Data Communications Networking Devices - 4th Ed.pdf

764 ___________________________________________________________ SPECIALIZED DEVICESOne of the most promising features of data compression is its ability to provide amechanism to alter the structure of a communications network. How such a changemight occur can be seen from a brief examination of a typical network application.Assume an existing multidrop line services eight terminal locations. Perhaps anexpansion to add two additional terminal locations to the network is planned.Assume that the network planner estimated the activity on the multidrop line basedupon its expansion to ten drops and determined that due to the additional datatraf®c the response time of the terminals would be unacceptable. As a result of thisanalysis, a separate line consisting of two drops might be installed. If datacompression is implemented there would be less total traf®c, reducing the time ittakes a computer to poll and service each terminal on a multidrop line. Thus, byimplementing data compression one might be able to service the two additionallocations by extending the multidrop line without increasing the response time ofthe terminals.Using compression performing devicesOften, you can obtain the bene®ts of data compression while avoiding the effortsrequired to analyze actual or potential data traf®c and develop software to performcompression. This can be accomplished by leasing or purchasing data compressionperforming devices that are speci®cally designed to be used in a particular networkingenvironment. We will thus examine several hardware products to obtain abetter understanding of the use of compression performing devices. The productscovered in this section were selected for illustrative purposes only and should not beconstrued as an endorsement of the device.Compression DSUsCodex Corporation, a subsidiary of Motorola Corporation, markets a compressionperformingData Service Unit DSU) designed to enhance the transmissioncapability of 56 kbps digital lines. The Codex Compression DSU provides avariable compression ratio which depends upon the susceptibility of data to thecompression algorithms built into the DSU. Under normal operations users canreasonably expect to achieve at least an average compression ratio of 2:1, resultingin an average throughput of 112 kbps over a 56 kbps transmission facility.Figure 7.23 illustrates the use of a pair of compression-performing CSUs to boostrouter-to-router transmission over a 56 kbps digital transmission facility. Note thatthe serial ports of the routers are con®gured to transfer data to the DSU at 256 kbps,and the DSU will use ¯ow control to temporarily terminate transmission from therouters when its buffers ®ll to the point where additional router transmission couldresult in the over¯ow of a buffer and loss of data. Once the DSU's buffer is emptiedto a prede®ned level of occupancy, it will then disable ¯ow control, allowing theattached router to resume transmission.Through the use of compression-performing DSUs you can avoid the expenseassociated with obtaining a fractional T1 line instead of a 56 kbps line. With a retail