G
LOSSARY AND
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BBREVIATIONS
8030HEPTA/NCD Network Clock Device - V04.00
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hopf
Elektronik GmbH
Nottebohmstr. 41
• D-58511 Lüdenscheid • Tel.: +49 (0)2351 9386-86 • Fax: +49 (0)2351 9386-93 • Internet: http://www.hopf.com • E-Mail: [email protected]
14.3 Definitions
An explanation of the terms used in this document.
14.3.1 DHCP (Dynamic Host Configuration Protocol)
DHCP makes it possible to integrate a new computer into an existing network with no addi-
tional configuration. It is only necessary to set the automatic reference of the IP address on
the client. Without DHCP, relatively complex settings need to be made. Beside the IP address,
further parameters such as network mask, gateway and DNS server have to be entered. A
DHCP server can assign these parameters automatically by DHCP when starting a new com-
puter (DHCP client).
DHCP is an extension of the BOOTP protocol. A valid IP address is allocated automatically if
a DHCP server is available on the network and DHCP is enabled.
See RFC 2131 Dynamic Host Configuration Protocol for further infor-
mation.
14.3.2 NTP (Network Time Protocol)
Network Time Protocol (NTP) is a standard for the synchronization of clocks in computer sys-
tems via packet-based communication networks. Although it is processed mainly over UDP, it
can also be transported by other layer 4 protocols such as TCP. It was specially developed to
facilitate reliable timing via networks with variable packet runtime.
NTP uses the Marzullo algorithm (devised by Keith Marzullo of the San Diego University in his
dissertation) with a UTC timescale and supports leap seconds from Version 4.0. NTP. It is one
of the oldest TCP/IP protocols still in use. It was developed by David Mills of the University of
Delaware and published in 1985. The protocol and UNIX implementation continue to be de-
veloped under his direction. Version 4 is the up to date version of the protocol. This uses UDP
Port 123.
NTPv4 can maintain the local time of a system to an accuracy of some 10 milliseconds via the
public Internet. Accuracies of 500 microseconds and better are possible under ideal conditions
on local networks.
With a sufficiently stable, local clock generator (oven-stabilised quartz, rubidium oscillator,
etc.) and using the kernel PLL (see above), the phase error between reference clock generator
and local clock can be reduced to something of the order of a few hundred microseconds. NTP
automatically compensates for the drift of the local clock.
NTP can be installed over firewalls and offers a range of security functions.
See RFC 5905 for further information.
