1
1
Ethernet 2.6
• Adapted from Peterson&Davie Book
and other universities
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Ethernet Overview
• History
– developed by Xerox PARC in mid-1970s
– roots in Aloha packet-radio network
– standardized by Xerox, DEC, and Intel in 1978
– similar to IEEE 802.3 standard
•CSMA/CD
– carrier sense
– multiple access
– collision detection
•Frame Format
Dest
addr
64 48 32
CRCPreamble Src
addr Type Body
1648
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Ethernet (cont)
• Addresses
– unique, 48-bit unicastaddress assigned to each adapter
– example: 8:0:e4:b1:2
– broadcast: all 1s
– multicast
• Bandwidth: 10Mbps, 100Mbps, 1Gbps
• Length: 2500m (500m segments with 4 repeaters)
• Problem: Distributed algorithm that provides fair access
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Multiple Access Communications
– Broadcast networks
• Also referred to as multiple access networks
• All information is received by all users, routing is not
necessary
• A flat addressing scheme is sufficient to indicate which
user a given packet is destined
• Medium access control protocol is required to
orchestrate the transmissions from the various users
• Main concern is interference form other users
5
Transmit Algorithm
• If line is idle…
– send immediately
– upper bound message size of 1500 bytes
– must wait 9.6us between back-to-back frames
• If line is busy…
– wait until idle and transmit immediately
– called 1-persistent (special case of p-persistent)
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Algorithm (cont)
• If collision…
– jam for 32 bits, then stop transmitting frame
– minimum frame is 64 bytes (header + 46 bytes of data)
– delay and try again
• 1st time: 0 or 51.2us
• 2nd time: 0, 51.2, 102.4, or 153.6us
• 3rd time: 0, 51.2, 102.4, 153.6, 204.8, 256.0, 307.2, or 358.4us
•nth time: k x51.2us, for randomly selected k=0..2m–1,where
m=min(10,n).
• give up after several tries (usually 16)
• exponential backoff
