1. Ethernet Neil Tang 9/17/2008
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2. Outline Major Concern for MAC Design Basic Architecture
Physical Properties
MAC Protocol
Strengths and Weaknesses
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3. Major Concerns for MAC Design
Throughput
Fairness
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4. Ethernet (802.3)
Basic Architecture: multiple hosts share a physical medium.
MAC Protocol: CSMA/CD
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5. Basic Architecture
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6. Physical Properties
Broadcast: Any signal placed on Ethernet is broadcast.
Encoding: Manchester
Error Detection: CRC
Physical Distance: 2500m
Number of Hosts: 1024
Data Rate: 10Mbps/100Mbps, 1Gbps
Cables: 10Base2(200m), 10Base5(500m), 10BaseT(100m)
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7. Frame Format
Min/Max Body Size: 46/1500 bytes
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8. Addressing 48-bit Address: e.g., 08:00:2b:e4:b1:02
The address is burned to the ROM of each adaptor. Every adaptor has a world-wide unique address.
Each manufacturer is allocated a different prefix.
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9. MAC Algorithm In the receiver
The receiver in an adaptor receives all frames but only accepts:
frames addressed to its own address
frames addressed to the broadcast address (FF:FF:FF:FF:FF:FF)
Frames addressed to a multicast address (e.g., 01:00:5E:80:00:00 -01:00:5E:FF:FF:FF (reserved by IANA)), if it is in that multicast group
All frames, if it is placed in promiscuous mode.
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10. MAC Algorithm In the transmitter
If it has data to send and the line is busy, it waits for the line to go idle and transmit immediately.
At the moment it detects collision, it transmits a 32-bit jamming sequence and stops transmission.
It waits a “certain” amount of time and tries again. It uses binary exponential backoff, delay k  51.2μs , where k=0…2n-1(random selection).
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11. MAC Algorithm 1-persistent Vs. p-persistent
1-persistent protocol (Ethernet): It transmits once the medium is idle
p-persistent protocol (Aloha): It transmits with probability p and defers until
next empty time with probability q=1-p. Make the same decision next time.
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12. MAC Algorithm Why min frame length 512 bits
Worst case propagation round trip delay (2500m, 4 repeaters) = μs
Transmission time = 512/10M = 51.2 μs
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13. Strength and Weakness Strength: Weakness:
Simple: No routing, no switching and very simple MAC protocol.
Low Cost: Cheap cables and cheap equipments
Weakness:
Inefficient in Throughput: Under heavy traffic condition, collisions happen frequently.
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