Wireless Communications

MOBILE COMMUNICATIONSLecture:7

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Tanvir Ahmad NiaziTanvir.niazi@mail.au.edu.pk

Air University, Islamabad

Objectives

• After completing this lesson you will be able to understand basics of :

• 3 sub-systems of a GSM system and their interfaces.• Different equipment in each GSM sub-system.

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Traffic/Signaling

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Traffic/Signaling

• The network can carry two types of information:

– Traffic: it concerns all the «user to user» information. It can be voice as well as data.

– Signaling: the network also requires to carry information for its own working. Their purposes are numerous: traffic data routing, maintenance, security... These data are usually not visible from user’s point of view.

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Traffic/Signaling• There exists several signaling types:

– PTS (Per-Trunk Signaling): signaling and voice component are transmitted on the same facility. PTS requires the voice component to be completely built, even if the call can’t be completed.

– CCS (Common Channel Signaling): two separate paths are used for information transfer (one for traffic, another for all-related signaling information). Thus, CCS allows the voice component to be built separately which allows resources to be saved. For instance, no voice facilities would be assigned to the call if the dialed number is busy.

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Network Overview

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Network Overview• A GSM system is basically designed as a combination of three major

subsystems: the Network SubSystem (NSS), the radio subsystem called the Base station SubSystem (BSS), and the Operation SubSystem (OSS).

• The Network SubSystem (NSS) includes the equipment and functions related to end-to-end-calls, management of subscribers, mobility, and interfaces with the fixed network (PSTN). It is built on the switch of the system called Mobile-services Switching Center (MSC).

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Network Overview• The Base station SubSystem (BSS) includes the equipment and functions

related to the management of the connection on the radio path. It mainly consists of Base Transceiver Stations (BTS) communicating with the Mobile Station (MS) and one Base Station Controller (BSC) managing the flow of information between the BTSs and the MSC.

• The Operation SubSystem (OSS) mainly contains Operation and Maintenance Center for NSS (OMC-S) and Operation and Maintenance Center devoted to the BSS (OMC-R). It is connected to all equipment in the switching system and to the BSC (BTSs are not connected to the OSS).

• Any mobile network or PLMN (Public Land Mobile Network) is related to a public fixed network, commonly to the PSTN (Public Switched Telephone Network).

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Mobile Station

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Mobile Station• The Mobile Station (MS) is composed of three parts:

• The handset includes the radio equipment (receiver-transmitter) and the Man-Machine Interface (MMI),

• The SIM card (Subscriber Identity Module-card): this smart card allows the identification of any subscriber (not only of his equipment) by the network. In particular, he can borrow any mobile without changing anything from the network point of view since he keeps the same SIM-card,

• The battery.

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SIM-Card and GSM Mobile Equipment

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SIM-Card and GSM Mobile Equipment

• The GSM committee has introduced an important powerful innovation by using a Smart Card in conjunction with a mobile telephone. Thus GSM subscribers are provided with a Subscriber Identity Module card (SIM-Card) with its unique identification at the very beginning of the service.

• The subscriber is identified within the system when he inserts the SIM-Card in the mobile equipment and switches it on. This provide a considerable amount of flexibility to the subscribers since they can use any GSM-specified mobile equipment.

• With the SIM-Card the idea of "personal communication" is already realized: the user only needs to take his smart card on a trip. You can rent a mobile equipment unit at the destination, even in other country, and insert your own SIM-Card. Any call you make will be charged to your home GSM account. Also the GSM system is able to reach you at the mobile unit you are currently using.

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SIM-Card and GSM Mobile Equipment

• The Mobile Station (MS) includes radio equipment and the man machine interface (MMI) that a subscriber needs in order to access the services provided by the GSM network.

• Mobile Stations can be installed in vehicles or can be portable or hand-held stations. The mobile station includes provisions for data communication as well as voice.

• Mobile Stations transmit and receive messages to and from the GSM over the air interface to establish and continue connection through the system.

• Each mobile station has an International Mobile Equipment Identity (IMEI) that is permanently stored in the mobile unit. Upon request, the MS sends this number over the signaling channel to the network. The IMEI is used to identify mobile units that are reported stolen or operating incorrectly.

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The SIM-Card Functions

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The SIM-Card Functions• The SIM-Card is a removable smart card, the size of a credit card, and

contains an integrated circuit chip with a microprocessor, random access memory, and read-only memory.

• When a mobile users want to make a call, they insert their SIM-Card and provide their Personal Identity Number (PIN), which is compared with a PIN stored within the SIM-CARD.

• The PIN can also be permanently bypassed by the subscribers if authorized by the service provider. Disabling the PIN code simplifies the call setup but reduces the protection of the user's account in the event of a stolen SIM-CARD.

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Subscriber Identification

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Subscriber Identification• The International Mobile Subscriber Identity (IMSI) is

the primary identification of the subscriber within the GSM network and is permanently assigned to him.

• The Mobile Subscriber ISDN Number (MSISDN) is the number that the calling party dials in order to reach the GSM subscriber. It is used by the land networks to route calls toward an appropriate GSM network. MSISDN is stored in HLR.

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Mobile Identification

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Mobile Identification• Stored inside the Mobile Equipment.• Used to replace IMSI or TMSI when both are unavailable (example:

Emergency calls without SIM-Card) or when required by the network (for maintenance).

• Can be used for EIR database updating (when existing):• TAC = 6 digits describing the type of equipment,• FAC = 2 digits for identification of the factory,• SNR = 6 digits for the serial number of the device.

• The type of MS must be given to the NSS at the beginning of each new connection, because this type can change between calls. The subscriber may insert this SIM-Card into another Mobile Equipment (ME).

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MS Classmark

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MS Classmark• The classmark of each MS can contain up to five parameters:

• Revision level,• RF power capability,• Encryption algorithm: A5/1, A5/2,• Frequency capability: P-GSM (2 x 25 MHz), E-GSM (2 x 35 MHz), R-GSM (2

x 4MHz), GSM 1800, GSM 1900,• Short message capability.

• This classmark is sent when the system establishes the radio link between MS and the Base Transceivers Stations.

• The power class information is the maximum power the MS is able to transmit and is used by the network for several procedures: selection, power control, handover.

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Trends in Mobile Station• Trends for MS are:

• Hands-free (2 W + booster 5 W).• Increasing autonomy:

– idle mode: 40 hours to 140 hours,– communication mode: 4 hours to 15 hours,

• Supplementary features (e.g. display of calling number).�• Additional features (e.g. voice recognition).�• Connection with terminals for data transmission:�

– Modem on PCMCIA board for Laptop PC.– Modem integrated.

• Dual-band terminal (GSM 900/1800 MHz).• Radio organizer (Nokia 9000).• Versatile terminal (under JAVA softwares): fax, internet, pager, organizer.

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BSS Architecture

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BSS Architecture• The Base Station SubSystem (BSS) is a set of equipment (aerials,

transceivers and a controller) that is viewed by the Mobile Switching Center through a single A interface as being the entity responsible for communicating with mobile telephones or Mobile Stations (MSs) in a certain area.

• The radio equipment of a BSS may be composed of one or more cells, such a BSS may contain one or more Base Transceiver Stations (BTSs).

• The interface between the BSC and the BTSs is called an Abis interface.

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BSS Architecture• The BSS includes two types of equipment:

• The Base Transceiver Station (BTS) in contact with the mobile stations through the radio interface,

• The BSC, the latter being in contact with the Mobile Switching Center.

• A BSS contains only one Base Station Controller (BSC).

• The function split is basically between a transmission equipment, the BTS, and the BSC.

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NSS Architecture

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NSS Architecture• BSCs of a same area are connected to a switch. In a GSM system this switch is

called MSC (Mobile Switching Center). MSCs are connected to each others.• Usually, each MSC is associated to four databases.

– The Visitor Location Register (VLR) memorizes information about the subscribers physically present in a geographic area. If a subscriber leaves this area, this information is stored in the VLR of another MSC.

– Each Home Location Register (HLR) is related to a precise number of subscribers. The information present in a subscriber’s HLR are quite similar to these contained in the VLR of the area where he is but, here, this information is static. Thus the VLR stands for a copy of the HLR more easily available (the VLR and the MS are in the same area). They are always linked, since the HLR memorizes the identity number of the VLR where it can find its subscriber.

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NSS Architecture• Usually, each MSC is associated to four databases (cont…).

• Authentication Center (AuC): Radio channel use sets a problem of communication safety. In particular operators have to pay attention to the fraudulent resources use. Therefore the network is provided with a system of user authentication.

• The Equipment Identity Register (EIR) is a list of all the Mobile Equipment: it contains valid and invalid mobile equipment.

• When a communication comes from the PSTN to a given subscriber, it enters the network in the MSC that contains the subscriber’s HLR. This MSC is called GMSC (Gateway MSC).

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