CommNet
CommNet is a relay and control communication system implemented first in KSP 1.2 prerelease. With CommNet enabled (as a difficulty setting), antennae have limited range from which control and transmission of science data is possible. Vessels without a Pilot require a link to a control source for full control (valid control sources are a vessel with an operational Probe Control Point, or more commonly the Deep Space Network on Kerbin). Locations may be linked either through a direct connection or indirectly via one or more relays.
The following table lists the maximum range for vessels with a single antenna signaling the Deep Space Network:
Antenna Type  Tracking Station Level (Rating)  

1 (2G)  2 (50G)  3 (250G)  






























Contents
Range calculation
Individual vessels do not have a fixed maximum range: whether a connection is possible between two vessels that are a given distance apart depends on the transmission strength of the vessels at both ends of the connection. Each antenna has a "power rating", measured as a unitless number, which contributes to the overall power rating of the vessel it is attached to. A connection can be established between two vessels when the geometric mean of their power ratings is greater than their separation as measured in meters. This is calculated as follows:
 is the maximum distance in meters at which two vessels can connect with each other.
 and are the power ratings for the vessels that are attempting to connect to each other.
Power ratings for antennae range from 5k for the integrated antenna available in every Command module, to 100G for the largest dishes. Kerbin's Deep Space Network can be treated as a vessel with a rating of 2G/50G/250G (depending on the level of the Tracking Station) for the purposes of calculating range.
Combining antennas
When a craft has multiple, combinable antennas, the antenna's power ratings stack in a way that provides diminishing returns using the following formula:
The vessel's Average Combinability Exponent is the average of the combinability exponent values of all antennas on the vessel, where each antenna is weighted proportionally to its respective power rating. Note that all antennas and relays have a combinability exponent value of 0.75 except for the Communotron 16 and the Communotron 16S, with values of 1.0 and 0.0 respectively. This means that the Average Combinability Exponent will always be 0.75 for a vessel containing neither of these antennae. For vessels with a Communotron 16 or 16S, the Average Combinability Exponent is calculated:
 are the power ratings of each antenna.
 are the combinability values of each respective antenna.
For example, a vessel with a Communotron 8888 (100G power, 0.75 combinability) and also a Communotron 16 (500k power, 1.00 combinability) would have the following as its Average Combinability Exponent:
Antennae combinability exponent values can be found under "antennaCombinableExponent" in the part configuration files. If the value is missing, the default value of 0.75 is used.
Signal strength
The length of a communication link relative to its maximum range is represented by signal strength. Signal strength for a singlehop link starts at 100% at a distance of zero and decreases as distance increases, down to a signal strength of 0% at the maximum range of the link. The relationship is nonlinear: the true value requires plotting the relative distance on a Bezier curve with the keys '0 0 0 0' and '1 1 0 0'. This can be calculated as follows:
 Calculate the relative distance between the vessels (the result should be between 0.0 and 1.0):
 Substitute the result above into the Bezier curve equation (cubic derivation): or
The result is the signal strength of the link at the specified distance. Note that the above applies to a singlehop link: if the signal is being bounced through one or more relays, the signal strength is the product of the signal strengths of each link making up the path to the control point.
Example
Calculating the Signal Strength between a Level 3 DSN Tracking Center and a probe around Jool at a distance of 73.4Gm from Kerbin with two Communotron 8888 antennas:
 Max Power of Lvl 3 DSN connection:
 Max Power of Probe:
 Max Range of Connection:
 Relative distance between Link Points:
 Signal Strength:
Therefore the signal strength for this connection would be 71%.
Connections through Relays
When considering the antenna power of your vessel, Direct Antennas and Relay Antennas combine to give you a 'Power' to calculate Signal Strength back to the KSC, however, if a different vessel is connecting through you to communicate with the KSC the Direct Antennas are disregarded in any calculations made.
Example
 A vessel with a Communotron 8888 Direct Antenna and a RA100 Relay Antenna orbiting around Eeloo at a distance of 105Gm from the KSC, with a Level 3 DSN, will connect back to the KSC with a Signal Strength of 48% as both antennas combine to give an increased antenna 'Power' for its own direct connection.
 A vessel rendezvoused with the above craft with a measly Communotron 16 will have to communicate back to the KSC through the relay as its antenna is not strong enough to communicate with the KSC directly. As this small, probe craft will only consider the RA100 Relay Antenna on the relay craft when calculating its connection back to the KSC, the power rating of the relay vessel drops and therefore the Signal Strength will drop to 26% (please note the calculations for these numbers have been obfuscated from the example for easy understanding of the concept).
Please keep the above in mind when creating relay vessels for you other craft to connect through in the depth of space.
DSN Strength
Each upgrade of the tracking center improves the strength of Kerbin's terrestrialbased Deep Space Network (DSN). Based on the provided figures, the Level 1 tracking station has an antenna strength of 2G, Level 2 is 50G, and Level 3 is 250G.
Science Transmission
Signal strength has no bearing on transmitting science directly to the KSC, but vessels transmitting through a relay will gain a 40% bonus to the data transmitted (to a maximum of 100% value). For example, a Gravity Scan from low Jool orbit would give 90*0.4=35 Science if transmitted directly, but if bounced through a relay with an overall strength of 100% would give 90*0.4*1.4=50.4 Science. This bonus decreases nonlinearly with signal strength.
Difficulty Settings
Range Modifier = The value of this slider is a multiplier value that is applied to the power levels of all Antenna and Relay parts.
DSN Modifier = The value of this slider is a multiplier value that is applied to the power level of the DSN Network.
Tutorials, Calculators and Examples
A Google Sheets document that can help calculate Signal Strength and Ranges for you can be found here: CommNet Signal Strength Calculator & Antenna Selector
Example orbits for setting up a planetary network can be found at: Tutorial:Ideal Orbits for Communication Satellites
One method of setting up a fullsystem network can be found at Tutorial:Setting up a CommNet system.