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Bandwidth depends on Device interface rather then physical link


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Bandwidth depends on Device interface rather then physical link


You have touched quite sensitive topics.

In networking, especially in IP networking, we are misusing the word "bandwidth". This word originally comes from signal processing and analog circuits theory, and describes the range of frequencies that can be carried through a channel (a medium) with an acceptable distortion so that they can be correctly received at the other end. Signals transmitted beyond this frequency range (either under or above the bounds) will be too strongly distorted, attenuated or oterwise damaged.

Now, according the the Shannon-Hartley formula, the total amount of information that can be carried over a channel within a certain time depends on the bandwidth (the frequency range) of the channel and the signal-to-noise ratio. The higher the bandwidth, the higher the speeds we can obtain from a communication channel. Hence the word bandwidth became synonymous with the data information carrying capacity of a channel without really referring to the frequency range.

But you are correct in your observation: the bandwidth of a medium itself may be higher than the data throughput of the devices that are communicating over this medium. I may have the same cabling, yet I can use 10M, 100M, 1G, 10G Ethernet over it if it meets the necessary transmission characteristics. Very often, new methods of efficient data coding and error correction must be invented to push the effective throughput higher and higher, but every medium, including copper and fiber, has its theoretical limits than can be approached to, but can never be exceeded. For example, the telephone lines themselves have the maximal throughput around 56 kbps - the modems never got faster, and will never get faster because considering the limited bandwidth and added noise, it is simply not possible.

So it is the technical capability of the device that connects to the medium that determines whether it is able to make full use of the data carrying capacity of the medium.

The medium - copper will transmit at the speed of light.

You are here confusing the speed of data transmission with the speed of signal propagation on a medium. An electromagnetic pulse propagates roughly at 90% of speed of light through a copper wire (although the insulation may further delay it - see this Wikipedia article). However, that has nothing to do with the amount of data that a medium can carry in a time interval. It is only related to a property called "propagation delay".

Best regards,


Super Bronze

Bandwidth depends on Device interface rather then physical link


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Effective bandwidth relies on both the device interface and the media.

Perhaps it might help your understanding of media and interface bandwidth is to think of media as a highway with a speedlimit (which is often the same for the same media type) and various number of lanes for bandwidth.  The latter is what makes cat3 copper cable "different" from cat5 copper cable.  Both have the same speedlimit, but cat5 offers more "lanes" than cat3.  Think of an interface as like a toll gate that also has a defined number of booths.

For maximum throughput the number of booths of the toll gate should correspond to the number of the highway's lanes.  It's possible to have more lanes than booths, then your traffic volume is limited by the number of booths (e.g. cat5 cable connected to a 10 Mbps Ethernet interface), but the converse doesn't generally work, i.e. more booths than lanes (e.g. cat3 cable connected to a gig Ethernet interface).


Bandwidth is often equated to speed but it's really a measure of capacity.  The first bit sent on 10 Mbps or gig Ethernet will arrive at the same, but subsequent bits (part of the same transmission package) will arrive sooner for gig than 10 Mbps.

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