Glossary of Terms
O to S
GOTO: A B C D E F G H I J K L
M N O P Q R S T U V W X Y Z
Object-oriented technology The
art and science of manipulating data, like programming, in the form of
"objects", streamlining ways of identifying and addressing business problems and
creating applications. Its applications are built up from objects containing both
information and the intelligence needed to process that data in a single unit;
particularly useful in workgroups where it lets a document contain its own security and
routing information. Standards are being discussed by several bodies including the Object
Management Group with its Object Management Architecture. Dogged by acronyms and competing
methodologies, object technology is a growing phenomenon.
OC-1 Optical Carrier level 1 The
lowest optical transmission rate in the incipient Sonet standard at 51.48Mbit/s.
OC-3 Optical Carrier level 3. The
second fastest optical rate in the incipient Sonet standard at 155.52Mbit/s.
OCR (Optical Character
Recognition) Software converting scanned images of documents into text files which can
Octet A grouping of eight
bits in packet switched networks similar, but not identical to, a byte.
ODBC (Open Database
Connectivity) Non-proprietary programming SQL interface specified by Microsoft for
database connectivity. It offers access to a variety of PC, minicomputer and mainframe
systems, plus Apple networks.
ODLI (Open Data Link Interface) A
Novell technique similar in concept to Microsoft's NDIS, it enables PC adapter cards to
support ODLI and so run multiple protocols to access various implementations of NetWare.
ODS Microsoft's Open Data
Services: the part of Wosa supporting access from Microsoft's SQL Server to a wide range
of data sources and formats, including information from major mainframe databases.
OEM (Original Equipment
Manufacturer) The maker of equipment marketed by another vendor, usually under the
name of the reseller. The OEM may make only certain components or complete devices, which
can then be configured with software and/or hardware by the reseller.
OLE, OLE2 (Object Linking and
Embedding) A Microsoft specification for implementing object-oriented software
developments. An improved form of DDE, it adopts a layered approach and allows a file or
program to be embedded as an object in another file. OLE2 (the most recent version) and
ODBC drivers altered without affecting core software software applications from different
vendors to be accessed seamlessly. More and more organizations are adopting this set-up.
ONMA (Open Network Management
Architecture) IBM's network management architecture comprised of Entry Points, Service
Points and Focal Points - implemented as NetView.
OS/2 An operating system
devised by Microsoft and IBM for PCs based on Intel's 80286 and 80386 microprocessors. It
supports multi-tasking and programs needing more than 640kbytes of memory, as well as
program to program communications. It is a building block on which to base distributed
processing. OS/2.2 is the current version. OS/2 EE (Extended Edition) - IBM's extended
version of the original OS/2 with enhanced communications facilities.
OSF (Open Software Foundation) A
consortium of hardware manufacturers aimed at setting common standards for open systems,
including operating systems and networks. The OSF has defined the Distributed Computing
OSI Basic Reference Model (Open
Systems Interconnection Reference Model) An architectural model describing how
communications can be achieved between different vendors' systems. It is a logical
structure for network operations standardized within ISO and containing seven primary
layers. It enables any OSI compliant computer or device to communicate with other OS
1-compliant equipment. The seven Layers, starting with the lowest are the Physical, Data
Link, Network, Transport, Session, Presentation and Applications.
OSN (Office Systems Node) An
IBM concept describing a set of functions and services provided to connected nodes in an
IBM office system. OSN has a central role in a Document Interchange Architecture-defined
office system. Office systems with more limited IBM function sets are termed Source or
Recipient Nodes. A typical OSN is a host computer running software as the IBM Distributed
Office Support System (Dioss), while an SRN is typically a text-processing PC. A user at a
workstation, through the use of commands defined in DIA, can request an OSN to supply
document library, document distribution, fiber transfer and applications processing
services. DIA enables an SNA network to build an office systems network with multiple
remote locations sharing an OSN.
OSPF (Open Shortest Path First) A
dynamic routing protocol based on a shortest path first algorithm, better suited to larger
networks than RIP.
Out-of-Band signaling An
extra signal transmitted alongside the information signal to monitor and control the
transmission. It uses a separate channel of the LAN and allows network management devices
to access LAN devices even when the LAN itself is not functioning, so providing an
additional layer of resilience. P
PABX (PBX) A Private
Automatic Branch eXchange/telephone exchange linked to the PSTN. It handles calls
automatically i.e., unattended. Normally a manual, user-owned exchange.
Packet A collection of bits,
including the address, data and control, that are switched and transmitted together. The
terms frame and packet are often used synonymously.
Packet Buffer Memory space
reserved for storing a packet awaiting transmission or for storing a received packet.
Packet Switching A method of
switching data in a network. Individual packets of a set size and format are accepted by
the network and delivered to their destination. The sequence of packets is maintained, and
destination established, by the exchange of control information (also contained in the
packets) between the sending terminal and the network before the transmission starts. The
network is open to all users, all the time, with packets from the various nodes being
interleaved throughout the network. The packets can be sent in any order, as the control
information sent at the beginning of the transmission ensures they are interpreted in the
correct order at the receiving end. Because each packet carries its own control
instructions, it can use any route to reach its destination. The link lasts only as long
as the trarsmission. An ITU-TS standard for packet switched networks information.
Predictive dialing Automated
dialing feature in which the systems predicts from pre-set algorithms when an
agent/operator will become free and makes the call in advance.
Preview dialing Automated
dialing feature in which the system determines the next call to be made but allows the
agent/operator as a supervisor to check and activate the call.
Presentation Layer The sixth
of seven layers in the OSI model. Its job is to ensure that two computers wanting to
communicate establish the ground rules for how they can interoperate.
PRA (Primary Rate Access) An
ISDN access method that uses maximum data rates of 2.048 Mbit/s in Europe, or 1.544 Mbit/s
in the US and Japan.
PRI (Primary Rate Interface) ISDN's
carrier or large user interface comprising 30BD channels in Europe, supporting around
Primary domain controller The
server at which the master copy of a domain's user accounts database is maintained. The
primary domain controller also validates logon requests. A LAN Manager term.
Print Server A computer
and/or software to provide users or a network with access to a central printer. The
printer server acts as a buffer, holding the information to be printed out in memory until
the printer is free. It is possible to program the printer server to print jobs in the
order they arrive or to give priority to particular users who, in effect, jump the queue.
The advantages of a printer server include maximum exploitation of expensive resources
like laser printers and not having to retry if the printer is initially busy.
Printer driver A program
that controls printing and sets options such as print quality and paper size for a
particular printer. For example, in LAN Manager, each printer queue has a single printer
driver associated with it.
Private (dedicated) line Term
for a dedicated voice or data circuit, usually leased from a Common Carrier.
Privilege level For
user-level security, one of three settings -user, admin or guest is assigned for each user
account. The privilege level defines interfaces was drawn up in 1980, known as
Recommendation X.25. Hence packet switched networks are often referred to as X.25. In some
cases X.25 is being superseded by frame relay and eventually by ATM.
Assembler/Disassembler) A device used in a packet switched network to convert data to
an appropriate format so that a non X.25 (non-packet switching) device can send
information over an X.25 (packet switched) network, The pad is responsible for call set up
and addressing routines.
Paging A one-way alert or
alphanumeric message service. Messages are normally left via a bureau with a human or
Parallel interface An
interface that can handle a group of bits transmitted at the same time, by sending each
bit over a separate wire. Probably the best known parallel interface of all is the one
developed by Centronics, a printer manufacturer, that has 36 pins and can handle a byte of
data at a time. The transmission of the data is controlled by a computer generated strobe
Parity Bit An extra bit
added to a group of bits, usually to a seven-bit byte. The parity bit can be of 0 or 1
value so that every byte will then add up to an odd or even number, depending on whether
odd or even parity is chosen. When the transmitting device frames a character, it counts
the numbers of 0s and 1s in the frame and attaches the appropriate parity bit. The
recipient counts all the 0s and 1s and compares the total to the odd or even message
contained in the parity bit. If there's a discrepancy the recipient asks for the data to
PCM (Pulse Code Modulation) One
of the most common ways of converting an analog signal to a digital signal. This is done
by sampling the signal and coding the sample. It usually samples at 8,000 times per
second; each sampling representing 8bits. This produces a transmission rate of 641Kbit/s,
with the traffic on the line in interleaved data format. At the receiving end, the
interleaved signals are separated and converted back to analog.
Phase Modulation One of the
three ways of modulating or altering a signal so that it is able to carry information. The
other two are amplitude and frequency modulation. Phase modulation is used in high speed
Polling Method of
controlling terminals on a multi-point line by which a computer acting as master calls or
polls each of the terminals in turn to find out if they have any data to send. This access
method is used in star topology networks but is generally being superseded by the
preferred interrupt method.
Port A computer interface
capable of attachment to another device, such as a modem for communicating with a remote
terminal, or, if the port is within a hub, to a workstation.
PPP (Point to Point Protocol) An
addition to the Internet protocol suite to help connect devices where dissimilar transport
protocols exist, for example for the transfer of routing the range of actions a user can
perform on the network.
Profile A set of parameters
defining the way in which a device acts. In the LAN world, a profile is often used by one
or more workstations to determine the connection they will have with other devices.
Profiles work like batch files, executing a number of commands to save a user time and
Protocol A set of rules
governing the information flow within a communications infrastructure, often known as
"data link control". Protocols control format, timing, error essential
correction and running order. They are essential for a device to be able to interpret
incoming information. Suites of protocols are often used in networks, with each protocol
responsible for one part of a communications function.
Protocol converter A device
that translates between two protocols to facilitate communications between different
computers or different systems.
Protocol Independent Routing A
proprietary routing method designed for IBM networks developed by General DataComm,
improving on Source Routing. The router checks all LAN packets for their source address
contained in each frame. Adopting the proprietary DSPF (Discover Shortest Path First)
router-to-router protocol, a PIR router finds the best path between pairs of LANs, rather
than simply between pairs of routers as OSPF (Open Shortest Path First). Because a router
can have several LANs connected to it, OSPF does not necessarily produce the best result,
whereas PIR load-balances across each available LAN link for improved throughput and
performance. From a Token Ring LAN, PIR looks like a Source Routing bridge; in an Ethernet
environment, PIR appears as a Transparent bridge.
Proxy agent A network
management agent that sits in front of a device attached to a network that needs to be
managed, but has no intrinsic network management capabilities. The proxy agent provides
network management "by proxy" for that device.
PSN (Packet Switching Node) The
name of an Arpanet packet switch; PSNs were formerly called IMPS.
PDN (Public Data Network) A
network established by a PTT, PTO or TA to provide data transmission services for public
PUP (Parc Universal Packet) In
the internet system developed by Xerox Corporation, a PUP is the fundamental unit of
transfer, just as the IP datagram in a TCP/IP internet. The name was derived from the name
of the laboratory at which the Xerox internet was developed, the Palo Alto Research Center
Punch-down block Telephony
term describing the connector arrangements for distributing and connecting UTP/STP wiring
inside a building. Typically found in phone wiring closets.
PVC (Permanent Virtual Circuit) A
fixed virtual circuit between two users: the public data network equivalent of a leased
line. No call setup or clearing procedures are needed. Q
Q.921/931 ITU-TS (formerly
CCITT) Q Series Recommendations describing Lap- D, the Layer Two protocol for an ISDN
Qbit (Qualifier bit) In X.25
networks, bit 8 in an octet of packet header. It is used to indicate if a packet contains
QLLC (Qualified Logical Link
Control) Routines to provide Synchronous Data Link Control.
Quartet Signaling Part of
the 100BaseVG-AnyLAN proposed standard involving transmission over all four wire pairs
simultaneously in voice-grade UTP cable, expanding its capacity to 100Mbit/s. R
RS standards Laid down by
the EIA, RS232c approximates to V.24/28; RS422 to V.11; RS423 to V.10 and RS449 to V.36.
RARP (Reverse Address Resolution
Protocol) The TCP/IP protocol a diskless machine uses at startup to find its IP
Redirect A packet switching
function that routes a call to an alternative network address if the link to the original
address is not working. It is carried out by end point switches.
Redirector A LAN device
driver that translates operating system requests into network events and transmits them to
the right protocol stack.
Regeneration A method of
boosting a digital signal. Remote bridge LAN-to-LAN bridge that links geographically
distant LANs across a wide area link.
RFC (Request For Comment) Document
series begun in 1969 describing the Internet suite of protocols and related experiments.
RPC (Remote Procedure Call) A
means of communication between two tasks running on separate machines linked by a LAN. One
machine can request a service, typically computation, from the other, by executing a high
level request known as a language procedure call. RPCs are an underlying architectural
element of distributed processing and client/server computing.
RPL (Remote Program Load) Starting
a computer and loading the operating system into memory when the operating system software
is provided by a server on the network.
Reconfiguration The process
of physically altering the location or functionality of network or system elements.
Automatic configuration describes the way sophisticated networks can readjust themselves
in the event of a link or device failing, enabling the network to continue operation.
Red Book The 1984
compilation of the ITU-TS's standards for telecommunications, including some for ISDN.
Redirect A packet switching
function that routes a call to an alternative network address if the link to the original
network is not available. It is carried out by the end point switches.
Redundancy Otherwise known
as fault-tolerance, in data transmission, this refers to characters and bits that can be
removed from a transmission without affecting the message. Ir data processing and data
communications, it means providing backup for components: should one of them fail, the
system continues to run without operation. Total redundancy is usually impractical, but
financial institutions and other organizations with mission-critical applications attempt
to install a high level of redundancy on the basis that downtime loses money, lives,
depending on the business.
Repeater A device that
extends the maximum length of cable that can be used in a single network. In fiber
networks, it is an optoelectrical module that receives an optical signal and converts it
into an electrical form.
Ring topology A layout
scheme in which the network takes the form of a closed loop with the devices attached into
the ring. Every workstation is linked to two others, one on each side. All data is passed
from node to node in one direction only, each PC acting as a repeater for the next one in
the loop. Response time is determined by the number of stations on the ring - the more
there are, the slower it works. If one PC fails, the loop is broken, though most rings
have self-healing capacity to reconfigure and continue operation. IBM's Token passing ring
ensures that the failed station is removed and its neighbors are then directly connected.
Ring in and Ring out Token
Ring connectors on the MSAU connecting the unit to trunk cabling. The Wrap feature is
implemented at these interfaces.
RMON (Remote Monitoring) SNMP
specification for multivendor statistics-gathering by a standards-based (de facto SNMP)
management station from de facto standard (RMON)-compliant devices.
Roaming The ability of a
mobile communications device to move freely from one part the whole of a network
operator's system or another.
RIP (Routing Information
Protocol) The protocol used by Berkeley 4BSD Unix systems to exchange routing
information among a small group of computers. Implemented by the Unix program
"routed", RIP is derived from an earlier protocol of the same name developed at
RJ11 Popular name for the
standard four-wire modular connector for phone connections.
RJ45 Popular name for the
eight-pin modular connector in the 10BaseT standard for UTP connections to workstations or
smart wiring hubs. The actual connector is described in ISO standard 8877.
Rlogin (Remote Login) The
service offered by Berkeley 4BSD Unix systems that allows users of one machine to connect
to other Unix systems across an internet and interact as if their terminals connected to
the machines directly. Although Rlogin offers essentially the same service as Telnet, it
is superior because the software passes information about the user's environment to the
interconnector device operating at OSI Network Layer (Level Three) that supports a
particular Network Layer protocol and related stack, such as TCP/IP, DecNet, XNS, SNA, OSI
IP, IPX. These days routers tend to support multiple protocols by a variety of methods,
such as Protocol Independent Routing. A router can be used to link LANs together locally
or remotely as part of a WAN. A network built using routers is often termed an
Routing Process of
delivering a message across one or more networks via the most appropriate path.
Routing table Information
stored within a router that contains network path and status information. It is used to
select the most appropriate route to forward information along.
RS232-C An EIA standard
which is the most common way of linking data devices together. An interface for linking
DCEs and DTES, it defines the electrical characteristics of the signals from such devices.
RS232 is suitable for both synchronous and asynchronous communications and specifies a
25-pin connector, traditionally the DB-25. 20 of the pins carry out routine system
operation while the rest are reserved for modem testing or unallocated. For this reason it
is never safe to assume that two RS232 devices will work together. Newer, more compact
RS232 interfaces have nine pins only and an adapter card is needed. It is functionally
similar to the ITU-TS's V.24 and V.28 standards.
RS422 EIA recommended
standard to extend the RS232 50ft limit. It is most commonly implemented on 25-pin
connectors (DB-25s) and is electrically compatible with the ITU-TS V.11 standard.
RS423 The EIA recommended
standard for cable lengths that extend the RS232 50ft limit. It was introduced in tandem
with RS422 but is not widely used. Electrically compatible with the ITU-TS's V.10
SAA (Systems Application
Architecture) IBM's common application development environment, enabling software to
be developed to run on a broad range of IBM hardware, under a range of operating systems:
MVS/ESA, VM/ESA, OS/400 and OS/2 EE. This is achieved through a set of standard interfaces
for applications (programming), user and communications.
SDH (Synchronous Digital
Hierarchy) ITU-TS synchronous transmission standards aligned with Sonet above
155Mbit/s, aimed at network operators. Designed with ATM in mind, SDH has many advantages
over existing transmission technologies including flexibility in managing the
transmission, reconfiguration and control and switching at data rates to 622Mbit/s and
beyond. SDH-based networks are being implemented by operators in various parts of Europe
and developed by major players such as Alcatel and Northern Telecom.
SDLC (Synchronous Data Link
Control) An IBM protocol for use in SNA (System Network Architecture) environments. It
controls an individual link while catering for other network needs and can operate over
full or half duplex lines. SDLC is a bit oriented protocol, using a variety of patterns to
flag the beginning and end of a frame or packet. Different bit patterns are used to convey
address, control and header fields which accompany the transmission and guide the frame to
Segment A bus LAN term
meaning an electrically continuous piece of the bus. Segments can be joined together using
repeaters or bridges.
Serial interface Hardware
for sending and receiving data one bit at a time.
Server A node that permits
other nodes on the LAN to access its resources. The server may be dedicated, in which case
this is its sole function, or non-dedicated, where the node can be used in other ways,
such as a workstation.
Service point IBM network
management term for a system that allows non-SNA equipment and network components to be
incorporated into an SNA management structure.
Services A general term for
resources made available by a server to other nodes on the network. In Microsoft LAN
Manager terminology, this concept is broadened to include all main components of the LAN
Session A logical connection
between two nodes on a network for the exchange of data; alternatively, any live link
between any two data devices, such as a minicomputer and a dumb terminal. A session is
also used simply to describe connection time. Session Layer Level Five of the OSI data
communications model, drawn up by the ISO, ensures the management of dialogue between
applications by providing the protocols for assembling physical messages into logical
messages to all dialogue between end systems.
SGMP (Simple Gateway Monitoring
Protocol) A predecessor of SNMP.
Shielded Twisted Pair A
twisted pair of cables surrounded by a wire gauze shield, commonly used by IBM.
Simplex A communications
system or link which can only carry a signal in one direction.
SIR (Speaker Independent [Voice]
Recognition) Technologies for the automated conversion of speech to accurate and
meaningful textual information, typically ASCII. It typically accepts input from callers
to voice processors where callers are using rotary dial instead of DTMF phones. SIR
products have deliberately limited vocabularies, but are increasing due to the vast
installed base of non-DTMF phones.
Slotted Ring A LAN
architecture that continually carries a constant number of fixed length packets or slots
round the ring. The nodes then use, by replacement, empty slots as they pass through, to
transmit data. All the nodes have the ability to recognize empty slots or addressed to
them. This network architecture is also known as a Cambridge Ring.
Smart wiring hub A network
concentrator enabling multiple media to be supported and managed from a central location.
When supporting structured wiring systems, smart hubs provide port management.
SMB (Server Message Block) A
Microsoft - originated distributed system which enables access to another computer's files
and peripherals over the network as if they were local.
SMDS (Switched Multimegabit Data
Services) A public packet-switched high-speed data service that extends LAN-like
performance to MANs and WANs with no distance limit. Although reflecting DQDB MAN
standards, SMDS can also run over ATM. It offers variable data packet size, Virtual
Private Network and Closed User Group features, transmission rates up to 34Mbit/s today
and 150Mbit/s in the future.
SMTP (Simple Mail Transfer
Protocol) The TCP/IP standard protocol for transferring electronic mail messages from
one machine to another. SMTP specifies how two mail systems interact and the format of
control messages they exchange to transfer mail.
SNA (Systems Network
Architecture) IBM's layered communications protocol for sending data between IBM
hardware and software. Shortly to celebrate its Pearl anniversary (it was first announced
way back in September 1974), it is defined today in terms of a stack of seven layers.
While there is some commonalty between the layers of OSI and SNA, it is difficult to make
a direct mapping. SNA has traditionally been a hierarchical network architecture for
homogeneous networking between IBM systems, but in the 'new' SNA IBM has added increasing
support for peer to peer networking through the development of Advanced Peer to Peer
Communications. SNA is managed through the NetView network management system.
SNADS (SNA Distribution
Services) A standardized asynchronous distribution service architecture for the
transmission of files or jobs around an IBM SNA network. It is implemented as a
transaction service of the SNA network.
SNI (SNA Network Interconnect) (In
IBM SNA) The way in which autonomous SNA networks can be connected, while still allowing
them to be independently managed.
SNMP (Simple Network Management
Protocol) A transmission protocol defined by the IAB in RFC1157 for TCP/IP-based
network management, widely accepted as a de facto standard for LAN network management.
SNMP is used to monitor IP gateways and their networks. It defines a set of variables that
the gateway must keep and specifies that all operations on the gateway are a side-effect
of fetching or storing to the data variables. It consists of three parts: structure of
management information (SMI), Management Information Base (MIB) and the protocol itself.
The SMI and MIB define and store the set of managed entities; SNMP itself conveys
information to and from these entities. The public domain standard is based on the
operational experience of TCP/IP internetworks within Darpa/NSFnet.
SNMPv2 A combination of two
proposed updates to SNMP: Secure SNMP and Simple Management Protocol. Its 12 documents and
400 pages define everything from SMI to a Manager-to-Manager MIB, plus much-needed
Sockets IBM term for
software interfaces that allow two Unix application programs to talk via TCP/IP protocols.
Sonet (Synchronous Optical
NETwork) Transport network for synchronously multiplexed tributary signals. The
standard defines a set of transmission rates, signals and interfaces for fiber optic
transmission. Sonet is also an emerging standard proposed by Bellcore (Bell Communications
Research, the standards organization of the US Bell Operating companies) for a synchronous
optical transmission protocol. The broadband fiber network it supports is formed by a
family of network elements conforming to the Sonet interface requirements. The basic
electrical signal runs at 51.840Mbit/s - approximately 51 times the bandwidth of a
standard US, Ti leased line (1.544Mbit/s). Sonet grows in multiples of the basic signal
into the multi-Gigabit range. It is intended to be able to add and drop lower bit rate
signals from the higher bit rate signal without needing electrical demultiplexing.
Source Routing IEEE standard
for 802.5 Token Ring environments from an IBM development. A workstation (source)
determines the router through the LAN internetwork. It sends a "discovery" or
explorer packet along all active paths in the network, eventually reaching the destination
host. Once received, the packet is sent back to the originator. Along the way, each bridge
traversed adds its designator to the discovery packet. When it arrives at the originator,
it contains complete routing information and the originator can send its data with that
information attached. SR-compliant bridges generally need less processing power since most
of the work is done by the originating workstation. A liability of SR is that the number
of discovery packets traversing the network increases dramatically as the number of Token
Ring LAN segments and internetworking devices between the stations grows.
Source Routing Transparent (SRT)
An algorithm under consideration by IEEE combining Transparent Bridging for Ethernet
networks and Source Routing of Token Ring networks for interconnectivity of the two LAN
types. Upward migration to FDDI is guaranteed. The bridge applies either TB or SR logic to
each frame according to frame type.
Spanning Tree Algorithm Part
of an IEEE standard. A mechanism for detecting and preventing loops from occurring in a
multi-bridged environment. When three or more LAN segments are connected by bridges, a
loop can occur. As a bridge forwards all packets which are not recognized as being local,
some packets can circulate for long periods of time, eventually degrading system
performance. This algorithm ensures only one path connects any pair of stations, selecting
one bridge as the 'root' bridge, with the highest priority one as identifier, from which
all paths should radiate.
Speech Concatenation Voice
processing term for economical digitized speech playback. It uses independently recorded
files of phrases or file segments linked under application program control to produce a
customized response in natural sounding language. Examples could be bank balances or bus
schedules. It is done for speed and economy, lending itself to limited, structured
vocabularies that are best stored in ram or readily accessible from disk.
Spool (Simultaneous Peripheral
Operation On Line) A program or device that controls the flow of data to an outputting
device such as a printer. Spooling means that a user can send data to a device which is
already occupied and can rest assured that the data will be passed onto the desired
destination as soon as possible. Spooling of printer output is achieved on a LAN by means
of a print server. It is also important in WANs for providing a buffer for time
non-critical applications such as Electronic Mail and File Transfer.
Spoofing A method of fooling
network end stations into believing that keep-alive signals have come from and return to
the host. Polls are received and returned locally at either end of the network and are
transmitted only over the open network if there is a condition change. The result is a
non-time critical network with a minimum of keep-alive traffic between deterministic end
stations, while retaining the opportunity to send flags should an end station alter its
state. The synchronous approach harks back to unreliable circuits and the need to check
end station existence at regular intervals. Now networks are more reliable, spoofing by
routers is an acceptable compromise.
SQE(T) (Signal Quality Error
(Test) Function of transceivers immediately acter its attached computer has
transmitted on to the LAN. The transceiver sends a simple test signal over the AUI back to
the computer, ensuring the computer knows it has a working connection. Because it acts as
a confidence check and "pulses" down the line, it is also called a heartbeat.
SQL (Structured Query Language) A
standardized query language that can be used for querying databases across a network in
client/server applications. It can be used to interrogate the database, and extension to
the language allow the database to be updated through it as well.
Standalone server In LAN
Manager terminology, a server that maintains its own user accounts database and does not
participate in logon security.
Star Topology A network
layout or design in which each node is connected to a central hub. The hub establishes,
maintains and terminates all connections between the nodes. In a LAN, the hub is likely to
be a workstation, whereas in larger multi-point networks the hub is more likely to a
SDTM (Statistical Time Division
Multiplexing) A technique whereby a multiplexer apportions time on a dynamic basis
only to those channels which are active.
Store and forward A
buffering technique in which messages or packets are received in their entirety by an
intermediary, ever if the ultimate recipient is not presently able to receive, This is
used exclusively in the context of electronic mail across wide area networks, and
occasionally for terminal I/O.
StreetTalk Banyan Systems'
directory system which provides a distributed database of names addresses accessible from
Sub-Area Network The
original hierarchical approach used in the construction of IBM SNA backbone networks. The
structure of a Sub-Area Network is predefined. The relationship between the components of
the network and the links is generated into the software in the host systems involved. The
network can, however, be modified without having to re-build the definition of the entire
network. A new node can de added by defining it in the adjacent nodes to which it is
directly attached. Such a network is constructed around a backbone of communications
controllers to which the host systems are attached.
SVC (Switched Virtual Circuit) A
temporary virtual circuit between two users.
Switch Equipment used to
connect and distribute communications between a trunk line or backbone and individual
Switched attachment The IBM
term for linking devices to host computers through a PBX capable of handling data
Synchronous transmission A
method of communication in which data is sent in blocks, without the reed for start and
stop bits between each byte. Synchronization is achieved by sending a clock signal along
with the data, and by sending special bit patterns to denote the start of each block.
SSCP (Systems Services Control
Point) In an IBM S NA environment the SSCP resides in a host processor and is
responsible for central control, directory services and operational functions in the
network; either alone or in combination with SSCPs in other processors.
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