1. CSCD 439/539 Wireless Networks and Security
Lecture 5
Physical Layer, and b,g,a Differences
Fall 2007

2. Introduction Physical Layer in 802.11 Differences between 802.11 b,g,a
Frequency ranges
Speed
Spread Spectrum Techniques
DSSS Spread Spectrum, b

3. 802.11 Bands 802.11 networks use microwave size wavelengths
But, its really a radio technology
Microwaves are radio waves with short wavelength
Wavelength of AM radio is 1000 feet
devices, wavelength is 12 cm

4. Frequency Band ISM: Industry, Science, Medicine
unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz

5. IEEE Frequency Band
Wavelength
and b/g
802.11a

6. 802.11b/g Channels Channel Width = 22 MHz
Channels – 12 – 14, not sanctioned by FCC
2400 – Each channel spaced 5 MHz apart
Only non-overlapping channels are 1, 6 and 11

7. Physical Layer 3 Layers proposed in 802.11 back in 1997
FH – Frequency Hopping Spread Spectrum
DS – Direct Sequence Spread Spectrum
IR – Infrared - Never developed – no products
3 Further Layers Developed
802.11a OFDM
802.11b High Rate DS or DSSS
802.11g Extended Rate or ERP
Newest one
802.11n – MIMO PHY – High Throughput PHY

8. Radio Communications How do you transmit Radio Signals reliably?
Classic approach ….
Confine information carrying signal to a narrow frequency band and pump as much power as possible into signal
Noise is naturally occurring distortion in frequency band
Overcome noise
Ensure power of signal > noise

9. Radio Communications
Legal authority must impose rules on how RG spectrum is used
FCC in US
European Radiocommunications Office (ERO)
European Telecommunications Standards Institute (ETSI)
Ministry of Internal Communications (MIC) in Japan
Worldwide harmonization work done under
International Telecommunications Union (ITU)
Must have license to transmit at given frequency except for certain bands …

10. Radio Communications There are unlicensed bands
Networks operate in bands which are license free, Industrial, Scientific and Medical (ISM)
Does require FCC oversight, requires manufacturer to file information with the FCC
Competing devices have been developed in 2.4 GHz range
products
Bluetooth
Cordless phones
X10 – Protocol for home automation

11. Radio Communications 2.4 GHz is Unlicensed but
Must obey FCC limitations on power, band use and purity of signal
No regulations specify coding or modulation
Thus, there is contention between devices
Solve the problems
Stop using device, amplify its power or move it
Can’t rely on FCC to step in

12. Radio Communications
Given multiple devices compete in ISM bands, how do you reliably transmit data?
Spread Spectrum is one of the answers
Radio signals are sent with as much power as allowed over a narrow band of frequency
Spread Spectrum
Used to transform radio for data
Uses math functions to diffuse signal over large range of frequencies
Makes transmissions look like noise to narrowband receiver

13. Radio Communications Spread Spectrum continued
On receiver side, signal is transformed back to narrow-band and noise is removed
Spread spectrum is a requirement for unlicensed devices
Minimize interference between unlicensed devices, FCC imposes limitations on power of transmissions

14. Radio Communications Trivia Question
Who patented spread spectrum transmission and when was it patented?

15. Spread Spectrum
uses three different Spread Spectrum technologies
FH – Frequency Hopping (FHSS)
Jumps from one frequency to another in random pattern
Transmits a short burst at each subchannel
2 Mbps FH or FHSS is the original spread spectrum technology developed in 1997 with the standard
However, it was quickly bypassed by more sophisticated spread spectrum technologies
We won’t cover it … link on CourseNotes page

16. Spread Spectrum
uses three different Spread Spectrum technologies
DS or DSSS Direct Sequence
Took over from FHSS and allowed for faster throughput
Used in b
Spreads out signal over a wider path
Uses frequency coding functions
OFDM – Orthogonal Frequency Division Multiplexing
Divides channel into several subchannels and encode a portion of signal across each subchannel in parallel
802.11a and g uses this technology
Allows for even faster throughput than DSSS

17. RF Propagation
As radio signals travel through space, they degrade over distance
Performance determined by signal to noise ratio (SNR)
Says how strong is my signal compared to noise?
Degradation of signal will limit signal to noise ratio of receiver
Noise floor stays the same over network
But, as station gets further from Access Point, signal level drops and SNR will be lower

18. RF Propagation
AP1
Received Signal
Noise
Distance

19. RF Propagation Signal Degradation
When no obstacles, signal degradation can be calculated by following equation
Depends on distance and frequency
Path loss (dB) = log F + log d
where F = GHz , d = distance in meters
Higher F leads to more path loss at equal distances
Explains why a has a shorter range
It operates in the 5 GHz frequency range

20. 802.11 Signal Propagation Techniques

21. Spread Spectrum Code Techniques
Spread-spectrum is a signal propagation technique
Employs several methods
Decrease potential interference to other receivers while achieving privacy
Generally makes use of noise-like signal structure to spread normally narrowband information signal over a relatively wideband (radio) band of frequencies
Receiver correlates received signals to retrieve original information signal

22. Spread Spectrum Code Techniques
Typical applications include
Satellite-positioning systems (GPS)
3G mobile telecommunications
W-LAN (IEEE802.11a, IEEE802.11b, IEE802.11g)
Bluetooth

23. Spread Spectrum Code Techniques
Three characteristics of Spread Spectrum techniques
1. Signal occupies bandwidth much greater than that which is necessary to send the information
- Many benefits, immunity to interference, jamming and multi-user access … talk about this later
2. Bandwidth is spread by means of code independent of data
- Independence of code distinguishes this from standard modulation schemes in which data modulation will always spread spectrum somewhat
3. Receiver synchronizes to code to recover the data
- Use of an independent code and synchronous reception allows multiple users to access the same frequency band at the same time

24. Spread Spectrum Code Techniques
Transmitted signal takes up more bandwidth than information signal that is being modulated
Name 'spread spectrum' comes from fact that carrier signals occur over full bandwidth (spectrum) of a device's transmitting frequency
Military has used Spread Spectrum for many years
Worry about signal interception and jamming
SS signals hard to detect on narrow band equipment because the signal's energy is spread over a bandwidth of maybe 100 times information bandwidth

25. Spread Spectrum Techniques
In a spread-spectrum system, signals spread across wide bandwidth, making them difficult to intercept and demodulate

26. Spread Spectrum Code Techniques
Spread Spectrum signals use fast codes
These special "Spreading" codes are called "Pseudo Random" or "Pseudo Noise" codes
Called "Pseudo" because they are not truly random distributed noise
Will look at an example of this later

27. Spread Spectrum Code Techniques
Same code must be known in advance at both ends of the transmission channel
Spreading
de-Spreading
Codes are what DSSS uses … talk about next

28. Spread Spectrum Code Techniques
Real advantage of SS
Intentional or un-intentional interference and jamming signals rejected … do not contain the SS key
Only desired signal, which has key, will be seen at receiver when despreading operation is exercised
Practically can ignore interference if it does not include key used in the despreading operation
That rejection also applies to other SS signals not having right key
Allows different SS communications to be active simultaneously in the same band
Each will have their own PN code

29. Spread Spectrum Code Techniques
Can see results of interference attempts, interferer signals are not recovered

30. DSSS and HR/DSSS

31. DSSS DSSS is a spread spectrum technique
Modulation scheme used to transmit signal over wider frequency bandwidth
Modulation is the altering of carrier wave in order to transmit a data signal (text, voice, audio, video, etc.) from one location to another via a discrete channel
Phase-modulates a sine wave pseudorandomly
Continuous string of pseudonoise (PN) code symbols called "chips“
Each of which has a much shorter duration than an information bit
Each information bit is modulated by a sequence of much faster chips

32. DSSS
DSSS Techniques
To a narrowband receiver, transmitted signal looks like noise
Original signal can be recovered through correlation that reverses the process
The ratio (in dB) between the spread baseband and the original signal is called processing gain
It is the ratio by which unwanted signals or interference can be suppressed relative to the desired signal when both share the same frequency channel
Typical SS processing gains run from
10dB to 60dB

33. DSSS
How DSSS works
Apply something called a “chipping” sequence to the data stream
Chip is a binary digit
But, spread-spectrum developers make distinction to separate encoding of data from the data itself
Talk about data is bits
Talk about encoding is chips or chipping sequence

34. DSSS Chipping sequence Also called Pseudorandom Noise Codes (PNC)
Must run at a higher rate than underlying data
At left, is a data bit 0 or 1
For each bit, chip sequence is used
Originally, the chip was an 11 bit code combined with a data bit to produce an 11 bit code
This gets transmitted to receiver

35. DSSS Chipping Sequence
Encoded Data
Correlation
Data
Spreading
Modulo 2
Subtract
Modulo 2
add 1 1
Spreading Code
Spreading Code

36. DSSS
Chipping stream
Receiver uses correlation recovers bits by looking at each 11 bit segment of stream
Compares it to chipping sequence which is static
If it matches, bit is a zero
If it doesn’t match, bit is a one
Result of using a high chip-to-bit signal if signal is spread out over a wider bandwidth

37. DSSS Chipping stream DS system is concerned with Spreading Ratio
Number of chips used to transmit a single bit
Higher spreading ratios improve ability to recover transmitted signal
Because, also, spreading out noise over a larger area
Ratio of noise to actual spread and data is less
Doubling spreading ratio requires doubling chipping rate and doubles required bandwidth too

38. DSSS Chipping stream Two costs to increased chipping ratio
Direct cost of more expensive RF components that operate at higher frequencies
Amount of bandwidth required

39. DSSS Encoding DS 802.11 originally adopted an 11-bit Barker word
Each bit encoded using entire Barker word or chipping sequence
Key attribute of Barker words
Have good autocorrelation properties
High signal recovery possible when signal distorted by noise
Correlation function operates over wide range of environments and is tolerant of propagation delay

40. DSSS Encoding DS Why 11 bits?
Regulatory authorities require a 10 dB processing gain in DS systems
Using an 11 bit spreading code for each bit let meet regulatory requirements
Recall
The ratio (in dB) between the spread baseband and the original signal is processing gain

41. DSSS Complementary Code Keying (CCK)
Different modulation scheme used to encode more bits per code word
In 1999, CCK was adopted to replace the Barker code in wireless digital networks
CCK divides chip stream up into 8-bit code symbols so underlying transmission based on series of million code symbols/sec

42. DSSS Complementary Code Keying (CCK)
Based on mathematical transforms allow use of 8-bit sequences to encode 4 or 8 bits per code word
Helped increase data throughput to 5.5 Mbps or 11 Mbps
CCK selected over competing modulation techniques as it utilized same bandwidth and could utilize same header as pre-existing 1 and 2 Mbit/s wireless networks
Guarantee interoperability

43. HR/DSSS High Rate DSSS PHY
Is actually a different form of the PHY layers
Came up with a new “short” framing format that improves protocol efficiency and throughput
Uses short headers cuts overhead by 14%
Some other details are discussed for HR/DSSS involving transmission speeds
See references in CourseNotes page

44. Finish Next time More about 802.11 a, g and OFDM
Briefly talk about what you found for tools