Dirk Grunwald Dept. of Computer Science, ECEE and ITP University of Colorado, Boulder

Goal & Outline Wireless networks are center point for insuring competitive Internet access market for national broadband plan We hear about gigabit wireless networks Problem solved? Tutorial on wireless networks How physics gets in the way How wireless interacts with Internet Protocol

Simple Radio Physics We have better intuition about sound Modulation Transmission power & receiver sensitivity Signal Noise Signal to Noise “Signal processing” is mainly statistics Correlations - Cocktail party effect

Frequency and coverage 700Mhz (new spectrum) 3500 Mhz (common WiMAX)

Loss / Attenuation Radio signals “attenuate” or lessen with distance Just like voice Absorption Diffraction Also affected by receiver These loses & variability are just part of life

“Multi-path” Multiple paths can also “cancel” signal Like Bose headphones Causes “fading” If you’re really smart, you can use the two paths as two distinct channels “MIMO”

Wireless Network Trick: coding “Coding” information converts bits to some kind of signal Also adds information to prevent loss High rate: error prone, but lots of bits / second Low rate: resistant to error, fewer bits / second

Multiple Rates & “Neutrality” 8% of area could use high- rate 5.2mb/s throughput 44% can only use 1.1 mb/s Consider “neutral” access to a 1024 byte web page Fast 1.5ms Slow 7.1ms Combined throughput is 1.9mb/s, or mb/s per user “better” connection suffers more 1024 bytes Also 1024 bytes

Co-Channel Interference Unintended communication becomes “noise” or “interference” Common in dense WiFi networks with “chaotic” deployment But, not all visible interference is that important Just because you can “see” other AP’s doesn’t mean they significantly influence your reception

Channel Planning Commercial networks typically use non- overlapping channels Reduces interference, but it means your “peak speed” will be less 30 Mhz of spectrum may become three 10Mhz bands 10 Mhz may be 5 up, 5 down Can also increase “spectrum reuse” by directional antennas

Existing commercial networks PCWorld study “A Day in the life of 3G” % “reliability”

Impact of Packet Loss TCP tolerates packet losses, but that doesn’t mean it likes them Works around exist (e.g PHY ack & retry) TCP also works better if latency is reasonably stable

Solutions? Exploit Diversity Physics matters Varying spatial scales of access and spatial reuse But this relies on wireline infrastructure The bands will vary and “cognition” will be needed on each 60Ghz 2.4Ghz 1800Mhz 700Mhz

Wireless as a third leg for network competition? Wireless is good because it doesn’t have wires That’s where the value is – mobility and ease of use

10% of the population using VoIP (on g) at once 9,100 people / sq. km. is an estimated threshold density for 10% of the population to be able to use VoIP concurrently.

100% of the population using VoIP (on g) at once 91,000 people / sq. km. is an estimated threshold density for 100% of the population to be able to use VoIP concurrently.