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DM_7_GSM_Protocol_Architecture.ppt 1
GSM System Protocol Architecture
References:Jörg Erbespächer, Hans Jörg Vogel, Christian Bettstetter, GSM: Switching services andprotocols, Wiley 2001, ISBN 0-471-499903-XSchiller: Mobile communications, 2nd ed. 512 pages, ISBN 0-321-12381-6, Addison-Wesley, 2003
Copyright © Jouko Kurki, 2006
DM_7_GSM_Protocol_Architecture.ppt 2
Protocol architecture planes
Three Planes: User Plane (Speech, data)Control plane (Signaling fro call setupetc.)Management Plane (management ofnetwork elements, like configuration,faultsUser Plane: Logical Channels at theService Access Point (SAP) of layer 1(Physical layer).
Left: Reference model fro the GSMUser-Network Interface (UNI) Um.User data across air interface usingTraffic Channels, TCH.Protocols in signaling Plane needed e.g.fro call set-up and network elementmanagement . The D channels(corresponding to ISDN D channels) inair IF are signaling channels
DM_7_GSM_Protocol_Architecture.ppt 3
MS BTS MSCBSC
Um Interface A-bis interface A interface
Layer 3
Layer 2
Layer 1
CM
MM
RR
RR
MM
CM
RR
LAPDm LAPDm LAPD LAPD
physicallayer
physicallayer
physicallayer
physicallayer
BTSMBTSM
BSSAP BSSAP
SCCP SCCP
MPT MTP
GSM Network Architecture (protocols)
DM_7_GSM_Protocol_Architecture.ppt 4
MS BTS MSCBSC
Um Interface A-bis interface A interface
Layer 3
Layer 2
Layer 1
CM
MM
RR
RR
MM
CM
RR
LAPDm LAPDm LAPD LAPD
physicallayer
physicallayer
physicallayer
physicallayer
BTSMBTSM
BSSAP BSSAP
SCCP SCCP
MPT MTP
GSM Network Signaling Architecture as OSI Model
DM_7_GSM_Protocol_Architecture.ppt 5
Protocol Architecture in User Plane
1. Connection between Reference points2. Connections are constructed of connection elements
• Radio Interface Connection Element• A Interface Connection Element
DM_7_GSM_Protocol_Architecture.ppt 6
•The data link layer (layer 2) over the radio link is based on amodified 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 LAPDfrom ISDN.
•The Message Transfer Part (MTP) level 2 of the SS& protocol isused at the A interface.
DM_7_GSM_Protocol_Architecture.ppt 7
User Data and Control at Air Interface
ISDN channels: 2 x B + D = 2x 64 kb/s + 16kb/s = 144 kb/s. D channel used for signalingand 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 beused for user data if capacity is available.GSM’s Short Message Service (SMS) usesthis. Cases:
•No call, SDCCH channel•During call, SACCH channel
DM_7_GSM_Protocol_Architecture.ppt 8
0 1 1 1 1 1 1 0
FCS octet 2
FCS octet 1
Layer 3 information
Control octet 2
Control octet 1
Address octet 2
Address octet 1
0 1 1 1 1 1 1 0
LAPD frame structure
Octet 1
Octet 2
Octet 3
Octet 4
Octet 5
Octet 6
Octet N-3
Octet N-2
Octet N-1
Octet N
Opening flag
The structure of the controlfiled depends on the frame type
Layer 3 information is onlypresented in Layer in Layer 2‘information frame’
Frame check sequence
Frame check sequence
Closing flag
DM_7_GSM_Protocol_Architecture.ppt 9
Speech transmission in GSM
Overview of protocols for speech:
GSC = GSM Speech CodecFEC = Forward error CorrectionMPX = (Sub)Multiplexing –3 x 13+3 kb/s inserted to 64 kb/sTRAU = Transcoding unit: GSM Speech 13 kb/s coded to 64 kb/s ISDN speech
DM_7_GSM_Protocol_Architecture.ppt 10
BTS architectures –with various positions of TRAU
DM_7_GSM_Protocol_Architecture.ppt 11
GSM protocol architecture for speech –Air IFTranscoding unit (TRAU) at BTS site
Logical flow of data
DM_7_GSM_Protocol_Architecture.ppt 12
Protocol architecture for speech
DM_7_GSM_Protocol_Architecture.ppt 13
Protocol architecture for transparent data
Provides transparent data transmission at 9.6 / 14.4 kb/s. Standard GSM error correction.
DM_7_GSM_Protocol_Architecture.ppt 14
Protocol architecture for nontransparent data
•Provides non-transparent data transmission. Error correction based on checksum andAutomatic Repeat Request (ARQ).
•Uses Radio Link Protocol (RLP). Fixed 240 bit frames, otherwise rather similar to HDLClink protocol. 24-bit Frame check sequence.
•Delay around 200 ms
DM_7_GSM_Protocol_Architecture.ppt 15
Frame structure for RLP protocol
DM_7_GSM_Protocol_Architecture.ppt 16
SIGNALING ARCHITECTURE
DM_7_GSM_Protocol_Architecture.ppt 17
A –bis interface
•CM and MM messages are not interpreted by the BSC or theBTS. They are transferred over the A-bis interface astransparent messages and over the A interface using theDirect Transfer Application Part (DTAP).
•RR messages are mapped to the BSS Application Part(BSSAP) in the BSC. In the BTS, most of them are handledas transparent by the BTS (e.g. random access, startciphering, paging).
•BTS Management (BTSM) is used to transfer all OAMrelated information to the BTS.
DM_7_GSM_Protocol_Architecture.ppt 18
A interface
•The Message Transfer Part (MPT) and the SCCP (Signalling connectionControl Point) are used to support the transfer of signalling messagesbetween the MSC and the BSS.
•The SCCP part is used to provide a referencing mechanism to identify aparticular transaction relating to, for instant, a particular call. The SCCPcan also be used to enhance message routing, operation and maintenanceinformation.
DM_7_GSM_Protocol_Architecture.ppt 19
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 inPublic telephone system and mobile telephone systems throughout the world. It enablescalls throughout the world.
•Signaling system is separate system from speech•Example of operations. Signaling in call set-up. Signaling is transferred as messages.
DM_7_GSM_Protocol_Architecture.ppt 20
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)
Layer 1
Layer 3
Layer 2
Physical Connections
Data Link Control
Transport of SignalingMessages within oneNetwork
DM_7_GSM_Protocol_Architecture.ppt 21
Telephone User Part (TUP) –call controlTUP is the entity that sends and receives messages used for actual call controlIn 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)
DM_7_GSM_Protocol_Architecture.ppt 22
Signaling Connection and Control Part - SCCP
Takes care of connections between networks allowing: virtual connection and connectionlesssignaling. This enables signaling between networks.
Summary:MTP is Message Transfer Part, responsible fortransferring messages between networkelements within same networkTUP is the user part for messages transferred byMTP. These messages deal with setting upsupervising and clearing call connections. Twovariations NUP (national) and ISUP (ISDN).SCCP is the Signaling Connection and ControlPart. Main function: virtual connection andconnectionless signaling.
DM_7_GSM_Protocol_Architecture.ppt 23
SS7 in GSM (1)
In GSM system more functionality is needed because of more functionally due to non-callrelated signaling (measurements, handover control etc.), user mobility and additional networkelements.The additional protocol layers are: BSSAP: Used for signaling between MSC
and BSC and MS. Since there is BSCbetween MSC and MS, there must be avirtual connection, thus SCCP is needed.Examples: Authentication, assigning newTMSI. BSSP need for signaling betweenMSC-BSC and MSC-MS.MAP is a GSM specific protocol, for non-call related applications between NetworkElements. MAP signaling includes e.g.messaging between MSC and HLR.TCAP, Transaction CapabilitiesApplication Part. The MAP transactionsneed a sort of “secretary”for managingthe transactions. This is the function ofTCAP.
DM_7_GSM_Protocol_Architecture.ppt 24
•The MTP part provides a mechanism for providing thereliable transfer of signalling messages. A subset of MTP isused between the BSS and the MSC.
•The BSSAP provides the channel switching and aerialfunctions, it performs RR management, and theinterworking functions between the data link protocols usedon the radio and the BSS-MSC side for transportingsignalling related messages.
SS7 in GSM (2): MTP and BSSAP
DM_7_GSM_Protocol_Architecture.ppt 25
SS7 Protocols in GSM (3)
Protocol FunctionName
DM_7_GSM_Protocol_Architecture.ppt 26
SS7 layers in GSM system elements –overview
DM_7_GSM_Protocol_Architecture.ppt 27
Signaling functions in various GSM elements
DM_7_GSM_Protocol_Architecture.ppt 28
Protocol interfaces in Mobile Network
DM_7_GSM_Protocol_Architecture.ppt 29
International signaling relations via ISDN
DM_7_GSM_Protocol_Architecture.ppt 30
Overview of signaling in GSM
DM_7_GSM_Protocol_Architecture.ppt 31
Protocol architecture for Signaling
DM_7_GSM_Protocol_Architecture.ppt 32
Layer 3 Functions are divided into 3 categories:
1. Radio Resource (RR) Management:Function relating to the establishment of physical connections forthe purpose of transmitting call-related signalling information.
2. Mobility Management (MM):Functions relating to location registration, paging,attachment/detachment, handover, dynamic channel allocationand management.
3. Connection Management (CM):Call control related functions, SMS and service handlingfunctions.
DM_7_GSM_Protocol_Architecture.ppt 33
Layer 3 Protocols at MS side
Main task of LAPDm is to transferlayer layer 3 protocols with specialsupport for:
RR = radio Resource ManagementMM = Mobility managementCC = Call Control
DM_7_GSM_Protocol_Architecture.ppt 34
Protocol architecture for SMS transfer
DM_7_GSM_Protocol_Architecture.ppt 35
Signaling at Air Interface
Layer 1 service interfaces
LAYER 1 SERVICES:•Layer 1 provides bit transportservice to logical channels.•Access Capabilities•Error Detection•Encryption
Physical layer connections.Logical channels defined.
RR hasdirect accessto Layer 1
DM_7_GSM_Protocol_Architecture.ppt 36
Layer 1 servicesMS Modes:•Idle•Dedicated
Layer 1 frame structure (SACCH block):•Transport of signaling messages thataccur as LAPDM frames at the ServiceAccess Point (SAP).
•SACCH frame also carreis power leveletc.
•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 37
Layer 2 signaling
Logical channels and modes forLayer 2 SAPIs
Specific SAPI values reserved frospecific functions:SAPI = 0 for signaling (CM, MM,RR)SAPI = 3 for SMS
L2 = Data link protocol for signalingchannels at air interface. Similar toHDLC except fixed frame length.Two modes:Unacknowledged, no flow and errorcontrolAcknowledged mode: Positiveacknowledgement, Error correctionbased on ARQ.
DM_7_GSM_Protocol_Architecture.ppt 38
LAPDm frame formats
USAGE:A, B:
SACCH, FACCH,SDCCH
Abis, Bbis:BCCH, PCH, AGCH
DM_7_GSM_Protocol_Architecture.ppt 39
Radio Resource management
Procedures for Radio Resource Management (RR) are signaling and control procedures forair interface (Um): Assignment, allocation and administration of radio resources. Format ofUm signaling message shown below.Message Type (MT), see next page.
Format of Layer 3 signaling message at air interface (Um)
DM_7_GSM_Protocol_Architecture.ppt 40
RR messages, channels and MT codes
MT = Message type code
DM_7_GSM_Protocol_Architecture.ppt 41
RR connection set-up and take-down
Measurement result
LAPDm held back until complete
BTS sends RRmessage:Physicalinformation, TA
DM_7_GSM_Protocol_Architecture.ppt 42
Channel change encryption and handover
DM_7_GSM_Protocol_Architecture.ppt 43
Mobility Management (MM)MM messages
DM_7_GSM_Protocol_Architecture.ppt 44
MM messages –category common
Specific: e.g Location Update, see Eberspächer
DM_7_GSM_Protocol_Architecture.ppt 45
Call Set-Up
Mobile originating and Mobileterminating
DM_7_GSM_Protocol_Architecture.ppt 46
OACSU
•Mobile initiated call setupusing late assignment(OACSU = Off Air Call Set-Up)
•Radio resource reserved onlyin last minute when call isfully routed
DM_7_GSM_Protocol_Architecture.ppt 47
Realization of Short Message Services (SMS)
DM_7_GSM_Protocol_Architecture.ppt 48
EXTRA
DM_7_GSM_Protocol_Architecture.ppt 49
•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 incomingcall between GMSC and serving MSC. It is not known to GSM users.
•Both MSISDN and IMSI contain a country identity and a networkidentity Within the country.
•MSRN can be contained in the HLR record if the serving MSC/VLRhas provided it when updating the location information.
DM_7_GSM_Protocol_Architecture.ppt 50
HLR
MSRNGMSC MSC / VLRIncoming Call
MSISDN
(1) MSISDN (2) IMSI
(4) MSRN (3) MSRN
An example of a call routing (to route an incoming call towards serving MSC)
MSISDN Mobile Station ISDN NumberIMSI International Mobile Subscriber IdentityMSRN Mobile Station Roaming Number
DM_7_GSM_Protocol_Architecture.ppt 51
MSRIL3-CC
HLR
GMSC
MAP/C
MAP/D
Protocols in the call control producers
RIL3-CC Radio Interface Layer 3 - Call ControlMAP Mobile Application Part
MSC/VLR
DM_7_GSM_Protocol_Architecture.ppt 52
Format of BTSM-RSL protocol messages
Format of BTSM-RSL protocol messagesLayer 3 between BSC and BTS