Intelligent Network (IN)

What is Intelligent Network?

Intelligent Network (IN):

Overview of Intelligent Network Architecture

Over the last thirty years, one of the major changes in the implementation of Public Switched Telephone Networks (PSTNs) has been the migration from analogue to digital switches. Coupled with this change has been the growth of intelligence in the switching nodes. From a customer’s and network provider’s point of view this has meant that new features could be offered and used.

Since the feature handling functionality was resident in the switches, the way in which new features were introduced into the network was by introducing changes in all the switches. This was time consuming and fraught with risk of malfunction because of proprietary feature handling in the individual switches.

To overcome these constraints the Intelligent Network architecture was evolved both as a network and service architecture.

In the (Intelligent Network) IN architecture, the service logic and service control functions are taken out of the individual switches and centralized in a special purpose computer. The interface between the switches and the central computer is standardised. The switches utilize the services of the specialized computer whenever a call involving a service feature is to be handled. The call is switched according to the advice received by the requesting switch from the computer. For normal call handling, the switches do not have to communicate with the central computer.

1 Objectives of the Intelligent Network

The main objectives of the IN (Intelligent Network) are the introduction and modification of new services in a manner which leads to substantial reduction in lead times and hence development costs, and to introduce more complex network functions.

An objective of IN (Intelligent Network) is also to allow the inclusion of the additional capabilities and flexibility to facilitate the provisioning of services independent of the underlying network’s details. Service independence allows the service providers to define their own services independent of the basic call handling implementation of the network owner.

The key needs that are driving the implementation of IN are :

  • Rapid Service Deployment

Most business today require faster response from their suppliers, including telecommunication operators. By separating the service logic from the underlying switch call processing software, IN enables operator to provide new services much more rapidly.

  • Reduced Deployment Risk

Prior to IN (Intelligent Network), the risk associated with the deployment of new services was substantial. Major investments had to be made in developing the software for the services and then deploying them in all of the switches.

With the service creation environment available, the IN services can be prototyped, tested and accessed by multiple switches simultaneously. The validated services can then be rolled out to other networks as well.

  • Cost Reduction

Because the IN (Intelligent Network) services are designed from the beginning to be reusable, many new services can be implemented by building on or modifying an existing service. Reusability reduces the overall cost of developing services. Also, IN is an architecture independent concept, i.e. it allows a network operator to choose suitable development hardware without having to redevelop a service in the event that the network configuration changes.

  • Customization

Prior to IN, due to complexity of switch based feature handling software, the considerable time frame required for service development prevented the provider from easily going back to redefine the service after the customer started to use it. With IN, the process of modifying the service or customization of service for a specific customer is much less expensive and time consuming.

The customization of services is further facilitated by the integration of advanced peripherals in the IN (Intelligent Network) through standard interfaces. Facilities such as voice response system, customized announcements and text to speech converters lead to better call completion rate and user-friendliness of the services.

IN Architecture

Building upon the discussion in the previous section, one can envisage that an IN would consist of the following nodes :

  • Specialized computer system for – holding service logic, feature control, service creation, customer data, and service management.
  • Switching nodes for basic call handling.
  • Specialized resources node.

The physical realization of the various nodes and the functions inherent in them is flexible. This accrues form the “open” nature of IN interfaces.

Let us now look at the nodes that are actually to be found in an IN implementation.

The service logic is concentrated in a central node called the Service Control Point (SCP0.

The switch with basic call handling capability and modified call processing model for querying the SCP is referred to as the Service Switching Point (SSP).

Intelligent Peripheral (IP) is also a central node and contains specialized resources required for IN (Intelligent Network) service call handling. It connects the requested resource towards a SSP upon the advice of the SCP.

Service Management Point (SMP0 is the management node which manages services logic, customers data and traffic and billing data. The concept of SMP was introduced in order to prevent possible SCP malfunction due to on-the-fly service logic or customer data modification. These are first validated at the SMP and then updated at the SCP during lean traffic hours. The user interface to the SCP is thus via the SMP.

All the nodes communicate via standard interfaces at which protocols have been defined by international standardization bodies. The distributed functional architecture, which is evident from the above discussion, and the underlying physical entities are best described in terms of layers or planes. The following sections are dedicated to the discussion of the physical and functional planes.

3 Physical Plane

Service Switching Point (SSP)

The SSP serves as an access point for IN (Intelligent Network) services. All IN services calls must first be routed through the PSTN to the “nearest” SSP. The SSP identifies the incoming call as an IN service call by analysing the initial digits (comprising the “Service Key”) dialled by the calling subscriber and launches a Transaction Capabilities Application Part (TCAP) query to the SCP after suspending further call processing. When a TCAP response is obtained from the SCP containing advice for further call processing, SSP resumes call processing.

Integrated Services Digital Network | ISDN

What is Integrated Services Digital Network (ISDN)?

Integrated Services Digital Network (ISDN):

The ISDN is an abbreviation of Integrated Services Digital Network. The current communications networks vary with the type of service, such as telephone network, telex network, and digital data transmission network. On the other hand, the ISDN is an integrated network for various types of communications services handling digitized voice (telephone) and non voice (data) information.

Fig.1 shows the current network configuration with individual networks, such as telephone network and a data network existing independently, and telephone sets, data terminals, etc. connected individually to each network

The Network Configuration Without and with ISDN 

ISDN Definition

The CCITT defines the ISDN as follows :

  • A complete, terminal-to-terminal digital network. Fig.3 shows the end-to-end digital connectivity.

 End-to-End Digital Connectivity

  • A network that provides both telephone and non-telephone services in the same network. Fig.4 shows the voice and non-voice services in the same network.

Voice and Non-Voice Service in the Same Network

  • A network based on a digital telephone network.
  • A network that utilizes Signaling System No. 7 (SS7) for signaling between switching systems. Fig. 5 shows the signaling connection between Switching Systems.

The Signaling Connection between Switching Systems

  • A network offers standard user network interface. Fig.6 shows the standard user network interface.

Standard User Network Interface

ISDN Services

1) A wide range of services

The ISDN provides the following functions, as shown in Fig.7.

  1. Packet switching service
  2. Circuit switching service
  3. Leased circuit service

A Wide Range of Services

Circuit switching service includes both telephone and data circuit switching.

  • As shown in the figure, ISDN can interface with various terminals, such as a telephone set, FAX, Video terminal or personal computer to provide a wide range of services.
  • The ISDN concept can be summarized by two statements :
  • ISDN offers a variety of services, such as telephone, data and image transmission through one network.
  • ISDN handles all information digitally.

2) Standard user-network interface. Fig.8 shows the user-terminal/network interface.

shows the user-terminal/network interface

  • The subscriber line is connected with an NT (Network Termination) installed at the customer premises.
  • Various terminals are connected to the NT. These terminals can include digital telephones, multi media terminal, digital facsimile machines, personal computers, etc. as shown in the figure.
  • The NT and terminals are connected by S or T interface (S/T interface), as recommended by the CCITT. Up to 8 terminals are connected to one S/T interface. The NT and terminals are connected using an 8-pin connector, which is also recommended by the CCITT.
  • As shown in this figure, the personal computer uses the RS232C interface that is different from the ISDN S/T interfaces, so a TA (Terminal Adapter) is provided to adapt the RS232C interface for use with the ISDN interfaces.

Fig. 9 shows operation of various terminals in the home.

Operations of various terminals in the home

  • Each terminal is connected to the NT through S/T interface which, in turn, is connected to the switching system through the subscriber line.
  • At the upper left of the figure a person is using a television telephone called a Video Phone, at the lower left, a person is watching a picture on a Videotext terminal.
  • At the upper right of the figure, a person is operating a personal computer, which requires the use of a TA to convert the computer’s RS232C interface to the S/T interfaces used by ISDN. At the lower right, a person is doing catalog shopping using a Videotex terminal.

3) Home Shopping and Home Banking

  • 10 shows home shopping and home banking services.
  • 10 shows a typical service made possible by ISDN. It shows something is being ordered to a department store, and then delivered

Home shopping and home banking service

  • The goods are ordered using the Videotex terminal, and an instruction is output to the bank to transfer the amount of the bill from your account.
  • The department store delivers the ordered goods.

4) Home Medical System

  • 11 shows home medical system.
  • 11 shows another service provided by ISDN : the receiving of medical care at home.

Home Medical System

  • The upper left shows the measuring of blood pressure, with the result shown on the videotex screen both at home and at a medical facility (show at the bottom right of the figure).
  • The lower left shows a consultation for medication using a TV telephone.

User Network Interface

ISDN User Network Interface Configuration

  • 12 shows the interface between the user and the network. Telephone service makes use of two wires for the subscriber line between the switching system and customer’s premises. These same two wires can be ued by ISDN to receive ISDN services.
  • An NT (Network Termination) is installed at the subscriber’s home and connected to the subscriber line.

The Interface between the User 

  • The Interface between the NT and the ISDN exchange (switching system) is called U interface. This interface has not been defined in the CCITT Recommendations because circumstances are different in each country. The point between the NT and the on-premises terminals is called the S or T reference point. The ISDN user/network interface refers to these S/T points, and is defined in the CCITT Recommendations.
  • The S/T interface uses four wires, two for sending and two for receiving. Since U interface uses two wires, the NT provides a two-wire/four-wire conversion function.
  • CCITT recommends the use of AMI (Alternative Mark Inversion) code at the S/T point. AMI code is a bipolar waveform.
  • As shown in the figure, the ISDN Terminal provides S/T interface that follows the CCITT Recommendations, and can be connected directly to the NT. Since the personal computer and the analog FAX utilize a different interface from S/T interface, they require protocol conversion by a TA (Terminal Adapter).

Service Access Points (Reference Points)

  • In the existing telephone network, a point at which a service is provided for a user, that is, a service access point is located at a rossete between the user’s telephone set and the subscriber line.

Since the ISDN provides various types of service other than telephone service through a plural number of terminals, various service access points are provided. Thus, service access points would have to be defined corresponding to the ISDN Services.

  • Fig 13 shows the user-network interface reference points which is based on the CCITT reference model and identifies the important reference points of the model.

User-Network Interface Reference Points

  • The following describes the user-access points and the function of each for basic user-network interface.

Network Termination (NT) :

  • The NT can be split into NT1 and NT2. NT1 and NT2 are terminating equipment for the network.
  • In this case, NT1 provides the Layer 1 functions, such as circuit termination, timing and supply of electricity, while NT2 provides the layer 2 functions, such as protocol, control and concentration functions.

Terminal Equipment (TE) :

  • The TE can be split into TE1 and TE2. TE1 is an ISDN terminal which is connected to ISDN via the S/T interface. TE2 is a non-ISDN terminal which is connected to ISDN via a Terminal Adapter (TA) such as personal computer or analog FAX as described in Fig. 12.

Terminal Adapter (TA) :

  • A TA is a physical device which is connected to a non-ISDN terminal (TE2) to permit access to ISDN.

S-Interface :

  • A 4-wire physical interface used for a single customer termination between a TA and NT2 or between TE1 and NT2.

T-Interface :

  • A 4-wire physical interface between NT1 and NT2.

R-Interface :

  • A physical interface used for single customer terminator between TE2 and TA.

U-Interface :

  • The subscriber line is called U-Interface and utilizes 2-wires.

ISDN User Network Interface Points

1) Requirements of User-Network Interface

Digital Switching

What is the Digital Switching System?

Digital Switching System:

A Digital switching system, in general, is one in which signals are switched in digital form. These signals may represent speech or data. The digital signals of several speech samples are time multiplexed on a common media before being switched through the system.

To connect any two subscribers, it is necessary to interconnect the time-slots of the two speech samples which may be on same or different PCM highways. The digitalised speech samples are switched in two modes, viz., Time Switching and Space Switching. This Time Division Multiplex Digital Switching System is popularly known as Digital Switching System.

In this handout, general principles of time and space switching are discussed. A practical digital switch, comprising of both time and space stages, is also explained.

4.1 Time and Space Switching

 Generally, a digital switching system several time division multiplexed (PCM) samples.  These PCM samples are conveyed on PCM highways (the common path over which many channels can pass with separation achieved by time division.). Switching of calls in this environment , requires placing digital samples from one time-slot of a PCM multiplex in the same or different time-slot of another PAM multiplex.

For example, PCM samples appearing in TS6 of I/C PCM HWY1 are transferred to TS18 of O/G PCM HWY2, via the digital switch, as shown in Fig1.


The interconnection of time-slots, i.e., switching of digital signals can be achieved using two different modes of operation. These modes are:  –

  1. Space Switching
  2. Time switching

Usually, a combination of both the modes is used.

In the space-switching mode, corresponding time-slots of I/C and O/G PCM highways are interconnected. A sample, in a given time-slot, TSi of an I/C HWY, say HWY1, is switched to same time-slot, TSi of an O/G HWY, SAY HWY2. Obviously there is no delay in switching of the sample from one highway to another highway since the sample transfer takes place in the same time-slot of the PCM frame.

Time Switching, on the other hand, involves the interconnection of different time-slots on the incoming and outgoing highways by re-assigning the channel sequence. For example, a time-slot TSx of an I/C Highway can be connected to a different time-slot., TSy, of the outgoing highway. In other words, a time switch is, basically, a time-slot changer.

4.2 Digital Space Switching


The Digital Space Switch consists of several input highways, X1, X2,…Xn  and several output highways, Y1, Y2,………….Ym, inter connected  by a crosspoint matrix of n rows and m columns. The individual crosspoint consists of electronic AND gates. The operation of an appropriate crosspoint connects any channel, a , of I/C PCM highway to the same channel, a, of O/G PCM highway, during each appropriate time-slot which occurs once per frame as shown in Fig 2. During other time-slots, the same crosspoint may be used to connect other channels. This crosspoint matrix works as a normal space divided matrix with full availability between incoming and outgoing highways during each time-slot.

Space switch

Each crosspoint column, associated with one O/G highway, is assigned a column of control memory. The control memory has as many words as there are time-slot per frame in the PCM signal. In practice, this number could range from 32 to 1024. Each crosspoint in the column is assigned a binary address, so that only one crosspoint per column is closed during each time-slot. The binary addresses are stored in the control memory, in the order of time-slots. The word size of the control memory is x bits, so that 2x = n, where n is the number of cross points in each column .

A new word is read from the control memory during each time-slot, in a cyclic order. Each word is read during its corresponding time-slot, i.e.,Word 0 (corresponding to TS0), followed by word 1 (corresponding to TS1) and so on. The word contents are contained on the vertical address lines for the duration of the time-slot. Thus, the cross point corresponding to the address, is operated during a particular time-slot. This cross point operates every time the particular time-slot appears at the inlet in successive frames. normally, a call may last for around a million frames.

As the next time-slot follows, the control memory is also advanced by one step, so that during each new time-slot new corresponding words are read from the various control memory columns. This results in operation of a completely different set of cross points being activated in different columns. Depending upon the number of time-slots in one frame, this time division action increases the utilisation of cross point 32 to 1024 times compared with that of conventional space-divided switch matrix.


Consider the transfer of a sample arriving in TS7 of I/C HWY X1 to O/G HWY Y3. Since this is a space switch, there will be no reordering of time i.e., the sample will be transferred without any time delay, via the appropriate cross point. In other words, the objective is to connect TS7 of HWY X1 and TS7 of HWY Y3.

The central control (CC) selects the control memory column corresponding output highway Y3. In this column, the memory location corresponding to the TS7 is chosen. The address of the cross point is written in this location, i.e., 1, in binary, is written in location 7, as shown in fig 2.This cross point remains operated for the duration of the time-slot TS7, in each successive frame till the call lasts.

For disconnection of call, the CC erases the contents of the control memory locations, corresponding to the concerned time-slots. The AND gates, therefore, are disabled and transfer of samples is halted.

Practical Space Switch

In a practical switch, the digital bits are transmitted in parallel rather than serially, through the switching matrix.

In a serial 32 time-slots PCM multiplex, 2048 Kb/s are carried on a single wire sequentially, i.e., all the bits of the various time-slots follow one another. This single wire stream of bits, when fed to Serial to Parallel Converter is converted into 8-wire parallel output. For example, all 8 bits corresponding to TS3 serial input are available simultaneously on eight output wires (one bit on each output wire), during just one bit period, as shown in fig.3. This parallel output on the eight wires is fed to the switching matrix. It can be seen that during one full time-slot period, only one bit is carried on the each output line, whereas 8 bits are carried on the input line during this period. Therefore, bit rate on individual output wires, is reduced to 1/8th of input bit rate=2048/8=256Kb/s

Basic Principles of Electronic Exchanges

Basic Principles of Electronic Exchanges:

The prime purpose of an exchange is to provide a temporary path for simultaneous. bi- directional transmission of speech between

(i) Subscriber lines connected to same exchange (local switching)

(ii) Subscriber lines and trunks to other exchange(outgoing trunk     call)

(iii) Subscriber lines and trunks from other exchanges(incoming trunk calls) and

(iv) Pairs of trunks towards different exchanges (transit switching)

These are also called the switching functions of an exchange and are implemented through the equipment called the switching network. An exchange, which can setup just the first three types of connections., is called a Subscriber or Local Exchange. If an exchange can setup only the fourth type of connections, it is called a Transit or Tandem Exchange. The other distinguished functions of an exchange are

i) Exchange of information with the external environment (Subscriber lines or other exchanges) i.e. signaling.

ii) Processing the signaling information and controlling the operation of signaling network, i.e. control, and

iii) Charging and billing

All these functions can be provided more efficiently using computer controlled electronic exchange, than by the conventional electromechanical exchanges.

This handout describes the basic principals of SPC exchanges and   explains how the exchange functions are achieved.

3.1 Stored Programme Controlled Exchange:            

In electromechanical switching, the various functions of the exchange are achieved by the operation and release of relays and switch (rotary or crossbar) contacts, under the direction of a Control Sub-System. These contracts are hard – wired in a predetermined way.  The exchange dependent data, such as, subscriber’s class of service, translation and routing, combination signaling characteristics, are achieved by hard-ware and logic, by a of relay sets, grouping of same type of lines, strapping on Main or Intermediate Distribution Frame or translation fields, etc. When the data is to be modified, for introduction of a new service, or change in services already available to a subscriber, the hardware change ranging from inconvenient to near impossible, are involved.

In an SPC exchange, a processor similar to a general purpose computer, is used to control the functions of the exchange. All the control functions, represented by a series of various instructions, are stored in the memory. Therefore the processor memories hold all exchange-dependent data. such as subscriber date, translation tables, routing and charging information and call records. For each call processing step. e.g. for taking a decision according to class of service, the stored data is referred to, Hence, this concept of switching. The memories are modifiable and the control program can always be rewritten if the behavior or the use of system is to be modified. This imparts and enormous flexibility in overall working of the exchange.

Digital computers have the capability of handling many tens of thousands of instructions every second, Hence, in addition to controlling the switching functions the same processor can handle other functions also. The immediate effect of holding both the control programme and the exchange data, in easily alterable memories, is that the administration can become much more responsive to subscriber requirements. both in terms of introducing new  services and modifying general services, or in responding to the demands of individual subscriber. For example, to restore service on payment of an overdue bill or to permit change from a dial instrument to a multi frequency sender, simply the appropriate entries in the subscriber data-file are to be amended. This can be done by typing- in simple instructions from a teletypewriter or visual display unit. The ability of the administration to respond rapidly and effectively to subscriber requirements is likely to become increasingly  important in the future.

The modifications and changes in services which were previously impossible  be achieved very simply in SPC exchange, by modifying the stored data suitably. In some cased, subscribers can also be given the  facility to modify their own data entries for supplementary services, such as on-demand call transfer, short code, (abbreviated ) dialing, etc.

The use of a central processor, also makes possible the connection of local and remote terminals to carry out man-machine dialogue with each exchange. Thus, the maintenance and administrative operations of all the SPC exchanges in a network can be performed from a single centralised place. The processor sends the information on the performance of the network, such as, traffic flow, billing information, faults, to the centre, which carries out remedial measures with the help of commands. Similarly, other modifications in services can also be carried out from the remote centre. This allows a better control on the overall performance of the network.

As the processor is capable of performing operations at a very high speed, it has got sufficient time to run routine test programmes to detect faults, automatically. Hence, there is no need to carry out time consuming manual routine tests.

In an SPC exchange, all control equipment can be replaced by a single processor. The processor must, therefore, be quite powerful, typically, it must process hundreds of calls per second, in addition to performing other administrative and maintenance tasks. However, totally centralised control has drawbacks. The software for such a central processor will be voluminous, complex, and difficult to develop reliably. Moreover, it is not a good arrangement from the point of view of system security, as the entire system will collapse with the failure of the processor. These difficulties can be overcome by decentralising the control. Some routine functions, such as scanning, signal distributing, marking, which are independent of call processing, can be delegated to          auxiliary or peripheral processors. These peripheral units, each with specialised function, are often themselves controlled by a small stored programmes processors, thus reducing the size and complexity at central control level. Since, they have to handle only one function, their programmes are less voluminous and far less subjected to change than those at central. Therefore, the associated programme memory need not be modifiable (generally, semiconductors ROM’s are used).

 3.2  Block Schematic of SPC Exchange

Despite the many difference between the electronic switching systems, and

all over the world there is a general similarity between most of the systems in terms of their functional subdivisions. In it’s simplest from. an SPC exchange consists of five main sub-systems, as shown in fig.

  • Terminal equipment, provides on individual basis for each subscriber line    and for interexchange trunk.
  • Switching network, may be space- division or time-division, uni-directional or bi-directional.
  • Switching processor, consisting mainly of processors and memories.
  • Switching peripherals ( Scanner, Distributor and Marker ), are Interface Circuits between control system terminal equipment and switching network.
  • Signaling interfaces depending on type of signaling used, and
  • Data Processing Peripherals  ( Tele – typewriters, Printers, etc. ) for man- machine dialogue for operation and maintenance of the exchange.


Basic Concept of Telephone Traffic

How to Define Telephone Traffic?

Basic Concept of Telephone Traffic:

 2.0 Introduction

Telephone traffic is originated by the individual needs of different subscribers and so it is beyond the control of telephone administration. Any and every subscriber can originate a call at any and every moment without giving any previous  information and the duration of calls  is also not previously  known. Although the individual telephone traffic originates at random, the average telephone  traffic  for a particular exchange follows the general pattern of activity in the exchange area. Normally there is a peak in morning, a dip during lunch period followed by a afternoon peak. In some localities the traffic has seasonal characteristic, for example at a holiday resort. A typical 24 hours variations in calling rate is shown below.

typical 24 hours variations in calling rate

2.1 Whatever be the nature of variation of traffic, a telephone engineer is interested in maximum traffic that occurs in an exchange.

Telephone traffic

The hour in which maximum traffic usually occurs in an exchange is known as Busy Hour.

Busy Hour Traffic is the average value of maximum traffic  in the busy hour. In computing Busy Hour Traffic the seasonal effects are also taken into account. Sometimes it is convenient to refer to Busy hour calling rate (BHCR). Busy hour calling rate is the number of calls originated per subscriber in the busy hour. This provides a simple means for designing the exchange with respect to the number of subscribers. It also provides probable growth of traffic to the estimated growth in number of subscribers. The busy hour calling rate may vary about 0.3 for a small country exchange and 1.5 or more for a busy exchange in business area in  a city.

When the volume of traffic is quoted in terms of number of calls originated in a given time, this is insufficient to determine the consequent occupancy of lines and equipment. Therefore, measurement of traffic should not only consider number of calls but also their duration. The duration during which equipments and circuits are held when a call is made is called HOLDING TIME. Normally, it is average holding time per call for the particular item of equipment that is taken into account, so far as the caller is concerned the useful time is during the conversation only. However, the total time during which equipments and circuits are held when a call is made also includes, the period during which call is being established and time taken to release the equipment after the call has concluded.

2.2 Measurement of Telephone Traffic.

The total cost of providing telephone service can be roughly divided into those charge which are constant and independent of volume of traffic and those, which are determined by the amount of traffic. The cost of subscriber’s line and instrument and certain individual equipment in the exchange is totally independent of the volume of traffic. The quantity of common switching equipment required is almost entirely dependent by volume of traffic. The quantity of such equipment is dependent not only on number of calls but also on duration of calls. Therefore  to determine the quantity of switching equipment in automatic exchange or staffing in manual exchange telephone traffic may be measured in terms  of both the number of calls and the duration of calls.

For certain purpose it is sufficient to specify a Traffic Volume which is product of number of calls occurred during the time concerned by their average duration. however for the purposes  of automatic exchange a more precise unit of traffic flow is required. this is called Traffic Intensity. Traffic intensity is the average number of calls simultaneously in progress. The unit of traffic intensity is Erlang.

A traffic intensity of one erlang is obtained in any specified period when the average number of calls simultaneously in progress during that period in unity. The specified period is always one hour and is taken as being the busy hour unless some other  period is indicated.

There is a more precise way to define traffic intensity. The average Traffic Intensity during a specified period T, carried by a group of circuits or equipments,  is given by the sum of the holding times divided by T.  The holding times and period T all being expressed in the same unit.

Sometime it is stated that the average traffic intensity is equal to the average number of calls, which originate during the average holding time.All the above three definitions give the same numerical result.

The foregoing relationships may be expressed symbolically as follows.

Let S be sum of holding times during a given period T , both expressed in hours. Then by definition.

   A  =  S/T                

Where A is the average traffic intensity. Let C be the total number of calls during the period T then the average holding time ‘t’ hours per call, is given by


Then       A = S/T 

Can also be written as

A = Ct/T

 It also follows that when the average call duration is known, the average call intensity can be obtained by determining the number of calls occurring during the period T. Also because A is equal to average number of calls simultaneously in progress, an approximate value of A can be obtained by counting the number of occupied circuits or equipments at uniform interval during the time T and finding the average value.


2.3  Grade of service.

Owing to the fact that calls originated in a pure chance manner, it is likely

that during the busy hour some calls may fail to mature due to insufficiency of switching equipment. To ensure that the number of calls so lost is reasonably small, it is the standard practice switching equipment such that on the average not more than one call out of  every 500 in the busy hour is lost at each switching stage, with the provision that loss does not fall below 1 in 100 with a 10 percent increase of traffic.

This allowable loss is termed the grade of service and is usually represented by the symbol ‘B’ with one lost call in 500 the grade of service is written as

Electronic Exchanges

What are the Electronic Exchanges? What are the advantages and types of Electronic Exchanges?

Electronic Exchanges:

To overcome the limitations of manual switching; automatic exchanges, having Electro-mechanical components, were developed.  Strowger exchange, the first automatic exchange having direct control feature, appeared in 1892 in La Porte (Indiana). Though it improved upon the performance of a manual exchange it still had a number of disadvantages, viz., a large number of mechanical parts, limited availability, inflexibility, bulky in size etc.

As a result of further research and development, Crossbar exchanges, having an indirect control system, appeared in 1926 in Sundsvall, Sweden. The Crossbar exchange improved upon many short- comings of the Strowger system. However, much more improvement was expected and the revolutionary change in field of electronics provided it. A large number of moving parts in Register, marker, Translator, etc., were replaced en-block by a single computer. This made the exchange smaller in size, volume and weight, faster and reliable, highly flexible, noise-free, easily manageable with no preventive maintenance etc.

1.1 The first electronic exchange employing Space-Division switching (Analog switching) was commissioned in 1965 at Succasunna, New Jersey. This exchange used one physical path for one call and, hence, full availability could still not be achieved. Further research resulted in development of Time-Division switching  (Digital Switching) which enabled sharing a single path by several calls, thus providing full availability. The first digital exchange was commissioned in 1970 in Brittany, France.

This handout reviews the evolution of the electronic exchanges, lists the chronological developments in this field and briefly describes the facilities provided to subscribers, administration and maintenance personnel.

Table 1 Chronological Development of Electronic Exchanges.


1965 No.1 ESS Local Bell Labs, USA
1972 D  10 Local and Transit NEC. Japan.
1973 Metaconta Local LMT. France
1974 No. 1 ESS Centrex Local and Transit Bell Labs. USA
1975 Proteo Local & Transit Proteo, Italy
1976 AXE Local PTT & LM Ericsson, Sweden
1976 No.4 ESS Transit Bell Labs, USA
1978 AXE Local LM Eiricsson, Sweden.

Advantages of Electronic Exchange Over Electro-mechanical Exchanges


Electromechanical Exchanges – Electronic Exchanges
Category, Analysis, Routing, translation, etc;, done by relays.


Any changes in facilities require addition of hardware and/or large amount of wiring change. Flexibility limited.


Testing is done manually externally and is time consuming. No logic analysis carried out.


Partial full-availability, hence blocking.

limited facilities to the subscribers.


Slow in speed. Dialing speed is max. 11 Ips and switching speed is in l milliseconds.


Switch room occupies large volume.



Lot of switching noise.


Long installation and testing time.

Large maintenance effort and preventive

maintenance necessary.


Translation, speech path Sub’s Facilities, etc., managed by MAP and other DATA.


Changes can be carried out by simple commands. A few changes can be made by Subs himself. Hence, highly flexible.


Testing carried out periodically automatically and analysis printed out.


Full availability, hence no blocking.     A large number of different types of services possible very easily.

Very fast. Dialing speed up to 11 digits /sec possible. Switching is achieved in a few microseconds.

Much lesser volume required floor space of switch room reduced to about one-sixth.


Almost noiseless.


Short installation and testing period.

Remedial maintenance is very easy due to plug-in type circuit boards. Preventive maintenance not required.

1.3 Influence of Electronics in Exchange Design.

When electronic devices were introduced in the switching systems, a new concept of switching evolved as a consequence of their extremely high operating speed compared to their former counter-parts, i.e., the Electro-mechanical systems, Relays, the logic elements in the electromechanical systems, have operate and release times which are roughly equal to the duration of telephone signals to maintain required accuracy. However, to achieve the requisite simultaneous call processing capacity, it became essential for such system to have number of such electrical control units (Called registers in a Cross-bar Exchange), in parallel, each handling one call at a time. In other words, it was necessary to have an individual control system to process each call.

Electronic Exchange

Electronic logic components on the other hand, can operate a thousand or ten thousand times during a telephone signal.  This led to a concept of using a single electronic control device to simultaneously process a number of calls on time-sharing basis. Though such centralisation of control is definitely more economical it has the disadvantage of making the switching system more vulnerable to total system failure. This can, however be overcome by having a standby control device.

Another major consequence of using electronics in control subsystems of a telephone exchange was to make it technically and economically feasible to realize powerful processing units employing complex sequence of instructions. Part of the control equipment capacity could then be employed for functions other than call processing, viz., exchange operation and maintenance. It resulted in greatly improved system reliability without excessively increasing system cost. This development led to a form of centralized control in which the same processor handled all the functions, i.e., call processing, operation and maintenance functions of the entire exchange.

In the earlier versions of electronic control equipment, the control system was of a very large size, fixed cost unit.  It lacked modularity. It was economically competitive for very large capacity exchanges.  Initially, small capacity processors were costlier due to high cost per bit of memory and logic gates. Therefore, for small exchanges, processor cost per line was too high. However, with the progressive development of the small size low cost processor using microprocessor, it became possible to employ electronic controls for all capacities. In addition control equipment could also be made modular aiding the future expansion.

The impact of electronics on exchanges is not static and it is still changing as a function of advances in electronic technology.

  • Phased Developments

 Many electronic switching systems, including the recent ones, had an electromechanical switching network and used miniature electromagnetic relays in junctors and subscriber line equipments None-the-less the trend is towards all electronic equipments for both public and private switching and the switching network has already been made fully electronic with the advent of digital switching.

Data Creation in Local Exchange

Data Creation in Local Exchange (LE)

There are basically different type of technologies used as local exchange. An RAX has been tested successfully with technologies like EWSD,OCB,5ESS and C-DOT. To facilitate the BSNL staff for connecting AN-RAX with above mentioned LEs, following are the commands and their parameters used in corresponding LE.

    Procedure for equipping VU and creating AN-Interface at C-DOT MAX If the VU has not been equipped then first equip VU by following process. The software version should be either 2_2_1_3 or 2_2_1_4 (all patches). It is not supported in 2_1_1_1 link.

    • Equip the VU frame by using the command EQUIP-FRAME with the following parameters;

MOD-NO = BM number


FRAME NO = Depending on the frame to be equipped.

TIC-ID = TIC ID of the frame to be equipped.


Data Creation in Local Exchange

  • Equip PHCs in the slots in which SHM card are Physically Present.
    Use the command EQUIP-TRML-CARD with the following parameters;


VER –NO = 1

CARD-SLOT = BM-rack-frame-slot,

where slot=7/8/9/10 and 17/18/19/20

Depending on the slot position of phc

Note: The command will be rejected if VU is not already equipped in the switch.

  • Initialize VU
    Use the command PUT-SWU-INS with the following parameters;

MOD-NO = BM Number in which VU is equipped

UNIT –ID =TIC ID of the VU

With the corresponding TIC becoming INS-ACT , the VU gets the path to the APC, critical alarm will now be raised for VU on the ADP. After successful code and patch loading VU comes up and unit status of VPC, VMU-0, VMU-1 is shown as IN SERVICE and VPC-1 as INS-SBY in DISPL-SYS-ALL command of the BM

  • Equip DTS card in DTU frame by the command equip-trml-card, using following parameters


VER-NO = 1

CARD-SLOT = BM-rack-frame-slot, depending on the slot

where card is to be Equipped.

AI-NUM * = AN Interface Number (1-100)

AI-NAME = Interface Name

AI-TYP = V5.2

VAR-ID * = (Maximum value 127)

AI-CTG = 1

ST-L3ADR = 0

AI-LNK = (PCM-ID1)-0 & (PCM-ID2)-1




Note :-

* The value should be same as defined at AN side ST-L3ADR preferred

to be define as 0

  • Creation of Subscriber is to be done by using command cre-sub using following parameters:



[AISUB-ID] * : (AI-NUM)-(L3ADR ) eg : 1-0




[SUB-PRI] : 1








[LIN-CAT] : 1

[SUB-CTG] : 1

[CAB-ID] : 1-1-1


[BS] : BS12





Note: All the parameters are to be defined same as in case of ordinary


accept AI-SUBID

TEN= None

  • The status of AN subscribers, AI channels and PHC terminals can be seen by using DISPL-TRM-STATUS command by using following




[TEN] :


Note: Ten value is to be given when TML-TYP = AICHNL/PHC and DIRNO

is to be given for TML-TYP = ANSUB.

    Testing of AN-RAX done with EWSD as LE at Karol Bagh exchange, New Delhi. Following are the commands and their details which are used in EWSD exchange. The patch used for V11 version is CJ144 and for version V13 is CJ145.



V5IF = interface number

V5IFID = interface ID


PROVAR = this should be same as variant ID at AN side

V5PORTS = total number of ports



V5IF = interface number

V5LINK =0-13-0 ( first zero is sort of BM no. 13 is LTG no.,and last zero

is PCM no.)

V5LINKID =it is logical number given to the PCM (say 0)

This command has to be given for standby link also. It is to be noted that the standby link cannot be of same LTG (13 in this case). In one LTG maximum of four PCM can be configured.



V5IF = interface number


V5TS =16



(PROTECTION GROUP=2 is used when TS15 Is used as communication


this command is given for both the links



V5IF = interface number

V5LINKID =0 (only primary link)

V5TS =16




V5IF = interface number




Now this command is given four times for all the protocols as the pathtype




This means that all the protocol of V5 will travel on TS 16 of V5CHANID 0,

physically on 0-13-0.



The parameter FORMAT in this command should be set as STDFRM to

disable the CRC.

    After creation of V5link, its status has to be made active by this command.


V5IF =interface number



That means status is first made MBL (like OOS) and then ACT. This

command is run for both the link, forV5LINKID=1 also.

    DISPV5IF = displays the data created for interface

DISPV5PORTS = displays the data created for subscriber

STATLTG = status of line trunk group is shown

STATV5PORT = status of subscriber is shown

DISPV5TS =data created for v5 time slot

STATV5LINK = status of v5 link is given (ACT, MBL, NAC, UNA)








LAC = local area code

DN = directory number

EQN* = equipment no.(10-0-0-1)


CAT = category




The fields of equipment no (EQN):

First field = 10 (V5IFID)

Second field =0 (fixed)

Third field = two bits

Software Architecture AN-RAX

Software Architecture AN-RAX

The Software architecture is completely modular. It comprises of entities which operate in a layered environment, with physical, data link and network layers, to support the communication between AN-RAX and LE. Most of the entities use an FSM based approach. The coding is done in C language. The entire software runs on the ARC card. The other processor based card in the system is the RTC card. The software for this card is reused from the RAX product.

    Maintaining modularity, the architecture has been conceived as comprising of two major modules: the V5 Module and the AN Module.

This comprises of entities/processes which handle the V5 protocol towards the Local Exchange(LE).

  • i) Core Protocols
    It consists of the processes for PSTN protocol (PSTNT), CONTROL protocol (CPT), Bearer Channel Connection (BCC), LINK CONTROL protocol (LCP), and PROTECTION protocol (PPT).
  • ii) System Management /Access Initialisation Task (AIT) It consists of the system level general management and the layer 3 management for the V5 protocols.

This comprises of entities/processes which handle the product related features.

  • Maintenance Software / Fail Safe Task (FST)
    It implements strategies for providing fail safe services to the ANRAX subscribers.
  • Man Machine Interface / Operation Administration Task (OAT)
    The user interface is provided through an RS232 interface. The MMI provides interface for the user to configure the V5 interface and perform the maintenance functions on subscriber ports and V5 links.
  • Port tester Task (PTT)
    This process handles the RTC (tester card) communication protocols and the port testing.
  • SPC Interface Task (SPT)
    This process handles the interrupts and subscriber events reported by SPC/ISP card from the line side. Also handles the ring cadence and metering pulse feeding.
  • Layer 2 management/Data Link Protocol Task (DLPT)
    Manages the data link entity. It also acts as a message parser and distributor for ANRAX system for message received on V5 links and IPCP links. The functionality regarding the management of V5 links is shared with protection protocol entity.
  • Data Link Entity/Data Link Control Task (DLCT)
    It implements the data link layer functionality for both : V5 protocol and Inter processor communication within ANRAX. It handles the error correction and ensures reliable communication over physical channels.
  • Driver/Serial Communication Control Task (SCCT) It is the interface between Data Link Entity and Communication channel.
  • Real time Operating System (XRTS)
    The operating System is real time, based upon Xinu Operating System (Xinu Real Time OS).
  • Database Task (DBT)
    This process takes care of maintaining and updating the V5, system and port related data in both active and standby ARCs. Any status change in active card is immediately updated to standby. Software Architecture AN-RAX Software Architecture AN-RAX

Process Interaction Diagram

    • V5 Messages
      As we know, V5 protocol is `message based’, i.e., any information between LE and AN is exchanged through messages available in different protocols. The list of messages available in different protocols is given below.

  Software Architecture AN-RAX, Software Architecture AN-RAX

i) Establish

ii) Establish Ack

iii) Signal

iv) Signal Ack

v) Status

vi) Status Enquiry

vii) Disconnect

viii) Disconnect Complete

  • Control Protocol

i) Port Control

ii) Port Control Ack

iii) Common Control

iv) Common Control Ack

  • BCC Protocol

i) Allocation

ii) Allocation Complete

iii) Allocation Reject

iv) De-allocation

v) De-allocation Comp

vi) De-allocation Reject

vii) Audit

viii) Audit Complete

ix) Protocol Error

  • Link Control Protocol

i) Link Control

ii) Link Control Ack

  • Protection Protocol

i) Switch Over Request

ii) Switch Over Ack

iii) Switch Over Com

iv) Switch Over Reject

v) Protocol Error

vi) Reset SN Com

vii) Reset SN Ack

  • Message Flow
    Message flow between AN and LE is explained in sec. & with the help of examples. Further, message flows in different call scenario is given at the end of this chapter.

    • Call Initiated From LE
      On receiving a call request from the network for a particular AN port, LE feeds call routing tone to calling subscriber and proceed to get a bearer channel for this call by sending an ALLOCATION message to AN and starts a timer. After getting an ALLOCATION COMPLETE message from AN, LE sends on ESTABLISH message to AN with cadenced ringing parameter to connect the ring to user port and starts a timer. AN sends ESTABLISH ACK message and call enter into ringing phase.
      In case AN subscriber has caller-id feature in which directory number of calling subscriber is to be sent to user’s equipment, LE shall send ESTABLISH message to AN without cadenced ringing parameter. LE shall send the digits in-band and thereafter send a SIGNAL message with Cadenced Ringing to AN to connect ring to user port. Call enters into conversation phase when answer is received from the AN subscriber, answer should be communicated across V5 interface by sending SIGNAL (Off Hook) message to the other end.

Various subscriber features can be initiated by the subscriber by doing Hook Switch Flash when the call is in the conversation phase.
If the release of the call is initiated from LE, parking tone should be fed to AN subscriber, parking tone timer shall be run at LE and disconnection from AN subscriber be awaited. AN subscriber disconnects before the expiry of parking tone timer, this indication comes in the form of SIGNAL (On Hook) message across V5 interface. Call clearing is started by sending DEALLOCATION message and on getting DEALLOCATION COMPLETE, PSTN protocol is cleared by DISCONNECTION / DISCONNECTION COMPLETE message.

    • Call Initiated From AN
      AN on detecting an origination from user port should send ESTABLISH message to LE. LE shall send ESTABLISH ACK message in response, gets a bearer channel by ALLOCATION/ALLOCATION COMPLETE and connect dial tone to the channel. When answer is received from PSTN subscriber, call will enter into conversation phase. For AN originated calls from subscribers with home metering facility, metering pulses shall be reported to AN in the form of SIGNAL (Meter Pulse) message over the V5 interface.


Hardware Architecture AN-RAX

Hardware Architecture AN-RAX

    The integrated circuits used in the C-DOT 256P AN-RAX hardware have low power dissipation and high operational reliability. The components used are based on Metal-Oxide Semiconductor (MOS), Complementary MOS (CMOS), Low-Power Schottky Transistor-Transistor Logic (LSTTL), and bipolar technologies. All the system circuitry has been packaged into seven card types. On the broad level these could be divided into following categories:
  • Terminal Interfaces

Subscriber Line Card (LCC/CCM)

  • Controller Cards

AN-RAX Controller Card (ARC)

AN-RAX Interface Card (ARI)

Signalling Processor Card (SPC) or Integrated Signalling Processor Card(ISP)

  • Service Cards

RAX Terminal Tester Card (RTC)

  • Power Supply Unit (PSU-I)


    C-DOT 256P AN-RAX uses Subscriber Line Card (LCC/CCM) to provide Analog Terminal Interface. Each terminal interface card caters to 8 terminations. Four cards make a Terminal Group (TG) which is associated with PCM 32 channel link towards the ARC card. Signalling information are multiplexed and placed on 4 wire ABCD signalling bus toward SPC/ISP card.Subscriber Line Card (LCC/CCM) (Ref. Fig. 4.1)
    Line Circuit Card (LCC) is used to interface ordinary subscriber lines. Fig. 4.1 gives the detailed block diagram of this card.
    The Line Circuit Card performs a set of functions collectively termed as BORSCHT, signifying:

B – Battery Feed

O – Overvoltage Protection

R – Ringing

S – Supervision

C – Coding

H – Hybrid Conversion

T – Testing

  • Battery Feed
    A -48V_+ 4V battery with current limiting facility is provided on each line for signalling purposes and for energising the microphone.
  • Overvoltage Protection
    A hybrid transformer and surge arresters across Tip and Ring provide protection against over voltages.
  • Ringing
    Ringing is extended to subscribers under the control of Signalling Processor (SPC/ISP card), through the contacts of an energized relay. The Ring is tripped when off-hook condition is detected.
  • Supervision
    On/Off-hook detection and dialling make/break are encoded and passed on to SPC/ISP card as the scan information from the subscriber lines.
  • Coding
    Coding refers to encoding of analog voice to digital form (8 bit, A-law PCM) through a coder/decoder (codec). Codec outputs of 32 codecs of each Terminal Group are time division multiplexed to form a PCM 32 channel at 2.048 Mbps.
  • Hybrid Conversion
    2-wire to 4-wire conversion is done before coding for full duplex (voice) operation.
  • Testing
    Metallic access is provided on subscriber lines for routine test. (Tests Access Relays)
  • Coin Collection Box (CCB) interface card is an ordinary LCC card with an additional reversal relay per subscriber to extend reversal on called party answer. This card is basically used to cater to special requirements of PCOs and PABXs. However, this card can also be used as line circuit card (LCC).
    Coin Collection Box with Metering (CCM) card is also same as LCC/CCB card except that it has got extra hardware to generate and feed 16 KHz pulses towards subscriber premise. This card is basically used to interface STD PCOs or special subscribers having home metering requirements. However, in CCM card out of eight ports only last two i.e., Port no. 7 and 8 are equipped with 16 KHz pulse generator. Therefore, only two subscribers per CCM card may have this provision. Rest of the ports are used for ordinary subscribers or coin collection box type. This card as a whole can be used as LCC.
    The ARC card functions as the main controller of the AN-RAX. It performs time switching of voice/data slots between line cards. Towards the line cards it gives the card select, subscriber select, clock and sync signals. It has an interface towards the SPC/ISP card for providing signalling interface to the line cards. It has an interface towards the ARI (AN-RAX Interface) card used in slave frame to support voice and signaling interface for the line cards in the slave frame. The card exists in copy duplication and occupies slots 12 and 15 of the master frame in 256P AN-RAX. It interfaces with RTC (RAX Terminal Tester) card for supporting terminal testing in AN-RAX.

Line Circuit card


The Functional Blocks of ARC are :

Processor and memory block

Time switch and service circuits block

SPC/ISP interface block

Digital trunk interface block

DT clock extraction and generation block

ARI interface block

PSU interface block

ARC Card Diagram

Processor Block & Memory Mapped Device

  • Processor and Memory Block
    This card is designed using Morotola’s 68392 processor in MASTER-SLAVE configuration as shown in the Fig. 4.3. The processor is clocked at 16.384 MHz. The processor clock is generated using a crystal oscillator. The reset circuitry uses a micro monitor chip, which asserts reset when VCC is out of range or when manual reset switch is activated or when strobe is missing. The 16.38 MHz processor input clock is divided by two and given as strobe to the micro monitor chip.
    Communication Block
    The master IMP’s SCC1 is used as HDLC link towards RTC card or ETT card. SCC will operate in NMSI mode at 64 Kbps for RTC card and in PCM mode at 2 Mbps for ETT card.
    Master IMP’s SCC2 is used as HDLC link towards duplicate ARC card. Speed of this link is 64Kbps.
    Master IMP’s SCC3 is used as debugging ACIA link. Speed of this link is software programmable and normally is 9600 baud.
    SCP of Master IMP is used to communicate with tester card in ARC card tester. Slave IMP’s SCC1 & SCC2 are used in PCM mode.
    SCC1 is used to handle HDLC messages (V5.2) on DT0 link. SCC2 is used to handle HDLC messages (V5.2) on DT1 link.
    Slave IMP’s SCC3 is used as an ACIA link for providing MMI through a dumb terminal. Speed of this link is S/W programmable and normally is 9600 baud.
    Slave IMP’s SCP is used to access DT ASIC (CPRAC) registers in order to control and monitor the DT links. In this communication, processor is always the master.
    Timers Block
    Master IMP’s Watchdog timer is used as software watchdog. The timer reference register is initialised with the time-out value. Software periodically resets the counter so that the timer count register never reaches the time-out count. If software fails to reset the timer count register within the stipulated time, timer count reaches the reference count and a level 7 interrupt is raised to IMP and also to mate ARC card.
    Timer 1 of master IMP is used as RTC (Real Time Clock). This timer can be programmed to periodically interrupt the processor at regular intervals.
    Timer 2 of master IMP is used as counter or timer in ARC card tester.
    Timer 1 of slave IMP is used as DT0 slip detector/counter. The counter can be programmed to count the number of slip’s occurring in DT link for statistical health monitoring of the DT link
    Timer 2 of slave IMP is used as DT1 slip detector/counter.
    Real Time Calendar
    The ARC card has been provided with one Real Time Clanedar chip, which can count the time, date, day of the week & Year. At present, this is not used.
    Control and Status Registers Block
    The Port A and Port B registers of master and slave processors are used as control and status registers. Some of the control and status registers are implemented externally using programmable devices. They are used to latch the status of all interrupts and to clear the latched status, program loop back bits and to latch ID bits from the back plane.
    Interrupt Logic Block
    This block receives all error interrupts and peripheral interrupts, prioritizes them and inputs to master IMP. Some interrupts are given directly to the Port B interrupt pins of master and slave at level 4. All the events are latched and the status is provided to the processor through status registers. The processor can clear the latched events by appropriately setting the corresponding bits in the control registers. Interrupt from SPC/ISP card master frame and slave frame are combined and presented at level 5. Error signals from Master and slave PSU cards are combined to generate a level 1 interrupt to the processor.
    Memory Block
    This card supports onboard memory of 1MB FLASH or 2MB EPROM, 1MB RAM and 64 KB/1MB NVRAM. Chip selects are generated using master and slave IMP’s chip select registers and glue logic. One jumper is provided to select either FLASH or EPROM and one more jumper is provided to select NVRAM capacity.
  • Time Switch and Service Circuits Block This card has a 2K by 2K time switch, implemented in FPGA. The time switch is operated at 8MHz speed and is used in 16 bit processor mode. One input link is programmed as conference link. Speed of the conference link is 8Mbps and it supports 32 Four party conferencing. 12 out of 16 possible I/O links are used as shown below.

    The input links of the time switch are :

  1. One conference link (8Mbps) for 32 four party conferencing
  2. One 8Mbps link from Tone, Announcement, MF and DTMF generation circuit.
  3. Eight 2Mbps links (TG (0)_IN to TG (7)_IN) from TGs.
  4. One 2Mbps link for DT and ETT messages
  5. Two 2Mbps links from DTs

The output links of the time switch are :

  1. One link for conferencing (8Mbps)
  2. Eight 2 Mbps links (TG (0)_OUT to TG (7)_OUT) towards TGs.
  3. One 2 Mbps link for DT and ETT messages
  4. Two 2 Mbps links towards DTs.

Time Switch input/Output Link UsageTone, Announcement, MF-DTMF Generation
The MF, DTMF, tone and announcement samples are stored in EPROMs. The EPROMs are addressed by free running counter chains, which are implemented in FPGAs. Bank control EPROMs are used to address different pages of the stored data. Parallel output of EPROMs are converted to serial link at 8 Mbps and connected to time switch as shown in Fig. 4.6.