GSM System Protocol Architecture
References:
Jörg Erbespächer, Hans Jörg Vogel, Christian Bettstetter, GSM: Switching services and
protocols, Wiley 2001, ISBN 0-471-499903-X
Schiller: Mobile communications, 2nd ed. 512 pages, ISBN 0-321-12381-6, AddisonWesley, 2003
DM_7_GSM_Protocol_Architecture.ppt
Copyright © Jouko Kurki, 2006
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Protocol architecture planes
Three Planes: User Plane (Speech, data)
Control plane (Signaling fro call setup
etc.)
Management Plane (management of
network elements, like configuration,
faults
User Plane: Logical Channels at the
Service Access Point (SAP) of layer 1
(Physical layer).
Left: Reference model fro the GSM
User-Network Interface (UNI) Um.
User data across air interface using
Traffic Channels, TCH.
Protocols in signaling Plane needed e.g.
fro call set-up and network element
management . The D channels
(corresponding to ISDN D channels) in
air IF are signaling channels
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GSM Network Architecture (protocols)
MS
BTS
MSC
BSC
CM
CM
MM
MM
Layer 3
RR
BSSAP
BSSAP
RR
RR
BTSM
BTSM
Layer 2
LAPDm
LAPDm
LAPD
LAPD
Layer 1
physical
layer
physical
layer
physical
layer
physical
layer
Um Interface
DM_7_GSM_Protocol_Architecture.ppt
A-bis interface
SCCP
SCCP
MPT
MTP
A interface
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GSM Network Signaling Architecture as OSI Model
MS
BTS
MSC
BSC
CM
CM
MM
MM
Layer 3
RR
BSSAP
BSSAP
RR
RR
BTSM
BTSM
Layer 2
LAPDm
LAPDm
LAPD
LAPD
Layer 1
physical
layer
physical
layer
physical
layer
physical
layer
Um Interface
DM_7_GSM_Protocol_Architecture.ppt
A-bis interface
SCCP
SCCP
MPT
MTP
A interface
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Protocol Architecture in User Plane
1.
2.
Connection between Reference points
Connections are constructed of connection elements
• Radio Interface Connection Element
• A Interface Connection Element
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•The data link layer (layer 2) over the radio link is based on a
modified LAPD (Link Access Protocol for the D channel)
referred to as LAPDm (m like mobile).
•On the A-bis interface, the layer 2 protocol is based on the LAPD
from ISDN.
•The Message Transfer Part (MTP) level 2 of the SS& protocol is
used at the A interface.
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User Data and Control at Air Interface
ISDN channels: 2 x B + D = 2x 64 kb/s + 16
kb/s = 144 kb/s. D channel used for signaling
and possibly some user data.
In GSM:
•Bm channel for traffic / user data
•Dm channel for signaling
As in ISDN the Dm channel in GSM can be
used for user data if capacity is available.
GSM’s Short Message Service (SMS) uses
this. Cases:
•No call, SDCCH channel
•During call, SACCH channel
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LAPD frame structure
Octet 1
0 1 1 1 1 1 1 0
Octet 2
Address octet 1
Octet 3
Address octet 2
Octet 4
Control octet 1
Octet 5
Control octet 2
Octet 6
Layer 3 information
Octet N-3
Opening flag
The structure of the control
filed depends on the frame type
Layer 3 information is only
presented in Layer in Layer 2
‘information frame’
Octet N-2
FCS octet 1
Frame check sequence
Octet N-1
FCS octet 2
Frame check sequence
Octet N
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0 1 1 1 1 1 1 0
Closing flag
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Speech transmission in GSM
Overview of protocols for speech:
GSC = GSM Speech Codec
FEC = Forward error Correction
MPX = (Sub)Multiplexing –3 x 13+3 kb/s inserted to 64 kb/s
TRAU = Transcoding unit: GSM Speech 13 kb/s coded to 64 kb/s ISDN speech
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BTS architectures –with various positions of TRAU
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GSM protocol architecture for speech –Air IF
Transcoding unit (TRAU) at BTS site
Logical flow of data
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Protocol architecture for speech
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Protocol architecture for transparent data
Provides transparent data transmission at 9.6 / 14.4 kb/s. Standard GSM error correction.
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Protocol architecture for nontransparent data
•Provides non-transparent data transmission. Error correction based on checksum and
Automatic Repeat Request (ARQ).
•Uses Radio Link Protocol (RLP). Fixed 240 bit frames, otherwise rather similar to HDLC
link protocol. 24-bit Frame check sequence.
•Delay around 200 ms
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Frame structure for RLP protocol
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SIGNALING ARCHITECTURE
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A –bis interface
•CM and MM messages are not interpreted by the BSC or the
BTS. They are transferred over the A-bis interface as
transparent messages and over the A interface using the
Direct Transfer Application Part (DTAP).
•RR messages are mapped to the BSS Application Part
(BSSAP) in the BSC. In the BTS, most of them are handled
as transparent by the BTS (e.g. random access, start
ciphering, paging).
•BTS Management (BTSM) is used to transfer all OAM
related information to the BTS.
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A interface
•The Message Transfer Part (MPT) and the SCCP (Signalling connection
Control Point) are used to support the transfer of signalling messages
between the MSC and the BSS.
•The SCCP part is used to provide a referencing mechanism to identify a
particular transaction relating to, for instant, a particular call. The SCCP
can also be used to enhance message routing, operation and maintenance
information.
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Signaling –Needed for call control
•Signaling is needed to setup and takedown of calls etc.
•The whole system is called Signaling System Number 7 (SS#7 or SS7) and it is used in
Public telephone system and mobile telephone systems throughout the world. It enables
calls throughout the world.
•Signaling system is separate system from speech
•Example of operations. Signaling in call set-up. Signaling is transferred as messages.
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Message Transfer Part -MTP
•MTP is the in infrastructure for transferring signaling messages between network elements
•The entire Signaling System number 7 (SS#7 or SST) is built on the foundation of MTP
•Layer 1 –Physical layer, voltages etc.
•Layer 2 –Datalink, error free transfer of signaling messages
•Layer 3- Transfer of signaling messages within one network (only)
Transport of Signaling
Messages within one
Network
Data Link Control
Physical Connections
DM_7_GSM_Protocol_Architecture.ppt
Layer 3
Layer 2
Layer 1
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Telephone User Part (TUP) –call control
TUP is the entity that sends and receives messages used for actual call control
In fixed network there are only these two parts. TUP can be replaced by National User Part
(NUP), allowing some minor variations.
In ISDN more functionality -> ISDN User Part (ISUP)
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Signaling Connection and Control Part - SCCP
Takes care of connections between networks allowing: virtual connection and connectionless
signaling. This enables signaling between networks.
Summary:
MTP is Message Transfer Part, responsible for
transferring messages between network
elements within same network
TUP is the user part for messages transferred by
MTP. These messages deal with setting up
supervising and clearing call connections. Two
variations NUP (national) and ISUP (ISDN).
SCCP is the Signaling Connection and Control
Part. Main function: virtual connection and
connectionless signaling.
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SS7 in GSM (1)
In GSM system more functionality is needed because of more functionally due to non-call
related signaling (measurements, handover control etc.), user mobility and additional network
elements.
The additional protocol layers are:
BSSAP: Used for signaling between MSC
and BSC and MS. Since there is BSC
between MSC and MS, there must be a
virtual connection, thus SCCP is needed.
Examples: Authentication, assigning new
TMSI. BSSP need for signaling between
MSC-BSC and MSC-MS.
MAP is a GSM specific protocol, for noncall related applications between Network
Elements. MAP signaling includes e.g.
messaging between MSC and HLR.
TCAP, Transaction Capabilities
Application Part. The MAP transactions
need a sort of “secretary”for managing
the transactions. This is the function of
TCAP.
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SS7 in GSM (2): MTP and BSSAP
•The MTP part provides a mechanism for providing the
reliable transfer of signalling messages. A subset of MTP is
used between the BSS and the MSC.
•The BSSAP provides the channel switching and aerial
functions, it performs RR management, and the
interworking functions between the data link protocols used
on the radio and the BSS-MSC side for transporting
signalling related messages.
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SS7 Protocols in GSM (3)
Nam
e
Protocol
Function
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SS7 layers in GSM system elements –overview
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Signaling functions in various GSM elements
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Protocol interfaces in Mobile Network
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International signaling relations via ISDN
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Overview of signaling in GSM
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Protocol architecture for Signaling
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Layer 3 Functions are divided into 3 categories:
1. Radio Resource (RR) Management:
Function relating to the establishment of physical connections for
the purpose of transmitting call-related signalling information.
2. Mobility Management (MM):
Functions relating to location registration, paging,
attachment/detachment, handover, dynamic channel allocation
and management.
3. Connection Management (CM):
Call control related functions, SMS and service handling
functions.
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Layer 3 Protocols at MS side
Main task of LAPDm is to transfer
layer layer 3 protocols with special
support for:
RR = radio Resource Management
MM = Mobility management
CC = Call Control
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Protocol architecture for SMS transfer
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Signaling at Air Interface
LAYER 1 SERVICES:
•Layer 1 provides bit transport
service to logical channels.
•Access Capabilities
•Error Detection
•Encryption
Physical layer connections.
Logical channels defined.
RR has
direct access
to Layer 1
Layer 1 service interfaces
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Layer 1 services
MS Modes:
•Idle
•Dedicated
•NULL: Power off
•BCH: Synchronized with one BTS, ready for
RACH
•Tuning DCH: Tries to synchronize with a
channel
State diagram of MS physical layer
DM_7_GSM_Protocol_Architecture.ppt
Layer 1 frame structure (SACCH block):
•Transport of signaling messages that
accur as LAPDM frames at the Service
Access Point (SAP).
•SACCH frame also carreis power level
etc.
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Layer 2 signaling
L2 = Data link protocol for signaling
channels at air interface. Similar to
HDLC except fixed frame length.
Two modes:
Unacknowledged, no flow and error
control
Acknowledged mode: Positive
acknowledgement, Error correction
based on ARQ.
Logical channels and modes for
Layer 2 SAPIs
Specific SAPI values reserved fro
specific functions:
SAPI = 0 for signaling (CM, MM,
RR)
SAPI = 3 for SMS
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LAPDm frame formats
USAGE:
A, B:
SACCH, FACCH,
SDCCH
Abis, Bbis:
BCCH, PCH, AGCH
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Radio Resource management
Procedures for Radio Resource Management (RR) are signaling and control procedures for
air interface (Um): Assignment, allocation and administration of radio resources. Format of
Um signaling message shown below.
Message Type (MT), see next page.
Format of Layer 3 signaling message at air interface (Um)
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RR messages, channels and MT codes
MT = Message type code
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RR connection set-up and take-down
LAPDm held back until complete
BTS sends RR
message:
Physical
information, TA
Measurement result
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Channel change encryption and handover
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Mobility Management (MM)
MM messages
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MM messages –category common
Specific: e.g Location Update, see Eberspächer
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Call Set-Up
Mobile originating and Mobile
terminating
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OACSU
•Mobile initiated call setup
using late assignment
(OACSU = Off Air Call SetUp)
•Radio resource reserved only
in last minute when call is
fully routed
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Realization of Short Message Services (SMS)
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EXTRA
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•MSISDN is the ‘directory number’used to call GSM subscribers.
•IMSI is the main subscriber number used internally within GSM.
•MSRN is the routing number used on the second leg of an incoming
call between GMSC and serving MSC. It is not known to GSM users.
•Both MSISDN and IMSI contain a country identity and a network
identity Within the country.
•MSRN can be contained in the HLR record if the serving MSC/VLR
has provided it when updating the location information.
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HLR
(1) MSISDN
(2) IMSI
(4) MSRN
Incoming Call
GMSC
(3) MSRN
MSRN
MSC / VLR
MSISDN
An example of a call routing (to route an incoming call towards serving MSC)
MSISDN
IMSI
MSRN
Mobile Station ISDN Number
International Mobile Subscriber Identity
Mobile Station Roaming Number
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Protocols in the call control producers
MAP/D
HLR
RIL3-CC
MSC/VLR
MS
MAP/C
GMSC
RIL3-CC
MAP
Radio Interface Layer 3 - Call Control
Mobile Application Part
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Format of BTSM-RSL protocol messages
Format of BTSM-RSL protocol messages
Layer 3 between BSC and BTS
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