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17 Fibre in the Loop (FITL) and Other Access Networks The advent of optical fibre communication has coincided with a worldwide trend towards deregulation of public telecommunication network services. This has caused rapid heavy investment in optical fibre networks, including access networks for the connection of customers. This in turn has brought not only focus onto the development of new modulation and multiplexing technologies for use in conjunction with optical fibres themselves, but also the development of new techniques to enable the better usage of existing copper and coaxial cable access network lineplant, as encumbent operators attempt to make the best of the installed lineplant. In this chapter we review some of the most important of these new technologies. 17.1 FIBRE ACCESS NETWORKS A number of new terms have appeared to describe different initiatives developing solutions for the deployment of fibre cables in business and residential customer access networks. These can all be classified as various forms of fibre in the loop (FZTL). Subcategories of FITL are jibre to the building (FTTB), providing direct fibre connection of business customers, office buildings of campus sites; fibre to the home (FZTH), providing video on demand (VoD), cable television and telephone services to residential premises andfibre to the curb (FTTC), whereby the fibre extends only as far as the streetside cabinet, from which existing copper or coaxial lineplant can be used to connect customer premises. Figure 17.1 illustrates these various concepts. 17.2 FIBRE TO THE BUILDING (FTTB) The main driver for FTTB (fibre to the building) has been the boom in demand from business telecommunications users for line capacity. It is nowadays usually most economic for network oeprators to lay fibre optic cable directly into large business 329 Networks and Telecommunications: Design and Operation, Second Edition. Martin P. Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic) 330 FIBRE IN THE LOOP (FITL) AND OTHER ACCESS NETWORKS copper mc and coax Figure 17.1 Fibre in the loop (FITL) premises rather than multiple pair copper cables. First, the fibre optic cable occupies far less valuable duct space; in addition, it does away with the need for amplifiers and other regenerative devices within the access network; finally, optical fibre provides plentiful capacity to meet future customer orders for bandwidth. In the simplest realization of FTTB only a standard multiplexor and an optical line terminating unit (OLTU) are required in the customer’s premises and at the exchange to provide a range of different line connections with different bitrates and line interfaces. A number of new metropolitan network operators have emerged which are building exactly such networks. Metropolitan Fibre Systems (MFS), City of London Telecommunications (COLT) and Teleport are examples. Their networks consist of fibre optic access networks, built in redundant multiple ring topologies, using SDH (synchronous digital hierarchy) transmission technology (Chapter 13). They, and similar network operators have existing networks in many large cities across the globe. The in-building multiplexor at the customer site may either be dedicated to a single customer and installed in his offices, or in many cases be shared by a number of different tenants of a large office complex; in this case being installed in a small equipment room rented by the network operator within the building as a common attachment point. 17.3 FIBRE TO THE CURB (FTTC) Fibre to the curb (FTTC) is a natural extension of the second case of fibre to the building. In fibre to the curb a shared mulitplexor is installed in a streetside cabinet rather than in an equipment room on a customer’s premises. From this point, existing copper lineplant is used to connect to individual customers. The main benefit of FTTC is the ability to rationalize the copper junction cabling (i.e. that between the streetside distribution cabinets and the exchange) as a first step in access network modernization, without requiring the upheaval or investment that would result from wholesale replacement of all copper lineplant. FIBRE TO THE HOME (FTTH) 331 Figure 17.2 A streetside cabinet providing for fibre to the curb (FTTC) (Courtesy of Siemens AG) The OPAL (optical access line) system of the Deutsche Telekom is an example of an FTTC system. The OPAL system was used extensively in the penetration and modern- ization of the former East German telephone network following the reunification of Germany in 1990. 17.4 FIBRE TO THE HOME (FTTH) The cabling of fibre directly into residential customers’ homes is usually carried out with the main objective of providing cable television or other video entertainment services like video on demand (VoD). Here, the emphasis of new passive optical networks (PON) has been the establishment of low cost, low maintenance access networks that do not require active electronic components installed in the street environment. 17.5 BROADBAND PASSIVE OPTICAL NETWORK Broadband Passive Optical Network (BPON) is a simple approach to broadband networking with a very clearly focused commercial application. It is a technology 332 FIBRE IN THE LOOP (FITL) AND OTHER ACCESS NETWORKS proposed and developed by British Telecom that is intended to bring fibre to the home. Basically, it is a network composed of monomode fibres (either in a ring or star topology) connecting telephone exchanges and peoples homes, allo’wing not only basic telephony but also the ‘broadcasting’ of cable television and video programmes. The foundation of BPON is a technique known as TPON (telephony passive optical network). This is the method by which telephone services are provided in residential homes by means of fibre connected back to the exchange (again in either a ring or star topology). Optical couplers enable the various fibre distribution joints to be made without active electronic components (Figure 15.3). Individual calls from customers are time division multiplexed (TDM) at the exchange and selectively demultiplexed by the appropriate subscriber’s receiver. By using TDM and a technique known as wavelength division multiplex (WDM, basically the use of another laser of a different wavelength light), other broadband signals may be carried over the same fibre network. Thus the broadcast of cable television and video services is possible simultaneously with the telephone operation. This is the principle of BPON (Figure 17.3). 17.6 ACCESS NETWORK INTERFACES The emergence of new active technology in the access network between customer premises and the exchange site has naturally brought with it new problems and opportunities. The problems arise from the need to devote effort to standardization of new interfaces, the opportunity is the new service functionality thereby made possible, together with the scope for network restructuring and cost optimization. Two types of interface are now being addressed by standardization work on trans- mission technology for the access network. These are local exchange (LE) to access network (AN) interfaces (designated V5 interfaces by ETSI) and the subscriber-network interface (SNI). Figure 17.4 illustrates these interfaces. 1300 nm 1550 nm Figure 17.3 Broadband passive optical network (BPON) access network (AN) Figure 17.4 Access network interfaces 17.7 ETSI V5 INTERFACES In conjunction with the modernization of the East German telephone network and the introduction of its OPAL (optical access line) technology, Deutsche Telekom recognized the potential for savings in access network lineplant, in the number of customer ports needed on telephone exchanges and in the number of telephone exchanges needed to supply a given region. This could be done by inclusion of concentration functions within the OPAL network. This lead to the development of the ETSI V5 interfaces. As Figure 17.5(a) illustrates, the access network need only support sufficient connections across itself for the actual number of telephone calls in progress. Historically, copper access networks had provided a permanent connection line for each end user (Figure 17.5(b)). This configuration requires many more connections within the access network (AN) and many more local exchange (LE) ports. In the example of Figure 17.6, ten end user terminals are connected to the local exchange. It is assumed that only a maximum of two of these terminals are in use at any one time. In the case of Figure 17.5(a), a concentrating function (i.e. simple switching function) within the access network ensures that only two through connections are required to be carried and only two ports are required at the exchange. In Figure 17.5(b), no concentration is undertaken by the access network, so that ten connections and ten exchange ports are necessary. Before the access network can undertake the concentration function, a new signalling procedure must first be defined, because it would otherwise no longer be possible for the exchange to know (merely by port of origin) which customer was wishing to make a call. The local exchange requires this information so that the correct customer is billed for the call. Similarly, for incoming calls, the local exchange must be able to signal to the access network which destination customer is to be connected. This signalling is defined in the ETSI specifications for its V5.1 and V5.2 interfaces. 334 FIBRE IN THE LOOP (FITL) AND OTHER ACCESS NETWORKS .H c) E! d V5.2 INTERFACE 335 17.8 V5.2 INTERFACE The V5.2 interface is defined in ETSI standard ETS 300 347. It defines a method for connecting up to 480 customer lines of 64 kbit/s capacity (480 simple telephone lines, 240 ISDN basic rate access lines or 16 ISDN primary rate access lines or an appropriate mix thereof) via an access network (AN) to a telephone or ISDN local exchange (LE). The access network may be connected using up to sixteen 2Mbit/s lines to the local exchange. Figure 17.6 illustrates the V5.2 interface. As the V5.2 interface provides for a concentration function (like Figure 17.5(a)) to be undertaken by the access network, the number of traffic-carrying channels at the V5.2 interface (between AN and LE) may be less than the number of customer connections required from the AN to customer premises. The protocol of V5.2 is complex and not covered in detail here. It bears some resemblance to ISDN D-channel signalling (ITU-T Q.931) and is OS1 model compliant. Main elements and terminology of the interface are as follows. Bearer channel: this is a channel with a bitrate of 64 kbit/s (or an integral multiple thereof) which is used to carry customer telephone signals or ISDN data services. Bearer channel connection (BCC) protocol: this is a protocol which allows the LE to control the AN in the allocation of bearer channels. It is one of the types of information which may be carried by an information path. Communication path (c-path): this is the path needed to carry signalling or data- type information across the V5.2 interface. Apart from the BCCprotocol, a c-path is also used for carriage of the ISDN D-channel signalling and packet or frame data originated by the various customer ISDN connections. Communication channel (c-channel): this is a 64kbit/s allocation at the V5.2 interface configured to carry a communication path. Logical communication channel (logical c-channel): this is a group of one or more c-paths. up to 480 customer connections of 64 kbitls local exchange (LE) Figure 17.6 V5.2 interface between local exchange and access network 336 FIBRE IN THE LOOP (FITL) AND OTHER ACCESS NETWORKS Physical communication channel (physical c-channel): this is an actual 64 kbit/s timeslot allocated at the V5.2 interface for carrying logical c-channels. A physical c-channel may is configured for communication and signalling and may not be used to carry bearer channels. Active c-channel: this is a physical c-channel which is currently carrying a logical c-channel. When not carrying a logical channel, the same physical c-channel becomes a standby c-channel. Standby c-channel: this is a physical c-channel which is not currently carrying a logical c-channel. Thus the 64kbit/s timeslots traversing the V5.2 interface are subdivided into bearer channels and c-channels by assignment (i.e. when the network is configured). The bearer channels serve to carry user telephone and ISDN or data connections. The c-channels serve (on an as-needed basis) to carry the BCCprotocol for allocation of bearer channels to individual calls and to carry the ISDN D-channel signalling and data information between the end user terminal and the local telephone or ISDN exchange. 17.9 V5.1 INTERFACE The V5.1 interface is a simpler version of the V5.2 interface in which the concentration feature (Figure 17.5(a)) is not included. V5.1 should be seen as the first step to V5.2. It allowed the adoption of new generation access network technology while the full specification and development of the concentration function (V5.2) took place. 17.10 SIGNIFICANCE OF THE V5.x INTERFACES The V5.1 and V5.2 intefaces (or V5.x interfaces, as they are collectively known) provide for a standard means of connecting remote switching units (RSUs) of ISDN or telephone exchanges back to a central main exchange site (Figure 17.7). exchange (site of main processor) Figure 17.7 Use of a V5 type interface to support remote switching units RE-USE OF EXISTING COPPER ACCESS NETWORKS 337 A network topology comprising central main exchange sites and remote switching units interlinked by standardized interfaces (V5) has significant benefits for public telephone network and ISDN operators. First, the number of exchange processor sites may be dramatically reduced (typically to a number of tens in a given country). This has significant investment and operational cost benefits. Second, the remote switching units (RSUs) may be purchased from multiple vendors, thus giving the network operator more leverage on price for these devices, which are needed in relatively high volume. This may lead to reluctance on behalf of the switch equipment manufacturers to developing it. 17.11 RE-USE OF EXISTING COPPER ACCESS NETWORKS The boom in demand for 2Mbit/s and higher bit rate connection services created by optical fibre technology has also had a spin-off in stimulating the development of technologies which attempt to re-use the existing copper and coaxial cable infrastructure. Three new technologies of particular interest are e HDSL (high bitrate digital subscriber line) e ADSL (asymmetric digital subscriber line) e HFC (hybridfibrelcoax networks) We discuss them in turn. 17.12 HDSL (HIGH BITRATE DIGITAL SUBSCRIBER LINE) HDSL is a technique providing for full duplex 2 Mbit/s access lines using two or three copper pairs. The technique is particularly designed to serve high speed business user needs over distances up to 3 km without having to replace the copper access network or lay new lineplant. As well as being used to provide the complete access line from customer site to exchange building, HDSL is also likely to play an important role as a complement to FTTC (Jibre to the curb) networks. In such usage HDSL provides for the final few metres from the FTTC street cabinet into the customer premises, thus potentially saving the need for a new cable or cableduct into the customer premises. 17.13 ADSL (ASYMMETRIC DIGITAL SUBSCRIBER LINE) ADSL uses technology similar to HDSL, but instead of providing 2 Mbit/s bitrates in both directions, an asymmetric pair of bitrates are provided. Downstream (i.e. from the exchange to the customer) a high bitrate of between 1.5 Mbit/s and 8 Mbit/s is intended to provided for boradcast and video-on-demand (VoD) services, as well as telephony. 338 FIBRE IN THE LOOP (FITL) AND OTHER ACCESS NETWORKS Figure 17.8 ADSL (asymmetric digital subscriber line) Upstream (i.e. from customer to exchange) the bitrate provided is much lower (between 16 and 450 kbit/s). This bitrate is only intended to be sufficient for telephony and for control of the network services (e.g. to say which video should be delivered). Figure 17.8 illustrates ADSL. 17.14 HYBRID FIBRE/COAX (HFC) NETWORKS There is considerable interest amongst coaxial cable TV companies to upgrade their networks for the needs of coming interactive video and multimedia services, and in the short term simply to offer public telephone service in addition to television broadcast service to their customers. This has led to a number of developments for telephony over coaxial cable TV networks and integration of coaxial cable networks (for attachment of customer premises) into fibre networks. These are sometimes referred to as fibre to the curb (FTTC) technologies, sometimes more specifically as HFC (hybridfibre/coax).