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H
H-Channel
The H-Channel is the
channel on an ISDN basic rate interface that is used for higher bit rates.
The bit rates associated with these channels are:
- H0: 384kbps
- H11: 1536kbps
- H12: 1920kbps
These channels could be used for high bit rate communications
such as video telephoning.
Half-Bridge
As an enterprise
network grows, it will tend to be dispersed over a wide geographical area.
Another important use of bridges is to connect LANs remotely. WAN bridges,
also called half-bridges, work together in pairs.
Half-Duplex
A leased line
typically has four wires (although it is possible to lease a two-wire
line). Half-duplex means that one wire-pair is used to transmit in one direction
and the other in the other direction, but transmission does not take place
in both directions at the same time. Half-duplex is faster than using a
single wire-pair for both directions because it's not necessary for the
modems to wait for the line to "turn around" each time the
direction of data transmission reverses.
Handshaking
Handshaking
refers to the initialization process that two or more computers go through
before they are able to communicate. It is the first part of each and every
data communications protocol. It is used to establish initial setup
parameters.
HDLC
HDLC stands for
High-Level Data Link Control. The HDLC protocol suite represents a wide
variety of Link Layer protocols such as SDLC (Synchronous Data Link
Control), LAPB (Link Access Protocol Balanced) and LAPD (Link Access
Protocol D).
Header
A header is part
of a message, packet or frame that contains information necessary to send a
unit of information from one node to another. The header normally contains
a field specifying the length of the encapsulated message together with at
least one field providing information about the message. If, for example,
the message is a segment of a larger message, the header might specify the
relative position of the segment in the complete message and probably the
total number of segments in the message.
Hexadecimal
The hexadecimal
number system is a base 16 numbering system. It is the numbering system
used to condense binary bytes into a compact form for printing or analysis
of computer data. It is composed of the numbers 0-9 and the letters A-F.
Each "nibble" (four bits) of a byte can be represented by one of
the 16 characters.
High Performance Parallel Interface
HPPI
stands for high performance parallel interface. It is also referred to as
HiPPI. HPPI is a high-speed physical layer standard that provides 25Mbps
per line on a 32-bit parallel bus for a total bit rate of 800Mbps. It is
limited to 25 meters
but in conjunction with fiber optic links it can be extended to two
kilometers. HPPI is also referred to as an ANSI standard X3T9.
Hop
When a packet or
datagram is being routed in a network, it may go through multiple routers
or packet switches on the way to the final destination. Each intermediate
node (router) is considered a hop. Hop count is the number of routers that
a packet goes through between source and destination nodes.
Host ID
Internet
(IP) addresses must all be unique. The IP address contains a network
portion and a host portion. The host ID portion of the address must be
unique for the particular network. Every node on the network must know how
to tell which bits in the Internet address correspond to their physical
network, or subnet. This is accomplished through a "subnet mask"
which is set through the software in each node. If the subnet mask is
incorrectly set by the user or system administrator the node will not be
able to recognize its address in messages on the LAN and will not be able
to communicate. The subnet mask, which must be consistent throughout the
network, is a 32-bit hexadecimal word which "masks out" the node
address.
Hub
A hub is a
network component that centralizes circuit connections. Hubs started out as
wiring concentrators but have developed into sophisticated switching
centers.
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