Transmission Technologies (sdh&pdh)

Introduction to Transmission Technologies A Little Bit of History. Until about 1960 all switching and transmission systems were analog, so voice conversations between telephone handsets were achived by carrying analogue signals with Frequency Division Multiplexing (FDM) used to combine signals on coaxial cables. In the early 1960s, digital transmission systems began to appear, utilising a method known as Pulse Code Modulation (PCM).The prinsiples of PCM:

PCM allows analogue waveform to be represented in binary form.So using PCM it was possible to represent a standart analogue telephone signals as a 64Kbps digital bit stream. The method used to combine multiple 64 Kbps channels into a single high speed bit stream is known as Time Division Multiplexing (TDM).

Multiplexing means taking a certain number of signals and putting them together. Theoreticaly, N into 1 multiplexer must provide transmission rate of NxV, where V - rate of single "tributary" stream. As a rule, a base rate digital signal (DS-0) transmited at rate of 64Kbps is used as a "tributary" stream. So with a help of a first order multiplexer (N:1) it's possible to form digital streams of rate at Nx64Kbps. Then using second, third and so on order multiplexers

it's possible to form transmission rate hierarchies. So in 1965 the US standart of 24 voice signals multiplexed together with one "framing" bit to form a 1,544Mbps signal called DS-1 was born. In 1968, Europeans deviced a similar standart E-1, with 30 voice channels plus a channel for "framing" and a channel for signalling, for a total of 32x64Kbps=2,048Mbps.As demand for voice telephony increased, and levels of traffic in the network grew ever higher, futher levels of multiplexing were added to the European standart at 8Mbps, 34Mbps, 140Mbps and 565Mbps to produce a full hierarchy of bit rates. The similar work occured in North America to develop their own hierarchy. The next figure compares the North America and Europe transmission hierarchies.

What is Plesiochronous Digital Hierarchy (PDH) ? The multiplexing hierarchy described above appears simple enough in principle, but in practice there are complications.At each step, the multiplexer has take into account the fact that the tributaries are likely to have been created by different pieces of equipment, so their clock are all slightly different. So the method called the Plesiochronous Digital Hierarchy was developed. "Plesiochronous" from Greek means "almost synchronous".

All the incoming channels must be brought up to the same bit rates.It achieved by adding "dummy" information bits or stuffing bits.The multiplexer also has the mechanism to signal to demultiplexer that it has performed stuffing, and so the demultiplexer must know which bit to throw out. There are two main problems with the PDH method: • One is that each time it is necessary to pick out or insert a stream from a high-order stream, it is necessary to perform all the operations of all multiplexers that created the high-order stream. This operation is called Add/Drop. For example, in order to access a single 2Mbps line within say a 140Mbps channel, the last must be completely demultiplexed to its 64 constituent 2Mbps lines via 34 and 8Mbps.Once the required 2Mbps line has been identified and extracted, the channels must then be remultiplexed back up to 140Mbps. This is shown in the next figure:

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Another problem is that these multiplexers create a network in which measuring performance, rerouting signals after network failures, and managing remote network elements from work centers are all extremely difficult.

Synchronous Digital Hierarchy. Contents. • •

STM-1 Structure. Payload Area • AU Pointer Area • Section Overhead Area • Higher-Order Multiplexing. • SDH Structure.(ETSI) In order to overcome the problems associated with PDH, in particular the inability of it to extract individual circuits from high capacity systems without having to demultiplex the whole system, the new method of multiplexing, called Synchronous Digital Hierarchy (SDH) in Europeand the Synchronous Optical Network (SONET) was developed.

STM-1 Structure The basic transmission rate defined in the SDH standarts is 155,520Mbps (STM-1). STM-1 (Synchronous Transport Module 1) is the format of the first (lowest) level of synchronous hierarchy.STM-1 frame consist of 2430 bytes which can be considered as a structure of 270 columns x 9 lines. This corresponds to a frame duration of 125ms. The bytes are transmitted one row at a time starting from the point labeled "0ms". The STM-1 frame is devided into three main sections:

Payload Area. AU Pointer Area. Section Overhead Area.

Payload Area. Signals from all levels of PDH can be accommondated in synchronous network.But PDH signals are not just copied into the STM-1 frame as they arrive. They are packaged into the appropriate synchronous "container". Then the Path Overhead (POH) is added to the "container" to form the relevant Virtual Container. There are many different VCs, one for each type of PDH signal. The POH is a single column of nine bytes and it provides information for use in end-to-end management of a synchronous path. The process of loading containers, and attaching overhead is repeated at several levels in the SDH, resulting in the "nesting" of a smaller VCs within larger ones, until the largest size of VC (VC-4 in Europe ) is filled and this is then loaded into the payload of the STM-1 frame.

AU Pointer Area. After the POH is added and the STM-1 payload area is filled by the largest unit available, a pointer is generated which indicates the start of the VC relative to the STM-1 frame. VC together with the pointer is called Tributary Unit (TU), if it carries lower order tributaries, or Administrative Unit (AU) for higher order. The use of pointers in the STM-1 frame structure means that PDH signals can be accommondated within the synchronous network without the use of buffers.This is because the tributaries to a multiplexer each have a frame that is not aligned in time with the other tributaries, nor with the frame of the output stream. In PDH, the multiplexer does not even need to know where this frame is in time;that is the task of the demultiplexer in the lower hierarchical level. This is exactly why Add/Drop operations are so expensive. To resolve this problem, the SDH multiplexer finds where the frame starts in each tributary. It calculates a pointer that tells where in the STM-1 frame it has placed the tributary frame. If the tributary frame slips with respect to the STM-1 frame, the system just changes the pointer. Next figure shows a 140Mbps E-4 signal in an STM-1 frame. It begins midway through the STM-1 frame and ends midway through the next one.A pointer indicates its position.

The result of this is that in any data stream it's possible to identify individual tributary channels,thus overcoming one of the main drawbacks of the PDH.

Section Overhead Area. The Section Overhead (SOH) bytes are used for communication between adjacent pieces of synchronous equipment.They perform a variety of management and administration facilities.

Higher-Order Multiplexing. When a higher transmission rate than 155,52Mbps of STM-1 is required, it's achieved by using a relatively straightforward byte-interleaved multiplexing scheme. Next figure shows how to construct the next level, called the STM-4 at 622Mbps.

The STM-16 is created in the same way as the STM-4, by interleaving four STM-4 signals.This is2,4Gbps rate, the highest one defined so far.

SDH structure(ETSI). Next figure shows the subset of the standarts used in countries which follow European Telecommunications Standarts Institute (ETSI) recommendations.

The European formats called C-12, C-11, C-3 and C-4 are mapped as shown in figure into Virtual Containers. The TU and TUG (Tributary Unit Group) are intermediate stages for reaching the final VC-4.

Differences between SDH and SONET.

The CCITT SDH recommendations can be viewed as the worldwide standarts for synchronous transmission. Within these standarts, however, there is some room for manoeuvre. As a result, the ETSI implementation of SDH, used in Europe, differs in some details from the North American implementation. This is because the North American implementation is developed from the ANSI SONET standarts. Thus the best way of viewing the differences between SONET and SDH is to consider SONET as a subset of the worldwide SDH standarts. The most significant differences between SONET and SDH occur at the sub STM-1 level.

The first level of the SONET hierarchy is referred to as STS-1 (for an electrical signal) or OC-1 (for an optical signal) and corresponds to a bit rate of 51,84Mbps.(STS=Synchronous Transport Signal, OC=Optical Carrier).In much the same way as an STM-1 frame can be considered as a 270 bytes x 9 line structure , an STS-1 frame can be viewed as 90 bytes x 9 lines. The section overhead constitutes the first three columns of the STS-1 frame. The following are the levels defined by the SONET hierarchy: OC-1/STS-1 51,84Mbps OC-3/STS-3 155,52Mbps ( = STM-1 ) OC-9/STS-9 466,56Mbps OC-12/STS-12 622,08Mbps ( = STM-4 ) OC-18/STS-18 933,12Mbps OC-24/STS-24 1244,16Mbps OC-36/STS-36 1866,24Mbps OC-48/STS-48 2488,32Mbps ( = STM-16 ) When the basic STS-1 signal is multiplexed to STS-3 it becomes identical in frame format to STM-1. There are, howerver, slight differences in pointer processing.

Benefits of a Synchronous Networks. Singhronous networks offers a number of benefits. • The more efficient Add/Drop operations, together with powerful network management capabilities, lead to greater ease in provisioning of high bandwidth lines for new multimedia services, as well as ubiquitous access to those services. • The network management capability enable the failure of links or even nodes to be identified immediately. • Synchrononous networks are fully software controllable. • In synchronous networks it's possible to dynamically allocate network capacity, or bandwidth, on demand.

CCITT Recommendation Relating to Synchronous Network. The CCITT recommendations which relate to synchronous network are listed below: G.707 - Synchronous Digital Hierarchy Bit Rates. G.708 - Network Node Interface for the Synchronous Digital Hierarchy.

G.709 - Synchronous Multiplexing Structure. G.781 - Structure of Recommendations on Multiplexing Equipment for the SDH G.782 - Types and General Characteristics of SDH Multiplexing Equipment. G.783 - Characteristics of SDH Multiplexing Equipment Functional Blocks. G.784 - SDH Management.