ATM Adaptation Layer type 1

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AAL1

AAL type 1 (AAL1) was defined for a class A service. AAL1 CS and SAR Sublayer functions include segmenting and reassembling user data (e.g., an E-1 or E-3 bit stream); handling variations in cell delay; handling lost (or mis-inserted cells) and bit errors; recovering clock information; and, when possible, correcting bit errors.

The AAL1 SAR-PDU contains a 1-octet header followed by 47 octets of user data. The SAR-PDU header contains two 4-bit fields called the sequence number (SN) and sequence number protection (SNP).

The SN field, which comprises two subfields, detects lost or mis-inserted cells and might carry control information. The Convergence Sublayer Indicator (CSI) is a single bit that carries service-specific CS information. The CSI bit is used differently in odd and even cells. In even-numbered cells, the CSI bit indicates whether a pointer specifies the offset into the current payload of the first octet in an Nx64 kbps Payload. If the pointer is present, it occupies the second octet of the Payload (thus, the cells only carry 46 octets of user data). In odd-numbered cells, the CSI bit carries a single bit of a 4-bit counter to ensure synchronization between sender and receiver clocks. The CS computes the clock drift, or residual time stamp (RTS), once every eight cells. The 4-bit value is placed into the next four outgoing “odd” cells. The remaining three bits form the Sequence Count, a modulo 8 sequence number incremented between cells.

Also composed of two subfields, the SNP field provides error detection information. The first three bits form the CRC Control, a three-bit cyclic redundancy check remainder that detects bit errors in the SN field. The CRC Control is followed by a single parity bit that provides an even parity check for the entire eight-bit SAR-PDU header.

Although no physical circuit is actually dedicated, AAL1 provides circuit emulation service. Therefore, the necessity for the receiver to recover clock information from the transmitter is of particular importance to AAL1 applications. AAL1 takes advantage of the fact that ATM will operate over digital facilities using inherently reliable network clocks, such as E-1, E-3, and SDH. The AAL1 RTS clock recovery scheme does not send explicit timing information; instead, it sends a measure of the frequency discrepancy between the network’s reference clock and the transmitter’s clock. If the clocks have a very high degree of accuracy, only a few bits of information are needed to convey this information.

AAL1 Applications

The principal application for the AAL1 protocol is continuous content—either streaming or framed. An example of the former is streaming video from a video or other analog source. An example of the latter is a data feed having an internal framing structure such as in DS-1 and DS-3 services.

Listed below are applications of AAL1 transport.

  • Circuit emulation services (CES) supporting either structured or unstructured (channelized or unchannelized) DS-1 or DS-3 service
  • Transporting video or voice signals from an analog-to-digital converter (could include signaling and other telephony functions)
  • Live data streams, typically unframed, from telemetry or weather observation equipment