USB
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Maximum useful distance USB 1.1 maximum cable length is 3 metres (9.8 ft).[26] USB 2.0 maximum cable length is 5 metres (16 ft). Maximum permitted hubs connected in series is 5. Although the device address is a 7 bit value, giving a theoretical 127 devices on the bus, the host's address and the bus reset address (a binary zero) must be subtracted, making the maximum permitted number of devices connected to a single host interface a total of 125.
Although a single cable is limited to 5 metres, the USB 2.0 specification permits up to five USB hubs in a long chain of cables and hubs. This allows for a maximum distance of 30 metres (98 ft) between host and device, using six cables 5 metres (16 ft) long and five hubs. In actual use, since some USB devices have built-in cables for connecting to the hub, the maximum achievable distance is 25 metres (82 ft) + the length of the device's cable. For longer lengths, USB extenders that use CAT5 cable can increase the distance between USB devices up to 50 metres (160 ft).
Since USB host interfaces (and powered hubs) are required to provide power for devices connected to them, a special type of USB extender cable was created, consisting of a miniature one-port USB hub molded onto one end of a 5-metre cable. These mini-hubs are fully self-contained within the cable, requiring no separate bulky hub device and no external power. They are as simple to use as plugging cables together, with each hub drawing power through all the previous single-port hubs in the chain. Because bus power is limited, the most practical arrangement consists of four single-port hub extender cables, one plain 5-metre cable and, at the very end, a powered multiport hub to support multiple USB devices.
Another method of Extending USB beyond 5 metres (16 ft) is by using low resistance cable. The higher cost of USB 2.0 Cat 5 extenders has urged some manufacturers to use other methods to extend USB, such as using built-in USB hubs, and custom low-resistance USB cable. It is important to note that devices which use more bus power, such as USB hard drives and USB scanners will require the use of a powered USB hub at the end of the extension, so that a constant connection will be achieved. If power and data does not have sufficient power then problems can result, such as no connection at all, or intermittent connections during use. USB 3.0 cable assembly may be of any length as long as all requirements defined in the specification are met. However, maximum speed can be achieved across a maximum cable length of approximately 3 metres.[27]
