CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 104 potx

Glossary of Key Terms 999
plenum An open ceiling that acts as an air-return duct for heating, ventilation, and
air-conditioning systems.
plug-in Software or a program that can easily be installed and used as part of a web
browser.
PM (phase modulation) Modulates the polarity (phase) of the wave.
poison reverse updates Updates that are necessary to defeat larger routing loops.
Generally speaking, increases in routing metrics indicate routing loops. Poison reverse
updates are then sent to remove the route and place it in holddown.
port In IP terminology, an upper-layer process that receives information from lower
layers. Ports are numbered, and many are associated with a specific process. For example,
SMTP is associated with port 25. A port number of this type is called a well-known
port or address.
power cord Connects an electrical device to an electrical outlet to provide power to
the device.
power supply Supplies power to a computer.
power-on self-test (POST) Set of hardware diagnostics that runs on a hardware device
when that device is powered up.
presentation layer Layer 6 of the OSI reference model. This layer ensures that infor-
mation sent by the application layer of one system can be read by the application layer
of another.
printed circuit board (PCB) A thin plate on which chips (integrated circuits) and other
electronic components are placed.
privileged mode Used for copying and managing entire configuration files.
propagation delay A simple measurement of how long it takes for a signal to travel
along the cable being tested.
protocol stack A set of related communications protocols that operate together and,
as a group, address communication at some or all of the seven layers of the OSI reference
model. Not every protocol stack covers each layer of the model, and often a single pro-
tocol in the stack addresses a number of layers at once. TCP/IP is a typical protocol
stack.

protocol suite A set of related communications protocols that operate together and,
as a group, address communication at some or all of the seven layers of the OSI refer-
ence model. Not every protocol stack covers each layer of the model, and often a single
protocol in the suite addresses a number of layers at once. TCP/IP is a typical protocol
suite.
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1000 Appendix B: Glossary of Key Terms
protocol A formal description of a set of rules and conventions that govern how
devices on a network exchange information.
PSELFEXT (power sum equal-level far-end crosstalk (PSELFEXT) The combined
effect of ELFEXT from all wire pairs.
PSNEXT (power sum near-end crosstalk) Measures the cumulative effect of NEXT
from all wire pairs in the cable.
pulse Pulses determine the value of the data being transmitted. If a disturbance is
deliberately caused and involves a fixed, predictable duration, it is called a pulse.
queuing A process by which ACLs can designate certain packets to be processed by
a router before other traffic, on the basis of some configurable parameter such as spec-
ifying a protocol.
raceway Channels that contain cables in an installation. Raceways include common
electrical conduit, specialized cable trays or ladder racks, in-floor duct systems, and
plastic or metal surface-mounted raceways.
RAM (random-access memory) Volatile memory that can be read and written by a
microprocessor.
random-access memory (RAM) Also known as read-write memory. Can have new
data written to it as well as stored data read from it.
range error A frame that had a legal-size value in the Length field but that did not
match the actual number of octets counted in the Data field of the received frame.
RARP (Reverse Address Resolution Protocol) Protocol in the TCP/IP stack that provides
a method for finding IP addresses based on MAC addresses.
RCP (remote copy protocol) Protocol that allows users to copy files to and from a file

system residing on a remote host or server on the network.
read-only memory (ROM) A type of computer memory in which data has been
prerecorded.
reflection Reflection is the photons of light striking a surface and leaving that surface
in an equal but opposite direction.
refraction Refraction is the change in direction of a beam of light when it enters
another medium.
repeater A networking device that exists at Layer 1, the physical layer, of the OSI
reference model. The purpose of a repeater is to regenerate and retime network signals
at the bit level, allowing them to travel a longer distance on the medium.
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Glossary of Key Terms 1001
resistance The property of a material that resists electron movement.
RFI (radio frequency interference) Noise from other signals being transmitted
nearby.
ring topology A topology in which hosts are connected in the form of a ring or circle.
Unlike the physical bus topology, the ring topology has no beginning or end that needs
to be terminated.
RIP (Routing Information Protocol) An IGP supplied with UNIX BSD systems. The
most common IGP in the Internet. RIP uses hop count as a routing metric.
RJ-45 A connector commonly used for finishing a twisted-pair cable.
routed protocol A protocol that can be routed by a router. A router must be capable
of interpreting the logical internetwork as specified by that routed protocol. Examples
of routed protocols are AppleTalk, IPX, and IP.
router A network layer device that uses one or more metrics to determine the optimal
path along which network traffic should be forwarded. Routers forward packets from
one network to another based on network-layer information contained in routing updates.
Occasionally called a gateway (although this definition of gateway is becoming
increasingly outdated).
routing The process of finding a path to a destination host. Routing is very complex

in large networks because of the many potential intermediate destinations that a
packet might traverse before reaching its destination host.
Routing Information Protocol (RIP) An IGP supplied with UNIX BSD systems. The
most common IGP in the Internet. RIP uses hop count as a routing metric.
routing metric A method by which a routing algorithm determines that one route is
better than another. This information is stored in routing tables and is sent in routing
updates. Metrics include bandwidth, communication cost, delay, hop count, load,
MTU, path cost, and reliability. Sometimes simply called a metric.
routing protocol A protocol that accomplishes routing through the implementation
of a specific routing algorithm. Examples of routing protocols are IGRP, OSPF, and RIP.
routing table A table stored in a router or some other internetworking device that
keeps track of routes to particular network destinations and, in some cases, metrics
associated with those routes.
routing update A message sent from a router to indicate network reachability and
associated cost information. Routing updates are typically sent at regular intervals
and after a change in network topology. Compare with flash update.
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1002 Appendix B: Glossary of Key Terms
SCS (structured cabling system) Integrated cable plans that include the connectors,
wiring, distribution centers, and installation techniques developed by specific companies.
segment A section of a network that is bounded by bridges, routers, or switches.
segment In the TCP specification, a logical information group at transport layers of
the OSI reference model.
serial port Can be used for serial communication in which only 1 bit is transmitted at
a time.
session layer Layer 5 of the OSI reference model. This layer establishes, manages,
and terminates sessions between applications and manages data exchange between
presentation layer entities.
shortest path first (SPF) algorithm A calculation performed on the database resulting
in the SPF tree.

simplex The capability for transmission in only one direction between a sending
station and a receiving station. Broadcast television is an example of a simplex
technology.
sine waves Graphs of mathematical functions of many natural occurrences that
change regularly over time, such as the distance from the earth to the sun, the distance
from the ground while riding a Ferris wheel, and the time of day that the sun rises.
single-mode Single-mode is an optical fiber that has only one mode of light transmis-
sion. Contrast with multimode.
SNAP (Subnetwork Access Protocol) Internet protocol that operates between a net-
work entity in the subnetwork and a network entity in the end system. SNAP specifies
a standard method of encapsulating IP datagrams and ARP messages on IEEE networks.
The SNAP entity in the end system makes use of the services of the subnetwork and
performs three key functions: data transfer, connection management, and QoS selection.
SNMP (Simple Network Management Protocol) A network-management protocol
used almost exclusively in TCP/IP networks. SNMP provides a means of monitoring
and controlling network devices and managing configurations, statistics collection,
performance, and security.
SNR (signal-to-noise ratio) The ratio of useable signal being transmitted to the
undesired signal (noise). It is a measure of transmission quality. The ratio of good
data (signal) to bad (noise) on a line, expressed in decibels (dB).
sound card An expansion board that handles all sound functions.
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Glossary of Key Terms 1003
spectrum analyzer An electronic device that creates graphs for frequency-domain
analysis. Engineers also use frequency-domain analysis to study signals.
split horizon A routing technique in which information about routes is prevented
from exiting the router interface through which that information was received. Split-
horizon updates are useful in preventing routing loops.
square waves Graphs that do not continuously vary with time. The values remain the
same for some time, then suddenly change, then remain the same, and then suddenly

return to the initial value.
SS (spread spectrum) A modulation technique developed in the 1940s that spreads a
transmission signal over a broad band of radio frequencies. The term spread spectrum
describes a modulation technique that sacrifices bandwidth to gain signal-to-noise
performance.
standard A set of rules or procedures that are either widely used or officially specified.
standard ACL ACL that compares source IP addresses to the rules defining a stan-
dard ACL.
star topology The most commonly used physical topology in Ethernet LANs. The
star topology is made up of a central connection point that is a device such as a hub,
switch, or router, where all the cabling segments meet.
static routing The process of manually defining and configuring the routes.
storage-area network (SAN) A dedicated, high-performance network that moves
data between servers and storage resources.
store-and-forward switching A packet-switching technique in which frames are pro-
cessed completely before being forwarded out the appropriate port. This processing
includes calculating the CRC and checking the destination address. In addition, frames
must be stored temporarily until network resources (such as an unused link) are avail-
able to forward the message.
STP (shielded twisted-pair) STP is a two-pair wiring medium used in a variety of
network implementations. STP cabling has a layer of shielded insulation to reduce EMI.
STP (Spanning Tree Protocol) Bridge protocol that uses the spanning tree algorithm,
enabling a learning bridge dynamically to work around loops in a network topology
by creating a spanning tree. Bridge exchange BPDU messages with other bridges to
detect loops and then remove the loops by shutting down selected bridge interfaces.
straight-through cable A cable that maintains the pin connection all the way through
the cable. Thus, the wire connected to pin 1 is the same on both ends of the cable.
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1004 Appendix B: Glossary of Key Terms
structured cabling A uniform cabling system with standards that define the actual

cable, cabling distances, type of cable, and type of terminating devices.
stub network An OSPF area that carries a default route, intra-area routes, and inter-
area routes, but that does not carry external routes.
subnet address A portion of an IP address that is specified as the subnetwork by the
subnet mask.
subnet mask A 32-bit address mask used in IP to indicate the bits of an IP address
that are used for the subnet address. Sometimes simply called a mask.
subnetting The method of dividing full network address classes into smaller pieces.
This has prevented complete IP address exhaustion.
subnetwork mask A 32-bit address mask used in IP to indicate the bits of an IP
address that are being used for the subnet address.
subnetwork 1. In IP networks, a network sharing a particular subnet address. Sub-
networks are networks arbitrarily segmented by a network administrator to provide
a multilevel, hierarchical routing structure while shielding the subnetwork from the
addressing complexity of attached networks. Sometimes called a subnet. 2. In OSI net-
works, a collection of ESs and ISs under the control of a single administrative domain
and using a single network access protocol.
switch A device that connects LAN segments, uses a table of MAC addresses to
determine the segment on which a frame needs to be transmitted, and reduces traffic.
Switches operate at much higher speeds than bridges.
switch Network device that filters, forwards, and floods frames based on the destina-
tion address of each frame. The switch operates at the data link layer of the OSI model.
system routes Routes to networks within an autonomous system. Cisco IOS Software
derives system routes from directly connected network interfaces and system route
information provided by other IGRP-speaking routers or access servers. System routes
do not include subnet information.
system unit The main component of a PC system.
TB (terabyte) Approximately 1 trillion bytes. Hard drive capacity on some high-end
computers is measured in TB.
Tbps (terabits per second) One trillion bits per second. A standard measurement of

the amount of data transferred over a network connection. Some high-speed core
Internet routers and switches operate at more than Tbps.
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Glossary of Key Terms 1005
TCP (Transmission Control Protocol) A connection-oriented transport-layer protocol
that provides reliable full-duplex data transmission. TCP is part of the TCP/IP proto-
col stack.
TCP (Transmission Control Protocol) Connection-oriented transport layer protocol
that provides reliable full-duplex data transmission. TCP is part of the TCP/IP proto-
col stack.
TCP/IP (Transmission Control Protocol/Internet Protocol) Common name for the
suite of protocols developed by the U.S. Department of Defense in the 1970s to sup-
port the construction of worldwide internetworks. TCP and IP are the two best-known
protocols in the suite.
TDR (time domain reflectometer) A device that can measure the lengths of cables
that are installed and locate faults. It does these tasks by sending a signal down a cable
and then measuring the amount of time that it takes for that signal to return.
Telnet A standard terminal emulation protocol in the TCP/IP protocol stack. Telnet
is used for remote terminal connection, enabling users to log in to remote systems and
use resources as if they were connected to a local system. Telnet is defined in RFC 854.
TFTP (Trivial File Transfer Protocol) Simplified version of FTP that allows files to be
transferred from one computer to another over a network, usually without the use of
client authentication (for example, username and password).
thicknet Thicknet is an early form of coaxial cable using 10BASE5 for networking.
Thicknet was once desirable because it could carry signals up to 500 meters.
Thinnet Thinnet is a simple, thin, coaxial network cable for the 10BASE2 system.
Thinnet can carry a signal only 185 meters, but was much easier to work with than
thicknet.
three-way handshake A sequence of messages exchanged between two or more net-
work devices to ensure transmission synchronization before sending user data.

throughput The rate of information arriving at or passing through a particular point
in a network system.
TIA (Telecommunications Industry Association) TIA is a standards association that
publishes standards for telecommunications.
TIA/EIA-568-B standard Specifies ten tests that a copper cable must pass if it is used
for modern, high-speed Ethernet LANs.
TLVs (Type Length Values) Blocks of information embedded in CDP advertisements.
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1006 Appendix B: Glossary of Key Terms
token passing An access method by which network devices access the physical
medium in an orderly fashion based on possession of a small frame called a token.
Token Ring A token-passing LAN developed and supported by IBM. Token Ring
runs at 4 or 16 Mbps over a ring topology.
topological database A collection of information gathered from LSAs.
TR (telecommunications room) The area within a building that houses the telecom-
munications cabling system equipment.
traceroute Program available on many systems that traces the path a packet takes to
a destination. It is used mostly to debug routing problems between hosts.
trailer Control information appended to data when encapsulating the data for net-
work transmission.
Transmission Control Protocol/Internet Protocol (TCP/IP) A common name for the
suite of protocols developed by the U.S. DoD in the 1970s to support the construction
of worldwide internetworks. TCP and IP are the two best-known protocols in the suite.
transport layer Layer 4 of the OSI reference model. This layer is responsible for reli-
able network communication between end nodes. The transport layer provides mecha-
nisms to establish, maintain, and terminate virtual circuits, transport fault detection and
recovery, and information flow control.
triggered update A triggered update is an update that is sent without waiting for the
update timer to expire.
UDP (User Datagram Protocol) A connectionless transport layer protocol in the

TCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams without
acknowledgments or guaranteed delivery, requiring that error processing and retrans-
mission be handled by other protocols. UDP is defined in RFC 768.
unicast Message sent to a single network destination.
universal serial bus (USB) port Lets peripheral devices such as mice, modems, key-
boards, scanners, and printers be plugged in and unplugged without resetting the system.
update timer Specifies how frequently routing update messages are sent. The IGRP
default for this variable is 90 seconds.
uptime The amount of time that the device is functional and in service relative to the
users’ requirements for its availability.
UTP (unshielded twisted-pair) UTP is a four-pair wire medium used in a variety of
networks.
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Glossary of Key Terms 1007
vertical cabling Backbone cables, intermediate and main cross-connects, mechanical
terminations, and patch cords or jumpers used for backbone-to-backbone cross-
connections.
video card A board that plugs into a PC to give it display capabilities.
virtual private network (VPN) A private network constructed within a public network
infrastructure such as the global Internet.
WAN (wide-area network) A data communications network spanning a large geo-
graphic area, such as a state, province or country. WANs often use transmission facili-
ties provided by common carriers, for example, telephone companies.
wave Energy traveling from one place to another.
wavelength The length of a wave measured from any point on one wave to the corre-
sponding point on the next wave. The wavelength of light is usually measured in
nanometers (nm).
WDM (wavelength-division multiplexing) Multiple optical wavelength can share
the same transmission fiber. The spectrum occupied by each channel must be separated
adequately from the other.

web browser A graphical user interface (GUI)-based hypertext client application, such
as Internet Explorer or Netscape Navigator, used to access hypertext documents and
other services located on remote servers throughout the WWW and the Internet.
well-known ports These ports are defined in RFC 1700 and are reserved in both TCP
and UDP. These well-known ports can identify applications that run above the trans-
port layer protocols.
WEP (wired equivalent privacy) A security mechanism, defined within the 802.11
standard, that is designed to protect the over-the-air transmission between wireless
LAN access points and NICs.
white noise Noise that affects all transmission frequencies equally.
wide-area network (WAN) A data communications network that serves users across
a broad geographic area and often uses transmission devices provided by common
carriers.
windowing A flow-control mechanism requiring that the source device receive an
acknowledgment from the destination after transmitting a certain amount of data.
work area A space where computers, telephones, and other network equipment is used.
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