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spanning-tree algorithm
Algorithm used by the STP to create a spanning tree. Sometimes abbreviated STA. See also spanning tree and
Spanning-Tree Protocol.
spanning-tree explorer packet
Follows a statically configured spanning tree when looking for paths in an SRB network. Also known as a
limited-route explorer packet or a single-route explorer packet. See also all-routes explorer.
Spanning-Tree Protocol
See STP.
SRAM
Static random access memory. Type of RAM that retains its contents for as long as power is supplied. SRAM
does not require constant refreshing, like DRAM. Compare with DRAM.
SRB
Source-route bridging. Method of bridging originated by IBM and popular in Token Ring networks. In a SRB
network, the entire route to a destination is predetermined, in real time, prior to the sending of data to the
destination. Contrast with transparent bridging.
SRT
Source-route transparent bridging. IBM bridging scheme that merges the two most prevalent bridging strategies,
SRB and transparent bridging. SRT employs both technologies in one device to satisfy the needs of all ENs. No
translation between bridging protocols is necessary. Compare with SR/TLB.
SR/TLB
Source-route translational bridging. Method of bridging where source-route stations can communicate with
transparent bridge stations with the help of an intermediate bridge that translates between the two bridge
protocols. Compare with SRT.
Glossary of Terms B-33
SSAP
Source service access point. The SAP of the network node designated in the Source field of a packet. Compare to
DSAP. See also SAP.
SSCS
Service specific convergence sublayer. One of the two sublayers of any AAL. SSCS, which is service dependent,
offers assured data transmission. The SSCS can be null as well, in classical IP over ATM or LAN emulation
implementations.
SSRP
Simple server redundancy protocol. A Cisco value-add that provides backup capability for LANE 1.0 servers,
including the LECS, LES, and BUS. LANE 1.0 did not specify a method for doing this.
standby monitor
Device placed in standby mode on a Token Ring network in case an active monitor fails. See also active monitor
and ring monitor.
star topology
LAN topology in which end points on a network are connected to a common central switch by point-to-point
links. A ring topology that is organized as a star implements a unidirectional closed-loop star, instead of
point-to-point links. Compare with ring topology.
store-and-forward
Frame forwarding technique in which frames are completely processed before being forwarded out the
appropriate port. This processing includes calculating the CRC and checking the destination address. In addition,
frames must be temporarily stored until network resources (such as an unused link) are available to forward the
message. Contrast with cut-through.
STP
1. Shielded twisted-pair. Two-pair wiring medium used in a variety of network implementations. STP cabling has
a layer of shielded insulation to reduce EMI. Compare with UTP. See also twisted pair.
2. Spanning-Tree Protocol. Bridge protocol that utilizes the spanning-tree algorithm, enabling a learning bridge
to dynamically work around loops in a network topology by creating a spanning tree. Bridges exchange BPDU
messages with other bridges to detect loops, and then remove the loops by shutting down selected bridge
interfaces. Refers to both the IEEE 802.1 Spanning-Tree Protocol standard and the earlier Digital Equipment
Corporation Spanning-Tree Protocol upon which it is based. The IEEE version supports bridge domains and
allows the bridge to construct a loop-free topology across an extended LAN. The IEEE version is generally
preferred over the Digital version. Sometimes abbreviated STP. See also BPDU, MAC address learning, spanning
tree, and spanning-tree algorithm.
subnet mask
32-bit address mask used in IP to indicate the bits of an IP address that are being used for the subnet address.
Sometimes referred to as mask.
Subnetwork Access Protocol
See SNAP.
B-34 Catalyst Token Ring Switching Implementation Guide
SVC
Switched virtual circuit. Virtual circuit that is dynamically established on demand and is torn down when
transmission is complete. SVCs are used in situations where data transmission is sporadic. Called a switched
virtual connection in ATM terminology. Compare with PVC.
switch
Network device that filters, forwards, and floods frames based on the destination address of each frame. The
switch operates at the data link layer of the OSI model.
switched LAN
LAN implemented with LAN switches. See LAN switch.
Switched Multimegabit Data Service
See SMDS.
Switched Port Analyzer
See span.
switched virtual circuit
See SVC.
T
TC
Transmission convergence. A sublayer of the ATM physical layer that transforms the flow of cells into a steady
flow of bits for transmission over the physical medium. When transmitting, the TC sublayer maps the cells into
the frame format, generates the HEC, and sends idle cells when there is nothing to send. When receiving, the TC
sublayer delineates individual cells in the received bit stream and uses HEC to detect and correct errors.
TCP/IP
Transmission Control Protocol/Internet Protocol. Common name for the suite of protocols developed by the U.S.
DoD in the 1970s to support the construction of worldwide internetworks. TCP and IP are the two best-known
protocols in the suite.
TDM
Time-division multiplexing. Technique in which information from multiple channels can be allocated bandwidth
on a single wire based on preassigned time slots. Bandwidth is allocated to each channel regardless of whether
the station has data to transmit.
Telnet
Standard terminal emulation protocol in the TCP/IP protocol stack. Telnet is used for remote terminal
connection, enabling users to log in to remote systems and use resources as if they were connected to a local
system. Telnet is defined in RFC 854.
TIA
Telecommunications Industry Association. Organization that develops standards relating to telecommunications
technologies. Together, the TIA and the EIA have formalized standards, such as EIA/TIA-232, for the electrical
characteristics of data transmission. See also EIA.
Glossary of Terms B-35
TIC
Token Ring interface coupler. Controller through which an FEP connects to a Token Ring.
timeout
Event that occurs when one network device expects to hear from another network device within a specified
period of time, but does not. The resulting timeout usually results in a retransmission of information or the
dissolving of the session between the two devices.
token
Frame that contains control information. Possession of the token allows a network device to transmit data onto
the network. See also token passing.
TokenChannel
A parallel configuration of 2 to 8 ports between two Catalyst 3900s. Can also be used with the Catalyst 3920.
token passing
Access method by which network devices access the physical medium in an orderly fashion based on possession
of a small frame called a token. Contrast with circuit switching. See also token.
Token Ring
Token-passing LAN developed and supported by IBM. Token Ring runs at 4 or 16 Mbps over a ring topology.
Similar to IEEE 802.5. See also IEEE 802.5, ring topology, and token passing.
Token Ring interface coupler
See TIC.
topology
Physical arrangement of network nodes and media within an enterprise networking structure.
traffic policing
Process used to measure the actual traffic flow across a given connection and compare it to the total admissible
traffic flow for that connection. Traffic outside of the agreed upon flow can be tagged (where the CLP bit is set
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