. 18
( 19)


outdated). Compare with gateway.
1) route processor. Processor module on the
Process of finding a path to a destination host.
Cisco 7000 series routers that contains the
Routing is very complex in large networks be-
CPU, system software, and most of the memory
cause of the many potential intermediate desti-
components that are used in the router. Route
nations a packet might traverse before reaching
processor is also known as supervisory proces-
its destination host.
routing domain 2) rendezvous point. Router specified in PIM
Group of end systems and intermediate systems sparse mode implementations to track member-
operating under the same set of administrative ship in multicast groups and to forward mes-
rules. Within each routing domain is one or sages to known multicast group addresses. See
more areas, each uniquely identified by an area also PIM sparse mode.
Routing Information Field remote-procedure call
See RIF. Technological foundation of client-server com-
puting. RPCs are procedure calls that are built
Routing Information Identifier or specified by clients and executed on servers,
See RII. with the results returned over the network to the
Routing Information Protocol
Reverse Path Multicasting
routing metric
Multicasting technique in which a multicast
Method by which a routing algorithm deter-
datagram is forwarded out of all but the receiv-
mines that one route is better than another. This
ing interface if the receiving interface is one
information is stored in routing tables. Metrics
used to forward unicast datagrams to the source
include bandwidth, communication cost, delay,
of the multicast datagram.
hop count, load, MTU, path cost, and reliabil-
ity. RP-TNC
Routing metric is also known as metric. A connector type unique to Cisco Aironet ra-
dios and antennas. Part 15.203 of the FCC rules
routing protocol
covering spread spectrum devices limits the
Protocol that accomplishes routing through the
types of antennas that may be used with trans-
implementation of a specific routing algorithm.
mission equipment. In compliance with this
Examples of routing protocols include IGRP,
rule, Cisco Aironet, like all other wireless LAN
OSPF, and RIP.
providers, equips its radios and antennas with a
unique connector to prevent attachment of non-
routing table
approved antennas to radios.
Table stored in a router or some other internet-
working device that keeps track of routes to
particular network destinations and, in some
cases, metrics associated with those routes.
Glossary 389

RS-232 RTP
Popular physical layer interface. 1) Routing Table Protocol. VINES routing pro-
RS-232 is known as EIA/TIA-232. tocol based on RIP. Distributes network topol-
ogy information and aids VINES servers in
RS-422 finding neighboring clients, servers, and
Balanced electrical implementation of routers. Uses delay as a routing metric. See also
EIA/TIA-449 for high-speed data transmission. SRTP.
RS-422 is referred to collectively with RS-423 2) Real-Time Transport Protocol. Commonly
as EIA-530. used with IP networks. RTP is designed to pro-
vide end-to-end network transport functions for
applications transmitting real-time data, such as
Unbalanced electrical implementation of
audio, video, or simulation data, over multicast
EIA/TIA-449 for EIA/TIA-232 compatibility.
or unicast network services. RTP provides such
RS-423 is referred to collectively with RS-422
services as payload type identification, se-
as EIA-530.
quence numbering, timestamping, and delivery
monitoring to real-time applications.
Popular physical layer interface.
RS-449 is known as EIA/TIA-449.
Ready To Send
EIA/TIA-232 control signal that requests a data
transmission on a communications line.
remote shell protocol
Protocol that allows a user to execute com-
mands on a remote system without having to
round-trip time
log in to the system. For example, rsh can be
Time required for a network communication to
used to remotely examine the status of a num-
travel from the source to the destination and
ber of access servers without connecting to
back. RTT includes the time required for the
each communication server, executing the com-
destination to process the message from the
mand, and then disconnecting from the commu-
source and generate a reply. RTT is used by
nication server.
some routing algorithms to aid in calculating
optimal routes.
Route/Switch Processor
Processor module used in the Cisco 7500 series
request/response unit
routers that integrates the functions of the RP
Request and response messages exchanged be-
and the SP.
tween NAUs in an SNA network.
run-time memory
remote source-route bridging
Memory accessed while a program runs. On a
SRB over WAN links.
LightStream 2020 ATM switch, this memory
contains configuration data that is accessed
while the switch operates.
Resource Reservation Protocol
Protocol that supports the reservation of re-
safety ground wire
sources across an IP network. Applications run-
Circuit wire that connects to a local earth
ning on IP end systems can use RSVP to
ground and the chassis of an electrical appli-
indicate to other nodes the nature (bandwidth,
ance or device via an electrical outlet and plug.
jitter, maximum burst, and so forth) of the
It is used to ensure that no voltage potential ex-
packet streams they want to receive.
ists between the chassis of the electrical device
Resource Reservation Protocol is also known as
and the earth ground.
Resource Reservation Setup Protocol.
390 CCNA Security Course Booklet, Version 1.0

sag pensate for clock phase shift on long cables.
When the DCE device uses SCTE instead of its
Any decrease of below 80% in the normal volt-
internal clock to sample data from the DTE, it
age carried by a power line. A sag is sometimes
is better able to sample the data without error
referred to as a brownout.
even if there is a phase shift in the cable.
sampling rate
Rate at which samples of a particular waveform
Synchronous Digital Hierarchy
amplitude are taken.
European standard that defines a set of rate and
SAP format standards that are transmitted using op-
1) service access point. Field defined by the tical signals over fiber. SDH is similar to
IEEE 802.2 specification that is part of an ad- SONET, with a basic SDH rate of 155.52
dress specification. Thus, the destination plus Mbps, designated at STM-1.
the DSAP define the recipient of a packet. The
same applies to the SSAP. See also DSAP and
Synchronous Data Link Control
SNA data link layer communications protocol.
2) Service Advertisement Protocol. IPX proto-
SDLC is a bit-oriented, full-duplex serial proto-
col that provides a means of informing network
col that has spawned numerous similar proto-
clients, via routers and servers, of available net-
cols, including HDLC and LAPB.
work resources and services. See also IPX.

SDLC Transport
satellite communication
Cisco router feature with which disparate envi-
Use of orbiting satellites to relay data between
ronments can be integrated into a single, high-
multiple earth-based stations. Satellite commu-
speed, enterprise-wide network. Native SDLC
nications offer high bandwidth and a cost that is
traffic can be passed through point-to-point se-
not related to distance between earth stations,
rial links with other protocol traffic multiplexed
long propagation delays, or broadcast capabil-
over the same links. Cisco routers can also en-
capsulate SDLC frames inside IP datagrams for
Sbus transport over arbitrary (non-SDLC) networks.
Bus technology used in Sun SPARC-based Replaces proxy polling.
workstations and servers. The SBus specifica-
tion has been adopted by the IEEE as a new bus
Feature that performs translation between
SDLC and IEEE 802.2 type 2.
Scattering is the physical phenomenon that oc-
Switched Multimegabit Data Service DSU
curs when radio frequency waves strike an un-
DSU for access to SMDS via HSSIs and other
even surface (for example, a rough surface) and
serial interfaces.
are reflected in many directions.

See secondary station.
sustainable cell rate
Parameter defined by the ATM Forum for ATM
secondary station
traffic management. For VBR connections,
In bit-synchronous data link layer protocols
SCR determines the long-term average cell rate
such as HDLC, a station that responds to com-
that can be transmitted.
mands from a primary station.
SCTE A secondary station is also known as a second-
serial clock transmit external
Timing signal that DTE echoes to DCE to
maintain clocking. SCTE is designed to com-
Glossary 391

security management service access point
One of five categories of network management See SAP.
defined by ISO for management of OSI net-
Service Advertisement Protocol
works. Security management subsystems are
See SAP.
responsible for controlling access to network
resources. service point
Interface between non-SNA devices and
NetView that sends alerts from equipment un-
1) Section of a network that is bounded by
known to the SNA environment.
bridges, routers, or switches.
2) In a LAN using a bus topology, a segment is Service Profile Identifier
a continuous electrical circuit that is often con-
nected to other such segments with repeaters.
3) Term used in the TCP specification to de- Service-Oriented Network Architecture
scribe a single transport layer unit of informa- See SONA.
tion. The terms datagram, frame, message, and
packet are also used to describe logical infor-
1) Related set of communications transactions
mation groupings at various layers of the OSI
between two or more network devices.
reference model and in various technology cir-
2) In SNA, a logical connection enabling two
NAUs to communicate.
sequence number
session layer
Number used to ensure correct sequencing of
Layer 5 of the OSI reference model. This layer
the arriving data.
establishes, manages, and terminates sessions
Sequenced Routing Update Protocol between applications and manages data ex-
See SRTP. change between presentation layer entities.
Corresponds to the data flow control layer of
serial clock transmit external
the SNA model.
Serial Interface Processor
Super Frame
See SIP.
Common framing type used on T1 circuits. SF
consists of 12 frames of 192 bits each, with the
Serial Line Internet Protocol
193rd bit providing error checking and other
functions. SF has been superseded by ESF, but
serial transmission is still widely used.
Method of data transmission in which the bits Super Frame is also known as D4 framing.
of a data character are transmitted sequentially
over a single channel. Compare with parallel
Simple Gateway Monitoring Protocol
Network management protocol that was consid-
serial tunnel ered for Internet standardization and later
See STUN. evolved into SNMP. Documented in RFC 1028.

server shaping
Node or software program that provides serv- See traffic shaping.
ices to clients.
shielded cable
Server Message Block Cable that has a layer of shielded insulation to
See SMB. reduce EMI.
392 CCNA Security Course Booklet, Version 1.0

shielded twisted-pair silicon switching engine
See STP. See SSE.

Shipworm Simple Gateway Monitoring Protocol
See Teredo. See SGMP.

shortest path first algorithm Simple Mail Transfer Protocol
See SPF. See SMTP.

shortest-path routing Simple Multicast Routing Protocol
Routing that minimizes distance or path cost See SMRP.
through application of an algorithm.
Simple Network Management Protocol
signal injector See SNMP.
Device used to measure attenuation of a signal
on a network.
Capability for data transmission in only one di-
signal reference ground rection between a sending station and a receiv-
Reference point used by computing devices to ing station. Compare with full duplex and half
measure and compare incoming digital signals. duplex.

signaling single-mode fiber
Process of sending a transmission signal over a Fiber-optic cabling with a narrow core that al-
physical medium for purposes of communica- lows light to enter only at a single angle. Such
tion. cabling has higher bandwidth than multimode
fiber, but requires a light source with a narrow
signaling packet spectral width (for example, a laser).
Generated by an ATM-connected device that Single-mode fiber is also known as monomode
wants to establish a connection with another fiber.
such device. The signaling packet contains the
ATM NSAP address of the desired ATM end-
point, as well as any QOS parameters required 1) SMDS Interface Protocol. Used in communi-
for the connection. If the endpoint can support cations between CPE and SMDS network
the desired QOS, it responds with an accept equipment. Allows the CPE to use SMDS serv-
message, and the connection is opened. ice for high-speed WAN internetworking.
Based on the IEEE 802.6 DQDB standard. See
Signaling System number 7 also DQDB.
See SS7. 2) Serial Interface Processor. Obsolete interface
processor for Cisco 7000 series routers that
provided either two or four channel-independ-
Stateless IP/ICMP Translation
ent ports for synchronous serial connections at
Algorithm used in NAT-PT that translates the
speeds from 2.4 Kbps to 4 Mbps. The SIP has
IP header fields. Compare with NAT-PT and
been replaced by the FSIP. Sometimes called
SX-SIP or Pre-FSIP. See also FSIP.
Silicon Switch Processor
site-local unicast address
See SSP.
An IPv6 address which is very similar in func-
tion to the IPv4 private address space that in-
silicon switching
cludes ranges. These addresses are meant for
Switching based on the SSE, which allows the
internal communications and are not routable
processing of packets independent of the Sili-
on the public Internet. Site-local addresses start
con Switch Processor (SSP) system processor.
with the prefix FEC0::/10. Compare with link-
Silicon switching provides high-speed, dedi-
local unicast address.
cated packet switching.
Glossary 393

sliding window SMI
Refers to the fact that the window size is nego- Structure of Management Information
tiated dynamically during the TCP session. Document (RFC 1155) specifying rules used to
define managed objects in the MIB.
sliding window flow control
Method of flow control in which a receiver
gives transmitter permission to transmit data See traffic shaping.
until a window is full. When the window is full,
the transmitter must stop transmitting until the
Simple Multicast Routing Protocol
receiver advertises a larger window. TCP, other
Specialized multicast network protocol for
transport protocols, and several data link layer
routing multimedia data streams on enterprise
protocols use this method of flow control.
networks. SMRP works in conjunction with
SLIP multicast extensions to the AppleTalk protocol.
Serial Line Internet Protocol
Standard protocol for point-to-point serial con-
Station Management
nections using a variation of TCP/IP. Predeces-
ANSI FDDI specification that defines how ring
sor of PPP.
stations are managed.
slotted ring
LAN architecture based on a ring topology in
Simple Mail Transfer Protocol
which the ring is divided into slots that circu-
Internet protocol providing electronic mail
late continuously. Slots can be either empty or
full, and transmissions must start at the begin-
ning of a slot. SNA
Systems Network Architecture
slow switching
Large, complex, feature-rich network architec-
Packet processing performed at process level
ture developed in the 1970s by IBM. Similar in
speeds, without the use of a route cache. Con-
some respects to the OSI reference model, but
trast with fast switching.
with a number of differences. SNA is essen-
SMAC tially composed of seven layers.
Source MAC
SNA Distribution Services
MAC address specified in the Source Address
field of a packet. Compare with DMAC.
SNA Network Interconnection
See SNI.
Server Message Block
File-system protocol used in LAN Manager and SNADS
similar NOSs to package data and exchange in-
SNA Distribution Services
formation with other systems.
Consists of a set of SNA transaction programs
that interconnect and cooperate to provide
asynchronous distribution of information be-
Switched Multimegabit Data Service
tween end users. One of three SNA transaction
High-speed, packet-switched, datagram-based
WAN networking technology offered by the
telephone companies. SNAP
Subnetwork Access Protocol
SMDS Interface Protocol
Internet protocol that operates between a net-
See SIP.
work entity in the subnetwork and a network
entity in the end system. SNAP specifies a stan-
dard method of encapsulating IP datagrams and
394 CCNA Security Course Booklet, Version 1.0

ARP messages on IEEE networks. The SNAP 3, NET or NSAP, address and is therefore usu-
entity in the end system makes use of the serv- ally a MAC address on a LAN or Virtual Cir-
ices of the subnetwork and performs three key cuit ID in X.25, Frame-Relay, or ATM.
functions: data transfer, connection manage-
ment, and QOS selection.
Software structure operating as a communica-
Snapshot routing tions end point within a network device.
Method of gathering routing information during
an active time, taking a snapshot of the infor-
Service-Oriented Network Architecture
mation and using that routing information for a
Architectural framework that guides the evolu-
configured length of time (referred to as the
tion of the network to an Intelligent Informa-
quiet time).
tion Network (IIN). It enables enterprises to
SNI optimize applications, processes, and resources
1) Subscriber Network Interface. Interface for to deliver greater business benefits.
SMDS-based networks that connects CPE and
an SMDS switch. See also UNI.
Synchronous Optical Network
2) SNA Network Interconnection. IBM gate-
High-speed (up to 2.5 Gbps) synchronous net-
way connecting multiple SNA networks.
work specification developed by Bellcore and
SNMP designed to run on optical fiber. STS-1 is the
Simple Network Management Protocol basic building block of SONET. Approved as
Network management protocol used almost ex- an international standard in 1988.
clusively in TCP/IP networks. SNMP provides
source address
a means to monitor and control network de-
Address of a network device that is sending
vices, and to manage configurations, statistics
collection, performance, and security.
source and destination IP addresses
SNMP communities
Field within an IP datagram that indicates the
Authentication scheme that enables an intelli-
32-bit source and destination IP addresses.
gent network device to validate SNMP requests
from sources such as the NMS. A LightStream source MAC
2020 ATM switch, for example, responds only
to SNMP requests that come from members of
known communities and that have the access source port
privileges required for that request. Number of the calling port.
SNMP2 source service access point
Simple Network Management Protocol version See SSAP.
source-route bridging
Version 2 of the popular network management
See SRB.
protocol. SNMP2 supports centralized as well
as distributed network management strategies,
source-route translational bridging
and includes improvements in the SMI, proto-
col operations, management architecture, and
security. source-route transparent bridging
See SRT.
subnetwork point of attachment address Southeastern Universities Research
SNPA address is the point at which subnetwork Association Network
services are provided. This is the equivalent of See SURAnet.
the Layer 2 address corresponding to the Layer
Glossary 395

SP scribes. The ISDN device uses the SPID when
accessing the switch that initializes the connec-
switch processor
tion to a service provider.
Cisco 7000-series processor module that acts as
the administrator for all CxBus activities.
Switch processor is also known as ciscoBus
Any power impulse lasting between .5 and 100
microseconds and possessing an amplitude over
SPAN 100% of peak power line voltage.
1) Switched Port Analyzer. Feature of the Cata-
split-horizon updates
lyst 5000 switch that extends the monitoring
Routing technique in which information about
abilities of existing network analyzers into a
routes is prevented from exiting the router in-
switched Ethernet environment. SPAN mirrors
terface through which that information was re-
the traffic at one switched segment onto a pre-
ceived. Split-horizon updates are useful in
defined SPAN port. A network analyzer at-
preventing routing loops.
tached to the SPAN port can monitor traffic
from any of the other Catalyst switched ports. spoofing
2) Full-duplex digital transmission line between 1) Scheme used by Cisco routers to cause a
two digital facilities. host to treat an interface as if it were up and
supporting a session. The router spoofs replies
spanning tree
to keepalive messages from the host in order to
Loop-free subset of a network topology.
convince that host that the session still exists.
Spanning Tree Protocol Spoofing is useful in routing environments such
as DDR, in which a circuit-switched link is
See STP.
taken down when there is no traffic to be sent
spanning-tree algorithm across it in order to save toll charges. See also
2) The act of a packet illegally claiming to be
spanning-tree protocol
from an address from which it was not actually
See STP.
sent. Spoofing is designed to foil network secu-
rity mechanisms such as filters and access lists.
sparse mode PIM
See PIM sparse mode.
spread spectrum
A radio transmission technology that spreads
speed matching
the user information over a much wider band-
Feature that provides sufficient buffering capa-
width than otherwise required in order to gain
bility in a destination device to allow a high-
benefits such as improved interference toler-
speed source to transmit data at its maximum
ance and unlicensed operation.
rate, even if the destination device is a lower-
speed device.
source route/translational bridging
Method of bridging where source-route stations
shortest path first algorithm
can communicate with transparent bridge sta-
Routing algorithm that iterates on length of
tions with the help of an intermediate bridge
path to determine a shortest-path spanning tree.
that translates between the two bridge proto-
Commonly used in link-state routing algo-
cols. Compare with SRT.
SPF is also known as Dijkstra™s algorithm.
Type of RAM that retains its contents for as
long as power is supplied. SRAM does not re-
Service Profile Identifier
quire constant refreshing, like DRAM. Com-
Number that some service providers use to de-
pare with DRAM.
fine the services to which an ISDN device sub-
396 CCNA Security Course Booklet, Version 1.0

source-route bridging silicon switching engine
Method of bridging originated by IBM and Routing and switching mechanism that com-
popular in Token Ring networks. In a SRB net- pares the data link or network layer header of
work, the entire route to a destination is prede- an incoming packet to a silicon-switching
termined, in real time, prior to the sending of cache, determines the appropriate action (rout-
data to the destination. Contrast with transpar- ing or bridging), and forwards the packet to the
ent bridging. proper interface. The SSE is directly encoded in
the hardware of the Silicon Switch Processor
SRT (SSP) of a Cisco 7000 series router. It can
source-route transparent bridging therefore perform switching independently of
IBM bridging scheme that merges the two most the system processor, making the execution of
prevalent bridging strategies, SRB and trans- routing decisions much quicker than if they
parent bridging. SRT employs both technolo- were encoded in software.
gies in one device to satisfy the needs of all
ENs. No translation between bridging protocols
is necessary. Compare with SR/TLB. service set identifier
The SSID is a code attached to all packets on a
SRTP wireless network to identify each packet as part
Sequenced Routing Update Protocol of that network. The code is a case sensitive
Protocol that assists VINES servers in finding text string which consists of a maximum of 32
neighboring clients, servers, and routers. alphanumeric characters. All wireless devices
attempting to communicate with each other
must share the same SSID. Apart from identify-
Signaling System number 7
ing each packet, SSID also serves to uniquely
Standard CCS system used with BISDN and
identify a group of wireless network devices
ISDN. Developed by Bellcore.
used in a given service set.
source service access point
1) Silicon Switch Processor. High-performance
The SAP of the network node designated in the
silicon switch for Cisco 7000 series routers that
Source field of a packet. Compare to DSAP.
provides distributed processing and control for
interface processors. The SSP leverages the
high-speed switching and routing capabilities
system services control points
of the SSE to dramatically increase aggregate
Focal points within an SNA network for man-
router performance, minimizing performance
aging network configuration, coordinating net-
bottlenecks at the interface points between the
work operator and problem determination
router and a high-speed backbone.
requests, and providing directory services and
2) Switch-to-Switch Protocol. Protocol speci-
other session services for network end users.
fied in the DLSw standard that routers use to
SSCP-PU session establish DLSw connections, locate resources,
system services control points - physical unit forward data, and handle flow control and error
session recovery.
Session used by SNA to allow an SSCP to man-
age the resources of a node through the PU.
Spanning Tree Algorithm
SSCPs can send requests to, and receive replies
Algorithm used by the Spanning Tree Protocol
from, individual nodes in order to control the
to create a spanning tree.
network configuration.
See protocol stack.
Glossary 397

standard single physical channel. Statistical multiplexing
dynamically allocates bandwidth only to active
Set of rules or procedures that are either widely
input channels, making better use of available
used or officially specified.
bandwidth and allowing more devices to be
star topology connected than with other multiplexing tech-
LAN topology in which end points on a net- niques. Also referred to as statistical time-divi-
work are connected to a common central switch sion multiplexing or stat mux. Compare with
by point-to-point links. A ring topology that is ATDM, FDM, and TDM.
organized as a star implements a unidirectional
statistical time-division multiplexing
closed-loop star, instead of point-to-point links.
Compare with bus topology, ring topology, and
tree topology.
StarLAN statistical time-division multiplexing
Technique whereby information from multiple
CSMA/CD LAN, based on IEEE 802.3, devel-
logical channels can be transmitted across a
oped by AT and T.
single physical channel. Statistical multiplexing
start-stop transmission dynamically allocates bandwidth only to active
See asynchronous transmission. input channels, making better use of available
bandwidth and allowing more devices to be
stat mux
connected than with other multiplexing tech-
See statistical multiplexing.
niques. Also referred to as statistical time-divi-
sion multiplexing or stat mux. Compare with
stateless autoconfiguration
Plug-and-play IPv6 feature that enables devices
to connect themselves to the network without
any configuration and without any servers (like
Synchronous Transport Module level 1
DHCP servers). This key feature enables de-
One of a number of SDH formats that specifies
ployment of new devices on the Internet, such
the frame structure for the 155.52-Mbps lines
as cellular phones, wireless devices, home ap-
used to carry ATM cells.
pliances, and home networks.
store and forward packet switching
Stateless IP/Internet Control Message
Packet-switching technique in which frames are
Protocol (ICMP)
completely processed before being forwarded
out the appropriate port. This processing in-
cludes calculating the CRC and checking the
static electricity
destination address. In addition, frames must be
Unpredictable electrical charges in the atmos-
temporarily stored until network resources
phere that interfere with radio reception, com-
(such as an unused link) are available to for-
puter networking, and the like.
ward the message. Contrast with cut-through
static route packet switching.
Route that is explicitly configured and entered
into the routing table. Static routes take prece-
1) shielded twisted-pair. Two-pair wiring
dence over routes chosen by dynamic routing
medium used in a variety of network imple-
mentations. STP cabling has a layer of shielded
Station Management insulation to reduce EMI. Compare with UTP.
See SMT. 2) Spanning Tree Protocol. Bridge protocol that
utilizes the spanning-tree algorithm, enabling a
statistical multiplexing
learning bridge to dynamically work around
Technique whereby information from multiple
loops in a network topology by creating a span-
logical channels can be transmitted across a
ning tree. Bridges exchange BPDU messages
398 CCNA Security Course Booklet, Version 1.0

with other bridges to detect loops, and then re- Router feature allowing two SDLC- or HDLC-
move the loops by shutting down selected compliant devices to connect to one another
bridge interfaces. Refers to both the IEEE through an arbitrary multiprotocol topology
802.1 Spanning-Tree Protocol standard and the (using Cisco routers) rather than through a di-
earlier Digital Equipment Corporation Span- rect serial link.
ning-Tree Protocol upon which it is based. The
IEEE version supports bridge domains and al-
Portion of an SNA network that consists of a
lows the bridge to construct a loop-free topol-
subarea node and any attached links and pe-
ogy across an extended LAN. The IEEE
ripheral nodes.
version is generally preferred over the Digital
version. subarea node
SNA communication controller or host that
StreamView network management
handles complete network addresses.
Cisco suite of SNMP-based network manage-
ment tools used in conjunction with the Light- subchannel
Stream 2020 ATM switch. The StreamView
In broadband terminology, a frequency-based
suite includes three GUI-driven applications: a
subdivision creating a separate communications
configuration program (the configurator), a net-
work topology map (the topology map), and a
node monitoring program (the monitor); and a subinterface
command-line interface (CLI). One of a number of virtual interfaces on a sin-
gle physical interface.
Structure of Management Information
See SMI. subnet
See subnetwork.
Synchronous Transport Signal level 1 subnet address
Basic building block signal of SONET, operat- Portion of an IP address that is specified as the
ing at 51.84 Mbps. Faster SONET rates are de- subnetwork by the subnet mask.
fined as STS-n, where n is a multiple of 51.84
subnet mask
32-bit address mask used in IP to indicate the
STS-3c bits of an IP address that are being used for the
Synchronous Transport Signal level 3, concate- subnet address.
nated Subnet mask is also known as mask.
SONET format that specifies the frame struc-
subnet mask field
ture for the 155.52-Mbps lines used to carry
The subnet mask field contains a 32-bit mask
ATM cells.
that identifies the network and subnet portion
stub area of the IP address. The addition of this field is
OSPF area that carries a default route, intra- the single most important change made to the
area routes, and interarea routes, but does not RIP v2 message structure.
carry external routes. Virtual links cannot be
configured across a stub area, and they cannot
1) In IP networks, a network sharing a particu-
contain an ASBR. Compare to non-stub area.
lar subnet address. Subnetworks are networks
stub network arbitrarily segmented by a network administra-
Network that has only a single connection to a tor in order to provide a multilevel, hierarchical
router. routing structure while shielding the subnet-
work from the addressing complexity of at-
tached networks. Subnetwork is also known as
serial tunnel
Glossary 399

subnet. See also IP address, subnet address, and SVC
subnet mask. switched virtual circuit
2) In OSI networks, a collection of ESs and ISs Virtual circuit that is dynamically established
under the control of a single administrative do- on demand and is torn down when transmission
main and using a single network access proto- is complete. SVCs are used in situations where
col. data transmission is sporadic.
Switched virtual circuit is also known as
Subnetwork Access Protocol
switched virtual connection in ATM terminol-
See SNAP. ogy.
subnetwork point of attachment switch
See SNPA. 1) Network device that filters, forwards, and
floods frames based on the destination address
Subscriber Network Interface
of each frame. The switch operates at the data
See SNI.
link layer of the OSI model.
subvector 2) General term applied to an electronic or me-
chanical device that allows a connection to be
A data segment of a vector in an SNA message.
established as necessary and terminated when
A subvector consists of a length field, a key that
there is no longer a session to support.
describes the subvector type, and subvector
specific data.
switch card
Super Frame Card on the LightStream 2020 ATM switch that
handles communication between the other
See SF.
cards on the switch. Each LightStream 2020
supernetting switch has one or two switch cards. The second
Aggregating IP network addresses advertised as card, if present, serves as a backup for the first.
a single classless network address. For exam-
Switch Processor
ple, given four Class C IP networks-,
See SP., and hav-
ing the intrinsic network mask of
switched LAN, one can advertise the address
LAN implemented with LAN switches. with a subnet mask of
Switched Multimegabit Data Service
supervisory processor
See RP (route processor).
Switched Port Analyzer
Southeastern Universities Research Association
Network switched virtual circuit
Network connecting universities and other or- See SVC.
ganizations in the Southeastern United States.
SURAnet, originally funded by the NSF and a switched virtual connection
part of the NSFNET, is now part of BBN See SVC.
Switch-to-Switch Protocol
surge See SSP.
Any voltage increase above 110% of the nor-
mal voltage carried by a power line.
Cisco SNMP-based network management soft-
sustainable cell rate ware, running on Microsoft Windows, that of-
See SCR. fers a powerful set of tools to manage an entire
network, including switches, hubs, routers, and
400 CCNA Security Course Booklet, Version 1.0

bridges. SwitchVision can automatically dis- system services control points
cover and map any SNMP device on the net- See SSCP.
work and show the status of network devices.
Systems Network Architecture
SwitchVision allows network administrators to
See SNA.
set event thresholds, activate actions when error
conditions occur, and set up custom tables and
graphs to view critical network variables.
Digital WAN carrier facility. T1 transmits DS-
1-formatted data at 1.544 Mbps through the
telephone-switching network, using AMI or
Establishment of common timing between
B8ZS coding. Compare with E1.
sender and receiver.
Synchronous Data Link Control
Digital WAN carrier facility. T3 transmits DS-
3-formatted data at 44.736 Mbps through the
Synchronous Digital Hierarchy telephone switching network. Compare with
See SDH. E3.

Synchronous Optical Network TAC
See SONET. 1) Terminal Access Controller. Internet host
that accepts terminal connections from dial-up
synchronous transmission
Term describing digital signals that are trans- 2) Technical Assistance Center. Cisco TACs
mitted with precise clocking. Such signals have provide technical assistance to partners and end
the same frequency, with individual characters users, and form the hub of Cisco global sup-
encapsulated in control bits (called start bits port.
and stop bits) that designate the beginning and
end of each character. Compare with asynchro- TACACS
nous transmission, isochronous transmission, Terminal Access Controller Access Control
and plesiochronous transmission. System
Authentication protocol, developed by the
Synchronous Transport Module level 1
DDN community, that provides remote access
See STM-1. authentication and related services, such as
event logging. User passwords are administered
Synchronous Transport Signal level 1
in a central database rather than in individual
See STM-1.
routers, providing an easily scalable network
Synchronous Transport Signal level 3, con- security solution.
See STS-3c.
Terminal Access Controller Access Control
sysgen System Plus
system generation Proprietary Cisco enhancement to TACACS.
Process of defining network resources in a net- Provides additional support for authentication,
work. authorization, and accounting.

system generation tagged traffic
See sysgen. ATM cells that have their CLP bit set to 1. If
the network is congested, tagged traffic can be
System ID
dropped to ensure delivery of higher-priority
System ID is a NSAP address field that identi- traffic.
fies an individual OSI device. In OSI, a device Tagged traffic is also known as discard eligible
has an address, just as it does in DECnet, while (DE).
in IP an interface has an address.
Glossary 401

Transparent Asynchronous Transmitter/Re- time domain reflectometer
ceiver Interface 4-byte/5-byte Device capable of sending signals through a
Encoding scheme used for FDDI LANs as well network medium to check cable continuity,
as for ATM. Supports speeds of up to 100 Mbps length, and other attributes. TDRs are used to
over multimode fiber. TAXI is the chipset that find physical layer network problems.
generates 4B/5B encoding on multimode fiber.
Technical Assistance Center
T-carrier See TAC.
TDM transmission method usually referring to
Technical Office Protocol
a line or cable carrying a DS-1 signal.
See TOP.
Transmission Control Protocol
Abbreviation for telephone company.
Connection-oriented transport layer protocol
that provides reliable full-duplex data transmis- telecommunications
sion. TCP is part of the TCP/IP protocol stack.
Term referring to communications (usually in-
volving computer systems) over the telephone
Transmission Control Protocol/Internet Proto-
col Telecommunications Industry Association
Common name for the suite of protocols devel-
See TIA.
oped by the U.S. DoD in the 1970s to support
the construction of worldwide internetworks. telephony
TCP and IP are the two best-known protocols Science of converting sound to electrical sig-
in the suite. nals and transmitting it between widely re-
moved points.
test and control system telepole
Independently-powered subsystem used to ini- Telescoping pole with a hook at one end. It is
tialize, monitor, and troubleshoot the hardware used to get cable across a ceiling or attic
on a LightStream 2020 ATM switch. The TCS quickly.
consists of a hub residing on the switch card
and slaves on NPs and line cards.
Work arrangement in which employees enjoy
TCU limited flexibility in working location and
trunk coupling unit hours. The daily commute to a central place of
In Token Ring networks, a physical device that work is replaced by telecommunication links.
enables a station to connect to the trunk cable. Teleworking is also known as a Branch of One,
telecommuting, e-commuting, telework, or
working from home (WFH).
time-division multiplexing
Technique in which information from multiple telex
channels can be allocated bandwidth on a sin- Teletypewriter service allowing subscribers to
gle wire based on preassigned time slots. Band- send messages over the PSTN.
width is allocated to each channel regardless of
whether the station has data to transmit. Com-
Command used to verify the application layer
pare with ATDM, FDM, and statistical multi-
software between source and destination sta-
tions. This is the most complete test mechanism
402 CCNA Security Course Booklet, Version 1.0

Tempest test and control system
U.S. military standard. Electronic products ad- See TCS.
hering to the Tempest specification are de-
Texas Higher Education Network
signed to withstand EMP.
See THEnet.
Teredo is a mechanism which tunnels IPv6
Trivial File Transfer Protocol
datagrams within IPv4 UDP. This method pro-
Simplified version of FTP that allows files to be
vides for private IPv4 address use and IPv4
transferred from one computer to another over
NAT traversal.
a network.
Teredo was formerly known as Shipworm.
transmission header
SNA cluster controller identification. Termid is
SNA header that is appended to the SNA basic
meaningful only for switched lines.
information unit (BIU). The TH uses one of a
Termid is also known as Xid.
number of available SNA header formats.
THC over X.25
Simple device at which data can be entered or
Feature providing TCP/IP header compression
retrieved from a network. Generally, terminals
over X.25 links, for purposes of link efficiency.
have a monitor and a keyboard, but no proces-
sor or local disk drive. THEnet
Texas Higher Education Network
Terminal Access Controller
Regional network comprising over 60 academic
See TAC.
and research institutions in the Texas (United
Terminal Access Controller Access System States) area.
terminal adapter Term used to define a thinner, less expensive
Device used to connect ISDN BRI connections version of the cable specified in the IEEE 802.3
to existing interfaces such as EIA/TIA-232. Es- 10BASE2 standard. Compare with Cheapernet.
sentially, an ISDN modem.
terminal emulation Rate of information arriving at, and possibly
Network application in which a computer runs passing through, a particular point in a network
software that makes it appear to a remote host system.
as a directly attached terminal.
terminal server Telecommunications Industry Association
Communications processor that connects asyn- Organization that develops standards relating to
chronous devices such as terminals, printers, telecommunications technologies. Together, the
hosts, and modems to any LAN or WAN that TIA and the EIA have formalized standards,
uses TCP/IP, X.25, or LAT protocols. Terminal such as EIA/TIA-232, for the electrical charac-
servers provide the internetwork intelligence teristics of data transmission.
that is not available in the connected devices.
terminator Plastic ties used for holding cables together or
Device that provides electrical resistance at the for holding cables in place.
end of a transmission line to absorb signals on
time domain reflectometer
the line, thereby keeping them from bouncing
See TDR.
back and being received again by network sta-
Glossary 403

time domain reflectometry TNotify
Technique of sending an electrical signal down Time Notify
a cable and then timing the signal™s reflection Specifies how often SMT initiates neighbor no-
back from the end of the cable. tification broadcasts.

Time Notify to switch unit
See TNotify. See TSU.

Time To Live token
See TTL. Frame that contains control information. Pos-
session of the token allows a network device to
time-division multiplexing transmit data onto the network.
See TDM.
token bus
time-out LAN architecture using token passing access
Event that occurs when one network device ex- over a bus topology. This LAN architecture is
pects to hear from another network device the basis for the IEEE 802.4 LAN specification.
within a specified period of time, but does not.
token passing
The resulting time-out usually results in a re-
transmission of information or the dissolving of Access method by which network devices ac-
the session between the two devices. cess the physical medium in an orderly fashion
based on possession of a small frame called a
TKIP token. Contrast with circuit switching and con-
Temporal Key Integrity Protocol tention.
TKIP is a WPA feature used to ensure integrity
Token Ring
in wireless data transmission.
Token-passing LAN developed and supported
TLV by IBM. Token Ring runs at 4 or 16 Mbps over
Type, Length, Value a ring topology. Similar to IEEE 802.5.
TLV is in the IS-IS and ES-IS PDUs that con-
tain variable-length fields, depending on the
function of the PDU. Each field contains a type Technical Office Protocol
code and length, followed by the appropriate OSI-based architecture developed for office
values. These fields are identified by one octet communications.
of type (T), one octet of length (L) and “L”
octets of value (V). The Type field indicates the
Physical arrangement of network nodes and
type of items in the Value field. The Length
media within an enterprise networking struc-
field indicates the length of the Value field. The
Value field is the data portion of the packet. Not
all router implementations support all TLVs, topology map
but they are required to ignore and retransmit
Tool for managing a LightStream 2020 ATM
the ignored types.
switch that examines a network and displays
the status of its nodes and trunks. The topology
map is an HP OpenView-based application that
Terminal emulation software that allows a ter-
runs on an NMS.
minal to appear to an IBM host as a 3278
Model 2 terminal. The Cisco TN3270 imple- ToS
mentation allows users to access an IBM host
type of service
without using a special IBM server or a UNIX
Field within an IP datagram that indicates how
host acting as a server.
the datagram should be handled.
404 CCNA Security Course Booklet, Version 1.0

total length traffic shaping
Field within an IP datagram that indicates total Use of queues to limit surges that can congest a
length of the header plus the data. network. Data is buffered and then sent into the
network in regulated amounts to ensure that the
Totally stub area traffic will fit within the promised traffic enve-
An area that does not accept external au- lope for the particular connection. Traffic shap-
tonomous system (AS) routes and summary ing is used in ATM, Frame Relay, and other
routes from other areas internal to the au- types of networks.
tonomous system. Instead, if the router needs to Traffic shaping is also known as metering,
send a packet to a network external to the area, shaping, or smoothing.
it sends it using a default route.
TPC Control information appended to data when en-
Transmit Power Control capsulating the data for network transmission.
TPC is an IEEE 802.11h specification which Compare with header.
has been used in the cellular telephone industry
for years. TPC sets the transmit power of the
access point and the client adapter to allow for Result-oriented unit of communication process-
different coverage area sizes and to conserve ing.
battery life.
transaction services layer
trace route Layer 7 in the SNA architectural model. Repre-
Program available on many systems that traces sents user application functions, such as spread-
the path a packet takes to a destination. It is sheets, word-processing, or electronic mail, by
mostly used to debug routing problems between which users interact with the network. Corre-
hosts. There is also a traceroute protocol de- sponds roughly with the application layer of the
fined in RFC 1393. OSI reference model.

traffic management transceiver
See ControlStream traffic management. See MAU.

traffic policing transceiver cable
Process used to measure the actual traffic flow See AUI.
across a given connection and compare it to the
transfer priority
total admissable traffic flow for that connec-
See transmit priority.
tion. Traffic outside of the agreed upon flow
can be tagged (where the CLP bit is set to 1) transit bridging
and can be discarded en route if congestion de-
Bridging that uses encapsulation to send a
velops. Traffic policing is used in ATM, Frame
frame between two similar networks over a dis-
Relay, and other types of networks. Also know
similar network.
as admission control, permit processing, rate
enforcement, and usage parameter control translational bridging
(UPC). Bridging between networks with dissimilar
MAC sublayer protocols. MAC information is
traffic profile
translated into the format of the destination net-
Set of COS attribute values assigned to a given
work at the bridge. Contrast with encapsulation
port on a LightStream 2020 ATM switch. The
profile affects numerous parameters for data
transmitted from the port including rate, cell transmission control layer
drop eligibility, transmit priority, and inactivity Layer 4 in the SNA architectural model. This
timer. layer is responsible for establishing, maintain-
ing, and terminating SNA sessions, sequencing
Glossary 405

data messages, and controlling session level ence of bridges is transparent to network end
flow. Corresponds to the transport layer of the nodes. Contrast with SRB.
OSI model.
transport layer
Transmission Control Protocol Layer 4 of the OSI reference model. This layer
See TCP. is responsible for reliable network communica-
tion between end nodes. The transport layer
Transmission Control Protocol/Internet provides mechanisms for the establishment,
maintenance, and termination of virtual cir-
See TCP/IP. cuits, transport fault detection and recovery,
and information flow control. Corresponds to
transmission group
the transmission control layer of the SNA
In SNA routing, one or more parallel communi-
cations links treated as one communications fa-
cility. trap
Message sent by an SNMP agent to an NMS,
transmission header
console, or terminal to indicate the occurrence
See TH.
of a significant event, such as a specifically de-
transmission link fined condition or a threshold that has been
See link.

tree topology
transmit power
LAN topology similar to a bus topology, except
A radio transmission technology that spreads
that tree networks can contain branches with
the user information over a much wider band-
multiple nodes. Transmissions from a station
width than otherwise required in order to gain
propagate the length of the medium and are re-
benefits such as improved interference toler-
ceived by all other stations. Compare with bus
ance and unlicensed operation.
topology, ring topology, and star topology.
transmit priority
Trivial File Transfer Protocol
Queuing scheme in which each internal TOS of
a LightStream 2020 ATM switch correlates to a
relative priority in queues in the ATM network.
This priority determines which traffic is serv-
Physical and logical connection between two
iced first in the case of contention for a network
ATM switches across which traffic in an ATM
network travels. An ATM backbone is com-
Transmit priority is also known as forwarding
posed of a number of trunks.
priority or transfer priority.
trunk card
Line card on a LightStream 2020 ATM switch
Major packet data network run by France Tele-
that is configured to communicate with other
ATM switches. LightStream 2020 trunk cards
Transparent Asynchronous offer a variety of interface types. CLCs, LSCs,
Transmitter/Receiver Interface 4-byte/5-byte and MSCs can operate as trunk cards.
See TAXI 4B/5B.
trunk coupling unit
transparent bridging See TCU.
Bridging scheme often used in Ethernet and
Trunk Up-Down
IEEE 802.3 networks in which bridges pass
See TUD.
frames along one hop at a time based on tables
associating end nodes with bridge ports. Trans-
parent bridging is so named because the pres-
406 CCNA Security Course Booklet, Version 1.0

TSU ceives data from a different secondary station
on the same serial line.
to switch unit
Subsystem on each LightStream 2020 ATM
Type 1 operation
switch line card that appends ATM routing in-
IEEE 802.2 (LLC) connectionless operation.
formation to outgoing cells and sends the cells
to the switch card. Type 2 operation
IEEE 802.2 (LLC) connection-oriented opera-
time to live
Field in an IP header that indicates how long a type of service
packet is considered valid. See ToS.
TUD Type, Length, Value
Trunk Up-Down See TLV.
Protocol used in ATM networks that monitors
trunks and detects when one goes down or
comes up. ATM switches send regular test mes- Universal Asynchronous Receiver/Transmitter
sages from each trunk port to test trunk line Integrated circuit, attached to the parallel bus of
quality. If a trunk misses a given number of a computer, used for serial communications.
these messages, TUD declares the trunk down. The UART translates between serial and paral-
When a trunk comes back up, TUD recognizes lel signals, provides transmission clocking, and
that the trunk is up, declares the trunk up, and buffers data sent to or from the computer.
returns it to service.
tunneling unspecified bit rate
Architecture that is designed to provide the QOS class defined by the ATM Forum for ATM
services necessary to implement any standard networks. UBR allows any amount of data up
point-to-point encapsulation scheme. to a specified maximum to be sent across the
network, but there are no guarantees in terms of
cell loss rate and delay. Compare with available
German test agency that certifies products to bit rate (ABR), CBR, and VBR.
European safety standards.
twisted pair
User Datagram Protocol
Relatively low-speed transmission medium Connectionless transport layer protocol in the
consisting of two insulated wires arranged in a TCP/IP protocol stack. UDP is a simple proto-
regular spiral pattern. The wires can be shielded col that exchanges datagrams without acknowl-
or unshielded. Twisted pair is common in te- edgments or guaranteed delivery, requiring that
lephony applications and is increasingly com- error processing and retransmission be handled
mon in data networks. by other protocols. UDP is defined in RFC 768.
two-way simultaneous UL


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