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Latency Versus Bandwidth

One of the most commonly misunderstood concepts in networking is speed and capacity, with most people believing they are the same thing.  For example it's common to hear "How fast is your connection?" with the invariable response of "3Mb", "7Mb" or similar.  These answers are actually referring to the bandwidth or capacity of the service, not the speed.

Speed and bandwidth are in fact interdependent.  The combination of latency and bandwidth gives users the perception of how quickly a webpage loads or a file is transferred.  It doesn't help that the broadband providers keep saying "get high speed access" when they probably should be saying "get high capacity access".  Note the word 'broadband' - it refers to how wide the pipe is, not how fast.

 

Latency

Latency is delay.  For our purposes, it is the amount of time it takes a packet of data to travel from source to destination.  Together, latency and bandwidth define the speed and capacity of a network.

Latency is normally expressed in milliseconds.  One of the most common methods to measure latency is the ping utility.  A small packet of data, typically 32 bytes, is sent to a host and the RTT (round-trip time) is measured.  This is the time it takes for the packet to leave the source host, travel to the destination host and return back to the source host.  See our separate Knowledge base article about Ping Command for more information on how to carry out a ping test.

The following are typical latencies of different types of circuits.  Please remember that latency is also affected by any routing that an ISP will perform (e.g. if your data has further to travel the latency will increase).

Ethernet  0.3 ms

ISDN       15-30 ms

ADSL 2+  20-30 ms

FTTC        10-15 ms

 

Bandwidth

Bandwidth is normally expressed in bits per second.  It's the amount of data that can be transferred during a second.

Solving bandwidth is easier than solving latency.  To solve bandwidth issues, more pipes are added.  For example, in early analogue modems it was possible to increase bandwidth by bonding two or more modems together.

Note that if the bandwidth is saturated then congestion occurs and latency increases.  However, if the bandwidth of a circuit is not at it's peak, the latency will not decrease.  Bandwidth can always be increased but latency cannot be decreased.  Latency is a function of the electrical characteristics of the circuit.

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