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Ethernet Neil Tang 9/17/2008

Ethernet Neil Tang 9/17/2008. Outline. Major Concern for MAC Design Basic Architecture Physical Properties MAC Protocol Strengths and Weaknesses. Major Concerns for MAC Design. Throughput Fairness. Ethernet (802.3). Basic Architecture: multiple hosts share a physical medium.

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Ethernet Neil Tang 9/17/2008

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  1. EthernetNeil Tang9/17/2008 CS440 Computer Networks

  2. Outline • Major Concern for MAC Design • Basic Architecture • Physical Properties • MAC Protocol • Strengths and Weaknesses CS440 Computer Networks

  3. Major Concerns for MAC Design • Throughput • Fairness CS440 Computer Networks

  4. Ethernet (802.3) • Basic Architecture: multiple hostsshare a physical medium. • MAC Protocol: CSMA/CD CS440 Computer Networks

  5. Basic Architecture CS440 Computer Networks

  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) CS440 Computer Networks

  7. Frame Format Min/Max Body Size: 46/1500 bytes CS440 Computer Networks

  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. CS440 Computer Networks

  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. CS440 Computer Networks

  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). CS440 Computer Networks

  11. MAC Algorithm 1-persistent Vs. p-persistent 1-persistent protocol (Ethernet): It transmits once the medium is idle p-persistent protocol (Aloha): Ittransmits with probability p and defers until nextempty timewith probability q=1-p. Make the same decision next time. CS440 Computer Networks

  12. MAC Algorithm Why min frame length 512 bits • Worst case propagation round trip delay (2500m, 4 repeaters) = 51.2 μs • Transmission time = 512/10M = 51.2 μs CS440 Computer Networks

  13. Strength and Weakness Strength: • 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. CS440 Computer Networks

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