Transcript WC Lecture 3-4

Wireless Communication
Introduction to Mobile Communication and Cellular System
Lecture 3-4
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Why Hexagon?
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Interfaces
Um
VLR
Abis
BSC
A
MSC
ISDN,
TUP
Abbreviations:
MSC – Mobile Switching Center
BSS – Base Station Sub-system
BSC – Base Station Controller
HLR – Home Location Register
BTS – Base Transceiver Station
VLR – Visitor Location Register
TRX – Transceiver
AuC – Authentication Center
MS – Mobile Station
EIR – Equipment Identity Register
OMC – Operations and Maintenance Center
PSTN – Public Switched Telephone Network
ISDN – Integrated Services Digital Network
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Mobile Station
The Mobile Station (MS) consists of the physical equipment used by a
subscriber to connect to the network. It comprises the Mobile Equipment
(ME) and the Subscriber Identity Module (SIM). The ME forms part of the
Mobile Termination (MT) which, depending on the application and
services, may also include various types of Terminal Equipment (TE) and
associated Terminal Adapter (TA).
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• The IMSI identifies the subscriber within the GSM network
while the MS ISDN is the actual telephone number a caller
(possibly in another network) uses to reach that person.
(The International Mobile Subscriber Identity or IMSI /ˈɪmziː/ is used to identify the
user of a cellular network )(MSISDN (Mobile Station International Subscriber Directory
Number) )
• Security is provided by the use of an authentication key
and by the transmission of a temporary subscriber identity
(TMSI) across the radio interface where possible to avoid
using the permanent IMSI identity.
• The IMEI (The International Mobile Equipment Identity)may be used
to block certain types of equipment from accessing the
network if they are unsuitable and also to check for stolen
equipment.
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MS and SIM
The mobile station consists of :
• mobile equipment (ME)
• subscriber identity module (SIM)
The mobile equipment has a unique International Mobile Equipment
Identity (IMEI), which is used by the EIR( Equipment Identity Register ).
The SIM stores permanent and temporary data about the
mobile, the subscriber and the network, including :
• The International Mobile Subscribers Identity (IMSI)
• MS ISDN number of subscriber
• Authentication key (Ki) and algorithms for authentication
check
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Base Station Subsystem (BSS)
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The BSS comprises:
• Base Station Controller (BSC)
• One or more Base Transceiver Stations (BTSs)
The purpose of the BTS is to:
• provide radio access to the mobile stations
• manage the radio access aspects of the system
BTS contains:
• Radio Transmitter/Receiver (TRX)
• Signal processing and control equipment
• Antennas and cables
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The BSC:
• allocates a channel for the duration of a call
• maintains the call:
monitors quality
controls the power transmitted by the BTS or MS
generates a handover to another cell when required
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Network Switching System (NSS)
The NSS combines the call routing switches
(MSCs and GMSC ) with database registers
required to keep track of subscribers’ movements
and use of the system. Call routing between MSCs
is taken via existing PSTN or ISDN networks.
Signaling between the registers uses Signaling
System No. 7 protocol.
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Functions of the MSC( Mobile Switching Center ):
• Logging calls, controlling calls and Switching calls
• Interface with PSTN, ISDN, PSPDN
• Mobility management over the radio network and other
networks
• Radio Resource management - handovers between BSCs
• Billing Information
PSPDN -> Packed-Switched Public Data Network
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Access Network
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BSS Configuration
• Collocated BTS
• Remote BTS
• Daisy Chain BTS
• Star Configuration
• Loop Configuration
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Collocated BTS: BTS is situated along with BSC or the MSC and no
additional E1 link is required.
BTS
BSC/MSC
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Remote BTS : BTS is situated in a stand alone position and additional E1
links are required to connect to BSC.
BSC
BTS
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Daisy Chain
BTS 3
BTS 4
BTS 1
BSC
BTS 2
MSC
Star Configuration
BTS 3
BTS 1
BSC
BTS 4
BTS 2
MSC
Loop Configuration
BTS 3
BTS 1
BTS 4
BSC
BTS 2
MSC
Comparison of Different Configurations
• Daisy Chain: Easy to implement, effective utilization of
transmission links but if one of the link fails, all the BTSs connected in
the chain will went off.
• Star Configuration: Easy to implement but poor utilization
of links. Each BTS require one E1 to connect to BSC. But if link goes
down only individual BTS will be affected.
• Loop Configuration: Slightly difficult to implement but
effective utilization of E1 links. Even if one link goes off BTS will
continue to communicate with the network from the other side.
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NSS Topology
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Objective
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Terminology used in Network Sub System
Protocols and Interfaces inside NSS
Call routing and circuit groups
Switching modules
Stand alone and integrated HLR
Authentication, Ciphering, OMC, Billing center
Transit Switch
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Introduction
Network Sub System can be considered as a heart of the GSM Network.
All the major activities like switching of calls, routing, security
functions, call handling, charging, operation & maintenance,
handover decisions, takes place within the entities of NSS.
Various kinds of interfaces are used to communicate between the
different entities. Different methods are used to optimize and
provide the quality network with the minimum operating cost.
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Network Switching System (NSS)
Key elements of the NSS:
• Mobile Switching Center (MSC)
• Visitor Location Register (VLR)
• Home Location Register (HLR)
• Authentication Center (AuC)
• Equipment Identity Register (EIR)
• Gateway MSC (GMSC)
These elements are interconnected by
means of an SS7 network
NSS Identifier
IMEI – International Mobile Equipment Identifier
The IMEI is an internationally-unique serial number allocated to the MS
hardware at the time of manufacture. It is registered by the network
operator and (optionally) stored in the AuC for validation purposes.
IMEI = TAC + FAC + SNR +sp
TAC = Type Approval Code by central GSM body
FAC = Final Assembly Code, identifies the manufacturer
SNR = Serial Number, unique six digit number
sp = spare for future use
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IMSI – International Mobile Subscriber Identifier
When a subscriber registers with a network operator, a unique subscriber
IMSI identifier is issued and stored in the SIM of the MS as well as in
the HLR . A MS can only function fully if it is operated with a valid SIM
inserted into an MS with a valid IMEI. IMSI consist of three parts:
IMSI = MCC + MNC + MSIN
MCC = Mobile Country Code (3 digit)
MNC = Mobile Network Code (3 digit)
MSIN = Mobile Station Identification Number (9 digit)
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TMSI –Temporary Mobile Subscriber Identity
A TMSI is used to protect the true identity (IMSI) of a subscriber. It is
issued by and stored within a VLR (not in the HLR) when an IMSI attach
takes place or a Location Area (LA) update takes place. At the MS it is
stored in the MS’s SIM. The issued TMSI only has validity within a
specific LA.
Since TMSI has local significance, the structure may be chosen by the
administration. It should not be more than four octets.
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MSISDN – Mobile Station ISDN Number
The MSISDN represents the ‘true’ or ‘dialed’ number associated with the
subscriber. It is assigned to the subscriber by the network operator at
registration and is stored in the SIM.
According to the CCITT recommendations, it is composed in the following
way:
MSISDN = CC + NDC + SN
CC = Country Code
NDC = National Destination Code
SN = Subscriber Number
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Section 3 – NSS Topology
Mobile Switching Center (MSC)
The Mobile Switching Center (MSC) performs the telephony switching
functions of the system. It also controls calls to and from other
telephony and data systems, such as the Public Switched Telephone
Network (PSTN) and Public Land Mobile Network (PLMN).
Difference between a MSC and an exchange in a fixed network is:
MSC has to take into account (‫) زمرے میں شمار کرنا‬the impact of the
allocation of radio resources and the mobile nature of the subscribers
and has to perform in addition, at least the following procedures:
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Section 3 – NSS Topology
• required for location registration
• procedures required for handover
An MSC can be connected to only one VLR. Therefore, all mobile stations
that move around under base stations connected to the MSC are
always managed by the same VLR.
An MSC would communicate typically with one EIR (Equipment Identity
Register). While it is possible for an MSC to communicate to multiple
EIRs, this is highly unlikely (‫) امکان نہیں‬since the EIR provides a
centralized and geographic independent function.
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Section 3 – NSS Topology
The MSC consults an HLR to determine how a call should be routed to a
given mobile station:
• For incoming calls to a mobile station, the MSC would typically consult
one HLR.
• For mobile-to-mobile calls in larger networks, a MSC could consult
HLRs of other systems to help minimize the trunk paths to the other
mobile station.
A given MSC can be interconnected to other MSCs to support inter-MSC
handovers
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