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CCNP Enterprise Core (350-401) Cisco Wireless

Wireless Signals and Modulation

With a wired link, a electrical signal is applied at one end and carried to the other end. This is simple for the wire as it’s continuous and conductive. There is no wire for a wireless link, so it is not as continuous or conductive.

A wireless link send signals in a steady up and down rhythm, like swinging and waving a rope.

Electromagnet waves do not travel in a predictable line like a wire. It travels by expanding in all directions away from the source of the signal, the antenna.

Once the signals reach the destination antenna, the destination device can read the waves that are received over the air. If the signal quality is good, the receiving device will be able to decipher the message from the originating antenna.

Waves from a wireless link can be measured in several ways. One property is the frequency of the wave, this is the amount of times a wave can complete an up and down cycle in one second. A frequency that completes five cycles in one second can be known as a 5 hertz (5Hz) signal.

Signals can range from 1 hertz, to 1000 hertz or a kilohertz, to 1,000,000 hertz, a Megahertz, or 1,000,000,000 Hz, a Gigahertz

The range from 3 kilohertz to 300 Gigahertz is known as the radio frequency, RF, range. It can include many types of communication from low-frequency radio to radar.

The RF range also include Microwave radio which is used in wireless networks, 2.4 Gigahertz and 5 Gigahertz

There are bands inside of these two ranges that are used specifically for wireless networks rather than the entire 2.4Ghz to 2.49Ghz frequency. These are 2.400 to 2.4835 Ghz for the 2.4Ghz band and 5.150 to 5.350, 5.470 to 5.825Ghz for the 5Ghz band.

The bands are divided up further into channels. Channels are known by a channel number and each channel number is assigned to a specific frequency.

The 2.4Ghz band is split up into 14 channels spaced 5Mhz apart (except channel 14).

The 5Mhz space between the channels is known as the channel width.

A device connecting to this channel will not be set to the specific channel, the RF signal is not narrow and will overspill to other neighbouring channels in the range due to channels being narrower than the signal bandwidth (for example, 20Mhz width on a 5Mhz channel)

RF signals are dependant on timing since they are always in motion. The phase of a signal is the measure of shift in time relative to the start of a cycle.

Phase

Phase is measured in degrees where 0 is the start of the cycle and 360 is the completion of a cycle.

The point which is halfway along the cycle is the 180 degrees mark.

If two identical signals are produced at exactly the same time and their cycles match up, they are said to be in phase with each other.

If there are two signals that are delayed from the other, they said to be out of phase with each other.

Phase is important when signals are received. If there are signals in phase they add together, signals that are 180 degrees out of phase tend to cancel each other out.

Wavelength

The wavelength of a signal is a measure of the physical distance that it travels over one complete cycle.

Wavelength is defined with the greek symbol for lambda, λ

A 2.4Ghz signal would travel 4.92 inches, a 5Ghz signal would travel around 2.36 inches

RF waves at a constant speed, in a vacuum it is the speed or light, outside of that in air it is slightly less.

The wavelength will decrease as the frequency increases. As the wave cycles get smaller they cover less distance.

RF Power / dB

For the RF signal to travel the distance required, it must be sent with enough power.

The strength of an RF signal is measured by its power in watts. A wireless network transmitter normally has a strength between 1mW and 100mW

When measured, it is considered to be an absolute power measurement. The decibel can be used to compare one absolute measurement to another.

It was originally used to compare sound intensity but can apply to power levels too.

There are laws to remember in dB:

The law of zero means that a value of 0 dB means two absolute power levels are equal.

The law of threes means a value of 3 dB means the power level is double the reference value. A value of -3 dB means the power level is half the reference value

The law of tens means a value of 10 dB means the power level is ten times the reference value. Again a value of -10 dB means the power level is one tenth of the reference value.

Comparing Power

The power and loss along a signal can be compared by using the measuring the power level leaving the transmitter and at the receiver where the power is being received. The difference in signal can show you the power that was lost.

Measuring Power Changes

If an antenna is connected to a transmitter, it can provide a signal gain boost to the resulting RF signal. This can increase the dB value of the signal above of what the transmitter can manage on it’s own.

The antenna does not generate any amount of absolute power. It makes it impossible to measure the antennas gain in dBm. An antennas gain is measured by comparing the performance of that with a reference antenna and computing its value in dB.

The reference antenna is an isotropic antenna, so gain is measured in dBi.

The isotropic antenna does not actually exist as its ideal in every way and radiates RF equally in every direction. There is not a physical antenna that exists that can do that.

Once a combination of transmitter power level, length of cable, and antenna gain is known The actual power level from the antenna can be worked out, known as the effective isotropic radiated power, EIRP, in dBm.

Effective isotropic radiated power is an important parameter as governments use it to regulate wireless networks. Limits are set on devices to ensure a system can not radiate signals higher than a certain EIRP.

For the EIRP to be calculated, the transmitter power level is added to the antenna gain and the cable loss is then subtracted:

A transmitter configured for 12 dBm, with a cable with 3dB loss connects to the antenna with 8dBi gain.

The calculation would be 12 dBm – 3dB + 8dBi = 17dBm

The EIRP is calculated in decibel-milliwatt, dBm.

Free Space Path Loss

When a signal passes through free space its amplitude decreases, even if there are no obstacles in the way. This is known as free space path loss. Free space path loss is a loss of signal over distance and frequency only

As an RF signal travels through the free space as a wave it expands. The expansion causes the signal strength to weaken

Even with an antenna that sharply focuses the transmitted energy it will still spread out over the distance it travels and lose energy.

Free space is largely lost quicker near the transmitter, but the rate of signal weakening will decrease the further away it gets.

Free space path loss is greater in 5Ghz signals in comparison to 2.4Ghz signals.

2.4Ghz signals have a greater range than that of 5Ghz signals as the frequency is lower, so less loss in Db.

Power Levels at the Receiver

With wireless LAN devices, the EIRP levels leaving the transmitter antenna range from 100mW to as low as 1mW. This can be from +20dBm to 0 dBm.

The receiver will pick up on a much weaker signal, from 1mW to the smallest fractions.

In terms of dBm, this can be between from 0 dBm down to -100dBm.

Receivers measure a signals power level according to RSSI, the received signal strength indicator scale. The RSSI value is defined in the 802.1 standard as a 1 byte value between 0 to 255. 0 being the weakest and 255 being the strongest.

RSSI can vary between hardware manufactures on the exact levels of what defines a 0 and what defines a 255, so it is not a compatible universal scale and can vary between different devices.

RSSI focus on the expected signal alone with no other insight into other signals that may be received. All other signals that are received are simply seen as noise. That noise level and its average signal strength of the noise is known as the noise floor.

Every receiver as a sensitivity level, its a threshold on the device that defines the line between a signal being intelligible or unintelligible.

As long the receiver gets signals that are above the sensitivity level, it will be able to recognise the signal and understand it correctly.

As long as the noise floor is below of the signal that is trying to be heard it will be received and understood correctly. The difference between signal and noise is also known as the signal to noise ratio which is measured in dB. A higher signal to noise ratio is preferred over a lower one.

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