How does distance play a role for total transmission time?

Question

I've found a similar question in this site but the answers were not convincing me enough so I'll make a small scenario of two routers to express my point:

Two routers connected by a link of rate 1 Mbps and distance of 10 Miles. The Router1's buffer has 5 packets to be sent to Router2 each of which has a size of 1 Megabit (let's ignore the processing delay and queueing delay, and no congestion occurs). According to this answer, at each second, a packet will be successfully transmitted to Router2 without being affected by the distance! As far as I know, the distance of the medium and speed of the signal must be taken into consideration when finding the total transmission time between two nodes (Of course I am not forgetting transmission delay). If I misunderstood, please enlighten me.

Thank you.

Answer 1

Of course distance affects transmission time. What it doesn't affect is bandwidth or the rate at which the data are transmitted or received. There is a difference between the time it takes from transmission start and reception end (transmission time), and the rate at which something is being transmitted.

The first device will transmit at 1 Mbps, and it will be done transmitting after five seconds. It's bandwidth is 1 Mbps.

The second device will have a delay due to distance before it starts receiving, but it will receive the data at 1 Mbps when the data gets there. It will receive at a bandwidth of 1 Mbps. Distance plays no part in the rate (bandwidth) of transmission or reception.

Answer 2

Signals move as waves, the exact propagation speed varies with the particular medium, but 2×10⁸ meters per second is a reasonable ballpark estimate.

10 miles is about 16×10³ meters so your propogation delay is approximately 8×10^-4 seconds or about 0.8 milliseconds.

So, assuming "router a" transmits continously, the total time to transmit the data from "router a" to "router b" is 10.0008 seconds.

In this particular example, the propagation delay was a negligible part of the total, but things can easilly swing the other way, if distances are longer, transmission speeds are faster, or the total amount of data is smaller.

Also in the real world, the sender often will generally not send all the data without waiting, since doing so could cause congestion and packet loss further along the path. It will instead start slowly and gradually speed up as the packets make it to their destination without packet loss. This process will take longer if the round trip time is larger.