Frame Relay Makes Its Voice Heard By Jeff Newman   Originally developed to accommodate the bursty traffic generated by LANs interconnected over WANs, frame relay eventually caught the eye of those seeking cost-effective consolidation of their disparate networks, such as legacy Systems Network Architecture (SNA) networks. Frame relay's low overhead, low cost and versatile bandwidth structure quickly turned that eye into the center of a hurricane that has swept the mainstream WAN market. And, whenever there's an explosive migration toward any specific technology, we all have a tendency to ride that wave until it breaks, watching it lea p and bound to meet the problems created by the growing demands. Frame relay has evolved from a solution for piping legacy data across the WAN to transmitting modern LAN protocols, to handling more delay-sensitive protocols, to the current challenge of delivering voice and fax across WANs to branch offices. If you've been searching for an integrated service offering that will let you consolidate data, voice and fax services into one neat package while eliminating the charges normally paid to carriers for traditional voice services between remote offices, frame relay may be just the answer. Freeze Frame Now here comes the but that withers the optimism created by all new technology promises. Although frame relay is popular for its flexible bandwidth, its popularity has created traffic jams in the public network. Carriers have had difficulty keeping up with the demands for new service, and many have been slow to develop or even follow technology advancements. In this respect, frame relay is no different than ISDN, high-speed modem service or ATM switched virtual circuit (SVC) services: Customers armed with low-cost equipment drive the carriers to new services. Given nothing more than the demand for higher bandwidth as additional organizations subsc ribe, it is a relatively simple solution for the public carriers to add switches and capacity to the backbone to accommodate the growing demand. But now that the bar has been raised by customers requesting lower latency services from their carriers--so that voice and fax can be carried--many carriers are struggling to increase switch density and get the latest switch software in place so that they can meet the demand. The quality of voice transmission is actually determined more by perception than by measurable quantity. The carriers know that voice transmissions can tolerate no more than 350 millisecond to 400 millisecond delays round-trip. This introduces an interesting turn of events as carriers and customer premises equipment (CPE) vendors at tempt to address this problem simultaneously. Luckily, occasional dropped packets aren't an issue, since the ear can tolerate small glitches without losing the continuity of the transmission, and the retransmission of dropped packets containing voice information often is self-defeating. CPE vendors seem to have been the ones tiptoeing around most of the sleeping carrier giants by realizing a growing interest in voice over frame relay, and by following up with fast-paced research and the development of voice-capable frame relay access devices (FRADs). There are tons of FRADs on the market. Not so willing to hinge their success on the advancement of the carriers, many vendors offer FRADS with advanced techniques to overcome delay problems. These techniques include: · Voice compression. ITU G.729 (also known as CS-ACELP) is an international standard that compresses the standard 64 K pulse-code modulation (PCM) traffic as used in typical voice transmission to as low as 8 Kbps. ITU G.728 (also known a s LD-CELP) is an international standard that compresses to 16 Kbps. Many vendors offer proprietary algorithms that drop the rate down as far as 4 Kbps. · Echo cancellation. ITU G.165 defines a method of avoiding echo, which occurs when delays cause voice traf fic to be reflected back to the transmission point. · Traffic management. Advanced protocol prioritization algorithms can place voice packets at a higher priority than data packets and can tune the size of the data packets to reduce delays for more intense voice traffic. · Continuity algorithms. These are designed to intelligently fill the void of missing or erroneous voice packets.

Making Your Server System Scale by Jay Milne Updated March 7, 1997