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Today, serial and parallel direct cable connections are considered obsolete ways to transfer data between computers. A USB direct cable connection, or DCC, is not all that popular in the enterprise, but this type of USB cable connection is still used by home users because of its easy setup and fast transfer speeds.
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USB DCC has been around for well over a decade, but has failed to become a popular method to transfer data because of the rapid growth of networks and their significant speeds. In most cases, users with Gigabit Ethernet ports on their workstations simply require a standard unshielded twisted pair straight-through cable to begin transferring data at double the rate of a USB DCC.
Here's a closer look at USB cable connections, as well as other USB interfaces, and how they work.
USB stands for universal serial bus. Today, it's the standard interface for mobile devices and all computer peripherals -- such as printers, faxes, CD-ROMs, mice, joysticks and so on.
The USB gives you a single, standardized, easy-to-use way to connect multiple devices to a computer. The USB port is also capable of providing power to the connected devices, but it should be noted that each USB port has a limit of delivering a maximum amount of power, depending on the USB version.
The initial specification of USB v1.1 was designed to deliver a maximum of 0.75 watts at 5 volts. The USB v2.0 specification increased the maximum power to an impressive 2.5 watts. And the USB v3.0 specification next came along and further increased the maximum power to 7.5 watts at 5 volts, allowing more power-demanding peripherals to connect and be supported by USB.
The latest USB specification, v3.1, delivers 100 watts of power at 5, 12 or 20 volts.
As mentioned previously, there are four different versions of USB specifications: v1.1, v2, v3.0 and v3.1. Most USB ports today on computers and laptops support the USB v3.0 specification, while peripheral devices with USB v3.0 interfaces are already available in the market.
USB v3.1 and v3.0 are fully backward-compatible with USB v2.0 and v1.1 peripherals, ensuring full compatibility with any USB device.
The table below compares the different versions of a USB cable connection currently available, along with their most important technical specifications.
Keep in mind, when you're using a USB DCC cable, you won't get such great speeds, but somewhere around 500 Kbps. This also depends on the type of CPU, OS, the quality of the cable, and electronic components and protocols running on your system.
When using a USB cable connection to transfer data between two computers, the effective throughput -- or speed -- achieved will depend on a variety of factors, such as the USB port version on both ends, USB direct cable version, CPU and hard disk drive (HDD) speeds, and how busy both systems are. A USB v2.0 cable connected to USBv2 ports is expected to achieve a 480 Mbps transfer rate; however, if the HDD on either side is unable to keep a sustained transfer speed, throughput will dramatically decrease.
Another important detail is the operating system used. Today, all Windows and Linux operating systems fully support USB ports; however, older operating systems are not completely compatible.
The standard USB cable connection
The standard USB cable connection uses A and B connectors to avoid confusion. A connectors head upstream toward the computer, while B connectors head downstream and connect to individual devices. This might seem baffling to some, but it was designed to avoid confusion between consumers because it would be more complicated for most people to try and figure out which end goes where.
This is what the USB cable and connector looks like:
The diagram below shows a standard USB cable connection with its internal four wires and their function. The shielding located at the far left is used to protect the cable from electromagnetic interference.
The USB direct connection cable
As previously mentioned, transferring data between two computers using the USB ports requires a USB direct connection cable. This cable is also known as a USB transfer, or data link cable. The DCC cable is not a simple crossover cable, but contains electronic circuits that make it possible to use it to transfer data between computers.
Many manufacturers offer USB DCCs; the vast majorities only sell USB 2.0 DCCs, which support a maximum transfer speed of 480 Mbps.
DCCs are plug-and-play, which means no drivers are needed, and they are powered directly by the USB port. In most cases, when the USB cable is plugged into the computer, the machine will see it as an external drive, containing a special executable application run on each computer to enable transferring files between them.
Catalog of all 10 cabling lessons
Cabling series, lesson 7: Direct cable connection
Cabling series, lesson 8: Serial direct cable connection
Cabling series, lesson 9: Parallel direct cable connection