Книга: Distributed operating systems

2.2.4. The ATM Adaptation Layer

2.2.4. The ATM Adaptation Layer

At 155 Mbps, a cell can arrive every 3 ?sec. Few, if any, current CPUs can handle in excess of 300,000 interrupts/sec. Thus a mechanism is needed to allow a computer to send a packet and to have the ATM hardware break it into cells, transmit the cells, and then have them reassembled at the other end, generating one interrupt per packet, not per cell. This disassembly/reassembly is the job of the adaptation layer. It is expected that most host adaptor boards will run the adaptation layer on the board and give one interrupt per incoming packet, not one per incoming cell.

Unfortunately, here too, the standards writers did not get it quite right. Originally adaptation layers were defined for four classes of traffic:

1. Constant bit rate traffic (for audio and video).

2. Variable bit rate traffic but with bounded delay.

3. Connection-oriented data traffic.

4. Connectionless data traffic.

Quickly it was discovered that classes 3 and 4 were essentially the same, so they were merged into a new class, 3/4. At that point the computer industry woke up from a short nap and noticed that none of the adaptation layers were suitable for data traffic, so they drafted AAL 5, for computer-to-computer traffic (Suzuki, 1994). Its nickname, SEAL (Simple and Efficient Adaptation Layer), hints at what its designers thought of the other three AAL layers. (In all fairness, it should be pointed out that getting people from two industries with very different traditions, telephony and computers, to agree to a standard at all was a nontrivial achievement.)

Let us focus on SEAL, due to its simplicity. It uses only one bit in the ATM header, one of the bits in the Payload type field. This bit is normally 0, but is set to 1 in the last cell of a packet. The last cell contains a trailer in the final 8 bytes. In most cases there will be some padding (with zeros) between the end of the packet and the start of the trailer. With SEAL, the destination just assembles incoming cells for each virtual circuit until it finds one with the end-of-packet bit set. Then it extracts and processes the trailer.

The trailer has four fields. The first two are each 1 byte long and are not used. Then comes a 2-byte field giving the packet length, and a 4-byte checksum over the packet, padding, and trailer.

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