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A.~.1 Wire Mediums and Terminals Wire mediums have more Man 100 years of successful communication deployments. A good overview is presented in Communications Handboolfor Traffic Control Systems, FHWA 1993. This section will not repeat Hat information, but will provide supplemental information, organized under He following categones: TWP cable plants; Wavelike modems; High speed TWP circuits (SDL`S/HDLS); and T! Hierarchy and digitized voice. A.~.~.1 TWP Cable Plants Twisted Wire Pair CTWP) cable plants have been deployed by He telephone industry for more than 100 years and Heir chata~enstics have been extensively studied for voice, data via analog wavelike modems, and T! circuits [2l, [31. More recently, motivated by He Internet and "video-on-demand," lower cost high speed techniques have been developed for deployment over existing telephone company cable plants [41. We will briefly discuss the characteristics of TWP, essentially adapting He discussion in [41. Communication capabilities and distances supportable by IWP are usually determined by the impairments presented In Table A.~.~.~-~. Prior to He 1984 divestiture, private lines were specified by FCC Tariff No. 260 which contained line conditioning specifications. Those intended for voice and data were defined as 3002 channels and specified a basic channel and types of "C" conditioning. An overview of these specifications is presented in Table A.~.~.~-2. These are end-to-end connection specifications with IWP often only serving He user-to-network portion of the link. Private ITS circuits consisting of TWP and private T1/SONET networks are usually of hider quality. These specifications are shU quoted although individual service providers may provide Heir own specifications. It is a good quality of service reference. ~:`NCHRP\Phase2~pt NCHRP 3-51 Phase 2 final Report A1-2

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Table A.~.~.~1 TWP Impairments | Propagation Loss | Attenuation of signal amplitude ty epically as a function of distance. Influenced by gauge of lWP. _.. . . Amplitude Distortion Signal distortion due to differing signal attenuation as a function of frequency over the signal bandwidth. The ideal channel would have constant attenuation over all frequencies in the signal bandwidth. . Phase Distortion Phase delay of the signal as a function of frequency. A equivalent is envelope (or group) delay which is the derivative of phase. Phase delay is expressed in seconds per km (or mile). The ideal would be linear phase delay as function of frequency or constant group delay. . . . _ . _ Cross Talk Interference Coupling of signals on different TWP within a cable. Near-end cross talk (NEXT) occurs near the transmitter. Far-End Cross Talk (FEXT) occurs near the receiver. In high count TWP cables, crosstalk is often the major determinant of throughput capacity. Gauge Changes The public telephone network typically changes to higher gauge and lower pair count cable on runs from Central Office (CO) to user. In private networks such as ITS systems, gauge changes are often not employed. Impedance Mismatches Occurs at endpoints of loops where impedance change. _ ..._ _ Temperature Changes Change is resistance and, to a lesser extent, inductance of a TWP as a result of temperature variation. Thermal Noise Browlan motion of electrons in copper. In high count TWP cables, cross talk is frequency Me major determinant of throughput capacity. TWP has been employed extensively in llS-related signal systems. TWP bandwidth and supportable bit rates decreases u ith length. Also attenuation increases win increasing frequency. Table A.-~.~.~-3 contains reference on TWP cable specification, TWP cable construction and installation, and TWP transmission systems. c;`NCHRP^ase:~p ~NCHRP 3-51 Phase 2 final Report A1-3

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Table A.~.~.~2 Line Conditioning Specifications . . _ . . 1 mplitude Distortion: AKenuationa I Phase Distortion: Envelope Delayb L . . Frequency Variation Frequency Variation Range (Hz) (dB) Range (Hz) (as) Basic 3002 300-500 -3 to +12 800-2600 1750 500-2500 -2 to +8 2500-3000 -3 to +12 C1 300 - 1000 -2 to +6 800-1000 1750 1000-2400 -1 to +3 1000-2400 1000 2400-2700 -2 to +6 2400-2600 1750 . ~ 2700-3000 ~ -3to+12 C2 300-500 -2 to +6 500-600 3000 500-2800 -1 to +3 600-1000 15050 2800-3000 -2 to +6 1000-2600 500 2600-2800 3000 , C4 300-500 -2 to +6 500-600 3000 500-3000 -2 to +3 600-800 1500 3000-3200 -2 to +6 800-1000 500 1000-2600 300 2600-2800 500 2800-3000 1500 The loss at 1004 Hz is specified as 16 + 4 dB, and losses at other frequencies are referenced to We 1004 Hz loss. Maximum delay variation within specified band. c:wc~.~t NC^P3-51 P~2F ~A1-4

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Table A.~.~.~3 TWP Standards and Related Specification [1] | Insulated Cable Engineers Association. Standard for Polyolefin Insulated Communication Cables for Outdoor Use. ANSI/ICEAS-56-434-1983. Available from ICEA. [2] Insulated Cable Engineers Association. Standards for Telecommunications Cable, Filled, Polyolefin Insulated, Copper Conductor Technical Requirements. ANSI/ICEA S-84-608-1988. Available from ICEA. [3] | Insulated able engineers association. Standard for Telecommunications Cable, Aircore, Polyolefin Insulated, Copper Conductor Technical Requirements. ANSI/ICEA S-85-625-1989. Available fro ICEA. [4] | Insulated able Engineers Association. Standard for Telecommunications Cable, Buried Distribution and Service Wire Technical Requirements. ANSI/ICEA S-86 634-1991. AvailablefromlCEA . . . [5] | REA Sp6 :ification for Filled Telephone Cables, Bulletin 345-67. PE-39, Jan. 1987. Available from Rural Electrification Administration (REA). [6] REA Specification for Filled Telephone Cables with Expanded Insulation, Bulletin 1753F-208, June 1993. [7] REA Specification for Aerial and Underground Telephone Cable, Bulletin 345-13. PE-22, Jan. 1983. [8] | REASpe,ificationforSelf-SupportingCable,Builetin345-29. PE-38,Feb.1982. [9] Generic Requirements for Metallic Telecommunications Cables. Bellcore Technical Reference TR-TSY-000421, Sept. 1991. [10] | PIC Fille ASP Cable. Bellcore Technical Reference TR-TSY-000100, June 1 988. [11] | Aircore IC ALPETH Cable. Bellcore Technical Reference TR-TSY-000101, June 1988. [12] | PIC Self upport Cable. Bellcore Technical Reference TR-TSY-000102, June 1988. [13] Underground Foam-Skin PIC Bonded STALPETH Cable. Bellcore Technical Reference TR-TSY-000106, June 1988. [14] | PlCFille Screened ASP Cable. BellcoreTechnicalReferenceTR-TSY-000109, June 1988. [15] PIC Riser Cable. Bellcore Technical Reference TR-TSY-0001 1 1, June 1988. [16] l Generi equirements for Multiple Pair Buned Wire. Bellcore Technical Reference TR-TSY-000124, Sept. 1991. [17~ ~Red TE&CM Section 635: Construction of Aerial Cable Plant. Feb. 1962, with 1966 and 1979 addenda. Available from REA. ;`NC~Phasc:.rp' NCHRP3-51e Phase2F~nalReport Al-S

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[18] [19] [20] Outside Plant Engineering Handbook. AT&T Document Development Organization, 1990. _ Hamsher, D. Communication System Engineering Handbook. McGraw-Hill Book Company, 1967 Transmission Systems for Communications. Bell Telephone Laboratories, Inc. 1982. Available from AT&T Customer Information Center. [21] REATE&CM Section 406: Transmission Facility Data. Aug. 1977. Available from REA. References rid Communications Handboolfor Traffic Control Systems, FlIWA Report No. FE1WA-SA- 93-052, April 1993. [2] Subscriber Loop Signaling and Transmission Handbook: Analog, Whitham D. Reeve, ~ k'F'.E Press, 1992. [3] Subscriber Loop Signaling aru] Transmission Handbook: Digital, Whitham D. Reeve, TIME Press, 1995. [4] 'Yhe H1)SL. Environment," HERE Journal on SelectedFAreas in Communications, Vol. 9, #6;Aug.l991. A.~.~.2 WireIine Modems Wireline modem technology has advanced significantly In the last 10 to 12 years coinciding roughly win We divestiture of Regional Bell Operating Companies (RBOCs) In 1984 and win somewhat earlier court decisions permuting attachment of customer equipment (e.g., modems, answenug machines, cordless phones, etc.) to commercial telephone lines. Equally important has been He emergence of He Personal Computer (PC) and He 'demand for remote access to data of all tYDes. Within As timeframe, low cost standard-compliant modems in the $100 to $500 puce range have evolved from speeds of 300/1200 bits per second (bps) to the current International Telecommunication Union (FLU) V.34 with an upper speed of 28,800 bps. It should be noted that some suppliers have developed propnetaTy modems, often for special t:\NCHRP`Phasc2.rp ~NCHRP 3-51 Phase 2 Anal Report A1-6

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military applications, but market demand has been minimal due to customer preferences for standards and multi-vendor interoperability. Primarily due to the extensive use of point-to-poirlt multidrop circuits in widely deployed signal systems, Me transportation community has not been able to take advantage of these advances. This section win present an oversew of current modem technology and He features and specifications that determine appropriate uses in traditional and emerging ITS applications. Recommendations on how ITS can better use modem technology win also be presented. A.~.~.2.! Modem Technology Overview MOdulate/DEModulate (Modem) technology was created to permit the transmission of digital data over the analog voice telephone network. A typical voice telephone connection provides a communication channel within the audio frequency band between approximately 200 Hz and 3700 Hz (3500 Hz bandwidths. A modem modulates a earner (i.e., a sine waveform) USA the digital data, using techniques generally similar to the FM modulation employed in commercial broadcasting but win different technical details. I;M modulation has carriers and modulation bands in He 88 - 108 MHZ RF band while modems are in He audio band. ~ technical communication terminology, modems are double sideband passband signals win symmetrical sidebands on each side of He camer frequency. The details of the camer, sidebands, and bandw~d~s of signals modulated by digital data is beyond He scope or Intent of this work; however, extensive literature is available on He theory and concepts of digital data modulation. Terminology Following are key modem definitions Hat define He applicability of a modem standard and compliant products for a given application: Bit rate (Rid or speed, is the number of bits per second (bps) Hat He modem is capable of supporting. It should be noted Cat baud rate is often incorrec~dy used interchangeably win bit rate. ~:`NCH~2.rp: NCHRP 3-51 Phase 2 final Report A1-7

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Symbol rate (Rs) sometimes caned baud rate, indicates symbols per second and the symbol penod Is (= I/ Rs) is Me time that each symbol is transmitted before changing to the next symbol. Each symbol may encode one or more bits per symbol (:Bs). Thus, Rb = Rs x Bs. Carrier frequency (fc) and modulation bandwidth 03W) define We audio spectrum required to transmit modulated data. The BW is principally determined by We symbol rate (Rs). The banded required is typicalRy 1 to 2 times We symbol rate and depends on We modulation format and We fiItenng employed in the modem transmitter. Additionally, Frequency Shift Key (FSK) modulation typically requires more bandwidth for a given symbol rate than phase modulation formats such as Phase Shift Key (PSK) and Quadrature Amplitude Modulation (QAM). The BW is symmetrical and centered at the camer frequency (fc). Modulation format defines the bits per symbol (Bs). The trend has been to increase the bits per symbol while holding the symbol rate relatively constant. Example modulation formats are presented in Table A.~.~.2.~-~. Table A.~.~.2.~1 Example Modulation Formats Modulation Format Frequency Shift Key (FSK) Binary(2) PSK (BPSK) Quadrature(4) PSK (QPSK) 16 QAM 1 Bits per symbol period, B' # Symbols = 2BS 1 2 Figure A.~.~.2.~-1 presents example FSK, PSK, and QAM modulation formats. FSK is an older modulation technique that typically employees two tones separated in frequency by an integer multiple (almost always I) of the symbol rate. FSKs bandwidth is significantly greater Han Me symbol rate. Although conceptually it can use more than two tones, in practice FSK typically uses two tones for one bit/symbol. FSK modulation is suitable for simple demodulation without canter synchronization which is advantageous in multidrop circuits where turnaround is important. Additionally, FSK and its variations have constant amplitude and are suitable for low-power linear amplifiers in battery powered mobile wireless applications. L:WC~RE~Pha=.rp ~NCHRP 3-51 Phase 2 F~1 Report A1-8

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It should be noted that PSK Ed QAM are more modem phase modulation formats and require less bandwidth Can FSK for equivalent symbol rates. Thus, PSK and QAM are He modulation formats extensively employed in higher speecI wireline modems and microwave transmissions. A specific signal-to-no~se ratio (SNR) is required to achieve an acceptable Bit-Error-Rate (BER). In modems, BERs in practice are typically io-3 to lo-6. Me required Sew to support an acceptable BER is most heavily influenced by the symbol rate and Be modulation format which collectively defines Be bit rate. A higher SNR is required to support higher symbol rates and greater bits per symbol. Representative SNRs for commercially available modems are presented in Tables A.~.~.4.~-! and A.~.~.4.~-2. The teens 2-wire or 4-w~re refer to Be number of twisted wire pairs that connect the modem to the communication network. Since each connection is typically fun duplex, a squire connection implies two physically separate end-to-end communication circuits between modems. It should be noted that public telephone voice networks typically have only one Twisted Wire Pair (TWP) between the network and the telephone handsets or modems on customer premises. This method is illustrated in Figure A.~.~.2.~-2. A hybrid balancing network is employed In each handset or modem to properly balance the levels of local and remote voice signals; however, this balancing network is inadequate for modem data signals. It should be noted Mat most public network connections mix Me transm~ttreceive signals only on a single TWP between the central office and the customer premises. More expensive, 4-Wire (2 TWP) circuits can often be procured from public telephone service providers and are common in private networks. Method of achieving two-way communication is an important consideration in modem technology. The important definitions are as follows: Simplex: One-way communication ondy (i.e., User A to User B) Half duplex: Two-way commu~cadon, but one-way at a time (i.e., alternates User A to User B. then User B to User A, etc.) L:~h~.~t NCHRP 3-51 PI 2 Few Ream A1-10

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Full duplex: Simultaneous two-way communication Full duplex actually requires two communication channels, one for each direction. Full duplex operation of data modems can be achieved in sever ways (see Figure A.~.~.2.~-3~: I. Split band modulation techniques over a single TWP where each direction occupies a different portion of the audio band. For example: one direction is between 400 and 1400 Hz, while the other direction is between 1600 and 3300. This works for lower-speed modems with symbol rates below approximately 600 symbols/second (or typically 2400 bits per second). At higher rates the spectrums overlap and corrupt each other. Thus, above approximately 2400 bps, only half duplex or simplex is possible by split band techniques. 2. Echo canceling (EC) techniques over a single TWP are employed for symbol rates above approximately 600 symbols/second. In EC, sophisticated signal processing filtering algorithms are employed to generate and subtract the approximately known outbound local signal from the received signal leaving a sufficiently dominant remote signal to be demodulated. EC requires a time consuming training sequence on the establishment of every connection. 3. 4-w~res (or 2 TWP) are employed for We entire communication link; one pair for each direction. This totally avoids We problem of two potentially interfenng signals on a single TWP and is usually We preferred solution in private networks, where extra pairs in cables typically do not add significant incremental cost. Furthermore, the need for the torte consuming training of an echo canceler is eliminated. Dial-up or dedicated typically refers to whether or not a modem has the capability to dial or answer data calls over the public network. Modems for dedicated circuits generally work over leased or private circuits that may be of higher (sometimes referred to as special) quality than He typical dial-up circuit. These dedicated circuits do not usually require dial or answer capabilities u\NCHRP\.Phas~.rpr NCHRP 3-51 Phase 2 Tonal Report A1-12

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above BR! full duplex at 160 kbps and the primary ISDN (PRI) at DS-1, 1,544 Mbps, rate. PRI uses repeatered T1 camer circuits or HDSL for subscriber links. Table A.~.~.3-2 presents a summary of high data rate circuits that are available to support ITS- related requirements. The above him speed digital circuits must accommodate certain common impairments: . Attenuation, dB/ft, dB/km, etc.; Near-end crosstalk (NEXT); Far-end crosstalk (FEXI); Amplitude and delay distortion; and ~ Noise. Cable plant design must account for these impairments and design methodologies are available. Careful consideration must be given to NEXT and FEXr when multiple circuits, consisting of Be same or different type canters, are included with multiple pair TWP cable. A.~.~.4 T] Digital Hierarchy and Digitized Voice Section A.1.2.3 discusses SONET which is a modern extension of the T1 hierarchy. Section A.2.6. 1 discusses the applicability of the T1 and SONET hierarchy to ITS applications. Origin, Funefiona/ify, end elements The ong~ns of the Digital Hierarchy, or T-Carrier Systems (or TI), were the result in New York City (NYC) of subscriber growth to He extent that analog Twisted Wire Pair (TWP) required for a single analog voice was growing beyond the capacity of existing and expandable conduit space for new installations. As a solution, early implementations of the digital hierarchy were conceived that digitized up to 24 analog voice signals into 24 digital voice signals at ~ bits~sample and 8,000 samples/sec to create a DS-O digital signal at 64,000 bps. The analog voice signal is filtered to limit He bandwidth to less than 3100-3500 Hz. These 24 DS-O digital voice signals are then Time Division Multiplexed (TDM) into a 1.544 Mbps digital signal. ~:mCHRP\Phase2 rip NCHRP 3-51 Phase 2 Fmal Report A1-44

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The concept is illustrated in Figure A.1.1.4-1. This TDM was found to be cheaper Han popular alternative Frequency Division Multiplexing (FDM) alternatives of the 1950s era. Thus, the 24 TWP for analog signals could be reduced to 2 TWP (full duplex, one for each direction). The equipment that interfaces to analog voice TWP is a channel bank and usually includes the standard telephone BORSCHT functions: Battery (normally 48 Vdc) Overvoltage Protection (lightning, etc.) Ringing (Voltage) Supervision (i.e. Dial Tone, On/Off Hook Detection, etc.) Coding (A/D, D/A Analog/Digital Conversions) Hybrid (i.e., 2 wire to 4 wire for fills duplex) Test There are several altemative channel bank options that can be selected depending on He application. The multiplexed digital signal consists of 24 DS-0 digital signals at 64 kbps and one frame bit for a total of 1.544 Mbps. This signal can be transmitted on TWP using 16 to 26 gauge cable to support repeaterIess distances of 6000 feet, the common telephone industry manhole spacing. Greater IMP repeater spacing distance and data rates are supportable with careful system design including cable plant design. The DS-! (often incorrectly used synonymously as TI) was very successful alla has expanded into He Digital TDM Hierarchy presented in Figure A.~.~.4-2 to accommodate greater multiplexing and higher data rates. The figure includes: The composite serial bit rate. Defined standard multiplexing between the venous digital signal levels. The number of 64 kbps DS-0 (64 kbps) channels multiplexed at each digital signal level. The typical transmission media. :\NCHRP\Phase:.rp ~NCHRP 3-51 Phase 2 Final Report A1-46

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As He cost of electronics decreased, the telephone industry developed cost-effective digital Central office (CO) switches so that most cans (local and L.D.) are maintained In digital form from the CO of the calling subscriber Trough all intermediate transmission, multiplexing, and switching facilities to He CO of He caned subscnber. The concept is illustrated in Figure A.1.1.4-3. Except for PBX requirements, a switching capability or CO is usually not required for ITS and over private network infrastructure requirements; however, He transmission, multiplexing, and cross-connect equipment provides excellent capabilities. Of significance to ITS applications is He availability of high volume cost- effective, equipment for ITS communications infrastructure development. The cross-connect depicted in Figure A.~.~.4-3 is an important element of modern networks. A cross-connect historically has been an electromechanical panel of jacks, plugs, and jumpers for He purpose of reconfiguring DS-O, DS-l, DS-IC, and DS-2 connections. With these panels and jumpers, the following useful system manual functions can be performed: Restoration of a failed circuit by using a spare. Rerouting for systems reconfiguration. Looping for testing during installation and maintenance. More recently, electronic cross-connects have been developed that operate at rates up to DS-3 and add significant advantage to digital transmission systems and networks by performing some of He following functions: Speed rearrangement of digital channels and circuits. Hubb~ng, grooming, and consolidation of channels and circuits. Fast rerouting of circuit, based on routine tune-of-day, or temporary requirements. Restore failed circuits quickly by rerouting if necessary. Increased flexibility by rapid electronic control. ~ Network management and centralized testing; integrated network management. . L.\NCHRP\Ph~c2.rpt NCHRP 3-51 Phase 2 Fmal Report A1-49

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Electronics cross-connects are referred to as digital access and cross-connect systems (DACS). Thus, the electromechanical DSX designators (see Figure A.~.~.4-2) become DCS. Subrate Mu/fip/exing The digital hierarchy provides standards for bit rates from 64 kbps (DS-O) to 274.176 Mbps (DISC. ITS has applications for bit rates below the DS-! and DS-O circuits. These are addressed by TI subrates to DS-1 or DS-O as depicted in Figure A.~.~.4-4 with common rates. Standards exist for subrate multiplexing, but are not as widely and consistently embraced by industry. Many non-standard, often proprietary, implementations are deployed. Of particular importance to ITS are the ELA-232 bit rates that are multiplexed in a DS-O frame. The bit rates supported are: 2.4 kbps 4.8 kbps 9.6 kbps 19.2 kbps 28.8 kbps 38.4 kbps Commercial services are available from telephone companies Mat support these DS-O subrates, typically under the name of digital data service (DDS) or subrate digital loop (SDRL). It should be noted that 56 kbps is a common available bit rate compared to the standard 64 kbps, DS-O bit rate. This lower rate is actually a DS-O frame, but with a "robbed bit" used for traditional signal functions (onion hook status). The bit rate 03R) is: Bit Rate = 7 bits~sample x 8,000 samples/sec = 56 kbps Many commercial services are available at 56 kbps, although Me modern trend is a fills "clear" 64 kbps, DS-O channel. ~:`NCHR~Phasc2 rpr NCHRP 3-51 Phase 2 Fmal Report A1-5 1

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T! Digifa/ Hierarchy Standards ant/ References The complete list of starboards for TI, Digital Hierarchy would be long. Below is an abbreviated list covering important topics. Expanded references are ~ncludecl in these documents. The U.S. T] digital hierarchy standards include: Amencan National Starboard for Telecommun~cabons. Digital Hierarchy-Formats Specifications. ANSITI.107-1988. Access Specification for High Capacity (DS]/DS3J Dedicated Digital Services. AT&T Technical Reference TR62415, June 1989. Available from AT&T Corporate Mailings. High Capacity Digital Service (] 544 Mb/sJ Interface Generic Requirements for End Users BeNcore Technical Reference TR-NPL-000054, Apn! 1989. Available from BeRcore Customer Service. Digroup Terminal and Digital Interface Frame Technical Reference and Compatibility Specification AT&T Compatibility Bulled 123, Aug. 1981. Available from AT&T Cotporate Mailings. Digital Channel Bank Requirements and! Objectives Bell System Transmission Engineenng, BeDcore Technical Reference TR4380l, Nov. 1982. Available from Beacon Customer Service. Amencan National Standard for Telecommun~cadons. Carrier-to-Customer Installation, DS] Metallic Interface ANST TI.403-1989. Requirements for Interfacing Digital Terminal Equipment to Services Employing the Exte~uled! Superframe Format AT&T Technical Reference TR54016, Sept. 1989. Available from AT&T Corporate Mailings. Data Communication Networks: Services and Facilities Interfaces Recommendations X.1- X.32. CCll-l Blue Book, Vol. VIII, Fascicle VIII.2, Geneva, 1989. Available from NITS. General Aspects of Digital Transmission Systems; Terminal Equipments Recommendations G.700-G.772. CCrrT Blue Book, Vol. m, Fascicle m.4, Geneva, 1989. Integrated Digital Loop Carrier System Generic Requirements, Objectives, and] Interface Belicore Technical Reference TR-TSY-000303, Sept. 1986, revision 4, Aug. 1991. :`NCHRP`Phase~rp' NCHRP 3-51 Phase 2 Final Report A1-53

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American National Standard for Telecommun~cabons. Integrated Services Digital Network (ISDN): Basic Access Interface for Use on MelaRic Loopsfor Application on the Network Side of the NT Player 1 SpecificationJ. A~ST Tl.601-1988. Digital Networks, Digital Sections and Digital Line Systems. CC~ Blue Book, vol. m, Fascicle m.s, Geneva, 1989. Available from NTIS, Order No. PBg9-143895. American National Standatd for Telecommun~cabons. Digital Hierarchy: Supplement to Formats Specifications (Synchronous Digital Data FormatJ. ANST TI.107b-1991. Generic Requirements for High-Bit-Rate Digital Subscriber Lines. Bedcore Technical Advisory TA-NWT-001210, act. 1991. Available from BeUc ore Customer Service. Amencan national Standard for Telecommun~cabons. Digital Hierarchy: Electrical Interfaces. ANST TI.102-1987. The subrate standards include: Subrate Data Multiplexing, A Service Function of DATAPHONE@' Digital Service. AT&T Technical Reference TR54075, Nov. 19SS. Secondary Channel in the Digital Data System: Channel Interface Requirements. BelIcore Technical Reference TR-NPL-000157, BelIcore, April 1986. Available from BeDcore Customer Service. Digital Data System Channel Interface Specifications. AT&T Technical Reference PUB 41021, March 1987. Available from AT&T Corporate Mailings. Digital Data System Channel Interface Specification. AT&T Technical Reference PIJB 62310, Nov. 1987; Addendum I, Jan. 19S8; Addendum 2, Oct. 1989; Addendum 3, Dec. 1989. Available from AT&T Corporate Mailings. D3 and D4 Subrate Dataport Channel Unit Technical Reference and Compatibility Specification. AT&T Compatibility Bulletin No. 126, April 1981. Available from AT&T Corporate Mailings. D3 and D4 56 KB Dataport Channel Unit Technical Reference and! Compatibility Specification. AT&T Compatibility Bulletin No. 141, April 1981. Available from AT&T Corporate Mailings. Digital Channel Banks: Requirements for Dataport Channel Unit Functions. Bellcore Technical Advisory TA-TSY-000077, 1986. Available from Bellcore Documents Registrar. ~:WCHRP`Phase:.rp' NCHRP3-51. Phase2FmalReport A1-54