I’ve started studying for the CCNA wireless exam and thought I would put my notes online. I always learn better when writing and hopefully my notes can be of assistance to someone else. These notes are based on reading the official certification guide “CCNA Wireless 200-355 Official Cert Guide“.
Basic wireless theory
Wireless LANs are based on the 802.11 standard.
Wireless LANs is a lot about Radio Frequency (RF) and planning of the RF environment.
When alternating current is sent through the antenna electric and magnetic fields propagate out and away as traveling waves. They travel along each other and are at right angles to each other.
Electromagnetic waves do not travel in a straight line. They travel by expanding in all directions away from the antenna.
When the electromagnetic waves reach the receiver’s antenna, they induce an electrical signal.
Frequency – The number of times a signal makes one complete up and down cycle in one second. Measured in Hertz (Hz)
The frequency range from 3 kHz to 300 GHz is commonly called RF. Types of devices in this frequency range is radar, radio, shortwave radio, television, FM radio, microwave etc. The main two frequency ranges used for wireless are 2.4 GHz and 5 GHz.
When a range of frequencies is used for a common purpose it is often referred to as a band of frequencies, like the 530 kHz to 1710 kHz AM broadcast band.
One of the two main frequency ranges used for wireless communication lies between 2.400 and 2.4835 GHz and is often called the 2.4 GHz band.
The 5 GHz band consists of four ranges:
- 5.150 to 5.250 GHz
- 5.250 to 5.350 GHz
- 5.470 to 5.725 GHz
- 5.725 to 5.825 GHz
The 5 GHz band contains several smaller bands and as you can see above there are some gaps. There are some efforts ongoing to reclaim frequences and repurpose them for wireless WLANs so that more channels can be available.
Bands are divided into a number of distinct channels. Each channel is known by a channel number and is assigned to a specific frequency.
Why do channels need to be spaced apart? The RF signal is not infinitely narrow, it spills above and below a center frequency to some extent meaning that it is occupying neighboring frequencies. The center frequency defines the channel location within the band. The frequency range needed for the transmitted signal is known as the signal bandwidth. A 22 MHz bandwidth signal is bounded at 11 MHz above and below the center frequency.
The phase of a signal is a measure of shift in time relative to the start of a cycle. Normally measured in degrees. The start of the cycle is 0 degrees. A complete cycle equals 360 degrees. Halfway along the cycle is at the 180 degree mark. When two identical signals are produced at the exactly same time , their cycles match up and are said to be in phase with each other. If one of signals is delayed from the other they will be out of phase. Signals that are in phase tend to add to each other while signals that are 180 degrees out of phase tend to cancel each other out.
The wavelength is the physical distance that a wave travels over one complete cycle. Regardless of the frequency, RF waves travel at constant speed. In a vacuum, radio waves travel at exactly speed of light. In air, the velocity is slightly slower than speed of light. The wavelength decreases as the frequency increases.
RF power and DB
The strength of a signal can be measured as the amplitude which is the height from the top peak to the bottom peak of the signal’s waveform. RF signals are normally measured in Watts (W). Wireless LAN transmitters normally have a power ranging from 1 mW to 100 mW.
Power measured in W or mW is considered to be an absolute power measurement.
Decibel (db) is a function that uses logarithms to compare one absolute measurement to another. When comparing two power values the following formula is used: dB = 10*log_10(P2/P1) where P2 is the source of interest and P1 is the reference value.
Important dB laws
Law of zero – A value of 0 dB means the two power values are equal.
Law of 3s – A value of 3 dB means the power of the value of interest is double the reference value. If the value is -3 dB the value of interest is half the reference.
Law of 10s – A value of 10 dB means the power of the value of interest is 10 times the reference value. Hence -10 dB means the value of interest is 1/10 of the reference value.
A = 4 mW
B = 8 mW
C = 16 mW
B = 3 dB higher than A because (8/4) = 2
C = 3 dB higher than B because (8/4) = 2
C = 6 db higher than A because (16/4) = 4 which is (2*2)
More complex example:
A = 5 mW
B = 200 mW
A * 10 = 50 mW (add 10 dB)
50 mW * 2 = 100 mW (add 3 dB)
100 mW * 2 = 200 mW (add 3 dB)
10 + 3 + 3 = 16 dB so B is 16 dB more than A.
It’s common to compare wireless signals to a reference value of 1 mW when comparing the strength of signals. This is called dBm. The dBm values for a received signal will be between 0 and -100 where 0 is the best.
Power changes and EIRP
An antenna connected to the transmitter adds some amount of gain to the signal. This increases the dB value above that of the transmitter alone. Gain can’t be measured in dBm, since without am antenna there is no power being pushed out of it. Instead performance is compared to isotropic antenna, which is an ideal antenna that doesn’t actually exist. The gain is then measured in dBi. Some power is lost when the signal travels through the cable between the transmitter and the antenna. Effective Isotropic Radiated Power (EIRP) is the power level that will be transmitted from the antenna and measured in dBm.
Transmitter sends with 10 dBm.
There is 5 dB cable loss.
The antenna has 8 dBi gain.
10 – 5 + 8 = 13 dBm EIRP
To calculate the received power level some more parameters need to be taken into account.
Rx signal = Tx power – Tx cable + Tx antenna – free space + Rx antenna – Rx cable
There is loss when the signal travels in free space. Also note that the receiving antenna has gain as well.
Power levels at the receiver
The receiver expects to find a signal on a predetermined frequency with enough power to contain useful data. The signals power is measured in dBm according to the Received Signal Strength Indicator (RSSI) scale.
EIRP level at transmitter’s antenna is normally between 100 mW down to 1 mW. In dBm this corresponds to +20 dBm down to 0 dBm. At the receiver the power levels are much lower, ranging from 1 mW to fractions of a milliwatt, approaching 0 mW. This corresponds to 0 dBm down to -100 dBm. The RSSI is then a range between 0 and -100 where 0 is the strongest.
Every receiver has a sensitivity level or a threshold that divides a useful signal from one that is too weak to be intelligible.
All other signals received on the frequency as the one you are trying to receive is viewed as noise. The noise level or the average signal strength of the noise is called the noise floor*. The RF signal must be **greater than the noise floor by a decent amount so that it can be received and understood correctly. The difference between the signal and the noise is called the Signal-to-Noise-Ratio (SNR) and is measured in dB. A higher SNR is preferred.
Noise floor is -90 dBm.
RSSI is -54 dBm.
SNR = -54 dBm-(-90 dBm) = 36 dB
Carrying Data over an RF Signal
The frequency needs to be steady because the receiver needs to tune to a known frequency to find the signal. The basic RF signal is called the carrier signal because it is used to carry other useful information.
To add data onto the RF signal, the frequency of the carrier signal must be preserved but some characteristic of the wave must be modified to signal if it’s an 1 or 0. The altering of the carrier signal is known as modulation. The receiver will then do demodulation.
The goals of the RF modulation scheme is generally:
- Carry data at a predefined rate
- Be reasonably immune to interference and noise
- Be practical to transmit and receive
The following properties of the RF signal can be modulated:
- Frequency but only by varying slightly above or below the carrier frequency
Some amount of bandwidth centered on the carrier frequency is required by using these modulation techniques. This is due to the rate of the data being carried as well as the overhead of encoding the data and manipulating the carrier signal. Narrowband transmissions such as carrying audio over AM or FM signal have a straightforward modulation and requires little bandwidth.
Wireless LANs must carry data at high bit rates which then requires more bandwidth for modulation. Data is sent across a range of frequencies known as spread spectrum. There are three main categories of spread spectrum.
- Frequency-Hopping Spread Spectrum (FHSS)
- Direct-Sequence Spread Spectrum (DSSS)
- Orthogonal Frequency Division Multiplexing (OFDM)
Early approach taking a compromise between avoiding RF interference and needing complex modulation. Wireless band was divided into a number of channels where each channel was 1 MHz wide. To avoid narrowband interference the transmissions would continously “hop” between different frequencies. The transmissions would “hop” through a predetermined sequence. This had to occur at regular intervals so that the receiver and transmitter can stay synchronized.
Drawbacks of using FHSS:
- Channel was only 1 MHz wide, limiting the data rate to 1 or 2 Mbps
- Multiple transmitters could eventually collide and interfere with each other on the same channels
For those reasons FHSS got replaced by DSSS.
DSSS uses a small number of fixed, wide channels that can support complex modulation. Each channel is 22 MHz wide. DSSS is used in 2.4 GHz band where there are 14 possible channels but only 3 non-overlapping.
Transmits data in a serial stream, each data bit is prepared for transmission one at a time. A wireless transmitter performs several functions to make the data stream less susceptible to being degraded on the transmission path.
Scrambler – Scrambles data in a predetermined manner to make the data into a randomized string of 0s and 1s instead of long sequences of 0 or 1 bits.
Coder – Each data bit is converted into multiple bits of information that have patterns designed to protect against errors due to noise or interference. The coded bit is called a chip. The complete group of chips is called a symbol. DSSS uses Barker codes and Complementary Code Keying (CCK).
Interleaver – The stream of symbols is spread into separate blocks so that bursts of interference don’t affect all of the blocks simultaneously.
Modulator – The bits in each symbol are used to alter or modulate the phase of the carrier signal. This enables the RF signal to carry the data.
1-Mbps Data Rate
Each bit of data encoded with Barker 11 meaning that each bit of data is encoded as a sequence of 11 bits. Up to 9 out of 11 bits in a chip can be lost before the original data can’t be restored. Each bit in a Barker chip can be transmitted by using Differential Binary Phase Shift Keying (DBPSK). The phase of the carrier is shifted or rotated according to the data bit being transmitted:
0 – Phase is not changed
1 – Phase is shifted or rotaded 180 degrees so that the signal is inverted.
2-Mbps Data Rate
Use Differential Quadrature Phase Shift Keying (DQPSK) to modify the carrier signal in four different ways:
00 – Phase is not changed
01 – Rotate 90 degrees
11 – Rotate 180 degrees
10 – Rotate 270 degrees
5.5 Mbps Data Rate
Uses CCK to increase the data rate where DQPSK is used to modulate.
11 Mbps Data Rate
CCK with DQPSK but puts more bits into each symbol.
Sends data bits in parallell over multiple frequencies, all contained in a single 20 MHz channel. Each channel is divided into 64 subcarriers (also called subchannels or tones) that are spaced 312.5 kHz apart. Subcarriers consist of the following types:
Guard – 12 subcarriers are used to help set one channel apart from another and to help receivers lock onto the channel.
Pilot – 4 subcarriers that are equally spaced and they are always transmitted to help the receivers evaulate the amount of noise on the channel.
Data – 48 subcarriers that are the ones actually carrying the data.
The rate of each subcarrier is relatively low but the sum of all carriers is high. Subcarriers overlap but are aligned so that most of the potential interference is cancelled.
Uses various modulation schemes. BSPK 1/2 means one half of the bits are new and **one half are repeated. BPSK 3/4 means three-fourths new data and one fourth is repeated. Generally speaking a higher fraction means a higher data rate but less tolerant to errors.
Quadrature Amplitude Modulation (QAM) combines QPSK with multiple amplitude levels which gives a greater number of variations of the signal. 16-QAM uses 2 bits to select the QPSK rotation and 2 bits to select the amplitude level, 4 bits equals 16 possible outcomes. QAM can be used with different coder ratios of data where 16-QAM with 1/2 means a data rate of 24 Mbps and 3/4 means a data rate of 36 Mbps.
|Modulation||DSS Data Rate (Mbps)||OFDM Data rate (Mbps)|
|OFDM BPSK 1/2||6|
|OFDM BPSK 3/4||9|
|OFDM QPSK 1/2||12|
|OFDM QPSK 3/4||18|
|OFDM 16-QAM 1/2||24|
|OFDM 16-QAM 3/4||36|
|OFDM 64-QAM 2/3||48|
|OFDM 64-QAM 3/4||54|
|OFDM 256-QAM 3/4||78|
|OFDM 256-QAM 5/6||86|
To pass data over the RF signal, the transmitter and receiver must agree on the modulation. The best data rate available should be used, given the current environment. If SNR is low or RSSI then a lower data rate may be better.
The transmitter and receiver negotiate the modulation dynamically as SNR and RSSI conditions will vary and sender and transmitter may be mobile.