Standard Notes Tsn Unit II

2 Telecommunication Switching and Networks

TIME DIVISION SWITCHING Space and time switching: A tandem switching centre or the route switch of a local exchange must be able to connect any channel on one of its incoming PCM highways to any channel of an outgoing PCM highway. The incoming and outgoing highways are spatially separate, so the connection requires space switching. A connection will occupy different time slots on the incoming and outgoing highways. Thus the switching network must be able to receive PCM samples from one time slot and re-transmit them in a different time slot. This is known as Time slot interchange or Time switching.

Space switches: Crosspoint matrix connects incoming and outgoing PCM highways. Different channels of an incoming PCM frame may need to be switched by different crosspoints in order to reach different destinations.Crosspoint is a 2 input AND gate. One input is connected to incoming PCM highway and another to connection store that produces a pulse at required instants. Figure below shows space switches with k incoming, m outgoing PCM highways carrying n channels. The connection store for each column of crosspoints is a memory with an address location for each time slot which stores the number of the crosspoint to be operated in that time slot. This number is written into the address by the controlling processor in order to set up the connection. The numbers are read out cyclically in synchronism with incoming PCM frame. In each time slot, the number stored at corresponding store address is read out and decoding logic converts this into a pulse on a single lead to operate relevant crosspoint. Since a crosspoint can make a different connection in each of n time slots, it is equivalent to n crosspoints in a space division network.

fig. Space switch

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Time switches: Time switch connects an incoming n-channel PCM highway to an outgoing nchannel PCM highway. Since any incoming channel can be connected to any outgoing channel, it is equivalent to a space-division crosspoint matrix with n incoming and outgoing trunks. Time slot interchange is carried out by means of two stores, each having a storage address for every channel of the PCM frame. Speech store consists of data of each of the incoming time slots (i.e. its speech sample) at a corresponding address. Each address of the connection store corresponds to a time slot on outgoing highway. It contains number of time slot on incoming highway whose sample is to be re-transmitted in that outgoing time slot. Information is read into the speech store cyclically in synchronism with the incoming PCM system; however random access read out is used. The connection store has cyclic read out, but writing is non-cyclic. To establish a connection, the number (X) of the time slot of an incoming channel is written into the connection store at the address corresponding to the selected outgoing channel (Y). During each cyclic scan of the speech store, the incoming PCM sample from channel X is written into address X. During each cyclic scan of the connection store, the number X is read out at the beginning of time slot Y. This is decoded to select address X of the speech store, whose contents are read out and sent over the outgoing highway. From input PCM highway 0 1

Cyclic write

To output PCM highway

X

31

Decode logic 0 1

Cyclic read Y

31

Fig. Time switch Time switching introduces delay. If Y>X the output sample occurs later in the same frame as the input sample. If Y 2 B + D Primary Rate Interface (PRI) -23B + D channel for T1 -30 B + D channel for E1 BRI is used in residential services. Data Rate for BRI in 2 x 64 + 16 + (overhead for synchronization and framing) =144 + (overhead for synchronization and framing) =192 kbps PRI is used for connections between a PBX and a CO. Data rate for PRI = 1.544 Mbps for T1 = 2.048 Mbps for E1

ISDN Architecture:

Reference points: Conceptual points used to separate groups of functions. Functional group: A group of functions that might be found in a typical device. Local exchange (LE): The ISDN central office. Terminal equipment functional group, TE1: Includes telephones, fax machines, computers. Terminal adapter functional group, TA: Provides the functions needed to attach a nonISDN device to ISDN. Network termination functional group, NT1: Connects between the signals/ wiring from ISDN device and the signalling/ electrical standards adhered to by the local office. NT2: A digital PBX or a LAN responsible for switching and multiplexing ISDN services. When NT1 and NT2 are used. The S and T interfaces are separated by the NT2 which is able to reallocate channels in the form of BRI or PRI connections.

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ISDN Call Control Signaling : ACM: Address Complete Message ANM: Answer Message IAM: Initial Address Message REL: Release RLC: Release Complete

The calling party starts the sequence by sending a SETUP message to the network. In the SETUP message, the user’s terminal includes the information that the network needs in order to establish a call. Examples of the information are the desired bearer service capability, the identity of the called party, the B-channel used for call etc. Bearer service capability is the essence of ISDN. Bearer capability can be offered in circuit, frame or packet mode.

Cellular Radio Networks The cellular radio network’s offers the mobile and portable telephone stations the same service provided fixed stations over conventional wired loops. It has the capacity to serve tens of thousands subscriber in a major metropolitan areas. The principle is illustrated in fig below, where the entire geographical area is divided into cells. A call is the basic geographical unit of a cellular network. Each cell uses one of the available frequencies. All cells have the same number of radio frequencies. Each cell consists of a Radio Base Station (RBS), Mobile Stations (MS), A mobile switching center (MSC). The Radio Base Stations (RBSS) in a group of cells are connected to a mobile switching center (MSC). The MSC’s are lined by fixed circuits and interfaced to the PSTN. In order to make a call , the mobile user access the cellular network through a base station. For a call to a mobile user, initially the directory number of the user must be known. Therefore for the network, it is necessary to determine in which cell the user is located. The network must keep track of the user during the period of conversation, because the user move form one cell to another cell. This process of moving from one cell to another cell is called handoff. The mobile telephone continuously monitors a control channel and so receives the information that identifier the area. If the received

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2 Telecommunication Switching and Networks signal strength falls below threshold, the mobile telephone automatically switches to another control channel. Each mobile telephone has a home switching center. This contains a home location register, which stores customers data, including the directory number, equipment serial number and class of service. There are number of cellular radio standards in use. These include 1) The advanced mobile phone system 2) The N order mobile Telephone Service (NMT) 3) Total Access Communication System (TACS) 4) The Nippon Automatic Mobile Telephone System (NAMTS)

Intelligent Network For many interconnected exchanges in a network, it is costly and the time consuming to make software upgrades and hardware changes. Consequently, these changes are made very infrequently and it is very difficult to introduce the new services. A possible solution to this problem is to separate the software that controls the basic function of exchange from the software required for providing more complex services. These include free phone services, calling card service, and value added services. The more complex services can be controlled by a centralized processor called a service control point (SCP). A telecommunication network that has been enhanced in this way is called intelligent network (IN). The exchange that makes the required connection is called a service switching point (SSP).

The architecture of intelligent network is shown in fig above. The SCP is a centralized processor and its software is organized in three levels. a) Node software : This software provides common facilities such as signaling, database access, transmission and alarm. b) Services logic program (SLP): These are program that control various services. c) The services logic execution environment (SLEE): This program hosts various SLP’s and network with the basic call control and switching of SSP. There is a Service Management System which is connected to all the SCP’s by data links. This allows the addition of new customers, updates of data and data reloads.

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2 Telecommunication Switching and Networks At present IN use a centralized structure as shown in fig.7.12. In future IN may use a decentralized structure. The decentralized IN may give faster response and reduce load on the network and it is cheaper and widely used services. However, centralized SCP control is better to enable services to be provided.

Private Networks Private networks are secured leased lines that were access protected by a unique user name and a password , private network encrypt data transmitted from an access point before it bits the network and decrypt all the received traffic before it gets the user. This basic functionality not the medium the PN spans, is the true definition of a PN. Fig below. shows the overall topology of a private network.

The private network data path A typical end to end PN data could contain: Several machines not under control of the corporation A security gateway An internal segment An external segment Private Data Networks Public data networks consists of two parts: 1)The data network identification code (DNIC) of 4 digits 2)The network terminal number (NTN) of 10 digits. DNIC consists of a data country code DCC of 3 digit followed by a network digit. By this means a single country can have upto ten different data networks. A country can have more than one DDC. The format user for the ten digits NTN can be determined by the network from another country.

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CONTROL OF SWITCHING SYSTEMS Switching systems were fast evolving from being manually controlled to being controlled by relays and electronically. The change from manual to Strowger system brought about a change from centralized to distributed control. Also it became possible to perform a number of functions using the same hardware by using different programs. This revolutionary technique is known as the stored program technique.

Call processing functions: For any call to be made, a sequence of operations takes place in which the calling and the called customers’ lines and the connections to them change from one state to another. The various states for a simple call between two customers whose lines terminate in the same exchange is described below. Idle state: Initially, the customer’s hand-set is in the ‘on-hook’ condition. The line is said to be idle (state 0). The exchange meanwhile is continuously monitoring the line to detect a change in state. Call request signal: When the customer lifts the handset, current flows into the line and a signal is sent to the exchange. This is also known as seize signal. Calling line identification: The exchange now detects the line in which the calling condition originated. For this equipment number (EN) to directory number (DN) translation occurs. Determination of originating class of service: The originating class of service (COS) corresponding to the range of services available to the calling customer. In an SPC exchange, the customer’s COS is stored as data. Identification of calling party: If the originating COS indicates a multi-party line, it is necessary to ensure that the correct party is billed for the call. Connection to the calling line: The exchange now makes a connection to the calling line. Proceed to send signal: The exchange now sends a signal (dial tone) to the calling party indicating to him to send the identity of the number he wants to call. The exchange now waits for this information (state 1). Address signal: The calling customer now dials the number of the person he wants to contact in his hand-set. This is the address signal to the exchange. Selection of outgoing line termination: The exchange now determines the required outgoing line termination from the address signal it just received. For this DN-EN translation is done. Determining the terminating class of service: Just as in the case of the caller, the COS of the called line must also be checked. Testing called line termination: The exchange first tests the called line before making a connection to it because it may be busy or out of service. Status signal: A status signal, called a call progress signal is now sent back to inform the caller regarding the progress of the call. If the busy tone or number unobtainable tone is sent, then the caller replaces the hand-set and connection is released. Idle (0) state is resumed.

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2 Telecommunication Switching and Networks Connection to called line: If the called line is obtainable and free, the exchange now makes a connection to it. Alerting called customer: The exchange now sends an alerting signal to the called line, i.e. the phone at the called line end now rings indicating the called party to answer the call. At the same time, it sends a ring-back tone to the caller. The exchange now waits for an answer (state 2). Answer signal: When the called customer answers by lifting the hand-set, the line is looped and current flows. This provides an answer signal to the exchange and thus it ceases to send the ringing tone back to the caller and the called line. If the customer does not reply, the calling line hangs up and idle (0) state is resumed. Completion of the connection: On receiving the answer signal from the called customer, the exchange completes the connection between the called and the calling parties. Conversational state: Since connection is complete, they can converse as long as they want (state 3). The exchange supervises the connection to detect the end of the call. Clear signal: When each customer replaces the hand-set, line current ceases and provides a clear signal to the exchange. Release of connection: The exchange then clears down the connection and hence idle (0) state is resumed. Since the calling party is billed, the connection is released after the calling party hangs up. Various other situations resulting in problems can occur in case one of them hangs up and the other does not. These problems are rectified using certain time based circuits.

Signal exchanges Signals sent in the direction away from the caller (towards the called line) are called forward signals. Signals sent towards the caller (and away from the called line) are called backward signals. Also each signal should produce a response in the opposite direction, thus verifying correct operation as follows: 1. The call request signal is answered by the proceed to send signal. 2. The address signal is answered by the call status signal. 3. The answer signal is a response to the alerting signal 4. The caller responds to the answer signal by commencing the conversation. 5. The backward clear signal is a response to the forward clear signal (or vice versa) For a call over a junction between two exchanges, the actions between the customer’s calling signal and connection to an outgoing line occur at the originating exchange. The exchange then sends a seize signal to the terminating exchange. After the originating exchange has sent the address information to the terminating exchange, the actions from receipt of address information to alerting the called customer take place at the terminating exchange. When the answer signal is got from the called customer, connection is made and after the conversation has ended, then connections are released.

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2 Telecommunication Switching and Networks

Idle

Idle Call request (seize)

E v e n t

Connect to calling terminal Proceed to send Address signal Connect to called terminal Status signal Alert called terminal Answer Connection Clear signal Disconnect

Time (not to scale) Fig. Timing of signals exchanged for a local call

Call request (seize)

Proceed to send Address Alert Calling terminal

Status

Switching system

Called terminal Answer

Answer Forward clear Forward clear

Backward clear Backward clear

Fig. Signal exchange diagram for a local call

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2 Telecommunication Switching and Networks

State transition diagrams The sequence of operations discussed previously can be represented by a graphical method known as state transition diagram (STD). The various symbols used are as follows: State boxes: They are labeled with a number and a title. Address information can also be included. Event boxes: Rectangular box. If action is that of sending a signal, then protruding arrow is used. Decision boxes: Diamond shaped boxes used in computing flowcharts. Below is an example of a STD for a local call:

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2 Telecommunication Switching and Networks

Common control A common control performs a specific call-processing function. Thus, the control of the switching system employs functional division. A common control is hence brought into play only when required and released when not. Switching networks are generally lost call systems. When common controls are busy, calls offered to them are not normally lost but are delayed. The traffic performance of common controls therefore can be discussed using queuing theory. Common control units have been designed using relays, electronic digital signals and stored program control (SPC). An SPC common control enables the same logic circuits to perform different tasks under the control of different programs. A separate small network using switches similar to those in the main switching network can be used to common control the trunks as required.

Control of the exchange is distributed among a number of processes, each performing a specific function. These communicate with each other and with customer’s connection by means of temporary connections made through the main switching network, as required. When it is necessary to exchange signals between numbers of functional units but the signal exchanges need only take place one at a time, the units can be connected by a common bus. The bus can be used both in serial as well as parallel modes. In scanning, electronic gates, forming the equivalent of a rotating switch, connect a common control to each trunk in turn. Since, at any time only one trunk can communicate with the common control, there can be no contention. However scanning presents the following requirements which may conflict, In every cycle, the scanner must connect the common control to each trunk for a sufficiently long period to exchange the required signals. The period of a complete scanning cycle must be sufficiently short for the common control to detect every change that occurs in the states of the trunks being scanned. ECE study materials [www.pbtstudies.blogspot.com]

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2 Telecommunication Switching and Networks In case of conflict between the requirements, then start-stop scanning is used. MMI I/O

Control bus

CPU 1

CPU 2

Read/ Write memory

Backing store

Fig. Functional units interconnected by a bus

Reliability, availability and security Switching systems must be reliable. The strowger system was very fault tolerant because of its distributed nature in common control and particularly centralized control, makes the operation of an exchange dependent on a small number of equipments. The ideal time interval between failures is fixed as a standard known as Mean Time Between Failures (MTBF). Once failure of equipment occurs, fault must be diagnosed and rectified. The longer the MTBF and the shorter the Mean Time To Repair (MTTR), then greater is the proportion of time for which an equipment provides service. This proportion is called the availability of the equipment. Availability = MTBF/ (MTBF + MTTR) This gives the probability that the equipment will operate correctly. Now unavailability is defined as 1 – availability = MTTR/(MTBF + MTTR)

Security The failure of an entire exchange is a very serious matter. So availability must be as close to unity as possible. If common control is used, an exchange that had the minimum possible configuration of equipment would be unlikely to obtain adequate availability, even if constructed with the most reliable components. The security measures used are as follows: Line circuits :none Switching network :none or partial duplication) Common controls :1 in n sparing Central processors :replication Explanation: Since a terminating unit for a customer’s line contains relatively few components, it should suffer faults less frequently than the line itself. Therefore no additional measures are needed.

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2 Telecommunication Switching and Networks For space division switching network, since a choice of many paths is there for a connection, a fault in one does not affect the entire system greatly. This is not so in the case of a time division network, less equipment is used, this saving in the cost can be used to provide more reliable equipment. For common control equipments, such as registers, 1 in n sparing is used. Here when only n equipments are actually needed for functioning, n+1 equipments are provided. This way, when one fails other is used. Before one more fails, the previous one can be repaired. If a single processor is handling all the calls, then its failure would result in mass inconvenience. Therefore two or more processors are provided. Also, when one is being upgraded, the others handle the traffic without any deterioration in the grade of service.

Stored program control Processor architecture In order to obtain adequate security, a switching system with a minimum of two processors are required. They can be configured in the following ways: Worker and Standby Load sharing Synchronous operation In a system with a worker and a standby, either a cold or a hot standby may be used. In cold standby, the spare processor’s memory is not updated resulting in disruption during change-over. Whereas in hot standby, there is no disruption because the spare processor’s memory is constantly updated. In load sharing, as the name itself suggests, both the processors perform different tasks for different calls. This causes problems during change-over. Also, contention must be prevented. Additional processors can be provided as the traffic grows. In synchronous dual operation, both processors receive identical inputs and operate in synchronism to produce the same output at the same time. Comparisons of outputs result in fault detection. A test program is used to detect the faulty one. This cannot be used in case of a software fault. In large exchanges, to handle more traffic, a multi-computer or a multi-processor is used. The load in a multi-computer is usually divided between them on a geographical basis, each handling processing for a different part of the exchange. In multi-processor system, the processors share programs and data stores; each one controls any part of the exchange, using any program. A software fault is rectified using ‘roll-back’, i.e. re-configuration.

Distributed processing: The reduction in the cost of processing brought about by the microprocessor has enabled the control of switching systems to be de-centralized. Here, routine tasks, associated with parts of the system or particular functions are delegated to separate small processors called regional processors. Since a connection involves a number of different

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2 Telecommunication Switching and Networks functions and passes through different parts of the system, a central processor is still required to direct the regional processors and also to perform complex functions. In early SPC systems, fully centralized systems placed a restriction on the amount of directly addressable memory that was available. With distributed microprocessors, each one with its own extensive RAM, limitations are removed. Since regional processors are used, the complexity is reduced and reliability and maintainability are improved. As a result of data being stored at the regional processors, no processor requires access to data that is beyond its direct addressing capability. However copies of all software are needed as back-up.

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