Specifications of xDSL Technologies

Table A-1 Technical specification of symmetric DSL technology

Index IDSL HDSL SDSL HDSL2

Standards ANSI T1.601

ETSI ETR 152

ITU-T G991.1 Proprietary ANSI

Table A-2 Comparison between ADSL, VDSL and SHDSL

Index ADSL VDSL SHDSL

Standards

ITU-T G.992.1 ITU-T G.992.2 ANSI T1.413,Issue2

No standard

currently ITU-T G.991.2

Interoperability Yes No No

Line Code DMT QAM TCPAM

Speeds Supported (bit/s)

Asymmetric, 6–8Mbit/s in downstream, and 640kbit/s–1Mbit/s in upstream

Symmetric, 12M Symmetric, 192k-2312k

Wire pairs 1 1 1 or 2

Maximum reach (26

AWG) 3–5 km 1.5 km 3–5 km

Rate adaptive Yes Yes Yes

Repeater

support Yes Yes Yes

In the following sections, the ADSL2+ and SHDSL technologies will be detailed.

A.2 ADSL2+

I. Overview

The full expression of ADSL2+ is the second generation full rate asymmetric digital subscriber line. In 2003, the ITU-T issued the new Recommendation G.992.5, which is also known as ADSL2+ recommendation. This recommendation, which is derived from the first generation one, provides more functions, higher access rate, and steadier performance for users.

II. New operation modes

There are three kinds of operating modes for ADSL.

z ADSL over POTS: ADSL annex A. In this mode, POTS and ADSL services are transmitted over the same pair of twisted pairs.

z ADSL over ISDN: ADSL annex B. In this mode, ISDN and ADSL services are transmitted over the same pair of twisted pairs.

z ADSL annex C: It is used under the TCM–ISDN crosstalk condition. This mode is mainly applied in Japan.

The following new operating modes are added to the ADSL2+.

z Annex I: This digital operating mode is compatible with Annex A in respect of spectrum. There is no POTS service transmitted on the line, and therefore, the ADSL2+ upstream spectrum is in the range of 3 kHz to 138 kHz. The number of sub-bands reaches 31, and the upstream bandwidth is over 1 Mbit/s.

z Annex J: This digital operating mode is compatible with Annex B in respect of spectrum. There is no ISDN service transmitted on the line. In this mode, the upstream frequency band ranges from 3 kHz to 276 kHz. A maximum of 64 upstream sub-bands are supported and the maximum upstream rate reaches 2.3 Mbit/s.

z Annex M: It extends upstream bandwidth of Annex A. The upstream sub-band starts from 6, and ends with 32, 36, 40, 44, …, 63 based on different requirements of bandwidth. At the same time, the total Tx power remains unchanged. In this way, Annex M can provide the upstream rate close to Annex J.

There are two kinds of downstream frequency bands, overlap and nonoverlap, which are the same as Annex B.

z Annex L: It extends the reach.

ADSL supports two kinds of interfaces, STM interface (PCM interface) and ATM interface (UTOPIA). Besides this, ADSL2+ also provides PTM (packet) interface, which bears HDLC over ADSL in non-ATM transmission mode.

III. Higher transmission rate

Because ADSL2+ uses wider frequency (tone 32–511) and more sub-bands (512), it supports the maximum downstream rate of 24 Mbit/s. Figure A-1 shows its frequency spectrum. In ADSL2+ Annex J or Annex M, the upstream rate is increased to 2.3 Mbit/s.

IV. Longer reach

The reach of ADSL is less than 5 km, while that of ADSL2+ is 6.5 km at least.

V. Lower power consumption

ADSL2+ provides the power management function to reduce the running power.

z ADSL2+ offers the low power consumption mode, L2 mode. In this mode, if no user data is transmitted, the system reduces the Tx power to 30% of that in normal running state, which is only used to transmit the necessary management messages and signal synchronization signals. When there is data to be transmitted, the normal power consumption is restored rapidly.

z The CO and the CPE of ADSL2+ have the function of “power cutback”, ranging from 0 to 40 dB. This function can reduce the Tx power of the system effectively during the normal operation. For ADSL, only the CO has such function, ranging from 0 to 12 dB.

VI. Steadier running and good frequency spectrum compatibility

ADSL2+ has steadier running capability and good frequency spectrum compatibility.

z The receiver determines the carrier sequencing according to the channel analysis result, which can reflect the channel state more reliably and select most appropriate tone as pilot. This helps improve the steadiness of the ADSL connection.

z In the training, the Rx end can test the distribution of RFI signals, and can control the Tx power of each tone through CO-MIB during the tone is closed. This avoids RFI and reduces the crosstalk to other pairs.

z ADSL2+ is of good dynamic adaptability. The enhanced bit swap and the function of changing line rate seamlessly and dynamically in the “show time”

state improve the adaptability to the line parameter variation.

z For ADSL2+, the receiver and Tx end have the function of “power cutback”, ranging from 0 to 40 dB. This function reduces the local end echo and crosstalk.

For ADSL, only the CO has such function in downstream direction, ranging from 0 to 12 dB.

z The Rx end determines pilot, which avoids that the line cannot be activated because of the interference of line bridge extractor and Am.

z The Tx and Rx ends control the initialization state length, which optimizes the function of transmitting and receiving signals.

z ADSL2+ provides shortened training process, during which the faulty connection can be restored to the normal state quickly.

VII. Line diagnosis function

ADSL2+ supports the dual ended loop test between the CO and the CPE to obtain the following parameters based on the special line test process.

z Line transfer function

The test result is given in two modes: linear expression and logarithm expression. The former indicates the information about the amplitude and the phase, and the latter gives the result convenient for calculation.

z Static line background noise power spectral density

When modem does not send any data to the line, it only detects the background noise on the line. Each tone has a test value.

z Signal-to-noise ratio

It indicates the ratio of the signal power to the noise power that is received by the receiver.

z Loop attenuation (average value)

It indicates the average value of the amplitudes of transfer function of all nodes.

z Signal attenuation

It indicates the ratio of the signal power received by the Rx end to the signal power transmitted from the Tx end. Actually, it is the line attenuation parameter of ADSL.

z SNR margin

This value is given during the training and can be refreshed on a timed basis during the normal operation. It indicates the capability of the line connection to resist noise.

z Maximum reachable rate

You can use the following formula to work out the maximum reachable rate of the line based on the parameters listed above.

s

It indicates the actual total Tx power of the local and the remote.

VIII. Seamless rate adaptive

A telephone cable is made up of multiple pairs of twisted pairs. The electric signals in one pair of twisted pairs are coupled to the other twisted pairs of the same telephone cable. Such phenomenon is called crosstalk, which has an influence on ADSL performance.