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2G Mobile Communication Systems
2G Review: GSM
Services
Architecture
Protocols
Call setup
Mobility management
Security
HSCSD
GPRS
EDGE
Cellular Communication Systems 2 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
Public Land Mobile Network (PLMN)
Definition:
a network established and operated by an administration to provide land-based mobile telecommunications services to the public
a PLMN may be regarded as an extension of a network (e.g. an ISDN)
a PLMN consists of a collection of areas within a common numbering plan (e.g. same National Destination Code) and a common routing plan
PLMNs are independent telecommunications entities
Source: 3GPP 23.002-5.5.0
o
Cellular Communication Systems 3 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
GSM: Mobile Services
GSM offers
several types of connections
voice connections
data connections
short message service
multi-service options (combination of basic services)
Three service domains (a “mobile” model of ISDN)
Bearer Services
Teleservices
Supplementary Services
GSM-PLMN
transit
network
(PSTN, ISDN)
source/
destination
network
TE TE
bearer services
teleservices
R, S (U, S, R) Um
MT
MS
PLMN: Public Land Mobile Network
PSTN: Public Switched Telephone Network
ISDN: Integrated Services Digital Network
MS: Mobile Station
MT: Mobile Termination (radio-specific part)
TE: Terminal
Cellular Communication Systems 4 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
Bearer Services
Telecommunication services to transfer data between access points
Specification of services up to the terminal interface (OSI layers 1-3)
Different data rates for voice and data (original standard)
data service (circuit switched)
synchronous: 2.4, 4.8 or 9.6 kbit/s
asynchronous: 300 - 1200 bit/s
data service (packet switched) –> superseded by GPRS
synchronous: 2.4, 4.8 or 9.6 kbit/s
asynchronous: 300 - 9600 bit/s
Cellular Communication Systems 5 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
Teleservices
Telecommunication services that enable voice communication via mobile phones
mobile telephony primary goal of GSM was to enable mobile telephony offering nearly ISDN quality (bandwidth of 7 kHz);
Today: Fullrate codec (FR–13kb/s), halfrate (HR-5.6kb/s), Enhanced Fullrate (EFR-12.2kb/s)
emergency number common number throughout Europe (112); mandatory for all service providers; free of charge; connection with the highest priority (preemption of other connections possible)
multinumbering several ISDN phone numbers per user possible
Non-Voice Teleservices
group 3 fax
voice mailbox (implemented in the GSM network)
Short Message Service (SMS) alphanumeric data transmission to/from the mobile terminal using the signaling channel, thus allowing simultaneous use of basic services and SMS
Cellular Communication Systems 6 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
Supplementary services
Services in addition to the basic services
cannot be offered stand-alone
similar to ISDN services besides lower bandwidth due to the radio link
may differ between different service providers, countries and protocol
versions
Important services
call forwarding
identification: forwarding of caller number
suppression of number forwarding (CLIP, CLIR)
automatic call-back
conferencing with up to 7 participants
locking of the mobile terminal (incoming or outgoing calls)
...
Cellular Communication Systems 7 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
Architecture of the GSM system
GSM is a PLMN (Public Land Mobile Network)
several providers setup mobile networks following the GSM standard within each country
GSM system comprises 3 subsystems
RSS (radio subsystem): covers all radio aspects
MS (mobile station)
BSS (base station subsystem) or RAN (radio access network)
BTS (base transeiver station)
BSC (base station controller)
NSS (network and switching subsystem): call forwarding, handover, switching
MSC (mobile services switching center)
LR (location register): HLR and VLR
OSS (operation subsystem): management of the network
OMC (operation and maintenance centre)
AuC (authentication centre)
EIR (equipment identity register)
Cellular Communication Systems 8 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
GSM: overview
fixed network
BSC
BSC
MSC MSC
GMSC
OMC, EIR,
AUC
VLR
HLR
NSS
with OSS
RSS
VLR
BTS BTS BTS BSC: n:1 (tree) BSC MSC: n:1 (tree) MSC – VLR: 1:1 MSC – MSC : meshed network
Cellular Communication Systems 9 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
GSM: elements and interfaces
NSS
MS MS
BTS
BSC
GMSC
IWF
OMC
BTS
BSC
MSC MSC
Abis
Um
EIR
HLR
VLR VLR
A
BSS
PDN
ISDN, PSTN
RSS
radio cell
radio cell
MS
AUC OSS
signaling
O
Um Interface (MS and BTS): radio, air interface
Abis Interface (BTS and BSC)
Interfaces B,...,H within NSS (between MSC, VLR and HLR)
A Interface (BSC and MSC)
o
Cellular Communication Systems 10 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
Radio subsystem
The Radio Subsystem (RSS) comprises the cellular mobile network up to the switching centers
Components
Base Station Subsystem (BSS)
Base Transceiver Station (BTS)
radio components including sender, receiver, antenna
one BTS can cover several cells
Base Station Controller (BSC)
switching between BTSs,
controlling BTSs,
managing of network resources,
mapping of radio channels (Um) onto terrestrial channels (A interface)
BSS = BSC + sum(BTS) + interconnection
Mobile Stations (MS)
Cellular Communication Systems 11 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
Base Transceiver Station and Base Station Controller
Tasks of a BSS are distributed over BSC and BTS
BTS comprises radio specific functions of lower layers (PHY, MAC)
BSC manages and controls the radio channels in the BTS and terrestrial
channels to BTS and MSC
Design Principle: “central intelligence” = BSC, “dumb radio station” = BTS
Functions BTS BSC
Management of radio channels X
Frequency hopping (FH) X X
Management of terrestrial channels X
Mapping of terrestrial onto radio channels X
Channel coding and decoding X
Rate adaptation X
Encryption and decryption X X
Paging X X
Uplink signal measurements X
Traffic measurement X
Authentication X
Location registry, location update X
Handover management X
Cellular Communication Systems 12 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
possible radio coverage of the cell
idealized shape of the cell cell
segmentation of the area into cells
GSM: cellular network
use of several carrier frequencies
not the same frequency in neighboring cells
cell radius varies from some 100 m up to 35 km depending on user density, geography, transceiver power etc.
hexagonal shape of cells is idealized (cells overlap, shapes depend on geography)
if a mobile user changes cells -> handover of the connection to the neighbor cell
Cellular Communication Systems 13 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
GSM: Air Interface
FDMA (Frequency Division Multiple Access) / FDD (Frequency Division Duplex)
123 124 . . .
890 MHz 915 MHz
123 124 . . .
935 MHz 960 MHz
200 kHz
Uplink Downlink
frequency
TDMA (Time Division Multiple Access)
time
Downlink
8 7 6 5 4 3 2 1
4,615 ms
= 1250 bit
Uplink
8 7 6 5 4 3 2 1
Cellular Communication Systems 14 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
Framing Modulation
(GMSK)
GSM: Voice Coding
Voice coding Channel coding
Framing Modulation
(GMSK)
114 bit/slot 114 + 42 bit
Guard (8.25 bits): avoid overlap with other time slots (different time offset of neighboring slot)
Training sequence: select the best radio path in the receiver and train equalizer
Tail: needed to enhance receiver performance
Flag S: indication for user data or control data
1 2 3 4 5 6 7 8
GSM TDMA frame
GSM time-slot (normal burst)
4.615 ms
546.5 µs 577 µs
tail user data Training S guard
space S user data tail guard
space
3 bits 57 bits 26 bits 57 bits 1 1 3
Cellular Communication Systems 15 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
GSM hierarchy of frames
0 1 2 2045 2046 2047 ... hyperframe
0 1 2 48 49 50 ...
superframe
0 1 6 7 ... frame
burst
slot
577 µs
4.615 ms
120 ms
6.12 s
3 h 28 min 53.76 s
traffic multiframe
0 1 24 25 ...
0 1 2 48 49 50 ... 235.4 ms control multiframe
0 1 24 25 ...
traffic multiframe: 24 frames (22.8 kbps) used for traffic channel (user data), or fast signaling
1 frame (950 bps) used for slow signaling, 1 frame unused
o
Cellular Communication Systems 16 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
Mobile station
Terminal for the use of GSM services
A mobile station (MS) comprises several functional groups
MT (Mobile Termination):
offers common functions used by all services the MS offers
corresponds to the network termination (NT) of an ISDN access
end-point of the radio interface (Um)
TA (Terminal Adapter):
terminal adaptation, hides radio specific characteristics
TE (Terminal Equipment):
peripheral device of the MS, offers services to a user
does not contain GSM specific functions
SIM (Subscriber Identity Module):
personalization of the mobile terminal, stores user parameters, and security algorithm
R S Um
TE TA MT
o
Cellular Communication Systems 17 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
Network and switching subsystem (NSS)
NSS is the main component of the public mobile network GSM
switching, mobility management, interconnection to other networks, system control
Components
Mobile Services Switching Center (MSC) controls all connections via a separated network to/from a mobile terminal within the domain of the MSC - several BSC can belong to a MSC
Databases (important: scalability, high capacity, low delay)
Home Location Register (HLR) central master database containing user data, permanent and semi-permanent data of all subscribers assigned to the HLR (one provider can have several HLRs)
Visitor Location Register (VLR) local database for a subset of user data, including data about all user currently in the domain of the VLR
Cellular Communication Systems 18 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
Operation subsystem
The OSS (Operation Subsystem) enables centralized operation,
management, and maintenance of all GSM subsystems
Components
Authentication Center (AUC)
generates user-specific authentication parameters on request of a VLR
authentication parameters used for authentication of mobile terminals
and encryption of user data on the air interface within the GSM system
Equipment Identity Register (EIR)
registers GSM mobile stations and user rights
stolen or malfunctioning mobile stations can be locked and sometimes
even localized
Operation and Maintenance Center (OMC)
different control capabilities for the radio subsystem and the network
subsystem
Basic Functions in GSM Systems
Connection Setup
Handover
Location management
Roaming
Authentication
Cellular Communication Systems 20 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
Connection Setup & Radio Resource Assignment
BS BSC MSC
Cellular Communication Systems 21 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
Mobile Terminated Call (MTC)
PSTN calling
station GMSC
HLR VLR
BSS BSS BSS
MSC
MS
1 2
3
4
5
6
7
8 9
10
11 12
13
16 10 10
11 11 11
14 15
17
1: calling a GSM subscriber
2: forwarding call to GMSC
3: signal call setup to HLR
4, 5: request MSRN from VLR
6: forward responsible MSC to GMSC
7: forward call to
current MSC
8, 9: get current status of MS
10, 11: paging of MS
12, 13: MS answers
14, 15: security checks
16, 17: set up connection
Cellular Communication Systems 22 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
Mobile Originated Call (MOC)
PSTN GMSC
VLR
BSS
MSC
MS 1
2
6 5
3 4
9
10
7 8
1, 2: connection request
3, 4: security check
5-8: check resources (free circuit)
9-10: set up call
Cellular Communication Systems 23 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
Handover
The problem:
Change the cell while communicating
Reasons for handover:
Quality of radio link deteriorates
Communication in other cell requires less radio resources
Supported radius is exceeded (e.g. Timing advance in GSM)
Overload in current cell
Maintenance
Lin
k q
ualit
y
Link to cell 1 Link to cell 2 time
cell 1
cell 2
Handover margin (avoid ping-pong effect)
cell 1 cell 2
Cellular Communication Systems 24 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
4 types of handover
(Anchor)
MSC MSC
BSC BSC BSC
BTS BTS BTS BTS
MS MS MS MS
1 2 3 4
• intra-cell handover: reason: quality, interference
• inter-cell handover/intra BSS: within same BSS, handled by BSC (reasons: mobility, receipt level, power budget, load)
• inter-cell handover/inter BSS: between BSC at the same MSC
• inter-cell handover/inter MSC: between BSC of different MSCs
(Anchor MSC: the initial MSC, which started the connection, keeps control)
GMSC
Cellular Communication Systems 25 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
X
BS BS
Before
X
BS BS
During
X
BS BS
After
GSM: Handover Principle
“Hard” handover, “make before break” Mobile assisted handoff/handover (MOHA):
MS sends regular measurement reports to network (own cell, neighbor cells, every 480 ms) Network (old BSC) decides upon handover (when, target cell) Network (old BSC) sets up new communication path Network (old BSC) instructs the MS to execute handover
Cellular Communication Systems 26 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
Handover procedure (change of BSC)
HO access
BTSold BSCnew
measurement
result
BSCold
Link establishment
MSC MS
measurement
report
HO decision
HO required
BTSnew
HO request
resource allocation
ch. activation
ch. activation ack HO request ack
HO command HO command
HO command
HO complete HO complete
clear command clear command
clear complete clear complete
„Make-before-break“ strategy
make
break
Cellular Communication Systems 27 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
Security in GSM
Security service
System was designed with a moderate level of security to authenticate the subscriber using a pre-shared key and challenge-response.
access control/authentication user SIM (Subscriber Identity Module): secret PIN (personal identification
number)
SIM network: challenge response method
no authentication of network!
confidentiality
voice and signaling encrypted on the wireless link (after successful authentication)
anonymity
temporary identity TMSI (Temporary Mobile Subscriber Identity)
newly assigned at each new location update
encrypted transmission
3 algorithms specified in GSM
A3 for authentication (“secret”, open interface)
A5 for encryption (standardized)
A8 for key generation (“secret”, open interface)
“secret”:
• A3 and A8
available in the
Internet
• network providers
can use stronger
mechanisms
Cellular Communication Systems 28 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
GSM - authentication
A3
RAND Ki
128 bit 128 bit
RAND
SRES* =? SRES
A3
RAND Ki
128 bit 128 bit
SRES 32 bit
SRES
Authentication Request (RAND)
Authentication Response (SRES 32 bit)
mobile network
AuC
MSC
SIM
Ki: individual subscriber authentication key SRES: signed response
SRES* 32 bit
Challenge-Response: • Authentication center provides RAND to Mobile
• AuC generates SRES using Ki of subscriber and
RAND via A3
• Mobile (SIM) generates SRES using Ki and RAND
• Mobile transmits SRES to network (MSC)
• network (MSC) compares received SRES with one
generated by AuC
Cellular Communication Systems 29 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
GSM - key generation and encryption
A8
RAND Ki
128 bit 128 bit
Kc
64 bit
A8
RAND Ki
128 bit 128 bit
SRES
RAND
encrypted
data
mobile network (BTS)
MS with SIM
AuC
BTS
SIM
A5
Kc
64 bit
A5
MS
data data
cipher
key
Ciphering: • Data sent on air interface ciphered for security • A8 algorithm used to generate cipher key • A5 algorithm used to cipher/decipher data • Ciphering Key is never transmitted on air
Cellular Communication Systems 30 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
2G+: GSM Evolution
Limits of GSM
limited capacity at the air interface:
data transmission standardized with only 9.6 kbit/s
advanced coding allows 14,4 kbit/s
not enough for Internet and multimedia applications
=> EDGE
inappropriateness for bursty and non-symmetrical data traffic
=> GPRS
Extensions
HSCSD (High-Speed Circuit Switched Data)
GPRS (General Packet Radio Service)
EDGE (Enhanced Data Rate for GSM Evolution)
EGPRS (EDGE und GPRS)
GERAN (GSM Interface to UMTS)
Cellular Communication Systems 31 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
HSCSD (High-Speed Circuit Switched Data)
continuous use of multiple time slots for a single user
(on a single carrier frequency)
asynchronous allocation of time slots between DL and UL
gain: net data rate up to 115,2 kbps (allocation of all 8 traffic channels)
mainly software update
additional HW needed if more than 3 slots are used
Uplink
Downlink
7 1 2 3 8 4 5 6 1 2
7 1 2 3 8 4 5 6 1 2
Cellular Communication Systems 32 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
GPRS (General Packet Radio Service)
Introducing packet switching in the network
Using shared radio channels for packet transmission over the air:
multiplexing multiple MS on one time slot
flexible (also multiple) allocation of timeslots to MS (scheduling by PCU Packet Control Unit in BSC or BTS)
using free slots only if data packets are ready to send (e.g., 115 kbit/s using 8 slots temporarily)
standardization 1998, introduction 2001
advantage: first step towards UMTS, flexible data services
carrier TS
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
Multiplexing Multislot capability
Cellular Communication Systems 33 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
connection-oriented packet switched core
GPRS architecture and interfaces
MS BSS GGSN SGSN
MSC
Um
EIR
HLR/
GR
VLR
PDN /
Internet
Gb Gn Gi
SGSN
Gn
o
Cellular Communication Systems 34 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
EDGE (Enhanced Data Rates for GSM Evolution)
Enhanced spectral efficiency depends on:
Size of frequency band
Duration of usage
Level of interference with others (power)
EDGE Technology:
EDGE can carry data speeds up to 236.8 kbit/s for 4 timeslots (theoretical maximum is 473.6 kbit/s for 8 timeslots)
Adaptation of modulation depending
on quality of radio path GMSK (GSM standard – 1 bit per symbol)
8-PSK (3 bits per symbol)
Adaptation of coding scheme (redundancy) depending
on quality of radio path (9 coding schemes)
Gain: data rate (gross) up to 69,2 kbps (compare to 22.8 kbps for GSM)
complex extension of GSM!
NodeB
UE 1
UE 2
Near-far problem
Cellular Communication Systems 35 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
2G to 3G Evolution: GSM - GPRS - UMTS
GSM
RAN
Base station
Base station controller
Base station
Base station
MSC
ISDN
GSM Core (Circuit switched)
HLR AuC EIR
GMSC
TransmissionATM based
GSM
Cellular Communication Systems 36 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
2G to 3G Evolution: GSM - GPRS - UMTS
GPRS Core (Packet Switched)
SGSN
GGSN
Inter-net
GSM
RAN
Base station
Base station controller
Base station
Base station
MSC
ISDN
GSM Core (Circuit switched)
HLR AuC EIR
GMSC
TransmissionATM based
GSM+GPRS
Cellular Communication Systems 37 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
2G to 3G Evolution: GSM - GPRS – UMTS R99
GPRS Core (Packet Switched)
SGSN
GGSN
Inter-net
GSM
RAN
Base station
Base station controller
Base station
Base station
UTRAN
Radio network controller
Base station Base station
Base station
MSC
ISDN
GSM Core (Circuit switched)
HLR AuC EIR
GMSC
TransmissionATM based
GSM+GPRS+UMTS R99
Cellular Communication Systems 38 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
2G to 3G Evolution: GSM - GPRS - UMTS R5 - IMS
GPRS Core (Packet Switched)
SGSN
GGSN
Inter-net
GSM
RAN
Base station
Base station controller
Base station
Base station
UTRAN
Radio network controller
Base station Base station
Base station
TransmissionIP based
3G Core
GERAN GERAN + UMTS R5 + IMS
Cellular Communication Systems 39 Andreas Mitschele-Thiel, Jens Mückenheim Oct-14
References
Jochen Schiller: Mobile Communications (German and English), Addison-Wesley, 2000
(most of the material covered in this chapter is based on the book)
Michel Mouly, Marie-Bernadette Pautet: The GSM System for Mobile Communications. Telecom Pub, Juni 1992
Jörg Eberspaecher, u. a.: GSM Switching, Services and Protocols. John Wiley and Sons Ltd, 2001
Siegmund Redl, u. a.: GSM and Personal Communications Handbook. Artech House, 1998
Gunnar Heine: GSM Networks: Protocols, Terminology, and Implementation. Artech House Mobile Communications Library. Artech House Publishers, 1998