Cisco Systems OL-14619-01 manual Integrating by Using SIP, 6-22

Chapter 6 Integrating Cisco Unity with the Phone System

Integrating by Using SIP

Integrating Cisco Unity with Cisco Unified Communications Manager Express in SRST Mode

Cisco Unity supports a topology with centralized call processing and distributed messaging, in which your Cisco Unity server is located at a remote site or branch office and registered with the Cisco Unified CM at a central site.

When the WAN link fails, the phones fall back to the Cisco Unified CM Express-as-SRST device. Cisco Unity can also fall back to the Cisco Unified CM Express-as-SRST device, which lets users at the remote site access their voice messages and see message waiting indicators (MWIs) during a WAN outage. Note that MWIs must be resynchronized from the Cisco Unity server whenever a failover happens from Cisco Unified CM to Cisco Unified CM Express-as-SRST or vice versa.

For information on setting up this configuration, see the Integrating Cisco Unity with Cisco Unified CME-as-SRSTapplication note at http://www.cisco.com/en/US/products/sw/voicesw/ps4625/prod_troubleshooting_guides_list.html.

Integrating by Using SIP

SIP (Session Initiation Protocol) is the Internet Engineering Task Force standard for multimedia calls over IP. SIP is a peer-to-peer, ASCII-based protocol that uses requests and responses to establish, maintain, and terminate calls (or sessions) between two or more end points. A SIP network uses the following components:

SIP proxy server	The proxy server is an intermediate device that receives SIP requests from a client
	and then forwards the requests on behalf of the client. 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.
Redirect server	Provides information to the client about the next hop or hops that a message should
	take. The client then contacts the next hop server or user-agent server directly.
Registrar server	Processes requests from user agent clients for registration of their current location.
	Registrar servers are often installed on the redirect or proxy server.
Phones	Can act as either a server or client. Softphones (PCs that have phone capabilities
	installed) and Cisco SIP IP phones can initiate SIP requests and respond to requests.
Gateways	Provide call control. Gateways provide many services; the most common is a
	translation function between SIP call endpoints and other terminal types. This
	function includes translation between transmission formats and between
	communications procedures. In addition, the gateway translates between audio and
	video codecs, and performs call setup and clearing on both the LAN side and the
	switched-circuit network side.

Cisco Unity accepts calls from a proxy server. Cisco Unity relies on a proxy server or call agent to authenticate calls.

SIP uses a request/response method to establish communications between various components in the network and to ultimately establish a conference (call or session) between two or more endpoints. A single call may involve several clients and servers.

Users in a SIP network are identified by:

•A unique phone or extension number.

	Design Guide for Cisco Unity Release 5.x
6-22	OL-14619-01