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// Unit eXchange Digital 5B //
// The Phinal Phile? //
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// By Keltic Phr0st //
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OK folks, here it is: The LAST ever UXD5b file from yours truly where
ALL will be revealed... But first a word of warning... The excerpt itself
is quite old, but nonetheless, technically accurate. And I havent seen
anyone else working on this type of switch or punting any of this info
around, so out it goes for consumption by the HP Masses.
(Excerpeted from British Telecommunications Engineering)
UXD5b : A 600 Line Digital Local Telephone Exchange
===================================================
The UXD5b is a Microprocessor controlled digital switching telephone
exchange designed for worlwide applications where the exchange capacity
is not expected to grow much above 600 lines. The System offers a wider
range of modern facilities over its predecessor, the UXD5A. This article
also decsribes a compact rural version of the exchange for applications
where the exchange availability is less stringent than that normally
specified for higher order exchanges.
INTRODUCTION
============
UXD5b is the latest addition to the UXD5 range of small digital local
telephone exchanges, and follows the UXD5A, which is now an established
system in the British Telecom (BT) Network serving rural communities
in Scotland. The UXD5A has already been described in earlier issues of
this Journal.
The UXD5A system provided basic telephony facilities suited
mainly to rural communities wher the exchange capacity did not exceed
150 connections. In the BT Network this confined its use to a modern
replacement for the aging Strowger Unit Automatic Exchange (UAX) 12.
The UXD5B extends the network modernisation program to cover UAX13's
14's. Whereas most UAX12's are located in Scotland, the larger UAX's
are found in most areas of the BT Network, particularly in Wales and
South West England. The costs of developing this larger size UXD5 system
have been kept to a minimum by interchanging many of the most recent design
advances with BT's Monarch PABX thereby continuing the practise adopted for
the UXD5A, which itself was based on the original PABX design.
The UXD5b is a BT development, which commenced mid-1980, with the
co-operation of Plessey and the General Electric Company, whose involvement
covered export-market requirements. The public service trial of the first
production exchange is now well advanced at MuckHart, near Dundee in
Scotland. BT's confidence in the design has led to substantial initial
supply contracts being placed with industry in advance of completion of
the trial.
This article describes the main features of the UXD5b and outlines
BT's program for its introduction into the network.
BACKGROUND TO THE DEVELOPMENT OF THE UXD5B
==========================================
The Dual BT/Export role of the UXD5b has been paramount from the
outset of the development. The product specification, formulated at the
start of the development, specified a system capable of operaying both
in the BT network and also in telecommunications networks worldwide with
the minimum of adaptive engineering. It was recognised that a wide range
of modern telecommunications facilities would be required and available
on an optional basis and that the system must be capable of interworking
in either a wholly analogue or mixed analogue/digital enviroment. The
concept of a flexible design approach has, therfore, been a major
consideration throughout the development.
The UXD 5a was taken as the base for the new design, and the
development of the UXD5b proceeded in 2 Distinct stages; namely, expanding
the exchange connection capacity and widening the range of facilities
offered. The Objective of the first stage was achieved, by the development
of a 96 port line shelf. By this means, the capacity of a single cabinet
containing 6 line shelves was doubled from 150 lines to 300 lines.
The extended capacity required an increase in processing power, and the 8085
microprocessor used as the main control in the UXD5A was replaced by the
more powerful 8088 device. At the same time, the principle of a main or
or central processing unit (CPU) and pre-processing unit (PPU) was retained.
Provision of a second 300-line cabinet and the interposition of a pulse
code modulation (PCM) interface card, specially developed for the UXD5B,
between the respective control units enables the two 300 line units
to function as a single 600-line exchange.
The second but parallel stage of development included the replacement
of standard electromechanical subscribers meters with an electronic bulk
billing system, provision of a wider range of maintennance and Diagnostic
(M and D) aids and man machine Interface (MMI) facilities. A Universal
signalling junction card was developed to give a greater flexibility in the
allocation of circuits to routes and signalling systems than had been
possible with the dedicated junction units developed for the UXD5A. This new
design also has the capability of interworking with signalling systems more
appropriate to the export enviroment. Again, for the export market, the
system can offer CCITT R2 signalling facilities. In common with other modern
nowbeing introduced in the BT network, the UXD5B offers 6 of the top 8 star
services (supplementary services).
The complexity of the devlopment and the extremely short timescales
set for its completion required that the work be partioned into convenient
development packages and allocated between BT and industry on a functional
basis. For example, industry assumed responsibility for export facilities
not required on BT applications, electronic bulk billing (Call Accounting
Sub-System), MMI, subscribers private meters (SPM) and ringer developments.
BT undertook the majority of main software and hardware development,
junction cards and new line cards, and cabinet unit development. A new
DC-DC convertor was developed under contract with a firm specialising
in this type of design.
UXD5B SYSTEM ORGANISATION
=========================
A system Block Diagram of the UXD5B is shown in Figure 1 (External
file UXDFIG1.ASC).
Line Shelves
------------
The combination of 2 types of line shelves and backplanes is unique
to the UXD5 Application. The 2 Types have 32-port and 96-port capacities
respectively.
The 32 port shelves accomodate 2-port and 4-port line cards providing
an analogue interface to subscribers 2-wire lines. The usual arrangement
on a shelf is six 4-port cards and four 2-port cards, though 2-port cards
can be fitted in the 4-port positions to provide other combinations if
required. The 4-port positions accomodate mostly PBX line cards, each of
which serves 4 PBX Lines. These can be configured on-board to cater for
either loop calling or earth calling PBX groups. The line card provides
digital speech and signalling at 72 Kbit/s, to and from the shelf multiplex
at which speech and signalling are seperated into 2.048Mbit/s and 256Kbit/s
highways respectively to the control shelf. Exceptionally, spare 4-port
positions can be equipped with ordinary-subscribers line cards of the type
use on the UXD5A if required. Lines allocated to the 4-port positions
would normally have calling rates well above average for this type of
exchange. The 2-port positions serve Coinbox (CCB) lines and junction units.
The CCB Line card is designed to interwork with the standard BT Pay-on
answer payphones and each card serves 2 lines. The BT Self contained
payphone is treated as an ordinary line with signalling from the exchange
by means of the 50hz signals used for SPM. The junction units provide 2
junction circuits on a card and these can be arranged as either 2 incoming,
2 Outgoing or 1 Bothway according to the traffic requirements of a particular
exchange application.
The 96-port line shelf accomodates only one type of line card,
namely the 8-port line card. Each card provides connection to eight 2-wire
customer's lines and a fully equipped shelf houses 12 cards serving a total
of 96 lines. Most subscribers lines are connected to this shelf type and
the originating and terminating traffic levels allow the 96 lines to be
concentrated on to 32 Channels to the control shelf.
In this case, each line unit converts to 2.048 Mbit/s PCM, which is
inserted directly into the appropriate timeslot under control of a shelf
interface unit. The particular speech channel/time-slot that is used is
freely allocated by the microprocessor-controlled interface unit, but it
is arranged to allocate low-order numbered time-slots first.
For a call terminating at a port on the shelf, the main processor
will allocate a time-slot starting at the higher ordered time-slots. Thus,
there is contention only between originating and terminating calls when the
32 Channel highway is nearly full. In this event, the terminating call
takes priority. When all time slots are full, an equipment engaged tone
is returned on a terminating call and silence is transmitted to originating
calls.
The actual concentration is performed by time-slot assignment codecs
whose active time-slots are controlled by a microprocessor on each line
card. This is instructed with which time slot to use by the shelf interface
unit. Signalling between the concentrating-shelf interface unit and the
line card uses an 8 bit paralell bidirectional bus that is time shared
between the line cards. Each line card is given a unique address determined
by the wiring on the back plane. Signalling and speech are passed to and
from the concentrating line shelf in the same format as for a
non-concentrating 32-line port shelf, enabling the same control algorithms
to be used in each case.
Control Shelves
---------------
The 4 identical control shelves on the exchange (two on each 300 line
unit) provide the main processing power and switching capability of the
exchange. Only one control shelf on each 300-line unit of a pair of control
shelves is active at any one time, the other control operates in the standby
mode.
Each control shelf includes a change-over card, which provides the
switching function neccesary to allow a pair of shelves to operate in active
and stand-by mode. All inputs to the control shelves are wired in parallel
to both shelves, but the outputs pass through the change-over card. The
cards are interlinked and the stand-by card can be activated under the
control of a microprocessor on its control shelf.
The change-over cards are also linked into a hardware watchdog circuit
on the control-shelf processor cards. If any control-shelf microprocessor
should should become inactive or faulty in its behaviour, this is detected
by the watchdog and the condition is signalled to the change-over card.
The change-over card is designed so that it will not allow its shelf to
become active if it is receiving a fault signal from any watchdog. Thus,
under fault conditions, control will automatically pass to the other shelf
only if all is in order. Change-over causes calls in progress to be lost.
Periodic routine change-over is effected under software control and is
therefore arranged to occur when no calls are in progress.
Digitally encoded speech the shelf interface units and shelf
multiplexers is applied to a non-blocking time switch which switches eight
2048 Kbit/s 32 Channel PCM highways. The timeswitch inputs also include
digitally encoded tones generated on the services card. This card provides
up to 12 tones which can be either BT Standard tones, CCITT standard or any
combination of tones required by an administration. The tones are programmed
in electrically programmable read-only memory (EPROM) and a logic gate
array formats the tones for a particular application under the control of
the EPROM. The services card also contains 6 press-button multi-frequency
(PBMF) Receivers for use with press-button telephones employing multi
frequency (MF) Signalling. (KpT : PBMF Receiver = KRD. Savvy?)
These telephones are allocated a special port type so that the
operating system automatically associates a PBMF receiver whenever MF
dialling is required. The receivers decode the received digits into a
form required by the main control. Where a high percentage of press-button
telephones are in use, additional receivers can be equipped on cards
accomodated on non-concentrating line shelves.
Several facilities on the exchange require interconnection of more
than 2 speech paths; for example, 3-party service, and trunk offering. This
requirement is met by using a conference bridge in which a digital summation
is performed on up to 4 speech paths giving 4 outputs. Each output excludes
one of the incoming speech paths such that it can be connected to a
subscriber without increasing his sidetone levels by excluding his own
speech. The sum is linear so that the same transmission rules apply for a
multi-party call as or a 2-party call. Four conference bridges are provided
on each services card.
Signalling from the shelf interface units and shelf multiplexers is
applied to the signalling-in card on each control shelf. This card stores
the information in a 256 byte random access memory (RAM) with each byte
containing the current signalling information from a port. The processors
scan the store for this information. The outgoing signalling is taken from
the signalling-out card, which is another 256 byte RAM store, that can be
written to by the processor. As the address of each byte corresponds to a
port number on the exchange, the microprocessors can readily read and write
to any port in sequence.
The exchange continually monitors the performance of the speech and
signalling paths by performing a loop-round test. To test the speech path,
a tone is switched to an IDLE line and, as the telephone is ON-HOOK, the
2-wire/4-wire hybrid balance circuit on the line card is mismatched
sufficiently to allow a proportion of the tone to be reflected back through
the system. A Circuit on the services card detects the reflected tone and,
by switching the output from the line card to the services card, the
analogue-to-digial functioning of the line card is verified. A similar
arrangement is used to check the signalling paths.
Each control shelf uses two 8088 microprocessors and the main program
is stored in two 128Kbyte EPROM Cards. Exchange configuration data and
subscribers class-of-service (COS) information is held in battery backed
RAM. As a back-up-facility, the battery-backed RAM card includes and EPROM
device mounted on an internal daughter board. This is automatically
programmed in situ under MMI Control.
The microprocessors are arranged as a mian processor and a pre-
processor. The pre-processor provides fast scanning of those ports which
require it and controls the input/output with the billing processors. The
main processor provides the main call billing function, and uses the
majority of the program store. The processors are closely coupled and share
a common bus. Both processors use the same design of board, the different
functions being implemented by means of links.
Two 300-line abinets are linked together to form a signle 600-line
exchange by means of a 30-channel PCM link. The Link uses standard CCITT
frame definitions and a common channel signalling protocol in time-slot
16. Low-level signal processing, synchronisation and alignment are
performedon-board under microprocessor control. The same basic card is also
used for standard 30-channel channel-associated junction PCM systems.
For the inter cabinet application, the card is located in a control-shelf
position. For interworking with a junction PCM line system, the card replaces
a shelf multiplex on a non-concentrating line shelf. The 32 available ports
on the shelf are allotted to 30 junction connections with ports 0 and 16 being
reserved for synchronisation and signalling operations respectively.
Billing Shelf
-------------
The billing, or call-accounting subsystem, is a duplicated
microprocessor-controlled system which calculates and records the charge
data for each call in progress on the exchange. The duplicated systems are
linked to each control shelf by a pair of serial asynchronous links.
The dialled-digit information, COS of the calling subscriber and time-of-day
are passed from the main system control active control shelf to both
billing planes. Each processor uses this information independently in a
complex algorithm to determine the correct charging rate for a call. At the
end of a call, the 2 values are compared and the final data is recorded on
the subscribers 'software' meter. Any discrepancy is in the recorded data
is conveyed to the M and D routine for analysis and fault reporting.
Billing records are stored in battery-backed CMOS RAM to maintain the
integrity of the data under power-failure conditions.
The system caters for a variety of charging schemes; for example,
singke shot; Variable time Intervale (VTI); and metering-over-junction,
which, if required, can also be sourced to a lower-order exchange;
allowing for local, national and International dialling. The billing system
contains its own accurate real-time clock for local generattion of charging
rates. For current BT applications, Bulk Billing information only is stored;
However, the interfaces are designed to allow for the introduction of
itemised billing. Full access to the system is available via the MMI and the
data is presented in tabular form for administrative use.
(KpT : I shall be releasing,at some point in the future, a guide on how to
decipher and read tabular billing records, as well as EMPIRE line
monitor readings [very similar to billing records] )
Main Control Software
---------------------
Most of the source code for the programs is written in CORAL using a
computer-aided design (CAD) facility called SX1. This is a code generator
which automatically produces CORAL code from Program flow charts. This
procedure has ensured that the code produced is easily maintainable and
reduced development time. Apporximately 80% of the software is written in
CORAL, and the rest in 8088 assembler language. The assembler is used where
either time is critical or input/output routines are being used. The majority
of the software is run by the main processor, and this is best described
under the following main process headings:
Operating System :
------------------
The Operating system provides the link between all the other programs run on
the main processor. It can start programs running either after responding to
an interrupt (known as foreground programs) or on request from a program
(known as background programs). Several programs perform a similar class
of function and in this case, they are given a similar class name, as,
for example, exchange scan, which has foreground programs that scan the
ports and a background program that provides the output function to the
ports. The operating system also provides general routine requests such as
providing the real time clock. It can start background programs after set
delays or at specific times. It also causes the the microprocessor to output
a pulse every 100ms so that the hardware watchdog can verify that it is still
running.
Exchange Scan :
---------------
There are 4 programs associated with exchange scanning. The main program is
run in foreground and scans the complete exchange every 128ms. Subsidiary
exchange-scan programs run at 8ms intervals and 24ms Intervals. These
subsidiary programs scan ports which need a faster response to their
signalling changes than the 128ms scan can provide.
Examples that require this are the detection of loop-disconnect pulses from
a standard telephone and the detection of digits received by the PBMF
receivers. The exchabge-scan backgorund program provides the software
interface to the signalling output to the ports. It also provides an
interface from other programs such as call processing and M and D to the
method of scanning the signalling input provided by the foreground exchange
scan programs.
Call Processing :
-----------------
Call Processing is the program that controls the main functions of the
exchange. It is a background program and acts in response to a request from
other programs. It responds mainly to exchange scan programs as its main
purpose is to interpret the changes in telephonic functions reported to it
and to decide what action the exchange should take.
Inter-Shelf Link :
------------------
Three Programs are associated with controlling the serial link between the
2 control shelves on the same 300-line rack. As for exchange scan, a
background program acts as the interface to other programs and controls
the output across the link, and 2 foreground programs control the reception
of data from the other shelf. At present, the link is used only to
communicate changes in the exchange configuration and changes in the fault
status from one control to another.
Billing :
---------
Billing also has 3 programs used to communicate between call processing and
the billing processors over the serial links from the pre-processor. The
programs are for a similar purpose to that decsribed above and comprise one
background program and 2-foreground programs.
Maintennance and Diagnostics :
------------------------------
This group of programs control the automatic testing and fault reporting
features of the exchange. The background program is started by the operating
system every 15s when it starts a test on one card on the exchange. As it
goes through a testing schedule automatically, the complete exchange is
tested every few hours. A control shelf that is in the NON-ACTIVE state
cannot test line cards, but it can perform tests on its own cards, and
sufficient spare processing power is available to perform more extensive
tests on areas such as the RAM. A fault found on the non-active shelf
is reported to the active shelf via the inter-shelf link programs so that
the neccesary alarm indication action can be taken. A foreground M and D
program runs every 128ms to provide validation of time critical events.
The background program can also be started under request from other
programs so that tests can be run independently of the routine 15 second
test and so that it can decide what action to take on errir conditions
being reported by other programs.
Man-Machine Language :
----------------------
This is a background program that provides the control and decides the
action to take in response to commands from the administration's terminal
that have been passed on to the control shelf by the MMI Processor.
List :
------
The 3 programs associated with the interface to the man-machine processor
are all referred to as list programs. They control the serial link to the
MMI processor and have a background program and 2 foreground programs.
Pre-processor Software :
------------------------
The pre-processor has a small operating system that schedules foreground
programs. The programs that run on the pre-processor are the 2 fast exchange
scan programs runningat 8 and 24ms and the fast 8ms billing program. The
firmware for these programs is contianed on-board the pre-processor so as to
minimise the activity of the pre-processor on the main-control-shelf
microprocessor bus.
Database Handler :
------------------
The function of this process is to carry out maintennance tasks on the
exchange configuration database both during intialistaion and during the
operational life of the exchange.
PHYSICAL REALISATION OF UXD5B
=============================
The UXD5B system consists of 3 cabinet units, 2 A-units holding the line
cards and central processing shelves and a B-uni holding ancilliary
equipment such as the billing system, MMI processor, SPM equipment, alarm
display and spare shelf positions for exchange specific equipment. The
overall dimensions of a 3-cabinet exchange are 1680mm X 600mm X 2080mm.
Up to about 300-line capacity, a single A-cabinet provides the necessary
line equipment accomodation which, together with the B-cabinet, gives
dimensions of 1120mm X 600mm X 2080mm. A front view of a 3-cabinet unit
is given in Fig. 2 (NOT shown)
A-Cabinet equipment :
---------------------
The A-Cabinet (The Outer Cabinets in Fig. 2) provides accomodation for 9
equipment shelves given positional designations A-J, reading upwards. The
power shelf is located at the bottom (Shelf Position A) and this houses
3 DC-DC convertors serving the 6 line shelves above. Two ringing units,
operating in main and standby mode, are also equipped on the power shelf.
The 6 line shelves are alternate 96-port and and 32-port types. The 2
control shelves serving this unit occupy shelf positions G and H. The
equipment cards on both types of line shelf are interconnected by means
of printed-wiring backplanes. These backplanes contain connectors for
terminating the plug-ended cabling to the MDF. The backplane used on the
control shelf is a wired type where the wiring is automatically wrapped
during production.The 2-A cabinets used in a 600-Line exchange have
identical equipment arrangements.
B-Cabinet equipment :
---------------------
The B-Cabinet provides accomodation for the exchange ancilliary equipment.
Referring to the middle cabinet shown in Fig. 2 and reading from bottom
to top, the equipped shelf positions are : SPM, MMI/Power, billing,
miscellaneous, fuse panel. The spare shelf positions are available for
optional facilities such as remote line testing, call logger or for use
as additional miscellaneous or SPM equipment positions. A visual alarm
display unit occupies 2 card positions on the miscellaneous shelf. This
shelf is also designed to house a range of exchange-specific equipment and,
to achieve maximum flexibility, each card position on the backplane is
fitted with a 64-way edge connector for wiring on and off shelf connections.
Unlike the earlier UXD5A, the main distribution frame (MDF) is not
included in the B-Cabinet. For BT applications a standard cable length of
15m has been adopted for cabling between the A-Cabinet Line shelves and
seperately sited MDF. Line shelf equipment is provided with secondary
surge voltage protection on-board. Primary protection, either in the form
of gas-discharge tubes or 3-terminal solid state devices, is required at the
MDF Line side. An intermediate distribution frame (IDF) is not required
since exchange number - directory number (EN-DN) translation is performed
by the exchange software and, under normal traffic conditions and line
card disposition, the exchange is not sensitive to the distribution of
traffic load between individual line units or shelves. Line cable pairs
are therefore jumpered from the line side of the MDF directly to the
exchange side where terminations are arranged in EN order.
Power Supplies :
----------------
The equipment is powered from a conventional -50v DC exchange battery
supply. All other voltages required are derived by using specially
designed efficient DC-DC convertors. Details of the construction of the
convertor can be seen in Fig. 3. Each convertor provides up to 150W of
output power made up of 102W at +5V, 24W at +12V and 24W at -12V.
The outputs are protected against inout voltage transients. Thirteen
convertors are distributed throughout the system to provide the required
exchange availability.
On the A-Cabinet, each control shelf is powered by a dedicated
convertor mounted on the shelf. Line shelves are powered on a paired
basis, the pairing arrangement being one 32-port and one 96-port shelf.
On the B-Cabinet, each call-accounting subsystem has its own convertor and
these are located on the adjacent MMI/Power Shelf. A third convertor on the
MMI/Power shelf supplies the miscellaneous equipemtn shelf and the SPM
shelf.
Typical power consumption figures for a 600-line exchange under
specific traffic loadings are given in Table 1.
TABLE 1
Typical Power Consumption
Traffic Loading (Erlangs) Power Consumption (W)
Idle 1400
40 1900
64 2200
100-LINE SINGLE-CABINET RURAL VERSION
=====================================
A 100-line single-cabinet system has been developed. This system uses a
single control shelf and is primarily intended for applications where the
system availability achieved with duplicated controls and multiple line
shelves is not a major requirement of the administration. With such an
arrangement the mean-time-between-failures resulting in a total loss of
service is expected to be in excess of 5 years. Fig 4. Shows the equipment
cabinet arrangement.
The reduced-height cabinet has provision for 7 shelves of standard
height. The power shelf occupies the bottom shelf position and the order
of shelves above this reading upwards is : MMI, Call Accounting, 32-port
line shelf, 96-port line shelf, control shelf and miscellaneous / SPM
shelf. A fuse panel is mounted to the rear of the power shelf and an
insulation-displacement type MDF is located at the rear of the line shelves.
In this arrangement, the duplicated call-accounting subsystem has been
retained to avoid the nedd for a special design for this application.
Each line shelf is seperately powered from a DC-DC convertor mounted
on the power shelf. The power shelf also includes a main and stand-by ringer
unit. The MMI shelf accomodates the call-accounting subsystem and
miscellaneous / SPM shelf convertors. The control shelf includes its own
dedicated convertor.
UXD5B FACILITIES
================
It has already been mentioned that the UXD5B provides a wider range of
facilities than was available on UXD5A as well as additional facilities
specifically for export requirements.
Subscribers Lines have the options of 2 Line-card types. The standard
8-port line card provides a nominal 25mA constant-current feed to line.
On-board gain adjustment is provided to cover lines with up to 10dB loss.
(The 4-port UXD5A long-line card is used on lines exceeding 10dB loss.)
Certain types of customer apparatus require line current in excess of 25mA.
To Cater for these a card is being developed giving a nominal 40mA line feed.
This card can also provide test access to the line or exchange where remote
line test equipment is fitted. Both card types provide for a limited number
of shared-serice subscribers.
A multiple-signalling junction card, controlled by an on-board
microprocessor, is the standard analogue junction circuit for the UXD5B. It
can be configured to interwork with all BT Local-Network Analogue junction
signalling systems and Caters for E and M signalling for export applications.
A standard message protocol is used for communication between the card and
the main processor and the system therefore does not need to know the type
of signalling system in use.
Where CCITT R2 signalling is required, 2 card types are provided :
The receiver, and the control/sender. Four receivers are mounted on a card
each handling both incoming and outgoing traffic. The control/sender card
provides the control functions (The software for which presides in PROM),
the MF tones and 4 Senders. The control handles 4 sender/receivers, and both
card types are fitted in 4-port positions on the non-concentrating line
shelf.
The UXD5B provides as options several advanced customer facilities.
These are provided by additional software in the call processing program and
are available to a maximum of 200 subscribers on the exchange. The facilities
offered and their function are described below.
Call Waiting Indication
-----------------------
A Tone is transmitted to a busy subscriber's line to indicate that another
subscriber is attempting to contact him. The other subscriber receives an
announcement informing him to wait.
Repeat Last Call
----------------
This enables the last digits dialled by a subscriber to be repeated by
using a short special code.
Call Diversion
--------------
The subscriber can have 3 forms of diversion which he can set up himself:
divert all calls, divert on no reply (his phone will ring for a few seconds
during which time he must answer or else the call will be diverted to another
number), and divert on busy (if he is already engaged on another call).
In all cases, diversions can be set up only to another number on the same
exchange.
3 Way Calling
-------------
While a subscriber is engaged on a call he may dial up another subscriber
and put the original call into a wait state. He then has the option of
speaking to both parties in a conference mode, or clearing down from one
party and continue speaking to the other. For this service to have satis-
-factory transmission standards the conference facility is allowed only to
have one party outside the exchange service area.
Call Barring (Subscriber Controlled)
------------------------------------
The subscriber can bar certain outgoing calls made from his telephone. This
is performed on recognition of the first few digits dialled and not
specifically on call type.
Code Calling
------------
A personal store is allocated to the subscriber in the exchange RAM for
dialling frequently used codes. By applying a short dialling code these
numbers are recalled and passed to the switching system as though he
had dialled the complete number.
EXCHANGE MAINTENNANCE AND DIAGNOSTICS
=====================================
The main software of the exchange contains control options for both routine
self diagnostics and fault finding and operator controlled fault finding.
The aim of the M and D facilities is to detect any faults on the exchange
and give their location down to plug-in unit level. On-site maintennance
is confined to plug-in unit replacement. Lower levels of maintennance and
repair are performed off-site.
The diagnostic software operates in 2-ways. For hardware that has no,
or limited, microprocessor control, all tests are performed by the main
control shelf processor. Those units that have more microprocessor control
instigate their own tests and report on failure of any test to the main
control shelf processor. Some units that can have catastrophic failure
(for example, a fuse blowing) report errors automatically to the main
processor via the howler and alarm board. Any faults found are entered into
the recent fault history list which gives details relating to the type of
fault, the unit that was faulty, the time the fault was found, the number
of occurences of the fault and the associated alarm category.
Another main action taken by the M and D software is the reporting of
faults. If an alarm-extension junction is provided, faults deemed serious
cause alarm conditions to be extended over the junction. Several categories
are catered for and for BT applications these are given the following
titles : Prompt alarm, deferred alarm, and in-station alarm. The category of
alarm determines what level of action should be taken.
MAN-MACHINE COMMUNICATIONS
==========================
The 4 Processors on the control shelves and 2 on the billing shelf have
communication links that require interface to an administration terminal.
An MMI processor is provided on the the MMI shelf to link one terminal to
any of the baove mentioned processors. To allow for multiple usage of the
MMI, several input lines are provided. Up to 4 Modem-interface lines can
be provided for remote interface to the MMI processor, one local terminal,
a transaction recorder and a printer so that the MMI processor gives the
administration complete flexibility on the type of interface it requires.
Several levels of interface to the processor are allowed:
A) Fault Status Level
---------------------
The administration can access the MMI processor such that a brief form of the
fault record for each control shelf on the exchange can be printed out
(referred to as the active alarm list)
B) Billing Access Level
-----------------------
By using different passwords (Which are rarely enforced... KpT) the
administration can gain access to the billing information and enable a dump
of the billing records to be given. At this level no access is permitted to
the main processor MMI.
C) Main MMI Level
-----------------
By using Different Passwords again (Which are rarely enforced... AGAIN!),
the administration can gain access to the man-machine language on the main
processors. By using this, full access is gained to the M and D facilities
on the exchange and the exchange configuration can be altered and listed.
D) Specialist User Level
------------------------
This allows designers to interrogate the system for debugging, etc. To avoid
conflict between simultaneous users, only one user is allowed access to
a particular exchange function at any one time.
Remote access to the exchange MMI is provided via Modems. These can be either
switched or on private wires. To prvide extra security on switched
connections, the logon procedure is as follows. A special code is dialled
into the exchange and the exchange recognises it as a special type requiring
MMI Access. The call is cleared down and the exchange dials out on a
pre-configured number and connects the modem when a satisfactory connection
has been achieved. Several of these numbers can be configured, one to each
MMI access Number, to allow access for a number of administration sites.
Full access is still not attained until the neccesary logging in procedure
has been followed.
FUTURE DEVELOPMENTS
===================
In conjunction with British Industry, The UXD5B development is continuing
further, mainly in the areas of customer services and administration
facilities.
Advanced Customer services such as call reminder and call-charge
advice require a voice guidance and announcement system to ensure that the
service is correctly set up and responding in the required manner. These
are currently under development for BT applications. A number of systems
already exist and work is currently in hand to adapt the most suitable to
UXD5B.
BT Have embarked on a trial program of fitting call-logging equipment
in all local exchanges. In the case of the UXD5B, the intention is to
produce an equipment having a hardware and software interface with the
exchange identical with those of the normal billing system so that it can
be fitted as an alternative on an optional basis. Some additional changes to
the main system software will be required for handling the message protocol
between associated equipment provided on a common basis in the network.
A development projected for the longer term is a new customer
interface card capable of providing Integrated Services Digital Network
(ISDN) facilities. New administration facilities include the provision of
the CCITT No. 7 common channel signalling system (ARRRRRGGGHHHH) and a
message transmission sub-system ((MTS). These will offer UXD5B customers
the use of co-operative supplementary services in common with System X in
an Integrated Digital Network.
CONCLUSION
==========
The development of the UXD5B is a joint BT and British Industry activity.
In the case of BT it continues the program of rural-network modernisation,
which began with the introduction of the UXD5A. The larger capacity UXD5B
will be used mainly as a replacement for the older UAX13s, 14s and SAX
exchanges in Scotland, Wales and South West England. To meet this program,
initial orders have been placed with industry for over 350 exchanges
covering a 3 year supply period. Not only is the UXD5B suitable for
integration into an eventually all digital network, the wide range of
advanced facilities it can offer, together with the continuing development
program, mean that it will meet the future needs of the customer for the
forseeable future with minimal change to the overall system architecture.
Much interest is being shown in the UXD5B by overseas administrations,
and BT's own commitment to it should enhance the success of the product in
export markets.