1. CSCD 439/539 Wireless Networks and Security
Lecture 6
Physical Layer Continued …
Fall 2007

2. Introduction
g, a and n
OFDM
MIMO and other enhancements, n

3. Intro to a
802.11a was approved in September 1999, two years after standard approved
Operates in 5 GHz unlicensed national information infrastructure (UNII) band
Spectrum is divided into three “domains,” each having restrictions imposed on the maximum allowed output power
First 100 MHz in the lower frequency portion is restricted to a maximum power output of 50 mW
Second 100 MHz has a higher 250 mW maximum
Third 100 MHz, which is mainly intended for outdoor applications, has a maximum of 1.0 W power output

4. Intro to a
802.11a
Offered an alternative to the overcrowded band 2.4 GHz, 5GHz
The 5GHz ISM bandwidth is not continuous
There are two areas 5.15GHz GHz and
5.725GHz Ghz
More details about a later …

5. Intro to OFDM 802.11a and 802.11g based on OFDM
Orthogonal Frequency Division Multiplexing
Revolutionized Wi-Fi and other cellular products by allowing faster throughput and more robustness
OFDM makes highly efficient use of the available spectrum
This characteristic will be important in coming years as wireless networks dominate especially in enterprise environments

6. OFDM Based on FDM Recall …
Frequency division multiplexing (FDM) is a technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system
Each signal travels within its own unique frequency range (carrier)

7. FDM
Comment:
FDM Access transmissions are the least efficient networks since each analog channel can only be used one user at a time
Each User has their own channel

8. OFDM based on FDM
In OFDM, data divided among large number of closely spaced carriers
The "frequency division multiplex" part of the name
The entire bandwidth is filled from a single source of data
Instead of transmitting data serially, data is transferred in a parallel
Divided among multiple subcarriers
Only a small amount of the data is carried on each carrier, which besides the obvious benefit of being parallel
Provides benefits related to the radio nature of wireless

9. OFDM An OFDM signal consists of
Several closely spaced modulated carriers
When modulation of any form - voice, data, etc. is applied to a carrier
Sidebands spread out on either side
A receiver must be able to receive the whole signal to be able to demodulate the data
So, when signals are transmitted close to one another they must be spaced with a guard band between them

10. Traditional View with Guards
Guard bands waste the spectrum
Receiver filter passband: one signal selected
Guards
Traditional view of signals carrying modulation

11. OFDM Making the subcarriers mathematically orthogonal
Breakthrough for OFDM
Enables OFDM receivers to separate subcarriers via an Fast Fourier Transform
Eliminate the guard bands
OFDM subcarriers can overlap to make full use of the spectrum
Peak of each subcarrier spectrum, power in all the other subcarriers is zero

12. OFDM
OFDM offers higher data capacity in a given spectrum while allowing a simpler system design
Power
Others are have zero power

13. OFDM
Shows parallel nature of subcarriers

14. Benefits of OFDM Radio signals are imperfect
General challenges of RF signals include
Signal-to-noise ratio
Self-interference (intersymbol interference or ISI)
Fading owing to multipath effects
Same signal arrives at a receiver via different paths
Briefly look at multipath fading …

15. Multipath Fading
The mobile or indoor radio channel is characterized by multipath reception
Sent signal contains not only a direct line-of-sight radio wave, but also a large number of reflected radio waves
Even worse in urban areas, the line-of-sight is often blocked by obstacles, and collection of differently delayed waves is received by a mobile antenna
These reflected waves interfere with direct wave, causes significant degradation link performance
Reason is that waves arrive at slightly different times, so they are out of phase with original wave
Will randomly boost or cancel out parts of the signal

16. Multipath Fading

17. Benefits of OFDM Main way to prevent Intersymbol Interference errors
Transmit a short block of data (a symbol)
Wait until all the multipath echoes fade before sending another symbol
Waiting time often referred to as guard interval

18. Benefits of OFDM
Longer the guard intervals - more robust system to multipath effects
But during guard interval, system gets no use from the available spectrum
Longer the wait, the lower the effective channel capacity
Some guard interval is necessary for any wireless system
Goal is to minimize that interval and maximize symbol transmission time

19. Benefits of OFDM
OFDM meets this challenge by dividing transmissions among multiple subcarriers.
Same guard interval can then be applied to each subcarrier, while the symbol transmission time is multiplied by the number of subcarriers
With a, there are 52 channels, so the system has 52 times the transmission capacity compared to single channel

20. Benefits of OFDM
Using multiple subcarriers also makes OFDM systems more robust to fading
Fading typically decreases received signal strength at particular frequencies, so problem affects only a few of the subcarriers at any given time and …
Error-correcting codes provide redundant information that enables OFDM receivers to restore information lost in these few erroneous subcarriers

21. 802.11a OFDM
802.11a specifies eight non-overlapping 20 MHz channels in the lower two bands
Each divided into 52 sub-carriers (four of which carry pilot data) of 300-kHz bandwidth each
Four non-overlapping 20 MHz channels are specified in the upper band
The receiver processes the 52 individual bit streams, reconstructing the original high-rate data stream
Four complex modulation methods are employed, depending on the data rate that can be supported by channel conditions between the transmitter and receiver.
Include BPSK, QPSK, 16-QAM, and 64-QAM.

22. Trying to Use 802.11a Advantage Disadvantage
Since 2.4 GHz band is heavily used, using 5 GHz band gives a the advantage of less interference
Disadvantage
However, high carrier frequency also brings disadvantages
It restricts use of a to almost line of sight, necessitating use of more access points
It also means that a cannot penetrate as far as b since it is absorbed more readily, other things (such as power) being equal.

23. Trying to Use 802.11a 802.11a products started shipping in 2001
Lagged b products slow availability of the 5 GHz components needed to implement products
802.11a was not widely adopted because b was already widely adopted
Because of a's disadvantages, poor initial product implementations, making its range even shorter, and because of regulations
Manufacturers of a equipment responded to lack of market success by improving the implementations
Plus making technology that can use more than one standard.
There are dual-band, or dual-mode or tri-mode cards that can automatically handle a and b, or a, b and g, as available
Similarly, there are mobile adapters and access points which can support all these standards simultaneously

24. Comparing a and b
The throughput of a is 2 to 4.5 times better than b up to a certain range …
Example: At 225 ft, a averages yielded 5.2 Mbps compared to 1.6 Mbps for b
Next slide shows this as a graph

25. Throughput Range Performance
Averaged throughput performance for 1500 byte packets: a thoughputs always better by 2 to 4.5 times up to 225 ft.

26. 802.11g June 2003, a third modulation standard ratified 802.11g
Works in 2.4 GHz band (like b) but operates at a maximum raw data rate of 54 Mbit/s, or about 24.7 Mbit/s net throughput like a
802.11g hardware will work with b hardware
Older networks, b node significantly reduces the speed of an g network

27. 802.11g The modulation scheme used in 802.11g
OFDM for data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s, and reverts to CCK, like b for 5.5 and 11 Mbit/s
DBPSK/DQPSK+DSSS for 1 and 2 Mbit/s
Even though g operates in same frequency band as b
Achieve higher data rates because of its similarities to a
The maximum range of g devices is slightly greater than that of b devices
Range in which a client can achieve full (54 Mbit/s) data rate speed is much shorter than that of b

28. Beyond a and b, g
Despite its major acceptance, g suffers from same interference as b in the already crowded 2.4 GHz range

29. 802.11n … A miracle or …

30. 802.11n Introduction
802.11n is the long anticipated update to Wi-Fi standards a/b/g
Expected to bring a 4x increase in throughput
Improvement in range
802.11n expected to be ratified by the IEEE within the next 18 months

31. 802.11n Timeline

32. 802.11n 802.11n utilizes a larger number of antennas
The number of antennas relates to the number of simultaneous streams
Two receivers and two transmitters (2x2) or four receivers and four transmitters (4x4)
The standards requirement is a 2x2 with a maximum two streams, but allows 4x4

33. 802.11n
Solutions based on the n standard will operate in the 2.4-GHz, the 5-GHz radio band, or both
Backward compatibility with preexisting a/b/g deployment
Majority of Wi-Fi devices and access points deployed are dual-band
Operate in both 2.4-GHz and 5-GHz frequencies

34. 802.11n Features
Wireless solutions based on n standard employ several techniques to improve throughput, reliability, and predictability of wireless
Three primary innovations are
Multiple Input Multiple Output (MIMO) technology
Channel bonding (40MHz Channels)
Packet aggregation
These techniques allow n solutions to achieve an approximate fivefold performance increase over current a/b/g networks

35. MIMO 802.11n builds on previous standards by adding
multiple-input multiple-output (MIMO)
MIMO uses multiple transmitter and receiver antennas to improve the system performance
MIMO uses additional signal paths from each antenna to transmit more information, recombine signals on the receiving end
Similar to our ability to localize, using just our two ears, origin of specific sounds or to isolate and understand one conversation fragment from midst of assorted cocktail party chatter

36. MIMO
802.11n access points and clients transmit two or more spatial streams, and employ multiple receive antennas and advanced signal processing to recover multiple transmitted data streams
MIMO-enabled access points use spatial multiplexing to transmit different bits of a message over separate antennas
Provide greater data throughput
Allow for more robust, resilient wireless LANs
Whereas previous wireless technologies had problems dealing with signal reflections, MIMO actually uses these reflections to increase the range and reduce "dead spots" in the wireless coverage area

37. MIMO Technology
Multiple independent streams are transmitted simultaneously to increase the data rate

38. MIMO Performance gain is result of MIMO smart antenna technology
Allows wireless access points to receive signals more reliably over greater distances than with standard diversity antennas
Example, distance from access point at which an a/g client communicating with a conventional access point might drop from 54 Mbps to 48 Mbps or 36 Mbps, same client communicating with a MIMO access point may be able to continue operating at 54 Mbps

39. Channel Bonding
Most straightforward way to increase capacity of a network is to increase the operating bandwidth
However, conventional wireless technologies limited to transmit over one of several 20-MHz channels
802.11n networks employ technique called channel bonding to combine two adjacent 20-MHz channels into a single 40-MHz channel
Technique more than doubles the channel bandwidth

40. Channel Bonding
Channel bonding most effective in 5-GHz frequency given greater number of available channels
2.4-GHz frequency has only 3 non-overlapping 20-MHz channels
Thus, bonding two 20-MHz channels uses two thirds of total frequency capacity
So, IEEE has rules on when a device can operate in 40MHz channels in 2.4GHz space to ensure optimal performance
Cisco expects greatest benefits of channel bonding will be realized in the 5-GHz frequency

41. Packet Aggregation In conventional wireless transmission methods
Amount of channel access overhead required to transmit each packet is fixed, regardless of the size of the packet itself
As data rates increase, time required to transmit each packet shrinks
Overhead cost remains same, potentially becoming much greater than packet itself at high speeds delivered with n

42. Packet Aggregation
802.11n technologies increase efficiency by aggregating multiple packets of application data into a single transmission frame
802.11n networks can send multiple data packets with the fixed overhead cost of just a single frame
Packet aggregation is more beneficial for certain types of applications such as file transfers because can aggregate packet content
Real-time applications (e.g. voice) don’t benefit from packet aggregation because its packets would be interspersed at regular intervals
And combining packets into larger payload would introduce unnecessary latency
Voice and other multimedia applications still benefit from other effects of MIMO

43. 802.11n Advantages
Applications benefit most from n wireless LANs include:
• Environments and applications that require sharing of large files, including anything from advanced design and engineering applications to users in a conference room collaborating on a large Microsoft PowerPoint presentation
• Voice and video applications that demand high-quality transmissions, such as video conferencing and IPTV services that use multiple streams of high-definition video
• Facilities with challenging RF characteristics including warehouses, manufacturing floors and retail locations
• Disaster recovery, backup, and storage applications

44. Summary Looked at 802.11 a,g and n
Big part of increase in speed was OFDM
Very important currently for other media transmission, HD TV, Cellular phone and Wi-Max networks
802.11n holds tremendous promise
Not there yet … mixed results in testing with various products
However, expected standard within the year
Should provide some improvement over existing Wi-Fi

45. Finish Next time Bluetooth and Wi-Max
Briefly talk about what you found for tools