Information Technology Reference
In-Depth Information
Implementing IP Phones in CUCM
The implementation of IP Phones is remarkably simple, considering the myriad of serv-
ices, protocols, and processes going on in the background to make the system work well.
This section reviews these “hidden” processes and details some of the administrative tasks
required to easily and reliably run IP Phones in CUCM.
Special Functions and Services Used by IP Phones
A variety of standards-based and proprietary protocols and services support IP Phones in
CUCM. In no particular order, they include the following:
Network Time Protocol (NTP)
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Cisco Discovery Protocol (CDP)
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Dynamic Host Configuration Protocol (DHCP)
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Power over Ethernet (PoE)
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Trivial File Transfer Protocol (TFTP)
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Domain Name System (DNS)
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The next section describes each of these services, how IP Phones use them, and how to
configure them in CUCM (or other systems as appropriate).
NTP
NTP is an IP standard that provides network-based time synchronization. There are many
good reasons to use NTP beyond the convenience and consistency of having the same
time on all devices. Call Detail Records (CDR) and Call Management Records (CMR) are
time stamped, as are log files. Comparing sequential events across multiple platforms is
much simpler and easier to understand if the relative time is exactly the same on all those
devices. Some functions and features can also be time (calendar) based, too, so time syn-
chronization is important for those functions to operate properly.
In a typical NTP implementation, a corporate router synchronizes its clock with an Inter-
net time server (such as a NIST atomic clock or a GPS satellite clock). Other devices in the
corporate network then sync to the router.
The Cisco Unified Communications Manager (CUCM) Publisher is one such device; dur-
ing installation, CUCM asks for the IP address of an NTP server. (Alternatively, it can use
its internal clock, which is not recommended because of its inaccuracy compared to NTP.)
The Subscriber servers then sync their clocks to the Publisher, and the IP Phones get their
time from their Subscribers via Skinny Client Control Protocol (SCCP) messages. Session
Initiation Protocol (SIP) phones need an NTP reference (detailed later), but in the absence
of one, they can get the time from the time stamp in the SIP OK response from the Sub-
scriber server.
