PM7375-SC datasheet(69/430 Pages) PMC

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PM7375 LASAR-155

DATA SHEET

PMC-931127

ISSUE 6

LOCAL ATM SAR & PHYSICAL LAYER

PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE

empty. In this way, the maximum burst is defined by the selectable bucket capacity.

When the bucket is empty, transmission continues at the SCR or the rate of token

generation.

9.7.3 CBRC Support

VBR VCs are subject to both PCR and SCR traffic shaping as described above.

However in addition to PCR and SCR shaping, for VCs that are subjected to CBRC,

the TATS maintains an aggregate credit count to allow the network to back pressure

transmission.

A user programmable number of credits are issues by the network using the second

most significant bit of the GFC field in the cell flow from the network to the

LASAR-155 device. After each cell transmission for a CBRC VC, the TATS

decrements the credit count. Cell transmission for all CBRC VCs continues until the

credit supply is exhausted at which point the TATS suspends transmission of all

VCs subjected to CBRC until the network issues more credits.

9.8

Transmit ATM and Adaptation Layer Cell Processor

The Transmit ATM and Adaptation Layer Cell Processor (TALP) performs ATM layer

and AAL (Type 5) layer processing. Please refer to the Operations Section for the

ATM header fields and the AAL-5 protocol data unit structures.

9.8.1 AAL Layer Processing

The TALP block performs packet segmentation with the help of the TATS and PCID

blocks. As described above, the TATS block schedules when cells from a

CPAAL5_PDU under segmentation should be sent. The PCID block actually

performs the segmentation and retrieves the packet's bytes from the host packet

memory. The TALP block aids in segmentation by calculating the CPAAL5_PDU's

CRC-32 field and adding the CPAAL5_PDU CRC-32 field to form the entire

CPAAL5_PDU. The CRC-32 polynomial used is:

x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1.

9.8.2 ATM Layer Processing

ATM Layer processing includes generating the GFC, VPI, VCI, PTI, CLP fields and

optionally generating the CRC-10 field for each cell transmitted. In addition, the

TALP provides the ability to multiplex cells in from the Multipurpose Port and to

enforce an aggregate peak cell rate (APCR).

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