Readers chime in, Part 1

Many readers ask questions and offer their own advice and perspectives on ATM as a convergence protocol. We'd like to share some of these over the next few issues.

First, one reader asks: " How does ATM's variable bit rate (VBR) differ from statistical multiplexing? "

Statistical multiplexing, time division multiplexing (TDM) and frequency division multiplexing are the three main ways to share networks. All ATM traffic is statistically multiplexed - including constant bit rate (CBR), VBR, available bit rate (ABR), unspecified bit rate (UBR) and guaranteed frame rate (GFR). However, CBR traffic in ATM emulates a TDM connection to the end points, whereas all other ATM service classes introduce variable delay.

In TDM, the network reserves a "time slot" in the data stream for each user's data. CBR acts much the same way, because even though it is statistically multiplexed, the statistical probability of getting traffic across a CBR channel without noticeable latency, with extremely low jitter and at a constant rate is essentially the same as with a TDM channel. In ATM, CBR reserves capacity in much the same way as TDM, and any reserved but unused capacity is wasted.

In ATM, VBR (and ABR, UBR and GFR) allows multiple users to share the same transmission channel and to "oversubscribe" the transmission speed. So multiple users, using multiple addresses across the same link can share the ATM connection. Their data is prioritized much like the way a postage stamp prioritizes how soon a letter will arrive. VBR uses statistical multiplexing and follows the postal stamp model; you get the network priority based on the kind of stamp you put on the envelope.

If you have questions you'd like addressed on topics we cover, please feel free to send them our way to larry@larryhettick.com

Next time, we'll reply to a few more reader queries.

Steve Taylor is President of Distributed Networking Associates and Publisher/Editor-in-Chief of Webtorials.Com. For more detailed information on most of the topics discussed in this newsletter, connect to 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.

Larry Hettick is an independent consultant, with 19 years of experience in telecommunications and data communications marketing and product management for service providers and equipment vendors. He can be reached at larry@larryhettick.com

You can reach the authors at taylor@webtorials.com or larry@larryhettick.com.

Convergence archive
Past newsletters.

ATM Forum

Media servers will bring new services
Network World, 10/08/01