rrays may be constructed having a variety of possible Cell buffering in the switching elements can be organised on three characteristics. In general, such networks may be expanded by possible bases: addition of stages to handle very large numbers of ports, but do not expand in total size as rapidly as the simple matrix when 1. Input buffering: One buffer dedicated to each switching ele- additional ports are added; also they usually offer a multiplicity ment input. Contents can be switched to one (or several in point of path possibilities between each pair of inputs and outputs and to multi-point operation) output. this offers advantages in connection with both traffic and se- Output buffering: One buffer dedicated to each switching ele- curity. The individual elements of such a matrix may them- ment output, with no blocking of cells from any of the switch- selves be implemented using topologies such as those described ing element inputs. 41/8 Asynchronous Transfer Mode Central buffering: one common buffer memory block is pro- 3. Ability to handle a range of user traffic source rates using the vided for all cells in the switching element. same technique and equipment. 4. Statistical multiplexing capability. 41.3.4 Traffic control and resource allocation 5. Flexibility in handling the mix of service demand. 6. Transport infrastructure is general purpose, being conceived to 41.3.4.1 Admission control handle by adaptation, a very wide range of traffic/service types. 7. Possibility of service integration on the same medium, an It is anticipated that the ATM based B-ISDN will carry a mixture of important consideration for a broadband network where service traffic sources varying from uniform rate to very bursty, from very types and demand are still ill-defined. short to very long duration. When a new connection is requested it is necessary to ensure that capacity can be allocated on the network resources such that the new connection will provide the required 41.4 Standards for ATM Quality of Service (QOS) and that the 