PART 3 Introduction to GSM

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First system:
NMT-450 (Nordic Mobile Telephone)
Scandinavian standard; adopted in most of Europe
450 MHZ band
First european system (Sweden, october 1981)
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RTMI (Radio Telefono Mobile Italiano)
Market: 1973
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RTMS (Radio Telefono Mobile di Seconda Generazione)
450 MHZ
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Total Access Communication System
900 MHZ
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First generation: 1980’s
Several competing standards in
different countries
NMT (Nordic Mobile Telephone)
Scandinavian standard; adopted in most
of Europe
First european system (Sweden, 1981)
TACS (Total Access Communication
Systems), starts in 1985
UK standard; A few of Europe, Asia,
Japan
AMPS (Advanced Mobile Phone
Service)
US standard
C-Netz (Only in Germany)
Radiocom 2000 (Only in France)
Giuseppe Bianchi
Analog transmission
Frequency modulation
Various bands:
NMT:
450 MHz first
900 MHz later
TACS
900 MHz
1230 bidirectional
channels (25KHz)
AMPS
800 MHz
Today still in use in lowtechnology countries
And not yet completely dismissed
in high-tech countries
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Circuit switched data communication
Uses up to 4 slots (1 slot = 9.6 or 14.4 Kbps)
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Packet data (use spectrum only when needed!)
Up to 115 Kbps (8 slots)
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Higher data rate available on radio interface (3x)
» Up to 384 Kbps (8 slots)
» Thanks to new modulation scheme (8PSK)
» May coexist with old GMSK
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ITU standard: IMT-2000 (International Mobile
Telecommunication – 2000)
UMTS forum created in 1996
Later on 3GPP forum (bears most of standardization
activities)
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But several other proposals accepted as “compatible”
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Much faster than projections!
August 2000: 372 GSM networks, 362M customers
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The network in the proper sense
MS: Mobile Station
operator
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Manages transmission path from MS
to NSS
External
Networks
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Communication and interconnection
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GSM network administration tools
Users
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Radio Interface (Um)
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GSM separates user mobility from equipment mobility,
by defining two distinct components
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The cellular telephone itself (or the vehicular
telephone)
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IMEI (International Mobile Equipment Identity)
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Fixed installed chip (plug-in SIM) or
exchangeable card (SIM card)
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IMSI (International Mobile Subscriber Identity)
MSISDN (Mobile Subscriber ISDN number)
» the telephone number!
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15x17mm
Mini UICC
12x15mm
(standardized feb 04)
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Terminal
Mobile
Mobile
Terminal
Equipment
Termination
Equipment Termination
Mobile Termination functions
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Terminal Equipment functions
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Terminal Adaptor functions
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Terminal
Equipment
Terminal
Adaptor
Mobile Equipment
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Mobile
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Uniquely identifies the mobile equipment
15 digits hierarchical address
assigned to ME during manifacturing and “type approval” testing
Type approval procedure: guarantees that the MS meets a
minimum standard, regardless of the manifacturer
IMEI structure:
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Before april 1, 2004:
TAC = Type Approval Code
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Details: http://www.numberingplans.com/
Includes IMEI analyser
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Protection against stolen and malfunctioning terminals
Italian “memorando di intesa” signed on maj 5, 2003
Equipment Identity Register (EIR): 1 DataBase for each operator; keeps:
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valid IMEIs
Corresponding MEs may be used in the GSM network
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IMEIs of all MEs that must be barred from using the GSM network
Exception: emergency calls (to a set of emergency numbers)
Black list periodically exchanged among different operators
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IMEIs that correspond to MEs that can be used, but that, for some
reason (malfunctioning, obsolete SW, evaluation terminals, etc), need
to be tracked by the operator
A call from a “gray” IMEI is reported to the operator personnel
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subscriber’s secret authentication key (Ki)
Authentication algorithm (“secret” algorithm - A3 – not unique)
Cipher key generation algorithm (A8)
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PIN (Personal Identification Number, 4-8 digits)
PUK (PIN Unblocking Key, 8 digits)
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Uniquely identifies the user (SIM card)
GSM-specific address
unlike MSISDN - normal phone number
15 digits hierarchical address
assigned by operator to SIM card upon subscription
IMSI structure (follows ITU-T E.212 numbering plan):
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MSISDN: the “usual” telephone number
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GSM is the first network to distinguish
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Separation IMSI-MSISDN protects confidentiality
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Separation IMSI-MSISDN allows
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single IMSI may be associated to several MSISDN numbers
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CC, NDC of the visited network
SN assigned by VLR
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has significance only in this area
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reduces problem of
“eavesdropping”
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Subscriber Number (SN)
assigned to provide routing
information towards actually
responsible MSC
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MS
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BTS
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MSC
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MSC
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Interface
BSS
A-bis
Interface
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Interface
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Transmitter and receiver devices, voice coding & decoding, rate adaptation for data
Provides signaling channels on the radio interface
Limited signal and protocol processing (error protection coding, link layer LAPDm)
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performs most important radio interface management functions:
Radio channels allocation and deallocation; handover management; …
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HF
Transmitter
Input
Filter
HF
Receiver
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Digital
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Transmission
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filter
Slow freq.
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(to MS)
Operation and Maintenance Functionality/clock distribution
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In essence, BTS is
a complex modem!
Abis Interface (to BSC)
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switch calls from MSC to
correct BTS
and conversely
Protocol and coding
conversion
for traffic (voice) &
signaling (GSM-specific
to ISDN-specific)
Manage MS mobility
Enforce power control
BTS:
-collects speech traffic
-Deciphers and removes error protection
-Result:
-13 kbps air-interface GSM speech-coded signal
MSC:
-A modified ISDN switch
-Needs to receive ISDN-coded speech
-64 kbps PCM format (A-law)
Rationale: re-use existing ISDN switches & protocols
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Transcoding and
Rate Adaptation
Unit (TRAU)
needed!
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On BTS
On BSC
13 kbit/s
BTS
13 kbit/s
13 kbit/s
BTS
MSC
BSC
TRAU
64 kbit/s
16 kbit/s
BSC
BTS
On MSC
64 kbit/s
64 kbit/s
MSC
TRAU
64 kbit/s
(4x16 sub-mux)
16 kbit/s
BSC
TRAU
MSC
Why 16 kbps instead of 13? Inband signalling needed for BTS control of TRAU
(TRAU needs to receive synchro & decoding information from BTS)
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handover execution and control
paging
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N-plicated for reliability reasons
In large operator networks, there may be 2+ HLR with distinct
information, although MSISDN-HLR association needs to be
introduced (e.g. first two digits of the Subscriber Number)
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Permanent information associated
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Temporary information associated
to the user
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Current VLR address (if avail)
Current MSC address (if avail)
MSRN (if user outside PLMN)
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Entry created when an MS enters the MSC area (registration)
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Tracking and routing information
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Network measurement and control functions
Monitored and initiated from the OMC
(Operation and Maintenance Center)
Basic functions
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configuration, operation, performance management, statistics
collection and analysis, network maintenance
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