ation. Proxy servers, located in gateways and firewalls, serve as intermediaries between callers and applications or endpoints, telephones, and other devices connected to the LAN. They are commonly used with Session Initiation Protocol (SIP) for audio and video conferencing. See the section "Session Initiation Protocol in UC" for an explanation of SIP. Assessing Network Quality by Using Voice Quality Measurements IT staff members can manage VoIP by using software management tools to assess voice and video quality to analyze the following. Packet loss This refers to the packets that are dropped when there is net- work congestion. Packet loss results in uneven voice quality. Voice conversa- tions "break up" when packet loss is too high. Latency This term refers to delays (in milliseconds) that are incurred when voice packets traverse the network. Latency results in long pauses within conversations, and clipped words. Jitter Jitter is uneven latency and packet loss, which results in noisy calls that contain pops and clicks or crackling sounds. IP Private Branch Exchange Architecture Echo This is the annoying effect of hearing your voice repeated, an issue with which SO many of us are familiar. It is often caused when voice data is translated from a circuit-switched format to the IP format. This is usually corrected during installation by special echo-canceling software. Prioritizing Voice and Video on a Virtual Local Area Network Organizations can program voice telephony as a separate Virtual Local Area Network (VLAN). These "virtual" networks act as separate LANs. Voice systems use compo- nents that are programmed together to decrease delay. These components use common protocols and control types of devices that are able to communicate with IP telephones and hold audio and video conferences. VLANs perform the following special treatment for IP endpoints. 802.1P protocols tag voice and multimedia packets to prioritize them for improved availability with less delay. The tag distinguishes voice and video packets. In a