ited bandwidth of the telephone system. As well as defining a specific PHY layer, ISDN also specifies Data Link and Network layer operation. LAP-D (Link Access Protocol D-channel) is a Data Link protocol, defined in ITU-T Q.920/921, that ensures error free transmission on the PHY layer. Two Network layer protocols are defined in ITU-T Q.930 and ITU-T Q.931 to establish, maintain and terminate user-to-user, circuit-switched, and packet-switched network connections. FireWire FireWire, also known as IEEE 1394 or i.Link, was developed by Apple Computer Inc. in the mid-1990s as a local area networking technology. At that time it provided a 100 Mbps data rate, well above the Universal Serial Bus (USB) speed of 12 Mbps, and it was soon taken up Table 2-7: 2B1Q Line Code Used in ISDN Input "DIBIT" Output "QUAT" Line voltage +.833 -.833 Wireless Network Logical Architecture Computer A Digital video camera Firewire Bridge Digital video recorder Firewire splitter Printer 1 Firewire repeater Printer 2 Computer B Figure 2-10: FireWire Network Topology: Daisy-chain and Tree Structures by a number of companies for applications such as connecting storage and optical drives. Fire Wire is now supported by many electronics and computer companies, often under the IEEE 1394 banner, because of its ability to reliably and inexpensively transmit digital video data at high speeds, over single cable lengths of up to 4.5 metres. The standard data rate is 400 Mbps, although a faster version is also available delivering 800 Mbps and with plans for 3.2 Gbps. Range can be extended up to 72 metres using signal repeaters in a 16-link daisy chain, and Fire Wire to fibre transceivers are also available that replace the copper cable by optical fibre and can extend range to 40 km. A generic Fire Wire topology is shown in Figure 2-10. The FireWire standard defines a serial input/output port and bus, a 4 or 6 wire dual-shielded copper cable that can carry both data and power, and the related Data Link, Network and Transport layer protocols. Fire Wire 