272 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



common to 10 concentrators interrupt the further transmission of the 

 vertical group pulse so that the line scanning is confined to the 5 lines 

 in the vertical group in which the call originated. In this way the cen- 

 tral office will receive a service request pulse at least every 0.010 sec as 

 a check that the call has not been abandoned while awaiting service. 

 Fig. 18(b) shows the detection of a call origination and the several 

 short scan cycles for abandoned call detection. 



b. Line Selection 



When the central office is ready to establish a connection at the con- 

 centrator a reset pulse is sent to return the counters to normal. In gen- 

 eral, the vertical group and vertical file pulses are sent simultaneously 

 to reduce holding time of the central office equipment and to minimize 

 marker delays caused by this operation. For this reason the VG and VF 

 pulses are each transmitted over different control pairs from the central 

 office. The same polarity is used. 



On originating calls it is desirable to make one last check that the 

 call has not been abandoned, while on terminating calls it is necessary 



L* 120MS >| 



■M k-2MS I 



PULSE ' 



012340123401234 0123401234 



VF - 



VG 

 LB 

 RS 



J__l I I I I I I I I I I I I I I 1 I I I I 1 I L 



1^ \1 ^ 



(a) REGULAR LINE SCANNING 



VF 



123401 23401 23401 2340123401 

 _l_l 1 I I I I I I I I I I I I I I I I I L_l I I I I 



,5 ,6 



VG 1 1 — 



LB- 



RS- 



SR- 



M- 



(b) CALL ORIGINATION SERVICE REQUEST FROM LINE 6/3 



12 3 1 

 I I I I I 



VG- 



LB 



RS — 1>- 



SR h 



1° 1' 1^ |3 1^ 1^ 1^ 



jLi. 



M - 



H^-l^-- 



"7 



RESULTS FROM CONC CONTROL RECEIVED 'OPERATE ""CROSSPOINT 'NORMAL SCANNING 



CKT AT CENTRAL OFFICE ONLY IF CROSSPOINT CLOSURE IS RESUMED 



RECEIVING FROM MKR VG , LINE 6/3 HAS INDICATION 



VF, HG INFORMATION BECOME BUSY 



(C) LINE SELECTION FOR LINE 6/3 



Fig. 18 — Pulse sequences, (a) Regular, (b) Call origination, (c) Line selection. 



