Cisco Unified Survivable Remote Site Telephony

Ensuring High-Availability IP Communications in Branch Offices

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Now Cisco® CallManager IP Communications can cost-effectively extend to small branch offices using Cisco CallManager Survivable Remote Site Telephony


IP telephony is undergoing tremendous growth. Most Fortune 500 corporations are already deploying the technology or are planning to. The number of enterprises likely to adopt this new technology will increase to more than 80 percent within four years, according to Phillips Group InfoTech 2000. Enterprises with more than 500 employees will represent the largest segment of this market. This rapid growth is driven by the many features that can be deployed at users' desktops using applications that improve individual and group productivity over traditional telephony solutions.
As enterprises extend their IP telephony deployments from central sites to remote offices, one vital consideration is whether backup redundancy can be provided cost effectively at small branch offices. The size and number of these small offices precludes most enterprises from deploying dedicated call-processing servers, unified messaging servers, or multiple WAN links to each site to achieve the high availability necessary. Additionally, small offices do not generally have expert IT personnel. Therefore, any solution must be simple to deploy, administer, and maintain. The Cisco CallManager IP telephony solution combined with the Cisco CallManager Survivable Remote Site Telephony (SRST) feature allows companies to extend
high-availability IP telephony to remote branch offices with a cost-effective solution that is extremely simple to deploy, administer, and maintain.


The evolution of IP telephony started with enterprises deploying the technology in small, controlled groups, according to studies by the Meta Group (Figure 1). Then enterprises steadily began to extend IP telephony throughout their campus locations and then across large branch offices (Meta Group, Voice & Data Convergence Study 2000).

Figure 1

Typical IP Telephony Deployment


As enterprises deploy IP telephony to their larger branch offices, they have added Cisco CallManager clusters while using existing WAN links and onsite IT resources. Because of the larger office size and the greater number of users at each site, this method of expanding the IP telephony network is cost effective.
Customers now want to extend high availability IP telephony to their small offices so they can take advantage of this technology's features, including next-generation call centers, unified messaging services, embedded directory services, mobility, and Cisco IP SoftPhones. These features improve employee productivity and allow corporations great flexibility in where they deploy workers to generate additional revenue and improve customer satisfaction. Cisco CallManager SRST allows high-availability IP telephony to be extended cost effectively to smaller branch offices by allowing customers to use their existing WAN infrastructures and voice-enabled Cisco routers.


Cisco CallManager SRST is unique in the industry because it provides backup redundancy for broad deployment of IP telephony to small branch offices. Small offices under normal operation can now take advantage of the call-processing servers, unified messaging servers, and other IP telephony equipment maintained at a central site and use the converged voice and data network to transmit the calls to distributed, intelligent Cisco IP phones. In the event of a WAN failure, SRST enables the local router to provide basic call processing and allows access to telephony features through the duration of the failure. "Companies can install one of these systems at a central location and use it to serve IP phones at remote locations over an IP network. ... This is one of the key economic advantages of IP telephony" (The Phillips Group InfoTech 2000).
Centralized IP telephony call processing delivers these benefits:

• Reduced administrative costs:

• Centralized IT staff

• Centralized call-processing equipment

– Availability under normal operations of Cisco CallManager features and associated applications across all branches and to all users, such as Web-enabled call centers, unified messaging services, embedded directory services, mobility, and Cisco IP SoftPhones

– Increased end-to-end quality of service (QoS) using the industry-leading capabilities embedded into Cisco IOS® Software


Cisco CallManager SRST is embedded in the Cisco IOS Software that runs on the local branch office IP telephony access router. In the event of a WAN link failure, SRST allows a Cisco router to perform backup call processing for a small office with 1 to 480 Cisco IP phones. (The number of supported phones depends upon which routing platform is used.) SRST automatically detects a failure in the network and, using Cisco Simple Network Automated Provisioning (SNAP), initiates a process to intelligently autoconfigure the router to provide call-processing backup redundancy for the Cisco IP phones in that office.
The router processes the calls for the duration of the failure so that the phone capabilities remain operational (Figure 2). When WAN connectivity is restored in the network, the system automatically shifts the call processing function to the primary Cisco CallManager. Configuration for this capability is done once at the Cisco CallManager in the central site, simplifying deployment, administration, and maintenance. No IT staff is needed at the remote sites to enable and disable this function.

Figure 2

Cisco CallManager SRST Provides Backup in the Small Branch Office



The initial Cisco IOS Software images available for Cisco 1751 and 1760 modular access routers, the Cisco Catalyst ® 2600 Series, the Cisco 3700 Series, and the Cisco Catalyst 4500 Access Gateway Module require Cisco CallManager v3.05 or later. The software acts independently of the WAN or metropolitan-area network (MAN) link type and requires no additional hardware modules. The router already acts as a local digital/analog voice gateway to allow communication between Cisco IP phones and analog devices such as telephones, fax machines, and teleconferencing systems.
These Cisco CallManager functions are supported in Cisco CallManager SRST v3.0:

• All features from versions 2.0 and 2.1

• E1 R2 support

• Secondary dial tone

• Dual-line appearance per button

• Three-party G711 unplanned conferencing

• Call transfer with consult

• Music-on-hold multicast

• Support for Cisco IP Phone 7905

• European date formats

• Enhanced dial-plan-pattern command

• Increase in directory number maximums

• Additional language options for Cisco IP phones

• Configurable system message

• Improved debugging features for phones

• Symmetric Session Initiation Protocol (SIP) gateway-to-gateway dual tone multifrequency (DTMF) relay

• Ringing timeout for phones Cisco recommends that all gateway routers in remote offices maintain direct links to the public switched telephone network (PSTN) for backup callout capability. This is particularly important for maintaining emergency calling capability.


The Cisco CallManager SRST software operates by taking advantage of the keep-alive packets emanating from both the centralized Cisco CallManager cluster and local Cisco IP phones. During normal operations, the Cisco CallManager receives keep-alive packets from the Cisco IP phones. Cisco CallManager performs call setup and processing, call maintenance, and call termination (Figure 3). The branch router is configured for SRST but has no awareness of the IP phones in normal mode.

Figure 3

IP Telephony in Branch During Normal Operations



When a WAN link fails, the Cisco IP phones detect that they are no longer receiving keep-alive packets from the Cisco CallManager (Figure 4). The Cisco IP phones then register with the router, which queries the phone about its configuration and then autoconfigures itself. In this instance, the Cisco CallManager SRST software is automatically activated and builds a local database of all Cisco IP phones attached to it (up to its stated maximum). The Cisco IP phones are configured to query the router as a backup call processing source when the central Cisco CallManager does not acknowledge keep-alive packets. The Cisco CallManager SRST router now performs call setup and processing, call maintenance, and call termination. The Cisco IP phones indicate on their displays that they are in "CM Fallback Operating Mode" for the duration of the failure.

Figure 4

IP Telephony in the Branch-WAN Failure and SRST Activation



When the WAN link is restored, the Cisco IP phones detect keep-alive packets from the central Cisco CallManager and revert to it for primary call setup and processing (Figure 5). As Cisco IP Phones re-home to the Cisco CallManager, the SRST router purges its call-processing database and reverts to standby mode. Calls in progress are not interrupted because they are managed by the router gateway function. Phones in use during WAN link recovery re-home to the Cisco CallManager after they return to idle state.

Figure 5

IP Telephony in the Branch-WAN Link Restored to Normal Operations




The Cisco CallManager SRST solution allows network managers to deploy more reliable IP telephony to smaller branch offices where users would not otherwise experience its many benefits. It is inexpensive, easy to deploy, and is automatically activated and deactivated. And it provides essential high-availability telephony services when a Cisco CallManager cannot communicate with a smaller office because of a WAN link failure.


To find out more about Cisco CallManager SRST software, go to: