DATA COMMUNICATION Principles The Network Performance The Network

DATA COMMUNICATION Principles : The Network Performance

The Network Performance Every bit of information sent to a communications network is a job assigned to it. The network is expected to deliver the information at another point such that it is reliable, readable and useful. Reliability prevents errors, readability makes it intelligible, and usefulness pays. The following example may help illustrate the point. If a stockbrokerage company allows its subscribers to get the latest stock market information online and buy shares live, it would need the underlying network to be: Reliable, so that the information can be trusted; Readable, so that the user can get the correct meaning from it; In time, so that it may not be too late to use the information. Reliability, readability and timeliness are three dimensions of performance. In the above example, the job assigned to the data network is to convey the stock information with certain metrics for these three dimensions. When a data packet travels from origination point through the layers of network, to the end user, it passes through many processes and obstacles. If a large number of data packets are in transit together, they may have to be stored in a storage facility, such as a queue of packets. Figure 9 -1 shows a simple depiction of such journey with a single switching node between the origination and the destination points.

It is obvious from the figure that, in order for the network to transport data packet to the destination, each link, layer and process has to deliver some performance so that the three dimensions of performance are kept within tolerable ranges. For examples, suppose a packet requires a reliability of 99%, readability of 95% and delay less than 0. 5 seconds. This requires that the accumulated packets in error should not be more than 1 in 100, the intelligibility of information contained in the packet should be 95 out of 100 and the total transmission, propagation and waiting time in queues should be less than 0. 5 seconds. Of the three, only two can be objectively measured, the reliability and the delay. Intelligibility is highly subjective and usually only the end-user can determine if there is an acceptable level of intelligibility or not. Luckily, for numerical data, intelligibility is easily related to reliability and delay. The exact definition of performance may be layer dependent still in most cases it may be regarded as the pair (r, d), where r stands for reliability and d stands for delay. Let’s use this pair to interpret the meaning of performance at various layers of the OSI-RM in the sections to follow.

REFERENCES • Ahmad A. - Data Communication Principles. For Fixed and Wireless Networks • Cornelius T. Leondes - Database and Data Communication Network Systems, Three-Volume Set_. . -Academic Press