in the ITU and the protocols developed in the IETF is that the protocols developed in the IETF are often ASCII-based and do not require any encoding to be read. The other major differ- ence is that SIP is a more flexible protocol, designed to deal with different types of media, including voice, video, and chat from initial conception. Because of this flexibility, many different methods often are used to implement the same functionality; for example, putting a voice call on hold can be accomplished by many different methods, though there is a recommended method for this. This flexibility causes many problems with inter- operability but does provide the capability for the protocol to be adopted for many dif- ferent purposes. H.325 is being proposed at the ITU as a third generation protocol that supports distrib- uted and media rich collaboration environments for initiating real-time communications Chapter 1: Overview of IP Telephony 5 as an alternative for SIP and H.323. The goal of this protocol is to quickly and reliably establish communications. The design goal of this protocol is to take the strengths of both SIP and H.323 to create a protocol that should be used for many different purposes. To date, no one has implemented this protocol, and at this time, there has been limited movement toward its standardization. Call Control Elements of IP Telephony The call control component of IP Telephony is usually one of the following: Endpoint-based user agents (such as IP Phones and analog to TDM gateways) Back-to-Back User Agents that contain the central call control component. Cisco Unified Communications Manager (Cisco UCM) is a back-to-back user agent that, as one of its functions, is to receive and interpret messages from IP Phones, generate SIP messages to other entities such as SIP gateways, and send messages to control a SIP Trunk to another device. Cisco UCM also provides the function of interworking with other call control protocols such as Media Gateway Control Protocol (MGCP) and H.323. The most common devi