EE535 - PROJECT 3
VDSL - Very High-Rate Digital Subscriber Lines

The development of the VDSL standard is a joint effort of:

• the ANSI (American National Standards Institute) T1/E1.4,
• the ETSI (European Telecom Standard Institute) TM6,
• and IEEE committees.

As of March of 1997, the different standardization committees have not published any papers on VDSL nor have they released any drafts of their working papers. The ETSI is expecting to publish a technical report on VDSL by the first half of 1998.

Issues in the Development of VDSL:

The following is a list of issues that have arisen in the discussions of the T1E1.4 working group about VDSL. The complete minutes of the T1E1.4 meetings can be obtained here in PDF format.

• - TDD vs. FDD. The type of division duplexing that will be used in VDSL is being discussed to be one of Time Division Duplexing or Frequency Division Duplexing. FDD seems to be a better choice because existing services are typically echo cancelled or FDD. Synchronization of the upstream and downstream channels can be easier with FDD, because all the systems need to have the same bandsplit frequencies, whereas with TDD, synchronization can be more complex.

• - Reference Model. The noise model to use for designing VDSL is being discussed. The noise characteristic on the line will vary not only with the type of line, but also with the installed base of the local loop. In some countries there is aerial drop cable, while in most other countries the cable is placed underground for the last drop to the customer. There is no agreement as of this date, although it has been noted that it may be necessary to have multiple noise models in a final technical report.

  A fair amount of work on the noise model is being done by the manufacturers. They consider the noise to be couples to the line code. The European committee (TM6) is in favor of waiting for results from the network operators, and separate the noise model from the line code.

• - Ham radio interference.. In the case of aerial local loop wiring, the VDSL signal on the cable will generate an electric field capable of interfering with ham radio bands. The opposite is also true: ham radio frequency bands that coincide with the frequency content of VDSL will damage the VDSL signal.

• - Radiating Aerial Cables. BT Labs has carried out some measurements in high frequency bands generated by cables that carry VDSL signals. Using TDD (Time Division Duplexing), a VDSL transmitter produces unwanted emissions that interfere with close-by amateur radio receivers. Other measurements included the total path loss from the launch point of the VDSL signal in the the UTP and to the ham radio receiver. It was found that a maximum PSD of -60 dBm/Hz for use with VDSL can lead to potential interference to some bands of the high frequency radio spectrum. VDSL systems that are based on TDD may require power control measures to prevent this kind of interference.

  It has been suggested that a power level of -80 dBm/Hz is used in the bands that are occupied by amateur radio, and -60 dBm/Hz elsewhere.

• - Asymmetric / Symmetric Operation. Asymmetric service is that in which the upstream and downstream channels have different bit rates. It has been agreed that asymmetric and symmetric systems are possible. Furthermore, mixing of the two systems in VDSL implementations seems possible. Symmetric VDSL is planned for short distances and asymmetric operation is planned for long range.

  For these short distances, symmetric VDSL is attractive because it can simplify the network interface in corporate LANs and it is in accordance with the future of FTTH (Fiber to the home). In a much closer future, systems like FTTC and FTTB (to the curb and to the building) can be used with a symmetric VDSL system that uses 26 Mbps, which is a healthy rate for ATM.

  On the other hand, an advantage of the asymmetric system is the fact that it simplifies the electronic equipment required an the home side of the loop.

• - Flexible transceivers. The European TM6 committee has commented that they would like to see VDSL transceivers that can be configured both for symmetric and asymmetric operation.

• - Line Code. The choice of a line code (CAP, DMT, DWMT) is also in the agendas of the standards committees.

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