SDH Technology : Amazing Information

SDH Technology : In this article of SDH technology, very special and basic information about SDH to be provided. So all of you must read the entire article carefully. In this article all the technical information will be given which are found practically in the field.

These topics will be covered in this SDH Technology article like- Plain Old Telephony System , PDH – Plesiochronous Digital Hierarchy, SDH – Synchronous Digital Hierarchy, Monitoring, Maintenance and Control Functions in SDH , Automatic Protection Switching , SDH Synchronization , Jitter and Wander etc.

SDH Technology : Amazing Information

Plain Old Telephony System

Earlier when telephony began more than approx 100 years ago, only one speech connection at a time could be made using a specific pair of copper wires. Speech was transmitted as analog electrical signals corresponding to its analog adaptations. 

As technology progressed, digitization was introduced into telephony, transmission reliability improved, and resulted in better use of cables.  However signals from subscribers are transmitted in analogue form, making a digitalization process necessary.

Old Telephony System

Pulse Code Modulation

Maybe you all know that the basis of analog to digital conversion is Shannon’s theory. This Shannon's theory states that original signal can be reproduced after the transmission of signals within certain limits from digital signal obtained by Sampling an analog signals at regular intervals the highest significant message frequency at a rate at least twice . The PCM Consists of 3 steps.

• Sampling
• Quantization
•  Coding

Sampling

Sampling is the periodic measurement of the value of an analog signal. A sample signal contains all the information if the sampling frequency is sampled at least twice the highest frequency of the signal.

As the analogue signals in telephony are band-limited from 300 to 3400Hz, .To allow the actual slopes of the filters used a sampling rate of 8 khz is required. After band limiting with a low-pass filter the analog signal is sampled and the samples obtained are digitally encoded.

The recommended sampling rate by ITU-T G.711 is 8000samples per second.8 bits that is one byte should be used per sample.The time taken for one sample is 125µ sec.This time of one sample is called one frame.

Sampling of signals

Quantization & Encoding

Digital data and voice transmission is based on 2.048Mbit / s with 30 time division multiplex (TDM) voice channels, each running at 64Kbps (known as E1) and two channels carrying additional control information. 

At E1 level, time is controlled to an accuracy of 1 in 1011 by synchronizing with a master cesium clock. Increasing traffic has demanded that more of these basic E1s be multiplexed together to provide increased capacity. 

The time rate has therefore increased to 8, 34 and 140Mbit / s. The highest capacity encountered by a PDH fiber optic link is 565Mbit / s, with each link having 7,680 base channels.

Quantization & Encoding

Conversion of voice into digital signal:

Voice Frequency - 4 KHz

Digital Sampling - 4 KHz x 2 = 8 KHz

Quantizing  = Amplitude is given a certain value.

Encoding - 8 KHz x 8 = 64 KHz

Line Coding.

CEPT countries use for Quantization, A- Law, some other countries use µ-law for Quantization.

The decoder output value number in µ-law  is 0 to 127 for positive and 0 to 127 for Negative.

The decoder output value number in A- Law , is 1 to128 for positive and 1 to 128 for negative. 

 

Quantizing  = Amplitude is given a certain value. Encoding   =   8 KHz * 8 = 64 KHz.

Non-Linear Quantization and Encoding

The amplitude of a specific telephone speech signal may vary from one speaker to another and more than the normal speaking range of a single person. In fact, the range of major variation can be found as great as 50 - 60dB.

Low-level signals are more important than high levels with a human voice, so using quantization levels that are closer together at a lower amplitude and become wider as amplitude increases and is more efficient. 

This process is known as non-linear encoding and has two CCITT recommendations 

A-law >> European >>E1 system

u-Law (mu-law) >> USA >> T1 system

In non linear coding used 8-bit/s and would require an equivalent 12-bit/s in linear coding.

Non Linear Quantization and Encoding

A question arises here what is the PCM signal data rate, I want to tell that PCM signal data is 8000 samples / second x 8 bits/sample = 64Kbits / second.

2Mb Frame Structure

PCM 30 MUX consisting of 30 time division multiplexed (TDM) voice channels, each running at 64Kbps (known as E1 and described by the CCITT G.702,G.703 specification) and two additional channels carrying control information. 0 & 16 Channel are carrying the control information. for frame alingnment signal is 0 and 16 is for Non frame alignment signal. 32 channels /Time slots  each of 125 micro seconds.

Each Time slot is divided in to 8 bytes and over all time period of 8 bytes is of 3.9 micro sec.Each byte is of 488 nsec time period .

SDH - 2Mb Frame Structure 

2 Mbit/s Frame Structures

From the picture given below, you can understand in a very simple way how the 2 Mbit / s frame structure occurs.

2 Mbit / s frame structure

Time Division Multiplexing (TDM)

As you saw earlier, an encoded telephone speech signal is transmitted at a rate of 64kbit / s (8 bits / sample; 8kHz sampling frequency).

However, long-distance telephone trunks are designed to handle data at a higher rate.

It therefore makes sense for multiple channels to share the same transmission link using the technique of time division multiplexing (TDM).

A separate encoder and decoder for a 4-channel multiplex system is shown in the diagram. The entire signal is divided into a repeated sequence of four consecutive time slots.

When broadcasting begins, time slot 1 is used to transmit 8 bit code for the first sample of channel 1; Time slot 2 is then used to send the first sample of channel 2. The process resumes after the four slots have finished, with time slot 1 being the second sample of channel 1.

Time Division Multiplexing

PDH (Plesiochronous Digital Hierarchy)

First we try to understand the meaning of word plesiochronos, Plesio means = nearly and Chronous means= Timing.

“Almost synchronous, as the bits are filled in as padding in the frame and the location of the call (signal) varies slightly - jitters - from frame to frame"

The first digital multiplex system was introduced in the early 1970s. The introduction of digital exchanges for 64kbit / s channels increased the pressure to group a large number of channels together for digital broadcasting. 

Three international multiplex hierarchies were generated. The bit rates of these hierarchies were gradually standardized. The primary level of the hierarchy is synchronous, but the synchronization technique depends on the network and the 64kbit / s interface of the primary multiplex.

Central clock synchronization is the dominant technology in telephone networks. This technique uses a central clock that is fed hierarchically to the switching devices.

PDH Systems Worldwide

PDH Multiplex / Demultiplex

What are Plesiochronous Tributaries?

ITU-T defines plesiochronous digital hierarchy tributaries with a fixed permissible deviation of bitrate. Each multiplex has its own clock source (oscillator), thus varying the accuracy of the output frequency. Tolerance ranges have been standardized for the bit rate accuracy. These systems are known as free running and networks based on such systems are plesiochronous. (plesio- comes from the Greek word for near since these systems are nearly synchronous.)

This slide shows the European PDH hierarchy. The bit rate of each tributary can vary from the nominal within specified limits as shown. All basic sytems & higher order mux‘s have their own independaent clock sources which are free running & plesiochronous.

Frequency differences between individual multiplex tributaries are compensated out using bit stuffing techniques. In the PDH system N/W clock synchronisation is performed only at the switching nodes. Transmission systems and higher order mux‘s all operate Plesiochronously.

PDH Multiplex and Demultiplex

PDH Bit Stuffing

In PDH multiplexer different bits must run at the same speed otherwise the bits cannot be interleaved. Higher order speeds are generated by an internal oscillator in the multiplex and not generated by the primary reference clock.

The possible  plasiochronous differences are met using a technique known as justification. Additional bits are added to the digital tributaries (stuffed) that effectively increase the speed of the tributary until they are all the same.

PDH Bit Stuffing

The number of stuffing bits depends not only on the speed of tributaries for the multiplex, but also on the speed of the higher order bit stream. The justification process is employed in all PDH multiplexers. At the far end of the transmission system, the justification bits are removed and the original digital signal is retrieved. Removing these justification bits causes a slight change in the clock phase. This variation is called jitter.

Plesiochronous Drop & Insert

key drawbacks with the PDH system:-

Drop and/or Insertion of 2 Mbit/s channels into, for example, a 140Mbit/s local line requires a minimum investment of 4 systems (3 multiplex systems and 1 terminal equipment.

Cross connects within the transmission segment of the network are typically done via 64kbps  patch-panels or similar mechanical methods . This makes any network configuration changes a  time consuming and costly process.

Although electronic routers are developed for 64kbit / s and 2 Mbit / s channels. The development costs associated with plesiochronous techniques are very high for upper hierarchy levels.

To reach a single channel of multiplex signals (eg .64 or 2048 kbit / s) it is necessary to go twice through the entire multiplex chain (hardware redundancy). - Bit interleaving and stuffing in each hierarchy means full demo is needed to access individual chambers. Little or no designed in TMN inhibits performance monitoring, protection switching and b/w management.

Plesiochronous Drop & Insert

Disadvantages of PDH

Plesiochronous Hierarchy based on 2Mbps primary rates allows multiplexing up to 140Mbps respectively.  Additional tools are required to convert from a hierarchical level to an ensemble. Special equipment is required to transmit multiplex signals (34/140 MB, etc.). Redirection (cross-connection) of channels must be done by hand on the DDF. Administrative connections require separate equipment to support supervision and security switching. Compatibility of transmission and administrative signals between different vendors can be troublesome. Already mentioned above in key drawbacks with the PDH system.

SDH – Synchronous Digital Hierarchy

SDH needed for the extensive network management capability is required within the hierarchy. Standard interface between devices.There is a need to inter-work between North American and European systems. Facility to directly connect or drop tributaries from high speed signals. There is a need for standardization of the equipment management process.

One question also can arise here that why needed SDH? In SDH it is very important that simple drop & insert of traffic channels (direct access to lower level systems without synchronization). 

It is also a characteristics of SDH is that simpler multiplexing (low SDH level can be directly identified from higher SDH level) and it allows mixing of ANSI and ETSI PDH systems.

In general SDH is widely open for new applications (It can carry PDH, ATM, HDTV, Ethernet, MAN, IP...). SDH also provides TMN (for centralized network control).

Synchronous Network Structure

Synchronous Network Structure

Here we must consider the telecommunications network overall. Users in the area of customer network nodes are connected to the Exchange (DSC) via the User Network Interface (UNI). Local cross connect (DXC) should be used instead of this central switching point. In PDH networks a fixed network is made by point to point link.

Channels are switched through these links. Signals from other networks use this transmission technology through flexible multiplexers up to 2 Mbit / s.

The increase in data traffic is much greater compared to voice communication. The biggest demand is in the area of   high bit rate access from the subscriber sector. Such transmission capacity should be available at a reasonable cost and at short notice. 

In Synchronous Network Structure Terminal multiplexers (TMs) with diverse interfaces feed this traffic directly into the SDH network or via add and drop multiplexers (ADMs) that are configured in a ring network (back bone). This ring is made by two fiber optical cables with different back-up switching possibilities. Network management (TMN) sets the required connection.

Layered Model of the SDH Network

Layered Model of the SDH Network

Here we see the layered model of the SDH network. Various networks provide the basic services in the Circuit Layer:

Line-switched services Packet-switched services Leased lines Broadband services

In the Path Layer two fully independent layers can be introduced due to the VC (Virtual Container) concept. The Transmission Layer encompasses the digital signal sections  and the physical medium.

Network Node Interface (NNI) and Path Denominations

Recommandation G.708 indicates the range of validity of the NNI along with its functions. (rec. G.707 specifies the SDH bit rates).

Tributary signals enter the SDH network via a synchronous multiplexer. Network elements are always connected via the NNI. This means that various transmission medias such as radio links and cables must include this interface. SDH systems always use framed signals since the basic element is the synchronous transport module (STM). 

In synchronous network cross connect systems are the heart and as such they determine the structure of the trunk network. The physical specifications for the NNI are contained in Re. G.703 (electrical) and G.957 (optical).

SDH path Denomination and NNI

STM-1 Frame Structure

Before discussing the frame structure of STM-1, we will see the frame structure of SDH here, then we will discuss the frame structure of STM-1.

Frame structure of SDH

Now about STM-1 frame structure, the STM-1 signal has a byte-oriented structure with 9 rows and 270 columns. A distinction is made between three areas:

The payload area, which uses 261 columns, is the pointer area section overhead, which divides the regenerator- and multiplex-section overhead into two parts.

Each byte of STM-1 corresponds to a 64kbit / s channel. After the calculation the overall bit rate of the STM-1 frame corresponds to 155.520 Mbit / s. The frame repetition time is 125µs.

STM 1 Frame Structure

SDH Frame Structure 

SDH Frame Structure Details

The STM-1 signal has a byte-oriented structure with 9 rows and 270 columns. A distinction is made between three areas:

The payload area, which uses 261 columns, is the pointer area section overhead, which divides the regenerator- and multiplex-section overhead into two parts.

In SDH system each byte corresponds to a 64kbit / s channel. After the total calculation the overall bit rate of the STM-1 frame corresponds to 155.520 Mbit / s. The frame repetition time is 125µs. Best structure of  STM-n frame is represented as a rectangle of 9 x 270 x n.

9 x n is the first column frame header and the rest of the frame is internal structure data i.e. payload (including data, indication bits, stuff bits, pointers and management).

All the signals of the STM-N frame is usually transmitted over an optical fiber. The frame is moved row by row (first the first line is then transmitted to the second and so on). At the beginning of each frame, synchronization bytes A1, A2 are transmitted.  

Here it is clear that the multiplexing method of 4 STM-1 streams in STM-1x4 is an interleaving of STM-1 streams to produce STM-4 currents.

A SDH frame with a bit rate of 155.52Mbps is defined in ITU-T Recommendation G.707. This frame is called synchronous transport module (STM), as it is the first level in the hierarchy known as STM-1. 

It is made up of a byte matrix of 9 rows and 270 columns. The transmission is row by row, beginning with a byte in the upper left corner and ending with a byte in the lower right corner. The frame repetition rate is 125us. After pay load calculation we find that each byte in the payload represents a 64kbps channel.

Best represented  STM-n frame structure is as a rectangle of 9 x 270 x n. 9 x n is the first column frame header and the rest of the frame is internal structure data i.e. payload (including data, indication bits, stuff bits, pointers and management).

The STM-N frame is usually transmitted over an optical fiber. The frame is moved row by row (first the first line is then transmitted to the second and so on). I have already mentioned above that at the beginning of each frame, synchronization bytes A1, A2 are transmitted.The multiplexing method of 4 STM-1 streams in STM-1x4 is an interleaving of STM-1 streams to produce STM-4 currents.

STM-1 Frame Structure

STM-1 Frame Structure

An STM-1 frame is constructed in the following way. A basic unit, known as a container (C), is composed of plesiochronous signals. Stuffing is used to give placicchronous signals a fixed bit rate. The clock frequency of the signal is optimized using positive or positive - zero - negative (bit) stuffing. The container bit rate itself is created through an additional fixed stuffing process. The container is primarily synchronized to the STM-N frame.

The insertion of a path overhead (POH) produces a virtual container (VC). The transmission paths through the SDH network are created by these VCs which are the smallest transport units in the SDH. This means that a VC is to be terminated at the end of a path at the SDH / PDH transition point.

VCs are coupled by STM-1 frame to pointers (PTRs). These pointers are used with stepping techniques (byte-stuffing) that compensate for unavoidable phase fluctuations and other disruptions in synchronization operating. Indicators and Vice Chancellors form the Administrative Unit (AU). Finally the Administrative Unit Group (AUG) or STM-1 is formed by adding SOH.

PDH/SDH multiplex procedure

We can explain the PDH/SDH multiplex procedure like below details:

Container C-n: (n=1-4)

By the basic information structure of synchronous system we get the synchronous payload. In this system the input data rate is adapted by fixed stuffing bits. Clock deviations compensated by a stuffing procedure similar to PDH.

Virtual Container VC-n: (n=1-4)

The virtual container (VC) is determined for the information structure with facilities for maintenance and supervising of the system. Virtual container VC also comprises the information (payload) and the POH. In SDH system maintenance signals are path related which spans from end-to-end.

Tributary Unit TU-n: (n=1-3); only for VC-1/2/3

In the SDH system the tributary unit is formed of the virtual container and a pointer to indicate the start of the VC. We get that the pointer position is fixed.

In the SDH system Triburary Unit Group TUG-n: (n=3,4); only if TU's are available. It is created by a group of identical TUs for further processing.

In the SDH system the Administration Unit AU-n: (n=3,4). This element of SDH system comprises a VC and an AU pointer. Within the STM-1 frame the pointer position is fixed.

PDH/SDH multiplex procedure

Bit Interleaved Parity (BIP)

In SDH network Parity bytes providing a means to supervise the transmission 

quality of a life STM-N signal!

The SDH system monitors transmission quality using a method called Bit Interleaved Parity (BIP).

A number of BIP types are used in SDH:

BIP-24 for B2 bytes is formed for every STM-1 frame w.o. RSOH.

BIP-8 for B1 byte for STM-N frame after scrambling and for B3 byte for the VC-3 and VC-4.

BIP-2 for the V5 byte for VC-11, VC-12 and VC-2.

On the transmit side a code word is formed using a fixed encoding rule for a specific bit stream within the STM module. The code word is transmitted in the parity bytes of the following module. The same rule is used to compute an identical word on the receive end. This code word is compared with the incoming code word in the B bytes of the next module. Mismatch indicates transmission error(s).

How to Built a Parity Byte ?

Bit interleaved data field structure of the area covered

Field width: BIP-24: 24 bits (B2)

                  BIP-8:    8 bits (B1, B3)

                  BIP-2:    2 bits (V5)

Column by column parity check for even numbers of "1"

BIP-X (Bit Interleaved Parity-X), X=8 for above example BIP-8. An error detection block is divided in to small sub-blocks with N bits. (For SDH, the block corresponds to STM-N or VC-n frame.)

Kth bits of all sub-blocks in the block are checked in sequence (K1,K2, ・ ・ ・ ,Ki, ・ ・ ・, Kn) and number of 1 is counted. When the counting result is even, the Kth bit of the designated sub-block in the following block is set to 0, for counting result of odd to 1. This process is called even parity. (For SDH, the designated sub-block is B1 or B2s or B3 ・ ・ ・ )

The same procedure is applied to all of X bits in sub-block in parallel. At the receiver, same check is done to the received signal and the result is compared to the indication of the designated sub-block received. Inconsistency means error detection.

Multiple errors in a sequence (K1,K2, ・ ・ ・ ,Ki, ・ ・ ・, Kn) result in no error or only one error for even and odd errors respectively. But the length of sequence is short enough to avoid such inconvenience under practical error rate and almost always the result is correct.

Computing area of B1 BIP is entire bits of STM-N frame. But B2 BIP excludes RSOH area. Bytes in RSOH might be accessesed by regenerators and changed. If RSOH is included, those changes are recognized as errors by B2.

Relationship with the scrambling function

Calculate the parity of B1 after scrambling.

Calculate the parity of B2 before scrambling.(Since scrambling function belongs to the Multiplex section, not to Regenerator section, it is included to the B2 monitoring but excluded from B1.)

Section Trace(J0)

Section Trace (J0)

J0 Section Trace is used for checking the optical fiber or the cable connection between the Nodes that terminate Regenerator section.

At RST block of each station, setting of J0 sending at TX side value and J0 expected value at RX side is necessary. Set Send value to a=abc at  Node A and Expected value to b=abc at Node B.

At Node B, received value a=abc and expected value b=abc are checked to see that they match.  If  they are much (a=b), all received data are outputted into downstream.

If not(a=abc, b=acb),  J0 TIM (Trace Indicator Mismatch) Alarm is reported and all 1 data (AIS) output downstream. In line protection configured Figure above,  set a different value with normal line and protection line.  If misconnection of optical fiber occurs, J0 TIM Alarm will reported, and checking of optical fiber connection can be done.

SDH section trace (J0)

We all know that when we work practically in the field on SDH system, we need Optical Interface Characteristic more than ever. An engineer working on a SDH system,in  practical solution wants to know how optical interface characterization is done. So now I will only talk about the  optical interface characteristic according to STM.

Optical Interface Characteristic

From the table below, you can understand all about optical interface characteristically very well. Now I will add table about optical interface characteristic according to the STMs.

STM-1-4-16 Optical Interface Characteristic

Optical Interface Characteristic

To get more information about SDH, you can open and read the link given below-

What is basic difference between SDH and SONET?

How to check error in SDH Network

SDH Ring Architecture and Switching