All new technology switching systems are based on Stored Program Control
concept. The call processing programmes are distributed over different control
organs of the system and are stored in ROM / RAM of different control units.
Processor in the control units by using the programme and data stored in
unit ROM / RAM process and handle calls. Handling or processing call means to
ultimately establish a connection in a switch between i/c and o/g ends. Depending
on the name and architecture of control units and switch may change but criterion
for switching remains more or less the same.
OVERVIEW OF OCB-283
OCB-283 is digital switching system which supports a variety of
communication needs like basic telephony, ISDN, etc. This system has been
developed by CIT ALCATEL of France and therefore has many similarities to its
predecessor E-10B (also known as OCB-181 in France).
SALIENT FEATURES OF THE SYSTEM:
1. It is a digital switching with single ‘T’ stage switch. A maximum of 2048
PCM’s can be connected.
2. It supports both analog and digital subscribers.
3. It supports all the existing signalling systems, like decadic, MF (R2), CAS
and also CCITT#7 signalling system.
4. It provides telephony, ISDN, Data communication, cellular radio, and other
value added services.
5. The system has ‘automatic’ recovery feature. When a serious fault occurs in
a control unit, it gives a message to SMM. The SMM puts this unit out of service,
loads the software of this unit in a back up unit and brings it into service.
Diagnostic programmes are run on the faulty unit and the diagnostics is printed on
6. It has a double remoting facility.Subscribers access unit can be placed at a
remote place and connected to the main exchange through PCM links. Further,
line concentrators can also be placed at a remote location and connected to the
CSNL or CSND through PCMs.
7. Various units of OCB 283 system are connected over token rings. This
enables fast exchange of information and avoids complicated links and wiring
between various units.
8. The charge accounts of subscribers are automatically saved in the disc, once
in a day. This avoids loss of revenue in case of battery failure.
9. The traffic handling capacity of the system is huge.
10. The exchange can be managed either locally or from an NMC through 64
11. All the control units are implemented on the same type of hardware. This is
called a station.
12. The system is made up of only 35 types of cards. This excludes the cards
required for CSN. Due to this, the number of spare cards to be kept for
maintenance, are drastically reduced.
13. The system has modular structure. The expansion can be very easily carried
out by adding necessary hardware and software.
14. The SMMs are duplicated with one active other standby. In case of faults,
switch over takes place automatically.
15. The hard disc is very small in size,compact and maintenance free. It has a
very huge memory capacity of 1.2 Giga bytes.
16. The space requirement is very small.
17. There is no fixed or rigid rack and suite configuration in the system.
SUBSCRIBERS FACILITY PROVIDED BY OCB-283
OCB-283 provides a large number of subscriber facilities. Some
facilities are available to only digital subscribers and as such they cannot be
availed by analog subscribers. To avail these facilities subscriber number are
given special categories by man machine commands.
Facilities to analogue subscribers-
• A line can be made only outgoing or incoming.
• Immediate hot line facility-
The subscriber is connected to another predetermined subscriber on lifting the
handset without dialling any number.
• Delayed hot line facility-
When subscriber lifts the handset, dial tone is provided he can dial any number. If
he does not dial a number, within a predetermined time, he is connected to
• Abbreviated dialling-
The subscriber can record a short code and its corresponding full number in the
memory. Later he dial this number, he has to only dial short code.
• Call forwarding-
When activated, incoming calls to the subscriber gets transferred to the number
mentioned by the subscriber while activating the facility.
• Conference between four subscribers-
Two subscribers while in conversation can include two more subscribers by
pressing button and dialling their numbers.
• Call waiting indication-
When a subscriber is engaged in conversation and if he gets an incoming call, an
indication is given in the form of tone. Hearing this, the subscriber has option,
either to hold the subscriber in conversation and attend the waiting call or to
disconnect this subscriber and attend the waiting call. In the former case, he can
revert back to the earlier subscriber.
• Automatic call back on busy
If this facility is activated and if the called subscriber is found busy, the calling
subscriber simply replaces the receiver. The system keeps watch on the called
subscriber and when it becomes free, a ring is given to both the subscribers. On
lifting they can talk to each other.
• Priority line-
Calls from this line are processed and put through even when the number of free
channels are within a threshold.
• Malicious call identification-
In this category, the number of calling subscriber is printed on the terminal
• Battery reversal- the system extends battery reversal when called subscriber
• Detailed billing-
The system provides detailed bills giving details of date, time, etc.
• Absent subscriber service-
When activated, the incoming calls are diverted to absent subscriber service for
suitable instructions or information.
Facilities to digital subscribers:
Digital subscribers are provided all the facilities available to analog
subscribers. In addition, they are provided following facilities which are called
ISDN services. An ISDN subscriber can use many electronic devices on its
telephone line and can utilize them for two or more simultaneous calls of either
The ISDN or Digital Subscribers of OCB-283 can be provided the following types
• 2B+D LINE :- 2 Voice Channel of 64kbps and 1 Data Channel of 16kbps
• 30 B+D LINE :- 30 Voice Channel of 64kbps and 1 Data Channel of 64kbps
The following is the list of some of the services to digital subscribers:
1. It provides 64kb/s digital connectivity between two subscribers for data
2. The system provides facsimile services.
3. It provides videotext services.
4. It provides display of calling subscriber number on called subscribers telephone.
5. It also provides the facility for restriction of the display of calling subscriber
number on the called subscriber’s terminal.
6. Charging advice - The system is capable of providing charging advice either in real
time or at the end of the call.
7. User to user signaling - The system permits transfer to mini messages between
calling and called subscribers during call set up and ringing phase.
8. Terminal portability during the call - A subscriber can unplug terminal, carry it to
some other place or room and resume the call within 3 minutes.
TIME SWITCH CONCEPT
The time switch comprises of a Speech Buffer Memory, A Control
Memory, An Incoming Highway Of Digital Speech In Parallel Bits and An
Outgoing Highway. This is an Input Associated Controlled Time Switch.
In this switch the Buffer Memory and Control Memory are controlled write type
i.e. the writing in it is controlled. The control function writes in the control
memory at the location corresponding to the Incoming Time Slot Number, the
location where it should be written in the Buffer Memory. Both these memories
are sequential read type. Reading of control memory gives the address in the
Buffer Memory for writing Incoming TS Byte. Thus reading of Buffer Memory
sequentially the TS will be read from the location given by the Control Memory.
Thus a one way Time switching has taken place. Similarly a both way switching
requires two sets of such switches.
The switching is done in OCB-283 in two fully duplicated branches
simultaneously. For this purpose from each connection units the LR links
originate in two parallel branches towards two parallel sets of switching matrices
called SMX A and SMX B. The branches of such network are called A and B
branches. Also the receive side LR links come from both the SMX’s A & B and
are terminated on the respective connection units. The duplicated branches of
switching have been designed to provide high reliability switching path for such
diverse purposes as data switching, video conference, ISDN applications etc.
With the duplicated paths of switching if there is error in one path the other path
which is good can be used continuous without interrupting the call in progress.
The connection units have their internal duplicated hardware which
is called Control Logic, which work in Pilot / Reserve arrangements. Also they
have non duplicated hardware such as subscriber cards and PCM termination
cards. The duplicated LR’s originate from a function in connection units called
SAB-Selection And Amplification Of Branches. It’s role is to generate two sets of
LR’s in trans-direction with calculation of parity etc. In receive direction it gets
data from both the branches which it checks for parity and compares to detect any
error in the two branches. In case of error the samples from only the good branch
are taken after automatic testing of the quality of transmission of both the
branches by the common control and the faulty branch is withdrawn from the
The connection units lr links are formed into group of 8 LR’s at the
factory into cables with both ends terminated with plugs for the convenience of
installation. Such groups of LR’s are called GLR.
COMMON CHANNEL SIGNALLING NO.7
The latest signalling being implemented world wide is now the Common Channel
Signalling. This type of signalling is essential for the setting up of the ISDN
In this type of signalling the signalling information is sent from one
exchange to other exchange in the form of message coded in binary which is
understandable by the intelligent devices available in both exchange. The CCITT
organization has recommended a standard protocol called CCITT signaling.
The signalling message travels over a single Time Slot of the PCM connecting
the two exchange . This Time Slot is called Common Channel for signalling,
hence the name Common Channel Signalling. The message over this common
channel carry all relevant data for any other time slots circuits which carry voice
or subscriber data. The channels for subscribers are called Voice Channels.
Signalling is often referred to as the Glue, which holds a network together. It
provides the ability to transfer information between subscribers, within networks
and between subscribers and networks. Without signalling, networks are inert. By
providing effective signalling systems, a network is transformed into a
tremendously powerful medium through which subscribers can communicate with
each other using a range of telecommunications services.