Subcategories of FITL are jibre to the building F T T B , providing direct fibre connection of business customers, office buildings of campus sites; fibre to the home FZTH, providing
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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 t o 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
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 ( F T T B ) , 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 ( F T T C ) , 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
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)
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copper m c
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 ( O L T U ) 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 ( C O L T ) 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 ( F T T C ) 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
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Figure 17.2 A streetside cabinet providing for fibre to the curb (FTTC) (Courtesy of Siemens A G )
The O P A L (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
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 ( P O N ) has
been the establishment of low cost, low maintenance access networks that do not require active electronic components installed in the street environment
Broadband Passive Optical Network ( B P O N ) is a simple approach to broadband networking with a very clearly focused commercial application It is a technology
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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 ( W D M ,
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 ( L E ) to access network ( A N ) interfaces (designated V5 interfaces by ETSI) and the subscriber-network
interface ( S N I ) Figure 17.4 illustrates these interfaces
1300 nm
1550 nm
Figure 17.3 Broadband passive optical network (BPON)
Trang 5access 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 O P A L (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 O P A L 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 ( A N ) and many more local exchange ( L E ) 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
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.H
c)
E!
d
Trang 7V5.2 INTERFACE 335
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 ( A N ) to a telephone or ISDN local exchange ( L E ) 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 A N 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
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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
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
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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
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 H D S L (high bitrate digital subscriber line)
e A D S L (asymmetric digital subscriber line)
e HFC (hybridfibrelcoax networks)
We discuss them in turn
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 ( V o D ) services, as well as telephony
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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 ( F T T C ) technologies, sometimes more specifically as HFC (hybridfibre/coax)