Overview of Plesiochronous Digital Hierarchy And of Synchronous Digital Hierarchy (PDH and SDH)
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 1.5 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 64 kbit/s. PDH and SDH PDH and SDH PDH and SDH PDH and SDH
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 1).
The main problems of PDH systems are:
- Homogeneity of equipment
- Problem of Channel segregation
- The problem cross connection of channels
- Inability to identify individual channels in a higher-order bit stream.
- Insufficient capacity for network management;
- Most PDH network management is proprietary.
- There’s no standardised definition of PDH bit rates greater than 140 Mbit/s.
- There are different hierarchies in use around the world. Specialized interface equipment is required to interwork the two hierarchies.
1988 SDH standard introduced with three major goals:
– Avoid the problems of PDH
– Achieve higher bit rates (Gbit/s)
– Better means for Operation, Administration, and Maintenance (OA&M)
SDH is an ITU-T standard for a high capacity telecom network. SDH is a synchronous digital transport system, aim to provide a simple, economical and flexible telecom infrastructure. The basis of Synchronous Digital Hierarchy (SDH) is synchronous multiplexing – data from multiple tributary sources is byte interleaved.
SDH brings the following advantages to network providers:
- High transmission rates
Transmission rates of up to 40 Gbit/s can be achieved in modern SDH systems. SDH is therefore the most suitable technology for backbones, which can be considered as being the super highways in today’s telecommunications networks.
- Simplified add & drop function
Compared with the older PDH system, it is much easier to extract and insert low-bit rate channels from or into the high-speed bit streams in SDH. It is no longer necessary to demultiplex and then remultiplex the plesiochronous structure.
- 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.
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.
- 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.
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.
Figure 2 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.
Current SDH networks are basically made up from four different types of network element. The topology (i.e. ring or mesh structure) is governed by the requirements of the network provider.
- 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.
- Terminal multiplexers Terminal multiplexers are used to combine plesiochronous and synchronous input signals into higher bit rate STM-N signals.
- Add/drop multiplexers (ADM) Plesiochronous and lower bit rate synchronous signals can be extracted from or inserted into high speed SDH bit streams by means of ADMs. This feature makes it possible to set up ring structures, which have the advantage that automatic back-up path switching is possible using elements in the ring in the event of a fault.
- Digital cross-connects (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.
- Network element management The 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. Network management is described in more detail in the section “TMN in the SDH network”
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.
The STM-1 frame format
The standardized SDH transmission frames, called Synchronous Transport Modules of Nth hierarchical level (STM-N).
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 2 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.