Ethernet Neil Tang 9/17/2008 CS440 Computer Networks.

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Presentation transcript:

Ethernet Neil Tang 9/17/2008 CS440 Computer Networks

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

Major Concerns for MAC Design Throughput Fairness CS440 Computer Networks

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

Basic Architecture CS440 Computer Networks

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

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

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

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

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

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. CS440 Computer Networks

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

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. CS440 Computer Networks