Introduction to Data Communications
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36. IEEE-802.3 Protocol

The IEEE-802.3 Protocol is based on the Xerox Network Standard (XNS) called Ethernet. The IEEE-802.3 Protocol is commonly called Ethernet but it is just 1 version. There are 4 versions or flavours of the Ethernet frame:

Ethernet_802.2Frame type used on Netware 3.12 & 4.01
Ethernet_802.3Frame type used on Netware 3.x & 2.x (raw)
Ethernet_IIFrame type used on DEC, TCP/IP
Ethernet_SNAPFrame type used on Appletalk (SubNet Access Protocol)

NOTE: The Source and Destination must have the same Ethernet Frame type in order to communicate.

36a. CSMA/CD (Carrier Sense Multiple Access/ Collision Detect)

Bus arbitration is performed on all versions of Ethernet using the CSMA/CD (Carrier Sense Multiple Access/ Collision Detect) protocol. Bus arbitration is another way of saying how to control who is allowed to talk on the (medium) and when. Put simply, it is used to determine who's turn it is to talk.

In CSMA/CD, all stations, on the same segment of cable, listen for the carrier signal. If they hear the carrier, then they know that someone else it talking on the wire. If they don't hear carrier then they know that they can talk. This is called the Carrier Sense portion of CSMA/CD.

All stations share the same segment of cable and can talk on it similar to a party line. This is the Multiple Access portion of CSMA/CD.

If 2 stations should attempt to talk at the same time, a collision is detected and both stations back off for a random amount of time and then try again. This is the Collision Detect portion of CSMA/CD.

Introduction to Data Communications
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