The document discusses the characteristics and performance limitations of wireless networks. It covers topics such as wireless network types and standards, factors that affect wireless throughput such as bandwidth and signal-to-noise ratio, and challenges such as interference from other networks and variable bandwidth. The key points are that wireless networks have limited and shared bandwidth, unpredictable latency, and throughput that varies based on environmental factors. Applications over wireless must be designed to adapt to these constraints.

3. • “The future is about ubiquitous connectivity where access to the
Internet is omnipresent.”
– Omnipresent: present everywhere at the same time
• Widespread wireless technologies in use:
– Operate on common principles
– Have common trade-offs
– Subject to common performance criteria and constraints.
Type Range Applications Standards
Personal area netw
ork (PAN)
Within reach of a p
erson
Cable replacement f
or peripherals
Bluetooth, ZigBee,
NFC
Local area network
(LAN)
Within a building or
campus
Wireless extension
of wired network
IEEE 802.11 (WiFi)
Metropolitan area n
etwork (MAN)
Within a city
Wireless inter-netw
ork connectivity
IEEE 802.15 (WiMAX
)
Wide area network
(WAN)
Worldwide
Wireless network ac
cess
Cellular (UMTS, LTE,
etc.)

4. • Shannon-Hartley theorem
– Theoretical tightest upper bound on the information rate
– C is the channel capacity and is measured in bits per second.
– BW is the available bandwidth, and is measured in hertz.
– S is signal and N is noise, and they are measured in watts.

5. • Federal Communications Commission
– FCC determines the frequency range
and its allocation in US
• Bandwidth: the size of freq. range
– Double of bandwidth (e.g. 20 to 40 MHz)
double the channel data rate
– 802.11n is improving its performance
over earlier WiFi standards
• Low-frequency signals
– Travel farther
– Large areas (macrocells)
– Requiring larger antennas and having
more clients competing for access.
• High-frequency signals
– Transfer more data
– Not travel as far
– Smaller coverage areas (microcells)
– Requiring more infrastructure.
FCC radio spectrum allocation for the 2,300–3,000 MHz band

6. http://rf-mw.org/images/United_States_Frequency_Allocations_Chart_2003_-_The_Radio_Spectrum.jpg

7. • Early days of radio, anyone could use any frequency range
• Radio Act of 1912 within the United States
– Mandated licensed use of the radio spectrum.
– Motivated by the investigation into the sinking of the Titanic partly.
– More lives could have been saved, if proper frequencies were monitored by
all nearby vessels
• The Communications Act of 1934
– The FCC has been responsible for managing the spectrum allocation
within the U.S.
– Effectively "zoning" it by subdividing into ever-smaller parcels designed for
exclusive use.
• Unlicensed : The "industrial, scientific, and medical" (ISM) bands
– Established at the International Telecommunications Conference in 1947,
– Both the 2.4–2.5 GHz (100 MHz) and 5.725–5.875 GHz (150 MHz) bands,
– “Unlicensed spectrum," which allow anyone to operate a wireless network
– Respects specified technical requirements (e.g., transmit power).
• Licensed Spectrum Auctions
– A license is sold to transmit signals over the specific bands.
– The UHD 700 MHz FCC auction took place in 2008 for mobile usage.

8. • Signal-power-to-noise-power, S/N ratio, or SNR.
– Signal power:
• the second fundamental limiting factor in all wireless
communication
– Noise
• Background noise or interference
• The larger the amount of background noise
The stronger the signal is required.
– All radio communication is done over a shared medium
• means that other devices may generate unwanted interference
• Microwave and Wifi
– To achieve the desired data rate where interference is
present
• Increase the transmit power, or
• Decrease the distance

9. • Cell-breathing
– The coverage area expands and shrinks based on the cumulative noise
and interference levels.
• Near-far problem
– A receiver captures a strong signal
– Impossible for the receiver to detect a weaker signal

10. • No Digital Signal
– Digital alphabet (1’s and 0’s), needs to be translated into
an analog signal (a radio wave).
• Modulation
– The process of digital-to-analog conversion
– Different "modulation alphabets" can be used to encode
the digital signal with different efficiency.
• Example
– Receiver and sender can process 1,000 pulses or symbols
per second: 1,000 baud
– Each transmitted symbol represents a different bit-
sequence, determined by the chosen alphabet (e.g., 2-bit
alphabet: 00, 01, 10, 11).
– The bit rate of the channel is 1,000 baud × 2 bits per
symbol, or 2,000 bps

11. • The performance of wireless network is limited by
– The amount of allocated bandwidth and
– The signal-to-noise ratio between receiver and sender.
• Further, all radio-powered communication is:
– Done over a shared communication medium (radio waves)
– Regulated to use specific bandwidth frequency ranges
– Regulated to use specific transmit power rates
– Subject to continuously changing background noise and
interference
– Subject to technical constraints of the chosen wireless
technology
– Subject to constraints of the device: form factor, power,
etc.

12. • All wireless technology Peak Data Rate
– 802.11g standard is capable of 54 Mbit/s
– 802.11n standard raises the bar up to 600 Mbit/s.
– LTE are advertising 100+ MBit/s throughput
• Factors that affect the performance:
– Amount of distance between receiver and sender
– Amount of background noise in current location
– Amount of interference from users in the same network
(intra-cell)
– Amount of interference from users in other, nearby
networks (inter-cell)
– Amount of available transmit power, both at receiver and
sender
– Amount of processing power and the chosen modulation
scheme

13. • How to increase throughput?
– Try to remove any noise and interference you can
control,
– Place your receiver and sender as close as possible,
– Give them all the power they desire, and make sure
both select the best modulation method.
– If you are bent on performance, just run a physical
wire between the two!

15. • Adaptation and an extension of Ethernet (802.3)
• Ethernet -> LAN, 802.11 family -> Wireless LAN

16. • CSMA/CD: For Ethernet
– Carrier Sense Multiple Access (CSMA) protocol,
• “Listen before you speak" algorithm
– Check whether anyone else is transmitting.
– If the channel is busy, listen until it is free.
– When the channel is free, transmit data immediately.
– Collision Detection
• Collisions can still occur.
– If a collision is detected,
– Then both parties stop transmitting immediately and sleep for a
random interval (with exponential backoff).
• CSMA/CA: Wifi
– Collision Avoidance
• sender attempts to avoid collisions by transmitting only when
the channel is sensed to be idle
– The channel load must be kept below 10%. If not,
collision increased and performance was degraded.

17. • "b" and "g" standards
– The most widely deployed and supported
– Use 2.4 GHz ISM band and 20 MHz of bandwidth,
• "n" and upcoming "ac" standards
– Doubling the bandwidth from 20 to 40 MHz per channel
– Using higher-order modulation, and MIMO
– Adding multiple radios to transmit multiple streams in parallel
 All combined, and in ideal conditions, this should enable gigabit-plus
throughput with the upcoming "ac" wireless standard.
802.11
protocol
Release Freq (GHz) Bandwidth (MHz)
Data rate per stream
(Mbit/s)
b Sep 1999 2.4 20 1, 2, 5.5, 11
g Jun 2003 2.4 20 6, 9, 12, 18, 24, 36, 48, 54
n Oct 2009 2.4 20
7.2, 14.4, 21.7, 28.9, 43.3,
57.8, 65, 72.2
n Oct 2009 5 40
15, 30, 45, 60, 90, 120,
135, 150
ac ~2014 5 20, 40, 80, 160 up to 866.7

18. • No central scheduler for WiFi
– No guarantees on throughput or latency
– New WiFi Multimedia (WMM) extension enables basic Quality of
Service (QoS)
– No control over traffic generated by other, nearby WiFi
networks.
inSSIDer visualization of overlapping WiFi networks (2.4 and 5 GHz bands)

19. • Throughput
– 802.11g client and router reachable up to 54 Mbps
– Other occupying the same channel, bandwidth is cut in half, or
worse.
• Latency
– With many overlapping networks, measured in tens and even
hundreds msec
– Because of back-off delay from competing medium.
Wikipedia illustration of WiFi channels in the 2.4 GHz band

20. • 5 GHz band
– Used by the new 802.11n and 802.11ac standards
– Offers both a much wider frequency range
– Still interference free in most environments
– A dual-band router supports 2.4 G and the 5 G
• Measuring Your WiFi First-Hop Latency
– Running a ping to your wireless gateway is a simple
way to estimate the latency
Latency difference between 2.4GHz and 5GHz WiFi bands
Freq (GHz) Median (ms) 95% (ms) 99% (ms)
2.4 6.22 34.87 58.91
5 0.90 1.58 7.89

21. • WiFi
– provides no bandwidth or latency guarantees or
assignment to its users.
– provides variable bandwidth based on signal-to-noise
in its environment.
– transmit power is limited to 200 mW, and likely less
in your region.
– has a limited amount of spectrum in 2.4 GHz and the
newer 5 GHz bands.
– access points overlap in their channel assignment by
design.
– access points and peers compete for access to the
same radio channel.

22. • High number of collisions
– Between multiple wireless peers in the area
– Retransmission and error correction mechanisms
– Hide these wireless collisions from higher layers
• Higher variability in packet arrival times
– Instead of direct TCP packet loss
– Due to the underlying collisions and retransmissions
• In-flight frame at any point in time
– Prior to 802.11n,
• WiFi protocol allowed at most one,
• Had to be ACKed by the link layer before the next frame is sent.
– New "frame aggregation" feature
• Allows multiple WiFi frames to be sent and ACKed at once.

23. • Leverage Unmetered Bandwidth
– Bandwidth limited by the available WAN bandwidth
• WiFi network extended to a wired LAN,
• Connected via DSL, cable, or fiber to the WAN
– When the "radio network weather" is nice!
• Adapt to Variable Bandwidth
– WiFi provides no bandwidth or latency guarantees.
• The user’s router may have some application-level QoS policies,
– Fairness to multiple peers on the same wireless network.
– WiFi radio interface itself has very limited support for QoS.
• No QoS policies between multiple, overlapping WiFi networks.
– The available bandwidth may change dramatically,
• On a second-to-second basis,
• Based on small changes in location,
• Activity of nearby wireless peers
• The general radio environment.

24. • Adapt to Variable Bandwidth
• Adapt to Variable Latency
– No guarantees on the latency of the first wireless hop.
– More unpredictable if multiple wireless hops are needed
– Latency varies:
• 1–10 msec median for the first wireless hop
• With a long latency tail: Occasional 10–50 msec delay,
• In the worst case, even as high as 100s msec
 Convert to unreliable UDP transport
Container Video resolution Encoding
Video bitrate (M
bit/s)
mp4 360p H.264 0.5
mp4 480p H.264 1–1.5
mp4 720p H.264 2–2.9
mp4 1080p H.264 3–4.3