
MT90503
Data Sheet
57
Zarlink Semiconductor Inc.
4.3 TX_SAR Module
4.3.1 Overview
The purpose of the TX_SAR Module is to assemble TDM data bytes into ATM cells to be transmitted to the UTOPIA
Module. The TX_SAR Module has no external interfaces, and does not control any of the MT90503’s pins.
However, it is the connecting block between the TDM module and the UTOPIA Module in the data transmission
direction.
When the TX_SAR assembles ATM cells, it reads bytes from the circular buffers in data memory and uses these to
assemble the ATM cells.
In order to transmit CBR ATM cells correctly, a timing algorithm is used to ensure that the ATM cells are assembled
and transmitted at the defined data rate. This timing algorithm is implemented using the transmit event scheduler.
The transmit event scheduler is a construct in control memory that identifies the time ATM cells need to be
transmitted, and the circular buffer from which data for the ATM cell is to be retrieved. ATM cells are capable of
carrying a variable number of virtual channels. Therefore, the rate at which cells are transmitted is variable.
4.3.1.1 Support and Trunking for Different Types of ATM Cells
The type of ATM cells the TX_SAR supports includes AAL1, CBR-AAL0, and AAL5-VTOA. The transmit event
scheduler is capable of supporting the different data rates which are inherent to the different types of ATM cells.
Figure 23 - ATM Cell Formats shows the different formats for the ATM cells that the TX_SAR Module supports.
The TX_SAR is also capable of supporting hyperchannels (trunking) of up to 2048 channels per VC. In addition, it
can also support AAL0 and AAL5 trunking, providing the number of channels is divisible by the payload bytes.
Figure 23 - ATM Cell Formats
GFC
VPI
VPI
VCI
VCI
VCI
PTI
CL
HEC
CSI SEQ CRC P
P
P-Byte
Payload Byte #0
Payload Byte #1
Payload Byte #44
Payload Byte #45
GFC
VPI
VPI
VCI
VCI
VCI
PTI
CL
HEC
CSI SEQ CRC P
Payload Byte #0
Payload Byte #1
Payload Byte #2
Payload Byte #45
Payload Byte #46
GFC
VPI
VPI
VCI
VCI
VCI
PTI
CL
HEC
Payload Byte #2
Payload Byte #3
Payload Byte #46
Payload Byte #47
Payload Byte #0
Payload Byte #1
GFC
VPI
VPI
VCI
VCI
VCI
PTI
CL
HEC
Payload Byte #2
Payload Byte #3
Payload Byte #0
Payload Byte #1
CRC32 [31:24]
CRC32 [23:16]
CRC32 [15:8]
CRC32 [7:0]
Length [15:8]
Length [7:0]
"00000000"
"00000000"
Payload Byte #39
AAL1 with Pointer Cell
46 Payload Bytes
AAL5 Cell
40 Payload Bytes
AAL0 Cell
48 Payload Bytes
AAL1 without Pointer Cell
47 Payload Bytes