CHAPTER 7 Transport Networks: Technology examples

CHAPTER 7 Transport Networks: Technology examples
In chapter 6, the main concepts of transport networks were discussed. To get a more concrete view on the matter, this chapter will concentrate on some examples of widely deployed transport network technologies. M. Pickavet and C. Develder

Outline Transport network examples SDH WDM 1.1 Belgacom 1.2 Telenet
1.3 Deutsche Telekom 1.4 International network SDH WDM First, some examples of existing transport networks will be highlighted. The transport network infrastructure of both Belgacom and Telenet will be shown and also an international network operator will be shown. Transport networks: Technology Examples 2

Belgacom Network Unique telephony operator in Belgium for a long time
Basic services: telephony, leased lines, public data network, etc. Since 1998, also competition possible on last monopoly: the telephone service Basic fixed telephone services expanded to new services (e.g. mobile telephony, fast Internet, digTV) Increase bandwidth down to the customer (xDSL techniques) Belgacom is the well-established national operator which had a monopoly position for telephony for a very long time in Belgium (until 1998). The basic services offered by Belgacom in the past were fixed telephony, leased lines, public data network (and during the last decade also mobile telephony using GSM, under Proximus, and Internet access). To cope with increasing bandwidth demand from the customers, xDSL (Digital Subscriber Loop) techniques are introduced together with the deployment of optical fiber closer to the customer. The most important xDSL techniques today are ADSL or Asymmetric DSL and VDSL or Very high bitrate DSL. Transport networks: Technology Examples 3

Belgacom: switching and transport network
Technology: mainly SDH (+PDH & WDM) LEX TEX TEX LEX DXC ADM ADM This figure shows the principle layout of the Belgacom network: both the switching and the transport network. We observe the telephone switches (LEX or Local EXchange and TEX or Transit EXchange). The local exchanges (LEXs) are interconnected with the transit exchanges and with some other nearby local exchanges via an SDH ring. This is called the regional transport network. This ring uses Add Drop Multiplexers (ADMs). The different transit exchanges are also interconnected via an SDH network (using digital cross-connects or DXCs in this example). This part is called the core transport network. Note: as indicated above, SDH is the main transport network technology in Belgacom. However, also PDH is still used at some places (mainly due to legacy equipment) and also WDM technology is applied. core transport network regional transport network Transport networks: Technology Examples 4

Belgacom: transport network architecture
ZTC ZTC: Zonal Transmission Center level 1: CORE network RING + MESH level 2: REGIONAL network LTC LTC: Local Transmission Center A more detailed view of the core and regional transport network is shown in the figure. The nodes in the core transport network are Zonal Transmission Centers (ZTCs). The nodes in the regional network are Local Transmission Centers (LTCs). The core network has a special architecture: a combination of rings and a mesh are used to transport the information between ZTCs or internationally. The regional transport network consists of “islands” consisting of a number of rings concentrated around a ZTC (clover leaf). In practice one has also rings which are connected to two ZTCs at the same time (for survivability reasons). RING Transport networks: Technology Examples 5

Belgacom: Zonal Transmission Center (ZTC)
ADM ring ... core DXC ... mesh TEX regional An example of a simplified ZTC is shown here. The upper part belongs to the core network (an ADM for the ring part and a DXC for the mesh part). The lower part is connected to the regional ring. Because of survivability reasons we observe two ADMs to connect to the regional ring. We observe also a transit exchange (telephone switch). ADM ring ... Transport networks: Technology Examples 6

Belgacom: CORE network topology
An (approximate) topology of the core transport network is shown in the figure. Note that the rings in the core network are not indicated. We also observe a number of international links. No regional rings are shown. ZTC: Zonal Transmission Center international links Transport networks: Technology Examples 7

1.3 Deutsche Telekom 1.4 International network SDH WDM Transport networks: Technology Examples 8

Telenet Network origin: reuse of existing coax cable (used for TV distribution by CATV operators) now: offer also interactive services (telephony, Internet access, digTV)  upgrade access network  construct a backbone network (switching and transmission) Telenet Operations is a relatively new operator in Flanders (northern part of Belgium), since Telenet reuses as much as possible the existing coaxial cable plant from the different CATV operators in Flanders. It is of prime importance to reuse the last copper part in the ground, close to the user, because it is extremely expensive to replace this (mainly digging costs). The main goal of Telenet is to offer telephony, high speed Internet access and other services (e.g. leased lines) to its customers. To build up this network, one had to upgrade the existing coax access network plant (introduce islands interconnected with fiber, go digital, make the coax plant bidirectional) and construct a backbone network including switching and transmission equipment. Transport networks: Technology Examples 9

Telenet: Topology Concept
Primary ring head-end Switch Secondary ring HFC loop Node Tree and branch 2-way coaxial network Subscriber premise The conceptual topology of the network is shown in the figure. Originally the CATV network consisted of head-ends (where the video signals were captured and put on the coax) of the different cable operators and a tree and branch coaxial network. There were a large number of head-ends because of the large number of CATV operators in Flanders. Each coax tree and branch network is now split in islands (each island is connected to a node, indicated by a dot on the figure) and these nodes are connected via a fiber ring to the original head-end (triangle on the figure). Each node is serving about 1000 CATV subscribers via the existing upgraded coax plant. The head-ends themselves are interconnected to the new switches (5 in total, represented by squares on the figure) by the use of 8 secondary rings connected to a primary ring. The actual topology is shown on the next figure. Transport networks: Technology Examples 10

Telenet: Physical Topology
The physical topology of the Telenet Operations transport network is shown above, with primary ring and secondary rings. In total, more than 1000 km (*) of optical fiber has been installed to realise this transport network topology. The Network Operations Center (NOC) is also indicated on the figure (Mechelen). (*) This number is only including the primary and secondary rings. Another 8000 km of fiber has been installed to interconnect the head-end with the optical nodes (see previous slide) and about km of coax cable is used for the interconnection of the optical nodes with the users at home. Transport networks: Technology Examples 11 Zie ook “Bedrijfsinfo”, “Het Netwerk”

Telenet: Basic Architecture
access multiplexer in head-end switch The goal of the transport network is (amongst others) to interconnect the different head-ends (47 in total) with the switches (5 in total). Each head-end has to be interconnected with one switch. The switches are also interconnected amongst each other by the transport network. These logical interconnections (network connections in the SDH path layer, not the physical topology) are shown in the figure. The transmission technology used is SDH (Synchronous Digital Hierarchy). This transmission technology is used to interconnect the switches and the head-ends by using the primary and secondary rings. Both the primary and the secondary rings run at 2.5 Gbit/s. The rings use SDH add-drop multiplexers and at the interconnection between the secondary rings and the primary ring there are also SDH digital cross-connects. SDH network connections (not the physical topology) Transport networks: Technology Examples 12

1.3 Deutsche Telekom 1.4 International network SDH WDM Transport networks: Technology Examples 13

Deutsche Telekom Unique telephony operator in Germany for a long time
cf. Belgacom in Belgium For mobile solutions: T-mobile cf. Proximus Largest European network operator Presence in countries: Germany, but also US, UK, eastern Europe Some numbers ( ): Total traffic volume: 2250 Gbit/s Number of lines: broadband: 12 million narrowband: 39 million The incumbent operator in Germany is Deutsche Telekom (situation similar to Belgacom in Belgium). DT is the largest European network operator today, active in (of course) Germany, but also with significant shares in the US, UK and several eastern European countries. Transport networks: Technology Examples 14

Deutsche Telekom: physical topology
50 core nodes Technology: WDM The physical topology of this network is shown in the picture above. Transport networks: Technology Examples 15

Deutsche Telekom: logical IP topology
63 PoPs Hierarchical topology: Core: 3 innercore nodes (fully meshed) 6 outercore nodes (dual homed with innercore nodes) Regio/metro The logical topology is shown in the picture above. Transport networks: Technology Examples 16

1.3 Deutsche Telekom 1.4 International network SDH WDM Of course, a large part of the traffic to be conveyed is no national traffic (e.g. think about the IP traffic streams generated by WWW surfing). Hence, high capacity international transport networks will be necessary to accommodate all this traffic. Transport networks: Technology Examples 17

International networks
Map of European Network Connectivity For example, in the figure above, a European map indicates how many bandwidth providers are offering services between major cities. As you can see, the competition can be fierce on a European scale. Transport networks: Technology Examples 18

International network: Level3
An example of an international network is shown on the figure above. Level3 is a multi-national communications and information services company (headquarters in Colorado) offering networking services at different network layers, mainly in the US and Europe. The underlying transport network is based mainly based on Wavelength Division Multiplexing technology, allowing to cope with the huge bandwidth requirements. Technology: WDM, SONET, … Transport networks: Technology Examples 19 Zie ook

Outline Transport network examples Synchronous Digital Hierarchy (SDH)
2.1 Introduction 2.2 Main layer structure 2.3 Detailed layer structure 2.4 Frame structure and synchronisation 2.5 Network elements 2.6 Network architectures WDM As illustrated by the transport network examples in section 1, Synchronous Digital Hierarchy or SDH is one of the main transport network technologies. In the current section, the main characteristics of this network technology will be discussed. The layering concept (see chapter 6) will be applied to model the distinct functionalities in subsections 2.2 and The structure of an SDH frame and the special measures to ensure synchronisation will be explained in subsection The different SDH network elements and their application in the most common SDH network architectures will be discussed in the two last subsections. For a study of the management, control and recovery features of SDH, the reader is referred to chapter 8. Transport networks: Technology Examples 20

Main features of SDH bi-directional transport network technology
multiplexing: TDM (Time Division Multiplexing) historically: mainly replacing PDH - based on direct synchronous multiplexing - about 5% of signal structure for ‘overhead’ - can accommodate both existing and future signals - can interconnect equipment from multiple vendors high bandwidth links (capacities: 155 Mbit/s, 622 Mbit/s, 2.5 Gbit/s and 10 Gbit/s) semi-permanent network connections (bitrates: 150 Mbit/s, 49.5 Mbit/s, 6.9 Mbit/s, 2.3 Mbit/s) central management (TMN) or distributed control (GMPLS) network elements: Digital Cross-Connect (DXC) Add-Drop Multiplexer (ADM) Terminal Multiplexer (TM) In this subsection we will briefly introduce the SDH transport network technology. SDH combines low bitrate signals into a high bitrate signal by multiplexing in the time domain. Historically, SDH has been deployed mainly to replace the older PDH (or Plesiochronous Digital Hierarchy) systems, mainly for telephony. Some of the main advantages of SDH over PDH are summarised above. SDH has different high bitrate link capacities: 155 Mbit/s, 622 Mbit/s, 2.5 Gbit/s and 10 Gbit/s. The network is supporting network connections of different bitrates: 150, 49.5, 6.9 and 2.3 Mbit/s. To steer the SDH network, several options are deployed nowadays. It can be based on a central management system like TMN (Telecommunications Management Network) or on a distributed control mechanism, based on GMPLS for instance. This will be discussed more thoroughly in chapter 8. Note: As SDH was originally developed for telephony, the technology is fully bi-directional in nature: all links and line systems are full duplex (e.g. a SDH connection of 10 Gbit/s means that one has a bandwidth of 10 Gbit/s in one direction and the same bandwidth in the opposite direction) and all network connections are also full duplex. In the remainder of the chapter, bi-directionality is always assumed. Transport networks: Technology Examples 21

Physical Layer physical layer: Synchronous Transport Module (STM-N)
link capacity: STM-1: 155 Mbit/s STM-4: 622 Mbit/s STM-16: 2.5 Gbit/s STM-64: 10 Gbit/s As indicated on the previous slide, SDH has different link capacity options: 155 Mbit/s, 622 Mbit/s, 2.5 Gbit/s and 10 Gbit/s. These links are called Synchronous Transport Modules (STM) with capacity N, denoted as STM-N. STM-1 corresponds to 155 Mbit/s, STM-4 to 622 Mbit/s, STM-16 to2.5 Gbit/s and STM-64 to 10 Gbit/s. We can consider the STM-N links as the lowest layer in the network (note: in reality it is more complex, see subsection 2.3). Transport networks: Technology Examples 22

SDH transport capabilities
Signals that can be transported by SDH: PDH: 2, (8,), 34, 140 Mbit/s (Europe) IP: Internet packets ATM: Asynchronous Transfer Mode FDDI: Fiber Distributed Data Interface etc… Several network connection types (virtual containers): VC-4 (150 Mbit/s) VC-3 (49.5 Mbit/s) VC-2 (6.9 Mbit/s) VC-12 (2.3 Mbit/s) VC-12 3x VC-2 OR 1x 7x VC-3 OR 1x 3x OR 1x All of the tributary signals which appear in a PDH network can be transported over SDH. This means that SDH is completely backwards compatible. SDH has been deployed, therefore, as an overlay network supporting the existing PDH network with greater flexibility. In addition, SDH transport capabilities have flexibility to accommodate the more advanced customer service signals. This list of signals includes : Internet Protocol (IP): a stream of IP packets can be transported via an SDH network connection Asynchronous Transfer Mode (ATM) Fiber Distributed Data Interface (FDDI): a high speed Local Area Network (LAN) standard Internet: Currently there is a lot of ongoing work to transport IP packets directly on SDH. To provide an efficient transport for the different bitrates of all these technologies, several network connection types were defined in SDH. Such a network connection is called a virtual container (VC) in SDH. The different VC sizes are summarised on the slide above. VC-4s and VC-12s are most commonly used. VC-4 Nx STM-N Transport networks: Technology Examples 23

SDH versus SONET SONET: - Synchronous Optical Network - introduced 1985 (USA, Japan) SDH: - CCITT 1988: SDH and SONET linked - ETSI 1990: SONET part of SDH Synchronous Transport Module (SDH) Synchronous Transport Signal (SONET) Line Rate Mbit/s - STM - 1 STM - 4 STM - 16 STM - 64 51.84 155.52 622.08 STS - 1 STS - 3 STS - 12 STS - 48 STS - 192 The first synchronous optical network was introduced in 1985 in the USA in order to cope with the problems encountered in PDH (incompatibility, inflexibility, difficult management). This network was called SONET and was able to carry the existing PDH signals. The lowest bitrate (STS-1) is Mbit/s and is able to carry the PDH DS3 signal ( Mbit/s). The lowest level was further multiplexed in STS-3 ( Mbit/s), STS-9 ( Mbit/s), STS-12, ..., STS-48. Because the PDH is different in Europe, a different lowest level SDH signal was defined: STM-1 at Mbit/s which could carry the PDH Mbit/s signal. SDH and SONET have been 'merged' making it a worldwide standard. The lowest level SDH signal is called the Synchronous Transport Module level 1 (STM-1) which has a signal rate of Mb/s. Higher level signals, obtained by byte-interleaved multiplexing lower level signals, are denoted by STM-N where N is an integer. The line rate of the higher level STM-N signal is N times Mb/s. Note: SONET and SDH are quite similar technologies, with only some minor differences. Therefore, in the remainder of this chapter, we will concentrate on the European situation (SDH), the main characteristics are the same for SONET anyway. Transport networks: Technology Examples 24

2.1 Introduction 2.2 Main layer structure 2.3 Detailed layer structure 2.4 Frame structure and synchronisation 2.5 Network elements 2.6 Network architectures WDM The layering concepts described in chapter 6 have been originally developed for SDH networks (ITU standard G.803). In this and the following subsection, the different network layers in an SDH network will be identified. Transport networks: Technology Examples 25

Higher Order Path Layer
VC-4 or Virtual Container-4 link connection VC-4 link connection capacity: 150 Mbit/s link capacity in # link connections: STM-1 ==> 1 x VC-4 STM-4 ==> 4 x VC-4 STM-16 ==> 16 x VC-4 STM-64 ==> 64 x VC-4 VC-4 link connection 4 TDM 1 N=4 1 4 N=4 VC-4 link The physical layer, consisting of different Synchronous Transport Modules (STM-1, STM-4, STM-16 and STM-64) was already discussed in subsection 2.1. We will now consider the Higher Order Path (HOP) layer. The HOP layer has link connections with a capacity of 150 Mbit/s. The characteristic information is a VC-4 or Virtual Container-4. A VC-4 link is supported by a network connection in the physical layer. This network connection in the physical layer corresponds directly to a link in the physical layer. This learns us that we have VC-4 links in the HOP layer with capacity of 1, 4, 16 or 64 VC-4 link connections (depending on the underlying physical layer: STM-1, STM-4, STM-16 or STM-64). It is interesting to observe that a VC-4 link connection has a capacity of 150 Mbit/s and this VC-4 link connection can be directly carried in an STM-1 link in the physical layer (with a capacity of 155 Mbit/s). The difference in bitrate (5 Mbit/s) is due to the adaptation and termination function between the HOP and physical layer. fiber (622 Mbit/s) STM-4 network connection Transport networks: Technology Examples 26

HOP Network Connection
matrix connection VC-4 matrix fiber (622 Mbit/s) 1 4 N=4 VC-4 link connection VC-4 link VC-4 link This example shows a VC-4 network connection as a concatenation of a VC-4 link connection, a VC-4 matrix connection and a VC-4 link connection. The VC-4 matrix is realised by a cross-connect. Transport networks: Technology Examples 27

Lower Order Path Layer LOP layer HOP layer Physical layer
VC-12 VC-2 VC-3 VC-4 = 63 VC-12 VC-4 = 21 VC-2 VC-4 = 3 VC-3 VC-4 = combination of VC-3, VC-2 and VC-12 63 VC-12 21 VC-2 3 VC-3 LOP layer 1 4 N=4 HOP layer SDH considers another layer above the HOP (Higher Order Path) layer: the LOP or Lower Order Path layer. This layer has different possibilities for the characteristic information: VC-12 (2.3 Mbit/s), VC-2 (6.9 Mbit/s) and VC-3 (49.5 Mbit/s). A mixture is also possible. This layer also has matrices (cross-connects) which will set up matrix connections at the different levels (VC-12, VC-2 and VC-3). Note: The Lower Order Path (LOP) layer is drawn as a higher layer (on top of HOP layer). This seems confusing but the LOP layer carries the lower bitrates (that is why it is called Lower). Physical layer Transport networks: Technology Examples 28

LOP Network Connection
VC-12 network connection VC-12 matrix connection VC-12 link connection VC-12 link connection VC-12 link VC-12 link fiber (622 Mbit/s) N=4 VC-4 link connection VC-4 matrix fiber (622 Mbit/s) N=4 VC-4 link connection VC-4 matrix In the example we observe a VC-12 connection as a concatenation of a VC-12 link connection, a VC-12 matrix connection and a VC-12 link connection. The VC-12 link connections are part of a VC-12 link. The VC-12 links are based on VC-4 network connections. Each VC-4 network connection is a concatenation of a VC-4 link connection, a VC-4 matrix connection and a VC-4 link connection. Transport networks: Technology Examples 29

Exercise VC-3 ADM VC-4 DXC switch STM-4 links full interconnection of switches with VC-3 network connections ring topology interconnection of VC-3 ADMs with VC-4 network connections use physical star topology In order to clarify the layer structure in SDH, we will use a simple example. We consider 4 telephone switches connected to a VC-3 ADM (LOP ADM). The VC-3 ADMs are connected to a central VC-4 cross-connect by using STM-4 links (physical star topology). We want to have a fully interconnected logical topology for the telephone switches. In the next slides we will show the different layers: physical layer, HOP layer, LOP layer and circuit layer. Transport networks: Technology Examples 30

Physical Layer Transport networks: Technology Examples 31
In the physical layer there is no difference between the link, link connection and network connection. There are no matrices. Transport networks: Technology Examples 31

HOP Layer Transport networks: Technology Examples 32
In the HOP layer we observe the links, matrices, link connections (4 in one link) and the matrix connections (4 in total). Transport networks: Technology Examples 32

LOP Layer Transport networks: Technology Examples 33
The LOP layer shows a ring topology: the VC-3 matrices are connected in a ring. This is because the network connections in the HOP layer provide this ring topology (see slide before). Transport networks: Technology Examples 33

Circuit Layer Transport networks: Technology Examples 34
The final result is a full mesh topology in the circuit layer. Each link has a VC-3 capacity. Transport networks: Technology Examples 34

Network Elements: MUX / ADM / DXC
LOP/HOP HOP/Physical MUX DXC ADM tributary ports (to higher layer) HOP or LOP matrix There are 3 important network elements in SDH: the terminal multiplexer, the add-drop multiplexer and the digital cross-connect. The terminal multiplexer is multiplexing a number of LOP signals in a HOP signal or a number of HOP signals in an STM link. The ADM (Add-Drop Multiplexer) is a matrix (HOP or LOP) connected to 2 links (in the HOP or in the LOP layer): these are the aggregate ports. There are also network connections (HOP or LOP) which are locally terminated: the tributary ports. The matrix has a limited flexibility: matrix connections can be set up between the two different links or between a link and a tributary port. The latter are the “add-drop” connections. The ADM is typically used in ring networks, although this is not necessary. The digital cross-connect (DXC) has the largest flexibility. There can be many (HOP or LOP) links connected to the (HOP or LOP) matrix and there are also tributary ports (terminating network connections in the HOP or LOP layer). The matrix has a full flexibility. aggregate ports (to lower layer) Transport networks: Technology Examples 35

2.1 Introduction 2.2 Main layer structure 2.3 Detailed layer structure 2.4 Frame structure and synchronisation 2.5 Network elements 2.6 Network architectures WDM While the main layer structure was described in the previous subsection 2.2, the ‘physical layer’ from subsection 2.1 needs in fact some refinement to be fully compatible with the SDH functionality. Transport networks: Technology Examples 36

Overview SDH transport layers circuit layer lower order path layer
higher order path layer multiplex section layer regenerator section layer This subsection will gradually build up the different layers in SDH. The layers considered are: physical layer, regenerator section layer, multiplex section layer, higher order path layer and lower order path layer. These layers will support different client layers (circuit layers) such as IP, ATM, PDH, etc. physical layer Transport networks: Technology Examples 37

Physical Layer optical fiber coaxial cable radio link fiber link tcp
electrical The physical layer will transport the information from one place to another. Different media can be used such as optical fiber, coaxial cable or a radio link. The latter 2 options are only used for the lower bitrate signal (STM-1 at 155 Mbit/s). The service offered by this layer to its client layer (=regenerator section layer) is the transport of an electrical signal (in case of fiber after conversion to an optical signal). If we inject a bit stream from the client to the server at a termination connection point (tcp), we will observe a degradation of the signal quality (introduction of amplitude noise, phase variations, etc.) at the other end (other tcp), due to the electro-optic conversions and the transmission on the fiber. optical Transport networks: Technology Examples 38

Physical Layer transfer of bits between two physical points
laser transmitter photodetector receiver optical fiber Transmission over optical fiber will use a laser diode to convert the electrical signal in an optical signal and a photodetector to convert the signal back to an electrical signal. During transmission the optical signal will degrade, mainly due to attenuation and dispersion in the fiber. The wavelengths used to transmit the signal are 1.3 and 1.55 mm (only one wavelength per fiber, the case of multiple wavelengths leads to WDM technology, see section 3). bitrates: 155 Mbit/s, 622 Mbit/s, 2.5 Gbit/s, 10 Gbit/s Transport networks: Technology Examples 39

Regenerator Section (RS) Layer
RS trail regenerator section layer RS link connection = RS network connection fiber link connection physical layer Due to the signal degradation in the physical layer, it is necessary to regenerate the signal. This is the major task of the regenerator section layer. This layer will however also provide the possibility to measure the signal quality (bit error rate or BER). If this quality becomes too bad, alarm signals will be sent around in the network. Transport networks: Technology Examples 40

Regenerator Section (RS) Layer
signal regeneration signal monitoring signaling regenerator fiber If the physical link between two network elements is too long (e.g. a transatlantic link between two cross-connects), regenerators have to be introduced (e.g. undersea repeaters). It will however be necessary to send and retrieve information to and from these regenerators (e.g. status of transmitter, power supply, etc.). This will be done by a dedicated digital signal which also forms part of the regenerator section layer. Transport networks: Technology Examples 41

Multiplexing Section (MS) Layer
MS trail multiplex section layer MS network connection cp tcp fiber RS network connection regenerator section layer physical layer MS link connection The regenerator section layer will transport a digital signal with high quality between two end points of a RS network connection. The goal of the next layer in the SDH transmission (or transport) network is to multiplex a number of signals together in a higher bitrate signal. This will be the goal of the multiplex section (MS) layer network. There will be no flexibility in the multiplex section layer (only “point to point network connections”). The capacity (bitrate) of the multiplex section layer will depend on the number of signals one wants to carry. The following possibilities exist: - STM-1: 155 Mbit/s - STM-4: 622 Mbit/s - STM-16: 2.5 Gbit/s - STM-64: 10 Gbit/s (STM-N or Synchronous Transport Module-N ; N=1,4,16 or 64) Note that also the Multiplex Section will provide the possibility to check the quality of the signal transfer over its network connection (again by measuring the bit error rate). Also signaling will be provided between the endpoints of a MS network connection. Transport networks: Technology Examples 42

Higher Order Path (HOP) Layer
.. HOP trail HOP network connection higher order path layer HOP link connection matrix connection multiplex section layer fiber regenerator physical layer HOP link HOP link The multiplex section layer will transport a multiplexed digital signal with high quality between two end points of an MS network connection. The goal of the next layer in the SDH transmission (or transport) network is to provide flexibility in the network (the cross-connect or add-drop functionality). This will be the goal of the Higher Order Path (HOP) layer network. The flexibility will be provided by the introduction of matrices in the HOP layer network. The characteristic information of this layer network is a VC-4 or Virtual Container 4 (with a bitrate of approximately 150 Mbit/s). The figure shows the HOP link, link connection, network connection, matrix connection and trail. Note that there is no separate regenerator shown (as in the previous figure) in order not to overload the figure. It is also important to note that the matrix shown in the figure is only to illustrate the principle because in general more ports will be provided: there will be more fibers connected and there also will be local drop ports. Transport networks: Technology Examples 43

Higher Order Path (HOP) Layer
higher layer HOP layer lower layers fiber It is important to note that the matrix shown in the previous figure is only to illustrate the principle because in general more ports will be provided: there will be more fibers connected and there will also be local ports to the higher layer. Transport networks: Technology Examples 44

Lower Order Path (LOP) Layer
.. LOP trail LOP network connection lower order path layer LOP link connection LOP link matrix connection fiber multiplex section layer regenerator physical layer ... higher order path layer .. The higher order path (HOP) layer will transport a digital signal with high quality between two end points of a HOP network connection. This HOP network connection may directly support a non-SDH layer (e.g. ATM, PDH, IP) but it is also possible to support a last SDH layer: the Lower Order Path (LOP) layer. This is shown in the figure The goal of this LOP layer in the SDH network is to provide additional flexibility in the network (the cross-connect or add-drop functionality) but with a higher granularity than the HOP layer. The flexibility will be provided by the introduction of matrices in the LOP layer network. The characteristic information of this layer network is a VC-3 or Virtual Container 3 (with a bitrate of approximately 49 Mbit/s), a VC-2 (6.9 Mbit/s) or a VC-12 (2.3 Mbit/s). Note that a mixture of these VCs can be combined in a VC-4 ! The figure shows the LOP link, link connection, network connection, matrix connection and trail. Note that there is no matrix in the HOP layer shown (as in the previous figure) in order not to overload the figure. The matrix in the LOP layer is also simplified (see previous slide for HOP layer). This LOP layer will support ATM, PDH, IP, telephony, ... Transport networks: Technology Examples 45

2.1 Introduction 2.2 Main layer structure 2.3 Detailed layer structure 2.4 Frame structure and synchronisation 2.5 Network elements 2.6 Network architectures WDM Now that we know the different layers and associated functionalities of an SDH transport network, it is time to study how the different tributary traffic streams will be multiplexed and combined in an SDH frame structure. The choice of this structure is also largely influenced by the issue of synchronisation: how do we ensure that traffic streams coming from different sources, with slightly different clock speeds, can be combined into a single aggregate traffic stream ? Transport networks: Technology Examples 46

STM-1 frame ... ... frame 1 frame 2 ... frame 1 270 columns ...
1 byte/frame = 64 kbit/s ... frame 1 125 msec frame 2 1 2 3 2430 ... frame columns 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 ... 270 271 540 272 273 2161 2430 2162 2163 frame columns 9 rows ... 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 270 271 540 272 273 In SDH, all traffic is ordered in a frame structure. The SDH frame is represented as a block of bytes. One STM-1 block has 9 rows and 270 columns that are sent through the network in a normal book reading order: from left to right and from top to bottom (1 frame corresponding to 1 book page). Every 125 msec the same block is repeated. A frame has in total 2430 bytes and this is repeated with a repetition frequency of 8000 kHz (or a period of 125 msec). It is interesting to note how this frame structure has been chosen. One byte in a frame corresponds to a 64 kbit/s signal, because the 8 bits timeslot is repeated every 125 msec. This 125 msec originates from the sampling frequency of analog voice (telephony) when converting it to the digital domain: a 3.4 kHz analog signal is sampled at 8 kHz with a resolution of 8 bits, resulting in the well-known 64 kbit/s digital voice signal. For higher order SDH frames, the number of bytes within one frame will be adjusted accordingly. For instance, an STM-4 frame will consist of 9 rows and 4x270 = 1080 columns, the frame duration is still 125 msec. 2161 2430 2162 2163 2430 bytes/frame x 8 bits/byte x 8000 frames/sec = Mbit/s Higher order frames (e.g. STM-4): byte interleaving (1 byte from first VC-4, 1 from second, …) Transport networks: Technology Examples 47

STM-1 frame 261 columns ... 9 rows RSOH MSOH pointer Virtual Container VC-4 The first 9 columns of the STM-1 frame form a block: Regenerator Section Overhead (RSOH), needed for signaling and integrity checking at the RS layer Multiplex Section Overhead (MSOH), needed for signaling and integrity checking at the MS layer A pointer, for synchronisation purposes (see further). NOTE that this is not a continuous stream of 9x9 bytes because after the first 9 bytes one will transmit 261 other bytes, then again 9 bytes of the RSOH, etc. The other 261 columns are used to transport the virtual container (VC-4) in the higher order path layer. This virtual container corresponds to a bitrate of Mbit/s. It is important to understand that it is this virtual container which will be cross-connected in the HOP layer (in the matrices in this layer)! This is why one is talking about a virtual container: it is like a real container which is transported in a railway network. Note also that the lower layers did not have any flexibility (no matrices). VC-4: 261 x 9 bytes every 125 msec ==> Mbit/s Transport networks: Technology Examples 48

STM-1 frame ... VC-4 VC-12 VC-2 VC-3 Transport networks:
The first column of the VC-4 virtual container is the HOP overhead (9 bytes, see slide above: grey fields). The rest of the Virtual Container is called “container” and abbreviated: C-4. As a result one has: VC-4 = HOP-OH + C-4. The C-4 container is the effective payload which can be used. The capacity is Mbit/s (normally one talks about a capacity of 150 Mbit/s, which is a rounded value). The Lower Order Path layer is supported by the Higher Order Path layer. This HOP layer is transporting VC-4s and therefore the characteristic information of the LOP (VC-3, VC-2 and VC-12) will be transported in these VC-4s. This is schematically represented in the figure where we observe that VC-3, VC-2 and/or VC-12 virtual containers are mapped in the C-4 container. The C-4 container will have a number of pointers pointing at the first byte of the LOP VCs that are contained in the C-4. This is very similar to the pointer used to indicate the position of a VC-4 in an SDH frame. The LOP virtual containers also have overhead: LOP overhead (indicated in grey on slide above). This is similar to the HOP overhead. ... VC-4 Transport networks: Technology Examples 49

Timing issues • frame synchronisation • clock synchronisation
reference clock clck . DXC . A . STM-1 STM-4 . . . DXC DXC STM-1 B Z The pointer mechanism in SDH (see previous slides) may seem like a useless complexity at first sight, but is in fact very useful to cope with a number of timing and synchronisation issues in SDH networks. An SDH network will consist of different network elements (e.g. DXCs or Digital Cross Connects) which are distributed over a geographical region and interconnected via STM line systems. This gives rise to two important timing issues : Frame synchronisation: if for example (see figure above) 3 STM-1 signals are leaving node A, B and C with their frames exactly aligned (at times t0, t µsec, …, t0 + n*125µsec, …), they will normally not arrive aligned in node Z. This is because the distances from A, B and C to Z will usually be different, resulting in different propagation delays. If one wants to map the STM-1s (at the input of Z) into an STM-4 frame (at the output of Z), one first has to frame-align the incoming STM-1s (because the outgoing STM-4 has only one frame). Clock synchronisation: the SDH network is a synchronous network which means that all the clocks are running (more or less) at the same frequency. This will be obtained by deriving the different network element clocks from a single reference clock (see figure above). Note that still some phase variations may occur. In special cases the link to the reference clock may be broken and therefore the network element will have to work on a local clock (which will deviate after a while). STM-1 . DXC . • frame synchronisation • clock synchronisation C Transport networks: Technology Examples 50

Frame Synchronisation
VC TIMING PHASE MAINTAINED SECTION OVERHEAD ALIGNED "A" in "B" in "C" in A4 A3 A2 A1 P7 F A5 A4 A3 A2 A1 P2 F "A" out "B" out "C" out FRAME SYNCHRONISATION MACHINE B3 B2 B1 P5 F B4 B3 B2 B1 P3 F Let us first consider the issue of frame synchronisation. Before any multiplexing can be carried out in SDH network equipment, individual SDH transport signals must be synchronised within the network equipment. On the input side of the SDH equipment, the individual SDH transport signals may be misaligned in timing phase (see figure above: frame boundaries ‘F’ not arriving on same moment at input side). Frame synchronisation should align the individual SDH transport signals. In the frame synchronisation process, the Section Overhead part and the VC-4 part of the transport signals are handled differently. The Section Overhead bytes for each of the transport signals are frame synchronised (i.e. each frame starts with 9 bytes of RSOH, 261 bytes of payload, etc…). The VC-4 bytes, on the other hand, maintain the same relative timing phase relationship with respect to each other (to avoid large payload buffering needs). This is achieved by re-calculating the Pointer value associated with each VC-4 in order to accommodate any adjustment in the timing phase of the Section Overhead due to frame synchronisation. This mechanism is the main motivation for the use of a pointer in the frame structure: a new container can start at any byte of the 261x9 bytes in the STM-1 frame (see green region 3 slides back), not only at the upper left byte. C1 P4 F C2 C1 P5 F A, B, C denote transport signals F denotes framing byte Px denotes pointer byte value x Transport networks: Technology Examples 51

Clock Synchronisation: pointer processing
positive justification : carrier clock is faster than received data clock J PT JI PT+1 frame N frame N+1 frame N+2 frame N+3 PT J than received data clock negative justification : carrier clock slower PT J The second problem to deal with is the non-perfect synchronisation between different clocks in the network. The pointer mechanism is also very useful in this case. The pointers will allow a justification process by allocating more or less bytes of payload within a frame. The examples in the figure above show the actions required for negative and positive justification. In the negative justification case, the carrier clock ( = clock used to send out the frames, i.e. the clock of the considered network node) is slower than the clock of the received data (based upon the clock of the upstream network node which has sent this data). This implies that there are more payload bits available than there can be put in a normal frame (without justification). The justification will wait until there are 24 bits which can not be transported in the normal way. These 24 bits are then put in 3 bytes of the overheads: J (3 bytes of the pointer). This will be indicated (JI = Justification Indication). For the next frame the pointer value will be adjusted. Positive justification works in a similar way. In this case there are not enough bits available from the received data. From the moment that the arriving bit stream runs out of bits, 3 empty bytes will be inserted in the payload area (again indicated by JI). The reset frame will also use an adjusted pointer value. JI J PT-1 J Transport networks: Technology Examples 52

2.1 Introduction 2.2 Main layer structure 2.3 Detailed layer structure 2.4 Frame structure and synchronisation 2.5 Network elements 2.6 Network architectures WDM Now that the SDH layer structure and the STM frame are introduced, the 3 general transport network elements (MUX, ADM and DXC) can be made more concrete in the specific case of SDH technology. Transport networks: Technology Examples 53

Multiplexer (MUX) 2Mb/s 63x • VC12 STM-1 STM-1 • STM-4 VC4 2Mb/s •
The multiplexer will multiplex different tributary signals (PDH, ATM, FDDI,...) into an STM-N frame. Depending on the functionality there can be a very simple multiplexing function or a very complex one (including grooming for optimal further transport of the signals). An example of an access-multiplexer is shown in the figure (top left) where only 2-Mbit/s signals are multiplexed or (bottom left) a mixture of 2, 34 and 140 Mbit/s signals. A more complex multiplexer will combine a number of STM-1 frames into a number of STM-4 frames. Here the grooming function can be very important in order to optimise the further transport of the signals. NOTE: The minimum granularity that the SDH network can handle is the bitrate of 2 Mbit/s. Therefore a lower bitrate (e.g. 64 kbit/s) is not allowed as an input to an SDH access multiplexer. These lower bitrate signals have to be multiplexed first (in a higher network layer). Transport networks: Technology Examples 54

Add-Drop Multiplexer (ADM)
• • • 2Mb/s e.g.: 10 tributaries STM-1 bidirectional • • • STM-16 140Mb/s 2Mb/s 34 Mb/s unidirectional Two examples of an ADM are shown in the figure (unidirectional and bidirectional). The tributary ports can be a mixture of different signals as shown in the second example. The signals are dropped from and added to the passing STM frames. Transport networks: Technology Examples 55

Digital Cross-Connects (DXC)
140 Mb/s Mb/s STM1 2Mb/s 34Mb/s 140Mb/s Two examples of digital cross-connects are shown: DXC 4/4 or AU-DXC or HO-DXC or broadband digital cross-connect, which will only cross-connect at the VC-4 level. DXC 4/3/1 or TU-DXC or LO-DXC or wideband digital cross-connect which will be able to cross-connect VC-4, VC-3 and VC-12 signals. Note that there is a wide range of possibilities for DXCs. Another example is a DXC 4/1 which is capable of cross-connecting at VC-4 and VC-12. Internally there is a full interconnection possibility (as shown on the figure). In practice however, there could be severe limitations due to the matrix itself or due to the limited physical space available for tributary cards in the cross-connect rack. It is clear that the DXC has the highest functionality but that it will also be the most expensive solution. The choice within a certain network will therefore depend on the requirements. broadband DXC AU-DXC HO-DXC DXC 4/4 wideband DXC TU-DXC LO-DXC DXC 4/3/1 Transport networks: Technology Examples 56

2.1 Introduction 2.2 Main layer structure 2.3 Detailed layer structure 2.4 Frame structure and synchronisation 2.5 Network elements 2.6 Network architectures WDM The choice of these network elements will largely depend on the network architecture that is deployed. In this last subsection on SDH, an overview of the SDH network architectures that are most common in practice will be described. Transport networks: Technology Examples 57

Network architectures
Ring network architecture Meshed network architecture Hybrid network architecture General architecture The following network architectures will be discussed: ring, mesh and hybrid. Specific aspects related to network recovery (e.g. self-healing rings) will be treated in the next chapter. Transport networks: Technology Examples 58

Single ring: unidirectional
ADM add transit transit One of the most important network architectures in SDH is the ring network. This is based on the use of an Add Drop Multiplexer (ADM). ADMs have become very popular in SDH networks. In the figure above, a unidirectional ring is shown: the traffic is running in the clockwise direction, which results in different routes for both directions. We observe that, for this type of ring, a bidirectional connection always occupies the whole ring. Note 1: In practice, SDH rings will always have protection capacities (they are called SDH Self Healing Rings or SHRs). Note 2: One can also use ADMs in a linear chain (no closed ring). This is e.g. useful when some traffic has to be added and dropped between two DXCs. drop Transport networks: Technology Examples 59

Single ring: unidirectional
STM-4 ring HOP ADMs An example is given of an STM-4 unidirectional ring with HOP ADMs (adding and dropping at VC-4 level). The ring is fully occupied by 4 VC-4 bidirectional connections. Transport networks: Technology Examples 60

Single ring: bidirectional
ADM A bidirectional SDH ring will have transmission in both directions on its links. As a result, both directions of a bidirectional connection can and will always be routed along the same side of the ring. An advantage compared to the unidirectional ring is that a connection does not occupy the whole ring (see figure). This may be interesting in case of neighbouring traffic load on the ring. It is not interesting if all traffic is routed to one “gateway” node. Transport networks: Technology Examples 61

Ring network cloverleaf hierarchical central ring RING 1 RING 2
ADM RING 2 RING 1 hierarchical In most cases, a ring network will consist of more than one ring. The basic idea is shown for a 2-ring network (figure left). Other possibilities are cloverleaf and hierarchical multi-ring networks (figures right). In the clover leaf node one can route directly between two rings. In the hierarchical case one always has to go through the central ring. central ring Transport networks: Technology Examples 62

Ring network: Interconnection between rings
1 2 ADM DXC ring 1 2 ring 1 2 ADM DXC Different node architectures (see figure) are possible to interconnect 2 rings. One can use a DXC (left), two ADMs back to back (middle), and two ADMs with a DXC in-between (right). In the case where rings are interconnected at 2 or more places, one can also use the same interconnecting node architectures. Transport networks: Technology Examples 63

Mesh network DXC Transport networks: Technology Examples 64
A meshed SDH network will use digital cross-connects (DXC) which are interconnected with a number of bidirectional links. A simple example is shown in the figure. In contrast to a ring network, the routing and management of a meshed network are much more complex. On the other hand it offers more flexibility. The use of DXCs will however result in a higher cost (cost DXC >> cost ADM). Transport networks: Technology Examples 64

Hybrid network: ring/mesh
ADM RING DXC MESH In practice one can combine mesh and ring networks (see example Belgacom). Special care should be taken when interconnecting the mesh and the ring: typically there is a double interconnection for survivability reasons. Transport networks: Technology Examples 65

General network architecture
STM-16 mesh core overlay ring STM-16 regional A general network architecture may have different levels: local, regional and core. Also different combinations of ring/mesh and different bitrates are possible. If we consider the Belgacom case, we have typically a core network (mesh with overlay rings) and regional network (with possible extention to local network level). Telenet Operations is using a regional + local architecture. STM-4 local Transport networks: Technology Examples 66

Outline Transport network examples SDH WDM 3.1 Introduction
3.2 WDM point-to-point transmission 3.3 Components for optical networking (OTN) 3.4 Network elements in OTN 3.5 Layer structure in OTN 3.6 Network architectures in OTN Until a few years ago, transport networks were based on TDM or Time Division Multiplexing techniques (e.g. SDH, PDH) to reach high bitrate transmission. The next step to higher capacity will be based on optical techniques, mainly using WDM or Wavelength Division Multiplexing. Transport networks: Technology Examples 67

WDM Principle fiber l mux l demux 6 x 10 Gb/s STM-64
. STM-64 The principle of WDM (Wavelength Division Multiplexing) is very simple. We will modulate different optical signals (all separate wavelengths !) with different electrical signals (e.g. STM-64 at 10 Gbit/s). We will multiplex the different modulated wavelengths together on a single fiber where the aggregate signal can be transported over long distances. At the receiving side we will first demultiplex the different wavelengths and then convert the optical signals back to the electrical domain. Transport networks: Technology Examples 68

Why WDM ? Technology evolution: PDH: up to 565 Mbit/s
world traffic volume IP ( % / y) Voice (+ 5 % / y) As indicated on the slide above, IP traffic requirements are growing considerably worldwide (around 50 to 100 % traffic increase per year !) and have already overtaken the voice traffic volume around (Note that this is the break-even point in terms of traffic volume ! In terms of revenues, the break-even point came a few years later, since voice services are typically much more expensive per bit than IP services). This huge and still ongoing increase in IP traffic requirements puts serious constraints on the network responsible for transporting these volumes. SDH is more and more uncapable to cope with this, the only viable transport network solution to keep up with this pace is based on WDM technology. year 1998 1999 2000 2001 2002 2003 2004 Technology evolution: PDH: up to 565 Mbit/s SDH: up to 10 Gbit/s WDM: up to Tbit/s Transport networks: Technology Examples 69

Technology History switch granularity link capacity WDM SDH PDH Teleph
Over the years one has observed an evolution from analog to digital transmission, from PDH to SDH and recently from SDH to WDM. If we define the capacity of the different technologies as the typical transmission bitrate (link bitrate) used and the granularity as the typical bitrate being switched in the flexible nodes (switches or cross-connects), then we can see a clear evolution as indicated in the figure. Initially one introduced digital transmission with a capacity of 2 Mbit/s (the primary multiplexers) and a granularity of 64 kbit/s. The 64 kbit/s still forms the basic bitrate in telephony networks. A next step was to improve the transmission efficiency by allowing higher bitrates and introducing cross-connection (mainly manual but in some countries such as France and UK also automatic). The capacity became 140 Mbit/s and the granularity was 2 Mbit/s. Note that there are also intermediate bitrates (such as 8 Mbit/s and 34 Mbit/s). This technology was called PDH or Plesiochronous Digital Hierarchy. A further step (in most countries during the last decennium) was the introduction of SDH or Synchronous Digital Hierarchy. Here we typically have a granularity of 150 Mb/s (e.g. HO DXC) and a capacity of 10 Gbit/s is possible. Note again that other intermediate bitrates are also possible. A next step will be the widespread use of WDM or Wavelength Division Multiplexing. Here one can envisage a capacity of 320 Gbit/s (32 wavelengths) or even more with a granularity of 10 Gbit/s. When we visualise these data on a logarithmic graph (see figure) it becomes clear that technology steps are typically introduced at the moment the required capacity increases are in the range of a factor 30 to 60. In order to fully support former technologies the granularity of the new technology typically coincides with the link capacity of the previous technology generation. 0.01 0.1 1 10 100 1000 10000 100000 Mbit/s Transport networks: Technology Examples 70

Technology Introduction
next generation 0,01 1 10 100 1000 10000 equipment Capacity increase at 100% rate per year volume of installed WDM equipment Capacity in Gbit/s volume of installed SDH equipment The evolution of the installation of new generations transmission equipment is schematically drawn against time in the figure above. The required line capacity increase, estimated at a yearly rate of 100%, is also set out in the diagram. When the line capacity, inherent to a certain technology, is no longer able to fulfill the capacity needed, a next technology step needs to be taken. The “next generation technology” capacity is set at 10Tbit/s in the graph. It is clear that in advance of reaching “volume installation” first trials and first users will appear (not indicated on the graph). At this moment, WDM equipment is being spread more and more worldwide. volume of installed PDH equipment 0,1 Year 1994 1998 2002 2006 2010 Transport networks: Technology Examples 71

3.2 WDM point-to-point transmission 3.3 Optical switching (OTN) 3.4 Network elements in OTN 3.5 Layer structure in OTN 3.6 Network architectures in OTN In a first phase, WDM technology was breaking through as a pure transmission technique, to provide high bandwidth point-to-point transmission between network nodes (e.g. IP routers, SDH XCs). The communication network nodes – providing flexibility in the network – were still fully based on electronics. This WDM transmission technique will be described in more detail in this subsection. Transport networks: Technology Examples 72

Optical Fiber 1 0.5 1.3 1.6 1.0 attenuation dB/km wavelength mm
1.3 1.6 1.0 attenuation dB/km The basis of the WDM networks is the use of monomode optical fiber as a transmission medium. This optical fiber has a very low attenuation (below 0.5 dB/km) and a very high bandwidth (>40 THz) in the range of 1300 to 1600 nm. In practice one will typically use transmission at wavelengths around 1300 nm and around 1550 nm. These are the second and third windows. There is also a first window with relative low attenuation (around 800 nm) but still too high for long distance transmission. There are however also negative features of optical fiber transmission: dispersion, non-linearity, etc. Dispersion will result in broadening of the pulses that are sent over the fiber (because the velocity of light with slightly different wavelengths is different). Non-linearities may result in mixing of different wavelengths in a WDM line transmission system. wavelength mm low attenuation (1.3 and 1.55 mm) high bandwidth dispersion non-linear effects Transport networks: Technology Examples 73

Optical components ... ... wavelength multiplexing
transmitter: laser diode (tunable / multi-wavelength) receiver: photodetector wavelength multiplexer / demultiplexer optical amplifier Tx Rx W D M W D M In order to transmit WDM signals over an optical fiber one requires a number of optoelectronic components: Transmitter: A laser diode will generate the light at the right wavelength. Different options are: tunable transmitters that will output one wavelength which is electrically tunable, multi-wavelength lasers that will emit light at different wavelengths simultaneously, etc. Receiver: A photodetector can be used to convert the optical signal to an electrical signal. Wavelength (de) multiplexer: This passive device will combine different wavelengths at the transmitter side or split them at the receiver side (cf. functionality of a prism) . Optical amplifier: An optical amplifier will amplify the optical signal (without requiring a conversion to the electrical domain). The most commonly used optical amplifier is an EDFA or Erbium Doped Fiber Amplifier. Note: In practice, WDM equipment is usually bi-directional. For instance a WDM line system will typically consist of 2 fibers, one for each direction, with on both sides a transmitter and a receiver. On each of the two fibers, optical amplifiers can be placed, in opposite directions. 1000 km 20 Gb/s Tx Rx OA OA OA ... ... Tx wavelength multiplexing Rx Transport networks: Technology Examples 74

Optical components: Optical Amplifier: EDFA
Erbium Doped Fiber Amplifier FIBER FIBER OPTCAL AMPLIFIER gain (dB) 20 e.g. 8 WDM (no modulation) with modulation limited wavelength band (around 1.54 mm) gain not flat no regeneration noise 10 We only give one example of an optoelectronic component: the optical fiber amplifier (EDFA). We observe its gain spectrum which is not flat and limited in its wavelength range. The amplifier introduces a lot of noise. Even with these negative characteristics, the EDFA was a real break-through in the optical transmission domain, as it paved the way for long-distance transmission, by placing EDFAs at regular distances along long-distance links. A typical example of a 8-wavelength WDM system is shown on the figure. 8 wavelengths are chosen according to a equidistant pattern. By modulating each of the 8 wavelengths with 10 Gbit/s signals, we get a broadening of the signal (in the order of 10 GHz). However, this broadening is much less than the distance between the subsequent wavelengths in the 8-wavelength system. (Check this by calculation !) l(nm) Transport networks: Technology Examples 75

Optical Fiber in Europe
This picture gives an impression of the amount of fiber installed in Europe. Transport networks: Technology Examples 76

Global Fiber Network Transport networks: Technology Examples 77
The figure above gives an overview of the submarine cables of optical fibers worldwide (situation of 2004). Transport networks: Technology Examples 77

3.2 WDM point-to-point transmission 3.3 Optical switching ( OTN) 3.4 Network elements in OTN 3.5 Layer structure in OTN 3.6 Network architectures in OTN It is possible to apply the WDM technique for point-to-point transmission only, but this implies that at each flexibility point (i.e. node) in the network the optical signals must be converted to the electrical domain again and vice versa. This Optical-electrical-optical (OEO) conversion is quite expensive and would be avoided if it would be possible to switch all-optically in the network nodes, without going back to the electrical domain. For instance, if it would be possible to switch at the wavelength (containing for instance a 2.5 or 10 Gbit/s information stream) granularity level, this would allow a very efficient switching in communication networks on a coarse level. In this subsection, we will introduce some basic components to realise this optical switching functionality and give some examples of possible optical switch realisations. This will turn the WDM technology into a real networking (instead of only transmission) technology: the Optical Transport Network (OTN). The further characteristics of the OTN will be discussed in the following subsections. Note: Today, real optical switching is still very much in the research phase. The proposed components and technologies have to cope with technical challenges and economical issues (difficult for new technology to compete with mature, existing technologies). Therefore, ‘optical’ switches in the OTN nowadays are in fact still based on electronics inside: all incoming wavelength channels are converted to the electrical domain and processed and switched electronically, and converted back to the optical domain at the output. These ‘optical’ switches with an electronic core are called opaque optical switches. Transport networks: Technology Examples 78

WDM pt-to-pt  OTN Transport networks: Technology Examples 79
This migration from WDM point-to-point towards an Optical Transport Network is illustrated on the figure above. Transport networks: Technology Examples 79

Basic Components required for OTN
power splitter / combiner optical space switch wavelength (de)multiplexer wavelength filter wavelength convertor optical amplifier (pre- and booster-) optical regenerator (1R, 2R, 3R) In order to make nodes with optical switching capability, specific optoelectronic devices are required: Power splitter / combiner: will split the incoming optical power in N different parts (input = 1 fiber, output = N fibers). Optical space switch: will be able to connect an incoming port to a particular outgoing port in a flexible way (similar to electronic switches). Wavelength (de)multiplexer: functionality similar to a prism. Wavelength filter: is capable of filtering out one wavelength from a wavelength combination. Wavelength convertor: will convert an incoming wavelength to a new outgoing wavelength. Optical amplifier: is typically used as a pre-amplifier (at the receiving side of the node) or a booster amplifier (at the sending side of the node). Optical regenerator: different types exist (in order of increasing fabrication complexity): 1R-regeneration: the optical signal is reamplified but not reshaped or retimed. 2R-regeneration: the optical signal is reamplified and the electrical pulses carried by the optical signals are reshaped, but there is no retiming. 3R-regeneration: the optical signal is reamplified, reshaped and retimed. Depending on the type of optical regeneration, there will be limits on the maximum distance that can be travelled by an optical signal through the network without going back to the electrical domain. Transport networks: Technology Examples 80

MicroElectroMechanical System (MEMS)
Principle: Microscopic mirror © Lucent A first possibility to realise optical switching is based on MEMS-technology. Tiny mechanical devices (size in the order of m) are built onto semiconductor chips and are controlled mechanically to deviate (if mirror is up) or pass (if mirror is down) the small light beam. If the different wavelengths (each modulated with an information stream, see principle of WDM) are separated physically by a WDM demultiplexer (could be done by an AWG for instance, see further), each individual wavelength channel can be sent to the desired output by a 2-dimensional matrix of microscopic mirrors. (Also 3-dimensional variants of MEMS-technology exist.) The main advantages of this technology are the limited light losses in the optical switch and the possibility to build a MEMS-switch in an integrated way (which improves the scalability considerably). On the down side, due to the mechanical instead of electrical control of the switch, the time to change the switch configuration is significant (in the order of ms to flap the mirror up or down). Characteristics: + limited loss + integration ( scalability) - slow reconfiguration Transport networks: Technology Examples 81

Broadcast-and-Select architecture with Semi-conductor Optical Amplifier (SOA) gates
Principle: . . . broadcast select Another possibility is to use SOA technology to create gates that can block or pass the incoming signal. This allows to create a broadcast-and-select architecture, as explained on the figure above. All incoming light signals are broadcast to all output modules (leading to severe split losses). Then these signals have to pass through two rows of SOA gates. The first row assures that only the (multi-wavelength) light from the correct input fiber is passed, light from all other input fibers is blocked. The second row of gates, together with the following WDM multiplexer, assure that only one wavelength from this multi-wavelength signal comes through, all other wavelengths are blocked. Hence, this two-stage selection allows to select exactly the preferred input wavelength channel for the correct output, by opening and closing the correct SOA gates. These gates are controlled electronically and allow a very fast reconfiguration (order of ns). Characteristics: - split losses (light signal must be regenerated / limited scalability) + multicast possible + fast reconfiguration Transport networks: Technology Examples 82

Arrayed Waveguide Grating (AWG) and Tunable Wavelength Convertors (TWCs)
Principle: TWC AWG An AWG is a passive optical component which one special feature: if a light signal enters at a particular input port, this light signal will be sent to another output port depending on its exact wavelength. This behaviour is similar to a prism (or rainbow): if white light falls into a prism, it is split in different colors (=wavelenghts) since every color sees a different refractive index. This characteristic allows to build an optical switch based on an AWG core. At the entrance, the multi-wavelength light signal coming from a fiber is split up in its individual wavelength signals. Each wavelength signal is going through a tunable wavelength convertor, that puts the information stream on another wavelength. This TWC output wavelength is chosen exactly to make sure that the signal is following the right path through the AWG, to arrive at the correct output fiber. Characteristics: + no splitt losses + switching element itself (AWG) is passive device fast reconfiguration if TWC fast tunable Transport networks: Technology Examples 83

3.2 WDM point-to-point transmission 3.3 Optical switching (OTN) 3.4 Network elements in OTN 3.5 Layer structure in OTN 3.6 Network architectures in OTN Regardless whether the optical switches in the OTN are real or opaque optical switches, their functionality is more or less the same. Similar to SDH, one also has add-drop multiplexers and cross-connects in OTN: denoted with the terms OADM (Optical ADM) and OXC (Optical XC). Transport networks: Technology Examples 84

Network Elements: Wavelength Routing ADM
space switch passive splitter passive combiner l1 A C l2 fiber l3 filter The basic functionality of an optical add-drop multiplexer (OADM) is shown in the figure above. This network element will drop and add one or more wavelengths (example with one wavelength shown). The space switch will select one wavelength to be dropped/added. This is very similar to a VC-4 ADM in SDH. drop add Transport networks: Technology Examples 85

Network Elements: Wavelength Routing OXC
passive splitter passive combiner l1 A C l2 fiber l3 l1 B D l2 A second network element is the wavelength routing optical cross-connect (WR-OXC). This network element will cross-connect one or more wavelengths. The space switch will make the necessary connections. Note that there is no wavelength conversion in this network element: an electrical signal carried on a certain wavelength will leave the cross-connect at the same wavelength. Note: In the figure we show no locally terminated wavelengths. This is a transit OXC. l3 fixed filter space switch Transport networks: Technology Examples 86

Network Elements: Wavelength Translating OXC
passive splitter passive combiner l1 l1 l2 B D l2 l2 l1 A third network element is the wavelength translating optical cross-connect (WT-OXC). This network element will cross-connect one or more wavelengths and will at the same time convert the wavelength to another wavelength. The space switch will make the necessary connections and the wavelength convertors will do the wavelength translation. Note that there is wavelength conversion in this network element: an electrical signal carried on a certain wavelength may leave the cross-connect at another wavelength. We observe that this is very similar to a VC-4 DXC in SDH. Note: Also in this figure the OXC is a transit OXC. l3 l3 l3 fixed filter space switch wavelength convertor Transport networks: Technology Examples 87

3.2 WDM point-to-point transmission 3.3 Optical switching (OTN) 3.4 Network elements in OTN 3.5 Layer structure in OTN 3.6 Network architectures in OTN As was already indicated in the previous subsection, the functionality of an OTN will be quite similar to SDH. This is also reflected in the layer structure to represent the hierarchy in OTNs. Transport networks: Technology Examples 88

OTN layer structure .. .. OC trail OC network connection channel layer
optical OC link connection OC link connection OC link OC link matrix connection optical multiplex section layer optical amplifier The detailed OTN layer structure is depicted above. We observe the physical layer (= fiber cable) and then we go to the optical transmission section layer, optical multiplex section layer and finally the optical channel layer. The characteristic information of the channel layer is a wavelength. The higher layers can for instance be the normal SDH layers: regenerator section layer, multiplex section layer and the path layers (HOP and LOP). The optical channel layer has matrices where wavelengths are switched (see previous subsection). It is also possible to have matrices in the optical multiplex section layer (where the fiber is the characteristic information). These matrices correspond to fiber space switches. We also observe an optical amplifier (similar to an electrical regenerator in SDH). transmission optical section layer physical layer fiber fiber fiber Transport networks: Technology Examples 89

OPTICAL CROSS-CONNECT
OTN layer structure OPTICAL MULTIPLEX SECTION OPTICAL MULTIPLEX SECTION optical transmission section optical transmission section optical transmission section TRIBUTARY SIGNALS (e.g. STM-64) WDM MULTI- PLEXER WDM MULTI- PLEXER TRIBUTARY SIGNALS (e.g. STM-64) optical amplifier optical amplifier OPTICAL CROSS-CONNECT The slide above gives an overview of the OTN sections (again very similar to the SDH situation). An optical channel is a network connection in the optical channel layer and it forms an end-to-end connection between 2 electrical access points to the network. These electrical access points will be in many cases SDH or IP electrical signals but other signals are also possible (for instance ATM). OPTICAL CHANNEL Transport networks: Technology Examples 90

OPTICAL CROSS-CONNECT
SDH over OTN SDH PATH SDH MULTI- PLEXER SDH regenerator SDH MULTI- PLEXER WDM MULTI- PLEXER OPTICAL CROSS-CONNECT optical amplifier OPTICAL CHANNEL SDH regenerator DIGITAL CROSS-CONNECT This figure illustrates how an optical channel can be used in an SDH network supporting a link between two SDH regenerators. Transport networks: Technology Examples 91

OTN Overhead no access to digital signal ! ? communication channel ??? ? BER measurement ??? ? … ??? monitoring: optical power, wavelength, spectrum, SNR signaling: separate wavelength, pilot tone, separate network SDH has a very good structured overhead with a lot of functionality (signaling, error detection, frame alignment, trace identification, error indication, alarm propagation, etc.). This is much more difficult in OTN because one has no access to the electrical signal. One can consider two major problems: signaling and monitoring. Signaling: One can use a separate wavelength which will be terminated (converted to the electrical domain) in each layer (or at each section or channel end-point). This wavelength will multiplex in the electrical domain the overhead information for the different OTN layers. A second option is to modulate each wavelength with a very low frequency signal (pilot tone). This is possible because the electrical signal from the client network (e.g. an SDH signal) typically has no power in the very low frequency range. The use of a pilot tone has the advantage that it is linked to its wavelength but it has the disadvantage that it has a very low capacity (low bitrate). A third option is to use a separate signaling network. Monitoring: Monitoring of the signal quality is very important in order to provide a reliable service to the client layers of the OTN network. Because one has no access to the electrical signal it is not possible to do a bit error rate (BER) measurement as in SDH. One has to use other techniques to know something about the signal: optical power measurement, wavelength, signal to noise ratio, etc. Transport networks: Technology Examples 92

3.2 WDM point-to-point transmission 3.3 Optical switching (OTN) 3.4 Network elements in OTN 3.5 Layer structure in OTN 3.6 Network architectures in OTN Also the network architectures in OTN will be quite similar to SDH. In this subsection, we will give a few examples. Transport networks: Technology Examples 93

Optical Ring Networks l1 l2 l3 l4 OTN ring network (unidirectional)
We show a unidirectional optical ring based on the use of OADMs. This is very similar to a VC-4 SDH STM-4 ring. Transport networks: Technology Examples 94

Optical Mesh Networks: Wavelength Routing
SDH l3 We show a meshed network based on the use of WR-OXCs. This means that there is no wavelength conversion in the network. This is schematically illustrated by representing each wavelength as a wavelength plane (in this example we consider 3 wavelengths). Note that one OXC is represented 3 times (in each wavelength plane). We consider a OTN network interconnecting two SDH cross-connects. If we leave one SDH DXC at a certain wavelength then we know that at the other side, where the optical channel is terminated, we will have the same wavelength (no wavelength conversion in the OTN network). This is equivalent to the fact that there is no possibility to change wavelength planes in the OXC’s. Transport networks: Technology Examples 95

Optical Mesh Networks: WavelengthTranslating
We show a meshed network based on the use of WT-OXCs. This means that there is wavelength conversion in the network. We consider again a OTN network interconnecting SDH cross-connects. If we leave one SDH DXC at a certain wavelength then we know that at the other side, where the optical channel is terminated, we can have another wavelength (wavelength conversion in the OTN network). This is equivalent to the fact that there is a possibility to go from one wavelength plane to another in the OXCs (indicated with the vertical lines in the nodes). SDH SDH l3 Transport networks: Technology Examples 96

CHAPTER 7 Transport Networks: Technology examples

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