Synchronous Digital Hierarchy (SDH)

Introduction to SDH:

Synchronous Digital Hierarchy is an international standard for high capacity telecommunication fiber optic networks.  It is a synchronous digital transport system aimed at providing a simpler, and more economical and flexible telecommunication network infrastructure.

SDH is essentially a transport protocol based on a common time reference (primary clock), which multiplexes different signals within a common flexible hierarchy and administers their transmission efficiently over fiber optic cable, with internal protection mechanisms.  As all network terminals have a stable reference clock, no type of stuffing or framing bits are needed during multiplexing of signals.

SDH is usually considered a physical transport layer protocol and, as such, is used to carry traffic in the form of packets of information, such as voice, video, multimedia, and data packets such as those generated by ATM, IP, or applications such as PDH. 

To that end, its role is essentially to manage the use of fiber optic infrastructure, which implies efficient broadband management while carrying different types of traffic, detecting errors, and recovering transmissions from them transparently for the higher layers.

Origin of SDH

Synchronous transmission systems were developed as a result of operators’ need to identify solutions to, principally, the following problems:

a)             Deployment of flexible and resistant networks;

b)            Definition of standard interfaces between the equipment of different manufacturers;

c)            Facilitation of network interconnection between North American and European transmission hierarchies.

In 1989, the standard culminated in ITU-T Recommendations G.707, G.708, and G.709, which define the Synchronous Digital Hierarchy.

These recommendations define a number of basic rates of transmission that can be used in SDH.  The first is 155.52 Mbps, normally referred to as an STM-1 (where STM means Synchronous Transport Module).  Higher transmission rates, such as STM-4 (622.08 Mbps), STM-16 (2488.32 Mbps), and STM-64 (9953.28 Mbps), have also been defined.

The recommendations define a multiplexing structure in which an STM-1 signal transports or accommodates a number of lower transmission rate signals, such as PDH signals of all levels, to transport the old frames into a new frame, packaging them together in its payload area, utilizing a multi-step process in a different number of structures.

Advantages:

a)          Simpler and more flexible multiplexing and demultiplexing, enabling circuits to be extracted and inserted without stopping the data stream, utilizing a simple multiplexer and enabling point-to-point services to be provided rapidly on request.

b)          Ease of migration toward higher orders of multiplexing, as they use the same working philosophy.

c)          Definition in the standard of network administration capacity.

d)          Main hubs that enable network Operation, Administration, and Maintenance (OAM) procedures to be improved.

e)          Administration and transport of a wide variety of fixed broadband services, such as PDH traffic signals at 2 Mbps, 34 Mbps, and 140 Mbps, G.702, ATM, Ethernet interfaces that take IP data or LAN data, analog voice interfaces, and ISDN/ADSL interfaces.

f)           Integrated protection mechanisms, with automatic re-routing of traffic without service interruption.

g)          Definition of an open standardized electrical and optical interface to permit interconnection with other equipment.

Fig.1: Traffic access in PDH vs. SDH

Characteristics of the SDH transport network:

a)   Digital multiplexing:  Analog communication signals are carried in digital format over the network.  Digital traffic may be carried much more efficiently and it permits monitoring of errors for quality purposes.

b)   Fiber optics: This is the physical medium commonly utilized on today’s transport networks owing to its higher traffic transport capacity, which tends to reduce costs associated with the transport of traffic.

c)   Protection schemes: These have been standardized to ensure traffic availability.  In case of possible errors or fiber breakage, traffic can be switched to an alternative route so that the end user does not experience any disruption of service.

d)   Ring topologies:  These are being increasingly deployed owing to possible loss of connection.  There is an alternative traffic route on the other side of the ring, minimizing connections and the amount of optical fiber deployed on the network.

e)   Network administration: Administration of such networks from a single remote location is an important service for operators.  Software has been developed that enables all nodes and traffic routes to be administered from a single computer.  One operator may administer a wide range of functions, such as providing capacity in response to customer demand or monitoring of a network’s quality.

f)    Synchronization: Network operators must provide synchronized timing for all network elements to ensure that information passing from one node to another is not lost.  Synchronization is becoming a critical point, making SDH an ideal network philosophy.

José Alberto Benitez Gómez
IPT - FIUNA
Center for Technological Innovation
of the Engineering School of Paraguay