gsm basics&call flow
TRANSCRIPT
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GB_BT01_E1_0 GSM Basics
Course Objectives:
Aware of the Development Background of GSM technology
Grasp GSM Network structure and Features
State GSM main interfaces
Aware GSM common Events
Describe basic calling process
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Contents
1 GSM Basic ......................................................................................................................................................1
1.1 2G Mobile Communication Technology Evolution ......................................................................................1
1.2 Mobile Communication Technology Development Trend ...........................................................................5
1.3 GSM History ..................................................................................................................................................6
1.4 GSM Features .................................................................................................................................................7
1.5 GSM Specifications .......................................................................................................................................8
1.6 GSM Network Structure ................................................................................................................................9
1.7 GSM Protocol Platform ...............................................................................................................................12
1.8 Available GSM Services ..............................................................................................................................15
1.8.1 Telecommunications Services Provided by the GSM ................................................................15
1.8.2 Supplementary Services of the GSM System .............................................................................16
1.9 Operation Band ............................................................................................................................................17
2 GSM Events ..................................................................................................................................................20
2.1 Status of Mobile Subscriber .........................................................................................................................20
2.1.1 Attach Flag upon MS Power-on ..................................................................................................20
2.1.2 Detach upon MS Power-off .........................................................................................................21
2.1.3 MS Busy .......................................................................................................................................21
2.1.4 Periodical Registration .................................................................................................................21
2.2 Location Update ...........................................................................................................................................21
2.2.1 Normal Location Update ............................................................................................................22
2.2.2 Periodical Location Update .........................................................................................................22
2.2.3 IMSI Attach ..................................................................................................................................22
2.3 Handover ......................................................................................................................................................22
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1 GSM Basic
1.1 2G Mobile Communication Technology Evolution
Brief History of Evolution
The outline of GSM history is shown below:
1979 - Europe wide frequency band reserved for cellular
1982 - Groupe Spcial Mobile (GSM) created within CEPT
1986 Eight proposals put forward by European countries after extensive
research and experiments accepted in Paris
1988 - ETSI took over GSM Committee
1990 - The phase 1 GSM recommendations frozen
1991 - GSM Committee renamed Special Mobile Group and GSM renamed as
Global System for Mobile Communication
1992 - GSM launched for commercial operations
1993 Major part of GSM phase 2 standard completed
1994 A new research phase (Phase 2+) added to improve GSM for mobile data
services
Mobile Communication during 1920 ~ 1940
In 1920, mobile communication system was first used by military while in1940s; it was
put in use for civil purpose.
Mobile communication started flourishing in recent decade. Its development phases are
as follows:
First generation (1G) mobile communication system
Second generation (2G) mobile communication system
Third generation (3G) mobile communication system
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1G during 1980s
Since 1980's, 1G analog mobile communication system adopts cellular networkingtechnology. Till 1982 Cellular Systems were exclusively Analog Radio Technology.
At the end of 1980s Analog System was unable to meet continuing demands due to:
Severely confined spectrum allocations
Interference in multipath fading environment
Incompatibility among various analog systems
Inability to substantially reduce the cost of mobile terminals and infrastructure
required
Easy to eavesdrop and misuse the subscribers account
Standards of First Generation
Different standards of first generation are shown in Table 1.1-1.
Table 1.1-1 Different Standards of First Generation
Standard Origin Frequency Band
Advanced Mobile Phone System (AMPS) North America 800 MHz
Nordic Mobile Telephone System-
450/900 (NMT-450/900)
North Europe
(Scandinavian)450 & 900 MHz
Total Access Communication System
(TACS)U.K. 900 MHz
2G during 1990s
During 1990s, Digital mobile communication system characterized by digital
transmission, Time Division Multiple Access (TDMA), and narrowband Code Division
Multiple Access (CDMA) were developed.
Standards of Second Generation
Different standards of second generation are:
GSM
CDMAIS95
Personal Digital Cellular (PDC)
Advantages of 2G
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Compared with 1G mobile communication system, 2G mobile communication system
has the following advantages:
Provides high spectrum utilization and large system capacity.
Provides diversified services (voice services and low-rate circuit-switched data
services).
Enables automatic roaming.
Provides better voice quality.
Provides good security.
Can be interconnected with ISDN and PSTN.
Basic structure of GSM network is shown in Fig 1.1-1.
Fig 1.1-1 Basic Structure of GSM Network
Discrepancies of 2G
2G mobile communication system has the following discrepancies:
Provides low-rate data services only and cannot support multi-media service. For
example, Internet data access speed of GSM MS can reach 9.6 kbps theoretically.
Different 2G mobile communication systems in the world use different
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1.2 Mobile Communication Technology Development Trend
3G Research during 1980s
3G research, development, and establishment started in mid 1980s.
IMT-2000
International Mobile Telecommunication 2000 (IMT-2000) established by International
Telecommunications Union (ITU) introduces 3G.
IMT-2000 introduces:
Mobile data service and some fixed high-speed data services through one or more
radio channels
Fixed network platform
A global standard
IMT-2000 services, which are compatible with other fixed network services
High quality
Use of common band in the world
Small terminals used in the world
Global roaming
Multi-media services and terminals
Higher frequency utilization
Flexibility for development to the next generation
High-speed hierarchical data rate
Rate up to 2 Mbps while stationary
Rate up to 384 kbps during walking speed
Rate up to 144 kbps while in vehicle
Instead of having pure technology, communication system is currently developing
into a mode featuring the combination of services and technology.
Communication technology is estimated to undergo the largest change in future. It
is strategic transition from voice services to data services from the aspect of
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market application and service demand. This change has deeply influenced the
development trend of communication technology.
4G Services
Some researchers and telecom operators describe fourth-generation (4G) mobile
communication system as a new world better than 3G, which can provide:
Many unimaginable applications
Over 100 Mbps data transmission rate, which is 10,000 times of current MSs and
50 times of 3G MSs
High-performance multi-media contents
Service as a personal identification device through ID application
Service for high-resolution movies and TV programs, acting as bridge of
combined broadcast and new telecommunication infrastructure
Some services such as 4G wireless instant connections, are cheaper than 3G
services.
1.3 GSM HistoryBecause analog mobile communication system had limited expansion capability, Global
System for Mobile Communication (GSM) was developed on demand for capacity
expansion which achieved global success. It operates at 900 MHz band within European
countries.
GSM Development process is as follows:
1982: Conference of European Posts and Telegraphs (CEPT) formed a study
group called the Group Special Mobile (GSM) to study and develop 2G mobile
communication system.
1986: Eight proposals put forward by European countries after massive research
and experiments were accepted in Paris, and on-site experiments were performed.
1987: After on-site test, demonstration, and comparison, GSM member countries
have reached an agreement that digital system adopts narrowband Time Division
Multiple Access (TDMA), Regular Pulse Excitation-Long Term Prediction (RPE-
LTP), voice coding, and Gaussian Minimum Shift Keying (GMSK) modulation.
1998: Eighteen European countries reached GSM Memorandum of Understanding
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(MOU).
1989: GSM took effect.
1991: First GSM network was deployed in Europe.
1992: GSM standard was frozen.
1993: Major part of GSM phase II standard was completed.
1994: A new research phase (Phase 2+) was added to further improvement of
GSM as a platform of mobile data services.
1.4 GSM Features
GSM system has the following features:
High Spectrum efficiency
GSM system features high spectrum efficiency due to the high-efficient modulator,
channel coding, interleaving, balancing, and voice coding technologies adopted.
Large capacity
Volumetric efficiency (number of channels/cell/MHz) of GSM system is three to five
times higher than that of Total Access Communication System (TACS).
High voice quality
Digital transmission technologies and GSM specifications, voice quality is irrelevant
with radio transmission quality.
Open interfaces epic
GSM standard provides open air interface, also open interfaces between networks and
those between network entities, such as A interface and Abis interface.
High security
MS identification code encryption makes eavesdropper unable to determine the MS
number, ensuring subscribers location security. Voice encryption, signaling data, and
identification codes make the eavesdropper unable to receive the communication
contents.
Interconnection with Integrated Services Digital Network (ISDN) and PSTN.
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GSM can interconnect with other networks through current standard interfaces, such as
Integrated Service User Part (ISUP) or Telephone User Part (TUP).
Roaming function
GSM supports roaming by introducing Subscriber Identity Module (SIM) card that
separates subscriber from the terminal equipment.
Diversified services
GSM provides diversified services, tele-services, bearer services, and supplementary
services.
Inter-cell handover
During conversation, MS continues to report the detailed radio environment of local cell
and neighboring cells to serving base station. If inter-cell handover is required, MS sends
a handover request to serving base station.
1.5 GSM Specifications
European Telecommunications Standards Institute (ETSI) initiated and made GSM
standard.
ETSI developed GSM in several phases and set up more Special Mobile Groups (SMG)
to make the related GSM standard.
GSM detailed specifications conform on functions and interfaces only, not on hardware.
Purpose is to reduce the restriction on designers, enabling the operators to purchase
equipment from different manufacturers.
GSM technical specifications consist of 12 fields:
Field 1: General
Field 2: Services
Field 3: Network Functions
Field 4: MS-BS Interfaces and Protocols
Field 5: Physical Layer on Radio Path
Field 6: Speech Coding
Field 7: MS Terminal Adaptor
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for voice and data.
NSS Main components are:
Mobile Switching Centre (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Equipment Identification Register (EIR)
Authentication Centre (AUC)
Short Message Centre (SMC)
Home Location Register - HLR is a central database of a system. HLR stores all the
information related to subscribers, including the roaming authority, basic services,
supplementary services, and current location information. It provides routing information
for MSC for call setup.HLR may cover several MSC service areas or even the whole
PLMN.
Visitor Location Register - VLR stores all subscriber information in its coverage area
and provides call setup conditions for the registered mobile subscribers.As a dynamic
database, VLR must exchange large volume of data with HLR to ensure data validity.
When an MS leaves the controlling area of a VLR, it registers in another VLR. The
original VLR deletes the temporary records of that subscriber.VLR integrated within
MSC.
Equipment Identification Register - EIR stores the parameters related to MS. It can
identify, monitor, and block the MS. ERI preventing unauthorized MS from accessing the
network.
Authentication Centre - AUC is a strictly protected database that stores subscriber
authentication information and encryption parameters. AUC integrated with HLR
physically.
Base Station Subsystem BSS serves as a bridge between NSS and MS. It performs
radio channel management and wireless reception and transmission. Base Station
Controller (BSC) and Base Transceiver Station (BTS) are main components of BSS.
Base Station Controller - Located between MSC and BTS, it controls and manages
more than one BTS. It performs radio channel assignments. BTS and MS transmit power
control, and inter-cell handover. BSC is also small a switch that converge and connects
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local network with the MSC through A interface. Abis interface connects BTS to BSC.
Base Transceiver Station - BTS is wireless transceiving equipment controlled by theBSC in BSS. BTS carries radio transmission. It performs wired-related wireless
conversion, radio diversity, radio channel encryption, and hopping. Um interface
connects BTS to MS.
Transcoding and Rate Adaptation Unit - TRAU Located between BSC and MSC,
TRAU transcodes between 16 kbps RPE-LTP codes and 64 kbps A law PCM codes.
Operation and Maintenance Subsystem OMS is operation & maintenance part of
GSM. Functional units in GSM are connected to OMS internal networks. OMS monitors
various functional units in GSM network, submits status report, and performs fault
diagnosis.
OMS consists of two parts: OMC System (OMC-S) and OMC-Radio (OMC-R). The
OMC-S performs operation and NSS maintenance, while OMC-R performs operation
and BSS maintenance.
Mobile Station
MS is subscriber equipment in GSM, it can be vehicle installed or hand portable. MS
consists of mobile equipment and SIM.
Mobile equipment processes voice signals, receives and transmits radio signals.
SIM stores all information required for identifying a subscriber and security information,
preventing unauthorized subscribers. Mobile equipment cannot access GSM network
without a SIM card.
Network Service Area
GSM service area refers to the total area covered by networks of all GSM operators.
Network consists of several MSC service areas, each of which consists of several cells.
Logically, several cells form a location area (LA).
MSC Service Area - A Public Land Mobile Network (PLMN) includes multiple MSC
service areas. MSC service area refers to the MSC coverage area, that is, the total area
covered by BTS under control of BSC connected to MSC. All MSs in the service area
table register in local VLR. Therefore, in actual network, MSC is always integrated with
VLR as a node.
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Location Area - Each MSC/VLR service area includes multiple of LAs. MS can move
freely without performing location update in LA. Hence, LA is the paging area of a
broadcast paging message.An LA belongs to one MSC/VLR only, that is, LA cannot
cross MSC/VLR.The system can identify different LA via LA Identity (LAI).
Cell - LA contains several cells. Each cell has a unique Cell Global Identification (CGI),
which indicates a basic radio coverage area in a network.
Fig 1.6-4shows the relationship among different coverage areas in a GSM network.
GSM service area
The total network coverage provided by all GSM operatorsPLMN service area
The network coverage provided by a GSM operator
MSC service areaThe area controlled by an MSC
Location areaAn area for location update and paging
CellA service area provided by a
specific BTS
Fig 1.6-4 Relationship among Coverage Areas in a GSM Network
1.7 GSM Protocol Platform
GSM technical specifications make clear and normative definition of interfaces and
protocols between subsystems and various functional entities. Interface refers to the
point where two adjacent entities are connected. Protocol defines the rules for
information exchange at the connection point.
GSM Interfaces
Fig 1.7-5 shows the GSM interfaces.
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MS BTS BSC MSC
VLR VLR
HLR
MSC
EIR
Sm Um Abis A
B
D
C
E F
G
Fig 1.7-5 shows the GSM interfaces.
Sm Interface: Man-machine interface implemented in MS. It is an interface between
subscribers and PLMN. MS consists of keyboard, LCD, and SIM card.
Um Interface: Radio interface between MS and BTS. It is an important interface in
PLMN. Digital mobile communication network has different radio interface as compared
to analogue mobile communication network.
A Interface: It is an interface between BSC and MSC. Base station management
information, call processing interface, mobility management information, and specific
communication information are transferred through A interface.
Abis Interface: It is an interface between BSC and BTS. Supports all services provided
to subscribers. Also supports the control of BTS radio equipment and management of
radio resources assigned.
B Interface: It is an interface between MSC and VLR.VLR is a database locating and
managing MS when MS roams in the related MSC control area. MSC can query the
current location of MS from VLR and update MS location.When subscriber uses a
special supplementary service or changes a relevant service, MSC notifies the VLR.
Sometime VLR also updates information in HLR.
C Interface: It is an interface between MSC and HLR. C interfacetransfers management
and route selection information.When a call finishes, MSC sends the billing information
to HLR.When PSTN cannot get location information of a mobile subscriber, the related
GMSC queries HLR of the subscriber to obtain the roaming number of the called MS,
and then transfers it to the PSTN.
D Interface: It is an interface between HLR and VLR. Exchanges MS location
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information and subscriber management information. To enable a mobile subscriber to
originate or receive calls in the whole service area, data must be exchanged between
HLR and VLR. VLR notifies HLR about the current location of MS belonging to HLR,
and then provides MS roaming number. HLR sends VLR all the data required to support
the services of the MS.When an MS roams to the service area of another VLR, HLR
notifies the previous VLR to delete the relevant MS information. When MS uses
supplementary services, or some parameters are changed, D interface is also used to
exchange the related information.
E Interface: It is an Interface between MSCs. It exchanges the handover information
between two MSCs.When MS in a conversation moves from one MSC service area to
another MSC service area, inter-cell handover occurs to maintain the conversation. At
that time, related MSCs exchange the handover information through E interface.
F Interface: It is an interface between MSC and EIR. It exchanges the MS management
information, such as IMEI, between MSC and EIR.
G Interface: It is an interface between VLRs. When MS uses a Temporary Mobile
Subscriber Identity (TMSI) to register with a new VLR, the relevant information is
exchanged between VLRs through G interface.This interface also searches IMSI of the
subscriber from VLR that registers TMSI.
GSM Protocol Structure and OSI
2G cellular mobile network GSM adopts Open System Interconnection (OSI) model to
define its protocol structure. Fig 1.7-6 shows GSM interface protocol model, which
defines the interfaces and protocols between MS and MSC.
Um interface Abis interface A interface
CM
MM
RRM
LAPDm
Radio
CM
MM
RRM
MTP
64
kbit/s
RRM
LAPDm
Radio
LAPD
64
kbit/s
RRM
LAPD
64
kbit/s
MTP
64
kbit/s
SCCP SCCP
MS BTS BSC MSC
Fig 1.7-6 GSM Interface Protocol Model
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OSI reference model is a hierarchical structure. According to the hierarchy concept,
communication process can be divided into several logical layers from lowest to highest
layer. In different systems, the entities in the same layer that exchange information for
the same purpose are called peer entities. Entities in adjacent layers interact with each
other through the common layer. The lower layers provide services to higher layers. The
services provided by layer N is a combination of the services and functions provided by
the layers below it.
First layer of Um interface protocol is physical layer, which is marked as L1 and it
is a lowest layer. L1 provides basic radio channels for the information
transmission of higher layers.
Second layer L2 is data link layer, which is marked as LAPDm. It covers various data
transmission structures and controls data transmission.
Application layer is the third highest layer L3. It covers various messages and programs,
and controls services. L3 includes Radio Resource Management (RRM), Mobility
Management (MM) and Call connection Management (CM).
Abis interface protocol is slightly different from Um interface protocol. Its
physical layer is 64 kbps land line, and link layer is LAPD.
First layer of A interface protocol is 64 kbps land line, and second layer is the
Message Transfer Part (MTP), which is part of Common Channel Signalling7
(CCS7) network. MTP consists of many network protocols and centralizes all
link layer protocols. Signaling connection control part (SCCP) and MTP together
represent a network layer protocol on A interface.
In BSC both MM and CM are transparently transmitted
1.8 Available GSM Services
1.8.1 Telecommunications Services Provided by the GSM
1. Circuit Services
1) Voice Service
Full-rate voice service
Half-rate voice service
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Enhanced full-rate voice service
2) Data service
14.4Kbit/s full-rate data service
9.6Kbit/s full-rate data service
4.8Kbit/s full-rate data service
2.4Kbit/s full-rate data service
2. SMS services(support Chinese short messages)
1) Point-to-point short message service
Point-to-point short message service with the mobile user serving as called
Point-to-point short message service with the mobile user serving as caller
2) Cell Broadcast Short Message
Cell broadcast service originated from the SMS center or the OMC-R.
3. Packet Services
1) GPRS service
2) EDGE service
At present, the point-to-point interactive telecom services are supported,
including:
Access to the database: Allocate service to users as needed, e.g. Internet, and
provide storing and forwarding, as well as information processing for user-to-user
communications.
Session service: Provide bi-directional user-to-user and port-to-port real time
information communication, e.g. Internet Telnet service.
Tele-action service: Applicable to small-volume data processing services, credit
card confirmations, lottery transactions, electronic monitoring, remote meter
reading (water, electricity and gas), monitoring systems, and so on.
1.8.2 Supplementary Services of the GSM System
GSM supplementary services are diversified, including:
Call Forwarding Unconditional: forward all incoming calls to the number specified by
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the subscriber.
Barring: barring of outgoing/coming calls.
Call Waiting: When a call is connected for a subscriber, indication of a new coming call
is given to the subscriber. The subscriber can accept, reject or ignore the waiting call.
Call Hold: A subscriber can suspend the connected call to do other things.
Multiparty Service: A simultaneous communication with up to six parties is allowed.
Closed User Group: The subscribers of CUG are restricted from outgoing and incoming
calls, but they can normally communicate with each other.
Hot Billing: The network generates an instant call billing message from the billing
manager. It is applicable to leased phone service, including all kinds of call modes. Bills
are generated and presented to the subscriber immediately after the call is ended.
1.9 Operation Band
1. Working band
Currently, the GSM communication system works at 900 MHz, extended 900
MHz and 1800 MHz, or 1900 MHz band in some countries.
1) 900 MHz band
Uplink (MS transmitting and BS receiving) frequency range: 890 MHz ~ 915
MHz
Downlink (BS transmitting and MS receiving) frequency range: 935 MHz ~
960MHz
2) Extended 900 MHz band
Uplink (MS transmitting and BS receiving) frequency range: 880 MHz ~ 915
MHz
Downlink (BS transmitting and MS receiving) frequency range: 925 MHz ~ 960
MHz
3) 1,800 MHz band
Uplink (MS transmitting and BS receiving) frequency range: 1,710 MHz ~ 1,785
MHz
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Downlink (BS transmitting and MS receiving) frequency range: 1,805 MHz ~
1,880 MHz
4) 1,900 MHz band
Uplink (MS transmitting and BS receiving) frequency range: 1,850 MHz ~ 1,910
MHz
Downlink (BS transmitting and MS receiving) frequency range: 1,930 MHz ~
1,990 MHz
2. Channel interval
The interval between two adjacent channels in any band is 200 kHz.
3. Channel configuration
All channels are configured with the same interval.
1) 900 MHz band
The channel numbers are in the range of 1 ~ 124. There are 124 frequency bands in
all.
The relationship between a channel number and nominal central frequency of a
frequency band is illustrated as follows:
Fu (n) = 890 + 0.2 n-512 (MHz), uplink
Fd (n) = Fu (n) + 45 (MHz), downlink
Where, 1 n 124, n is a channel number, or an Absolute Radio Frequency
Channel Number (ARFCN).
2) Extended 900MHz band
The channel numbers are in the range of 0 ~ 124 and 975 ~ 1023. There are 174
frequency bands in all.
The relationship between a channel number and nominal central frequency of a
frequency band is illustrated as follows:
Fu (n) = 890 + 0.2 n (MHz), 0 n 124
Fu (n) = 890 + 0.2 (n-1024) (MHz), 975 n 1023
Fd (n) = Fu (n) + 45 (MHz)
3) 1,800 MHz band
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The channel numbers are in the range of 512 ~ 885. There are 374 frequency bands
in all.
The relationship between a channel number and nominal central frequency of a
frequency band is illustrated as follows:
Fu (n) = 1710.2 + 0.2 (n-512) (MHz)
Fd (n) = Fu (n) + 95 (MHz)
512 n 885
4) 1,900 MHz band
The channel numbers are in the range of 512 ~ 811. There are 300 frequency bands
in all.
The relationship between a channel number and nominal central frequency of a
frequency band is illustrated as follows:
Fu (n) = 1850.2 + 0.2 (n-512) (MHz)
Fd (n) = Fu (n) + 80 (MHz)
512 n 811
4. Duplex transceiving interval
1) 900 MHz band
The duplex transceiving interval is 45 MHz.
2) Extended 900 MHz band
The duplex transceiving interval is 45MHz.
3) 1,800 MHz band
The duplex transceiving interval is 95 MHz.
4) 1,900 MHz band
The duplex transceiving interval is 80 MHz.
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original LAI stored in the SIM card. VLR only adds Attach flag to this subscriber.
2.1.2 Detach upon MS Power-off
After the MS is powered off, the MS sends a Detach Request to the MSC. After the MSC
receives the request, it informs VLR to add the Detach flag to IMSI of this MS. At this
time, HLR does not receive the notice indicating that this subscriber is detached from the
network. After this subscriber is paged, the HLR requests the MSRN from the
MSC/VLR. At this time, the VLR informs the HLR that this MS is powered off.
2.1.3 MS Busy
In this case, the MS is allocated with a traffic channel to transmit the voice or data and
the IMSI of the subscriber is marked as Busy.
2.1.4 Periodical Registration
When the MS sends the IMSI Detach message to the network, it is possible that the GSM
system cannot decode properly due to the poor radio quality or other reasons and still
believes that MS is in Attach status. Or when the MS is powered on but has roamed
beyond the service coverage, i.e., a blind area, the GSM system does not know it and still
believes that the MS is in Attach status. In both cases, if the subscriber is paged, the
system will keep sending paging messages, wasting radio resources.
To solve the above problems, the measure of forced registration is taken in the GSM
system: The MS must make registration at a regular interval. This is called periodical
location update. If the GSM system does not receive the periodical registration
information of the MS, the VLR where the MS resides records the Implicit Detach status
of the MS. When the correct periodical registration information is received again, the
status is changed into Attach status.
2.2 Location Update
When the MS changes the location area, it finds out that the LAI in its SIM card is
inconsistent with the LAI received. Thus, it registers the location information. This flow
is called location update. Location update is originated by the MS. There are three type
of location update:
Normal Location Update.
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2.3.2 Classification of Handover
According to the scope of handover , it can be divided into the following types
1. Intra-cell hand-over
2. Inter-cell hand-over
3. Inter-BSC hand-over of same MSC
4. Inter-MSCs hand-over
According to the synchronous relationship between MS and BTS when handover
happens, there are three type of handover:
1. Synchronous: MS use the same TA both in destination and target cell. This usually
applies to hand-over of same cell or different sectors within the same cell. This is
the hand-over with highest speed.
2. Asynchronous: MS dont know the TA to be used in target cell. When either of
the two cells doesnt synchronize with BSC, this mode should be used. The hand-
over speed is low.
3. Pseudo-synchronous: MS is able to calculate out the TA it should use in the target
cell. When both cells have synchronized with BSC, this mode may be used. The
hand-over speed is fast.
2.4 Cell selection and Reselection
2.4.1 Cell selection
After a MS is turned on, it will attempt to contact a common GSM PLMN, so the MS
will select an appropriate cell, and extract from it the parameters of the control channel
and the prerequisite system information. Such a selection process is referred to as cellselection. The quality of a radio channel is an important factor of cell selection. The
GSM specification defines the path loss criterion C1, and such appropriate cell must
ensure that C1>0. The C1 is calculated according to the following formula:
C1=RXLEV-RXLEV_ACCESS_MIN-MAX((MS_TXPWR_MAX_CCH-P), 0)
Where:
The RXLEV is the average reception level.
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The RXLEV_ACCESS_MIN is the minimum level at which the MS is allowed to
access.
The MS_TXPWR_MAX_CCH is the maximum power level of the CCH.
The P is the maximum transmitted power of the MS.
MAX (X, Y) = X; If X Y.
MAX (X, Y) = Y; If Y X.
After the MS selects a cell, it will stay in the selected cell if no major changes have
occurred to various conditions.
2.4.2 Cell reselection
After a MS selects a cell, the MS will stay in the selected cell as long as no major
changes occur to various conditions. At the same time, the MS starts to measure the
signal level of the BCCH carrier of the adjacent cells, records the six adjacent cells with
the highest signal levels, and extracts from them the various system messages and
control messages of each adjacent cell. When the appropriate conditions are met, the MS
will switch from the current cell to another cell, a process known as cell reselection.
Such appropriate conditions include multiple factors, including cell priority, and whether
the cell is prohibited from access. Among them, an important factor is the quality of the
radio channel. When the signal quality of the adjacent cell exceeds that of the current
cell, cell reselection is triggered. For cell reselection, the channel quality criterion is
determined by the C2 parameter, which is calculated according to the following formula:
2.5 Authentication
Fig. 2.5-1 shows the authentication process, where RAND is the question asked by the
network side and only the legal subscriber can give the correct answer SRES.
RAND is generated by the random number generator of the AUC on the network side. It
is 128 bits in length. The value of RAND is obtained in a random manner from the range
of 0~21281.
SRES is called a signed response. It is obtained through the calculation of subscribers
unique key parameter Ki. It is 32 bits in length.
Ki is stored in the SIM card and AUC in a very confidential way. Even the subscribers do
not know their own Ki. Ki can be of any format and any length.
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A3 algorithm is the authentication algorithm determined by the carrier. It is also
confidential. The only restriction of the A3 algorithm is the length of the input parameter
(RAND is 128 bits in length) and the size of the output parameter (SRES must be 32
bits).
Mobile Terminal Network
A3 algorithm
Random number generatorKi RAND
SRES'
SRES
Ki
A3 algorithm
Fig. 2.5-1 Authentication Process
2.6 Encryption
In the GSM, the position of encryption and decryption over the transmission link allows
the transmitting data in all dedicated modes to use the same protection method. The
transmitting method can be the subscriber information (such as voice and data),
subscriber-specific signaling (such as message carrying the called number), or even the
system-specific signaling (such as the message carrying radio measurement result for the
handover).
Encryption and decryption are the exclusive or operation (this algorithm is called the A5
algorithm) of 114 radio burst pulse code bits and one 114-bit encryption sequence
generated by a special algorithm. To obtain each burst encryption sequence, A5
calculates on two inputs: One is the frame number and the other is the key (Kc) agreed
upon by the MS and network, as shown in Fig. 2.6-2. Two different sequences are used
over the uplink and downlink. For each burst, one sequence is used for the encryption
inside the MS and meanwhile used as the decryption sequence in BTS. The other
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sequence is used for the encryption of BTS and meanwhile used as the decryption
sequence in MS.
A5
Frame No.
(22-bit)Kc (64-bit)
A5
S1
(114-bit)
S2 S1 S2
MS BTS
Frame No.
(22-bit)Kc (64-bit)
(114-bit) (114-bit) (114-bit)
Fig. 2.6-2 Encryption Algorithm
1. Frame number: Frame number is encoded into a serials of three values, which are
22 bits in total.
Frame number of each burst varies with the type of radio channel. Each burst
dedicated for communication on the same direction uses different encryption
sequence.
2. A5 algorithm
A5 algorithm must be defined in the global range. This algorithm can be describes
into the two 114-bit sequence black boxes generated by a 22-bit parameter (frame
number) and a 64-bit parameter (Kc).
3. Kc
Before the encryption, Kc must be agreed upon by both the MS and network. In
the GSM, the Kc is calculated during the authentication and then stored in the
SIM card permanently. On the network side, this potential key is also stored in the
visited MSC/VLR and ready for use in the encryption.
The algorithm that uses the RAND (same with the one used for authentication)
and Ki to calculate the Kc is called A8 algorithm. Like the A3 algorithm that
calculate the SRES using RAND and Ki, the A8 algorithm also needs to be
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determined by the carrier.
Fig. 2.6-3 shows how the Kc is calculated.
Mobile Terminal Network
A8 algorithm
Random number
generatorKiRAND
Kc
Ki
Kc
A8 algorithm
Fig. 2.6-3 Kc Calculation Method
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3 GSM basic calling process
Though basically similar, the call process of mobile subscribers and that of ordinary
fixed subscribers are different in the following aspects:
Before a mobile subscriber originates a call, he should first input the number, ensure that
no modification is needed, and then send the call.
Before the number is sent out and the call is connected, there is some additional
information that should be transferred between mobile stations (MS) and the network.
Such operations are automatically performed by the equipment, with no need for user
interference, but it results in a certain delay.
3.1 Initialization
Initialization is a random access process. It starts from MS which sends a channel
request message on RACH.
After receiving this message, BTS notifies BSC, and attaches BTSs estimation of the
transmission delay (TA) from this MS to BTS and the cause for the current access.
BSC will select an idle and dedicated channel SDCCH to notify BTS to activate it
according to the access cause and the current information.
Access causes mainly include: location updating; response to a paging call; and
subscriber service application, such as a call, sending one short message.
After BTS completes the activiation of the designated channel, BSC sends the
immediate allocation message on AGCH via BTS, including the description of the
SDCCH channel assigned by BSC to MS, TA, the maximum initialization transmission
power and the access random reference value.
When MS correctly receives its initialized allocation, it will, according to channel
description, adjust itself to this channel, set up a signaling transmission link, and send the
first initialized message on the dedicated channel, including subscribers identification
number (such as IMSI), cause for the current access, registration, and authentication. If
BSC has no idle channels to allocate, BSC will send to MS the immediate allocation
rejected message.
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3.2 Location update
(1) MS moves from one area (belonging to the coverage of MSC-B) to another area
(belonging to the coverage of MSC-A).
(2) By detecting the broadcasting information sent persistently by the base station BS,
MS finds out that the newly received location area identification is different from the
currently used location area identification.
(3)(4) MS sends the location updating request with the message of Im here via this
base station to MSC-A.
(5) MSC-A sends the location updating message that contains the MSC-A identifier and
MS identification number to HLR (the authentication or encryption calculation process
will start from here, though not shown in the diagram).
(6) HLR sends back the response message, including all the related subscriber data.
(7)(8) Subscriber data registration in the visited VLR.
(9) Sending related location updating response message via the base station to MS (if
TMSI is re-assigned, it is sent together to MS).
(10) Notifying the original VLR to delete subscriber data related to this MS.
3.3 Outgoing call flow from MS to PSTN
(1) Within the service cell, once the mobile subscriber dials, the mobile station will
request the base station for the random access channel.
(2) The setup process to set up signaling connection between the mobile station MS and
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the mobile service switching center MSC.
(3) Authentication of the mobile stations identification number; if encryption is needed,then it sets the encryption mode and enter the call setup starting phase.
(4) Service channel allocation
(5) Adopting the No.7 signaling user part ISUP/TUP to set up a channel from the fixed
network (ISDN/PSTN) to the called subscriber, send ringing to the called subscriber, and
send back the call connection acknowledgment signal to the mobile station.
(6)The called subscriber offhooks to reply, in which case a response (connection)
message is sent to the mobile station, thus entering the ultimate call session phase.
3.4 Incoming Call Flow from PSTN to MS
(1) Through the No.7 signaling user part ISUP/TUP to enter MSC(GMSC) and
receive a call from the fixed network (ISDN/PSTN).
(2) GMSC requests HLR for the MSC address (i.e., MSRN) visited by the related
called mobile subscriber.
(3) HLR requests the visited VLR to assign MSRN which is assigned and notified by
VLR to HLR in each call.
(4) After GMSC obtains MSRN from HLR, it can re-search for routes to set up
connection to the visited MSC.
(5)(6) The visited MSC obtains related subscriber data from VLR.
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(7)(8) MSC sends paging messages to the mobile station through all base
stations( BS) in the location area.
(9)(10) The mobile station of the called mobile subscriber sends back the paging
response messages, then carries out the same steps of (1), (2), (3), (4) as shown
in the above outgoing call flow till the mobile station rings,
then sends back the call connection acknowledgment signal (omitted in the diagram) to
the calling subscriber.
(11) The mobile subscriber offhooks to answer, thus the response (connection)
message is sent back to the fixed network to signal calling and called parties enter
final call session.
3.5 Call Flow Between Two Mobile Subscribers
MS1 is served by MSC1/VLR1, and MS2 is served by MSC2/VLR2 and belongs to
HLR/AUC.
1. MS1 dials the phone number of MS2. BSS informs MSC1 of the call.
2. MSC2 analyzes the phone number of MS2, finds out the home HLR of MSC2 and
sends the route application to HLR.
3. HLR queries the current location information of MS2 and obtains the
MSC2/VLR2 that serves the MS2. HLR requests the route information from the
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MSC2/VLR2.
4. MSC2/VLR2 allocates the route information, that is, MSRN and submits theMSRN to the HLR.
5. HLR sends the MSRN to the MSC1.
6. MSC2 sets up the call with MSC2 according to the MSRN.
7. MSC2/VLR2 sends the paging message to MS2.
8. MSC2/VLR2 receives the message, indicating the access of MS2 is allowed.
9. The call between MSC2 and MSC1 is set up.
10. MSC1 sends the successful connection signal to MS1. MS1 and MS2 can talk
over the phone.
Fig. 3.5-4 shows the call flow.
Fig. 3.5-4 Call Flow