The link layer in this section will comprise all protocols which support the ATM layer. The wireless data link layer will be adaptive to provide an appropriate degree of data rate versus reliability in order to properly support the various types of ATM traffic. For example, we may want to drop voice packets, which are very time sensitive, but retry data packets. The edge interface unit has some knowledge of the requirements of each traffic stream. For example this knowledge can be obtained from VC identification. The efficiency of the data link layer will be improved by using this knowledge.
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 in an HDLC-like frame. 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, while TCP/IP can cope with variable delays . The HDLC-like protocol can change in response to traffic requirements. ATM end-to-end provides the following benefits:
The link layer must also maintain cell order; this will be critical during handoff of an RN from one ES to another. This is covered further in Section 10. Figures 5 and 6 show the architecture for the link/ATM level implementation. Below the device independent ATM layer are the high speed packet radio (PR) specific layers. The RN/ES interface bridges at the cell level, not the AAL layer. No link level addressing is used since the ATM level specifies the address.
Figure 5: RN Network Protocol Stack.
Figure 6: ES-Network Protocol Stack