PRIORITIES ATM networks can feature two forms of priority mechanism: space and time. Both forms relate to how an ATM buffer operates, and these are illustrated in Figure . Space priority addresses whether or not a cell is admitted into the finite waiting area of the buffer. Time priority deals with the order in which cells leave the waiting area and enter the server for onward transmission. Thus the main focus for the space priority mechanism is to distinguish different levels of cell loss performance, whereas for time priority the focus is on the delay performance | Introduction to IP and ATM Design Performance With Applications Analysis Software Second Edition. J M Pitts J A Schormans Copyright 2000 John Wiley Sons Ltd ISBNs 0-471-49187-X Hardback 0-470-84166-4 Electronic 13 Priority Control the customer comes first PRIORITIES ATM networks can feature two forms of priority mechanism space and time. Both forms relate to how an ATM buffer operates and these are illustrated in Figure . Space priority addresses whether or not a cell is admitted into the finite waiting area of the buffer. Time priority deals with the order in which cells leave the waiting area and enter the server for onward transmission. Thus the main focus for the space priority mechanism is to distinguish different levels of cell loss performance whereas for time priority the focus is on the delay performance. For both forms of priority the waiting area can be organized in different ways depending on the specific priority algorithm being implemented. The ATM standards explicitly support space priority by the provision of a cell loss priority bit in the ATM cell header. High priority is indicated by the cell loss priority bit having a value of 0 low priority with a value of 1. Different levels of time priority however are not explicitly supported in the standards. One way they can be organized is by assigning different levels of time priority to particular VPI VCI values or ranges of values. SPACE PRIORITY AND THE CELL LOSS PRIORITY BIT An ATM terminal distinguishes the level of space priority for the traffic flows it is generating by setting the value of the cell loss priority bit. Within the network if buffer overflow occurs the network elements may selectively discard cells of the lower-priority flow in order to maintain the performance objectives required of both the high- and low-priority traffic. For example a terminal producing compressed video can use high priority for the important synchronization information. This then avoids the need to operate .
