Introduction to Data Communications
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Start Delimiter (SD)

The Start Delimiter (SD) is used to:

• Inform the receiving station that a new frame is arriving.
• Synchronize the transmit and recieve clocks

The SD is 8 bits long and consists of :

	J K 0 J K 0 0 0

Where J & K are Manchester coding violations and have no change of state during the half bit. The J code violation is a steady high state and the K code violation is a steady low state.

Access Control (AC)

The Access Control (AC) field is used to:

• Set priority of Token
• Indicate if it is a Token or Information Frame
• To stop frames from continuously circulating the ring

The AC field consists of 8 bits:

	P P P T M r r r

Where:

• P = Priority Bits
• T = Token Bit (0 - Token , 1 - Information)
• M = Monitor Bit
• R = Reservation Bits

The Priority Bits are used to indicate the priority of the Token. Each workstation is assigned a priority for their transmissions: 000 is the lowest and 111 is the highest (7 levels of priorities). The Lan administrator sets the priority levels. For a workstation to claim a Token, it must have a priority equal to or greater than the priority of the Token.

It is the responsibility of the node when finished transmitting data, to release the Token and to return the priority bits to the Reservation Bits.

The Reservation Bits are used to negotiate the priority of the next token as a transmission passes by. When a Token or Token Frame goes by, a node is allowed to reserve the priority of the next Token to be released by placing its priority in the Reservation Bits. In order to change the Reservation Bits, the node's priority must be greater than the existing Reservation bits.

The Token Bit is used to indicate whether the frame is a Token or Token Frame (information frame). T = 0 indicates a Token, T=1 indicates a Token Frame.

The Monitor Bit is used by the Active Monitor (AM) to stop frames from continuously circulating the ring. The Monitor bit is set to M=0 by the Source and when a frame passes by the AM, the monitor bit is set to M=1. If the AM receives a frame with the monitor bit set to M=1. The frame is removed from the ring, purges the ring and issues a new token.

Introduction to Data Communications
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