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11: Physical layer - what is bandwidth, throughput, and goodput part 3


Data is carried on media at a different speed depending on the technology used. So to analyze the speed of the transmission of data on a given medium, we need to rely on three ways. Bandwidth, throughput, and goodput.

These three variations have different functions in terms of measuring  data transfer. Each of them measures an aspect of data transmission.  Besides, all of which insist in the same standard that is bits per second.

Bandwidth

To start with, bandwidth means the amount of data that can be carried in a given amount of time; or, in other words, the capacity of a medium to carry data in a specific period of time. As stated previously, data transfer is measured using bits per second (bps). Thanks to the technological development, bandwidth is now commonly referred to in kilobits per second and megabits per second, and in case using highly sophisticated media, bandwidth can reach gigabits per second and terabits per second. These speeds can be achieved relying on the physical  characteristics of media and the signaling method being applied. Thus if one is to calculate the bandwidth of a given medium, the physical properties as well as the signaling method must be taken into consideration.
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To make things much clearer, it is of pivotal importance to provide an example of analogy. Suppose that the medium through which data frames to be transmitted is analogous to a tube and the bandwidth is an amount of water.

As you can see in the above figure, the amount of water that can be passed through tube 1 to reach the end in one second  is not the amount of water that tube 2 allows flowing towards to the end in one second. Tube 1 carries much more water than tube 2 in one second, because of the fact that tube 1 is broader and larger than tube 2 and thus resulting in a far great amount of water being accumulated at the end of the tube 1 than tube 2. From these differences wa can infer that the amount of data frames that are transmitted through a medium that is very sophisticated and works at high speed is considerably dissimilar from the volume of data frames that are transferred through an antiquated medium that operates at a low speed. Now it can be said that tube 1 allows more bandwidth than tube 2. Thus bandwidth depends largely on media used.

The following units of bandwidth and their equivalence in bits may be useful in measurement.

Throughput

The next term which is equally significant is throughput.  It can be defined as the real or actual data transfer rate in a given amount of time. We said that bandwidth is the capacity of a medium for data transmisstion, however, this capacity is hardly attained in real data transfer because of a variety of factors such as interference, noise, and errors in data transmission. For this reason, when we talk about bandwidth, we are actually taking about a theoretical rate of speed which is not same when data frames are really transmitted, whereas when we refer to the speed rate of a given medium using a throughput value, we are actually utilizing the real value at which data frames are carried. This means that throughput is bandwidth minus interference and errors, and therefore bandwidth is theoretical and throughput is practical.

In addition, when designing a network, the designer must take into account all what can keep the network from attaining the real speed rate. In other words, throughput is to be taken into consideration rather than bandwidth when planning networks. In the same way as bandwidth, throughput is measured in bits per second.

Significantly, throughput can also be influenced by many factors that might prevent it from attaining the expected rate of speed. The factors that may affect the throughput vary. The amount of traffic is one key factor. A medium being used by devices that use internet immensely  results in reducing the throughput. Besides, the type of traffic may make inroads on throughput as well. For example, watching videos with high quality online is not like navigating a website such as facebook. The former case is more throughput-consuming than the latter. Another main factor that has a mammoth influence on throughput is the number of network machines being connected to the medium being measured. Logically, the throughput of a medium gets degraded if the number of devices increases. A very good example to consider is multiaccess topology using Ethernet technology when nodes compete media access. See the followinf figure.
Goodput

The final crucial measuring component is goodput.  It denotes  the transfer rate of the actual usable data bits; or, to put it another way, goodput measures the final data that the user benefits from or use, for data such as a picture, text, or video does not travel on media alone, but rather there is another control information, which is generally considered as data, or what is referred to as overhead. Its purpose is to make sure that data is safe, undammaged, unrepeated, and to guide data up to the destination. Thus there is no need to include the overhead in goodput measuring as long as it is not actually used by the user. The scenario is that when data is generated by the user at the application layer, it passes by all of the lower layers, and each layer attaches its overhead information to the data. The following figure clarifies overhead encapsulation.
By and large, it can be said that goodput is throughput minus overhead traffic. Such traffic can be there for establishing a session between two ends , acknowledgments, and data encapsulation. All of these processes consume a considerable amount of throughput. As such including overhead in the calculation of goodput is not useful.
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A rather simplified scenario comprising all of the three measuring components might remove ambiguities. For instance,  A local area network has two devices connected to each other. One device sends a file to the other one, let it be a video. The bandwidth of the LAN network is 100 Mbps. But because of the fact that the devices share the media, the throughput between the devices is merely 60 Mbps. By removing the overhead from the actual data, which has been encapsulated throughout the layers,  the real rate of the data received by the receiving device, goodput, is only 40 Mbps.
11: Physical layer - what is bandwidth, throughput, and goodput part 3 Reviewed by BOUFTIRA on 12:02:00 PM Rating: 5

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