commonly called a SIP proxy, a proxy server sends requests on a client's behalf if the client doesn't know the address of the recipient in advance. A SIP proxy is sort of like a computerized phone operator. Redirect server Redirect servers send message back to the user agent if the recipient has been moved or deleted. Registrar Phones (user agent clients in SIP-speak) have to register, and the registrar's job is to process these registrations. On Cisco IP PBXs, skinny phones register with their MAC addresses. SIP clients register with their IP addresses. Location server When a registrar processes a registration, the locations of the clients are stored in a location server. Usually, all these server types are combined into a single server for ease of connection by end users. Unless you're building SIP systems, you'll probably see only a SIP proxy, or if you need to enter multiple SIP server designations (as we will), they will likely all be the same server address. SIP call setup is pretty straightforward at a high level. SIP messages may include the following: REGISTER Register a client's location to the server. INVITE When you dial a phone, it sends a SIP INVITE. Small-Office VolP Example Acknowledge a SIP message. CANCEL Cancel an INVITE before the call has been established. This is like hanging up the phone while the far end is ringing. Used for signaling during a call. OPTIONS Ask SIP server for information about its capabilities. Terminate a connection. When you hang up a SIP phone, it sends a BYE message. Figure 30-5 shows how two small offices might connect if each had Cisco CME and SIP trunks provided by different SIP providers. Each phone will register using SCCP to the local CME-enabled router. Each router's CME will register as a SIP client to provider's SIP servers. Figure 30-5. Two offices with CME and SIP trunks The beauty of most SIP providers is that they often provide complete PBX functionality, negating the need for a CME (or any other) IP PBX at the site. This also negates the need for people l