EXFO CableSHARK P3, User Manual, Page: 15 / 321

CableSHARK
P3
User Guide
3
Section 2 Digital Subscriber Line (DSL) Overview
The need for faster and cheaper Internet access, video-on-demand, simplex video, remote LAN access and
interactive multimedia by residential and business customers alike has been a driving force in the creation of
Digital Subscriber Line (DSL) technologies.
xDSL (the "x" in xDSL stands for the various kinds of digital subscriber line technologies) is a digital public
end-to-end network technology that delivers high bandwidth over conventional copper local loop wiring
(currently used for plain old telephone service (POTS)) at limited distances. By utilizing higher frequencies
than that of a POTS network, xDSL can encode more information to achieve higher data rates than would
otherwise be possible. This is a more economical venture than the use of fiber optic technology to get the
speed necessary for large bandwidth applications.
There are several different kinds of DSL technologies such as ADSL and HDSL. These technologies all are
provisioned via modem pairs, with one modem located at the Central Office (CO) and the other at the
customer premise. ISDN (Integrated Services Digital Network) is very similar to that of xDSL in that it is a
digital technology and uses the existing telephone company copper cabling infrastructure. However, ISDN
differs from pure xDSL technologies in that it is a public switched service, whereas xDSL is a point-to-point
digital access service. Some of the more popular xDSL technologies are described here.
Integrated Services Digital Network
- ISDN can be considered one of the first xDSL technologies
developed to simultaneously carry digital voice, data, and images over conventional copper cables. Basic
Rate ISDN is comprised of three logical channels operating over a single copper pair. Two bearer channels
(B Channels) carry voice, data, and images while the one D channel is used for signaling; commonly referred
to as 2B+D. Basic Rate ISDN offers speeds of up to 160 kbps symmetrically. ISDN is also available with a
Primary Rate Interface (PRI). The PRI interface offers data / voice / image transfers of up to 1.544 Mbps
(T1) over 23 B channels and one D channel or up to 2.048 Mbps (E1) over 30 B channels and one D
channel. Each channel in PRI operates at 64 kbps and is commonly referred to as 23B+D or 30B+D.
Figure 2A – Basic Rate ISDN Architecture