GSM : The Network Layer
Rasha Morsi
Presentation Outline:
Radio Resource Management
Mobility Management
Call Control Management
Layer 3 Protocols
Mobile Application Part protocol
Flow chart of initial mobile acquisition
Mobile originating call procedure
Structure of a layer 3 message
Why GSM?
Acronyms:
TMSI : Temporary Mobile Subscriber Identity
IMSI : International Mobile Subscriber Identity
BCCH : Broadcast Control Channel
FCCH : Frequency Correction Channel
SCH : Synchronization Channel
RACH : Random Access Channel
AGCH :Access Grant Channel
SDCH : Stand-alone Dedicated Control Channel
SABM: Set Asynchronous Balanced Mode
Acronyms (cnt’d)
RIL3 : Radio Interface Layer 3
MAP : Mobile Application Part
UA : Unnumbered Acknowledge
TUP : Telephone User Part
ISUP : ISDN User Part
TCH : Tariff Channel
ACCH: Associated Control Channel
BSSMAP : BSS Management Part
SCCP : Signaling Connection Control Part
Air (Um)
Interface
A-bis
Interface
CCM
RIL3 - CC
MM
RR
RIL3 - MM
RIL3 - RR
DATP
RSM
A
B
C,D HLR/AuC
MAP/D
BSSMAP
Distribution Protocol
MAP/C
MAP/E
MAP/G
TUP,
ISUP
O
S
I
Component Sublayer
TCAP
Transaction Sublayer
L
A
Y
E
R
S
Presentation
Layer *
Session
Layer *
Transport
Layer *
Network
Layer
Data Link
Physical
SCCP
SCCP
SCCP
SCCP
MTP3
MTP3
MTP3
MTP3
MTP3
MTP2
MTP2
MTP2
MTP1
MTP1
MTP1
LAP-D
LAP-D
MTP2
MTP2
Radio
64kbps ch
MTP1
MTP1
Radio Resource Management sublayer:
Tasks closely related to physical layer
management of frequency spectrum
everything to do with maintaining a clear
channel between PLMN and MS
Handles all procedures necessary to establish,
maintain and release dedicated radio channels
Procedures in RR layer:
Channel assignment
Channel release
Channel change and handover
Change of channel frequencies, hopping sequences
(algorithms) and frequency tables
Measurement reports from the MS
Power control and Time advance
Modification of channel modes (speech and data)
Cipher mode setting
Mobility Management sublayer:
Cope with all effects of handling a mobile user
that are not directly related to radio functions.
Support of user mobility, registration, and
management of mobility data
checking user and equipment identity
user allowed to use services ? Extra services
allowed ?
Support user confidentiality ( register user under a
TMSI)
Provision of user security
Provision of an MM connection, based on
existing RR connection, to the CM sublayer
Procedures in MM sublayer :
Location Update
Periodic updating
Authentication procedure
IMSI attach procedure (on power up a MS will
present its IMSI to network and get a TMSI)
IMSI detach ( on power off of a MS, detach
procedure to tell network it is no longer in service.
TMSI reallocation
Identification
Call Control sublayer:
Manages all functions necessary for circuit-switched
call control in GSM PLMN
includes several protocol entities e.g. CC itself, SS,
and SMS.
CC entity includes
control functions defined for ISDN network signaling,
messages and procedures required for
call establishment,
call clearing
call information etc.
Procedures in CC sublayer:
Call establishment for mobile-originated calls
Call establishment for mobile-terminated calls
Changes of transmission mode during an
ongoing call ( incall modification)
Call reestablishment after interruption of an
MM connection
Dual-tone multifrequency (DTMF) control
procedure for DTMF transmission
Air (Um)
Interface
A-bis
Interface
CCM
RIL3 - CC
MM
RR
RIL3 - MM
RIL3 - RR
DATP
RSM
A
B
C,D HLR/AuC
MAP/D
BSSMAP
Distribution Protocol
MAP/C
MAP/E
MAP/G
TUP,
ISUP
O
S
I
Component Sublayer
TCAP
Transaction Sublayer
L
A
Y
E
R
S
Presentation
Layer *
Session
Layer *
Transport
Layer *
Network
Layer
Data Link
Physical
SCCP
SCCP
SCCP
SCCP
MTP3
MTP3
MTP3
MTP3
MTP3
MTP2
MTP2
MTP2
MTP1
MTP1
MTP1
LAP-D
LAP-D
MTP2
MTP2
Radio
64kbps ch
MTP1
MTP1
Layer 3 Protocols:
RIL3-RR :
establish and release radio connections between
an MS and various BSCs for the duration of a call
despite user movements.
Provide system information broadcasting
provide inter- and intracell change of channels
provides ciphering mode setting
Radio Subsystem Management ( RSM ):
provides RR functions between BTS and BSC
Direct Transfer Application Part ( DTAP ):
provide RR messages between MS and MSC
BSSMAP :
provides RR messages between BSC and MSC
Distribution Protocol : distinguishes between
BSSMAP and DTAP
RIL3-MM :
deals with MS location management
location updating
IMSI attach
periodic updating
security aspects of system
authentication
TMSI reallocation
identification.
RIL3-CC :
deals with mobile originating and terminating call
establishment
SS
SMS
All MM and CC functions reside in MSC. All
CC and MM messages are not interpreted by
either the BTS or BSC. An RR connection
needs to be established before communication
between MS and MSC can occur.
Air (Um)
Interface
A-bis
Interface
CCM
RIL3 - CC
MM
RR
RIL3 - MM
RIL3 - RR
DATP
RSM
A
B
C,D HLR/AuC
MAP/D
BSSMAP
Distribution Protocol
MAP/C
MAP/E
MAP/G
TUP,
ISUP
O
S
I
Component Sublayer
TCAP
Transaction Sublayer
L
A
Y
E
R
S
Presentation
Layer *
Session
Layer *
Transport
Layer *
Network
Layer
Data Link
Physical
SCCP
SCCP
SCCP
SCCP
MTP3
MTP3
MTP3
MTP3
MTP3
MTP2
MTP2
MTP2
MTP1
MTP1
MTP1
LAP-D
LAP-D
MTP2
MTP2
Radio
64kbps ch
MTP1
MTP1
Transaction Capabilities Application Part
(TCAP) :
provides correlation between individual
operations and structured exchanges building up a
complete transaction.
Transaction sublayer : manages transactions on an
end-to-end basis.
Component sublayer : correlates commands and
responses within a dialog.
MAP Protocol :
Handles non-call related signaling between
different network parts
designed to interact with MSC, VLR/HLR, AUC,
and GMSC so they can communicate with each
other.
Query - response
contains a number of Application System Elements
(ASE)
all messages contain either mandatory or optional
application parameters.
Functions can be grouped into two categories:
MM and
 basic service support.
MAP Protocol Connections :
RIL3
BSSMAP
MAP/F
MSC
EIR
MAP/B
VLR
MAP/I
MAP/D
BSS
HLR
MAP/C
MAP/E
MAP/G
MAP/C
GMSC
VLR
MSC
MAP/B
MAP/H
SMS
Gateway
MAP Communication between peerto-peer nodes :
ASE 1
ASE1
MAP
MAP
ASE n
TCAP
ASE n
TCAP
Physical Connection
Flow Chart of Initial Mobile Acquisition:
Power on
Select channel with highest
RF level among control ch.s
Scan channels,
monitor RF
levels
Scan ch. For frequency
correction burst (FCCH)
Select ch. With next highest
RF level from control list
NO
Is
FCCH detected?
YES
Scan ch. For timing sync.
Burst ( SCH )
NO
NO
From the ch data update the
control ch list
is
SCH detected ?
YES
Read data from BCCH ch and determine if
the ch is a control ch ( BCCH )
is
the current
BCCH incl.
YES
Camp on BCCH and start
decoding
Mobile Originating Call Procedure:
MS
Press send
BSS
MSC
VLR
RIL3-RR Ch. Request
RACH
RIL3-RR IMM SABM
AGCH
< identity of message >
UA
Service Request
TMSI, Call Setup
SDCH (Up.Lk)
SDCH (Dn. Lk.)
SDCH (Up. Lk.)
Service Request
TMSI, Call Setup
MAP/B Service
Request, TMSI,
Call Setup
Authentication, Ciphering, and TMSI assignment
HLR/EIR/AUC PSTN/ISDN
MS
BSS
MSC
RIL3-CC Setup ( Call Information )
VLR
HLR/EIR/AUC
MAP/B Send Call
setup information
Call complete MAP/B
RIL3-CC call Proceeding
Subscriber
Phone
Rings
RIL3-CC
Assignment cmd
RIL3-CC assign.
SDCH (Up.Lk.)
complete
RIL3-CC assign
complete
TUP/ISUP Initial Address Message ( IAM )
TUP/ISUP Address Complete Message ( ACM )
RIL3-CC alerting
TCH/ACCH
PSTN/ISDN
RIL3-CC connect
Answer Message
RIL3-CC Connect Ack.
Conversation Starts
“ TUP/ISUP
Answer
Signal”
Structure of a layer 3 message:
8
7
6
5
4
3
2
1
Octet
Address field
1
Control field
2
Length indicator field
3
Bit
8
TI
flag
0
7
6
5
4
3
2
1
Protocol
Discriminator
TI
Message Type
4
Information field
CRC
Layer 2 Format type B
.
.
.
10
11
12
Information Elements
“ mandatory ”
Information Elements
“ optional ”
Structure of a GSM layer 3 message
Octet
Transaction Identifier ( TI ) :
Used to distinguish between possible (multiple) parallel
CC connections and between various transactions taking
place over these simultaneous CC connections.
TI not relevant for RR and MM connections ( coded “
0000” )
TI flag : indicates originator of a CC message ‘0’ by
originator and coded ‘1’ by its peer entity
originator fills in TI value. Kept and serves as a label
during transaction
Protocol Discriminator ( PD )
Links layer 3 protocol to the entity the
message is addressed to.
Identifies 6 protocols
Protocol
RRM
MM
CC
SMS
SS
Test procedure
All other values reserved
PD Binary
0110
0101
0011
1001
1011
1111
Message Type :
MT indicates the function of layer 3 message.
Uses lower 6 bits ( 64 possible message types)
for a protocol type indicated by PD.
MT is part of a set of messages in a protocol
Bit 8 = 0 ( reserved )
Bit 7 = send sequence variable and may be
used for MM and CC messages.
Information Elements (IE):
Depending on MT, may
or may not have one or
more IE
Types of IE :
 Mandatory fixed length
( MF )
Mandatory variable
length ( MV)
Optional Fixed Length
(OF)
OV
TI
flag
0
Protocol
Discriminator
TI
Message Type
Information Elements
“ mandatory ”
Information Elements
“ optional ”
Structure of a GSM layer 3 message
Why GSM ?
Digital
High audio quality and link integrity
High spectral efficiency
Identical system in all countries
Intersystem Roaming
High degree of flexibility ( open architecture
that will allow new services to be introduced
in the future.
Economy in both sparsely and heavily
populated areas
Integration with ISDN
Other security features
SMS and use of facsimile features
Easy to introduce the system
Low-cost infrastructure
References:
GSM System Engineering, Asha Mehrotra,
Artech House Publishers, 1997
An Introduction To GSM, Redl, Weber,
Oliphant, Artech House Publishers, 1995