The basic stucture of the No. 5 crossbar system is shown in Fig. 4.14. It consists of line link frame, trunk link frame and common control equipment. The No. 5 system employs a single switching train to handle incoming, outgoing or switch through. Also, the connection of the subscriber to the dial register circuit is also made through this switching train. Subscriber lines and incoming trunks terminate on the line link frames, trunks and orginating registers termination on the trunk link frames. The line link and trunk link frames are interconected by "junctors" which give each link frame access to all of the trunk link frames.
Line link frames. The basic line link frame is a 2 bay frame work with each bay mounting ten 200 point, 3 wire switches. The ten switches on one bay are used as combined line and junctor switches and provide terminations for 100 junctors and 100 lines. The ten switches on the other bay are line switches which provide terminations for 190 additional lines and ten "no test'' verticals used to obtain access to busy lines. Thus a line link frames provides 290 line terminations and 100 junctor terminations.
A feature of this line link frame is that the same frame can serve customers who have various classes of service. Like coin telephone, emergency line facility, hot line facility, a maximum of thirty classes of service can be served on a frame.
Junctors. Each line link frame has 100 junctor terminations which are used to connect to all the trunk link frames in the office. The number of junctors in a group depends on the number of trunk line frames in the office. For example, in an office with eight trunk link frames and sixteen line link frames, each junctor group contains either twelve or thirteen junctors.
Trunk link frames. The trunk link frame is made up of trunk switches, junctor switches and relays for marker access to the frame. Trunks and originating registers, which register the called number are connected to the trunk switches. The trunk links run from vertical to vertical, the junctors being connected to the horizontals of the junctor switches and the trunks to the horizontals of the trunk switches. Thus, the trunk is accessible to all the junctors on the frame but only to either, the left or right junctors on one channel test.
Common control equipments : The common control equipments used in AT & T crossbar system are originating and incoming registers, markers, translators, senders and connectors. These equipments are explained briefly below.
Marker. It is the most active part of common control equipment. All the markers and their associated equipment serves up to a maximum of 20,000 members make a marker group. There are three types of markers. The combined marker performs dialtone, alloting the jobs between markers. The dialtone marker is used exclusively on dialtone jobs. Completing marker performs all the other jobs. The principal functions of the marker are to respond demands for dialtone, determine the proper route, establish connection, determine the class of service, recognize the status of the connection etc.
Outgoing registers. It furnish dialtone to subscribers and record the digits that are dialed, and then the called number is transmitted from register to marker. The originating circuit may be assigned to seize the pretranslator after either the second or third digit has been dialed.
Pretranslator. The pretranslator determines, how many digits the register should expect before seizing a marker. A pretranslator can be placed in the outgoing register. For complex numbering plans, a separate pretranslator circuit is provided. The pretranslator determines how many more digits should be dialed and instructs the register, the time it must wait to make connection to marker.
Outgoing Sender. The marker transfers the required digits of the called number to a sender which is connected to an outgoing trunk. The sender type is based on the switching system used. In crossbar No. 5 office, four different types of outgoing senders provided are Dial pulse (DP), Multi frequency (MF), Reventive pulse (RP) Panel call indicator (PCI) This four types of outgoing senders and intermarker group senders may be located on one sender link frame.
Connectors. It is used for interconnecting two equipment elements for short interval of time. If more than one type of equipment originate action toward another type, the connector is named according to both the originating and terminating action :
Important features of No. 5 crossbar system :
(i) The use of precious metal, non-sliding contacts results in noise free conversations.
(ii) Various options of charging methods such as AMA or message resister and coin.
(iii) Provision of toll and tandem switching features with the same common control equipment as is used for local traffic.
(iv) Good trouble shooting procedures and additional features like eleven digit capacity, alternate routing, code conversion, marker pulse conversion etc.
(v) Improved distribution of usage over various equipment units by means of rotating sequence and memory circuits.
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