Chapter 7

Cross-Layer Adaptation for Multimedia Services in 802.11-Type Embedded Communications Systems 1

7.1. Introduction

At present, communicating systems architecture is based mainly on the TCP/IP model. This model is characterized by its modular structure implementing several distinct and totally isolated layers that communicate through well-defined interfaces. To provide a particular service, the lower layer uses the encapsulation to put the data in the upper layer in the data field. Thus, in a vertical view, each layer augments the useful data with headers containing service information specific to each layer in the TCP/IP stack. Conversely, from a horizontal view and on the level of remote/communicating systems, the layers in the same level coordinate themselves and communicate with the help of the service fields (headers) in order to implement advanced congestion control services (receipt and loss reports), control errors (Checksum and CRC), etc.

Throughout the evolution of the TCP/IP model, several mechanisms and protocols have been added to the various layers in order to implement the major requirements in terms of quality of service (QoS), security, mobility, etc. However, several mechanisms have been duplicated uselessly on several layers, or in contrast located in their entirety on some protocol layers. A typical case is the error control mechanism, which is done simultaneously at the “data link” layer, but also at the “transport” layer in order to overcome the same ...

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