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- 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)
17
Fibre in the Loop (FITL)
and Other Access Networks
Theadvent ofopticalfibrecommunicationhascoincidedwithaworldwidetrendtowards
deregulation public
of telecommunicationnetwork services. This caused
has rapid heavy
investment in optical fibre networks, including access networks for the connection of customers.
This turn brought only
in has not focus the
onto development of new modulationand
multiplexingtechnologies for use inconjunctionwithopticalfibresthemselves,butalsothe
development of new techniques to enable the better usage of existing copper and coaxial cable
accessnetworklineplant, as encumbentoperatorsattempttomakethe bestof the installed
lineplant. In this chapter we review some of the most important of these new technologies.
17.1
FIBRE ACCESS NETWORKS
A numberof new termshave appearedto describe different initiatives developing
solutions for the deployment offibre cables in business and residential customer access
networks.Thesecan all be classified asvariousforms of fibre in theloop (FZTL).
Subcategories of FITL are jibre theto 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 canbe used to
connect customer premises. Figure 17.1 illustrates these various concepts.
17.2 FIBRE TOTHEBUILDING (FTTB)
The main driver for FTTB (fibre to the building) has been the boom in demand from
businesstelecommunications users for line capacity. It is nowadaysusuallymost
economic for network oeprators to lay fibre optic cable directly into large business
329
- 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 ( O L T U ) are required in the customer’s premises and at the exchange
toproviderange
a of different connections different
line with 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.Theirnetworksconsist of
fibre optic access networks, built in redundant multiple ring topologies, using SDH
(synchronous digital
hierarchy) transmissiontechnology (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
customerand installed in his offices, or in many casesbeshared by anumber of
different tenants of alarge office complex;inthiscasebeinginstalled in asmall
equipment room rented by the network operator within the building as a common
attachment point.
17.3 FIBRE TO THE CURB(FTTC)
Fibre tothecurb ( F T T C ) is a natural extension of the secondcaseof 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 upheaval
the or investment that would result wholesale
from
replacement of all copper lineplant.
- (FTTH)
FIBRE TO THE HOME 331
Figure 17.2 A streetside cabinet providing fibre to the curb(FTTC) (Courtesy o Siemens A G )
for f
The O P A L (optical access line)system of the Deutsche Telekomis 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 TOTHEHOME (FTTH)
The cabling of fibre directlyinto residential customers’ homes usually carried out with
is
the mainobjective of providing cable television or othervideo entertainment services like
video on demand (VoD). Here, the emphasisof new passive optical networks ( P O N ) has
been the establishment low cost, low maintenance
of access networks that do not require
active electronic components installed in the street environment.
17.5 BROADBANDPASSIVE OPTICAL NETWORK
BroadbandPassiveOpticalNetwork ( B P O N ) is a simple approachtobroadband
networking with a very clearly focused commercial application. It is a technology
- 332 FIBRE IN THE LOOP (FITL) AND OTHER ACCESS NETWORKS
proposed and developedby British Telecom that is intended to bring fibre to the home.
Basically,itis anetworkcomposed of monomodefibres(eitherinaringorstar
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
a
homes by means of fibre connected back to the exchange (again in either 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 wavelengthdivisionmultiplex (WDM,
basically the use of another laser of a different wavelength light), other broadband
signalsmaybecarried overthesamefibrenetwork.Thusthebroadcast 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
Theemergence of new activetechnologyinthe access networkbetweencustomer
premises andtheexchange site hasnaturallybroughtwithit new problemsand
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 thesubscriber-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 Accessnetworkinterfaces
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
recognizedthe potentialforsavingsin accessnetworklineplant,inthenumberof
customerportsneededontelephoneexchangesandinthenumberoftelephone
exchangesneededtosupplyagivenregion.Thiscould be done by inclusion of
concentration functions within the O P A L network. This lead to the developmentof the
ETSI V5 interfaces.
As Figure 17.5(a)
illustrates, access
the network only
need support sufficient
connections across itself for actual
the number of
telephone in
calls 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
anyonetime.Inthe caseofFigure17.5(a), aconcentratingfunction (i.e. simple
switchingfunction)within access
the networkensures only through
that two
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 networkcan undertake the concentration function, new signalling
a
no
procedure must first defined, because it would otherwise longer be possible for the
be
exchange to know (merely by port of origin) which customer was wishing to make a
call. The local exchange requires this information that the correct customer is billed
so
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 I THE LOOP (FITL) AND OTHER ACCESS NETWORKS
N
E!
.H
c)
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 ( 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. Mainelements 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 a protocol which allows the LE to
is
controlthe A N intheallocationof bearerchannels. 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 theV5.2 interface. Apart from theBCCprotocol, 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 local
connections exchange
of 64 kbitls (LE)
Figure 1 . V5.2 interface between local exchange and
76 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 carryinga 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 theBCCprotocol 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 theadoption of new generation access networktechnology 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 meansofconnecting remote switching units (RSUs) of ISDNor
telephone exchanges back to a central main exchange site (Figure 17.7).
exchange
(siteof main
processor)
Figure 17.7 Use of a V5 type interface to support remote switching units
- NETWORKS COPPER
ACCESS OF EXISTING
RE-USE 337
A network topology comprising central main exchange sites and remote switching
units interlinked by standardizedinterfaces (V5) has significantbenefits for public
telephone network and ISDN operators. First,the number of exchange processor sites
may be dramatically reduced (typically a number of tens in agiven country). This has
to
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.
Thismaylead to reluctance on behalf of theswitchequipmentmanufacturers 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
opticalfibretechnologyhasalsohadaspin-off in stimulatingthedevelopment of
technologies
which attempt
to re-useexisting
the 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(hybridfibrelcoaxnetworks)
We discuss them in turn.
17.12 HDSL (HIGH BITRATEDIGITAL 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 havingto replace the copper access network or
lay new lineplant. Aswell as beingused to provide the complete access linefrom
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 bitrateof 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.
- 338 FIBRE IN THE LOOP (FITL) AND OTHER ACCESS
NETWORKS
Figure 17.8 ADSL (asymmetricdigitalsubscriber 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 to a number of developments for telephony over
led
coaxial cable TV networks and integration coaxial cable networks (forattachment of
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).
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