 AMR speech codec consists of a multirate speech codec, a source controlled rate scheme including a voice activity detector, a com- fort noise generation system, and an error concealment mechanism to combat the effects of transmission errors and lost packets. The multirate speech codec is a single integrated speech codec with eight source rates from 4.75 kbps to 12.2 kbps and a low rate background noise encoding mode. For AMR, the sampling rate is 8 kHz. The usage of AMR requires an optimized link adaptation that selects the best codec mode to meet the local radio channel and capacity requirements. If the radio conditions are bad, source coding is reduced and channel coding is increased. In addition to an AMR audio codec, an extension of it is an AMR-wideband (AMR-WB) audio codec. The sampling rate for AMR-WB is 16 kHz, which is dou- ble the sampling rate of AMR. Therefore, AMR is often abbreviated as AMR-NB (narrowband). AMR-WB is supported if 16-kbps sampling for the audio is used in the UE. Similarly, like AMR-NB, AMR-WB operates with various bit rates. For AMR-WB the bit rates range from 6.60 kbps to 23.85 kbps. It is emphasized that both AMR-NB and AMR-WB are used in HSPA and LTE radio systems. A more detailed description of AMR-NB and AMR-WB can be found in [10]. General functionality of the AMR codec is illustrated in Figure 10.4. During voice activity periods there is one voice frame generated every 20 ms (number of VolP packets on the active period SID packets on the silent period 20 ms 160 ms Headers Payload Figure 10.4 Illustration of VolP traffic. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology bits/frame depending on the AMR codec mode), whereas during the silent periods Silence Descriptor (SID) frames are generated once every 160 ms. 10.3.2 Delay Requirements Voice over Internet Protocol (VoIP) is a conversational class service, and the packet delay should be strictly maintained under reasonable limits. The maximum accept- able mouth-to-ear delay for