comt 3911 cellular and pcs technical issues comt 391
TRANSCRIPT
COMT 391
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Cellular and PCS Technical Issues
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Overview
• Cell Layout• Signaling• Mobile Handset Operation
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System Block Diagram
Mobile BTS BSC MTSO PSTN
Data Bases
forward
reverse
BTS Base Tranceiver Station
BSC Base Station Controller
MTSOMobile Telephone Switching Office
Radio Link Voice Trunk
Data Trunk
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Frequency Reuse Pattern
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Standards
• AMPS - United States• TACS - UK• NTT - Japan• NMT - Scandanavia• MATS-E - France• C-450 - Germany• GSM - global
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North American AMPS system
• 50MHz spectrum• 30kHz per channel per direction• 832 total full-duplex channels• 4, 7, and 12 cell clusters used
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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
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Antenna Placement - high
• Good Coverage• Co-Channel Interference Problems
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Buildings as Boundaries
• Cell Limits Defined• Possible Adjacent Channel
Interference
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Interference
• Co-Channel Interference• Adjacent Channel Interference• Outside Sources
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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
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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.
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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
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Mobile-Originated Call
Mobile BTS BSC MTSO PSTN
RCC MIN, ESN, dialed no
validation
FCC Ch No, SAT code, Power
FVC SAT
RVC SAT
FVCRVC
Conversation
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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
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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
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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
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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.
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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.
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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)
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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
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Hand-Off
MTSO
BSBS
MSMS
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Roaming
Mobile
MTSO
Mobile
Mobile
Mobile
Mobile
MTSO
Mobile
Mobile
Mobile
Mobile
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Intersystem Hand-Off
MTSO
BSBS
MSMS
MTSO
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The “Shoe-Lace” Effect
BS
BS
BS
BS
BS
BS
BS
BS
MS
MS
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Multi-System Tandem
MTSO
BSBS
MSMS
MTSOMTSOMTSO
MTSO
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Automatic Roaming
MTSO
BS
MS
MTSO
HLR
HLR VLR
Request
Response
Billing Information
Service Profile
Current Location
Temporary Directory Number
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The “Trombone” Effect
MTSO
BS
MS
MTSO
Fixed Station
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Security/Fraud Issues
• Security– Privacy of Conversations– Denial of Service
• Fraud– Illegally obtained ESN/MINs– Roaming-related (“tumbling” ESNs)
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Classes of Standards
• Analog Cellular– FDMA - Frequency Division Multiple
Access
• Digital Cellular– FDMA– TDMA - Time Division Multiple Access– CDMA - Code Division Multiple Access
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Access MethodsFr
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Time
FDMA
TDMA
CDMA
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Standards Content
• Frequency Bands• Channel Assignments• Voice Encoding• Access Method and Frame
Structure• Signaling Structure