1. Introduction to SDH

Before 1970 most of the worlds telephony systems were based on single line, voice frequency, connections over twisted copper pair.

In the early 1970's digital transmission systems began to appear using Pulse Code Modulation (PCM). Alec Reeves of Standard telephone cables (STC) had first proposed this system of transmission in 1937.
PCM enables analogue waveforms such as speech to be converted into a binary format suitable for transmission over long distances via digital systems.
PCM works by sampling the analogue signal at regular intervals, assigning a value to the sample and then transmitting this value as a binary stream.
This process is still in use today and forms the basis of virtually all the transmission systems that we currently use.

Fig 1.1 PCM Block diagram

Engineers soon saw the potential to produce more effective transmission systems by combining several PCM channels together over the same copper pair.

In Europe a standard was adopted where thirty-two, 64kbit/s channels were combined together in a process called "multiplexing", to produce a structure with a transmission rate of 2.048 Mbit/s (usually referred to as 2 Mbit/s).

As demand for telephony services grew, it soon became apparent that the standard
2 Mbit/s signal was not sufficient to cope with the demands of the growing network, and so a further level of multiplexing was devised.
Four, 2 Mbit/s signals were combined together to form an 8 Mbit/s signal (actually 8.448 Mbit/s).
As the need arose, additional levels of multiplexing structure were added to include rates of 34 Mbit/s (34.368) and 140 Mbit/s (139.264).

These transmission speeds are called Plesiochronous Digital Hierarchy or PDH rates.

1.1. Comparison of hierarchical PDH rates

Whilst the European hierarchy was being developed a similar system was being devised in America. Although the same principal was used, a different hierarchical structure was adopted.

Europe North America
Primary 2.048 Mbit/s Primary 1.544 Mbit/s
8.44 Mbit/s 6.132 Mbit/s
34.368 Mbit/s 44.736 Mbit/s
139.264 Mbit/s

Although each of the systems works fine as a stand-alone hierarchy, it does make international inter-connection very difficult and costly.
This was the major reason for the development of a new internationally agreed standard.

1.2. PDH ‘n' Suffix

The PDH rates are often referred to by an ‘n' suffix.
This suffix is also used within SDH to refer to the various different PDH input signals.
The table below shows these suffixes and there associated rates.

‘n' Suffix Bit rate (Kbit/s)
11 1,544
12 2,048
21 6,312
22 8,448
31 34,368
32 44,736
4 139,264

1.3. Disadvantages of PDH networks

Because of the way that PDH signal is structured, it is impossible to extract a single 2 Mbit/s signal from within a higher order (say 140 Mbit/s) stream.
Therefore when a 2 Mbit/s signal needs to be cross-connected between one transmission system and another, it must be de-multiplexed back down to its primary rate first. This forms what is referred to a multiplexer mountain.

As we can see, the multiplexer mountain means that we need to have a lot of expensive equipment just to connect 2 Megs together.
This means that:

· Valuable space is taken up in racks in node sites and more equipment means more maintenance-associated problems.

· Each of the equipment levels is synchronised from a different source and at a different rate. This can lead to clocking problems that can cause errors.

· This equipment must also be jumperd not only at the 2 Mbit/s level for customer interconnection, but also between the various multiplexers that make up the individual transmission system. This leads to large amounts of coax wiring, which is physically very bulky and also relatively high maintenance, due to the fact that the terminating plugs work on a mechanical nature.

· An advantage of PDH is the small overhead of the system. This leads to efficient use of bandwidth. Unfortunately because of this lack of overhead in the structure, management facilities in PDH are severely limited:

· There is no automatic storage of route information so comprehensive and accurate paper records must be kept to avoid problems.

· There is no ability to remotely configure equipment and the alarm monitoring is rudimentary, effectively only reporting loss of inputs.

· Protection of the transmission paths is generally only available using 1+1 protection at the higher PDH levels i.e.140 Mbit/s and above, leaving customer 2 Mbit/s circuit vulnerable to failure.

1.4. Overview of PDH Limitations

· Interconnection between national (European/North American) systems difficult.

· PDH 'multiplexer mountain' is costly and inflexible.

· All hierarchy levels are clocked individually, so slips possible.

· Protection of paths is at higher rates only.

· Management is very limited.

· Relatively prone to faults (by today's standards).

2. Origins of SDH

As can be seen from the previous chapter PDH is a workable but flawed system.
At its conception it used the best available technology and was a giant leap forward in transmission, but with the advent of silicon chips and integrated microprocessors, customer demand soon provided the need to introduce a new and better system.
This new system needed to solve the existing limitations of PDH, but also provide for applications of the future.

The first of the working systems to be introduced was the SYNTRAN (Synchronous Transmission) system from Bellcore. This was however realy only a development system and was soon replaced with SONET (Synchronous Optical Network).
Initially SONET could only carry the ANSI (American National Standards Institute) bit rates i.e. 1.5, 6, 45 Mbit/s.
Since the aim of the project was to provide easier international interconnection, SONET was modified to carry the European standard bit rates of 2, 8, 34 & 140 Mbit/s.

In 1989 the ITU-T (International Telecommunications Union - Telecommunication's standardisation section), published recommendations which covered the standards for SDH. These were adopted in North America by ANSI (SONET is now thought of as a subset of SDH), making SDH a truly global standard.

2.1. Features and Advantages of SDH

· SDH permits the mixing of the existing European and North American PDH bit rates.

· SDH is synchronous. All SDH equipment is based on the use of a single master reference clock source.

· Compatible with the majority of existing PDH bit rates

· SDH provides for extraction/insertion, of a lower order bit rate from a higher order aggregate stream, without the need to de-multiplex in stages.

· SDH allows for integrated management using a centralised network control.

· SDH provides for a standard optical interface thus allowing the inter-working of different manufacturers equipment.

· Increase in network reliability due to reduction of necessary equipment/jumpering.

2.2. Basic SDH Network Topology

In COLT, SDH networks are usually deployed in protected rings. This has the advantage of giving protection to the data, by providing an alternate route for it to travel over in the event of equipment or network failure.

Each side of the ring (known as A and B, or sometimes, East and West), consists of an individual transmit and receive fibre. These fibres will take diverse physical paths to the distant end equipment to minimise the risk of both routes failing at the same time.

The SDH equipment can detect when there is a problem and will automatically switch to the alternate route.

To speed up switching times, although SDH multiplexers transmit on both sides of the ring simultaneously, they only receive on one side at any time. This means that only the receiving end needs to switch, thus reducing the impact of a fault on the customers' data.

3. SDH Principles

3.1. Overview

The SDH standard defines a number of 'Containers' each corresponding to an existing PDH input rate. Information from the incoming PDH signal is placed into the relevant container.
Each container then has some control information known as the 'Path Overhead' (POH) and stuffing bits added to it. The path overhead bytes allow the system operator to achieve end to end monitoring of areas such as error indication, alarm indication and performance monitoring data. Together the container and the path overhead form a 'Virtual Container' (VC).

Due to clock phase differences, the start of the customers' PDH data may not coincide with the start of the SDH frame. Identification of the start of the PDH data is achieved by adding a 'Pointer'.
The VC and its relevant pointer together form a 'Tributary Unit' (TU).

Tributary units are then multiplexed together in stages (Tributary User Group 2 (TUG-2) - Tributary User Group 3 (TUG-3) - Virtual Container 4 (VC-4)), to form an Administrative Unit 4 (AU-4). Additional stuffing, pointers and overheads are added during this procedure.
This AU-4 in effect contains 63 x 2 Mbit/s channels and all the control information that is required.

Finally, Section Overheads (SOH) are added to the AU-4.
These SOH's contain the control bytes for the STM-1 section comprising of framing, section performance monitoring, maintenance and operational control information.
An AU-4 plus its SOH's together form an STM-1 transport frame.

3.2. STM Hierarchy and Container Bit Rates

The first hierarchy level for SDH is set at 155,520 kbit/s/s.
This is known as a Synchronous Transport Module 1 (STM-1).
Higher levels are simply multiples of the first level, which are denoted by the number after the ‘-‘
At present the SDH hierarchy is as follows:

· STM-1: 155,520 kbit/s. (155 Mbit/s)
· STM-4: 622,080 kbit/s. (620 Mbit/s)
· STM-16: 2,488,320 kbit/s. (2.5 Gbit/s)
· STM-64: 9,953,280 kbit/s. (10 Gbit/s)

SDH allows for various PDH input rates to be mapped into containers as shown below:

· Container C11: 1544 kbit/s (1.5 Mbit/s)
· Container C12: 2048 kbit/s (2 Mbit/s)
· Container C2: 6312 kbit/s (6 Mbit/s)
· Container C3: 49,536 kbit/s (45 & 34 Mbit/s)
· Container C4: 139,264 kbit/s (140 Mbit/s)

As can be seen from this chart, the only PDH rate that is not supported by SDH is 8 Mbit/s.

3.3. Full SDH Multiplexing Structure

The diagram shows the complete SDH multiplexing structure. PDH signals enter on the right into the relevant container and progress across to the left through the various processes.

3.4. 2 Mbit/s Multiplexing Structure

The diagram below shows the structure for a 2 Mbit/s circuit. The relative bit rate and process is shown for each stage

3.5. Graphical SDH Multiplexing Structure

3.6. STM-1 Frame Structure

The STM-1 transport frame has a duration of 125ms. It contains 2430 bytes of information. Each byte contains 8 data bits (i.e. a 64kbit/s channel).
The number of frames per second is 1 second / 125ms = 8000 Frames per second.
Therefore the rate transmitted to line is: -

8 bits x 2430 bytes x 8000 per second = 155,520,000 bits/s or 155 Mbit/s.

As each frame consists of 2430 bytes, this would prove very difficult to show as a diagram on a page. To get round this, we show the frame chopped up into 9 segments, stacked on top of each other as shown in the diagram below.
The bits start at the top left with byte number one and are read from left to right and top to bottom. They are arranged as 270 columns across and 9 rows down.
Therefore byte 270 is the byte in column 270, row 1. Byte 271 is in column 1, row 2 and byte 2430 is located at column 270, row 9 etc.

Further explanations of the areas within the STM-1 frame are given in Appendix A.

3.7. SDH Concatenation

The SDH frame can be thought of as an articulated lorry. The data to be transported is placed in the VC-4 'Container'. This is then hitched to the SOH 'Cab unit' that 'drives' the data to its destination.

The maximum carrying capacity of the vehicle is determined by the size of the 'container'. Therefore although the SDH signal is 155 Mbit/s in size, the largest single circuit that can be transmitted at any one time by the customer is limited to the size of the VC-4 i.e. 140 Mbit/s.

When using higher rates of SDH (STM-4, STM-16 etc), multiple 'containers' and 'cabs' are added one after another, to form a bigger vehicle. The customer is still limited to a single circuit size of 140 Mbit/s however, because each individual 'container' is still the same size (140 Mbit/s). They can however transmit multiple 140 Mbit/s circuits simultaneously.
The diagram below represents the standard STM-4 structure

This limitation of 140 Mbit/s per individual circuit is not a particularly efficient way of managing bandwidth and a method of combining 'containers' together has been developed which is called 'Concatenation'.
The diagram below represents an STM-4 concatenated structure (VC-4-4C).

Concatenated paths are commonly defined as VC-4-xC circuits (where x is size of the concatenation), as shown below:

· STM-4 concatenation (written as VC-4-4c), provides a single circuit with a bit rate of approximately 600M (actually 599.04 Mbit/s)

· STM-16 concatenation (written as VC-4-16c), provides a single circuit with a bit rate of approximately 2.2G (actually 2.2396160 Gbit/s)

· STM-64 concatenation (written as VC-4-64c), provides a single circuit with a bit rate of approximately 10G (actually 9.584640 Gbit/s)

· STM-256 concatenation (written as VC-4-256c), provides a single circuit with a bit rate of approximately 38G (actually 38.338560 Gbit/s)

4. Appendix A - SDH Structure Details

The following page's shows an overview of the process followed by a 2 Mbit/s PDH input signal until it becomes part of an STM-1 frame.
It details the individual stages and should be used with reference to the preceding SDH structure diagrams

4.1. Mapping of a 2 Mbit/s PDH signal into a C-12.

The 2 Mbit/s PDH input signal is mapped into a Container 12 (C-12). The input frame consists of 32 bytes of information and this fits directly into the C-12 as shown.

4.2. Mapping of a C-12 into a VC-12.

When mapping a C-12 into a VC-12, we need to add four bytes of overhead control information. This process takes place over 4 consecutive frames as we can only add one byte per frame of customers' data: -

Frame number One has two bytes of fixed stuffing added to it. One byte is added at the start and one byte at the end. It then has one byte of overhead control information added to the start. This byte of over head is called the V5 byte and is known as the VC-12 Path OverHead (POH).

Two of the more important features of the V5 byte are:

· BIP-2 is Bit Interleaved Parity Check-2. This looks at the data in the C-12. It counts all of the binary one's that it sees in the odd bit positions (i.e. bits 1,3,5,7 etc) and then it counts all of the binary one's that it sees in the even bit positions (i.e. bits 2,4,6,8 etc). This BIP-2 is then recalculated at the distant end. If the count is different, then some bit corruption has occurred.

· FEBE is Far End Bit errors. This bit is set correspondingly to the result of the BIP-2 check. If errors are received at the distant end then there needs to be a mechanism for informing the sender of the problem.

Frame number Two has two bytes of fixed stuffing added to it. One byte is added at the start and one byte at the end. It then has one byte of overhead control information added to the start. This control byte in frame 2 is the Lower Order Path Trace or J2 byte. J2 is used to check continuity of a 2 Mbit/s path. It is currently not supported by most manufacturers.

Frame number Three has two bytes of fixed stuffing added to it. One byte is added at the start and one byte at the end. It then has one byte of overhead control information added to the start. This control byte N2, in frame 3 is called the Network Operator or Tandem Control byte. N2 is used to transmit performance-monitoring information where the circuit spans differing vendors networks (i.e. Colt to BT).

Frame number Four has one byte of fixed stuffing added to the end. It also has one byte of variable stuffing added to the start. It then has one byte of overhead control information added to the start. This control byte in frame 4 is called K4.
K4 is used for 2 Mbit/s Automatic Protection Switching or APS.
APS is used to automatically switch a single 2 Mbit/s circuit to its alternate path if a fault condition occurs.

4.3. Mapping of a VC-12 into a TU-12 signal.

The V5 byte must be seen by the distant end for it to detect the start of the 2 Mbit/s signal and hence the start of the customers data.
There must be some mechanism therefore to ensure that the distant end can detect V5.
This is achieved by adding four overhead bytes to the multiframe, which together form a calculated byte count to the start of V5. This is called a pointer value and is known as the TU Pointer.

There are four pointer bytes called V1, V2, V3 and V4, which are used to calculate the location of V5.

4.4. Multiplexing of TU-12 into a TUG-2

As can be seen from the previous section, each VC-12 consists of 36 bytes of information. These 36 bytes fill up exactly 4 columns of the STM-1 frame.
3 separate TU-12's are directly mapped together to form a TUG-2.

The 3 TU-12's will fit exactly into 12 columns of the STM-1 frame as shown below:

4.5. Mapping of a TUG-2 into a TUG-3 signal.

The mapping of TUG-2's into TUG-3's uses fixed column interleaving and is shown below.

4.6. Mapping of a TUG-3 into a VC-4 signal.

SDH provides for fixed mapping from TUG-3 into a VC-4 container as shown in the diagram.

The three TUG-3's are column interleaved to form the VC-4 payload.
At this point two columns of fixed stuffing are added and the 'VC-4 Path Overhead' is added to the start.

4.7. VC-4 Path Overhead.

The VC-4 Path Overhead consists of one whole column of nine bytes as shown below. It forms the start of the VC-4 payload area.
The POH contains control and status messages (similar to the V5 byte) at the higher order.

The function of some of the more important bytes is as follows:

· J1 - Higher Order Path trace. This byte is used to provide a fixed length user configurable string, which can be used to verify the connectivity of 140 Mbit/s connections. It is not supported by all manufactures.

· B3 - Bit Interleaved Parity Check (BIP-8). This byte provides an error monitoring function for the VC-4 payload.

· G1 - Higher Order Path Status. This byte is used to transmit back to the distant end, the results of the BIP-8 check in the B3 byte

· K3 -Automatic protection Switching (APS). K3 provides for automatic protection switching control with VC-4 payloads. Similar to the K4 bits in the 2 Mbit/s overheads

4.8. Mapping of a VC-4 into an STM-1 frame.

An AU pointer is added to the VC-4 to form an AU-4 or Administrative Unit -4.
The AU pointers are in a fixed position within the STM-1 frame and are used to show the location of the first byte of the VC-4 POH.

The AU-4 is then mapped directly into an AUG or Administrative Unit Group, which then has the Section Overheads or SOH, added to it.
These section overheads provide STM-1 framing, section performance monitoring and other maintenance functions pertaining to the section path.

The VC-4 payload, plus AU pointers and Section Overheads, together form the complete STM-1 transport frame.

4.9. STM-1 Section Overheads

The STM-1 Section Overhead (SOH) consists of nine columns by nine rows as shown below. It forms the start of the STM-1 frame.
The SOH contains control and status messages (similar to the V5 and VC-4 POH), at the optical fibre level.

Some of the most important bytes within the Section Overheads are:

· A1 & A2 - STM-1 Frame Alignment. These 6 bytes are used for STM-1 frame alignment. They are the first bytes transmitted.
Frame alignment takes place over three STM-1 frames.

· J0 - STM-1 Section Path Trace. This byte is used to provide a fixed length user configurable string, which can be used to verify network topology connections.
It is not supported by all manufactures.

· B1 - Byte Interleaved Parity Check 8 (BIP-8). This byte provides an error monitoring function for the entire STM-1 frame after encoding.

· B2 - Byte Interleaved Parity Check 24 (BIP-24). These 3 bytes provide an error monitoring function for the STM-1 frame before encoding.
A comparison between the BIP-8 and BIP-24 checks reveal if there were any encoding errors.

· D1 to D12 - Data Communications Channel (DCC). These bytes provide a data channel for the use of network management systems.

· K1 - Automatic protection Switching (APS). This byte is used to perform automatic protection switching of the optical fibre.

· X - Reserved. These bytes are reserved for national use.

· All Unmarked bytes are reserved for future international standardisation.

5. Appendix B - Circuit Labelling and Rates look up table

6. Appendix C - The Electromagnetic Spectrum

Fiber optic transmission makes use of three optical windows (850, 1300, 1550 nm), where the attenuation characteristics of silica fibers are the lowest.
670nm light is used for visible fault location, with 780 nm and 1625 nm lasers used for shorthaul and long haul applications respectivly.