9 extending lans fibre modems repeaters bridges and switches

8/3/2019 9 Extending LANs Fibre Modems Repeaters Bridges and Switches

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11/11/2011 1

Data Communications & Networking: DIT 230

Extending LANs: Fibre Modems, Repeaters,Bridges and Switches

UNIT: 09



11/11/2011 2

Introduction

MAC protocols such as CSMA/CD require

time proportional to the length of the cable

Electrical signal weakens with distance

Network designers choose a combination of

capacity, delay and distance that can beachieved within a given cost hence:

LAN technologies are designed with

constraints of speed, distance and costs.

Typical LAN technology can span, at most, a

few hundred meters.



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LAN Design for Distance

LANs use shared medium - Ethernet, token

ring

Length of medium affects fair, shared accessto medium

CSMA/CD - delay between frames, minimum

frame length

Token passing - circulation time for token

Length of medium affects strength of electricalsignals and noise immunity



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LAN Extensions

Several techniques extend diameter of LAN

medium

Most techniques use additional hardware LAN signals relayed between LAN segments

Resulting mixed technology stays withinoriginal engineering constraints while

spanning greater distance



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Fiber Optic Extensions

Fiber modems extend connection betweencomputer and transceiver

Can extend connection to a computer using

fiber optic cable

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 on different

buildings



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Example of Fibre Modem Connections



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Repeaters

We may want to extend the LAN medium:

Ethernet - timing constraints allow longer

medium Signal strength constraints limit length

Repeater – a bidirectional, analog amplifier that retransmits analog signals.

One repeater can effectively double the

length of an LAN segment.



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Figure below shows



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Ethernet Repeaters

Simply copy signals between segments

Do not understand frame formats

Do not have hardware addresses

Any Ethernet segment is limited to 500

meters Repeater can double to 1,000 meters



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Limits on Repeaters

• Can't extend Ethernet with repeaters

indefinitely.

• CSMA/CD requires low delay; if mediumis too long, CSMA/CD won't work.

• Ethernet standard includes limit of 4repeaters between any two Ethernet

stations.



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Cont’d



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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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Network Bridges

A network bridge is a device that connectsmultiple network segments (Also connect twoLAN 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

Bridging is most suitable to be used for LANnetwork traffic data load.



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Advantages of Network Bridges

Bridges are simple to use and they are

relatively inexpensive.

Bridges have simple configuration modes

Bridges help to lower the data load over the

data link layer. Bridges are more reliable if one wants to

lower the bandwidth utilization.



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Disadvantages of Network Bridges

All bridges are unable to read specific IP

address

Bridges cannot help to build a communicationnetwork between the networks of different

architectures

Unable to handle more complex and variable

data load such as occurring from WAN.

Bridges transfer all types of broadcastmessages



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Bridged LAN Segments



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Filtering Bridges

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 multicastpackets

This kind of bridge is also called a transparent

bridge, since it requires no special setup.



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Frame Filtering

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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How Does A Bridge Set Up Its Forwarding Table?

A bridge examines the source address in

each arriving frame.

It adds an entry to the list for the LANsegment from which the frame was received

(essentially recording from which “direction”

the frame arrived).

The bridge forwards each frame to each

interface that does not have the destinationaddress in its list.



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Filtering Example



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Startup Behavior of Filtering Bridges

Initially, the forwarding tables in all bridgesare empty.

The 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. This behavior may result in a burst of traffic

after some events like power failures.



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Designing with Filtering Bridges

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

• U and V can exchange frames at the sametime X and Y exchange frames.

• Designers identify patterns of local

communication and isolate groups of communicating computers with bridges.



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Bridging Between Buildings

• This is similar to extending AUI with fiber modems.

• We can put a bridge in one building with a long

connection to a LAN segment in a differentbuilding.

• This avoids an extended AUI connection for each computer in the remote building.



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Bridging Across Longer Distances

• Can use leased line, microwave, laser or

satellite to connect two bridges and LAN

segments• Using two bridges instead of one:

– Filters at both ends, reducing traffic across slow link

– Provides buffering at both ends, matching dissimilar

transmission speeds



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Bridges and Cycles• We can use multiple bridges to interconnect

many LAN segments.

• Stations on segment c sends frames tostations on segment g through B2, B1, B3and B6

• Broadcasts are forwarded through all bridges.

• Suppose another bridge connects g and f ?



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Cycles

• A circular path through bridged networks iscalled a cycle

• Adding B4 creates a cycle



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Eliminating Broadcast Cycles

Bridges must cooperate to broadcast framesexactly once on each segment.

The solution is from graph theory - spanning

trees - used to determine which bridges willforward broadcasts.

As each bridge joins the network, itcommunicates with the other bridges usingspecial hardware (typically multicast)addresses

The set of bridges learn the network topology;

performs the spanning tree computation; and

determines if the new bridge will result in a cycle.



11/11/2011 28

Switching

A network switch is a small hardware device that joins multiple computers together within one localarea network (LAN).

• Effectively a separate LAN segment for each port.

• Similar to hub - hub shares single segment amongall ports.

• With switching, multiple stations can transmit

simultaneously.• Switching provides much higher aggregate

bandwidth.



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Switches and Hubs

Network switches inspect data packets asthey are received, determining the source and

destination device of that packet, and

forwarding it appropriately.

A hub sends all packets out through all ports

A switch only sends the packet a packet outthrough the port that actually needs it. Cuts

down on broadcast traffic.

A switch is effectively a higher-performancealternative to a hub.



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Adv & Disad of Switches

Advantages : Reduces the number of Broadcast domains

Supports VLAN's

Intelligent device Compared to Bridges ,Switches are more H/w oriented

therefore operations are less CPU intense [Basicoperations]

Disadvantages: Not as good as a router in limiting Broadcasts

Handling Multicast packets needs quite a bit of

configuration & proper designing. At times switches when in Promiscuous mode is a

opening for Security attacks [Spoofing Ip address or capturing Ethernet Frames using ethereal]

9 extending lans fibre modems repeaters bridges and switches

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