The acronym xDSL stands for the family of Digital Subscriber Line technologies, which allow high-speed access to the Internet and multimedia services over the local loop, which connects the CP (customer premises) to the CO (central office), that is over simple twisted pair cables. An xDSL transceiver at the CO communicates with an xDSL transceiver at the CP over the local loop.
Since decades the local loop, which is a transmission line consisting of two twisted copper wires, also called unshielded twisted pair (UTP), has given the customer access to POTS (Plain Old Telephony Service). The POTS signal, transmitted over the local loop, is analog and contained in the frequency band up to 4 kHz, which corresponds to the spectral content of speech.
xDSL exploits the frequency band above 4 kHz up to several MHz, which is not used by POTS. However as the legacy local loops have been engineered for voice-band transmission, there are no guarantees about the quality of the local loop with respect to transmission in this higher frequency band. The signal-to-noise ratio (SNR) as a function of the frequency at the receiver at the CP, respectively the CO, for the downstream transmission (from the CO to the CP), respectively the upstream transmission (from the CP to the CO) plays an important role. The SNR at the receiver at the CP, respectively the CO, is determined as a function of the frequency by the transfer function of the loop between the CO and the CP and the noise PSD (Power Spectral Density) at the CP, respectively the CO given the PSD of the transmitted signal at the CO, respectively the CP.
                    C        =                              ∫            0            W                    ⁢                                                    log                2                            ⁡                              (                                  1                  +                                                            P                      ⁡                                              (                        f                        )                                                                                    N                      ⁡                                              (                        f                        )                                                                                            )                                      ⁢                          ⅆ              f                                                          (        1        )            P(ƒ)=|H(ƒ)|2S(ƒ)  (2)
In general the local loop consists of a network of transmission lines. Every line in the network is a UTP characterized by its length and type. The line type specifies the cross-sectional geometrical dimensions, such as the wire diameter (also called wire gauge), and the material physical constants, such as the electrical permittivity of the dielectric separating the 2 copper wires. The most used wire diameters are 0.4 mm, 0.5 mm, and 0.6 mm. Polyethylene (PE) is the most occurring insulator but other materials are also used such as paper and PVC.
The network topology of the local loop is limited to a tree structure. The simplest topology is a single line. The magnitude of the transfer function reflects the attenuation of the line, which gets worse with increasing frequency and line length. Another topology that exists for long loops is a cascade of 2 or more lines with increasing wire diameter from the CO to the CP. For this topology reflections are caused by the change of the wire diameter at the splices connecting 2 lines. A topology that is also frequently encountered, especially in the USA, is a loop with 1, 2 or more bridged taps. A bridged tap is an open-ended line spliced to the main line. Reflections appear for this topology at the splice connecting the bridged tap to the loop and at the open end of the bridged tap. Reflections have a negative impact on the transfer function, because they interfere with the signal propagating along the direct path. For those frequencies for which the interference is destructive, the magnitude of the transfer function reduces. Such reductions rarely appear in the voice band because the bridged taps are usually not too long.
As the twisted pairs constituting the local loop are unshielded, external electromagnetic waves may couple into the loop and propagate towards the CO and the CP causing noise at the receiver. The electromagnetic coupling is reduced by the twisting of the 2 wires, because adjacent segments of the twisted pair experience electromagnetic waves with opposite polarity. In addition the twisting improves the balance of the line. A line is balanced when the 2 conductors have an equal impedance towards the earth. The balancing of the line prevents a common-mode signal from transforming into a differential-mode signal. In the case of a common-mode signal the 2 wires carry equal currents and the return path of the current is the ground. For a differential-mode signal the 2 wires carry opposite currents (out of phase currents). Electromagnetic waves may couple into the line because of imperfect twisting, and the common-mode signal that they cause, may transform into a differential-mode signal because of imperfect balancing, which is correlated with the twisting. Balance decreases with increasing frequency.
The noise is divided into 2 different types according to the origin of the external electromagnetic waves coupling into the loop. The first type of noise is crosstalk, which is the electromagnetic coupling between twisted pairs in the same cable or between cables. The cables leaving the CO contain thousands of twisted pairs. The closer to the CP the less pairs there are present in a cable. A difference is made between near-end crosstalk (NEXT) and far-end crosstalk (FEXT). The transmitters at the CO, respectively CP, are the source of NEXT for the near-end receivers at the CO, respectively the CP, and are the source of FEXT for the far-end receivers at the CP, respectively the CO. In general crosstalk gets worse with increasing frequency.
A second type of noise is radio-frequency interference (RFI), which is caused by radio waves coupling into the local loop, that acts as an antenna, especially if there are aerial lines. There are 2 major sources of radio waves in the frequency band of xDSL namely AM radio and amateur radio.
Hence, the local loop has several impairments for transmission in the frequency band of xDSL, which are not present for voice-band transmission.
Various proposals have been made to calculate or estimate FEXT at a port of a transmission line. The methods of T1.413 [ANSI, 1995] assume that all loops are symmetrical, i.e. that the receivers can be replaced by the transmitters and vice versa. This may be unreasonable because:    a) It is very unlikely that the loop under test and an interfering loop will have the same tolopogies, e.g. the same number of bridge taps. A bridge tap in only the test loop will reduce performance, a bridge only in the interfering loop will improve performance.    b) Since most bridge taps are closer to the CP than the CO this is most important in the upstream direction. Not all CP's are the same distance away. Short cables will have a serious affect on long cables at the CO.
In the early 1980's many hundreds of thousands of measurements of crosstalk were made; mostly on 50 pair binder groups that are used for interexchange transmission of T1 and T2 signals. These included pair-to-pair measurements and pairs to one measurements. From these measurements cumulative probability density functions were plotted and some worst-case probabilities estimated. Analysis of this type has resulted in various empirical formulae which are supposed to provide reasonable estimations. However, such empirical formulae are unsatisfactory especially when systems are considered that differ greatly from those from which the empirical results have been obtained. Also when applying xDSL to legacy networks the topology of the loops is often not known so that no assessment can be made as to whether the model used is suitable.
Upstream power backoff (UPBO) guarantees spectral compatibility between long and short loops that are operating in the same cable binder. UPBO is a method by which the upstream power spectrum density (PSDtransmit) being generated by a transmitter on a short disturber line is controlled in order to limit excessive crosstalk into neighbouring lines. The crosstalk measured at the line terminator of the victim loop impacts on the bit rate performance of the victim loop. A current method of UPBO is based on an estimation of the length of the loop. The electrical length is obtained by comparing the attenuation of the loop under consideration with the attenuation of a reference loop. This reference loop consists of a single line segment. The PSDtransmit is determined by (PSDref)/|TF| where TF is the transfer function of the disturber loop which is a function of the electrical length of this loop. This method is only useful for single segment loops, i.e. with no bridge tap, no mixed wire gauges nor imperfect terminations. Particularly, bridge taps can result in imperfect performance as indicated above.
It is an object of the present invention to provide a method and apparatus for the upstream backoff of combinations of transmission lines which are more accurate and/or flexible than conventional methods and apparatus.
It is a further object of the present invention to provide a method and apparatus for simulation of crosstalk-related parameters in transmission lines which are more accurate and/or flexible than conventional methods and apparatus.