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Critical Lines are phone lines which are maintained even when there are disruptions to the local electrical power, the local network, the University network or the University telephony system. They have four hours of battery backup, and will fail when the battery runs down. There is an SLA of  4 hours on the kit supported by ATOS, not the cabling. If the fault is proved to the cabling we will enagage A&M Comms who are our cabling contractors.There is no formal SLA for their attendance but if the unit or College would like next day attendance we can invoke a callout. This is chargeable at £290.40 with a further hourly rate of £36.30. The unit or College must agree to pay the charges before we will call them out.

Each branch location has 2 BT lines for outgoing calls, which critical lines use during outages.  If both lines are in use, the outgoing call will get a busy signal. During normal operation they can receive incoming calls as well.

There are several scenarios for failure of the Chorus system.

1) Local failure of power or network in a unit

All of the VoIP phones in a unit will fail.  The critical lines will dial out over the copper lines and then BT lines. 

Calls can be made outbound from a critical line, but it is not possible to receive calls inbound.  If the branch room (where the critical line equipment lives) is on the same site and has the same power supply, the critical line may suffer a power outage after 4 hours (when the UPS runs down).  However, a divert can be placed on the Chorus system (manually at the time, or automatically when a particular phone is offline) to a mobile or other non-Chorus phone line.

It is also possible to divert calls at the telephony carrier level (Gamma), although internet connectivity is required to make changes to this at the time.  This is a chargeable option, please request by emailing chorus@it.ox.ac.uk.

Anyone using the Chorus client on a smartphone (OSMO) would still be able to make and receive calls, as long as they had wifi coverage or were using cellular coverage, as connectivity is direct to the central system.

Recommendations for mitigation:

Set up important phones such as reception or lodge lines to divert to a mobile or a separate BT line in the case of phone failure or power outage. This is called a dependable divert, and can be requested by emailing chorus@it.ox.ac.uk.  Hunt groups can make this more flexible as new phones/numbers can be added easily via Phoneman.

If a large number of calls are expected, set up a contact centre which can play messages and manage calls in a queue.

2) Failure of the entire Chorus system (e.g., failure of both datacentres or data corruption)

All of the VoIP phones in a unit will fail. The critical lines will dial out over the copper and BT lines.

In this failure scenario, while calls can be made outbound from a critical line, it is not possible to receive calls inbound. Also, any Chorus diverts will fail, the only diversion that will succeed is one placed at the telephony carrier level (Gamma), which is a chargeable option, please request by emailing chorus@it.ox.ac.uk.  Internet connectivity is required to make changes to this at the time.

The Chorus client on a smartphone will fail, as there is no connectivity to the central system.

Recommendations for mitigation:

Set up important numbers at the telephony carrier level (Gamma) to divert to mobiles or a separate BT line on failure or manually (both are possible).  This is a chargeable option, please request by emailing chorus@it.ox.ac.uk.

3) Failure of the Janet internet connection

The Chorus system has backup ISDN lines, so although existing calls will be disconnected, new calls will be able to be made after a short delay.

Recommendations for mitigation:

Set up important numbers at the telephony carrier level (Gamma) to divert to mobiles on failure or manually (both are possible). This is a chargeable option, please request by emailing chorus@it.ox.ac.uk.

 

To request a critical line, please use the critical line service request.

 

See information about analogue lines which don't need to critical.

Written by IT Services. Latest revision 14 November 2018