ADCCP (Advanced Data Communications Control Procedures) - A bit-oriented, ANSI-standard communications link-layer protocol.
Actuator - The mechanism which moves the read/write head in a hard disk drive.
Address Mask - A bit mask used to select bits from an Internet address for subnet addressing. Also known as Subnet Mask.
AFP (AppleTalk File Protocol) - Apple?s network protocol provides file server/client access in an AppleShare network.
ALAP (AppleTalk Link Access Protocol) - A link access layer (or data link layer) protocol that governs packet transmission on LocalTalk.
APPC (Advanced Program-to-Program Communications) - A high-level communications protocol from IBM that enables one program to interact with another anywhere on the network. It supplies commands for managing a session, sending and receiving data, and transaction security and integrity (two-phase commit).
APPN (Advanced Peer-to-Peer Networking) - An addition to IBM?s SNA communication, APPN provides the most efficient route for establishing direct communication between users anywhere on the network.
ARA 2.0 (Apple Remote Access) - Communication software designed to offer remote access to an AppleTalk-compatible network via an ARA server.
ARP (Address Resolution Protocol) - A TCP/IP protocol used to obtain a node's physical address when only its logical IP address is known. An ARP request with the IP address is broadcast onto the network, and the node with that address responds by sending back its hardware address so that packets can be transmitted. The protocol translates IP addresses into physical network addresses, such as Ethernet IDs, by converting the 32-bit IP addresses into the 48-bit physical network Ethernet addresses. Another TCP/IP protocol which plays a less visible, but equally important role in the operation of TCP/IP networks. Reverse ARP, or RARP, is used by a diskless workstation to obtain its logical IP address.
AUP (Acceptable Use Policy) - Procedures used to govern the appropriate usage of a network or service.
Bisync or BSC (Binary SYNChronous) - A synchronous communications protocol that transmits binary-coded data between two devices by using a set of control characters and control character sequences. Unlike asynchronous transmissions, Bisync requires that both sending and receiving devices are synchronized before transmission of data is started.
Bandwidth - The capacity or amount of data that is transmitted over a network, processor bus, cache bus, or I/O bus. Normally expressed in bits/second, bytes/second, or cycles/second.
Bad Block Table - An area on the hard drive disk reserveded for keeping information on failed sectors on the device.
BNC (British Naval) Connector - A connector commonly used with coaxial cable.
Boost - The addition of UPS battery power to utility power to increase voltage to an acceptable level (usually 120 volts).
BOOTP (Bootstrap Protocol) - Protocol that allows an Internet node to discover certain startup information, such as its IP address. An alternate to RARP.
Brownout - A condition in which utility voltage falls by more than 10 percent.
Buck - The blocking of overly high-voltage utility power by a UPS before it reaches the attached devices.
Cache Flush - The act of writing all the data in the cache to either system memory or the hard drive.
Channel - A direct path from one resource to another. For example, a direct path to a set of hard disk drives or a direct path from a NIC to another NIC.
CHAP/PAP (Challenge Handshake and Authentication Protocol/Password Authentication Protocol) - Standard authentication protocol for PPP connections.
CLNP (Connectionless Network Protocol) - An OSI network layer protocol that does not require a circuit to be established before data is transmitted. An example of this type of protocol is UDP.
Clustering - Linking multiple servers to ensure system resources will be made available to all users. A clustered system is transparent to users, seeing no difference between a cluster and a stand-alone server.
Collision - The loss of electronic signals, or packets, that results when two workstations or two computers attempt to transmit data simultaneously across a shared medium. Data must be re-sent as a consequence.
Command Queuing - The issuing of multiple commands and allowing the disk controller to execute these commands out of sequence.
Concurrent Reconstruction - The act of reconstructing data of a failed hard drive while the server is still executing applications and servicing client users.
Consistency Check - In the case of hard drives, this is the act of checking the hard drives with a shared parity drive to see if their XOR?d data matches.
Cookie - A piece of information sent by a Web server to a Web browser. The browser software is then expected to save the data and send it back to the server whenever the browser makes additional requests from the server.
Crossover Cable - A type of networking cable in which some wires are reversed from one end to the other to join two computers or two hubs.
Data Avallability - The ability to access data from a hard drive, memory, or cache when required by the processor or resource controller.
Data Reliability - The ability of the server to operate and deliver data without failure over an extended period of time.
DDCMP (Digital Data Communications Message Protocol) - A byte-oriented, link layer protocol that supports half- or full-duplex modes and either point-to-point or multipoint lines in a Digital Network Architecture network.
DHCP (Dynamic Host Configuration Protocol) - A protocol for automatic TCP/IP configuration that provides static and dynamic address allocation and management.
Disconnect - The act of freeing the SCSI bus while a hard drive is seeking and reconnecting when the drive is ready to transfer data.
DNS (Domain Naming System) - A hierarchical system for assigning unique names to Internet hosts. Special hosts running DNS server software accept host name queries and return either the IP number of the desired host or a pointer to another DNS server that "knows" about the desired host. From the user's point of view, this resolution of host names to machine addresses usually occurs transparently and almost instantaneously, allowing the user to reference Internet hosts and resources symbolically rather than via arbitrary numeric identifiers.
DTDS (Disaster Tolerant Disk Systems) - A new term from the RAID advisory board which describes server systems which must be divisible into two or more zones which cooperate to protect against loss of access to data in the event of one system?s complete failure. Protection also is provided against massive power outages, cooling system failures, and disconnection of power or bus cables. DTDS+ adds a distance between zones of at least 1Km.
Domain - An element of the naming hierarchy on the Internet.
Domain Name - The unique name that identifies an Internet site.
EEC (error correction code) - An algorithm which tests for and promptly corrects errors (in memory) on the fly. The algorithm normally used will take checksums generated by hardware circuitry to correct any memory errors greater than one bit.
E-Commerce - Conducting business transactions through electronic transmissions between computers. Typically used in reference to doing business over the internet.
Encryption - A type of network security in which information sent over the network is encoded so that only the intended users can access the information.
Enhanced Error Recovery - The ability to keep track of partial failures during the lifetime of a unit and analyze those errors to locate early potentially fatal failures.
Error Logging - the ability to store errors between service periods. Normally stored in a Flash Memory device.
Ethernet - A LAN cable-and-access protocol that uses twisted-pair or coaxial cables and CSMA/CD (Carrier Sense Multiple Access with Collision Detection), a method for sharing devices over a common medium. Ethernet runs at 10 Mbps; Fast Ethernet runs at 100 Mbps. Ethernet is the most common type of LAN.
Fault Tolerance - The measurement of a system?s ability to recover from a failure.
Firewall - Hardware and/or software used to divide a LAN into two or more parts for security reasons.
FRDS (Failure Resistant Data Systems) - A new term from the RAID advisory board which describes server systems which are designed to protect against data loss due to any single component failure within the system and against loss of data to single disk failure. FRDS+ adds automatic hot disk swap and protection against data loss due to cache failure or external power failure.
FTDS (Failure Tolerant Data Systems) - A new term from the RAID advisory board which describes server systems which must offer continuous data availability in the event of any single system component failure. FTDS+ adds data access protection against host and host I/O bus failure and external power failure. FTDS+ also requires hot swapping of any major component and the ability to connect at least two hosts using separate I/O buses.
FTP (File Transfer Protocol) - A file-sharing protocol that requires users to log in their name for verification and security purposes. It allows users to transfer text and binary files to and from a PC, list directories on the foreign host, delete and rename files on the foreign host, and perform wildcard transfers between hosts. Designed to work with TCP/IP.
Full-Duplex - Using simultaneous two-way communication between network cards, which effectively doubles the available bandwidth by transmitting and receiving simultaneously. For example the bandwidth would double from 10 to 20 Mb per second in Ethernet or from 100 to 200 Mb per second in Fast Ethernet when in full-duplex mode.
Hamming Code - A well known type of error correcting algorithm used for detecting and correcting memory errors. The algorithm was developed by R.W. Hamming and is able to detect single-bit errors and correct them. The algorithm is also able to detect double-bit errors and nibble-bit errors, but not able to correct them on the fly.
HDLC (High-level Data Link Control) - An ISO communications protocol standard for point-to-point and multi-point communications, HDLC is used in x.25 packet switching networks and provides error correction at the data link layer. HDLC is a precursor to current ADSC standards.
Hop - Term used to describe the data link between two gateways or routers that a packet must travel to reach its destination.
Host - Term used in the internet community to describe any device attached to the network that provides application-level service.
Hot-Spare - Normally in reference to a hard drive. A spare disk drive which is kept operational in the server system, in case a different hard drive unit fails. After the hard drive failure, the spare unit replaces the failed unit.
Hot-Swap - Ability to remove and add hardware to a computer system without powering off the system.
HTTP (Hypertext Transfer Protocol) - The protocol for transfer of hypermedia documents that is the foundation of the World-Wide Web. In the simplest sense, the protocol consists mainly of "send me this resource" requests from a Web browser to a Web server, and a "here it is" or "can't find it" or "not right now" reply from the server to the browser.
HTML (Hypertext Markup Language) - A simple-hypertext publishing language based on computing's lowest common denominator; ASCII. The hypertext document-encoding scheme is used for resources published on World-Wide Web servers. Much like the Rich Text Format (RTF), an HTML document is a mixture of ASCII text and special reserveded character sequences called tags that control formatting of the text. HTML is a subset of the Standard Generalized Markup Language (SGML).
Hub - A hardware device that serves as the junction where individual PCs and other network devices connect to each other. It most cases, up to four hubs can be connected per LAN segment to increase the number of available ports.
Hyperlink - A "hot spot" within an HTML document. Hyperlinks can be clicked by the viewer to cause a "jump" to another document or other resource on the same or another Web server. The hyperlink has two components: the text or graphic that is displayed by the Web browser and the URL of the corresponding resource. The user-visible portion of a hyperlink is usually marked with color or underlining or both (if text) or surrounded by a special border (if a bitmap image).
ICMP (Internet Control Message Protocol)- Allows IP routers to send error and control messages to other IP routers and hosts. ICMP messages travel in the data fields of IP datagrams and are a required part of all IP implementations. If a router is unable to forward an IP datagram, for example, it uses ICMP to inform the sender that there's a problem.
IGMP (Internet Group Management Protocol) - Permits IP datagrams to be broadcast, or multicast, to computers that belong to multicast groups.
IGRP (Interior Gateway Routing Protocol) - A distance vector routing protocol developed by Cisco Systems for use in large, heterogeneous networks.
Internet - A conglomeration of networks connected together forming a world-wide network. Uses the TCP/IP protocol as the backbone. Originally developed for military purposes, but now used more for commercial use.
IP (Internet Protocol) - The TCP/IP standard protocol that defines the IP datagram. A low-level protocol that routes packets of data across separate networks tied together by routers to form the Internet or an intranet. Tracks the Internet address of nodes, routes outgoing messages and recognizes incoming messages. Used in gateways to connect networks at OSI network layer Level 3 and above.
IP Number - Each machine or "host" that participates in any Internet transaction, whether it be a PC or a Cray supercomputer, is identified by a unique 32-bit value called the IP number. The IP number is used as either a return address or a destination address in all Internet transactions. IP numbers are commonly written out as "dotted octets" with the decimal values of the 4 bytes separated by periods--126.96.36.199, for example.
IPX (Internet Packet Exchange) - A Novell NetWare communications protocol used to route messages from one node to another. IPX is not able to always deliver a complete message because an IPX packet can occasionally get lost when crossing networks. The application has to provide that control or NetWare's SPX protocol must be used.
ISP (Internet Service Provider) - An organization which provides access to the Internet typically for a fee. All major metropolitan areas in the world have numerous individual ISPs which will provide Internet service.
JBOD (Just a Bunch Of Drives)- A term used concerning a specific combination of hard drives on a SCSI controller.
Joule - A unit of energy. The surge suppression ability of a UPS is often quoted in joules.
LAN (Local Area Network) - A communications network that links PCs and other devices in a single office or small campus. In Client/server LANs, users?PCs (the clients) access shared files and sometimes applications stores on a dedicated PC that acts as the server. In peer-to-peer networks, any connected PC can serve as both a client and a server.
Line Conditioning (or power conditioning) - A process by which UPSs filter utility power to ensure that the proper voltage is sent to devices.
Line-Interactive - Refers to a design in which a UPS monitors and filters utility power before passing it to the attached devices. Battery power is used only when the incoming voltage dips below or rises above acceptable levels. Most server-level UPSs are line-interactive units, as these are generally less expensive than on-line UPSs and provide adequate protection for devices such as PCs.
Load Shedding - A procedure by which the UPS switches off selected devices to save power or prolong battery runtime.
LLC (Logical Link Control) 802.2 - An Ethernet-based protocol developed for data-link-level transmission control. The upper sublayer of the IEEE Layer2 (OSI) protocol complements the MAC protocol, includes end-system addressing and error checking, and provides a common access control standard. 802.2 governs the assembly of data packets and their exchange between data stations independent of how the packets are transmitted on the LAN.
LUN (Logical Unit Number) - A physical number given to a SCSI device (normally hard drives) on the same SCSI channel. Each SCSI controller keeps track of a device on that chain through the LUN.
MID-II (Management Information DataBase II) - A collection of objects that can be accessed via a network management protocol such as SNMP.
Mirroring - A technique used with hard drive arrays where the data on each drive is duplicated on a mirrored (or exact copy) drive.
MTBDL (Mean Time Between Data Loss) - A mathematical equation which predicts the average time when a hard drive will operate without losing data.
MTBF (Mean Time Between Failure) - A mathematical equation which predicts the average time when a component or any electrical resource will work without failure. The equation takes the number of failures divided by the hours under intense testing and observation.
MTDA (Mean Time of Data Availability) - A mathematical equation which predicts the average time when the hard drive will be accessible and the data on the hard drive will be made available without data loss or failure.
MTTF (Mean Time to Failure) - A mathematical equation which predicts the average time when a component or electrical resource will fail. Somewhat based on the MTBF value.
NDIS (Network Device Interface Specification) - a hardware- and protocol-independent driver specification for network adapter cards developed by Microsoft and 3Com and supported by many NIC vendors and OSs.
NetBEUI (NetBIOS Extended User Interface) - An enhanced version of the NetBIOS protocol used by network operating systems such as LAN Manager, LAN Server, Windows for Workgroups and Windows NT. It formalizes the transport frame and adds more functions.
NetBIOS (Network Basic Input/Output System) - A network operating protocol that comes in two modes ?Data-gram and Session. Datagram, which is the faster but less reliable of the two, uses a self-contained packet with send and receive name, usually limited to 512 bytes. The Session establishes a mode until broken. Unlike Datagram, Session guarantees delivery of messages up to 64KB long. The most common version of NetBIOS in use today is NetBEUI, which is a part of all Microsoft Windows systems. NetBIOS differs from IPX and IP in that it is not routable and is only on a single network segment.
NFS (Network File System) - A file-system protocol that distributes files within a diverse network. Allows non-local networked computers to use the files and peripherals of the connecting networked computer. Primarily used in Unix, but has migrated somewhat to other off the shelf operating systems.
NIC (Network Interface Card) - Another name for the adapter card that goes into your PC and connects it to a LAN.
Noise - A phenomenon caused by lighting, load switching, generators, and other sources of interference that disrupts the utility power?s smooth sine wave and may product glitches and introduce errors in programs and data files. Also known as electromagnetic interference (RFI).
ODI (Novell?s Open Data-link Interface) - A device driver specification for network adapter cards. Like NDIS, ODI is protocol-independent and is supported by many NIC vendors and Operating Systems.
On-line - Refers to a design in which the UPS first converts utility power to low-voltage direct current (DC), then converts it back to the higher-voltage alternating current (AC) the attached devices require. The batteries are always on-line, so power is never interrupted by switching, even during a blackout. This type of design is more expensive to implement but is preferable for sensitive electronic equipment that might be damaged by the brief switching interval of a line-interactive UPS.
OSPF (Open Shortest Path First) - A link-state routing protocol used to determine the least expensive path for routing a message by examining the number of routers, transmission speed, delays and route cost.
Packet- An individual bundle of data that is transmitted across the network. Each packet includes information about its size, origin, and destination, as well as error detection and correction bits, in addition to the data being sent.
PAP (Password Authentication Protocol) - A protocol used for identifying and authenticating a user?s username and password.
Parity - The data that is used to detect/correct single bit failures. Normally associated with hard drives or system memory resources.
Partial Drive Failure - The failure of a single sector on a hard drive.
PPP (Point-to-Point Protocol) - A protocol that allows the user to connect directly onto the internet using a standard telephone line and high-speed modem. Features error detection and data protection, unlike older internet working systems such as SLIP.
Power Event - Any significant alteration in the flow of 120V utility power, such as a spike, surge, brownout, or blackout.
RAID (Redundant Array of Independent Disks) - A group of two or more hard drives which provide increased performance with various levels of error recovery and fault tolerance. Can be implemented in software operating systems, by software with standard disk controllers, or designed in a disk controller itself.
RARP (Reverse Address Resolution Protocol) - Converts physical network address into IP addresses. Does the opposite of ARP, converting the 48-bit physical network addresses into a 32-bit IP addresses. Another TCP/IP protocol which plays a less visible, but equally important role in the operation of TCP/IP networks.
Rebuild - The act of reconstructing the data in a spare/replaced hard drive.
Recovery - The act of reconstructing the data from the remaining hard drive array after a hard drive failure.
Redundancy - Having one more than is needed to operate. In case of a failure, the “spare?component item can take over operation with no user downtime.
RIP (Routing Information Protocol) - The simplest routing protocol, RIP finds the shortest path between two points on a network in terms of “hops.?OSPF AND IGRP are more advanced routing protocols.
RJ-45 Port - The standard 8-pin connector found in hubs and NICs. Twisted -pair cable runs between two RJ-45 ports.
Router - A computer or network device which transfers packets of data from one network to another.
TCP (Transmission Control Protocol) - A connection-oriented protocol that transmits data in byte streams. Data is transmitted in packets called TCP segments, which contain TCP headers and data. TCP is a “reliable?protocol because it uses check-sums to verify data integrity and hand-shaking to make sure transmitted data is received intact.
TCP/IP (Transmission Control Protocol/Internet Protocol) - This is actually two acronyms, but they are almost always seen together. They stand for Transport Control Protocol and Internet Protocol respectively and are the two fundamental packet-oriented network communication protocols of the Internet. This communications protocol is used for data exchange between diverse computer hardware and operating systems. TCP/IP software stacks are available for nearly every flavor of processor and operating system, and most of them are either built into the operating system itself or available for free. Other Internet protocols for terminal emulation, file transfer, e-mail, news, and information publishing are layered on top of TCP/IP. TCP/IP uses a sliding window to maximize speed and adjusts to slower circuits and delays in route.
Trickle Charging - A common method of battery recharging that continually provides a small amount of current to the battery to keep it fully powered.
UDP (User Datagram Protocol) - A TCP/IP connection-less mode protocol that allows an application to send a message to one of several applications running on a remote or local machine. UDP is an ?unreliable?protocol because the sender receives no information indicating whether a datagram was actually received.
Uplink Port - A jack on a hub that connects slower devices and faster backbones, such as a 10-Mbps Ethernet LAN and a 100-Mbps Fast Ethernet backbone.
URL (Uniform Resource Locator) - A method for specifying the exact location of an Internet resource (typically a file) and the network protocol necessary to retrieve and interpret the resource. For example, the URL http://www.peds.csmc.edu/readme.html indicates a file named README.HTML that resides on an Internet host named www.peds.csmc.edu and tells you that the resource can be retrieved and viewed with a program that supports the HTTP protocol (in other words, a Web browser).
Volt-Amperes (VA), and Kilovolt-Amperes (kVA) - Two ways of expressing a power measurement derived when volts are multiplied by amps. A VA rating is used to indicate a UPS?s output capacity, as well as the input power requirement of a computer or other AC load.
Web Browser - A program that can retrieve HTML documents from Web servers using the HTTP protocol and format them for display. A browser also knows how to interpret hyperlinks within the body of an HTML document and use them to navigate from one HTML document to another on the same or another Web server. Cello, Lynx, Mosaic, Microsoft Internet Explorer and Netscape Navigator are examples of Web browsers.
Web Client - A common presentation Internet navigator, normally an Intel-based personal computer or notebook attached to an Internet connection. A web front end is a GUI-based, hypertext network browser that makes Internet navigation a mouse click away. It lets users jump from one information source to another, anywhere in the world, get data, set off applications, view the results, or just communicate in real time across the whole internet. Web clients are available for literally every form of computer-from dumb terminals on MVS or VMS, to X Windows, NextStep, OS/2, Windows 3.1, Windows95 and Windows NT, even MS-DOS. Major operating system venders normally ship a web browser with their software.
Web Server - An Internet-based distributed-processing system. The Web server is conducting a background process called the Hypertext Transfer Protocol (HTTP), which implements a reduced version of the Internet's FTP (File Transfer Protocol) commonly used for packaging and transferring files across the Internet.
Web Server Program - A program that understands the HTTP protocol and responds to resource requests from Web browsers using that protocol. A Web server program must run on an Internet host that is addressable with an IP number, found via its corresponding DNS host name. The machine that runs the Web server program is often referred to informally as a Web server as well, although the machine may well be running many other programs at the same time (including other types of server programs).
WWW - A colloquial abbreviation for the World-Wide Web. The Web, which was invented by physicists at the European Community's particle physics research center in Switzerland (CERN), is more of a conceptual construct than a physical entity. All the Web servers on the Internet taken together constitute the World-Wide Web, but there is no central administration or coordination of servers. Each server is identified by a DNS host name; each document or other resource on a Web server is designated by a URL.
X.25 - The protocol that provides devices with a direct connection to a packet switched network. The devices usually include large computers such as mainframes and mini-computers. Word processors, PCs, workstations and dumb terminals, and so forth do not support X.25 packet switching protocols unless they are connected to the network via PADs ?Packet Assembler / Dis-assemblers.
XNS (Xerox Network Services) - A distributed-file-system protocol that allows network stations to use other non-local computers?files and peripherals. It is a five-layer architecture of protocols and was the foundation of the OSI seven-layer model.
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