| Key Point When implementing VoIP using traditional phones, components include the phones, a PBX, and a voice-enabled router, as illustrated earlier in the VoIP portion of Figure 7-2.
Traditional phones and PBXs have been installed in businesses for many years. The addition of a voice-enabled router, also called a voice gateway, can be the first step toward a fully converged voice and data network. The voice-enabled router contains digital signal processors (DSPs) to perform its functions in hardware, which is much faster than doing it in software. To support VoIP, an underlying IP infrastructure must be functioning and robustin other words, don't even think of adding voice to a network that is experiencing other problems, such as congestion or network failures. Voice-specific recommendations for the IP infrastructure are as follows: QoS, as described earlier and detailed in Chapter 6, should be implemented within the network. Queuing should be deployed on any link (WAN or LAN) that could potentially experience congestion, to meet delay and jitter requirements for the voice traffic. WANs for voice networks should use a hub-and-spoke topology, with a central hub site and multiple remote spoke sites connected to the central hub site.[3] Each remote site is then one WAN link hop away from the central site and two WAN link hops away from all other spoke sites. This ensures that call admission controllimiting the number of calls allowed over the network so that the quality of all calls is within acceptable limitsat the central site can keep track of the bandwidth available between any two sites. Redundancy should be implemented within the network. For example, WAN links should be redundant to ensure the availability of the voice traffic between sites, and redundant devices and links should be deployed within the campus where necessary to ensure the availability within each site. Compression of voice traffic over WANs should be implemented to reduce the required bandwidth. The various compression techniques are described later in the "Voice Coding and Compression Techniques" sectionG.729 compression is recommended for WAN links.
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