Cisco ATA 191 Quick Start Manual page 3

Cisco ATA 191 Analog Telephone Adapter Overview
Figure 2: SIP Architecture
SIP Clients
SIP clients include:
• Gateways—Provide call control. Gateways provide many services, the most common being a translation
• Phones—Can act as either a UAS or UAC. The ATA 191 can initiate SIP requests and respond to requests.
SIP Servers
SIP servers include:
• Proxy server—The proxy server is an intermediate device that receives SIP requests from a client and
• Redirect server—Receives SIP requests, strips out the address in the request, checks its address tables
• Registrar server—Processes requests from UACs for registration of their current location. Registrar
function between SIP conferencing endpoints and other terminal types. This function includes translation
between transmission formats and between communications procedures. In addition, the gateway also
translates between audio and video codecs and performs call setup and clearing on both the LAN side
and the switched-circuit network side.
then forwards the requests on the client's behalf. Proxy servers receive SIP messages and forward them
to the next SIP server in the network. Proxy servers can provide functions such as authentication,
authorization, network access control, routing, reliable request retransmission, and security.
for any other addresses that may be mapped to the address in the request, and then returns the results of
the address mapping to the client. Redirect servers provide the client with information about the next
hop or hops that a message should take, then the client contacts the next hop server or UAS directly.
servers are often colocated with a redirect or proxy server.
Cisco ATA 191 Analog Telephone Adapter Overview
SIP Clients
3