TRANSMISSION NETWORKS

Plesiochronous Digital Hierarchy (PDH) . . . .3.1 PDH Multiplexing Stages . . . .3.2 Capacities, Bit Rates and Tolerances within PDH . . . .3.3 Primary-Level Devices . . . .3.4 64 kbit/s Cross Connection in the PDH . . . .3.5 8 Mbit/s and 34 Mbit/s Leased Services over the PDH . . . .3.6 The Synchronous Digital Hierarchy (SDH) . . . .3.7 Advantages of SDH . . . .3.8 SDH Payloads . . . .3.9 Concept of Virtual Containers (VC) in SDH . . . .3.10 End-to-End Connection and Virtual Containers . . . .3.11 Multiplexing Virtual Containers . . . .3.12 The Synchronous Transport Module (STM) . . . .3.13 Synchronous Multiplexers . . . .3.14 SDH Rings . . . .3.15 SDH Local Access Ring . . . .3.16 Concatenation . . . .3.17 Contiguous Concatenation Example . . . .3.18 Next Generation SDH . . . .3.19 Virtual Concatenation (VCAT) . . . .3.20 Comparison of Communication Media . . . .3.21 Radio Fundamentals – The EM (Electromagnetic) Wave . . . .3.22 Properties of EM Waves . . . .3.23 Radio Fundamentals – The EM Spectrum . . . .3.24

II © Wray Castle Limited

General Propagation Concepts . . . .3.25 Fading . . . .3.26 Modulation Techniques . . . .3.27 Radio System Parameters . . . .3.28 A Typical Digital Radio System . . . .3.29 PTP Radio Access . . . .3.30 SDH Radio Access Alternate Routing . . . .3.31 Optical Fibre . . . .3.32 Types of Optical Fibre . . . .3.33 Options for Increasing Capacity . . . .3.34 Wavelength Division Multiplexing (WDM) . . . .3.35 Coarse WDM (CWDM) and Dense WDM (DWDM) . . . .3.37 Optical Cross-Connects and Optical Switching . . . .3.38 Optical Transport Network (OTN) . . . .3.39 Client Signals in the OTN – Example . . . .3.40

IV © Wray Castle Limited

At the end of this section you will be able to:

 describe the operating principles of the Plesiochronous Digital Hierarchy (PDH)

 identify the main functions of the Synchronous Digital Hierarchy (SDH)

 outline the advantages of SDH compared to PDH

 identify the structure of typical SDH networks

 explain the reasons for developing next-generation SDH

 provide examples of how radio is used in PDH and SDH networks

 identify the main types of optical fibre and where they are used

 explain how optical fibre can provide high-capacity transmission and how this may be extended with Dense Wavelength Division Multiplexing (DWDM)

 describe the main functions of an Optical Transport Network (OTN)

OBJECTIVES

VI © Wray Castle Limited

TY2600/v4.1 © Wray Castle Limited 3.1 Plesiochronous Digital Hierarchy (PDH)

PDH networks employ a hierarchy of multiplexing stages to create high-capacity systems. ITU-T Recommendation G.703 specifies this hierarchy of frame-structured digital signals.

Primary level Multiplexers (PMUX) use Time Division Multiplexing (TDM) to generate a byte-interleaved frame structure – either E1 or T1. Before the development of a digitally switched network, E1 signals operated between PMUXs and the receiver recovered its timing (clock) from the transmitting PMUX.

Each PMUX was independently operated from a quartz timing source, which meant that all the E1s in a network were operating at a slightly different speed. The term used to describe this is ‘plesiochronous’.

The variation of the E1 speed was limited to +/– 50ppm (parts per million).

When Higher Order Multiplexers (HOMs) were developed, it could not be guaranteed that all the E1 inputs were running at exactly the same speed. When multiplexing takes place it is very important to ensure that no bits are ‘lost’. Therefore, if four plesiochronous E1 signals (2.048 Mbit/s) are to be multiplexed in time, then some extra bits are added to the higher speed signal to ensure that no ‘real’

data bits are lost. This means that the output bit rate is more than four times the input. This process of adding the extra bits is known as ‘justification’.

The HOMs also employ TDM, but generate bit-interleaved frame structures. They are unaware of the frame structures of the lower speed tributaries and just take each bit and multiplex it into the higher speed signal. If there are justification bits or traffic bits in the tributary signal, the HOM is unaware of these – it just multiplexes every bit in turn.

A hierarchy of multiplexers was developed over the years, each one building on the principles of justification. This is known as the Plesiochronous Digital Hierarchy (PDH). The PDH may be considered as a mechanism for transporting 2.048 Mbit/s, or indeed any other PDH signal rate.

With the development of the digital switch, all the E1 inputs and outputs to the switch had to be time synchronized. This means that now, the vast majority of E1’s and primary level equipment operates synchronously, i.e. their clocks are referenced to an accurate common source (an atomic clock).

However, the PDH still carries out justification at each multiplexing stage.

It is important to recognize that the E1 signal is the only signal in the hierarchy where there are no justification bits present.

PMUX

30 analogue speech band

signals

E1 signal (2.048 Mbit/s +/- 50ppm)

2-8 HOM

4 x E1 signals (+/- 50ppm)

E2 signal (8.448 Mbit/s +/- 30ppm)

Note: the output bit rate is more than the sum of the inputs as justification is

carried out in the HOM.

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3.2 © Wray Castle Limited

PDH Multiplexing Stages

A 2.048 Mbit/s path is implemented via connections through the HOM environment. A 2.048 Mbit/s signal is multiplexed via a 2–8 Digital Multiplexer (DM), an 8–34 DM and a 34–140 DM equipment to occupy a position in the 140 Mbit/s signal. The demultiplexing process via the 140–34 DM, 34–8 DM and 8–22 DM equipment results in the presentation of a 2.048 Mbit/s tributary signal whose average rate is the same as its corresponding input tributary.

Even though the HOM environment is plesiochronous, it maintains the timing characteristics of the primary level input signals (E1s).

A main aim of the PDH is to ensure that for a given 2.048 Mbit/s path through the network, the output tributary signal operates at the same average rate as its corresponding input signal. This is also true of higher-rate signals.

In the diagram, it can be seen that the 140 Mbit/s signal is passed over a transmission line system. This transmission system could be either a cable or radio network.

As each digital multiplexer operates independently of the one above or below, it is not possible to find the E1 signal inside the 140 Mbit/s bit stream. The signal must be completely demultiplexed back to the E1 (2.084 Mbit/s) level.

34.368 Mbit/s ± 20 ppm

Same average rate

Virtual 2.048 Mbit/s path

Multiplexing Demultiplexing

Virtual 8.448 Mbit/s ± 30 ppm Virtual 34 Mbit/s ± 20 ppm

TE TE

140 34-140 DM

TY2600/v4.1 © Wray Castle Limited 3.3 Capacities, Bit Rates and Tolerances within PDH

Although the digital rates in the CEPT hierarchy are defined by the ITU-T, these are nominal rates and a slight variation is permitted.

The allowance for rate variations results in a series of higher multiplexers (those above the primary rate), which do not operate on four synchronous input streams. The inputs to these multiplexers all have the same nominal rate, and are therefore termed ‘plesiochronous’ (nearly synchronous). The HOMs are described as plesiochronous multiplexers. In this situation, all HOM clocks are free running, i.e. not synchronized, but operate within a predetermined range.

The diagram also indicates the number of 64 kbit/s channels which may be carried at each level of the PDH.

Hierarchy

Level Multiplexer O/P Bit Rate (Mbit/s)

Currently there are no standalone fifth-order multiplexers in use, this stage of multiplexing is performed inside Optical Line Terminating Equipment (OLTE).

# This number represents the number of simultaneous telephone calls possible at each level of the PDH hierarchy.

Tributary signals flexibly assigned to timeslots

Digital Access Cross Connect System (DACCS)

Manually configured

Aggregate E1 E11 Aggregate Aggregate E12

Manually configured (64 kbit/s – cross connect)

Voice digitized any timeslot to any timeslot

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3.4 © Wray Castle Limited

Primary-Level Devices

The timeslot interchange function enables the development of a number of primary level equipment types: terminal multiplexers, Add and Drop Multiplexers (ADM), and Digital Access Cross Connect Systems (DACCS).

A terminal normally consists of a single E1 aggregate and a range of interface cards to enable the support of voice and data services. Traffic applied to an interface card will be mapped to an available timeslot on the aggregate. Speech signals applied to a voice card are converted to 64 kbit/s PCM.

Add and drop multiplexers, also called drop and insert multiplexers, are versatile devices typically deployed in access networks. They can support multiple E1 aggregates and a range of tributary cards to support voice and data services. Traffic from any tributary card can be flexibly assigned to any available timeslot on any aggregate. This assignment is bidirectional, i.e. input to a tributary is added (inserted) to an assigned timeslot in the transmit direction, while traffic received in the assigned timeslot is dropped to the appropriate tributary card. In addition to add and drop connections it is also to through-connect the contents of a timeslot on one aggregate to an available timeslot on another aggregate. This is known as a 64 kbit/s cross connect. The connection map for the device is manually configured and statically stored in local memory. This can be reconfigured from time to time by the operator to provision new circuits or to re-route existing ones. It is possible to equip an ADM with an E1 tributary card. In this case it is possible to establish 64 kbit/s cross connect between a timeslot on the tributary card and any available timeslot on any aggregate.

DACCS devices may terminate a high density of E1 bearers. However, they do not provide access to tributary cards. They are normally used in an operator’s core network to route and groom traffic; for example, they can be used to provide 64 kbit/s leased services. The DACCS device is able to establish a 64 kbit/s cross connect between any timeslot on any E1 bearer to any other available timeslot on any other or the same E1 bearer. The connection map for the device is manually configured and statically stored in local memory. This can be reconfigured from time to time by the operator to provision new circuits or to re-route existing ones.

The DACCS and ADM are 64 kbit/s cross connect devices. While it is possible to build N x 64 kbit/s (N=1 to 30) circuits, they cannot cross connect an entire E1 bearer.

TY2600/v4.1 © Wray Castle Limited 3.5 64 kbit/s Cross Connection in the PDH

Leased lines at various bit rates can be provided for customers. For example, a 64 kbit/s leased line can be implemented across the network using a range of primary layer transmission equipment including terminal multiplexers, ADMs and DACCS systems. Terminal multiplexers map tributary inputs to a nominated timeslot while the ADMs and DACCS devices cross connect the contents of a timeslot from one E1 bearer into another timeslot in another E1 bearer.

In the diagram, a corporate customer is linking two offices at A and B using a 64 kbit/s leased line with timeslot mappings and cross connections between E1 bearers. E1 bearers between network sites are connected via a PDH HOM network. The 64 kit/s connection will remain in place for the duration of the lease, which could be many years.

The cross connects can be controlled remotely from a network management system, which allows a leased line to be provisioned relatively quickly across the network.

TS3 TS6

64 kbit/s Tributary signal assigned to Timeslot 2

64 kbit/s Tributary signal assigned to Timeslot 5

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3.6 © Wray Castle Limited

8 Mbit/s and 34 Mbit/s Leased Services over the PDH

To enable flexibility in the provision of digital signals through the PDH network, Digital Distribution Frames (DDFs) are employed between each multiplexing stage. These DDFs contain coaxial jumper cables used to interconnect the multiplexers and other equipment at each stage.

In the figure, there are two customers, one with an 8 Mbit/s leased line and another with a 34 Mbit/s leased line.

For the 8 Mbit/s leased line, the Customer Premises Equipment (CPE) produces the 8 Mbit/s signal and in this case an 8 M radio access link is used to access the network. The signal is then connected to an 8–34 DM (in this case using tributary four). The DDF is used to make this connection.

The 34 Mbit/s signal is then passed to a 34 M DDF to be connected to tributary one on the 34–140 DM.

The 140 Mbit/s signal is then passed to the 140 M DDF and in this case the signal is passed to an Optical Line Terminating Equipment (OLTE). The OLTE converts the electrical signal to optical and transmits it over an optical fibre to a distant location where the process is reversed down to the 8 Mbit/s level. This signal in this case is then transferred to the customer’s premises over an optical fibre access link.

For the 34 Mbit/s leased line, the signal from the customer is taken to the 34 M DDF and then connected to tributary three on the 34–140 DM.

Circuit provisioning for these leased lines can be time consuming as the DDFs must be manually configured using coaxial jumper cables.

It is important to recognize that in the PDH, justification is carried out at each stage of multiplexing. This means that it is not possible to drop out an 8 Mbit/s signal directly from a 140 Mbit/s signal. It must be demultiplexed one stage at a time until the 8 Mbit/s signal can be recovered.

8 Mbit/s

TY2600/v4.1 © Wray Castle Limited 3.7 The Synchronous Digital Hierarchy (SDH)

As defined by the ITU-T in Recommendation G.708, ‘The SDH is a set of hierarchical transport structures, designed to transport suitably adapted payloads in a physical (managed) transmission network.’

From the user’s perspective, the SDH is a transmission network that provides information pipes connected from input to output. As in PDH, these pipes may operate at a number of different bit rates.

However, the bit rates used in SDH are far higher than PDH.

In SDH the information pipes are referred to as payloads. SDH may also be used to carry several types of payload, including PDH signals. From the point of view of the operator, SDH is a high-capacity, flexible network with management and protection facilities built in, and backward compatibility with PDH. It is future-proofed to interwork with high-speed services.

By examining the relationships between each of the three key elements of the formal ITU-T definition, it is possible to build a simple conceptual model of an SDH transmission network and the facilities that such a network could be designed to support.

Transmission NetworkSDH PDH

Input

PDH Output 2 Mbit/s

34 Mbit/s 140 Mbit/s

2 Mbit/s 34 Mbit/s 140 Mbit/s

high capacity flexible

built-in management and protection backward compatibility with PDH future-proofed

TY2600/v4.1

3.8 © Wray Castle Limited

Advantages of SDH

SDH contains many functions that overcome the shortcomings of PDH systems, including network management capability, flexibility and resilience.

The SDH transmission structures contain sufficient overhead elements to allow the monitoring of lines, sections and paths, so that each of these entities can be managed separately.

SDH is based on a synchronous multiplexing technique, which means that justification is only carried out once, when the tributary signal is inserted in the frame. This means that the lower-order payloads can be identified and directly dropped out of or added into the higher bit rate signal. Flexible multiplexing arrangements include add-and-drop facilities, and along with remote configuration capabilities, it means that SDH is more efficient than PDH systems, which had a very static configuration.

With the SDH, reconfiguration is carried out under software control, but in the PDH, reconfiguration was carried out by manually moving jumpers on the DDFs.

SDH incorporates sophisticated protection mechanisms which can be used to protect traffic at both the circuit and line level.

Managed Network

Extensive overhead facilities permit in-service monitoring of:

– lines – section – paths

Flexibility

SDH offers flexibility of operation by:

– remote configuration/reconfiguration

(via control channels embedded in overheads) – add-and-drop and cross-connect functionality

Network Resilience

Built-in protection mechanisms are implemented at:

– media level (line system) – path level (individual circuit) – equipment redundancy

SDH frame structures have been optimized to transport structured PDH payloads, LAN/MAN and Broadband ISDN (B-ISDN) service payloads.

The SDH frame structures have been designed to transport payloads structured in accordance with both CEPT and ANSI PDH standards. The exception to this is the 8.448 Mbit/s signal (E2), which is not supported as an SDH payload.

A number of levels have been defined in a hierarchical set of transport structures. Each level is defined in terms of a specific transmission, or transport, bit rate and by a unique frame-structured signal referred to as a Synchronous Transport Module (STM).

The nominal transmission bit rates for each of the hierarchical levels 1 to 4 are specified in ITU-T Recommendation G.707. It is interesting to note the simple harmonic relationship between each hierarchical level of the SDH, and to compare this with the relationships that exist between the various levels of the conventional PDH. For example, an STM-4 is exactly four times the bit rate of an STM-1 and no justification is carried out between the higher STM levels. This can be achieved because all the STM signals in the network are derived from the same atomic clock and are therefore accurately synchronized.

STM-0 (51.84 Mbit/s) of the SDH was defined in an annex of the 1993 issue of ITU-T Recommendation G.708 in response to a requirement for a transport structure to support light to medium aggregate traffic in an SDH network.

LAN and MAN Payloads (e.g. FDDI and IEEE 802.6)

TY2600/v4.1

Concept of Virtual Containers (VC) in SDH

The frame structure of an SDH signal is based on the concept of Virtual Containers (VCs). All virtual containers in a network are derived from the same clock source and are therefore synchronous.

Different sizes of VC have been designed to carry the various bit rates of the PDH. For example, a VC-12 is designed to carry the first level of the PDH, second option which is E1. A VC-11 is designed to carry the first level of the PDH, first option which is T1.

Other containers are available to carry most of the PDH payloads from both the European and North American hierarchies. These are shown in the diagram.

Note that a VC-2 has been designed to carry 6.312 Mbit/s from the North American PDH but there is no facility to carry an 8 Mbit/s signal from the European PDH.

The bit rates of all the VCs are synchronized, i.e. all VC-12s have a bit rate of 2.24 Mbit/s. This means that the E1 tributary signal can have a variation of +/– 50 pmm but the container VC-12 will always be 2.24 Mbit/s. Justification bits are therefore added to ensure the tributary signal fills up the VC-12. If the E1 signal is slow, then more justification bits are added, and if the E1 signal is fast then fewer justification bits are added.

All VCs also carry a Path Overhead (POH). This is a set of labels containing information about the VC such as its identifier, whether there are errors present, various alarms, etc.

In SDH, it is the VC which is cross-connected through the network.

Payload

Justification Bits

VC-11

1.544 Mbit/s (T1)

Payload

Justification Bits

VC-12 (2.24 Mbit/s)

2.048 Mbit/s (E1)

Payload

Justification Bits

6.312 Mbit/s (T2) VC-2

Payload

Justification Bits

VC-3

34.368 Mbit/s (E3) or

44.736 Mbit/s (T3)

Payload

Justification Bits

VC-4 (150.336 Mbit/s)

139.264 Mbit/s (E4)

A Virtual Container of order N (VC-N) may be considered as the unit of transport for SDH networks. All payload types are connected to a tributary card on an SDH device such as an ADM and are mapped into the appropriate VC-N, as shown above. The VC-N is then transported end to end across the SDH network.

Intermediate SDH nodes such as other ADMs and Cross-Connects (X-con) cross connect the VC-N from one STM signal to another. When the VC-N reaches its destination the terminating SDH device recovers the payload from the VC-N and transmits it out of the appropriate tributary card.

VC-N cross-connected into appropriate slots

within the STM-N

TY2600/v4.1

Multiplexing Virtual Containers

As all virtual containers are derived from a common clock source, they are synchronous in nature.

Although there are containers of different bit rates, these bit rates are directly related and remain

In document Introduction to Telecoms (Page 83-107)

CONTENTS

CONTENTS

OBJECTIVES

PMUX

2-8 HOM