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مشاهدة النسخة كاملة : What is ABR?



A7med Baraka
04-15-2009, 08:52 PM
The ATM Forum Traffic Management (TM) subworking group is working on
the definition of a new ATM service type called ABR which stands for
Available Bit Rate. Using ABR traffic is not characterized using
peak cell rate, burst tolerance, et.al., and bandwidth reseverations are not
made. Instead traffic is allowed into the network throttled by a flow
control type mechanism. The idea is to provide fair sharing of network
bandwidth resources.

Currently in the ATM Forum there are two approaches being discussed (fought)
to implement ABR. One is a "Credit Based" scheme which keeps count of the
occupied buffers for each VC at each downstream hop(s). At each hop, the
upstream switch (or terminal) stops transmitting on an ABR
circuit if its downstream buffer occupancy reaches a per-VC limit.
At the downstream end, the switch (or terminal) sends back a "credit"
after forwarding some number of cells on each circuit.

Unlike the hop-by-hop Credit scheme, the "Rate Based" approach is an
end-to-end congestion avoidance mechanism. An end system would be allowed
to send cells into the network up until a "congestion indicated" cell is
received from the network. This congestion cell could be generated by
any component along the route which the VC follows. The end system would
respond by stopping its cell flow and then do some kind of V-J slow-start
similar to that done with TCP today.

The consumer hope is that the ATM Forum specify a SINGLE approach and not
fail by specifying both approaches as "options" for the market to sort out
(and end up with something like the VHS and BETA video tape format battle).
Then again, there are always folks like IBM who will move their own standards
forward, like the 25mhz ATM interface format. So maby there is a limit to
what an industry form can do.