basic gsm
DESCRIPTION
gsmTRANSCRIPT
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Objectives
At the end of this module, the trainee should be able to:– Define wireless technology– Define basic terms like :
• cell, frequency reuse, handover, Access scheme – Explain the network components in GSM network
Draw flows for key GSM procedures like• Location Update• Originating Call• Terminating Call
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What is Wireless network?
Communication between two network elements via a radio interface.
Switch
Access Network
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Access without wire
How does user access network in absence of wire ?
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Radio Access Schemes
TDMA: Time Division Multiple Access
Time
Frequency
Single User Time Slice
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Radio Access Schemes
• FDMA: Frequency Division Multiple Access
Time
Frequency
Single User Frequency Slice
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Radio Access Schemes
• Hybrid FDMA/TDMA
Time
Frequency
Single User Bandwidth
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Concept of Cell/Frequency reuse
– Cell is a small geographic area– Every cell is covered by one base station having a set of frequencies– With usage of multiple cells a large geographic area can be covered – Frequency is reused to maximize the use of allotted spectrum in such
a way that interference is minimized
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Categorization of Cells
Depending upon cell size, the cell is given a specific name:
– Large cell: 10 - 35 KM• In rural areas, coverage is key
– Small Cell: 1 - 3 KM• In urban areas, capacity is key
– Micro Cell: 100 - 300 M
– Pico Cell: 10 - 30 M
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Consequences of removing wire -Mobility
Location Management
• Handover • When a mobile moves into a different cell while a
conversation is in progress, the call is transferred to new cell
• Roaming
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MSC/VLR
BSC
BSC
BSC
MSC/VLR
BTS
BTS
BTS
BTS
Handover
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Soft Handover :- Make-before-break type Connection is established first. After that old channel is released
Hard Handover :- Break-before-make type
Connection with old cell is broken before establishing new connection
Type Of HandOver (HO)
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Intra Cell Handover :- Handover from one channel/timeslot to another within same cell
Type Of Handover
Inter Cell Handover :- Handover from one cell to another cell
Inter BSC Handover :-Handover when both the source and destination cells are controlled by different BSC
Inter MSC Handover :- Handover when both the source and destination cells are controlled by different MSC
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Roaming
Network A Network B
When subscriber of network A moves to another geographical area and receives services from another network B
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Wireless Networks
Started with First Generation Networks ( Early1980s )– No or minimum standardization.– Analog– Deployed and used within regional boundaries
Evolved to Second Generation Networks– Standards defined. – Digital– Roaming across network boundaries possible.– GSM: one of the most prevalent standard for
2G mobile networks
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Issues in wireless networks
Multipath propagation Path loss Radio Signal Interference Network Security
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Advantages
Wireless Network Advantages:– Provides Mobility– Easy installation. No cabling required.
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GSM Radio interface
GSM radio interface is mix of FDMA and TDMA
FDMA divides the frequency spectrum into small slices– Carrier Separation is 200 kHz
TDMA divides each channel into 8 timeslots such that each carrier is shared by 8 users
The basic radio resource is a time slot with a duration of 577 µs
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Various GSM based Systems Bands
P-GSM ( Primary GSM ) up-link 890-915MHz 200KHz Carrier down-link 935-960MHz
E-GSM ( Extended GSM ) up-link 880-915MHz 200KHz Carrier down-link 925-960MHz
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Services provided by GSM:User perspective
Speech Services Data services SMS
– PTP– Cell Broadcast
Supplementary services– Call barring ,call forwarding etc
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GSM Network Architecture - Nodes
MS
MS
BTS
BTS
BTS
BSC
BSC
MSC
VLR GMSC
HLR
PSTN
EIR
AuC
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GSM Network Components
Mobile Station– SIM– Mobile Equipment
Access Network – Base Transceiver Station (BTS)– BSC (Base Station Controller)
Core Network– Home Location Register (HLR)– Authentication Center (AuC)– Equipment Identity Register (EIR)– Short Message – Service Center (SM-SC)– Mobile Switching Center (MSC)/Visitor Location Register (VLR) – Gateway Mobile Switching Center (GMSC)
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Mobile Station
Comprises of two separate units, viz. SIM and ME. The Structure provides terminal mobility. Subscriber Identity Module (SIM): Belongs to the
operator (e.g. Bharti, Hutch). – Contains the permanent data of subscriber (e.g. IMSI).
Mobile Equipment is developed by equipment manufacturers (e.g. Nokia, Alcatel).– Contains Mobile Termination and User Interface functions.– Communicates with SIM
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Access Network: Overview
All the radio-related functions activities have been separated from the MSC and concentrated in the BSS.
The BSS consists of two components– Base Transceiver Station – Base Station Controller
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Base Transceiver Station (BTS)
It takes care of – air interface signaling – speech processing – ciphering
It consists of – radio transmitter & receiver,– antennas and signal processing specific to
radio interface
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Base Station Controller (BSC)
Controls the radio network Main responsibilities are
– Controls all BTS under its purview. – Connection establishment between MS and
NSS– Handover Management
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Core Network: Overview
Core Network provides core functionality including:– Mobility management,
– Call handling,
– Subscriber data management
– Authentication, etc.
Includes– Home Location Register (HLR)
– Authentication Center (AuC)
– Equipment Identity Register (EIR)
– Short Message Service Center (SMSC)
– Mobile Switching Center (MSC)/Visitor Location Register (VLR)
– Gateway Mobile Switching Center (GMSC)
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Home Location Register: Overview
HLR is the master database for a subscriber (i.e. it holds the subscription data).
– Each subscriber is assigned to only one HLR.
HLR maintains and provides subscriber data to other network entities on demand (i.e. pulled by network entities).
– E.g. During Location Update In certain cases, the subscriber data is sent to the
network entities (e.g. when the data is modified by operator, the HLR pushes the data to network entities).
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Authentication Center (AuC)
AuC holds authentication information. – This information is used for authentication and other security functions.
At the center of all AuC functions is a secret key that is shared between the AuC and the SIM of the UE. – This secret key facilitates roaming of a subscriber because even in a
visited network, the MSC/VLR can authenticate the user by obtaining security information from AuC.
The AuC itself does not take part in the authentication. – This function is performed by the MSC/VLR. – Role of the AuC is to provide MSC/VLR with the necessary information
that can be used by MSC/VLR for authentication.
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Equipment Identity Register (EIR)
EIR monitor the legitimacy of a User Equipment (UE) used in the UMTS network,
EIR holds the list of IMEI used in the GSM network.
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Short Message Service Center
SMSC is responsible for relaying store‑and‑forwarding
of short messages between an MS and a Short Message Entity
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Mobile Switching Center (MSC)
MSC performs all functions necessary to handle the circuit switched services to and from the MS.
It is customary to represent VLR and MSC as MSC/VLR. – They are generally integrated.
Like HLR, Visitor Location Register (VLR) is a repository of information. VLR tracks the location of the MS
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Gateway Mobile Switching Center
Similar to MSC Switches voice cells. Provides external interface with PSTN and other mobile
networks.
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Hierarchical Organization of GSM N/w (1)
LA1 LA2
LA3 LA4
PLMN
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Hierarchical Organization of GSM N/w(2) Public Land Mobile Network (PLMN): It is at the
highest level of the hierarchy.
PLMN ID = MCC+MNC
Location Area (LA): LA is defined as an area in which an MS may move freely without updating its current location at the VLR. Group of cells are given a single Location Area ID (LAI)
LAI = PLMN ID + LAC
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Hierarchical Organization of GSM N/w (3)
Cells: At the lowest level of hierarchy is the cell. Each cell is
identified by the Cell Identity (CI). A CI is unique within a
location area. To identify a cell uniquely across PLMNs, an
identity called the Cell Global Identity (CGI) is defined. CGI
is obtained by the concatenation of LAI and the CI.
– CGI = LAI+CI
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Addresses and Identifiers
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GSM Procedures
Key Procedures in GSM Location Update & Mobile Registration Mobile Originated Call Paging Mobile Terminated Call
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Mobile Registration in the network
MS BTS BSC (G)MSC VLR HLR Action
Channel Request
Channel Assignment
Location Update Request
Authentication Request
Authentication Response
Comparison of Authentication params
Accept LUP and alloc TMSI
Ack of LUP and TMSI
Entry of new area and identity into VLR and HLR
Channel Release
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Location Registration
MSC/VLR
HLR
PreviousVLR
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Location Registration (2)
MSC/VLR
HLR
PreviousVLR
1. LU request<TMSI, LAI>
2. Request IMSI
5. Subscriber Data req <1..N>
4. Update Location req
3. IMSI
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Location Registration (3)
MSC/VLR
HLR
PreviousVLR
8. LU Ack<New TMSI>(Store TMSI)
6. Subscriber Data ack <1..N>
7. Update Location ack
9. Cancel Location
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Mobile Termination for Roaming
VMSC
HLR
Gateway MSC
3. Provide Roaming Identification (PRI) req
<IMSI>
2. Send Routing Information req<MSISDN>
4. PRI Ack<MSRN>
1. Initial Address Message (IAM)
<MSISDN>
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Mobile Terminations and Paging
VMSC
HLR
Gateway MSC
6. Initial Address Message <MSRN>
5. Send Routing Information ack<MSRN>
7. Paging<TMSI>
8. Paging Resp.<Cell ID> 9. SETUP
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Mobile Originated Call
MS
MS
BTS
BTS
BTS
BSC
BSC
MSC
VLR
GMSC
HLR
PSTN
EIR
AuC
Req for dedicated channel for communication (RACH)Alloc of dedicated channel(AGCH)Call establishment request(SDCCH)Authentication request(SDCCH)Authentication response(SDCCH)
Authentication response(SDCCH)
Ciphering command(SDCCH)Ciphering response(SDCCH)
Assignment of TCHCall Setup
Assignment of TCH
Call Setup
Alerting(TCH) Alerting
Alerting(TCH)
Connected(TCH) Connected
Connected(TCH)
Connected
Exchange ofCommunication
Exchange of communication(TCH)
Exchange ofCommunication
Call release
Call release(TCH)
Call release
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Mobile Terminated Call
MS
MS
BTS
BTS
BTS
BSC
BSC
MSC
VLR
GMSC
HLR
PSTN
EIR
AuC
Land to Mobile call
Query forVLR info
Reply
Routeto MSC
Query VLR for LAC andTMSIPaging
the area
Paging
Paging
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References
The GSM system for mobile communications - Mouly, M., Pautet, M.
GSM System Engineering - Asha Mehrotra ETSI GSM Technical Specifications – www.etsi.org GSM presentation by Ashok Kumar GSM presentation by Sumit/Nishit
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Acknowledgements
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Disclaimer
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