Motorola MPC8260 - Manual (Page 800)

29-20 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA
Part IV. Communications Processor Module
29.5 Available Bit Rate (ABR) Flow Control
While CBR service provides a Þxed bandwidth and is useful for real-time applications with
strictly bounded end-to-end cell transfer delay and cell-delay variation, ABR service is
intended for data applications that can adapt to time-varying bandwidth and can tolerate
signiÞcant cell transfer delay and cell delay variation. The MPC8260 implements the two
following mechanisms deÞned by the ATM Forum TM 4.0 rate-based ßow control.
¥ Explicit forward congestion indication (EFCI). The network supplies binary
indication of whether congestion occurred along the connection path. This
information is carried in the PTI Þeld of the ATM cell header (similar to that used in
frame relay). The source initially clears each ATM cellÕs EFCI bit, but as the cell
passes through the connection, any congested node can set it. The MPC8260 detects
this indication and sets the congestion indication (CI) bit in the next backwards RM
cell to signal the source end station to reduce its transmission rate.
¥ Explicit rate (ER) feedback. The network carries explicit bandwidth information, to
allow the source to adjust its rate. The source sends forward RM cells specifying its
chosen transmit rate (source ER). A congested switch along the network may
decrease ER to the exact rate it can support. The destination receives forward RM
cells and returns them to the source as backward RM cells. The MPC8260
implements source behavior by adjusting the rate according to each returning
backward RM cellÕs ER.
Explicit rate feedback has several advantages over binary feedback (EFCI). Explicit rate
feedback allows immediate source rate adaptation, eliminating rate oscillation caused by
incremental rate changes. Using the information in RM cells, the network can allocate
bandwidth evenly among active ABR channels.
29.5.1 The ABR Model
Figure 29-10 shows the MPC8260Õs ABR model.
Figure 29-10. MPC8260Õs ABR Basic Model
Source Behavior Destination Behavior
Nrm Data Cells
F-RM Cell
B-RM Cell
Update Rate
CCR,ER
ER, CI, NI
F-RM Cell
B-RM Cell
Set CI, NI
Turn-around
Reduce ER