Blocks of phone numbers may be allocated to specific SIP service providers
or to IP PBXs accessible from the Internet. This may have wide-ranging regu-
latory and business implications for the deployment of ENUM that are, how-
ever, beyond the scope of this book.
Call Setup Delay
Note the large number of DNS lookups required (1) for registration and (2) for
finding the remote SIP server. The DNS lookup can be an important factor in
long call setup delays.
DNS-Based Routing Service Using SIP
There are some problems with personal addresses. With the proliferation of
communication services provided by the PSTN, wireless phone systems, and
various Internet services, it becomes quite difficult to track the increasing
number of addresses for contacts of people we are interested in. The frequent
changes of IP addresses by many Internet access service providers make this
problem even harder.
Contacting a called party is a difficult problem because of the number of
communication devices (such as: home PCs and home faxes, palmtop com-
puters, laptops, office PCs and faxes, pagers, cell phones, IP printers, vehicles,
boats, and so on). It is thus possible to require a choice among up to 10 devices
or more, to reach the called party.
Users may also want to indicate temporary contact addresses, such as the
phone number of a secretary, restaurant, or a hotel when traveling. Besides the
contact address, users may also want to indicate the mode by which they are
to be contacted. A user attending a meeting may indicate, for example, that
text chat is appropriate to convey urgent messages in a nonintrusive manner.
SIP URI or Telephone Number?
A SIP URI in the form of sip:[email protected] a very powerful
single address, since it can support user mobility (see Chapter 15) as well as
user preferences.
In the transition period where most users still have PSTN and/or mobile
telephone service, another solution is to provide an E.164 phone number as the
single contact address on a business card. When a PSTN telephone number
includes a country code and conforms to certain rules, it is known as an E.164
number, which refers to the ITU-T document that describes telephone num-
ber’s structure. Phone numbers, especially the home phone number or the
mobile phone number, change less frequently and are, thus, well suited to be
stable contact addresses.
DNS and ENUM 67