8. Gsm and Cdma Basics

BSNL RTTC Ahmedabad 1

Bharat Sanchar Nigam Limited

Regional Telecom Training Centre, Jagatpur, Ahmedabad-382481

GSM and CDMA Basics


GSM AND CDMA BASICS

CHAPTER

SUBJECT PAGE NO.

1 MOBILE INTRODUCTION 3

2 Cellular Concept

5

3 GSM ARCHITECTURE

10

4 Call Management Overview 22

5 CDMA Concept 79

6 GPRS 85

7 EDGE 115

8 3G COMMUNICATION 131

9 IMT-2000 149

Modulation Techniques in Mobile

Communication

155


MOBILE INTRODUCTION

The Global System for Mobile communications is a digital cellular communications system. It was developed in order to create a common European mobile telephone standard but it has been rapidly accepted worldwide. GSM was designed to be compatible with ISDN services.

HISTORY OF THE CELLULAR MOBILE RADIO AND GSM

The idea of cell-based mobile radio systems appeared at Bell Laboratories (in USA) in the early 1970s. However, mobile cellular systems were not introduced for commercial use until the 1980s. During the early 1980s, analog cellular telephone systems experienced a very rapid growth in Europe, particularly in Scandinavia and the United Kingdom. Today cellular systems still represent one of the fastest growing telecommunications systems.

But in the beginnings of cellular systems, each country developed its own system, which was an undesirable situation for the following reasons:

The equipment was limited to operate only within the boundaries of each country. The market for each mobile equipment was limited.

In order to overcome these problems, the Conference of European Posts and Telecommunications (CEPT) formed, in 1982, the Group Special Mobile (GSM) in order to develop a pan-European mobile cellular radio system (the GSM acronym became later the acronym for Global System for Mobile communications). The standardized system had to meet certain criteria:

Spectrum efficiency International roaming Low mobile and base stations costs Good subjective voice quality Compatibility with other systems such as ISDN (Integrated Services Digital Network) Ability to support new services

Unlike the existing cellular systems, which were developed using an analog technology, the GSM system was developed using a digital technology. The reasons for this choice are explained in section 3.

In 1989 the responsibility for the GSM specifications passed from the CEPT to the European Telecommunications Standards Institute (ETSI). The aim of the GSM specifications is to describe the functionality and the interface for each component of the system, and to provide guidance on the design of the system. These specifications will then standardize the system in order to guarantee the proper inter-working between the different elements of the GSM system. In 1990, the phase I of the GSM specifications was published but the commercial use of GSM did not start until mid-1991.


The most important events in the development of the GSM system are presented in the table 1.

Year Events

1982 CEPT establishes a GSM group in order to develop the standards for a pan-European cellular mobile system 1985 Adoption of a list of recommendations to be generated by the group

1986 Field tests were performed in order to test the different radio techniques proposed for the air interface

1987 TDMA is chosen as access method (in fact, it will be used with FDMA) Initial Memorandum of Understanding (MoU) signed by telecommunication operators (representing 12 countries)

1988 Validation of the GSM system 1989 The responsibility of the GSM specifications is passed to the ETSI 1990 Appearance of the phase 1 of the GSM specifications 1991 Commercial launch of the GSM service

1992 Enlargement of the countries that signed the GSM- MoU> Coverage of larger cities/airports

1993 Coverage of main roads GSM services start outside Europe 1995 Phase 2 of the GSM specifications Coverage of rural areas

Table 1: Events in the development of GSM

From the evolution of GSM, it is clear that GSM is not anymore only a European standard. GSM networks are operational or planned in over 80 countries around the world. The rapid and increasing acceptance of the GSM system is illustrated with the following figures:

1.3 million GSM subscribers worldwide in the beginning of 1994. Over 5 million GSM subscribers worldwide in the beginning of 1995. Over 10 million GSM subscribers only in Europe by December 1995.

Since the appearance of GSM, other digital mobile systems have been developed. The table 2 charts the different mobile cellular systems developed since the commercial launch of cellular systems

Year Mobile Cellular System 1981 Nordic Mobile Telephony (NMT), 450> 1983 American Mobile Phone System (AMPS) 1985 Total Access Communication System (TACS) Radiocom 2000 C-Netz 1986 Nordic Mobile Telephony (NMT), 900> 1991 Global System for Mobile communications> North American Digital Cellular (NADC) 1992 Digital Cellular System (DCS) 1800 1994 Personal Digital Cellular (PDC) or Japanese Digital Cellular (JDC) 1995 Personal Communications Systems (PCS) 1900- Canada> 1996 PCS-United States of America>


Cellular Concept Traditional mobile service was structured similar to television broadcasting: One very powerful transmitter located at the highest spot in an area would broadcast in a radius of up to fifty kilometers. The Cellular concept structured the mobile telephone network in a different way. Instead of using one powerful transmitter many low-powered transmitter were placed through out a coverage area. For example, by dividing metropolitan region into one hundred different areas (cells) with low power transmitters using twelve conversation (channels) each, the system capacity could theoretically be increased from twelve conversations using one hundred low power transmitters.

The cellular concept employs variable low power levels, which allows cells to be sized according to subscriber density and demand of a given area. As the populations grows, cells can be added to accommodate that growth. Frequencies used in one cell cluster can be reused in other cells. Conversations can be handed over from cell to cell to maintain constant phone service as the user moves between cells.

The cellular system design was pioneered by during70s by Bell Laboratories in the United States, and the initial realization was known as AMPS (Advanced Mobile Phone Service). The AMPS cellular service was available in United States in 1983. AMPS is essentially generation 1 analog cellular system in contrast to generation 2 digital cellular systems of GSM and CDMA (1S-95).

Cells : A cell is the basic geographic unit of cellular system. The term cellular comes from the honeycomb areas into which a coverage region is divided. Cells are base stations transmitting over small geographic areas that are represented as hexagons. Each cell size varies depending upon landscape. Because of constraint imposed by natural terrain and man-made structures, the true shape of cell is not a perfect hexagon. A group of cells is called a cluster. No frequencies are reused in a cluster. Features of Digital Cellular Systems:

Small cells Frequency reuse Small, battery-powered handsets Performance of handovers

\


Cellular System Characteristics General

Cellular radio systems allow the subscriber to place and receive telephone calls over the wire-line telephone network where ever cellular coverage is provided. Roaming capabilities extend service to users traveling outside their outside home service areas.

characteristics of digital cellular systems

The distinguishing features of digital cellular systems compared to other mobile radio systems are: Small cells

A cellular system uses many base stations with relatively small coverage radii (on the order of a 100 m to 30 km).

Frequency reuse The spectrum allocated for a cellular network is limited. As

a result there is a limit to the number of channels or frequencies that can be used. For this reason each frequency is used simultaneously by multiple base-mobile pairs. This frequency reuse allows a much higher subscriber density per MHz of spectrum than other systems. System capacity can be further increased by reducing the cell size (the coverage area of a single base station), down to radii as small as 200 m.

Small, battery-powered handsets In addition to supporting much higher densities than previous systems, this approach enables the use of small, battery-powered handsets with a radio frequency that is lower than the large mobile units used in earlier systems.

Performance of handovers In cellular systems, continuous coverage is achieved by executing a handover (the seamless transfer of the call from one base station to another) as the mobile unit crosses cell boundaries. This requires the mobile to change frequencies under control of the cellular network.

Frequency Reuse : Why frequency reuse

The spectrum allocated for a cellular network is limited. As a result there is a limit to the number of frequencies or channels that can be used. A cellular network can only provide service to a large number of subscribers, if the channels allocated to it can be reused. Channel reuse is implemented by using the same channels within cells located at different positions in the cellular network service area. Radio channels can be reused provided the separation between cells containing the same channel set is far enough apart so that co-channel interference can be kept below acceptable levels most of the time. Cells using the same channel set are called co-channel cells.

Cell clustering The figure on the opposite page shows an example. Within the service area (PLMN), specific channel sets are reused at a different location (another cell). In the example, there are 7 channel sets: A through G. Neighboring cells are not allowed to use the same frequencies. For this reason all channel sets are used in a cluster of neighboring cells. As


there are 7 channel sets, the PLMN can be divided into clusters of 7 cells each. The figure shows three clusters. The number of channel sets is called K. K is also called the reuse factor. In the figure, K=7. Valid values of K can be found using equation (where i and j are integers): K=i+j+I*j Explaining this equation is beyond the scope of this course. Some constraints to K are provided later in this chapter. Note that in the example: Cells are shaped ideally (hexagons). The distance between cells using the same channel set is always the same.

Other cell clusters

The figure on the opposite page shows some examples of possible clusters. The more cells in a cluster, the greater the separation between co-channel cells when Other clusters are deployed. The idea is to keep co-channel cell separation the same throughout the system area for cells of the same size. Some valid cluster sizes that allow this are: 1, 3, 4, 7, 9 and 12.

Procedure for locating co-channel cells

It is always possible to find cells using the same channel set, if only the value of K is known. The following procedure is used. In the figure on the opposite page an example is shown with K = 19.

Signal attenuation With distance

Frequencies can be reused throughout a service area because radio signals typically attenuate with distance to the base station (or mobile station). When the distance between cells using the same frequencies becomes too small, co-channel Interference might occur and lead to service interruption or unacceptable quality of service.

Step Action 1 Use the integer values i and j from the equation, and start

With the upper left cell. Through this cell, draw the j-axis. 2 Draw the i-axis. To find the starting point for the i-axis, count j cells

down the j-axis. In the example, one has to count 2 cells down (j=2). The positive direction of the i-axis is always two cell faces (120 degrees) relative to the positive direction of the j-axis.

3 Find the first co-channel cell. It is found by counting i cells in the positive i-axis direction. In the example, i = 3.

4 Find the other co-locating cells by repeating the previous steps. The Starting point is again at the upper left cell, but now choose another Direction for the j-axis (e.g. rotate the j-axis with 60 degrees, which is one cell face). As each cell has 6 faces, one will find 6 co-channel cells around the starting cells. These are the nearest located co-channel cells.

Capacity/Performance Trade-offs : n If K increases, then performance increases n If K increases, then call capacity decreases per cell


The number of sites to cover a given area with a given high traffic density, and hence the cost of the infrastructure, is determined directly by the reuse factor and the number of traffic channels that can be extracted from the available spectrum. These two factors are compounded in what is called spectral efficiency of the system. Not all systems allow the same performance in this domain: they depend in particular on the robustness of the radio transmission scheme against interference, but also on the use of a number of technical tricks, such as reducing transmission during the silences of a speech communication. The spectral efficiency, together with the constraints on the cell size, determines also the possible compromises between the capacity and the cost of the infrastructure. All this explains the importance given to spectral efficiency. Many technical tricks to improve spectral efficiency were conceived during the system design and have been introduced in GSM. They increase the complexity, but this is balanced by the economical advantages of a better efficiency. The major points are the following: The control of the transmitted power on the radio path aims at minimizing the average power broadcast by mobile stations as well as by base stations, whilst keeping transmission quality above a given threshold. This reduces the level of interference caused to the other communications; Frequency hopping improves transmission quality at slow speeds through frequency diversity, and improves spectral efficiency through interferer diversity; Discontinuous transmission, where by transmission is suppressed when possible, allows a reduction in the interference level of other communications. Depending on the type of user information transmitted, it is possible to derive the need for effective transmission. In the case of speech, the mechanism called VAD (Voice Activity Detection) allows transmission requirements to be reduced by an important factor (typically, reduced by half); The mobile assisted handover, whereby the mobile station provides measurements concerning neighboring cells, enables efficient handover decision algorithms aimed at minimizing the interference generated by the cell (whilst keeping the transmission quality above some threshold). References:1. The GSM system for mobile communication-Michel Mouly & Marie- Bernadette Pautet. 2. GSM system Engineering-Asha Mehrotra (Artech House Publisher).


TYPES OF CELLS The density of population in a country is so varied that different types of cells are used:

Macro cells The macro cells are large cells for remote and sparsely populated areas

Micro cells These cells are used for densely populated areas. By splitting the existing areas into smaller cells, the number of channels available is increased as well as the capacity of the cells. The power level of the transmitters used in these cells is then decreased, reducing the possibility of interference between neighboring cells.

Selective cells It is not always useful to define a cell with a full coverage of 360 degrees. In some cases, cells with a particular shape and coverage are needed. These cells are called selective cells. Typical examples of selective cells are the cells that may be located at the entrances of tunnels where coverage of 360 degrees is not needed. In this case, a selective cell with coverage of 120 degrees is used.

Umbrella cells A freeway crossing very small cells produces an important number of handovers among the different small neighboring cells. In order to solve this problem, the concept of umbrella cells is introduced. An umbrella cell covers several micro cells. The power level inside an umbrella cell is increased comparing to the power levels used in the micro cells that form the umbrella cell. When the speed of the mobile is too high, the mobile is handed off to the umbrella cell. The mobile will then stay longer in the same cell (in this case the umbrella cell). This will reduce the number of handovers and the work of the network.

A too important number of handover demands and the propagation characteristics of a mobile can help to detect its high speed


GSM ARCHITECTURE

INTRODUCTION

A GSM system is basically designed as a combination of three major subsystems: the network subsystem, the radio subsystem, and the operation support subsystem. In order to ensure that network operators will have several sources of cellular infrastructure equipment, GSM decided to specify not only the air interface, but also the main interfaces that identify different parts. There are three dominant interfaces, namely, an interface between MSC and the base Transceiver Station (BTS), and an Um interface between the BTS and MS.

GSM NETWORK STRUCTURE

Every telephone network needs a well-designed structure in order to route incoming called to the correct exchange and finally to the called subscriber. In a mobile network, this structure is of great importance because of the mobility of all its subscribers [1-4]. In the GSM system, the network is divided into the following partitioned areas.

GSM service area; PLMN service area; MSC service area; Location area; Cells.

The GSM service is the total area served by the combination of all member countries where a mobile can be serviced. The next level is the PLMN service area. There can be several within a country, based on its size. The links between a GSM/PLMN network and other PSTN, ISDN, or PLMN network will be on the level of international or national transit exchange. All incoming calls for a GSM/PLMN network will be routed to a gateway MSC. A gateway MSC works as an incoming transit exchange for the GSM/PLMN. In a GSM/PLMN network, all mobile-terminated calls will be routed to a gateway MSC. Call connections between PLMNs, or to fixed networks, must be routed through certain designated MSCs called a gateway MSC. The gateway MSC contains the interworking functions to make these connections. They also route incoming calls to the proper MSC within the network. The next level of division is the MSC/VLR service area. In one PLMN there can be several MSC/VLR service area. MSC/VLR is a role controller of calls within its jurisdiction. In order to route a call to a mobile subscriber, the path through links to the MSC in the MSC area where the subscriber is currently located. The mobile location can be uniquely identified since the MS is registered in a VLR, which is generally associated with an MSC.

The next division level is that of the LAs within a MSC/VLR combination. There are several LAs within one MSc/VLR combination. A LA is a part of the


MSC/VLR service area in which a MS may move freely without updating location information to the MSC/VLR exchange that control the LA. Within a LA a paging message is broadcast in order to find the called mobile subscriber. The LA can be identified by the system using the Location Area Identity (LAI). The LA is used by the GSM system to search for a subscriber in a active state.

Lastly, a LA is divided into many cells. A cell is an identity served by one BTS. The MS distinguishes between cells using the Base Station Identification code (BSIC) that the cell site broadcast over the air.

GSM Architecture

MOBILE STATION

The MS includes radio equipment and the man machine interface (MMI) that a subscribe needs in order to access the services provided by the GSM PLMN. MS can be installed in Vehicles or can be portable or handheld stations. The MS may include provisions for data communication as well as voice. A mobile transmits and receives message to and from the GSM system over the air interface to establish and continue connections through the system .

Different type of MSs can provide different type of data interfaces. To provide a common model for describing these different MS configuration, reference configuration for MS, similar to those defined for ISDN land stations, has been defined.

Each MS is identified by an IMEI that is permanently stored in the mobile unit. Upon request, the MS sends this number over the signaling channel to the MSC. The IMEI can be used to identify mobile units that are reported stolen or operating incorrectly.

OMC

MSC BSC HLR A

MS

Other MSCs

BTS AUC

Other Networks EIR

Other MSCs VLRs VLR

BSS B

C D

E F

G

UM

Abis


Just as the IMEI identities the mobile equipment, other numbers are used to identity the mobile subscriber. Different subscriber identities are used in different phases of call setup. The Mobile Subscriber ISDN Number (MSISDN) is the number that the calling party dials in order to reach the subscriber. It is used by the land network to route calls toward an appropriate MSC. The international mobile subscribe identity (IMSI) is the primary function of the subscriber within the mobile network and is permanently assigned to him. The GSM system can also assign a Temporary Mobile Subscriber Identity (TMSI) to identity a mobile. This number can be periodically changed by the system and protect the subscriber from being identified by those attempting to monitor the radio channel.

Functions of MS

The primary functions of MS are to transmit and receive voice and data over the air interface of the GSM system. MS performs the signal processing function of digitizing, encoding, error protecting, encrypting, and modulating the transmitted signals. It also performs the inverse functions on the received signals from the BS.

In order to transmit voice and data signals, the mobile must be in synchronization with the system so that the messages are the transmitted and received by the mobile at the correct instant. To achieve this, the MS automatically tunes and synchronizes to the frequency and TDMA timeslot specified by the BSC. This message is received over a dedicated timeslot several times within a multiframe period of 51 frames. We shall discuss the details of this in the next chapter. The exact synchronization will also include adjusting the timing advance to compensate for varying distance of the mobile from the BTS.

The MS monitors the power level and signal quality, determined by the BER for known receiver bit sequences (synchronization sequence), from both its current BTS and up to six surrounding BTSs. This data is received on the downlink broadcast control channel. The MS determines and send to the current BTS a list of the six best-received BTS signals. The measurement results from MS on downlink quality and surrounding BTS signal levels are sent to BSC and processed within the BSC. The system then uses this list for best cell handover decisions.

MS keeps the GSM network informed of its location during both national and international roaming, even when it is inactive. This enables the System to page in its present LA.

The MS includes an equalizer that compensates for multi-path distortion on the received signal. This reduces inter-symbol interface that would otherwise degrade the BER.

Finally, the MS can store and display short received alphanumeric messages on the liquid crystal display (LCD) that is used to show call dialing and status information. These messages are limited to 160 characters in length.


Power Levels

These are five different categories of mobile telephone units specified by the European GSM system: 20W, 8W, 5W, 2W, and 0.8W. These correspond to 43-dBm, 39-dBm, 37-dBm, 33-dBm, and 29-dBm power levels. The 20-W and 8-W units (peak power) are either for vehicle-mounted or portable station use.

The MS power is adjustable in 2-dB steps from its nominal value down to 20mW (13 dBm). This is done automatically under remote control from the BTS, which monitors the received power and adjusts the MS transmitter to the minimum power setting necessary for reliable transmission.

SIM Card

As described in the first chapter, GSM subscribers are provided with a SIM card with its unique identification at the very beginning of the service. By divorcing the subscriber ID from the equipment ID, the subscriber may never own the GSM mobile equipment set. The subscriber is identified in the system when he inserts the SIM card in the mobile equipment. This provides an enormous amount of flexibility to the subscribers since they can now use any GSM-specified mobile equipment. Thus with a SIM card the idea of Personalize the equipment currently in use and the respective information used by the network (location information) needs to be updated. The smart card SIM is portable between Mobile Equipment (ME) units. The user only needs to take his smart card on a trip. He can then rent a ME unit at the destination, even in another country, and insert his own SIM. Any calls he makes will be charged to his home GSM account. Also, the GSM system will be able to reach him at the ME unit he is currently using.

The SIM is a removable SC, the size of a credit card, and contains an integrated circuit chip with a microprocessor, random access memory (RAM), and read only memory (ROM). It is inserted in the MS unit by the subscriber when he or she wants to use the MS to make or receive a call. As stated, a SIM also comes in a modular from that can be mounted in the subscribers equipment.

When a mobile subscriber wants to use the system, he or she mounts their SIM card and provide their Personal Identification Number(PIN), which is compared with a PIN stored within the SIM. If the user enters three incorrect PIN codes, the SIM is disabled. The PIN can also be permanently bypassed by the service provider if requested by the subscriber. Disabling the PIN code simplifies the call setup but reduces the protection of the users account in the event of a stolen SIM.

International Mobile Subscriber Identity.

An IMSI is assigned to each authorized GSM user. It consists of a mobile country code (MSC), mobile network code (MNC), and a PLMN unique mobile subscriber identification number (MSIN). The IMSI is not hardware-specific. Instead, it is maintained on a SC by an authorized subscriber and is the only absolute identity that a subscriber has within the GSM system. The IMSI consists of the MCC followed by the NMSI and shall not exceed 15 digits.


Temporary Mobile Subscriber Identity

A TMSI is a MSC-VLR specific alias that is designed to maintain user confidentiality. It is assigned only after successful subscriber authentication. The correlation of a TMSI to an IMSI only occurs during a mobile subscribers initial transaction with an MSC (for example, location updating). Under certain condition (such as traffic system disruption and malfunctioning of the system), the MSC can direct individual TMSIs to provide the MSC with their IMSI.

Mobile Station ISDN Number

The MS international number must be dialed after the international prefix in order to obtain a mobile subscriber in another country. The MSISDN numbers is composed of the country code (CC) followed by the National Significant Number (N(S)N), which shall not exceed 15 digits.

The Mobile Station Roaming Number (MSRN)

The MSRN is allocated on temporary basis when the MS roams into another numbering area. The MSRN number is used by the HLR for rerouting calls to the MS. It is assigned upon demand by the HLR on a per-call basis. The MSRN for PSTN/ISDN routing shall have the same structure as international ISDN numbers in the area in which the MSRN is allocated. The HLR knows in what MSC/VLR service area the subscriber is located. At the reception of the MSRN, HLR sends it to the GMSC, which can now route the call to the MSC/VLR exchange where the called subscriber is currently registered.

International Mobile Equipment Identity

The IMEI is the unique identity of the equipment used by a subscriber by each PLMN and is used to determine authorized (white), unauthorized (black), and malfunctioning (gray) GSM hardware. In conjunction with the IMSI, it is used to ensure that only authorized usera are granted access to the system. An IMEI is never sent in cipher mode by MS.

BASE STATION SYSTEM

The BSS is a set of BS equipment (such as transceivers and controllers) that is in view by the MSC through a single A interface as being the entity responsible for communicating with MSs in a certain area. The radio equipment of a BSS may be composed of one or more cells. A BSS may consist of one or more BS. The interface between BSC and BTS is designed as an A-bis interface. The BSS includes two types of machines: the BTS in contact with the MSs through the radio interface and the BSC, the latter being in contact with the MSC. The function split is basically between transmission equipment, the BTS, and managing equipment at the BSC. A BTS compares radio transmission and reception devices, up to and including the antennas, and also all the signal processing specific to the radio interface. A single transceiver within BTS supports eight basic radio channels of the same TDM frame. A BSC is a network component in the PLMN that function for control of one or more BTS. It is a functional entity that handles common control functions within a BTS.


A BTS is a network component that serves one cell and is controlled by a BSC. BTS is typically able to handle three to five radio carries, carrying between 24 and 40 simultaneous communication. Reducing the BTS volume is important to keeping down the cost of the cell sites.

An important component of the BSS that is considered in the GSM architecture as a part of the BTS is the Transcoder/Rate Adapter Unit (TRAU). The TRAU is the equipment in which coding and decoding is carried out as well as rate adoption in case of data. Although the specifications consider the TRAU as a subpart of the BTS, it can be sited away from the BTS (at MSC), and even between the BSC and the MSC.

The interface between the MSC and the BSS is a standardized SS7 interface (A-interface) that, as stated before, is fully defined in the GSM recommendations. This allows the system operator to purchase switching equipment from one supplier and radio equipment and the controller from another. The interface between the BSC and a remote BTS likewise is a standard the A-bis. In splitting the BSS functions between BTS and BSC, the main principle was that only such functions that had to reside close to the radio transmitters/receivers should be placed in BTS. This will also help reduce the complexity of the BTS.

Functions of BTS

As stated, the primary responsibility of the BTS is to transmit and receive radio signals from a mobile unit over an air interface. To perform this function completely, the signals are encoded, encrypted, multiplexed, modulated, and then fed to the antenna system at the cell site. Trans-coding to bring 13-kbps speech to a standard data rate of 16 kbps and then combining four of these signals to 64 kbps is essentially a part of BTS, though, it can be done at BSC or at MSC. The voice communication can be either at a full or half rate over logical speech channel. In order to keep the mobile synchronized, BTS transmits frequency and time synchronization signals over frequency correction channel (FCCH and BCCH logical channels. The received signal from the mobile is decoded, decrypted, and equalized for channel impairments.

Random access detection is made by BTS, which then sends the message to BSC. The channel subsequent assignment is made by BSC. Timing advance is determined by BTS. BTS signals the mobile for proper timing adjustment. Uplink radio channel measurement corresponding to the downlink measurements made by MS has to be made by BTS.

BTS-BSC Configurations

There are several BTS-BSC configurations: single site; single cell; single site; multicell; and multisite, multicell. These configurations are chosen based on the rular or urban application. These configurations make the GSM system economical since the operation has options to adapt the best layout based on the traffic requirement. Thus, in some sense, system optimization is possible by the proper choice of the configuration. These include omni directional rural configuration where the BSC and


BTS are on the same site; chain and multidrop loop configuration in which several BTSs are controlled by a single remote BSC with a chain or ring connection topology; rural star configuration in which several BTSs are connected by individual lines to the same BSC; and sectorized urban configuration in which three BTSs share the same site amd are controlled by either a collocated or remote BSC.

In rural areas, most BSs are installed to provide maximum coverage rather then maximum capacity.

Transcoder

Depending on the relative costs of a transmission plant for a particular cellular operator, there may be some benefit, for larger cells and certain network topologies, in having the transcoder either at the BTS, BSC or MSC location. If the trascoder is located at MSC, they are still considered functionally a part of the BSS. This approach allows for the maximum of flexibility and innovation in optimizing the transmission between MSC and BTS. The transcoder is the device that takes 13-Kbps speech or 3.6/6/12-Kbps data multiplexes and four of them to convert into standard 64-Kbps data. First, the 13 Kbps or the data at 3.6/6/12 Kbps are brought up to the level of 16 Kpbs by inserting additional synchronizing data to make up the difference between a 13-Kbps speech or lower rate data, and then four of them are combined in the transcoder to provide 64 Kpbs channel within the BSS. Four traffic channel can then be multiplexed on one 64-Kpbs circuit. Thus, the TRAU output data rate is 64 Kpbs. Then, up to 30 such 64-Kpbs channels are multiplexed onto a 2.048 Mpbs if a CEPT1 channel is provided on the A-bis interface. This channel can carry up to 120-(16x 120) traffic and control signals. Since the data rate to the PSTN is normally at 2 Mbps, which is the result of combining 30-Kbps by 64-Kbph channels, or 120- Kbps by 16-Kpbs channels.

BSC

The BSC, as discussed, is connected to the MSC on one side and to the BTS on the other. The BSC performs the Radio Resource (RR) management for the cells under its control. It assigns and release frequencies and timeslots for all MSs in its own area. The BSC performs the intercell handover for MSs moving between BTS in its control. It also reallocates frequencies to the BTSs in its area to meet locally heavy demands during peak hours or on special events. The BSC controls the power transmission of both BSSs and MSs in its area. The minimum power level for a mobile unit is broadcast over the BCCH. The BSC provides the time and frequency synchronization reference signals broadcast by its BTSs. The BSC also measures the time delay of received MS signals relative to the BTS clock. If the received MS signal is not centered in its assigned timeslot at the BTS, The BSC can direct the BTS to notify the MS to advance the timing such that proper synchronization takes place. The functions of BSC are as follows.

The BSC may also perform traffic concentration to reduce the number of transmission lines from the BSC to its BTSs, as discussed in the last section.


SWITCHING SUBSYSTEMS: MOBILE SWITCHING CENTER AND GATEWAY SWITCHING CENTER

The network and the switching subsystem together include the main switching functions of GSM as well as the databases needed for subscriber data and mobility management (VLR). The main role of the MSC is to manage the communications between the GSM users and other telecommunication network users. The basic switching function of performed by the MSC, whose main function is to coordinate setting up calls to and from GSM users. The MSC has interface with the BSS on one side (through which MSC VLR is in contact with GSM users) and the external networks on the other (ISDN/PSTN/PSPDN). The main difference between a MSC and an exchange in a fixed network is that the MSC has to take into account the impact of the allocation of RRs and the mobile nature of the subscribers and has to perform, in addition, at least, activities required for the location registration and handover.

The MSC is a telephony switch that performs all the switching functions for MSs located in a geographical area as the MSC area. The MSC must also handle different types of numbers and identities related to the same MS and contained in different registers: IMSI, TMSI,ISDN number, and MSRN. In general identities are used in the interface between the MSC and the MS, while numbers are used in the fixed part of the network, such as, for routing.

Functions of MSC

As stated, the main function of the MSC is to coordinate the set up of calls between GSM mobile and PSTN users. Specifically, it performs functions such as paging, resource allocation, location registration, and encryption.

Specifically, the call-handling function of paging is controlled by MSC. MSC coordinates the set up of call to and from all GSM subscribers operating in its areas. The dynamics allocation of access resources is done in coordination with the BSS. More specifically, the MSC decides when and which types of channels should be assigned to which MS. The channel identity and related radio parameters are the responsibility of the BSS, The MSC provides the control of interworking with different networks. It is transparent for the subscriber authentication procedure. The MSC supervises the connection transfer between different BSSs for MSs, with an active call, moving from one call to another. This is ensured if the two BSSs are connected to the same MSC but also when they are not . In this latter case the procedure is more complex, since more then one MSC in involved. The MSC performs billing on calls for all subscribers based in its areas. When the subscriber is roaming elsewhere, the MSC obtains data for the call billing from the visited MSC. Encryption parameters transfers from VLR to BSS to facilitate ciphering on the radio interface are done by MSC. The exchange of signaling information on the various interface toward the other network elements and the management of the interface themselves are all controlled by the MSC. Finally, the MSC serves as a SMS gateway to forward SMS messages from Short Message Service Centers (SMSC) to the subscribers and from the subscribers to the SMSCs. It thus acts as a message mailbox and delivery system.


VLR

The VLR is collocated with an MSC. A MS roaming in an MSC area is controlled by the VLR responsible for that area. When a MS appears in a LA, it starts a registration procedure. The MSC for that area notices this registration and transfers to the VLR the identify of the LA where the MS is situated. A VLR may be in charge of one or several MSC LAs. The VLR constitutes the databases that support the MSC in the storage and retrieval of the data of subscribers present in its area. When an MS enters the MSC area borders, it signals its arrival to the MSC that stores its identify in the VLR. The information necessary to manage the MS is contained in the HLR and is transferred to the VLR so that they can be easily retrieved if so required.

Data Stored in VLR

The data contained in the VLR and in the HLR are more or less the same. Nevertheless the data are present in the VLR only as long as the MS is registered in the area related to that VLR. Data associated with the movement of mobile are IMSI, MSISDN, MSRN, and TMSI. The terms permanent and temporary, in this case, are meaningful only during that time interval. Some data are mandatory, others are optional.

HOME LOCATION REGISTER

The HLR is a database that permanently stores data related to a given set of subscribers. The HLR is the reference database for subscriber parameters. Various identification numbers and addresses as well as authentication parameters, services subscribed, and special routing information are stored. Current subscriber status including a subscribers temporary roaming number and associated VLR if the mobile is roaming, are maintained.

The HLR provides data needed to route calls to all MS-SIMs home based in its MSC area, even when they are roaming out of area or in other GSM networks. The HLR provides the current location data needed to support searching for and paging the MS-SIM for incoming calls, wherever the MS-SIM may be. The HLR is responsible for storage and provision of SIM authentication and encryption parameters needed by the MSC where the MS-SIM is operating. It obtains these parameters from the AUC.

The HLR maintains record of which supplementary service each user has subscribed to and provides permission control in granting services. The HLR stores the identification of SMS gateways that have messages for the subscriber under the SMS until they can be transmitted to the subscriber and receipt is knowledge. Some data are mandatory, other data are optional. Both the HLR and the VLR can be implemented in the same equipment in an MSC (collocated). A PLMN may contain one or several HLRs.


AUTHENTICATION CENTER

The AUC stores information that is necessary to protect communication through the air interface against intrusions, to which the mobile is vulnerable. The legitimacy of the subscriber is established through authentication and ciphering, which protects the user information against unwanted disclosure. Authentication information and ciphering keys are stored in a database within the AUC, which protects the user information against unwanted disclosure and access. In the authentication procedure, the key Ki is never transmitted to the mobile over the air path, only a random number is sent. In order to gain access to the system, the mobile must provide the correct Signed Response (SRES) in answer to a random number (RAND) generated by AUC. Also, Ki and the cipher key Kc are never transmitted across the air interface between the BTS and the MS. Only the random challenge and the calculated response are transmitted. Thus, the value of Ki and Kc are kept secure. The cipher key, on the other hand, is transmitted on the SS7 link between the home HLR/AUC and the visited MSC, which is a point of potential vulnerability. On the other hand, the random number and cipher key is supposed to change with each phone call, so finding them on one call will not benefit using them on the next call. The HLR is also responsible for the authentication of the subscriber each time he makes or receives a call. The AUC, which actually performs this function, is a separate GSM entity that will often be physically included with the HLR. Being separate, it will use separate processing equipment for the AUC database functions.

EQUIPMENT IDENTIFY REGISTER

EIR is a database that stores the IMEI numbers for all registered ME units. The IMEI uniquely identifies all registered ME. There is generally one EIR per PLMN. It interfaces to the various HLR in the PLMN. The EIR keeps track of all ME units in the PLMN. It maintains various lists of message. The database stores the ME identification and has nothing do with subscriber who is receiving or originating call. There are three classes of ME that are stored in the database, and each group has different characteristics.

White List: contains those IMEIs that are known to have been assigned to valid MSs. This is the category of genuine equipment.

Black List: contains IMEIs of mobiles that have been reported stolen. Gray List: contains IMEIs of mobiles that have problems (for example, faulty

software, wrong make of the equipment). This list contains all MEs with faults not important enough for barring.


INTERWORKING FUNCTION

GSM provided a wide range of data services to its subscribers. The GSM system interface with the various forms of public and private data networks currently available. It is the job of the IWF to provide this interfacing capability.

The IWF, which in essence is a part of MSC, provides the subscriber with access to data rate and protocol conversion facilities so that data can be transmitted between GSM Data Terminal Equipment (DTE) and a land-line DTE.

ECHO CANCELER

EC is used on the PSTN side of the MSC for all voice circuits. The EC is required at the MSC PSTN interface to reduce the effect of GSM delay when the mobile is connected to the PSTN circuit. The total round-trip delay introduced by the GSM system, which is the result of speech encoding, decoding and signal processing, is of the order of 180 ms. Normally this delay would not be an annoying factor to the mobile, except when communicating to PSTN as it requires a two-wire to four-wire hybrid transformer in the circuit. This hybrid is required at the local switching office because the standard local loop is a two-wire circuit. Due to the presence of this hybrid, some of the energy at its four-wire receive side from the mobile is coupled to the four-wire transmit side and thus retransmitted to the mobile. This causes the echo, which does not effect the land subscriber but is an annoying factor to the mobile. The standard EC cancels about 70 ms of delay.

During a normal PSTN (land-to-land call), no echo is apparent because the delay is too short and the land user is unable to distinguish between the echo and the normal telephone side tones However, with the GSM round-trip delay added and without the EC, the effect would be irritating to the MS subscriber.

OPERATION AND MAINTENANCE CENTER

The OMC provides alarm-handling functions to report and log alarms generated by the other network entities. The maintenance personnel at the OMC can define that criticality of the alarm. Maintenance cover both technical and administrative actions to maintain and correct the system operation, or to restore normal operations after a breakdown, in the shortest possible time.

The fault management functions of the OMC allow network devices to be manually or automatically removed from or restored to service. The status of network devices can be checked, and tests and diagnostics on various devices can be invoked. For example, diagnostics may be initiated remotely by the OMC. A mobile call trace facility can also be invoked. The performance management functions included collecting traffic statistics from the GSM network entities and archiving them in disk files or displaying them for analysis. Because a potential to collect large amounts of data exists, maintenance personal can select which of the detailed


statistics to be collected based on personal interests and past experience. As a result of performance analysis, if necessary, an alarm can be set remotely.

The OMC provides system change control for the software revisions and configuration data bases in the network entities or uploaded to the OMC. The OMC also keeps track of the different software versions running on different subsystem of the GSM.

References: [1] The GSM system for mobile communication-Michel Mouly & Marie- Bernadette Pautet.

[2] GSM system Engineering-Asha Mehrotra (Artech House Publisher).

[3] haug, T.,Developing GSM standard, pan-European Digital Cellular Radio Conf., Nice, France, 1991.

[4] Mouly, M., and pautet Marie-Bernadette,Current Evolution of the GSM system, IEEE Personal Communications, October 1995, PP.9-19.

[5] Beddoes, E, W., GSM Network Architecture, GSm Seminar, Budapest, October 1990, Session 2.1.


4 Call Management Overview

OBJECTIVE

After completion of this lesson the student is able to: Describe the steps of a mobile originate Call Describe the steps of a mobile terminated Call.

MOBILE-TO-LAND CALL SCENARIO

The following table lists the phases of a mobile-to-land call. Stage Description

1 Request for service, the MS request to setup a call.

2 Authentication; the MSC/ VLR request the AUC for authentication parameters. Using these parameters the MS is authenticated.

3 Ciphering: Using the parameters that were made available earlier during the authentication the uplink and the downlink are ciphered.

4 Equipment Validation: the MSC/VLR requests the EIR to check the IMEI for validity.

5 Call Setup; the MSC established a connection to the MS. 6 Handovers 7 Call release, the speech path is released.


The following figure shows the Phases 1, 4 and 5.

Fig : 1 OPTIONAL PHASES

The authentication, ciphering, equipment validation and handover phases are optional; the service provider may decide that some of these phases might not take place in a

mobile-to-land call. STEPS IN REQUEST FOR SERVICE PHASE The following is an example of a scenario of a mobile-to-land call. It is assumed that the MS is already registered with the system and has been allocated a Temporary Mobile

subscriber Identity Number (TMSI). A Mobile originated call starts by the user entering the director number digits, associated with the person to be called, on the MS handset. The user presses the Sent key after all

digits have been entered.

MS transmits a channel request message over the Random Access Channel (RACH)

Once the BSS receives the Channel Request message, it allocated a Stand-alone Dedicated Control Channel (SDCCH) and forwards this channel assignment information to the MS over the Access Grant channel (AGCH). It is over the SDCCH


that the MS will communicate with the BSS and MSC until a traffic channel is assigned.

The MS transmits a service request message to the BSS over the SDCCH. Included in this message are the MS TMSI and Location Are Identification (LAI). The BSS forwards the service request message to the MSC/ VLR.

The following figure shows the request for service phase.

Fig : 2

AUTHENTICATION AND CIPHERING PHASES The Authentication and ciphering phase that might be performed here to setup a mobile-

to-land call are exactly the same as seen before in the location update scenarios. EQUIPMENT VALIDATION The Mobile-equipment validation process is the means by which a specific piece of mobile

equipment can be identified regardless of the user. It is needed to prevent the use of stolen, unauthorized, or malfunctioning equipment in the network.

Each piece of mobile equipment is uniquely identified by an International Mobile Equipment identify (IMEI) code. The IMEI code which is incorporated into the

equipment by the manufacturer has three components: a Type Approval Code (TAC), a Final Assembly Code (FAC), and a Serial Number (SNR). The IMEI code is secure and

physically protected against unauthorized change.

MS BSS MSC/ VLR

Channel Request

SDCCH Assignment

Service Request Authentication Ciphering Equipment Validation Call set up Handover(s) Call release

Um A

RACH

AGCH

SDCCH

1

2

3


The Equipment Identify Register (EIR) is responsible for storing the IMEI codes that identify the mobile equipment deployed in the GSM system.

STEPS IN EQUIPMENT VALIDATION PHASE At this point in time, the MS has been authenticated and the radio channel is being

encrypted. The MSC will interrogate the MS for its equipment number and checks the equipments against information in the Equipment Identify Register (EIR).

The MSC transmits a request to the MS requesting it to respond with its International Mobile Equipment Identity (IMEI) The MS upon receiving this request, reads its equipment serial number and

returns this value to the MSC The MSC then requests the EIR to check the IMEI for validity. The EIR will first

check to see if the IMEI value is within a valid range. If, so it then checks to see if the IMEI is on a suspect or know list of invalid equipment.

The EIR returns to the MSC the results of the IMEI validation. If the results are negative, the MSC might abort the call or possibly let the call continue but inform the network service provider of the event. In this scenario, well assume that the IMEI is valid.


The following figure shows the equipment validation phase.

Fig : 3 STEPS IN CALL SETUP PHASE WITH THE MS The call is setup a voice path is phase with MS created between the MS and the MSC by

allocating a radio traffic channel and a voice trunk

The MS transmits a call setup request message to the MSC/ VLR after it has ciphered the radio channel. Included in this request message are the dialed digits. The MSC, upon receiving the call setup request message, will request the VLR to supply subscriber parameters necessary for handling the call. The VLR will check for call barring conditions, such as the MS being barred from making specific outgoing call (e.g. international calls), or possibly if some supplementary services are active which prevent the call from being granted. If the VLR determines that the call cannot be processed, the VLR will provide the reason to the MSC. In this scenario, well assume that his procedure is successful. The VLR returns a message to the MSC containing the service parameters for the particular subscriber.

The MSC informs the MS that the call is proceeding.

Request for service Authentication Ciphering

MS BSS MSC/ VLR

IME I Request

Um A

SDCCH

SDCCH

4

7

5

3

IMEI Response (IMEI)

EIR

F

IMEI Check Results

Check IMEI

Call setup Handover(s) Call release

6

3

9

3


The following figure shows the call setup phase.

Fig : 4 VOICE PATH ESTABLISHMENT

The next four steps involve establishing a voice path between the MSC and the MS. The MSC allocates a trunk to the BSS currently serving the MS. The MSC sends a

message to the BSS supplying it with the trunk number allocated (TN), and requests the BSS to allocate a radio traffic channel (TCH) for the MS

The BSS allocates a radio traffic channel and transmits this assignments to the MS over the SDCCH

The MS tunes to the assigned radio traffic channel and transmits an acknowledgment to the BSS.

The BSS connects the radio traffic channel to the assigned trunk of the MSC. Since a small portion of a radio traffic channel is available for out-of-band signaling, the SDCCH is no longer used for signaling between the BSS and MS. The BSS de-allocates the SDCCH. The BSS then transmits a trunk and radio assignment complete message to the MSC.

STEPS IN CALL SETUP PHASE WITH LAND NETWORK At this point in time a voice path has been established between the MS and the MSC. The MS user hears silence since the complete voice path has not yet been established. The last

phase of setting up a mobile-originated call involves the MSC establishing a voice path from the MSC to the Public Switched Telephone Network (PSTN)

The MSC sends a network setup message to the PSTN requesting that a call be setup. Included in the message are the MS dialed digits (DD) and details specifying which trunk should be used for the call.

MS BSS MSC/ VLR

Call Setup Request

Request for service Authentication Ciphering Equipment Validation

Um A

SDCCH

SDCCH

8

9Call Proceeding


The PSTN may involve several switching exchanges before finally reaching the final local exchange responsible for applying the ringing tone to the destination phone. The local exchange will generate the ringing tone over the trunk, or series of trunks (if several intermediate switching exchanges are involved), to the MSC. At this point in time, the MS will hear ringing tone. The PSTN notifies the MSC with a network alerting message when this event occurs.

The MSC informs the MS that the destination number is being altered. Note: this is primarily a status message to the MS. The MS will hear a ringing tone from the destination local exchange through the established voice path.

When the destination party goes off-hook, the PSTN will inform the MSC of this event. This event usually triggers the beginning of billing. At this point. The MS will be connected to the destination party.

The MSC informs the MS that the connection has been established. The MS acknowledges the receipt of the connect message.

The following figure shows the call setup with land network phase.


Fig : 5

Request for Service Authentication

Ciphering Equipment validation

MS BSS MSC/ VLR

PSTN Um A

Network setup (DD ....)

Network Alerting

Connect (answer)

Start Billing

Alerting

FACCH

Connect

FACCH Connect Acknowledgement

14

15

16

17

18

19

FACCH

Handover(s) Call release


STEPS IN RELEASE PHASE MS INITIATED Under normal conditions, the termination of a call is: MS initiated or network initiated.

In this scenario, well assume that the MS initiated the release of the call. A network initiated release is illustrated in the land-to-mobile call scenario in the upcoming pages.

The mobile user initiated the release of the call by pressing the end button (the button might be labeled with a different term on the MS. The MS sends a Disconnect message to the MSC).

The other party (The PSTN party) is notified of the termination of the call by a Release message from the MSC. The end-to-end connection is terminated.

When the MSC determines that the call has no more reason to exist (no side tasks to complete, e.g. charging indication) a Release message is sent to the MS.

The MS answers back with a Release complete message. At this stage the lower connections are released (unless they are used for something else in parallel)

The voice trunk between the MSC and the BSS is released. The traffic channel is cleared. The release of the resources is completed.

The following figure shows the release phase MS initiated.


Fig : 6

Land-To-Mobile Call Scenario

The following table lists the phases of a mobile call Stage Description

Routing Analysis: the MS terminated call is routed to the visited MSC using information from the HLR and VLR.

Paging : the MSC initiates a communication with the MS. Authentication: the MSC/VLR requests the AUC for authentication

parameters. Using these parameters the MS is authenticated. Ciphering: using the parameters which where made available earlier

during the authentication the uplink and the downlink are ciphered. Equipment Validation: the MSC/VLR requests the EIR to check the

IMEI for validity. Call Setup: the MSC establishes a connection to the MS. Handover (s) Call release: The speech path is released.

MS BSS MSC/ VLR PSTN

Um A

Disconnect

Network Release

Stop Billing

Clear

Release

Release complete

Clear Complete

FACCH

FACCH

Channel Release

FACCH

20

21

22

23

24

25

FACCH

26


The following figure shows the phases of a land-to-mobile call.

Fig : 7 OPTIONAL PHASES

The authentication, ciphering, equipment validation and handover phases are optional; the service provider may decide that some of these phases might not take place in a land-

to-mobile call.

STEPS IN ROUTING ANALYSIS PHASE The following is a scenario of a mobile terminating call. It is assumed that the MS is

already registered with the system and has been allocated a Temporary Mobile Identity Number (TMSI). It is also assumed that a land subscriber dials the directory number of a

mobile subscriber and the call enters the GSM network via a Gateway MSC (GMSC).

I. The PSTN routes the call to the GMSC of this directory number, based on the Mobile Subscriber ISDN number (MSISDN)

II. The GMSC, not knowing whether this MS is roaming in its own service area or not, sends a message, with the MSISDN in it, to the HLR.

III. The HLR requests the MSC/ VLR to provide routing information about this MS. IV. The MSC/ VLR returns to the GMSC via the HLR, a director number where the MS can

be reached, which is referred to as the MS Roaming Number (MSRN) V. The call is routed from the GMSC to the visited MSC.


The following figure shows Land-to-mobile call- Routing Analysis

Fig : 8

MSC/ VLR

Relea

Paging Authentication Ciphering Equipment validation Call setup Handover (s) Call release

Incoming Call (MSISDN)

Routing info (MSRN)

Get Route (MSISDN)

Get Route (IMSI)

1

2

3

4

5 Incoming Call (MSISDN)

Pa

HLR

GMSC

PSTN

Routing info (MSRN)


STEPS IN PAGING PHASE I. The MSC uses the location area identity, provided by the VLR, to determine which

BSSs should page the MS. The MSC transmits a message to each of these BSSs requesting that a page to be performed. Included in the message is the TMSI of the MS.

II. Each of the BSSs broadcasts the TMSI of the mobile in a page message on the paging channel (PCH)

III. When a MS detects its TMSI, or IMSI, broadcasts on the paging channel, it responds with a channel request message over a common access channel. Random Access Channel (RACH)

IV. Once the BSS receives the Channel Request message, it allocates a Stand-alone Dedicated Control Channel (SDCCH) and forwards this channel assignment information to the MS over the Access Grant Channel (AGCH). It is over the SDCCH that the MS will communicate with the BSS and MSC until a traffic channel is assigned.

V. The MS transmits a page response message to the BSS over the SDCCH. Included in this message is the MS TMSI and Location Area Identification (LAI)

VI. The BSS forwards the page response message to the MSC. The MSC informs its VLR that a particular MS is responding to a page.


The following figure shows the Land-to-mobile call- Paging Phase.

Fig : 9

MS BSS MSC/ VLR

Um A

ReleaAuthentication Ciphering Equipment validation Call setup Handover (s) Call release

Routing analysis

PCH

SDCCH Assignment

Perform Page (TMSI)

Channel Request

6

7

8

9

10

AGCH

Page Response (TMSI, LAI)

SDCCH

Page Response 11

Page

RACH


AUTHENTICATION, CIPHERING AND EQUIPMENT VALIDATION PHASES The Authentication and ciphering that might be performed here to phases setup a mobile-to-land call are the same as seen before in the location update scenario. The equipment validation phase is done in the same way as in the mobile-to-land scenario.

STEPS IN CALL SETUP PHASE WITH MS The call with the mobile is setup; a voice path is created between the MS and a voice

trunk: I. After the MSC receives the Encipher complete message from the MS, the MS is

informed that a call will be setup via a setup message. II. The MS, upon receiving a setup message, performs compatibility checking before

responding to the setup message- it is possible that the MS might be incompatible for certain types of call setups. Assuming that the MS passes compatibility checking. It acknowledges the call setup with a setup confirm message.

III. The MSC selects a trunk (terrestrial channel) to the BSS. The MSC then send an assignment request message to the BSS requesting it to assign radio resources. Included in the message are attributes describing the type of radio resource to be allocated and the trunk (terrestrial channel) to be used.

IV. The BSS upon receiving an Assignment Request message allocates an appropriate radio traffic channel and transmits an Assignment Command over the SDCCH to the MS informing in to tune to a new radio channel configuration.

V. The MS tunes to the specified traffic channel and transmits an assignments complete message back to the BSS. The MS then begins alerting the user (i.e. the phone rings). Prior to this point in time, the MS user in unaware that he/she is receiving a call. The MS no longer uses the SDCCH after receiving a traffic channel assignment.

VI. The BSS upon receiving the assignment complete message connects the assigned traffic channel to the trunk (terrestrial channel) that was allocated by the MSC. The BSS places the SDCCH on a free list and transmits an assignment complete message to the MSC.

It was assumed in the past three steps that the BSS had no complications in assigning and connecting a radio traffic channel to the specified trunk. Several possible errors include: no radio resource available. Equipment failure, requested transcoding/ rate adaptation unavailable, and terrestrial resource already allocated if any of these or other errors occur, the BSS would send an assignment failure message to the MSC.


The following figure shows the Land-to-mobile call- Call Setup with MS phase

Fig : 10

MS BSS MSC/ VLR

Um A

Relea

Handover (s) Call release

Routing analysis Paging Authentication Ciphering Equipment validation

SDCCH

Assign Trunk & Radio Channel

Call Setup Confirm

SDCCH

Call Setup

Radio Assignment Complete

12

13

14

15

16

SDCCH

Assign Radio Channel

SDCCH

Trunk & Radio Assignment Complete

17


STEPS IN CALL SETUP PHASE WITH LAND NETWORK

o As discussed with the previous land figure, the MS will begin alerting the user after it receives a traffic channel assignment. Once alerting has begun, the MS sends an alerting message to the MSC.

o The MSC, upon receiving an Alerting indication from the MS, would begin generating audible ringing to the calling party and send a network alerting via the GMSC to the PSTN. Prior to this point, the calling party heard silence.

o At this point in the call, the MS is alerting the called party by generating an audible tone to the calling party. One of three events can occur; calling party hangs-up, mobile subscriber answers the phone, or the MSC times-out waiting for the mobile subscriber to answer. Since a radio traffic channel is a valuable resource, GSM does not allow a MS to ring forever.

o In this scenario it is assumed that the mobile subscriber answers the phone, the MS in response to this action stops alerting and sends a connect message to the MSC.

o The MSC removes audible to the PSTN and connects the PSTN trunk to the BSS trunk (terrestrial channel), and sends a connect message via the GMSC to the PSTN. The caller and called party now have a complete talk path. This event typically denotes the beginning of the call for billing purposes.

o The MSC sends the MS a connect acknowledgement message.

The following figure shows the Land-to-mobile Call-Call Setup with Land Network phase.


Fig : 11

MS BSS MSC/ VLR

Um A

Connect

FACCH

Network Altering

FACCH

Connect (Off - hook)

FACCH

Connect

HLR GMSC PSTN

Mobile Alerting

Relea

Connect Acknowledge

Handover (s) Call release

18

19

20

21

22

Routing analysis Paging Authentication Ciphering Equipment validation


STEPS IN RELEASE PHASE NETWORK INITIATED

The release triggered by the land user is done in a similar way as the release triggered by the mobile user.

I. 1 The MSC receives a Release message from the network to terminate the end-to-end connection.

II. This causes the sending of a Disconnect message towards the MS

III. The MS answers by a release message, the MSC release the connection to the PSTN.

IV. This is acknowledged by a Release Complete from the MSC

V. The Voice trunk between the MSC and the BSS is cleared VI. The traffic channel is released.

VII. The resources are completely released.

The following figure shows the Land-to-mobile Call- Release Network Initiate phase.


Fig : 12

MS BSS MSC/ VLR

Routing analysis Paging Authentication Ciphering Equipment validation Call setup Handover(s)

Um A

Release Complete

FACCH

Network Release

FACCH

Release Complete

FACCH

Clear Command

HLR GMSC PSTN

Disconnect

Release

Channel Release

Clear Complete

FACCH

23

24

25

26

27

28

29


Mobile-To-Mobile Call Scenario

The Mobile-to-mobile call is established using the same phases as seen earlier.

As shown on the opposite page, the mobile-to-mobile call phases can be subdivided in to two parts:

The originating mobile part where the phases are the same as those of a mobile-to-land call, except that the call setup phase is partially performed. This means that only the call setup with Mobile is done.

The terminating mobile part consist of the same phases as the land-to-mobile call scenario except again that the call setup phase performs only call setup with mobile.

ORIGINATING MOBILE The phases of an Originating mobile are:

Request for service

Authentication (Optional) Ciphering (optional) Equipment validation (optional) Call setup

Release.

TERMINATING MOBILE The phases of a Terminating mobile are:

Routing analysis

Paging

Authentication (Optional) Ciphering (optional) Equipment validation (optional) Call setup

Release.

Radio Resource Management Overview

OBJECTIVES This lesson describes the different Radio resource management aspects.

After completion of this lesson the student is able to: Describe all the Radio resource management functions:


This lesson is applicable to GSM-900, GSM-1800 and GSM-1900. When the term GSM is mentioned, it includes GSM-900, GSM-1800 or GSM 1900. Unless they are referred to as separate systems explicitly.

OVERVIEW OF RADIO RESOURCES

MANAGEMENT Radio Resource management is a group of functions concerned with the management of transmission resources on the radio path (Um interface). It must cope with limited radio resources (and the corresponding terrestrial resources) and share them dynamically between all demands.

The role of the Radio Resources management is to:

Establish stable connections between the mobile stations and the BSC (Base Station Controller)

Maintain them despite user movement for the duration of a call. Release the connections between the mobile stations and the BSC.

FUNCTIONS PERFORMED BY BSC AND MS The mobile station and the BSC mainly perform the functions of the Radio Resource management.

Radio resources Management Functions The Radio Resource management functions are:

Power Control

Handover

Discontinuous transmission

Call re-establishment

Frequency hopping.

POWER CONTROL Power control enables the mobile station and/or the BTS to increase or decrease the transmission power on a per-radio link basis. Power Control is separately performed for the uplink and downlink. In both cases the BSC is responsible for initiating Power Control; the mobile station and the BTS adopt transmit power according to the BSC Power Control commands.

MEASUREMENTS While a mobile station is active on a call, it has the responsibility of providing measurement data about the performance of the air-interface to its serving BTS so that the serving BSC can decide if a power control should be performed. Also the serving BTS measures the performance of the air-interface. Whereas the mobile station measures the performance of the downlink, the BTS measures the performance of the uplink.


DOWNLINK MEASUREMENTS The mobile station measures and reports the following measurements to the BSC regarding the Performance of the downlink. Strength of the signal being receiving from its serving BTS (in dBm) Quality of the signal being received from its serving BTS (in bit error rate). UPLINK MEASUREMENTS The BTS measures and reports the following measurements to the BSC regarding the performance of the uplink: Strength of the signal being received from the mobile station. Quality of the signal being received from the mobile station. PERIODICAL MEASURING The mobile station measures periodically the performance of the downlink, and sends the Measurements in the SACCH (Slow Associated Control Channel) via the serving BTS to the BSC every SACCH multi-frame. This corresponds to the transmission of data every 104 TDMA frames or 480 ms. The base station measures the quality of the uplink. Also, it transfers the measurements in the SACCH to the BSC every 480ms.

SIGNAL STRENGTH When the BSC notices that the signal strength of a particular radio link measured on the uplink becomes below the lower pre-defined threshold because the mobile station moves away from the BTS, it sends a Power Control Command to the mobile station to increase its transmits power (MS_TXPWR) by a pre-defined step (typically 2 dB). The transmit power of the mobile station can be increased until a maximum defined level is reached. The BSC can also send a Power Control command to the mobile station to reduce transmits power when it notices that the signal strength measured becomes above the upper pre-defined threshold. The downlink Power Control process is similar to the uplink Power Control process.


The following figure shows the uplink Power Control process.

Fig : 13 SIGNAL STRENGTH AND SIGNAL QUALITY In the description of Power Control given up to this point, the reason for Power Control is that the uplink/downlink signal strength measured is higher or lower than thresholds specified. Another reason for activating Power Control is an uplink/ downlink signal quality measured which is higher or lower than thresholds specified. In the optimum area, the area delimited by the different pre-defined thresholds. No Power Control actions are taken. If signal quality and/ or strength are beneath the specified thresholds, Power Control will increase power (indicated by +). If signal quality and/ or strength are above the specified thresholds, Power Control will reduce transmits power (indicated by -)


The following figure shows the optimum area in the uplink Power Control process.

Fig : 14 GSM FEATURE Power Control is a GSM feature that can be enabled or disabled on a per cell basis. If power Control is disabled for the mobile stations, the mobile stations will always transmit at maximum power level. The same is applicable for the BTS: if Power Control is disabled for the BTS. It will always transmit at maximum power level.

REASONS FOR POWER CONTROL One reason to enable Power Control is to save mobile station battery power. However, the main reason for Power Control is improving the carrier-to-interference ratio within the cellular network, reducing power on the BTS or the mobile station, while keeping similar signal quality received, and decrease interference caused on the other calls in the surrounding area.

HANDOVER Handover is the process of automatically switching a call in progress from one traffic channel to another to neutralize the adverse effects of user movements. The switch can be made either to a TCH within the same cell or in another cell. Usually, handovers take place on the TCH, when the call is in the speech stage. However, in rare cases it may be necessary to perform a handover when, for example, the call is still in build up stage. In that case the SDCCH will be handed over to another frequency or time slot. This type of handover is more likely to take place during transmission of short messages by the point-to-point Short Message Service (SMS)

Note that the handover process will normally only be started if power control is not helpful anymore.


The following figure shows the handover process.

Fig : 15 MEASUREMENTS To decide if a handover should be performed, the BSC receives measurements data about the performance of the air interface from its serving BTSs and mobile stations. The BSC uses the same measurements are those used in the power control process.

DOWNLINK MEASUREMENTS Quality of the signal being received from its serving BTS (in bit error rate) Signal strength of the 6 best neighboring BTS downlink control channels (candidate list). UPLINK MEASUREMENTS The BTS measures and reports the following measurements to the BSC regarding the performance of the uplink: Strength of the signal being receiving from the mobile station, Quality of the signal being received from the mobile station. Distance between the serving BTS and the mobile station (in meters)


HANDOVER PROCESS As a mobile station moves away from its serving BTS towards the coverage area of neighboring BTSs, the mobile station measurement reports will show a gradual decrease in signal strength from its serving BTS while showing an increase in measured signal strength from one or more neighboring BTSs. It is the responsibility of the serving BSC to analyze the measurement reports from the mobile station and to decide when a handover should be performed. If it is determined that there is a better BTS to serve the call, the serving BSC initiates the handover procedure.

HANDOVER TYPES The types of handover procedure executed depends on what level of switching must be performed in order to move the call from the serving BTS to the new candidate BTS. There are basically four types of handovers: Internal or intra-BSS handover, which can be:

Intra cell handover Inter-cell handover.

External or inter-BSS handover, which can be: Intra-MSC handover. Inter-MSC handover.

If the serving and candidate BTSs reside within the same BSS, the BSC for the BSS can perform the handover without the involvement of the MSC thus termed internal or intra BSS handover. This type of handover can also be sub-divided into intra-cell and inter-cell handovers. An intra-cell handover is an intra-BSS handover within the same BTS. An inter-cell handover is a handover between different BTSs. If the serving and candidate BTSs do not reside within the same BSS, then an inter-BSS handover is performed, which requires the MSC to serving BTS and the candidate BTS. This type of handover can also be divided into intra-MSC and inter-MSC handovers.

EXAMPLES OF DIFFERENT HAND OVER TYPES

Different types of handover are explained using the examples of a system consisting of two MSCs and three BSSs. Also depicted is cell coverage Areas with example Cell Global Identification codes for each BSS. Assume that the mobile and land stations are active in a cell, the call is being controlled by MSC A, and the mobile station is currently in cell area 234-01-100-51.


The following figure shows four types of handovers.

Fig : 16 INTRA-BSS INTRA CELL HANDOVER For this type of handover, the mobile station is handed over to a different radio channel within the same cell area: 234-01-100-51. This is actually an unusual type of handover, since it is not triggered by poor signal strength (if it was, the candidate base station would be different from the serving base station). A probably cause for this type of h

Date post:	22-Nov-2015
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