To provide the new B-ISDN services, use of a technology called asynchronous transfer mode (ATM) is specified by ITU. ATM is a connection-oriented technique based on the use of fixed size packets termed cells. These cells are 53 octets in size, with 5 octets used for the cell header, and the remaining 48 octets for data

Figure: An ATM cell

The term asynchronous transfer mode needs some explanation. The words transfer mode say that this technology is specific way of transmitting and switching through the network. The term asynchronous refers to the fact that the packets are transmitted using asynchronous techniques, and the two end-points need not have synchronised clocks. Also, the use and allocation of cells and their subsequent multiplexing and transmission through the network is determined in an asynchronous fashion, e.g. on demand, and is independent of the user. ATM will support both circuit switched and packet switched (sometimes referred to as circuit mode and packet mode, respectively) services.

Figure: The principle of asynchronous transfer mode

There is much to know about ATM, however, here we will consider only the transmission aspects, by looking at the Physical layer and the ATM layer. There are several types of cells to consider, some are passed all the way through to the

ATM layer, while others never proceed beyond the Physical layer:

· Assigned Cell. (ATM Layer) A cell which is used to provide the service to the

higher layers. This is the cell type that is used in transporting the higher level

PDU.

· Unassigned cell. (ATM Layer) An ATM layer cell which is not an assigned cell.

· Valid cell. (Physical layer) Any cell that does not contain header errors, either

through successful transmission or after being corrected by the physical layer.

· Invalid cell. (Physical layer) A cell with an error in its header that can not be

corrected. It will be discarded by the physical layer.

· Idle cell. (Physical layer) A cell that is inserted/extracted by the physical layer for the purposes of payload capacity adaptation.

As explained earlier, there are two interfaces to the B-ISDN to consider -- the user-tonetwork interface (UNI) and the network-to-network interface (NNI). For these two interfaces the internal structure of the ATM cell header is slightly different, as shown in Figure

The various parts of the cell header are:

· Generic flow control (GFC). (4 bits) Default value 0000. The exact use of this

field is, as yet, not fully defined. This is used only in assigned or unassigned (i.e.

ATM layer cells).

· Virtual path identifier (VPI). (8 or 12 bits) This is used for physical layer

routing, together with the;

· Virtual circuit identifier (VCI). (16 bits) This is used in conjunction with the

VPI field to provide physical level routing. Many cells with different VCIs may

have the same VPI. Some VCI/VPI values are pre-assigned for special purposes

· Payload type (PT). (3 bits) Some PT values -- PT identifiers (PTIs) -- are preassigned for special purposes.

· Cell loss priority (CLP). (1 bit) Flag stating whether or not this cell can be

dropped in the prescence of network congestion or other network difficulties. A

value of 0 means that this cell has high priority and should not be discarded.

· Header error control (HEC). (8 bits) This is used by the physical layer for

detecting and correcting errors in the cell header.