CLC/TR 50083-10-1:2009
(Main)Cable networks for television signals, sound signals and interactive services - Part 10-1: Guidelines for the implementation of return paths in cable networks
Cable networks for television signals, sound signals and interactive services - Part 10-1: Guidelines for the implementation of return paths in cable networks
1.1 General Standards of the EN 50083 and EN 60728 series deal with cable networks including equipment and associated methods of measurement for headend reception, processing and distribution of television signals, sound signals and their associated data signals and for processing, interfacing and transmitting all kinds of signals for interactive services using all applicable transmission media. This includes • CATV )-networks; • MATV-networks and SMATV-networks; • individual receiving networks; and all kinds of equipment, systems and installations installed in such networks. The extent of this standardization work is from the antennas and/or special signal source inputs to the headend or other interface points to the network up to the terminal input. The standardization of any user terminals (i.e., tuners, receivers, decoders, multimedia terminals, etc.) as well as of any coaxial, balanced and optical cables and accessories thereof is excluded. 1.2 Specific scope of this Technical Report This document is intended to provide guidance to network designers on the issues which should be addressed, when considering the design of a CATV (HFC) return path. Items such as return path architecture & design, channel performance, channel planning & sources of interference, measurements, segmentation & re-segmentation, in home networks, distortion and commissioning are included. This document is not intended as a design reference but provides details which need to be addressed on individual issues relating to the design of the CATV/HFC return path.
Kabelnetze für Fernsehsignale, Tonsignale und interaktive Dienste - Teil 10-1: Leitfaden für die Einrichtung von Rückkanälen in Kabelnetzen
Réseaux de distribution par câbles pour signaux de télévision, signaux de radiodiffusion sonore et services interactifs - Partie 10-1: Lignes directrices relatives à la mise en oeuvre de la voie de retour dans les réseaux câblés
Kabelska omrežja za televizijske in zvokovne signale ter interaktivne elemente - 10-1. del: Smernice za uporabo povratnih poti v kabelskih omrežjih
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
01-april-2009
Kabelska omrežja za televizijske in zvokovne signale ter interaktivne elemente - 10
-1. del: Smernice za uporabo povratnih poti v kabelskih omrežjih
Cable networks for television signals, sound signals and interactive services - Part 10-1:
Guidelines for the implementation of return paths in cable networks
Kabelnetze für Fernsehsignale, Tonsignale und interaktive Dienste - Teil 10-1: Leitfaden
für die Einrichtung von Rückkanälen in Kabelnetzen
Réseaux de distribution par câbles pour signaux de télévision, signaux de radiodiffusion
sonore et services interactifs - Partie 10-1: Lignes directrices relatives à la mise en
oeuvre de la voie de retour dans les réseaux câblés
Ta slovenski standard je istoveten z: CLC/TR 50083-10-1:2009
ICS:
33.060.40 Kabelski razdelilni sistemi Cabled distribution systems
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
TECHNICAL REPORT
CLC/TR 50083-10-1
RAPPORT TECHNIQUE
February 2009
TECHNISCHER BERICHT
ICS 33.060.40
English version
Cable networks for television signals,
sound signals and interactive services -
Part 10-1: Guidelines for the implementation of return paths
in cable networks
Réseaux de distribution par câbles Kabelnetze für Fernsehsignale,
pour signaux de télévision, Tonsignale und interaktive Dienste -
signaux de radiodiffusion sonore Teil 10-1: Leitfaden für die Einrichtung
et services interactifs - von Rückkanälen in Kabelnetzen
Partie 10-1: Lignes directrices relatives
à la mise en oeuvre de la voie de retour
dans les réseaux câblés
This Technical Report was approved by CENELEC on 2008-12-05.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Cyprus, the
Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland and the United Kingdom.
CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
Central Secretariat: avenue Marnix 17, B - 1000 Brussels
© 2009 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. CLC/TR 50083-10-1:2009 E
Foreword
This Technical Report was prepared by the Technical Committee CENELEC TC 209, Cable networks for
television signals, sound signals and interactive services.
The text of the draft was submitted to the vote in accordance with the Internal Regulations, Part 2,
Subclause 11.4.3.3 (simple majority) and was approved by CENELEC as CLC/TR 50083-10-1 on
2008-12-05.
__________
- 3 - CLC/TR 50083-10-1:2009
Contents
1 Scope . 7
1.1 General. 7
1.2 Specific scope of this Technical Report . 7
2 Normative references . 7
3 Terms, definitions, symbols and abbreviations . 8
3.1 Terms and definitions . 9
3.2 Symbols .11
3.3 Abbreviations .12
4 Network architecture .14
4.1 HFC architecture .14
4.2 Upgrade alternatives .16
4.3 Active or passive return path .26
4.4 In building network .26
4.5 In home network .27
5 Network design .28
5.1 Considerations .28
5.2 Return path loss, path loss difference and return path slope .28
5.3 Noise and nonlinearity, optimising signal levels .32
5.4 Isolation between outlets.35
5.5 Equalisation and filtering in return paths .35
6 Channel planning .38
6.1 Purpose of this section .38
6.2 Introduction .38
6.3 Summary .38
6.4 Considerations for channel planning .39
6.5 Common path distortion products .44
6.6 European upstream bandwidths .44
6.7 Channel width .44
6.8 QPSK/16QAM operation and channel widths .44
6.9 Available return path spectrum (Table 8) .45
6.10 Channel plans .46
6.11 Network radiation .48
7 Equipment for return path implementation .48
7.1 General.48
7.2 Return path amplifiers .48
7.3 Equipment for fibre optic return links .51
8 Installation and maintenance .51
8.1 Signal level adjustment .51
8.2 Monitoring and measurements .56
Annex A (informative) Interference on return path .59
A.1 Multiple interference .59
A.2 Impulse interference .69
A.3 Interference from home terminals .71
A.4 Hum modulation .71
A.5 Common path distortion (CPD) .72
Annex B (informative) Methods of measurement .82
B.1 Noise power ratio (NPR) measurement on return path .82
B.2 10-Tone measurement .88
B.3 Modulation error ratio (MER) measurement on return path .91
B.4 Null packet and PRBS definitions .93
Annex C (informative) ITU DWDM grid .95
Bibliography .97
Figures
Figure 1 – Typical HFC topology . 14
Figure 2 – Regional network . 15
Figure 3 – Trunk-and-distribution architecture using only coaxial equipment . 16
Figure 4 – HFC system . 17
Figure 5 – Generic diagram showing the mapping of nodes and CMTS(s) to segments . 18
Figure 6 – Segment comprising a single CMTS to six optical nodes . 18
Figure 7 – Spectrum allocation bandwidth . 19
Figure 8 – Basic node architecture . 20
Figure 9 – Re-arranged feeds (two CMTS serving four nodes) . 21
Figure 10 – Optical node with frequency stacking . 22
Figure 11 – Divided node . 22
Figure 12 – Return path segmentation . 23
Figure 13 – Division of the node areas using additional fibres. . 23
Figure 14 – DWDM (CWDM) return path transmission . 24
Figure 15 – Digital return technology basic concept . 25
Figure 16 – Two return paths multiplexed to the transmission stream . 25
Figure 17 – Optical node segmentation . 26
Figure 18 – In house structures for transparent return path transmission . 27
Figure 19 – Example of forward and return path network with operating levels for the drop and
in home parts of the network . 29
Figure 20 – Example of a block diagram of return path amplifier. 49
Figure 21 – Commissioning of the forward path. 52
Figure 22 – Commissioning of the return path amplifiers using the same method as on the
forward path . 52
Figure 23 – Problem when commissioning return path amplifiers following the method used for
downstream amplifiers (standard output levels) . 53
Figure 24 – Unity gain method . 54
Figure 25 – Optical reverse path . 54
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Figure 26 – Optical node with reverse transmitter . 55
Figures in annexes
Figure A.1 – Typical spectrum of a return path . 59
Figure A.2 – Noise funnelling . 60
Figure A.3 – Average noise level vs. the number of subscribers and the return path frequency [6] . 61
Figure A.4 – Simplified equivalent circuit of a drop cable . 61
Figure A.5 – Screening effectiveness of a coaxial cable vs. frequency . 63
Figure A.6 – Spectrogram of noise level vs. frequency and time (example) . 65
Figure A.7 – Maximum, minimum and average noise levels vs. frequency (example) . 66
Figure A.8 – Centile analysis of noise levels vs. frequency (example) . 67
Figure A.9 – Temporal evolution of the -10 dB(mV) threshold crossing occurrence (example) . 68
Figure A.10 – Frequency evolution of the -10 dB(mV) threshold crossing occurrence (example) . 68
Figure A.11 – Illustration of impulse noise measurement according to the method described in
EN 60728-10 . 70
Figure A.12 – Example for the use of the return path frequency range . 72
Figure A.13 – Test set-up for CPD simulation . 74
Figure A.14 – Intermodulation products with 8 MHz spacing . 75
Figure A.15 – Contact resistance as function of contact pressure . 76
Figure A.16 – Upstream pass-band characterisation . 77
Figure A.17 – Set-up of test signals . 78
Figure A.18 – Test set-up for passive devices . 78
Figure A.19 – Test set-up for power passing devices . 79
Figure A.20 – Thermal cycle profile . 79
Figure A.21 – Spectral response with CPD in the return path . 80
Figure B.1 – Band-pass and band-stop filters response . 82
Figure B.2 – NPR test set up . 83
Figure B.3 – NPR versus RF power density applied at input of optical transmitter
and determination of OMI 100 % . 84
Figure B.4 – Example of the frequency response of the optional band-pass filter. 85
Figure B.5 – Correction factor versus level difference . 86
Figure B.6 – Example of NPR dynamic range. 88
Figure B.7 – Dynamic Range (dB) plotted versus NPR (dB) . 88
Figure B.8 – Alternative NPR measurement principle . 89
Figure B.9 – Relationship between classical NPR method and multi-tone method . 90
Figure B.10 – Test set-up for Modulation Error Ratio (MER) measurement . 91
Figure B.11 – Example of constellation diagram for a 64QAM modulation format . 92
Tables
Table 1 – Summary of in home return path losses . 31
Table 2 – Calculation of return path versus temperature . 32
Table 3 – Broadcasting allocations between 5 MHz and 42 MHz . 41
Table 4 – Amateur and Citizens Band allocations between 5 MHz and 42 MHz . 41
Table 5 – DOCSIS/EuroDOCSIS symbol rates and channel widths . 42
Table 6 – Data carriers in the gaps between broadcasting bands . 43
Table 7 – Data carriers in the gaps between broadcasting, amateur and CB bands . 43
Table 8 – Available spectrum between 5 MHz and 65 MHz . 45
Table 9 – Example of a 1,6 MHz wide channel plan (avoiding CPD products) . 46
Table 10 – Example of a 3,2 MHz wide channel plan . 47
Table 11 – Permitted radiation 0,3 MHz to 30 MHz (A-Deviation for Great Britain). 48
Table 12 – Permitted radiation 30 MHz to 68 MHz (A-Deviation for Great Britain). 48
Table 13 – Split frequencies used in Europe . 50
Table 14 – Alarm thresholds for upstream monitoring (example) . 58
Tables in annexes
Table A.1 – European EMC standards applicable to home terminals . 71
Table B.1 – Band-stop filter notch frequencies . 82
Table B.2 – Noise correction factor . 86
Table B.3 – Null transport stream packet definition . 93
Table C.1 – ITU DWDM grid. 95
- 7 - CLC/TR 50083-10-1:2009
1 Scope
1.1 General
Standards of the EN 50083 and EN 60728 series deal with cable networks including equipment and
associated methods of measurement for headend reception, processing and distribution of television
signals, sound signals and their associated data signals and for processing, interfacing and
transmitting all kinds of signals for interactive services using all applicable transmission media.
This includes
1)
• CATV -networks;
• MATV-networks and SMATV-networks;
• individual receiving networks;
and all kinds of equipment, systems and installations installed in such networks.
The extent of this standardization work is from the antennas and/or special signal source inputs to
the headend or other interface points to the network up to the terminal input.
The standardization of any user terminals (i.e., tuners, receivers, decoders, multimedia terminals,
etc.) as well as of any coaxial, balanced and optical cables and accessories thereof is excluded.
1.2 Specific scope of this Technical Report
This document is intended to provide guidance to network designers on the issues which should be
addressed, when considering the design of a CATV (HFC) return path.
Items such as return path architecture & design, channel performance, channel planning & sources
of interference, measurements, segmentation & re-segmentation, in home networks, distortion and
commissioning are included. This document is not intended as a design reference but provides
details which need to be addressed on individual issues relating to the design of the CATV/HFC
return path.
2 Normative references
The following referenced documents are indispensable for the application of this document. For
dated references, only the edition cited applies. For undated references, the latest edition of the
referenced document (including any amendments) applies.
EN 50083-2 Cable networks for television signals, sound signals and interactive services -
Part 2: Electromagnetic compatibility for equipment
EN 50083-8 Cable networks for television signals, sound signals and interactive services -
Part 8: Electromagnetic compatibility for networks
EN 55013 Sound and television broadcast receivers and associated equipment - Radio
+ A1 disturbance characteristics - Limits and methods of measurement
+ A2 (CISPR 13, mod. + A1 + A2)
EN 55020 Sound and television broadcast receivers and associated equipment - Immunity
characteristics - Limits and methods of measurement (CISPR 20)
———————
1)
This word encompasses the HFC networks used nowadays to provide telecommunications services, voice, data, audio and video,
both broadcast and narrowcast.
EN 55022 Information technology equipment - Radio disturbance characteristics - Limits and
+ A1 methods of measurement (CISPR 22, mod. + A1)
EN 55024 Information technology equipment - Immunity characteristics - Limits and methods of
+ A1 measurement (CISPR 24, mod. + A1 + A2)
+ A2
EN 60728-1 Cable networks for television signals, sound signals and interactive services -
Part 1: System performance (IEC 60728-1)
EN 60728-3 Cable networks for television signals, sound signals and interactive services -
Part 3: Active wideband equipment for coaxial cable networks (IEC 60728-3)
EN 60728-4 Cable networks for television signals, sound signals and interactive services -
Part 4: Passive wideband equipment for coaxial cable networks (IEC 60728-4)
EN 60728-5 Cable networks for television signals, sound signals and interactive services -
Part 5: Headend equipment (IEC 60728-5)
EN 60728-6 Cable networks for television signals, sound signals and interactive services -
Part 6: Optical equipment (IEC 60728-6)
EN 60728-10 Cable networks for television signals, sound signals and interactive services -
Part 10: System performance for return paths (IEC 60728-10)
EN 61280-2-2 Fibre optic communication subsystem test procedures -
Part 2-2: Digital systems - Optical eye pattern, waveform and extinction ratio
measurement (IEC 61280-2-2)
ETSI EN 300 386 Electromagnetic compatibility and Radio spectrum Matters (ERM);
Telecommunication network equipment; Electromagnetic Compatibility (EMC)
requirements
ETSI ES 201 488-1 Access and Terminals (AT); Data Over Cable Systems;
Part 1: General
ETSI ES 201 488-2 Access and Terminals (AT); Data Over Cable Systems;
Part 2: Radio Frequency Interface Specification
ETSI ES 202 488-1 Access and Terminals (AT); Second Generation Transmission Systems for
Interactive Cable Television Services - IP Cable Modems;
Part 1: General
ETSI ES 202 488-2 Access and Terminals (AT); Second Generation Transmission Systems for
Interactive Cable Television Services - IP Cable Modems;
Part 2: Radio frequency interface specification
ETSI ETS 300 800 Digital Video Broadcasting (DVB);Interaction channel for Cable TV distribution
systems (CATV)
IEC 60050 International Electrotechnical Vocabulary (IEV)
IEC 60617 database Graphical symbols for diagrams
IEC/TR 61931 Fibre optic - Terminology
ISO/IEC 13818-1 Information technology - Generic coding of moving pictures and associated audio
information: Systems
- 9 - CLC/TR 50083-10-1:2009
3 Terms, definitions, symbols and abbreviations
3.1 Terms and definitions
For the purposes of this technical report, the terms and definitions listed hereafter apply. As far as
possible the available terms and definitions are taken from IEC 60050 series and are repeated
below. The relevant IEV-numbers or other references are given in rectangular brackets after the
definition text.
3.1.1
common path distortion
intermodulation distortion of downstream signals, mainly due to nonlinearities found at metallic junctions. The
distortions are manifest as a series of beats (caused by analogue downstream channels) or a band(s) of
noise (caused by digital downstream channels) most noticeably in the upstream path. CPD may also be
present in the downstream path, but since it adds with other downstream distortions (i.e. CTB and CSO),
caused by active components, it is difficult to differentiate between the two. The nonlinear behaviour found at
passive junctions may be due to a number of reasons including corrosion, typically from exposure to the
elements, dissimilar metals, contact pressure, and junctions involving connectors contaminated with
carbonaceous materials
3.1.2
downstream direction
direction of signal flow in a cable network from the headend or any other central point (node) of a cable
network towards the subscriber
[EN 60728-10, modified]
3.1.3
forward path (downstream)
physical part of a cable network by which signals are distributed in the downstream direction from the
headend or any other central point (node) of a cable network towards the subscriber
[EN 60728-10, modified]
3.1.4
gateway
functional unit that connects two computer networks with different network architectures
EXAMPLES – LAN gateway, mail gateway
NOTE The computer networks may be either local area networks, wide area networks or other types of networks.
[IEV 732-01-16]
3.1.5
headend
assembly of equipment feeding signals into a cable network from local or external sources, including
equipment for reception and signal processing
[IEV 723-09-11, modified]
NOTE The headend may, for example, comprise antenna amplifiers, frequency converters, combiners, separators and generators.
3.1.6
hub
local area distribution point for the insertion and recovery of two-way narrowcast signals such as
DOCSIS/EuroDOCSIS with broadcast transmissions from the headend in the RF domain (frequency
multiplexing)
3.1.7
hybrid fibre coaxial network
HFC network
cable network which comprises optical equipment and cables and coaxial equipment and cables in different
parts
[EN 60728-10]
3.1.8
ingress noise
noise which is caused by electromagnetic interference into cable networks. Its power decreases with
increasing frequency. It is permanently present but slowly varies in its intensity as a function of time
[EN 60728-10]
3.1.9
(network) segment
part of a cable network comprising a set of functions and/or a specific extent of the complete cable network
[EN 60728-10]
3.1.10
network termination unit
NTU
equipment for access to the cable network connected between home network interface (HNI) and system
outlet
3.1.11
node
any point in a cable network where two or more links are interconnected
[IEV 715-08-06, modified]
3.1.12
Optical Modulation Index
OMI
the Optical Modulation Index is defined as:
-
φ φ
h l
m =
φ + φ
h l
where φ is the highest and φ is the lowest instantaneous optical power of the intensity modulated optical
h l
signal. This term is mainly used for analogue systems
[EN 60728-6]
NOTE This definition doesn’t apply to systems where the input signals are converted and transported as digital baseband signals. In
this case the terms modulation depth or extinction ratio defined in 2.6.79 and 2.7.46 of IEC/TR 61931 have to be used. A test procedure
for extinction ratio is described in EN 61280-2-2.
3.1.13
return path (upstream)
physical part of a cable network by which signals are transmitted from any subscriber, connected to the
network, to the headend or any other central point (node) of a cable network
[EN 60728-10, modified]
3.1.14
upstream direction
direction of signal flow in a cable network from a subscriber towards the headend or any other central point
(node) of a cable network
[EN 60728-10, modified]
- 11 - CLC/TR 50083-10-1:2009
3.2 Symbols
The following graphical symbols are used in the figures of this Technical Report. These symbols are
either listed in IEC 60617 or based on symbols defined in IEC 60617:
Symbol Function Symbol Function
Optical transmitter Optical transmitter
forward path return path
Optical receiver Optical receiver
forward path return path
Analogue-Digital Converter Digital-Analogue Converter
Multiplexer De-multiplexer
[IEC 60617-S01626] [IEC 60617-S01626, modified]
Attenuator (fixed) Adjustable attenuator
[IEC 60617-S01244] [IEC 60617-S01245]
Tap-off (n ports) Multi-tap (n ports)
with terminated feeder line
Distribution network Splitter
Combiner
(in the reverse direction)
Symbol Function Symbol Function
Amplifier, one-way Amplifier, two-way
[IEC 60617-S01239] [IEC 60617-S00433]
Low-pass filter High-pass filter
[IEC 60617-S01248] [IEC 60617-S01247]
Diplexer Band-pass filter
[IEC 60617-S01249]
Fibre cable Multiplier
Equalizer
3.3 Abbreviations
For the purposes of this document, the following abbreviations apply.
AC Alternating current
ADC Analogue-to-digital converter
ALSC Automatic level & slope control
AM Amplitude modulation
BNI Building network interface
BNTU Building network termination unit
C/NLD Carrier to non linear distortion ratio
CATV Community antenna television
CB Citizens band
CF Centre frequency
CMTS(s) Cable modem termination system(s)
CPD Common path distortion
CPE Customer premises equipment
- 13 - CLC/TR 50083-10-1:2009
CSO Composite second order
CTB Composite triple beat
CW Continuous wave
CWDM Coarse wavelength division multiplex
DAC Digital-to-analogue converter
DAVIC Digital Audio Visual Council
DC Direct Current
DeMUX De-multiplexer
DFB Distributed feedback (laser)
DOCSIS Data-over-cable service interface specification
DS Downstream
DVB Digital video broadcasting
DWDM Dense wavelength division multiplex
EDFA Erbium doped fibre amplifier
EMS Element management system
EuroDOCSIS European data-over-cable service interface specification
EUT Equipment under test
FM Frequency modulation
FP Fabry-Perot (laser)
FSK Frequency shift keying
HE Headend
HF High frequency
HFC Hybrid-fibre-coax
HNI Home network interface
IP Internet protocol
ISF Ingress suppression filter
MATV Master antenna television
MDU Multiple dwelling unit
MER Modulation error ratio
MUX Multiplexer
NGN Next generation network
NLD Non-linear distortion
NPR Noise power ratio
NTU Network termination unit
OMI Optical modulation index
PID Packet identifier
PIM Passive intermodulation
PMD Polarization mode dispersion
PRBS Pseudo random bit sequence
PSTN Public switched telephone network
QAM Quadrature amplitude modulation
QPSK Quadrature phase shift keying
RF Radio frequency
RP Return path
Rx Receiver
SBC Session border controller
SDH Synchronous digital hierarchy
SDU Single dwelling unit
SIP Session initiation protocol
SIR Signal to ingress ratio
SMATV Satellite master antenna television
STB Set-top box
SUT System under test
Tx Transmitter
US Upstream
VOD Video on demand
VoIP Voice-over-IP
VSB Vestigial side-band
4 Network architecture
4.1 HFC architecture
Access networks today are required to carry a multitude of different services. As a result, network
engineers and architects are challenged to build infrastructures that are able to deliver these new
services.
CATV networks, using HFC technologies have become the standard for providing both broadcast
and interactive services. In an HFC network the forward path and return path portions are closely
linked together. Where needed, for a better understanding, both portions will be considered in this
document. This document will however focus primarily on return transmission. The return path
portion of such networks is related to the corresponding standard document (EN 60728-10). The
return path frequency range of such networks (Figure 1, network portion between reference points)
is typically specified up to a maximum frequency range of 5 MHz to 65 MHz; (other frequency
ranges may apply).
NTNTUU
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VoVoicicee
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CMTSCMTS
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Figure 1 – Typical HFC topology
- 15 - CLC/TR 50083-10-1:2009
Most of the guidelines in this document are also valid for legacy networks, consisting only of an RF
distribution networks. To enable interactive services the CATV network must be made “bidirectional”
by adding diplex filters and active or passive return path hardware. The return path portion in the RF
distribution network is to be considered active, when at least one amplifier station is equipped with a
return path amplifier module; otherwise it should be considered as passive.
It is assumed that in the HFC network optical links are feeding optical nodes, which are followed by
the RF coaxial distribution network. It is also assumed that the trunk and distribution networks are
subdivided in nodal areas, which provides a more reliable and manageable topology. Greater
reliability is achieved due to the fact that an interruption in the signal path will at most affect only the
nodal area. It is also more manageable because upgrades and interventions need not be extended
over the whole network, but can be performed on a “per nodal area” and therefore phased as
needed.
The subdivision into nodal areas has an additional benefit of providing serving zones that can be
managed individually and provides greater flexibility with regards to traffic capacity and capacity
planning. At the subscriber site typically a network termination unit (NTU) is connected to the
reference point of the given network interface (HNI). An NTU (network termination unit) provides the
necessary interfaces to the system outlets.
In practice real networks are more extensive and sophisticated. Therefore the issues to be
considered have to be broadened to include possible multiple headends, hubs, cascaded optical
links and interfaces for service interconnection. Several serving zones could be combined to
connect with a regional network by means of (a) transport network(s) (Figure 2). Different
technologies are available for the transport networks. If the distance is short an RF analogue
transport network may be used. For longer distances optical transport may be employed using,
either at 1 310 nm or 1 550 nm optical links.
NOTE At 1 550 nm the fibre attenuation is lower than at 1 310 nm and also optical amplification of the signals using optical amplifiers
(EDFAs) is possible.
For long distances and high data rates high speed digital transport standards e.g. SDH, Gigabit
Ethernet, 10 Gigabit Ethernet are possible transport technologies.
MasMastterer
HHeadendeadend
PSTPSTNN
TrTraannsspport Networkort Network
InInteternernett
LLoocalcal
HHeadendeadendss
HubHub
HubHub
HuHubb
NodeNode
NodeNode
NTNTUU
NodeNode WirWireelelessss
ViVidedeoo
VoiVoiccee
NoNoddee
DaDattaa
NOTE In Next Generation Networks (NGN) PSTN gateways will be replaced by IP gateways and SIP session border controllers (SBC).
Figure 2 – Regional network
4.2 Upgrade alternatives
4.2.1 Introduction
HeadendHeadend
CoaxiCoaxiaall cablcablee and tand trrunkunk aammppllififieierr
LLiinnee a anndd d diistrstriibbuutiotionn aammpplilifiefierr
Figure 3 – Trunk-and-distribution architecture using only coaxial equipment
The HFC architecture is typically derived by means of traditional trunk-and-distribution architectures
as shown in Figure 3. During any upgrade often some of the amplifiers are replaced by optical
nodes and connected by two-way optical fibre links directly to the local headend (see Figure 4).
- 17 - CLC/TR 50083-10-1:2009
NoNodede NodeNode
HeHeadadenendd
TTwwo-wo-waayy oopptticalical
fibrefibre ccablingabling
NoNodede
CCooaaxxiiaall ccaabbllee and trand truunnkk aammpplifierlifier
LineLine and dis and disttrriibubutiotionn aammpplifilifieerr
Figure 4 – HFC system
In an early phase of any upgrade 500 ÷ 5 000 homes passed per optical node is a typical starting
value for network engineers. A “segment” serving between 500 and 20 000 homes passed consists
of several optical nodes and is configured to handle the initial data requirements. However, with the
potential requirements of future services, and increased number of connections, there is usually a
need to increase the upstream data capacity either by the use of smaller segment sizes (fewer
optical nodes per segment) or by adding more data channels. As a result a single segment could be
served by one or more downstream DOCSIS/EuroDOCSIS channels together with the associated
upstream channels.
One of the tasks for network engineers is to connect optical nodes to any termination system, in a
way which guarantees the required bandwidth to all subscribers. Most networks have been
configured with a single segment (500 to 20 000 homes passed) being served by a single
downstream feed together with a single upstream feed (Figure 6). The downstream feed typically
carries a single DOCSIS/EuroDOCSIS QAM channel with the associated upstream link carrying
between 1 and 6 upstream RF channels.
Figure 5 shows a theoretical segment for interactive traffic. This segment can consist of a number of
nodes and be served by a number of CMTS units. On “day 1” a single CMTS may serve 6 or
8 optical nodes. As traffic requirements increase this may change to a single CMTS serving a single
optical node and eventually say 4 CMTS units feeding a single node. This process is known as
“re-segmentation”.
Further options and considerations for re-segmentation are given in the following sections.
NOTE 1 Only one segment is shown in the diagram for clarity
NOTE 2 As protection against equipment failures is of paramount importance, more segments, hubs and CMTS cards are added as
required for the number of homes passed, penetration and traffic requirements
Figure 5 – Generic diagram showing the mapping of nodes and CMTS(s) to segments
Headend /Hub
OpticaOpticall NodeNodess
DownstreamDownstream
CMTSCMTS
OO
TraTrannsmittersmitter
EE
fds1fds1
EE
OO NodeNode 11
OO
EE
NN
RxRx11
fus1fus1
EE
OO
OO
EE
RxRxNN NodeNode NN
fusNfusN
EE OO
OO EE
ReReturturnn Pa Pathth
ReRececeiviveerr
fds = downstream frequency, fus = upstream frequency
Figure 6 – Segment comprising a single CMTS to six optical nodes
(no standby protection; see Figure 19 for a protection scheme)
FM radio
- 19 - CLC/TR 50083-10-1:2009
Different ways, depending on the cost and the structure of the existing network, could be used to
re-arrange this connectivity.
Historically the return path of HFC networks has been used for low speed data transmission using
FSK or QPSK modulation. In recent years the return path has been upgraded to enable the carriage
of multiple return path DOCSIS and Euro-DOCSIS channels, which allow modulations with higher
bandwidth efficiency (QPSK, 16QAM and 64QAM). Nevertheless in the coaxial portion of the
network the available bandwidth is limited. The frequency allocation (Figure 7), defined by
EN 60728-10, allows 60 MHz (at the very maximum, limited by group delay) to be shared by all
users.
Digital Video, Telephony, Data,.
Data, Telephony,.
Analogue Video
MHz
565 862
Upstream
Downstream Services
Services
Figure 7 – Spectrum allocation bandwidth
A much higher bandwidth (> 200 MHz) could be used in the return path optical link even with low
cost optical lasers and receivers. Several return path frequency ranges are up converted at different
suitable frequencies and multiplexed together in the optical node (frequency stacking). A further
possibility is the analogue-to-digital conversion of each return path frequency range in the optical
node and time domain multiplexing of several digitised return path frequency ranges (digital return).
Further detail on these options is given below.
When an interactive service is implemented, traffic is usually low, both in the forward and return
path. As more subscribers are connected and/or the levels of service are increased, there is
insufficient network capacity to handle the increase in traffic. When coaxial upstream capacity is
exhausted changes need to be made to the network, either as a whole or individual part, in order to
address this issue. (This exercise is usually referred to as “re-segmentation”).
Physically splitting the network and adding more equipment and fibres reduces the number of
subscribers served per “segment” thus increasing the available bandwidth to each subscriber. One
advantage of carrying out this option for re-segmentation is that the
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