Basic
Rate Interface
One of several defined
user interfaces between an ISDN device and the ISDN switch. The basic rate
interface is comprised of
two B-channels (64 Kbps each) and one D-channel (16 Kbps), providing a
user data rate of 144 Kbps.
Also referred to as 2B+D
B-channel
Bearer services are carried
on the B-channel at the rate of 64 Kbps. Since ISDN protocols do not
specify the format of this
data, literally any service may be transmitted over the B-channel,
including voice, video, audio,
and data signals. The B-channel is never used to communicate with
the ISDN network (to send
service requests); such signaling information is always carried on the
D-channel.
Bearer Service
One of the services obtained
through a request sent on the D-channel. Bearer services provide data
transfer at layers 1, 2,
and 3 of the OSI reference model, but do not verify the compatibility at
a higher
layer of the connected devices.
Bus Configuration
In the ISDN context,
the type of connection between the ISDN device and the network termination
equipment. Either point-to-point,
meaning that only one ISDN device is connected, or point-to-multipoint,
meaning that up to eight different ISDN devices share the basic rate interface.
Among the considerations
arising when more than one device shares the BRI are the method of
multiplexing data on the
B-channel for transmission to the other ends of the connections, as well
as the
method of multiplexing control information on the D-channel for transmission
to the ISDN switch.
Circuit-switching
A physical circuit is
established on demand between two communications devices and maintained
exclusively for their use
until the call is terminated. Public telephone networks are circuit-switched.
Channel Bundling
Combining two or more
B-channels into a single, higher capacity channel. Depending on the ISDN
equipment, channel bundling,
also called inverse multiplexing, may be limited to one card, or may
include B-channels from more
than one card.
Compression
A process whereby the
effective throughput is increased by encoding data in such a way that fewer
bits are required to represent
it. For example, a text file might be compressed by representing
common words with single
characters; thus if the word "the" is represented by "@", it will be
transmitted three times faster.
Popular compression algorithms typically reduce the size of the data
by 50%, effectively doubling
the rate at which it is transmitted. "Lossy compression" is a technique
which permanently sacrifices
some of the original information to achieve a greater degree of
compression (suitable for
video or voice transmission), while "lossless compression" guarantees that
the information will be intact
after decompression (as required for text transmissions, financial
transactions, and so on).
Customer Premises Equipment
(CPE)
Equipment installed at
an individual user's home or business, and capable of attaching to a
telecommunications service
provider's network. A telephone, PC, and fax machine are examples of
CPEs.
D-channel
Signaling messages sent
between the ISDN device and the ISDN network are always carried on the
D-channel at the rate of
16 Kbps for the basic rate interface, and at 64 Kbps for primary rate
interface. These include
requests to set up a call to another ISDN device and information about
the type
and number of B-channels to be used. In some implementations of ISDN, unused
bandwidth on the
D-channel can also carry low-priority X.25 packets at a typical throughput
of 4-8 Kbps.
Frame Relay
A high speed, multiplexing
protocol designed to take advantage of the very low incidence of errors
on digital
and fiber circuits. Conceptually, Frame Relay is similar to X.25 with up
to 80% less internode error-checking,
and with a correspondingly higher transmission rate. The name "frame relay"
reflects the primary task
of each node in the virtual circuit, i.e., to relay data frames on to the
next node
as quickly as possible. Responsibility for flow control, sequencing, additional
error detection, and all
error correction is assumed by the end-user (more specifically, by the
higher layer protocols- transport
through application-in the OSI reference model).
ISDN
Integrated Services Digital
Network.
The term ISDN describes
(1) the collection of protocols, functional
devices, and interfaces between them used to implement a network capable
of providing end-to-end
digital connectivity, and (2) the interface through which a user accesses
digital services available
on such a network. The user-network interface specifies the number of data
channels available,
their transmission speed, and the signaling protocol used to set up and
maintain calls. Current
uses of ISDN include: wide area network connectivity, high-speed facsimile,
video conferencing,
medical imaging, modem pooling, database access, electronic document distribution,
and many others.
ISDN Device
See Terminal Equipment.
ISDN Number
The telephone number
of a remote ISDN device. Unlike an ordinary telephone number, an ISDN
number can include extra
addressing information to identify a specific device on the called line.
LAPD
Link Access Procedures on
the D-channel.
The data link layer protocol
specified for the D-channel. LAPD
provides the interface between an ISDN device and the network termination
equipment. Among
the functions performed by LAPD are addressing data frames, maintaining
their order, error-checking,
and flow control. It is a serial, synchronous, full-duplex protocol.
Local Access and Transport
Area (LATA)
The local telephone calling
area.
NI-1
The ISDN protocol used
in North America.
NT1 (Network Termination
1)
A device which serves
as the interface between the ISDN network and your personal ISDN
equipment. The ISDN line
installed by your service provider connects to an NT1 device, which is
responsible for line performance
monitoring, timing, power transfer, and multiplexing the B- and
D-channels onto the line.
In most countries outside of the United States and Canada, the NT1 is
considered a component of
the carrier's service.
NT2 (Network Termination
2)
A device which provides
customer site switching, multiplexing, and concentration, such as a PBX,
LAN, mainframe computer,
and other equipment for voice and data switching. An NT2 device is not
needed in every installation,
such as residential services.
Numbering Plan
The order and significance
of the digits in a telephone number. For example, in the North American
numbering plan the first
three digits of a telephone number identify the area code, the next three
identify the central office,
and the final four identify the user.
OSI Reference Model
Open System Interconnection
Reference Model. A networking model developed by the International
Organization for Standardization
(ISO). The OSI model divides data transmission into seven discrete
but interacting levels, or
layers, each with its own protocols and standards. Each layer is responsible
for a different aspect of
moving a block of data over a network. For example, layer 1 is responsible
for the bare mechanical and
electrical aspects of transmitting the bits, while layer 7 serves as the
bridge between the network
and the applications. Each layer depends on the transparent operation of
lower layers, and is in turn
essential to the successful operation of higher layers. From 1 to 7, the
layers are: physical, data
link, network, transport, session, presentation, and application.
Packet Switching
When communicating over
a packet-switching network, no direct physical circuit exists between
the two users. Instead, data
is separated into segments with additional addressing and control
information. These data "packets"
are routed through nodes on the network toward their destination,
where they are reassembled
into the original block of data. Packet-switching allows multiple users
to simultaneously
share the same physical transmission lines; but it is unsuitable for time-sensitive
applications because the
rate of throughput is not guaranteed.
Primary Rate Interface
One of several defined
user interfaces between an ISDN device and the ISDN switch. The primary
rate interface is comprised
of 23 or 30 B-channels (64 Kbps each) and one D-channel (64 Kbps),
providing a user data rate
of 1.544 Mbps or 2.048 Mbps. Also referred to as 23B+D and 30B+D. The
primary rate interface provides
sufficient channels to allow a host computer or other switching
device connected to multiple
ISDN devices to interface with the ISDN network.
Protocol
A collection of procedures,
data formats, and rules used to describe how two devices communicating
at a peer level will understand
one another. Different protocols are needed at every level of data
transmission, from defining
the specific electrical signal signifying a bit to sharing data in a
sophisticated manner between
applications.
Q.931
The D-channel network
layer protocol specified by the ITU.
Rate Adaptation
Also Rate Adaption. The
process of converting a user's actual bit rate, which may be 56 Kbps
synchronous or 9.6 Kbps asynchronous,
to the 64 Kbps speed of the B-channel.
RIP
Routing Information Protocol.
Used to update routing
tables in the many routers in a router internetwork.
S interface
Also S0 bus. This reference
point defines the point at which an ISDN NIC attaches to an NT-1 or a
PBX.
SAP
Service Advertising Protocol.
This protocol is used to advertise the services provided by a server to
all other
connected servers.
Subaddress
Additional addressing
information appended to the ISDN number. ISDN subaddresses are not needed
by the network to route the
call; instead, they are used by the called terminal equipment (such as
a PBX)
to direct the call to a specific device (such as an ISDN telephone).
Switching
The process of receiving
information over one transmission link and transmitting it over the next
transmission link. Switching
may also include the process of setting up and tearing down
information paths, or calls,
between users. See Circuit-Switching and Packet-Switching.
Terminal Adapter (TA)
The ISDN customer premises
equipment (CPE) device which allows non-ISDN equipment to be
attached to the ISDN network.
Terminal Endpoint Identifier
(TEI)
A unique identifier associated
with each ISDN device in operation. The TEI is used to differentiate
between devices whose control
signals are being multiplexed onto the same D-channel. Some devices
request a TEI from the network
switch upon being connected, while others request permission to use
a specific TEI assigned when
the ISDN subscription was negotiated.
Terminal Equipment (TE)
In the ISDN context,
any device which supports ISDN protocols, such as ISDN telephones. These
devices provide integrated
access to ISDN services; for example, an ISDN telephone offers advanced
call management features,
while an ISDN workstation may allow direct PC-to-PC screen sharing
or high speed file transfer.
Virtual Circuit
Computers connected through
packet-switched networks are unaware that their transmissions are
following an ad hoc route
through the network; the term virtual circuit refers to the single switched
circuit emulated by network.
A permanent virtual circuit is one which is always assigned to a
specific device and which
is always open. A switched virtual circuit is formed when one computer
places a call to another,
and is terminated when the computers hang up.
X.25
A packet-switching protocol
defined by the CCITT, X.25 is designed to carry high volumes of data
with no errors. More specifically,
X.25 defines the interface between user data terminal equipment
and the packet-switching
network equipment.