PBX system with assignable relationships between network loops and ports

A peripheral controller for a PBX system accepts PCM data from a plurality of channels. The channels may be processed in time and space switches of more than one network loop. A channel-merge device merges the PCM data into a combined PCM data stream for connection to a time switch. The time switch is controlled by a connection memory to direct data on specific channels to specific terminals.

BACKGROUND OF THE INVENTION 
The present invention relates to PBX systems and, more particularly, to 
digital PBX systems in which a plurality of signals are 
pulse-code-modulated for transmission on a data bus. 
A conventional digital PBX system such as, for example, one disclosed in 
U.S. Pat. No. 4,069,399, employs at least one bus carrying a plurality of 
multiplexed digital data signals for delivery to, and/or for reception 
from, a plurality of terminals (telephone instruments, data sets, central 
office trunks, T1 carrier facilities). The data on the bus is sorted by 
time and space switches in a plurality of network modules or loops to 
yield a plurality of data streams. Each data stream is made up of a first 
plurality of time slots, conventionally 32 time slots in a frame, and a 
second plurality of frames, conventionally 8 frames, making up a main 
frame. A data word is an 8-bit word consisting of one bit from a 
corresponding time slot in each of the 8 frames making up a main frame. 
Normally, one of the 32 time slots is devoted to signalling information, 
and a second one is reserved, thus leaving 30 time slots for the 
transmission of data. 
The interface between the network and its terminals takes place in line 
cards. Each line card is capable of serving from one to about 8 terminals. 
The total number of terminals that can be served by a loop depends on the 
quality of service that is desired, as is described below. 
A conventional measure of throughput in a PBX system is in hundreds of 
channel seconds per hour, usually abbreviated CCS. Each channel provides 
36 CCS (there are 3600 seconds per hour). The loop is capable of providing 
36 CCS times the number of time slots available for communication in the 
loop. In the case of a loop having 30 time slots available (32 time slots 
less one signalling time slot and one reserved time slot), the loop 
capacity is 36.times.30=1080 CCS. If fully non-blocking operation is 
desired, then each terminal requires 36 CCS. In such a case, only 30 
terminals can be accommodated. In fact, however, many applications require 
far less than 36 CCS. In motel/hotel use, for example, a typical terminal 
may require only 2 or 3 CCS. Other applications may require channels using 
anywhere from one or two to 36 CCS. Accordingly, it is possible for one 
conventional loop to serve many more than 30 terminals. 
If somewhat less than full non-blocking operation is required, some 
degradation in service is anticipated since there is a probability that 
communications may be desired when no channel capacity remains available. 
Service degradation can be predicted with some degree of accuracy, 
depending on the applications of the particular terminals. As long as it 
is infrequent enough to be no more than a minor inconvenience, such 
service degradation can be tolerated. It is thus possible, and indeed is 
conventional, to continue to add terminals to a loop until a prediction of 
service degradation reaches a predetermined level. 
A measure of grade of service (GOS) degradation is the probability of 
blocking. For a probability of blocking of about 0.01, the 30 channels of 
a conventional loop provide a total capacity of about 660 CCS. It will be 
noted that the total capacity is decreased from 1080 to about 660 CCS, in 
return for which the number of terminals which can be serviced is 
increased by a factor that depends on the type of service required on the 
terminals. In some applications, the number of terminals exceeds the 
number of channels (or available time slots) by a factor of 8. 
In practice, line cards are packaged in peripheral equipment shelves. Each 
peripheral equipment shelf is capable of containing a predetermined 
maximum number of line cards such as, for example, 10 or 16. All line 
cards in a peripheral equipment shelf communicate with the same network 
loop. One network loop may communicate with the line cards in two or more 
peripheral equipment shelves. For economies in peripheral equipment 
shelves, as well as space and power conservation, it is desirable to 
employ substantially all of the capacity of peripheral equipment shelves. 
The design of prior-art equipment contains no provision for matching the 
traffic capacity of its network loops (called loop capacity or channel 
capacity) with the needs of the terminals serviced by line cards in a 
peripheral equipment shelf. Normally, line cards are added to a peripheral 
equipment shelf until the loop capacity is utilized by an amount 
determined by the grade of service selected. If the loop capacity is not 
fully absorbed by the terminals connected to the line cards, a decision 
must be made whether to add a second peripheral equipment shelf to hold 
additional line cards to absorb the remainder of the loop capacity. If the 
remaining channel capacity is less than that which can be absorbed by 
terminals connected to a second full set of line cards in a second 
peripheral equipment shelf, then the decision entails either 
under-utilizing a peripheral equipment shelf, or under-utilizing channel 
capacity. If the second peripheral equipment shelf is supplied, then less 
than all of its full complement of line cards is required, thus 
under-utilizing the peripheral equipment shelf and increasing the usage of 
space and electricity. If the second peripheral equipment shelf is 
omitted, then the remaining unused channel capacity of the network loop is 
wasted. Neither of these alternatives is desirable. 
A large PBX system, such as disclosed in the referenced patent, includes a 
plurality of network loops. In the prior art, each network loop is served 
by its own set of one or more peripheral equipment shelves. Thus, the 
inefficiencies in under-utilized peripheral equipment shelves or 
under-utilized channel capacity outlined above are multiplied by the 
number of network loops. The total number of unused areas in peripheral 
equipment shelves, and/or the total unused channel capacity due to 
decisions omitting additional partly utilized peripheral equipment 
shelves, can result in a significant reduction in PBX system performance. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of the invention to provide a PBX system wherein 
utilization of peripheral equipment shelves and network channel capacity 
is improved. 
It is a further object of the invention to provide a PBX network switching 
system wherein a peripheral controller includes a time switch for 
permitting data to be communicated between more than one network loop and 
line cards on a single peripheral equipment shelf. In this manner, if the 
channel capacity of one network loop is fully consumed when a peripheral 
equipment shelf is only partly filled, additional channel capacity from a 
second loop can be fed to line cards in the peripheral equipment shelf so 
that the peripheral equipment shelf can be fully populated. If any channel 
capacity remains when a peripheral equipment shelf is fully populated, the 
remaining channel capacity can be absorbed by line cards in a subsequent 
peripheral equipment shelf. 
Briefly stated, the present invention provides a peripheral controller for 
a PBX system which accepts PCM data from a plurality of channels. The 
channels may be processed in time and space switches of more than one 
network loop. A channel-merge device merges the PCM data into a combined 
PCM data stream for connection to a time switch. The time switch is 
controlled by a connection memory to direct data on specific channels to 
specific terminals. 
According to an embodiment of the invention, there is provided a PBX system 
of a type including a bus for carrying digital data, comprising: at least 
first and second network loops, each of the first and second network loops 
including a first time switch and a space switch for communicating the 
digital data with the bus, at least one peripheral shelf, the at least one 
peripheral shelf including a peripheral controller, first means for 
communicating first digital data between the first network loop and the 
peripheral controller, means in the peripheral controller for combining 
the first and second digital data into a single third digital data, a 
plurality of terminals, a second time switch in the peripheral controller 
for directing portions of the third digital data to specific ones of the 
terminals, and a connection memory for relating the portions of the third 
digital data with the specific ones of the plurality of terminals, whereby 
a desired communications path is attainable. 
According to a feature of the invention, there is provided a peripheral 
controller for a PBX system comprising: means for accepting a first 
plurality of channels of first digital data from a first network loop, 
means for accepting a second plurality of second digital data from a 
second network loop, means for combining the first plurality of channels 
with the second plurality of channels to produce a third digital data, 
means for communication portions of the digital data with at least first 
and second terminals, and the means for communicating including a time 
switch. 
According to a further feature of the invention, there is provided a PBX 
system of a type including: a bus for carrying digital data, at least 
first and second network loops, each of the first and second network loops 
including a first time switch and a space switch for communicating the 
digital data with the bus, at least one peripheral shelf, the at least one 
peripheral shelf including a peripheral controller, first means for 
communicating first digital data between the first network loop and the 
peripheral controller, means in the peripheral controller for combining 
the first and second digital data into a single third digital data, a 
plurality of terminals, wherein the improvement comprises: a second time 
switch in the peripheral controller for directing portions of the third 
digital data to specific ones of the terminals, and a connection memory 
for relating the portions of the third digital data with the specific ones 
of the plurality of terminals, whereby a desired communications path is 
attainable. 
The above, and other objects, features and advantages of the present 
invention will become apparent from the following description read in 
conjunction with the accompanying drawings, in which like reference 
numerals designate the same elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is shown, generally at 10, a PBX system 
according to the prior art. For reference, PBX system 10 may be considered 
to be represented by the system disclosed in the above-reference patent. 
However, it is contemplated that other systems may equally provide the 
environment for the invention. Conventional elements not necessary to an 
understanding of the present invention are omitted from FIG. 1 and from 
the description thereof to reduce clutter and to make the present 
disclosure easier to understand. Such omitted elements include 
synchronizing and supervisory circuits and signals. If the reader wishes 
to review the relationships of such circuits and signals to the present 
invention, the reader may do so by reviewing the above-referenced patent, 
whose disclosure is herein incorporated by reference for background 
material. 
A network bus 12 carries pulse-code-modulated (PCM), 
time-division-multiplexed (TDM) data at a high frequency of, for example, 
about 2.048 Megabits per second (Mbps). A plurality of network loops 14, 
16, 18 and 20, receive PCM data from, and provide PCM data to, network bus 
12. 
All network loops are identical. Thus, only the content of network loop 14 
is shown. 
A time switch 22 selects PCM signals from network bus 12 that identify 
desired connections between pairs of its terminals. The selected signals 
from time switch 22 are connected to a space switch 24 wherein the PCM 
signals addressed to particular terminals are sorted out to feed PCM 
signals on a PCM loop 26 to a buffer 30 in a peripheral equipment shelf 
34. One buffer 30 is provided per peripheral equipment shelf 34. Outputs 
of buffer 30 are connected to terminal interface units 36 through 38. Each 
terminal interface unit converts the PCM data it receives to a form 
compatible with its particular terminal 40. If, for example, terminal 40 
is an analog telephone, terminal interface unit 36 converts between PCM 
signals available from buffer 30 to analog signals required by terminal 
40. 
PCM loop 26 is also connected to a second peripheral equipment shelf 42 
which communicates, in turn, with a further plurality of terminals 44. 
In a conventional system, the CCS requirements of each terminal 40 
connected to peripheral equipment shelf 34 are added up. Further buffers, 
terminal interface units and terminals are added until the full CCS 
capacity, at the specified GOS is consumed. Then, corresponding equipment 
is added to peripheral equipment shelf 42 until the capacity of peripheral 
equipment shelf 42, or the channel capacity of network loop 14, is 
completely consumed. As noted in the description of the background of the 
invention, if the remaining channel capacity requires only part of the 
space in peripheral equipment shelf 42, it is clear that the costs of 
equipment, space and electrical power incurred by the addition of 
peripheral equipment shelf 42 is only partly utilized. This raises the 
need for a choice to be made between partial utilization of hardware and 
waste of channel capacity. In a system having a large number of network 
loops, the total cost consequences of such a choice, repeated for each 
network loop, can be considerable. 
Referring now to FIG. 2, there is shown, generally at 46, a PBX system 
according to an embodiment of the invention. A plurality of network loops 
14, 16, 18 and 20 communicate PCM signals with network bus 12, as in the 
prior-art embodiment. A peripheral equipment shelf 48 includes a 
peripheral controller 50 receiving PCM signals on PCM lines 52, 54 and 56 
from network loops 14 and 16, as well as from other network loops. In 
turn, peripheral controller 50 communicates with all terminal interface 
units 36-57 in peripheral equipment shelf 48. As in the prior embodiment, 
each of terminal interface units 36-57 communicates with one of terminals 
40-58. It will be noted that peripheral controller 50 substitutes for the 
peripheral buffer in the prior-art embodiment in FIG. 1. 
For purposes of the following description, it is assumed that peripheral 
controller 50 communicates with three network loops, and that the 
communications capacity of 70 channels, selected from three network loops, 
is served by peripheral equipment shelf 48. 
Referring now to FIG. 3, a network interface 58 receives all of the PCM 
data in channels (time slots) addressed to terminals 40-58 for a total of 
three network loops (not shown in FIG. 3). Such channels may be, for 
example, all the thirty channels from each of the first and second network 
loops, and ten channels from the third network loop, for a total of 70 
channels. The remaining 20 channels of the third network loop (along with 
additional channels from other network loops) can be utilized on the next 
peripheral equipment shelf. The data bits of the three loops are applied 
to a channel-merge device 60, wherein the data bits are buffered and 
merged into a contiguous data stream for application on a line 62 to a 
time switch 64. A connection memory 66 retains a data library relating the 
addresses of particular terminals with the channel sources. Time switch 
64, under guidance of connection memory 66, makes the appropriate 
connections of channels from the three network loops with appropriate ones 
of terminals via terminal interface units 36-57. As in the prior-art 
embodiment, each terminal interface unit serves the requirements of one or 
more terminals. 
Connection memory 66 may be implemented using any convenient technology. In 
the preferred embodiment, connection memory 66 is a read/write random 
access memory which is updated by call processing software whenever a call 
is established or torn down. The software also ensures that any channel on 
the third loop which is used on peripheral equipment shelf 48 is not used 
on peripheral equipment shelf 49, and vice versa. 
It is foreseen that the relationship between specific channels of network 
loops and particular peripheral equipment shelves will be established upon 
installation of PBX system 46, and will remain substantially unchanged 
through the useful life of PBX system 46. 
The foregoing should not be taken to mean that the allocation of 
communications resources cannot be changed. On the contrary, if historic 
usage of PBX system 46 indicates a GOS on a peripheral equipment shelf 
that is substantially above or below an acceptable level, the number of 
network loops, and thus channels serving that peripheral equipment shelf, 
may be changed until a target GOS is attained. 
From a system standpoint, the present invention, as shown in FIGS. 2 and 3, 
and described above, substantially eliminates lost channel capacity and 
wasted cost, space and power in peripheral equipment shelves. That is, 
channel assignments can be selected from those available on network bus 
12, which are capable of using all available space on peripheral equipment 
shelf 48. Once the CCS of terminals serviced by peripheral equipment shelf 
48 are completely assigned, other channels are assigned to the next 
peripheral equipment shelf 49 until its CCS capacity is completely 
assigned. This process continues until the last peripheral equipment shelf 
51. This last peripheral equipment shelf 51 is assigned the remaining 
channel capacity. 
It will be evident to one skilled in the art that space in last peripheral 
equipment shelf 51 may be underused. However, this underutilization takes 
place only once per system rather than once per network loop. This 
represents a substantial improvement in the utilization of resources. 
Having described preferred embodiments of the invention with reference to 
the accompanying drawings, it is to be understood that the invention is 
not limited to those precise embodiments, and that various changes and 
modifications may be effected therein by one skilled in the art without 
departing from the scope or spirit of the invention as defined in the 
appended claims.