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Fall 2005

Extending LANs

Qutaibah MalluhiCSE DepartmentQatar University

Repeaters, Hubs, Bridges, Fiber Modems, and Switches

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LAN Distance Limitation

LAN technologies are designed with constraints of speed, distance and costs

Typical LAN technology can span, at most, a few hundred meters

Why limit the distance?– Length of medium affects fair, shared access to

medium » CSMA/CD - delay between frames, minimum frame length » Token passing - circulation time for token

– Length of medium affects strength of electrical signals and noise immunity

How can a network be extended to cover longer distances; e.g., the QU campus?

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LAN Extension

Most techniques use additional hardware to relay LAN signals between LAN segments

Resulting mixed technology– Must not violate design assumptions– Often part of original design

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Repeaters

May want to extend LAN medium – Ethernet - timing constraints allow longer medium – Signal strength constraints limit length

Repeater - bidirectional, analog amplifier that retransmits analog signals

Simply copy signals between segments – Do not understand frame formats – Do not have hardware addresses

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Ethernet Repeater

One repeater can effectively double the length of an LAN segment

Any Ethernet segment is limited to 500 meters Repeater can double to 1,000 meters

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Repeaters Limitations

Can't extend Ethernet with repeaters indefinitely CSMA/CD requires low delay; if medium is too long,

CSMA/CD won't work Ethernet standard includes limit of 4 repeaters between

any two Ethernet stations – With four repeaters, can extend Ethernet through a building

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Characteristics of Repeaters

Very easy to use - just plug in Repeaters simply re-transmit analog signals

– Collisions affect entire network – Transient problems - noise - propagates throughout

network

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Hubs Physically

– Small electronic device– Has connections for several computers (e.g., 4 or 20)

Logically– Operates on signals– Propagates each incoming signal to all connections– Similar to connecting segments with repeaters– Does not understand frames

Extremely low cost Considering the Ethernet 4 repeater limitation, a

hub counts as a repeater

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Bridges

Connect two LAN segments Retransmits frames from one segment on other

segment(s) Handles complete frame

– Uses NIC like any other station – Performs some processing on frame

Invisible to other attached computers Relatively easy to use - just plug in Does not forward noise or collisions

– Isolate collisions, noise

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Frame Filtering

Bridges can do additional processing – Don't forward collisions, noise – Only forward frames where necessary

Bridge performs frame filtering and forwards frames along LAN segments to destination – Learns location of stations by watching frames – Forwards all broadcast and multicast packets

Bridge checks destination of each incoming frame Looks up destination in list of known stations

– Forwards frame to next interface on path to destination – Doesn't forward frame if destination on LAN segment

from which frame was received

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Setting up Bridge Forwarding Table

Bridge examines source address in each frame Adds entry to list for LAN segment from which

frame was received Must forward any frame whose destination is not

in the list on every interface Startup bridge behavior

– Initially, the forwarding tables in all bridges are empty – First frame from each station on LAN is forwarded to

all LAN segments – After all stations have been identified, frames are only

forwarded as needed – May result in burst of traffic after, e.g., power failure

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Filtering Example

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Design with Filtering Bridges

Filtering bridge allows concurrent use of different LAN segments if traffic is local

U and V can exchange frames at the same time X and Y exchange frames

Can improve performance by careful design– Designers identify patterns of local communication and

isolate groups of communicating computers with bridges

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Bridging Between Buildings

Similar to extending AUI with fiber modems Can put bridge in one building with long

connection to LAN segment in different building

Avoids the costly extended AUI connection for each computer in remote building

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Cycles Of Bridges

Complex bridge connections may not be apparent– Adding one more bridge may inadvertently introduce a cycle– A circular path through bridged networks is called a cycle

Consider broadcasting when a cycle of bridges exists

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Breaking Bridge Cycles

Bridges must cooperate to broadcast frames exactly once on each segment

Use a Distributed Spanning Tree (DST) algorithm– Used to determine which bridges will forward broadcasts– Results in a unique path from each source to each

destination– Used by all bridges to

» Discover one another» Break cycle(s)

As each bridge joins the network, it communicates with other bridges on special hardware (typically multicast) address – Learns network topology – Performs spanning tree computation – Determines if bridge will form a cycle

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Spanning Tree Example

Initial logical broadcast topology

Broadcast topology after running the DST algorithm

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Fiber Optic Extensions

Can extend connection to a computer using fiber optic cable – Insert fiber modems and fiber optic cable into AUI cable

Optical fiber– Has low delay– Has high bandwidth

Fiber modems: – Convert AUI signals to digital signal – Transmit digital signals via fiber optic cable to other

modem

Most often used to connect two LANs - typically through a bridge - different buildings (see next slide)

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Bridging Between Buildings

Similar to extending AUI with fiber modems Can put bridge in one building with long

connection to LAN segment in different building

Avoids the costly extended AUI connection for each computer in remote building

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Bridging Over Long Distances

Can use leased line, microwave, laser or satellite to connect two bridges and LAN segments

Using two bridges instead of one – Why? – WAN links are expensive and usually slow

» T1: 1.54 Mbps» Satellite 56 Kbps

– Filters at both ends, reducing traffic across slow link – Provides buffering at both ends, matching dissimilar

speeds

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Switches

Effectively a separate LAN segment for each port

Physically similar to a hub– hub shares single segment among all port

Logically similar to a bridge– Operates on packets– Understands addresses– Only forwards when necessary

With switching, multiple stations can transmit simultaneously

Provides much higher aggregate bandwidth

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Switches And Hubs

Switches are more expensive per port May make more sense economically to use hubs

for some stations and switches for others

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Summary LANs

– Have distance limitations– Can be extended

Optical fiber and modems can be used to extend AUI for single station

Repeater– Connects two LAN segments– Repeats and amplifies all signals– Forwards noise and collisions

Bridge– Understands and forwards entire frames– Uses addresses– Does not forward noise or collisions– Filtering bridges forward frames only as needed

» Allows simultaneous transmission on the segments – Therefore, careful design can improve the performance.

» Forwards all broadcast and multicast frames– DST used to break cycles

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Summary (cont’d)

Hub– Central facility in star-shaped network– Operates like a repeater

Switch– Central facility in star-shaped network– Operates like a set of bridged segments– More expensive that a hub but provide higher aggregate

bandwidth