Cisco Unified Survivable Remote Site Telephony

Cisco Unified Survivable Remote Site Telephony for Cisco Hosted Collaboration Solution - A Cloud Connector

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Cisco® Hosted Collaboration Solution (Cisco HCS) is a widely accepted cloud service that enterprise customers are adopting. Cisco Unified Survivable Remote Site Telephony (Cisco Unified SRST) is a software and hardware solution that allows customers to maintain business connectivity during a time of lost telephony connectivity between their premises and the cloud service.

Customers are adopting Cisco HCS because it allows them to gain all the functions of a Cisco Unified Communications Manager environment with an effective pay-per-use service that the cloud can provide. For many, this means they can enjoy the benefits of the rich collaboration feature set and broad set of endpoint choices that are often missing in a traditional IP Centrex environment. Overall, the service is a win for both partners that can offer this new and effective cloud-based service and enterprise customers who can take advantage of the cloud without a compromise on reliability (Figure 1).

Figure 1. Cisco Hosted Collaboration Solution

Cloud Challenges with Hosted Collaboration Services

As enterprises adopt Cisco HCS, certain challenges arise, including how to ensure continuity of services when connectivity to the cloud is lost. Voice - and soon video - communications are such an essential part of a communications solution that businesses cannot continue without them. With this level of importance of a specific service, enterprises must consider the idea of resiliency in the face of a cloud outage. Critical challenges include the following:

• Availability: The enterprise must ensure continuity of features when WAN connectivity to the partner is compromised. A reliant and redundant WAN architecture with multiple paths to ensure HCS connectivity is essential for business continuity. Cisco Unified SRST aims to provide even more resiliency, allowing for the survivability of call control when all connectivity to the cloud-based voice services is lost.

• Productivity: Workers need to access applications such as unified communications whether or not WAN connectivity is problematic. In a compromised situation, such as loss of connectivity to the cloud, maintaining telephony access is essential to ensure productivity.

• Complex operations: Managing resiliency needs to be as automated as possible as customers move to cloud-based solutions. In cloud-based solutions, partners cannot afford to do much manual maintenance - automation is an essential component of an effective cloud service.

Cisco Unified SRST for Cisco HCS

Cisco Unified SRST is an example of a cloud connector, and it supports the Cisco HCS solution. In the Cisco HCS solution, telephony survivability can be accomplished with standard Cisco SRST configurations on the customer premises equipment (CPE)-based Cisco Integrated Services Routers Generation 2 (ISR G2) routers.
Cisco Unified SRST has two parts:

• Cisco Unified SRST uses the existing remote site network to provide feature-rich call-processing redundancy for centralized Cisco call-processing deployments such as Cisco HCS. If the WAN link to the remote location fails and the connection to the Cisco HCS is lost, the phones are automatically redirected to the Cisco Unified SRST router, which takes over and provides the functions of Cisco HCS. When the disrupted WAN link is restored, the phones automatically re-register with Cisco HCS - no manual intervention is required.

• A simple configuration is required in the cloud-based Cisco Unified Communications Manager (Cisco HCS) to point the phones at a particular site to the Cisco Unified SRST device that supports that site.

Cisco Unified SRST is deployed at the customer premises and is orderable and supported on the Cisco ISR G2 routers. The capacity of the SRST functions starts from as low as 5 users on the Cisco 800 Series Integrated Services Routers and scales up to 1500 users for the Cisco 3945E Integrated Services Router. For more information, please refer to Cisco Unified Survivable Remote Site Telephony.

How to Obtain Cisco Unified SRST

Cisco Unified SRST, generally offered by Cisco HCS partners, can be enabled with minimal configuration to the premises-based ISR router and the Cisco HCS. Cisco Unified SRST can be provisioned by either:

• Ensuring that a new Cisco ISR is provisioned for Cisco Unified SRST

• Reconfiguring an existing Cisco ISR to ensure that it has the Cisco Unified SRST enabled

In both of these options, enterprises should request the Cisco Unified SRST as part of their Cisco HCS deployments.

Cisco Unified SRST: An Example of a Cisco Cloud Connector

Cisco Cloud Connectors are software components that improve the security, resiliency, or performance of cloud-based solutions. Cisco HCS is a leading infrastructure-as-a-service (IaaS) offering that eliminates much of the overhead and support costs for an enterprise voice and video collaboration deployment. Cisco HCS provides dedicated virtual appliances in the cloud to deliver collaboration services without the expense of purchasing and maintaining physical servers. Cisco Unified SRST provides an additional improvement of this cloud-based solution by providing ongoing telephony survivability when access to the cloud is lost (Figure 2).

Figure 2. Continuity During Cloud Outage

Platforms and Capacities

Now consider some of the specifics about platforms and capacities that are available with Cisco Unified SRST.
For capacity planning the size of the router is used to determine the correct equipment to place at the customer premises to optimize the cloud experience (refer to Table 1).

Table 1. Capacity by Cisco ISR Platform for Cisco Unified SRST



Cisco Unified SRST

Cisco 2911

Small offices

50 phones

Cisco 2921

Medium offices

100 phones

Cisco 2951

Large offices

250 phones

Cisco 3925

Regional offices

730 phones

Cisco 3945

Very large offices

1200 phones

Cisco 3945E

Largest offices

1500 phones


Cisco Unified SRST is an important addition to cloud-based communications services. Cisco Unified SRST is a feature in Cisco IOS ® Software that is enabled by command-line interface (CLI) commands on the Cisco ISR and a basic configuration on the hosted Cisco Unified Communications Manager. Cisco Unified SRST is an example of a Cloud Connector that is part of a Cloud Intelligent Network to provide customers an enhanced experience when using cloud services.