RS8234
2.0 Architecture Overview
ATM ServiceSAR Plus with xBR Traffic Management
2.5 Advanced xBR Traffic Management
N8234DSC
PRELIMINARY
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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
scheduling allows VBR traffic to be statistically multiplexed onto the ATM line,
resulting 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 PCR and CDVT. VBR2 controls PCR and CDVT, as
well as Sustained Cell Rate (SCR) and Burst Tolerance (BT). VBRC (also called
VBR3) controls PCR and CDVT on all cells, but controls SCR 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
independently.
The RS8234 employs an internal feedback control loop mechanism to enable
the TM4.0 ATM Source specification. The SAR utilizes the dynamic rate
adjustment 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 channel.
The RS8234 also responds to an incoming ABR cell stream as an ABR
Destination. 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
Turnaround 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
templates. The templates control such behaviors as the size of the additive rate
increase factors or multiplicative rate decrease steps. Each VCC’s rate varies
across the template independently.