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# lec19 - ECE 333 Introduction to Communication Networks Fall...

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1 ECE 333: Introduction to Communication Networks Fall 2002 Lecture 19: Medium Access Control VII More on token ring networks LAN bridges and switches. 2 More on token rings In the last lecture we began discussing token ring networks, in particular the IEEE 802.5 token ring standard. In such networks, a token is passed from node to node. Each node can only transmit when it posses the token. In the description of 802.5 last time we specified that a node will release the token only after it has finishing sending a packet and the first part of the packet has propagated all the way around the ring. In this case the maximum throughput of the token ring at high loads (i.e. every node backlogged) is given by: 1 1 1 / 1 a if a if a throughput max where packet of on time transmissi ime rotation t a Here the rotation time refers to the time it takes a signal to propagate around the ring, where this includes the delay incurred at each station. The above suggests that, as with CSMA/CD, if we increase the data rate, and want to still have good performance, then either the length of the ring must be decreased or the minimum packet size must increase.

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3 For example consider a token ring where the rotation time is 1msec. If an average frame contains 2000 bytes (16000 bits), then at 4 Mbps we have 25 . ) 10 4 /( ) 16000 ( 10 1 6 3 a Notice in this case at any time the ring contains at most 3 3 6 10 4 10 1 10 4 bits or 1/4 of a frame. However at 100Mbps, we have 25 . 6 ) 10 100 /( ) 16000 ( 10 1 6 3 a This results in a total throughput at high loads of approximately (100 Mbps)/(6.25) = 16 Mbps. In this case the ring could contain up to 6.25 frames at one time. But because of the protocol, it can only contain 1. This is because each node waits to release the token until the start of the frame has returned. A simple solution to this is to allow nodes to release the token immediately after sending a packet - this is called early token release . (Note after a node releases the token, it must still remove the message it
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lec19 - ECE 333 Introduction to Communication Networks Fall...

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