Download SDH NETWORK IN TELECOM and more Summaries Telecommunications Engineering in PDF only on Docsity!
E1 E2EB Version 3.0 April 2021 Page 15 of 147
2 SDH
2.1 LEARNING OBJECTIVE
After reading this unit, you should be able to understand:
- Limitation of PDH signals.
- Concept of SDH.
- Multiplexing Structure of STM.
2.2 INTRODUCTION
With the introduction of PCM technology in the 1960s, communications networks were gradually converted to digital technology over the next few years. To cope with the demand for ever higher bit rates, a multiplex hierarchy called the Plesiochronous digital hierarchy (PDH) evolved. The bit rates start with the basic multiplex rate of 2 Mbit/s with further stages of 8, 34 and 140 Mbit/s. In North America and Japan, the primary rate is 1. Mbit/s. Hierarchy stages of 6 and 44 Mbit/s developed from this. Because of these very different developments, gateways between one network and another were very difficult and expensive to realize. PCM allows multiple use of a single line by means of digital time- domain multiplexing. The analog telephone signal is sampled at a bandwidth of 3.1 kHz, quantized and encoded and then transmitted at a bit rate of 64kbit/s. A transmission rate of 2048 kbit/s results, when 30 such coded channels are collected together into a frame along with the necessary signaling information. This so-called primary rate is used throughout the world. Only the USA, Canada and Japan use a primary rate of 1544 kbit/s, formed by combining 24 channels instead of 30. The growing demand for more bandwidth meant that more stages of multiplexing were needed throughout the world. A practically synchronous (or, to give it its proper name: plesiochronous) digital hierarchy is the result. Slight differences in timing signals mean that justification or stuffing is necessary when forming the multiplexed signals. Inserting or dropping an individual 64 kbit/s channel to or from a higher digital hierarchy requires a considerable amount of complex multiplexer equipment.
Traditionally, digital transmission systems and hierarchies have been based on multiplexing signals which are plesiochronous (running at almost the same speed). Also, various parts of the world use different hierarchies which lead to problems of international interworking; for example, between those countries using 1.544 Mbit/s systems (U.S.A. and Japan) and those using the 2.048 Mbit/s system. To recover a 64 kbit/s channel from a 140 Mbit/s PDH signal, it’s necessary to demultiplex the signal all the way down to the 2 Mbit/s level before the location of the 64 kbit/s channel can be identified. PDH requires “steps” (140-34, 34-8, 8-2 demultiplex; 2-8, 8-34, 34-140 multiplex) to drop out or add an individual speech or data channel (see Figure 1A).
2.3 PLESIOCHRONOUS DIGITAL MULTIPLEXING
PDH technology (Plesiochronous Digital Hierarchy) is based on pulse code modulation (PCM). In pulse code modulation a multiple-shift usage of a transmission link is enabled by TDM (time division multiplexing). PDH technology enables with its hierarchical structures the implementation of networks with transmission capacities of up to 140 Mbit/s. In applications with cross connecting on bit-level or with a demand of special interfaces, PDH system technology is in use even today.
E1 E2EB Version 3.0 April 2021 Page 16 of 147
Traditionally, transmission systems have been asynchronous, with each terminal in the network running on its own clock. In digital systems, clocking (timing) is one of the most important considerations. Timing means using a series of repetitive pulses to keep the bit rate of the data stream constant and to indicate where the ones and zeros are located in a data stream. Because these clocks are free running and not synchronized, large variations occur in the clock rate and thus the signal bit rate.
Asynchronous multiplexing uses multiple stages; lower-rate signals are multiplexed, and extra bits are added (bit-stuffing) to account for the variations of each individual stream and combined with other bits (framing bits) to form higher-level bit rates. Then bit-stuffing is used again to produce even higher bit rates. At the higher asynchronous rate, it is impossible to access these signals without multiplexing.
Fig. 1(A) Plesiochronous Digital Hierarchies (PDH) The Plesiochronous Digital Hierarchy (PDH) signals have the essential characteristics of time scales or signals such that their corresponding significant instants occur at nominally the same rate. The prefix plesio, which is of Greek origin, means “almost equal but not exactly,” meaning that the higher levels in the CCITT (ITU today) hierarchy are not an exact multiple of the lower level. Any variation in rate is constrained within specified limits. The PDH systems belong to the first generation of digital terrestrial telecommunication systems in commercial use.
Before SDH transmission networks were based on the PDH hierarchy. 2 Mbit/s service signals are multiplexed to 140 Mbit/s for transmission over optical fiber or radio. Multiplexing of 2 Mbit/s to 140 Mbit/s requires two intermediate multiplexing stages of 8 Mbit/s and 34 Mbit/s. Multiplexing of 2 Mbit/s to 140 Mbit/s requires multiplex equipment known as 2nd, 3rd and 4th order multiplexer.
E1 E2EB Version 3.0 April 2021 Page 18 of 147
3. HIGH AVAILABILITY AND CAPACITY MATCHING
With SDH, network providers can react quickly and easily to the requirements of their customers. For example, leased lines can be switched in a matter of minutes. The network provider can use standardized network elements that can be controlled and monitored from a central location by means of a telecommunications network management (TMN) system.
4. RELIABILITY
Modern SDH networks include various automatic back-up and repair mechanisms to cope with system faults. Failure of a link or a network element does not lead to failure of the entire network which could be a financial disaster for the network provider. These back-up circuits are also monitored by a management system.
5. FUTURE-PROOF PLATFORM FOR NEW SERVICES
Right now, SDH is the ideal platform for services ranging from POTS, ISDN and mobile radio through to data communications (LAN, WAN, etc.), and it is able to handle the very latest services, such as video on demand and digital video broadcasting via ATM that are gradually becoming established.
6. INTERCONNECTION
SDH makes it much easier to set up gateways between different network providers and to SONET systems. The SDH interfaces are globally standardized, making it possible to combine network elements from different manufacturers into a network. The result is a reduction in equipment costs as compared with PDH.
7. SUPPORT PDH PAYLOADS
SDH supports the transmission of existing PDH payloads, other than 8Mbit/s. Most importantly, because each type of payload is transmitted in containers synchronous with the STM-1 frame, selected payloads may be inserted or extracted from the STM-1 or STM-N aggregate without the need to fully hierarchically de-multiplex as with PDH systems.
2.5 SDH RATES
SDH is a transport hierarchy based on multiples of 155.52 Mbit/s. The basic unit of SDH is STM-1. Different SDH rates are given below:
STM-1 = 155.52 Mbit/s STM-4 = 622.08 Mbit/s STM-16 = 2588.32 Mbit/s STM-64 = 9953.28 Mbit/s
Each rate is an exact multiple of the lower rate therefore the hierarchy is synchronous.
2.6 THE STM-1 FRAME FORMAT
The S.D.H. standards exploit one common characteristic of all PDH networks namely 125 micro seconds duration, i.e. sampling rate of audio signals (time for 1 byte in 64 k bit per second). This is the time for one frame of SDH. The frame structure of the SDH is represented using matrix of rows in byte units as shown. As the speed increases, the number
E1 E2EB Version 3.0 April 2021 Page 19 of 147
of bits increases and the single line is insufficient to show the information on Frame structure. Therefore, this representation method is adopted. How the bits are transmitted on the line is indicated on the top of the figure.
The Frame structure contains 9 rows and number of columns depending upon synchronous transfer mode level (STM). In STM-1, there are 9 rows and 270 columns. The reason for 9 rows arranged in every 125 micro seconds is as follows:
For 1.544 Mbit PDH signal (North America and Japan Standard), there are 25 bytes in 125 micro second and for 2.048 Mbit per second signal, there are 32 bytes in 125 micro second. Taking some additional bytes for supervisory purposes, 27 bytes can be allotted for holding 1.544 Mbit per second signal, i.e. 9 rows x 3 columns. Similarly, for 2.048 Mbit per second signal, 36 bytes are allotted in 125 micro seconds, i.e. 9 rows x 4 columns. Therefore, it could be said 9 rows are matched to both hierarchies.
The standardized SDH transmission frames, called Synchronous Transport Modules of Nth hierarchical level (STM-N). The STM-1 frame is the basic transmission format for SDH. The frame lasts for 125 microseconds; therefore, there are 8000 frames per second.
A frame with a bit rate of 155.52 Mbit/s is defined in ITU-T Recommendation G.707. This frame is called the synchronous transport module (STM). Since the frame is the first level of the synchronous digital hierarchy, it is known as STM-1. Figure 4 shows the format of this frame. It is made up from a byte matrix of 9 rows and 270 columns. Transmission is row by row, starting with the byte in the upper left corner and ending with the byte in the lower right corner. The frame repetition rate is 125 ms., each byte in the payload represents a 64 kbit/s channel. The STM-1 frame is capable of transporting any PDH tributary signal.
The first 9 bytes in each of the 9 rows are called the overhead. G.707 makes a distinction between the regenerator section overhead (RSOH) and the multiplex section overhead (MSOH). The reason for this is to be able to couple the functions of certain overhead bytes to the network architecture. The table below describes the individual functions of the bytes.
E1 E2EB Version 3.0 April 2021 Page 21 of 147
Fig. 3 Section Overhead
The table below describes the individual functions of the bytes.
Table 1: Overhead bytes and their functions
PAYLOAD AREA
Information payload area is the place where information about various services is stored in the SDH frame structure. Horizontal columns 10 × N~270 × N, and vertical rows 1~9 belong to the information payload area. In it, there are still some Path Overhead (POH) bytes transmitted as part of the payload in a network and these bytes are mainly used for the monitor, management and control of the path performance.
ADMINISTRATIVE UNIT POINTER (AU-PTR) AREA
AU PTR is a kind of indicator, mainly used to indicate the accurate position of the first byte of information payload in the STM-N frame, so that the information can be correctly decomposed at the receiving end. It is located at the fourth row of 1~9 × N columns in the STM-N frame structure. The adoption of the pointer mode is an innovation of SDH. It can perform multiplex synchronization and STM-N signal frame locating in the quasi- synchronization environment.
E1 E2EB Version 3.0 April 2021 Page 22 of 147
PATH OVERHEAD
Path Overhead (POH) bytes are mainly used for the monitor, management and control of the path performance. A distinction is made between two different POH types:
VC-11/12 POH
The VC-11/12 POH is used for the low-order path. ATM signals and bit rates of 1. Mbit/s and 2.048 Mbit/s are transported within this path.
VC-3/4 POH
The VC-3/4 POH is the high-order path overhead. This path is for transporting 140 Mbit/s, 34 Mbit/s and ATM signals.
2.7 NETWORK ELEMENTS OF SDH
Figure 4 is a schematic diagram of a SDH ring structure with various tributaries. The mixture of different applications is typical of the data transported by SDH. Synchronous networks must be able to transmit plesiochronous signals and at the same time be capable of handling future services such as ATM.
E1 E2EB Version 3.0 April 2021 Page 24 of 147
Fig. 6: ADM
3. REGENERATORS
Regenerators as the name implies, have the job of regenerating the clock and amplitude relationships of the incoming data signals that have been attenuated and distorted by dispersion. They derive their clock signals from the incoming data stream. Messages are received by extracting various 64 kbit/s channels (e.g. service channels E1, F1) in the RSOH (regenerator section overhead). Messages can also be output using these channels.
Fig. 7: Regenerator
4. DIGITAL CROSS-CONNECT (DXC)
This network element has the widest range of functions. It allows mapping of PDH tributary signals into virtual containers as well as switching of various containers up to and including VC-4. It permits switching of Transmission lines with different bit rates.
Fig.8: DXC
5. NETWORK ELEMENT MANAGER
Telecommunications management network (TMN) is considered as a further element in the synchronous network. All the SDH network elements mentioned so far are software- controlled. This means that they can be monitored and remotely controlled, one of the most important features of SDH.
