BT Monarch Shelf Multiplex (non-concentrating)

ASU1A1/SA20039

Inter-PBX circuits and console line circuits terminate on Line Units in non-concentrating shelf positions which are served by a Shelf Multiplex in place of the CSI.

Depending on the type of Line Unit concerned, each inter-PBX line circuit has one or two

separate paths in each direction to and from the Shelf Multiplex provided for its exclusive use,

and each console line circuit always has two separate paths in each direction. Each plug-in unit

accommodates 2 inter-PBX line circuits (at some early installations, possibly one, depending on

the type of signalling), or one console line circuit. The resultant modifications to the block

schematic diagram in Figure 4, necessary to show the Shelf Multiplex and inter-PBX/console

Line Unit arrangements are indicated in Figure 5. The transmission paths in each direction, between each line circuit and the Shelf Multiplex, carry 8 bit PCM encoded speech plus a single bit per sample for signalling purposes, ie a total of 9 bits per sample. With a sampling rate of 8kHz, the total bit rate on these transmission paths is 72 kbit/s (9 X 8000).

The Shelf Multiplex is designed to serve up to 32 transmissions paths to and from the Line

Units, although in Monarch 120B Compact this number of transmission paths is not required

and the Shelf Multiplex is not used to its full capacity. At the Shelf Multiplex circuit, combined

speech and signalling bits from all the line circuits enter the equipment over the 72 kbit/s paths.

The signalling bits are separated from the speech bits and the latter are multiplexed to form a

standard 32 time slot PCM group which is extended over a 2.04 8Mbit/s (8 X 8000 X 32) highway

to the Speech Switch. The signalling bits are extended over a separate 256 kbit/s (1 X 8000 X 32)

highway to the Signalling Circuit. The speech and signalling formats at this point are illustrated

in Figure 6. In the opposite direction of transmission, speech bits are received over a 2.04 8 Mbit/s

highway from the Speech Switch, and signalling bits are received over a 256 kbit/s highway

from the Signalling Circuit. These are then demultiplexed and combined to provide speech and

signalling at 72 kbit/s from the shelf multiplex to each line circuit. The formats on the speech and

signalling highways to and from the Shelf Multiplex are identical to those on the highways to and

from the Concentrating Shelf Interface used for the concentrating shelf positions and so far as the

Speech Switch and Signalling Circuit are concerned, the highways are inter-changeable.

The Speech Switch and Signalling Circuit are multi-purpose designs which each have a

capacity for up to 8 highways. In Monarch 120B Compact, 3 highways are used in the basic unit;

one for the concentrating shelf positions, one for the non-concentrating shelf positions and one

for the Services Card, and in the add-on unit 2 highways are used; one for the concentrating shelf

positions, and one for the non-concentrating shelf positions. Thus, when both units are provided, use is made of 5 of the 8 high ways and the remaining 3 are spare.

The descriptions of the operation of the Speech Switch and Signalling Circuit given in the following paragraphs assume

that all 8 highways are used and relate to the concentrating line shelf positions, where a channel is

allocated to a port for each call, as already described, and different channels are allocated to the

same port for different calls. Since in the Monarch 120B Compact application less than 8

highways are used, the surplus input and output highway terminals are left spare. In the case of

the non-concentrating shelf positions, each port is permanently allocated a channel for its

exclusive use. Thus, in this latter case, the storage locations in the Speech Switch and Signalling

Circuit each represent not only a channel but a particular port. The resultant slight modifications

to the operational description in the following paragraphs should be readily apparent.

3.1.7 The 8 highways connected to the Speech Switch are collectively capable of carrying

speech samples corresponding to 256 (8 X 32) channels. Within the Speech Switch each of these

channels is permanently allocated an 8 bit location in a 256 X 8 bit speech store which takes the

form of a Random Access Memory (RAM). The bits representing the speech samples arrive in

serial form over the highways, and after conversion to parallel form are entered into the

appropriate parts of the speech store. After each frame period, the bits stored against a particular

channel are replaced by the next sample. When a call is in progress, the control equipment has

already received the keyed, or dialled, digits and recorded the identities of the calling and called line circuits, and the channels which have been connected to these line circuits. The identities of the 2 channels which need to be interconnected to maintain the connection are passed from the

CPU to another 256 X 8 bit RAM (the Connection Store) in the Speech Switch. In Figure 4 it is

assumed that line circuit A is connected to line circuit B. The information in the connection store

causes the speech sample held in the speech store of the channel to which line circuit A is

connected to be transferred to the output of the Speech Switch at a time which will cause it to be

sent out in the timeslot of the channel to which line circuit B is connected. Similarly, the

information held in the speech store of the channel to which line circuit B is connected, is

transferred to the output of the Speech Switch at a time which will cause it to be sent out in the

timeslot of the channel to which line circuit A is connected. Thus the Speech Switch, under the

control of the connection store provides a single stage 256 X 256 non-blocking, full availability

switch for inter-connecting channels in the 8 incoming and 8 outgoing highways connected to it.

3.1.8 Every channel in the eight 256 kbit/s signalling-in highways connected to the Signalling

Circuit is permanently allocated an 8 bit store in one half of a 512 X 8 bit RAM in this circuit.

After conversion from serial to parallel form, each successive byte of signalling information from

the channel is placed in this half of the store. Thus, a change of signalling condition is registered in

the store within 1 ms (See paragraph 3.1.3). The control equipment scans this signalling-in half of

the store and on detecting a change of condition, takes appropriate action. In the case of channels

which are not in use, the scanning interval is 128 ms, but while a channel is connected to a line

circuit in the dialling state the interval is reduced to 8 ms.

3.1.9 The other half of the RAM in the Signalling Circuit has a permanently allocated 8 bit

location for each of the 256 channels provided by the 8 signalling-out highways. When the

control equipment requires to send a signalling instruction to a Concentrating Shelf Interface

(usually for onward transmission to a line circuit) it places an appropriately coded signalling byte

in the store location corresponding to the channel concerned. The Signalling Circuit, after

converting the information from parallel to serial form, sends the byte to the CSI during the

channel timeslot period.