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Chapter 13
Enterprise and Wide Area Networks
Instructor: Nhan Nguyen Phuong
Contents
1. Creating Larger Networks
2. Wide Area Network (WAN) Transmission Technologies
3. WAN Implementation Basics
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1. Creating Larger Networks
1.1. Repeaters
1.2. Bridges
1.3. Switches
1.4. Routers
1.5. Gateways
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• 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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1.1. Repeaters
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1.2. 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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1.3. 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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1.4. 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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1.5. 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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2. Wide Area Network (WAN)
Transmission Technologies
2.1. Analog Connectivity
2.2. Digital Connectivity
2.3. Packet-Switching Networks
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• 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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2.1. Analog Connectivity
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• 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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2.1.1. 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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2.1.2. 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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• 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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2.2. 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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2.2.1. 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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• 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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2.2.2. 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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2.2.3. 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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2.2.4. Switched 56K
• Switched 56K leased lines are older, digital, pointto-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; ondemand 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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2.2.5. 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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2.3. 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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2.3.1. 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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2.3.2. X.25
• Developed in the mid-1970s, the X.25 specification
provided an interface between public packetswitching 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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2.3.3. Frame Relay
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3. WAN Implementation Basics
3.1. Customer Equipment
3.2. Provider Equipment
3.3. Going the Last Mile
3.4. Remote Access Networking
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• 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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3.1. 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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3.2. 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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3.3. Going the Last Mile
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3.4. Remote Access Networking
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3.4.1. 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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3.4.2. 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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• 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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