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Link Layer

 

In this architecture ATM will be carried end-to-end. However, at the edge between the wired (high-speed) network and wireless links, multiple ATM cells will be combined into an HDLC-like frame. These frames comprise the Adaptive HDLC (AHDLC) protocol. The wireless data link layer is adaptive to provide an appropriate trade-off between data rate and reliability in order to support the various services. For example, we may want to drop voice packets, which are time sensitive, but retry data packets. The edge interface unit makes this decision based on knowledge of the requirements of each traffic stream, possibly based on virtual circuit number.

For some types of traffic, error correction may be achieved using retransmission. Here, delay is increased for this class of traffic to prevent cell losses. It is well known that even a few cell losses can have a significant impact on the performance of TCP/IP (Transmission Control Protocol/Internet Protocol), while TCP/IP can cope with variable delays [12]. The Adaptive High Level Data Link Control (AHDLC) protocol can change in response to traffic requirements. ATM end-to-end provides the following benefits:

  1. Moderate cut-through, e.g. an IP segment may contain 8192 bytes or about 170 cells, while one ATM HDLC-like frame will contain on the order of 3-20 cells
  2. ATM is a standard protocol.
  3. ATM can incorporate standardized Quality of Service (QoS) parameters which could be based on the virtual circuit identifier.

The link layer must also maintain cell order; this will be critical during hand-off of an RN from one ES to another. Details of the Adaptive HDLC protocol and frame structures can be found in [10] and [11] .



Steve Bush
Tue Nov 12 11:48:31 CST 1996