28236-DSH-001-B_15 Datasheet, PDF (53/443 Pages) M/A-COM Technology Solutions, Inc. – ATM ServiceSAR Plus with xBR Traffic Management

CN8236
ATM ServiceSAR Plus with xBR Traffic Management
2.0 Architecture Overview
2.5 Advanced xBR Traffic Management
2.5.3 ABR Traffic
The CN8236 implements the ATM Forum ABR flow control algorithms. The
CN8236 acts as a fully compliant ABR Source and Destination, as defined in the
TM 4.1 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 CN8236 employs an internal feedback control loop mechanism to enable
the TM 4.1 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 CN8236 injects an in-rate stream of Forward RM cells for each
ABR VCC. When these cells return to the CN8236’s receive port as Backward
RM cells after a round trip through the network, the CN8236 processes these cells
and uses the data returned as feedback to dynamically adjust the rates on each
ABR channel.
The CN8236 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.
2.5.4 UBR Traffic
The UBR service category is intended for nonreal-time applications that do not
require tightly constrained delay and delay variation, such as traditional computer
communications applications like file transfer and e-mail.
Those VCCs which have not been assigned to one of the other service
categories covered previously are scheduled as UBR traffic. All UBR channels
within a priority are scheduled on a round-robin basis. To limit the bandwidth that
a UBR priority consumes, the system designer should use a CBR tunnel in that
priority level.
2.5.5 GFR Traffic
Guaranteed Frame Rate is a new service category defined by the ATM Forum to
provide a MCR QoS guarantee for AAL5 CPCS-PDUs not exceeding a specified
frame length. A GFR service connection is treated as UBR with a guaranteed
MCR.
The CN8236 implements GFR by scheduling/shaping the connections using
both the VBR1 scheduling procedure (for the MCR rate value) and a UBR
priority queue, thereby providing fair sharing for all GFR connections to excess
bandwidth.
28236-DSH-001-B
Mindspeed Technologies™
2-19