Summary

ATM WANs are a connection-oriented service, meaning that a communications path must be provisioned between endpoints prior to the exchange of traffic across the circuit. Interfaces to these ATM WANs are categorized as any one of the following five types: UNI, NNI, PNNI, DXI, and B-ICI.

ATM WAN connections are classified by their CoS, defined in Table 9-1.

Table 9-1. ATM CoS Description

QoS	Description and Application Use
CBR	Provides constant bandwidth across the virtual connection. Used for private line emulated services, such as multimedia, CAD/CAM, or medical imaging.
VBRrt	Provides a specified average bandwidth across the virtual connection. Used for transporting delay-sensitive applications, such as voice or video.
VBRnrt	Provides a specified average bandwidth across the virtual connection. Used for transporting non-delay-sensitive applications, such as data information.
UBR	Best-effort service, meaning no bandwidth is specified across the connection or delivery of service guaranteed.
ABR	Best-effort service implemented in a different fashion than UBR. This service provides continuing feedback indicating how much bandwidth is available for use. By throttling back as necessary, sending hosts avoid network congestion, preventing traffic from being sent, but thrown away before it reaches its intended destination. Used to support data applications where delivery is important, but not necessarily in a near-real-time environment.

Table 9-2 demonstrates user applications with the most appropriate ATM CoS to support communications for these applications.

Table 9-2. Application Areas for ATM Service Categories

Application Area		CBR	VBRrt	VBRnrt	ABR	UBR
Critical Data		• •	•	• • •	•	- - -
LAN Interconnection/LAN Emulation		•	•	• •	• • •	• •
Data transport/Internetworking (IP/Frame Relay)		•	•	• •	• • •	• •
Circuit Emulation (PBX)		• • •	• •	- - -	- - -	- - -
POTS/ISDN Video Conference		• • •	•	•	- - -	- - -
Compressed Audio		•	• • •	• •	• •	•
Video Distribution		• • •	• •	•	- - -	- - -
Interactive Multimedia		• • •	• • •	• •	• •	•
Legend:
• • • = Optimum	• • = Good	• = Fair		- - - = Not Suitable

ATM CoS are defined by one (or more) of the following parameters, dependant on the CoS implemented (for example, CBR, VBRrt, etc.):

• PCR Maximum bit rate that can be transmitted

• SCR Upper limit for the average cell rate that can be transmitted

• MBS/Burst Tolerance (BT) Maximum number of cells the source can transmit at the Peak Cell Rate (PCR)

• MCR Minimum cell rate guaranteed by the ATM network

• CDVT Tolerance in CDV (referred to as the PCR)

ATM WAN Architecture uses two traffic control parameters and mechanisms for congestion control: CAC and UPC. CAC is the process the ATM network uses during the VP/VC setup to determine whether a connection can be made. UPC, or policing, is the ATM network process monitoring and controlling the traffic at the UNI.