Synchronous Digital Hierarchy (SDH)

Synchronous Digital Hierarchy (SDH) as the name implies, really is in sync. There's only one clock speed within the network, so you don’t need a buffer zone. As a result, SDH circuits are much easier to access and don’t require the number of multiplexors PDH networks do.

The speeds of an SDH’s core network and the multiplication of channels are different from PDH systems, too. The box at right details the characteristics.

SDH is a much more efficient method of providing circuits to customers. Because there is usually only one clock source in the entire network, the clock speed is passed to each individual and enables total continuity across all the links.

Troubleshooting
You may have to be on the lookout for problems that may occur during the SDH multiplexing. Here are some things to keep in mind when troubleshooting:

• If a multiplexor appears to be resetting itself after 20 or 30 seconds, you have to check the Cyclic Redundancy Check-4 (CRC-4) setup. A bad CRC-4 configuration causes the units at each end of a link to not talk to each other, effectively taking the link down.
• If you’re using voice, you must check the setup of the PBX.
• If you get random errors, you need to check the ground termination on the G.703.
• Is the Network LED on? If so, then the unit is receiving valid HDB3 (or AMI/B8ZS) encoding. If not, check the incoming line for valid data.
• Is the Framing LED on? This indicates that the unit is expecting valid G.704 framing and is indeed receiving it. If it isn’t on, ensure that you don’t have an unstructured service.
• Are the timeslots at each end of the link configured the same way?
• If you’re losing data, check the clock source—there should only be one clock in the network.