Basically, delay. Different from bandwidth, and the source of unending confusion and argument because of that.
Latency affects everything from hard drives to the internet. It's often neglected when comparing relative performance, probably sometimes from ignorance and other times to deliberately confuse.
If there's a overall, everybody gets it way to explain latency, I sure don't know it - what makes sense to me might not make sense to you. For some examples of people trying to explain it, see
A long comment has been merged in here:
From the first article: "The latency is exactly the same thing. It's the minimum time between asking for a piece of data and getting it, just like the seek time of a disk, and it's just as important." Now I'm sure the author thinks that the above is a suitable definition for latency, but I completely disagree, especially since he brought seek time of a disk into the picture (seek time is typically used to measure how fast the read/write head assembly can be positioned). Now, when I was in school studying electronics (this was about 40 years ago), the passage of time between data request and data receipt was broken down into three more-or-less independent elements: 1) Latency: the time required for a device to react to a command to do something. Most often, latency measured the mechanical performance of the device, not its electical performance, since the latter had little effect in those days. That's changed, of course, with today's stuff. <Smile> 2) Propagation delay: the time required for the signal representing the data to traverse the interconnecting medium. It was assumed for the purpose of discussion that transmission speed through copper occurred at about 70 percent of the speed of light in a vacuum (186,282 miles per second), which meant an electrical impulse traveled at about 130,000 miles per second. 3) Bandwidth: the theoretical capacity of the medium as a function of time. Then, as now, bandwidth is measured in some multiple of bits per second (which *IS NOT* the same as baud rate). An example in one of the articles refered to the performance of a satellite link in terms of latency. In fact, propagation delay, not latency, accounts for the bulk of the time required to get data from the satellite to the receiver. Compared to the performance of the satellite and receiver electronics, the medium (electromagnetic transmission) is terribly slow due to the linear distance between endpoints (about 23,000 miles one way for a geo-stationary satellite). Also, some references to the performance of analog modems used latency to describe what are actually internal bandwidth limitations. A modem, like any other clocked device, can only process so many bits of data per second, especially when on-the-fly data compression gets into the picture. This is not latency, as the speed at which the modem's circuits react is the same as in any other electrical device. The limitation is in how rapidly circuits toggle from one state to the other, which is determined by the system clock. Incidentally, inexpensive TTL devices -- the type of silicon found in a typical modem -- can switch between states at better than 20 million times per second. A good example of the interelationship between latency, propagation delay and bandwidth is in mass storage. A disk, being a mechanical device, experiences latency due to the time required for the mechanism to react to commands from the host machine -- including the time required for a given sector to appear under a read/write head. Propagation time is expended in moving the data signals from the drive to the host, and the bandwidth of the interconnecting bus (e.g., IDE, SCSI, SATA, etc.) determines how often bits of data can be passed. Bandwidth, in turn, is determined by how many bits that can be simultaneously moved on the bus and how rapidly data line transitions can occur. The limitation on data line transition speed gets lower as the bus gets longer... So my opinion is that the term "latency" is, like many other words in today's version of English, being improperly used to describe the elapsed time between request and receipt. This, of course, is coming from the same crop of people who tell you to buy the latest computer so you can "grow your business." By the way, how does one grow a business? Do you sprinkle fertilizer on the office roof and pray for rain? Is it better to use chemical fertilizer or male bovine excrement? --BigDumbDinosaur
Which, I think, illustrates my point: people think of latency differently (if they think of it at all), and disagree about it, have different ways to explain it, etc.
Got something to add? Send me email.
More Articles by Tony Lawrence © 2011-07-06 Tony Lawrence