Monthly Archives: July 2014

Operation and Maintenance

A modern telecommunications exchange should offer op­eration and maintenance functions that guarantee a high quality of service to the operators and the subscribers. Op­eration is the normal everyday running of the exchange. This includes activities to adapt the exchange to continu­ously changing demands. Examples of operational activi­ties are:

• Connection and disconnection of subscribers

• Change of subscriber data

• Collection of charging data

• Collection of statistics

Maintenance is the prevention, detection, localization, and correction of faults. The faults can be detected auto­matically by the exchange or reported to the operator by the subscribers or other exchanges in the network. Exam­ples of maintenance activities are:

• Fault detection, testing, and repair of exchange hard­ware, for example, trunk lines or subscriber lines

• Fault detection, auditing, recovery and correction of exchange software and data

• Checking of disturbance indicators in various parts of the exchange such as the power system or the control system

In a modern telecommunications exchange there are several approaches to maintenance. One is preventive maintenance that involves a set of routine tasks to check for faults before they occur and requires a high level of effort to achieve a certain grade of service. Another is cor­rective maintenance where faults are dealt with as they occur. This requires a more selective and limited effort but may result in a less consistent quality of service. The best is to have a balance between the preventive and corrective maintenance.

Statistics are used to supervise traffic and performance in the network and to reconfigure an exchange to handle more or less traffic. Particularly for the configuration of location areas and cells within a mobile network, large amounts of traffic data are used as facts to support cor­rective actions.

All operation and maintenance activities should have minimal impact on the traffic handling of the exchange. The ideal situation is an exchange where every subscriber can make a call at any time no matter what happens to the system. In order to achieve this, the system should be ro­bust to operator errors and allow the performance of main­tenance and software and hardware upgrades without af­fecting the execution of traffic events.

An exchange should be able to be operated and main­tained remotely. The ability to access exchanges remotely using an operation and support system (OSS) ensures a high level of service to subscribers and low costs for the op­erator. A centralized operation also contributes to a more reliable operation of the exchange and a reduction in per­sonnel.

Subscriber Services

When an exchange receives digital data and is computer – controlled, almost any communication service can be per­formed, and a large number have also evolved. The service software is located in the terminals, in the ordinary ex­changes, service control points and network databases. The most cost-efficient location depends on the type of service. For some services where the logic is local (such as abbrevi­ated number, also called speed dialing), it is most efficient to store the translation between abbreviated number and real number in the calling terminal. For more intelligent network services such as virtual private networks, freep – hones, or universal personal numbers, it is preferable to locate the service logic in a network node, in either (1) the ordinary exchanges such as the mobile switching center or local exchange or (2) a network database such as the SCP or HLR. The advantage of central service control is that the introduction of new services and features is simplified. In addition, some features require consistent data for the en­tire network, such as the information in the HLR regarding where a called mobile subscriber is located.

More powerful protocols enable more advanced services to be implemented in the network. At the same time, there are an increased number of services that are implemented in the terminals, and related data is sent transparently through the network between the end users. As an exam­ple, ISDN has spread slowly while data traffic over the telecom network has increased much more rapidly, where the services are executed in the end-users’ computers.

A few common telephony subscriber services, imple­mented in an exchange, are as follows:

• Freephone. The call is free of charge and paid by the called party. Often the freephone service can be di­rected to various physical subscribers depending on time, date, and traffic.

• Conference Call. More than two parties take part in a call.

• Transfer Services. The call is transferred to another telephone immediately or when the called number is busy or not replying.

• Universal Personal Number. One phone number is used regardless of which mobile or fixed physical con­nection a person is using.

• Call Completion Services. When the called party is no longer busy or nonreplying, the call is reinitiated.

• Virtual Private Network. A group of subscribers, for instance a corporation, form a private network with their own charging and telephone numbers.

These services are also effective over a network, not only when both parties reside in one exchange.

Cellular Mobile Telephony

Cellular mobile telephony differs from basic telephony since the subscribers can move freely within areas covered by the radio access network. As a result, the exchanges in a cellular system must keep track of where the sub­scribers are located and find free radio channels to use for new calls and during calls since these are shared among all subscribers in an area.

The radio frequency spectrum made available for mo­bile telephony is a scarce resource that is reused by di­viding the space in small areas called cells. Usually the frequency spectrum is also divided into frequency bands, and these bands can in turn be time or code divided into channels. Cells that are not too close to each other can, due to the use of limited power levels, share frequency bands and channels without disturbing each other.

Handover. Handover means to change or switch connec­tion from one cell to another with better radio transmission quality during an ongoing call. The handover decision is based on measurements of received signal quality in up and down links. Handover can be made several times dur­ing a call. This and the fact that handover decisions require the collection and analysis ofmeasurement data contribute to making cellular mobile telephony quite processing re­source consuming compared to fixed telephony.

Channel Allocation. Channel allocation, aims at finding free frequency, time and/or code divided channels within a cell and then allocating such free channels to calls. The allo­cation logic gives an ongoing call higher priority than a new call. Channel allocation is closely related to the handover function and especially intercell handover, which does han­dover between channels in the same cell.

Location Update. When the mobile phone is turned on and running in idle mode, it listens to control messages indicating which area the closest base station sending con­trol messages belongs to. When a border is passed it sends a message to the mobile switching center (MSC), indicat­ing that the subscriber has moved to another location. The information is stored in the MSC, in a visitor location regis­ter (VLR), and it is also stored in the home location register (HLR) if the area is handled by a new MSC.

Paging. Locating a mobile subscriber within the net­work is called paging. This is done by requesting the HLR in which VLR/MSC and location area where the subscriber is located, and then sending a paging message on the page channel to the cells in that area.

Roaming. When subscribers move to another operator’s network than their own, the network can page the sub­scriber and then set up a call to their new location.


The telecommunications exchange is a multi application digital switching product that offers its main services to its subscribers but also services to the operator of the ex­change. One example of a service offered to the subscribers is telephony calls, and a service offered to the operator is the ability to charge for such services by registering of charging data in the exchange.

A telecommunications network offers various services to the users and the operator. ITU-T has divided these ser­vices into two main categories:

• A bearer service for transport of speech or data in the network between the user interfaces. The transport of speech should be done in real time and without distor­tion or alteration. The function of the bearer service corresponds to the OSI levels 1-3 for transport, rout­ing, and safeguarding of the information through the network.

• A teleservice is a complete communication service that combines the information transfer of the bearer service with terminal services, such as information processing functions. A teleservice corresponds to the OSI levels 1-7. Some teleservices are tied to a spe­cial bearer service, whereas others can utilize differ­ent bearer services. Examples of teleservices are tele­phony, facsimile, and computer connection.

The bearer services and the teleservices are divided into basic and supplementary services. Telephony is an exam­ple of a basic teleservice, and call waiting is an example of a supplementary service that gives users additional func­tionality. In general, the supplementary services provide additional capabilities that rely on basic services to be used.

Examples of teleservices are:

• Telephony. The normal two-way voice communication between two users is the most fundamental service.

• Facsimile. This teleservice allows the connection of facsimile machines.

• Voice Mail. This service offers the subscribers the pos­sibility to forward calls to a central location in the net­work. The subscriber can later check the system for unanswered calls and listen to recorded voice mes­sages.

Basic Telephony

Below is a brief description of the main functions required by the exchange in order to set up, maintain, and discon­nect a basic telephone call between two mobile or fixed sub­scribers.

Subscriber Signaling. In order to set up a call, the call­ing subscriber alerts the exchange that there is a new call attempt and then sends the dialed number. For a digital (mobile or ISDN) access, the alert and the digits are all sent in one message, in order to save bandwidth resources and decrease the delay for call setup (see Fig. 3). For an analog access, the alert is made by lifting the handset, and the exchange replies by a dial tone. The dialed numbers are then sent one by one. In both cases, the exchange replies with a tone to the calling subscriber when the status of the called subscriber has been checked.

Number Analysis. The A-number (the calling subscriber) and the B-number (the called subscriber) are analyzed. The result is then used as input to charging and routing anal­ysis. For analog subscribers, each digit in the B-number arrives as the subscriber dials, whereas all digits can be sent at once for digital mobile or fixed subscribers.

Subscriber Category and Service Analysis. The exchange must check whether the calling subscriber has any particu­lar service invoked. Some of the services must be analyzed early in the call setup, such as blocking of outgoing calls.

The services implemented in software (programs and data) are executed either in the local exchanges used by the calling and called subscribers, in other cases in a tran­sit exchange or in a separate exchange that handles intel­ligent network (IN) services, a service control point (SCP). In the latter case, the local exchange will need to check whether to request the service from the SCP or not.

Charging. There are two classic charging methods, pulse metering and detailed billing. Detailed billing, also called toll ticketing, enables an operator to specify the character­istics of each call very extensively.

Charging can be divided into two steps: analysis and output. The result of the analysis is the charging method (toll ticketing, pulse metering or flat rate) and the charging rate, depending on a number of call data set by the operator. The output includes formatting of the charging data along with output on a reliable storage medium, locally or in a charging and maintenance center.

Figure 3. Subscriber signaling, ISDN.

Routing Analysis. Finding a path from source to destina­tion is called routing and is made mainly on the B-number. Usually there are two or three (and sometimes up to ten) alternative routes to select among. The selection of a route (also called trunk group) is guided by priority and load sta­tus information. If the first route fails or is overloaded, the next alternative is selected. There are more sophisticatedrouting algorithms that dynamically choose a link in or­der to minimize the congestion in the network. These dy­namic routing algorithms can be either local or central; the local provides results by using data available in its own exchange such as previous success rates on different link choices, while the central algorithms collect input data from other exchanges in the network.

Connection. The connection is the through-connection of two normally 64 kb/s circuits, one in each direction, in the hardware devices, and particularly in the switch fabric. The connection is required to be with limited probability of blocking, from end-to-end. This means that the switch fabric must add very low blocking probabilities, in order to fulfill the end-to-end requirements for calls that pass several transit exchanges. The connection also must be well synchronized with the rest of the exchange and with the rest of the network, in order to handle the digital speech connections properly.

Trunk Signaling. Trunk signaling enables a call to be con­nected between subscribers in separate exchanges. The ba­sic data in all trunk signaling systems are alert messages that a call is to be connected or disconnected, along with routing information, mainly the relevant parts of the dialed digits. Modern signaling systems can transmit all types of data—for instance, in order to support detailed billing, ad­vanced network services, and transparent user data.

Early signaling was made on the same line where the speech was transmitted, first by decadic pulses and later by tones of different frequencies. A still common such in­band signaling system is multi-frequency signaling, where a combination of two tones is sent to a tone-receiver in the exchange. Modern signaling is based on digital mes­sage passing. The globally dominant signaling system is the Signaling System No. 7.

Network Standards

The classic telecommunications network available all over the world is the public switched telephone network (PSTN). It is basically designed to allow the transmission of speech between two or more users and services related to that. However, this network is also used for facsimile traffic and data traffic via modems. Examples of services are alarm calls and abbreviated dialing; call forwarding and three – party calls.

Integrated services digital network (ISDN) is an evo­lution of PSTN that gives the subscribers access to in­tegrated or combined services. ISDN integrates different telecommunication services into the same network that transports voice and data in digital form between net­work access points. The main advantage of the evolution from analog to digital end-to-end communication is safer and more flexible transfer of information. ISDN provides a wide range of services divided into bearer services and teleservices. ISDN is based on the digital telephony net­work using ordinary two-wire subscriber lines, 24-or 32- channel PCM link structures, and Signaling System No. 7. Integrated access implies that the user has access to both voice and non-voice services through a single subscriber line, whereas combined access implies the use of several subscriber lines. Services include voice, facsimile, and com­puter connections.

There are two types of user-network accesses defined by ITU-T:

• Basic Rate Access. A basic rate access is used for low traffic load. It normally includes one 16 kbps signaling channel (D) and two 64kbps communication channels (B).

• Primary Rate Access (T1/E1). The primary rate access handles higher traffic loads. It normally includes 23 or 30 communication channels (B) and one signaling channel (D).

The public land mobile network (PLMN) is used here as an acronym for a set of networks based on standards such as advanced mobile phone system (AMPS) and its digital version (DAMPS), Nordic mobile telephony (NMT), global system for mobile communication (GSM), and personal dig­ital cellular (PDC) with the primary objective to provide communication to and from mobile subscribers connected to the fixed network via radio. The radio interface is imple­mented by the mobile terminals and the base stations. The base stations are end points in the fixed (wired) network.

Analog systems such as AMPS and NMT are sometimes called first generation mobile networks while DAMPS, GSM and PDC are referred to as second generation mobile networks, and are all based on cellular digital technology.

The third generation systems based on code division multiple access (CDMA) and wideband code division mul­tiple access (WCDMA) IMT-2000 and UMTS are designed to support variable speed multimedia communication.

Typical network nodes in the switching part of the PLMN are:

• The mobile services switching center (MSC) controls the radio base stations and the calls within the PLMN and calls to and from other telephony and data com­munication networks such as PSTN and ISDN.

• The home location register (HLR) contains subscriber information such as which supplementary services are activated and information regarding in which MSC-area the subscriber is currently located.

• The visitor location register (VLR) is a database with information of the locations of the mobile stations in the area controlled by the MSC. The VLR also fetches information from the HLR so that the call setup can be performed without using the HLR each time.

• The media gateway (MGW) is a switching/routing/ transferring node in the UMTS transport network, to facilitate communication between RNCs, between RNCs and the core network nodes, and between RNCs and O&M nodes.

• The base station controller (BSC), called radio net­work controller (RNC) in UMTS, coordinates and con­trols a number of radio resources usually located in base stations and some interwork functions such as handover between the cells, covered by the these base stations.

• The Radio Base Station (RBS) is responsible for radio transmission/reception in one or more cells to/from the User Equipment (UE).

The intelligent network (IN) is an architecture aimed at making a telecom network work as one uniform system where new network services can be easily developed, intro­duced, and made available in the network from a central service control point (SCP). IN aims at a logical separa­tion of signaling, call, connection, and transport. The idea is that local and transit exchanges use number analysis and ask the SCP to handle the call, unless it is a simple IN service that can be handled locally. The SCP then executes a corresponding service script that results in orders to the exchange regarding how to proceed with the call. For this communication the intelligent network application proto­col (INAP) is used. New services are described as service scripts or building blocks consisting of functional compo­nents and developed by the operator in a service creation environment.

The telecommunication management network (TMN) is an architecture and standard portfolio in the area of op­eration and maintenance. It defines functional areas and protocols for management in general terms and also more specific information models for how to monitor and managethese areas.

The open systems interconnection (OSI) reference model is a standardized layered model of how computer systems can be interconnected and interoperate that has had an influence on the way signaling networks and proto­cols are viewed and built. The model defines seven layers: application, presentation, session, transport, network, link, and physical.

The telecommunication information networking archi­tecture (TINA) is an international collaboration for defin­ing an open architecture for telecommunication systems. It focuses on the software architecture. To some extent this effort can be seen as an attempt to put together some other standardization efforts such as OSI, IN, and TMN from a software architecture point of view.

Purpose of Networks and Network Design

As discussed above, there are several types of networks seen from a functional point of view. A purpose of this par­titioning or layering into several functional networks is to help us to build network architecture with well-defined functional areas and interfaces between these that will simplify extensions and modifications to be done. Another purpose is to support the building of cost-effective net­works, both from a purchase and from a life-cycle point of view, with low operation and maintenance costs.

The end users of a telephone network have a more holis­tic view: They want to be able to contact each other so that they can talk when they are physically far away. Hence, the main purpose of a telephone network is to establish com­munication between people. In a mobile multimedia net­work this interest extends to having mobile access wher­ever the user is located also to media servers including ordinary databases and file systems as well as audio and video stream content.

End users of telephony not only want to talk, they also want to talk inexpensively and get a good quality of service. This requires efficient use of resources and a good network design. The costs to operate a network depend much on how the subscribers are distributed in the geographical area that the network is aimed to cover. Charging policy, traffic intensity, and traffic patterns such as length and locality of calls are other factors that all will influence the design of the network. Most networks are built in a more or less hi­erarchical structure to collect, concentrate, transmit, and connect the traffic such that higher-capacity transmission and switching equipment can be utilized. This reduces the costs for transmission and switching since many users can then use the equipment more efficiently and with less risk for blocking due to statistical multiplexing effects. The de­gree of concentration that can be utilized is influenced by the traffic intensity and the desired service quality level.

Another important cost factor is that the total physical length of the transmission path can be reduced in this way, and this is important since the cost for digging ditches and use of pipes is a large contributor to the overall network costs. Microwave links are used to reduce cost where it is too expensive to dig ditches. For high traffic routes, direct routes are often used, especially if the distances are short, such as in metropolitan areas. Alternative routes are also used to improve reliability, but these increase the cost and for that reason are usually not used in the access network.

Using the structural perspective of networks, one can for a specific area identify a distribution of subscribers that must be interconnected in a reasonably efficient way—for example, in a star – or ring-structured physical access net­work. One can then allocate a number of network nodes of different basic types that are needed, both to deal with ex­pected maximum traffic loads and to interconnect these in a way that keeps transmission costs close to a minimum. Typical nodes in fixed and mobile telecommunication net­works are illustrated in Fig. 2.

We can now go back to the external functional network perspective and hence to the different functions and ex­amine how these are distributed in the physical network. This distribution can be guided by different principles. One principle that has to do with costs is that simple and cheap functions that are used often and for long periods of time should be located close to the subscribers. Complex expen­sive functions used seldom and for short periods of time should be placed more centrally in the network and thus be shared by many users. However, with today’s technol­ogy and using mainstream components, complex must not mean expensive and hence a complex function if imple­mented as an integrated circuit can often be placed close to the subscribers, for example voice coders. Furthermore, the effects of distribution on reliability, signaling, and mainte­nance must also be considered when deciding on function distribution over network nodes.

From the above discussion one can see that there is no absolute definition of how networks shall be implemented or what a network node shall contain. Rather, there are several possible configurations that can fulfill the require­ments, because not only does the distribution of subscribers as well as their quality of service (QoS) requirements and traffic patterns differ significantly but also implementa­tion costs for different types of solutions differ and change over time.

Figure 2. Typical nodes in a telecom network.

Going back to and analyzing what an exchange is, one can see that many of the functions carried out in the dif­ferent logical networks have traditionally been placed in an exchange node and can still be located in such a node. On the other hand, these functions and hence the exchange
can also be more or less distributed and placed in more spe­cialized networks and nodes. With well-defined functional areas and interfaces between these it is possible to config­ure networks and network nodes in many ways. However, for a node or cluster of nodes to be regarded as an exchange it must have some basic call access, control, switching and connection handling capability.

Routing and Switching Techniques

The task of finding a path from source to destination is called routing. The task to follow such a route from source to destination guided by end-to-end address information is called route selection or forwarding. Each node that partic­ipates in this task does not know the whole path but must be able to analyze the destination address and find out in which direction the path should go and also find such a path with a trunk that is free to use.

To forward information (a bit, a voice sample, a message, a cell, a packet or a frame) from a fixed or mobile input ter­minal (address, channel, line or trunk) to a selected output terminal (address, channel, line or trunk) of an exchange where the route selection or forwarding is controlled by local address information (based on the selected path) is called switching.

The telephone exchange will use network topology or routing information to prepare a connection. Hence, rout­ing precedes the establishment of a full circuit switched connection and is done only once while switching is done for each voice sample (using traditional circuit switching) during a connection.

To guarantee that all resources needed to enable com­munication in real time are available, they can be reserved by setting up a circuit connection along a route between the circuit end points. Information (voice and data) to be exchanged between such end points in a network can then be sent along the circuit connection that is routed and set up before the actual information exchange starts and then released when the information exchange ends. This guar­antees that all resources needed to enable voice and data communication in real time are reserved beforehand.

For historical reasons, circuit connection techniques are often associated with the synchronous transfer mode (STM) and pulse code modulation (PCM) using a 125 Ds frame rate for transmission of byte (8-bit) encoded voice samples.

Another technique, called asynchronous transfer mode (ATM), uses small packets called cells with a given fixed size (53 bytes) divided into a small header (5 bytes) and payload (48 bytes). The header does in this case not contain the address but rather path and channel identifiers, where a path is a bundle of channels. Routing is done in a setup mode where the destination address is sent as payload in a cell (hence used to carry control information). The traffic can then be switched at the path or channel level cell by cell in each node along the paths and channels routed and set up in advance from sources to destinations.

A third technique usually used for data communication is to send information in packets of a reasonable size, e. g. as short Internet Protocol (IP) packets, add addressing in­formation and other control information to the packet, and send it via a route toward its destination address. Each node on the way to the destination participates in the for­warding of the packet. For information that needs to be divided into several packets, forwarding is done for each packet in each node. To get deterministic packet transport delays circuit like reserved routes can be established using multi protocol label switching (MPLS) techniques.