Lecture 9

This lecture is divided into hyperlinked sections

Plesiochronous Digital Hierarchy PDH
Advantages of SONET/ SDH
STS-1 and STM-1 Comparison
Payload Envelopes
SONET Virtual Tributaries
SDH Virtual Containers
SONET Overhead
Signal Labels
Payload Pointers
High Rate SONET/ SDH
Concatenated STS
SONET Optical Carrier Levels
Comparison of SONET and SDH
SONET/ SDH Network Hardware
SONET/ SDH Management
Performance Management
Automatic Protection Switching
Very High Datarate SONET/ SDH
Private SONET/ SDH networks
SONET Private lines
Fibre optic signals and properties
Noise on Transmission Lines
Signal Propagation in FO links


This lecture will introduce the idea of SONET/ SDH and why it was needed.
It will examine the structuring of the data on the fibre optic cabling.
It will see differences between the North American networks and the rest of the world.
It will look at the hardware needed to form an optical network.
It will look at a few of the properties of fibre optic cabling.

Plesiochronous Digital Hierarchy PDH

Before SONET/ SDH there was a mismatch of cable data-rates across the world and this made inter-continental data transfer a complex process. This was known as the plesiochronous digital hierarchy or PDH. The older PDH technologies were optimised for speech transmission rather than data which often called for separate networks for digital and analogue signals.

The different networks had grown independently of the other networks operating in different parts of the world. This meant that each network had its own hierarchy of line capacities. For data transport within a network, there was no problem, but sending data from Europe to America or Japan was a problem because the differing line capacities would not match gracefully at inter-network interfaces, thus a scalable solution was sought for by the public switched network (PSN) providers.

Figure 9.1 The PDH incompatibility at the mid-span meet

What the PSN providers wanted was a universal digital signaling interface to resolve difficulties with compatibility between North America, Europe and the Far East.

To illustrate the mismatch of PDH across the world, an example of line capacities around the world can be seen in the table below.

Table 9.1 Plesiochronous Digital Hierarchy

Synchronous Optical Network/ Synchronous Digital Hierarchy

The solution reached by the carrier organisations was SONET/ SDH and is the name given to the fibre-optical carrier system used today in most of the world. It provided a scalable digital hierarchy. SONET/ SDH is SONET/ SDH was developed to provide

SONET stands for Synchronous Optical NETwork and was developed by the North American carriers (ANSI is the standards body). SDH is Synchronous Digital Hierarchy (ITU-T is the standards body).

The table below shows how the data capacities now relate.

Table 9.2 The scalable digital hierarchy

Above 150 Mbit/sec, SONET and SDH are basically the same, however the names given to the link types are different. Below 150 Mbit/sec there are major differences in data rates.

The interoperability provided by the SONET/ SDH scheme has accelerated the use of fibre-optic technology which provides extremely long inter-repeater lengths and virtually unlimited bandwidth. At the midspan meet (where SONET meets SDH) the fibre optic signals are totally compatible .The compatibility also allows for multivendor working too.

Advantages of SONET/ SDH

· Scalability is provided in terms of user number, size and data rate and there is the capability to adapt to new needs and services.
· There is provision for standardised Operations, Administration and Management (OAM) functions and procedures.
· SONET/ SDH can provide transparent transport for digital signals of any kind, isochronous and non real-time data.
· Individual signals may be inserted or removed from the line at any point. This was not possible with the older PDH TDM technology. (Synchronous multiplexing is used to allow the use of Add/ Drop Multiplexers (ADMs) which are devices inserted in the fibre links to extract individual signals from the multiplexed signal stream). 


SONET/ SDH is predominantly a physical layer technology, thus other network functions must be provided by protocol stacks running above the SONET/ SDH layers.

Figure 9.2 Depicting both the OSI model and ATM using SONET/ SDH 


The SONET and SDH reference models are divided into sublayers. SONET and SDH are conceptually identical but use different terms to describe the same things.

Figure 9.3 SONET/ SDH Reference Model

SONET/ SDH links are bi-directional and are subdivided into parts connected with specific network equipment. The goal here is to minimise cost and maximise efficiency. Each piece of equipment is designed to perform only the functions necessary.

The Path represents the end-to-end data flow. Path Terminating Equipment (PTE) is a service adapter, terminal or multiplexer and it interfaces the synchronous network with the users.

The multiplexer section (USA - line) represents a link along which the multiplexed structure is unchanged. Line terminating equipment is an ADM (add/ drop multiplexer) or cross-connect switch.

Regenerator Sections (USA - sections) are connected with simple repeaters.


SONET/ SDH frames can be viewed as two-dimensional structures. They are always nine rows high with ninety or more columns width. Each of the cells thus produced holds one byte of data. Each frame takes 125 ms to transmit and there are no gaps between adjacent frames.

Figure 9.4  SDS-1 frame, the basic building block

STS-1 and STM-1 Comparison

STS-1 (Synchronous Transport Signal) is the basic SONET building block. It can be considered as in fig 9.4 with the first 3 columns being reserved for section and line overhead. This leaves 87 columns for the STS-1 Synchronous Payload Envelope SPE.

Basic SONET rate    = 9 rows X 90 columns X 64 kbit/sec  = 59.84 Mbit/sec
Overhead                 = 9 rows X 3 columns = 27 bytes/125 ms = 9.728 Mbit/sec
SPE                         = 9 rows X 87 columns = 783 bytes/125 ms = 50.112 Mbit/sec
Information capacity  = 9 rows X 86 columns = 774 bytes/125 ms = 49.536 Mbit/sec

STM-1 (Synchronous Transport Module) is the basic SDH building block. It can be considered as in fig 9.5 with a width of 270 columns, with the first 9 columns being reserved for regenerator section and multiplexer section overhead.

Basic SDH rate   = 9 rows X 270 columns X 8 bits X 8000 = 155.520 Mbit/sec
Overhead           = 9 rows X 9 columns    = 5.184 Mbit/sec
SPE                  = 9 rows X 261 columns   = 150.336 Mbit/sec
Data capacity     = 9 rows X 260 columns   = 149.760 Mbit/sec

Figure 9.5  STM basic building block

Payload Envelopes

All SONET/ SDH payloads are loaded into the Synchronous Packet Envelope (SPE). The SPE is a very flexible structure designed to carry:

· Pre-SONET/ SDH (plesiochronous) signals i.e. T1 to T3 and E1 to E4
· Asynchronous traffic e.g. FDDI, frame relay, SMDS etc.
· Synchronous data
· Any foreseeable future data streams e.g. ATM cells

Table 9.3 Payload capacities of SONET vs. SDH

Note from the table above that fractional cells may be inserted into the SPE. This means that any cell may ‘spill over’ into the subsequent SPE. 


Mappings of legacy (PDH) carrier signals into SONET/ SDH SPEs have been standardised with the overall result of efficient and economical evolution to SONET/ SDH. The standardisation also has lowered hardware cost and facilitates multivendor networks.

SPEs can be subdivided into standard data containers. These containers are always nine rows high but have variable column width. They fit nicely into STS-1/ STM-1 frames.

In SONET, T-x signals are mapped into containers called virtual tributaries (VTs).
In SDH, E-x signals are mapped into containers called virtual containers (VCs).

SONET Virtual Tributaries

Figure 9.6 SONET Virtual Tributaries

SDH Virtual Containers

Figure 9.7 SDH Virtual Containers

SONET Overhead

A 4% overhead was included in the standard. This overhead is divided into section, line and path overheads. It is used for maintenance, troubleshooting and alarms. It can carry operation, administration and management (OAM) information.

Figure 9.8 Depiction of section, line and path

The 1-byte compartments of the section, line and path overheads are individually labelled and reserved for various functions. The path overhead in SONET is carried end to end whereas the line and section overheads are regenerated at repeaters and LTEs. 

Signal Labels

Within the path overhead of both SONET and SDH are bytes labelled C2 so that the SONET/ SDH source can identify the payload map used by the source and thus the destination equipment can identify the original signal(s).

Table 9.4 Depiction of some payload labels

Payload Pointers

To identify the starting positions of the data within a frame, the line overhead includes bytes labelled H to point at the beginning of the SPE within the frame. It is not necessary for a SPE to be contained totally within one frame, they can cross frame boundaries.

This gives easy access to synchronous payloads and avoids the need for 125 ms buffers to synchronise arriving data to the frames. This accommodates slight frequency variations between payload signals. It also allows unsynchronised signals to be carried by the synchronous network.

The payload pointer allows the SPE to float within the frame thus the SPE is not locked into position in the STS-1 or STM-1 frame.

Small networks can use locked operation where the SPE is locked into a specific place within the frame. This is often used in ATM interfaces.

High Rate SONET/ SDH

Higher SONET/ SDH data rates are obtained by multiplexing the STS-1/ STM-1 frames. This is accomplished in two steps:

· The SPEs are synchronised by adjusting the payload pointers
· The tributary STS-1 frames are byte interleaved with each other

Concatenated STS

Concatenated signals have payload combined into one indivisible SPE with a single path overhead occupying one column. These signals are optimised for transport of high data rate payloads i.e. >50 Mbit/sec. An example is E4 (139.264 Mbit/sec). As these represent single entities they must be switched, multiplexed and transported as one entity.

Concatenated STS-n signals are denoted by the suffix c. Thus STS-3c is a concatenated STS-3 signal.

For example, let us take STS-3c. Like the STS-3, the frame is 9 rows X 270 columns (155.52 Mbit/sec)
The overhead is section and line and is 9 rows X 9 columns; path overhead is one column. The SPE-3c is 9 rows X 260 columns i.e. 149.76 Mbit/sec.

SONET Optical Carrier Levels

Optical carrier levels (OC-n) are carried along the optical fibres in a SONET network. OC-n is produced in two steps:

· Firstly the STS-n multiplexed signals are scrambled. This is to shape the characteristics of the light pulses that are injected onto the cabling
· Then they are converted into optical form and transmitted onto the fibre by laser diode optical transmitters.

Figure 9.9 Production of optical carrier levels

The function of the scrambler is to remove long strings of 1s and 0s and this improves clock recovery and Bit Error Rate (BER).

Comparison of SONET and SDH

ITU-T defines SDH. The main interest was in carrying E-3 and higher signals. The basic building block of SDH is the synchronous transport module (STM-1).

ANSI defined SONET and its interest was in carrying T3 signals or less. The basic building block of SONET is the synchronous transport signal (STS-1).

The optical carrier levels (OC-n) are identical and this provides for the midspan meet. These OC-n signals are obtained after scrambling STM-n, STS-nc, STS-n signals.

SDH and SONET can be incompatible if different SPE mappings are used, but this is not a problem with ATM interfaces where the SPE is locked into a specific place within the frame.

Table 9.5 SDH compared with SONET

SONET/ SDH Network Hardware

SONET standards describe five types of equipment:

1. Fibre-Optic Transmission System (FOTS). FOTS multiplex STS-1s into STS-ns and also convert electrical STS-s into OC-s.

2. Terminal Multiplexers (TMs). These multiplex T1s to STS-1s (or higher). Both TMs and FOTS are often combined into one unit.

3. Add/ Drop Multiplexers (ADMs). These are placed in series along the route of the FO cable .An ADM is a single-stage device that is used to add or extract individual signals from the FO without having to demultiplex all the signals.

4. Access Multiplexers (AMs). These use the virtual tributary structure and are similar to ADMs but they extract or add individual T1s. The T1s then may be demultiplexed to access individual channels (calls).

5. Digital Cross Connects (DXCs)

Broadband Cross Connect (BBX). These are used in SONET/ SDH to rearrange STS/ STMs within an OC-n.
Wideband Cross Connect (WBX). These are used in SONET to rearrange virtual tributaries (VTs)

Using the above equipment allows for the construction of very flexible networks.

SONET/ SDH Management

Operations, Administration and Management (OAM) have always been an integral part of the SONET/ SDH standard. In the past all that existed to perform this function were the proprietary systems such as AT&T, MCI and Sprint, all of which were incompatible.

The key features of OAM include:

· OAM channels are set aside in the section, line and path overhead
· OAM information is carried through SONET equipment and allows single-point control of the entire network
· OAM allows for remote configuration of network capacity with circuits, tributaries, virtual tributaries being able to be remotely connected, disconnected and reassigned. Dynamic traffic balancing is also realised
· Supports existing functions and allows evolution to greater functionality such as T1's OAM and ITU-T's Transmission Management Network (TMN)

Performance Management

SONET/ SDH includes standard performance monitoring with continuous error monitoring Bit-Interleaved Parity (BIP) BIP-8 bytes in overhead fields. Any problems such as unusually high BER are indicated using standard bi-directional alarms. Alarm Indication Signals (AISs) are sent to upstream equipment.

Downstream path (end-to-end) alarms include:

· Far-end receive failure (FERF) if signal is lost
· Far-end block error (FEBE) if error rate is too high
· Loss of pointer (LOP) if payload is lost

Downstream line (multiplexer section) alarms include path alarms and loss of frame (LOF) if multiplexing alignment is lost.

Automatic Protection Switching

SONET/ SDH includes standard mechanisms for Automatic Protection Switching (APS). APS has the benefits of:

· Faster restoration of service when failure occurs (or service deteriorates) if e.g. the optical path is severed or the electrical equipment fails or loses power.
· Standardisation allows APS in a multivendor environment
· Protection switching may be used during maintenance or testing

For APS to function there must already exist a pre-provisioned protection facility i.e. a backup route.

APS can be included in cross connects or ADMs (LTE) and operates using section (multiplexer section) APS channels

With a 1:1 APS there is a dedicated backup for each route, both routes being uses and the one displaying best performance being chosen.

1:n APS has one backup route to protect several working facilities and the backup path remains unused until required.

A common use of APS is in self healing rings. A unidirectional self-healing ring (USHR) is made using two fibres between each of the SONET/ SDH nodes with one working fibre and one protection fibre.

Far-End Receive Failure (FERF) alarms initiate the self-healing process. When a failure does occur, the affected station issued a FERF. Other stations react to the FERF and re-route the signal around the failure. More complex self-healing rings can be constructed using more fibres. A bi-directional self healing ring (BSHR) uses four fibres.

Very High Data-rate SONET/ SDH

Dense Wavelength Division Multiplexing (DWDM) is being deployed and uses 120 channels. WDM couples several different colours of light onto one fibre, with each colour being an independent data path. The term Dense relates to the colours being very close to each other.

Figure 9.10 An example of an 8-DWDM

MCI has upgraded part of its Southern California network to give 80 Gbit/sec on a single SMF using 8-DWDM with OC-192 on each colour.

From its inception in 1990, SONET is expected to have fully penetrated the US network by 2005.

Private SONET/ SDH networks

SONET/ SDH was originally viewed as the technology for PSNs. Now large private networks are beginning to include SONET/ SDH equipment. The advantages are:

· International standard
· Very high bandwidth
· Built in upward migration, fibre to STS-1 to STS-3 to STS-12 etc.
· This positions the owner for future BISDN and gigabit LANs

Some Local Asynchronous Transfer Mode (LATM) interfaces use SONET/ SDH framing.

SONET Private lines

SONET leased lines are now becoming available from MAN and WAN providers. This service is aimed at high capacity users and applications such as:

· CAD and CAM
· Bulk data transfer
· Voice/ data aggregation

In the US, OC-3 (155 Mbit/sec) and OC-12 (622 Mbit/sec) leased lines are available from PSNs. Access to these lines is via an ADM which is either leased from the service provider or alternatively privately purchased.

Fibre optic signals and properties

As SONET and SDH use fibre optic cabling, it is important to realise why fibre optic cable provides such a good quality signaling medium.

Noise on Transmission Lines

Noise on transmission lines falls into four categories:
· Thermal noise
· Intermodulation noise
· Crosstalk
· Impulse noise

Crosstalk and impulse noise do not affect FO links so we only need to consider thermal and intermodulation noise. Intermodulation noise is produced when there is some non-linearity in the transmitter, receiver or transmission line and is unique to the system. This non-linearity can be caused by component malfunction or the use of excessive signal strength.

The only noise that affects FO systems is thermal noise.

Signal Propagation in FO links

In multimode propagation, rays of light entering the fibre at shallow angles are reflected and propagated along the fibre. Other rays at wider angles are absorbed by the surrounding material. Because there are multiple paths through the fibre, each has a different path length and this allows signal elements to spread out in time and limits the rate and distance over which data can be accurately received.

At the other extreme is single-mode propagation where the radius of the core is reduced to the order of a single wavelength so only a single mode or angle may pass, the axial ray. Having only one path gives single-mode propagation superior performance over multimode as signal elements cannot spread out in time and so such distortion cannot occur.

Two different light sources are used to transmit onto FO, to convert the electrical energy into light energy. The Light Emitting Diode, LED and Injection Laser Diode, ILD. LED links are cheaper, operate over a greater temperature range and have a longer operational life than ILDs.

The ILD is more efficient and supports higher data rates than LED, however it is much more expensive and needs to be temperature controlled to keep output frequency stable. 


SONET with SDH are essentially the same above 155 Mbit/sec. Below that, SONET and SDH have their own schemes for presenting data streams to the cabling.

The frames of data are sent synchronously, once every 125 mseconds. All frames are 9 rows high. The width of the frame will vary as the data rate increases.

The frames have overhead information at the start of the frame. This contains
· Payload pointer to inform the receiver where the data starts
· Payload label to inform receiver how to interpret the data
· Bytes for signaling between networks

Within the data part of the frame is carried the SPE (synchronous payload envelope). This is to carry the data.

Data to be carried is formed into containers or tributaries (USA) of fixed size to fit exactly into the SPEs.

Concatenated STS is used to carry a high data rate signal e.g. video and is a single entity. When carried over SONET/SDH STS-3c cannot be split to 3 STS-1s.

When data are passed to the optical network, it is ‘scrambled’ to shape the signals for the cabling. This removes long strings of 1s and 0s and improves clock recovery and Bit Error Rate (BER).

Fibre optic is only susceptible to thermal noise so has very good properties for high speed, reliable data transport.

(c) M Clements 2000

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