Bits and signals
A computer stores everything as bits: each bit is a 1 or a 0. The letter A, for example, is the 8 bits 01000001. To send data, the computer must turn those bits into something that can travel.
On a copper cable, the sender changes the voltage on the wire many times a second. The receiver measures those changes and turns them back into 1s and 0s.
In a fibre-optic cable, bits travel as flashes of light inside a thin strand of glass. A laser switches the light on and off, or brighter and dimmer, and a sensor at the far end reads it.
Wi-Fi and phones need no cable. The sender shapes a radio wave to carry the bits, the wave spreads through the air, and the antenna on the receiver picks it up.
Two kinds of speed
A signal is fast but not instant. Light in glass moves at about two-thirds of its speed in a vacuum: roughly 200,000 km per second, or about 5 microseconds (millionths of a second) per kilometre.
Latency is how long one bit takes to get from sender to receiver. Across 1000 km of fibre that is about 5.0 ms (thousandths of a second) one way, and a faster link does not change it.
Bandwidth is how many bits the sender can put on the wire each second. At 1 Gb/s (a billion bits per second) a new bit leaves every nanosecond. At 10 Mb/s, one leaves every 100 nanoseconds.
Data travels in packets. A big packet is 1,500 bytes, the most a normal Ethernet frame carries: that is 12,000 bits. At 1 Gb/s the sender needs just 12 µs to push them all onto the wire.
On a 10 Mb/s link the same 12,000 bits take 1.2 ms to push out: 100 times longer. The bits leave more slowly, but each one still travels down the wire at the same speed.
Putting it together
Think of a pipe. Bandwidth is its width: how much fits through each second. Latency is its length: how long the trip takes. A wider pipe finishes big downloads sooner, but the first bit still needs 5.0 ms across 1000 km.
The last bit arrives after push-out time plus travel time. Over 1000 km at 1 Gb/s: 12 µs + 5.00 ms ≈ 5.02 ms. At 10 Mb/s: 1.2 ms + 5.00 ms ≈ 6.20 ms.
The numbers
;; step 1/11 · bits "A" in ASCII = 0x41 = 65 decimal bits (most significant first) 01000001 8 bits = 1 byte Ethernet sends each byte least-significant bit first, so the rightmost 1 is the first bit on the wire.
In the figure one bead stands for one bit in the first steps and for about 1000 bits in the packet steps; timings are slowed down millions of times but keep their ratios. Speed in fibre is taken as 2/3 of c (199,862 km/s, 5.00 µs per km); real single-mode fibre is about c / 1.47, a little faster, and real cable routes are longer than the straight-line distance.