Computer Interface Speeds and Distances – CompTIA A+ 220-801: 1.7

| October 3, 2012


Your computer has many different interfaces, and each interface type has a different set of speed and distance standards. In this video, you’ll learn about the speeds and distances of USB, FireWire, PATA, SATA, and wireless standards.

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The universal serial bus is a very common way to connect devices into our computer. If you’ve got the older style USB, you’re probably using a USB standard called 1.1. That older standard has two speeds, a low speed and a full speed. The low speed runs at 1.5 megabits per second, and the distance of the cable that you can use is about three meters. The full speed, 12 megabits per second, has a distance limitation of about five meters.

If you’re using the more modern USB, you’re probably using USB 2.0, which runs at 480 megabits per second. You can see that’s quite an increase over the slower USB 1.1. And again, you have a maximum length of about five meters of that cable that you can use on a USB connection.

The latest version of USB is USB 3.0. We’re starting to see USB 3.0 devices roll out. That’s something called SuperSpeed, and that runs at 4.8 gigabits per second, a significant increase over at the slower 480 megabits in USB 2.0. But notice that the distance of the cable is a little bit shorter. Your maximum distance there is about three meters long.

Another type of interface connection we use is one called FireWire. This is also referred to as IEEE 1394. FireWire is what Apple calls it, but other manufactures may call it iLink or Lynx. And it allows you to connect up to 63 devices, daisy chained on to one single FireWire interface on your computer.

There are two different types of FireWire that you’ll commonly see. The first is FireWire 400, and it runs at 100, 200, or 400 megabits in half duplex. The distance limitation that you generally see with a FireWire 400 is about 4 and 1/2 meters, about 15 feet long, and they are 72 meters maximum. Each one of these FireWire devices is a repeater, so you can have a distance of cable that’s 15 feet and a FireWire device, another distance of cable coming off of that device that’s 15 feet and another FireWire device, all the way up to ultimately 63 different devices. And the total distance in all of those that you could go is a maximum of 72 meters, so you can only go so far when you start stringing these out. You may not be able to do 4 and 1/2 meters for every single one, but as long as you stay within a distance of 72 meters maximum, you’ll be able to use FireWire 400.

FireWire 800 was an update to the standard that allowed for faster speeds. In fact, FireWire 800 goes at 800 megabits per second, but notice that it goes at full duplex. That means you have 800 megabits in one direction and 800 megabits in the other direction all at the same time. There are other types of connections you can use for FireWire 800, and one of those is an optical connection that allows you to go up to 100 meters. For your copper connections, you still have some of those older standards of 4 and 1/2 meters of a distance limitation and 72 meters max, but if you’re using some of those optical specifications, you can have much larger distances on your FireWire connections.

Some of the legacy storage devices we would use will connect up through a parallel ATA interface, and those interfaces had different speeds as the standards changed through the years. There’s an ATA-4, ATA-5, ATA-6, and ATA-7, which also correlated back to the ATAPI standards as well. These used an ultra direct memory access mechanism for two, four, five, and six, and those standards also correlated back to different transfer rates so that the drive could talk directly to memory as fast as possible. On the ATA-4, we had about 33 megabytes per second as a maximum transfer rate, and you can see with the ATA-7, we have 133 megabytes per second as the transfer rates associated with that.

The serial ATA, or SATA standard, is one that’s commonly seen on today’s computers, and the first revision of SATA was one that allowed 1.5 gigabits per second of throughput, and the maximum size of the cable that you could use between the SATA interface on your motherboard and the drive itself maxed out at about one meter. There was an updated version of SATA to revision 2.0 that increased the speed up to 3.0 gigabits per second, still kept the maximum length of that cable at one meter. And we didn’t change the size of the cable on revision 3.0 either, but we did double the speed again, up to speeds of 6.0 gigabits per second. External SATA is used for drives that are outside of your computer’s case, and generally the speeds of eSATA will match the SATA type that you have inside of your computer. The distance of the cable, however, can be a little bit longer so that you can move it away from your computer by as much as two meters.

Wireless technologies are a little bit more difficult to be able to determine lengths and distances because of all the variables involved, but we can come pretty close. For instance, with infrared technologies, IrDA, those tend to run at about 4 megabits per second. You have a line of sight. They’re commonly used between laptops and printers.

We don’t see them used much any longer, but they’re still on some of the legacy systems that are out there, and you can see the distance is very, very short. It’s about one meter, about three feet, and beyond that, it’s not able to communicate any further. Bluetooth gave us a lot more flexibility with the devices that we would use, and it also extended the distances, for Bluetooth 2.0 for instance, with something called EDR. It ran at a speed of three megabits per second, and this allowed us to go up to 10 meters, 30 feet away from our devices for class two devices, and one meter, which is about three feet, for the class three Bluetooth devices.

One of the most common wireless standards you’ll see, of course, is 802.11 wireless, and there have been four major versions of that. For 802.11a, we can run up to 54 megabits per second, and those will generally get you all about 120 meters, although there are some special licensing that you can get from the US government that can increase the power up to 5,000 meters. We don’t generally see that on common 802.11a, but the capability does exist in the standard.

For 802.11b, we can run a maximum of 11 megabits per second up to about 140 meters or so. 802.11g runs the same distance, but you can increase the speed up to 54 megabits per second, and 802.11n allowed us to have a maximum theoretical throughput per stream of 150 megabits per second over about a 250 meter distance. But notice, we can have multiple data streams at the same time, which would increase the total available throughput of an 802.11n up to 600 megabits per second.

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Category: CompTIA A+ 220-801

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