What exactly is the difference between bandwidth and speed?

Question

I am currently studying QoS and I am often confused and tend to mix the terms "Bandwidth" and "Speed".

If I am not mistaken, bandwidth is the capacity of the link, correct? While the speed defines how fast the data is transmitted?

If that's the case, I am not quite sure if I understand what speed means here. If we have a link with a speed of 2 gbit/s, would it transmit data (lets say, 100 mbits) faster than a 1 gbit/s link? Or what if we (let's use our imagination here for a second) had a link with a BW of 1000 mbits but the speed would only be 500 mbit/s? The link's capacity would not be reached but it still wouldn't be able to transmit all 1000 mbits within a second, right?

Thank you for your time.

Answer 1

It's all about context. Basically, "bandwidth" is (as you write) usually about the capacity of a link or path, and "speed" can mean all sorts of things.

If we have a link with a speed of 2 gbit/s, would it transmit data (lets say, 100 mbits) faster than a 1 gbit/s link?

Here, "speed" is the link's nominal link rate. An Ethernet link with a nominal link rate of 1 Gbit/s (e.g. 1000BASE-T) already factors in the physical layer overhead, but it doesn't factor in the overheads of the higher layers.

For TCP over IPv4 with maximum-sized frames/packets, the calculation goes like this:

• maximum Ethernet frame size including IPG = 1538 bytes
• maximum Ethernet frame payload = IP MTU = 1500 bytes
• IPv4 overhead (without options) = 20 bytes
• TCP overhead (without options) = 20 bytes
• effective application payload = 1500-20-20 = 1460 bytes
• effective application bandwidth = 1000 Mbit/s / 8 bit/byte / 1538 byte * 1460 byte = 118.7 Mbyte/s.

Now, "speed" could be applied to that bandwidth as well, as application throughput/speed. It could also mean a practically achievable speed in a given situation, regardless of potentially available network bandwidth (e.g. a given backup speed might be limited by I/O, not by SAN link speed).

So, if you need to be precise, make sure you include the exact layer or aspect where your bandwidth or speed should be applied.

Answer 2

Depending on context, "speed" might also be related to "latency". A link that has a higher bandwidth does not necessarily mean less latency, and might thus seem slower.

As an example, imagine two pieces of equipment 1 km apart. Too far for Ethernet, but to connect them you have the options a) laying a 9600 bps rs-232 cable or b) using a satellite service. As long as you're sending a few bytes per second, the 9600 bps option will always seem faster because it has much less latency - you can't get around the speed of light with the satellite service. Once you ramp up the (used) bandwidth, this changes.