Chapter 13 Enterprise and Wide Area Networks Instructor: Nhan Nguyen Phuong

Chapter 13

Enterprise and Wide Area Networks

Instructor: Nhan Nguyen Phuong

Guide to Networking Essentials, Fifth Edition 2

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



• 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


1.1. Repeaters



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



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




1.4. 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




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



• 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


2.1. 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


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



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




• 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



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)



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


• 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


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



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


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


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


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


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)


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


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



• 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


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)


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


3.3. Going the Last Mile


3.4. Remote Access Networking


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


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)


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


• 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

Documents

Chapter 13 Enterprise and Wide Area Networks Instructor: Nhan Nguyen Phuong