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Numeric 10Base-2: An adaptation of the Ethernet standard that uses thin coaxial cable and provides data transfer rates of up to 10 Mbps. The maximum effective distance for 10Base-2 is 185 meters. 10Base-2 is also known as thinnet.
10Base-5: The original Ethernet standard that uses thick coaxial cable and provides data transfer rates of up to 10 Mbps. The maximum effective distance for 10Base-5 is 500 meters. 10Base-5 is also known as thicknet.
10Base-F: An adaptation of the Ethernet standard that uses optical fiber cable and provides data transfer rates of up to 10 Mbps. 10Base-F can transmit data over very long distances with little loss of data integrity.
10BaseT: Ethernet local area network (LAN) that uses twisted-pair wiring. This is the most common Ethernet implementation. 10BaseT networks are physically laid out in a star topology and the wiring is connected to devices using a plug that resembles a phone jack called an RJ-45.
10 Gigabit Ethernet: An emerging high-speed network standard that will use high-capacity cabling to provide data transfer rates up to 10,000 Mbps.
32-bit: Describes the number of bits used by an operating system to perform an operation. The term also describes the microprocessor on which the operating system runs.
100Base-T (Fast Ethernet): A high-speed network standard, based on Ethernet, that provides data transfer rates as high as 100 Mbps.
1000Base-T (Gigabit Ethernet): An emerging high-speed network standard, based on Ethernet, that provides data transfer rates as high as 1000 Mbps.
A address: There are three types of addresses in common use within the Internet: e-mail addresses; IP, internet or Internet addresses and hardware or MAC addresses.
agent: In the client-server model, the agent is the part of the system that performs information preparation and exchange on behalf of a client or server application.
alias: A name, usually short and easy to remember, that is translated into another name, usually long and difficult to remember.
anonymous FTP: Anonymous FTP allows a user to retrieve documents, files, programs and other archived data from anywhere on the Internet without having to establish a user id and password. By using the special user id of "anonymous," the network user will bypass local security checks and will have access to publicly accessible files on the remote system.
ANSI (American National Standards Institute): This organization is responsible for approving U.S. standards in many areas, including computers and communications. Standards approved by this organization are often called ANSI standards. ANSI is a member of ISO.
Appletalk: This is a network protocol for linking Macintosh computers and peripherals. There are two implementations: LocalTalk (230.4 Kbps) and EtherTalk (10 Mbps).
asynchronous: Transmission by individual bytes, not related to specific timing on the transmitting end.
ATM (Asynchronous Transfer Mode): This is high-speed transmission technology suitable for voice, data and video using a fixed- size packet (called a cell). ATM is also known as "fast packet."
authentication: The verification of the identity of a person or process.
B backbone: The top level in a hierarchical network. Stub and transit networks which connect to the same backbone are guaranteed to be interconnected.
bandwidth: The amount of data that can be sent through a given communications circuit. Usually measured in bits per second for digital circuits, or hertz for analog circuits.
baseband: A transmission medium through which digital signals are sent without complicated frequency shifting. In general, only one communication channel is available at any given time. Ethernet is an example of a baseband network.
bridge: A communication device that connects two or more networks and forwards traffic between network segments based on datalink layer information. These segments would have a common network layer address.
broadband: Transmission medium capable of supporting a wide range of frequencies. It can carry multiple signals by dividing the total capacity of the medium into multiple, independent bandwidth channels where each channel operates only on a specific range of frequencies.
bus topology: A network topology in which all of the workstations are connected to one main line of cable. A single cable, running around the office, is linked to individual computers via drop cables. Bus networks are common because they are easy to install and use less cable. An advantage of this configuration is that one malfunctioning computer does not hamper the rest of the network. A major drawback to this type of network, however, is that a single cable break can bring down the entire network. Trouble-shooting a bus network is like trying to find a dead light bulb on a string of Christmas tree lights-often very difficult.
C Category 3, 4, 5: These indicate the quality of wire for data rates and reliability. Category 3 will handle voice and LAN traffic up to 10 Mbps, Category 4 handles data up to 16Mbps, Category 5 handles newer 100 Mbps networks.
Category 3 wiring: Also sometimes called level 3. Category 3 is a type of twisted pair wiring that is no longer being used for new installations. Instead, Category 5 wiring is being used.
Category 5 wiring: Also sometimes called level 5. Category 5 is a type of twisted pair wiring in which there is a certain number of twists per foot. This high number of twists prevents interference and enables Category 5 cable to carry higher bandwidths than others such as Category 3. According to Universal Wiring Plan specifications, Category 5 wiring is being used in all new installations.
circuit switching: A communications structure in which a dedicated communication path is established between two hosts and on which all packets travel. The telephone system is an example of a circuit switched network.
client: In a LAN, a computer that retrieves information from, or uses resources provided by, the server. (i.e., A workstation requesting the contents of a file from a file server is a client of the file server)
coaxial cable: A connecting cable consisting of two insulating layers and two conductors. A central conductor wire is surrounded by the first layer of insulation. An outer shielding conductor is laid over this insulation and then covered with the second layer of insulation.
congestion: Congestion occurs when the communication load exceeds the capacity of a data communication path.
connection-oriented: The data communication method in which communication proceeds through three well-defined phases: connection establishment, data transfer and connection release. TCP is a connection-oriented protocol.
connectionless: The data communication method in which communication occurs between hosts with no previous setup. Packets between two hosts may take different routes, as each is independent of the other. UDP is a connectionless protocol.
cross-connect: A group of connection points (wall or rack mounted) used to mechanically terminate and administer building wiring. Preferred method when there are a large number of circuits.
CSMA/CD (Carrier Sense Multiple Access with Collision Detection): The access method used by local area networking technologies such as Ethernet.
D default route: A routing table entry which is used to direct packets addressed to networks not explicitly listed in the routing table.
dialup: A temporary, as opposed to dedicated, connection between machines established over a standard phone line.
DNS (Domain Naming System): The DNS is a general purpose distributed, replicated, data query service. The principal use is the lookup of host IP addresses based on host names. The style of host names now used in the Internet are called "domain names," because they are the style of names used to look up anything in the DNS. Some important domains are: .com (commercial), .edu (educational), .net (network operations), .gov (U.S. government) and .mil (U.S. military). Most countries also have a domain. For example, .us (United States), .uk (United Kingdom) and .au (Australia).
DSx (digital signal X): Digital signal X is a term for the series of standard digital transmission rates or levels based on DS0, a transmission rate of 64 Kbps, the bandwidth normally used for one telephone voice channel. Both the North American T-carrier system system and the European E-carrier systems of transmission operate using the DS series as a base multiple. The digital signal is what is carried inside the carrier system.
DS0 is the base for the digital signal X series. DS1, used as the signal in the T-1 carrier, is 24 DS0 (64 Kbps) signals transmitted using pulse-code modulation (PCM) and time-division multiplexing (TDM). DS2 is four DS1 signals multiplexed together to produce a rate of 6.312 Mbps. DS3, the signal in the T-3 carrier, carries a multiple of 28 DS1 signals or 672 DS0s or 44.736 Mbps.
Digital signal X is based on the ANSI T1.107 guidelines. The ITU-TS guidelines differ somewhat. The following table summarizes the set of signals and relates them to the T-carrier and E-carrier systems.
Digital Signal Designator
Data Rate
DS0 Multiple
T-Carrier
E-Carrier
DS0
64 Kbps
1
-
-
DS1
1.544 Mbps
24
T-1
-
-
2.048 Mbps
32
-
E1
DS1C
3.152 Mbps
48
-
-
DS2
6.312 Mbps
96
T-2
-
-
8.448 Mbps
128
-
E2
-
34.368 Mbps
512
-
E3
DS3
44.736 Mbps
672
T-3
-
-
139.264 Mbps
2048
-
E4
DS4/NA
139.264 Mbps
2176
-
-
DS4
274.176 Mbps
4032
-
-
-
565.148 Mbps
4 E4 channels
-
E5
DSL (Digital Subscriber Line): This technology is used to transmit digital data over copper phone lines. With DSL, the analog voice phone calls and digital signals can coexist on the same wires.
E encryption: Encryption is the manipulation of a packet's data in order to prevent any but the intended recipient from reading that data . There are many types of data encryption and they are the basis of network security.
ethernet: A LAN that connects computers and peripherals and operates over twisted-pair or coaxial cable at speeds up to 10 Mbps. All hosts are connected to a coaxial cable where they contend for network access using a Carrier Sense Multiple Access with Collision Detection (CSMA/CD) paradigm.
F FDDI (Fibre Distributed Data Interface): A LAN standard for using fiber-optic cable which carries a light source generated by a laser or LED. FDDI standards govern high-speed (100 Mbps) fiber-optic connections.
fiber-optic cable: A thin, flexible, medium capable of conducting modulated light transmission. Compared to other transmission media, fiber-optic cable is more expensive, not susceptible to EMI, and is capable of higher data rates.
file transfer: The copying of a file from one computer to another over a computer network.
File Transfer Protocol (FTP): A protocol which allows a user on one host to access and transfer files to and from another host over a network. Also, FTP is usually the name of the program the user invokes to execute the protocol.
finger: A program that displays information about a particular user, or all users, logged on the local system or on a remote system. It typically shows full name, last login time, idle time, terminal line and terminal location (where applicable). It may also display plan and project files left by the user.
firewall: Firewalls are systems that establish access control policies among networks. They can block information from entering a network, or from getting out of the network, and they can permit different users the ability to perform different operations based on the users' authorizations.
FQDN (Fully Qualified Domain Name): The FQDN is the full name of a system, rather than just its hostname. For example, "venera" is a hostname and "venera.isi.edu" is an FQDN.
Frame Relay: A network technology that uses packet-switching and high quality phone lines to provide data transfer rates from 56 Kbps to 1.544 Mbps. Frame relay checks packets for errors but does not correct them; end devices, like LAN servers and clients, make corrections when necessary.
H hop: A term used in routing. A path to a destination on a network is a series of hops (through routers) away from the origin.
host: A computer that allows users to communicate with other host computers on a network. Individual users communicate by using application programs, such as electronic mail, Telnet and FTP.
hostname: The name given to a machine.
HTML (hypertext markup language): The standard language of the World Wide Web, HTML is used to create and recognize hypermedia documents. HTML documents are standard ASCII files with formatting codes that contain information about layout, such as text styles, document titles, paragraphs, lists and hyperlinks.
HTTP (Hypertext Transfer Protocol): The language that Web browsers and servers use to communicate with each other. HTTP can handle multiple application protocols that allow users access to many Internet protocols. It also supports multimedia functions.
hub: A device connected to several other devices. In ARCnet, a hub is used to connect several computers together. In a message handling service, a hub is used for the transfer of messages across the network.
I IEEE: Institute of Electrical and Electronics Engineers
IETF (Internet Engineering Task Force): The IETF is a large, open community of network designers, operators, vendors and researchers whose purpose is to coordinate the operation, management and evolution of the Internet and to resolve short-range and mid-range protocol and architectural issues.
internet: While an internet is a network, the term "internet" is usually used to refer to a collection of networks interconnected with routers.
Internet: (Capital "I") The Internet is the largest internet in the world. It is a three level hierarchy composed of backbone networks (e.g., NSFNET, MILNET), mid-level networks and stub networks. The Internet is a multiprotocol internet.
internet address: An IP address that uniquely identifies a node on an internet.
IP (Internet Protocol): The network layer for the TCP/IP Protocol Suite. It is a connectionless, best-effort packet switching protocol.
IP address: The 32-bit address. It is usually represented in dotted decimal notation, i.e., 147.31.254.130.
iSCSI (Internet SCSI):Pronounced eye skuzzy. is an IP-based standard for linking data storage devices over a network and transferring data by carrying SCSI commands over IP networks. iSCSI supports a Gigabit Ethernetinterface at the physical layer, which allows systems supporting iSCSI interfaces to connect directly to standard Gigabit Ethernet switches and/or IP routers. When an operating system receives a request it generates the SCSI command and then sends an IP packet over an Ethernet connection. At the receiving end, the SCSI commands are separated from the request, and the SCSI commands and data are sent to the SCSI controller and then to the SCSI storage device. iSCSI will also return a response to the request using the same protocol
IPV 6 (Internet Protocol Version 6):
IPv6 (Internet Protocol Version 6) is the latest level of the Internet Protocol (IP) and is now included as part of IP support in many products including the major computer operating systems. IPv6 has also been called "IPng" (IP Next Generation). Formally, IPv6 is a set of specifications from the Internet Engineering Task Force (IETF). IPv6 was designed as an evolutionary set of improvements to the current IP Version 4. Network hosts and intermediate nodes with either IPv4 or IPv6 can handle packets formatted for either level of the Internet Protocol. Users and service providers can update to IPv6 independently without having to coordinate with each other.
The most obvious improvement in IPv6 over the IPv4 is that IP addresses are lengthened from 32 bits to 128 bits. This extension anticipates considerable future growth of the Internet and provides relief for what was perceived as an impending shortage of network addresses.
IPv6 describes rules for three types of addressing: unicast (one host to one other host), anycast (one host to the nearest of multiple hosts), and multicast (one host to multiple hosts). Additional advantages of IPv6 are:
Options are specified in an extension to the header that is examined only at the destination, thus speeding up overall network performance.
The introduction of an "anycast" address provides the possibility of sending a message to the nearest of several possible gateway hosts with the idea that any one of them can manage the forwarding of the packet to others. Anycast messages can be used to update routing tables along the line.
Packets can be identified as belonging to a particular "flow" so that packets that are part of a multimedia presentation that needs to arrive in "real time" can be provided a higher quality-of-service relative to other customers.
The IPv6 header now includes extensions that allow a packet to specify a mechanism for authenticating its origin, for ensuring data integrity, and for ensuring privacy.
IPX (Internetwork Packet eXchange): Novell's protocol used by Netware. A router with IPX routing can interconnect LANs so that Novell Netware clients and servers can communicate.
ISDN (Integrated Services Digital Network): ISDN combines voice and digital network services in a single medium, making it possible to offer customers digital data services as well as voice connections through a single "wire."
ISO (International Organization for Standardization): A voluntary, non-treaty organization founded in 1946 which is responsible for creating international standards in many areas, including computers and communications. Its members are the national standards organizations of the 89 member countries, including ANSI for the U.S.
L LAN (Local Area Network): A data network intended to serve an area of only a few square kilometers or less. Because the network is known to cover only a small area, optimizations can be made in the network signal protocols that permit data rates up to 100Mb/s.
layer: Communication networks for computers may be organized as a set of more or less independent protocols, each in a different layer (also called level). The lowest layer governs direct host-to-host communication between the hardware at different hosts; the highest consists of user applications. Each layer builds on the layer beneath it. For each layer, programs at different hosts use protocols appropriate to the layer to communicate with each other. TCP/IP has five layers of protocols; OSI has seven. The advantages of different layers of protocols is that the methods of passing information from one layer to another are specified clearly as part of the protocol suite, and changes within a protocol layer are prevented from affecting the other layers. This greatly simplifies the task of designing and maintaining communication programs.
LLC (Logical Link Control): The upper portion of the datalink layer. The LLC sublayer presents a uniform interface to the user of the datalink service, usually the network layer. Beneath the LLC sublayer is the MAC sublayer.
On a local area network (LAN) or other network, the MAC (Media Access Control) address is your computer's unique hardware number. (On an Ethernet LAN, it's the same as your Ethernet address.) When you're connected to the Internet from your computer (or host as the Internet protocol thinks of it), a correspondence table relates your IP address to your computer's physical (MAC) address on the LAN.
The MAC address is used by the Media Access Control sublayer of the Data-Link Layer (DLC) layer of telecommunication protocol. There is a different MAC sublayer for each physical device type. The other sublayer level in the DLC layer is the Logical Link Control sublayer.
MAN (Metropolitan Area Network): A network that serves an area anywhere from a few miles to more than 30 miles in size, such as a city or university. A MAN is larger than a LAN, which serves one building or complex, but smaller than a WAN, which may have worldwide coverage. A MAN links two or more LANs.
MAC (Media Access Control): The lower portion of the datalink layer. The MAC differs for various physical media.
medium: The material used to support the transmission of data. This can be copper wire, coaxial cable, optical fibre, or electromagnetic wave (as in microwave).
MIB (Management Information Base): The set of parameters an SNMP management station can query or set in the SNMP agent of a network device (e.g., router). Standard, minimal MIBs have been defined, and vendors often have private enterprise MIBs. In theory, any SNMP manager can talk to any SNMP agent with a properly defined MIB.
modem: A modem is a device that converts digital data, originating from a terminal or computer, to analog signals used by voice communication systems, such as telephones. At the origination point, modems convert digital pulses to audible tones. At the receiving end, they convert the audio tones back to digital pulses. The word modem is taken from "modulator-demodulator."
MTU (Maximum Transmission Unit): The largest frame length which may be sent on a physical medium.
multimode fiber: Fiber-optic cabling consisting of several fibers and supporting the propagation of multiple electromagnetic field patterns. The minimum bandwidth is 500 MHz-km at 1300 nm.
The translation of an Internet Protocol address (IP address) used within one network to a different IP address known within another network. One network is designated the inside network and the other is the outside. Typically, a company maps its local inside network addresses to one or more global outside IP addresses and unmaps the global IP addresses on incoming packets back into local IP addresses. This helps ensure security since each outgoing or incoming request must go through a translation process that also offers the opportunity to qualify or authenticate the request or match it to a previous request. NAT also conserves on the number of global IP addresses that a company needs and it lets the company use a single IP address in its communication with the world.
NAT is included as part of a router and is often part of a corporate firewall. Network administrators create a NAT table that does the global-to-local and local-to-global IP address mapping. NAT can also be used in conjunction with policy routing. NAT can be statically defined or it can be set up to dynamically translate from and to a pool of IP addresses. Cisco's version of NAT lets an administrator create tables that map:
A local IP address to one global IP address statically
A local IP address to any of a rotating pool of global IP addresses that a company may have
A local IP address plus a particular TCP port to a global IP address or one in a pool of them
A global IP address to any of a pool of local IP addresses on a round-robin basis
NAT is described in general terms in RFC 1631. which discusses NAT's relationship to Classless Interdomain Routing (CIDR) as a way to reduce the IP address depletion problem. NAT reduces the need for a large amount of publicly known IP addresses by creating a separation between publicly known and privately known IP addresses. CIDR aggregates publicly known IP addresses into blocks so that fewer IP addresses are wasted. In the end, both extend the use of IPv4 IP addresses for a few more years before IPv6 is generally supported.
NetBIOS (Network Basic Input Output System): The standard interface to networks on IBM PC and compatible systems.
network: A computer network is a data communications system which interconnects computer systems at various sites. A network may be comprised of any combination of LANs, MANs or WANs.
network address: The network portion of an IP address. For a class A network, the network address is the first byte of the IP address. For a class B network, it is the first two bytes of the IP address. For a class C network, the network address is the first three bytes of the IP address. In each case, the remainder is the host address. In the Internet, assigned network addresses are globally unique.
NOC (Network Operations Center): A location from which the operation of a network or internet is monitored. Additionally, this center usually serves as a clearinghouse for connectivity problems and efforts to resolve those problems.
node: An addressable device attached to a computer network.
O OSI Model: Open System Interconnection. This is an international framework of standards for communications between different systems. It consists of seven layers, each of which specify particular network functions. The highest layer (application) is closest to the user while the lowest layer (physical) is closest to the media technology. The seven layers are:
7--Application: Application layer functions include the user interface to e-mail, file transfer, and terminal emulation.
6--Presentation: The presentation layer is concerned with the syntax of the data exchanged between two application-layer entities. Data compression and cryptography are implemented in this layer.
5--Session: The session layer controls communication between the applications such as application-level error control and dialog control.
4--Transport: The transport layer is responsible for reliable network communications between end nodes. It implements flow and error control and often uses virtual circuits to ensure reliable data delivery.
3--Network: The network layer controls how packets are routed from their source to a destination.
2--Data Link: The data link layer ensures error free transmissions. It handles breaking data into frames, sequential transmission, and processing acknowledgements.
1--Physical: The physical layer defines the electrical, mechanical, and physical interfaces to the network and the details of how 1's and 0's get from one place to another. The OSI Model is a theory. It does not apply directly in practice. Some protocols do not use all seven layers.
OSPF: Open Shortest-Path First Interior Gateway Protocol
P packet: The unit of data sent across a network. "Packet" is a generic term used to describe a unit of data at all levels of the protocol stack, but it is most correctly used to describe application data units.
packet switching: A communications paradigm in which packets (messages) are individually routed between hosts, with no previously established communication path.
PBX (Private Branch Exchange): A private switching system usually serving an organization, such as a business or government agency, which is located on the customerÕs premises. It switches calls both inside a building and outside to the telephone network.
peer-to-peer network: A network typically found in small companies in which all computers are equal. A file server is not required. Peer-to-peer networks are extremely popular and many new operating systems allow peer-to-peer networking right out of the box.
physical media: Any means in the physical world for transferring signals between OSI systems. Considered to be outside the OSI Model, and therefore sometimes referred to as "Layer 0," the physical connector to the media can be considered as defining the bottom interface of the Physical Layer, i.e., the bottom of the OSI Reference Model.
PING (Packet InterNet Groper): A program used to test reachability of destinations by sending them an ICMP echo request and waiting for a reply. The term is used as a verb: "Ping host X to see if it is up!"
plenum cable: Plenum is a space used to circulate air back to the heating or cooling system in a building (i.e., suspended ceiling or raised floor.) Plenum cables use TEFLONÒ or HARLARÒ in the cable jacket to give low flame and low smoke producing properties, which enables them (by code) to be placed in plenum areas. Any cable not plenum rated by code would need to be run in conduit in a plenum area.
POP (Point of Presence): A site where a collection of telecommunications equipment, usually digital leased lines and multi-protocol routers, exist.
POPx (Post Office Protocol): A format for storing and retrieving e-mail messages used by mail servers and clients. POP works with simple mail transfer protocol (SMTP), which enables e-mail messages to be sent from one system to another. There are three versions: POP, POP2, and POP3. Latter versions are NOT compatible with earlier versions.
polling: Connecting to another system to check for things like mail or news.
PPP (Point-to-Point Protocol): provides a method for transmitting packets over serial point-to-point links.
PPPoE (Point-to-Point Protocol over Ethernet): PPPoE relies on two widely accepted standards: PPP and Ethernet. PPPoE is a specification for connecting the users on an Ethernet to the Internet through a common broadband medium, such as a single DSL line, wireless device or cable modem. All the users over the Ethernet share a common connection, so the Ethernet principles supporting multiple users in a LAN combine with the principles of PPP, which apply to serial connections.protocol stack: A layered set of protocols which work together to provide a set of network functions.
proxy: The mechanism whereby one system "fronts for" another system in responding to protocol requests. Proxy systems are used in network management to avoid having to implement full protocol stacks in simple devices, such as modems.
Q QoS: On the Internet and in other networks, QoS (Quality of Service) is the idea that transmission rates, error rates, and other characteristics can be measured, improved, and, to some extent, guaranteed in advance. QoS is of particular concern for the continuous transmission of high-bandwidth video and multimedia information. Transmitting this kind of content dependably is difficult in public networks using ordinary "best effort" protocols. Using the Internet's Resource Reservation Protocol (RSVP), packets passing through a gateway host can be expedited based on policy and reservation criteria arranged in advance. Using ATM, which also lets a company or user preselect a level of quality in terms of service, QoS can be measured and guaranteed in terms of the average delay at a gateway, the variation in delay in a group of cells (cells are 53-byte transmission units), cell losses, and the transmission error rate. The Common Open Policy Service (COPS) is a relatively new protocol that allows router and layer 3 switches to get QoS policy information from the network policy server.
R RBOC: Regional Bell Operating Company. One of seven telephone companies created after the AT&T divestiture in 1984. Also sometimes called Regional Bell Holding Companies. RBOC is pronounced "R-bock."
repeater: A device which propagates electrical signals from one cable to another.
RFC (Request For Comments): The document series, begun in 1969, which describes the Internet suite of protocols and related experiments. Not all (in fact very few) RFCs describe Internet standards, but all Internet standards are written up as RFCs.
ring topology: A network topology that takes the shape of a circle. Each computer in the network is connected to the next with small sections of cable. Messages sent from one computer on the network travel around the ring until they reach the correct computer. Communications only travel in one direction around the ring. Advantages of this type of system are the ability to self correct if the cable is broken, and little attenuation due to signal regeneration at each station. Disadvantages include large cable requirements and short wiring distances between each node.
route: The path that network traffic takes from its source to its destination. Also, a possible path from a given host to another host or destination.
router: A device which forwards traffic between networks. Routers act like bridges but have greater functionality. They are used to build wide area networks (WANs).
routing: The process of selecting the correct interface and next hop for a packet being forwarded.
RPC (Remote Procedure Call): An easy and popular paradigm for implementing the client-server model of distributed computing. In general, a request is sent to a remote system to execute a designated procedure, using arguments supplied, and the result returned to the caller. There are many variations and subtleties in various implementations, resulting in a variety of different (incompatible) RPC protocols.
SCSI interfaces provide for faster data transmission rates (up to 80 megabytes per second) than standard serial and parallel ports. In addition, you can attach many devices to a single SCSI port, so that SCSI is really an I/Obus rather than simply an interface.
Although SCSI is an ANSI standard, there are many variations of it, so two SCSI interfaces may be incompatible. For example, SCSI supports several types of connectors.
While SCSI has been the standard interface for Macintoshes, the iMac comes with IDE, a less expensive interface, in which the controller is integrated into the disk or CD-ROM drive. Other interfaces supported by PCs include enhanced IDE and ESDI for mass storage devices, and Centronics for printers. You can, however, attach SCSI devices to a PC by inserting a SCSI board in one of the expansion slots. Many high-end new PCs come with SCSI built in. Note, however, that the lack of a single SCSI standard means that some devices may not work with some SCSI boards.
The following varieties of SCSI are currently implemented:
server: A server is a dedicated machine shared by users on the LAN. Servers can have many functions such as controlling print jobs, being used as a data repository and as the access control to e-mail and voice mail.
singlemode fiber: Fiber with nominal 8.3/125 mm core/cladding diameter, which allows the propagation of only one mode Ð wide bandwidth. Typically used in long haul and high speed applications.
SLIP (Serial Line IP): A protocol used to run IP over serial lines, such as telephone circuits or RS-232 cables, interconnecting two systems.
SLA (Service Level Agreement): Service level agreements are an important way to objectively measure an outsourcing supplier's performance. The most common metrics of quality, speed and price clearly let each partner assess the current level of service the buyer is receiving. If performance slides, the service level agreement may trigger cost reductions for the buyer. Putting these metrics in writing is important because it provides the legal basis for contract termination.
SMTP (Simple Mail Transfer Protocol): A communications protocol that directs e-mail exchange on TCP/IP networks. It is a server-to-server protocol, so other protocols are used to access the messages.
SNMP (Simple Network Management Protocol): The Internet standard protocol developed to manage nodes on an IP network.
SPYWARE (Also called Adware):
Also called adware, spyware is any software that covertly gathers user information through the user's Internet connection without his or her knowledge, usually for advertising purposes. Spyware applications are typically bundled as a hidden component of freeware or shareware programs that can be downloaded from the Internet. Once installed, the spyware monitors user activity on the Internet and transmits that information in the background to someone else. Spyware can also gather information about e-mail addresses and even passwords and credit card numbers.
Spyware is similar to a Trojan horse in that users unwittingly install the product when they install something else. A common way to become a victim of spyware is to download certain peer-to-peer file swapping products that are available today.
Aside from the questions of ethics and privacy, spyware steals from the user by using the computer's memory resources and also by eating bandwidth as it sends information back to the spyware's home base via the user's Internet connection. Because spyware is using memory and system resources, the applications running in the background can lead to system crashes or general system instability.
Because spyware exists as independent executable programs, they have the ability to monitor keystrokes, scan files on the hard drive, snoop other applications, such as chat programs or word processors, install other spyware programs, read cookies, change the defaulthome page on the Web browser, consistently relaying this information back to the spyware author who will either use it for advertising/marketing purposes or sell the information to another party.
Licensing agreements that accompany software downloads sometimes warn the user that a spyware program will be installed along with the requested software, but the licensing agreements may not always be read completely because the notice of a spyware installation is often couched in obtuse, hard-to-read legal disclaimers.
SQL (Structured Query Language): The international standard language for defining and accessing relational databases.
star topology: This topology uses a hub or concentrator to connect to workstations. Each computer uses a single cable to attach to the central hub. This topology gets its name because connections radiate out from the hub in a star fashion. Unlike networks that use the bus topology, a connection break will not bring down an entire network. However, the initial cost of a star network is higher because they use more cable and large portions of a network will stop functioning if a hub fails.
subnet: A portion of a network, which may be a physically independent network segment, which shares a network address with other portions of the network and is distinguished by a subnet number. A subnet is to a network what a network is to an internet.
subnet address: The subnet portion of an IP address. In a subnetted network, the host portion of an IP address is split into a subnet portion and a host portion using an address (subnet) mask.
subnetwork: A collection of OSI end systems and intermediate systems under the control of a single administrative domain and utilizing a single network access protocol. Examples:- private X.25 networks, collection of bridged LANs.
synchronous: Data communications in which transmissions are sent at a fixed rate, with the sending and receiving devices synchronized.
T T1: A term for a digital carrier facility used to transmit a DS-1 formatted digital signal at 1.544 megabits per second.
T3: A term for a digital carrier facility used to transmit a DS-3 formatted digital signal at 44.746 megabits per second.
TCP/IP (Transmission Control Protocol/Internet Protocol): A protocol used to link dissimilar computers across many kinds of networks. Developed in the 1970s by the United States Department of Defense, TCP/IP is the transport protocol employed by the Internet.
Telnet: Telnet is the Internet standard protocol for remote terminal connection service
terminal emulator: A program that allows a computer to emulate a terminal. The workstation thus appears as a terminal to the remote host.
terminal server: A device which connects many terminals to a LAN through one network connection. A terminal server can also connect many network users to its asynchronous ports for dial-out capabilities and printer access.
TL (Time to Live): A field in the IP header which indicates how long this packet should be allowed to survive before being discarded. It is primarily used as a hop count.
token ring: A ring type of network, which uses a supervisory "token" that is passed from station to station around the ring. Computers must have the token in order to transmit, and do so using the entire bandwidth of the communications media.
topology: The physical configuration of a network that determines how the network's computers are connected. A network layer must stay abreast of the current network topology to be able to route packets to their final destination. Common configurations include bus, star and ring topologies.
tunneling: Tunneling refers to the encapsulation of protocol A within protocol B, such that A treats B as though it were a datalink layer. Tunneling is used to get data between administrative domains which use a protocol that is not supported by the Internet
twisted pair: Two insulated copper wires twisted around each other. Several sets of twisted pair wires can be enclosed in one cable. The twists in the wire reduce induction and thus interference from one wire to another.
U UDP (User Datagram Protocol): An Internet standard transport layer protocol. It is a connectionless protocol which adds a level of reliability and multiplexing to IP.
V virtual circuit: A network service which provides connection-oriented service regardless of the underlying network structure.
virus: A program which replicates itself on computer systems by incorporating itself into other programs which are shared among computer systems.
VoIP: VoIP (voice over IP - that is, voice delivered using the Internet Protocol) is a term used in IP telephony for a set of facilities for managing the delivery of voice information using the Internet Protocol (IP). In general, this means sending voice information in digital form in discrete packets rather than in the traditional circuit-committed protocols of the public switched telephone network (PSTN). A major advantage of VoIP and Internet telephony is that it avoids the tolls charged by ordinary telephone service. VoIP, now used somewhat generally, derives from the VoIP Forum, an effort by major equipment providers to promote the use of ITU-T H.323, the standard for sending voice (audio) and video using IP on the public Internet and within an intranet. The Forum also promotes the user of directory service standards so that users can locate other users and the use of touch-tone signals for automatic call distribution and voice mail. In addition to IP, VoIP uses the real-time protocol (RTP) to help ensure that packets get delivered in a timely way. Using public networks, it is currently difficult to guarantee Quality of Service (QoS). Better service is possible with private networks managed by an enterprise or by an Internet telephony service provider (ITSP).
W WAN (Wide Area Network): A network of computers connected to each other over a geographic area.
WHOIS: An Internet program which allows users to query a database of people and other Internet entities, such as domains, networks and hosts, kept at the DDN NIC. The information for people shows a person's company name, address, phone number and e-mail address.
WAIS (Wide Area Information Servers): A distributed information service which offers simple natural language input, indexed searching for fast retrieval and a "relevance feedback" mechanism which allows the results of initial searches to influence future searches.
