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Cellular and PCS Technical Issues

Cellular and PCS Technical Issues. COMT 391. Overview. Cell Layout Signaling Mobile Handset Operation. System Block Diagram. forward. Mobile. BTS. BSC. MTSO. PSTN. reverse. Data Bases. BTS Base Tranceiver Station BSC Base Station Controller MTSO Mobile Telephone Switching Office.

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Cellular and PCS Technical Issues

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  1. Cellular and PCS Technical Issues COMT 391

  2. Overview • Cell Layout • Signaling • Mobile Handset Operation

  3. System Block Diagram forward Mobile BTS BSC MTSO PSTN reverse Data Bases BTS Base Tranceiver Station BSC Base Station Controller MTSO Mobile Telephone Switching Office Radio Link Voice Trunk Data Trunk

  4. Frequency Reuse Pattern

  5. Standards • AMPS - United States • TACS - UK • NTT - Japan • NMT - Scandanavia • MATS-E - France • C-450 - Germany • GSM - global

  6. North American AMPS system • 50MHz spectrum • 30kHz per channel per direction • 832 total full-duplex channels • 4, 7, and 12 cell clusters used

  7. AMPS Specifics • 832 channels (2 frequencies per channel) • Forward: 869-894 MHz • Reverse: 824-849MHz • Forward/Reverse Spacing: 45MHz • Adjacent Channel Spacing: 30kHz • 21 Control Channels per group • Cell Sizes: 2-20km

  8. Antenna Placement - high • Good Coverage • Co-Channel Interference Problems

  9. Buildings as Boundaries • Cell Limits Defined • Possible Adjacent Channel Interference

  10. Interference • Co-Channel Interference • Adjacent Channel Interference • Outside Sources

  11. Signaling Channels • Dedicated Control Channels (CC) • Mobile will scan for the strongest control channel when first switched on • System Identification • Number of Paging and Access Channels • Paging Channels (PC) • Used to Initiate Calls to a Mobile • Access Channels (AC) • Used to Initiate Calls from a Mobile

  12. Call Setup from Mobile • Mobile receives AC number over PC • User dialed digits are stored in the Mobile • Mobile finds the strongest AC • Mobile sends the dialed digits over the AC to the Base Station and the MTSO • MTSO transmits the Voice Channel No.

  13. Call Setup From Mobile cont… • Mobile tunes to the selected voice channel • Mobile loops the supervisory audio tone (shows voice channel active) • MTSO sends digits to the PSTN • PSTN completes call

  14. Mobile-Originated Call Mobile BTS BSC MTSO PSTN RCC MIN, ESN, dialed no validation FCC Ch No, SAT code, Power SAT FVC SAT RVC FVC Conversation RVC

  15. Call Setup to Mobile • PSTN delivers call to MTSO • MTSO transmits Mobile ID over all Paging Channels • Mobile acknowledges on the strongest Access Channel • MTSO selects the voice channel to use • MTSO starts supervisory tone and sends voice channel number to mobile

  16. Setup to Mobile Cont. • Mobile loops the supervisory tone • MTSO instructs mobile to begin ringing • Mobile rings and sends 10kHz alert tone to the MTSO • Alert tone stops when mobile picks up, MTSO completes call connection

  17. Speech Channel Control Signals (AMPS) • Signaling Tone (10kHz) from Mobile to MTSO • Alerting • Flash-hook, disconnect, etc. • Supervisory Audio Tones (6kHz+/-30Hz) (there are three of these) • Base Station to Mobile, looped back by the mobile

  18. Call Handoff • Base Stations monitor the signal strength • If strength drops below hand-off level, all base stations are asked to look for the mobile; strongest becomes new BS • MTSO instructs the new base station to activate a voice channel to receive the handoff; BS starts transmitting the Supervisory Audio Tone.

  19. Call Handoff cont… • Current base station at MTSO command sends signal to mobile giving the new channel assignment. • Mobile send a signaling tone burst, and tunes to the new channel. • Previous base station clears the call path.

  20. Mobile Handset • ESN - Electronic Serial Number • Programmable NAM (Number Assignment Module) • Contains among other things • System ID • First control channel to scan • A/B system selection • MIN (Mobile Identification Number)

  21. Mobile Identification Number • MIN1 - Mobile System Id (3 digits) • US: Area Code • International: Mobil Carrier Identifcation • MIN2 - Station Number • US: NXX and line number • International: internal mobile number

  22. Hand-Off MS MS BS BS MTSO

  23. Roaming MTSO Mobile MTSO Mobile Mobile Mobile Mobile Mobile Mobile Mobile Mobile

  24. Intersystem Hand-Off MS MS BS BS MTSO MTSO

  25. The “Shoe-Lace” Effect MS BS BS BS BS BS BS BS BS MS

  26. Multi-System Tandem MS MS BS BS MTSO MTSO MTSO MTSO MTSO

  27. Automatic Roaming MS BS Request MTSO MTSO Response HLR HLR VLR Current Location Temporary Directory Number Billing Information Service Profile

  28. The “Trombone” Effect MS BS MTSO MTSO Fixed Station

  29. Security/Fraud Issues • Security • Privacy of Conversations • Denial of Service • Fraud • Illegally obtained ESN/MINs • Roaming-related (“tumbling” ESNs)

  30. Classes of Standards • Analog Cellular • FDMA - Frequency Division Multiple Access • Digital Cellular • FDMA • TDMA - Time Division Multiple Access • CDMA - Code Division Multiple Access

  31. Frequency Frequency Time Time Frequency Time Access Methods FDMA TDMA CDMA

  32. Standards Content • Frequency Bands • Channel Assignments • Voice Encoding • Access Method and Frame Structure • Signaling Structure

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