Frame Relay is a data-only service, right? Wrong, according to Dowty Group Plc’s Dowty Information Systems, which has been revealing the details of its FPX 2000, a fast packet switch that will start shipping late next year. Fast packet, of course, is a fairly nebulous term – Dowty uses it, not to refer, as is usual, to Asynchronous Transfer Mode technology with fixed packet lengths, but as a generic term that encompasses Frame Relay, with its variable packet lengths. So when it says fast packet in this context, Dowty really means Frame Relay. That piece of obfuscation aside, the company’s new piece of kit will have two methods of transporting speech traffic, depending on the quality of the service required. For good quality speech, able to meet the Network Code of Practice requirements, Dowty splits its circuits up using time division multiplexed techniques and then reserves slots for the isochronous traffic. The voice call travels across the link in circuit-switched mode, while the rest of the link’s capacity is made available for Frame Relay traffic, which can include speech. The company recommends packetising speech only where Code of Practice restrictions do not apply – that is, when both ends of the conversation terminate within a private network, and even then care needs to be taken to avoid some of the more obvious problems entailed by such an endeavour. One of those is what happens if two people are carrying out an animated conversation over the packetised link when, at the same time, someone begins a large file transfer and starts dropping 4Kb data packets onto the network? The company’s answer is a number of tweaks and work arounds which are designed to avoid the problems of network congestion. First, voice packet size is kept very small, a mere 48 bytes; secondly, it is possible to set up the switch so that speech packets get priority over data; and, third, it is also possible to limit the length of the data frames that the local net can place on the network. Both the circuit-switched and the packetised voice channels terminate in a 2Mbps standard PABX interface, according to David Loe, Dowty’s integrated network product marketing manager. Just how well the technology will work in practice, though, remains to be seen. While the switch has been tested in the lab, it won’t enter the first user test site until early next year. There are also questions as to its interoperability with other vendors’ equipment. Loe says that since the FPX 2000 will use standard Frame Relay for inter-switch traffic there should be no problems – but the Frame Relay forum has not so far held substantive discussions about speech.
