VDSL

VDSL, The Very-high-speed Digital Subscriber Line technology makes possible delivery of information to speeds up to 52 Mb/s in asymmetric mode. The high-frequency band used (up to 20 MHz) raises many challenges not existing in the present DSLs, among them spectrum allocation, transmission in a FEXT (far-end-crosstalk) noise environment, and RF interference sources.  However, as technology has been taking some big steps over the past few years, we can discuss VDSL technology as perhaps an alternative to ADSL.

The following chapter will summarize the VDSL infrastructure, and then discuss how VDSL technology stands against the ADSL technology.

VDSL deployment configurations
Due to the large attenuation of high frequency signals on twisted-pair lines, the deployment of VDSL is limited to a loop length of less than 4500 feet (1500m) from the signal source. This limited length is the downside of VDSL technology.  we present two configurations:
For customers close to the central office (CO), VDSL can be deployed over copper wiring from CO (the central office),  this configuration is called fiber to exchange (FTTEx).  However, when co-existing with ADSL infrastructure, VDSL's performance can be damaged.

 
Figure 1. FTTEx configuration

For more-distant customers the fiber is run to an optical network unit (ONU), from which the data is distributed using the existing infrastructure. This configuration is called fiber to the cabinet (FTTCab).
In this configuration, the opposite from FTTEx is occurred :  VDSL's performance remains stable, but noise generated by its signals may cause ADSL's downstream to be attenuated from CO to ONU (see figure below). 

 
Figure 2. FTTCab configuration

Data rates, spectrum allocation, and multiplexing
Data rates for symmetric/asymmetric services are shown in the following table:

Frequency-division multiplexing (FDM) was chosen as the multiplexing method for separating the upstream and downstream data transmission.  Using Asymmetric services the ratio between d/l and u/l streams is about 10:1 in favor of d/l.  Using Symmetric  the bandwidth should be equally allocated.
It is a requirement of the standard that both types of services should coexist on the same cable (this is not trivial, but we will not discuss here the technical details of how it is done).

Known Issues & Problems , and Solutions

• Crosstalk noise sources - as mentioned before, the phenomenon of noise generated between signals traveling in opposite directions (called near-end crosstalk or NEXT) and noise generated between signals traveling in the same direction in a cable (called far-end crosstalk, or FEXT).
  NEXT can be resolved by allocating different frequency bands for upstream and downstream direction.
  FEXT is the more dominant noise source, and is more complicated to deal with.  This problem occurs only in the upload streams, whose transmitter are scattered and not gathered near the ONU/CO as the download transmitters are.  This causes FEXT noise to be injected from transmitters closer to the ONU, to other upload transmitters located further.  The solution consists in reducing the transmit power, depending on the distance from the transmitter location to the ONU/CO.
• RF interference - Both the European and US standards allow six relatively narrow (100-200-kHz) amateur radio bands within the VDSL spectrum, causing interference between VDSL's and the amateur radio stations signals (usually due physical mishandles such as improper shielding , etc.).  This problem needs dealing with in 3 fronts:
  1.egress suppression - on the side of the radio amateur bands, VDSL interference can be blocked by 
      limiting the power spectral density (PSD) of the VDSL signal in these bands to 80 dBm/Hz.
  2. ingress suppression - The amateur-radio signal interference can be as high as 0 dBm at the VDSL 
      receiver input. It is highly desirable to attenuate such interference before converting to digital.
  3. the amateur radio signal is a non-stationary signal, characterized by ON/OFF periods.  Dealing with this
      kind of  interference is not an easy task, and we will not discuss it in this context.

Modulation
VDSL can use either QAM, DMT, and even FMT method for modulation.
using DMT requires extended range for the transmitter buffer and increased power consumption in the analog front end, due to DMT's sharp and high amplitude level. It also requires frame synchronization and supervision, meaning employ timing recovery techniques, which is a whole lot of mess. 
using QAM does not require timing in any way. However, it requires two QAM transmitters/receivers in each direction.
Also, DMT solves the egress suppression problem more easily, and have more flexibility in controlling power spectral density (PSD) , which can stand to its advantage.

ADSL vs. VDSL

 VDSL differs from ADSL in that it was designed to support both symmetric and asymmetric configurations. Also, it was designed to provide faster data transmission over shorter distances.  It therefore sustains a different network topology then ADSL relying on remotely fiber-fed access nodes located closer to the consumers.
However, currently technological improvements (see ADSL2 & re-ADSL) allows ADSL to offer higher downstream data rate, and enhanced upstream capabilities that can theoretically and practically overtake VDSL's performance (or at least offer some good competition).

Figure 3. Downstream comparision between ADSL, ADSL2, VDSL

In spite of their differences, ADSL and VDSL are technically similar.  They both employs advanced transmission techniques and forward error correction (although, surprisingly, ADSL has the more complex transmission technology since it most cope with much larger dynamic ranges than VDSL), but they do it in different rates over different range.  They can be considered as complements to each other by co-existing (as shown in the figures above), each trying to provide the best transmission tool over existing telephone copper wiring.

Next: What does the future have in store for us?