802.
11n Wi-Fi Basics
Objectives
Understand 802.11n technology Identify WiFi network elements Common issues in implementing an outdoor WiFi network
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Agenda
Physical layer:
Network Layer:
Channels MIMO Beamforming Modes Data Rates
Access Method Beacons VAP 802.11n modes
RF links Common issues in WIFI
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802.11n Main Features
Multiple antennas on Tx and Rx
Range Increase Throughput Increase As opposed to only 20 MHz in 11b/g On Rx On Tx (optional) Frame aggregation Block ACK
20 MHz 802.11a/b/g/n 40 MHz 802.11n
Multiple Data Streams
20 MHz and 40 MHz channels
Beamforming
MAC enhancements- less overhead, more data
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WiFi Channels 802.11n 2.4Ghz band
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Typical 802.11n channels 5Ghz band
Channel 148: 5740 MHz Channel 149: 5745 MHz (*) Channel 150: 5750 MHz Channel 151: 5755 MHz Channel 152: 5760 MHz Channel 153: 5765 MHz (*) Channel 154: 5770 MHz Channel 155: 5775 MHz Channel 156: 5780 MHz Channel 157: 5785 MHz (*) Channel 158: 5790 MHz
Channel 159: 5795 MHz Channel 160: 5800 MHz Channel 161: 5805 MHz (*) Channel 162: 5810 MHz Channel 163: 5815 MHz Channel 164: 5820 MHz Channel 165: 5825 MHz (*) Channel 166: 5830 MHz Channel 167: 5835 MHz
(*) 802.11 typical channels
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MIMO communication
Data is spited into multiple streams Each stream is transmitted by a different antenna
Each receive antenna receives all the streams
The modem reconstructs the original streams to a single data stream
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MIMO - Multiple in Multiple Out
DOUBLES the Capacity Not Improving coverage Different Data streams over the air
11011
010
00000
0101000101
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MIMO Terminology
Multiple antennas at both AP and client 11n device is characterized by 3 number:
Number of Tx antenna (Tx) Number of RX antenna (Rx) Number of streams (St)
Tx x Rx: St
3 x 3:3
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What is Beam Forming ?
General signal processing technique
Controls the directionality
Transducer array
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Beamforming (TxBF)
TxBF gain can range from 4.7 dB (array gain) up to 7-8 dB (array + diversity gain) TxBF works with one-stream rates only TxBF works when the CPE supports sounding
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Beam Forming limitations
No MIMO enabled
Either MIMO or BF Loss for Sounding
Reduce UL capacity
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Beam Forming Sounding
CPE sends Sounding
Sounding is received with different strength on different antenna
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Beam Forming Beam creation
CPE Receives higher SNR
BS calculates signal and builds the beam
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802.11n modes
Enabling coexistence with legacy 802.11a/b/g devices is critical This is accomplished using HT (High Throughput) Protection and Coexistence mechanisms. There are three 802.11n operating modes:
HT: (Greenfield mode) 802.11n only. Legacy stations cannot communicate with the 802.11n AP Non-HT: 802.11b/g only mode. Legacy stations can communicate with the AP. 802.11n stations will work in 802.11b/g mode HT Mixed: provides backwards compatibility while maintaining a better performance for 802.11n stations.
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MCS set- one spatial stream
R - is the proportion of the data-stream that is useful (non-redundant). That is, if the code rate is k/n, for every k bits of useful information, the coder generates totally n bits of data, of which n-k are redundant.
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MCS set- two spatial streams
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MCS set- three spatial streams
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Rate Adaptation
Throughput
Green 3 antenna 11n client, 2SS Dual-stream rates Blue Red 2 antenna 11n client, 2SS single antenna 11g client
Single-stream rates
The range of dual-stream is bigger for 3-antenna client!
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Range
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Access Method
Access protocol : CSMA/CA- Carrier Sense Multiple Access/Collision Avoidance
Carrier Sense: device tries to detect the presence of an encoded signal from another station before attempting to transmit. If a carrier is sensed, the station waits for the transmission in progress to finish before initiating its own transmission. Multiple Access: multiple stations send and receive on the medium. Transmissions by one node are generally received by all other stations using the medium.
Collision Avoidance: If the channel is sensed busy before transmission then the transmission is deferred for a "random" interval. This reduces the probability of collisions on the channel
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Beacon
Beacon is a message from the AP, which carries the parameters an STA should support in order to connect to a specific VAP An AP can transmit multiple beacons for multiple VAP
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VAP
VAP (Virtual Access Point) is the name of the network STA listen for VAP, not for channels STA (generally) can associate to only one VAP The access to an VAP can be open or through authentication ( WEP, WPA, WPA2) The authentication can be performed locally or through a RADIUS server VAPs can be broadcasted through beacons or not (hidden VAP). A STA must know the parameters of a hidden VAP in order to associate to it (it cannot be discovered)
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RF link
Received signal:
Tx Power+Tx Antenna Gain-Link Attenuation+Rx Antenna gain
Signal strength decreases proportionally to the square of the distance between two points The link attenuation is the same uplink or downlink between two designated points
Generally, the AP has a more powerful Tx Power, this creates a situation where the STA hears the AP but not the opposite Then, the link will be limited by the Tx power of the STA
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Noise Level
Noise Floor of WBS:
Thermal Floor: -174 dBm/Hz Bandwidth of channel :22MHz Noise level in channel: -174dBm+22MHz[dB]=-174dBm+73dB=-101dBm
WBS Noise Figure=5dB Noise floor of channel in WBS: noise level in channel + WBS noise figure: -101dBm+5dB= -96dBm
Other RF transmitter rise the noise floor
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SNR
SNR = received signal strength (dBm)-Noise Level (dBm) The ability to decode a signal modulated to a specific rate depends on its SNR rather than on its level (RSSI) High noise levels will limit the distance allowed for signal fading to reach a specific SNR
Received Signal Level
SNR
-80 <High Noise Level< -70 -95 <Low Noise Level< -80
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Data Transmission
If STA A does not receive ACK it assumes packet loss due to collision or receive error and retransmits After many retransmissions, the data rate is dropped The transmission data rate is set by the transmitter!!
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Common issues in Wi-Fi
Interference
High air activity: Consumes air-time
Reduces system capacity Regardless of RF values Embedded in CSMA/CA Protocol Noise
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Hidden node problem
No Collision detection in wireless node. Hidden Node CPE-A fails to sense the presence of a potential competitor for medium because the competitor is far from it collision occurs.
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The solution: RTS/CTS
Transmitter first sends a RTS (Request To Send) frame to the receiver that typically contains the sender's address and the size of the packet it wishes to transmit. The receiver replies with CTS (Clear To Send) frame. The CTS frame contains the data length (copied from the RTS frame). Upon receipt of the CTS frame, sender begins transmission. The hidden node problem is solved here as a hidden node also receives a CTS and becomes aware of the impending transmission. Hence it defers its own transmission. RTS/CTS is a CPE parameter!!!
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Using RTS/CTS
With RTS/CTS the system works in CSMA/CD, adding overhead but gaining free air-time Without using RTS/CTS Need to set the minimal Tx-PDU which will trigger an RTS message in the CPE
Collisions
Using RTS/CTS
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Thank you
