Introduction to ATM QoS
 
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When we began our series on quality of service, we alluded to the differences between QoS and class of service, or CoS. We'd like to reintroduce these terms and begin a more complete discussion of how ATM natively supports each and why ATM is so well suited as a convergence technology.
Our readers will recall that while QoS and CoS measure service levels, QoS is measured by engineering specifications, while CoS is a relative measure. The ATM Forum defines QoS in terms of the following attributes of the end-to-end ATM connection:
* Cell loss ratio
* Cell transfer delay
* Cell delay variation
ATM protocols were originally based on the ITU Broadband ISDN Protocol Model, which laid out two broad service categories: the connection-oriented vs. the connectionless service, and the constant bit rate (CBR) vs. the variable bit rate (VBR) service. The model also looked at how various applications affect the network. These were eventually developed into numbered ATM adaptation layers (AAL). For example:
* AAL 1 is for CBR traffic, like traditional voice and video, for which there needs to be a source-destination timing relationship maintained.
* AAL 2 is for VBR traffic, like packet voice and video, which still needs to maintain a source-destination timing relationship.
* AAL 3/4 is for VBR traffic. It is primarily for data, and is optimized for connectionless traffic. (This AAL was originally two AAL types encompassing connection-oriented and connectionless, which have been combined.) AAL 3/4 is the basis for Switched Multimegabit Data Service (SMDS). With the demise of SMDS services, interest in this AAL has severely declined.
* AAL 5 is for VBR data traffic, and is optimized for connection-oriented traffic. In many ways, AAL 5 is similar to frame relay, and ATM-frame relay interworking is based on AAL 5.
Each of these AALs are designed to serve as transport for another application or service and require differentiated and specific measurable QoS. Next time, we'll look at some other attributes that will serve as a baseline for our ongoing discussion about ATM and QoS.
RELATED LINKS
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.
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