ATM is a network mechanism intended to accommodate real-time traffic as well as bulk data transfer. We present ATM here as a LAN layer, for which it is still sometimes used, but it was originally proposed as a replacement for the IP layer as well, and, to an extent, the Transport layer. These broader plans were not greeted with universal enthusiasm within the IETF. When used as a LAN layer, IP packets are transmitted over ATM as in 3.5.1 ATM Segmentation and Reassembly.
A distinctive feature of ATM is its small packet size. ATM has its roots in the telephone industry, and was therefore particularly intended to support voice. A significant source of delay in voice traffic is the packet fill time: at DS0 speeds (64 kbps), voice data accumulates at 8 bytes/ms. If we are sending 1 kB packets, this means voice is delayed by about 1/8 second, meaning in turn that when one person stops speaking, the earliest they can hear the other’s response is 1/4 second later. Slightly smaller levels of voice delay can introduce an annoying echo. Smaller packets reduce the fill time and thus the delay: when voice is sent over IP (VoIP), one common method is to send 160 bytes every 20 ms.
ATM took this small-packet strategy even further: packets have 48 bytes of data, plus 5 bytes of header. Such small packets are often called cells. To manage such a small header, virtual-circuit routing is a necessity. IP packets of such small size would likely consume more than 50% of the bandwidth on headers, if the LAN header were included.
Aside from reduced voice fill-time, other benefits to small cells are reduced store-and-forward delay and minimal queuing delay, at least for high-priority traffic. Prioritizing traffic and giving precedence to high-priority traffic is standard, but high-priority traffic is never allowed to interrupt transmission already begun of a low-priority packet. If you have a high-priority voice cell, and someone else has a 1500-byte packet just started, your cell has to wait about 30 cell times, because 1500 bytes is about 30 cells. However, if their low-priority traffic is instead made up of 30 cells, you have only to wait for their first cell to finish; the delay is 1/30 as much.
ATM also made the decision to require fixed-size cells. The penalty for one partially used cell among many is small. Having a fixed cell size simplifies hardware design, and, in theory, allows it easier to design for parallelism.
Unfortunately, the designers of ATM also chose to mandate no cell reordering. This means cells can use a smaller sequence-number field, but also makes parallel switches much harder to build. A typical parallel switch design might involve distributing incoming cells among any of several input queues; the queues would then handle the VCI lookups in parallel and forward the cells to the appropriate output queues. With such an architecture, avoiding reordering is difficult. It is not clear to what extent the no-reordering decision was related to the later decline of ATM in the marketplace.
ATM cells have 48 bytes of data and a 5-byte header. The header contains up to 28 bits of VCI information, three “type” bits, one cell-loss priority, or CLP, bit, and an 8-bit checksum over the header only. The VCI is divided into 8-12 bits of Virtual Path Identifier and 16 bits of Virtual Channel Identifier, the latter supposedly for customer use to separate out multiple connections between two endpoints. Forwarding is by full switching only, and there is no mechanism for physical (LAN) broadcast.
3.5.1 ATM Segmentation and Reassembly
Due to the small packet size, ATM defines its own mechanisms for segmentation and reassembly of larger packets. Thus, individual ATM links in an IP network are quite practical. These mechanisms are called ATM Adaptation Layers, and there are four of them: AALs 1, 2, 3/4 and 5 (AAL 3 and AAL 4 were once separate layers, which merged). AALs 1 and 2 are used only for voice-type traffic; we will not consider them further.
The ATM segmentation-and-reassembly mechanism defined here is intended to apply only to large data; no cells are ever further subdivided. Furthermore, segmentation is always applied at the point where the data enters the network; reassembly is done at exit from the ATM path. IPv4 fragmentation, on the other hand, applies conceptually to IP packets, and may be performed by routers within the network.
For AAL 3/4, we first define a high-level “wrapper” for an IP packet, called the CS-PDU (Convergence Sublayer - Protocol Data Unit). This prefixes 32 bits on the front and another 32 bits (plus padding) on the rear. We then chop this into as many 44-byte chunks as are needed; each chunk goes into a 48-byte ATM payload, along with the following 32 bits worth of additional header/trailer:
- 2-bit type field:
- 10: begin new CS-PDU
- 00: continue CS-PDU
- 01: end of CS-PDU
- 11: single-segment CS-PDU
- 4-bit sequence number, 0-15, good for catching up to 15 dropped cells
- 10-bit MessageID field
- CRC-10 checksum.
We now have a total of 9 bytes of header for 44 bytes of data; this is more than 20% overhead. This did not sit well with the IP-over-ATM community (such as it was), and so AAL 5 was developed.
AAL 5 moved the checksum to the CS-PDU and increased it to 32 bits from 10 bits. The MID field was discarded, as no one used it, anyway (if you wanted to send several different types of messages, you simply created several virtual circuits). A bit from the ATM header was taken over and used to indicate:
- 1: start of new CS-PDU
- 0: continuation of an existing CS-PDU
The CS-PDU is now chopped into 48-byte chunks, which are then used as the entire body of each ATM cell. With 5 bytes of header for 48 bytes of data, overhead is down to 10%. Errors are detected by the CS-PDU CRC-32. This also detects lost cells (impossible with a per-cell CRC!), as we no longer have any cell sequence number.
For both AAL3/4 and AAL5, reassembly is simply a matter of stringing together consecutive cells in order of arrival, starting a new CS-PDU whenever the appropriate bits indicate this. For AAL3/4 the receiver has to strip off the 4-byte AAL3/4 headers; for AAL5 the receiver has to verify the CRC-32 checksum once all cells are received. Different cells from different virtual circuits can be jumbled together on the ATM “backbone”, but on any one virtual circuit the cells from one higher-level packet must be sent one right after the other.
A typical IP packet divides into about 20 cells. For AAL 3/4, this means a total of 200 bits devoted to CRC codes, versus only 32 bits for AAL 5. It might seem that AAL 3/4 would be more reliable because of this, but, paradoxically, it was not! The reason for this is that errors are rare, and so we typically have one or at most two per CS-PDU. Suppose we have only a single error, ie a single cluster of corrupted bits small enough that it is likely confined to a single cell. In AAL 3/4 the CRC-10 checksum will fail to detect that error (that is, the checksum of the corrupted packet will by chance happen to equal the checksum of the original packet) with probability 1/210. The AAL 5 CRC-32 checksum, however, will fail to detect the error with probability 1/232. Even if there are enough errors that two cells are corrupted, the two CRC-10s together will fail to detect the error with probability 1/220; the CRC-32 is better. AAL 3/4 is more reliable only when we have errors in at least four cells, at which point we might do better to switch to an error-correcting code.
Moral: one checksum over the entire message is often better than multiple shorter checksums over parts of the message.