Trunk Interconnection Within Modern Mobile Phone Network Computer Science Essay

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With the introduction of first generation mobile systems during the 1980's, the mobile communications industry has seen exponential growth. This can be seen by the fact that there is a wide range of devices available in the current climate.

The first such wireless mobile systems were known as 1G or could also be referred to as first generation. The system was introduced around 1980s and continued up until it was replaced by 2G. the difference between 1G and 2G being that the signals used by 1G were analogue and 2G was digital. Although 1G and 2G systems both used digital signalling to connect the radio towers to the rest of the network.

In 1991 2G was launched it was the second generation wireless mobile system. The main benefits over 1G being was calls were encrypted, 2G used the radio spectrum more efficiently and 2G introduced data services for mobiles, such as text messages which were referred to as SMS.

2G could be divided into two such technologies. TDMA and CDMA based standards. This depended on the type of multiplexing used. GSM the TDMA based standard, used in Europe and almost all other continents. Operators also use CDMA2000 in the 450 MHz frequency band CDMA450.

Prior to the leap from 2G to 3G wireless networks, the standard 2.5G was introduced as a provisional standard that bridged the gap. 2.5G enabled high speed data transfer over upgraded existing 2G networks.

The evolution of 2G networks to 3G happened with the introduction of GPRS. GPRS could provide data rates from 56 Kbit/s up to 115 Kbit/s. It was used for services such as WAP, MMS, and for services such as email and surfing the internet.

2.5G was followed by 3G which allowed faster data transmission speeds this meant you could use your mobile in more mobile data fashioned way. This meant you could use services like streaming video, movie trailers, television, audio and much more.

This advance came about from the increasing amount of network traffic and data needs. The industry was required to design and develop new systems to solve growing demands of users. This led to the development of 3G.

Currently 3G has many standards. It is probably one of the most used wireless technologies while mobile. 3G allowed the use of many services while mobile wirelessly but usually delivers slower speeds than expected. Systems today can offer up to 14 Mbit/s downstream and up to 6 Mbit/s upstream.

Problem definition

With 4G rapidly approaching and poor economic climate mobile networks are finding it difficult to strike the balance between investing in new technologies and improving their infrastructure in current technologies such as 3G. With customers striking to get a better deal in the current economic climate and pushing the regulators to bring down prices mobile network companies are finding it hard to get a head start in new markets such as 4G. With these issues in hand mobile operators are finding it hard to upgrade their networks to offer more bandwidth as customer demand increases.


Although there are many standards of Mobile Networks, this report is based exclusively on the standards used in the UK.

It is possible to do this as 4G has not yet been implemented in the UK as of yet and looking at the whole European telecommunications market can be ignored as a whole.


The reason for conducting this report was based on the fact that modern mobile phone networks have varied and a wide range of applications. With mobile network companies trialling new 4G technologies and the current economic climate when the technology rolls out mobile networks may charge customers at a much greater cost then 3G. This project will outline the history of mobile networks, a detailed overview of mobile networks and show how current technologies are connected to provide the modern service. The reason of this is to show that modern mobile networks aren't outdated as of yet and that customers can benefit from current technologies rather than investing in new technologies and paying a greater cost.


To elaborate on current technologies which are used to interconnect a modern mobile phone network.


The objectives for this report are the following:

Review history of the development of 1G to 3G

Detailed overview of trunk systems

Analyse advantages and drawbacks of trunk systems

Investigate in to future development and technologies


First Generation (1G)

The first mobile networks which emerged were the Nordic mobile telephone (NMT) network. It was deployed in Scandinavian countries around 1981. In 1983 came the advanced mobile phone service (AMPS). This mobile network was deployed in the US and shortly followed by the rest of the world by similar analogue systems. The total access communications (TACS) was another one of these analogue systems. These mobile systems were the first of such analogue systems. They were referred to as first generation or 1G.

These analogue systems used frequency division multiple access (FDMA). These radio systems each had a user channel which had a dedicated carrier band. The AMPS system used a 30 KHz wide carrier band for each user band. Then came an improvement to the AMPS system, it was known as Narrow AMPS (NAMPS). NAMPS had a carrier band which was only 10 KHz wide which was more efficient then AMPS. This meant that NAMPS could support three times the amount of subscribers.

A development came to the AMPS system in the 1990s which allowed data transmission. It was known as cellular digital packet data (CDPD). CDPD allowed the transmission of packet data over an analogue channel. It did this by using idle channels. This allowed data speeds of up to 19.2 Kbit/s.

Second Generation (2G)

As the number of mobile network subscribers rose, so did the need for increased capacity of the network. This brought on the invention of digital systems. In Europe there was the global system for mobile communications (GSM) and in the US code division multiple access (CDMA). cdmaOne was the 2G version of CDMA. These systems formed 2G ushering in digital systems. The European Telecommunications Standard institute (ETSI) developed GSM. cdmaOne was the brand name for Interim Standard 95 (IS-95). IS-95 was the first digital based CDMA standard developed. cdmaOne was pioneered by Telecommunications Industry Association (TIA).

GSM was a TDMA based radio system which had carrier bands which were 200 KHz wide. These bands were made up of eight bearer slots. This system was based on a circuit switched system which a dedicated bearer slot is given to a voice communication so up to eight subscribers could be supported by a carrier band. Cell sites would usually support many carrier bands. For GSM radio frequencies which were used for carrier bands could be reused in different cells. This was fine as long as radio towers which used similar frequencies were not adjacent to one another. This was implemented in a pattern which was referred to as frequency plan. This plan was engineered in such a way to reduce radio interference.

Another example of a TDMA system was known as the integrated digital enhanced network (IDEN). It was a standard made to work with special frequencies specified for analogue specialised mobile radio (SMR). IDEN gave the capability of a system known as push to talk (PTT). It allowed mobile phones to have the capability similar to that of a walkie talkie. It worked along a wide area network as long as the mobiles were used in the same cell sites.

cdmaOne uses a spread spectrum a where it is divided into carriers which are around 1.23 MHz wide. With cdmaOne, the voice channel has a exclusive code contained by the carrier and then the voice signal is transmitted in a spread to a rate around 1.23 Mbit/s. In a given cell site all users share the same channel band. The only way to tell the difference between the calls is by the unique code given to each voice channel. The exclusive code is used to spread the signal and is then decoded at the receiving end. cdmaOne uses universal frequency reuse. It grants the ability to reuse the same frequency in every cell and it's the exclusive code which determines the voice channels. This gives cdmaOne the ability to have better network capacity compared to TDMA systems and more efficient use of the spectrum.

GSM Network Components

The gsm network is divide into two systems. Each of these systems are comprised of a number of fundemental units which are individual components of the mobile network. The two systems are:

Switching System (SS)

Base Station System (BSS)

In addition, as with all telecommunications networks, GSM networksare operated, maintaned and managed from computerized centres.


The SS is responsible for performing call processing and subscriber related functions. It includes the following functional units:

Mobile services Switching Centre (MSC)

Home Location Register (HLR)

Visitor Location Register (VLR)

Authentication Centre (AUC)

The BSS performs all the radio related functions. The BSS is comprised of the following functional units:

Base Station Controller (BSC)

Base Transceiver Station (BTS)

The OMC performs all the operation and maintenance tasks for the network such as monitoring network traffic and network alarms. The OMC has access to both the SS and the BSS.

Switching System (SS) Components

Mobile Services Switching Centre (MSC)

The MSC performs the telephony switching functions for the mobile network. It controls calls to and from other telephony and data systems, such as the Public Switched Telephone Network (PSTN), Integrated Services Digital Network (ISDN), public data networks, private networks and other mobile networks.

Gateway functionality

Gateway functionality enables an MSC to interrogate a network's HLR in order to route a call to a Mobile Station (MS). Such an MSC is called a Gateway MSC (GMSC). For example, if a person connected to the PSTN wants to make a call to a GSM mobile subscriber, then the PSTN exchange will access the GSM network by first connecting the call to a GMSC. The same is true for a call from an MS to another MS.

Any MSC in the mobile network can function as a gateway by integration of the appropriate software.

HOME Location Register (HLR)

The HLR is a centralized network database that stores and manages all mobile subscriptions belonging to a specific operator. It acts as a permanent store for a person's subscription information until that subscription ends. The information store is made up of:

Subscriber identity

Subscriber supplemenentary services

Subscriber local Information

Subscriber authentication information

The HLR can be implemented in the same network node as the MSC or as a stand alone database. If the capacity of a HLR is exceeded by the number of subscribers, additional HLRs may be added.

Base Station System (BSS) Components

Base Station Controller (BSC)

The BSC manages all the radio related functions of a GSM network. It is a high capacity switch that provides functions such as MS handover, Radio channel assignment and the collection of cell configuration data. A number of BSCs may be controlled by each MSC.

Base Transceiver Station (BTS)

The BTS controls the radio interface to the MS. The BTS comprises the radio equipment such as transceivers and antennas which are needed to serve each cell in the network. A group of BTSs are controlled by a BSC.

Network Monitoring Centres

Operation and Maintenance Centre (OMC)

An OMC is a computerized monitoring centre which is connected to other network components such as MSCs and BSCs via data network links. In the OMC, staff are presented with the information about the status of the network and can monitor and control a variety of system parameters. There may be one or several OMCs within a network depending on the network size.

Network Management Centre (NMC)

Centralized control of a network is done at a Network Management Centre (NMC). Only one NMC is required for a network and this controls the subordinate OMCs. The advantage of this hierarchical approach is that staff at the NMC can concentrate on long term system wide issues, whereas local personnel at each OMC can concentrate on short term, regional issues.

OMC and NMC functionality can be combined in the same physical network node or implemented at different locations.

Mobile Station (MS)

An MS is used by a mobile subscriber to communicate with the mobile network. Several type of MSs exist, each allowing the subscriber to make and receive calls. Manufacturers of MSs offer a variety of Designs and features to meet the needs of different markets.

The range of a coverage area of an MS depends on the output power of the MS. Different types of MSs have different output power capabilities and consequently different ranges. For example, hand held MSs have a lower output power and a shorter range than car installed MSs with roof mounted antennas.


GSM MSs consist of:

A mobile terminal

A Subscriber Identity Module (SIM)

Unlike other standards, in GSM the subscriber is separated from the mobile. Each subscriber's information is stored as a "smart card" SIM. The SIM can be plugged in to any GSM mobile terminal. This brings the advantages of security and portability for subscribers. For example, subscriber A's mobile may have been stolen. However, subscriber A's own SIM can be used in another person's mobile and the calls would be charged to subscriber A.

GSM Geographical Network Structure

Every telephone network needs a specific structure to route incoming calls to the correct exchange and then on to the subscriber. In a mobile network, this structure is very important because the subscribers are mobile. As subscribers move through the network, these structures are used to monitor their location.


A cell is the basic unit of a cellular system and is defined as the area of radio coverage given by one BS antenna system. Each cell is assigned a unique number called Cell Global Identity (CGI). In a complete network covering an entire country, the number of cells can be quite high.


Location Area (LA)

A Location Area (LA) is defined as a group of cells. Within the network, a subscriber's location is known by the LA which they are in. The identity of the LA in which an MS is currently located is stored in the VLR.

When an MS crosses a boundary from a cell belonging to one LA into a cell belonging to another LA, it must report its new location to the network. This only occurs when the MS is idle. When the MS is on a call, its location is not updated even if it changes LAs. When an MS crosses a cell boundary within an LA, it does need to report its new location to the network. When there is call for an MS, a paging message is broadcast within all cells belonging to an LA.

MSC Service Area

An MSC service area is made up of a number of LAs and represents the geographical part of the network controlled by one MSC. In order to be able to route a call to an MS, the subscriber's MSC service area is also recorded and monitored. The subscriber's MSC service area is stored in the HLR.


PLMN Service Area

A Public Land Mobile Network (PLMN) service area is the entire set of cells served by one network operator and is defined as the area in which an operator offers radio coverage and access to its network. In any one country there may be several PLMN service areas, one for each mobile operator's network.

GSM Service Area

The GSM service Area is the entire geographical area in which a subscriber can gain access to a GSM network. The GSM service area increases as more operators sign contracts agreeing to work together. Currently, the GSM service spans countries across the world.

International Roaming is the term applied when an MS moves from one PLMN to another.


Second and a Half Generation (2.5G)

2G systems maintained simple data services. 2G offered restricted capacity as a single voice channel was used for data transfer. As one bearer slot in GSM is designated to data transmission the rate is restricted to 9.6 Kbit/s. in this case the subscriber is billed as a subscriber would for a voice call, on a time connection system.

The system was improved when the availability of a high speed circuit switched data (HSCSD). This made it possible for many bearer slots to be available per call. The disadvantage of this being that there were no extra bearer slots offered to other calls during the data call.

ETSI provided better support for such data services by developing the system general packet radio service (GPRS). This was a system which transmitted packets. It overlaid GSM with external data network like the internet. GPRS was known as a 2.5G system.

GPRS gave each mobile an IP address. The IP address given could be static or dynamically assigned by the choice of the mobile operator. This was done on a per connection basis. It worked whenever the mobile was switched on GPRS was at all times connected. Subscribers were only billed for the data transmitted, not on the connection time basis as was done for voice calls. Such GPRS enabled devices could utilize from one to eight wirelessly used bearer slots of the GSM carrier band. These slots were dynamically offered to a user when packets were transmitted. The more slots offered the higher the data transmission with speeds reaching 115 Kbit/s.

Figure displays a GSM with an overlaid GPRS network. Labels such as the elements of the network and interfaces are in accordance with ETSI standards.

The BSC transmits voice calls to the MSC and contains the packet control unit (PCU) for handling data traffic to the GPRS network.


With the new GPRS network comes two new nodes. The first new node being the Serving GPRS support node (SGSN) and the second new node being the gateway GPRS support node (GGSN). The traffic transmitted from the mobile is separated at the BSC. The voice being sent to the MSC and the packet data is sent to the SGSN. The SGSN node has the job of tracing the GPRS mobiles in its area and for transmitting packet data down the correct routes for mobile terminals. It also keeps a record of which BSC area the mobile is assigned. The GGSN node works as a router which works between the internet, other packet data networks and the GPRS network. The GGSN assigns IP addresses to mobile terminals which are allocated dynamically, and assigns the routes that mobile designated packets are designated to the correct SGSN.

Third Generation (3G)