PDF of this tutorialDXC/DCS can occur in two main types. Higher order cross-connects are generally used to route bulk traffic in blocks of nominally 155 Mbps for network provisioning or restoration (including disaster recovery). They are designated as DXC 4/4. The first "4" refers to 155 Mbps transmission ports on the cross-connect, and the second "4" indicates that the whole payload within the 155 Mbps is switched as an entity. Lower order cross-connects (DXC 4/1 or 1/1, the "1" denoting primary rate at 1.5 or 2 Mbps) are used for time switching leased lines, consolidation, and service restoration. They switch traffic components down to primary rate, usually having options to switch alternatively at the intermediate rate of 34 or 45 Mbps. The capabilities and applications of these two cross-connect families may overlap, with some designs capable of parallel operation, for example at 4/4, 4/1, and 1/1.
Those ADMs and hub multiplexers, which include time-slot interchange, also can be used as small nonblocking DCSs. A ring of several ADMs can be managed as a distributed cross-connect but typically will experience some blocking, which must be anticipated in network planning.
Some cross-connect designs allow all traffic interfaces to be in PDH form for compatibility with existing equipment. In particular, these designs might allow the p hierarchical level in a DXC p/q cross-connect to be at either 34 or 140 Mbps in PDH format, as an alternative to 155 Mbps, so that network flexibility becomes available where SDH infrastructure does not yet exist. In these cross-connects, a port at 34 or 140 Mbps can include an embedded PDH multiplex equipment for internal conversion to and from 2 Mbps, which provides a transmultiplexer function between PDH and SDH areas of the network.
ADMs conventionally allow traffic to be in PDH form, such as at 2 or 34 Mbps on their add-drop ports, and also may provide the transmultiplexer function. The through traffic ports are in SDH form.
