What are bandwidth, latency, and speed?

Bandwidth and latency are attributes that characterize the speed of a network.

Bandwidth

Bandwidth, typically measured in bits, kilobits, or megabits per second, is the rate at which data flows over the network. This is a measure of throughput (amount per second) rather than speed (distance traveled per second). Just as more water flows through a wide river than a small, narrow creek, a high bandwidth network generally can deliver more information than a low bandwidth network given the same amount of a time. Because this can make the network feel faster, high bandwidth networks and connections often are called "high-speed". Residential cable and DSL Internet connections often are advertised as high-speed connections, even though the actual speed of the information traveling from one end to another is roughly the same for cable, DSL, and normal phone connections.

Latency

Latency, usually measured in milliseconds, is the time that elapses between a request for information and its arrival. A high latency can degrade the performance of even the largest capacity network to a tremendous degree. Because it takes time for a signal to pass through wire, some latency will always be present, but slow servers, inefficient data packing, and excessive network hopping can collectively increase transmission delay.

Excess latency gives a network a low-speed feel. If a connection takes three or four seconds to respond, many users will complain the connection is "slow", even though the bandwidth is high, and even though the data comes in such a large chunk that it appears to arrive all at once.

Speed

A network's speed is essentially a subjective evaluation of the combination of bandwidth and latency. As mentioned above, the term is often used in place of bandwidth, even by technicians and professionals; many times a network administrator or hardware technician will talk about a 10BaseT, 100BaseT, or gigabit "speed" in reference to products or networks with 10KB/sec, 100KB/sec, or 1,000KB/sec bandwidths.

Most vendors advertise the theoretical bandwidths of their networking products, but due to bottlenecks, hardware problems, and high processing loads, the effective throughput is usually much less. A gigabit (1000BaseT) Ethernet card will be lucky to achieve 800Mb/s in real-world use, and in many situations will achieve far less than that.

For a scorecard of bandwidths and other networking characteristics, see:

http://www.csgnetwork.com/bandwidth.html

The page above doesn't address latencies, but modems typically have values of around 100 milliseconds, vastly higher than any of the technologies reviewed on the scorecard. For a good overview of latency and its effects, read Stuart Cheshire's It's the Latency, Stupid at:

http://rescomp.stanford.edu/~cheshire/rants/Latency.html