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276 Glossary of Terms
Term Definition
program on a computer, NTP sends periodic time requests to servers, obtaining server
time stamps and using them to adjust the client's clock.
OFDM (Orthogonal Frequency Division Multiplexing) An FDM modulation technique for
transmitting large amounts of digital data over a radio wave. OFDM works by splitting
the radio signal into multiple smaller sub-signals that are then transmitted
simultaneously at different frequencies to the receiver. OFDM reduces the amount of
crosstalk in signal transmissions. 802.11a WLAN technology uses OFDM.
OSPF (Open Shortest Path First) This routing protocol was developed for IP networks based
on the shortest path first or link- state algorithm. Routers use link-state algorithms to
send routing information to all nodes on a network by calculating the shortest path to
each node based on a topography of the Internet constructed by each node. Routers
send that portion of the routing table (keeping track of routes to particular network
destinations) that describes the state of its own links, and it also sends the complete
routing structure (topography). The advantage of shortest path first algorithms is that
they result in smaller more frequent updates everywhere. They converge quickly, thus
preventing such problems as routing loops and count-to-infinity (when routers
continuously increment the hop count to a particular network). This makes for a stable
network. OSPF (version 2) is defined in RFC 1583 and is rapidly replacing RIP on the
Internet as the preferred routing protocol. See also, RFC and Router.
Packet How data is distributed over the Internet. A packet contains the source and destination
addresses, as well as the data. An ethernet packet is normally 1,518 bytes. In IP
networks, packets are often called datagrams. See also, Forwarding Rate, Packet
Switching Network, pps, and Throughput.
Packet Switching
Network Refers to protocols in which messages are divided into packets before they are sent.
Each packet is then transmitted individually and can even follow different routes to its
destination. Once all the packets forming a message arrive at its destination, they are
recompiled into the original message. Most modern Wide Area Network (WAN)
protocols, including TCP/IP, X.25, and Frame Relay, are based on packet-switching
technologies. By contrast, normal telephone services use a circuit-switching technology
in which a dedicated line is allocated for transmission between two parties. Circuit-
switching is ideal for fast data transmissions where the data must arrive in the same
order in which it is sent. This is the case with most real-time data, such as live audio and
video. Packet switching is more efficient and robust for data that can withstand some
delays in transmission, such as e-mail messages and Web pages. See also, Forwarding
Rate, Packet, pps, and Throughput.
PDF (Portable Document Format) A type of file format developed by Adobe Systems© that
displays documents identically on any computer system. PDF files retain their original
formatted design, unlike HTML documents which adjust the format depending on the
users viewing medium (for example, monitor size).
Ping (Packet INternet Groper) A program that transmits a signal to a host and expects a
response within a predetermined time. This is useful when troubleshooting network
transmission problems. See also, ICMP.
Portal A portal is a Web site. The portal consists of a collection of links to the most popular
Web services on the Internet. Generally speaking, a portal is a door to the Internet. See
also, Internet.
PPP (Point-to-Point Protocol) PPP has superseded SLIP as the standard protocol for serial
data communications over the Internet. See also, SLIP.