Chapter Nine New secondhalf

Chapter Nine New secondhalf - CSC 311 Chapter Nine Token...

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CSC 311 Chapter Nine Token Ring Lans: IEEE Standard 802.5
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CSC 311 Chapter Nine Like Ethernet, the token ring is a MAC protocol sitting between the logical link control and the physical layer. Data rates are listed at 4 Mbps and 1 Mbps but IBM token rings can run at 4, 16, or 100Mbps. Differential Manchester Encoding is used. Devices are configured in a ring. Any particular device can send only to its neighbors, in most cases only one neighbor as data flow is usually unidirectional. Unlike Ethernet, collisions do not occur in token ring networks, contention is controlled by use of a special packet called a token. Only one token circulates the ring, and a device wishing to send must have possession of the token in order to send.
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CSC 311 Chapter Nine In some token ring configurations, the failure of a single device can cause failure of the entire network. Each device, in turns checks the destination address of a data frame, If the address matches the device address, the frame is copied, a few bits changed and the frame is passed to the next station. If the address does not match, the frame is simply passed to the next station.
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CSC 311 Chapter Nine The token is a special frame. Both token and frame formats are shown below:
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CSC 311 Chapter Nine Each frame has a starting delimiter SD and an ending delimiter ED. The SD has a special signal pattern: JK0JK000 How to you send a J or K? By violating differential Manchester signal rules;
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CSC 311 Chapter Nine Sending a J or K We simply violate differential Manchester encoding rules; remember, both Manchester and differential Manchester encoding require a signal transition in the middle of the bit period: To send J: start out like you are sending a 0, ie, transition at the beginning of the bit period, but do not transition in the middle of the bit period. To send K: start out like you are sending a 1, ie, no transition at the beginning of the bit period, but do not transition in the middle. The signals, J and K, will not appear naturally in data, so there is no confusion. 1 0 J 1 K 1
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Chapter Nine Notice that the ending delimiter sequence is: JK1JK11E, the “E” bit is an error bit. Remember that the frame check sequence using CRC uses modulo 2 arithmetic to check for errors, since this is just the EXOR of the received bits and the divisor bits, AND, since the error check operation can begin as soon as a string of bits equal in length to the divisor function has been received and the next step in the operation is performed as soon as the next single bit arrives, by the time the last bit arrives, the error check is complete and it is a simple matter to set the E bit, error bit, to indicate whether or not an error has occurred. The frame status field, FS has two bits which can be set by the receiving
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This note was uploaded on 02/10/2012 for the course CSC 311 taught by Professor Whitston,h during the Spring '08 term at S. Alabama.

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Chapter Nine New secondhalf - CSC 311 Chapter Nine Token...

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