Appendix B
Glossary of Key Terms
1000BASE-LX 1000-Mbps baseband Gigabit Ethernet specification using a long wave-
length for a long-haul fiber-optic cable for a maximum length of 10,000 (32808.4 ft.).
1000BASE-SX 1000-Mbps baseband Gigabit Ethernet specification using a short laser
wavelength on multimode fiber-optic cable for a maximum length of 550m (1804.5 ft.).
1000BASE-T 1000-Mbps baseband Gigabit Ethernet specification using four pairs of
Category 5 UTP cable for a maximum length of 100m (328 ft.).
100BASE-FX 100-Mbps baseband Fast Ethernet specification using two strands of
multimode fiber-optic cable per link. To guarantee proper signal timing, a 100BASE-FX
link cannot exceed 400m (1312 ft.) in length. It is based on the IEEE 802.3 standard.
100BASE-TX 100-Mbps baseband Fast Ethernet specification using two pairs of either
UTP or STP wiring. The first pair of wires is used to receive data; the second is used to
transmit. To guarantee proper signal timing, a 100BASE-TX segment cannot exceed
100m (328 ft.) in length. It is based on the IEEE 802.3 standard.
10BASE2 10-Mbps baseband Ethernet specification using 50-ohm thin coaxial cable.
10BASE2, which is part of the IEEE 802.3 specification, has a distance limit of 185m
(606 ft.) per segment.
10BASE5 10-Mbps baseband Ethernet specification using standard (thick) 50-ohm
baseband coaxial cable. 10BASE5, which is part of the IEEE 802.3 baseband physical
layer specification, has a distance limit of 500m (1640 ft.) per segment.
10BASE-T 10-Mbps baseband Ethernet specification using two pairs of twisted-pair
cabling (Category 3, 4, or 5): one pair for transmitting data and the other for receiving
data. 10BASE-T, which is part of the IEEE 802.3 specification, has a distance limit of
approximately 100m (328 ft.) per segment.
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980 Appendix B: Glossary of Key Terms
10-Gb Ethernet Built on the Ethernet technology used in most of today’s LANs, 10-Gb
Ethernet is described as a technology that offers a more efficient and less expensive
approach to moving data on backbone connections between networks, while also
providing a consistent technology end to end. Ethernet now can step up to offering

data speeds at 10 Gbps.
4D-PAM5 The symbol-encoding method used in 1000BASE-T. The four-dimensional
quinary symbols (4D) received from the 8B1Q4 data encoding are transmitted using
five voltage levels (PAM5). Four symbols are transmitted in parallel each symbol period.
8B1Q4 For IEEE802.3, the data-encoding technique used by 1000BASE-T when
converting GMII data (8B-8 bits) to four quinary symbols (Q4) that are transmitted
during one clock (1Q4).
acknowledgment Notification sent from one network device to another to acknowl-
edge that some event (for example, receipt of a message) occurred. Sometimes abbrevi-
ated ACK.
ACL (access control list) A means of controlling or limiting network traffic that com-
pares different criteria to a defined rule set.
active hub Must be plugged into an electrical outlet because it needs power to
amplify the incoming signal before passing it out to the other ports.
adjacent neighbor Two directly connected routers that participate in the exchange of
routing information are said to be adjacent.
administrative distance A rating that shows trustworthiness of a routing information
source. This value is shown as a numeric value between 0 and 255. The higher the
value is, the lower the trustworthiness rating is.
algorithm A well-defined rule or process for arriving at a solution to a problem. In
networking, algorithms are commonly used to determine the best route for traffic from
a particular source to a particular destination.
alien crosstalk When crosstalk is caused by a signal from outside the cable.
alignment error A message that does not end on an octet boundary.
AM (amplitude modulation) Modulates the height of the carrier wave.
American Standard Code for Information Interchange (ASCII) The most commonly
used code for representing alphanumeric data in a computer. Uses binary digits (bits)
to represent the symbols typed on the keyboard.
amplitude The amplitude of an electrical signal represents its height, but it is measured
in volts instead of meters.

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Glossary of Key Terms 981
analog bandwidth Typically refers to the frequency range of an analog electronic
system. Analog bandwidth can be used to describe the range of frequencies transmitted
by a radio station or an electronic amplifier.
angle of incidence Angle of incidence is the angle at which the ray hits the glass surface.
angle of reflection The angle between the reflected ray and the normal.
ANSI American National Standards Institute.
application layer Layer 7 of the OSI reference model. This layer provides services
to application processes (such as e-mail, file transfer, and terminal emulation) that are
outside the OSI reference model. The application layer identifies and establishes the
availability of intended communication partners (and the resources required to con-
nect with them), synchronizes cooperating applications, and establishes agreement on
procedures for error recovery and control of data integrity.
application Interprets the data and displays the information in a comprehensible
format as the last part of an Internet connection. Applications work with protocols
to send and receive data across the Internet.
ARP (Address Resolution Protocol) Internet protocol used to map an IP address to a
MAC address.
ASCII (American Standard Code for Information Interchange) An 8-bit code (7 bits
plus parity) for character representation.
attenuation The decrease in signal amplitude over the length of a link.
AUI (attachment unit interface) The 15-pin physical connector interface between a
computer’s NIC and Ethernet cable.
autonomous system A network or set of networks that are under the administrative
control of a single entity, such as the cisco.com domain.
backbone A backbone is the part of a network that acts as the primary path for traffic
that is most often sourced from, and destined for, other networks.
backoff The retransmission delay enforced when a collision occurs.
backplane A large circuit board that contains sockets for expansion cards.

balanced hybrid routing protocol Routing protocols that utilize elements of distance
vector and link-state routing protocols.
bandwidth The amount of information that can flow through a network connection
in a given period of time.
binary A number system characterized by 1s and 0s (1 = on, and 0 = off).
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982 Appendix B: Glossary of Key Terms
bit The smallest unit of data in a computer. A bit equals 1 or 0. It is the binary for-
mat in which data is processed, stored, and transmitted by computers. In a computer,
bits are represented by on/off switches or the presence or absence of electrical charges,
light pulses, or radio waves.
bit bucket The destination of discarded bits (dropped packets), as determined by the
router.
blueprint An architectural plan or technical drawing that provides details of a con-
struction project or an existing structure.
Boolean logic In computer operation with binary values, Boolean logic can describe
electromagnetically charged memory locations or circuit states that are either charged
(1 or true) or not charged (0 or false). The computer can use an AND gate or an OR
gate operation to obtain a result that can be used for further processing.
bootstrap The protocol used by a network node to determine the IP address of its
Ethernet interfaces to affect network booting.
border router A router situated at the edges or end of the network boundary, which
provides basic security from the outside network or from a less controlled area of the
network into a more private area of the network.
BPDU (bridge protocol data unit) Spanning Tree Protocol hello packet that is sent
out at configurable intervals to exchange information among bridges in the network.
bridge A Layer 2 device designed to create two or more LAN segments, each of
which is a separate collision domain.
broadcast A data packet that is sent to all nodes on a network. Broadcasts are identi-
fied by a broadcast address.

broadcast address Used to broadcast packets to all the devices on a network.
broadcast domain A set of all devices that receive broadcast frames originating from
any device within the set. Broadcast domains are typically bounded by routers (or, in a
switched network, by VLANs) because routers do not forward broadcast frames.
bullwheel A large-diameter pulley that is used in a mechanical cable-pulling process.
bus topology Commonly called a linear bus, this topology connects all the devices
with a single cable. This cable proceeds from one computer to the next like a bus line
going through a city.
bus A collection of circuits through which data is transmitted from one part of a
computer to another.
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Glossary of Key Terms 983
byte A unit of measure that describes the size of a data file, the amount of space on
a disk or another storage medium, or the amount of data being sent over a network.
1 byte equals 8 bits of data.
cable tree A device that supports a number of small reels of cable. This enables the
cable installer to pull multiple runs of cable simultaneously.
CDP (Cisco Discovery Protocol) Used to obtain information about neighboring
devices, such as the types of devices connected, the router interfaces they are connected
to, the interfaces used to make the connections, and the model numbers of the devices.
CD-ROM drive An optical drive that can read information from a CD-ROM.
cell-switched services Provide a dedicated-connection switching technology that
organizes digital data into cell units and transmits them over a physical medium using
digital signal technology.
CENELEC Comité Europe en de Normalisation Electrotechnique, or European
Committee for Electrotechnical Standardization.
central processing unit (CPU) The computer’s “brain,” where most of the calculations
take place.
circuit switching A WAN switching method in which a dedicated physical circuit
through a carrier network is established, maintained, and terminated for each commu-

nication session. ISDN is an example of a circuit-switched WAN technology.
Cisco Internetwork Operating System (IOS) Software Software stored as an image
file in Flash memory on the router that, when loaded into RAM, provides the operating
system that runs the router.
Class A address Designed to support extremely large networks. A Class A IP address
uses only the first octet to indicate the network address. The remaining three octets
enumerate host addresses.
Class B address Designed to support the needs of moderate- to large-sized networks.
A Class B IP address uses two of the four octets to indicate the network address. The
other two octets specify host addresses.
Class C address The most commonly used of the original address classes. This address
space was intended to support a lot of small networks.
Class D address Created to enable multicasting in an IP address.
Class E address The IETF reserves these addresses for its own research. Therefore,
no Class E addresses have been released for use in the Internet.
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984 Appendix B: Glossary of Key Terms
classless interdomain routing (CIDR) A technique supported by BGP and based on
route aggregation. CIDR allows routers to group routes to cut down on the quantity
of routing information carried by the core routers. With CIDR, several IP networks
appear to networks outside the group as a single, larger entity.
CLI (command-line interface) An interface that enables the user to interact with the
operating system by entering commands and optional arguments.
coaxial cable A coaxial cable is a cable consisting of a hollow outer cylindrical con-
ductor that surrounds a single inner wire conductor.
collision In Ethernet, the result of two nodes transmitting simultaneously. The frames
from each device impact and are damaged when they meet on the physical media. See
also collision domain.
collision domain In Ethernet, the network area within which frames that have collided
are propagated. Repeaters and hubs propagate collisions; LAN switches, bridges, and

routers do not.
command-line interface (CLI) An interface that enables the user to interact with the
operating system by entering commands and optional arguments.
connectionless Data transfer without the existence of a virtual circuit.
connection-oriented Data transfer that requires the establishment of a virtual circuit.
convergence The speed and capability of a group of internetworking devices running
a specific routing protocol to agree on the topology of an internetwork after a change
in that topology.
count to infinity A problem that can occur in routing algorithms that are slow to
converge in which routers continuously increment the hop count to particular networks.
Typically, some arbitrary hop-count limit is imposed to prevent this problem.
crossover cable A cable that crosses the critical pair to properly align, transmit, and
receive signals on the device with line connections.
crosstalk The transmission of signals from one wire pair to nearby pairs. Adjacent
wire pairs in the cable act like antennas generating a weaker but similar electrical signal
onto the nearby wire pairs. This crosstalk causes interference with data that might be
present on the adjacent wires.
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Glossary of Key Terms 985
CSMA/CD (carrier sense multiple access collision detect) A media-access mechanism
wherein devices ready to transmit data first check the channel for a carrier. If no carrier
is sensed for a specific period of time, a device can transmit. If two devices transmit at
once, a collision occurs and is detected by all colliding devices. This collision subsequently
delays retransmissions from those devices for some random length of time. CSMA/CD
access is used by Ethernet and IEEE 802.3.
cut-through switching A packet-switching approach that streams data through a switch
so that the leading edge of a packet exits the switch at the output port before the packet
finishes entering the input port. A device using cut-through packet switching reads,
processes, and forwards packets as soon as the destination address is looked up and
the outgoing port is determined. See also store-and-forward switching.

data center A globally coordinated network of devices designed to accelerate the
delivery of information over the Internet infrastructure.
data link layer Layer 2 of the OSI reference model. Provides transit of data across a
physical link. The data link layer is concerned with physical addressing, network topol-
ogy, line discipline, error notification, ordered delivery of frames, and flow control.
datagram A logical grouping of information sent as a network layer unit over a
transmission medium without prior establishment of a virtual circuit. IP datagrams are
the primary information units in the Internet. The terms cell, frame, message, packet,
and segment also describe logical information groupings at various layers of the OSI
reference model and in various technology circles.
daughter card Similar to an expansion board, but it accesses the motherboard com-
ponents (memory and CPU) directly instead of sending data through the slower expansion
bus.
DCE (data circuit-terminating equipment) (ITU-T expansion) Devices and connections
of a communications network that comprise the network end of the user-to-network
interface. The DCE provides a physical connection to the network, forwards traffic,
and provides a clocking signal used to synchronize data transmission between DCE
and DTE devices. Modems and interface cards are examples of DCE.
debugging To find and remove errors (bugs) from a program or design.
decibel An important way of describing networking signals as a unit that measures
the loss or gain of the power of a wave. Decibels are usually negative numbers repre-
senting a loss in power as the wave travels, but can also be positive values representing
a gain in power if the signal is amplified.
de-encapsulation Unwrapping data in a particular protocol header.
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986 Appendix B: Glossary of Key Terms
delay skew The propagation delays of different wire pairs in a single cable can differ
slightly because of differences in the number of twists and electrical properties of each
wire pair. Delay skew is the delay difference between pairs.
demarc The point at which the service provider’s cable interfaces with the building

distribution cabling.
demarcation point Usually the point at which the access provider’s facilities stop and
the customer-owned structured cabling begins.
digital bandwidth Measures how much information can flow from one place to
another in a given amount of time.
dispersion Dispersion is the broadening of light signals along the length of the fiber.
distance vector routing protocol A class of routing algorithms that iterate on the
number of hops in a route to find a shortest-path spanning tree. Distance vector routing
algorithms call for each router to send its entire routing table in each update, but only
to its neighbors. Distance vector routing algorithms can be prone to routing loops but
are computationally simpler than link-state routing algorithms. Also called a Bellman-
Ford routing algorithm.
distance-vector routing A class of routing algorithms that iterate on the number of
hops in a route to find a shortest-path spanning tree. Distance-vector routing algorithms
call for each router to send its entire routing table in each update, but only to its neigh-
bors. Distance-vector routing algorithms can be prone to routing loops but are compu-
tationally simpler than link-state routing algorithms. Also called the Bellman-Ford
routing algorithm.
DNS (Domain Name System) The system used in the Internet for translating names
of network nodes into addresses.
DoS (denial-of-service) Type of attack on a network that is designed to bring the net-
work to its knees by flooding it with useless traffic.
dotted-decimal format In this notation, each IP address is written as four parts sepa-
rated by periods, or dots.
dotted-decimal notation A syntactic representation for a 32-bit integer that consists
of four 8-bit numbers written in base 10 with periods (dots) separating them. Used to
represent IP addresses on the Internet, as in 192.67.67.20.
DSSS (direct-sequence spread spectrum) A technology in which transmissions are
more reliable because each bit (1 or 0) is represented by a string of 1s and 0s, called
a chipping sequence.

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Glossary of Key Terms 987
DTE (data terminal equipment) Device at the user end of a user-network interface
that serves as a data source, destination, or both. DTE connects to a data network
through a DCE device (for example, a modem) and typically uses clocking signals gen-
erated by the DCE. DTE includes such devices as computers, protocol translators, and
multiplexers.
dynamic routing Routing that adjusts automatically to network topology or traffic
changes. Also called adaptive routing. Requires that a routing protocol be run between
routers.
EIA (Electronic Industries Association) EIA is a group that specifies electrical trans-
mission standards. The EIA and TIA have developed numerous well-know communication
standards.
ELFEXT (equal-level far-end crosstalk) A test that measures FEXT.
EMI (electromagnetic interference) EMI is an electromagnetic field that has the
potential to disrupt the operation of electronic components, devices, and systems in its
vicinity.
encapsulation Wrapping of data in a particular protocol header. For example, upper-
layer data is wrapped in a specific Ethernet header before network transit. Also, when
bridging dissimilar networks, the entire frame from one network simply can be placed
behind the header used by the data link layer protocol of the other network.
encoding Process by which bits are represented by voltages.
equipment room Space for equipment. Also can be used as a telecommunications
room.
Ethernet A baseband LAN specification invented by Xerox Corporation and devel-
oped jointly by Xerox, Intel, and Digital Equipment Corporation. Ethernet networks
use CSMA/CD and run over a variety of cable types at 10, 100, and 1000 Mbps.
Ethernet is similar to the IEEE 802.3 series of standards.
expansion slot An opening in a computer, usually on the motherboard, where an
expansion card can be inserted to add new capabilities to the computer.

extended ACL Compares source IP address, destination IP address, TCP/UDP port
number, and other criteria to the rules defining an extended ACL.
extended-star topology A network in which a star network is expanded to include
an additional networking device that is connected to the main networking device.
exterior gateway protocol (EGP) A routing protocol designed for use between
networks that are controlled by two different organizations.
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988 Appendix B: Glossary of Key Terms
Exterior Gateway Protocol (EGP) An Internet protocol used to exchange routing
information between autonomous systems. Border Gateway Protocol (BGP) is the
most common EGP.
exterior router In firewall architecture, the router that is connected to the Internet
is referred to as the exterior router. It forces all incoming traffic to pass through the
application gateway.
exterior routes Routes to networks outside the autonomous system that are consid-
ered when identifying a gateway of last resort.
extranet Intranet-based applications and services that employ extended, secure access
to external users or enterprises.
Fast Ethernet Any of a number of 100-Mbps Ethernet specifications. Fast Ethernet
offers a speed increase 10 times that of the 10BASE-T Ethernet specification, while
preserving such qualities as frame format, MAC mechanisms, and MTU. Such similar-
ities allow the use of existing 10BASE-T applications and network-management tools
on Fast Ethernet networks. Fast Ethernet is based on an extension to the IEEE 802.3
specification.
FDDI (Fiber Distributed Data Interface) A LAN standard, defined by American
National Standards Institute (ANSI) 3T9.5, specifying a 100-Mbps token-passing net-
work using fiber-optic cable, with transmission distances of up to 2 km. FDDI uses a
dual-ring architecture to provide redundancy.
FEXT (far-end crosstalk) Crosstalk that occurs when signals on one twisted pair are
coupled to another pair as they arrive at the far end of a multipair cable system.

FHSS (frequency-hopping spread spectrum) A technology in which transmissions
hop from one frequency to another in random patterns. This feature enables the trans-
missions to hop around narrowband interference, resulting in a clearer signal and
higher reliability of the transmission.
fiber-optic cable A fiber-optic cable is a physical medium capable of conducting
modulated light transmission. Compared with other transmission media, fiber-optic
cable is more expensive but is not susceptible to electromagnetic interference. Some-
times called optical fiber.
File Transfer Protocol An application protocol, part of the TCP/IP protocol suite,
used to transfer files between network hosts.
firewall One or more network devices, such as routers or access servers, designated
as a buffer between any connected public networks and a private network. A firewall
router uses access control lists and other methods to ensure the security of the private
network.
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