Metricom's Ricochet Network: alternative new technology for traffic signals

The third phase of the City's Advanced Signal System is currently being implemented. This third phase will implement a wireless communications to remote traffic controllers via an integration of Metricom's Wireless Ricochet Network with Econolite's ASC/2s traffic controllers. The City's central system will utilize this packet radio network operating under the Star Mode, a proprietary, connectionless service, via a virtual private network configuration. The modems operate in the 900 MHz range, and the network handles all routing, contention, and retransmissions. The City's Advanced Signal System will utilize a new communications protocol that is "event driven". This protocol dictates that a message is sent only when a change in status occurs, or when a request for information is received. This is very different from the typical polling based protocols used for nearly all of today's traffic signal systems. The City's system will use the State of California's AB3418E (extended) message sets for system communications. Since the network only needs addresses and timestamps for communications, any modem can be easily relocated to a new location with relatively little effort. Future enhancements to the protocol include support for NTCIP Class B messages, and transmission of controller event logs. The City will implement modems at traffic signal locations that will ultimately be connected over fiber. As each of these signals is connected with fiber, the modems will be relocated to other more remote locations in the City. This paper will discuss the basic technology, the versatility of this packet radio network for communications with traffic signals, the new communications protocols being developed, and the future enhancements to this technology.