As the enterprise extends its IP telephony deployments from central sites to remote offices, one of the critical factors in achieving a successful deployment is the ability to support backup call control at the remote branch office. Cisco® Unified Survivable Remote Site Telephony (SRST) provides a cost-effective solution for supporting redundant call control in the remote branch office.
Cisco Unified Communications is a comprehensive IP communications system of voice, video, data, and mobility products and applications. It enables more effective, more secure, more personal communications that directly affect both sales and profitability. It brings people together by enabling a new way of communicating-where your business moves with you, security is everywhere, and information is always available...whenever and wherever it is needed. Cisco Unified Communications is part of an integrated solution that includes network infrastructure, security, mobility, network management products, lifecycle services, flexible deployment and outsourced management options, end-user and partner financing packages, and third-party communications applications.
Benefits of Centralized Call-Processing Architecture
Cisco Unified Survivable Remote Site Telephony is a critical component of a centralized call-processing architecture in which a Cisco Unified Communications Manager cluster, located at a central site, provides telephony services for all sites of a corporation. The architecture provides numerous benefits to enterprises, including centralized and simplified management. Table 1 lists the benefits of a centralized call-processing architecture.
Table 1. Benefits of Centralized Call-Processing Architecture
However, centralized call-processing architecture must include a strategy for survivability of telephony service at the branch office when access to the centralized call processing is interrupted because of WAN outage or other factors. Call-processing redundancy in the branch office is particularly critical in an emergency-which may be the cause of a WAN outage in the first place.
Components of Centralized Call-Processing Architecture
The Cisco Unified Communications system uses Cisco Unified Communications Manager in combination with Cisco Unified Survivable Remote Site Telephony which is embedded within Cisco IOS® Software, to help provide high-availability IP telephony to branch offices. When access to Cisco Unified Communications Manager from the branch office is impeded, for example, as a result of a WAN link failure, Cisco Unified Survivable Remote Site Telephony provides telephony backup services to help ensure that the branch office has continuous telephony service over the Cisco network infrastructure deployed in the branch. The enhanced reliability provided by Cisco Unified Survivable Remote Site Telephony makes the Cisco Unified Communications system a cost-effective solution to ensure telephony operation to all users in an organization, whether they are located in the headquarters or in a branch office.
Furthermore, in certain environments, the security of telephony communication is a critical requirement. The Cisco Unified Communications system supports secure telephony communication between any two phones in the network, whether those phones are in the headquarters facility or at a branch office. Cisco Unified Survivable Remote Site Telephony contributes to this secure telephony communication solution by supporting the same secure telephony protocols in the branch office when the branch loses communication with the centralized Cisco Unified Communications Manager.
How It Works
Cisco developed Cisco Unified Survivable Remote Site Telephony technology for all Cisco IOS Software platforms that support voice (refer to Table 3 for a complete list). The Cisco Unified Survivable Remote Site Telephony feature integrates network intelligence into Cisco IOS Software, which acts as the call-processing engine for IP phones located in the branch office during a WAN outage (Figure 1).
Figure 1. Centralized Cisco Unified Communications Manager Deployment with Remote Site Experiencing WAN Failure; Cisco Router Using Cisco Unified Survivable Remote Site Telephony
Cisco Unified Survivable Remote Site Telephony functions in the branch-office router to automatically detect a failure in the network and initiate a process to autoconfigure the router, providing call-processing backup redundancy for the IP phones in that office and helping ensure that the telephony capabilities stay operational. Upon restoration of WAN connectivity, the system automatically shifts call processing back to the primary Cisco Unified Communications Manager cluster. The Cisco Unified Survivable Remote Site Telephony configuration needs to be completed only once during install, simplifying deployment, administration, and maintenance. No IT staff is required at the remote sites to manage the Cisco Unified Survivable Remote Site Telephony feature.
Cisco routers with Cisco Unified Survivable Remote Site Telephony also offer secure voice mode with Cisco Unified Survivable Remote Site Telephony 3.3 and higher. If you deploy secure voice with Cisco Unified Communications Manager at your main site, secure Cisco Unified Survivable Remote Site Telephony gives you the option to keep calls secure during Cisco Unified Survivable Remote Site Telephony mode with transparent layer security (TLS) and Secure Real-Time Transport Protocol (SRTP) for signaling and media encryption, respectively. When the WAN link or Cisco Unified Communications Manager is restored, Cisco Unified Communications Manager resumes secure call-handling capabilities.
Cisco Unified Survivable Remote Site Telephony Version 3.4 and higher supports Session Initiation Protocol (SIP) for Cisco Unified IP phones, which provide basic telephony functions when the network SIP proxy or Cisco Unified Communications Manager is no longer available. The Cisco Unified Survivable Remote Site Telephony router with SIP enabled provides SIP registrar services during the outage and supports a back-to-back user agent, allowing for supplementary features such as call transfer and forwarding. Cisco Unified IP phones using SIP register to the Cisco Unified Survivable Remote Site Telephony enabled router when the WAN link is out of service.
Cisco Unified Survivable Remote Site Telephony offers fault monitoring using Simple Network Management Protocol (SNMP) with the SRST Management Information Base (MIB), which gives you the ability to remotely monitor the Cisco Unified Survivable Remote Site Telephony site using existing SNMP tools or CiscoWorks. The CISCO-SRST-MIB provides the network operations center details about Cisco Unified Survivable Remote Site Telephony activity, including duration of SRST usage, IP phones registered or registration failure, and calls processed during Survivable Remote Site Telephony mode. A backup WAN link connection is required to receive CISCO-SRST-MIB data to the central site during Survivable Remote Site Telephony mode. Table 1 lists part numbers for Cisco Unified Survivable Remote Site Telephony. Cisco Unified Survivable Remote Site Telephony Platform Density and Feature License Part Numbers
* The Cisco Catalyst 6500 Series CMM supports Cisco Unified Survivable Remote Site Telephony 4.0 with Cisco IOS Software Release 12.4 and supports Cisco Unified Survivable Remote Site Telephony 2.1 with Cisco IOS Software Release 12.2(13)ZC.
** The Cisco uBR7200 Series supports Cisco Unified Survivable Remote Site Telephony 2.1 only with Cisco IOS Software Release 12.3 Mainline.
*** The maximum number of SIP phones supported in Survivable Remote Site Telephony mode in this release is different as follows: Cisco 1751, Cisco 1760, Cisco 2610XM, Cisco 2611XM, Cisco 2620XM, Cisco 2621XM and Cisco 2801-up to 24 phones; Cisco 2650XM and Cisco 2811-up to 36 phones; Cisco 2821-up to 48 phones; Cisco 2691-up to 72 phones; Cisco 2851-up to 96 phones; Cisco 3725-up to 144 phones; Cisco 3825-up to 168 phones; Cisco 3745-up to 192 phones; and Cisco 3845-up to 480 phones.
Cisco Unified Survivable Remote Site Telephony Platform Information
Cisco platforms with Cisco Unified Survivable Remote Site Telephony support from 24 to 720 phones. Details about currently supported platforms and the number of phones per platform is provided in the Cisco Unified Survivable Remote Site Telephony Specifications Sheet for each version, which can be viewed online at: http://www.cisco.com/en/US/products/sw/voicesw/ps2169/products_documentation_roadmap09186a008018912f.html.
Cisco offers integrated services router bundles with Cisco Unified Survivable Remote Site Telephony at a discount when compared to purchasing bundle components separately. These bundles are listed in Table 2.
Table 2. Cisco Unified Survivable Remote Site Telephony Bundles
Cisco Unified IP Phone Support
Cisco Unified Survivable Remote Site Telephony is supported with Cisco Unified CallManager Version 3.01 and greater. Cisco Unified Survivable Remote Site Telephony is not dependent on Cisco Unified CallManager versions but on IP phone loads.
Table 3 lists the Cisco Unified IP phones supported by Cisco Unified Survivable Remote Site Telephony with Skinny Call Control Protocol (SCCP) phone loads.
Table 3. Cisco Unified IP Phone Support Using SCCP
Table 4 lists the Cisco Unified IP phones supported by Cisco Unified Survivable Remote Site Telephony with Session Initiated Protocol (SIP) phone loads.
Table 4. Cisco Unified IP Phone Support Using SIP
Cisco IOS Software Image Support
Table 5 summarizes the correlation between Cisco Unified Survivable Remote Site Telephony version and Cisco IOS Software.
Secure Cisco Unified Survivable Remote Site Telephony is available with Cisco Unified Survivable Remote Site Telephony 3.3 and higher for Cisco Unified IP phones using SCCP and also requires Cisco Unified CallManager 4.1(2) and higher.
Cisco Unified Survivable Remote Site Telephony for SIP phones is supported with Cisco Unified Survivable Remote Site Telephony 3.4 and higher and only with Cisco Unified IP phones.
For the latest Cisco IOS Software release and features, consult the Feature Navigator at: http://www.cisco.com/go/fn.
Table 5. Cisco IOS Software Release(s)
Supported Features
Cisco Unified Survivable Remote Site Telephony provides robust support for many IP phone features through the duration of the WAN failure, a feature that is not available from other traditional telephony solutions. Table 6 lists the features supported during failure.
Table 6. Cisco Unified Survivable Remote Site Telephony Features