GSM cellular phones use a codec specifically designed for low-bandwidth, lower power consumption. There are two modes of this codec: GSM-FR and GSM-EFR. The FR codec provides the default codec used normally if the higher fidelity EFR codec either is not available at the other endpoint or if the codec negotiation falls back to GSM-FR. Prior to Cisco CallManager 3.1, IP phones has no access to GSM codecs, neither directly nor through transcoders. Therefore, whenever an enterprise GSM phone user wanted to talk to an IP phone when inside the enterprise buildings, the call had to be hairpinned through an AS5300 gateway for transcoding. While this allowed a connection, its implementation was costly, and the call was subject to low quality due to multiple transcoding legs. Cisco CallManager 3.1 implements GSM-FR and -EFR codecs in the Catalyst 6608-T1 and E1 Digital Signal Processor (DSP) services cards. All eight segments on these cards can be configured as transcoder resources conference bridges or trunk gateways. A call from a GSM phone may be made through a transcoder or conference bridge directly to an IP phone or directly to the WS-6608 segment configured as a trunk gateway. Cisco CallManager 3.1 administration interface supports plans for implementation in a low-cost standalone gateway at a later release.
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