CIS-325
Data Communications
Dr. L. G. Williams, Instructor
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Chapter Eleven
Wireless Networks
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Advanced Mobile Phone Service
 First generation of cell phones
 3 basic devices
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mobile
base transceiver
mobile telephone switching office (MTSO)
 Voice and Control Channels
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Operations
 Outgoing from mobile
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input phone number and press send
mobile links to base xcvr via control channel
base to MTSO to POTS
MTSO routes connection back to mobile via
voice channel
mobile shifts from control to voice
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Operations
 Incoming to mobile
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call goes from POTS to MTSO
on control channel, MTSO searches for mobile
by PAGING every active mobile
if found, MTSO rings it and establishes voice
channel connection
uses xcvr with strongest signal from mobile
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Cells
 In a city, there might be one MTSO
 There will be many xcvrs, each in its own
area or CELL
 Cell is hexagonal, with dia. of 6 miles or
less
 See fig 11.1, page 318
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Channels
 Every mobile in a cell who is talking must
have its own channel
 Otherwise, there will be interference
 More channels = more users
 Rule of thumb - one channel can support 20
users
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Spectral Allocation
 In U.S., we use two 25mHz bands
 one band from mobile to xcvr
 one band from xcvr to mobile
 Each channel uses 30 kHz
 So - about 832 channels for a city
 But - channels split between two companies
 21 for control, 395 for voice
 So - can support about 7900 users
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Frequency Reuse
 Also called Spatial Allocation
 Cells are small, so signals can be low power
 Can use same freq. in a cell that is far away,
but not one nearby
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at least 7 cells must intervene
 So - each cell can use about 57 channels
 So - each cell can support 1140 users
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Hand-offs
 Mobile phones move from cell to cell
 As signal fades, mobile asks MTSO for a
new base xcvr
 MTSO polls all base xcvrs to see which has
strongest signal with mobile
 Mobile shifts to new channel coming from
new base
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Roaming
 Mobile has moved to a different city or
subscriber area
 MTSO will check with your provider, via
land-line
 Then will allow you access
 Keeps track of billing data
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Global System for Mobile
Communications (GMS)
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Second-generation system
Widely used in Europe
Most of the same concepts as AMPS
System is digital, not analog
Uses fewer channels, but they are
multiplexed
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Subscriber Identity Card
 Plug this into any mobile and it becomes
‘yours’
 Allows very flexible use of system and
components
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Mobile Services Switching
Center (MSSC)
 Replaces MTSO, but has same functions
 Controls many Base Station Controllers
 Each Controller may control one or more
base transceivers
 MSSC connects system to POTS
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Multiple Access
 Use spectrum more efficiently by
multiplexing channels
 Frequency-division multiplexing
 Time-division multiplexing
 Space-division multiplexing
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essentially frequency re-use
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Code-division Multiplexing
 Move message to higher bandwidth
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more room
 Encode each signal
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one bit may now have a 6-bit code
 Combine several of these channels into
same signal, then un-mix at destination
 can put up to 100 signals on one channel
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Third Generation Wireless
Communications
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Universal Personal Communications
Identify yourself to any system, anywhere
PCS - Personal Communications Services
Global standards for terminals, cell phones
Smaller or micro-cells
Much greater bandwidth (150 mHz)
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Satellite Communications
 Geosynchronous
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22,300 miles above
Simple to track and find
Needs strong signal to cover distance
Slight delay in signal due to distance
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Satellite Communications
 Highly Elliptic Orbiting Satellites (HEOS)
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GEO can’t reach poles very well due to oblique
angles
Russian’s use HEOS
65 degrees off equater
Elliptic orbit
• close at South, far/high in North
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Need 3 satellites for coverage
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Satellite Communications
 Low Earth Orbiting Satellites (LEOS)
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Constellation of small, “inexpensive” satellites
200-700 miles high
Need >60 for 24 hour coverage
Sophisticated hand-off between satellites
Tables 11-1 and 11-2, pg. 333 lists proposed
systems
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Iridium
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A proposed LEOS system
Motorola
Voice, data, fax, paging
15-20 ground stations
Will not replace cellular system
Satellites at 420 nm
Phone call at $3/minute
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