Dr. Huda Naji

Information Technology \Information Networks Dept.

SONET/SDH

The predominant client layers in backbone networks today

are SONET/SDH, Ethernet, and the Optical Transport

Network (OTN).

These protocols would correspond to the physical layer in

the OSI hierarchy.

SONET/SDH as part of the first generation of optical

networks was the earliest to be deployed in backbone

networks and has been very successful over the years.

An important feature of SONET/SDH is that it provides

carrier grade service of high availability.

SONET (Synchronous Optical Network) is the

current transmission and multiplexing

standard for high-speed signals within the carrier

infrastructure in North America.

A closely related standard, SDH (Synchronous

Digital Hierarchy), has been adopted in Europe

and Japan and for most submarine links.

In order to understand the factors underlying the

evolution and standardization of SONET and SDH,

we need to look back in time and understand how

multiplexing was done in the public network.

Prior to SONET and SDH, the existing infrastructure

was based on the plesiochronous digital hierarchy

(PDH), dating back to the mid-1960s. (North American

operators refer to PDH as the asynchronous digital

hierarchy.)

PDH suffered from several problems, which led to seek a new transmission and multiplexing standard in the late 1980s.

This resulted in the SONET/SDH standards, which solved many problems associated with PDH.

We explain some of the benefits of SONET/SDH below and contrast it with

PDH:

Multiplexing simplification: In asynchronous

multiplexing, each terminal in the network runs its own clock, and

while we can specify a nominal clock rate for the signal, there can

be significant differences in the actual rates between different clocks. In synchronous, all the clocks in the network are perfectly synchronized to a single master clock.

Management: The SONET and SDH standards incorporate

extensive management information for managing the network,

including extensive performance monitoring, identification of

connectivity and traffic type, identification and reporting of failures,

and a data communication channel for transporting Management

information between the nodes. This is mostly lacking in the PDH

standards

Interoperability: Although PDH defined multiplexing methods,

it did not define a standard format on the transmission link.

Thus different vendors used different line coding, optical

interfaces, and so forth to optimize their products, which

made it very difficult to connect one vendor’s equipment to

another’s via a transmission link. SONET and SDH avoid this

problem by defining standard optical interfaces that enable

interoperability between equipment from different vendors

on the link..

Network availability: The SONET and SDH standards have

evolved to incorporate specific network topologies and

specific protection techniques and associated protocols to

provide high -availability services. As a consequence, the

service restoration time after a failure with SONET and SDH

is much smaller less than 60 ms than the restoration time in

PDH networks, which typically took several seconds to

minutes

In a set of synchronous signals, the digital transitions in the signals occur at exactly the same rate. There may, however, be a phase difference between the transitions of the two signals, and this would lie within specified limits.

If two digital signals are plesiochronous, their transitions occur at almost the same rate, with any variation being constrained within tight limits.

Asynchronous, in this case, means that the difference between two clocks is much greater than a plesiochronous difference.

In telecommunication, the phrase multiplexing is used to denote the process of combining two or more channels into a single channel.

For instance, in encoding a video stream, we need to multiplex audio and video on single channel.

One of the ways of doing it is known as time division multiplexing The basic idea of the process is as follows: Suppose we have three individual users in Mumbai who wish to send low stream data to three users in Delhi . Assigning physical lines for each will be expensive and such expense would grow exponentially with increasing number of users. If instead, we used a higher bit rate channel, we could use different time slots for the different data set. The figure below illustrates how this is achieved.

In sixties, the International Telecommunication Union (ITU) defined what is now known as a T-1carrier. The analog voice data was digitized by sampling at a rate which is twice that of the maximum frequency component in the signal. This is known as Pulse Code Modulation (PCM). The T-1 standard was an universal agreement on a sampling rate of 8 kHz (voiceband or narrowband telephone calls limit audio frequencies to the range of 300 Hz to 3.4 kHz) and a channel rate of 64 kilo-bit per second. The single voice channel is known as DS-0 signal (DS= Digital Signal). Thus, if we return back to the example of three users, with each transmitting at a rate of 64 kbps, we could transmit them as a sequential stream of data over a single channel capable of transmitting at 192 kbit/sec. . The way to achieve this is to divide the high rate channel into a series of time slots and the time slots could be assigned to the individual data stream. T-1 trunk could transmit 24 telephone calls at a time, because it used a digital carrier signal called Digital Signal 1 (DS-1)

Example : The T-1 line (T= Transmitted) is a channel capable of transmitting at a speed of 1.544 Mbit/sec. The voice is sampled at 8 kbits/sec, so that the time occupied by a bit is 125 ms. The interval is subdivided into 24 time slots with each time slot coding 8 bits of data for a channel. Thus in each 125 ms there are 192 bits. Adding one bit to mark the beginning, there are 192 bits transmitted in 125 ms which gives the speed to be 1.544 Mbits/sec (192*8000(1000000/=1.544). For mathematical reasons, a voice channel was selected to be at 64 Kbps.

24 of these channels is a composite of 1.536 Mbps, not 1.544 Mbps! Why is there a difference? The reason is that after a byte (8 bits) of data is sent from each channel (24 * 8 = 192 bits) there is an extra bit used for synchronizing called a Frame bit - hence 193 bits are sent and this increase of 1 bit per 192 causes the speed to increase to 1.544 Mbps.

The signal formed by interleaving 24 DS-0 signal is known as DS-1 signal and the corresponding transmitted signal is T-1. Proceeding further, four DS-1 signals are interleaved to give a 6.3 Mbit/sec DS-2 signal (transmitted as T-2) and seven DS-2 give rise to a 45 Mbit/sec DS-3 transmitted over a T-3 line.

Following are the key difference between TDM and WDM systems. • In TDM, resulting capacity is the aggregate or sum of all the input signals/channels. In WDM, each signal is transmitted independent of the others and hence each channel will have its own dedicated bandwidth. • In WDM, all signals will arrive at the same time while in TDM they will arrive one after the other. This is because in TDM the signals are broken up and multiplexed time wise before transmission. This is same as TDMA frame, where in data from various stations are multiplexed and then transmitted.

https://en.wikipedia.org/wiki/Multiplexing#/media/File:Telephony_multiplexer_system.gif https://en.wikipedia.org/wiki/Multiplexing