PDF of this tutorialTraffic Interfaces
SDH defines traffic interfaces that are independent of vendors. At 155 Mbps they are defined for both optical and copper interfaces and at higher rates for optical ones only. These higher rates are defined as integer multiples of 155.52 Mbps in an n x 4 sequence, giving for example, 622.08 Mbps (622 Mbps) and 2488.32 Mbps (2.5 Gbps). To support network growth and the demand for broadband services, multiplexing to even higher rates such as 10 Gbps continues in the same way, with upper limits set by technology rather than by lack of standards as was the case with PDH.
Each interface rate contains overheads to support a range of facilities and a payload capacity for traffic. Both the overhead and payload areas can be fully or partially filled. Rates below 155 Mbps can be supported by using a 155 Mbps interface with only a partially filled payload area. An example of this is a radio system whose spectrum allocation limits it to a capacity less than the full SDH payload, but whose terminal traffic ports are to be connected to 155 Mbps ports on a cross-connect. Interfaces are sometimes available at a lower synchronous rate for access applications. North America has for some time used 51.84 Mbps SONET, and ETSI has defined a 34 Mbps SDH interface—now being deployed—whose data rate is identical to that of 34 Mbps PDH.
SDH Layers
In the multiplexing process, payloads are layered into lower-order and higher-order virtual containers, each including a range of overhead functions for management and error monitoring. Transmission is then supported by the attachment of further layers of overheads. This layering of functions in SDH, both for traffic and management, suits the layered concept of a service-based network better than the transmission-oriented PDH standards.
Management Functions
To support a range of operations, SDH includes a management layer whose communications are transported within dedicated data communications channel (DCC) time slots inside the interface rate. These have a standard profile for the structure of network-management messages, irrespective of vendor or operator. However, there has been no agreement on the definition of the message sets to be carried, so there is no interworking of management channels between equipment vendors at the SDH interface.
Elsewhere, at the network-management interface to each node, which is typically via a local-area network (LAN), there has been more agreement. ITU—TS standards define a Q3em interface between an SDH equipment and its manager; SDH vendors are migrating their software to be compatible with this interface.
