Gateway :
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A network station designed to interconnect two otherwise incompatible devices or networks. Occasionally, a gateway may perform protocol conversion and packet assembly/disassembly ( PAD) functions. Gateways operate at the fourth through seventh layers of the OSI model. Contrast with bridge .
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Guard Band, Guard Frequency :
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Two definitions are commonly found in data communications:
1 - The unused frequencies between sub channels in FDM systems used to separate channels, thereby preventing crosstalk.
2 - A single carrier tone used to indicate that a communications line is prepared to transmit data.
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Point-to-Point :
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A data communications circuit or path connecting two points.
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Presentation Layer :
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The sixth layer in the OSI model that is responsible for format and code conversion.
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Profile :
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A term used in packet-switched environments to describe a set of parameters or values defining a hardware device (printer, terminal) that would be stored and later recalled as a group for later use.
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Propagation Delay :
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The time for transit of a signal from one point on a circuit, link, network, or system to the other point.
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Protocol :
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The method of "line discipline" used to control the orderly exchange of information between the stations on a data link or on a data communications network or system.
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Start-Stop System :
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A system that uses both a start bit as well as a stop bit.
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Switch, Switching :
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Several definitions exist in normal usage: 1) In central office facilities, switching refers to the process of transferring calls from one device to another via circuit connection. 2) A data switcher, or data PBX may sometimes be informally referred to as a "switch". See data PBX, data switcher. 3) In a packet-switched environment, the "switch" is installed on the packet networks backbone, at one of the nodes and serves to direct packets, also known as a switching processor.
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Synchronous, Synchronous Transmission :
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Using a constant time interval between bits or characters to insure that all data communications equipment is in step with each other. The opposite of asynchronous transmission, synchronous transmission, does not use start-stop bits surrounding each byte, but relies on the transmitter and receiver to be functioning at a fixed rate of transmission, thereby being a more efficient method of data communication. Contrast with asynchronous transmission.
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Security :
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Any method or technique designed to prevent unauthorized physical access to information. Wiretapping or electromagnetic eavesdropping is a security threat whenever data travels through the air or over wires that are not in a secure area. Most data communication networks use common carrier facilities, and this presents problems. Sensitive data that is to be transmitted from one location to another should be encrypted to make it private. Privacy transformations involving static methods of coding require a certain amount of work to break, but can usually be decoded after some effort. The best coding techniques involve keys that are as long as the data to be encrypted. By using different starting values, and different related sequences of random numbers, it becomes very difficult to determine the generating algorithm from eavesdropping, so the required work to break the code is very extensive.
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Serial Transmission :
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A "normal" mode of information transfer in data communications in which the bits comprising a character are sent in sequence, one at a time. Contrast with parallel transmission, normally used between a computer and its peripherals.
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Session :
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A term used to define the logical link-up between two stations allowing them to communicate.
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Session Layer :
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The fifth layer of the OSI model that addresses establishing, managing, and terminating connections for individual application programs, and interfacing with the transport layer.
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Token Ring :
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A LAN environment topology that uses a token for explicit access. Unlike token bus, the token is passed from station to station, in sequential order, so that the next logical station that receives the token will also be the next physical location.
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Topology :
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The arrangement of stations and the links connecting the stations of a data communications network.
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Traffic :
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The quantity and movement of messages through a data communications system.
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Transmission :
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The sending of information over a data communications medium.
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Transmission Speed :
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A measure of how quickly data is being transmit ted, usually expressed in bps, although commonly, but incorrectly, expressed in baud.
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Timeout :
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A method used to improve and minimize user response times by allocating a terminal or other computer device for a predefined time period, after which if no activity is present on the terminal session or connection is terminated.
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Timesharing, Time Sharing :
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A method of operating a computer so that two or more users are simultaneously able to present problems to the machine and retrieve information from it usually by doing some computation of processing for one user, putting it aside, then doing some computation for a different user. A clocking mechanism would send the CPU interrupts at a predefined interval to provide a timesharing environment for connected terminals.
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Token, Token Passing :
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A term used in a LAN environment to describe the special "message" called a token, that allows a station to control the transmission medium. The token is transmit ted from node to node. When the receiving station is in possession of the token, it may transmit messages before passing the token on to the next node. See token bus, token ring.
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Token Bus :
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A LAN environment bus topology using a token for explicit access and all stations attached to the bus listen for the token. A station must first receive the token before transmitting. Contrast with token ring.
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Router :
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A router is a computer networking device that forwards data packets across a network toward their destinations, through a process known as routing. Routing occurs at Layer 3 (the network layer i.e. Internet Protocol (IP)) of the OSI seven-layer protocol stack.
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Passive Hubs :
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These hubs are nothing more than point contacts for the wires that make up the physical network. An example of this would be a punchdown block that is a simple plastic, unpowered box used to plug network cables into it.
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Active Hubs :
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Active hubs are a little smarter than the passive hubs we saw. All You might also come across the term "concentrators" which are basically active hubs, concentrating and strengthening a signal as it enters and exits the hub.
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