (max. 4 repeaters

2000–3000 using 1 km FOIRL

100 Mbps

— 205 2000

FOIRL = fiber optic inter-repeater linkUTP = unshielded twisted pair

Gigabit Ethernet (GbE)

Gigabit operation standardized in 1998– After Fibre Channel became established

Used Fibre Channel physical layer chips for 1-Gbps duplex operation– Needs fiber or enhanced quality copper (UTP-5e or better)

Shared (CSMA/CD) GbE added in 1999– Restricted to single hub– Extended Ethernet collision window to 4096 bit times (4.096µsec)

Introduced compatibility issues with 10Base-T and 100Base-T

– Has proved unpopular compared to switched GbE

STP = shielded twisted pair

Data rate Distance limitations (meters)

STP 4-pair

UTP 4-pair

MMF SMF

1000 Mbps

25 100

50/125: up to 55062.5/125up to 440

10,000

Link Aggregation

Initially, a proprietary switch hardware feature– Available on Fast Ethernet and Gigabit Ethernet

interfaces Now standardized as IEEE 802.3ad

– Further modifies LAN Layer 2 Example

– Aggregating four 100-Mbps inter-switch linksgives aggregated bandwidth of 400 Mbps

– Is cheaper than upgrading switches to GbE Link Aggregation Control Protocol (LACP) negotiates load

sharing– Fat pipes are treated as a single link by Spanning Tree Protocol

MACcontrol

MAC

PHY

New! MAC

control

MAC

PHY

…

LLC

Link Aggregation

LLC = logical link controlMAC = media access controlPHY = physical

Fat 400 Mbit/s pipe

10 Gigabit Ethernet (10GbE)

Standardized in 2003 as IEEE 802.3ae Initially aimed at MAN/WAN links and storage area networks

– Not designed for use in LAN Switched full duplex only

– CSMA/CD neither supported nor required Current standard only supports optical fiber

– Copper versions under investigation Poses new engineering challenges!

Framing is compatible with earlier versions of Ethernet Novel additions include

– WAN interface definition for connecting to SDH/SONET MANs and WANs– New, high-specification multimode fiber type and PHY-PMD interface

MAN = metropolitan area networkPMD = physical medium dependent

10GbE Architecture

Full duplex MACFull duplex MAC

XGMII or XAUIXGMII or XAUI

WWDM PHY(8B/10B coding)

WWDM PHY(8B/10B coding)

WWDM PMD(1310nm)

WWDM PMD(1310nm)

Serial LAN PHY(64/66B coding)Serial LAN PHY(64/66B coding)

Serial WAN PHY(64/66B coding + WIS)

Serial WAN PHY(64/66B coding + WIS)

SerialPMD

850nm

SerialPMD

850nm

SerialPMD

1310nm

SerialPMD

1310nm

SerialPMD

1500nm

SerialPMD

1500nm

SerialPMD

850nm

SerialPMD

850nm

SerialPMD

1310nm

SerialPMD

1310nm

SerialPMD

1500nm

SerialPMD

1500nm

10GBASE-CX410GBASE

-SR -LR -ER10GBASE

-SW-LW -EW

WIS = WAN interface sub-layer XAUI = 10Gbps attachment user interfaceWWDM = wideband wave division multiplexing XGMII = 10Gbps medium independent interface

10GbE Operating Distances

10GbE specification supports four fiber types– Two MMF types and two SMF types

Also allows three wavelengths: 850, 1310, and 1550 nm Leads to a number of operating distances!

– Table shows selection of distances (in meters) Shorter distance is with 62.5/125-µ cable, longer one with 50/125-µ

cable

Wavelength Fiber type

850 nm 1310 nm 1550 nm

10GBASE-S 26–300 ― ―

10GBASE-L ― 10,000

10GBASE-E ― ―30,000–40,000

10GBASE-LX4 300 300 10,000

Switched Ethernet

Ethernet Evolution

Shared vs. Switched LANs

Transparent Learning

Spanning Tree Protocol

Tutorial Questions

Additional Notes

Hubs, Bridges and Switches

Hubs extend the collision domain– They are Layer 1 devices

Operate on bits

Bridges and switches are bothLayer 2 LAN devices

– Operate on MAC frames Early bridges had limited connectivity

– Often operated on one frame at a time Switches have considerable connectivity potential

– Can operate on several frames in parallel

Full Duplex Operation

Traditional CSMA/CD operates in shared (half duplex) mode– Host can either write to network or read from network

But not do both at same time Traditional CSMA/CD hubs have CSMA/CD LAN ‘inside the box’

– Therefore are inherently half duplex Extension to standard allows simultaneous reading and writing

– Full duplex operation– Conditional on being supported by network device and host NIC

And there being only one NIC attached to device port

NIC = network interface card

Conventional hubs:inherently half duplex

Systems on dedicated switch ports:could be half- or full duplex

Modern CSMA/CD NICs available in two main forms– 100/100 or 10/100/1000

Notation means they are capable of selecting– Speed at which they operate (10, 100 or 1000 Mbit/s)– And the mode at which they operate

Half- or full-duplex Selection process carried out by an auto-negotiation protocol

– Runs between NIC and switch– Also works with some hubs

Protocol attempts to negotiate highest throughput first– 1000Mbit/s or 100Mbit/s full-duplex operation

Then works down through list to lowest throughput– 10Mbit/s half-duplex

Full duplex operation is collision-free

Auto-Negotiation Protocol

Bridges and Switches I

Bridges and LAN switches are MAC Layer relays– Used to interconnect LANs of same type– Use LAN MAC addressing– Operate on LAN frames

802.3,5,11, etc

Physical: to matchData Link Protocol

Bridge/LAN switch

Layer 2relay

Bridges and Switches II

Each port connects to a different collision domain– Supports parallel activity on each attached LAN

segment Transparent to routers and host operating systems

layers 5/6/7:Application

TCP, UDP

IP

802.3,5,11,..

Physical

802.3,5,11,..

Physical

802.3,5,11,..

Physical

802.3,5,11,..

Physical

layers 5/6/7:Application

TCP, UDP

IP

802.3,5,11,..

Physical

Shared LANs

Shared LANs developed for– File and print sharing (often on local servers)– Networked applications (may be on remote servers)– Bandwidth sharing, where dedicated bandwidth not required

Or dedicated bandwidth too expensive to justify for one system LANs have used structured cabling systems since early 1990s

– High specification cable interconnecting ‘wiring closets’ and network devices

– Facilitate high transmission rates Many of today’s wired LANs are CSMA/CD

– Operating at 10/100 or 10/100/1000Mbit/s– Fully switched LANs increasingly popular

Wireless LANs (11 and 54 Mbit/s) also becoming common– Operate as shared media LANs

Discussed later in course Switched LANs offer significant throughput increase over shared LANs

Switched vs. Shared Bandwidth

Example: Twelve users and fourservers share 100Mbit/s LAN

– All in same collision domain– Access time to shared channel

increases as usage increases Solution to increasing congestion:

replace shared LAN with 10/100Mbit/s switch– Users divided into smaller

collision domains Each receives larger portion

of bandwidth– Switch throughput at least

port speed ½ number of ports Eight-port switch supports

up to 400Mbit/s throughput

Switched LANs

Switches commonly used for LAN-LAN interconnection– Usually interconnect same technologies

For example, CSMA/CD to CSMA/CD– Falling switch prices mean end of hub market

Switches available for all versions of CSMA/CD– 10, 100, 1000 and 10000Mbit/s

Rapidly decreasing support for older technologies– Token Ring (16 & 100Mbit/s), FDDI and ATM (25, 155 &

622Mbit/s) CSMA/CD switches have become widespread due to

– Their versatility Support of different bit rates and media types

– Their lower per-port cost than alternative technologies

Ethernet Switches

Began to appear in mid-1990s as combined hub/bridges– Lower latency and lower per-port cost than bridges

Now commodity items– Multiple 10/100 interfaces at very low per-port cost

Unmanaged switches have become cost-efficient hub replacements

– Many manufacturers no longer make hubs Managed switches have CPU, memory and multitasking

operating system– Support many additional features

VLANs QoS Multicast And, of course, network management

QoS = quality of serviceVLAN = virtual LAN

Ethernet Switches(continued)

Late 1980s

Mid-1990s Late 1990s

Bridge

Hub Hub

SwitchSwitch

Hub Hub

Multilayer Switches

Multilayer switches have both switching and routing modules

– Operate at Layer 2 and Layer 3– Often very high-speed and expensive devices

Typically equipped with hardware acceleration– Used in backbone (or ‘distribution’) networks

Multilayer switch

Modern LAN Structure

Wiringcloset

Workgroupservers

Fiberlinks

10/100switch

Multilayerswitch

Site backboneGigabit Ethernet/ATM

Hub

Hub

Hub

Workgroups connected to small switches

– Workgroup servers get dedicated ports

– 10 and 100Mbit/s connections

Workgroup switches interconnected by multilayer switches

– The backbone or distribution network

– 100 and 1000Mbit/s connections used

Switched Ethernet

Ethernet Evolution

Shared vs. Switched LANs

Transparent Learning

Spanning Tree Protocol

Tutorial Questions

Additional Notes

Bridge(switch) interfaces conform to specific IEEE LAN standard– For example, IEEE 802.3 Ethernet family

And operate according to the IEEE bridging standards– 802.1D: CDMA/CD transparent bridging and Spanning Tree– 802.1Q: Virtual LANs (VLANs)– 802.1p: Traffic classes and prioritization

The term switch is not standardized– Used in different ways to suit manufacturers’ marketing policies

For example, ‘multilayer switch’ The term bridge defines a generic MAC-layer relay

– Literature uses this term– We shall also use it in when discussing

these devices in the context of a standard

Relay logic

MAC1 MAC2

Phys1 Phys2

Standards and Terminology

Transparent Learning Bridges

Each interface operates in promiscuous mode– Receives and processes all frames from LAN

attached to that interface Devices build a MAC address table

– Inspect each frame header– Record frame’s source MAC

address (MACSA) and the portnumber on which the frameentered

Process is calledtransparent learning

– Determines how dataframes processedsubsequently

Port 1

Port 2

MACA

MACN

MACG

MACK

MAC address table

MACSA = AMACDA = K

IPSA = AIPDA = K

Upper layerinfo

G

A

B

C T

K N

1

2

B

The Three F’s

Bridge extracts MACDA from incoming frame headers and looks it up in MAC address table

– Device then makes forwarding decision Forwards frame to systems on different ports

– To ensure frame reaches correct LAN segment Filters frames between systems on same port

– Since systems on same port, normal MAC-level addressing ensures that frame reaches destination

– No further action required by bridge Floods broadcast and multicast frames

– Bridge also required to flood frames with unknown MAC destination addresses

– Flooding means sending a copy of the incoming frame to all other ports T unknown:

flooded

B G: filtered

A K: forwarded

G

A

B

C T

H K N

1

2

Static Filtering

Older bridges could be programmed with static filters Examples

– Filter frames to system G arriving on port 1– Filter AppleTalk frames arriving at port 2

This function eventually taken over by routers

All G: filtered

G

A

B

C T

H K N

1

2All AppleTalk: filtered

Port 1

Port 2

Port 3

L R A

M S B

N T C

G

H

MAC Address Tables

Identify the port on which known systems can be reached– Could be via other switches

RM

A

B C G H

L

NS

T

SW1

SW2

1 2

3

1 2

3

Port 1

Port 2

Port 3

A G L

B H M

C N

R

S

T

Switched Ethernet

Ethernet Evolution

Shared vs. Switched LANs

Transparent Learning

Spanning Tree Protocol

Tutorial Questions

Additional Notes

Single Points of Failure

A single bridge interconnecting two LANs constitutes a single point of failure

– More than one user affected if device fails

LAN 2

LAN 1

LAN 5

LAN 3 LAN 4

G

A

B

C T

K N

1

2

B

What About Using ExtraBridges for Resilience?

Bridges learn MAC addresses as usual– For example, a frame from H to P

Each bridge queues a copy of the frame for forwarding to the other LAN segment

Other port on each bridge receives copy of frame– Notes new port for H– Queues frame for forwarding to original LAN– And looping starts to occur

… but when does it stop? Suppose you were to use two extra bridges

instead of just one?– Or there is more than one loop?

Bridges use the Spanning Tree Protocol to resolve the problems of looping frames

B1

1 2

B2

1 2N

P

F

G

H Q

Supported by all MAC-Layer bridges (switches)– Runs automatically (unless disabled)

Bridges send each other topology messages (‘BPDUs’) to build the spanning tree

– A loop-free topology When the protocol has run, certain bridge ports

will forward MAC data frames– Other ports will be blocked *– Leaves a single path between any two LANs

Inter-switch links treated as LANs in this context

Once configured, protocol periodically re-affirms topology

– Reconfigures spanning tree if topology fails

The Spanning Tree Protocol(IEEE 802.1D)

B1

1 2

B2

1 2

N

P

F

G

H Q

BPDU = bridge protocol data unit* All ports continue to receive BPDUs, even if blocked for data frames

Motivation for Spanning Tree A collection of LANs that are interconnected by a set of bridges, The interconnection may have redundant to increase reliability, Redundancy introduce loops which defeats the objective because multiple reception

of same packets (diff. routes) and routing back to source are possible, A spanning tree create a single virtual route, although multiple physical routes are

there.

Setting up a Spanning Tree The bridge (B) with lowest ID is selected as Root Bridge (RB), The root port (RP) of B, other than RB, is a port of B such that: the link(RP,RB) is

least among all ports of B, where least cost means minimum number of hops, least delay, or maximum bandwidth,

There should be one designated B (DB) for each LAN-X. DB is defined as a B directly connected to LAN-X such that Path(LAN,RB) is least is route is done through DB.

There should be a designated port (DP/DB) connecting DB to LAN-X, if ties then lowest ID.

Make Root Ports (RPs) and Designated Ports (DPs/DB) as forwarding ports and all the other as blocking ports.

The results: packets flowing through RPs and DPs/DB follow a spanning tree route without loops.

The Spanning Tree Protocol(IEEE 802.1D)

Spanning Tree Overview

Spanning Tree operation based on two device parameters– Bridge identifier, or BID– Notional cost of leaving bridge on given

port Bridges flood BPDUs to determine bridge with

lowest BID– This bridge becomes Root Bridge

Other bridges then1. Identify their root ports

Those with lowest cost path to Root Bridge

2. Identify any designated ports Those responsible for forwarding

frames away from the Root3. Block their non-designated ports

Those with higher-cost path to Root

10 15

10 5

bridge 1priority 10

1 2

1 2

N

P

F

G

H Q

bridge 2priority 1

Port costs

Bridge Identifiers

Two fields concatenated into 64-bit number

– 16-bit priority and 48-bit MAC address Priority is at most significant end of number

Manufacturers normally select default value for priority

– Standard recommends default of 32768 Reducing priority increases bridge’s Root eligibility

– For example, reducing the priority to 32700 MAC address acts as tie-breaker if all priorities equal

PrioritySending port’sMAC address

MAC = 0060.6475.6bc0 MAC = 0060.6475.6b00 MAC = 0060.6475.6d05

Port cost

Every bridge/switch port has outgoing port cost

– Older bridges used 109/bandwidth by default

– Newer versions use non-linear scale ofIEEE 802.1D

– Port costs can usually be set manually Root path cost is the sum of (outgoing) path

costs to Root from current bridge– Cost is 0 for all Root ports

Bandwidth Cost

10 Mbps 100

16 Mbps 62

45 Mbps 39

100 Mbps 19

155 Mbps 14

622 Mbps 6

1 Gbps 4

10 Gbps 2

100 19 19

Root pathcost = 138

Outgoingcost = 19

Outgoingcost = 100

100 Mbps

100 Mbps

10 Mbps

10 Mbps

Root bridge

LAN Switching

Hubs and Media

Bridges and Switches

Transparent Learning

Spanning Tree Protocol

Spanning Tree Examples

Note on VLANs

Two-Switch ExampleStage I: Root Discovery

Both bridges flood BPDUs withRoot Bridge = self for a preset period

At end of this stage, B2 found to havelowest BID

– Priorities same (32768)– B2’s MAC address lower than B1’s

B2 becomes Root Bridge

32768

32768 0060.6475.6b00

0060.6475.6bc0B1’s BID

B2’s BID

B1Priority 32768

0060.6475.6bc0

1 2

1 2

N

P

F

G

H Q

LAN 1(10Mbps)

B2Priority 32768

0060.6475.6b00

LAN 2(10Mbps)

Stage II: Find lowest cost root path

Non-root bridge(s) determinelowest-cost path to root

Equal-lowest-cost paths decided byseries of tie-breakers

1. Upstream bridge with lowest BID Where more than one bridge

available

2. Lowest port ID on bridge Where bridge has lowest

cost path from more than one port Defaults to lowest port number

B1 has two lowest cost paths to root– Port 1 is lower port number than port 2

B1 port 1 becomes root port– Alternate path via Port 2 will be blocked

Becomes a non-designated port

LAN 1(10Mbps)

LAN 2(10Mbps)

B1Priority 32768

0060.6475.6bc0

1 2

1 2

F

G

H

B2Priority 32768

0060.6475.6b00

Root pathCost = 100

Root pathCost = 100

N

P

Q

Stage III: Finalize Spanning Tree

Root Bridge– Places all active ports in forwarding

state All active ports on root bridge

are designated ports Non-root bridges modify port states

– Root port placed into forwarding state B1, port 1

– Designated port(s) placed into forwarding state

In this example, B1 has no designated ports

– Non-designated ports moved to blocking state

B1, port 2 Root bridge periodically sends out Root

BPDU to reaffirm topologyLAN 1

(10Mbps)LAN 2

(10Mbps)

B1Priority 32768

0060.6475.6bc0

1 2

1 2

F

G

H

B2Priority 32768

0060.6475.6b00 N

P

Q

RP(F) NDP(B)

DP(F) DP(F)

X

Further Example: Before

LAN 2

C = 10

C = 10

Bridge 3

C = 5

C = 5

Bridge 4

C = 5

C = 5

Bridge 5

C = 10

C = 10

Bridge 1

C = 10

Bridge 2

C =

5 C

= 5

LAN 1

LAN 5

LAN 4LAN 3

Further Example: Before

LAN 2

C = 10

C = 10

Bridge 3

C = 5

C = 5

Bridge 4

C = 5

C = 5

Bridge 5

C = 10

C = 10

Bridge 1

C = 10

Bridge 2

C =

5 C

= 5

LAN 1

LAN 5

LAN 4LAN 3

Stage I: Identify Root Bridge

Bridge with lowest BID becomes

Root

LAN 2

C = 10

C = 10

Bridge 3

C = 5

C = 5

Bridge 4

C = 5

C = 5

Bridge 5

C = 10

Bridge 2

C =

5 C

= 5

LAN 1

LAN 5

LAN 4LAN 3

C = 10

C = 10

Bridge 1

Stage II: Identify Forwarding Ports

Notes1. RPC = root path cost2. Bridge 2 has only one

root path3. Bridges 4 & 5 tie on

lowest path to Root for LAN 5– Lowest BID

is tie-breaker (4)4. All Root Bridge ports

are designated portswith RPC of zero

Notes1. RPC = root path cost2. Bridge 2 has only one

root path3. Bridges 4 & 5 tie on

lowest path to Root for LAN 5– Lowest BID

is tie-breaker (4)4. All Root Bridge ports

are designated portswith RPC of zero

LAN 2

C = 10

C = 10

Bridge 3

C = 5

C = 5

Bridge 4

C = 5

C = 5

Bridge 5

C = 10

C = 10

Bridge 1

C = 10

Bridge 2

C =

5 C

= 5

LAN 1

LAN 5

LAN 4LAN 3

RPC = 10

RPC = 15

RPC = 5

RPC = 10

RPC = 10 or 15

RPC = 5

RPC = 0

RPC = 0

RPC = 10

(No path to root)

Stage III: Finalize Spanning Tree

LAN 2

C = 10

C = 10

Bridge 3

C = 5

C = 5

Bridge 4

C = 5

C = 5

Bridge 5

C = 10

Bridge 2

C =

5 C

= 5

LAN 1

LAN 5

LAN 4LAN 3

C = 10

C = 10

Bridge 1X

X

B1

The Spanning Tree

B2 B4B5 B3

L1

L4 L5

L2

L3

Key

= Forwarding

= Blocking

Key

= Forwarding

= BlockingX

Summary

Variety of media are used on CSMA/CD LANs– But the most common media today are twisted pair and optical fibre

LAN switches have replaced bridges– Behave exactly like bridges– But have higher connectivity and throughput

Bridges and switches divide LANs into separate collision domains– But interconnected LANs are still a single broadcast domain

Modern LANs must have some degree of fault tolerance– Provided by installing additional switches and links

CSMA/CD bridges use the Spanning Tree Protocol to create aloop-free topology that spans whole Layer 2 domain

Virtual LANs provide additional traffic control in switched LANs

Switched Ethernet

Ethernet Evolution

Shared vs. Switched LANs

Transparent Learning

Spanning Tree Protocol

Tutorial Questions

Additional Notes

Tutorial Questions

1. At which OSI layers do bridges and switches operate?

2. What is meant by a transparent learning bridge?

3. How, and why, are frames to unknown destination addresses treated like broadcasts and multicasts?

4. What is the reason for using Spanning Tree and why is it required?

5. Full duplex operation is likely to be most beneficial for what types of host?

Tutorial Questions(continued)

6. In the diagram below, there are three hosts systems, A, B & C and one server, D all connected to 10Mbit/s LAN segments

6. Briefly describe how a frame from A reaches D, assuming that all systems have just been switched on; include a description of how the ARP from A is processed by the bridges.

7. Show the entries of the port tables in bridges B1 and B2, once the location of all three systems have been determined.

A

B

C

D

B1 B2LAN 2

(10Mbit/s)

LAN 1(10Mbit/s)

LAN 3(10Mbit/s)

Tutorial Questions(continued)

7. Here is the LAN diagram again, but an additional bridge, B3, has been connected as shown, requiring the use of Spanning Tree. (The diagram shows the MAC address of each bridge, all of which have equal priority.)

Re-draw the diagram showing the Root Bridge and labelling all bridge ports as root ports, designated ports or non-designated ports and showing which ports are forwarding and which are blocking.Show one way in which you could connect the above LANs and server, with another LAN segment and three further servers into a single eight-port switch.

A

B

C

D

B10000.0c07.ac01

B20004.2875.c860

B30006.28c3.03c0

LAN 1(10Mbit/s)

LAN 2(10Mbit/s)

LAN 3(10Mbit/s)

Switched Ethernet

Ethernet Evolution

Shared vs. Switched LANs

Transparent Learning

Spanning Tree Protocol

Tutorial Questions

Additional Notes

Repeaters and Hubs

Operate at the Physical Layer– Physical Layer relays– Unit of transfer is the bit

Extend domain of MAC protocol– The collision domain– Repeat incoming bits to other

ports MAC frames seen by all

systems Systems contend for

extended communication channel

Support a variety of media types– Allows old style shared coaxial

segments to be connected to modern twisted pair segments

Most hubs just multi-port repeaters

relay logicPhys

1

Phys

2

Coaxial LANsegment

hub

Ph1 Ph1 Ph1 Ph1 Ph1 Ph1 Ph1

relay logic

10BaseT Hubs

Have separate 10BaseT ports for each system – Enhances LAN resilience– Works in conjunction with structured

cabling systems– Can be cascaded to interconnect multiple

LAN segments Maximum number of hubs/repeaters allowed

between any two systems varies with media type and bit rate

– 10BaseT No more than 4 (same as for coax

cable)– 100BaseT

No more than 2 with twisted pair cable

Coaxial LANsegment

CSMA/CD Media Types & Limitations

LAN segment lengths limited by two factors– The operation of the CSMA/CD protocol– The media type

Strictly, the bandwidth of the media CSMA/CD collision window sets maximum amount of time for

detecting a collision– Specified by the 10Base5 standard as 51.2µs at 10 Mbit/s

Equal to 512 bit times– At 100Mbit/s the collision window becomes 5.12µs (still 512 bit

times) Different media have different transmission qualities

– Structured cabling systems specify maximum distance from wiring closet to desktop system of 100 metres

Standards committees meet this criterion for all desktop LAN speeds

CSMA/CD = carrier sense multiple access with collision detection

CSMA/CD Media Types & Limitations (continued)

Media Type

Data rate(Mbit/s)

Max. cable length

(metres)

Max. Number

of stations per cable

Twisted Pair

10, 100, 1000

100 Two

Thin Coax.

10 185 30

Thick Coax

10 500 100

Optical Fibre

10, 100, 1000, 10000

Depends on fibre type and data rate

Two

Evolving Technologies forEthernet LAN Interconnection

1980 – 1984 Shared Ethernet deployedInternational LAN standards developed

1985 – 1989 Bridges used for LAN interconnection to limit size of collision domains, with Spanning Tree facilitating bridge redundancy; routers used for LAN-WAN interconnection

1990 - 1994 High-speed, low-cost routers become alternatives to bridges‘Backbone routers’ developed for site interconnections

1995 - 1999 VLAN-capable switches replace bridges and LAN routers100Mbit/s Ethernet becomes common, GbE developed

2000 - Switched access and VLAN deployment become common10GbE developed, Ethernet switches become QoS-enabled

The Rise and Fallof the LAN Router

In early 1990s, small routers introduced to limit sizeof broadcast domains

– Became cheap, and fast, enough to use in LANs But routers operate at Network Layer

– Require configuration (are not plug-and-play)– Have higher per-port cost than equivalent bridge

LAN switches began to replace bridges in mid-1990s– Still operate at Layer 2– Have much lower per-port cost than routers– Can be operated in plug-and-play mode or configured

For example with management and VLAN information Routers still required for inter-site and inter-VLAN

communication– Particularly suitable for interconnecting different

technologies For example, CSMA/CD & Frame Relay, CSMA/CD &

Token Ring