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GSM - The Network Layer
The document discusses the network layer in GSM systems. It describes the main sublayers and protocols in the network layer, including radio resource management (RR), mobility management (MM), and call control (CC). It explains the functions and procedures handled by each sublayer, such as channel assignment and handover (RR), location updating and authentication (MM), and call establishment and clearing (CC). It also outlines some of the key layer 3 protocols used, including RIL3-RR, RIL3-MM, RIL3-CC, and MAP, and provides examples of signaling flows for initial mobile acquisition and a mobile-originating call.
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GSM - The Network Layer
- 1. GSM : TheNetwork Layer Rasha Morsi
- 2. Presentation Outline: Radio ResourceManagement 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?
- 3. Acronyms: TMSI : TemporaryMobile 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
- 4. 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
- 5. RIL3 - CC RIL3- MM CCM MM RR Air (Um) Interface A-bis Interface MAP/E MAP/G MAP/D DATP RIL3 - RR RSM BSSMAP Distribution Protocol TCAP Component Sublayer Transaction Sublayer A B C,D HLR/AuC MAP/C Presentation Layer * Session Layer * Transport Layer * Network Layer Physical Data Link LAP-D Radio LAP-D 64kbps ch SCCP MTP3 MTP2 MTP1 SCCP MTP3 MTP2 MTP1 SCCP MTP3 MTP2 MTP1 SCCP MTP3 MTP2 MTP1 MTP3 MTP2 MTP1 TUP, ISUP O S I L A Y E R S
- 6. Radio Resource Managementsublayer: 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
- 7. Procedures in RRlayer: 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
- 8. Mobility Management sublayer: Copewith 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)
- 9. Provision of usersecurity Provision of an MM connection, based on existing RR connection, to the CM sublayer
- 10. Procedures in MMsublayer : 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
- 11. Call Control sublayer: Managesall 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.
- 12. Procedures in CCsublayer: 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
- 13. RIL3 - CC RIL3- MM CCM MM RR Air (Um) Interface A-bis Interface MAP/E MAP/G MAP/D DATP RIL3 - RR RSM BSSMAP Distribution Protocol TCAP Component Sublayer Transaction Sublayer A B C,D HLR/AuC MAP/C Presentation Layer * Session Layer * Transport Layer * Network Layer Physical Data Link LAP-D Radio LAP-D 64kbps ch SCCP MTP3 MTP2 MTP1 SCCP MTP3 MTP2 MTP1 SCCP MTP3 MTP2 MTP1 SCCP MTP3 MTP2 MTP1 MTP3 MTP2 MTP1 TUP, ISUP O S I L A Y E R S
- 14. 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
- 15. 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
- 16. RIL3-MM : deals withMS location management location updating IMSI attach periodic updating security aspects of system authentication TMSI reallocation identification.
- 17. RIL3-CC : deals withmobile 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.
- 18. RIL3 - CC RIL3- MM CCM MM RR Air (Um) Interface A-bis Interface MAP/E MAP/G MAP/D DATP RIL3 - RR RSM BSSMAP Distribution Protocol TCAP Component Sublayer Transaction Sublayer A B C,D HLR/AuC MAP/C Presentation Layer * Session Layer * Transport Layer * Network Layer Physical Data Link LAP-D Radio LAP-D 64kbps ch SCCP MTP3 MTP2 MTP1 SCCP MTP3 MTP2 MTP1 SCCP MTP3 MTP2 MTP1 SCCP MTP3 MTP2 MTP1 MTP3 MTP2 MTP1 TUP, ISUP O S I L A Y E R S
- 19. Transaction Capabilities ApplicationPart (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.
- 20. MAP Protocol : Handlesnon-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)
- 21. all messages containeither mandatory or optional application parameters. Functions can be grouped into two categories: MM and basic service support.
- 22. MAP Protocol Connections: RIL3 BSS BSSMAP MAP/B VLR MSC EIR HLR GMSC SMS Gateway MAP/C MAP/C MAP/I MAP/D MAP/F VLR MSC MAP/H MAP/B MAP/E MAP/G
- 23. MAP Communication betweenpeer- to-peer nodes : ASE1 ASE n ASE 1 ASE n MAP MAP TCAP TCAP Physical Connection
- 24. Flow Chart ofInitial Mobile Acquisition: Power on Scan channels, monitor RF levels Select channel with highest RF level among control ch.s Scan ch. For frequency correction burst (FCCH) Is FCCH detected? NO YES Scan ch. For timing sync. Burst ( SCH ) Select ch. With next highest RF level from control list is SCH detected ? NO YES Read data from BCCH ch and determine if the ch is a control ch ( BCCH ) is the current BCCH incl. YES NO From the ch data update the control ch list Camp on BCCH and start decoding
- 25. Mobile Originating CallProcedure: MS BSS MSC VLR HLR/EIR/AUC PSTN/ISDN Press send RIL3-RR Ch. Request RACH RIL3-RR IMM SABM AGCH < identity of message > SDCH (Up.Lk) UA SDCH (Dn. Lk.) Service Request TMSI, Call Setup SDCH (Up. Lk.) Service Request TMSI, Call Setup MAP/B Service Request, TMSI, Call Setup Authentication, Ciphering, and TMSI assignment
- 26. MS BSS MSCVLR HLR/EIR/AUC PSTN/ISDN RIL3-CC Setup ( Call Information ) MAP/B Send Call setup information Call complete MAP/B RIL3-CC Assignment cmd RIL3-CC assign. complete SDCH (Up.Lk.) RIL3-CC assign complete TUP/ISUP Initial Address Message ( IAM ) “ TUP/ISUP Answer Signal” TUP/ISUP Address Complete Message ( ACM ) RIL3-CC alerting Answer Message RIL3-CC connectTCH/ACCH RIL3-CC Connect Ack. Conversation Starts RIL3-CC call Proceeding Subscriber Phone Rings
- 27. Structure of alayer 3 message: CRC Address field Control field Length indicator field Information field 1 2 3 4 . . . 10 11 12 Layer 2 Format type B 8 7 6 5 4 3 2 1 Octet Bit 8 7 6 5 4 3 2 1 Octet Information Elements “ mandatory ” Information Elements “ optional ” 0 Message Type TI flag TI Protocol Discriminator Structure of a GSM layer 3 message
- 28. 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
- 29. Protocol Discriminator (PD ) Links layer 3 protocol to the entity the message is addressed to. Identifies 6 protocols Protocol PD Binary RRM 0110 MM 0101 CC 0011 SMS 1001 SS 1011 Test procedure 1111 All other values reserved
- 30. Message Type : MTindicates 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.
- 31. Information Elements (IE): Dependingon 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 Information Elements “ mandatory ” Information Elements “ optional ” 0 Message Type TI flag TI Protocol Discriminator Structure of a GSM layer 3 message
- 32. Why GSM ? Digital Highaudio 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.
- 33. Economy in bothsparsely and heavily populated areas Integration with ISDN Other security features SMS and use of facsimile features Easy to introduce the system Low-cost infrastructure
- 34. References: GSM System Engineering,Asha Mehrotra, Artech House Publishers, 1997 An Introduction To GSM, Redl, Weber, Oliphant, Artech House Publishers, 1995
Editor's Notes
- #25 1 ) MS scans 124 RF channels may take several seconds ( average of sig. Strength is needed. 2 ) FCCH present ?? 1st step to freq. Synch. Frequency correction burst is unique and easily recognisable It’s a long sine wave that is offset by 67.7 kHz from the carrier freq. The cell transmits all zero’s for this burst . MS has to extract the offset before an estimate of carrier freq. Can be made. 3 ) Occurs 8 burst periods later than FCCH SCH message contains the current frame number the MS is synchronized to.
- #27 RIL3 CC setup message contains : MS-ISDN number number of called party type of service requested. MAP/B Send Call setup information contains : information of what is desired If Ms authorized, VLR responds with MAP/B Call complete message listing : MS capabilities subscribed services parameters needed bu network to properly allocate a channel for the call. TCH assignment through RIL3-CC assignment cmd. RIL3-CC alerting : informs that desired party is ringing MSC establishes the two way communication.
- #32 There are distinct definitions for the different MT s each decides which IE s if any must be present in a message. Optional IE s if present are after the mandatory IE s Optional IE s always have to have an information element identifier ( IEI ) tells the receiver of the message the purpose of the info. Contained therein. Inside IE field : page mode channel description request reference timing advance mobile allocation