SDH Networks - An introduction

Synchronous Digital Hierarchy ( SDH Networks)

Digital  Telephony Networks evolved from  Pulse Code Modulation (PCM) & a multiplexing TDM hierarchy called Plesiochronous Digital Hierarchy (PDH). PDH multiplexing had an advantage of higher complexity and cost both as for dropping any signal from the bit stream at the ADD – Drop node each time the entire signal had to be De-multiplexed  - Multiplexed. Also accommodating different kind of traffic such as ATM, Ethernet etc was a problem.

SDH is synchronous implying that a distributed reference clock determines the basic bit frequency in the network.  Multiplexing, grooming, cross connecting etc can be done without buffers unlike PDH.

-          Multiplexing forms higher bit-rate channels out of lower bit rate channels typically in order to achieve more cost-effective transmission.

-          Grooming is done to fill up (in order to reduce the number of) higher bit rate channels in the network.

-          Cross connecting is needed for connection setup, protection, restoration and grooming, among other functions.

SDH networks are optimised for 2 Mb/s and 140 Mb/s tributary channels but can also be used for 1.5, 34 and 45 Mb/s channels.

These channels are then synchronized to the network and put in virtual containers (VC-11, 12, 3, 4)

Channels below VC-4 are called lower-order paths while VC-4 is called higher-order path.

SDH add/drop multiplexers (ADMs) and cross connects (DXC) exist for different path levels.

DXCs are labelled with m/n with m the highest access level and n the lowest cross connection level. Typical DXC are 4/1 or 4/4.

For transport, one or more VC-n’s are put into transmission frames called Synchronous Transfer Modules (STM). These frames or transport modules come with bitrates in multiples of 4: STM-1/4/16/64/256 at 155/622/2488/9953/39813 Mb/s, according to the current standard.

STM -1 Electrical versions exist for STM-1 while the rest are only defined for optical transmission. In reference to the DXC labeling above, an OXC with port-speeds and cross connection at STM-16 would be called a 6/6 DXC (with the difference that the OXC does not alter the overhead. Thus, there’s no fundamental difference in moving from a 4/1 DXC to a 4/4 DXC compared with moving from a 4/4 DXC to an OXC handling STM-16 channels.

The OAM features of SDH are extensive including QoS parameters, in-band management and identifiers for all channels.

Since all the features of SONET and SDH is the same in the context of this text, the term “SDH” is collectively used for both SONET and SDH to avoid writing SONET/SDH whenever used.

The main clock is normally (dual) redundant.