MT90500 Mitel Semiconductor, MT90500 Datasheet - Page 43

• Manufacturer Part Number: MT90500
• Description: Multi-Channel ATM AAL1 SAR
• Manufacturer: Mitel Semiconductor

n =1; and over partially-filled cells (where n is less than, or equal to, the payload size). The cell parameters are
configured through the microprocessor port. Figure 12 gives examples of the ATM AAL1 cell formats.
The MT90500 meets the ITU I.363.1 standard for SDT for 1 to 96 octets per structure (1 to 96 TDM channels
per VC). The MT90500 meets the ANSI.630 standard for SDT for 1 to 122 octets per structure (1 to 122 TDM
channels per VC).
TDM traffic over AAL0 is also supported (referred to in this document as CBR-AAL0). AAL0 is the “bare” or
“null” adaptation layer (5 bytes of header plus 48 bytes of direct user payload). Figure 13 shows CBR-AAL0
and AAL1 partially-filled cell formats. In addition, TDM traffic over AAL5 can also be transmitted (referred to in
this document as CBR-AAL5). The cell format for CBR-AAL5 is shown in Figure 14.
The AAL1 cell differs from the CBR-AAL0 cell in that it contains an AAL1 byte, and it may also contain a pointer
byte. The AAL1 byte contains a sequence count (0 to 7), a CSI bit and the SNP (Sequence Number Protection)
field. The sequence count is used to identify cell ordering, and to aid in the detection of lost cells. The CSI bit
indicates the presence (CSI = 1) or absence (CSI = 0) of a pointer byte in even-numbered cells. In odd-
numbered cells, the CSI bits serve to carry the SRTS nibble. The pointer byte indicates the start of the next
structure (since structures may be shorter or longer than 47 bytes, and therefore move through the AAL1
payload). Also worth noting is the high-order bit of the header’s PTI field, which can be set to indicate an OAM
cell (a cell whose payload contains signalling rather than TDM data).
The CBR-AAL5 cell differs from AAL1 and CBR-AAL0 in that its TDM payload is 40 bytes (where TDM data can
occupy 8, 16, 24, 32, or 40 payload bytes), and the remaining 8 bytes are devoted to AAL5 overhead. Every cell
in a CBR-AAL5 VC is an end-of-frame cell, as identified by the LSB of the PTI field in the cell header being set.
In addition, each cell contains a CPCS-UU byte and a CPI byte (both unused here), two length bytes and four
CRC bytes (all inserted by the MT90500). The MT90500 supports TDM trunking over AAL5, for n = 1, 8, 16, 24,
32, or 40. For n = 1, the MT90500 meets the cell format in the ATM Forum standard AF-VTOA-0083.000.
CSI
8
GFC / VPI
Partially-Filled AAL1 Cell
Figure 13 - Partially-Filled AAL1 and CBR-AAL0 Cell Formats
Pad Byte or Payload Byte #46
7
Sequence
VPI
VCI
< 47 Payload Bytes
Count
6
Payload Byte #3
Payload Byte #1
Payload Byte #2
Pad Byte
5
HEC
VCI
4
PTI
CRC
field
3
VPI
VCI
2
Parity
Even
CLP
1
8
GFC / VPI
Pad Byte or Payload Byte #47
Pad Byte or Payload Byte #48
7
VPI
VCI
1 - 48 Payload Bytes
6
CBR-AAL0 Cell
Payload Byte #2
Payload Byte #3
Payload Byte #1
Payload Byte #4
5
HEC
VCI
4
PTI
3
VPI
VCI
2
CLP
1
MT90500
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