Enterprise and Wide Area Networks

munchdrabNetworking and Communications

Oct 30, 2013 (3 years and 7 months ago)

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Chapter 13


Enterprise and Wide Area Networks

Instructor: Nhan Nguyen Phuong

Guide to Networking Essentials, Fifth Edition

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Contents

1. Creating Larger Networks


2. Wide Area Network (WAN) Transmission Technologies


3. WAN Implementation Basics


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

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, 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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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 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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2.3.3. Frame Relay

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