Wireless 103: The Long Link

Long link defined: A long link is 300 meters or 1,000 feet. 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.

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

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

RF Link budget calculator

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. The power is increased in one direction at the expense of another.

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:


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

Example 1:

You live in the last house that gets DSL. Your friend 2000 feet away across the river gets dial-up or nothing. You have a standard home router; he has a wireless bridge. Both put out 28 mW = 14 dBm. You both have standard rubber duck antennas attached to the router. Minimum usable receive signal strength is -83 dBm.

14 dBm TX power, no line loss, 2.1 dB antenna gain,



Two new terms have showed up here:

EIRP: Equivalent Isotropically Radiated Power. FCC limits this to 36 dBm for systems with omnidirectional antennas.

SOM: Signal Operating Margin. The difference between what we receive and what we need. We want this to be 20 dB for a reliable link. The signal strength we have in example 1 may give us a slow connection.

We need another 10 dB to get the signal strength up to our minimum SOM. Splitting that 10 dB equally between the two systems, we add 5 dB to each antenna and run that through the calculator:



We have the SOM we want, but the signal strength may not support DSL speeds. Let's run the numbers again with 14 dB antennas:



This signal strength should give us all the speed the hardware can handle.

Example 2:

A fire watch platform on a distant mountain. 5 miles of clear mountain air.

Same numbers as example 1: 14 dBm TX power, no line loss, 2.1 dB antenna gain



Negative signal operating margin. Not happening.

Let's try 24 dB gain antennas. 20 feet of LMR 400 to get the RF up the mast:



Robust signal strength. But EIRP is over 36! That will be explained in the next lesson.

Long Link 802.11 equipment


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.

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.

A long link is a specialized network requiring specialized equipment. An example of a specialized tool designed for long links is the Ubiquiti Bullet:



The Bullet is an AP or client, as selected on the configuration page. It is designed for outdoor use. The RF side connects directly to the N connector of an antenna. The CAT 5 side receives power via POE; Power Over Ethernet

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 mW ) versions.

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

At the time this was written, Bullet2s sell for ~$40, and POE injectors cost ~$15. Linksys WRT-54Gs sell for about $65. The Bullet does it better, for less, and you don't have to install third party firmware.

Excellent! Where is 102??