The lumped model of a wire (or any other element) is an approximation. It's not entirely accurate, but it can be useful in many circumstances.
The distributed transmission line model given by the telegrapher's equations is another approximation. It's useful in some circumstances where the lumped model becomes very inaccurate. But it will itself break down and be inaccurate in some cases.
If you are worried that the lumped approximation might not be accurate enough in your situation, you should solve your problem both ways (or maybe a small piece of your problem that will reveal the scale of the discrepancy between the models). If the difference is big enough to affect the final conclusions you are trying to draw from the model, then you should use the transmission line model. If the differences are too small to affect your conclusions, then you may be able to save some time and effort by using the lumped-element model.
SPI uses clock and data. From the sending (master) end, clock and data fly down their respective cables in sync (but delayed by the cable) and they reach the slave end and hey presto, the slave clocks in the data and does what it has to do but, what if it has to send back some data such as a value of something.
OK, it transmits its data synchronized to the local clock it is receiving and doesn't worry any more BUT, that local clock it receives is delayed by the cable and, the data the slave sends back to the master is further delayed by the cable and what happens at the master (when receiving data) is a mess unless the data rate is slow or the cable is short.
The main problem is that data sent from a slave is "timed" to the clock edges at the slave. The data received by the master is clocked into the master by the local clock at the master - the slave clock and master clock are not aligned due to the cable delay.
The OP has changed the question so here's some extra things about cable: -
Longer cables attenuate more - think about powering a motor from a battery - it works fine up close on short leads but if you make the leads longer the terminal voltage seen on the motor gets smaller and smaller as cable length increases. Copper is not zero-ohms.
It gets worse as frequency rises due to a phenomena called skin effect. skin effect reduces the conductivity of a copper wire by forcing currents to only be present in the skin of the conductor. This means smaller cross sectional area for the current hence higher resistance hence greater losses.
Dielectric loss in cable is proportional to frequency - basically energy is stolen from the signal to heat-up the insulating material between the two wires that form the transmission line or cable. This is what wiki says: -
Attenuation (loss) per unit length, in decibels per meter. This is
dependent on the loss in the dielectric material filling the cable,
and resistive losses in the center conductor and outer shield. These
losses are frequency dependent, the losses becoming higher as the
frequency increases. Skin effect losses in the conductors can be
reduced by increasing the diameter of the cable. A cable with twice
the diameter will have half the skin effect resistance. Ignoring
dielectric and other losses, the larger cable would halve the dB/meter
loss. In designing a system, engineers consider not only the loss in
the cable but also the loss in the connectors.
If losses are proportional to frequency then the likelihood of data corruption is also proportional to faster signals. Above a certain point there is another mechanism when cable (such as coax) starts to acts like a waveguide. Again wiki has the word: -
In radio-frequency applications up to a few gigahertz, the wave
propagates primarily in the transverse electric magnetic (TEM) mode,
which means that the electric and magnetic fields are both
perpendicular to the direction of propagation. However, above a
certain cutoff frequency, transverse electric (TE) or transverse
magnetic (TM) modes can also propagate, as they do in a waveguide. It
is usually undesirable to transmit signals above the cutoff frequency,
since it may cause multiple modes with different phase velocities to
propagate, interfering with each other. The outer diameter is roughly
inversely proportional to the cutoff frequency. A propagating
surface-wave mode that does not involve or require the outer shield
but only a single central conductor also exists in coax but this mode
is effectively suppressed in coax of conventional geometry and common
impedance. Electric field lines for this [TM] mode have a longitudinal
component and require line lengths of a half-wavelength or longer.
Best Answer
The electrical length is \$\frac{\beta{}l}{2\pi}360^\circ\$ (assuming ADS wants this parameter given in degrees, a detail I don't recall). For example, if the transmission line length is equal to one wavelength long, you'd specify the electrical length as \$360^\circ\$.
But ADS will assume the physical length of the line is not magically changing depending what signal is sent through it, meaning that the electrical length changes in proportion to the frequency. Therefore you must also tell it at which frequency you are specifying the electrical length, so that it knows how to adjust this parameter when simulating at any frequency.
For example, say you know the electrical length is 360 degrees at 100 MHz. You specify in the model that the electrical length is 360 degrees and the reference frequency is 100 MHz. Then ADS knows that that means 180 degrees at 50 MHz or 720 degrees at 200 MHz.