Técnicas/Estrategias para el Control de Señales

4.4 SHDSL AND HDSL4

See Chapter 6 for a discussion of SHDSL (Single-pair High-speed Digital Subscriber Line) and HDSL4 (extended reach DSL1 transmission).

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References

[1] G.703, ITU-T Recommendation "Physical/Electrical Characteristics of Hierarchical Digital Interfaces," November 2001.

[2] G.704, ITU-T Recommendation "Synchronous Frame Structures Used at 1544, 6312, 2048, 8448 and 44 736 kbit/s Hierarchical Levels," October 1998.

[3] Committee T1, "ISDN Basic Access Interface for Use on Metallic Loops for Application at the Network Side of NT, Layer 1 Specification," Document T1.601-1999.

[4] Committee T1, "High-Bit-Rate Digital Subscriber Line (HDSL)," Technical Report TR-28, February 1994.

[5] Bellcore TA-1210 on HDSL.

[6] Committee T1, Working Group T1E1.4, "A Draft Technical Report on High-Bit-Rate Digital Subscriber Line (HDSL)," T1E1.4/96-006, April 22, 1996.

[7] ETSI TS 101 135 V1.5.3 (2000-09), "Transmission and Multiplexing (TM); High Bit-Rate Digital Subscriber Line (HDSL) Transmission Systems on Metallic Local Lines; HDSL Core Specification and Applications for Combined ISDN-BA and 2,048 kbit/s transmission," September 2000.

[8] vCommittee T1 for Telecommunications—High Bit Rate Digital Subscriber Line—2nd Generation (HDSL2), T1.418-2000.

[9] ITU-T Recommendation G.991.1, "High Bit Rate Digital Subscriber Line (HDSL) Transceivers,"

October 1998.

[10] AT&T Tech Pub 62411.

[11] ANSI T1.403-1999, "Network and Customer Installation Interfaces—DS1 Electrical Interface."

[12] ANSI T1.413-1998, "Network to Customer Installation Interfaces—Asymmetric Digital

Subscriber Line (ADSL) Metallic Interface."

[13] ITU-T Recommendation G.991.2, "Single-Pair High-Speed Digital Subscriber Line (SHDSL) Transceivers," February 2001.

[14] L. F. Wei, "Trellis-Coded Modulation with Multidimensional Constellations," IEEE Transactions on Information Theory IT-33 (1987): 483.

[15] ITU-T Recommendation V.32, "A Family of 2-Wire, Duplex Modems Operating at Data Signalling Rates of Up to 9600 bit/s for Use on the General Switched Telephone Network and on Leased Telephone-Type Circuits," March 1993.

[16] Ameritech, "Third Generation HDSL," T1E1.4/95-044, June 5, 1995.

[17] Ameritech, "Second Generation HDSL," T1E1.4/96-094, April 22, 1996.

[18] Ameritech, "HDSL2 crosstalk Interferer Model," T1E1.4/96-095, April 22, 1996.

[19] Pairgain Technologies, "Normative Text for Spectral Compatibility Evaluations," T1E1.4/97-180, June 30, 1997.

[20] Pairgain Technologies, "On the Importance of Crosstalk from Mixed Sources," T1E1.4/97-181, May 12, 1997.

[21] ADC, Adtran, Level One, Pairgain, "Performance Requirements for HDSL2 Systems,"

T1E1.4/97-469,December 8, 1997.

[22] ADC Telecommunications, "Measured Spectral Compatibility of HDSL2 with Deployed HDSL," T1E1.4/97-434, December 8, 1997.

[23] Adtran, "Test Results of HDSL Units with HDSL2 Noise Generator," T1E1.4/97-440R1, December 8, 1997.

[24] Adtran, "Simulated Performance of HDSL2 Transceivers," T1E1.4/97-444, December 8, 1997.

[25] Pairgain Technologies, "A Modulation Strategy for HDSL2," T1E1.4/96-340, November 11, 1996.

[26] Adtran, "A Modulation Technique for CSA Range HDSL2," T1E1.4/97-073, February 3, 1997.

[27] Pairgain, "Performance and Spectral Compatibility Comparison of POET PAM and OverCAPped Transmission for HDSL2," T1E1.4/97-179, May 15, 1997.

[28] Pairgain, "Performance and Spectral Compatibility of OPTIS HDSL2," T1E1.4/97-237, June 30, 1997.

[29] Level One, ADC, Pairgain, "OPTIS PSD Mask and Power Specification for HDSL2,"

T1E1.4/97-320, September 22, 1997.

[30] Cicada Semiconductor, "Performance and Spectral Compatibility of MONET-PAM HDSL2,"

T1E1.4/97-307, September 22, 1997.

[31] Cicada Semiconductor, "Performance and Spectral Compatibiity of MONET (R1) HDSL2,"

be used, and linearity requirements are essentially about 1 bit less than the CO side. Low noise is particularly important at the CPE end where it can extend the range of usable frequencies and thereby range at any data rate downstream. Echo cancellation and adaptive hybrids can also be used to reduce requirements.

The prefilter before the ADC is usually a set of filters with increasing amounts of high-frequency boost for increasing long lines. This has the effect of altering the channel signal and noise levels with respect to the ADC noise floor at higher frequencies, effectively making the AFE look as if its ADC has more effective bits. For a complete analysis of this type of filter, see Chapter 7 in VDSL, where it is also used.

The receiver analog filter (Figure 3.16) needs to have very low noise. Often, this can happen only with the use of very special design of gain control circuits. On a very long line, the signal needs to be amplified after the prefiltering to significant signal levels. However, on a short line or null loop, such amplification is unnecessary and would lead to saturation of the ADC circuit. The low noise floor is necessary only on the long loop. Thus gain control circuits and design need exhibit the low noise floor (-140 dBm/Hz or lower) only on long loops.

T1E1.4/97-412R1, December 8, 1997.

[32] Adtran, Cicada, Siemens, Tellabs, Westell, "Proposal to Break the FEC Logjam for HDSL2,"

T1E1.4/97-443, December 8, 1997.

[33] G. Ungerboeck, "Channel Coding with Multilevel/Phase Signals," IEEE Transactions on Information Theory IT-28, No.1, January 1982.

[34] G. Ungerboeck, "Trellis-Coded Modulation with Redundant Signal Sets Part I: Introduction,"

IEEE Communications 25 no. 2 (February 1987): 5–11.

[35] G. Ungerboeck, "Trellis-Coded Modulation with Redundant Signal Sets Part II: State of the Art," IEEE Communications 25 no. 2 (February 1987): 12–26.

[36] Pairgain Technologies, "A 512-State PAM TCM Code for HDSL2," T1E1.4/97-300, September 22, 1997.

[37] Adtran, "Performance and Characteristics of One-Dimensional Codes for HDSL2," T1E1.4/97-337 September 25, 1997.

[38] Adtran, "Single-Loop HDSL CAP/PAM Comparison," T1E1.4/95-107, August 21, 1995.

[39] AT&T Network Systems, "Performance of CAP and PAM for Single Pair HDSL in Presence of SNEXT," T1E1.4/95-106, August 21, 1995.

[40] AT&T Network Systems, "Text for HDSL Technical Report Issue 2: CAP Interoperability,"

June 5, 1995.

[41] Adtran, "Proposal for Single-Loop HDSL Using Simple Coded PAM," T1E1.4/96-037, January 26, 1996.

[42] AT&T Network Systems, "Single Pair T1 HDSL: Spectral Compatibility with ADSL &

Implementation Issues," T1E1.4/96-038, January 26, 1996.

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Perhaps the greatest single noise source often overlooked by new ADSL designers is the noise of the other ADSL components themselves. Layout of components in an ADSL receiver is very important.

Digital electronics needs to be isolated well from analog components, because the noise associated with digital power supplies and digital logic is often well above the levels needed for ADSL. This is why AFE components are most often separate from digital-signal-processing components.^[14]

Optical isolation of the analog and digital sections is extremely effective but expensive, although some groups have been able to achieve the same type of effect with clever proprietary designs.

Analog sections and ground planes should be isolated as best as possible from digital ground planes.

Additionally, the inside of a PC is a very harsh environment and requires careful attention to the radiation of energy from other parts of the computer into the ADSL board. Special metal (or mu metal) enclosure of critical analog components (analog transformer, and early filter stage

components) may be necessary to ensure that noise levels are not artificially high. After engineers have labored for years to design effective ADSL standards, it is self-defeating for the final

component layout and enclosure to cause more noise and distortion than the telephone line itself.

[14] Despite warnings and understanding of the problem, one major early modem supplier attempted a single-chip ADSL modem and was late-to-market due to noise problems, and consequently dropped from the ADSL market.

Although this area is somewhat of a "black art" (i.e., one dominated by those who know special secrets and tricks that remain proprietary to the employing company), good design practice and careful consideration of the AFE may lead to enormous advantage of one vendor's ADSL product over another. With range being particularly important to phone companies because of the labor costs associated with special service visits, it may not be the cheapest component that actually is the lowest cost for the ADSL system. This can be a lesson hard learned for some, and ADSL designers are well advised not to underestimate this area.

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d of responding with ACK(1) as in basic transaction A, the CO unit requests that the capabilities list be exchanged. Once the two units exchange capabilities and negotiate mode of operation, a follow-up transaction is needed for mode selection.

In transaction B:C, the CP unit requests that the CO unit select the mode of operation via a MR message, but instead of the expect MS response of basic transaction B, the CO unit requests a capabilities list request, where both units exchange their capabilities list and negotiate the mode selection. A follow-up transaction is needed for the mode selection.

In transaction D:C, the CP unit proposes a mode of operation via the MP message and requests that the CO unit select the operating mode. Instead of the expected MS message from the CO unit of basic transaction D, the CO unit requests a capabilities list request, where both units exchange their capabilities list and negotiate the mode selection. A follow-up transaction is needed for the mode selection.

5.11.3 Message Segmentation

Note that in the above transactions, some messages can become rather large when passing messages that contain the identification and standard information parameter fields. Excluding the two FCS

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octets and any octets inserted for transparency, the maximum message length in a frame is 64 octets.

If a message is longer than 64 octets, then it must be segmented into two or more messages. The message types that can be segmented are those that contain the parameter octets, namely, CL, CLR, MP, and MS.

When a receiving station is parsing a segmented message, the receiving station sends an ACK(2), which indicates to the sending station that it is ready to receive the remainder of the message. Once the complete message is received, the receiving station responds with an ACK(1) or other

appropriate response.

5.11.4 Example Transactions

In this section, we provide some example G.994.1 sessions to demonstrate the handshake process.

Example 1: Table 5.7 shows the use of a transaction sequence that combines basic transaction C with A. The CP unit first initiates a capabilities list request where the two units exchange and negotiate capabilities via the CLR and CL messages. The CP unit then selects the mode of operation via the MS command (basic transaction A).

Example 2: Table 5.8 shows an example transaction that combines extended transaction A:C with basic transaction A. First the CP unit selects a mode of operation and requests that the CO select this mode. Instead the CO unit requests the CP unit for a capabilities list request. The two modems then exchange capabilities lists and negotiate operating modes. Once the exchange and negotiation are complete, the CP unit selects the mode of operation via the MS message.

Table 5.7. Example 1—Basic Transaction C Followed by Basic Transaction A

CP Unit CO Unit CP Unit CP Unit CO Unit

CLR_ CL_ ACK(1) MS_ ACK(1)

Table 5.8. Example 2 – Extended Transaction A:C Followed by Basic Transaction A

CP Unit CO Unit CP Unit CO Unit CP Unit CP Unit CO Unit

MS_ REQ-CLR_ CLR_ CL_ ACK(1) MS_ ACK(1)