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

framework. The complete framework has been treated in the chapter introducing
ATM tra c management 2.
4.3 Binary Feedback Schemes
4.3.1 Key Techniques
Binary feedback schemes essentially use a single bit feedback. The initial binary
feedback algorithm used a \negative polarity of feedback" in the sense that RM cells
are sent only to decrease the source rate, and no RM cells are required to increase
the rate. A \positive polarity," on the other hand, would require sending RM cells
for increase but not on decrease. If RM cells are sent for both increase and decrease,
the algorithm would be called \bipolar."
The problem with negative polarity is that if the RM cells are lost due to heavy
congestion in the reverse path, the sources will keep increasing their load on the
forward path and eventually overload it.
This problem was xed in the next version by using positive polarity. The sources
set EFCI on every cell except the nth cell. The destination will send an \increase" RM
cell to source if they receive any cells with the EFCI o . The sources keep decreasing
theirrateuntil they receive a positive feedback. Since the sources decrease their rate
proportional to the current rate, this scheme was called \proportional rate control
algorithm (PRCA)."
PRCA was found to have a fairness problem. Given the same level of congestion
at all switches, the VCs traveling more hops have a higher probabilityofhaving EFCI
set than those traveling smaller number of hops. If p is the probability ofEFCI being
set on one hop, then the probability of it being set for an n-hop VC is1; (1 ; p) n or
57