# | Video | Duration |
---|---|---|
1 | Subnets creation | 06:57 |
2 | Transmission and propagation time | 04:22 |
3 | Transmission delay of a packet | 07:10 |
4 | File Transfer from end to end | 08:32 |
Part II: Transmission and propagation time
On this second example, we will go in depth on a couple of concepts that, even though they are very simple, you have to know them and they have to be clear, you have to know how to use and apply them. The two concepts are simply Transmission Time and Propagation Time. So, if we take the example we have on the slides from the same network we had before, notice that we had a Router in the middle, and we had the computer A, B, and C Host. Each link we have: link 1, link 2, link 3, has a set of features on each link: transmission rate, its capacity, its length, the distance, and the type of means that defines in this case the propagation speed. On link 1 and link 2 the propagation speed is the velocity of the light, therefore, they can be optical fibers. And on link 3, what we have is that it can be a copper link or a coaxial cable, metallic in any case, and that is why the propagation speed of this signal link is inferior, it is typically 2.8 * 10^8 m/s.
We take, for instance, the first link and we are going to see on this link how to calculate Transmission Time. We can refer to Transmission Time as Rtx, a delay or a Transmission Time that is defined as L (the total length of the packet) divided by C (the link capacity). L, I insist, total packet length, including the headers and the data, and C, is the link capacity. And this produces a value that will have second units, given the fact that it is a delay or a time.
How do we have to interpret this result? This is the time that a packet takes, a packet sent by node A, with a length of L, to be transmitted, to the means. Remember that the links can be seen as pipes, as tubes, and the time we need to place a packet of length L inside the link is the Transmission Time.
We begin putting a first bit, then another bit, a third bit, to the last bit, L, in the link. And the time we take to put all these bits inside this link, is the transmission time. All right? Simple, but it is an important concept
Then, once a bit has entered the link, this bit travels inside this tube, inside this link, until the end and it takes a certain time, a time that depends on the distance of the link, as well as on the propagation speed, that is: how fast this bits moves in this link. How can we calculate this?: propagation delay, which is equal to the distance divided by the speed of the bit in this case. What unities will it have? Well, if these are meters, and these are meters per second, we will have again, seconds, since it is a delay.
Remember that this is a travel time; it is the time that a bit takes since it enters a link until it leaves of the link. And so, if we look at it from a bit point of view, when we will have put the first bit, this bit will take this time to leave the link. The second bit will take the same time, and so on for every bit we have in the packet.
Obra amb llicència Creative Commons Reconeixement-NoComercial-NoDerivats 3.0 Unported License.