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What is basic difference between SDH and SONET?

What is basic difference between SDH and SONET?: Many telecom friends ask the question that “What is the basic difference between SDH and SONET?”Yes there are differences in both technology .As per my best knowledge I will try to explain in details. We know that SDH stand for synchronous digital hierarchy.SDH is the synchronous technology and it is used everywhere in this world but except the few countries like US, Canada and Japan. Difference between SDH and SONET are based on the different asynchronous bit rate that must be mapped in to them.

In developing both SDH and SONET technology, network operators had a strong need to gradually integrate existing transmission technologies to enable SONET and SDH. Highest order multiplex signal commonly used in N.A is 45Mb/s, 51Mb/s was sufficient synchronous primary rate for virtually any SONET application. In the rest of the world 140Mb/s mux signals was very common and 155Mb/s (STM-1) was chosen as the primary synchronous mux rate.

Difference between SDH and SONET signal and bit rate
SDH and SONET Signal
Above hierarchy levels basically match the plesiochronous bit rates commonly used in these countries. Table indicates, there are points where transition between SDH and SONET system are possible. Matching is relatively simple, as gateway issues where taken into consideration during development of SDH. Only minor adjustment need to be made to certain overhead bytes. SDH terminology is however, quite different with the packing unit for example referred to a virtual container (VC-n) as oppose to virtual tributary.


In 1985, Bellcore proposed the idea of ​​an optical carrier-to-carrier interface that would allow the interconnection of optical devices of different manufacturers. It was based on a hierarchy of digital rates, all of which were created by interleaving of a basic rate signal. The idea of ​​a synchronous optical network (SONET) attracted career, Regional Bell Operating Companies (RBOCs), and manufacturers equally and achieved rapid growth.

Interested in the SONET by CCITT (now International Telecommunication Union - ITU-T) expanded its scope from domestic to an international standard, and by the 1988 The ANSI Committee successfully integrated the changes requested by ITU-T, and were well on their way towards issuing new standards.

SONET specification defines optical carriers (OC) interfaces and their electrical counterparts to allow Transmission of low-rate signals at a normal synchronous rate. One of the major benefits of SONET signal, with any standard, is that it allows multiple vendors to provide compatible transmission equipment in the same span.

SONET also allows for dynamic drop and insert capability on the payload without delay, and with high-speed signals with additional hardware connected with demotiplexing and remoliplexing.

Because overhead is relatively free of payload in SONET system, SONET system is capable of integrating new services like Asynchronous Transfer Mode (ATM) and Fiber Distributed Data Interface (FDDI), also in addition to existing DS3 and DS1 services.

Other major and big advantage of SONET is that the purpose of operations, administration, maintenance and provision (OAM & P) capabilities are made possible directly in the signal overhead to allow network maintenance from a central location.

Multiplexing Process in SONET

SONET multiplexing creates a building block with low-speed digital signals such as DS1, DS1C, E1, DS2 and DS3, requiring overhead, which is called the synchronous transport signal level one (STS-1). The STS-1 frame, which is arranged in 9 rows by 90 columns of bytes. This is transmitted row first, the most important bit (MSB) of each byte is transmitted first.
SONET Multiplexing process in STS-1 Frame
STS-1 Frame

A formula that calculates the bit rate of a framed digital signal:

bit rate = frame rate x frame capacity

For SONET to easily integrate existing digital services into its hierarchy, it was defined to operate at the original rate of 8 kHz or 125 microseconds per frame, so the frame rate is 8,000 frames per second.

The frame capacity is the number of bits contained within a single frame.
Capacity of Frame in SONET = 90 bytes/row x 9 rows/frame x 8 bits/byte = 6,480 bits/frame
Now we can get the bit rate of the SONET STS-1 signal is calculated as follows:
bit rate = 8,000 frames/second x 6,480 bits/frame = 51.840 Mbps.

In SONET system higher-rate signals are formed by combining multiples of the STS-1 block by interleaving a byte from each STS-1 to form an STS-3,

Function of a SONET add/drop multiplexer

Just for your information about the SONET add/drop multiplexer I would like to attach a image to understand easily. The origin of the signal is an important factor used to isolate the scars of trouble, the SONET signal provides a method of tagging every STS-1 with information about its location.
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What is SDH

What is difference between SDH and SONET
What is SDH

During 1990s Synchronous Digital Hierarchy SDH was a new transmission technology in the telecom sector for fiber optic transmission standardized by ITU-T. A standard way of assembling single channels (64 Kbps) into time division multiplexed streams of many channels; STM-16 = 2.48832 Gbps or 32,256 channels Synchronous format means time slots well defined.

Allows for channel groups from the old hierarchy (2, 34, 139 Mbps) as virtual tributaries.
The synchronous digital hierarchy (SDH) is synchronous multiplexing - data of many Tributary interfaces PDH (E1, E3, E4), SDH (STM-N) source is byte interleaved. SDH is a complete set of transport network and standards for handling, administration and maintenance (OAM).
Multiplexed channels are in fixed locations in SDH relative to the framing byte or In SDH there are fixed locations of Multiplexed channels relative to framing bit.
Demultiplexing is obtained by taking out required bytes from the digital stream.
SDH allows any channel to 'drop' from the data stream without demultiplexing intermediate rates as is required in the PDH.
A single SDH frame is called the synchronous transmission module (STM-1). Transmitted in the period of 125 μs, frames contain 2430 octets, which are arranged as 9 rows of 270 octets. In a SDH frame, a single octet represents a 64 kbps channel (125 bits each), many octets can be collected to form a container for large data rates.


Add/drop individual channels from the higher order stream without having to disassemble/reassemble it
Standardize signal content to allow interconnection of two suppliers terminals; so called “midspan meet”
Easier interchange of traffic, universal.

What Does SDH Do

Provides cost-effective, flexible networking based on direct synchronous multiplexing
Supports advanced network management and maintenance techniques - Nearly 5% of signal bandwidth allocated for this purpose
Accommodates both existing and future service signals
Allows a single telecom network infrastructure

Advantages of SDH

Worldwide standard
 First standard for bitrates higher than 45/140 Mbps
 Standardized optical interfaces. Easily and simple coding and decoding of the electrical signal into the line signal (scrambling). Integration of multiplexer and line equipment
 Heavy overhead capacity for error monitoring and for management objectives that called the (TMN)
 Direct access to tributary channels
 Only required small buffers in SDH------ small group delay figures
 Provides concept for clock/synchronization, protection switching etc.
The ATM is suitable for the physical layer of the network

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