MC92501 Datasheet, PDF (19/52 Pages) Motorola, Inc – ATM Cell Processor

SECTION 6. CLP TRANSPARENCY
6.1. Overview
The trafÞc management speciÞcation deÞnes two network
operation models with relation to CLP = 1 ßow: CLP
transparent and CLP signiÞcant. A connection which is CLP
transparent does not have different Cell Loss Ratio (CLR) for
CLP = 0 or CLP = 1 trafÞc, and therefore does not prefer
discarding CLP = 1 over CLP = 0 cells on congestion. Current
switch fabrics do distinguish globally between CLP = 0 trafÞc
and CLP = 1 trafÞc with the CLP = 1 trafÞc being more
susceptible to discarding in case of congestion.
The MC92501 solves the problem in the following manner:
Â¥ If a cell belongs to a connection which supports CLP
transparency (the ICTEÑIngress CLP Transparency
Enable bit in the Common Parameters Extension Word is
set), then the MC92501 moves the CLP from the cellÕs
header to IOCLPÑIngress Overhead CLP bit in the cellÕs
overhead and assigns 0 to the header CLP.
Â¥ The cell is forwarded to the switch fabric. The switch fabric
considers the cell as if it has CLP = 0.
Â¥ On the egress side, the MC92501 reconstructs the header
CLP from the EOCLPÑEgress Overhead CLP bit in the
cellÕs overhead.
In order to support other applications, the MC92501 function
is extended as follows. It assigns the ICTVÑIngress CLP
Transparency Value bit (deÞned in the Common Parameters
Extension Word) to the cellÕs header instead of 0. If the ICTVÑ
Ingress CLP Transparency Value bit = 0 then we are back at
the CLP transparency application. CLP Transparency is
enabled globally by the IPCVÑIngress Features Enable bit in
the ACR register.
ICTVÑIngress CLP Transparency Value
NOTE 1
Overhead
Header
Payload
NOTE 2
Overhead
Header
Payload
Switch Fabric
MC92501 #1
MC92501 #2
NOTES:
1. If a cell belongs to a connection which supports CLP transparency (the ICTEÑIngress CLP Transparency Enable bit
is set), then MC92501 moves the CLP from the cellÕs header to the IOCLPÑIngress Overhead CLP bit in the cellÕs
overhead and assigns the ICTVÑIngress CLP Transparency Value bit to the header CLP.
2. If the CIFSÑConnection Ingress Flow Status bit is set then the MC92501 reconstructs the header CLP from the
EOCLPÑEgress Overhead CLP bit in the cellÕs overhead.
Figure 16. CLP Transparency with a CLP Significant Switch Fabric
The IOCLPÑIngress Overhead CLP bit location is
programmable using the OCBLÑIOCLP Bit Location Þeld in
the Ingress Switch Interface ConÞguration Register (ISWCR).
The EOCLPÑEgress Overhead bit location is
programmable using the EOBYÑEOCLP Byte Location Þeld
and the EOBIÑEOCLP Bit Location Þeld in the Egress Switch
Overhead Information Register 1 (ESOIR1).
MOTOROLA
MC92501
19