OPEN-WIRE CROSSTALK 



27 



apparatus since the latter is a one-way device. In practice, to obtain 

 two-way circuits each of these one-way channels is associated with 

 another one-way channel transmitting in the opposite direction over 

 the same pair of wires. These return channels utilize a different band 

 of carrier frequencies and the near-end crosstalk current is largely 

 excluded from this frequency band by selective filters. The far-end 

 crosstalk is, therefore, the sole consideration with such a carrier system. 

 Use is not made of the same carrier frequencies in both directions on a 

 toll line largely because of difficulties in controlling the near-end 

 crosstalk. 



(A) 



'n (B) 'n 



Fig. 3 — Crosstalk between two one-way carrier frequency channels. 



In connection with the arrangement of Fig. 3A, there is a type of 

 crosstalk of considerable practical importance known as ''reflection 

 crosstalk.'' The theory of this is indicated by Fig. 3B which shows 

 the same two one-way carrier channels. Transmission from left to 

 right on circuit a is assumed. When the transmission current I 

 arrives at point B, a certain portion of it will be reflected if there is 

 any deviation of the input impedance of the terminal apparatus from 

 the characteristic impedance of circuit a. This reflected current Ir 

 causes a near-end crosstalk current in at point B in the disturbed 

 circuit. Similarly, a part of the near-end crosstalk current in at point 

 A in the disturbed circuit may be reflected and transmitted to point B. 

 Therefore, two additional crosstalk currents may result from these 

 two reflections and such currents can enter the terminal apparatus at 

 B and pass through to the output of this apparatus. 



