REMOTE CONTROLLED LINE CONCENTRATOR 255 



1. High speed pulsing (500 pulses per second) of information between 

 switching units. 



2. The use of plug-in packages employing printed wiring and encap- 

 sulation. (Fig. 2 shows a representative group of these units.) 



3. Line scanning for supervision with a passive line circuit. In present 

 systems each line is equipped with a relay circuit for detecting call orig- 

 inations (service requests) and another relay (or switch magnet) for 

 indicating the busy or idle condition of the line, as shown in Fig. 3(a). 

 The line concentrator utilizes a circuit consisting of resistors and semi- 

 conductor diodes in pulse gates to provide these same indications. This 

 circuit is shown in Fig. 3(b). Its operation is described later. The pulses 

 for each line appear at a different time with respect to one another. 

 These pulses are said to represent "time slots." Thus a different line is 

 examined each .002 second for a total cycle time (for 60 lines) of .120 

 second. This process is known as "line scanning" and the portion of the 

 circuit which produces these pulses is known as the scanner. Each of the 

 circuits perform the same functions, viz., to indicate to the central office 

 equipment when the customer originates a call and for terminating calls 

 to indicate if the line is busy. 



4. The lines are divided for control and identification purposes into 

 twelve groups of five lines each. Each group of five lines has a different 

 pattern of access to the trunks which connect to the central office. The 

 ten trunks to the central office are divided into two groups as shown in 

 Fig. 4. One trunk group, called the random access group, is arranged in 

 a random multiple fashion, so that each of these trunks is available to 

 approximately one-half of the lines. The other group, consisting of two 

 trunks, is available to all lines and is therefore called the full access 

 group. The control circuitry is arranged to first select a trunk of the 

 random access group which is idle and available to the particular line to 

 which a connection is to be made. If all of the trunks of this random ac- 

 cess group are busy to a line to which a connection is desired, an attempt 

 is then made to select a trunk of the full access group. The preference 

 order for selecting cross-points in the random access group is different 

 for each line group, as shown in the table on Fig. 4. By this means, each 

 trunk serves a number of lines on a different priority basis. Random ac- 

 cess is used to reduce by 40 per cent the number of individual reed relay 

 crosspoints which would otherwise be needed to maintain the quality 

 of service desired, as indicated by a theory presented some years ago.^^ 



5. Built-in magnetic tape means for recording usage data and making 

 call delay measurements. The gathering of this data is greatly facilitated 

 by the line scanning technique. 



