A cellular repeater, cell phone repeater, or wireless cellular signal booster, a type of bi-irectional amplifier (BDA) as commonly named in the wireless telecommunications industry, is a device used to boost the cell phone reception to the local area by the usage of a reception antenna, a signal amplifier and an internal rebroadcast antenna. These are similar to the cellular broadcast towers used to broadcast by the network providers, but are much smaller, usually intended for use by one building. Modern cellular repeater amplifiers work by rebroadcasting cellular signals inside the building. The systems usually use an external, directional antenna to collect the best cellular signal, which is then transmitted to an amplifier unit which amplifies the signal, and retransmits it locally, providing significantly improved signal strength. The more advanced models often also allow multiple cell phones to use the same repeater at the same time, so are suitable for commercial as well as home use.