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See TWS. Underwriters Laboratories
Independent agency within the United States
that tests product safety.
two-way simultaneous
Mode that allows a router configured as a pri- ULP
mary SDLC station to achieve better utilization upper-layer protocol
of a full-duplex serial line. When TWS is en- Protocol that operates at a higher layer in the
abled in a multidrop environment, the router OSI reference model, relative to other layers.
can poll a secondary station and receive data ULP is sometimes used to refer to the next-
from that station while it sends data to or re-
Glossary 407

highest protocol (relative to a particular proto- unnumbered frames
col) in a protocol stack. HDLC frames used for various control and
management purposes, including link startup
unbalanced configuration
and shutdown, and mode specification.
HDLC configuration with one primary station
and multiple secondary stations. unshielded twisted-pair
See UTP.
Underwriters Laboratories
See UL. unspecified bit rate
See UBR.
User-Network Interface UPC
ATM Forum specification that defines an inter- usage parameter control. See traffic policing.
operability standard for the interface between
upper-layer protocol
ATM-based products (a router or an ATM
See ULP.
switch) located in a private network and the
ATM switches located within the public carrier
networks. Also used to describe similar connec-
uninterruptable power supply
tions in Frame Relay networks.
Backup device designed to provide an uninter-
rupted power source in the event of a power
failure. They are commonly installed on all file
Message sent to a single network destination.
servers and wiring hubs.
Compare with broadcast and multicast.
Urgent Pointer
unicast address
Indicates the end of the urgent data.
Address specifying a single network device.
Compare with broadcast address and multicast
Universal Resource Locator
Standardized addressing scheme for accessing
uninsured traffic
hypertext documents and other services using a
Traffic within the excess rate (the difference be-
WWW browser.
tween the insured rate and maximum rate) for a
VCC. This traffic can be dropped by the net-
usage parameter control
work if congestion occurs.
See UPC.
Universal Asynchronous
Initiated in 1979, one of the oldest and largest
cooperative networks, with over 10,000 hosts
and a quarter of a million users. Its primary
Universal Resource Locator
service is a distributed conferencing service
See URL.
called news.
User Datagram Protocol
Operating system developed in 1969 at Bell
See UDP.
Laboratories. UNIX has gone through several
iterations since its inception. These include
User-Network Interface
UNIX 4.3 BSD (Berkeley Standard Distribu-
See UNI.
tion), developed at the University of California
at Berkeley, and UNIX System V, Release 4.0, UTP
developed by AT and T. unshielded twisted-pair
Four-pair wire medium used in a variety of net-
works. UTP does not require the fixed spacing
between connections that is necessary with
408 CCNA Security Course Booklet, Version 1.0

coaxial-type connections. There are five types VCC
of UTP cabling commonly used: Category 1 ca- virtual channel connection
bling, Category 2 cabling, Category 3 cabling, Logical circuit, made up of VCLs, that carries
Category 4 cabling, and Category 5 cabling. data between two end points in an ATM net-
Compare with STP. work.
Virtual channel connection is also known as
virtual circuit connection.
ITU-T standard for a physical layer interface
between DTE and DCE. V.24 is essentially the VCI
same as the EIA/TIA-232 standard. virtual channel identifier
16-bit field in the header of an ATM cell. The
VCI, together with the VPI, is used to identify
ITU-T standard describing a synchronous, the next destination of a cell as it passes
physical layer protocol used for communica- through a series of ATM switches on its way to
tions between a network access device and a its destination. ATM switches use the VPI/VCI
packet network. V.35 is most commonly used in fields to identify the next network VCL that a
the United States and in Europe, and is recom- cell needs to transit on its way to its final desti-
mended for speeds up to 48 Kbps. nation. The function of the VCI is similar to
that of the DLCI in Frame Relay. Compare to
ITU-T standard protocol for error correction
using LAPM. VCL
virtual channel link
variable bit rate
Connection between two ATM devices. A VCC
See VBR.
is made up of one or more VCLs.
Variable Length Subnet Masking
virtual circuit number
variable-length subnet mask 12-bit field in an X.25 PLP header that identi-
fies an X.25 virtual circuit. Allows DCE to de-
termine how to route a packet through the X.25
VBR network.
variable bit rate Virtual circuit number is also known as logical
QOS class defined by the ATM Forum for ATM channel identifier (LCI) or logical channel
networks. VBR is subdivided into a real time number (LCN).
(RT) class and non-real time (NRT) class. VBR
(RT) is used for connections in which there is a
Data segment of an SNA message. A vector
fixed timing relationship between samples.
consists of a length field, a key that describes
VBR (NRT) is used for connections in which
the vector type, and vector-specific data.
there is no fixed timing relationship between
samples, but that still need a guaranteed QOS.
Compare with ABR, CBR, and UBR.
Version number field with in an IP datagram.
Versatile Interface Processor
virtual circuit
See VIP.
Logical circuit created to ensure reliable com-
munication between two network devices. A vertical cabling
virtual circuit is defined by a VPI/VCI pair, and See backbone cabling.
can be either permanent (PVC) or switched
(SVC). Virtual circuits are used in Frame Relay video on demand
and X.25. In ATM, a virtual circuit is called a See VoD.
virtual channel.
Glossary 409

VINES across switches and offers traffic segmentation
and access control.
Virtual Integrated Network Service
NOS developed and marketed by Banyan
virtual path
Logical grouping of virtual circuits that connect
VIP two sites.
1) Versatile Interface Processor. Interface card
virtual path connection
used in Cisco 7000 and Cisco 7500 series
See VPC.
routers. The VIP provides multilayer switching
and runs the Cisco IOS software. virtual path identifier
2) Virtual IP. Function that enables the creation See VPI.
of logically separated switched IP workgroups
virtual path identifier/virtual channel identi-
across the switch ports of a Catalyst 5000 run-
ning Virtual Networking Services software.
virtual address
virtual path link
See network address.
See VPL.
virtual channel
virtual ring
See virtual circuit (VC).
Entity in an SRB network that logically con-
virtual channel connection nects two or more physical rings together either
See VCC. locally or remotely. The concept of virtual rings
can be expanded across router boundaries.
virtual channel identifier
See VCI. virtual route
In SNA, a logical connection between subarea
virtual channel link
nodes that is physically realized as a particular
See VCL.
explicit route. SNA terminology for virtual cir-
cuit. See also virtual circuit.
virtual circuit
See VC.
virtual telecommunications access method
virtual circuit connection
See VCC.
Virtual Terminal Protocol
See VTP.
virtual circuit number
See VCN.
virtual LAN
Virtual Integrated Network Service
Group of devices on a LAN that are configured
(using management software) so that they can
virtual IP communicate as if they were attached to the
See VIP. same wire, when in fact they are located on a
number of different LAN segments. Because
virtual LAN
VLANs are based on logical instead of physical
connections, they are extremely flexible.
virtual LAN internetwork
See VLI.
virtual LAN
Group of devices on a LAN that are configured
Virtual Networking Services
(using management software) so that they can
Software on some Catalyst 5000 switches that
communicate as if they were attached to the
enables multiple workgroups to be defined
same wire, when in fact they are located on a
410 CCNA Security Course Booklet, Version 1.0

number of different LAN segments. Because VPI/VCI
VLANs are based on logical instead of physical virtual path identifier/virtual channel identifier
connections, they are extremely flexible. See VCI and VPI.

variable-length subnet mask virtual path link
Ability to specify a different subnet mask for Within a virtual path, a group of unidirectional
the same network number on different subnets. VCLs with the same end points. Grouping
VLSM can help optimize available address VCLs into VPLs reduces the number of con-
space. nections to be managed, thereby decreasing
network control overhead and cost. A VPC is
made up of one or more VPLs.
video on demand
Systems that allow users to select and watch VRRP
video content over a network as part of an in- Virtual Router Redundancy Protocol
teractive television system. VoD systems either VRRP is a vendor neutral alternative to HSRP
“stream” content, allowing viewing while the and GLBP, providing router redundancy for
video is being downloaded, or “download” it in traffic exiting a LAN environment. VRRP al-
which the program is brought in its entirety to a lows a group of routers to form a single virtual
set-top box before viewing starts. router. One router is elected to handle all re-
quests sent to the virtual IP address. A VRRP
group has one master router and one or more
Voice over IP backup routers.
The capability to carry normal telephony-style
voice over an IP-based internet with POTS-like VTAM
functionality, reliability, and voice quality. VoIP virtual telecommunications access method
enables a router to carry voice traffic (for exam- Set of programs that control communication
ple, telephone calls and faxes) over an IP net- between LUs. VTAM controls data transmis-
work. In VoIP, the DSP segments the voice sion between channel-attached devices and per-
signal into frames, which then are coupled in forms routing functions.
groups of two and stored in voice packets.
These voice packets are transported using IP in
1) Virtual Terminal Protocol. ISO application
compliance with ITU-T specification H.323.
for establishing a virtual terminal connection
VPC across a network.
virtual path connection 2) VLAN Trunking Protocol. A Cisco propri-
Grouping of VCCs that share one or more con- etary protocol that uses Layer 2 trunk frames to
tiguous VPLs. communicate VLAN information among a
group of switches and to manage the addition,
deletion, and renaming of VLANs across the
virtual path identifier network from a central point of control.
8-bit field in the header of an ATM cell. The
VPI, together with the VCI, is used to identify WAN
the next destination of a cell as it passes Wide Area Network
through a series of ATM switches on its way to Data communications network that serves users
its destination. ATM switches use the VPI/VCI across a broad geographic area and often uses
fields to identify the next VCL that a cell needs transmission devices provided by common car-
to transit on its way to its final destination. The riers. Frame Relay, SMDS, and X.25 are exam-
function of the VPI is similar to that of the ples of WANs.
DLCI in Frame Relay. Compare with DLCI.
waveform coding
Electrical techniques used to convey binary signals.
Glossary 411

WCS wildcard mask
Wireless Control System 32-bit quantity used in conjunction with an IP
WCS allows the centralized configuration of address to determine which bits in an IP ad-
Cisco WLAN controllers in conjunction with dress should be ignored when comparing that
lightweight access points (centralized WLAN address with another IP address. A wildcard
model). mask is specified when setting up access lists.

WDS window
Wireless Domain Services Number of octets that the receiver is willing to
An access point providing WDS on your wire- accept.
less LAN maintains a cache of credentials for
window size
CCKM-capable client devices on your wireless
Refers to the number of messages that can be
LAN. When a CCKM-capable client roams
transmitted while awaiting an acknowledgment.
from one access point to another, the WDS ac-
cess point forwards the client™s credentials to wire map
the new access point with the multicast key.
Feature provided by most cable testers. Used to
Only two packets pass between the client and
test twisted pair cable installations, it shows
the new access point, greatly shortening the re-
which wire pairs connect to what pins on the
association time.
plugs and sockets.
Weighted fair queuing
wireless controller
See WFQ.
A wireless controller is a device used in a cen-
tralized WLAN topology which handles au-
thentication, association, mobility, and frame
Wired Equivalent Privacy
translation and bridging.
An optional security mechanism defined within
the 802.11 standard designed to make the link wiring closet
integrity of wireless devices equal to that of a
Specially designed room used for wiring a data
or voice network. Wiring closets serve as a cen-
tral junction point for the wiring and wiring
equipment that is used for interconnecting de-
weighted fair queuing
Queuing method that prioritizes interactive traf-
fic over file transfers in order to ensure satisfac- WLAN
tory response time for common user
wireless LAN
A WLAN a wireless local area network, which
permits a network connection between two or
wide-area network
more computers without using wires. It uses
See WAN.
radio communication to accomplish the same
wideband functionality that a wired LAN has.
See broadband.
Wi-Fi Alliance Wireless LAN Solution Engine
The Wi-Fi Alliance offers certification for inter- WLSE is a CiscoWorks option which allows
operability between vendors of 802.11 prod- centralized configuration and monitoring of the
ucts. It helps to market a WLAN technology by Cisco Aironet autonomous access points and
promoting interoperability between vendors. provides RF management, rogue access point
Certification includes all three 802.11 RF tech- detection, and interference detection. WLSE is
nologies and WPA. used with autonomous access points in the dis-
tributive WLAN model.
Wi-Fi Protected Access
See WPA.
412 CCNA Security Course Booklet, Version 1.0

workgroup X.25
Collection of workstations and servers on a ITU-T standard that defines how connections
LAN that are designed to communicate and ex- between DTE and DCE are maintained for re-
change data with one another. mote terminal access and computer communi-
cations in PDNs. X.25 specifies LAPB, a data
World Wide Web link layer protocol, and PLP, a network layer
See WWW. protocol. Frame Relay has to some degree su-
perseded X.25.
Wi-Fi Protected Access X.25 Level 3
WPA is a security model for WLANs released See PLP.
in 2003, based on the IEEE 802.11i standard. It
X.25 Protocol
is a standards-based, interoperable security en-
hancement that strongly increases the level of See PLP.
data protection and access control for existing
and future wireless LAN systems. It is derived
ITU-T recommendation that defines the termi-
from and will be forward-compatible with the
nal-to-PAD interface in X.25 networks.
upcoming IEEE 802.11i standard. WPA lever-
ages Temporal Key Integrity Protocol (TKIP) X.29
for data protection and 802.1X for authenti-
ITU-T recommendation that defines the form
cated key management.
for control information in the terminal-to-PAD
interface used in X.25 networks.
World Wide Web X.3
Large network of Internet servers providing hy-
ITU-T recommendation that defines various
pertext and other services to terminals running
PAD parameters used in X.25 networks.
client applications such as a WWW browser.
WWW browser
ITU-T recommendation specifying a standard
World Wide Web browser
for electronic mail transfer.
GUI-based hypertext client application, such as
Mosaic, used to access hypertext documents X.500
and other services located on innumerable re- ITU-T recommendation specifying a standard
mote servers throughout the WWW and Inter- for distributed maintenance of files and directo-
net. ries.
X.121 X.75
ITU-T standard describing an addressing ITU-T specification that defines the signalling
scheme used in X.25 networks. X.121 ad- system between two PDNs. X.75 is essentially
dresses are sometimes called IDNs (Interna- an NNI.
tional Data Numbers).
X.21 Number assigned to the ANSI Task Group of
ITU-T standard for serial communications over Accredited Standards Committee for their inter-
synchronous digital lines. The X.21 protocol is nal, working document describing FDDI.
used primarily in Europe and Japan.

ITU-T standard that defines the physical layer
protocol for communication between DCE and
DTE in an X.25 network. Virtually equivalent
to EIA/TIA-232.
Glossary 413

1) exchange identification. Request and re-
sponse packets exchanged prior to a session be-
tween a router and a Token Ring host. If the
parameters of the serial device contained in the
XID packet do not match the configuration of
the host, the session is dropped.
2) See termid.

eXtensible Markup Language
A standard maintained by the World Wide Web
Consortium (W3C). It defines a syntax that lets
you create markup languages to specify infor-
mation structures. Information structures define
the type of information, for example, subscriber
name or address, not how the information looks
(bold, italic, and so on). External processes can
manipulate these information structures and
publish them in a variety of formats. Text
markup language designed to enable the use of
SGML on the World Wide Web. XML allows
you to define your own customized markup lan-

Xerox Network Systems
Protocol suite originally designed by PARC.
Many PC networking companies, such as
3Com, Banyan, Novell, and UB Networks used
or currently use a variation of XNS as their pri-
mary transport protocol.

zero code suppression
Line coding scheme used for transmission
clocking. Zero line suppression substitutes a
one in the seventh bit of a string of eight con-
secutive zeros.


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