Alternatively, the adaptor board can use SONET (Synchronous Optical NETwork) in the physical layer, putting its cells into the payload portion of SONET frames. The virtue of this approach is compatibility with the internal transmission system of AT&T and other carriers that use SONET. In Europe, a system called SDH (Synchronous Digital Hierarchy) that is closely patterned after SONET is available in some countries.
In SONET, the basic unit (analogous to a 193-bit T1 frame) is a 9×90 array of bytes called a frame. Of these 810 bytes, 36 bytes are overhead, leaving 774 bytes of payload. One frame is transmitted every 125 μsec, to match the telephone system's standard sampling rate of 8000 samples/sec, so the gross data rate (including overhead) is 51.840 Mbps and the net data rate (excluding overhead) is 49.536 Mbps.
These parameters were chosen after five years of tortuous negotiation between U.S., European, Japanese, and other telephone companies in order to handle the U.S. T3 data stream (44.736 Mbps) and the standards used by other countries. The computer industry did not play a significant role here (a 9×90 array with 36 bytes of overhead is not something a computer scientist is likely to propose).
The basic 51.840-Mbps channel is called OC-1. It is possible to send a group of n OC-1 frames as a group, which is designated OC-n when it is used for n independent OC-1 channels and OC-n c (for concatenated) when used for a single high-speed channel. Standards have been established for OC-3, OC-12, OC-48, and OC-192. The most important of these for ATM are OC-3c, at 155.520 Mbps and OC-12c, at 622.080 Mbps, because computers can probably produce data at these rates in the near future. For long-haul transmission within the telephone system, OC-12 and OC-48 are the most widely used at present.
OC-3c SONET adaptors for computers are now available to allow a computer to output SONET frames directly. OC-12c is expected shortly. Since even OC-1 is overkill for a telephone, it is unlikely that many audio telephones will ever speak ATM or SONET directly (ISDN will be used instead), but for videophones ATM and SONET are ideal.
2.2.3. The ATM Layer
When ATM was being developed, two factions developed within the standards committee. The Europeans wanted 32-byte cells because these had a small enough delay that echo suppressors would not be needed in most European countries. The Americans, who already had echo suppressors, wanted 64-byte cells due to their greater efficiency for data traffic.
The end result was a 48-byte cell, which no one really liked. It is too big for voice and too small for data. To make it even worse, a 5-byte header was added, giving a 53-byte cell containing a 48-byte data field. Note that a 53-byte cell is not a good match for a 774-byte SONET payload, so ATM cells will span SONET frames. Two separate levels of synchronization are thus needed: one to detect the start of a SONET frame, and one to detect the start of the first full ATM cell within the SONET payload. However, a standard for packing ATM cells into SONET frames exists, and the entire layer can be done in hardware.
The layout of a cell header from a computer to the first ATM switch is shown in Fig. 2-5. Unfortunately, the layout of a cell header between two ATM switches is different, with the GFC field being replaced by four more bits for the VPI field. In the view of many, this is unfortunate, since it introduces an unnecessary distinction between computer-to-switch and switch-to-switch cells and hence adaptor hardware. Both kinds of cells have 48-byte payloads directly following the header.
Fig. 2-5. User-to-network cell header layout.
The GFC may some day be used for flow control, if an agreement on how to do it can be achieved. The VPI and VCI fields together identify which path and virtual circuit a cell belongs to. Routing tables along the way use this information for routing. These fields are modified at each hop along the path. The purpose of the VPI field is to group together a collection of virtual circuits for the same destination and make it possible for a carrier to reroute all of them without having to examine the VCI field.
The Payload type field distinguishes data cells from control cells, and further identifies several kinds of control cells. The CLP field can be used to mark some cells as less important than others, so if congestion occurs, the least important ones will be the ones dropped. Finally, there is a 1-byte checksum over the header (but not the data).
2.2.4. The ATM Adaptation Layer
At 155 Mbps, a cell can arrive every 3 μsec. 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.
2.2.5. ATM Switching
ATM networks are built up of copper or optical cables and switches. Figure 2-6(a) illustrates a network with four switches. Cells originating at any of the eight computers attached to the system can be switched to any of the other computers by traversing one or more switches. Each of these switches has four ports, each used for both input and output.
The inside of a generic switch is illustrated in Fig. 2-6(b). It has input lines and output lines and a parallel switching fabric that connects them. Because a cell has to be switched in 3 (μsec (at OC-3), and as many cells as there are input lines can arrive at once, parallel switching is essential.