Glossary of Key Terms 989
Flash memory A special type of EEPROM that can be erased and reprogrammed in
blocks instead of one byte at a time. Many modern PCs have their BIOS stored on a
Flash memory chip so that it can be updated easily if necessary. Such a BIOS is some-
times called a Flash BIOS. Flash memory is also popular in modems because it enables
the modem manufacturer to support new protocols as they become standardized.
flooding A traffic-passing technique used by switches and bridges in which traffic
received on an interface is sent out all that device’s interfaces except the interface on
which the information was received originally.
floppy disk drive Reads and writes to floppy disks.
flow control A technique for ensuring that a transmitting entity does not overwhelm
a receiving entity with data. When the buffers on the receiving device are full, a message
is sent to the sending device to suspend the transmission until the data in the buffers
has been processed. In IBM networks, this technique is called pacing.
flush timer Indicates how much time passes before a route is flushed from the routing
table. The IGRP default is seven times the routing update timer.
FM (frequency modulation) Modulates the frequency of the wave.
fragment-free switching Switching that filters out collision fragments, which are the
majority of packet errors, before forwarding begins.
frame A logical grouping of information sent as a data link layer unit over a trans-
mission medium.
frequency The amount of time between each wave.
FTP (File Transfer Protocol) An application protocol, part of the TCP/IP protocol
stack, used for transferring files between network nodes. FTP is defined in RFC 959.
full duplex The capability for simultaneous data transmission between a sending
station and a receiving station.
full-mesh topology Connects all devices (nodes) to each other for redundancy and
fault tolerance.
GB (gigabyte) Approximately 1 billion bytes. Sometimes called a “gig.” Hard drive
capacity on most PCs is typically measured in GB.
GBIC (Gigabit Interface Converter) A hot-swappable input/output device that plugs

into a Gigabit Ethernet port.
Gbps (gigabits per second) One billion bits per second. A standard measurement of
the amount of data transferred over a network connection. 10G or 10 Gigabit Ether-
net operates at 10 Gbps.
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990 Appendix B: Glossary of Key Terms
ghost Fluke Networks coined this new term to mean energy (noise) detected on the
cable that appears to be a frame but that lacks a valid SFD. To qualify as a ghost, this
“frame” must be at least 72 octets long (including preamble); otherwise, it is classified
as a remote collision.
Gigabit Ethernet Standard for a high-speed Ethernet, approved by the IEEE 802.3z
standards committee in 1996.
global configuration mode Used for one-line commands and commands that make
global changes to the router configuration.
half duplex A capability for data transmission in only one direction at a time
between a sending station and a receiving station.
hard disk drive Reads and writes data on a hard disk. The primary storage device in
the computer.
HC (horizontal cross-connect) Area where patch panels or punchdown blocks are
used to connect cables from work areas to the MC. The HC can be a telecommunica-
tions room, a cabinet, or a floor or ceiling installation.
header Control information placed before data when encapsulating that data for net-
work transmission.
hertz The unit of measure for the frequency of an electrical signal in the number of
complete cycles per second.
hierarchical topology Created similar to an extended-star topology. The primary dif-
ference is that it does not use a central node. Instead, it uses a trunk node from which
it branches to other nodes.
holddowns Prevent regular update messages from inappropriately reinstating a route
that might not be up.

hold-time timer Specifies the amount of time for which information about better
routes is ignored. The IGRP default for this variable is 3 times the update timer period
plus 10 seconds.
hop count A routing metric used to measure the distance between a source and a des-
tination. RIP uses hop count as its sole metric.
hop The passage of a data packet from one network node, typically a router, to
another.
HTML (Hypertext Markup Language) A simple hypertext document formatting lan-
guage that uses tags to indicate how a given part of a document should be interpreted
by a viewing application, such as a Web browser.
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Glossary of Key Terms 991
HTTP (Hypertext Transfer Protocol) The protocol used by Web browsers and Web
servers to transfer files, such as text and graphics files.
hub A common connection point for devices in a network. Hubs commonly connect
segments of a LAN. A hub contains multiple ports. When a packet arrives at one port,
it is copied to the other ports so that all the segments of the LAN can see all the packets.
hyperlink A computer program command that points to other HTML files on a web
server or other places on the same documents. Provides shortcuts to other web pages
and files.
Hypertext Markup Language (HTML) A simple hypertext document-formatting lan-
guage that uses tags to indicate how a given part of a document should be interpreted
by a viewing application, such as a web browser.
IC (intermediate cross-connect) The connection point between a backbone cable that
extends from the MC to the HC.
ICMP (Internet Control Message Protocol) Network layer Internet protocol that
reports errors and provides other information relevant to IP packet processing. It is
documented in RFC 792.
IEEE (Institute of Electrical and Electronic Engineers) A professional organization
whose activities include the development of communications and network standards.

IEEE LAN standards are the predominant LAN standards today.
IEEE (Institute of Electrical and Electronic Engineers) IEEE is professional organi-
zation whose activities include the development of communications and network
standards. IEEE LAN standards are the predominant LAN standards.
IEEE 802.1X/Extensible Authentication Protocol (EAP) An alternative WLAN security
approach to WEP as specified by IEEE 802.11. IEEE 802.1X/EAP focuses on develop-
ing a framework for providing centralized authentication and dynamic key distribution.
IEEE 802.2 An IEEE LAN protocol that specifies an implementation of the LLC sub-
layer of the data link layer. IEEE 802.2 handles errors, framing, flow control, and the
network layer (Layer 3) service interface.
IEEE 802.3 An IEEE LAN protocol that specifies an implementation of the physical
layer and the MAC sublayer of the data link layer. IEEE 802.3 uses CSMA/CD access
at a variety of speeds over a variety of physical media. Extensions to the IEEE 802.3
standard specify implementations for Fast Ethernet. Physical variations of the origi-
nal IEEE 802.3 specification include 10BASE2, 10BASE5, 10BASE-F, 10BASE-T,
and 10BROAD36. Physical variations for Fast Ethernet include 100BASE-TX and
100BASE-FX.
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992 Appendix B: Glossary of Key Terms
IGRP (Interior Gateway Routing Protocol) An IGP developed by Cisco to address
the problems associated with routing in large, heterogeneous networks. Compare with
EIGRP. See also IGP, OSPF, and RIP.
impedance A measurement of the resistance of the cable to AC and is measured in ohms.
insertion loss The combination of the effects of signal attenuation and impedance
discontinuities on a communications link.
intelligent hub Sometimes called “smart hubs.” These devices basically function as
active hubs, but also include a microprocessor chip and diagnostic capabilities. They
are more expensive than active hubs, but are useful in troubleshooting situations.
interface Connection between two systems or devices. In routing terminology, a net-
work connection.

Interior Gateway Protocol (IGP) An Internet protocol used to exchange routing
information within an autonomous system. Examples of common Internet IGPs are
IGRP, OSPF, and RIP.
Interior Gateway Routing Protocol (IGRP) An IGP developed by Cisco to address
the problems associated with routing in large, heterogeneous networks.
interior router The router that is connected to the internal network. The interior
router accepts packets only from the application gateway. The gateway controls the
delivery of network-based services both to and from the internal network.
interior routes Routes between subnets of a network attached to a router interface.
If the network attached to a router is not subnetted, IGRP does not advertise interior
routes.
Internet Protocol (IP) A network layer protocol in the TCP/IP protocol suite offering
a connectionless internetwork service.
Internet The largest global internetwork, connecting tens of thousands of networks
worldwide and having a culture that focuses on research and standardization based on
real-life use.
intranet A common LAN configuration. Intranets are designed to be accessed by
users who have access privileges to an organization’s internal LAN.
invalid timer Specifies how long a router waits in the absence of routing-update mes-
sages about a specific route before declaring that route invalid. The IGRP default for
this variable is three times the update period.
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Glossary of Key Terms 993
IP address A 32-bit address assigned to hosts using TCP/IP. An IP address belongs to
one of five classes (A, B, C, D, or E) and is written as four octets separated by periods
(that is, dotted-decimal format). Each address consists of a network number, an optional
subnetwork number, and a host number. The network and subnetwork numbers together
are used for routing, and the host number is used to address an individual host within
the network or subnetwork. A subnet mask is used to extract network and subnetwork
information from the IP address. CIDR provides a new way to represent IP addresses

and subnet masks. Also called an Internet address.
IP address class A 32-bit IP address is broken into a network part and a host part. A
bit or bit sequence at the start of each address determines the address’s class.
IP Version 6 (IPv6) The replacement for the current version of IP (Version 4). IPv6
includes support for flow ID in the packet header, which can be used to identify flows.
Formerly called IPng (IP next generation).
jabber Defined several places in the 802.3 standard as being a transmission of at least
20,000 to 50,000 bit-times in duration. However, most diagnostic tools report jabber
whenever a detected transmission exceeds the maximum legal frame size—which is
considerably smaller than 20,000 to 50,000 bit-times.
KB (kilobyte) Approximately 1000 bytes (1024 bytes exactly).
kbps (kilobits per second) One thousand bits per second. A standard measurement of
the amount of data transferred over a network connection.
kBps (kilobytes per second) One thousand bytes per second. A standard measurement
of the amount of data transferred over a network connection.
keepalive Message sent by one network device to inform another network device
that the virtual circuit between the two is still active.
keyboard port Connects a keyboard to a PC.
latency Delay between the time when a device receives a frame and the time when
that frame is forwarded out the destination port.
light-emitting diode (LED) Semiconductor device that emits light produced by con-
verting electrical energy. Status lights on hardware devices are typically LEDs.
link-state advertisements (LSAs) Small packets of routing information that are sent
between routers.
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994 Appendix B: Glossary of Key Terms
link-state routing protocol A routing algorithm in which each router broadcasts or
multicasts information regarding the cost of reaching each of its neighbors to all nodes
in the internetwork. Link-state algorithms create a consistent view of the network and
are therefore not prone to routing loops. However, they achieve this at the cost of rela-

tively greater computational difficulty and more widespread traffic than do distance
vector routing algorithms.
LLC (Logical Link Control) The higher of the two data link layer sublayers defined
by the IEEE. The LLC sublayer handles error control, flow control, framing, and
MAC-sublayer addressing. The most prevalent LLC protocol is IEEE 802.2, which
includes both connectionless and connection-oriented variants.
load sharing When dynamic routing protocols direct traffic from the same session
over different paths in a network for better performance.
local-area network (LAN) A high-speed, low-error data network covering a relatively
small geographic area (up to a few thousand meters). LANs connect workstations,
peripherals, terminals, and other devices in a single building or another geographically
limited area.
logarithms Equals the exponent that a given number has to be raised to in order to
generate a certain value.
logical connection Uses standards called protocols.
long frame A frame that is longer than the maximum legal size and that takes into
consideration whether the frame was tagged.
MAC (Media Access Control The lower of the two sublayers of the data link layer
defined by the IEEE. The MAC sublayer handles access to shared media, such as whether
token passing or contention will be used. See also LLC.
MAC address A standardized data link layer address that is required for every device
that connects to a LAN. Other devices in the network use these addresses to locate
specific devices in the network and to create and update routing tables and data struc-
tures. MAC addresses are 6 bytes long and are controlled by the IEEE. Also called a
hardware address, MAC-layer address, or physical address.
Manchester encoding Digital encoding scheme, used by IEEE 802.3 and Ethernet, in
which a mid[nd]bit-time transition is used for clocking; a 1 is denoted by a high level
during the first half of the bit time.
Mb (megabit) Approximately 1 million bits.
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Glossary of Key Terms 995
MB (megabyte) Approximately 1 million bytes (1,048,576 bytes exactly). A megabyte
is sometimes called a “meg.” The amount of RAM in most PCs is typically measured
in MB. Large files are typically some number of MB in size.
Mbps (megabits per second) One million bits per second. A standard measurement
of the amount of data transferred over a network connection. Basic Ethernet operates
at 10 Mbps.
MBps (megabytes per second) One million bytes per second. A standard measurement
of the amount of data transferred over a network connection.
MC (main cross-connect) The cross-connect that normally is located in the primary
telecommunications room or the equipment room.
Media Access Control (MAC) A hardware address that uniquely identifies each node
of a network. This address controls data communication for the host on the network.
Media Access Control (MAC) address A standardized data link layer address that is
required for every port of devices that connect to a LAN. Other devices in the network
use these addresses to locate specific ports in the network and to create and update
routing tables and data structures. MAC addresses are 6 bytes long and are controlled
by the IEEE.
media Media is the plural of medium. Media refers to various physical environments
through which transmission signals pass. Common network media include twisted-
pair, coaxial, fiber-optic cable, and the atmosphere (through which microwave, laser,
and infrared transmission occurs).
memory chips RAM chips on memory cards plug into the motherboard.
metric An algorithm that generates a number for each path through the network.
Typically, the smaller the metric number is, the better the path is.
metropolitan-area network (MAN) A network that spans a metropolitan area. Gen-
erally, a MAN spans a larger geographic area than a LAN but a smaller geographic
area than a WAN.
microprocessor A silicon chip that contains a CPU.
microsegmentation Allows the creation of private or dedicated segments—one host

per segment. Each host receives instant access to the full bandwidth and does not have
to compete for available bandwidth with other hosts.
modal dispersion When multiple modes of light propagating through fiber travel
different distances, depending on their entry angles, which causes them to arrive at the
destination (receiving end of the cable) at slightly different times.
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996 Appendix B: Glossary of Key Terms
modem Device that converts digital and analog signals. At the source, a modem con-
verts digital signals to a form suitable for transmission over analog communication
facilities. At the destination, the analog signals are returned to their digital form. Modems
allow data to be transmitted over voice-grade telephone lines. The term modem is also
used to describe various digital devices such as CSU/DSUs and ISDN terminal adapters.
motherboard A computer’s main circuit board.
mouse port Connects a mouse to a PC.
MSDS (material safety data sheet) A document that contains information on the
use, storage, and handling of a hazardous material. It provides detailed information on
the potential health effects of exposure and how to work safely with the material.
MTU (Maximum Transmission Unit) The maximum packet size, in bytes, that a par-
ticular interface can handle.
multicast Single packets copied by the network and sent to a specific subset of net-
work addresses.
multicast address A unique network address that directs packets that have that desti-
nation address to predefined groups of IP addresses.
multimeter A testing tool used to ensure that no voltage is on the telecommunica-
tions line. Most of these devices can measure AC/DC voltage, current, resistance,
diode, continuity, and transistor.
multimode Multimode is a type of fiber-optic cable that transmits more than one
light path.
MUTOA (multiuser telecommunications outlet assembly) A device that holds multi-
ple outlets for use in work areas where modular furniture is used. Computers and tele-

phones are plugged directly into the MUTOA.
named ACL ACL that allows standard and extended ACLs to be given names.
narrowband interference Noise that only affects small ranges of frequencies.
NEC National Electrical Code.
NetBIOS Extended User Interface (NetBEUI) An enhanced version of the NetBIOS
protocol used by network operating systems such as LAN Manager, LAN Server,
Windows for Workgroups, and Windows NT. NetBEUI formalizes the transport frame
and adds functions. NetBEUI implements the OSI LLC2 protocol.
network access layer The layer that is concerned with all the issues that an IP packet
requires to make a physical link to the network medium.
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Glossary of Key Terms 997
network interface card (NIC) A printed circuit board that fits into the expansion slot
of a bus on a computer motherboard. Also can be a peripheral device.
network layer Layer 3 of the OSI reference model. This layer provides connectivity
and path selection between two end systems. The network layer is the layer at which
routing occurs.
NEXT (near-end crosstalk) Computed as the ratio in voltage amplitude between the
test signal and the crosstalk signal when measured from the same end of the link.
NFPA National Fire Protection Association.
noise As related to communications, noise refers to undesirable signals. Noise can
originate from natural and technological sources and is added to the data signals in
communications systems.
nonvolatile RAM (NVRAM) RAM that retains its contents when a unit is powered off.
NRZ (nonreturn to zero) Signals that maintain constant voltage levels with no signal
transitions (no return to a 0V level) during a bit interval.
NRZI (nonreturn to zero inverted) Signal that maintains constant voltage levels with
no signal transitions (no return to a 0V level). It interprets the presence of data at the
beginning of a bit interval as a signal transition and interprets the absence of data as
no transition.

NVRAM (nonvolatile random-access memory) RAM that retains its contents when a
unit is powered off.
octet 8 bits. In networking, the term octet often is used (rather than byte) because
some machine architectures employ bytes that are not 8 bits long.
Open System Interconnection (OSI) reference model A network architectural model
developed by the ISO. This model consists of seven layers, each of which specifies par-
ticular network functions, such as addressing, flow control, error control, encapsulation,
and reliable message transfer. The OSI reference model is used universally as a method
for teaching and understanding network functionality.
oscilloscope An important electronic device used to view electrical signals such as
voltage waves and pulses.
OSHA Occupational Safety and Health Administration.
OUI (organizationally unique identifier) Three octets assigned by the IEEE in a
block of 48-bit LAN addresses.
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998 Appendix B: Glossary of Key Terms
packet A logical grouping of information that includes a header containing control
information and (usually) user data. Packets most often refer to network-layer units of
data. The terms datagram, frame, message, and segment also describe logical informa-
tion groupings at various layers of the OSI reference model and in various technology
circles.
packet-switched services Routes small units of data called packets through a network
based on the destination address contained within each packet.
parallel port An interface that can transfer more than 1 bit simultaneously. It is used
to connect external devices, such as printers.
partial-mesh topology At least one device maintains multiple connections to others
without being fully meshed. A partial-mesh topology still provides redundancy by having
several alternative routes.
patch cable A cable with a connector on each end, used to transfer signals from one
piece of network equipment to another.

patch cord A length of cable used to connect links at the cross-connect.
patch panel A device used to interconnect data networking or voice systems to the
physical cable network. The rear of the patch panel has network cables that are
punched down. The front of the patch panel has a factory-terminated interface of
some type.
peer-to-peer communication A form of communication in which each layer of the
OSI model at the source must communicate with its peer layer at the destination.
peer-to-peer network Networked computers act as equal partners, or peers, to each
other. As peers, each computer can take on the client function or the server function.
Personal Computer Memory Card International Association (PCMCIA) An organi-
zation that has developed a standard for small credit card-sized devices called PCM-
CIA cards (or PC cards). Originally designed to add memory to portable computers,
the PCMCIA standard has been expanded several times and is now suitable for many
types of devices.
physical connection A connection to a network that is made by connecting a special-
ized expansion card, such as a modem or NIC, from a PC with a cable to a network.
physical layer Layer 1 of the OSI reference model. The physical layer defines the
electrical, mechanical, procedural, and functional specifications for activating, main-
taining, and deactivating the physical link between end systems.
ping (Packet Internet Groper) ICMP echo message and its reply. Often used in IP
networks to test the reachability of a network device.
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