Centralized Deployment Model
In such model survivability is not more of a concern. Branch reliability becomes somewhat more of an issue than it is in a centralized Unified CVP model because WANs are typically less reliable than LAN links. But it is a good practice to configure the Survivability.TCL script on the ingress gateway in order to gracefully disconnect the call in case of some problem (Software/Hardware/Network etc.)
Distributed Deployments
Distributed deployments require design considerations for other voice services that are being run at the branch. For example, the branch is typically a remote Unified CM site supporting both ACD agent and non-agent phones. Typically in such deployment the PSTN gateway is used not only for ingress of Unified CVP calls but also for ingress/egress of the regular non-ACD phone calls.
Therefore, you must provide mechanisms that are local to the branch to gracefully handle calls that are impacted by loss of a WAN link to the central site. Call survivability must be considered for both the Unified CVP and non-CVP calls. For the Unified CM endpoint phones, survivability is accomplished via a Cisco IOS feature known as Survivable Remote Site Telephony (SRST)
For Unified CVP calls, survivability is handled by a combination of services from a TCL script (survivability.tcl) and SRST functions.
The survivability TCL script is used to monitor the H.225 or SIP connection for all calls that ingress through the remote gateway. If a signaling failure occurs, the TCL script takes control of the call and redirects it to a configurable destination. The destination choices for the TCL script are configured as parameters in the Cisco IOS Gateway configuration.
Alternative destinations for this transfer could be another IP destination (including the SRST call agent at the remote site), *8 TNT, or hookflash. With transfers to the SRST call agent at the remote site, the most common target is an SRST alias or a Basic ACD hunt group.
NOTE
Call survivability is supported in all Unified CVP call flow models except the VRU-only call flow model.
Survivability.TCL Script
For Unified CVP calls, survivability is handled by a combination of services from a TCL script (survivability.tcl) and SRST functions. The survivability TCL script is used to monitor the H.225 or SIP connection for all calls that ingress through the remote gateway. If a signaling failure occurs, the TCL script takes control of the call and redirects it to a configurable destination. The destination choices for the TCL script are configured as parameters in the Cisco IOS Gateway configuration.
This service should be placed on the incoming pots dial-peer that is destined for CVP. In the event of critical CVP application errors or a WAN failure that would normally disconnect the caller, this script allows the gateway to attempt a transfer to some alternate location after the failure occurs instead of disconnecting the caller. In the event that the call cannot be transferred to an alternate agent, the script will play a "call-back-later" message and disconnect.
This script provides the following capabilities:
- Ability to do multiple types of transfer in call failure conditions:
- "*8" network takeback
- Two B-Channel Transfer (TBCT)
- hookflash takeback
- hairpin
- SRST
- Ability to differentiate call recovery behavior by incoming DNIS
- Ability to differentiate call recovery behavior by incoming DNIS and how long the call had been in CVP prior to failure.
- Ability to differentiate call recovery behavior based on time of day and date.
- Separation of ANI and DNIS into their correct fields to provide a workaround for a PSTN switch anomaly causing the concatenation of ANI and DNIS.
- Ability to hand off to the CME auto-attendant service for enhanced recovery procedures such as caller queuing.
For details on different transfer options available, please check the URL:
http://www.cisco.com/univercd/cc/td/doc/product/icm/isn/cvp40/cvp40sdg/vp40trns.pdf
SIP Refer Transfer
In some scenarios, it is desirable for Unified CVP to transfer a call to a SIP destination and not have Unified ICM and Unified CVP retain any ability for further call control.
Unified CVP can perform a SIP Refer transfer, which allows Unified CVP to remove itself from the call, thus freeing up licensed Unified CVP ports. The Ingress Voice Gateway port remains in use until the caller or the terminating equipment releases the call.
A SIP Refer transfer can be invoked by either of the following methods:
- Unified ICM sends Unified CVP a routing label with a format of rfXXXX (For example, rf5551000).
- An application-controlled alternative is to set an ECC variable (user.sip.referertransfer) to the value "y" in the Unified ICM script, and then send that variable to Unified CVP.
Conditions
- SIP Refer transfers may be used in both Comprehensive and Call Director deployments.
- The SIP Refer transfer can be invoked after Unified CVP queue treatment has been provided to a caller. SIP Refer transfers can be made to Cisco Unified Communications Manager or other SIP endpoints, such as a SIP-enabled ACD.
H.323 Refer Transfer
Unified CVP 4.0(2) introduces a new transfer mechanism for H.323 calls that behaves in a similar manner to SIP Refer. This feature allows Unified CVP to remove itself from the call, thus freeing up call control ports. Using this feature, the call can be queued at the VoiceXML gateway and then sent to an agent on Cisco Unified Communications Manager or other H.323 endpoints such as an ACD.
The H.323 Refer transfer can be invoked by either of the following methods:
- Unified ICM sends Unified CVP a routing label with a format of RF88#xxxx# (For example, RF88#5551000#)
- An application-controlled alternative is to set an ECC variable (user.h323.rftransfer) to the value "y" in the Unified ICM script, and then send that variable to Unified CVP. The CVP H.323 service will modify the received label automatically to conform to the format given above.
- The Unified CVP Survivability service should be enabled to execute the H323 Refer transfers by using the following parameter:
param icm-rf 1
Conditions
- Unified CVP cannot execute further call control operations after this kind of transfer has been executed; however, Unified CVP Survivability can still be used for failure recovery in this scenario.
- This feature can be used in both Comprehensive and Call Director call flow models, and it is available only for PSTN-originated calls via a Cisco IOS gateway running the Unified CVP Survivability service.
