Sunday, December 19, 2010

Ethernet Tutorial 2

  1.  Ethernet Frames Format
  2. Ethernet Frames
    Field DIX Frame IEEE 802.3 Frame
    Preamble    8 bytes    7 bytes
    Start Frame Delimiter       1 bytes
    Destination Address    6 bytes    6 bytes
    Source Address   6 bytes    6 bytes
    Length      2 bytes
    Type    2 bytes   
    Data  1500 bytes     Yes
    CRC/FCS  4 bytes    4 bytes
    Total  1518 bytes    1518bytes
    Note: 100/1000Mbs Ethernet Systems do not need preamble or start frame delimiter fields.
  3.  MAC address
  4. Each NIC has a unique, 48-bit MAC address; The first 24 bits identify the manufacture while the 2nd half is assigned by the manufacture.
  5. Ethernet Access 
  6. Before Gigabit Ethernet, CSMA/CD is designed to allow fair access on shared media: Listen to avoid collision before transmitting. If collision occurs, retransmission is necessary. In 1997 IEEE 802.3x introduces full-duplex Ethernet, MAC control protocol and PAUSE.
  7. Ethernet Equipment
    • Hubs/Repeaters  Hubs/Repeaters are the central devices in a star topology  and enhance the incoming signals to avoid degradation before transmitting them out. Hubs/Repeaters also support SNMP.
    • Bridges Bridges transfer MAC layer packets from one network to another; and help prevent collisions by creating separate collision domain. To control the traffic, bridges need to build MAC tables and prevent loops using Spanning Tree Protocol
    • Switches Switches link multiple networks compared to Bridges. Two categories of switches are commonly in use today: Cut through  and Store-and-forward. Cut through provide lower latency while store-and-forward provides more intelligent processing of MAC layer packets.
    • Routers Routers filter out unnecessary traffic by looking up IP address instead of MAC address in bridges/switches. Routers works like firewall to some extent and provide security functionality.


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