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The SNP field contains the following two subfields:
• CRC-3: It is computed over the CSI and sequence count fields.
• Parity: Even parity bit used calculated over the CSI, sequence count, and CRC-3 fields.
The transmitting SAR computes the FCS for the first four bits of the header and inserts it into the CRC-3 field. The pattern used to compute the FCS is given by the polynomial: x3 + x + 1. After completing the CRC operation, the transmitting AAL calculates the even parity bit on the first seven bits of the header and inserts the result in the parity field.
The receiving SAR examines each SAR-PDU header by checking the FCS and the even parity bit. The state machine that controls the receiver’s error detection and correction scheme is the same as the header error control scheme used for the ATM header (see Section 3.2; see also Figure 3.4). At initialization, the state machine is set to the correction mode. Each time an SAR-PDU comes in, the FCS and the parity bit are checked. If no errors are found, the SN field is declared as valid and the state machine remains in the correction mode. If a single-bit error is detected, then it is corrected and the SN field is declared as valid, but the state machine switches to detection mode. If a multi-bit error is detected, then SN field is declared as invalid and the state machine switches to detection mode. In detection mode, the FCS and the parity bit are checked each time an SAR-PDU comes in; if a single-bit or a multi-bit error is detected, then the SN field is declared as
SAR Header payload
1 bit 3 bits 3 bits 1 bit
Figure 3.14 The SAR encapsulation for AAL 1.
THE ATM ADAPTATION LAYER
invalid and the state machine remains in detection mode. If no errors are detected, then the SN field is declared as valid and the state machine returns to the correction mode.
The receiving SAR sublayer conveys the sequence count, the CS indication, and the status of the SN field (i.e., valid or invalid) to the receiving CS.
This error detection and correction scheme runs in addition to the error detection and correction scheme for the ATM header. However, these two mechanisms apply to two different fields of the ATM cell. The header error control mechanism applies to the first four bytes of cell’s header, whereas the above scheme applies to the SN field.
The AAL 1 CS
The convergence sublayer performs various functions, such as handling cell delay variation, processing the sequence count, processing forward error correction, handling performance monitoring, transferring structured and unstructured data, and transferring timing information. Below, we describe each of these functions.
Handling cell delay variation
For AAL 1 to support constant bit rate applications, it has to deliver the data stream to the receiving application at the same bit rate at which it was transmitted. The ATM cells, which carry the data stream, might have to traverse multiple ATM switches before they reach their destination. In view of this, the arrival of these cells might sometimes be delayed because of network congestion. Also, the opposite might occur. That is, a group of cells might arrive closer to each other than they were originally transmitted.
To compensate for this variability in the arrival of the cells, CS writes the incoming SAR-PDUs into a buffer, from where it delivers the data stream to the receiving AAL application at a constant bit rate. (A similar method is used, for instance, when we listen to a radio station over the Internet.) In the event of buffer underflow, CS might need to insert dummy bits in order to maintain bit count integrity. Also, if there is a buffer overflow, then CS drops the appropriate number of bits.
Processing the sequence count
CS processes the sequence count values in order to detect lost or misinserted cells. Any detected misinserted cells are discarded. To maintain bit count integrity of the AAL user information, it might be necessary to compensate for lost cells by inserting dummy SAR-PDU payloads.
Forward error correction
For video, forward error correction might be performed in order to protect against bit errors. For enhanced error-protection, forward error correction can be combined with interleaving of AAL user bits.
CS can generate reports giving the status of end-to-end performance, as deduced by the AAL. The performance measures can be based on events of: lost or misinserted cells, buffer underflows and overflows, or bit errors.
Structured and unstructured data transfers
Two CS-PDU formats have been defined: the CS-PDU non-P format and the CS-PDU P format. The CS-PDU non-P format is constructed from 47 bytes of information supplied by the AAL user. The CS-PDU P format, on the other hand, is constructed from a 1-byte header and 46 bytes of information supplied by the AAL user. The 1-byte header contains an even parity bit and a 7-bit field that contains the Structured Data Transfer (SDT) pointer, which is used to point to the beginning of a block of data. The CS-PDU P format or non-P format becomes the payload of the SAR-PDU.