Wireless repeater

A wireless repeater (also called wireless range extender) takes an existing signal from a wireless router or wireless access point and rebroadcasts it to create a second network. When two or more hosts have to be connected with one another over the IEEE 802.11 protocol and the distance is too long for a direct connection to be established, a wireless repeater is used to bridge the gap. It can be a specialized stand alone computer networking device. Also, some Wireless network interface controllers (WNIC)s optionally support operating in such a mode. Those outside of the primary network will be able to connect through the new "repeated" network. However, as far as the original router or access point is concerned, only the repeater MAC is connected, making it necessary to enable safety features on the wireless repeater. Wireless repeaters are commonly used to improve signal range and strength within homes and small offices.

Since only one wireless device can transmit at once, wireless transmissions are doubled (router to repeater and then repeater to client versus just router to client), and so:

Some wireless range extending devices connect via a USB port. These USB adapters add Wi-Fi capability to desktop PCs and other devices that have standard USB ports. USB supports not only the data transfers required for networking, but it also supplies a power source so that these adapters do not require electrical plugs.

There are wireless range extending devices that conform to all 802.11 protocols. Most 802.11 compliant devices are backwards compatible. However, 802.11ac runs at 5 GHz and requires an access point capable of 5 GHz operation. 802.11ac is the most recent and third-generation Wi-Fi standard for wireless home networking. 802.11ac equipment is backward compatible with 802.11n, 802.11g or 802.11b equipment.

An older range extender will not be able to repeat the signal of a newer generation router. Security encryption compatibility also matters and must be at the same level of compatibility for the signal to be extended. For example, an older range extender which supports WEP and WPA will not be able to boost a WPA2-encrypted signal from a router.

Most wireless repeaters (or range extenders) are purpose built, but certain wireless routers can be flashed with custom firmware such as DD-WRT to give them a 'range extender' option.

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Wikipedia