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Clock synchronization is important in VoIP systems for accurate Call Detail Records (used for billing), easier troubleshooting
and debugging, and for good voice performance. Network Time Protocol (NTP) is used on all Cisco devices
to sync the system clock to a master clock. IP Phones get their time from the call agent (CM, CM Business Edition, CM
Express, or SBCS). The call agent(s) are configured to get their time from a master clock, usually a highly accurate
atomic or radio clock external to the network.
R o u t e r ( c o n f i g ) # c l o c k timezone pst -8
! S p e c i f i e s the l o c a l timezone as PST (8 hours behind GMT)
R o u t e r ( c o n f i g ) # c l o c k summer-time zone r e c u r r i n g f i r s t Sunday a p r i l 02:00 l a s t Sunday October 02:00
! A c t i v a t e s Summer Time change in the s p e c i f i e d date range
!
R o u t e r ( c o n f i g ) # n t p server 10.1.2.3
! I d e n t i f i e s the NTP master clock address
© 2008 Cisco Systems Inc. All rights reserved. This publication is protected by copyright. Please see page 147 for more details.
© 2008 Cisco Systems Inc. All rights reserved. This publication is protected by copyright. Please see page 147 for more details.
Cisco IP Phone Firmware and XML Configuration Files
Cisco IP Phones need the following three separate files to function:
• The firmware file: This file is loaded into nonvolatile memory and is persistent across reboots. To make the
firmware files available to the phones, use the router command tftp-server flash:firmware-file-name. The command
load phone-type firmware-file is also required to associate the model of IP phone with the appropriate firmware file.
• SEPAAAABBBBCCCC.cnf.xml: This is the device-specific XML configuration file (AAAABBBBCCCC is the
MAC address of the phone), which specifies the IP address, port, firmware, locale, directory URL, and many other
pieces of information. This file is created when the IP Phone has been added to the configuration.
• XMLDefault.cnf.xml: This is the XML configuration file that devices use if their specific SEP<MAC> file is not
available (typically if they have not registered before or if they have been factory reset).
These files are downloaded by the phone during its boot process.
Power over Ethernet
Power over Ethernet (PoE) is a desirable option because it eliminates the cost and clutter of power bricks for the IP
Phones. There are two methods of PoE delivery:
• Cisco prestandard (inline power)
• 802.3af standard
Extra care should be taken to ensure the following:
• RJ-45 cabling is tested and meets the required standard.
• The IP Phone and the switch have a common PoE delivery method.
• The PoE switch has a suitable UPS backup to provide power continuance in the event of a power failure.
© 2008 Cisco Systems Inc. All rights reserved. This publication is protected by copyright. Please see page 147 for more details.
Alternatively, each IP Phone may be powered by its own cable and transformer, or a variety of power injectors are available.
The Cisco prestandard PoE method works as follows:
1. The switch sends a special tone, called a Fast Link Pulse (FLP), out of the port. The FLP goes to the powered
device, in this case an IP phone.
2. When unpowered, the PoE device has a physical link between the pin on which the FLP arrives and a pin that goes
back to the switch. This creates a circuit, resulting in the FLP arriving back at the switch. Non-PoE devices will not
have this link; the switch will therefore never receive the FLP from a device that does not require PoE.
3. When the switch receives the returning FLP, it applies power to the line.
4. The link comes up within 5 seconds.
5. The powered device (IP phone) boots.
6. Using CDP, the IP Phone tells the switch exactly how much power it needs. (Power requirements vary from device
to device.)
The 802.3af PoE standard works slightly differently. The standard requires that all eight pins in the RJ-45 cable be
present and punched down. The following describes the 802.3af PoE negotiation steps:
1. The switch applies constant DC power to all ports that may require PoE.
2. An 802.3af-compliant device will apply 25 ohms resistance across the DC circuit.
3. The switch detects the resistance and applies low-power PoE to the link.
4. The powered device (the phone) boots.
5. The phone uses CDP to specify its power needs.