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MTNL was set up on 1st April, 1986 by the Government of India to upgrade the quality of telecom services, expand the telecom network, introduce new services and to raise revenue for telecom development needs of India’s key metros – Delhi, the political capital and Mumbai, the business capital of India. In the past 17 years, the company has taken rapid strides to emerge as India’s leading and one of Asia’s largest telecom operating companies. Besides having a strong financial base, MTNL has achieved a market share of approximately 13% of the Indian telecommunication network with a customer base of over 4.74 million lines.
The company has also been in the forefront of technology induction by converting 100% of its telephone exchange network into the state-of-the-art digital mode.
The Govt. of India currently holds 56.25% stake in the company.
In the year 2002-03, the company has not only consolidated the gains but also focused on new areas of enterprise viz. Joint Ventures for projects outside India, widened the cellular and CDMA-based WLL customer base , set up internet and allied services on all India basis
INTRODUCTION
The field of telecommunications has evolved from a stage when signs, drumbeats and semaphores were used for long distance communication to a stage when electrical, radio and electro- optical signals are being used. Telecommunication networks carry information signals among entities, which are geographically far apart. An entity may be a computer, a human being, a facsimile machine, a teleprinter, a data terminal, and so on. Billions of such entities the world over are involved in the process of information transfer which may be in the form of a telephone conversation or a file transfer between two computers or a message transfer between two terminals. In telephone conversation, the one who initiates the call is referred to as the calling subscriber and the one for whom the call is destined is the called subscriber. In other cases of information transfer, the communicating entities are known as source and destination respectively.
The full potential of telecommunications is realized only when any entity in one part of the world can communicate with any other entity in another part of the world. Modern telecommunication networks attempt to make this idea of ‘universal connectivity’ a reality. Connectivity in telecommunication networks is achieved by use of switching systems.
TELEPHONE NETWORKS
Public switched telephone network (PSTN) or the plain old telephone system (POTS) is perhaps the most stupendous telecommunication network in existence today. There are over 400 million telephone connections and over 60,000 telephone exchanges the world over. The length of telephone wire pairs buried underground exceeds a billion kilometers.
TYPES OF SIGNALING
Telephony started with the invention of magneto telephones, which uses a magneto to generate the ringing current, the only signal, sent over a dedicated time between two subscribers. As the switching technology has undergone a vast change from manual switching to Digital switching, the inter-exchange signaling techniques have also progressively changed from loop no loop signaling to MF signaling & finally digital signaling. The first digital switching developed was channel associated signaling in which the signaling information is conveyed on a separate channel which is rigidly associated channel. The utilization of such a dedicated channel for each speech channel is highly inefficient, as it remains idle during entire speech phase. Hence a new signaling system was developed which is capable of providing all new services & is internationally standardized is known as Common Channel Signaling (CCS#).
Signaling in Pulse Code Modulation
Pulse code modulation (PCM) is a method of converting information from an analogue form to a digital form for transfer over a digital transmission systems, the technique involves sampling the analogue waveform & coding the result in the digital format. Successive sampling allows the analog waveform to be represented by a series of 8 bit code. 8 bit codes from numerous speech channels are assembled into blocks for transmission by inserting into time slots. The technique is called Time Division Multiplexing (TDM).
The bandwidth required to transmit signals is much less than that for speech, so the signaling for several speech channels in a PCM system can be handled by a small portion of the bandwidth. The signaling capacity can be used for CAS or CSS, the means of identifying to which speech channel a particular signal refers is to divide the signaling capacity into dedicated bit locations. Signals pertinent to a particular speech path are always transmitted in signaling bit locations dedicated to that speech channel. The means of conveying CCS is to compound the signaling capacity into a signaling channel that is available as & when required.
The CCITT has defined PCM standard for 30 channel & 24 channel systems. The capacity available for signaling in these two standards is different as a result of differing constraints applied by the PCM standards.
In 30 channel PCM system, the 8 bit codes relating to 30 speech channels are time division multiplexed into a frame. Each 8 bit code is inserted into time slot within the frame. Time slot 0 is used for alignment, time slots 1-15 and 17-31 are used for encoded speech relating to 30 channels. Time slot 16 is dedicated for the use of signaling.
The tenet of CAS system is dedicated signaling capacity is available for each speech circuit. This is achieved in 30 channel PCM systems by allocating 4 bits in each 16 frame multi-frame to signaling for each speech channel.
PRINCIPLES OF CCS
In common channel signaling (CCS) systems, the physical tie between the signaling path and the traffic circuit is removed. All signaling transfer relating to a transmission page link takes place over a dedicated signaling channel. Hence, a common-signaling channel handles the transfer of signaling information for numerous traffic circuits. Signaling capacity is not reserved for each traffic circuit, but signaling capacity is allocated dynamically as and when required. Exchanges A & B are connected by numerous speech circuits, denoted by solid lines. All the signaling that relates to the speech circuits is transferred between the exchanges using the common-signaling path. The common-signaling path can be regarded as a pipe between two exchanges, typically operating at 64 Kbits, into which all signaling information is funneled. Similarly, all signaling information pertaining to the speech circuits between each subscriber and Exchange A is transferred via the access signaling channel.
The transfer of signaling information is achieved by sending message down the common-signaling path. The use of messages in CCS systems opens up a whole range of flexibility that is not present in CAS systems. Instead of being limited to a small no. of meanings for signals messages can be designed to cover a maltitude of stations and services.
The signaling activity when setting up & releasing a circuit is high, however on an average the signaling activity for a circuit is low because there are no signaling when the calls are not being made & during the conversation phase of the call. Hence, a single CCS channel can be used to handle numerous traffic circuits. The theoretical limit of the number of traffic circuits handled by a CCS channel is very high, but a typical practical value is 2000 traffic circuits. The picture becomes more complex when non-circuit related signaling activity is taken into account. Non circuit related signaling can be intermittent (e.g. it is used during call establishment to interrogate a database) or it can exhibit a high signaling activity (e.g. if it is used to transfer large amount of management data between nodes in a network)
MODES OF OPERATION
CCS system can operate in a number of modes within telecommunications network. An exchange in telecommunications network that operates CCS is termed as a ‘signaling point’. Any two signaling points with the possibility of signaling communication are said to have ‘signaling relation’. The realization of the signaling relation is by sending signaling messages between the two exchanges. The path taken by the signaling messages is determined by mode of operation. Hence the modes of operation determine how signaling messages are routed between signaling points. The modes of operation can be ‘associated’, ‘non-associated’ or ‘quasi-associated’.
In the associated mode of operation, the signaling messages transferred over transmission page link directly connecting the relevant signaling points. In quasi-associated modes of signaling, the messages pertinent to a particular signaling relation are not transferred over transmission links directly connecting the relevant signaling points. Instead, the messages are transferred using intermediate signaling points. In this mode of signaling, the path taken by the messages through the signaling network is predetermined by information assigned by the network.
Exchanges A & B having signaling relation and are interconnected by speech paths. However the signaling path used to implement the signaling relations via exchange C and not directly between A & B. in the case of failure of the A-B signaling link, the A-C-B signaling can be used to control the speech path between exchanges A & B. quasi-associated modes of operation illustrate great flexibility and powerful nature of CCS system.
REQUIREMENTS OF CCS
The CCS has the following addition requirements introduced in three areas:
1. Reliability and security
2. Speech continuity
3. Processing Overhead.
1. A signaling channel carried on a 64k bites/sec. page link has the practical capacity to control approximately 2000 traffic circuits. Hence the failure of an inter exchange signaling page link would cause the loss of a significant amount of speech traffic. For access signaling, the loss of the signaling page link would mean isolation of the subscriber from the local exchange. It is therefore essential to take exceptional precautions to avoid such losses.
2. Signaling security can be improved by developing the signaling network itself. In the access network, it is possible to provide two signaling links to a subscriber (preferably on physically-diverse transmission links) & to switch all switching traffic to one page link when other links interrupted. Similar arrangements can be made for inter exchange signaling with automatic reconfiguration of signaling paths, even via different exchanges to maintain a signaling continuity in the event of the failure of a signaling link.
3. The unavailability of signaling communication between two exchanges is specified as a maximum of 10 minutes per year.
4. CAS systems that use the speech path to transfer signaling information provide the inherent feature of checking the continuity of speech path being established before conversation begins. If continuity is not achieved, the signaling transfer is not successful the call is aborted or a further attempt is made to connect call. The inherent continuity check is absent in CCS systems, owing to separation of speech & signaling paths if considered desirable, separate speech-continuity checks can be provided.
5. The flexible manner in which CCS systems are structured & the implementation of complex network features mean that extra processing is necessary to operate CCS. Even the inherent concept of funneling all signaling on a transmission page link in to CCS means that messages must be analyzed to determine to which circuit they refer. However, this extra processing overhead is more than out-weighted by the benefits of CCS systems.
SALIENT FEATURES OF CCS
1. Signaling information for a number of circuits is sent on a single channel. The physical tie between the signaling path & traffic circuit is removed. All signaling transfer relating to transmission page link takes place over a dedicated signaling channel. Hence, it handles information for numerous traffic circuits.
2. Signaling is in the form of data. A data channel operating at 64 K bits/sec. rate is provided
3. Signaling is very fast. All the required digits can be simultaneously sent on data in the form of message.
4. It can also work on 4.8 K bits/sec analog page link by providing modems.
5. It is very economical between the exchanges where large number of circuits are provided.
6. It can cater for all new services such as Videotex, Data communication, Fascimile and ISDN services etc.
7. In CAS systems, signaling capacity is dedicated to a traffic circuit. Limitations exist on when signals can be sent, depending on the status of the call. For example it is not possible to send voice frequency signals during the speech phase of a telephone call in some CAS system, unless special measures are taken (e.g. provision of filters)., because subs. are able to hear tones. However with these constraints, it is not possible to send signals instantaneously. In CCS each message takes up whole of the signaling channel for a short length of time. It is possible for an exchange to send two messages relating to two circuit from a transmission page link at exactly same time, each message may take a few microseconds to transmit. For this reason “buffers” are provided at each are generated by an exchange, they are stored in a buffer & transmitted in a specified order. When there are no messages to transmit, there is a need to maintain synchronization of the signaling channel between the two exchanges. This is achieved by continuously transferring synchronization information until a new message is ready for transmission.
8. CCS systems are specified in terms of “formats” & “procedures”. The specification of the formats defines the structure of messages used & meaning of each field within the message. The specification of the procedures defines the logical sequences in which messages can be sent. The procedures of CCS circuit related systems are closely linked to functions within exchanges that control the set up & release outgoing calls. There is therefore a close relationship between CCS procedures & exchange call control & a major element in defining CCS systems is the need to achieve an optimum between these factors.
9. The drive to provide an unrestricted communication capability between exchange procedures eliminates per circuit signaling termination costs. These costs are inevitable in per circuit. CAS systems but by funneling all signaling information into a single common channel, only one signaling termination cost is incurred for each transmission link. These are cost penalties for CCS systems, the messages received by an exchange have to be analyzed resulting in a processing overheads. However these are covered by increased scope of inter processor communication & more efficient processor activity.
10. CAS systems process limited information transfer capability.
11. The restricted number of conditions that can be applied (e.g. the limited variations that can be applied to a DC loop or limited number of frequency combinations that can be implemented in a voice frequency system).
12. The limited number of opportunity to transfer signals (e.g. it is not possible to transmit voice frequency signals during the conversation phase of call without inconveniencing the subs or taking special measures).
13. Neither of these restrictions apply to CCS the flexible message-based approach allows a vast range of information to be defined & the information can be sent during any stage of a call. Hence, the repertoire of CCS is far greater than channel associated versions & messages can be transferred at any stage of a call without affecting the calling & called subs.
14. CCS system transfer signals very quickly. A message used to establish a call in a CCS system can contain all the address digits in an information field.
15. Techniques used in modern CCS systems can be further improve the flexibility proved to subs. ‘User-to-user’ signaling is a technique whereby messages can be transferred from one subs to another without undergoing a full analysis at each exchange in the network. Similarly ‘end-to-end’ signaling allows exchanges to transfer information to each other without intermediate exchanges having to fully process the messages.
16. One of the problems that prompted the development of CCS systems was ‘speech clipping’ in the international network. In some CAS systems, it is necessary to split the speech path during call set-up to avoid tones being heard by the calling subscriber. This results in a slow return of the answer signal and, if the called subs start speaking immediately after answer, then the first part of the statement by the called subs is lost. As the first statement is usually the identity of the called subs, this causes a great deal of confusion and inconvenience. CCS system avoids this problem by transferring the answer signal quickly.
17. As a result of the processing ability of CCS system, a high degree of reliability can be applied with a resulting high confidence in the transfer of uncorrupted information in the case of an intermediate exchange failure, rerouting can take place within the signaling network, enabling signaling transferred to be continued.
LEVEL STRUCTURE
CCS#7 is optimized for use in a digital environment, but it can be used in any transmission medium. CCS is highly flexible, facilitates the evolutionary process & supports a variety of services & network features. These attributes result from an early decision to specify CCS in a 4-layer structure.
A prime objective when formulating the design of CCS channel was to ensure that the signaling system flexibly handle the requirements for circuit-related applications. These applications include telephones & circuit-switched data (i.e. Data using circuit within transmission links in a similar way to telephone calls). The functions performed by the 4 layers are
Level 1- Physical Function
Any node with the capability of handling CCS is termed as “Signaling Points”. The direct inter connection of the two signaling points with CCS#7 use one or more signaling links. Early versions will use variation of the installed copper local loops for connections between the local exchange & the subs premises, later version uses fibre optic technology. Level 1 of the 4 level structure defines the physical & electrical functional characteristics of the signaling links. defining such characteristics within level 1 means that rest of the signaling system can be independent of the transmission medium adopted. By keeping the interface between levels 1 & 2 constant any changes within level 1 do not affects the higher levels. In a digital environment the usual physical page link is a 64 K bits/sec channel. This is typically within a digital transmission system using PCM. However other types of links can be used without affecting levels 2 to 4.
Level 2 – Signaling Link Functions
Level 2 defines the functions that are relevant to an individual signaling link, including error control & page link monitoring. Thus level 2 is responsible for the reliable transfer of signaling information between two directly connected signaling point. If error occurs during transmission of the signaling information, it is the responsibility of level 2 to invoke procedures to correct the errors. Such characteristics can be optimized without affecting the rest of signaling systems, provided that the interface to levels 1 & 3 remains constant. This function is achieved by
(a) Initial page link alignment, synchronization & proving to ensure that the links error rate performance is satisfactory.
(b) Continuous page link error rate monitoring by sending fill in signal units during the idle time.
© Error detection by means of the FCS field in each signal units & error correction by retransmission of the message signal unit.
The above measures ensure that there are not more than 1 in 10^10 signal units with undetected errors & not more than 1 in 10^7 lost signal units.
Level 3 – Signaling Network Functions
The functions that are common to more than one signaling page link i.e. signaling network function are defined in Level 3: this includes ‘message handling” functions & “signaling network management” function. When a message is transferred between two exchanges, there are usually several routes that the message can take, including via a signal transfer point. The message handling functions are responsible for the routing of the message through the signaling network to the correct exchange. Signaling network management functions control the configuration in response to status changes in the network, e.g. if an exchange within a signaling network fails, the level 3 of CCS#7 can reroute messages and avoid the exchange that has failed.
MTP Level 1 & 3 constitute a transfer mechanism that is responsible for transferring information in messages from signaling point to another. The combination of level 1 to 3 is known as message transfer part (MTP). MTP does not understand the meaning of the messages being transferred, but it controls a number of signaling messages, page link & network messages. This means that messages are delivered to appropriate exchange in an uncorrupted form & in the sequence that they were sent, even under failure conditions in the networks.
Level 4 – Users part
Layer 4 comprise the user parts, user is not confused with a subscriber. User part form level 4 of the layered structure and include messages, message coding and protocols necessary to handle basic telephony & ISDN services. A key feature is that many different user parts may be the standardized MTP. Three user parts have been defined, the Telephone user TUP, the ISDN user part & the Data user part DUP. The user parts are defined in terms of message formats & procedures. The message formats defined meaning of a particular message & specify coding to be used. The procedures define the sequence of messages to be exchanges to be followed are defined for each application.
APPLICATION OF LEVEL STRUCTURE
Exchanges A & B are directly connected by speech circuits. A signaling page link is also available between Exchanges A & B. It is shown that level 4 is closely associated with control function of the exchange.
If the control function of exchange A needs to communicate with control function of exchange (e.g. to initiate the set up of a speech circuit between the exchanges), the control function of exchange A request the level 4 functions to formulate an appropriate message. Level 4 then requests the message-transfer part (level 1 to 3) to transport the message to exchange B. Level 3 analyses the request & determines the means of routing the message to exchange B. The message is then transported via level 1 & 2.
Upon receipt of the message by MTP of the exchange B, levels 1 & 2 deliver the message to level 3. Level 3 at exchange B recognizes that the message has arrived at the correct exchange and passes message to level 4. Level 4 in exchange B then interacts with the control function to determine appropriate action & response. If problem arises in the transmission process between exchange A & B causing message corruption, the level 2 functions are responsible for detecting the corruption & retransmitting the information. If the signaling page link between exchange A & B is not available (e.g. failure of the link), the level 3 functions are responsible for rerouting the information through the signaling network to exchange B.
Using these techniques, exchange A & B can send each other appropriate messages until the need to communicate on a particular transaction ceases (speech circuit between exchanges is released).
