I am aware that punch-down junction boxes and other solutions are the recommended methods for continuing an UTP cable that is too short, but you are not in control of replacing (for example, solid-built in the wall).
Having a bit of electrical background in my past, I am really perplexed by the fact that carefully soldering UTP cable is seen as bad for high speeds (usually cited as an 'impedance bump'), but junction boxes which add potential resistance and risk of oxidation at the edges, are seen as good.
What are the exact details of why soldering is considered bad for this type of operation?
Later edit: I did a small, fast and not-the-most-scientific experiment (video of testing it) and cut a patch cord in two and soldered it back. I used then www.speedtest.net to measure the speed and it was the same as before (300 Mbps as in my subscription's specifications).
Best Answer
You cannot splice or otherwise extend a UTP cable that is too short. If a UTP cable is too short, you must run a new cable of the proper length, not to exceed the maximum length allowed (properly installed cabling will have a pull-string, or you could use the old cable for a pull-string). Splices, taps, couplers, etc. are expressly forbidden by the ANSI/TIA/EIA 568, Commercial Building Telecommunications Cabling Standard. Splicing the cable adds impedance and decreases the return loss.
UTP cabling is limited to 100 meters. That length assumes up to 90 meters of solid-core (better performance, but fragile) horizontal cable, and no more than 10 meters of stranded (poor performance, but less fragile) patch cord combined between both ends.
The frequencies used by modern networking require adherence to tight specifications.Each cable run must be tested to meet or exceed all the tests in the test suite for the cable category. Simple connectivity from end-to-end (like for telephone cable) is not sufficient. The high, modern networking frequencies require tight specifications to be met. The primary tests are:
Wire Map - Checks for proper pin to pin termination, and for each of the 8 conductors the wire map checks for: Continuity to the far end, Shorts between any two or more conductors, Reversed Pairs, Split Pairs, Transposed Pairs, Any other miswiring.
Length - The physical length of the cable is the actual length derived by measurement of the cable(s) between the two end points. The electrical length is the length derived from the propagation delay of the signal and depends on the construction of the cable. The maximum physical length of the horizontal cable (permanent link) one end of the cable to the other is 90 meters. The maximum length of the channel model is 100 meters.
Insertion Loss - Insertion loss is the loss derived from inserting a device into a transmission line. The insertion loss for both the permanent link and the channel models are the total insertion losses of all the components.
Near End Cross Talk (NEXT) - Pair to pair NEXT loss is the measurement of signal coupling from one pair to another. The result is based on the worst pair to pair measurement.
Power Sum Near End Cross Talk (PSNEXT) - Power sum NEXT takes into account the statistical crosstalk between all pairs while energized. This is a calculated amount derived by adding up the crosstalk results between all pair combinations.
Equal Level Far End Cross Talk (ELFEXT) - FEXT is the unwanted coupling of a signal induced by a transmitter at the near end, measured on the disturbed pair at the far end. ELFEXT is the same measurement of FEXT, less the effect of attenuation.
Power Sum Equal Level Far End Crosstalk (PSELFEXT) - As in Power Sum NEXT, these are computed values based on the sum of all the possible pair combinations under the respective tests.
Return Loss - Return loss is the value of energy reflected by impedance variations when devices are inserted into the cabling system.
Propagation Delay - Is the time it takes the signal to travel from one end of the cable/system to the other. The maximum channel propagation delay is 555ns (nanoseconds) and for the link it is 498 ns, both measured at 10Mhz.
Delay Skew - Delay skew is the signalling delay difference in time (nanoseconds) between the fastest pair and the slowest pair. The maximum channel delay skew is 50 ns, and in the permanent link it is 44 ns.
Any tests that are out of specification will fail the test, and the condition must be corrected and the test suite performed again until the cable passes or is replaced.
You can also permanently damage a cable by exceeding the pulling tension or minimum bend radius when installing the cable run.