3 High-speed subscriber lines according to G.fast standard
3.5 Advantages and disadvantages of VDMT

The main advantage of the modulation principle VDMT has already been mentioned.

The purpose of the modulation principle VDMT is to eliminate crosstalk interference and thus to achieve higher transmission speeds reachable by xDSL connections.

The process is called as crosstalk coordination. We talk about the coordination of individual broadcast connections in a bundle copper cable in connection woth modulation VDMT.

Since it is possible the principles and parameters of the analytical models describe the connections may be relatively simple to calculate and benefits VDMT modulation. Depending on the number of connections increases coordinated achievable transmission rate monitored connection. The following simulation was performed after a VDSL2 connection with the frequency plan B8-13, which has a bandwidth up to 30 MHz. The frequency plan allows routinely achieve transmission speeds of 100 Mbit/s. Graphical interpretations of increasing transmission speeds for the upstream and downstream direction is shown in the following figures.

For the purposes of the simulation was considered metallic cable bundle with 50 symmetrical pairs, with a diameter of 0.4 mm wire (cable TCEPKPFLE 50×4×0,4 mm). The benefits of coordination are modeled as depending on the length of the subscriber line. It ranged from 0.1 km to 2 km with step 0.1 km. Number of coordinated connections gradually increased from 0 to 49. Taking connection with the fiftieth monitor the performance of transmission (reachable bit rate). Volume copper cable connections VDSL2 is therefore fully occupied.

The benefits of coordination for the upstream direction and VDSL2 connection B8-13.
The benefits of coordination for the downstream direction and VDSL2 connection B8-13.

When analyzing the benefits of VDMT be used for this test case indicate that the length of the local loop 0.5 kilometers is possible without any coordination (full cast all couples xDSL connections) will reach in the direction downstream transmission rate of 16.676 Mbit/s. If we coordinate our broadcast connection and a further connection, which causes the most crosstalk our reference, we are able to achieve data rates in the downstream direction of 19.84 Mbit/s. If we coordinate broadcasts from five surrounding connections, reach 27.960 Mbit/s. In the coordination of twenty seven speeds reach 50.868 Mbit/s. If we carry out the full coordination of connections reaches our tracked in the direction downstream connection speed of 170.720 Mbit/s. VDMT benefits are so obvious.

Modulation VDMT has also the significant disadvantages too.

Disadvantage of modulation VDMT is the high computational complexity in coordinating of the broadcast.

VDMT principle is to modify each currently transmitted DMT symbol on each symmetric pair with respect to the current parameters of the transmission environment. It is therefore obvious that for full coordination of all connections with DMT modulation, it is necessary to know the function describing crosstalk links between all pairs of lines, the transmission function of each line and the currently transmitted DMT symbols all coordinated connections. For copper cables with several hundred pairs of symmetric implementation will be fully operational coordination very difficult calculations associated with the adjustment of DMT symbols and calculations related to the acquisition parameters transmission environment. It can be calculated that, for example, only 50 connections of VDSL2 with the 4096 subchannels and modulation rate 4 kBaud will have to make every second in the respective DSLAM only when editing the transmitted DMT symbols 40,960E9 mathematical operations. Additional computing power then you need when establishing a connection, control and direction of current connections, etc.

Modulation VDMT has benefits only in a situation where the propagation medium is used by all systems with DMT modulation. This requirement follows from the principle of VDMT.