What are the limitations?
The limitations of serial and parallel transmission is distance due to RLC. Serial travels further than parallel because it has more energy, +/- 15V, but is slower, only one pathway, limit 50 metres. Parallel travels less than serial because it has less energy, 1 V, but is faster as it has multiple pathways, limit 10 metres.

What is RLC?
Resistance, Inductance and Capacitance. Resistance plus inductance plus capacitance adds up to a term called Impedance. Impedance occurs when we have complex signals such as a.c. signals or high speed data signals. The impedance of structured cables is 100 ohms plus or minus 15 ohms. The plus or minus is due to the fact that impedance changes with frequency.

What to do to go further?
Increase the energy. Use data coding techniques to limit the effects of RLC attenuation. Make a better pathway, radio, copper, fibre. Match a signal to the pathway for least attenuation. Use better electronics to improve on how quickly we may change from a one to a zero thus we may send more data in less time.

How cable impedance is defined if you really want to know?

Characteristic impedance of the cable is the ratio of the electric field strength to the magnetic field strength for waves propagating in the cable (Volts/m / Amps/m = Ohms).

Ohm's Law states that if a voltage (E) is applied to a pair of terminals and a current (I) is measured in this circuit, the following equation can be used to determine the magnitude of the impedance (Z). The following formula will hold truth:

Z = E / I

This relationship holds true whether talking about direct current (DC) or alternating current (AC). Characteristic Impedance and is usually designated Zo or "Zed nought". When the cable is carrying RF power, without standing waves, Zo also equals the ratio of the voltage across the line to the current flowing in the line conductors. So the characteristic impedance is defined with the formula:

Zo = E / I

The voltages and currents depend on the inductive reactance and capacitive reactance in the cable. So the characteristic impedance formula can be written in the following format:

Zo = sqrt ( (R + 2 * pi * f * L ) / (G + j * 2 * pi * f * c) )

Where:
R = The series resistance of the conductor in ohms per unit length (DC resistance)
G = The shunt conductance in mhos per unit length
j = A symbol indicating that the term has a phase angle of +90 degrees (imaginary number)
pi = 3.1416
L = Cable inductance per unit lenght
C = Cable capacitance per unit lenght
sqrt = square root function

For materials commonly used for cable insulation, G is small enough that it can be neglected when compared with 2(3.1416) f C. At low frequencies, 2(3.1416) f L is so small compared with R that it can be neglected. Therefore, at low frequencies, the following equation can be used:

Zo = sqrt ( R / (j * 2 * pi * f * L))

If the capacitance does not vary with frequency, the Zo varies inversely with the square root of the frequency and has a phase angle which is -45^o near DC and decreases to 0^o as frequency increases. Polyvinyl chloride and rubber decrease somewhat in capacitance as frequency increases, while polyethylene, polypropylene, and Teflon* do not vary significantly. When f becomes large enough, the two terms containing f become so large that R and G may be neglected and the resultant equation is:

Zo = sqrt ( (j * 2 * pi * f * L) / (j * 2 * pi * f * C) )

Which can be simplified to the form:

Zo = sqrt ( L / C )

Cables characteristics at high frequencies

At high frequencies you can't look at the cable as a usual cable. With higher frequency it works as a waveguide. Characteristic impedance is specific resistance for electro-magnetic waves. So: It's the load the cable poses at high frequencies. The high frequency goes (dependent of cable of course) usually from 100kHz and upwards.

If you feed a sinusoidal electrical AC signal of reasonable frequency into one end of the cable, then the signal travels as an electrical wave down the cable. If the cable length is an extremely large number of wave-lengths at the frequency of that AC signal, and you measure the ratio of AC Voltage to AC current in that traveling wave, then that ratio is called the characteristic impedance of the cable.

In practical cables the characteristic impedance is determined by cable geometry and dielectric. The cable length has no effect on it's characteristic impedance.