ATM defines the set of traffic classes and traffic parameters to characterize incoming traffic. The traffic classes include Constant Bit Rate CBR, Variable Bit Rate VBR, Available Bit Rate ABR and
Unspecified Bit Rate UBR. The traffic parameters include Peak Cell Rate PCR, Sustained Cell Rate SCR, Minimum Cell Rate MCR, and Maximum Burst Size MBS. These traffic classes and
parameters are used to define a traffic contract between the ATM network and the user. Through monitoring these parameters, ATM network enforces traffic contracts. These concepts are described in
ATM Traffic Management specifications. We discuss these concepts in more detail in Chapter 8.
We next look at the traffic classes offered by ATM to handle different bandwidth demands.
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by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC
ISBN: 0849331390 Pub Date: 110198
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III. Constant Bit Rate
The Constant Bit Rate CBR service class is used for delay-sensitive, real-time traffic such as voice and video. It is also used for carrying traditional synchronous transport such as T1 traffic over ATM
networks. This is called Circuit Emulation Service CES. The AAL1 ATM adaptation layer handles the circuit emulation function.
It is necessary to use the CBR service class to guarantee certain delay, delay variance and jitter limits associated with the synchronous traffic. Only one parameter, PCR, is used to describe and monitor user
traffic for the CBR class. If the user traffic exceeds the PCR limit, then the user cells are marked for possible discard by setting the CLP parameter in the ATM cell header to 1. This behavior is illustrated in
Figure 6-2.
The CBR service class allocates bandwidth according to the peak rate of incoming traffic. The user is guaranteed full access to the allocated peak bandwidth throughout the established connection and is not
influenced by the bandwidth usage patterns of other users. Hence, it is possible to carry both synchronous traffic in the circuit emulation mode and very bursty LAN traffic on the same physical
medium. This is one of the most desirable characteristics of ATM.
Figure 6-2 Monitoring of CBR traffic: cells above the PCR line are marked for possible discard.
IV. Variable Bit Rate
The variable Bit Rate VBR service class primarily is used for handling bursty with varying bandwidth usage over time. For VBR, two subclasses are defined: Real Time VBR rt-VBR and Non-Real-Time
VBR nrt-VBR. The rt-VBR is used if the traffic source is operating in real-time and it is delay sensitive such as compressed voice and MPEG video traffic. For example, a voice application with
silence suppression and voice compression capabilities can use rt-VBR instead of CBR to reduce its bandwidth usage while still maintaining the delay requirement. If the traffic source is not a real-time
application, then the nrt-VBR service class is used. The difference is that the real-time traffic is less tolerant to delay than the non-real-time traffic. For example, for video conferencing rt-VBR is more
appropriate. On the other hand, nrt-VBR service is more suitable when the application is not delay bound. The nrt-VBR class traffic can tolerate delay through buffering. The nrt-VBR can be used for data
transfer which is not delay sensitive but requires low CLR.
The VBR service class requires several traffic parameters to describe the incoming traffic. These parameters include PCR, SBR, MBS. In addition, rt-VBR is delay bound with the max CTD parameter.
The bandwidth characteristics of VBR class are shown in Figure 6-3. As long as the VBR traffic stays within the given bandwidth limits bandwidth profile, it is not subjected to cell drop by the ATM
network, i.e., the assigned CLR level is maintained by the network.
Figure 6-3 Monitoring of VBR: cells above the PCR line or exceeding MBS limit above SCR line are
marked for possible discard.
V. Unspecified Bit Rate
The Unspecified Bit Rate UBR service category was primarily defined for handling LAN traffic to take advantage of the remaining available bandwidth. UBR does not guarantee any level of bandwidth. It
allows a source to use max remaining available bandwidth. Therefore, during a low traffic period, a LAN source can take advantage of the full line speed. However, during very high bandwidth usage, it is
possible that the UBR source will not receive any bandwidth at all. For UBR traffic class, ATM network does not guarantee any of the QoS parameters CLR, CTD. The UBR service is, in a sense, considered
as the Best Effort attempt by the ATM network. As one can realize, this behavior is very similar to the Ethernet LAN environment. In fact, UBR was defined to match this type of traffic pattern. In addition to
LAN traffic, UBR can also be used to take advantage of the available background bandwidth for applications such as transferring news and weather pictures, file transfer and email. ATM does not
consider congestion control for the UBR traffic. It is assumed that it is done by a higher layer on a end- to-end basis.
