S. M. Munir Ahmed


Since the introduction of the first digital PCM systems 30 years ago, the main objective of digital transmission has been to increase the capacity of existing cable links. At that time, main use of telecommunications was for voice communications and as such most of the digital standards are based upon a frequency bandwidth that will pass intelligible voice. This equates to a bit rate of 64kbit/s.

The solution was to multiplex the signals so that they could be transmitted along one ‘wire’. This is possible because pulses (bits) can be a lot shorter than 1/64000 of second, so the time in between pulses is taken up with pulses from other channels. Hence capacity is determined by how short the pulses can be made in order to increase the number of channels in the same time frame.


The above diagram shows four signals have been combined into a single new signal. This process can be repeated to combine four of the new signals into another new signal, thus combining 16 of the original signals as required.

Each stage combines 4 lower rate signals to form one higher rate one. These stages, or levels, were defined by the CCITT’ in 1962 and are known as the Plesiochronous Digital Hierarchy or PDH.

Historically PDH was developed for network operators to transfer large amounts of traffic between two specific locations, not to occasionally ‘drop’ a signal off en route.

As time progressed, people and especially business required more than just phone line and quickly a need developed to supply comparatively large amounts of capacity directly into customer’s premises.

Unfortunately, PDH is not ideally suited to this requirements and can be seen in diagram below. The large amount of multiplexers required to simply drop off, say, one 2Mbitls signal from a l4OMbitls bearer is costly.


* Ideas such as ring networks, traffic management and on-line fault reporting were not practically considered when PDH was introduced.

1.1 The fundamental idea behind SDH is to have a common standard for transmission system throughout the world. This means that any transmission rate can be multiplexed to a common format so that all demultiplexers can decode it.

It order to achieve this, one of the requirements is that every piece of equipment in the network must be clocked synchronously. Present technology has allowed every accurate cesium sources to be made and with the increased use of optical fiber distribution, it has allowed a clock source to be distributed over long distances without degradation.

The result is that it is now possible to time all the equipment in a network synchronously (i.e. from the same source).

Synchronous liming thus allows individual tributaries to he multiplexed directly into high rate hearers without needing to go sequentially through intermediate multiplexing levels (as done in PDH). SDH enables signal drop oils and networks to evolve.

The figure shows a typical ring network serving customers, using SDH


1.2 In 1986, the CCITT developed a series of recommendations for a new transmission system based on the North American SONET synchronous transmission principles. These were mainly set to overcome the problems that existed with the present PDH system, but they also added new services and features that PDH could not offer.

These recommendations formed the standard that is now known as the Synchronous Digitl Hierarchy or SDH.

2. Definition of Synchronous Digital Hierarchy (SDH)

2.1 SDH is an International Standard for high speed synchronous optical telecommunication network and more specifically, it is a synchronous digital transport system aimed at providing a more simple economical and flexible telecommunication network infrastructure.

2.2 The concept of a synchronous transport system, based on SDH standards, goes beyond the basic needs of a point to point transmission system to include the requirement of telecommunication networking switching, transmission and network application areas: local network and long haul network. SDH, therefore, provides a unified telecommunication network infrastructure.

2.3 SDH is primarily seen as a flexible multiplexing technique, which can be used to improve utilization of line plant by grooming and consolidating traffic for transmission.

3. Advantage of SDH

3.1 Direct Synchronous Multiplexing Add/Drop capability without the "Multiplexer mountain."

Direct Synchronous Multiplexing

Direct synchronous multiplexing preserves the accessibility of individual tributary signals within the structure of the multiplexed signal. The basic of real flexibility in telecommunication networking comes from being able to integrate the functionality of both synchronous multiplexing and digital switching within a single network element. A new type of Network Element (NE), the Add-Drop Multiplexer (ADM), has become viable through integrating synchronous multiplexing and digital switching. Because SDH units can directly multiplex signals into higher rates, less equipment will be needed. This will result in reduced procurement, installation and maintenance costs.

3.2 Integrated Network Management and Maintenance: The SDH signals has overhead facility that allows every signal and piece of equipment to be maintained and controlled automatically. Computer aided control of the network is essential but much of the management and maintenance can be delegated and built-in to the individual network elements of the SDH. For example, performance monitoring capabilities built-in to the SDH signal, provides the network operators with a sound basis for ensuring at the quality of service provided to the customers is maintained. In addition, should the quality of service start to deteriorate, for some reason or other, the relevant information can be relayed back to the network management computer via the data communication channels also built into the SDH signals. If appropriate, corrective action then be initiated and quickly communicated back to SDH network equipment from the network computer over data communications channel. On line management of SDH Network Bandwidth(bw) becomes a real possibility. In SDH, comprehensive fault detection together with self healing ring architecture allows any failure to be invisible on end to end path basis.

3.3 Transport Capabilities.

All of the tributary signals viz 2,34,140 Mb/s, DS1, DS2 and D83 signals which appear in today’s plesiochronous network can be transported over SDH.

In addition, SDH transport capabilities have flexibility to accommodate the more advanced customer service signals expected in the future. This will include:

Asynchronous Transfer Mode (ATM)

- The standard for Broadband ISDN(BB-ISDN)

Fibre Distributed Data Interface (FDDI)

- A high speed local area network (LAN) standard

Distributed Queue Dual Bus (DQDB)

- A Metropolitan Area Network (MAN) standard

3.4 A Single Network Infrastructure

A Single Network Infrastructure

SDH signal structure can be applied to provide simple, economical and flexible management of signals in all three traditional network application areas: Long haul network, Loop plant and Local network. A single SDH network infrastructure will be possible between the three major telecommunication application areas. SDH also provides a standardized network interface, referred to as the Network Node Interface (NNI), allows direct interconnection of transmission equipment's from different vendors.

4. SDH Signal Hierarchy, Line Rates & Channel Capacity.

The lowest level SDH signal is called the Synchronous Transport Module level I (STM-l). A 2-diniensional map for the STM-1 signal frame comprises 9 rows by 270 columns giving a total signal capacity of 19440 bit per frame. The frame repetition rate or "frame rate" is 8000 frame per second so that the duration of each frame

is 125 microsecs. The frame dimensions and repetition rate, result in the bit rate of the basic SDH structure being 155.52 Mb/s (i.e. 2430 bytes/frame X 8 bit/byte X 8000 frame/sec = 155.52 Mb/s)

* Both Electrical and Optical Interfaces

At 8000 frames/sec. each byte within the SDH signal structure represents a channel bandwidth of 64 Kb/s.(i.e. 8 bit/byte X 8000 byte/sec = 64 Kb/s) This is the bit rate as PCM voice channel. Four STMs are interleaved to form the STM-4 signal at 622.08 Mb/s and sixteen STMs to form the STM-16 at 2488.32 Mb/s.

5. SDH Network Segments

In SDH, the transmission path is viewed as comprising three segments - the Path, the Multiplexer Section and the Regenerator Section. Each segment is provided with its own overhead, hence three categories of overhead. Each overhead provides the support and maintenance signals (such as alarm monitoring bit error monitoring and data communication channel) associated with transmission across that segment.

5.1 SDH Network "Path"

"Path" is the logical connection between the point at which a tributary signal (such as a 140 Mb/s tributary signal) assembled into its Virtual Container and the point at which it is disassembled from the Virtual Container. The VC is assembled and disassembled only once, ever though it may be transferred from one transport system to another many times on its route through the network.

5.2. SDH Network "Multiplexer Section"

"Multiplexer" span comprises the transmission medium together with the associated equipment (including regenerators), that provide the means of transporting information between two consecutive network nodes (e.g. SDH multiplexers, SDH cross-connect switches). One of the network nodes originates the Multiplexer Section Overhead (MSOH) while the other terminates this overhead.

In the SDH network, the Multiplexer Section has special significance. It is the level at which the SDH network provides protection against equipment failure or deterioration in performance.

5.3 SDH Network "Regenerator Section"

"Regenerator Section" span comprises the transmission medium and associated equipment between:

a. A network element and regenerator, or

b. Two regenerators.

5.4 SDH key words

POINTER : Total network synchronization within a minimum delay time.

VIRTUAL CONTAINER : Flexible multiplexing for different speed signals.

OVER HEAD BYTE (SOH, POH) : Classification of transmission network and planly of O & M information.

6. Synchronization and Timing of SDH Networks.

The switching principle used in digital switches requires that all the switches in the network are operating synchronously with one another. This implies that each node receives its synchronization reference from a single source. In practice all the major operators provide their own primary reference source (PRS) and synchronization network of slave clocks which are used to synchronize individual switching offices. Synchronization reference information is distributed in this network mainly by primary rate signals of the PDH. PRSs are specified to very close tolerance of 1x10’ 1 so that they are plesynchronous with one another (i.e. nearly synchronous)

There are two techniques to establish a synchronization network: master-slave synchronization, and mutual synchronization. The first technique (master-slave) applies a unique primary reference clock for synchronization of the first hierarchical level of nodes. These nodes give their derived clocks to the next level nodes, and so on. In the second technique (mutual synchronization) all nodes are at peer level interconnected by the existing digital

links. Each node calculates the mean phase value of some incoming clocks and its own internal clock. Master slave synchronization is by far the most common but both techniques are applied.

7. Network of Bangladesh Telegraph and Telephone Board (BTTB) - its improvement & expansion and application of SDLI Technology.

7.1 The national telephone network of Bangladesh is a two hierarchy system consisting of local exchanges and primary centers. The primary centers are the seven digital long distance Trunk Automatic Exchanges (TAXs). Two in Dhaka, two in Chittagong and one each in Khulna & Bogra and Sylhet. There are two International exchanges (ITX) at Dhaka and all international calls are routed through these two ITXs. The local exchanges are located in the 64 district headquarters areas. Five more new TAXs have been planned for installation.

The private operators have started providing telephone service in 390 thana areas and the replacement of manual switchboards with digital PBX is now going on. When the local exchanges will be upgraded after completion of the replacement of the analog exchanges with digital ones, they will be connected to digital PBX for the expansion of nationwide dialing service.

7.2 Synchronization plan in BTTB Network.

The digital exchanges presently introduced by BTTB are synchronized by the buih in clock oscillator. In view of the planned introduction of large quantities of SDH transmission equipment and digital exchange, a standard clock oscillator will be equipped for the national network synchronization by the master-slave system in which il will be used as the master oscillator and supply the clock consecutively to lower exchange classes.

Specifically, the cesium standard oscillator for ITX, newly installed at Mohakhali, will be used as the master oscillator for synchronization of network as shown in Figure below:

VCXO : Voltage Controlled ______ :Main Distributing Clock      Crystal Oscillator

TAX : Trunk Automatic ______ :Standby Distributing Clock      Exchange

SE : Satellite Exchange

7.3 Transmission Facilities.

BTTB has planned to build a digital network that can provide not only high quality telephone service but also cater ever increasing traffic in inter district routes. Optical fibre system or a digital radio transmission system will be installed to meet the requirements. Major back bone transmission links will be replaced by digital ones to secure the required transmission capacity. Dhaka-Khulna and Dhaka-Sylhet links are already digital.

Digital exchanges at Dhaka and Chittagong have interexchange junction through optical fibres. Inter exchange junctions of 8 fibres, 34 Mb/s, 140 Mb/s and 560 Mb/s are being installed in Dhaka, Sylhet, Rajshahi and Khulna under 150 Kilo line Digital Telephone Installation Project.

In the Dhaka Chittagong section, the optical fibre system is proposed to be introduced because this section requires a large transmission capacity and traffic in the section is expected to he increased sharply in the future. Dhaka-Chittagong optical fibre link is proposed to be laid on 263 Km road alignment. SDH system (STM-4) 622 Mb/s (7,560 channels) is being considered to get higher capacity and transmission flexibility with future expansion upto 30,000 cct. (STM-16) In other sections, the digital radio system wilt he introduced to make effective use of existing facilities.

The existing digital links will be operated without changing its PDH (Plesiochronous digital hierarchy), and SDH will be introduced when the new system is installed.


1. Principles of SDH - a training document of TELKOM-Malayasia.

2. Electrical Communication, 4th Quarter, 1993-ALCATEL

3. APT, 1995, Seminar Paper on SDH Technology - A. S. MENON, VSNL, India.

4. APT, 1995, Seminar Paper on SDH - SM. Munir Ahmed, Bangladesh.

Engr S. M. Munir Ahmed was born in 1950 in Feni. He received his B.Sc. Engg. (Electrical) degree from BUET in 1970. Mr Munir joined BTTB in 1972 and was promoted to Divisional Engineer in 1976 & Director in 1994. He is now Director, Telecom Staff College.