ATM Technology

Asynchronous transfer mode or ATM technology has been developed for carrying broadband services. The technology has been standardized for broadband ISDN Jeffery, 1994; De Prycker, 1991; Cuthbert and Sapanel, 1993. The primary rate of transmission supported is 155 Mbps, in the short term lower rates will be used, and in the long term, higher rates such as 622 Mbps may be used.

Incoming information is segmented in small fixed length packets or cells and transported individually across a network at very high speeds. Incoming data streams are sliced into ATM cells and transmitted so as to meet the desired quality of service. Thus circuit mode transmission is supported by transmitting continuous streams of cells, whereas transmission of data streams can be discontinuous. Each cell comprises 53 octets, eight of which comprise the header, the remainder carrying information. The size has been selected as a compromise between opposing stipulations of the data services and telephone services communities. Larger packet lengths achieve higher link efficiency for data transmissions, whereas short packets minimize network transmission delay, a desirable property for telephony.

The header part of an ATM cell contains a number of fields, which contain network information associated with the user data. The header for the user network interface (UNI) is slightly different from the network node interface, due to the need for the UNI to arbitrate usage between sharing terminals at the access point. The header provides a number of messages for controlling the flow of information in the network.

ATM cells are transported within the network through ATM switch nodes. Each switch has a translation table, which contains an entry for VPI/VCI supported within it. ATM calls are initiated on a shared virtual signalling channel the metasignalling channel. In response to a request on the metasignalling channel, a virtual signalling channel is assigned, which is used by the terminal for further signalling. The ATM exchange then determines a route to the destination by seeking information from the ATM layer. Once the route has been found, the exchange generates a message to the next node using a CCITT signalling No. 7 protocol. The process is repeated at subsequent nodes until the destination is reached. A new route is determined at each node and a new value of VPI VCI is assigned. When the called party has answered a call, a signalling message is sent back along the line of exchanges, causing each to switch the connection. The call can then progress along the virtual route until completion, when entries on all exchanges are cleared.

Four classes of service have been identified. Service classes are identified by their bit rate behaviour, i.e. constant bit rate, or variable bit rate, and by the manner in which connections are established, i.e. connection oriented or connectionless.

A service is specified by the traffic descriptor and peak cell rate (PCR). The variable bit rate (VBR) service permits a guaranteed rate but allows sharing of network resources, thereby improving resource utilization through statistical multiplexing. The traffic descriptors associated with VBR are PCR, sustainable cell rate (SCR) and maximum burst size (MBS). These parameters can be monitored using an algorithm called the Generic Cell Rate Algorithm (GCRA) or 'leaky bucket'. The available bit rate (ABR) service has associated with it the service descriptors PCR and minimum cell rate (MCR). The ABR service is meant to support non real time service, and hence there are no specifications on delay requirements. Finally, unspecified bit rate (UBR) is a best effort service, with no guarantees regarding its quality of service or associated traffic descriptor.

To transfer the cells, each service has to add some underlying transport functions. The ATM adaptation layer (AAL) is used for the purpose. The AAL information is embedded in the user information field and is therefore carried transparently across the network, which leaves the choice of the preferred AAL to the communicating terminals. Various types of AAL have been standardized or are being studied. For example, type I AAL is intended to carry Class A services, which are essentially constant bit rate data streams over established connections (e.g. telephony).

An ATM network requires extensive network management and control. Intelligent bandwidth management is essential to support variable rate services, as otherwise a bursty message may suffer loss when a high burst of activity occurs. Quality of service must be ensured for each service. Delay and cell loss are important parameters for wideband transmissions. In particular, queueing delay becomes important for interactive services and due care must be exercised in allocating bandwidth to a session. Delay variations caused by variations in arrival rate, due to queueing in the path, can cause annoyance. As an example, in a video transmission, audio and video should be synchronized jitter can cause loss of synchronization between sound and video. When a queue is too large, incoming cells are likely to be lost. Cells are therefore marked as 'low priority' in the priority bit, giving unmarked cells a higher probability of arrival, The ATM network also requires monitoring to ensure that users are not violating their requested allocation by using more network resources than requested. Monitoring also provides a means of determining faulty terminals, etc.

Call charging in an ATM network can be complicated, as ATM calls have variables which have not been used traditionally these include requested bandwidth, requested quality of service, cell count, and quantity of successful delivery.