Browsing: H
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half duplex |
Capability for data transmission in only one direction at a time
between a sending station and a receiving station. Compare with
full duplex and
simplex.
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hammer drill |
Tool resembling an oversized electric drill used for drilling into
masonry. As it turns the bit, it hammers rapidly.
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handshake |
Sequence of messages exchanged between two or more network devices
to ensure transmission synchronization.
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hardware address |
See
MAC address.
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HBD3 |
Line code type used on E1 circuits.
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HCC |
Horizontal cross-connect. Wiring closet where the
horizontal cabling connects to a patch panel which is connected by
backbone cabling to the main distribution facility.
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H channel |
High-speed channel. Full-duplex ISDN primary rate channel
operating at 384 Kbps. Compare with B
channel,
D channel, and
E channel.
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HDLC |
High-Level Data Link Control. Bit-oriented synchronous data link
layer protocol developed by ISO. Derived from SDLC, HDLC specifies a
data encapsulation method on synchronous serial links using frame
characters and checksums. See also SDLC.
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headend |
The end point of a broadband network. All stations transmit toward
the headend; the headend then transmits toward the destination
stations.
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header |
Control information placed before data when encapsulating that data
for network transmission. Compare with
trailer. See also
PCI.
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header checksum |
Field within an IP datagram that indicates the integrity check on
the header.
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heartbeat |
See
SQE.
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HELLO |
Interior routing protocol used principally by NSFnet nodes. HELLO
allows particular packet switches to discover minimal delay routes.
Not to be confused with the Hello protocol.
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hello packet |
Multicast packet that is used by routers for neighbor discovery and
recovery. Hello packets also indicate that a client is still
operating and network-ready.
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Hello protocol |
Protocol used by OSPF systems for establishing and maintaining
neighbor relationships. Not to be confused with HELLO.
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helper address |
Address configured on an interface to which broadcasts received on
that interface will be sent.
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HEPnet |
High-Energy Physics Network. Research network that
originated in the United States, but that has spread to most places
involved in high-energy physics. Well-known sites include Argonne
National Laboratory, Brookhaven National Laboratory, Lawrence
Berkeley Laboratory, and the Stanford Linear Accelerator Center (SLAC).
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hertz |
Measure of frequency, abbreviated Hz. Synonymous with
cycles per second.
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heterogeneous network |
Network consisting of dissimilar devices that run dissimilar
protocols and in many cases support dissimilar functions or
applications.
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hexadecimal |
Base 16. A number representation using the digits 0 through 9, with
their usual meaning, plus the letters A through F to represent
hexadecimal digits with values of 10 to 15. The right-most digit
counts ones, the next counts multiples of 16, then 16^2=256, etc.
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hierarchical routing |
Routing based on a hierarchical addressing system. For example, IP
routing algorithms use IP addresses, which contain network numbers,
subnet numbers, and host numbers.
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hierarchical star topology |
Extended star topology where a central hub is connected by vertical
cabling to other hubs that are dependent on it.
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High-Energy Physics Network |
See
HEPnet.
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High-Level Data Link Control |
See
HDLC.
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High Performance Computing and Communications |
See
HPCC.
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High Performance Computing Systems |
See
HPCS. |
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High-Performance Parallel Interface |
See
HIPPI.
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High Performance Routing |
See
HPR.
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High-Speed Communications Interface |
See
HSCI.
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High-Speed Serial Interface |
See
HSSI.
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highway |
See
bus.
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HIP |
HSSI Interface Processor. Interface processor on the Cisco
7000 series routers. The HIP provides one HSSI port that supports
connections to ATM, SMDS, Frame Relay, or private lines at speeds up
to T3 or E3.
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HIPPI |
High-Performance Parallel Interface. High-performance
interface standard defined by ANSI. HIPPI is typically used to
connect supercomputers to peripherals and other devices.
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HLEN |
Number of 32-bit words in the header.
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HODSP |
High-Order DSP (HODSP) is a NSAP address
field that is used for subdividing the domain into areas. This is
roughly equivalent to a subnet in IP. See also
NSAP Address.
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holddown |
State into which a route is placed so that routers will neither
advertise the route nor accept advertisements about the route for a
specific length of time (the holddown period). Holddown is used to
flush bad information about a route from all routers in the network.
A route is typically placed in holddown when a link in that route
fails.
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homologation |
Conformity of a product or specification to international standards,
such as ITU-T, CSA, TUV, UL, or VCCI. Enables portability across
company and international boundaries.
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hop |
Term describing the passage of a data packet between two network
nodes (for example, between two routers). See also
hop count.
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hop count |
Routing metric used to measure the distance between a source and a
destination. RIP uses hop count as its sole metric. See also
hop and
RIP.
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horizontal cross connect |
See
HCC.
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host |
Computer system on a network. Similar to the term node except
that host usually implies a computer system, whereas node
generally applies to any networked system, including access servers
and routers. See also
node.
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host address |
See
host number.
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host node |
SNA subarea node that contains an SSCP.
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host number |
Part of an IP address that designates which node on the subnetwork
is being addressed. Also called a host address.
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Hot Standby Router Protocol |
See
HSRP.
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hot swapping |
See
OIR and
power-on servicing.
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hot wire |
Ungrounded lead wire that connects the transformer and electrical
devices or appliances via an electrical outlet and power plug.
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HPCC
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High Performance Computing and Communications. U.S.
government funded program advocating advances in computing,
communications, and related fields. The HPCC is designed to ensure
U.S. leadership in these fields through education, research and
development, industry collaboration, and implementation of
high-performance technology. The five components of the HPCC are
ASTA,
BRHR,
HPCS,
IITA, and
NREN.
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HPCS |
High Performance Computing Systems. Component of the HPCC
program designed to ensure U.S. technological leadership in
high-performance computing through research and development of
computing systems and related software. See also
HPCC.
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HPR |
High Performance Routing. Second-generation routing
algorithm for APPN. HPR provides a connectionless layer with
nondisruptive routing of sessions around link failures, and a
connection-oriented layer with end-to-end flow control, error
control, and sequencing. Compare to
ISR. See also
APPN.
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HSCI |
High-Speed Communications Interface. Single-port interface,
developed by Cisco, providing full-duplex synchronous serial
communications capability at speeds up to 52 Mbps.
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HSRP |
Hot Standby Router Protocol. Provides high network
availability and transparent network topology changes. HSRP creates
a Hot Standby router group with a lead router that services all
packets sent to the Hot Standby address. The lead router is
monitored by other routers in the group, and if it fails, one of
these standby routers inherits the lead position and the Hot Standby
group address.
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HSSI |
High-Speed Serial Interface. Network standard for
high-speed (up to 52 Mbps) serial connections over WAN links.
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HSSI Interface Processor
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See
HIP. |
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HTML |
Hypertext markup language. Simple hypertext document
formatting language that uses tags to indicate how a given part of a
document should be interpreted by a viewing application, such as a
WWW browser. See also
hypertext and
WWW browser.
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HTTP |
Hypertext Transfer Protocol. The protocol used by Web browsers and
Web servers to transfer files, such as text and graphics files.
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hub |
1. Generally, a term used to describe a device that serves as the
center of a star-topology network.
2. Hardware or software device that contains multiple independent
but connected modules of network and internetwork equipment. Hubs
can be active (where they repeat signals sent through them) or
passive (where they do not repeat, but merely split, signals sent
through them).
3. In Ethernet and IEEE 802.3, an Ethernet multiport repeater,
sometimes referred to as a concentrator.
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hybrid network |
Internetwork made up of more than one type of network technology,
including LANs and WANs.
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hypertext |
Electronically-stored text that allows direct access to other texts
by way of encoded links. Hypertext documents can be created using
HTML, and often integrate images, sound, and other media that are
commonly viewed using a WWW browser. See also
HTML and WWW browser.
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hypertext markup language |
See
HTML.
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Hz |
See
hertz.
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