DSL Glossary
Since DSL is such a comprehensive technology,
we have compiled a list of common DSL terms and brief explanations of each to
better educate our customers and prospective subscribers.
100BaseT. A 100-Mbps local area network that maintains
backward compatibility with 10BaseT networks running at 10 Mbps.
10BaseT. A 10-Mpbs Ethernet local area network that
runs over twisted pair wiring. This network interface was originally designed to
run over ordinary twisted pair (phone wiring) but is predominantly used with
Category 3 or 5 cabling.
access rate. The transmission speed of the physical
access circuit between the end user location and the local network. This is
generally measured in bits per second. Also called "access speed."
adapter card. Circuit board or other hardware that
provides the physical interface to a communications network; an electronics
board installed in a computer that provides network communication capabilities
to and from that computer; a card that connects the DTE to the network. Also
called a "network interface card" (NIC). See also data terminal equipment
and network interface card.
ADSL Forum. The organization that develops and defines
xDSL standards, including those affecting ADSL, SDSL, HDSL, and VDSL. On the
Internet, visit at http://www.adsl.com/.
asynchronous transmission. Data transmission of
one character at a time to the receiving device, with intervals of varying
lengths between transmittals, and with start bits at the beginning and stop bits
at the end of each character, to control the transmission. In xDSL and in most
dial-up modem communications, asynchronous communications are often found in
Internet access and remote office applications. See also synchronous
transmission.
ATM. Asynchronous Transfer Mode. A protocol that packs
digital information into 53-byte cells (5-byte header and 48-byte payload) that
are switched throughout a network over virtual circuits. Standardized by the ITU-T
in 1988 to create a Broadband Integrated Services Digital Network (B-ISDN). Its
ability to accommodate multiple types of media (voice, video, data) makes it a
likely player for full service networks based on ADSL and VDSL.
ATM Forum. The organization tasked with developing and
defining ATM standards. On the Internet, visit at
http://www.atmforum.com for more info.
bandwidth. This is a reflection of the size or
the capacity of a given transmission channel. In digital transmission, bandwidth
is normally described in bits per second.
broadband. A type of transmission that shares
the bandwidth of a medium--such as copper or fiber optic cable--to carry more
than one signal. Broadband facilities have a bandwidth (capacity) greater than a
voice grade line of 3 kHz. Such a broadband facility--typically coaxial
cable--may carry numerous voice, video and data channels simultaneously. Each
"channel" will take up a different frequency on the cable. "Guardbands" (empty
spaces) exist between the channels to make sure that each channel does not
interfere with its neighbor. A coaxial CATV cable is the "classic" broadband
channel. Simultaneously it carries many TV channels. Broadband cables are used
in some office LANs. But more common are the baseband variety, which have the
capacity for one channel only. Everything on that cable to be transmitted or
received must use that one channel. That one channel is very fast, so each
device needs only to use that high speed channel for only a little of the time.
(The problem is getting on the channel.) See also baseband.
central office (CO). A circuit switch that terminates
all the local access lines in a particular geographic serving area; a physical
building where the local switching equipment is found. xDSL lines running from a
subscriber's home connect at their serving central office.
channel. A generic term for a communications
path on a given medium; multiplexing techniques allow providers to put multiple
channels over a single medium. See also multiplexer.
dedicated line. A transmission circuit that is
reserved by the provider for the full-time use of the subscriber. Also called a
"private line."
dial up. The process of initiating a switched
connection through the network; when used as an adjective, this is a type of
communication that is established by a switched-circuit connection.
digital subscriber line access multiplexer (DSLAM).
The technical description of the Hitchhiker system. Also called "service
access multiplexer." The DSLAM uses digital subscriber line (xDSL) and
asynchronous transfer mode (ATM) technologies to deliver high speed data rates
over the existing copper network. See also Hitchhiker, xDSL, and
ATM. For an extensive description of the Hitchhiker system, refer to
The Hitchhiker System manual from Diamond Lane Communications Corporation.
downstream/upstream
- downstream. In xDSL, the communications
from the network towards the customer premises.
- upstream. In xDSL, the communications from
the customer site up into the telecommunications network.
DSL. Digital Subscriber Line. A general term for
any local network loop that is digital in nature; technically, DSL equates to
ISDN BRI, but this is decreasingly enforced terminology. DSL technology is
available in several varieties. See also ADSL, HDSL, IDSL,
RADSL, SDSL, VDSL, xDSL.
- ADSL. Asymmetric Digital Subscriber Line. A
term for one-way T1 transmission of signals to the home over the plain old,
single twisted-pair wiring already going to homes. ADSL modems attach to
twisted pair copper wiring. ADSL is often provisioned with greater
downstream than upstream rates (hence "asymmetric"). These rates are
dependent on the distance a user is from the central office and may vary
from as high as 9 Mbps to as low as 384 Kbps.
- HDSL. High bit-rate Digital Subscriber
Line. The oldest of the DSL technologies, HDSL continues to be used by
telephone companies deploying T1 lines at 1.5 Mbps and requires two twisted
pairs.
- IDSL. ISDN Digital Subscriber Line. IDSL provides
up to 144-Kbps transfer rates in each direction and can be provisioned on
any ISDN capable phone line. Unlike ADSL and other DSL technologies, IDSL
can be deployed regardless of the distance the user is from the central
office.
- RADSL. Rate Adaptive Digital Subscriber
Line. Using modified ADSL software, RADSL makes it possible for modems
automatically and dynamically to adjust their transmission speeds. This
often allows for good data rates for customers residing greater distances
from the CO.
- SDSL. Single-line Digital Subscriber Line
or Symmetric Digital Subscriber Line. A modified HDSL software technology,
SDSL is intended to provide 1.5 Mbps in both directions over a single
twisted pair. However, the distance over which this can be achieved is less
than 8,000 feet.
- VDSL. Very high-rate Digital Subscriber Line. The
newest of the DSL technologies, VDSL can offer speeds up to 25 Mbps
downstream and 3 Mbps upstream. Similar to SDSL, the gain in speed can be
achieved only at short distances. These maximum speeds can be achieved only
up to 1,000 feet. Sometimes also called broadband digital subscriber line
(BDSL).
- xDSL. A generic term for the suite of
digital subscriber line (DSL) services, where the "x" can be replaced with
any of a number of letters. See also DSL, ADSL, HDSL,
IDSL, MDSL, RADSL, SDSL, VDSL.
DSU/CSU. Data Service Unit/Channel Service Unit. The
devices used to access digital data channels are called DSU/CSUs (Data Service
Unit/Channel Service Units). At the customer's end of the telephone connection,
these devices perform much the same function for digital circuits that modems
provide for analog connections. For example, DSU/CSUs take data from terminals
and computers, encode it, and transmit it down the link. At the receive end,
another DSU/CSU equalizes the received signal, filters it, and decodes it for
interpretation by the end-user.
DS1/DS3. Like T1 or T3 connections.
- T1. A digital transmission link with a capacity of
1.544 Mbps. T1 uses two pairs of normal twisted wires, the same as found in
most residences. T1 normally handles 24 voice conversations, each one
digitized at 64 Kbps. But, with more advanced digital voice encoding
techniques, it can handle more voice channels. T1 is a standard for digital
transmission in the United States. T1 lines are used to connect networks
across remote distances. Bridges and routers are used to connect LANs over
T1 networks. There are faster services available. T1 links can often be
connected directly to new PBXs and many new forms of short haul
transmission, such as short haul microwave systems.
T3. A T3 line consists of 28 T1 lines or 44.736
million bits per second (commonly referred to as 45 Mbps). A T3 line can handle
672 voice conversations. T3 runs on fiber optic and is typically called FT3. See
also T1.
ELEC. Enterprise Local Exchange Carrier.
Generally, a larger corporation or organization (e.g., university) that operates
as its own local exchange carrier (LEC) as a means of obtaining better carrier
rates for itself, possibly selling services to others for a profit. ELECs could
be considered a subset of CLECs. See also LEC, ILEC, and ELEC.
enterprise network. A term for a widely dispersed,
multifaceted telecommunications network for a particular purpose or
organization; a term for all of an organization's telecommunications networking
services and equipment.
Ethernet. A LAN used to connect devices within a
single building or campus at speeds up to 10 Mbps. Within the OSI model,
Ethernet is defined at layer one (physical) and layer two (data link). Based on
Carrier Sense Multiple Access/Collision Detection (CSMA/CD), Ethernet works by
simply checking the wire before sending data. Sometimes two stations send at
precisely the same time in which case a collision is detected and retransmission
is attempted. See also 10BaseT.
Fast Ethernet. A LAN used to connect devices
within a single building or campus at speeds up to 100 Mbps. Within the OSI
model, Fast Ethernet is defined at layer one (physical) and layer two (data
link). Like Ethernet, Fast Ethernet uses CSMA/CD.
frame relay (FR). A high-speed packet-switched
data communications service, similar to X.25. Frame relay is a leading contender
for LAN-to-LAN interconnect services, and is well suited to the bursty demands
of LAN environments. See also permanent virtual circuit and switched
virtual circuit.
ILEC. Incumbent Local Exchange Carrier. A new term that
describes traditional local telephone companies that, prior to deregulation of
the telephone industry, had the exclusive right and responsibility to provide
local telephone service. ILEC delineates these service providers from the new
competitive providers (CLECs) and enterprise providers (ELECs). The term "local
exchange carrier" (LEC) is used as the generic term for all three. See LEC,
CLEC, and ELEC.
Integrated Services Digital Network (ISDN). ISDN
provides standard interfaces for digital communications networks and is capable
of carrying data, voice, and video over digital circuits. ISDN protocols are
used worldwide for connections to public ISDN networks or to attach ISDN devices
to ISDN-capable PBX systems (ISPBXs).
Developed by the International Telecommunications Union (ITU
[previously the CCITT]), ISDN includes two user-to-network interfaces: basic
rate interface (BRI) and primary rate interface (PRI). Note: See separate
entries for basic rate interface and primary rate interface.
An ISDN interface contains one signaling channel (D-channel)
and a number of information channels ("bearer" or B channels). The D-channel is
used for call setup, control, and call clearing on the B-channels. It also
transports feature information while calls are in progress. The B-channels carry
the voice, data, or video information.
interface. A point of connection between two
systems, networks, or devices.
Internet. The world's largest computer network.
The Internet originated from a research effort initiated by the U.S. Government
and was initially used to connect defense contractors and U.S. universities.
Today, its nature is more commercial, and it is becoming the preferred method of
linking businesses and individuals' computers to one another.
Internet Service Provider (ISP). A
telecommunications company that provides subscriber access to the Internet.
- Internet access. The physical telephone
circuit connection between the subscriber and the nearest Internet access
node.
Kbps, Gbps, Mbps
- kilobits per second (Kbps). A measure of
bandwidth capacity or transmission speed. It represents a thousand bits per
second.
- gigabits per second (Gbps). A measure of
bandwidth capacity or transmission speed. It represents a billion bits per
second.
- megabits per second (Mbps). A measure of
bandwidth capacity or transmission speed. It stands for a million bits per
second.
Intranet. A local network, for example, an office
network, where there are internal web servers accessable to computers in the
office, but not accessable from outside the company. Many intranets are
protected from exterior access by various security devices, like firewalls.
latency. A measure of the temporal delay.
Typically, in xDSL, latency refers to the delay in time between the sending of a
unit of data at the originating end of a connection and the reception of that
unit at the destination end.
LEC. Local Exchange Carrier. A local telephone company
(either a Bell Operating Company [BOC] or an independent [e.g., GTE]) that
traditionally had the exclusive, franchised right and responsibility to provide
local transmission and switching services. Prior to divestiture, the LECs were
called telephone companies or telcos. With the advent of deregulation and
competition, LECs are now known as ILECs (incumbent LECs). This terminology
delineates them from CLECs (competitive LECs) and ELECs (enterprise LECs). See
CLEC and ELEC.
local area network (LAN). A data communications
network covering a small area, usually within the confines of a building or
floors within a building; a relatively high-speed computer communications
network for in-building data transfer and applications. Common LAN protocols are
Ethernet and Token Ring. See also WAN.
local loop. A generic term for the connection
between the customer's premises (home, office, etc.) and the provider's serving
central office. Historically, this has been a wire connection; however, wireless
options are increasingly available for local loop capacity. Also colloquially
referred to as "the last mile" (even though the actual distance can vary).
long distance. The communication of information
to a destination outside the local calling area. Also called "long haul"
traffic.
modem . MOdulator/DEModulator. Equipment that converts
digital signals to analog signals and vice versa. Modems are used to send data
signals (digital) over the telephone network, which is usually analog. The modem
modulates the 1s and 0s into tones that can be carried by the phone network. At
the other end, the demodulator part of the modem converts the tones back into
digital 1s and 0s.
network interface card (NIC). The circuit board or
other form of computer hardware that serves as the interface between a computer
(or other form of data terminal equipment) and the communications network; in
ADSL, a common NIC is an Ethernet NIC, which serves as the interface to the ADSL
modem from the computer. See also adapter .
network interface device (NID). A device that
terminates a copper pair from the serving central office to the user's
destination. The NID is typically found installed on the exterior premises of
the destination location.
NOC . Network Operating Center. The NOC provides the
customer's corporate IT staff with access to automated information regarding
issues or network outages that affect the customer's teleworkers.
packet switched network . A network that does
not establish a dedicated path through the network for the duration of a session
but, instead, transmits data in units called packets in a connectionless manner.
Data streams are broken into packets at the front end of a transmission, sent
over the best available network connection, and then reassembled in their
original order at the destination endpoint.
- packet . A sub-unit of a data stream; a grouping
of information that includes a header (containing information such as
address destination) and, in most cases, user data.
- packet switching . A switching system that uses a
physical communications connection only long enough to transmit a data
message; data messages are disassembled into packets and reassembled at the
receiving end of the communications link; packets may travel over many
diverse communications links to get to the common endpoint. Packet switching
is most often contrasted with circuit switching in data communications,
where all data messages transmitted during a session are transmitted over
the same path for the duration of the session. See also circuit switching
.
PCLEC . Packet Competitive Local Exchange Carrier.
Covad's coined this term to align itself with the rapid innovation and service
deployment found in the PC industry, and strives to provide new services at
speeds and customer satisfaction levels not found in the current
telecommunications industry.
permanent virtual circuit (PVC). A term found in frame
relay and ATM networking in which a virtual connection between two fixed
end-points is established through the network. See also switched virtual
circuit .
plain old telephone service (POTS). This term commonly
refers to standard telephony, as in placing and receiving telephone calls.
PPP . Point-to-Point Protocol. This protocol allows a
computer to connect to the Internet with a standard dial-up telephone line and a
high-speed modem and enjoy most of the benefits of a direct connection,
including the ability to use graphical front ends such as a Mosaic and Netscape.
PPP is considered to be better than SLIP, because it features error detection,
data compression, and other elements of modem communications protocols which
SLIP, the older Internet protocol, lacks. See also SLIP .
modem . MOdulator/DEModulator. Equipment that converts
digital signals to analog signals and vice versa. Modems are used to send data
signals (digital) over the telephone network, which is usually analog. The modem
modulates the 1s and 0s into tones that can be carried by the phone network. At
the other end, the demodulator part of the modem converts the tones back into
digital 1s and 0s.
network interface card (NIC). The circuit board or
other form of computer hardware that serves as the interface between a computer
(or other form of data terminal equipment) and the communications network; in
ADSL, a common NIC is an Ethernet NIC, which serves as the interface to the ADSL
modem from the computer. See also adapter .
network interface device (NID). A device that
terminates a copper pair from the serving central office to the user's
destination. The NID is typically found installed on the exterior premises of
the destination location.
NOC . Network Operating Center. The NOC provides the
customer's corporate IT staff with access to automated information regarding
issues or network outages that affect the customer's teleworkers.
packet switched network . A network that does
not establish a dedicated path through the network for the duration of a session
but, instead, transmits data in units called packets in a connectionless manner.
Data streams are broken into packets at the front end of a transmission, sent
over the best available network connection, and then reassembled in their
original order at the destination endpoint.
- packet . A sub-unit of a data stream; a grouping
of information that includes a header (containing information such as
address destination) and, in most cases, user data.
- packet switching . A switching system that uses a
physical communications connection only long enough to transmit a data
message; data messages are disassembled into packets and reassembled at the
receiving end of the communications link; packets may travel over many
diverse communications links to get to the common endpoint. Packet switching
is most often contrasted with circuit switching in data communications,
where all data messages transmitted during a session are transmitted over
the same path for the duration of the session. See also circuit switching
.
RBOC . Regional Bell Operating Company. There are seven
RBOCs, each of which owns two or more BOCs (Bell Operating Companies). The RBOCs
were carved out of the old AT&T/Bell System as a result of the divestiture of
the Bell operating companies from AT&T at the end of 1983.
RJ-45 . An 8-pin connector used to attach data
transmission devices to standard telephone wiring. Commonly used in 10BaseT
connections.
router . The device that connects multiple computer
networks by reading OSI Layer 3 addressing on incoming and outgoing packets.
Packet information is read, and the packets are then forwarded to the
appropriate end station. While routers are like bridges, they work differently.
Routers provide more functionality than bridges. For example, they can find the
best route between any two networks, even if there are several different
networks in between. Routers provide network management capabilities such as
load balancing, partitioning of the network, use statistics, communication
priority, and trouble shooting tools that allow network managers to detect and
correct problems even in a complex network of networks. Given these
capabilities, routers are often used in building wide area or enterprise wide
networks. Some routers are protocol-dependent, and some are
protocol-independent.
switched virtual circuit (SVC). A virtual circuit
connection established across a network on an as-needed basis and lasting only
for the duration of the transfer. It is the datacom equivalent of a dialed phone
call. The specific path provided in support of the SVC is determined on a
call-by-call basis and in consideration of both the end points and the level of
congestion in the network. SVCs are used extensively in X.25 networks. SVCs also
are provided for in Frame Relay network theory, but have not been implemented,
because permanent virtual circuits (PVCs) provide the same function and are much
less complex to provision. See also permanent virtual circuit .
synchronous transmission . Data transmission using
synchronization bytes, instead of start/stop bits, to control the transmission.
In xDSL, video streams are considered to be synchronous in nature. See
asynchronous transmission .
telco . A generic term for the local telephone company
operator in a given area. In the U.S., the major telcos are the seven regional
Bell operating companies and the leading independent telcos, GTE, SNET, and
Sprint; in Europe, Asia and elsewhere, the term "telco" generally refers to the
incumbent monopoly, but increasingly refers to competing local providers as
well.
Transmission Control Protocol/Internet Protocol
(TCP/IP). The TCP/IP protocol is a networking protocol that provides
communication across interconnected networks, between computers with diverse
hardware architectures and various operating systems. TCP (Transmission Control
Protocol) and IP (Internet Protocol) are only two protocols in the family of
Internet protocols. Over time, however, "TCP/IP" has been used in industry to
denote the family of common Internet protocols.
twisted pair (TP). A common form of copper cabling
used for telephony and data communications. It consists of two copper lines
twisted around each other; the twisting protects the communications from
electromagnetic frequency and radio frequency interference. See also
unshielded twisted pair .
UAWG . Universal ADSL Working Group. The UAWG is a
consortium of telecommunications service providers, PC and network equipment
manufacturers, and ADSL equipment manufacturers. These companies plan to develop
a set of technical specifications to be submitted to international standard
setting bodies for ADSL products. These specifications will describe ADSL
products that will hopefully meet the price points and installation requirements
necessary to promote high speed Internet access for the consumer market. The
UAWG was initiated by Microsoft, Intel, and Compaq. See their website at
http://www.uawg.org/.
unshielded twisted pair (UTP). A cable with one or
more twisted copper wires bound in a plastic sheath. Preferred method to
transport data and voice to business workstations and telephones. Unshielded
wire is preferred for transporting high speed data, because, at higher speeds,
radiation is created. If shielded cabling is used, the radiation is not released
and creates interference.
WAN . Wide Area Network. A data network typically
extending a LAN (local area network) outside the building, over telephone common
carrier lines to link to other LANs in remote buildings in possibly remote
cities. A WAN typically uses common-carrier lines. A LAN doesn't. WANs typically
run over leased phone lines--from an analog phone line to T1 (1.544 Mbps). The
jump between a local area network and a WAN is made through a device called a
bridge or a router.
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