rties of the switching layers in N-ISDN and B-ISDN are very dif- ferent. The N-ISDN switching layer offers services as single duplex synchronous connections with a few discrete levels of transfer rates from 64kbps up to 2 Mbps. The ATM switching layer offers simplex asynchronous virtual channels (i.e., VCs), in order to support different transfer rates in the two directions (many media serv- ices require low transfer rate in one direction but high transfer rate in the other). The circuit-switching layer requires no particular properties to be defined when ordering a connection. On-demand VCs (and VPs) are ordered by the controlling actors of media services strata (such as the B-ISDN control actor, see Fig. 6.9). 6.4 ATM Cell Switching Ordering ATM virtual channels is considerably more complex (for a service-con- trolling actor). Two virtual channels (one in each direction) have to be established for most end-user services. For most channels, a large number of parameters must be given. These are parameters that define the load profile (maximum, minimum, and average transfer rate needed), the burst character of the traffic and a large number of quality of service parameters. The switching layer adapts the resources it allocates (buffers, timing supervision, etc.) to a channel to meet the requested properties. To simplify channel establishment, the standard defines a number of basic types of channels that the switching layer must be able to offer, such as: CBR ("constant bit rate" or "circuit-emulating channel"). This is a channel type with good real-time properties (both low delay and jitter), provided that the transfer rate is the same and rather constant in both direction. The behavior of the channel is therefore rather similar to a circuit-switched channel. It is the type of channel that is used for isochronous media services, such as interactive, uncompressed voice and video. VBR ("variable bit rate"). This is a channel that can handle large variations of transfer rate with low jitter. It is the type of channel