AAL-1 Supports Class A Constant Bit Rate Traffic
In support of synchronous networking, AAL-1 provides a constant rate bit-stream between the two ends of an ATM connection. The data stream is locked to a fixed timing reference. While providing steady rate transfer is a relatively simple matter over a single synchronous interface, the asynchronous nature of ATM provides a significant challenge. Variations in a network's ability to deliver ATM traffic can result in significant jitter that impedes orderly reassembly of the synchronous traffic stream. Figure below shows the format of the AAL-1
Some of the most interesting aspects of AAL-1 are involved in clock recovery. Send too much information and data must be dropped. Send too little (too slow), and information must be padded. Two general approaches to resolving these issues have been proposed, Synchronous Residual Time Stamp where the clocks on both ends of a network connection are synchronized, and Adaptive Clock, in which the receiver adjusts the speed of its clock on the basis of the amount of information available in its receive buffers.
AAL-5 Simple Data Transfer
AAL-5, originally coined the Simple and Efficient Adaptation Layer (SEAL) was designed to provide similar services at lower overhead. This AAL takes advantage of the ATM End of Message (EOM) flag to signal the end of a single message. Significant overhead is elimated by removing the SAR header and trailer. Processing involves construction of a CPCS-PDU shown in Figure below that can carry between 1 and 65535 octets, that is segmented into a series of 48 octet SAR-PDUs for ATM transmission.
Components of the AAL-5 CPCS-PDU are:
- Pad - rounds the data length to an even multiple of 4-octets.
- UU - user to user indication. Used to pass information between CPCS users.
- CPI - always zero, and provides boundary alignment
- Length - indicates the number of legitimate octets within the PDU.
- CRC (32-bits).
