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D
D Channel
In ISDN
the D Channel is the Data Channel. For the ISDN Basic Rate Interface the D
channel is 16Kbps. In the Primary Rate Interface the D channel is 64Kbps.
Daisy Chain
You
create a daisy chain network by linking multiple devices together via a
cabling system. In an AppleTalk network the daisy chain topology is created
using phonenet connectors and twisted pair wiring (regular phone wire). A
daisy chain configuration must be terminated at both ends using terminating
resistors.
Dark Fiber
Dark fiber
is fiber optic cable that does not have light traveling through it. Fiber
is said to "go dark," which means light (or a signal composed of
light) is no longer present.
Data Link Control
Data
Link controls are the characters used to control the transfer of data
frames (such as MAC frames) between two nodes. These control codes perform
such functions as flow control and sequencing.
Data Link Layer
The
Data Link Layer is the second layer of the OSI model. It resides between
the Physical Layer and the Network Layer. It is responsible for
point-to-point transfer of data frames.
Data Set
The
term data set refers to a modem (modulator/demodulator).
Data Set Ready
DSR
stands for Data Set Ready. It is one of the control pins on an RS232 cable.
The modems that communicate using the RS232 interface use the DSR signal to
indicate that they are ready to communicate. It is located on pin 6.
Data Circuit-Terminating Equipment
DCE
stands for Data Circuit-Terminating Equipment. In a network there are two
broad categories of devices, DCEs and DTEs. DTE stands for data terminal
equipment. The difference between the two is that one (DTE) is the end
device in a network and the other (DCE) is the device or devices in the
network that transmit and receive the DTE data.
Data Terminal Equipment
DTE
stands for Data Terminal Equipment. The difference between the two is that
one (DTE) is the end device in a network and the other (DCE) is the device
or devices in the network that transmit and receive the DTE data.
Datagram
A datagram is a
packet that is transferred independently of all other packets. The term
datagram is usually used in referenced to an IP layer packet that uses
connectionless delivery of packets (datagrams). In other words a session is
not established before sending a datagram from one node to another.
DB-25
DB-25 is the
standard connector used for the RS232 interface cable.
DDS
DDS stands for
Digital Data Service (also Dataphone Digital Service). It is a series of
services provided by the telephone company providing digital facilities for
data communications. DDS comes in several speeds, 2.4Kbps, 4.8Kbps, 9.6Kbps
and 56Kbps.
Dedicated Circuits
When
the telephone network was an analog network, electro-mechanical switches could
inject a certain amount of noise into circuits. Other instruments in the
transmission path, such as multiplexers, could corrupt the signal further.
At lower data rates, say 300 or 1200bps, modems can correctly transmit data
with few if any errors even when the line is noisy, but as data rates
increase, noise causes more problems. As a practical matter, in the analog
network, data could not be transmitted reliably over long distances on
switched lines at rates over 4800bps. Therefore, it became common practice
for the telephone companies to "lease" lines to companies for
continuous, unswitched use. These leased lines are also referred to as
"dedicated circuits."
Destination Address
The
destination address is the address of the node in a network that a message
is intended for. The destination address and source addresses are sent so
that the sending and receiving devices can communicate with one another and
so that intermediate nodes know where to send a frame or packet.
Destination addresses can be physical (i.e., the Ethernet card in a
workstation) or logical (one of multiple processes in a computer). The
destination node is the node that will receive the message; the source node
is the node that sends the data.
Demodulation
Modulation is the
process of modifying the form of a carrier wave (electrical signal) so that
it can carry intelligent information on some sort of communications medium.
Digital computer signals (baseband) are converted to analog signal for
transmission over analog facilities (such as the local loop). The opposite
process, converting analog signal back into their original digital state is
referred to as demodulation.
Device Driver
A
device driver is a program that controls devices attached to a computer
such as a printer or a hard disk drive.
Dialup Line
A
dial-up line is circuit that exists between two nodes that uses the
switched telephone network to communicate. Dialup lines provide the
following capabilities:
- 300 - 56Kbps transfer rates
- any-to-any connectivity (one at a time)
- requires compatible modems at each end
- before transmission can take place, call
initialization must occur
- inexpensive
Differential Manchester Encoding
Manchester code is a commonly found Physical
Layer coding scheme found in local area networks such as Ethernet and Token
Ring. This self-clocking code provides a signal state change in every bit
transmitted. Differential Manchester
encoding is a similar technique and is also used in LANs.
Director, ESCON
The
Director, sometimes referred to as a "dynamic switch," is a
switch that acts as a communications hub for ESCON channels. It provides
the capability to physically interconnect any two links that are attached
to it. Such a connection between two ports provides simultaneous, two-way
information transfer.
Distributed Processing
Distributed
processing refers to the practice of having several computer systems share
the data processing load. This is in contrast to central processing where
all processing takes place on a centralized host.
DNA
DNA stands for
Digital Network Architecture. It is the networking architectures designed
and implemented by Digital Equipment Corporation.
DNS (Domain Name Server)
In a
TCP/IP network, a user can communicate with another user by specifying a
domain name, such as johnd@engr.company.com. TCP and IP require Internet
addresses for messages, so one must be translated to the other. That's the
job of the name server-given a domain name, they will return an Internet
address (but not the reverse). Recall that domain names are hierarchical.
There is a corresponding hierarchy of name servers. Given the domain name
kdr@mcdata.com, DNS will resolve this from right to left. DNS will first
locate a server on the Internet that knows all of the locations for
companies. That DNS will then locate the company McDATA and ask for the
address of kdr.
Domain Name System
The
Domain Name System (DNS) is a hierarchical, distributed method of
organizing the name space of the Internet. The DNS administratively groups
hosts into a hierarchy of authority that allows addressing and other
information to be widely distributed and maintained. A big advantage to the
DNS is that using it eliminates dependence on a centrally maintained file
that maps host names to addresses.
Dotted Decimal Notation
The
syntactic representation for a 32-bit integer that consists of four 8-bit
numbers written in base 10 with periods (dots) separating them. Used to
represent IP addresses in the Internet as in 192.67.67.20.
Downlink
Downlink refers
to the signal path that extends from a satellite down to the earth. Uplink
is just the opposite; it is the path from the earth up to a satellite.
Downstream
Downstream refers
to the relative position of a Token Ring LAN device. A station that
receives the token after another device is said to be downstream of that
device. A downstream node is a node on a LAN that is downstream from the
host in the network.
DQDB
DQDB stands for
Distributed Queue Dual Bus. It is the IEEE 802.6 standard for metropolitan
area networks (MANs).
Dumb Terminal
A dumb
terminal is a terminal that is solely dependent on a host computer for
processing capabilities. Dumb terminals typically do not have a processor,
hard drive or floppy drives, only a keyboard, monitor and a method of
communicating to a host (usually through some type of controller).
Intelligent terminals on the other hand contain their own processor and
storage devices as well as software programs.
Duplex
Duplex refers to
the process of transmitting data in two directions at the same time. This
is also referred to as duplex transmission or duplex operation. There are
three modes of transmission: simplex, half duplex and duplex. Simplex
transmission is when a signal can only be transmitted in one direction.
With half duplex transmission the signal can travel in both directions, but
not simultaneously.
DVMRP
Distance
Vector Multicast Routing Protocol (DVMRP) is an Internet routing protocol
that provides an efficient mechanism for connectionless datagram delivery
to a group of hosts across an internetwork. It is a distributed protocol
that dynamically generates IP multicast delivery trees using a technique
called Reverse Path Multicasting
DVMRP combines many of the features of RIP with the Truncated Reverse Path
Broadcasting (TRPB) algorithm. DVMRP is developed based upon RIP because an
implementation was available and distance vector algorithms are simple, as
compared to link-state algorithms. In addition, to allow experiments to
traverse networks that do not support multicasting, a mechanism called
tunneling was developed.
DVMRP differs from RIP in one very important way. RIP routes and forwards
datagrams to a particular destination. The purpose of DVMRP is to keep
track of the return paths to the source of multicast datagrams. To make the
explanation of DVMRP more consistent with RIP, the term destination is used
instead of the more proper term source, however, datagrams are not
forwarded to these destinations, but rather, originate from them.
DVMRP packets are encapsulated in IP datagrams, with an IP protocol number
of 2 (IGMP). All fields are transmitted in Network Byte Order. DVMRP
packets use a common protocol header that specifies the IGMP Packet Type as
DVMRP. DVMRP protocol packets should be sent with the Precedence field in
the IP header set to Internetwork Control (hexadecimal 0xc0 for the Type of
Service Octet).
Dynamic Bandwidth Allocation
Dynamic
bandwidth allocation is the process used by some multiplexers which allows many
slower speed devices (circuits) to share a common high speed circuit, even
when the sum of all of the slower speed devices is greater than the
bandwidth of the high speed circuit.
Dynamic Routing
Dynamic
routing is a process where routing is adjusted automatically when the
topology of a network changes or the conditions within a network change
(such as congestion).
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