![]() Unacceptable for general network planning purposes. Table 4.1 Delay Specifications Range in MillisecondsĪcceptable provided that administrators are aware of the transmission time and the impact it has on the transmission quality of user applications. This recommendation defines three bands of one-way delay as shown in Table 4.1. The International Telecommunication Union (ITU) considers network delay for voice applications in Recommendation G.114. The look ahead is discussed in Algorithmic Delay. The analog voice stream is digitized into PCM samples and delivered to the compression algorithm in 10 ms increments. An example of how a G.729 compression system works is shown in Figure 3-1. For example, the basic block size used by a G.729 algorithm is 10 ms whereas the basic block size used by the G.723.1 algorithms is 30ms. These blocks vary in length based on the coder. The high complexity compression algorithms used in Cisco router/gateways analyze a block of PCM samples delivered by the Voice codec. If a digital PBX is used, the PBX performs the codec function and the Router processes the PCM samples passed to it by the PBX. Figure 2-2 Codec Function in Router/Gateway For example, if an analog voice system is used, then the router/gateway performs the CODEC function and the compression function as shown in Figure 2-2. Figure 2-1 End-to-End Voice Flowīased on how the network is configured, the router/gateway can perform both the codec and compression functions or only one of them. On the far side of the cloud the exact same functions are performed in reverse order. The PCM samples are then passed to the compression algorithm which compresses the voice into a packet format for transmission across the WAN. The analog signal from the telephone is digitized into pulse code modulation (PCM) signals by the voice coder-decoder (codec). The flow of a compressed voice circuit is shown in this diagram. Though the examples are geared to Frame Relay, the concepts are applicable to Voice over IP (VoIP) and Voice over ATM (VoATM) networks as well. This paper explains the sources of delay when you use Cisco router/gateways over packet networks. Overall voice quality is a function of many factors that include the compression algorithm, errors and frame loss, echo cancellation, and delay. If you account correctly for all potential delays, it ensures that overall network performance is acceptable. When you design networks that transport voice over packet, frame, or cell infrastructures, it is important to understand and account for the delay components in the network.
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