chapter 13 enterprise and wide area networkssjkuyath/etee3281/ch13.pdf · –supports dynamic...

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


Repeaters


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


Bridges


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


Switches


Routers


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


Routing Tables


Routers


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


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


Analog Connectivity


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


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


Modems in Network Communications


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


Types of Modems


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


Types of Modems


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)


Digital Connectivity


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


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


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


T1


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


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


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


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


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)


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


Frame Relay


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


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)


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


Going the Last Mile


Remote Access Networking


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


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)


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


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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