Olivetti’s Radio ATM LAN Virtual Connection Tree

WIRELESS ATM SWITCHES 342 w x Hardware and software implementation details are given in 12 for the second version of the WATMnet system operating at 25 Mbitrs in the 5 GHz w x ISM band. Some experimental results in 13, 14, 15 show the validation of major protocol and software aspects, including DLC, wireless control, and mobility signaling for handoffs.

12.2.2 Olivetti’s Radio ATM LAN

w x The work reported in 4 presents the design issues of a wireless ATM LAN that was developed at Cambridge-Olivetti Research Laboratories. All the base stations operate at 10 Mbitrs, using a quaternary phase-shift keying Ž . Ž QPSK radio at 2.45 GHz 10-MHz bandwidth available for short-range data . links in buildings in the United Kingdom . The network uses low-end ATM switches operating at 100 Mbitrs, and has a picocellular structure without frequency reuse. A picocell has a radius of about 10 m. The Olivetti radio ATM system employs slotted ALOHA with exponential backoff as the MAC layer protocol, which was implemented using Xilinx reprogramable gate arrays. Packets over the wireless link are one ATM cell long, and cell headers are appropriately altered to accommodate QoS, with the VPIrVCI field compressed. A CRC field of 16 bits is used for error Ž . detection, and retransmissions up to 10 are used for error correction. The next version of this testbed will have a data rate of 25 Mbitrs, operate in the w x 5-GHz band, and have a range up to 30 m 16 .

12.2.3 Virtual Connection Tree

In a paper that deals with topology issues in a cellular radio access network with ATM transport technology, some authors have proposed a structure w x called the virtual tree architecture 17 . The virtual connection tree totally decentralizes handoff operations, with each mobile responsible for managing its own handoff events without requiring intervention of the admission controller. They showed that this approach can support a very high rate of handoffs between cells. This capability in turn enables very small size cells, therefore very high capacity. The virtual connection tree consists of ATM switches and cellular base stations connected via some of the ATM switches. The switches are con- nected via some physical network topology, on top of which is a virtual tree. The root of the tree is a fixed ATM switch node, and the leaves are base stations. When a mobile terminal establishes a connection to the ATM Ž . networks, a connection tree is formed from a root an ATM switch to a set of base stations to which a mobile terminal may move. Whenever the mobile terminal moves into one of those base stations, it uses one of the preestab- lished virtual channels from the root node to the corresponding base station. The goal of a virtual connection tree is to reduce handoff latency by removing connection setup time for the new connection subpath. However, WIRELESS ATM SYSTEMS 343 this method uses preestablished multiple virtual channels, which require more bandwidth even though only one of them is used at a time.

12.2.4 BAHAMA Wireless ATM LAN