22 Solution – ADSL system
In this lab, The focus is to investigate the frequency response of local loops. The frequency response of these loops is critical to the performance of ADSL systems. Even though sections of local loop might use higher quality cables, but there are sections of local loops that use lower quality cables. Before ADSL can be used, the low pass filters in the switching offices must be removed from the lines. Finally, the effects of the UTP lines, bridge taps, and inline transformers and capacitors must be taken into account.
Transformers and capacitors, installed on the lines to prevent large DC voltages from damaging the equipment, act as a high pass filter. While we did not numerically examine the effect of these devices in this lab, it is clear that they will affect the frequency response of the system. Capacitors will block DC, and transformers will not transfer DC power. DC and very low frequencies, therefore, will be heavily attenuated. This will add a lower cutoff frequency to the system. Since we know that the local loops are used to carry baseband voice signals, however, we know that the ADSL signals at higher frequencies will not be affected by this cutoff.
The UTP lines themselves, both the inline sections from the telephone company to the street and the drop lines from the street to each house, contribute significantly to the frequency response of the system. These cables will cause signals to experience an attenuation that depends both on the length of the line and the frequency of the signal. This attenuation is what ultimately sets the upper cutoff frequency for the system, and we determined that frequency to be around 1MHz. This is similar to the 1.1MHz upper limit for ADSL that was discussed in class, and this bandwidth limit determines the speed that ADSL can achieve.
The last important factor in frequency response is the presence or absence of bridge taps. These are open circuited pieces of transmission line that connect to the local loop. They are a result of poorly-executed maintenance work, either when transitioning from party lines to individual lines or when repairing broken lines. These taps were left connected instead of removing them, and they end up appearing as stubs on the line. This causes an impedance mismatch, which in turn causes reflections which constructively or destructively interfere with the signal. The type and degree of interference is dependent on the frequency of the signal, and the interference causes the oscillations seen on the frequency response curve.
Ordinarily stubs are used for impedance matching, but in the case of bridge taps they are both unintended and of varying and unpredictable length. This can cause serious performance problems, and these issues become more severe if there are multiple taps on the line. Bridge taps are one of the most serious problems faced by ADSL, and systems are designed to calculate the frequency response of the line and compensate by using channels that are in a frequency range that is not severely attenuated by these taps.