By Guy Trotter
Network World, 9/15/97

One of ATM's greatest advantages is its built-in ability to support user-specified quality-of- service (QoS) levels.

To deliver QoS, the ATM Forum has defined five service classes, including available bit rate (ABR) service.

ATM allows users (or applications) to obtain end-to-end connections at varying bandwidth levels, making ABR well suited for the flexible transfer of data, voice and video. Defined service classes allow users to trade off bandwidth efficiency, delay and potential cell loss to achieve an acceptable QoS level.

For example, unspecified bit rate service promises best-effort delivery but may be subject to high cell loss rates and unpredictable network management capabilities.

Constant bit rate provides minimal delay but requires that a fixed amount of bandwidth be allocated for the entire session. This is an appropriate approach for voice but inefficient for many data applications.

Variable bit rate services come in two flavors: real time and nonreal time. These services offer somewhat more flexibility with service guarantees, but users must specify in advance a range of parameters that describe their potential traffic 'profile,' such as peak and sustained cell rates or maximum burst sizes.

Unfortunately, it is difficult to strictly characterize many of today's unpredictable applications within these parameters because there may be long periods of low data transfer followed by large bursts of data.

Moreover, if actual traffic falls outside the profile - as unpredictable bursty traffic often will - cells may be discarded, resulting in the corruption of application-layer information and a sharp decrease in throughput.

What is needed, then, is a bandwidth-efficient, low-cell-loss service with the flexibility to handle large data bursts.

Enter ABR

ABR operates as a feedback mechanism that utilizes special ATM cells, called Resource Management (RM) cells.

RM cells carry requests through the network to determine if higher transmission rates can be supported. RM cells then deliver an answer to the originating user.

Here's how: The user begins by transmitting data cells at an initial rate, interspersed with RM cells. If the switch wishes to increase its transmission data rate, it indicates the request via an RM cell transmitted into the network.

The RM cell is then pro-cessed and passed progressively through each switch in the connection.

Every switch examines the RM cell and determines whether it can support the requested rate increase, taking into account a fair allocation of bandwidth among all other active ABR sources.

Each switch can choose one of three options. If the switch can support the request, the RM cell is passed along unchanged.

If the switch is experiencing congestion but can still accommodate the current user traffic rate, it modifies the RM cell to indicate that the source should not change its rate of transmission.

If the switch is experiencing congestion that is slowing the network, it can modify the RM cell to indicate that the source should reduce its user traffic rate. Or the switch can tell it to transmit at a specific data rate.

Finally, the RM cell is looped back to the source, which takes the specified action.

It increases, maintains or decreases its rate of transmission or resets its rate to the level specifically indicated by the RM cell.

As long as the user transmits at the approved rate, cell loss rates are guaranteed to be low. Additional bandwidth can be dynamically requested and ob-tained, maintaining the low-cell-loss rate.

On the other hand, ABR also helps manage overall network resources by instructing users to reduce transmission rates if needed and releasing bandwidth to the network for use by other sources.

Increased efficiency

Most ATM equipment manufacturers currently are implementing ABR support in their switches. Based on normal testing, evaluation and deployment cycles, it is reasonable to expect that network users should begin to see ABR services by mid-1998.

Given the growth of applications with varying nonreal-time transmission requirements, ABR should become a key building block for ATM network managers in the future.