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CIS-325
Data Communications
Dr. L. G. Williams, Instructor
CIS-325 Data Communications
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Chapter Eleven
Wireless Networks
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Advanced Mobile Phone Service
First generation of cell phones
3 basic devices
mobile
base transceiver
mobile telephone switching office (MTSO)
Voice and Control Channels
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Operations
Outgoing from mobile
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
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
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)
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
essentially frequency re-use
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Code-division Multiplexing
Move message to higher bandwidth
more room
Encode each signal
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
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
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)
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
Need 3 satellites for coverage
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Satellite Communications
Low Earth Orbiting Satellites (LEOS)
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
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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