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Chapter 13
Enterprise and Wide Area Networks
Objectives
• Explain how large networks can be
implemented with a variety of devices
• Discuss the technologies used in
constructing WANs
• Explain some terminology used in
implementing WANs
• Configure and describe remote access
protocols
Guide to Networking Essentials, Fifth Edition 2
Creating Larger Networks
• Ways to stretch or expand network
capabilities– Physically expanding to support additional computers
– Segmenting the network into smaller pieces to filter
and manage network traffic
– Extending the network to connect separate LANs
– Connecting two or more disjointed networking
environments
• Many devices can accomplish these tasks– Repeaters, bridges, switches, routers, and gateways
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Repeaters
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Repeaters
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Bridges
• Bridges can: limit traffic on each segment; reduce bottlenecks; connect different network architectures; and forward frames between segments
• Transparent bridges build a bridging table as they receive frames
• Source-routing bridges (token ring networks) rely on the frame’s source to include path information
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Bridges
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Switches
• A switch is really a high-speed multiport
bridge, an intelligent device that maintains a
switching table and keeps track of which
hardware addresses are located on which
network segments
• Can dedicate bandwidth to each port on the
switch
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Switches
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Switches
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Routers
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Routing Tables
• Routing can be static or dynamic
• A router chooses best path for packet in two
ways
– Using a distance-vector algorithm
– Using a link-state algorithm
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Routing Tables
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Routers
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Gateways
• Gateway: translates information between two
dissimilar network architectures or data formats
– Often connects PCs to mainframe computers
• Other types are found in smaller networks
– When packets arrive at gateway, the SW strips the
networking information, leaving only the raw data
• It then translates the data into the new format and sends it back
down the OSI layers using the destination system’s networking
protocols
– Operates at Application, Network, or Session layer
– Harder to install, slower, and more expensive
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Wide Area Network (WAN) Transmission
Technologies
• WANs are often constructed by linking LANs
– Connections established using communication
devices with communication lines from ISP or
telco
– Special communication links to construct WANs• Packet-switching networks
• Fiber-optic cable
• Microwave transmitters
• Satellite links
• Cable television coax systems
– Most organizations lease WAN links
– Technologies: analog, digital, packet switching
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Analog Connectivity
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Analog Connectivity
• One way to improve the quality of a PSTN connection is to lease a dedicated line or circuit
– Line conditioning improves overall signal quality and reduces interference and noise
• When deciding between a dial-up or dedicated PSTN connection, consider a number of factors
– Length of connection time required
– Cost of service and usage levels
– Availability of dedicated circuits, conditioning, or other quality improvements
– Assessment of the need for a 24-7 connection
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Modems in Network Communications
• A modem is a device for making an analog
connection between computers over a
telephone line, effectively making a WAN
connection between computers or networks
– Modulates/demodulates signals
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Modems in Network Communications
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Types of Modems
• Types of modems: asynchronous and
synchronous
– Type used depends on phone lines and
requirements
– When continuous network connections are
needed, digital technologies such as DSL or
cable modems offer higher bandwidth and
better communication capabilities at little or
no extra cost
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Types of Modems
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Types of Modems
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Types of Modems
• Because synchronous modems have so
little overhead in terms of error checking,
they are much faster than asynchronous
modems
• Synchronous modems were not designed
for use over regular phone lines
– Found in dedicated, leased-line environments
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Types of Modems
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Digital Connectivity
• Because computers and LANs transmit data
digitally, using digital techniques to connect LANs
over long distances to form a WAN makes more
sense than using digital-to-analog conversion
• Digital Data Service (DDS) lines are direct or point-
to-point synchronous communication links with 2.4,
4.8, 9.6, or 56 Kbps transmission rates
– e.g., ISDN, T1, T3, and switched 56K
• DDS uses a communication device called Channel
Service Unit/Data Service Unit (CSU/DSU)
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Digital Connectivity
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Digital Modems
• The interface for ISDN is sometimes called a
digital modem– Consists of network termination (NT) device and terminal
adapter (TA) equipment
• Cable TV operators and telcos that offer
digital connections for SOHO also use the
term modem
– Technically, both uses of term “modem” are
incorrect• Some CATV systems do indeed use analog signaling, so the term
“cable modem” is correct in these cases
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Digital Modems
• Cable modems transmit signals to/from Internet points of presence using broadband CATV network– Provide shared media access bandwidth
– Security was a concern in early networks (users could eavesdrop other communication sessions)
• DSL uses the same twisted-pair phone lines that deliver voice services– Connections are not shared (guaranteed bandwidth)
– Disadvantage: distance limitation between the user’s location and the nearest central office
– Most common types: ADSL and SDSL
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T1
• T1 is a DDS technology that uses two two-wire pairs to transmit full-duplex data signals at a maximum rate of 1.544 Mbps– Digital link that organizations purchase or lease
– Subscribing to one or more channels instead of an entire T1 is possible with fractional T1
– In some countries, the E1 technology is used
• Multiplexing enables several communication streams to travel simultaneously over the same cable segment– Can increase DS-1 rates up to DS-4 speeds
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T1
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T3
• A T3 line has 28 T1s or 672 channels and
supports a data rate of 44.736 Mbps
• Many large service providers offer both T3
and fractional T3 leased lines with
transmission rates of 6 Mbps and up
• A single T3 commonly replaces several T1
lines
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Switched 56K
• Switched 56K leased lines are older,
digital, point-to-point communication links
offered by local and long-distance telcos– They offered the best alternative to PSTN
connections, particularly given their on-demand
structure
– A circuit was not dedicated to a single customer; on-
demand pathways established for users
– Lease terms were based on per-minute use charges
– Today, used when multiple 56 Kbps channels are
aggregated for frame relay services or when other
specialized dedicated digital leased lines are needed
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Integrated Services Digital Network
• Digital communications technology developed in
1984 to replace the analog telephone system
• Available in many metropolitan areas of the
United States, as well as most of Western Europe
• Defines single-channel links of 64 Kbps
• Enjoys some popularity in WANs as a backup line
• Available in two formats or rates
– Basic Rate Interface (BRI): 128 Kbps
– Primary Rate Interface (PRI): same bandwidth as T1
• B-ISDN supports much higher data rates
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Packet-Switching Networks
• Fast, efficient, and highly reliable technology
– Breaks data into packets before transmitting
them• e.g., the Internet
– Data delivery doesn’t depend on any single
pathway• Packets may take different routes
– Packets may need to be rearranged on delivery
– Packets are small• If a packet fails to arrive at destination, retransmission request
can be serviced with minimal time loss
• Reduces the time each switch or host needs to receive,
analyze, and retransmit packets
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Virtual Circuits
• Many packet-switching networks use virtual
circuits to provide temporarily “dedicated”
pathways between two points
– Created after devices at both ends of the
connection agree on bandwidth requirements
and request a pathway
– Incorporate communication parameters that
govern receipt acknowledgements, flow control,
and error control
– Two types: switched (SVCs) and permanent
(PVCs)
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X.25
• Developed in the mid-1970s, the X.25
specification provided an interface between
public packet-switching networks and their
customers– Used most often to connect remote terminals with
centralized mainframes
– SVC network
– Originally, used POTS lines as communication links
• Error checking and retransmission schemes improved success
of transmissions but dampened speed
– Usually associated with public data networks (PDNs)
instead of public or private networks
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Frame Relay
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WAN Implementation Basics
• You have already learned some terms for
the technologies that make WANs work,
such as POTS, ISDN, and frame relay
• This section discusses how WANs are
implemented
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Customer Equipment
• Customer: organization building the WAN
• The equipment at the customer site that’s
usually the responsibility of the customer is
called the CPE– Customer might own or lease the equipment from the provider
– Usually includes devices such as routers, modems (analog), and
CSU/DSUs (digital)
• Demarcation point: point at which the CPE
ends and the provider’s responsibility begins– Junction where the physical WAN connection is made from the
customer to the telco or ISP (the provider)
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Provider Equipment
• Provider location nearest the customer site
is often referred to as the central office (CO)– A cable runs from the customer site demarcation point
to the CO of the WAN service provider
• Usually copper or fiber-optic; provider’s responsibility
• For a wireless connection to the provider, a wireless transmitter
is usually mounted on customer’s building
• The connection between the demarcation
point and the CO is called the local loop or
last mile– The equipment specific to the WAN technology usually
resides at the CO
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Going the Last Mile
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Remote Access Networking
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Serial Line Internet Protocol (SLIP)
• Serial Line Internet Protocol (SLIP): older
protocol used primarily by PCs to connect to
the Internet via a modem– Data Link layer protocol that provides connectivity
across telephone lines and no error correction
– Relies on hardware for error checking and correction
– Supports connections only for TCP/IP and requires no
addressing because a connection is made only between
two machines
– Compressed SLIP (CSLIP) supports compression
– Not used much in today’s environment
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Point-to-Point Protocol (PPP)
• PPP provides a more dynamic connection between computers than SLIP– Provides both Physical and Data Link layer services
• Effectively turns a modem into a NIC
– Supports multiple protocols (e.g., IP, IPX, NetBEUI)
– Inherently supports compression and error checking
– Supports dynamic assignment of IP addresses• Can assign a block of addresses to RRAS modems
– Has replaced SLIP as the remote protocol of choice for TCP/IP connections
• The only dial-up connections that RRAS supports require PPP (or a direct Internet connection for VPNs)
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Summary
• Several devices can be used to expand a network
– A repeater increases the length of your network by
eliminating the effect of attenuation on the signal
– A bridge installed between two network segments filters
traffic according to HW destination addresses
– Switches, similar to bridges, can handle more network
segments and switch frames much faster
– A router connects several independent networks to form
a complex internetwork
• Analog WAN connections use conventional PSTN
phone lines and offer little reliability or speed
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Summary
• Low-cost, medium-bandwidth technologies (e.g.,
DSL, cable modem) are taking over for SOHO
connections
• T1 and similar lines are collections of pairs of
cables, so fractions of these links can be leased
• Packet-switching networks are fast, efficient, and
reliable WAN connection technologies
– Frame relay: 56 Kbps-1.544 Mbps, no error checking
• Equipment at WAN customer site is called CPE
• Windows RRAS enables up to 256 remote clients
to dial in if the hardware is available
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