�Module 2 Part 1 � GSM System Overview

TEXT BOOKS

Text Books:

1. “FUNDAMENTALS OF LTE”, Arunabha Ghosh, Jan Zhang, Jefferey Andrews, Riaz Mohammed, Pearson education (Formerly Prentice Hall, Communications Engg. and Emerging Technologies) ISBN-13: 978-0-13-703311-9.
2. “Introduction to Wireless Telecommunications Systems and Network”, Gary Mullet, First Edition, Cengaga learning India Pvt Ltd., 2006, ISBN-13: 978-81-315-0559-5.

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Reference Books:

1. “Wireless Communications: Principles and Practice” Theodore Rappaport, 2^nd Edition, Prentice Hall Communications engineering and emerging Technologies Series, 2002, ISBN 0-13-042232-0.
2. “LTE for UMTS Evolution to LTE-Advanced” Harri Holma and Antti Toskala, Second Edition - 2011, John Wiley & Sons, Ltd. Print ISBN: 9780470660003.2

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� ���

• Basic services offered by GSM cellular and the frequency band of operation

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• Network component of a GSM system and the basic function of MS,BSS,NSS

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• Concept of GSM network interfaces and protocols, and their relationship to the OSI model

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• GSM channel concept.

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• Functions of the GSM logical channels

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• TDMA concept and how it is implemented in GSM.

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• Mapping of logical channels on to the GSM physical channels

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Objectives

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• GSM uses narrowband Time Division Multiple Access (TDMA) for voice and Short Messaging Service (SMS).
• What is GSM?
• GSM stands for Global System for Mobile Communication and is an open, digital cellular technology used for transmitting mobile voice and data services.

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• The GSM emerged from the idea of cell-based mobile radio systems at Bell Laboratories in the early 1970s.

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• The GSM is the name of a standardization group established in 1982 to create a common European mobile telephone standard.

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• It is widely accepted standard and is implemented globally.

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• GSM operates in the 900MHz and 1.8GHz bands in Europe and the 1.9GHz and 850MHz bands in the US.

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Introduction to GSM

� ���Introduction to GSM

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• The GSM is owning a market share of more than 70 percent of the world's digital cellular subscribers.

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• The GSM makes use of narrowband TDMA technique for transmitting signals.

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• The GSM was developed using digital technology.

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• It has an ability to carry 64 kbps to 120 Mbps of data rates.

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• Presently GSM support more than one billion mobile subscribers in more than 210 countries throughout of the world.

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Introduction to GSM

• The GSM provides basic to advanced voice and data services including Roaming service.

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• Roaming is the ability to use your GSM phone number in another GSM network.

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• A GSM digitizes and compresses data, then sends it down through a channel with two other streams of user data, each in its own time slot.

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• It operates at either the 900 MHz or 1,800 MHz frequency band

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� Introduction to GSM

Why GSM?

• The GSM study group aimed to provide the followings through the GSM:
• Improved spectrum efficiency.
• International roaming.
• Low-cost mobile sets and base stations (BSs)
• High-quality speech
• Compatibility with other telephone company services.
• Support for new services.

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• Three basic types of services offered through GSM
• Teleservices (also referred to as Telephony Services)
• Bearer services (also referred to as Data Services )
• Supplementary Services

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FIG: Relationship of teleservices and bearer services to the GSM system

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GSM Network

End User

Transmission network

End User

GSM Teleservices

GSM Bearer services

GSM Services

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� � GSM Services (cont…)

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Teleservices or Telephony Services:

• A Teleservice utilises the capabilities of a Bearer Service to transport data, defining which capabilities are required and how they should be set up.

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• Voice Calls:
• The most basic Teleservice supported by GSM is telephony.

• This includes Full-rate speech at 13 Kbps and emergency calls, where the nearest emergency- service provider is notified by dialing three digits.

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• A very basic example of emergency service is 911 service available in USA.

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Teleservices or Telephony Services:�

• Videotext
• Another group of teleservices includes Videotext access, Teletex transmission, etc.

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• Short Text Messages:
• SMS (Short Messaging Service) service is a text messaging which allow you to send and receive text messages on your GSM Mobile phone.

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• text message services - include news, sport, financial, language and location based services, stocks and share prices, mobile banking facilities.

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GSM Services (cont…)

Bearer Services or Data Services:

• Using GSM phone to receive and send data is the essential building block leading to widespread mobile Internet access and mobile data transfer.

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• GSM currently has a data transfer rate of 9.6kbps.

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• New developments that will push up data transfer rates for GSM users are HSCSD (high speed circuit switched data) and GPRS (general packet radio service).

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GSM Services (cont…)

Supplementary Services

• Supplementary services are provided on top of teleservices or bearer services, and include features such as caller identification, call forwarding, call waiting, multi-party conversations, and barring of outgoing (international) calls.

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• Multiparty Service or conferencing: The multiparty service allows a mobile subscriber to establish a multiparty conversation.

GSM Services (cont…)

• Call Waiting:
• This service allows a mobile subscriber to be notified of an incoming call during a conversation.
• The subscriber can answer, reject, or ignore the incoming call.
• Call waiting is applicable to all GSM telecommunications services using a circuit-switched connection.

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• Call Hold:
• This service allows a subscriber to put an incoming call on hold and then resume this call.

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GSM Services (cont…)

• Call Forwarding:
• The Call Forwarding Supplementary Service is used to divert calls from the original recipient to another number, and is normally set up by the subscriber himself.

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• It can be used by the subscriber to divert calls from the Mobile Station when the subscriber is not available, and so to ensure that calls are not lost.

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• A typical scenario would be a salesperson turns off his mobile phone during a meeting with customers, but does not with to lose potential sales leads while he is unavailable.

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GSM Services (cont…)

• Call Barring:
• The concept of barring certain types of calls might seem to be a supplementary sevice rather than service.

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• However, there are times when the subscriber is not the actual user of the Mobile Station, and as a consequence may wish to limit its functionality, so as to limit the charges incurred.

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• Alternatively, if the subscriber and user are one and the same, the Call Barring may be useful to stop calls being routed to international destinations when they are routed.

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• The reason for this is because it is expected that the roaming subscriber will pay the charges incurred for international re-routing of calls.

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• So, GSM devised some flexible services that enable the subscriber to conditionally bar calls.

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GSM Services (cont…)

• Number Identification: There are following supplementary services related to number identification:
• Calling Line Identification Presentation:
• This service deals with the presentation of the calling party's telephone number.
• The concept is for this number to be presented, at the start of the phone ringing, so that the called person can determine who is ringing prior to answering.

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• Calling Line Identification Restriction:
• A person not wishing their number to be presented to others subscribes to this service.
• In the normal course of event, the restriction service overrides the presentation service.

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GSM Services (cont…)

• Connected Line Identification Presentation:
• This service is provided to give the calling party the telephone number of the person to whom they are connected.
• This may seem strange since the person making the call should know the number they dialled, but there are situations (such as forwardings) where the number connected is not the number dialled. The person subscribing to the service is the calling party.

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• Connected Line Identification Restriction:
• There are times when the person called does not wish to have their number presented and so they would subscribe to this person. Normally, this overrides the presentation service.

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� � GSM Radio Frequency Carriers��

• For GSM cellular systems the air interface consists of channels that have a frequency separation of 200kHz .

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GSM frequency bands

• GSM frequency bands or frequency ranges are the cellular frequency designated by the ITU for the operation of  GSM mobile phone.

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• In GSM cellular networks, an absolute radio-frequency channel number (ARFCN) is a code that specifies a pair of physical radio carriers and channels used for transmission and reception on the Um interface, one for the uplink signal and one for the downlink signal

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• ARFCNs use a channel spacing of 200 kHz in any given GSM band. Uplink-downlink spacing is typically 45 or 50 MHz 

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GSM frequency bands

� � GSM frequency bands (cont…)��

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Uplink frequencies

Downlink frequencies

1850 MHz

15 MHz

1910 MHz

1930 MHz

5 MHz

1990 MHZ

• A and B(15Mhz) bands are for Major Trade Areas (MTAs)
• C,D,E and F(5MHz) bands are for Basic Trade Areas (BTAs)

Figure: GSM frequency allocations in the 1900- MHz PCS bands

Figure: GSM timeslot in a TDMA frame

TDMA Frame

Physical channel

(One Timeslot)

� � �2 . GSM NETWORK AND SYSTEM ARCHITECTURE (cont.…)�� ��GSM Network and System Architecture

• Figure 5.4 shows the basic system architecture for a GSM wireless cellular network.
• The GSM network can be divided into following broad parts.
• The Mobile (MS)
• The Base Station Subsystem (BSS)
• The Network Switching Subsystem (NSS)
• The Operation Support Subsystem(OSS)

1. Mobile Station (MS):

• The MS is a multifunctional device that provides the radio link between the GSM subscriber and the wireless mobile network.
• MS also have large amount of signal and data processing power.

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• In GSM, MS provides the subscriber the means to control their access to the PSTN and PDN and also to facilitate their mobility once connected to the network.
• MS constantly monitors message being broadcast from the base transceiver system (BTS) to clear radio signals used for the transmission of various subscriber traffic.
• MS is constantly performing power and bit error rate (BER) measurements on signals being received from the BTS that it is attached to and neighboring BTS in MS’s vicinity.
• These measurements along with handover algorithms performed by the BSS enable MS mobility.

• GSM also uses SIM (Subscriber Identity Module ) when inserted into the MS makes it functional
• SIM is a smart card that is issued to the subscriber when the subscriber signs up for service with the wireless network operator.
• This card contains IMSI number, mobile MSISDN number, a SIM personal identification number (PIN), security authentication parameter, address book contact information stored by the subscriber.
• The Mobile Station is made up of two entities:
• Mobile Equipment (ME) – which is phone
• Subscriber Identity Module (SIM)
• Mobile is being constantly redesigned to incorporate new features.

� � � �Base station system (BSS)�� � ��Base Station System

• The base station system (BSS) is the link between the MS and the GSM mobile-services switching center (MSC).
• Base Station Subsystem is composed of two parts :
• Base Transceiver Station (BTS)
• Base Station Controller (BSC)
• The BTS communicates with the MS over the air interface using various protocols designed for the wireless channel.
• The BSC communicates with the MSC through the use of standard wireless protocols.
• The BSC and BTS communicate with each other using LAPD protocol which is the datalink protocol used in ISDN.

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• The two elements of BSC may be physically implemented by either 2 or 3 hardware system depending upon the GSM hardware vendor.
• The BTS is BSS air interface device that corresponds to subscribers MS. It provides radio link to MS over the air interface.
• The basic components of BSS are radio transceiver units, a switching and distribution unit, RF power combining and distribution unit, an environmental control unit, a power system, processing and database storage units.
• BTS is physically located near the antenna for the cell site.
• The functional elements required by a base station controller to implement its operation may be located at single physical unit or split out into several separate units.
• The basic BSC components are input and output interface multiplexers, a timeslot interchange group switch , a subrate switch, speech coder/decoders, transcoders and rate adaptors, SS7 signal points, environmental control units, power supply and power distribution units, various signal and control processors.

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• The transcoder and rate adaptation unit is sometimes split out from the BSC to be a stand alone unit known as transcoder controller(TRC).
• Some system economies for suburban and rural areas can be gained through the separate BSCs and a shared transcoder controller.
• Urban and heavy traffic are best served by a combined BSC/TRC.

Network Switching System (NSS)

• Wireless cellular NSS provides the interface for the connection of the wireless network to other networks.
• It provides mobility for the GSM subscriber within GSM network.
• The switching system maintains databases that are used to store information about the systems subscribers and facilitate the connection of mobile to the system as long as it has connection privileges.
• The GSM switching system was designed to communicate with PSTN through ISDN protocol.

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• The basic components of the network switching system include Mobile service switching center (MSC), a gateway MSC, visitor and home local register, the equipment identity register and the authentication center. In addition flexible number register and working location register is present to provide additional functionality.
• To handle SMS, SMS gateway MSC (SMS-GMSC) and an SMS- interworking MSC (SMS-IWMSC).
• Implementation of GPRS required additional switching system elements: i) Serving GPRS support node (SGSN)
• ii) GPRS support node (SGSN)

� � � �Network Switching System (NSS)� � ��

• The MSC with database listed previously performs necessary switching functions required to route incoming mobile terminated telephone calls to correct cell site and connect mobile originated calls to the correct network. ( PSTN or PLMN)
• The MSC communicates with PSTN and other MSCs using the SS7 protocol.
• The MSC connected to PSTN is refered to as the gateway MSC (GMSC).
• MSC is instrumental in supervision and administration of mobility and connection management, authentication and encryption.
• Visitor location Register (VRL) is a temporary database used to hold information about the mobile subscribers within the coverage area of particular MSC.

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Functions of each component of NSS :

Home Location Registers (HLR)

• permanent database about mobile subscribers in a large service area(generally one per GSM network operator)
• database contains IMSI, MSISDN, prepaid/post-paid, roaming restrictions, supplementary services.

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Visitor Location Registers (VLR)

• Temporary database which updates whenever new MS enters its area, by HLR database
• Controls those mobiles roaming in its area
• Reduces number of queries to HLR
• Database contains IMSI, TMSI, MSISDN, MSRN, Location Area, authentication key

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Functions of each component of NSS :

Authentication Center (AUC)

• Protects against intruders in air interface
• Maintains authentication keys and algorithms and provides security triplets ( RAND,SRES,Kc)
• Generally associated with HLR

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Equipment Identity Register (EIR)

• Database that is used to track handsets using the IMEI (International Mobile Equipment Identity)
• Made up of three sub-classes: The White List, The Black List and the Gray List
• Only one EIR per PLMN

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Functions of each component of NSS :

Other components:

• To handle short message service (SMS) the wireless switching system will need to have an SMS gateway MSC (SMS-GMSC) and an SMS-interworking MSC (SMS-IWMSC).

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• The implementation GPRS)for high-speed data transmission and reception requires the use of two additional switching system elements: a serving GPRS support node (SGSN) and a gateway GPRS support node (GGSN). These last two units connect to IP networks

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• The flexible numbering register (FNR) is used by the GSM system to provide number portability to a subscriber. With this feature a subscriber may change GSM operators and still maintain the same MSISDN number.

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�Operations and support system(OSS) and other nodes�

• Entire GSM wireless network is monitored and controlled by an operation and support system (OSS)

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• This centralized system can be used to provide surveillance of the complete network and thus provide the operator a means to support operation and maintenance of the entire network.

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• Usually there are several sublevels to the management function that cover the circuit packet and radio network portions of the GSM network.

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• The OSS software usually provides the system operator with the ability to perform configuration performance evaluation and security management of each portion of the wireless network along with the traditional display of alarms or fault indicators for specific system elements.

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• The other nodes are: billing gateway and service order gateway

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GSM network interfaces and protocol

GSM interfaces

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• The GSM standard specifies the various interfaces between the GSM elements.
• Fig 5-5 shows these GSM interfaces.
• The air interface between the MS and the BTS is the Um interface.
• The physical interface between the BTS and the BSC is known as the Abis interface,
• Interface between the BSC and the MSC is known as the A interface.
• The MSC has various interfaces between it and the other network switching system elements or other MSCs.

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Layered Structure/OSI Model:

• The OSI model views communications between user application processes as being partitioned into self-contained layers that contain tasks that can be implemented independently tasks in other layer
• A message sent between two network nodes travels downward in the protocol stack of the sending node.
• As the message is propagated through layers, information is added to the original message at the each layer.
• After transmission to the receiving network node, the message propagated upward through the receiving node protocol stack.
• At each layer information added by the sending node is stripped off the message and analyzed.

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GSM protocols and signaling model:

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GSM network interfaces and protocols

• Figure 5.6 shows signalling model for GSM system.
• Signalling scheme used by the MS communicates with the MSC to provide system connection, mobility and radio resource management by the sending messages back and forth over the air interface from the MS to the BTS , between the BTS and the BSC, and between the BSC and the MSC .
• The Figure 5.6 indicates the various protocols that are used between the different GSM interfaces and at the different OSI layer levels.

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GSM protocols and signaling model:

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Physical layer

Datalink layer

GSM signalling

protocol layer

GSM network interfaces and protocols

GSM protocols and signaling model:�

The signalling protocol in GSM is structured into three general layers, depending on the interface.

• Layer 1: The physical layer, which uses the channel structures over the air interface.

• Layer 2: The data-link layer. Across the Um interface, the data-link layer is a modified version of the Link access protocol for the D channel (LAP-D) protocol used in ISDN, called Link access protocol on the Dm channel (LAP-Dm). Across the A interface, the Message Transfer Part (MTP), Layer 2 of SS7 is used.

• Layer 3: The third layer of the GSM signalling protocol is divided into three sublayers:

o Radio Resource management (RR)

o Mobility Management (MM) and

o Connection Management (CM).

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GSM protocols

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GSM network interfaces and protocols

GSM Interfaces

Um Air Interface

• The Layer 1 "air" or radio interface standard that is used for exchanges between a mobile (ME) and a base station (BTS / BSC).
• The layer 2 protocol used on interface is LAPDm a modified version of the ISDN protocol LAPD.
• The major difference between LAPD and LAPDm protocol are the following :For LAPDm no error detection is employed since it has been built into layer 1 signalling and LAPDm messages are segmented into shorter messages than LAPD to be compatible with TDMA frame length used in GSM.

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GSM network interfaces and protocols

• Abis-Interface
• Abis interface exists between the BSC and the BTS.
• Layer 2 protocol used on the Abis interface is LAPD.
• At layer3 level, most of the messages just pass through the BTS transparently.
• There are radio resource management messages that are closely linked to the system radio hardware that must be handled by BTS.
• The BTS Management (BTSM) entities manage these messages.
• Ex: radio resource messages involve encryption.

GSM Interfaces)

A-Interface

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GSM network interfaces and protocols

A-Interface

• The A interface is used to provide communication between the BSC and the MSC and it exist between the BSC and the MSC.
• Signaling over A interface is done according to base station signalling application part (BSSAP).
• In MSC, in the direction of MS, layer 3 is subdivided into 3 parts: Radio Resource Management (RR), Mobility management (MM), Connection Management (CM).
• The protocol used to transfer CM and MM messages is BBSAP.
• BBSAP protocols has 2 subparts: BSSAP(Base Station System Application Part) and DTAP (Direct Transfer Application Part).

�GSM network interfaces and protocols�

• DTAP is used to send CM and MM messages between the MSC and the MS transparently through BSS.
• BSSAMP Protocols is used for sending messages between MSC and BSC

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GSM Interfaces)

Ater- Interface:

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GSM network interfaces and protocols

Ater- Interface:

• It is only exists in GSM system that has separate controller and BSC.
• Signalling between the BSC and the TRC is performed by use of BSC/TRC application part (BTAP) protocol over ater interface.
• The figure 5.8 indicates how BSSAP signalling is sent transparently through the TRC node.

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MSC interfaces

• The GSM signalling model Figure 5-6 shows two protocol stacks within the MSC node.
• The protocol stack on the left-hand side is associated with the A interface.
• The right-hand protocol stack corresponds to the MSC network interfaces to the VLR, HLR, GMSC, and the PSTN or other PLMNs.
• Within the network interface stack these are the following, protocols: MTP, SCCP, TCAP. MAP. and ISUP/TUP.
• Message transfer part (MTP) is used to transport messages and for routing and addressing.
• MTP corresponds to OSI Layers I, 2, and parts of 3.

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GSM network interfaces and protocols

�GSM network interfaces and protocols�

• MSC interfaces (conti)
• Signaling connection control part (SCCP) adds functions to SS7

signalling to provide for more extensive addressing and routing.

• Together, MTP and SCCP form the network service part (NSP) and correspond to Layers 1-3 in the OSI model.
• Transfer capabilities application part (TCAP) and mobile application part (MAP) are Layer 7 protocols.
• TCAP provides services based on connectionless network services.
• MAP is a protocol specifically designed for mobile communications.
• It is used for the signalling between databases (HLR, VLR, EIR, AUC, etc.)

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�GSM network interfaces and protocols�

MSC interfaces (conti….)

• ISDN-user part (ISDN-UP) and temporary user part (TUP) are used from Layer 3 up to Layer 7 and are used between the MSC and the ISDN/PSTN for call setup and supervision.

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• There are various control and traffic channels to carry out necessary operations inorder to setup subscriber link for transmission of data and voice.
• GSM system is based on TDMA (Time Division Multiple Access) to provide additional capacity over a limited amount of radio frequency spectrum.
• As in figure 5.9 GSM system divides the radio link connection time into eight (8) equal and repeating time slots known as frames for both uplink and downlink transmissions.
• The timeslots can be considered logical channels.
• From system point of view, each timeslot may carry either subscriber traffic or signalling and control information required for the management of radio link and other system resources.

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GSM CHANNEL CONCEPT ***�

• The system can use several different types of repeating frame structure known as multiframes depending upon the type of information being transmitted.

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GSM CHANNEL CONCEPT ***�

• The logical channels may carry either subscriber traffic or signalling and control information to facilitate subscriber mobility.
• There are three types of traffic channels (TCH).
• The full rate traffic channel (TCH/F or Bm) carries one conversation by using one timeslot.
• The transmitted voice signal is encoded at a 13-Kbps rate, but it is sent with additional overhead bits.
• This information plus additional channel overhead bits yields a final channel data rate of 22.8 Kbps.
• The full rate traffic channel may also carry data at rates of 14.4, 9.6, 4.8 and 2.4 kbps.

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GSM CHANNEL CONCEPT ***

Logical Channels�

• The half rate traffic channel (TCH/H or Lm) carries voice encoded at 6.5 kbps or data at rates of 4.8 or 2.4 kbps.
• With additional overhead bit the total data rate for TCH/H becomes 11.4 kbps.
• Therefore two conversation or conversation and a data transfer or two data transfers may be transmitted over one channel at the same time.
• Enhanced full rate (EFR) traffic, encodes voice at a 12.2 kbps rate and like TCH/F adds overhead bits to yield a 22.8 kbps channel data rate. The EFR channel may also transmit data at the TCH/F rates.

• The signaling and control channels consists of three(3) channel sub-categories.
• Broadcast channel
• Broadcast control channel
• Frequency correction channel
• Synchronization channel
• Common Control channel
• Paging channel
• Random access channel
• Access grant channel
• Dedicated control Channel
• Stand alone dedicated control channel
• Slow associated channel
• Fast associated channel
• Cell broadcast channel

1) Broadcast Control Channel (BCH)

• GSM cellular system uses broadcast channels (BCHs) to provide information to the mobile station about various system parameters and also information about the location area identity (LAI).
• Three (3) types of BCH are
• Broadcast Control Channel
• Frequency Correction Channel
• Synchronization Channel
• Using the information transmitted over these three BCHs, the MS can tune to a particular base transceiver system (BTS) and synchronize its timing with the frame structure and timing in that cell.
• Each time the MS attaches to a new BTS it must listen to these three BCHs.

i) Broadcast Control Channel

• The Broadcast Control channel (BCCH) contains information that is needed by the MS concerning the cell that it is attached to in order for the MS to be able to start making or receiving calls, or to start roaming.
• The type of information broadcast on the BCCH includes the LAI, the maximum output power allowed in the cell and the BCCH carrier frequencies for the neighboring cells.
• The information is used by the MS to allow it to monitor the neighboring cells in anticipation of a possible handover operation that might be needed as the MS moves about.
• The BCCH is only transmitted on the downlink from BTS and MS .

ii) Frequency Correction Channel (FCCH)

• Frequency Correction Channel (FCCH) transmits bursts of zeros (this is an unmodulated carrier signal) to the MS.
• This signalling is done for two reasons: the MS can use this signal to synchronize itself to the correct frequency and the MS can verify that this is the BCCH carrier.
• Again, the FCCH is only broadcast on the downlink.

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Broadcast channel(BCH)( cont.…)

iii) Synchronization channel (SCH)

• SCH is used to transmit the required information for the MS to synchronize itself with the timing within a particular cell.
• By listening to the SCH, the MS can learn about the frame number in this cell and about the base station identity code (BSIC) of the BTS it is attached to.
• The BSIC can only be decoded if the BTS belongs to the GSM network.
• Again, SCH is only transmitted in the downlink direction.

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2. Common Control Channel (CCCH)

• CCCHs provide paging messages to the MS and a means by which the mobile can request a signalling channel that it can use to contact the network.
• The three CCCHs are:
1. Paging Channel (PCH)
2. Random Access Channel (RACH)
3. Access Grant Channel (AGCH)

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i).Paging Channel (PCH):

• The PCH is a downlink channel .
• PCH is used by all BTSs to send paging messages to the MS attached to the cell.
• The MS listens to the PCH at certain time intervals to learn if the network wants to make contact with it.
• The mobile will be paged whenever the network has an incoming call ready for the mobile or some type of message (e.g., short message or multimedia message) to deliver to the mobile.
• The information transmitted on the PCH will consist of a paging message and the mobile's identity number (e.g., ISMI or TMSI).

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Common Control Channel (CCCH)

ii). Random Access Channel (RACH):

• It is a Uplink channel
• It is used by the mobile to respond to a paging message(PCH).
• If the mobile receives a page on the PCH, it will reply on the RACH with a request for a signalling channel.
• The RACH can also be used by the mobile if it wants to set up a mobile originated call.
• The format of the signal sent on the RACH provides enough information to the wireless network to allow it to calculate the distance of the mobile from the BTS.
• The measured time delay is translated into TA –Timing advance that is sent to the MS.

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iii). Access Grant Channel (AGCH)

• It is downlink point to point channel
• It is used to assign signalling channel to MS.
• After the mobile requests a signalling channel over the RANCH the network will assign a channel to the mobile by transmitting this information over the AGCH.
• It is only transmitted in downlink direction.

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3)Dedicated Control Channel (DCCH)

• These dedicated channel are used for specific call setup, handover, measurement and short message delivery functions.
• The 4 DCCH are Stand-alone dedicated control channel (SDCCH), the slow associated control channel (SACCH), the fast associated control channel (FACCH) and cell broadcast channel (CBCH).

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i) Stand-alone Dedicated Control Channel (SDCCH)

• The call setup procedure is performed on the SDCCH.
• The SDCCH is transmitted in both the uplink and downlink directions.
• When the call setup procedure is complete, both the mobile and the BTS switch to a preassigned available traffic.

ii) Slow Associated Control Channel (SACCH)

• The slow associated control channel (SACCH) is used to transmit information about measurements made by the MS or instructions from the BTS about the mobiles parameters of operation.
• In the uplink direction, the mobile sends measurements of the received signal strength from its own BTS and those of neighboring BTSs.
• In the downlink direction, the MS receives information from the BTS about the mobiles output power level and the timing advance that the mobile needs to use.
• The SACCH is transmitted in both the uplink and downlink directions over the same physical channels as the SDCCH or the TCH.

iii) Fast Associated Control Channel (FACCH)

• Fast Associated Control Channel (FACCH) is used to facilitate the handover operation in GSM system.
• If handover is required, the necessary handover signaling information is transmitted instead of 20ms segment of speech over TCH.
• This operation is known as stealing mode since time allotted for the voice conversation is stolen from the system for a short period.
• The subscriber is usually not aware of this loss of speech coder in the mobile simply repeats the last received voice block during this process.

iv) Cell Broadcast Channel (CBCH)

• Cell Broadcast Channel (CBCH) is used to deliver short message service in the downlink direction.
• It uses the same physical channel as the SDCCH.

GSM Speech Processing **

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• In the mobile speech is digitized and broken up into 20ms segments.
• It is then coded to reduce the bit rate and to control errors. This process produces 8000 samples of 13 bits per sample per second or 160 samples of 13 bits per sample per 20ms.
• The speech coder yields 260 bits per 20 ms or 13 kbps whereas channel coding yields 456 bits per 20ms or a 22. kbps data rate. Interleaving, ciphering and burst formatting yields 156.25 bits per timeslot.
• This yields an overall data transfer of 270.8 kbps over a GSM channel.

• The receiver works as:
• Signal bursts are received and used to create a channel model.
• The channel model is created in the equalizer where an estimated bit sequence is calculated for a received signal.
• After all of the bursts containing information about a 20ms segment of speech have been received and deciphered, they are reassembled into the 456 bit message.
• This sequence is then decoded to detect and correct any errors that occurred during transmission.

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Time slots and TDMA Frame

• In GSM both traffic and signaling and control information are transmitted over the same physical frequency channel, TDMA is used to accomplish this.
• The physical channel of the system used for transmission of traffic are distinguished by virtue of their particular time slot with the TDMA frame and the system signaling and control information is organized interms of both the specific timeslot within the TDMA frame (multiframes).
• The relationship between timeslots and TDMA multiframes is shown in figure 5.11

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Time slot and TDMA Frames

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Time slot and TDMA Frames

TDMA Frame Structure

• In GSM, each TDMA frame divided into 8 time slots.
• The system assigns number to the frame sequentially from 0 to 2,715,648 and the repeats the process.
• The grouping of TDMA frame is known as hyperframe that takes 3 hr 28 min 53 sec and 760 millisec to complete.
• 2048 superframes (2,715,648 frames) are combined into hyperframe.
• Each superframe consists of 1326 TDMA frames that take 6.12 sec to complete.
• TDMA frames are grouped into two types of multiframes formats
• One form of superframe consists of 51 (26-frame )multiframe.
• another format has 51-frame multiframe for control channels
• Within TDMA frame there are 8 eight time slots that take approximately 4.615ms to complete

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• GSM Frame Structure
• • 1 hyperframe = 2048 superframes (~3.5hr)
• • For speech
• – 1 superframe = 51 multiframes = 6.12s
• – 1 multiframe = 26 frames = 120ms
• •For Signalling
• – 1 superframe = 26 multiframes
• – 1 multiframe = 51 frames
• • 1 frame = 8 time slots = 4.615 ms
• • 1 time slot = 156.25 bit duration = 0.577ms

Time slot and TDMA Frames

• Time slots: transmitted digital bits within the air time slot itself can take on several different format depending upon the type of information being transmitted i.e. voice, data or video
• GSM air interface time slots is as shown in figure below,

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0.577 msec or 156.25 bit periods

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• Typical burst = 0.577 msec or 156.25 bits period
• Data burst = 0.546 msec or 146bits period
• Overall bit rate = 270.8kbps
• The uplink TDMA frame and down link TDMA frames are offset by three timeslots periods
• This results:
• Longer battery life
• Easier for mobiles hardware

Implementation

• RF proper system functioning

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Data Burst-0.5 msec or 148 Bit period

• The start of the TDMA frame on the uplink is delayed by three timeslot periods from the downlink frame as shown in fig 5.14.
• The purpose of the delay is so that the same timeslot may be used on both the downlink and uplink radio paths without the need for the MS to receive and transmit at the same time.
• This extends the battery life and makes it easier for the mobiles hardware to implement the RF operations needed for proper system functioning.

GSM Timeslots Bursts

Timeslots Bursts:

• The transmission of noramal and other types of burst are as shown in the figure 5.15.

• Normal Burst

• Frequency Correction Burst

• Synchronisation Burst

• Dummy Burst

• Access Burst

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GSM Timeslots Bursts

1.Normal Burst:(uplink and downlink)

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• This GSM burst is used for the standard communications between the basestation and the mobile, and typically transfers the digitised voice data.
• The structure of the normal GSM burst is exactly defined and follows a common format. It contains data that provides a number of different functions:
• 3 tail bits:   These tail bits at the start of the GSM burst give time for the transmitter to ramp up its power
• 57 data bits:   This block of data is used to carry information, and most often contains the digitised voice data although on occasions it may be replaced with signalling information in the form of the Fast Associated Control Channel (FACCH). The type of data is indicated by the flag that follows the data field

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• 1 bit flag:   This bit within the GSM burst indicates the type of data in the previous field.
• 26 bits training sequence:   This training sequence is used as a timing reference and for equalisation. There is a total of eight different bit sequences that may be used, each 26 bits long. The same sequence is used in each GSM slot, but nearby base stations using the same radio frequency channels will use different ones, and this enables the mobile to differentiate between the various cells using the same frequency.
• 3 tail bits   These final bits within the GSM burst are used to enable the transmitter power to ramp down. They are often called final tail bits, or just tail bits.
• 8.25 bits guard time:   At the end of the GSM burst there is a guard period. This is introduced to prevent transmitted bursts from different mobiles overlapping. As a result of their differing distances from the base station.

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GSM Timeslots Bursts

2. GSM frequency correction burst(down link)

• It is used by the MS to obtain frequency synchronization

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• 3 tail bits: Again, these tail bits at the start of the GSM burst give time for the transmitter to ramp up its power.
• 142 bits all set to zero:
• 3 tail bits: Again these are to enable the transmitter power to ramp down.
• 8.25 bits guard time: to act as a guard interval.�

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3.GSM synchronisation burst (downlink)

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• The purpose of this form of GSM burst is to provide synchronisation for the mobiles on the network.
• 3 tail bits: Again, these tail bits at the start of the GSM burst give time for the transmitter to ramp up its power
• 39 bits of information:
• 64 bits of a Long Training Sequence:
• 39 bits Information:
• 3 tail bits: Again these are to enable the transmitter power to ramp down.
• 8.25 bits guard time: to act as a guard interval.

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GSM Timeslots Bursts

4.GSM random access burst (uplink)

• This form of GSM burst used when accessing the network and it is shortened in terms of the data carried, having a much longer guard period.

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7 tail bits: The increased number of tail bits is included to provide additional margin when accessing the network.

41 training bits:

36 data bits:

3 tail bits Again these are to enable the transmitter power to ramp down.

69.25 bits guard time: The additional guard time, filling the remaining time of the GSM burst provides for large timing differences.

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GSM Timeslots Bursts

5. GSM Dummy burst:

• It is transmitted on the radio frequency designated as c₀ when no other type of burst signal is being transmitted.
• The purpose of the dummy burst is to ensure that the base station is always transmitting on the frequency carrying the system information. This affords the mobile the ability to make power measurements on the strongest BTS in its location

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• 3 tail bits: Again, these tail bits at the start of the GSM burst give time for the transmitter to ramp up its power
• 58 mixed bits
• 26 bits of a Training Sequence:
• 3 tail bits: Again these are to enable the transmitter power to ramp down.
• 8.25 bits guard time

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Mapping of Logical channels to physical channel

• The system needs to be able to transmit both data ,voice and signaling and control information to the subscriber.
• The subscriber needs to be able to access the system and request radio resources to setup a call or to send data.
•  Several of the above-mentioned types of logical channels can be transmitted over one single physical channel (timeslot).
• The GSM specifications 05.02 specify several combinations of channel types(the sequence of logical channels is fixed). The order of the logical channels depends on the channel combination.
• Several logic channels combine together in some way to form some specific types of channel to transmit user data or signaling information.
• They are called combined channels. One combined channel can be mapped to a physical channel.

There are the following combined channels:

a TCH/F + FACCH/F + SACCH/TF

b TCH/H(0,1) + FACCH/H(0,1) + SACCH/TH(0,1)

c TCH/H(0,0) + FACCH/H(0,1) + SACCH/TH(0,1) + TCH/H(1,1)

d FCCH + SCH + BCCH + CCCH

e FCCH + SCH + BCCH + CCCH + SDCCH/4(0...3) + SACCH/C4(0...3)

f BCCH + CCCH

g SDCCH/8(0 ..7) + SACCH/C8(0 .. 7)

CCCH = PCH + RACH + AGCH

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Mapping of Logical channels to physical channel

• For example for proper system operation there is a standard combination of logical channel that must be transmitted during timeslot 0 of the designated downlink radio frequency channel i.e. C₀ with in the cell it is:
• FCCH + SCH + BCCH + CCCH or

FCCH + SCH + BCCH + CCCH + SDCCH + SACCH

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• As shown in above Figure the sequence of FCCH, SCH, BCCH. and CCCH repeats every fifty-one TDMA frames (a multiframe).
• The last frame of the sequence (Frame #50) is an idle frame and carries no information.
• The nine groups of four frames carrying CCCH information are called paging blocks and the one group of four frames that carry BCCH information is needed due to the large amount of overhead information transmitted by the BTS over the BCCH.
• In the uplink direction, Timeslot 0 is reserved for use by the mobile for access to the GSM system (over the random access channel or RACh.

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Mapping of Logical channels to physical channel

• The second combination of channels that includes the SDCCH and SACCH channels along with the BCHs and CCCHs (known as a combined control channel) is implemented in the GSM multiframe structure as shown in Figure below.
• In this case, one can see that only three paging blocks are present but four SDCCH and two SACCH channels are available.
• This type of channel combination (known as SDCCH/4) is effective in a rural cell where little traffic is expected to be generated.

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Mapping of Logical channels to physical channel

Transmission of Short Messages

• A cell broadcast channel (CBCH) is required for the transmission of short message service in the downlink direction. One of the SDCCH subchannels will be assigned for this purpose. Only one CBCH can be supported within a cell.
• Traffic Channels
• With the channel combinations already mentioned. typically, Timeslot 0 and Timeslot 2 was used by broadcast and control channels and the dedicated control channels.
• This leaves six timeslots i.e. TS3-TS7 free for use by traffic channels (TCH).
• Traffic channels are mapped onto physical channels (timeslots) along with a SACCH channel.
• The repetition of the TCHs and the SACCH occurs over a sequence of twenty-six TDMA frames. Figure 5-19 shows this situation. The FACCH channel may also be used in this sequence by stealing timeslots from bursts of speech as shown.

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Mapping of Logical channels to physical channel

Half-Rate Traffic Channels: The GSM system can use half-rate channels to double system capacity since two users share the same physical channel.

• Figure 5-20 shows both full-rate and half-rate frame structures. Using half-rate channels, the idle frame used in full rate will be used for the SACCH signalling for the second MS.
• Since a mobile uses only every other timeslot for a call, the multiframe will contain thirteen idle frames for each mobile. Therefore, the mobile could be allocated two traffic channels or a speech and a data channel.

Paging Groups

• The mobile will be assigned by the network to a particular paging group through an algorithm (refer to GSM TS 05.02) that uses the mobile's IMSI number and other system information.
• From system information messages, the MSs attached to a cell will receive information about the type of combined mapping of logical channels that the cell supports and the type of multiframe structure used between transmissions of paging messages to the same paging group.
• Using this information and its own IMSI number, the mobile will calculate which CCCH and which paging group that it belongs to.
• From its calculations, the MS will only listen for pages and make random accesses on a specific CCCH.
• The number of paging groups using noncombined mapping is greater than with combined mapping of the logical channels.

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Thank You