The communication between basestation and each R3 is discussed as follows in terms of OSI layers.
At the physical layer, a pair of Proxim radio modems is dedicated as a wireless serial link between each R3 and its basestation. The link can be viewed logically as a standard RS-232 cable, with a data rate of 19.2 Kbps. Based on spread-spectrum technology, the radio bandwidth is multiplexed over seven radio channels. Each modem can select one of the channels to transmit or receive data, but only four radio channels can be concurrently used in the same area. To support more radio connections, each radio channel can be further divided into 64K logical sub-channels. Contention for the sub-channels is handled at the MAC (medium access control) layer by using the CSMA/CA (carrier-sense multiple access with collision avoidance) protocol. However, due to the inherent delay caused by CSMA/CA, throughput degrades as the number of active sub-channels increases.
In our configuration, we have used the maximum of four independent channels, statically assigning radio modems to sub-channels to accommodate more than four simultaneous connections. We are currently running PPP (Point-to-Point Protocol) on the wireless links to provide TCP/IP functionality. The basestation therefore can communicate with the Unix workstation on-board each R3 by using any of the facilities of the TCP/IP protocol suite. At the same time, the basestation has an Ethernet connection to the Internet, which provides high bandwidth connectivity to other Internet hosts.