Politecnico di Milano
Facoltà di Ingegneria dell’Informazione
MRN – 6 – GSM part 1
Mobile Radio Networks
Prof. Antonio Capone
A. Capone: Mobile Radio Networks
1
General characteristics of the
system
A. Capone: Mobile Radio Networks
2
History (1)
o  1982: the CEPT (Conférence Européenne des
Administrations des Postes et des
Télécommunications) creates special interest
group for the definition of a pan-European
cellular system: the Groupe Spécial Mobile, GSM
o  1985: Definition of the list of recommendations
(standard specifications) that the GSM will work
on (they ended up to be about 130: 1500 pages
in 12 volumes! ... plus all those of the evolution,
phases 2+ and 3 of GSM)
A. Capone: Mobile Radio Networks
3
History (2)
o  1988: the ETSI (European Telecommunication
Standards Institute) is created by the CEPT and the
European Commission, and the GSM standardization
activity is moved in the new organization
o  1990: GSM specifications are extended to DCS1800
(Digital Cellular System on 1800 MHz),
o  1992: The final version of the standard is released,
with the new meaning of the acronym Global System
for Mobile Communications
o  1992: Commercial launch of GSM (initially planned
for 1991 and then delayed because of the lack of
mobile terminals)
A. Capone: Mobile Radio Networks
4
History (3)
o  1994-95: Introduction of the SMS
o  1995-97: Commercial introduction of DCS1800
(operating at 1800 MHz)
o  1996: Standardization of enhanced voice codecs
o  1997: Dual-band terminals
o  1999: Standardization of GPRS
o  2000/01: Commercial introduction of GPRS
A. Capone: Mobile Radio Networks
5
General characteristics
o  Second Generation Digital System
(2G)
o  Multiple access scheme based on
multicarrier TDMA
o  Fixed frequency reuse
o  Services
n  Telephony with supplementary services
n  Circuit data services (single channel and
multiple channles)
n  Packet data (GPRS – General Packet
Radio Service)
A. Capone: Mobile Radio Networks
6
Frequencies
GSM /900
DCS/1800
downlink
uplink
downlink
uplink
F[ MHz ]
915
880 890
125+49 carriers
935
925
960
125+49 carriers
esteso
uplink
esteso
downlink
1785 1805
1710
374 carriers
1880
374 carroers
o  In UK and USA different frequencies around 1900 MHz
are used for DCS (1850÷1910 uplink, 1930÷1990
downlink).
A. Capone: Mobile Radio Networks
7
Radio carriers
o  Radio carriers are spaced of 200 kHz
200 kHz
f
o  On each carrier the transmission rate is 270.833
Kb/s
o  Carriers are identified by a ARFCN (Absolute Radio
Frequency Channel Number)
o  Modulation GMSK (Gaussian Minimum Shift Keying)
o  Each pair of frequencies for uplink and downlink are
spaced of 45 MHz in GSM 900 and 95 MHz in DCS
1800
A. Capone: Mobile Radio Networks
8
TDMA Frame
o  On each carrier the TDMA scheme allows
to create up to 8 channels for the
transmission of coded voice at 13 Kb/s
TDM Frame - 4.615 ms
5
6
7
0
1
2
3
4
5
6
7
0
1
2
3
BTS Transmits fdown
2
3
4
5
6
7
0
1
2
3
4
5
6
7
0
MS Transmits f up
Time slot = 577 µs
A. Capone: Mobile Radio Networks
9
Other characteristics
o  Power Control
n  Power emitted by base station and
mobile terminals is regulated with a
closed loop control
o  Discontinuous Transmission
n  During voice breaks, coding is paused for
reducing interference and energy
consumption
A. Capone: Mobile Radio Networks
10
System Architecture
A. Capone: Mobile Radio Networks
11
Network architecture
Um
NSS
BSC
VLR
Um
BTS
Network Switching Subsystem
HLR
A
AuC
Abis
Um
Abis
Um
BSC
EIR
A
BTS
GMSC
Abis
BSC
A
E
OMC
Abis
BTS
BTS
Radio
BSS
OMSS
Operation and
Maintenence Subsystem
MSC
VLR
A
RS
Fixed Telephone
Network
ISDN/PSTN
BSC
Base Station
Subsystem
A. Capone: Mobile Radio Networks
12
Mobile Station (MS)
o  Three categories with different nominal power:
n  vehicular: up to 20 W
n  portable: up to 8 W
n  personal (hand-terminal): up to 2 W (the
only category currently used)
Classe
1
2
3
4
5
Potenza massima nominale [W]
GSM 900 MHz
DCS 1800 MHz
.
1
8
0,25
5
4
2
.
0,8
.
A. Capone: Mobile Radio Networks
Potenza media nominale [mW]
GSM 900 MHz DCS 1800 MHz
.
120
960
30
600
480
240
.
96
.
13
Mobile Station (MS)
o  Characteristics
n  MS multi-band: can operate in different
bands (900, 1800, 1900, …)
n  MS multi-slot: can operate activating
multiple slots simultaneously (GPRS only)
o  MS includes ME (Mobile Equipment) and
SIM (Subscriber Identity Module)
n  ME is the main part of the mobile terminal
which includes all hw and sw modules.
Identified by an IMEI (International Mobile
Equipment Identifier)
n  SIM include the functions for identifying the
user and executing security procedures
A. Capone: Mobile Radio Networks
14
Subscriber Identity Module
(SIM)
o  It’s a smart card (with micro processor and memory)
which is essential to activate any ME
o  Multi be inserted in the reader slot of the ME
o  Serial number
n  Identify the SIM card
o  International Mobile Subscriber Identity (IMSI)
n  Identify the user in the network
o  Security authentication and cyphering information
n  A3 and A8 algorithm (used for authentication and
cyphering)
n  Ki, Kc (authentication and cyphering keys)
A. Capone: Mobile Radio Networks
15
Subscriber Identity Module
(SIM)
o  Temporary Network information
n  LAI (Location Area Identifier), last visited
location area
n  TMSI (Temporary Mobile Subscriber
Identity), temporary identifier used on
behalf of the IMSI
o  Subscribed services
o  Personal Identity Number (PIN)
o  Personal Unblocking Number (PUK)
o  Access rights
o  Prohibited networks
o  Call messages
o  Phone numbers
A. Capone: Mobile Radio Networks
16
Base Station System (BSS)
o  The BSS includes the functional units that deal
with the radio access
n  Radio coverage and communication with the
MSs through the radio interface
n  Radio resource management
o  The BSS includes:
n  Base Transceiver Station (BTS)
o  Implements the low level protocols of the radio
interfaces. It executes control commands sent by
the BSC
n  Base Station Controller (BSC)
o  Implements high level protocols controlling a
group of BTSs. Receive information on radio
interfance status from BTS and issues
configuration and management commands
A. Capone: Mobile Radio Networks
17
Network Switching
Subsystem (NSS)
o  The NSS is the circuit switching network
of the mobile system
o  It is used for circuit based services only
(voice)
o  It is basically a telephone network with
additional functionalities, nodes and
databases for managing mobile users
A. Capone: Mobile Radio Networks
18
Network Switching
Subsystem (NSS)
o  the NSS includes:
n  Mobile Switching Center (MSC):
o  Telephone switching station for mobile systems
n  Visitor Location Register (VLR):
o  It’s a database (usually integrated into the MSC) including
temporary information on users visiting the MSC area
n  Home Location Register (HLR):
o  It’s the main database of the network that includes all
information on mobile users. Includes also the
information on the currently visited VLR
n  Authentication Center (AuC):
o  Usually associated with the HLR. It is in charge of
authentication functions
n  Equipment Identity Register (EIR):
o  Include the IMEI of authorized devices
A. Capone: Mobile Radio Networks
19
Operation and Maintenence
Subsystem (OMSS)
o  The OMSS includes the units in
charge of controlling and monitoring
network elements by remote
o  The OMSS can
n  configure the functionalities of all
network devices
n  Visualize alarms on failures and
anomalous behavior
n  Visualize traffic statistical data
n  etc.
A. Capone: Mobile Radio Networks
20
Areas defined in GSM
o  PLMN (Public Land Mobile Network) Area:
n  Service area of an operator
o  MSC/VLR Area:
n  Area managed by a MSC. Data of users in this area
are stored in the VLR associated with the MSC
o  Location Area:
n  A MSC/VLR area includes one or more Location
Areas (LA). LA is identified by a LAI (Location Area
Identifier), transmitted by all BTSs in the area on
the broadcast channel
o  Cell:
n  area covered by a BTS. Identified by a BSIC (Base
Station Identity Code), transmitted on the
broadcast channel
A. Capone: Mobile Radio Networks
21
Base Transceiver Station
(BTS)
o  The BTS is in charge of implementing the low
level protocols of the radio interface
o  It transmits and receives signals from the MSs
implementing modulation, coding and
multiplexing functions on physical channels
o  It has the task of performing quality
measurements on physical channels and to
collect those performed by MSs (all
measurements are reported to the BSC)
o  It interfaces with the BSC with PCM channels at
64 kbit/s
o  Connects PCM channels with those of the radio
interface (traffic and signaling)
A. Capone: Mobile Radio Networks
22
Transcoder Rate Adaptation
Unit (TRAU)
o  GSM voice coding is at 13 Kbit/s while PCM is
at 64 Kbit/s
o  The transcoding is performed by the TRAU
o  The TRAU can be in the BTS, or (more often) in
the BSC
o  In this last case 13 Kbit/s flows must be
transmitted over 64 Kbit/s channels
o  On each 64 Kbit/s 4 flows at 13 Kbit/s are
multiplexed (they are first transformed into 16
Kbit/s flows with some padding)
o  For each GSM carrier (8 channels at 13 Kbit/s)
3 PCM channels at 64 Kbit/s are necessary
n  One for the signaling transported with the
link protocol LAPD
n  2 for the 8 traffic channels
A. Capone: Mobile Radio Networks
23
Transcoder Rate Adaptation
Unit (TRAU)
signaling
Frame delimiter
channel
1
2
30 31
0
1
2
…
15
16
15 16 17
…
29
30
30 31
0
1
125 µs
PCM channels
(64 Kbit/s)
GSM channels
(13 kbit/s)
PCM channels
(64 kbit/s)
TRAU
MSC
BSC
1 PCM
channel
per voice
circuit
A. Capone: Mobile Radio Networks
BTS
1 PCM
channel
for 4 voice
circuits
24
BTS structure
o  BTS is usually divided into
n  TRX (Transceiver)
o  Radio elements for transmission and
reception on a single radio carrier
n  BCF (Base Common Function)
o  Control element of a set of TRX that also
control connection interface with the BSC
A. Capone: Mobile Radio Networks
25
BTS structure
one TX
antenna Combiner
two RX
antennas
Splitter
….
….
TX
RX
Signal
Processing
TRX
Controller
TRX
TX
RX
Signal
Processing
TRX
Controller
TRX
……………....
A-bis Interface
PCM line or Radio
system 2 Mb/s
BSC
A. Capone: Mobile Radio Networks
26
Base Station Controller
(BSC)
o  A BSC controls a large number of BTSs: from a
few tens to hundreds
o  Main BSC tasks include:
n  Cell configuration with assignment of traffic
and control channels
n  Setup and release of channels at A and Abis
interfaces
n  Handover management
n  Paging management
n  Analysis of measurements received by BTSs
and MSs for the handover activation
A. Capone: Mobile Radio Networks
27
Base Station Controller
(BSC)
o  BSC is in charge of the Radio Resource
Management (RRM)
o  It is also a switching node
n  Even if it does not implement routing (this is
done by the MSC)
n  It connects BTS circuits with MSC circuits
performing (often) the trans-coding (TRAU)
n  It switches circuits during handover (intraBSC)
o  The BSCs can be co-located with the MSC, or
located in different sites
A. Capone: Mobile Radio Networks
28
Mobile Switching Centre
(MSC)
o  The MSC is the telephone switching station of
the mobile radio network and it has additional
functionalities for mobility management
o  It is associated with a VLR for storing
information on visiting users
o  The MSC is connected to its BSCs and other
MSC in the network
n  PCM channels (multiplexed with PDH or
SDH) are used for the connection
n  Some of the resources are used for the
signaling network SS7
o  One or more MSC (called Gateway MSC) per
PLMN are connected to the external telephone
networks
A. Capone: Mobile Radio Networks
29
Mobile Switching Centre
(MSC)
o  A MS can be reached by fixed telephone lines
through the number MSISDN
o  The call is routed to the GMSC which identifies
the HLR, retrieves from it user information
through the MSISDN
o  The HLR provides the MSRN (Mobile Station
Roaming Number)
n  MSRN is a temporary telephone number
(same structure of the MSISDN) assigned by
the visited VLR
n  MSRN allows the GMSC to route call to the
MSC currently visited by the user
A. Capone: Mobile Radio Networks
30
Mobile Switching Centre
(MSC)
o  The MSC is the main element of the signaling network
(we’ll see signaling later on)
o  The MSC implements the functions
n  CM (Connection Management)
o  originating call, terminating call, gateway
n  MM (Mobility Management)
o  location updating, periodic registration, authentication, …
o  Signaling protocol for communication with other
network elements:
n  DTAP (Direct Transfer Application Part) for direct
communication with MS
n  BSSMAP (BSS Management Application Part) for
communication with BSC
n  MAP (Mobile Application Part) for communication with other
MSC, VLR, HLR, EIR, AuC
A. Capone: Mobile Radio Networks
31
Home Location Register
(HLR)
HLR
o  It is a database permanently associated with a
GMSC
o  It stores data on all MSs associated with GMSC
(through the MSISDN)
o  The HLR stores permanent information like
IMSI MSISDN, registered services, etc.
o  The HLR also stores temporary information like
the currently visited VLR address,
authentication and encryption temporary keys,
etc.
A. Capone: Mobile Radio Networks
32
Visitor Location Register
(VLR)
VLR
o  It’s a database for storing temporary
information on users in the area of a MSC
o  In the VLR all relevant data of visiting users are
duplicated from the HLR
o  The IMSI is mapped into a TMSI (Temporary
Mobile Subscriber Identity) for avoiding
transmitting it in clear text over the air
interface
A. Capone: Mobile Radio Networks
33
Security procedures
o  Authentication:
n  It is in charge of verifying user identity
and protect the system from non
authorized access
o  Encryption:
n  It is in charge of coding the flow of data
so that it cannot decoded by others
o  In GSM authentication and encryption
are strongly related for the key
management
A. Capone: Mobile Radio Networks
34
Security procedures
o  Ingredients:
n  Ki
o  Authentication key of 128 bits stored in the AuC
and in the SIM
n  RAND
o  Random number of 128 bist generated by the AuC
and then sent to the MSC
n  A3
o  Algorithm for the authentication stored in the AuC
and the SIM
n  A8
o  Algorithm for generating the encryption key Kc,
stored in the AuC and the SIM
  Procedure results:
Multiple Triples
  Kc encryption key
(RAND, SRES, Kc)
  SRES result of the
Are generated for each IMSI
authentication algorithm
and stored in the HLR
A. Capone: Mobile Radio Networks
35
Security procedures
o  Authentication:
MS
Ki
Network
RAND
Ki
A3
RAND
Random
number
generator
A3
SRES(network)
SRES(ms)
Yes
Access
granted
A. Capone: Mobile Radio Networks
equal?
No
Access
denied
36
Security procedures
o  Encryption:
Network
MS
Ki
Ki
RAND
A8
A8
Kc
A5
RAND
Random
number
generator
Kc
informazioni cifrate
A. Capone: Mobile Radio Networks
A5
37
Security procedures:
Roles of network elements
o  Authentication Centre (AuC) AuC
n  It stores secret keys Ki of all users
n  Generated random numbers and calculates SRES and
encryption keys Kc
n  Provides triples to the other network elements
richiesta
(TMSI)
MS
RAND
SRES
A. Capone: Mobile Radio Networks
MSC/VLR
VLR
IMSI
triplette
IMSI
HLR
AuC
triplette
RAND,
SRES,
Ki
38
Security procedures:
Roles of network elements
o  Role of the BSS in the encryption:
MS
Ki
A8
MSC/VLR
request
RAND
SRES
BSS
Kc
A5
Encrypted text
A. Capone: Mobile Radio Networks
A5
Kc
VLR
Clear text
39
Security procedures:
TMSI allocation
o  All communications are initiate by the MS that
transmits its ID (IMSI) for activating the
authentication procedure
o  For avoiding transmitting the IMSI in clear over
the air interface the VLR allocates to each MS a
TMSI (Temporary Mobile Subscriber Identity)
o  The IMSI is used only when a TMSI is not
available
o  At each location update the VLR can allocate a
new TMSI to MS
A. Capone: Mobile Radio Networks
40
IMSI
o  It’s the internal user identification number
o  It is composed of three fields:
n  MCC: Mobile Country Code (3 digits)
n  MNC: Mobile Network Code (2 digits)
n  MSIC: Mobile Subscriber Identification
Number (up to 10 digits)
o  For example 222 01 4572228769, identifies an
Italian SIM (222) of the TIM mobile operator
(01)
o  The telephone number (MSISDN) is completely
independent from the IMSI
A. Capone: Mobile Radio Networks
41
Equipment Identity Register
(EIR)
EIR
o  It is an optional network database
o  It includes the ID (IMEI) and the characteristics of
all mobile equipments (manufacturer, country,
etc.)
o  It can be used for preventing the use of stolen
devises
MS
Conn. request
MSC/VLR
IMEI request
IMEI reply
IMEI check
EIR
reply
A. Capone: Mobile Radio Networks
42
Operation and Maintenance
Subsystem (OMSS)
o  It is the system in charge of all
management and monitoring functions
(both technical and administrative) of
the network
o  It performs billing, traffic control,
alarm messages management, BTS
and BSC monitoring, configuration, etc.
o  It allows to configure by remote all
network elements and to check their
correct operation
A. Capone: Mobile Radio Networks
43
Operation and Maintenance
Subsystem (OMSS)
o  It is based on a hierarchical architecture
n  OMC (Operation & Maintenance Centre) regional
n  NMC (Network Management Centre)
NMC
OMC
OMC
HLR
HLR
EIR
MSC
MSC
BSC
A. Capone: Mobile Radio Networks
BSC
BSC
EIR
BSC
44
Radio Interface
A. Capone: Mobile Radio Networks
45
Radio interface
FDD (Frequency Division Duplexing)
45 MHz di separazione uplink/downlink
downlink
uplink
f
200 kHz
915
880 890
6
7
0
1
2
3
4
5
6
7
0
960
925
TDM Frame - 4.615 ms
5
935
1
2
3
BTS Transmits fdown
3 slot offset uplink/downlink
2
3
4
5
6
7
0
1
2
3
4
5
6
7
0
MS Transmits f up
Time slot = 577 µs
A. Capone: Mobile Radio Networks
46
Radio interface
o  Modulation:
n  MGSK (Minimum Gaussian Shift Keying)
o  Phase continuous modulation with Gaussian pulse (intersymbol
interference)
o  Channel coding:
n  Convolutional
o  With several different rates
o  Equalization:
n  Known bit sequences transmitted in each physical burst
n  Equalization filter synthetized at the receiver
o  Voice coding:
n  13 Kbit/s (RPE codec - full rate)
n  12.2 Kbit/s (CELP codec - enhanced full rate)
A. Capone: Mobile Radio Networks
47
Transmission and reception
Voice
13 Kb/s
22.8 Kb/s
Channel
coding
Source
coding
22.8 kb/s
Timing
Modulation
GMSK
32 kb/s
Control
Voice
Decoder
Channel
Decoder
Equalizator
A/D
Converter
Demodulator
o  From 13 Kbit/s of the codec we get to 32 Kbit/s on the
radio channel
A. Capone: Mobile Radio Networks
48
Frequency Hopping
o  The impact of multipath fading depends on the
signal frequency
o  There may be carriers with low attenuation and
carriers with high attenuation
f
o  Since transmission are protected by FEC
coding, it is more convenient to spread errors
o  GSM uses a frequency hopping mechanism that
changes frequency slot by slot
A. Capone: Mobile Radio Networks
49
Power Control
o  MS power is controlled by the BTS, and BTS
power (traffic carriers only) is controlled by the
MS
o  The power control mechanism is based on a
closed loop scheme with up/down commands
send in both directions
o  The power control step is 2 dB
o  The objective is keeping received power
constant
o  The power control reduces interference and
energy consumed by MSs
A. Capone: Mobile Radio Networks
50
Synchronization
o  Carrier synchronization
n  Frequency synchronization
o  Slot synchronization
n  Time slot synchronization
o  Frame synchronization
n  Frame number
o  Base station synchronization (optional)
n  Slots and frame
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51
Carrier synchronization
o  The carrier frequency synchronization is
achieved by the MS listening on the broadcast
channel transmitted by the BTS
o  On this channel, at periodic intervals, the BTS
transmit a special slot in which a known
sequence is transmitted
o  This is used by the MS to adjust the local
frequency
A. Capone: Mobile Radio Networks
52
Slot and Frame
synchronization
o  GSM channels are based on a multi-frame
scheme (for instance, the broadcast channel is
transmitted only in some frames of the multiframe)
o  Also the Frequency Hopping sequence depends
on the multi-frame
o  All MSs must know the frame number in the
multi-frame to decode the TDMA channels
correctly
o  The BTS transmits on the broadcast channel
some information that allows MSs to obtain the
slot synchronization and the Frame Number
A. Capone: Mobile Radio Networks
53
Timing advance
o  The TDMA transmissions require
guard times due to propagation
delays
2τ
Time
Reference
Propagation delay
τ=dv
d - distance
v – light speed
Tg = max(2τ i )
i
A. Capone: Mobile Radio Networks
54
Timing advance
o  GSM is designed for cells with a radius of up to
Rmax=37.8 Km
o  The guard time should then be 2τ = 2
x 108 = 233 µs
x
35 / 3
o  Which is equivalent to 68,25 bits at carrier rate
of 270.8 Kb/s
A. Capone: Mobile Radio Networks
55
Timing advance
o  For reducing the guard time the BTS estimates
the delay and send it to the MS on a control
channel.
o  The MS can then compensate the delay and
reduce guard time
o  Guard time of normal slots in GSM is 8.25 bits
(30.46 µsec)
2) Delay
4) Other
Transmissions
estimation
1)  First
Transmission
3) Timing
Advance command
A. Capone: Mobile Radio Networks
56
Physical block (Burst)
o  The burst is the physical layer PDU transmitted in a
time slot
o  Due to TDMA scheme each burst is an autonomous
transmission with its own power profile
dB
+4
+1
-1
-6
-30
-70
10
8 10
A. Capone: Mobile Radio Networks
542.8
10
8 10
µs
57
Bursts
o
Normal Burst
n
o
Used for data transmission in traffic
channels
Access Burst
n  Used for the first access on the Random
Access Channel (RACH)
n  It has a long guard time since the timing
advance mechanism is still not active
A. Capone: Mobile Radio Networks
58
Bursts
o
Frequency Correction Burst
n  Used for frequency synchronization on the
Frequency Correction Channel (FCCH) - 142
bits all set to “1”
o
Synchronization Burst
n  Used for transmitting the slot and frame
synchronization information
o
Dummy Burst
n  Padding burst
n  Used in downlink only for ensuring a
constant power profile to broadcast carrier
A. Capone: Mobile Radio Networks
59
Normal Burst
148 bit = 546.12 µs
T
3
•
•
•
•
•
Coded bits
57
Training
S Sequence S
1
1
26
Coded bits
57
Guard
T Period
3 8.25
577 µs
T-bits: set to 0, used as frame delimiters
S-bits: (stealing bits) indicate is the burst has user data
or signaling
Coded Data: user data (voice, data, etc.), 114 bits
Training Sequence: control bits for the channel
equalization
GP: Guard time
A. Capone: Mobile Radio Networks
60
Access Burst
T
8
Synchronisation
41
Coded bits
36
T
3
Guard Period
68.25
577 µs
o  First transmission in uplink
o  It has 156.25 bits
n  8 tailing bits
n  41 synchronization sequence
n  36 coded bits
n  3 tailing bits
n  68.25 bits of guard time
A. Capone: Mobile Radio Networks
61
Frequency Correction Burst
T
3
Fixed bit sequences
142
Guard
T Period
3 8.25
577 µs
o  It includes 148 + 8.25 bits
n  2 x 3 tail control bits
n  142 fixed bit sequences
o  All set to 0
o  They allows frequency synchronization
n  8,25 bits of guard period
A. Capone: Mobile Radio Networks
62
Synchronization Burst
T
3
Coded bits
39
Training sequence
64
Coded bits
39
Guard
T Period
3 8.25
577 µs
o  It includes 148 + 8.25 bits
n  2 x 3 tail control bits
n  2 x 39 coded bits
o  25 bits of information
o  that become are 78 coded bits
o  Divided into to fields of 39 bits each
n  64 bits of training sequence
n  8.25 bits of guard period
A. Capone: Mobile Radio Networks
63
Dummy Burst
T
3
Pseudo-random sequence
142
Guard
T Period
3 8.25
577 µs
o  Used in downlink on the carrier where the
control channels are allocated
o  It ensures that power profile is constant
o  Power control is not used on this carrier
o  It includes 148 + 8.25 bits
n  2 x 3 tail control bits
n  142 pseudo-random sequence
n  8.25 bits of guard period
A. Capone: Mobile Radio Networks
64
Logical channels
o  “Logical channels” in GSM identify
different types of information
transmitted over the air interface:
n  Signaling
n  Traffic data
o  They are divided into:
n  Traffic channels and control channels
n  Common channels and dedicated
channels
A. Capone: Mobile Radio Networks
65
Logical channels
LOGICAL
CHANNELS
COMMON
CHANNELS
Broadcast
CONTROL
CHANNELS
FCCH
SCH
DEDICATED
CHANNELS
Common
CONTROL
CHANNELS
BCCH
PCH
RACH
Dedicated
CONTROL
CHANNELS
AGCH
FCCH=Frequency Correction CHannel
SCH=Synchronisation Channel
BCCH=Broadcast Control CHannel
PCH=Paging CHannel
RACH=Random Access CHannel
AGCH=Access Grant CHannel
A. Capone: Mobile Radio Networks
SDCCH
SACCH
TRAFFIC
CHANNELS
FACCH
TCH/F TCH/H TCH/E
SDCCH=Stand-alone Dedicated Control
CHannel
SACCH=Slow Associated Control CHannel
FACCH=Fast Associated Control CHannel
TCH/F=Traffic CHannel Full rate
TCH/H=Traffic CHannel Half rate TCH/
E=Traffic CHannel Enhanced Full rate
66
Traffic Channels (TCH)
o  Channels transporting user data
(voice or data)
o  They can be:
n  Full Rate channels: 22,8 Kb/s
n  Half Rate channels: 11,4 Kb/s
trama 1
Full Rate
Half Rate
Tf
trama 2
Tf
Tf
t
Th Th
Th Th
t
slot
A. Capone: Mobile Radio Networks
67
Control Channels (CCH)
o  Used for transporting signaling of different
types (14 types of control channels)
o  Three categories of CCHs
n  Broadcast Channels (BCH): system
information transmitted in the downlink
n  Common Control Channels (CCCH): shared
channels for connection initialization
n  Dedicated Control Channels (DCCH):
signaling channels dedicated to specific
traffic connections
Broadcast
CONTROL
CHANNELS
FCCH
SCH
Common
CONTROL
CHANNELS
BCCH
A. Capone: Mobile Radio Networks
PCH
RACH
Dedicated
CONTROL
CHANNELS
AGCH
SDCCH
SACCH
FACCH
68
Broadcast Channels (BCH)
Broadcast
CONTROL
CHANNELS
o  FCCH (Frequency Correction Channel):
Channel for frequency synchronization
(frequency correction bursts only)
o  SCH (Synchronization Channel): it
includes BTS id (BSIC) frame number
(FN)
o  BCCH (Broadcast Control Channel):
General system information
A. Capone: Mobile Radio Networks
69
BCCH (Broadcast Control
Channel)
Broadcast
CONTROL
CHANNELS
o  Number of common control channels
(2 bits)
o  1 bit for the common control TDMA
scheme type
o  Number of block reserved to AGCH (3
bits)
o  Periodicity of paging di messages (3
bits)
o  Frequency Hopping parameters
A. Capone: Mobile Radio Networks
70
Common Control Channels
(CCCH)
Common
CONTROL
CHANNELS
o  PCH (Paging Channel): downlink used
by the BTS per informing MSs of
incoming calls, broadcasted over the LA
o  RACH (Random Access Channel): uplink
used by MSs for accessing the network
(new calls, location update, etc.). It’s
contention based.
o  AGCH (Access Grant Channel):
downlink, used for replies to RACH
requests
A. Capone: Mobile Radio Networks
71
Dedicated Control Channels
(DCCH)
Dedicated
CONTROL
CHANNELS
o  SACCH (Slow Associated Control Channel):
exchange of measurements during connection
between MS and BTS (signal strengths, quality,
….). Multiplexed with the traffic channel (184
bits every 20 ms of measurements)
o  FACCH (Fast Associated Control Channel): used
for signaling during handover. It partially
replaces traffic channel in the TDMA structure.
o  SDCCH (Stand-alone Dedicated Channel):
Signaling channel assigned in the first phase of
call setup after the RACH/AGCH message
exchange (identification, authentication, call
set-up, …)
A. Capone: Mobile Radio Networks
72
Slow Associated Control
Channel (SACCH)
Dedicated
CONTROL
CHANNELS
o  In downlink:
n  Power control commands
n  The same BCCH info (they cannot be decoded by
the MS because it is listening on the traffic
channel)
n  BCCH-FREQ-NCELL “N” (# carriers of BCCH of
neighboring cells)
n  BSIC-NCELL “N” (BSIC of neighboring cells)
o  In uplink: measurements from the MS:
n  RXLEV-SERVING-CELL (signal strength from the
serving BTS)
n  RXQUAL-SERVING-CELL (BER measured on the
downlink)
n  RXLEV-NCELL “N” (signal strength from
neighboring cells)
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73
Connection setup
MS
BS
BCCH
Broadcast info
RACH
Access request
AGCH
SDCCH assignment
SDCCH
TCH + SACCH
A. Capone: Mobile Radio Networks
Signaling traffic +
TCH assignment
traffic + signaling
74
Random Access (RACH)
o  On the RACH a random access scheme is used
o  Collisions can occur
o  The correct reception of the access request is
acknowledged with a reply message on the
AGCH
o  In the RACH a temporary random identifier is
included which is also reported in the AGCH
reply
o  The RACH access protocol is a simple SlottedALOHA
A. Capone: Mobile Radio Networks
75
Mapping of logical channel
into physical channels
o  For using different rates for different logical
channels the mapping into physical channels
(TDMA structure) is based on a multi-frame
Frame 1
0 1 2
Frame 2
31 32 33 34
63 64 65 66
0 1 2
Multi-Frame
A. Capone: Mobile Radio Networks
76
Example: SACCH
o  A normal burst has 114 bits of data
148 bit = 546.12 µs
T
3
Coded bits
57
Training
S Sequence S
1
1
26
Coded bits
57
Guard
T Period
3 8.25
577 µs
o  Using one slot per frame we have 114 [bits]/
4.6 [ms]=24.7 Kb/s
o  The rate of coded voice is however 22,8 Kb/s
o  We have additional 1,9 Kb/s equivalent to 1
SLOT every 13 frames
o  SACCH: 1 SLOT every 26 frames for a speed
of 950 bits/sec.
A. Capone: Mobile Radio Networks
77
TCH+SACCH
0
12
25
T T T T T T T T T T T T AT T T T T T T T T T T T -
o  Traffic CHannels (T) and Slow Associated
Control Channel (SACCH) (A) are multiplexed
together
o  In a multi-frame of 26 frames (120 ms)
A. Capone: Mobile Radio Networks
78
TCH+SACCH
Downlink, Uplink
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
S
A
C
C
H
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
T
C
H
/
F
R
I
D
L
E
0 1 2 3 4 5 6 7 8 9 10 1112 131415 1617181920 2122 2324 25
0
1
2
3
4
5
6
7
Normal Normal Normal Normal Normal Normal Normal Normal
burst burst burst
burst burst burst
burst burst
577 µs
4,615 ms
A. Capone: Mobile Radio Networks
79
Common control channels
o  One slot (slot 0) an a carrier (carrier 0) is
assigned to common control channels with a
multi-frame of 51 frames (235.38 ms)
trama 0
7 0 1 2
trama 2
7 0 1 2
trama 50
7 0 1 2
7 0 1 2
supertrama
A. Capone: Mobile Radio Networks
80
Common control channels
o  In the downlink
•
•
•
•
FCH
SCH
BCCH
CCH
0
FS
o
50 frame.
B
C
FS
B
C
FS
B
C
-
In the uplink Random Access Channel (RACH)
RRRRRRRRRRRRRRRRRRRR
A. Capone: Mobile Radio Networks
RRRRRRRRRR
81
SDCCH
o  Another slot is used for creating 8 Stand-Alone
Dedicated Control Channel (SDCCH) (S)
channels
o  Used for call setup and other signaling (LA,
SMS)
o  The 8 channels are assigned 3 slots each in a
26 frames multi-frame
0
12
25
SSSSSSSSSSSSASSSSSSSSSSSS -
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82
Voice coding: 13 Kb/s
o  Every 20 ms codec generates 260 bits that are divided
into 3 groups (class 1a – 50 bits, class 1b - 132 bits,
class 2 – 78 bits)
Class 1a
50 bit
Class 1b
132 bit
Class 2
78 bit
Cyclic code (53, 50)
Class 1a
53 bit
Class 1b
132 bit
Tail
4 bit
Class 2
78 bit
Convolutional code 1/2
Class 1a + Class 1b
378 bit
Class 2
78 bit
456 bit

456 bits in 20 ms equivalent to a rate of 22.8 Kb/s
A. Capone: Mobile Radio Networks
83
Voice coding: 13 Kb/s
20 ms coded voice
456 bit
Fragment 1
Fragment 2
Fragment 3
Fragment 4
114 bit
114 bit
114 bit
114 bit
T
3
Coded bits
57
Training
S Sequence S
1
1
26
Coded bits
57
T
3
Normal Burst
o  4 Normal Burst transmitted into 4 frames 4.6*4 = 18.4 ms
o  But one every 13 is for signaling therefore on average
18.4*13/12=20 ms
A. Capone: Mobile Radio Networks
84
Voice coding: 13 Kb/s
o  Interleaving: 4 blocks of 114 bits
B(i,1)
B(i,2)
B(i,3)
B(i,4)
B(i+1,1) B(i+1,2) B(i+1,3) B(i+1,4)
...
B(i+3,3) B(i+3,4)
Interleaving
B(i,1)
B(i+1,1) B(i+2,1) B(i+3,1)
B(i,2)
A. Capone: Mobile Radio Networks
B(i+1,2) B(i+2,2) B(i+3,2)
...
B(i,4)
B(i+1,4) B(i+2,4) B(i+3,4)
85
Data channel coding:
9.6 Kb/s
12
48 bit
60 bit
60 bit
60 bit
60 bit
Tail
4 bit
240 bit
codice convoluzionale 1/2
488 bit
puncturing di 32 bit
456 bit
A. Capone: Mobile Radio Networks
86
Signaling coding:
SDCCH, BCCH, PCH, AGCH
184 bit
codice FIRE (224, 184)
184 bit
40 bit
Tail
4 bit
codice convoluzionale 1/2
456 bit
A. Capone: Mobile Radio Networks
87