ENUM (Electronic Number Mapping) is a DNS-based architecture and protocol for mapping an ITU-T (International Telecommunications Union, Telecommunications Standards Section) recommendation E.164 telephone number into an URI.
A URI is a string of characters that identifies resources such as documents, images, files, databases, e-mail addresses or other resources or services in a common structured format.
The ENUM mechanism converts telephone numbers into service specific URIs.
The operator ENUM feature provides support for the DNS-based architecture and protocol for mapping an E.164-compliant number into a service-specific URI, which is dictated by IETF RFC 3761. This mechanism is only applicable for interswitch connection with SIP gateways.
Lotus Sametime Unified Telephony uses this mechanism to route calls to other networks through IP-based means instead of through the PSTN (Public Switched Telephone Network). Up to six ENUM server pairs and at least 100 simultaneous queries are supported. Each server pair can contain a secondary server, which provides a failover mechanism if the primary server cannot process a query.
The administrator defines the following:
The primary ENUM server and an optional secondary server
- The default tier 0 zone, along with information about the primary ENUM server
Whether ENUM routing is always performed, or if it is performed only to registered endpoints
An ENUM query is launched when one of the following takes place:
An E.164 route marked as an ENUM route is reached.
The office classmark indicates that Lotus Sametime Unified Telephony must perform ENUM queries on all calls to non-hosted subscribers.
The query resolves the number into a sorted list of URIs.
System specific information
In this scenario, an enterprise might own three Lotus Sametime Unified Telephony systems with on-net subscribers on each, and has set up the office classmark to perform ENUM queries on all calls. Whenever a subscriber dials a telephone number that is not on the same Lotus Sametime Unified Telephony system, Lotus Sametime Unified Telephony performs an ENUM query to determine if the call can be routed via an IP network — in this case, to a subscriber on one of their other Lotus Sametime Unified Telephony systems — or if it has to be delivered to the PSTN.
If the primary server does not respond to repeated ENUM queries, all subsequent queries are sent to the secondary server until the primary server becomes available again. Alarming is provided when an ENUM server is not responding to queries, and when a complete ENUM server pair is not responding.
For calls involving ENUM server transactions, additional time, in the order of milliseconds, will be consumed for each ENUM server transaction, thereby increasing the call processing time.
Lotus Sametime Unified Telephony does not supply an ENUM server product. If the network designer elects to use the ENUM feature, the designer must ensure that a DNS ENUM server is available with adequate capacity and fast enough response time. Round-trip delay, from query to response, should be under 100 milliseconds. Longer response times can result in call setup delay and lost calls. The designer should also define the Lotus Sametime Unified Telephony routing database in a way that ensures that outbound calls can continue to be routed if an ENUM server fails.
The following is an example of a simple SIP call flow that includes an ENUM query:
Figure 1. Simple SIP Call Flow with ENUM Query
- User A dials user B, who is not located on the same Lotus Sametime Unified Telephony hosting user A.
- Lotus Sametime Unified Telephony launches a query to the ENUM server to determine how to route the call to user B.
- The ENUM server returns the Naming Authority Pointer (NAPTR) record(s) that contains the routing to user B. If the server cannot return the applicable NAPTR record, it can return an indication that vacant code treatment is to be provided.
- Lotus Sametime Unified Telephony determines that it can route the call on-net (and therefore does not need to go into the PSTN), and sends an INVITE request to the proxy for user B.
- The far-end switch (most likely another Lotus Sametime Unified Telephony system) presents the call to user B.
How to Configure Office-wide ENUM SettingsParent topic: Numbering Plan
This feature specifies whether or not ENUM queries are performed only on specific configured number ranges (checkbox unselected) or on all Off-net numbers (checkbox selected).