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2.0 Architecture Overview
2.5 Advanced xBR Traffic Management
RS8234
Datasheet for RS8234 xBR ServiceSAR
N8234DS1B
2.5.2 VBR Traffic
The RS8234 takes advantage of the asynchronous nature of ATM by reserving
bandwidth for VBR channels at average cell transmission rates without pre-
assigning “hardcoded” schedule slots, as with CBR traffic. This dynamic sched-
uling allows VBR traffic to be statistically multiplexed onto the ATM line, result-
ing in better utilization of the shared bandwidth resources. The xBR Scheduler
supports multiple priority levels for VBR traffic. Through the combination of
VBR parameters and priorities, it is possible to support real-time VBR services.
The outgoing cell stream for each VBR VCC is scheduled according to the
GCRA algorithm. The GCRA I and L parameters control the per-VCC Peak Cell
Rate (PCR) and Cell Delay Variation Tolerance (CDVT) of the outgoing cell
stream on any channel. This guarantees compliance to policing algorithms
applied at the network ingress point. The user can control the granularity of rate
by dictating the number of schedule slots in the schedule table.
Channels can be rate shaped as VCs or VPs according to one of three VBR
definitions. VBR1 controls Peak Cell Rate (PCR) and Cell Delay Variation Toler-
ance (CDVT). VBR2 controls PCR and CDVT, as well as Sustained Cell Rate
(SCR) and Burst Tolerance (BT). VBRC controls PCR and CDVT on all cells but
controls Sustained Cell Rate on only CLP=0 (i.e., high priority) cells.
2.5.3 ABR Traffic
The RS8234 implements the ATM Forum ABR flow control algorithms. The
RS8234 acts as both a fully compliant ABR Source and Destination, as defined in
the TM4.0 specification. The ABR service category effectively allows low cell
loss transmission through the ATM network by regulating transmission based
upon network feedback. The ABR algorithms regulate the rate of each VCC inde-
pendently.
The RS8234 employs an internal feedback control loop mechanism to enable
the TM4.0 ATM Source specification. The SAR utilizes the dynamic rate adjust-
ment capability of the xBR Scheduler as the ATM Source’s variable traffic rate
shaper. The RS8234 injects an in-rate stream of Forward RM cells for each ABR
VCC. When these cells return to the RS8234’s receive port as Backward RM cells
after a round trip through the network, the RS8234 processes these cells and uses
the data returned as feedback to dynamically adjust the rates on each ABR chan-
nel.
The RS8234 also responds to an incoming ABR cell stream as an ABR Desti-
nation. The reassembly coprocessor processes received Forward RM cells. It
“turns around” this incoming information to the segmentation coprocessor, which
formats Backward RM cells containing this information and inserts these Turn-
around RM cells into the transmit cell stream.
The exact performance of the rate-shaper is governed by one or more ABR
Templates in SAR shared memory. Each VCC is assigned to one of these tem-
plates. The templates control such behaviors as the size of the additive rate
increase factors or multiplicative rate decrease steps. Each VCCs rate varies
across the template independently.