12.2 Role of ATM

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Ensuring the predictable delivery of increasingly diverse applications and services running over packet-based networks has become the key challenge faced by companies seeking to consolidate networks, put more applications on the network, or adopt an electronic model of doing business. The ability to set QoS parameters is an inherent feature of ATM (see Table 12.1) and offers the most effective means of transporting multiple traffic types through the network without impacting the overall performance of individual applications.

Table 12.1: QoS Mechanisms Included with ATM QoS Mechanism Description Applications

QoS Mechanism	Description	Applications
CBR	Provides always-available bandwidth at a CBR.	Suited for real-time applications such as circuit emulation for PBX-to-PBX trunks, scheduled multicasts, distance learning, and videoconferencing. Also used for very large bit-rate applications such as streaming audio or video and the transfer of medical images, CAD/CAM files, and archival storage.
VBR	Provides bandwidth that can fluctuate according to the applications being used.	Suited for compressed voice (VBR-real time), or “bursty” LAN connectivity (VBR-nonreal time).
ABR	Provides bandwidth to the applications as it becomes available.	Suited for routine applications that are not time-sensitive such as file transfers and e-mail, which can wait for access bandwidth to become free.
UBR	Provides bandwidth to the applications on a besteffort basis.	Suited for nonessential traffic such as e-mail “messagedelivered” or “message-read” acknowledgments and Web lookups.

For example, LAN and video traffic can share the same physical link into the ATM network (see Figure 12.1). The aggregate bandwidth of a T3 link, for example, may be partitioned between the two applications, with each type of traffic assigned its own QoS. The video stream requires a CBR and is assigned to a highpriority (QoS 1) partition of the switching fabric, whereas “bursty” LAN traffic is admitted to the network at a VBR and is assigned to a low-priority (QoS 2) partition of the switching fabric. This scheme is referred to as QoS reservation. The QoS reservation can be set in the following ways:

Figure 12.1: In this depiction of QoS reservation, bursty VBR traffic does not cause cell delay for CBR traffic because it is assigned a low-priority (QoS 2) partition of the switching fabric. In this scheme, time-sensitive video traffic always has the right of way.

• When set to 0%, there is no distinction made between connections. The system sends ATM cells through the switching fabric on an equal opportunity, round-robin basis.

• When set to 25%, the available switch bandwidth is divided so that 25% of it is reserved for QoS 1 traffic and 75% reserved for QoS 2 traffic.

• When set to 50%, the available switch bandwidth is divided so that 50% of it is reserved for QoS 1 traffic and 50% reserved for QoS 2 traffic.

The service provider configures the QoS parameters and connection priorities for each type of traffic the customer has. The service provider may even supply and manage an IAD for each customer location in order to consolidate the different traffic types over virtual circuits (VCs) that are provisioned over multiple T1 access links. These links can be bonded together to support applications at higher speeds, which is an economical alternative to T3 access lines.

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