When designing networks that transport voice over packet, frame, or cell infrastructures, it is important to understand and account for the network delay components. Correctly accounting for all potential delays ensures that overall network performance is acceptable. | ClíCO Stỉiemỉ the Internet ÜHNESLATeON- Understanding Delay in Packet Voice Networks Introduction When designing networks that transport voice over packet frame or cell infrastructures it is important to understand and account for the network delay components. Correctly accounting for all potential delays ensures that overall network performance is acceptable. Overall voice quality is a function of many factors including the compression algorithm errors and frame loss echo cancellation and delay. This paper explains the delay sources when using Cisco router gateways over packet networks. Though the examples are geared to Frame Relay the concepts are applicable to Voice over IP VoIP and Asynchronous Transfer Mode ATM networks as well. Basic Voice Flow The compressed voice circuit flow is shown in Figure 1. The analog signal from the telephone is digitized into pulse code modulation PCM signals by the voice CODEC. The PCM samples are then passed to the compression algorithm that compresses the voice into a packet format for transmission across the WAN. On the far side of the cloud the exact same functions are performed in reverse order. Flow- Figure 1 End-to-End Voice Flow Depending on how the network is configured the router gateway can perform the CODEC and compression functions or only one of them. For example when using an analog voice system the router gateway performs the CODEC function and the compression function as shown in Figure 2. Figure 2 CODEC Function in Router Gateway When using a digital PBX the PBX performs the CODEC function and the MC3810 processes the PCM samples passed to it by the PBX. An example is shown in Figure 3. Figure 3 CODEC Function in PBX 04 02 2000 2000 Cisco Systems Inc. 2 Cisco Confidential How Voice Compression Works The high complexity compression algorithms used in Cisco router gateways work by analyzing a block of PCM samples delivered by the Voice CODEC. These blocks vary in length depending on the coder. For example the basic
