Packet Voice

With the converging of data and voice networks, the introduction of packet voice technology provides the means for companies to save toll charges on voice telephone calls. Voice is digitized into packets, cells, or frames, is sent as data throughout the networks, and is then converted back to analog voice (see Figure 3-10). Calls are placed on telephones attached to routers. The routers digitize the voice and then forward the calls on the WAN links. If it is determined that sufficient bandwidth is not available on the data network, the call is forwarded over the Public Switched Telephone Network (PSTN). Calls placed outside the company's intranet are forwarded to the PSTN.

Figure 3-10 Packet Voice

Router B

Figure 3-10 Packet Voice

Router B

With Voice over IP (VoIP) technology, voice is digitized into IP packets. A dial plan is created to list the IP destinations of VoIP telephones and outbound telephones. When a number is dialed on a telephone attached to a router, it looks at the digits and routes the call to the appropriate destination. For example, in Figure 3-10, if a call to 8-1234 is placed from telephone 6-1234, Router A will forward the packets with voice data to Router B. Router B will convert the digitized voice into analog signals and send them out its voice interface to telephone 8-1234. Table 3-1 shows the dial plan for Router A in Figure 3-10.

Table 3-1 Router a Dial Plan

Phone Number


Calls to 8-xxxx

Forward to IP:

Calls to 7-xxxx

Forward to IP:

Outside calls

Forward to PSTN


Voice port 1/0


Voice port 1/1

In VoIP, Real-Time Transport Protocol (RTP) is used to transport audio streams. RTP runs over User Datagram Protocol (UDP), but there is no specific port assigned. The only requirement is that the data is transported on an even port and RTCP (RTP Control Protocol) is carried on the next odd port. However, ports 5004 and 5005 have been reserved for those applications that choose to use them.

UDP, in contrast to TCP, is a connectionless protocol that relies on upper layer protocols for acknowledgement and error checking. UDP is preferred to TCP for voice due to the timesensitive nature of voice transmissions. Since voice applications are sensitive to the accumulation of delay, any Quality of Service techniques used on the network must prioritize these RTP/UDP IP packets over other traffic, such as FTP and HTTP. RTP header compression can also be used on WAN links to reduce the size of voice packets.

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