Chapter 7:
WANs and Remote Connectivity
Objectives
Identify network applications that require
WAN technology
Explain various WAN topologies, including
their advantages and disadvantages
Describe a variety of WAN transmission and
connection methods, including PSTN, ISDN,
T-carriers, DSL, broadband cable, and
SONET
Objectives (continued)
Assess WAN implementation options based
on speed, security, and reliability
Understand the hardware and software
requirements for remotely connecting to a
network
WANs and Remote Connectivity
• A WAN is a network that connects two or more
geographically distinct LANs
• Remote connectivity and WANs are significant
concerns for organizations attempting to meet the
needs of telecommuting workers, global business
partners, and Internet-based commerce
WAN Essentials
• A WAN is a network that traverses some distance
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and usually connects LANs, whether across the
city or across the nation
The internet is the largest WAN in existence
today7
LANs use a building’s internal cabling, such as
twisted-pair, that runs from work area to the wall,
through plenum areas and to a
telecommunications closet
WAN Essentials (continued)
• WANs typically send data over publicly available
communications networks, which are owned by
local and long-distance telecommunications
carriers
• Such carriers, which are privately owned
corporations, are also known as network service
providers (NSPs)
WAN Essentials (continued)
• For better throughput, an organization might lease
a dedicated line, or a continuously available
communications channel, from a
telecommunications provider, such as a local
telephone company or ISP
• A WAN link is a connection between one WAN site
(or point) and another site (or point)
WAN Topologies
• Bus
• A WAN in which each site is directly connected to
no more than two other sites in a serial fashion is
known as a bus topology WAN
• A bus topology WAN is similar to a bus topology
LAN in that each site depends on every other site in
the network to transmit and receive its traffic
• The WAN bus topology uses different locations,
each one connected to another one through pointto-point links
WAN Topologies (continued)
• A bus topology WAN is often the best option for
organizations with only a few sites and the
capability to use dedicated circuits
• Bus WAN topologies are suitable for only small
WANs
• A single failure on a bus topology WAN can take
down communications between all sites
WAN Topologies (continued)
• Ring
• In a ring topology WAN, each site is connected to
two other sites so that the entire WAN forms a ring
pattern
• This architecture is similar to the simple ring
topology used on a LAN, except that a WAN ring
topology connects locations rather than local
nodes and in most WANs, a ring topology uses
two parallel paths for data
WAN Topologies (continued)
• A ring topology WAN cannot not be taken down by
the loss of one site; instead, if one site fails, data
can be rerouted around the WAN in a different
direction
• WANs that use the ring topology are only practical
for connecting fewer than four or five locations
WAN Topologies (continued)
• Star
• The star topology WAN mimics the arrangement of
a star topology LAN
• A single site acts as the central connection point
for several other points
WAN Topologies (continued)
• If a single connection fails, only one location loses
WAN access
• When all of its dedicated circuits are functioning, a
star WAN provides shorter data paths between
any two sites
WAN Topologies (continued)
• Mesh
• A mesh topology WAN incorporates many directly
interconnected sites
• Because every site is interconnected, data can
travel directly from its origin to its destination
• Mesh WANs are the most fault-tolerant type of
WAN because they provide multiple routes for
data to follow between any two points
WAN Topologies (continued)
• The type of mesh topology in which every WAN
site is directly connected to every other site is
called a full mesh WAN
• Partial mesh WAN are used when only critical
WAN sites are directly interconnected and
secondary sites are connected through star or ring
topologies
• Partial mesh WANs are more common in today’s
business world than full mesh WANs because they
are more economical
WAN Topologies (continued)
• Tiered
• In a tiered topology WAN, sites connected in star
or ring formations are interconnected at different
levels, with the interconnection points being
organized into layers to form hierarchical
groupings
WAN Topologies (continued)
• Tiered systems allow for easy expansion and
inclusion of redundant links to support growth
• Their enormous flexibility means that creation of
tiered WANs requires careful consideration of
geography, usage patterns, and growth potential
PSTN
• Stands for Public Switched Telephone Network
• Refers to the network of typical telephone lines
and carrier equipment that service most homes
• PSTN may also be called plain old telephone
service (POTS)
• The PSTN comprises the entire telephone system,
from the lines that connect homes and businesses
to the network centers that connect different
regions of a country
PSTN (continued)
• The PSTN is often used by individuals connecting
to a WAN (such as the Internet) via a dial-up
connection
• A dial-up connection is one in which a user
connects, via a modem, to a distant network from
a computer and stays connected for a finite period
of time
PSTN (continued)
• A central office is the place where a telephone
company terminates lines and switches calls
between different locations
• The portion of the PSTN that connects your house
to the nearest central office is known as the local
loop, or the last mile
X.25
• X.25 is an analog, packet-switched technology
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designed for long-distance data transmission
The X.25 standard specifies protocols at the
Physical, Data Link, and Network layers of the OSI
Model
The X.25 provides excellent flow control and
ensures data reliability over long distances by
verifying the transmission at every node
X.25 checks for errors and, in the case of an error,
either corrects the damaged data or retransmits
the original data
Frame Relay
• An updated, digital version of X.25 that also relies
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•
on packet switching
Frame Relay protocols operate at the Data Link
layer of the OSI Model and can support multiple
different Network and Transport layer protocols
The name is derived from the fact that data is
separated into frames, which are then relayed
from one node to another without any verification
or processing
Frame Relay does not guarantee reliable delivery
of data
X.25 and Frame Relay
• Both X.25 and Frame Relay may be configured as
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switched virtual circuits (SVCs) or permanent
virtual circuits (PVCs)
SVCs are connections that are established when
parties need to transmit, then terminated once the
transmission is complete
PVCs are connections that are established before
data needs to be transmitted and maintained after
the transmission is complete and they are not
dedicated, individual links
The service provider guarantees a minimum
amount of bandwidth, called the committed
information rate (CIR)
ISDN
• Integrated Services Digital Network is an
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international standard for transmitting digital data
over the PSTN
ISDN specifies protocols at the Physical, Data
Link, and Transport layers of the OSI Model
ISDN relies on the PSTN for its transmission
medium
ISDN is distinguished because it can
simultaneously carry as many as two voice calls
and one data connection on a single line
ISDN (continued)
• All ISDN connections are based on two types of
channels: B channels and D channels
• The B channel is the “bearer” channel, employing
circuit-switching techniques to carry voice, video,
audio, and other types of data over the ISDN
connection
• The D channel is the “data” channel, employing
packet-switching techniques to carry information
about the call, such as session initiation and
termination signals, caller identity, call forwarding,
and conference calling signals
ISDN (continued)
• In North America, two types of ISDN connections
are commonly used: Basic Rate Interface (BRI)
and Primary Rate Interface (PRI)
• BRI (Basic Rate Interface) uses two B channels
and one D channel
• In a process called bonding, these two 64-Kbps B
channels can be combined to achieve an effective
throughput of 128 Kbps
ISDN (continued)
• PRI (Primary Rate Interface) uses 23 B channels
and one 64-Kbps D channel
• PRI is less commonly used by individual
subscribers than BRI is, but it may be selected by
businesses and other organizations that need
more throughput
• PRI link can carry voice and data, independently
of each other or bonded together
T-Carriers
• T-carrier standards specify a method of signaling,
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which means they belong to the Physical layer of
the OSI Model
A T-carrier uses time division multiplexing (TDM)
over two wire pairs (one for transmitting and one
for receiving) to divide a single channel into
multiple channels
Each channel may carry data, voice, or video
signals
The medium used for T-carrier signaling can be
ordinary telephone wire, fiber-optic cable, or
wireless links
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T-Carriers (continued)
• Types of T-Carriers
• T1 circuit can carry the equivalent of 24 voice or
data channels, giving a maximum data throughput
of 1.544 Mbps
• A T3 circuit can carry the equivalent of 672 voice
or data channels, giving a maximum data
throughput of 44.736 Mbps
T-Carriers (continued)
• A fractional T1 lease allows organizations to use
only some of the channels on a T1 line and be
charged according to the number of channels they
use
• The signal level refers to the T-carrier’s Physical
layer electrical signaling characteristics
• DS0 (digital signal, level 0) is the equivalent of one
data or voice channel
T-Carriers (continued)
•
T-Carrier Connectivity
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•
Every T-carrier line requires connectivity hardware at both
the customer site and the local telecommunications
provider’s switching facility
T-carrier lines require specialized connectivity hardware that
cannot be used with other WAN transmission methods
T-carrier lines require different media depending on their
throughput
• Wiring
• T1 technology can use unshielded or shielded
twisted-pair (UTP or STP) copper wiring
• STP is preferable to UTP
T-Carriers (continued)
• CSU/DSU (Channel Service Unit/Data
Service Unit)
• The CSU/DSU is the connection point for a T1 line
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at the customer’s site
The CSU provides termination for the digital signal
and ensures connection integrity through error
correction and line monitoring
The DSU converts the T-carrier frames into frames
the LAN can interpret and vice versa
After being demultiplexed, an incoming T-carrier
signal passes on to devices collectively known as
terminal equipment
T-Carriers (continued)
• Terminal Equipment
• On a typical T1-connected data network, the
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terminal equipment will consist of switches,
routers, or bridges
Usually, a router or Layer 3 or higher switch is the
best option, because these devices can translate
between different Layer 3 protocols that might be
used on the WAN and LAN
On some implementations, the CSU/DSU is not a
separate device, but is integrated with the router
or switch as an expansion card
DSL
• Digital subscriber line (DSL) is a WAN connection
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method introduced by researchers at Bell
Laboratories in the mid-1990s
DSL can span only limited distances without the
help of repeaters and is therefore best suited to
the local loop portion of a WAN link
DSL can support multiple data and voice channels
over a single line
DSL uses advanced data modulation techniques
DSL (continued)
• Types of DSL
• The term xDSL refers to all DSL varieties, of which
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at least eight currently exist
DSL types can be divided into two categories:
asymmetrical and symmetrical
The term downstream refers to data traveling from
the carrier’s switching facility to the customer
Upstream refers to data traveling from the
customer to the carrier’s switching facility
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DSL (continued)
• A technology that offers more throughput in one
direction than in the other is considered
asymmetrical
• In asymmetrical communications, downstream
throughput is higher than upstream throughput
• Symmetrical technology provides equal capacity
for data traveling both upstream and downstream
• Symmetrical transmission is suited to users who
both upload and download significant amounts of
data
DSL (continued)
• DSL Connectivity
• A DSL modem is a device that modulates
outgoing signals and demodulates incoming
DSL signals
• The DSL modem may be external to the computer
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and connect to a computer’s Ethernet NIC via an
RJ-45,USB, or wireless interface
DSL access multiplexer (DSLAM) aggregates
multiple DSL subscriber lines and connects them
to a larger carrier or to the Internet backbone
DSL is not available in all areas of the United
States
Cable
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Cable connections require that the customer use a
special cable modem, a device that modulates and
demodulates signals for transmission and reception via
cable wiring
Cable modems operate at the Physical and Data Link
layer of the OSI Model, and therefore do not
manipulate higher-layer protocols such as IP or IPX
To provide Internet access through its network, the
cable company must upgrade its existing equipment to
support bidirectional, digital communications
The cable company’s network wiring must be replaced
with hybrid fiber-coax (HFC), an expensive fiber-optic
link that can support high frequencies
Cable (continued)
• Either fiber-optic or coaxial cable may connect the
node to the customer’s business or residence via a
connection known as a cable drop
• These nodes then connect to the cable company’s
central office, which is known as its head-end
SONET
• SONET (Synchronous Optical Network) is a highbandwidth WAN signaling technique
• SONET specifies framing and multiplexing
techniques at the Physical layer of the OSI Model
SONET (continued)
• Its four key strengths are that it: can integrate
many other WAN technologies; offers fast data
transfer rates; allows for simple link additions and
removals; and provides a high degree of fault
tolerance
• The word synchronous means that data being
transmitted and received by nodes must conform
to a timing scheme
SONET (continued)
• SONET provides interoperability
• SONET is often used to aggregate multiple T1s,
T3s, or ISDN lines
• SONET is also used as the underlying technology
for ATM transmission
• Internationally, SONET is known as SDH
(Synchronous Digital Hierarchy)
• SONET’s extraordinary fault tolerance results from
•
its use of a double-ring topology over fiber-optic
cable
Self-healing is a characteristic of dual-ring
topologies that allows them to automatically
reroute traffic along the backup ring if the primary
ring becomes severed
SONET (continued)
• Most SONET multiplexers allow for easy additions
or removals of connections to the SONET ring,
which makes this technology easily adaptable to
growing and changing networks
• The data rate of a particular SONET ring is
indicated by its Optical Carrier (OC) level
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WAN Implementation
• Reliability
•
A WAN’s reliability depends partly on the transmission
medium it uses and partly on its topology and
transmission methods
• Not very reliable, suited to individual or unimportant
transmissions: PSTN dial-up
• Sufficiently reliable, suited for day-to-day
transmissions: ISDN,T1, fractional T1, T3, DSL, cable,
X.25, and Frame Relay
• Very reliable, suited to mission-critical applications:
SONET
WAN Implementation (continued)
• Security
• Fiber optic media are the most secure
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transmission media
WAN security depends in part on the encryption
measures each carrier provides for its lines
Enforce password-based authorization for LAN
and WAN access and teach users how to choose
difficult-to-decrypt passwords
Take the time to develop, publish, and enforce a
security policy for users in your organization
Maintain restricted access to network equipment
rooms and data centers
Remote Connectivity
• As a remote user, you must connect to a LAN
via remote access, which can be accomplished
in one of several ways: dial-up networking,
remote control, terminal services, Web portals,
or a virtual private network (VPN)
Remote Connectivity (continued)
• Dial-up Networking
• Dial-up networking refers to dialing into a private
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network’s or ISP’s remote access server to log on
to a network
The remote access server (a server designed to
accept incoming client connections) is attached to
a group of modems, all of which are associated
with one phone number
An advantage to using this remote access option
are that the technology is well-understood and its
software comes with virtually every operating
system
Dialing into a remote access server can be slow
because it relies on the PSTN
Remote Connectivity (continued)
• One well known program used to provide remote
access is the Microsoft Routing and Remote
Access Service (RRAS), which is available with
the Windows Server 2003 network operating
system
• The Serial Line Internet Protocol (SLIP) and Pointto-Point Protocol (PPP) are two protocols that
enable a workstation to connect to another
computer using a serial connection
Remote Connectivity (continued)
• Remote Control
• Remote control allows the remote user to “take
over” a computer that’s directly connected to the
LAN
• Remote control is not difficult to configure, but
suffers from the same slow throughput as dialing
into a remote access server
• Another disadvantage to this solution is that it
allows only one connection to the LAN at any
given time
Remote Connectivity (continued)
• Terminal Services
• In terminal services, multiple remote computers
can connect to a terminal server on the LAN
• A terminal server is a computer that runs
specialized software that allows it to act as a host
and supply applications and resource sharing to
remote clients
Remote Connectivity (continued)
• Many companies have created software to supply
terminal services
• Microsoft’s version of this solution is called Terminal
Services
• Citrix System, Inc.’s version is Metaframe and
remote workstations rely on software known as an
ICA (Independent Computing Architecture) client
Remote Connectivity (continued)
• Web Portals
• A Web portal is simply a secure, Web-based
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interface to an application
Any type of Internet connection is sufficient for
using Web portals
On the host side, a Web server supplies the
application to multiple users upon request
The use of Web portals calls for secure
transmission protocols
Virtual Private Networks (VPN)
• Virtual private networks (VPNs) are wide area
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networks logically defined over public transmission
systems
To allow access to only authorized users, traffic on
a VPN is isolated from other traffic on the same
public lines
The software required to establish VPNs is usually
inexpensive, and in some cases is being included
with other widely used software
RRAS allows you to create a simple VPN by
turning a Windows server into an access server
and allowing clients to dial into it
Virtual Private Networks (VPN)
(continued)
• Two important considerations when designing a
VPN are interoperability and security
• To make sure a VPN can carry all types of data in
a private manner over any kind of connection,
special VPN protocols encapsulate higher-layer
protocols in a process known as tunneling
• A VPN tunneling protocol operates at the Data
Link layer and encapsulates Network layer
packets
Virtual Private Networks (VPN)
(continued)
• Two major types of tunneling protocols are used
on contemporary VPNs: PPTP or L2TP
• PPTP (Point-to-Point Tunneling Protocol) is a
protocol developed by Microsoft that expands on
PPP by encapsulating it so that any type of PPP
data can traverse the Internet masked as an IP or
IPX transmission
• Another VPN tunneling protocol is L2TP (Layer 2
Tunneling Protocol), based on technology
developed by Cisco and standardized by the IETF
Summary
• Network applications that require WAN
technology
• WAN topologies, including their
advantages and disadvantages
• WAN transmission and connection
methods, including PSTN, ISDN, Tcarriers, DSL, broadband cable, and
SONET
Summary (continued)
• WAN implementation options based on
speed, security, and reliability
• Hardware and software requirements for
remotely connecting to a network