INTERNATIONAL TELECOMMUNICATION UNION
`
`ITU-T
`
`TELECOMMUNICATION
`STANDARDIZATION SECTOR
`OF ITU
`
`G.991.1
`
`(10/98)
`
`SERIES G: TRANSMISSION SYSTEMS AND MEDIA,
`DIGITAL SYSTEMS AND NETWORKS
`Digital transmission systems – Digital sections and digital
`line system – Access networks
`
`High bit rate Digital Subscriber Line (HDSL)
`transceivers
`
`ITU-T Recommendation G.991.1
`
`(Previously CCITT Recommendation)
`
`Netflix 1018 - Page 1
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`

`
`ITU-T G-SERIES RECOMMENDATIONS
`TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS
`
`INTERNATIONAL TELEPHONE CONNECTIONS AND CIRCUITS
`INTERNATIONAL ANALOGUE CARRIER SYSTEM
`GENERAL CHARACTERISTICS COMMON TO ALL ANALOGUE CARRIER-
`TRANSMISSION SYSTEMS
`INDIVIDUAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE
`SYSTEMS ON METALLIC LINES
`GENERAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE
`SYSTEMS ON RADIO-RELAY OR SATELLITE LINKS AND INTERCONNECTION
`WITH METALLIC LINES
`COORDINATION OF RADIOTELEPHONY AND LINE TELEPHONY
`Radiotelephone circuits
`Links with mobile stations
`TESTING EQUIPMENTS
`TRANSMISSION MEDIA CHARACTERISTICS
`DIGITAL TRANSMISSION SYSTEMS
`TERMINAL EQUIPMENTS
`DIGITAL NETWORKS
`DIGITAL SECTIONS AND DIGITAL LINE SYSTEM
`General
`Parameters for optical fibre cable systems
`Digital sections at hierarchical bit rates based on a bit rate of 2048 kbit/s
`Digital line transmission systems on cable at non-hierarchical bit rates
`Digital line systems provided by FDM transmission bearers
`Digital line systems
`Digital section and digital transmission systems for customer access to ISDN
`Optical fibre submarine cable systems
`Optical line systems for local and access networks
`Access networks
`
`For further details, please refer to ITU-T List of Recommendations.
`
`G.100–G.199
`
`G.200–G.299
`
`G.300–G.399
`
`G.400–G.449
`
`G.450–G.499
`G.450–G.469
`G.470–G.499
`
`G.600–G.699
`
`G.700–G.799
`G.800–G.899
`G.900–G.999
`G.900–G.909
`G.910–G.919
`G.920–G.929
`G.930–G.939
`G.940–G.949
`G.950–G.959
`G.960–G.969
`G.970–G.979
`G.980–G.989
`G.990–G.999
`
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`

`
`ITU-T RECOMMENDATION G.991.1
`
`HIGH BIT RATE DIGITAL SUBSCRIBER LINE (HDSL) TRANSCEIVERS
`
`Summary
`This Recommendation specifies a High bit rate Digital Subscriber Line (HDSL) which is a
`bidirectional and symmetrical transmission system that allows the transport of signals with a bit rate
`of 1544 kbit/s or 2048 kbit/s on the copper twisted pairs of an access network. The basic work has
`been carried out in the ANSI T1 committee for 1544 kbit/s signals. The results of this work were
`taken by TM6 of ETSI and adopted for 2048 kbit/s signals in a technical specification, which built
`the basis for this Recommendation.
`
`The HDSL system uses echo cancellation technique for the separation of the directions of
`transmission, so that one twisted pair can carry both directions. Two different options for the line
`code are recommended, the Pulse Amplitude Modulation 2B1Q and the Carrierless Amplitude/Phase
`Modulation CAP. CAP is applicable for 2048 kbit/s only, while for 2B1Q two different frames for
`1544 kbit/s and 2048 kbit/s are defined.
`
`The 2B1Q for 2048 kbit/s caters for both duplex transmission on a single pair and parallel
`transmission on two or three-pairs. This allows for the distribution of the signal to several pairs and
`for reduction of the symbol rate and an increase of the line length. CAP is defined for one- or
`two-pairs only and the 1544 kbit/s 2B1Q for two-pairs only.
`
`Source
`ITU-T Recommendation G.991.1 was prepared by ITU-T Study Group 15 (1997-2000) and was
`approved under the WTSC Resolution No. 1 procedure on the 13th of October 1998.
`
`Recommendation G.991.1 (10/98)
`
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`
`FOREWORD
`
`ITU (International Telecommunication Union) is the United Nations Specialized Agency in the field of
`telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of
`the ITU. The ITU-T is responsible for studying technical, operating and tariff questions and issuing
`Recommendations on them with a view to standardizing telecommunications on a worldwide basis.
`
`The World Telecommunication Standardization Conference (WTSC), which meets every four years,
`establishes the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations
`on these topics.
`
`The approval of Recommendations by the Members of the ITU-T is covered by the procedure laid down in
`WTSC Resolution No. 1.
`
`In some areas of information technology which fall within ITU-T’s purview, the necessary standards are
`prepared on a collaborative basis with ISO and IEC.
`
`In this Recommendation the term recognized operating agency (ROA) includes any individual, company,
`corporation or governmental organization that operates a public correspondence service. The terms
`Administration, ROA and public correspondence are defined in the Constitution of the ITU (Geneva, 1992).
`
`NOTE
`
`INTELLECTUAL PROPERTY RIGHTS
`
`The ITU draws attention to the possibility that the practice or implementation of this Recommendation may
`involve the use of a claimed Intellectual Property Right. The ITU takes no position concerning the evidence,
`validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others
`outside of the Recommendation development process.
`
`As of the date of approval of this Recommendation, the ITU had received notice of intellectual property,
`protected by patents, which may be required to implement this Recommendation. However, implementors are
`cautioned that this may not represent the latest information and are therefore strongly urged to consult the
`TSB patent database.
`
`All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means,
`electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU.
`
` ITU 1999
`
`ii
`
`Recommendation G.991.1 (10/98)
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`ª
`

`
`CONTENTS
`
`Page
`
`1
`
`2
`
`3
`
`4
`
`5
`
`5.1
`
`5.2
`
`5.3
`
`Scope ..........................................................................................................................
`
`References ..................................................................................................................
`
`Abbreviations .............................................................................................................
`
`Reference configuration and functional description...................................................
`
`HDSL core specification ............................................................................................
`
`Functions ....................................................................................................................
`5.1.1
`Transparent transport of core frames ............................................................
`5.1.2
`Stuffing and destuffing..................................................................................
`5.1.3 CRC-6 procedures and transmission error detection ....................................
`5.1.4
`Error reporting...............................................................................................
`5.1.5
`Failure detection............................................................................................
`5.1.6
`Failure reporting............................................................................................
`5.1.7 Bit timing ......................................................................................................
`5.1.8
`Frame alignment............................................................................................
`5.1.9 HDSL transceiver autonomous start-up control............................................
`5.1.10 Loopback control and coordination...............................................................
`5.1.11 Mapping between core frames and HDSL frames ........................................
`5.1.12 Control of the maintenance channel..............................................................
`5.1.13 Synchronization and coordination of HDSL transceivers.............................
`5.1.14 Identification of pairs ....................................................................................
`5.1.15 Correction of pair identification....................................................................
`5.1.16 Remote power feeding ..................................................................................
`5.1.17 Wetting current..............................................................................................
`
`Transmission medium ................................................................................................
`5.2.1 Description ....................................................................................................
`5.2.2 Minimum Digital Local Line (DLL) requirements for HDSL applications..
`5.2.3 DLL physical characteristics.........................................................................
`5.2.4 DLL electrical characteristics........................................................................
`
`Transmission method .................................................................................................
`5.3.1 General ..........................................................................................................
`5.3.2
`Transmission on three-pairs ..........................................................................
`5.3.3
`Transmission on two-pairs ............................................................................
`5.3.4
`Transmission on one-pair..............................................................................
`5.3.5
`Transmission on four-pairs ...........................................................................
`5.3.6
`Line code.......................................................................................................
`5.3.7
`Line symbol rate............................................................................................
`
`1
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`2
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`5.4
`
`5.5
`
`5.6
`
`5.7
`
`5.8
`
`5.9
`
`6
`
`6.1
`
`Frame structure...........................................................................................................
`5.4.1 Core frame.....................................................................................................
`5.4.2
`2B1Q HDSL frame........................................................................................
`5.4.3
`Scrambling method .......................................................................................
`
`HDSL embedded operations channel (eoc)................................................................
`5.5.1
`Functions of the HDSL eoc...........................................................................
`5.5.2 HDSL eoc acknowledgement protocol .........................................................
`5.5.3
`The HDSL eoc data read/write mode............................................................
`5.5.4 HDSL eoc-message list.................................................................................
`5.5.5 HDSL eoc-message set requirements............................................................
`5.5.6 Data registers in the NTU and in regenerators..............................................
`5.5.7 Noise margin .................................................................................................
`
`Start-up procedure ......................................................................................................
`5.6.1 General ..........................................................................................................
`5.6.2 Control and status signals .............................................................................
`5.6.3
`Transmitted signals .......................................................................................
`5.6.4
`Timers ...........................................................................................................
`5.6.5 Activation state diagrams..............................................................................
`5.6.6 Regenerator related procedures.....................................................................
`
`Operation and maintenance........................................................................................
`5.7.1
`Functions at the LTU external OAM reference point ...................................
`5.7.2
`Functions at the NTU external OAM reference point...................................
`5.7.3 OAM messages and functions supported by the HDSL core........................
`5.7.4
`Power feeding related OAM functions..........................................................
`5.7.5 Regenerator behaviour ..................................................................................
`
`Electrical characteristics of a single 2B1Q transceiver..............................................
`5.8.1 General ..........................................................................................................
`5.8.2
`Transmitter/receiver impedance and return loss ...........................................
`5.8.3
`Transceiver reference clock ..........................................................................
`5.8.4
`Transmitter output characteristics.................................................................
`5.8.5 Unbalance about earth...................................................................................
`
`Performance of individual HDSL transceivers...........................................................
`5.9.1
`Performance requirements ............................................................................
`5.9.2 DLL physical models (test loops) .................................................................
`5.9.3
`Jitter and wander ...........................................................................................
`
`Common circuitry specification .................................................................................
`
`Delay difference buffer...............................................................................................
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`6.2
`
`6.3
`
`7
`
`7.1
`
`7.2
`
`7.3
`
`8
`
`8.1
`
`8.2
`
`8.3
`
`8.4
`
`The pair identification mechanism.............................................................................
`6.2.1
`Pair identification initial values ....................................................................
`6.2.2
`Pair identification at the NTU.......................................................................
`6.2.3
`Pair identification at the LTU .......................................................................
`
`Laboratory performance measurements .....................................................................
`6.3.1 General ..........................................................................................................
`6.3.2
`Test configuration .........................................................................................
`6.3.3
`Test procedure with shaped noise .................................................................
`6.3.4
`Test procedure for impulse noise ..................................................................
`6.3.5 Common mode rejection test ........................................................................
`6.3.6 Micro interruption test ..................................................................................
`
`Power feeding.............................................................................................................
`
`General .......................................................................................................................
`
`Wetting current...........................................................................................................
`
`Remote power feeding aspects...................................................................................
`7.3.1 Remote power feeding aspects at the LTU ...................................................
`7.3.2 Remote power feeding aspects at the NTU...................................................
`7.3.3 Remote power feeding aspects at the regenerator.........................................
`
`Environmental requirements ......................................................................................
`
`Climatic conditions ....................................................................................................
`
`Safety..........................................................................................................................
`
`Overvoltage protection...............................................................................................
`
`Electromagnetic compatibility (EMC) .......................................................................
`
`Annex A – Transmission system for 1544 kbit/s two-pair system application.......................
`
`A.1
`
`Frame structure of the two-pair system for 784 kbit/s................................................
`
`Annex B – High bit rate Digital Subscriber Line (HDSL) CAP based system.......................
`
`B.1
`
`B.2
`
`B.3
`
`B.4
`
`B.5
`
`Scope and general information...................................................................................
`B.1.1
`Scope.............................................................................................................
`
`References ..................................................................................................................
`
`Abbreviations .............................................................................................................
`
`Reference configuration and functional description...................................................
`
`HDSL core specification ............................................................................................
`B.5.1
`Functions.......................................................................................................
`B.5.2 Transmission medium ...................................................................................
`B.5.3 Transmission method ....................................................................................
`B.5.4
`Frame structure..............................................................................................
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`B.5.5 HDSL embedded operations channel (eoc)...................................................
`B.5.6
`Start-up procedure.........................................................................................
`B.5.7 Operation and maintenance...........................................................................
`B.5.8 Electrical characteristics of CAP-based transceivers....................................
`B.5.9
`Performance of individual HDSL transceivers .............................................
`
`B.6
`
`B.7
`
`B.8
`
`Common circuitry specification .................................................................................
`B.6.1 Delay difference buffer .................................................................................
`B.6.2 Laboratory performance measurement tests..................................................
`
`Power feeding.............................................................................................................
`
`Environmental requirements ......................................................................................
`
`Appendix I – Application specific examples...........................................................................
`
`I.1
`
`I.2
`
`I.3
`
`I.4
`
`I.5
`
`Application specific requirements for ISDN PRA with 2048 kbit/s..........................
`I.1.1 Mapping of 2048 kbit/s to HDSL..................................................................
`I.1.2 Mapping of HDSL maintenance functions to the interface...........................
`I.1.3
`Performance ..................................................................................................
`
`Application specific requirements for the European 2048 kbit/s digital unstructured
`leased line (D2048U)..................................................................................................
`I.2.1
`Application interfaces ...................................................................................
`I.2.2 Mapping of the D2048U signal to HDSL .....................................................
`I.2.3 Mapping of HDSL maintenance functions to the interface...........................
`I.2.4
`Performance ..................................................................................................
`
`Application specific requirements for the European 2048 kbit/s digital structured
`leased line (D2048S) ..................................................................................................
`I.3.1
`Application interfaces ...................................................................................
`I.3.2 Mapping of the D2048S signal to HDSL......................................................
`I.3.3 Mapping of HDSL maintenance functions to the interface...........................
`I.3.4
`Performance ..................................................................................................
`
`Application specific requirements for fractional installation.....................................
`I.4.1 Mapping of fractional services to HDSL ......................................................
`I.4.2 Mapping of HDSL maintenance functions to the interface...........................
`I.4.3
`Performance ..................................................................................................
`
`Application specific requirements for partial operation.............................................
`I.5.1 Mapping of the application frame for partial operation application .............
`I.5.2 Mapping of HDSL maintenance functions to the interface...........................
`I.5.3
`Performance ..................................................................................................
`I.5.4
`Remote power feeding ..................................................................................
`I.5.5
`Partial failure criteria.....................................................................................
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`I.6
`
`I.5.6
`I.5.7
`
`Action following partial failure.....................................................................
`Time slot prioritization/reallocation..............................................................
`
`Application specific requirements for the 2048 kbit/s mapped into TU-12 structure
`I.6.1
`Reference Configuration ...............................................................................
`I.6.2
`Application Interfaces ...................................................................................
`I.6.3 Mapping of application frame into HDSL using TU-12 structure................
`I.6.4 Mapping of HDSL maintenance functions to the interface...........................
`I.6.5
`Performance ..................................................................................................
`
`Appendix II – Detailed definition of cable characteristics and test loops...............................
`
`II.1
`
`II.2
`
`Typical characteristics of cables.................................................................................
`
`Theoretical characteristics of test loops for Y = 31 dB at 150 kHz ...........................
`
`Appendix III – Bibliography....................................................................................................
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`Recommendation G.991.1
`
`HIGH BIT RATE DIGITAL SUBSCRIBER LINE (HDSL) TRANSCEIVERS
`(Geneva, 1998)
`
`Scope
`1
`This Recommendation describes a transmission technique called High bit rate Digital Subscriber
`Line (HDSL), as a means for the transportation of several types of applications. This
`Recommendation defines the requirements for the individual HDSL transmission system, the
`transmission performance, the HDSL maintenance requirements and procedures.
`
`An individual HDSL transceiver system is a two-wire bidirectional transceiver for metallic wires
`using the echo cancellation method. Three systems may be utilized, one transporting a bit rate of
`784 kbit/s over each of two or three-pairs used in parallel, a second with an increased bit rate of
`1168 kbit/s and two-pairs in parallel only, and a third with a more increased bit rate of 2320 kbit/s on
`one-pair only.
`
`The line code of systems specified in this Recommendation is 2B1Q and CAP. The implementor
`may choose one or the other of these alternatives, only one line code has to be realized in a
`transmission system.
`
`In the main body of this Recommendation, system with 2B1Q for 2048 kbit/s applications are
`described. In Annex A, the Committee T1 recommendation for the frame structure of 1544 kbit/s
`applications on two-pairs is described. Systems using a CAP line code are covered in Annex B.
`
`This Recommendation defines the common circuitry for combining and controlling one, two or three
`HDSL transceiver systems, depending on the bit rate of the transceiver system used. The common
`circuitry and the necessary number of HDSL transceiver systems form the HDSL core, which is
`independent from the possible applications.
`
`This Recommendation does not specify all the requirements for the implementation of NTU, LTU or
`REG. It serves only to describe the functionality needed.
`
`Examples for applications of HDSL are given in Appendix I.
`
`References
`2
`The following ITU-T Recommendations and other references contain provisions which, through
`reference in this text, constitute provisions of this Recommendation. At the time of publication, the
`editions indicated were valid. All Recommendations and other references are subject to revision; all
`users of this Recommendation are therefore encouraged to investigate the possibility of applying the
`most recent edition of the Recommendations and other references listed below. A list of the currently
`valid ITU-T Recommendations is regularly published.
`
`[1]
`
`[2]
`
`[3]
`
`[4]
`
`ITU-T Recommendation G.962 (1993), Access digital section for ISDN primary rate at
`2048 kbit/s.
`
`ITU-T Recommendation G.960 (1993), Access digital section for ISDN basic rate access.
`
`ITU-T Recommendation I.431 (1993), Primary rate user-network interface; Layer 1
`specification.
`
`CCITT Fascicle I.3 (1988), Terms and definitions.
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`Recommendation G.991.1 (10/98)
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`[5]
`
`[6]
`
`[7]
`
`[8]
`
`[9]
`
`IEC 60721-3 (1984), Classification of environmental conditions. Part 3: Classification of
`groups of environmental parameters and their severities. Introduction.
`
`IEC 60950 (1991), Safety of information technology equipment.
`
`CCITT Recommendation K.17 (1998), Tests on power-fed repeaters using solid-state
`devices in order to check the arrangement for protection from external interference.
`
`ITU-T Recommendation K.20 (1996), Resistibility of
`equipment to overvoltages and overcurrents.
`
`telecommunication switching
`
`ITU-T Recommendation K.21 (1996), Resistibility of subscriber’s terminal to overvoltages
`and overcurrents.
`
`[10]
`
`CISPR 22 (1997), Information technology – Radio disturbance characteristics – Limits and
`methods of measurement.
`
`Abbreviations
`3
`This Recommendation uses the following abbreviations:
`
`2B1Q
`
`AIS
`
`BER
`
`two binary one quaternary line code
`
`Alarm Indication Signal
`
`Bit Error Ratio
`
`BERTS
`
`Bit Error Ratio Test Set
`
`BT
`
`CAP
`
`CRC
`
`DC
`
`DLL
`
`EMC
`
`eoc
`
`ETS
`
`ETSI
`
`HDSL
`
`HOH
`
`Bridged Tap, an unterminated twisted pair section bridged across the line
`
`Carrierless Amplitude/Phase modulation
`
`Cyclic Redundancy Check
`
`Direct Current
`
`Digital Local Line
`
`Electromagnetic Compatibility
`
`Embedded Operation Channel
`
`European Telecommunication Standard
`
`European Telecommunication Standards Institute
`
`High bit rate Digital Subscriber Line
`
`HDSL Overhead
`
`ISDN-BA
`
`Integrated Services Digital Network – Basic Access
`
`ISDN-PRA
`
`Integrated Services Digital Network Primary Rate Access
`
`ITU-T
`
`IUT
`
`LCL
`
`LFA
`
`LOS
`
`LSB
`
`International Telecommunication Union – Telecommunication Standardization
`Sector
`
`Item Under Test
`
`Longitudinal Conversion Loss
`
`Loss of Frame Alignment
`
`Loss Of Signal
`
`Least Significant Bit
`
`2
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`
`LTU
`
`MSB
`
`MTIE
`
`NEXT
`
`NNI
`
`NTU
`
`OAM
`
`PRBS
`
`PSD
`
`PSL
`
`r.m.s.
`
`REG
`
`REG-C
`
`REG-R
`
`SDH
`
`TMN
`
`TS
`
`TU-12
`
`UI
`
`UNI
`
`UTC
`
`Line Termination Unit
`
`Most Significant Bit
`
`Maximum Time Interval Error
`
`Near-End crosstalk
`
`Network Node Interface
`
`Network Termination Unit
`
`Operation And Maintenance
`
`Pseudo-Random Bit Sequence
`
`Power Spectral Density
`
`Power Sum Loss
`
`Root mean square
`
`Regenerator
`
`NTU side of the regenerator
`
`LTU side of the regenerator
`
`Synchronous Digital Hierarchy
`
`Telecommunication Management Network
`
`Time slot
`
`Tributary Unit-12
`
`Unit Interval
`
`User Network Interface
`
`Unable to comply
`
`VC-12
`
`Virtual Container-12
`
`Reference configuration and functional description
`4
`An access digital section which uses HDSL technology can be considered as a number of functional
`blocks, see Figure 1. Depending upon the HDSL transceiver (H) transmission rate, a fully equipped
`HDSL core consists of one 2320 kbit/s, two 1168 kbit/s, or two or three 784 kbit/s HDSL transceiver
`pairs connected by Digital Local Lines (DLLs) [which are linked by some common circuitry (C)].
`The HDSL core is application independent. Operation with a non-fully equipped HDSL core is also
`permitted.
`
`If enhanced transmission range is required the HDSL core may contain optional regenerators (REGs).
`The overall insertion loss of the HDSL core with regenerator shall be less than 1.8 times the value Y
`of the non-regenerated HDSL core. The regenerator may be inserted at any convenient intermediate
`point in the HDSL core with the limitation that the insertion loss of each part-DLL shall be less than
`0.9 times Y. In addition there may be further restrictions in line length due to power feeding.
`
`An application is defined by the interface (I) and mapping and maintenance (M) functionalities.
`
`Recommendation G.991.1 (10/98)
`
`3
`
`Netflix 1018 - Page 13
`
`

`
`The functionalities at the exchange side constitute the Line Termination Unit (LTU) and act as
`master to the (slave) customer side functionalities, which collectively form the Network Termination
`Unit (NTU) and the REGs where applicable.
`
`Application
`interface
`
`Customer side
`
`Access digital section
`
`HDSL Core
`
`DLL
`
`H
`
`REG
`
`DLL
`
`H
`
`Application
`interface
`
`MI
`
`C
`
`C
`
`IM
`
`(Note)
`
`NTU
`(Network Termination Unit)
`
`LTU
`(Line Termination Unit)
`
`T1528820-98
`
`Description of functional blocks:
`C
`Common circuitry
`H
`HDSL transceiver
`I
`Interface
`M
`Mapping
`REG
`Regenerator
`DLL
`Digital Local Line
`
`NOTE – A fully equipped HDSL core consists of one, two or three H, REG and DLL combinations depending on HDSL transceiver
`data transmission rate. REGs are optional.
`
`Figure 1/G.991.1 – Access digital section employing HDSL technology
`(simplified configuration)
`
`It should be noted that throughout this Recommendation, reference is made to the terms REG-C,
`REG-R and individual HDSL transmission systems. REG-R identifies functionalities located at the
`LTU side of the regenerator, REG-C identifies functionalities located at the NTU side of the
`regenerator.
`
`Figure 2 describes the maintenance and other communication functionalities more clearly.
`
`4
`
`Recommendation G.991.1 (10/98)
`
`Netflix 1018 - Page 14
`
`

`
`Access digital section
`
`Application
`interface
`
`HDSL core
`transparent to core frame payload
`Clock and synchronization information
`
`Inter-
`face
`
`Mapping
`
`Maintenance
`
`Common
`circuitry
`
`HDSL
`transceiver
`
`Main-
`tenance
`
`Main-
`tenance
`
`REG
`
`Main-
`tenance
`
`HDSL
`transceiver
`
`Common
`circuitry
`
`Mapping
`
`Main-
`tenance
`
`Main-
`tenance
`
`Maintenance
`
`Application
`Interface
`
`Inter-
`face
`
`Maintenance
`(Reference
`Point)
`
`Maintenance
`channel
`144 byte
`Core frame
`
`(Note)
`
`(Note)
`
`(Note)
`
`NTU
`
`LTU
`
`Digital Local
`Lines (DLLs)
`
`Bidirectional transmission
`Functional block
`
`Maintenance
`(Reference
`Point)
`
`Network side
`
`T1528830-98
`
`NOTE – A fully equipped HDSL core consists of one, two or three H, REG, DLL combinations dependi