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Alcatel-Lucent | ENUM Use and Management for the Successful Deployment of ENUM-Enabled Services
For the case where an IMS subscriber calls a PSTN subscriber, the main steps to setup and route the
call are depicted in the figure below.
Use of ENUM by VoIP Peering Partners
Location of DNS Records for VoIP Peering Partners
Service providers may choose to partner together and interconnect their VoIP networks in order to
bypass the PSTN. In this case, peering using ENUM and DNS can be done between partners, and
DNS records that will be used by partners can be located in DNS servers in various locations. The
main records of interest here are the NAPTR records that map E.164 numbers to SIP addresses of
partners’ session boarder controllers (SBCs) used to access their networks, and address records that
map the host portion of SIP addresses of partners’ SBCs to their IP addresses. Three possible locations
for these two types of records are as follows:
Option 1: In DNS servers located in a centralized VoIP services data center that can be queried by
all partners
Option 2: In DNS servers located on the premises of the service provider that owns the SBC, with the
centralized DNS servers delegating authority for domains to these servers
Option 3: In centralized master DNS servers, and in slave DNS servers located on partners’ premises
Since the location of the NAPTR and address records is independent, nine combinations exist here.
A key consideration here is the ability to quickly access the records so as to minimize call/session
setup time. With this criterion, Option 3 is the best option, because a DNS server on an partner’s
premises will be able to resolve client queries from that partner, and will not need to send queries
across the partner’s network and a backbone network to get to another DNS server. However, this
option has the disadvantages of having the most zone update traffic and the longest zone propagation
update times during provisioning.
Figure 3: IMS Subscriber-to-PSTN Subscriber Call
In this case, the following steps are carried
out to setup and route the call.
1. User of SIP Client A calls E.164 number of
1-222-333-5555, which is on the PSTN.
2. The S-CSCF communicates with the
Application Server to provide subscriber services.
3. The S-CSCF queries the DNS/ENUM server with
the E.164 number, and it does not return a SIP
URI, since the server does not contain a NAPTR
record for this PSTN number.
4. The call is routed to a PSTN Gateway.
5. The call is routed through the PSTN network
to
1-222-333-5555.
