GE MDS iNET 900, Справочное руководство

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164 iNET Series Reference Manual 05-2806A01, Rev. J
from the number and order of bits in a data string. This value is com-
pared with a locally-generated value and a match indicates that the mes-
sage is unchanged, and therefore valid.
Datagram —A data string consisting of an IP header and the IP message
within.
dBi —Decibels referenced to an “ideal” isotropic radiator in free space.
Frequently used to express antenna gain.
dBm —Decibels referenced to one milliwatt. An absolute unit used to
measure signal power, as in transmitter power output, or received signal
strength.
DCE —Data Circuit-terminating Equipment (or Data Communications
Equipment). In data communications terminology, this is the “modem”
side of a computer-to-modem connection.
COM1
Port of the transceiver
is set as DCE.
Decibel (dB) —A measure of the ratio between two signal levels. Fre-
quently used to express the gain (or loss) of a system.
Delimiter —A flag that marks the beginning and end of a data packet.
DHCP (Dynamic Host Configuration Protocol) —An Internet stan-
dard that allows a client (i.e. any computer or network device) to obtain
an IP address from a server on the network. This allows network admin-
istrators to avoid the tedious process of manually configuring and man-
aging IP addresses for a large number of users and devices. When a
network device powers on, if it is configured to use DHCP, it will con-
tact a DHCP server on the network and request an IP address.
The DHCP server will provide an address from a pool of addresses allo-
cated by the network administrator. The network device may use this
address on a “time lease” basis or indefinitely depending on the policy
set by the network administrator. The DHCP server can restrict alloca-
tion of IP addresses based on security policies. An Access Point may be
configured by the system administrator to act as a DHCP server if one
is not available on the wired network.
DTE —Data Terminal Equipment. A device that provides data in the
form of digital signals at its output. Connects to the DCE device.
Encapsulation —Process in by which, a complete data packet, such as
Modbus frame or any other polled asynchronous protocol frame, is
placed in the data portion of another protocol frame (in this case IP) to
be transported over a network. Typically this action is done at the receiv-
ing end, before being sent as an IP packet to a network. A similar re-
versed process is applied at the other end of the network extracting the
data from the IP envelope, resulting in the original packet in the original
protocol.