Traffic Management for the Available Bit Rate (ABR) Service in ...

ATM data users found that their packets were being dropped indiscriminately by the
network. The reason was that due to fragmentation, even a single cell loss resulted
in a packet loss.
So, there was a need for a service which provides control over cell loss. ABR was
initially designed to meet this need through the use of feedback control. Basically,
the network explicitly distributes the \left over" capacity among the active ABR
sources. During the development of the ABR service, it was realized that feedback
control could also be used to provide high throughput, low delay and fairness among
contending sources. The tradeo was performance versus complexity. There was
the debate between credit-based framework and the rate-based framework, and the
latter was standardized because it mandated lesser required complexity. The standard
requires the network interface card (NIC) manufacturers to implement a set of source
and destination end system rules. The switches minimally need to give some kind of
feedback. They can set EFCI bits on data cells and/or process RM (control) cells sent
by the sources once every Nrm cell times to give feedback. Target ABR applications
are le transfer, WWW, email, variable quality video and voice.
The UBR service is \unspeci ed" in the sense that the only standard support
required from switches is the capability to accept a UBR connection request from the
source. By default, there is no resource to be reserved and the connection admission
control (CAC) procedure is very simple. Another implication of the service being un-
speci ed is that nothing is guaranteed. In particular, if network gets congested, UBR
cells may be dropped. The network switches may provide an enhanced UBR service
by using techniques like intelligent drop policies, bu er allocation and scheduling [36].
Network monitoring tra c, email and news are examples of the UBR applications.
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