Gsm architecture and signalling techniques


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Gsm architecture and signalling techniques

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Gsm architecture and signalling techniques

  1. 1. • • • • • Improved spectrum efficiency. International roaming. Low-cost mobile sets and base stations (BSs). High-quality speech. Compatibility with Integrated Services Digital Network (ISDN) and other telephone company services. • Support for new services.
  2. 2. Tele-services Bearer or Data Services Supplementary Services
  3. 3. • Telecommunication services that enable voice communication via mobile phones • Offered services • Mobile telephony • Emergency calling
  4. 4. • Include various data services for information transfer between GSM and other networks like PSTN, ISDN etc at rates from 300 to 9600 bps • Short Message Service (SMS) –up to 160 character alphanumeric data transmission to/from the mobile terminal • Unified Messaging Services(UMS) • Group 3 fax • Voice mailbox • Electronic mail
  5. 5. • Call related services : Call Waiting- Notification of an incoming call while on the handset. Call Hold- Put a caller on hold to take another call Call Barring- All calls, outgoing calls, or incoming calls Call Forwarding- Calls can be sent to various numbers defined by the user • Multi Party Call Conferencing - Link multiple calls together • CLIP – Caller line identification presentation • CLIR – Caller line identification restriction • CUG – Closed user group
  6. 6. • A GSM network consists of several functional entities, whose functions and interfaces are defined. The GSM network can be divided into following broad parts. • The Mobile Station (MS) • The Base Station Subsystem (BSS) • The Network Switching Subsystem (NSS) • The Operation Support Subsystem (OSS)
  7. 7. • Mobile Station (MS) Mobile Equipment (ME) Subscriber Identity Module (SIM) • Base Station Subsystem (BSS) Base Transceiver Station (BTS) Base Station Controller (BSC) • Network Switching Subsystem(NSS) Mobile Switching Center (MSC) Home Location Register (HLR) Visitor Location Register (VLR) Authentication Center (AUC) Equipment Identity Register (EIR)
  8. 8. Uniquely identified by an IMEI (International Mobile Equipment Identity).The Mobile Station is made up of two entities: • Mobile Equipment (ME):There are different types of terminals (MN) distinguished principally by their power and application: Portable , vehicle mounted, hand held device with Power level : 0.8W – 20 W 2. Subscriber Identity Module(SIM): It is protected by a four-digit Personal Identification Number (PIN). Allows user to send and receive calls and receive other subscribed services.
  9. 9. Base Station Subsystem is composed of two parts that communicate across the standardized Abis interface allowing Operation between components made by different suppliers 1. Base Transceiver Station (BTS): • Encodes,encrypts,multiplexes,modulates and feeds the RF signals to the antenna. • Frequency hopping • Communicates with Mobile station and BSC • Consists of Transceivers (TRX) units
  10. 10. 2. Base Station Controller (BSC) • Manages Radio resources for BTS • Assigns Frequency and time slots for all MS’s in its area • Handles call set up • Transcoding and rate adaptation functionality • Handover for each MS • Radio Power control • It communicates with MSC and BTS
  11. 11. Mobile Switching Center (MSC) • Heart of the network • Manages communication between GSM and other networks • Call setup function and basic switching • Call routing • Billing information and collection • Mobility management - Registration - Location Updating - Inter BSS and inter MSC call handoff • MSC does gateway function while its customer roams to other network by using HLR/VLR
  12. 12. • Home Location Registers (HLR) - permanent database about mobile subscribers in a large service area(generally one per GSM network operator) - database contains IMSI,MSISDN,prepaid/postpaid,roaming restrictions,supplementary services. • Visitor Location Registers (VLR) - Temporary database which updates whenever new MS enters its area, by HLR database - Controls those mobiles roaming in its area - Reduces number of queries to HLR - Database contains IMSI,TMSI,MSISDN,MSRN,Location Area,authentication key
  13. 13. • Authentication Center (AUC) - Protects against intruders in air interface - Maintains authentication keys and algorithms and provides security triplets ( RAND,SRES,Kc) - Generally associated with HLR • Equipment Identity Register (EIR) - Database that is used to track handsets using the IMEI (International Mobile Equipment Identity) - Made up of three sub-classes: The White List, The Black List and the Gray List - Only one EIR per PLMN
  14. 14. • In a GSM network, the following areas are defined: Cell: Cell is the basic service area: one BTS covers one cell. Each cell is given a Cell Global Identity (CGI), a number that uniquely identifies the cell. Location Area: A group of cells form a Location Area. This is the area that is paged when a subscriber gets an incoming call. Each Location Area is assigned a Location Area Identity (LAI). Each Location Area is served by one or more BSCs. MSC/VLR Service Area: The area covered by one MSC is called the MSC/VLR service area. PLMN: The area covered by one network operator is called PLMN. A PLMN can contain one or more MSCs.
  15. 15. • GSM uses TDMA and FDMA to let everybody talk. • FDMA: 25MHz freq. is divided into 124 carrier frequencies. Each base station gets few of those. • TDMA: Each carrier frequency is divided into bursts [0.577 ms]. 8 bursts are a frame.
  16. 16. • RF Spectrum GSM 900 Mobile to BTS (uplink): 890-915 Mhz BTS to Mobile(downlink):935-960 Mhz Bandwidth : 2* 25 Mhz GSM 1800 Mobile to BTS (uplink): 1710-1785 Mhz BTS to Mobile(downlink) 1805-1880 Mhz Bandwidth : 2* 75 Mhz
  17. 17. • • • • • • Carrier Separation : 200 Khz Duplex Distance : 45 Mhz No. of RF carriers : 124 Access Method : TDMA/FDMA Modulation Method : GMSK Modulation data rate : 270.833 Kbps
  18. 18. When a mobile subscriber makes a call to a PSTN telephone subscriber, the following sequence of events takes place: • The MSC/VLR receives the message of a call request. • The MSC/VLR checks if the mobile station is authorized to access the network. If so, the mobile station is activated. If the mobile station is not authorized, service will be denied. • MSC/VLR analyzes the number and initiates a call setup with the PSTN. • MSC/VLR asks the corresponding BSC to allocate a traffic channel (a radio channel and a time slot).
  19. 19. • The BSC allocates the traffic channel and passes the information to the mobile station. • The called party answers the call and the conversation takes place. • The mobile station keeps on taking measurements of the radio channels in the present cell and neighbouring cells and passes the information to the BSC. The BSC decides if handover is required, if so, a new traffic channel is allocated to the mobile station and the handover is performed. If handover is not required, the mobile station continues to transmit in the same frequency.
  20. 20. When a PSTN subscriber calls a mobile station, the sequence of events is as follows: • The Gateway MSC receives the call and queries the HLR for the information needed to route the call to the serving MSC/VLR. • The GMSC routes the call to the MSC/VLR. • The MSC checks the VLR for the location area of the MS. • The MSC contacts the MS via the BSC through a broadcast message, that is, through a paging request. • The MS responds to the page request. • The BSC allocates a traffic channel and sends a message to the MS to tune to the channel. The MS generates a ringing signal and, after the subscriber answers, the speech connection is established.
  21. 21. 1. MS sends dialed number to BSS 2. BSS sends dialed number to MSC 3,4 MSC checks VLR if MS is allowed the requested service. If so, MSC asks BSS to allocate resources for call. 5 MSC routes the call to GMSC 6 GMSC routes the call to local exchange of called user 7, 8,9,10 Answer back(ring back) tone is routed from called user to MS via GMSC,MSC,BSS
  22. 22. 1. 2. 3. 4. 6. Calling a GSM subscribers Forwarding call to GSMC Signal Setup to HLR 5. Request MSRN from VLR Forward responsible MSC to GMSC 7. Forward Call to current MSC 8. 9. Get current status of MS 10.11. Paging of MS 12.13. MS answers 14.15. Security checks 16.17. Set up connection
  23. 23. • the use of signals for controlling communications • the information exchange concerning the establishment and control of a telecommunication circuit and the management of the network, in contrast to user information transfer • the sending of a signal from the transmitting end of a telecommunication circuit to inform a user at the receiving end that a message is to be sent.
  24. 24. • • • • • 1.In-band versus out-of-band signaling 2. Line versus register 3 .Channel-associated versus common-channel signaling 4 .Compelled signaling 5 .Subscriber versus trunk signaling
  25. 25. • In the public switched telephone network (PSTN), in-band signaling is the exchange of call control information within the same channel that the telephone call itself is using. An example is dual-tone multi-frequency signaling (DTMF), which is used on most telephone lines to customer premises. • Out-of-band signaling is telecommunication signaling on a dedicated channel separate from that used for the telephone call. Out-of-band signaling is used in Signalling System No. 7 (SS7), the standard for signaling among exchanges that have controlled most of the world's calls for over twenty years
  26. 26. • Line signaling is concerned with conveying information on the state of the line or channel, such as on-hook, off-hook , ringing current (alerting), and recall. • Register signaling is concerned with conveying addressing information, such as the calling and/or called telephone number.
  27. 27. • Channel-associated signaling (CAS) employs a signaling channel which is dedicated to a specific bearer channel. • Common-channel signaling (CCS) employs a signaling channel which conveys signaling information relating to multiple bearer channels. These bearer channels therefore have their signaling channel in common.
  28. 28. • Compelled signaling refers to signaling where receipt of each signal from an originating register needs to be explicitly acknowledged before the next signal is able to be sent. • The term is only relevant in the case of signaling systems that use discrete signals , as opposed to signaling systems which are message-oriented where each message is able to convey multiple items of formation.
  29. 29. • Subscriber signaling refers to signaling between the telephone and the telephone exchange. Trunk signaling is signaling between exchanges