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PostSubject: [BASICS] LONG RANGE WIRELESS AND BACKHAULS   Sat Sep 08, 2012 10:53 am

Long link defined: A long link is 300 meters or 1,000 feet, or longer. Beyond that, you encounter the problems described below.

Why you don't want your friend down the road to connect to your home network.

Your home network is an access network. It is designed and optimized to connect to multiple clients in a small area. These clients communicate through the AP, and they can hear each other.

If a client needs to transfer data, it can listen for other users on the frequency. If the frequency is in use the client waits for a while and checks again before transmitting. This minimizes data collisions, which corrupt data and require retransmission of the corrupted packets.

The friend up the road with the big parabolic dish can't hear your local network. If his network feels the need to speak, it speaks. Trampling all over your local traffic. Forcing retransmission of packets. Which get trampled again...

This slows the entire network down. Your friend gets more benefit from your network than you do. sick

You need to change a network parameter for long links: ACK timing. 802.11 requires an ACKnowledge receipt for every packet sent. The round trip time for a long link may exceed the default time of an access router. If you change this to accommodate a distant client, you slow down the whole network.

This network behavior has a name. It's called Near / Far. The cure for Near / Far is to isolate the Far client in a separate network designed and tuned for the distance involved. A dedicated point to point link

[b]The point to point link

Point to point wireless links differ from links in access networks in several ways:

One AP connects to one client
It is a continuous, permanent connection
directional antennas are used
network parameters need to be configured for the specific link
specialized equipment may be required

What you need to know to design a long point to point link.

A long point to point link requires study and planning. You need to know:

the distance between antennas
the effect of any obstacles between the antennas
the signal strength required at each end
the power required to achieve the required signal strength
the equipment you need to achieve the power you need

Distance between antennas:

Google Earth, Garmin Map Source, et cetera. Mapping software will let you determine the distance between two points.

The effect of any obstacles between the antennas

Obstacles in the RF path can cause delays in part of the RF signal. This delayed signal combines with the signal that came directly to the other end and creates a distorted signal. Obstacles in the path affect signal strength.

The power required to achieve the required signal strength

Transmitter power is measured in milliwatts, watts or decibels. A description of these terms is beyond the scope of this tutorial.

We will use dBm; decibels referenced to one milliwatt, in this tutorial. The benefit of using decibels is simplicity. Decibels are based on logarithms. Using logarithms, we can add and subtract to calculate the effects of multiplying and dividing power.

More is better. Typical 802.11 gear can not receive signals below -83 dbm. To get the highest possible speed from the link you will want -50 dBm received signal strength or more.

The RF Link Budget


There are many factors that determine signal strength at the receiver:

The transmitter power
RF energy lost in the cable to the antenna
The gain of the transmitting antenna
The signal lost in the air between antennas
The gain of the receiving antenna
RF energy lost in the cable from the antenna

Transmitter Power

Typical home routers put out 28 milliwatts. This is ~14 dBm.
The specialized long link router described below puts out 100, 200 or 600 milliwatts. That is 20, 23 or 28 dBm.

The FCC defines a device that deliberately creates RF energy as an Intentional Radiator. Intentional radiators are limited to one watt of RF power by the FCC.

RF energy lost in the cable to ( or from ) the antenna

A coaxial cable is a low pass filter. High frequencies are attenuated when they travel down a cable. The farther they travel, the more they are attenuated. LMR-400 cable attenuates 2.4 gHz 802.11 b, g and n signals at 5.5 dB per 100 feet.

The gain of the transmitting ( or receiving ) antenna

Antenna gain is passive. The antenna does not create more power than it receives. Antenna gain comes from reshaping the pattern of RF energy leaving the antenna.

Antenna gain is bidirectional. Transmit gain and receive gain are affected equally when gain antennas are used.

The signal lost in the air between antennas

Free Space Loss is the signal strength you lose by sending a radio signal through space.

Calculated as:

20* log[10]( Frequency in mHz ) + 20 *log[10]( Distance in Miles ) + 36.6

Putting it all together:

The signal strength at the receiving end is:

The signal strength generated by the intentional radiator
MINUS cable loss
PLUS antenna gain
MINUS free space loss
PLUS receiving antenna gain
MINUS cable loss

If different equipment is used on both ends, the calculation must be done twice, to determine the signal that will be received at the client and the AP

Long Link 802.11 equipment

The most important thing to know about an 802.11 Transceiver:

It's a two way radio.

Everybody thinks they can hang an amplifier on the thing and make it go farther. You can, but it won't do you any good. The signal that goes farther due to amplification will reach clients that can't reply, because the client does not have an equally powerful amplifier.

Some amplifier vendors claim that their amplifier also receives better. This would require special and very expensive transistors designed for microwave work. These transistors would be overloaded by a low powered home WiFi router nearby. To get the performance some vendors claim you would need to cool the front end transistors with liquid nitrogen. WTF You wont get that thing applied usually ... yeah me in Bihar lol!

Increasing RF power by amplification is only effective if it is done at both ends of a link

Note “both ends”. Not “at every node in a system”. Amplifiers are for point to point links.

Specialized equipment for long links

SOHO routers are designed to provide network access to computers in one house or office. The power output of the radio transmitter is limited, to allow other routers nearby to use the same frequencies. The receiver has to listen for clients with laptops. There is no need to generate a strong RF signal that transmits so far that a laptop can hear, but not reply.

Typical SOHO routers generate 28 mW ( 14 dBm ) of RF power. The Bullet comes in 100 mW ( 20 dBm ), 200 mW ( 23 dBm ) and 600 mW ( 28 dBm ) versions.

When used in pairs, Bullets communicate with each other and automatically negotiate parameters like ACK timing to optimize link performance.

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