cellular mobile systems and services 07
TRANSCRIPT
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GSM ARCHITECTURE
A GSM system is basically designed as a combination of three major subsystems: the network
subsystem, the radio subsystem, and the operation support subsystem. In order to ensure that
network operators will have several sources of cellular infrastructure equipment, GSM decided
to specify not only the air interface, but also the main interfaces that identify different parts.
There are three dominant interfaces, namely, an interface between MSC and base transceiver
station, and an Um interface between the BTS and MS.
GSM Network Structure
Every telephone network need a well designed structure in order to route the incoming call to
the correct exchange and finally to the called subscriber. In a mobile network this structure is
of great importance because of the mobility of all its subscribers. In the GSM system the
network is divided in to the following partitioned areas.
y GSM service area
ell
Location Area
M
S
C
PL
M
N
GSM service Area
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y PLMN service area
y MSC service area
y Location area
y Cells
The Mobile Station is carried by the subscriber; the Base Station Subsystem controls the radio
link with the Mobile Station. The Network Subsystem, the main part of which is the Mobile
services Switching Center, performs the switching of calls between the mobile and other fixed
or mobile network users, as well as management of mobile services, such as
authentication. Not shown is the Operations and Maintenance center, which oversees the
proper operation and setup of the network. The Mobile Station and the Base Station
Subsystem communicate across the Um interface, also known as the air interface or radio
link. The Base Station Subsystem communicates with the Mobile service Switching Center
across the A interface.
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The above figure depicts a typical GSM network (called, Public Land Mobile Network or
PLMN) infrastructure.
AUC- Authentication Center
BSC -Base Station Controller
BSS -Base Station Subsystem
BTS -Base Transceiver System (Antenna System + Radio Base Station)
EIR -Equipment Identification Register (for IMEI verification)
IMEI -International Mobile Equipment Identity
FNR -Flexible Numbering Register (for number portability)
GMSC- Gateway MSC
HLR -Home Location Register
ISDN -Integrated Services Digital Network
IWF -Interworking Function
ILR -Interworking Location Register (for roaming between AMPS and GSM system)
IWMSC -Interworking MSC
MS -Mobile Station
MSC- Mobile Switching Center
NSS -Network Switching Subsystem
OSS -Operation and Support System
PDN -Public Data Network
PSTN -Public Switched Telephone Network
SMS -Short Message Service
VLR -Visitor Location Register
The GSM divides the infrastructure into the following three parts.
y Network Switching Subsystems (NSS)
y Base Station Subsystem (BSS)
y Network Management Subsystem (NMS)
If we count the Mobile Station (MS) or cell-phone is the 4th element.
The figure below depicts only the basic elements of the network architecture.
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.
Any telecommunications network requires some kind of NMS. A part of NMS is generic for
any telecom system. The billing and messaging are two examples. The core of the NSS is the
MSC (Mobile Switching Center) which is basically a PSTN switch with mobility management
related enhancement/add-on. The BSS is entirely new (compared to PSTN) that are required
for wireless access and mobility. The following sections of this document provide an overview
of the network elements and their functions. The role of these elements will be clearer as we
learn more.
Network Switching Subsystem (NSS)
Fundamentally, the network and switching subsystems (NSS) is responsible for call
connection, supervision and release operations between calling and called stations, where oneor both of them are mobile stations (MS). Other functions include:
y Handling short messages and packet data (email, fax and a variety of notifications)
y Providing µbearer¶ channel for data communications
y Maintaining database of its own users as well as visitors
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y Variety of authentication and encryption
y Gateway to PSTN, other mobile networks and data networks including the Internet
Home Location Register (HLR)
The home location register (HLR) is a database used for storing and managing subscriptions.
Generally a PLMN (Public Land Mobile Network) consists of several HLRs. The first two
digits of the mobile directory number (e.g. 0171 2620757) are the number of the HLR where
the mobile subscriber is stored. The data includes permanent data on subscribers (such as
subscriber's service profile) as well as dynamic data (such as current location and activity
status). When an individual buys a subscription from one of the GSM operators, he or she is
registered in the HLR of that operator.Data Elements (Subscriber) Examples:
Mobile Station¶s Identities:
y IMSI (International Mobile Subscriber Identity) (the primary Key),
y Current TMSI (Temporary IMSI)
y IMEI (International Mobile Equipment Identity)
y Mobile Station¶s Telephone number
y MSISDN (Mobile Stations ISDN number)
y Current MSRN (Mobile Station Roaming Number), if assigned
y Name and address of the subscriber
y Current service subscription profile
y Current location (MSC/VRL address)
y Authentication and encryption keys
y Individual Subscriber Authentication Key (KI)
y Mobile Country Code (MCC) and MNC (Mobile Network Code)
y List of MSC/VLR that belongs to this HLR
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Mobile Switching Center and Visitor Location Register (MSC/VLR)
The mobile switching center (MSC) performs the telephony switching function. A mobile
station must be attached to a single MSC at a time (either homed or visitor), if it is currently
active (not switched off). The visitor location register (VLR) is a database attached to an MSC
to contain information about its currently associated mobile stations (not just for visitors). A
basic switch (that is a PSTN/ISDN switch) already has a database for its telephone
connections. However, it is not designed to include visitors since a visitor has telephone
number that does not belong to this switch. That is why a separate VLR is needed. An MSC,
with the help of the HLR, allocates a visitor a µlocal¶ telephone number (the MSRN), which is
not currently allocated to anyone. This allocation is temporary (like visitor ID card). The VLR
stores the MSRN as mobile station¶s telephone number (along with other information).
However, VLR also stores some information like µsecurity triple¶ (authentication and
encryption information) for each mobile station that are currently attached to the MSC. A VLR
stores such information not only for its visitors but also for the homed mobile stations. From
this perspective VLR is for homed mobile stations as well.
Visitor Location Register contains selected administrative information from the HLR,
necessary for call control and provision of the subscribed services, for each mobile currently
located in the geographical area controlled by the VLR. Although each functional entity can be
implemented as an independent unit, most manufacturers of switching equipment implement
one VLR together with one MSC, so that the geographical area controlled by the MSC
corresponds to that controlled by the VLR, simplifying the signaling required.
Information of currently attached mobile stations
y IMSI/TMSI numbers
y MSISDN/MSRN numbers
y Security triples (authentication and encryption information)
y Location Area Identity (where the mobile station is currently located)
y List of base stations that belong to this MSC/VLR (by their BSIC or Base Station
Identity Code)
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y List of location areas that belong to this MSC/VLR (by their LAI or Location Area
Identity code)
Authentication Center The authentication center (AUC) provides authentication and encryption parameters that verify
the user's identity and ensure the confidentiality of each call. The AUC protects network
operators from different types of fraud found in today's cellular world. The GSM has standard
encryption and authentication algorithm which are used to dynamically compute challenge
keys and encryptions keys for a call. In the authentication procedure, the key Ki is never
transmitted to the mobile over the air path, only a random number is sent. In order to gain
access to the system, the mobile must provided the correct Signed Response (SRES) in answer
to a random number (RAND) generated by AUC.The HLR is also responsible for the ³authentication´ of the subscriber each time he makes or
receives a call. The AUC, which performs this function, is a separate GSM entity that will
often be physically included with the HLR. Being separate, it will use separate processing
equipment for the AUC database functions.
Equipment Identity Register (EIR)
The equipment identity register (EIR) is a database that contains information about the identity
of mobile equipment (ME) that prevents calls from stolen, unauthorized, or defective mobile
stations. The AUC and EIR can be implemented as stand-alone nodes or as a combined
AUC/EIR node. There is generally one EIR per PLMN. There are three classes of ME that are
stored in the database, and each group has different characteristics.
y White List: contains those IMEIs that are known to have been assigned to valid MS¶s.
This is the category of genuine equipment.
y Black List: contains IMEIs of mobiles that have been reported stolen.
y Gray List: contains IMEIs of mobiles that have problems (faulty software, wrong make
of the equipment etc). This list contains all MEs with faults not important enough for
barring.
Gateway MSC (GMSC)
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The Gateway MSC (GMSC) is an MSC that connects the PLMN (Public Land Mobile
Network) to a PSTN/ISDN. This is the only MSC in the network connected to the HLR. If a
network delivering a call to the PLMN cannot interrogate the HLR, the call is routed to the
GMSC. The GMSC will interrogate the HLR and then route the call to the MSC where the MS
is currently located.
Base Station Subsystem (BSS)
All radio-related functions between mobile stations and network are performed in the base
station subsystem (BSS). The radio equipment of BSS may be composed of one or more cells.
The BSS consists of:
y One base station controller (BSC) and
y All base transceiver stations (BTS) under the BSC
Base Transceiver Station
A Base Station Transceiver (BTS) is a radio transceivers station that communicates with the
mobile stations. Its backend is connected to the BSC. A BTS is usually placed at the center of a
cell. Its transmitting power defines the size of a cell. The primary responsibility of BTS is to
transmit and receive radio signals from a mobile unit over an air interface. To perform this
function completely, the signals are encoded, encrypted, multiplexed, modulated, and then fed
to the antenna system at the cell site. Random access detection is made by BTS which then
sends the message to BSC. Timing advance is determined by BTS. BTS signals the mobile for
proper timing adjustments. Uplink radio channel measurement corresponding to the downlink
measurements made by MS has to be made by BTS.
Base Station Controller
A Base Station Controller (BSC) is a high-capacity switch with radio communication and
mobility control capabilities. The functions of a BSC include radio channel allocation, location
update, handover, timing advance, power control and paging.
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Database associated with Subscriber
y Associated IMSI (Primary key)
y Paging Groups
Database associated with Network
y MSC Address
y List of Paging Groups
y List of LAI
y List of BSIC
y List of Cell ID (CI)
The BSC performs the inter cell hand over for MS moving between BTS in its control. It also
reallocates frequencies to the BTSs in its area to meet locally heavy demands during peak
hours or on special events. It controls the power transmission of both BSSs and MSs in its area.
The BSC also measures the time delay of received MS signals relative to the BTS clock. The
BSC may also perform traffic concentration to reduce the number of transmission lines from
the BSC to its BTSs.
Transcoder Rate Adapter Unit
The Transcoder/Rate Adaptation Unit (TRAU) is the data rate conversion unit. The
PSTN/ISDN switch is a switch for 64 kbps voice. Current technology permits to decrease the
bit-rate (in GSM radio interface it is 13 kbps for full rate and 6.5 kbps for half rate). Since
MSC is basically a PSTN/ISDN switch its bit-rate is still 64 kbps. That is why a rate
conversion is required in between the BSC and MSC.
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Mobile Station
The mobile station (MS) consists of the physical equipment, such as the radio transceiver,
display and digital signal processors, and a smart card called the Subscriber Identity Module
(SIM). The SIM provides personal mobility, so that the user can have access to all subscribed
services irrespective of both the location of the terminal and the use of a specific terminal. By
inserting the SIM card into another GSM cellular phone, the user is able to receive calls at that
phone, make calls from that phone, or receive other subscribed services.
The mobile equipment is uniquely identified by the International Mobile Equipment Identity
(IMEI). The SIM card contains the International Mobile Subscriber Identity (IMSI),
identifying the subscriber, a secret key for authentication, and other user information. The
IMEI and the IMSI are independent, thereby providing personal mobility. The SIM card may
be protected against unauthorized use by a password or personal identity number. The MS
monitors the power level and signal quality for known receiver bit sequence from its current
BTS and up to six surrounding BTSs.
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GSM INTERFACES
The Radio interface (MS to BTS)
The Um radio interface (between MS and base transceiver stations [BTS]) is the most
important in any mobile radio system, in that it addresses the demanding characteristics of the
radio environment. The physical layer interfaces to the data link layer and radio resource
management sub layer in the MS and BS and to other functional units in the MS and network
subsystem (which includes the BSS and MSC) for supporting traffic channels. The physical
interface comprises a set of physical channels accessible through FDMA and TDMA. Each
physical channel supports a number of logical channels used for user traffic and signaling. The
physical layer (or layer 1) supports the functions required for the transmission of bit streams onthe air interface. Layer 1 also provides access capabilities to upper layers. The physical layer is
described in the GSM Recommendation 05 series (part of the ETSI documentation for GSM).
At the physical level, most signaling messages carried on the radio path are in 23-octet blocks.
The data link layer functions are multiplexing, error detection and correction, flow control, and
segmentation to allow for long messages on the upper layers. The radio interface uses the Link
Access Protocol on Dm channel (LAPDm). This protocol is based on the principles of the
ISDN Link Access Protocol on the D channel (LAPD) protocol. Layer 2 is described in GSM
Recommendations. The following logical channels are supported.
Speech traffic channels (TCH)
y Full-rate TCH (TCH/F)
y Half-rate TCH (TCH/H)
Broadcast channels (BCCH)
y Frequency correction channel (FCCH)
y Synchronization channel (SCH)
y Broadcast control channel (BCCH)
Common control channels (CCCH)
y Paging channel (PCH)
y Random access channel (RACH)
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y Access grant channel (AGCH)
Cell broadcast channel (CBCH)
y Cell broadcast channel (CBCH) (the CBCH uses the same physical channel as the
DCCH)
Dedicated control channels (DCCH)
y Slow associated control channel (SACCH)
y Stand-alone dedicated control channel (SDCCH)
y Fast associated control channel (FACCH)
y The radio resource layer manages the dialog between
Abis Interface (BTS to BSC)
The interconnection between the BTS and the BSC is through a standard interface, Abis (most
Abis interfaces are vendor specific). The primary functions carried over this interface are
traffic channel transmission, terrestrial channel management, and radio channel management.
This interface supports two types of communications links: traffic channels at 64 kbps carrying
speech or user data for a full- or half-rate radio traffic channel and signaling channels at 16
kbps carrying information for BSC-BTS and BSC-MSC signaling. The BSC handles the LAPD
channel signaling for every BTS carrier. The first three layers are based on the following
OSI/ITU-T recommendations:
y Physical layer: ITU-T Recommendation G.703 and GSM Recommendation 0-8.54
y Data link layer: GSM Recommendation 08.56 (LAPD)
y Network layer: GSM Recommendation 08.58
There are two types of messages handled by the traffic management procedure part of the
signaling interface²transparent and nontransparent. Transparent messages are between the
MS and BSC-MSC and do not require analysis by the BTS. Nontransparent messages do
require BTS analysis.
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A Interface (BSC to MSC)
The A interface allows interconnection between the BSS radio base subsystem and the MSC.
The physical layer of the A interface is a 2-Mbps standard Consultative Committee on
Telephone and Telegraph (CCITT) digital connection. The signaling transport uses Message
Transfer Part (MTP) and Signaling Connection Control Part (SCCP) of SS7. Error-free
transport is handled by a subset of the MTP, and logical connection is handled by a subset of
the SCCP. The application parts are divided between the BSS application part (BSSAP) and
BSS operation and maintenance application part (BSSOMAP). The BSSAP is further divided into
Direct Transfer Application Part (DTAP) and BSS management application part (BSSMAP).
The DTAP is used to transfer layer 3 messages between the MS and the MSC without BSC
involvement. The BSSMAP is responsible for all aspects of radio resource handling at the
BSS. The BSSMAP supports all the operation and maintenance communications of BSS.
Interfaces between Other GSM Entities
Information transfer between GSM PLMN entities uses the MAP. The MAP contains a mobile
application and several Application Service Elements (ASEs). It uses the service of the
Transaction Capabilities Application Part (TCAP) of SS7. It employs the SCCP to offer the
necessary signaling functions required to provide services such as setting mobile facilities for
voice and nonvoice application in a mobile network. The major procedures supported by
MAP is
y Location registration and cancellation
y Handover procedures
y Handling supplementary services
y Retrieval of subscriber parameters during call setup
y Authentication procedures.
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METHODOLOGY
Hardware Analysis
Analysis of potential hardware problems in the network not detected by µnormal¶ fault
management methods.
Performance Statistics
Analysis of performance statistics with standard graphical information sheet for each cell.
Analysis of potential hardware problems in the network not detected by µnormal¶ fault
management methods.
Call Trace Analysis
Detects problems with antenna tilts.
Detects problems with Base Transceiver Subsystem (BTS) output power.
Frequency Planning Optimization
Re-definition of handovers and assigned frequencies.
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NEED FOR OPTIMIZATION
Network issues that may generate a requirement
Perceived reduction in network quality.
- Indications from network performance monitoring.
- Subscriber¶s experience of using the network
Maximizing the use of existing infra structure
- Operators wants to ensure best returns on investment
Introduction of new services
- Maximize existing resources to accommodate new services such as GPRS.
- Change in original design parameters.
1. Flawed original design in format.
2. Original design information has changed.