Net essentials6e ch12

Guide to Networking Essentials,
6th
Edition
Chapter 12: Wide Area Network
Essentials

Copyright © 2012 Cengage Learning. All rights reserved. 2
Objectives
2
• Describe the fundamentals of WAN operation and
devices
• Discuss the methods used to connect to WANs
• Configure and describe remote access protocols
• Describe the three major areas of cloud computing

Wide Area Network Fundamentals
• Internetworks and WANs can be described as two
or more LANs connected together
• Most obvious difference between internetworks and
WANs is the distance between the LANs being
connected.
• They also differ in two other areas:
– WANs use the services of carriers or service providers (phone
companies and ISPs) for network connection
– WANs use serial communication that can span miles compared
to LAN technologies that span distances measured in hundreds
of meters

WAN Devices
• WANs operate at the Data Link and Physical layers
(Layers 2 and 1) of the OSI model
• Several types of devices are likely to be used in
WANS for media access, signal transmission, and
reception and to connect a WAN to a LAN:
– Modems
– Channel service units/data service units
– Routers

Modems
• A modem is a device that allows a computer
(which works with digital signals) to communicate
over lines that analog signals
• A digital signal is a series of binary 1s and 0s
represented by some type of signal that has two
possible states (0v or 5v)
• An analog signal varies over time continually and
smoothly (transitions from 0v to 5v)

CSU/DSUs
• A channel service unit/data service unit
(CSU/DSU) is a device that creates a digital
connection between a LAN device (router) and
the WAN link from the service provider
• The WAN link is usually a T-carrier technology,
such as a T1 or T3 (discussed later)
• Similar to a modem, only all signals are digital
– Converts one type of digital signal to another type of digital
signal

Routers
• A router is responsible for getting packets from one
network to another
• In a WAN, it is usually the device connecting a LAN
to the WAN service provider via a modem or
CSU/DSU

WAN Connection Methods
• Many WAN technologies are available and differ in
speed, level of security and reliability, and cost
• Four most common connection methods:
– Circuit-switched
– Leased line
– Packet-switched
– VPN over the Internet

Circuit-Switched WANs
• A circuit-switched WAN creates a temporary
dedicated connection between sender and receiver
on demand
• Analog example: a phone line connection from the
PSTN, also known as plain old telephone service
(POTS)
• Digital example: Integrated Services Digital
Network (ISDN)
• Not as common today due to faster technologies
but still in use in some areas

 Plain Old Telephone Service (POTS) – limited in
bandwidth due to the digital-to-analog conversion
that is performed, usually by modems
 The conversion process degrades signal quality and limits
data transfer speeds to about 56 Kbps
 The most common modem standard for connecting to the
Internet is V.92
 V.92 modems use a technique called pulse code modulation
(PCM) that digitizes analog signals and introduces less noise
into the signal

Modem communication using the V.92 standard

• Integrated Services Digital Network (ISDN) – a digital
communication technology developed in 1984 to
replace the analog phone system
– Was not as popular as expected but can still be found in many
US metropolitan areas and Western Europe
– Defines communication channels of 64 Kbps
– Two formats or rates:
• BRI – Basic Rate Interface: consists of two B-channels (64 Kbps)
and a D-channel (16 Kbps). B-channels are used for data transfer
so BRI can operate at up to 128 Kbps
• PRI – Primary Rate Interface (PRI): consists of 23 B-channels
and a D-channel. Can provide up to 1.544 Mbps

Leased Lines
• A leased line provides a dedicated point-to-point
connection from the customer’s LAN through the
provider’s network and the destination network
– Provides permanent, secure, and dedicated bandwidth limited only by
the provider’s technology and how much the customer is willing to
spend
• Most expensive WAN connectivity because it is
dedicated
• Should be considered:
– When high quality, 24/7 access is needed
– For mission-critical applications
– When fast upstream as well as downstream communication is required

Leased Lines
• Leased lines are based on one of two types of digital
technology: T-carriers and SONET
• T-carriers
– Typical lines are T1 and T3 that operate at 1.544 Mbps and
44 Mbps, respectively
– Derived from multiple 64 Kbps channels, making a T1
connection a grouping of 24 channels, and a T3 connection a
grouping of 672 channels
– Uses a signaling method called time division multiplexing
(TDM): Allocates a time slot for each channel
• If a portion of a T-carrier line is used for one purpose and a
different portion for another purpose, the line has been
fractionalized

Leased Lines
• T-Carriers (cont.)
– Multiplexing:enables several communication streams to
travel simultaneously over the same cable segment
– Require a CSU/DSU at each end of the link to convert the
signals used by the T-carrier line into signals used by the
LAN
– T1 lines can use twisted-pair, coaxial or fiber-optic cabling
– T3 lines can use coaxial or fiber-optic cabling
– T1 lines are the most common WAN connection method in
the US

Leased Lines

Leased Lines
• Synchronous Optical Network (SONET) –
flexible, highly fault-tolerant technology that can
operate at different capacities over fiber cabling
• Typical SONET rates are OC-3 (155 Mbps), OC-12
(622 Mbps), OC-48 (almost 2.5 Gbps), OC-192,
and OC-768 (used by large ISPs)
• SONET networks can carry traffic from a variety of
other network types, such as T-carrier and ATM
• SONET uses a dual-ring topology (like FDDI),
making it very fault-tolerant

Packet-Switched WANs
• A packet-switched WAN does not create a
dedicated connection between sender and receiver
– Each packet is transmitted through the provider’s network
independently (similar to LAN traffic)
– Data shares bandwidth with your provider’s other customers
– Most common packet-switched networks are:
• X.25
• Frame relay
• ATM
• MPLS

Packet-Switched WANs
• Virtual Circuits – a logical connection created
between two devices in a shared network
– No single cable exists between the two endpoints
– Maps a path through the network of switches between two
points
– The pathway is created after sender and receiver agree on
bandwidth requirements and request a pathway
– Switched virtual circuit (SVCs): established when needed
and then terminated when the transmission is completed
– Permanent virtual circuit (PVCs): pathway between two
communication points is established as a permanent logical
connection (more expensive than SVCs)

X.25 Networks
• Packet-switching technology developed in the
mid-1970s running over older copper phone lines
• Offer both SVCs and PVCs – although not all X.25
providers offer PVCs
• Earlier X.25 could only operate at 64 Kbps
– A 1992 specification revision improved the maximum
throughput of X.25 to 2 Mbps, but the new version was not
widely deployed
• Even though X.25 offers reliable and error-free
communications, this technology has been largely
replaced by other higher-speed technologies

Frame Relay Networks
• A PVC packet-switching technology that offers
WAN communication over a fast, reliable digital
link
• Can maintain transmission rates from 64 Kbps to
44 Mbps (T3 speed)
• Allows customers to specify the bandwidth
needed
– Charges depend on the PVC’s bandwidth allocation
(known as Committed Information Rate [CIR])
– CIR is the guaranteed minimum transmission rate
• Connection is established by using a pair of
CSU/DSUs with a router or bridge at each end to
direct traffic on and off the WAN link

Frame Relay Networks

ATM Networks
• Asynchronous Transfer Mode (ATM) – high-speed
network technology designed for both LAN and WAN
use
• ATM bandwidth can be as low as a few Mbps up to
622 Mbps, but the most common speed is 155 Mbps
• Cell-based packet switching technology
– Cells are of a fixed length rather than typical packet-based systems
that use variable length packets
– Fixed length cells can be switched more efficiently than variable length
packets
• ATM is used quite heavily for the backbone and
infrastructure in large communications companies

Multiprotocol Label Switching (MPLS)
• MPLS runs over ATM, frame relay, SONET, and
even Ethernet
• Creates a connection-oriented virtual circuit using
labels assigned to each packet
– The label is used to make packet-forwarding decisions
within the MPLS network, making it unnecessary to view
the contents of the packet
• Capable of supporting different Layer 3 protocols, it
is currently used exclusively in IP networks
– Supports both IPv4 and IPv6

WANs over the Internet
• Using VPN connections over inexpensive Internet
connections is becoming a popular WAN alternative
• VPNs offer the following advantages over other WAN
methods discussed:
– Inexpensive: Cost of Internet access is much lower than leased
lines or packet-switched WAN connections
– Convenience: A VPN can be configured as soon as Internet
access is established
– Security: Advanced authentication and encryption protocols
protect the integrity and privacy of VPN traffic
– Flexibility: After a corporate VPN infrastructure is in place, it is
available for WAN connections from branch offices as well as
mobile users and telecommuters

WAN Connections Methods

WAN Equipment
• Customer Equipment
– Customer Premises Equipment (CPE): The equipment at the
customer site that’s usually the responsibility of the customer
– Customer might own or lease the equipment from the provider
– Usually includes routers, modems and CSU/DSUs
– The demarcation point is the point at which the CPE ends and
the provider’s responsibility begins (where the WAN connection
is made)

WAN Equipment
• Provider Equipment
– The provider location nearest the customer site is usually
referred to as the central office (CO)
– Media (usually coax or fiber) runs from the customer site
demarcation point to the CO of the WAN service provider
– The connection between the demarcation point and the CO is
called the local loop or last mile

WAN Equipment
• Going the Last Mile
– The device that sends data to the local loop is called the data
circuit-terminating equipment (DCE): The CSU/DSU or
modem
– The device that passes data from the customer LAN to the
DCE is called the data terminal equipment (DTE): Router or
bridge that has one connection to the customer LAN and
another connection to the DCE that makes the WAN
connection

WAN Equipment
A WAN connection showing the CPE, demarcation point, and
local loop

Remote Access Networking
• Windows server OSs include the Routing and
Remote Access Service (RRAS) that supports both
dial-up remote access and VPN remote access
• Users can dial in over POTS or use a VPN from
any type of Internet connection

Making a VPN Connection in Windows
• In Windows 7, you create a new connection from the
Network and Sharing Center by selecting “Set up a new
connection or network”
• This will start the “Set Up a Connection or Network
Wizard”

Making a Dial-Up Connection
• All versions of Windows, starting with Windows 95,
include Dial-Up Networking (DUN) software to make an
RRAS connection
• The protocol used is Point-to-Point Protocol (PPP) and
is used to carry a variety of protocols over different
types of network connections
• Two protocols that are integral to PPP:
– Link Control Protocol (LCP): Sets up the PPP connection and defines
communications parameters and authentication protocols
– Network Control Protocol (NCP): Encapsulates higher layer protocols
such as IP and provides services such as dynamic IP addressing

Remote Access Networking
via the Web
• Another remote access model is remote control of the
desktop of your office computer using a Web browser
• Several online services connect your Web browser to
your desktop, including LogMeIn and GoToMyPC
– A client component is installed on your computer and then log
on to the online service which connects you
– Uses authentication and encryption to maintain a secure
connection
• Third party software can also be used
– Microsoft’s Terminal Services Gateway (TSG) allows remote
connections by using SSL, the protocol that secures
communication between Web browsers and Web servers

Cloud Computing
• Cloud computing is a computer networking model
in which data, applications, and processing power
are managed by servers on the Internet, and users
of resources pay for what they use rather than for
the equipment and software needed to provide the
resources
• Benefits:
– Reduced physical plant costs
– Reduced upfront costs
– Reduced personnel costs

Cloud Computing
• There are three primary categories of cloud
computing:
– Hosted applications
– Hosted platforms
– Hosted infrastructure

Hosted Applications
• Hosted applications are also referred to as on-demand
applications or software as a service (SaaS)
• Usually offered as a subscription based on the number
of users
• Customers can take advantage of new software
editions much faster
• Available anywhere the customer has a connection to
the Internet
• Most well-known example is Google Apps

Hosted Platforms
• Hosted platform or platform as a service (PAAS)
– A customer develops applications using the service providers
development tools and infrastructure
– Once developed, the applications can be delivered to the
customer’s users from the provider’s servers
– Most common hosted platforms available are Force.com’s
Apex, Azure for Windows, Google’s AppEngine for Phython and
Java, WaveMaker for Ajax, and Engine Yard for Ruby on Rails

Hosted Infrastructure
• Hosted infrastructure or infrastructure as a service
(IaaS) allows a company to use storage or entire
virtual servers
• If a customer needs another 100 GB of space, they
can pay for the space without worrying about how
that space is actually provided
• If a customer needs another server they pay for the
amount of processing and storage the additional
server actually requires
• Customers rent the resources they are using

Chapter Summary
• The most obvious difference between internetworks
and WANs is the distance between the LANs being
connected
• Several types of devices are likely to be used in WANs
for media access, signal transmission, and reception
and to connect a WAN to a LAN: Modems, CSU/DSU,
and Routers
• The methods used to make a WAN connection often
dictate the technologies that can be used and the
connection’s properties. The four most common are
circuit-switched, leased line, packet-switched, and VPN
over the Internet

Chapter Summary
• WAN equipment can be categorized into customer
equipment, provider equipment, and the circuit that
makes the connections between the demarcation point
and the central office; called the last mile or local loop
• Large and small businesses alike are leveraging fast,
affordable remote access technologies that allow
employees to access their office desktops and
corporate resources from home and on the road

Chapter Summary
• Cloud computing is a computer networking model in
which data, applications, and processing power are
managed by servers on the Internet, and users pay for
what they use rather than for the equipment and
software needed to provide the resources
• There are three primary categories of cloud computing:
hosted applications, hosted platforms, and hosted
infrastructure

Net essentials6e ch12

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