Survivability Overview

Survivability is a collection of features that enables the system to monitor the status of a SIP service provider’s network and effect a failover of traffic from one provider SBC to another. In the event of a WAN failure or complete SIP service provider outage, the Edgemarc can handle SIP traffic locally for the purposes of calling between local endpoints and calls between local endpoints and the PSTN via PRI or FXO lines.

Survivability provides the following capabilities:

• Availability—Detects when a SIP server is unreachable.
• Redundancy—
• Can switch to an alternate SIP server supplied by the provider.
• Makes and receives station-to-station calls while all provider SIP servers are unreachable.
• Makes and receives PSTN calls while all provider servers are unreachable.

The provider SIP server may be unreachable for the following reasons:

• The server is unresponsive to SIP Options messages or otherwise signaling is unavailable.
• A router on the path to the server is malfunctioning.
• Local access is malfunctioning.

How Survivability Works

The process of configuring the survivability function begins with providing a list of provider SBCs to the Edgemarc. The SBCs may manually provisioned or may be obtained via DNS lookup of an SRV record. The SBCs may be provisioned by IP address or FQDN. Endpoints obtained by SRV record lookup will be used according to the priority received in the SRV record. Calls will always be directed to the highest priority SBC unless that SBC is deemed to be out of service.

An SBC is considered available after it responds to several SIP Options messages sent by the Edgemarc. After the primary SBC is marked available, the Edgemarc will keep track of the status of the SBC by monitoring the progress of calls being processed to and from the SBC. If an abnormal SIP call is observed, the Edgemarc will switch call processing to either the next SBC in the configured list, or take local control of calls if no SBC is available.

SBCs other than the primary SBC are monitored by SIP options message exchanges.

The Edgemarc must go through survivability mode in order to recover from use of the secondary SBC to the primary SBC.

Survivability in a Hosted Environment

The EdgeMarc’s survivability function provides additional services in a hosted IP PBX environment.

The Edgemarc provides a local registration proxy function that allows it to build a list of known local SIP endpoints that it can use to route calls between LAN side phones or between phones and a locally attached PSTN connection in the event of a complete inability to reach the SIP provider. This function also enables several additional capabilities to be offered:

• The Edgemarc can rate pace SIP registration messages out to the SIP provider. The Edgemarc can also override registration timers to and from the service provider
• When the Edgemarc declares a softswitch out of service, the Edgemarc can force local IP phones to immediately re-register with their alternate softswitch
• The Edgemarc can override a phone’s softswitch configuration.

Survivability with a Legacy PBX

When the Edgemarc is used with a PBX that is connected via PRI, the Edgemarc may be optionally configured to disable the PRI in the event the SIP provider’s network becomes unreachable. Otherwise, the same services are provided by the Edgemarc as they are in a hosted IP PBX network.

A survivability license is only required for this feature in VOS version 15.8.4. It is not required in VOS version 16.3.

Routing Calls to the PSTN

The Edgemarc supports routing calls to PSTN connections via FXO ports or PRI when in survivability mode and the appropriate call routing constructs are configured. Emergency call preemption is supported. Basic inbound DID manipulation is also supported.

Configuration

The following tasks need to be completed:

1. Configure the list of SIP servers either manually or by specifying an FQDN for SRV lookup
2. Configure settings for entering and leaving survivability mode
3. For hosted environments, configure registration control settings

ISDN PRI and Survivability

Edgemarc models 4806, 4808, and 6000 may be configured with a PRI that provides support for a legacy PBX. When the Edgemarc is in local mode, this PRI may act in one of these two ways:

1. The EdgeMarc can route calls from the PBX to an FXO for emergency outbound calling. By default, calls may be received on an FXO port and forwarded to the PRI.
2. The EdgeMarc can shut down the PRI thus signaling to the PBX to use an alternate route. If this behavior is desired, then complete the below steps.

Configuring the PRI to follow the call control state of the Edgemarc

The ENABLE_PRI_DCHAN_SURV flag in the file /etc/config/sipua_pri_defs.conf controls how the PRI state follows the call control state of the Edgemarc. Possible values are off and on, with off being the default setting. If the parameter is not present in the file, the setting is set to off. In the “off” state, the PRI D channel will remain up regardless of whether the Edgemarc is in local or remote call control mode.

To change the behavior of the Edgemarc such that the PRI goes down when the Edgemarc enters local mode, set the value to “on” by completing the following steps:

1. Open an SSH session to the Edgemarc

2. Change to directories to the configuration directory:

   cd /etc/config

3. Display the file sipua_pri_defs.conf:
   

cat sipua_pri_defs.conf

       4. Check the file to see if a line PRI_DCHAN_SURV is present and note the value.

       5. If the line is not present, then run the command:

echo “ENABLE_PRI_CHAN_SURV=on” >> sipua_pri_defs.conf

       6. Save the changes:

cfg_commit

        7. Restart the UA services for the change to take effect.

killall ewua

         8. If parameter is already configured but not set to the desired value, then copy the contents of sipua_pri_defs.conf to a text editor, set the value, and replace the new contents on the EdgeMarc. Then save and restart the UA per the above steps.