ATM Service Classes

Although it would be ideal to have one AAL in its entirety for all applications of ATM networking, the one AAL chosen would be a watered-down version of what is required by all applications. The restriction might be so limiting that certain applications, no matter how you tried to fit them in, would never work out. At the very least, certain features that optimize the performance of an application might interfere with the performance of another. By omitting these features, all applications work, but at least one works not as well. ATM service classes give insight into the broad, yet finite, range of traffic that ATM can support, each with slightly different needs.

The two ends of an ATM physical link can be configured with different service classes, with respect to the same VC. The problem that arises is that because the service class is not communicated within the ATM cell header, the only way switching devices, which look only at the cell header, know what kind of traffic the cell represents is if the switch's configuration for the VC, over which the cell is traveling, conforms to a certain service class (i.e., a certain connection-traffic descriptor).

For example, a PVC carrying voice traffic, which is highly sensitive to delay and jitter (delay variation), would not handle data traffic with the efficiency of a PVC that is configured for such traffic. Conversely, the switch would not know that traffic on a data PVC was actually high-priority uncompressed voice traffic that needed to be sent through the switching fabric and out onto the next link as quickly as possible. Therefore, even the concept of effective queuing depends on service-class configuration. Again, for maximum effectiveness, settings for the same VC should be identical at both ends of the physical link (although not required) and appropriate for the traffic.

The two broad categories of service class are real-time and non-real-time. The following list shows the standard ATM service classes, grouped by these categories:

• Real-time service classes

• Non-real-time service classes

— Unspecified bit rate (UBR)

Figure 10-11 shows a matrix of both traffic and QoS parameters that apply to each of the service classes discussed here. The following descriptions for each service class reference these parameters in varying degrees. In deciphering Figure 10-11, understand that "n/a" is used with traffic parameters to mean that the particular source-traffic descriptor (remember, a collection of traffic parameters) does not apply to that service class. For QoS parameters, the term unspecified indicates that that particular parameter is not negotiated for that service class. The reason that unspecified is not used for the traffic parameters is because some of the traffic parameters might be specified, but not as part of that particular source-traffic descriptor grouping. Although subtle, the difference exists.

Figure 10-11 Service Classes by Parameter

ATM Layer Service Category

Attribute

CBR

rt-VBR

nrt-VBR

UBR

ABR

GFR

End-to-End Timing

Required

Not Required

Bit Rate

Constant

Variable

Traffic Parameters4:

PCR and CDVT5

Specified

Specified 2

Specified 3

Specified

SCR, MBS, CDVT5

n/a

Specified

n/a

MCR

n/a

Specified

n/a

MCR, MBS, MFS, CDVT5

n/a

Specified

QoS Parameters4:

Peak-to-Peak CDV

Specified

Unspecified

maxCTD

Specified

Unspecified

CLR

Specified

Unspecified

See Note 1

1. CLR is low for sources that adjust cell flow in response to control information, whether a quantitative value for CLR is specified is network specific.

2. Might not be subject to CAC and UPC procedures.

3. Represents the maximum rate at which the ABR source can send. The actual rate is subject to the control information.

4. These parameters are either explicitly or implicitly specified for PVCs or SVCs.

5. CDVT refers to the Cell Delay Variation Tolerance (see Section 4.4.1). CDVT is not signaled. In general, CDVT need not have a unique value for a connection. Different values can apply at each interface along the path of a connection.

6. CLR is low for frames that are eligible for the service guarantee, whether a quantitative value for CLR is specified is network specific.

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