Syngress - Eleventh Hour Network+ Exam N10-004 Study Guide (11 ...

108 CHAPTER 7 Wide Area Networking
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WAN technologies generally function at the lower three layers of the Open
Systems Interconnection (OSI) model: the physical layer, the data link layer,
and the network layer.
SWITCHING METHODS
WANs are based on one of two types of switching methods: circuit switching and
packet switching.
Although almost all WAN protocols in use today are packet switched, there are
still some old networks out there using circuit-switched technologies. Packetswitching
technologies, such as X.25 and Frame Relay, are always available, so
they do not have to be set up every time they are used. However, circuit-switching
technologies are not. Circuit switching requires a separate setup for each connection
session. This is the biggest difference between these two types of switching
methods.
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Circuit switching Circuit-switched networks are networks that are dialed
on demand. The connection must be initiated before transmission can take
place. Example: Integrated Services Digital Network (ISDN).
Packet switching Packet-switched networks are “always on.” When you
set the carrier’s link up, it stays up. Examples: Frame Relay, asynchronous
transfer mode (ATM), switched multimegabit data services (SMDSes),
and X.25. Additional characteristics of packet-switched networks are as
follows:
■ They divide the transmitting data into packets in which each is sent
individually from the source to the destination.
■ All packets are given sequence numbers so that they can all be put back
together again in the right order at the destination.
■ Each packet can take a different route to get to its destination.
EXAM WARNING
The telephone service provided by your carrier is most likely based on a circuit-switching
technology. Circuit switching is ideal when data must be transmitted quickly and must
arrive in the same order in which it’s sent. Packet switching is the opposite of circuit
switching. Packet switching is more efficient and robust, and it is commonly used for data
that can withstand some delays in transmission.
WAN PROTOCOLS AND PROPERTIES
You must be able to understand and respond to questions about the speeds,
capacity, transmission media, and distance for WAN protocols and standards
such as terrestrial (T) carriers, ISDN, and Fiber Distributed Data Interface (FDDI).