networking - Explanation on network transfer bit rate R

Bit Rate R usually refers to:

Number of bits transferred in unit of network time on a link L.

I am curious on the sending direction of R. Because data on link L has two directions. Does R means aggregate bits transferred in the two directions?

Another question is link L can send data in a single direction?

I have bought a network wire of 100 Mbps. So this means that I can send/receive at 100Mbps rate?

Also we usually heard of bi-directional working like FTP protocol. What does this bi-directional mean?

Bit rates are almost always specified in terms of the rate from the sender to the receiver, with the implication that to get the maximum round-trip bit rate, you double the result. When this isn't correct (e.g., Asymmertic DSL, satellite or cable Internet), the bit rates will always be specified for both directions.

Links are almost always conceptually bi-directional, although not always (e.g., satellite Internet access sometimes is download-only, with upload via telephone lines), but from a physical perspective, a link is always unidirectional at a single point in time, and often always unidirectional.

Your "network wire of 100 Mbps" doesn't exist. What you bought was a CAT-5 Ethernet cable, which contains 8 wires. No single one of them can carry a 100Mbps signal, but they are used in combination to build up the full signal rate.

As to FTP, that's a completely different question, and the "bi-directional" is unrelated to all of this.