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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
