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Hubs, Switches and Bridges

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Hubs

4 ports Physically star/Logically Bus Flooding always happens Unintelligent device

Number of ports: 4, 8, 12, 32 Layer 1 device

Active: increases the signal strength Passive: Without rising the strength of the signal

Half Duplex Types of Addressing

Unicast Multicast

Broadcast

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Hubs

active central element of star layout

each station connected to hub by two UTP lines

hub acts as a repeater

limited to about 100 m by UTP properties

optical fiber may be used out to 500m

transmission from a station seen by all others

if two stations transmit at the same time have acollision

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Two Level Hub Topology

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Switches

Hardware based device

ASIC Chip used

Maintain MAC table Performance faster

Number of ports: 8, 12, 24, 48, 400, 800

High end switch

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Bridges

connects similar LANs

identical physical / link layer protocols

minimal processing

can map between MAC formats

reasons for use

reliability

performance

security

geography

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Bridge Function

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r ges on Intelligent device Software based device

Performance is slow Maintains MAC table MAC Table is filled dynamically Initially the MAC table is empty

Bridge forwards the frames by looking into the MACtable Layer 2 device Max Number of ports: 16

MAC table is also called as CAM table (ContentAddressable Memory) Destination address is not known, it will be forwarded

to all ports. Sending is broadcast and receiving is multicast. Ageing time is 300 seconds.

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Bridge Design Aspects

no modification to frame content or format

no encapsulation

exact bitwise copy of frameminimal buffering to meet peak demand

contains routing and address intelligence

may connect more than two LANs

bridging is transparent to stations

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Bridge Protocol Architecture

IEEE 802.1D

MAC level

bridge does not need LLC layer

can pass frame over external comms system

capture frame

encapsulate it

forward it across link remove encapsulation and forward over LAN link

e.g. WAN link

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Bridges andLANs

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Loop of Bridges

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Shared

MediumBus andHub

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Switch vs Bridge

switch can be viewed as full-duplex hub

differences between switches & bridges: bridge frame handling done in software

switch performs frame forwarding in hardware bridge analyzes and forwards one frame at a time

switch can handle multiple frames at a time

bridge uses store-and-forward operation

switch can have cut-through operation hence bridge have suffered commercially

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TypicalLarge

LANOrganizationDiagram