 signals from 64,000 bits per seconds to 8,000 bits per second. The header brings the total bandwidth to about 18,000 bits per second. H.323 H.323 is a family of signaling standards for multimedia trans- missions over packet networks adopted by the International Telecommunications Union (ITU). Microsoft Corporation and Intel Corporation adopted the standard in 1996 for sending voice over packet networks. H.323 includes standards for com- pressing calls and for signaling. It has higher overhead than the newer signaling protocol, SIP. Presence theft This refers to the impersonation of a legitimate IP telephone or other device by a hacker. Proxy server A proxy server screens communications between endpoints to verify that the called and calling parties are who they say they are and that no virus will be sent. A proxy server might sit between a VoIP server and external devices that request an audio conference or videoconference session. This is referred to as intermediating sessions. Proxy servers are also used between IP telephones and the Internet. Real-time Transport Protocol is an Internet Engineering Task Force (IETF) standardized protocol for transmitting multime- dia in IP networks. RTP is used for the "bearer" channels-the actual voice, video, and image content. SIP is commonly used for the signaling to set up and tear down sessions. Skinny Client Control Protocol is a Cisco proprietary signaling protocol for sending signals between devices in Cisco tele- phone systems. It is also referred to as "Cisco Skinny." Data Centers and IP Private Branch Exchanges Table 2-1 (Continued) Protocols and Terms for VolP Service Description Session Initiation Protocol establishes sessions over IP net- works, such as those for telephone calls, audio conferencing, click-to-dial from the Web, and instant messaging exchanges between devices. It is also used to link IP telephones from dif- ferent manufacturers to SIP-compatible IP telephone systems. It is used in landline and mobile networks. Quality of Service guarantees a parti