Transcript Wireless and going mobile

Wireless and
going mobile
Browsing via low energy
photons
Radio Basics
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Radio power fades as 1/distance^2
from where it’s transmitted.
Power needed is proportional to
bandwidth.
Range and bandwidth limited by
noise.
Signals can bounce causing
multipath.
Low frequency signals can go
around things.
Modulation
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Information carried by changing a
radio signal, or carrier.
 Can
change frequency (FM)
 Can change Amplitude or Strength
(AM)
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There are lots of complex
modulation schemes.
Point to Point
Communication
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One radio transmits signals to the
other
Can enable (transmit) its carrier all
the time, or part of the time
Can have one radio Rx on one
frequency, Tx on another (duplex)
Can share one frequency (simplex)
Multipoint
Communication
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One radio talks to a bunch of other
radios.
Need to share frequencies, control
access to radio waves
Need to address communication to
correct radios
Like Ethernet!
Cell phones!
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Lots of cells covering region
 Reduces
range to radios
 Increase bandwidth
 Decreases power needed in radio.
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Cells linked together by
communication links, and back to
phone system.
Cellular systems
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AMPS (Advanced Mobile Phone
Service): Not so advanced. Analog
sound transmitted over radio
waves with some digital data.
Works by using different
frequencies for users.
TDMA (Time Division Multiplexed
Access): Constantly rotate
between different users. Each gets
a different time slot.
More Cellular systems!
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GSM (Global System for Mobile
Comms): Yet another TDMA!
Narrowband TDMA -- multiple
frequencies, with time division, 8
channels, on top. LOTS of data
capability.
CDMA (Code Division Multiple
Access): Spread spectrum comms!
Constantly hops from one
frequency to another. Very
efficient!
Wireless Ethernet
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Ethernet packets broadcast over a
radio channel
Can have a Point to Point network
connection: like a wireless CAT-5
cable.
Often want multipoint network:
more like a bunch of nodes on a
network. Need to share system.
Sharing the radio waves
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Frequency Hopping (FH): Jump
occasionally. 75 or more
frequencies used. Up to 2 Mbps.
Digital Sequential Spread
Spectrum (DSSS): Jumping
frequency LOTS of times per
second. Can go way faster. Needs
way more power.
DSSS
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Doesn’t use frequency very
efficiently
Needs more radios - more cells
Has some resistance to multipath.
Good range.
Works by actually transmitting a far
higher bandwidth signal than the
data!
The new kid in town OFDM
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Orthogonal Frequency Division
Multiplexing
Stuff the frequencies used as close
as physics allows
Channels actually overlap, but can
be separated by complex math
done by the radio.
Needs really good radios to make
it work.
OFDM rules!
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Extremely multipath independent.
Considered to be effectively non-line-ofsite, if used at enough power. Can use
bounces, not just resist them.
In principle, can be low power, once
made small enough.
Fourth Generation (4G) communications,
including cell phones.
Darned fast!!!! 30 Mbps or more!
Mars!
Wireless networking
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Access point connected to the
network
Wireless cards in computers, or
computers hooked to other access
points.
Access point controls
communication over network.
Roaming on ethernet
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IEEE 802.11, the ethernet wireless
standard (WiFi):
 Defines
basic radio communication
methods. DSSS primarily used so
far.
 Defines how to make the radio links
secure: Wired-Equivalent Privacy.
 Defines how to move from one
access point to another
 Cells!
The Network,
everywhere
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As wireless systems start to
appear everywhere, we’ll need far
more advanced networks to deal
with issues:
 Who
can access the network
 How to allocate IP numbers
 How to access users services back
home
IPv6, the next
generation Internet!
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WAY larger address space: 128
bits instead of 32! 16 numbers!
2 x 10^20 addresses per square
cm of land on Earth!
Address split into pieces: local
address and global address.
Automatic allocation of addresses
is mandatory. No manual setting of
local addresses.
Die NAT Die!
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Network Address Translators:
convert local addresses to just one
address on the outside. Simple
firewall. Share IP numbers.
New protocols cannot operate
through NAT, as it’s very fixed.
IPv6 gets away from needing NAT.
Local Address
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Built from a unique address,
usually the ethernet address
Something like
fe80::240:12ce:c3a0:80. Lower 64
bits are local
Unique identification of machine.
Mobile IP
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Don’t confuse with Mobile Internet
(which is about cellphones)!
You plug your machine into a network
Your machine sends a request back to
your home network giving it your local
address
A “Mobile Agent” transmits network
packets back from your network to
wherever you are in the local network.
Local net in upper part of address.
The network of
tomorrow
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You’ll just access a network
Your identity, and permission to
use the local network, will be
verified.
You’ll see your services from your
home network, and from other
locations on the network.
You’ll never need to program in
your IP number.
The network will be everywhere.