Page 915 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

Achieving Fault Tolerance with Carrier Network

Connections

To obtain fault tolerance with leased lines or with connections to
carrier networks (that is, Frame Relay, ATM, SONET, SMDS, X.25,
and so on), you must deploy two redundant connections. For even
greater redundancy, you should purchase the connections from

two different telcos or service providers. However, when you’re
using two different service providers, be sure they don’t connect to
the same regional backbone or share any major pipeline. The
physical location of multiple communication lines leading from
your building is also of concern because a single disaster or human

error (e.g., a misguided backhoe) could cause multiple lines to fail
at once. If you cannot afford to deploy an exact duplicate of your
primary leased line, consider a nondedicated DSL, ISDN, or cable
modem connection. These less-expensive options may still provide
partial availability in the event of a primary leased line failure.

Standard modems, DSL, and ISDN are examples of nondedicated
lines. Digital subscriber line (DSL) is a technology that exploits the
upgraded telephone network to grant consumers speeds from 144

Kbps to 20 Mbps (or more). There are numerous formats of DSL, such
as ADSL, xDSL, CDSL, HDSL, SDSL, RASDSL, IDSL, and VDSL. Each
format varies as to the specific downstream and upstream bandwidth
provided.

For the exam, just worry about the general idea of DSL

instead of trying to memorize all the details about the various DSL
subformats.

The maximum distance a DSL line can be from a central office (that is,
a specific type of distribution node of the telephone network) is
approximately 5,000 meters.

Integrated Services Digital Network (ISDN) is a fully digital