
RS8234
2.0 Architecture Overview
ATM ServiceSAR Plus with xBR Traffic Management
2.5 Advanced xBR Traffic Management
N8234DSC
PRELIMINARY
2-15
2.5 Advanced xBR Traffic Management
The RS8234 implements ATM’s inherent robust traffic management capabilities
for CBR, VBR, ABR, UBR, GFR and GFC. The RS8234 manages each VCC
independently and dynamically.
The user assigns each connection a service class, a priority level, and a rate
if applicable. Then, the on-chip traffic controller, the xBR Traffic Man-
ager, optimizes usage of the line bandwidth according to the VCC’s traffic
parameters and control information stored in SAR shared memory. The
xBR Traffic Manager guarantees the compliance of each VCC to its ser-
vice contract with the ATM network at the UNI ingress point. It schedules
all data traffic by acting as a master to the segmentation coprocessor.
One of the functional components of the xBR Traffic Manager is the
xBR Scheduler. The xBR Traffic Manager assigns segmentation traffic
from active VCCs to schedule ‘slots’, which the segmentation coprocessor
then complies to by segmenting VCC traffic in the sequence/schedule dic-
tated by the xBR Scheduler.
In addition to reserved CBR bandwidth, the RS8234 provides 16 segmen-
tation priorities. The user configures these priorities for the remaining ser-
vice categories, including the TM4.0 defined ABR class.
The RS8234’s xBR Traffic Manager implements multiple functional
levels of traffic prioritizing. This is illustrated
in Figure 2-9.
The host submits data to be sent by writing entries to the transmit queues
for segmentation. The SAR processes these transmit queues either in round
robin order (transmit queue zero through thirty-one, looped back to zero),
or in priority order (with transmit queue thirty-one having highest prior-
ity). This scheme gives the user or system designer some control of the
delay between the host submitting traffic and the SAR starting to process
that traffic. For instance, the user could assign CBR traffic to the highest
priority transmit queue in order to minimize any delay in processing and
scheduling that traffic.
The RS8234 then submits this traffic demand to the xBR Traffic Manager
for scheduling. Traffic is scheduled based on the traffic class plus certain
parameters from the segmentation VCC Table entries (primarily the GCRA
I and L parameters). And if the service category is ABR, the SAR also uses
certain parameters from the ABR templates to help determine that traffic’s
placement on the Schedule Table.
The conforming traffic to be transmitted is further groomed in internal pri-
ority queues. Each virtual channel is prioritized according to its assigned
scheduling priority. CBR channels are given pre-assigned segmentation
bandwidth, and channels for the remaining service categories scheduled
according to their priority number (priority zero being the lowest priority
and priority fifteen being highest).
Figure 2-9 shows this prioritization as a
global set of queues, but it is actually maintained on a per-transmit oppor-
tunity basis. In this way, high priority traffic will be transmitted up to its
GCRA limits but will not block lower priority traffic when idle.