Novel’s encyclopedia of Networking (by Kevin Shafer) defines ATM as ‘A connection-oriented, packet-switched networking architecture based on broadband ISDN technology’. To understand ATM, we have to know the terms of ‘cell’, ‘connection-oriented’ and ‘packet switching’.

ATM is a packet technology that directs traffic using a label contained in the packet header. Unlike other packet technologies, ATM uses short, fixed-length packets called cells. Each cell is 53 bytes long: 48 bytes for the information field and 5 bytes for the preceding header. The header field contains information about the virtual channel and virtual path in use, payload type and cell loss priority.

The basic idea of ATM is to segment data in small cells and then transfer them by the use of cell-switching. Such cells have a uniform layout and a fixed size of 53 bytes, which greatly simplifies switching. Being more complex, packet-switching is not nearly fast enough to be of use for isochronous data (i.e. real-time video and sound). Cell-switching gives maximum utilization of the physical resources.

The basic principles of this technology were first formulated by AT&T and the French Telecompany in the first half of the 80's. Researchers found that if data was segmented in small, fixed size cells, switching could be done with simple, specially designed ICs. With sufficient intelligence to handle routing information in each cell, such ICs could be used to build very fast switch-matrixes. By connecting several switch-matrixes, highly efficient networks with small and predictable transmission delays can be built. The fact that ATM can be used efficiently in both WANs and LANs shows how powerful ATM is.

When two participants wants to talk to each other, they first have to set up a connection between them. This connection remains open during the whole session, and when the session is over the connection is closed. To the user it appears that the connection is permanent once established, but it's actually not. Cells are switched through the network so quickly that it appears as if there is a direct connection.

To be able to transmit a message to a host by ATM, a connection needs first to be made. ATM is thus connection-oriented.

An ATM cell has 5 bytes of header (administrative information) and 48 bytes of payload (data), counting a total of 53 bytes. The choice of the cell size caused much controversy during CCITT standardization. The US computer branch wanted 64 bytes on payload, because they were considering the bandwidth utilization for data networks and efficient memory transfer (length of payload should be a power of 2 or at least a multiple of 4). 64 bytes fit both requirements.

Representants from the European and Japanese telephony branch was taking voice applications into consideration and wanted smaller cells. At cell sizes greater than 151, there is a talker echo problem. Cell sizes between 32 and 152 result in listener echo. Cell sizes less than or equal to 32 overcome both problems, under ideal conditions. Europeans telecommunication companies had no desire to invest money in echo cancelling equipment, and thus went for 32 bytes of payload.

However, because of big distances the US telecommunication companies had already installed echo cancellers across the country, and didn't consider this to be a problem. After a lot of discussion 48 bytes of payload was agreed on. As far as the header goes, 10% of payload was perceived as an upper bound on the acceptable overhead, so 5 bytes was chosen. Although the header of 5 bytes is much for a 53-byte cell, the flexibility is worth the cost.

The header of 5 bytes contains 24 bits of VCI label, 8 bits of control field, and 8 bits of checksum.

GFC    Generic Flow Control.

VPI      Virtual Path Identifier.

VCI      Virtual Channel Identifier.

PT        Payload Type.

             Indicates what the cell contains. Also used for flow control.

CLP      Cell Loss Priority.

             Indicates whether the cell should have high or low priority in the case of overload.

HEC     Header Error Correction.

For error detection and correction. Only 1 bit errors can be corrected.

The 48 bytes of payload may optionally contain a 4-byte ATM adoption layer and 44 bytes of actual data, or all 48 bytes may be data. This depends on a bit in the control field of the header. This enables fragmentation and reassemble of cells into larger packets at the source and destination respectively. The control field may also contain a bit to specify whether there is a flow control cell or an ordinary cell, an advisory bit to indicate whether a cell is droppable in the face of congestion in the network or not.

It is interesting to note that there is no sync character in the ATM cell. In order to stay in sync with the data stream, the receiving host calculates the checksum for incoming cells on the fly. If connection has been lost, the host starts computing the checksum for everything received from the wire. When the checksum has been computed correctly for 16 subsequent cells it is reasonable to assume that the host is in sync.

reference - http://www.atmforum.com