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Um interface
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<p>The Um interface is the <a href="/facts/Air_interface/rXkUSM2p">air interface</a> for the <a href="/facts/GSM/Sw8QYaLE">GSM</a> mobile telephone standard. It is the interface between the <a href="/facts/Mobile_phone/H3EcqD9h">mobile station</a> (MS) and the <a href="/facts/Base_transceiver_station/dLDhUMEn">Base transceiver station</a> (BTS). It is called Um because it is the mobile analog to the <a href="/facts/U_interface/xdl8HjNT">U interface</a> of <a href="/facts/ISDN/zcV99rUb">ISDN</a>. Um is defined in the GSM 04.xx and 05.xx series of specifications. Um can also support <a href="/facts/GPRS/c4SJD7ov">GPRS</a> packet-oriented communication. </p>