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