Relation between insertion losses of subscriber loops at different frequencies

Relation between insertion losses of subscriber loops at different frequencies JERZY SIUZDAK, TOMASZ CZARNECKI Institute of Telecommunications Warsaw University of TechnologyWarsaw, POLAND

Presentation Plan • Introduction • Theory • Results • Measurements • Conclusions

Introduction • Broadband services (such as ADSL) require more from cables than POTS as the bandwidth is much greater • Broadband services may not work correctly whereas POTS operates well over the same cable • Before introducing ADSL the subscriber line must be checked (line qualification)

Introduction Line qualification methods: • Data bases on subscribers and their loops • Measurements from the central office • Measurements from both ends of the subscriber loop • Employment of voice band modems

Theory Relation between line parameters at high and low frequencies must be established: - SNR defines the line bit rate - interference is not correlated at HF and LF - the signal level is the only factor that have correlation between HF and LF - the input signal level is limited - the relation between the line attenuation at HF and LF has to be found

Theory: the chain matrix defines the loop insertion loss

Theory: the chain matrix defines the loop insertion loss Fundamental relations:

Theory • The chain matrix of a cascade is a product of chain matrices of the individual four poles • The chain matrices of typical subscriber loops in Poland (ETSI loops subset) were calculated • Their attenuations were calculated simultaneously for a voice band frequency and for 300 kHz (typical frequency of ADSL)

Theory: example of an ETSI loop

Theory • The chain matrices of a transmission line, and bridged tap should be known • Example: the chain matrix of the transmission line

Results

Results • Example of an approximate linear relation between attenuations at different frequencies:

Results • There is an approximate linear relation between loop insertion losses at voice band and high (300 kHz) frequencies • This relation depends neither on the loop type nor on its length • The greater the voice band frequency the more distinct the relation • This frequency is limited by the receiver and transmitter filters

Results The linear relation is mostly due to the unit attenuations of the line at high and low frequency

Measurements • V.34 (voice band) modems during connection initialization test many parameters • Among them is the loop frequency response • These results may be accessed from the operating system • The slope of the voice band frequency response may be related to the 300 kHz loss in the same way as the attenuation

Measurements • Experimental system was built based on the commercially available V.34 modems • A pair of modems is necessary: -one at the subscriber premises, -the second at the central office (need not be located at the subscriber central office- it may be far away at a completely different place) -in fact in one of the experiments it was located 300 km from the subscriber

Measurements: exemplary results

Conclusions • ADSL system bit rate depends on the subscriber loop attenuation • We are able to estimate this attenuation by means of V.34 modems measurements • Thus we may use V.34 modems to qualify loops for broadband services • Example: Sapphire system of Telcordia • Extensive tests are necessary before practical implementation

Acknowledgments This work was financed by Polish Scientific Research Committee (KBN) under the grant no 4T11D01525

Relation between insertion losses of subscriber loops at different frequencies