tailieunhanh - Nhiều giao thức truy cập đối với truyền thông di động P5

MODELS FOR THE PHYSICAL LAYER AND FOR USER TRAFFIC GENERATION In general, sophisticated communication systems are designed according to the concept of layering, often adhering to the OSI layering approach. The designer of a certain layer can then consider other layers as black boxes, which provide certain services defined in terms of functional relations between their respective inputs and outputs. She does not have to worry about the details of implementation of the next lower layer, but must only be aware of the services it provides | Multiple Access Protocols for Mobile Communications GPRS UMTS and Beyond Alex Brand Hamid Aghvami Copyright 2002 John Wiley Sons Ltd ISBNs 0-471-49877-7 Hardback 0-470-84622-4 Electronic 5 MODELS FOR THE PHYSICAL LAYER AND FOR USER TRAFFIC GENERATION In general sophisticated communication systems are designed according to the concept of layering often adhering to the OSI layering approach. The designer of a certain layer can then consider other layers as black boxes which provide certain services defined in terms of functional relations between their respective inputs and outputs. She does not have to worry about the details of implementation of the next lower layer but must only be aware of the services it provides. Conversely she has to be sure that the layer being designed will cater for the services required by the next higher layer. As we want to investigate the performance of multiple access protocols we are interested in the MAC sub-layer which will make use of the services provided by the physical layer. We must therefore assess the performance of the latter or indeed establish the relevant functional relations. Several options on how to model physical layer performance will be discussed and the models chosen for the performance assessment of a few multiple access protocols presented in Chapters 7 to 9 outlined in the following. Regarding the relationship between the MAC sub-layer and higher sub- layers of major concern here is the traffic coming from the latter which has to be handled by the MAC making the best possible use of the available physical link s . Where exactly in terms of layers this traffic is generated depends very much on the service considered but is not of interest here. What is relevant is only the quantity and the temporal characteristics of this traffic as seen by the MAC sub-layer and hence as delivered by the RLC sub-layer . For this purpose traffic models are defined which will then be used for the performance assessment of the MAC .