Section 8.1
• Explain the role of carriers in WAN transmissions
• Compare and contrast DUN and VPN connections
• Explain the benefits of circuit switching
• Describe how message switching works
• Describe packet switching
Section 8.2
• Describe how networks transmit data using the X.25
protocol
• Explain how frame relay transmits data
• Describe how ATM works
• Identify the uses of B-ISDN
Section 8.3
• Show how FDDI transmits data
• Describe how SONET transmits data
• Explain the benefits of SMDS
pp.
8.1
224-230
How a WAN Works
Guide to Reading
Main Ideas
Key Terms
Circuit-switched networks
use a direct, physical
connection to transmit
data back and forth.
Message-switched
networks send entire
transmissions between
intermediaries. Packetswitched networks route
packets through virtual
circuits.
carrier services
remote access
dial-up networking (DUN)
Virtual Private Network
(VPN)
circuit-switched network
message switching
packet switching
Packet Switching
Exchange (PSE)
pp.
8.1
224-230
How a WAN Works
Carriers
After a network grows
beyond the confines of the
LAN, a carrier service is
required.
carrier services Data
transfer services provided by
telecommunications
companies, such as AT&T,
Sprint, or Verizon. (p. 224)
Carriers provide a variety
of services which are
connected through
remote access.
remote access A direct
connection to a network
using regular dial-up lines.
After the connection is made,
the user can work on the
network as if in the office.
(p. 224)
pp.
8.1
224-230
How a WAN Works
Carriers
Remote access
connections are
established through either
a dial-up network (DUN)
or virtual private
network (VPN).
The advantages of a VPN
are that users benefit from
faster network
connections, privacy, and
security.
dial-up networking (DUN)
A client service using regular
telephone lines to connect to
a network. (p. 224)
Virtual Private Network
(VPN) An alternative to
using a dial-up network
connection. Is a network
connection between two
computers. With a VPN, a
remote user can securely
access the internal network
from a remote location.
(p. 226)
pp.
8.1
224-230
How a WAN Works
Switching Technologies
Data in the WAN is
transported through
various technologies:
• Circuit switching
• Message switching
circuit-switched network A
network that creates a direct
physical connection between
the sender and receiver.
(p. 228)
message switching A
technology used to route an
entire message from one
system to another. That
message is routed through
intermediate (go-between)
station and does not involve
a direct physical connection
between the sender and
receiver. (p. 228)
pp.
8.1
How a WAN Works
Switching Technologies
Message switching sends the entire message to
intermediary stations that decide which route to use.
224-230
pp.
8.1
224-230
How a WAN Works
Switching Technologies
Packet switching is
commonly found in LANs.
Packets are routed
through Packet
Switching Exchanges
until they reach their
destination.
packet switching A
transmission method in
which all transmissions are
broken into small units and
sent over the network.
Packets are reassembled at
the destination computer.
(p. 229)
Packet Switching
Exchange (PSE) Gobetweens in the packet
switching process. The PSE
inspects the packet’s
destination address, consults
a routing table, and forwards
the packet at the highest
possible speed. (p. 229)
pp.
8.1
224-230
How a WAN Works
Switching Technologies
Packet-switching networks send packets along the best
route available.
pp.
8.1
How a WAN Works
You Try It
• Activity 8A – Setting Up a DUN Connection (p. 225)
• Activity 8B – Setting Up a VPN Connection (p. 226)
• Activity 8C – Tracing Packets (p. 229)
224-230
pp.
8.2
232-236
Sending Data across a WAN
Guide to Reading
Main Ideas
Key Terms
X.25 is a packet-switching
protocol originally designed
for analog telephone lines.
Frame relay uses
permanent, virtual circuits
and reliable, digital lines to
transmit data. ATM transmits
data, voice, and video over
many types of lines.
X.25
Data Terminal Equipment
(DTE)
packet assembler/
disassembler (PAD)
Data Circuit-terminating
Equipment (DCE)
frame relay
Asynchronous Transfer
Mode (ATM)
cell relay
broadband ISDN (B-ISDN)
pp.
8.2
232-236
Sending Data across a WAN
X.25 Packet-Switching Protocol
Computers on an X.25
network can receive and
transmit data at the same
time. This is also called
full-duplex
communication.
X.25 A connection-oriented,
packet-switching protocol
designed for use on analog
telephone lines. Computers
on an X.25 network can
receive and transmit data at
the same time. (p. 232)
pp.
8.2
232-236
Sending Data across a WAN
X.25 Packet-Switching Protocol
The X.52 protocol
concentrates on the
interface through which
transmission flows. The
interface is made up of
the following devices:
• Data Terminal Equipment
(DTE)
• Packet
assembler/dissembler
(PAD)
Data Terminal Equipment
(DTE) X.25 device
terminology. There are no
computers, hosts, or nodes.
Instead, sending and
receiving computers are
referred to simply as this.
(p. 232)
packet
assembler/disassembler
(PAD) A device in X.25
transmissions that prepares
packets for transmission and
disassembles the packets
that come in. (p. 232)
pp.
8.2
232-236
Sending Data across a WAN
X.25 Packet-Switching Protocol
• Data Circuit-terminating
Equipment (DCE)
• Packet switching
exchanges (PSEs)
Data Circuit-Terminating
Equipment (DCE) Packets
are sent here from the DTE
or PAD. A DCE might be a
modem or packet switch. A
DCE is located at each end
of the connection. (p. 232)
pp.
8.2
232-236
Sending Data across a WAN
Frame Relay
Frame relay is faster than
the X.52 protocol. Frame
relay networks connecting
LANs to a WAN rely on
routers and switching
equipment.
frame relay A newer form of
packet switching that relies
on digital technologies, such
as fiber optic and ISDN.
(p. 233)
pp.
8.2
232-236
Sending Data across a WAN
ATM
Another type of switching
technology is
Asynchronous Transfer
Mode (ATM).
Like a frame relay, ATM is
designed to take
advantage of digital
media.
Asynchronous Transfer
Mode (ATM) A connectionoriented networking
technology. Like frame relay,
ATM is designed to take
advantage of digital media.
(p. 234)
pp.
8.2
232-236
Sending Data across a WAN
ATM
ATM is useful for highspeed LAN and WAN
networking over a range
of media types and relies
on cell relay.
ATM transmits data in
53-byte cells.
cell relay Used with ATM
transmissions, a high-speed
transmission method based
on fixed-size units that are
known as cells. (p. 235)
pp.
8.2
232-236
Sending Data across a WAN
ATM
ATM takes incoming data and packages the contents in
uniform, 53-byte cells. At the receiving end, ATM sends it
cells out onto a WAN in a steady stream for delivery.
pp.
8.2
232-236
Sending Data across a WAN
B-ISDN
ATM was created for
broadband ISDN (BISDN) and B-ISDN
services are divided into
two categories:
• Interactive services
• Distributed services
broadband ISDN (B-ISDN)
The next generation of ISDN
that is capable of delivering
high quantities of data.
(p. 236)
pp.
8.3
238-244
Developing Technologies
Guide to Reading
Main Ideas
Key Terms
FDDI transmits data over
fiber optic cabling using a
ring topology. SONET
multiplexes incoming
signals for transmission
over very-high-speed
optical cabling. SMDS
transmits data in any
format between networks.
Synchronous Optical
NETwork (SONET)
Synchronous Digital
Hierarchy (SDH)
optical carrier (OC)
Synchronous Transport
Signal (STS)
Switched Multimegabit
Data Services (SMDS)
pp.
8.3
238-244
Developing Technologies
FDDI
FDDI was developed based on ring topology and token
passing. It uses fiber optic as its medium of transmission.
FDDI was developed for two primary reasons. First, it was
designed to support and extend the capabilities of older
LANs, such as Ethernet and Token Ring. Second, it
provides a reliable infrastructure for businesses moving
mission-critical applications to networks.
pp.
8.3
238-244
Developing Technologies
FDDI
Although FDDI was developed specifically for use with
fiber optic cable, it can operate on a variety of cable types.
• Multimode fiber optic cable
• Single-mode fiber optic cable
• Category 5 Unshielded Twisted Pair copper wiring
• IBM Type 1 Shielded Twisted Pair copper wiring
pp.
8.3
Developing Technologies
FDDI
FDDI topology is similar to
Token Ring, except that it
operates using optical
transmissions.
Another important
distinction is that FDDI is
characterized by two
counter-rotating rings
(known as a dual-ring
topology). An FDDI
topology by itself usually
does not make use of
hubs.
238-244
pp.
8.3
238-244
Developing Technologies
FDDI
FDDI networks are limited to rings no longer than 100
kilometers apiece. They are also limited to no more than
500 nodes per ring.
FDDI is a high-speed, high-bandwidth network based on
optical transmissions. It is most often used as a network
backbone for connecting high-end computers.
pp.
8.3
238-244
Developing Technologies
SONET
Synchronous Optical
NETwork (SONET) is
transmitted over the fiber
optic cables that are used
by long-distance carriers.
The European standard is
Synchronous Digital
Hierarchy (SDH).
Synchronous Optical
NETwork (SONET) An
American National Standards
Institute (ANSI) standard for
the transmission of different
types of digital information
using optical media—data,
voice, and video. (p. 240)
Synchronous Digital
Hierarchy (SDH) A
European standard for the
transmission of different
types of digital information
using optical media—data,
voice, and video. (p. 240)
pp.
8.3
238-244
Developing Technologies
SONET
SONET multiplexes many incoming signals for high-speed
transport.
pp.
8.3
238-244
Developing Technologies
SONET
Two signal definitions lie
at the heart of the SONET
standard regarding
multiplexing capability:
• Optical carrier (OC)
• Synchronous Transport
Signals (STSs)
optical carrier (OC) A
signal definition in the
SONET standard used by
fiber optic media and
translated roughly to speed
and carrying capacity.
(p. 241)
Synchronous Transport
Signal (STS) The electrical
equivalents of OC levels and
are used by nonfiber media.
See also optical carrier.
(p. 241)
pp.
8.3
238-244
Developing Technologies
SONET
SONET converts electrical (STS) signals to optical (OC)
levels for transport. It also “unconverts” them (OC to STS)
at the point in which the transmissions leave the SONET
media. SONET frames are sent continuously – whether
they contain data or not.
pp.
8.3
238-244
Developing Technologies
SMDS
Switched Multimegabit
Data Services (SMDS) is a
means for businesses to
connect to LANs in separate
locations.
It is a less expensive
alternative to dedicated
leased lines. SMDS is a
connectionless, packetswitched technology wellsuited to the type of “bursty”
traffic characteristic of LAN
(or LAN-to-LAN)
communications.
Switched Multimegabit
Data Services (SMDS) A
broadband public networking
service offered by
communications carriers.
(p. 243)
pp.
8.3
238-244
Developing Technologies
SMDS
SMDS is a connectionless service that provides WAN
transmissions on an as-needed basis.
Chapter 8
Resources
For more resources on this chapter, go to the Introduction
to Networks and Networking Web site at
http://networking.glencoe.com.