Modifications to the TG1 MAC for Use in TG4 Systems
Subir Varma, Aperto Networks · 2001
• The Wireless Transport Layer which is responsible for the actual transfer of bits from one end of the wireless link to the other. Topics such as encapsulation of higher layer packets, ARQ, Request-Grant mechanisms in the upstream time base management and frame level control fall within its boundaries. The Wireless Link Management Protocol, which is responsible for overall control of the wireless link itself. Topics such as system initialization, encryption, flow management, registration, security etc. fall within its boundaries. The current TG1 MAC protocol is quite adequate for TG3 in the area of the Link Management Protocol, but lacks several features that are essential for a wireless transport operating in the lower frequency bands. The sub-11 GHz bands have the following characteristics that are not found in the higher bands: The wireless link is subject to larger delay spreads and thus a higher degree of multipath related fading, as opposed to the higher bands that are affected more due to longer term fading caused by environmental factors. This has obvious implications on the PHY layer, but even the MAC layer needs to be more agile, and have the ability to recover quickly from transient channel error conditions. There is a greater degree of co-channel interference due to the fact that the wireless signals propagate farther in the lower bands. The actions taken by the wireless transport layer have a profound effect on the amount of interference that is caused by a transmitter, as well as techniques that the system can use to recover from interference. The system is oriented more towards residential and SOHO markets, so that each wireless channel has to support a large number of users, each of whom may be generating sporadic traffic, with low long term average bit rate. Web Surfing would be dominant application running on these systems, so the protocol should be able to handle the traffic generated by TCP applications efficiently.