IBM SC30-3681-08 manual Fragment. See fragmentation, Level

¯ash memory. A data storage device that is programmable, erasable, and does not require continuous power. The chief advantage of ¯ash memory over other programmable and erasable data storage devices is that it can be reprogrammed without being removed from the circuit board.

¯ow control. (1) In SNA, the process of managing the rate at which data traffic passes between components of the network. The purpose of ¯ow control is to optimize the rate of ¯ow of message units with minimum congestion in the network; that is, to neither over¯ow the buffers at the receiver or at intermediate routing nodes, nor leave the receiver waiting for more message units. (2) See also pacing.

fragment. See fragmentation.

fragmentation. (1) The process of dividing a datagram into smaller parts, or fragments, to match the capabilities of the physical medium over which it is to be transmitted. (2) See also segmenting.

frame. (1) In Open Systems Interconnection architecture, a data structure pertaining to a particular area of knowledge and consisting of slots that can accept the values of speci®c attributes and from which inferences can be drawn by appropriate procedural attachments. (T) (2) The unit of transmission in some local area networks, including the IBM Token-Ring Network. It includes delimiters, control characters, information, and checking characters. (3) In SDLC, the vehicle for every command, every response, and all information that is transmitted using SDLC procedures.

frame level. Synonymous with data link level. See link

level.

frame relay. (1) An interface standard describing the boundary between a user's equipment and a fast-packet network. In frame-relay systems, ¯awed frames are discarded; recovery comes end-to-end rather than hop-by-hop. (2) A technique derived from the integrated services digital network (ISDN) D channel standard. It assumes that connections are reliable and dispenses with the overhead of error detection and control within the network.

front-end processor. A processor such as the IBM 3745 or 3174, that relieves a main frame from the communication control tasks.

G

gateway. (1) A functional unit that interconnects two computer networks with different network architectures. A gateway connects networks or systems of different architectures. A bridge interconnects networks or systems with the same or similar architectures. (T)

(2)In the IBM Token-Ring Network, a device and its associated software that connect a local area network to

another local area network or a host that uses different logical link protocols. (3) In TCP/IP, synonym for router.

general data stream (GDS). The data stream used for

conversations in LU 6.2 sessions.

general data stream (GDS) variable. A type of RU substructure that is preceded by an identi®er and a length ®eld and includes either application data, user control data, or SNA-de®ned control data.

H

header. (1) System-de®ned control information that precedes user data. (2) The portion of a message that contains control information for the message such as one or more destination ®elds, name of the originating station, input sequence number, character string indicating the type of message, and priority level for the message.

heap memory. The amount of RAM used to dynamically allocate data structures.

Hello. A protocol used by a group of cooperating, trusting routers to allow them to discover minimal delay routes.

hello message. (1) A message sent periodically to establish and test reachability between routers or between routers and hosts. (2) In the Internet suite of protocols, a message de®ned by the Hello protocol as an Interior Gateway Protocol (IGP).

heuristic. Pertaining to exploratory methods of problem solving in which solutions are discovered by evaluation of the progress made toward the ®nal result.

high-level data link control (HDLC). In data communication, the use of a speci®ed series of bits to control data links in accordance with the International Standards for HDLC: ISO 3309 Frame Structure and ISO 4335 Elements of Procedures.

high-performance routing (HPR). An addition to the Advanced Peer-to-Peer Networking (APPN) architecture that enhances data routing performance and reliability, especially when using high-speed links.

hop. (1) In APPN, a portion of a route that has no intermediate nodes. It consists of only a single transmission group connecting adjacent nodes. (2) To the routing layer, the logical distance between two nodes in a network.

hop count. (1) A metric or measure of distance between two points. (2) In Internet communications, the number of routers that a datagram passes through on its way to its destination. (3) In SNA, a measure of the number of links to be traversed in a path to a destination.

Glossary 699