2 cissp ® Official Study Guide Eighth Edition

Technology
Connection Type
Speed
Digital Signal Level 0 (DS-0)
Partial T1
64 Kbps up to 1.544 Mbps
Digital Signal Level 1 (DS-1)
T1
1.544 Mbps
Digital Signal Level 3 (DS-3)
T3
44.736 Mbps
European digital transmission format 1
El
2.108 Mbps
European digital transmission format 3
E3
34.368 Mbps
Cable modem or cable routers
10+ Mbps
A 
nondedicated line
is one that requires a connection to be established before data 
transmission can occur. A nondedicated line can be used to connect with any remote 
system that uses the same type of nondedicated line.

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WAN Technologies 
557
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 specifi c 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 offi ce (that is, a specifi c type of 
distribution node of the telephone network) is approximately 5,000 meters. 
Integrated Services Digital Network (ISDN)
is a fully digital telephone network that 
supports both voice and high-speed data communications. There are two standard classes, 
or formats, of ISDN service: