 DTMF Relay via KPML over SIP SUBCRIBE/NOTIFY sip -notify DTMF Relay via SIP NOTIFY messages Most DTMF relay mechanisms travel in the signaling path and are therefore available for manipulation by all call agents, soft switches, and proxies. RFC-2833 DTMF relay, how- ever, travels in the media path, as does any in-band tones. Manipulating these forms of DTMF is available only to network elements that handle the media path. Interpreting (that is, capturing and converting) in-band tones to DTMF relay requires a DSP on the CUBE. All other DTMF mechanisms can be converted in software only. Codecs Codecs specify how to encode and decode speech using various parameters and com- pression techniques. Common codecs in use for VoIP include G.711 and G.729. Narrowband codecs encode speech in the range 400 Hz to 3500 Hz, whereas wideband codecs such as G.722 encode 50 to 7000 Hz thereby offering much higher fidelity voice quality. Newer narrowband codecs such as (internet Low Bitrate Codec (iLBC) has better voice quality resiliency over networks with packet loss than the traditional G.711 and G.729A codecs, but it is not currently widely supported or used. Wideband codecs are becoming increasingly popular in end-to-end VoIP systems because the traditional PSTN (generally capable of nothing better than G.711) could not Chapter 8: Interworking 151 provide high fidelity voice. With SIP trunking this lowest common denominator limita- tion has been removed. Most current SIP trunk providers do not yet offer wideband codecs as a commercial offering, but this is bound to appear in the not-too-distant future. Consider several aspects of codecs in network interworking scenarios: Payload types Codec filtering or stripping Transcoding (meaning to change the codec used by a single voice call in the middle of the stream; for example, one endpoint uses G.711 whereas the other endpoint on the same call uses G.729A, and a device in the middle of the stream transcodes the RTP stream from G.711 to G.729A) Transrating (meaning to change the pac