Oversubscription and contention
 
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The subject of oversubscribed circuits - and to what extent these circuits are oversubscribed - remains a hot topic. It's a simple fact that there's little advantage to using any type of packet network - Ethernet, frame relay, ATM, SNA, or IP - unless the traffic is oversubscribed at some point.
The only example that stands out as an exception to this assertion is the ATM constant-bit-rate class of service. In this case, the advantages come from supporting this traffic in a converged voice/data network and from the enhanced control of using packet transmission.
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Oversubscription means that various services contend for bandwidth, and the use of oversubscription is a given in nearly all networks. However, the network location where the contention takes place varies widely among implementations.
Starting near the workstation, Ethernet packets contend for bandwidth on the transmission media. While Ethernet switching has lessened the contention by providing more segments with fewer workstations per segment, the connection between devices is still shared on a first-come, first-served basis.
Deeper into the access network, contention has become an often discussed - and often misrepresented - topic in the cable modem vs. DSL debate. In the case of cable modem services, the transmission facilities to the home or office are shared much like the office Ethernet (just listen to the DSL providers' digs to this effect in their TV commercials). On the other hand, with DSL, while there is a dedicated copper loop for each subscriber, multiple subscribers are concentrated in the terminating DSL Access Multiplexer (DSLAM), so there's little real difference here. (Note the recent negative press about SBC's miscalculated oversubscription of DSL resources).
As for frame relay and ATM services, again, there's oversubscription in the service providers' networks. And, as we've been discussing in previous newsletters, subscribers may choose to oversubscribe the aggregate of committed information rate across all permanent virtual circuits that terminate at a common port to gain "bursting" benefits through the network when spare capacity is available.
The question is not whether a given service or technology is oversubscribed. What matters how the oversubscription is handled and how the network is engineered to provide a given level of service.
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Steven Taylor, consultant and broadband packet evangelist, and Joanie Wexler, an independent networking technology editor and writer, team up to bring you this analysis and commentary. Taylor specializes in education and market analysis, and Wexler adds incisive reporting and research. For more detailed information on most of the topics discussed in this newsletter, connect to www.webtorials.com, the first Web site dedicated exclusively to market studies and technology tutorials in the Broadband Packet areas of Frame Relay, ATM, and IP.
Feedback and additional topic ideas are welcome. Please contact taylor@webtorials.com or joanie@jwexler.com.
Frame Relay archive
Past newsletters.
IDC: No knockout punch for cable vs. DSL
IDG News Service, 07/11/00
WatchGuard to safeguard DSL, cable modem links
Network World, 02/07/00
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"Playing the averages" with oversubscription
Network World on Frame Relay, 06/07/99
DSL oversubscription
Network World ISP Newsletter, 05/22/00
Frame Relay Forum: Technical brief on Frame Relay
VPNs and IP - VPNs
