31 Days Before Your CCNA Exam

18 31 Days Before Your CCNA ExamThe Impact of Voice and Video on the NetworkCurrently, voice and video are in the midst of a migration to traditional IP data networks. Beforethe late 1990s, voice and video used separate networking facilities. Most companies today areeither migrating or plan to migrate to IP phones, which pass voice data over the data networkinside IP packets using application protocols generally referred to as voice over IP (VoIP).Figure 30-2 show a few details of how VoIP works from a home high-speed Internet connection,with a generic voice adapter (VA) converting the analog signal from a normal telephone to an IPpacket.Figure 30-2Converting from Sound to Packets with a VAVolIP PacketIP UDP RTP Digital Voice Bits3412 Analog Electricity CODECHumanSpeechVAPhone #1R1Cable orDSLVoIP does not place a demand on the network for additional capacity. A voice call typically consumesless than 30 kbps of bandwidth. However, VoIP is sensitive to delay, jitter, and packet loss:■■■Low delay: VoIP requires a very low delay between the sending phone and the receivingphone—typically less than 200 milliseconds (.2 seconds). This is a much lower delay thanwhat is required by typical data applications.Low jitter: Jitter is the variation in delay. VoIP requires very low jitter as well, whereas dataapplications can tolerate much higher jitter. For example, the jitter for consecutive VoIP packetsshould not exceed 30 milliseconds (.03 seconds) or the quality degrades.Loss: If a VoIP packet is lost in transit because of errors or because a router doesn’t haveroom to store the packet while waiting to send it, the lost VoIP packet is not retransmittedacross the network. Lost packets can sound like a break in the sound of the VoIP call.Video over IP has the same performance issues as voice. However, video requires a lot more bandwidth—anywherefrom 300 kbps to 10 Mbps, depending on the quality demanded.To support the QoS requirements of voice, video, and other quality or time-sensitive applications,routers and switches can be configured with a variety of QoS tools. These configurations arebeyond the scope of the CCNA exam topics.