OCB Digital Switching System


All new technology switching systems are based on Stored Program Controlconcept. The call processing programmes are distributed over different controlorgans 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 inunit ROM / RAM process and handle calls. Handling or processing call means toultimately establish a connection in a switch between i/c and o/g ends. Dependingon the name and architecture of control units and switch may change but criterionfor switching remains more or less the same.



OCB-283 is digital switching system which supports a variety ofcommunication needs like basic telephony, ISDN, etc. This system has beendeveloped by CIT ALCATEL of France and therefore has many similarities to itspredecessor E-10B (also known as OCB-181 in France).

SILENT FEATURES OF THE SYSTEM:1. It is a digital switching with single ‘T’ stage switch. A maximum of 2048PCM’s can be connected.2. It supports both analog and digital subscribers.3. It supports all the existing signalling systems, like decadic, MF (R2), CASand also CCITT#7 signalling system.4. It provides telephony, ISDN, Data communication, cellular radio, and othervalue added services.5. The system has ‘automatic’ recovery feature. When a serious fault occurs ina 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 ona terminal.6. It has a double remoting facility.Subscribers access unit can be placed at aremote place and connected to the main exchange through PCM links. Further,line concentrators can also be placed at a remote location and connected to theCSNL or CSND through PCMs.7. Various units of OCB 283 system are connected over token rings. Thisenables fast exchange of information and avoids complicated links and wiringbetween various units.8. The charge accounts of subscribers are automatically saved in the disc, oncein 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 64kb/s link.11. All the control units are implemented on the same type of hardware. This iscalled a station.12. The system is made up of only 35 types of cards. This excludes the cardsrequired for CSN. Due to this, the number of spare cards to be kept formaintenance, are drastically reduced.13. The system has modular structure. The expansion can be very easily carriedout 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 avery 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-283OCB-283 provides a large number of subscriber facilities. Somefacilities are available to only digital subscribers and as such they cannot beavailed by analog subscribers. To avail these facilities subscriber number aregiven 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 thehandset without dialling any number.• Delayed hot line facility-When subscriber lifts the handset, dial tone is provided he can dial any number. Ifhe does not dial a number, within a predetermined time, he is connected topredetermined number.• Abbreviated dialling-The subscriber can record a short code and its corresponding full number in thememory. 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 numbermentioned by the subscriber while activating the facility.• Conference between four subscribers-Two subscribers while in conversation can include two more subscribers bypressing button and dialling their numbers.• Call waiting indication-When a subscriber is engaged in conversation and if he gets an incoming call, anindication 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 todisconnect this subscriber and attend the waiting call. In the former case, he canrevert back to the earlier subscriber.• Automatic call back on busyIf this facility is activated and if the called subscriber is found busy, the callingsubscriber simply replaces the receiver. The system keeps watch on the calledsubscriber and when it becomes free, a ring is given to both the subscribers. Onlifting they can talk to each other.• Priority line-Calls from this line are processed and put through even when the number of freechannels 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 subscriberanswers. • 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 forsuitable instructions or information.Facilities to digital subscribers:Digital subscribers are provided all the facilities available to analogsubscribers. In addition, they are provided following facilities which are calledISDN services. An ISDN subscriber can use many electronic devices on itstelephone line and can utilize them for two or more simultaneous calls of either• Voice• Data• VideoThe ISDN or Digital Subscribers of OCB-283 can be provided the following typesof connections• 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 datacommunication.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 subscribernumber on the called subscriber’s terminal.6. Charging advice – The system is capable of providing charging advice either in realtime or at the end of the call.7. User to user signaling – The system permits transfer to mini messages betweencalling and called subscribers during call set up and ringing phase.8. Terminal portability during the call – A subscriber can unplug terminal, carry it tosome other place or room and resume the call within 3 minutes.

TIME SWITCH CONCEPT: The time switch comprises of a Speech Buffer Memory, A ControlMemory, An Incoming Highway Of Digital Speech In Parallel Bits and AnOutgoing Highway. This is an Input Associated Controlled Time Switch.In this switch the Buffer Memory and Control Memory are controlled write typei.e. the writing in it is controlled. The control function writes in the controlmemory at the location corresponding to the Incoming Time Slot Number, thelocation where it should be written in the Buffer Memory. Both these memoriesare sequential read type. Reading of control memory gives the address in theBuffer Memory for writing Incoming TS Byte. Thus reading of Buffer Memorysequentially 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 switchingrequires two sets of such switches.

DUPLICATED SWITCHING: The switching is done in OCB-283 in two fully duplicated branchessimultaneously. For this purpose from each connection units the LR linksoriginate in two parallel branches towards two parallel sets of switching matricescalled SMX A and SMX B. The branches of such network are called A and Bbranches. Also the receive side LR links come from both the SMX’s A & B andare terminated on the respective connection units. The duplicated branches ofswitching have been designed to provide high reliability switching path for suchdiverse purposes as data switching, video conference, ISDN applications etc.With the duplicated paths of switching if there is error in one path the other pathwhich is good can be used continuous without interrupting the call in progress.

SAB FUNCTION: The connection units have their internal duplicated hardware whichis called Control Logic, which work in Pilot / Reserve arrangements. Also theyhave non duplicated hardware such as subscriber cards and PCM terminationcards. The duplicated LR’s originate from a function in connection units calledSAB-Selection And Amplification Of Branches. It’s role is to generate two sets ofLR’s in trans-direction with calculation of parity etc. In receive direction it getsdata from both the branches which it checks for parity and compares to detect anyerror in the two branches. In case of error the samples from only the good branchare taken after automatic testing of the quality of transmission of both thebranches by the common control and the faulty branch is withdrawn from theservice.The connection units lr links are formed into group of 8 LR’s at thefactory into cables with both ends terminated with plugs for the convenience ofinstallation. Such groups of LR’s are called GLR.

COMMON CHANNEL SIGNALLING NO.7: The latest signalling being implemented world wide is now the Common ChannelSignalling. This type of signalling is essential for the setting up of the ISDNnetwork.In this type of signalling the signalling information is sent from oneexchange to other exchange in the form of message coded in binary which isunderstandable by the intelligent devices available in both exchange. The CCITTorganization has recommended a standard protocol called CCITT signaling.The signalling message travels over a single Time Slot of the PCM connectingthe two exchange . This Time Slot is called Common Channel for signalling,hence the name Common Channel Signalling. The message over this commonchannel carry all relevant data for any other time slots circuits which carry voiceor subscriber data. The channels for subscribers are called Voice Channels.Signalling is often referred to as the Glue, which holds a network together. Itprovides the ability to transfer information between subscribers, within networksand between subscribers and networks. Without signalling, networks are inert. Byproviding effective signalling systems, a network is transformed into atremendously powerful medium through which subscribers can communicate witheach other using a range of telecommunications services.

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