Transcript Cellular and PCS Technical Issues

Cellular and PCS Technical
Issues
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Overview
• Cell Layout
• Signaling
• Mobile Handset Operation
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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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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)
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– 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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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
–
–
–
–
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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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Roaming
MTSO
Mobile
MTSO
Mobile
Mobile
Mobile
Mobile
Mobile
Mobile
Mobile
Mobile
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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 Methods
Frequency
Frequency
FDMA
Time
Frequency
Time
CDMA
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TDMA
Time
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Standards Content
• Frequency Bands
• Channel Assignments
• Voice Encoding
• Access Method and Frame Structure
• Signaling Structure
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