SDH Brief Overview


SDH is the freight train of the communications protocol and the STM frame is its boxcar. Chugging along at one fixed size freight car every 125µs, the train never stops or varies its pace; all passengers and cargo have to board on the fly. Even when the platform is empty and there is no freight to move, the train continues relentlessly. Processing freight cars is simplified because the freight cars arrive at their destination with such exacting punctuality and they all carry the same payload. The amount of resources required to process them does not vary, regardless of the amount of freight being carried.

STM-16: Our SDH Frame Size

The STM-16 frame is the frame size we chose to implement because the PL3 interface we chose for the LINK interface of the chip corresponds to an OC-48 data rate, as does the STM-16 frame format of SDH. Each STM-16 frame is comprised of an AUG-16 as well as Repeater Section Overhead (RSOH) and Multiplexer Section Overhead (MSOH). The STM-16 frame is shown below in a diagram that relates the various overhead portions to sections of the network.

An STM-16 frame has 16 times as many columns as an STM-1. STM-16 frames are transmitted at 16 times the data rate so the frame transmits in only 125µs.

The Pieces of an SDH Frame

A conceptual diagram of the STM-1 frame can be seen below. This diagram shows how a Container-4 maps into a Virtual Container-4 and how a VC-4 subsequently maps into an Administrative Unit-4 and then into an STM-1 frame.

Click on any of the three outlined sections below for more information.