# | 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 III: Transmission delay of a packet
On the last example we have simply clarified o redefined transmission time and propagation time. We will apply these concepts to a third example. In this case we will calculate the time that a packet takes since it leaves node A, until it arrives to node C. The values of each link, or the features of each link, as well as the length of the packet, are on the attached slide. And notice that the processing time is on each node. When that node receives a packet, how much time does it take to read the contents of that packet and to know, for instance, if it has to be resent, if it is for that node or if it has mistakes or not.
We have our network, which is always the same, with our router, node C, node B and node A here and they ask us: how long does a packet with a length of 12000 bits take since it leaves A, until it starts to be transmitted, since it gets to B? Basically, we will have to apply the concepts that we have seen on the last example to calculate the transmission time on this link (on link 1), the propagation time for this link, the processing time on the Router, the transmission and propagation time on link 2, and the processing time on computer B.
But before doing these calculations, we are going to draw how the packet would evolve inside this network. Basically we can draw the timing diagrams that we usually use to represent these problems. And in this case we have the three nodes that are part of the network through which this packet passes. Notice that every vertical shaft represents the time, that is, the time starts here and increases, and we take a reference time: t=0, from which packet A starts being transmitted. In this case, we draw what it would be when the first bit starts being transmitted. This bits travels through the link, it spreads. And this point right here after a certain propagation time on link 1 arrives to the Router, which would be this element right here. If the first bit starts being put in the link here, here is where the last bit finishes being put, and this time right here will be the transmission time on link 1.
Therefore on this point right here, the whole packet has arrived to the Router. And this Router takes a certain time, which we have on the slides, to process this packet and see what has to be resent to B. Depending on the headers that this packet contains, where the destination address is included, the Router decides whether to send it to C or B. In this case, according to this information, it decides to send it to B. Therefore, once it has been processed and it has decided to send it towards B, this packet is transmitted to B. And here, this first bit travels through this link, until it arrives to B, on this point right here. Therefore, this would be the propagation delay on link 2. This would be link 1, and this would be link 2.
Clearly the drawing is not to scale, for instance, this propagation delay is smaller than this transmission delay, more or less, but in any case we could draw this one right here a little bit more realistic. Notice that the capacity of link 2 is way smaller than the capacity of link 1. Therefore this would mean that the transmission time of this packet will be bigger on link 2 than on link 1. Therefore we could represent it this way, where this time here is the transmission delay on link 1, and this one here, on link 2. This time right here would be the transmission delay on link 2.
Therefore, on this point here, the packet arrives to B and then it has to be processed and it would take a certain time, until here, to process the packet. This would be the Router processing time.
What is the total time that they were asking for on this example? The time from here, from t=0, until this packet arrives and is processed in B, which would be the total time. We will not put the numbers on this example, you can solve it yourselves, but we will write the different terms for the record.
For instance, this total delay will simply have a first term, a second term, a third term, a fourth term, which would be the propagation delay, a fifth term, and a sixth term. In detail, it would be L divided by the capacity of link 1: the transmission time of this one right here, plus the propagation time, which would be the distance of link 1 divided by the propagation speed of link 1 and is the Router processing time. This is a value given, we can calculate it. And this would be the transmission of this packet to link 1 where we would have to simply add the same terms on link 2, that we could add here.
Notice that the packet is the same, therefore it has the same length, propagation time will be different, because either length or distance of link 2 compared to link 1 are different, in fact, notice that link 2 is way longer than link 1, according to the values on the slides. And finally, we will have to place the processing delay of the receiver, of node B in this case.
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