Chapter 12 Wide Area Networks

Chapter 12Wide Area Networks

Information Technology in TheoryBy Pelin Aksoy and Laura DeNardis

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Objectives

• Understand the concept of a wide area network• Identify the main technical components of a wide

area network• Distinguish between packet switching and circuit

switching• Understand virtual private networks (VPNs)

Information Technology in Theory

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Objectives (continued)

• Gain familiarity with the most important commercial WAN services

• Understand WAN access technologies, including dedicated lines, xDSL, cable modem access, and WiMAX

• Identify important network management functions


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WAN Background

• Industry convention describes a WAN as a network that spans a large geographical distance

• The largest example of a WAN is the public Internet, but many other types of WANs exist

• Wide area networking is sometimes referred to as enterprise networking


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WAN Background (continued)

• Wide area networking developed from the way businesses first used computer networks to exchange information internally, beginning in the mid-1970s

• Several architectural features distinguished these networks from modern WANs

• WANs now support voice, data, and multimedia information; use open network protocols; and often are offered over a public network such as the Internet


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Packet Switching

• Information sent over the Internet is broken into small segments called packets

• Each packet contains the actual information content to be transmitted, as well as the order of the packet, the sender’s binary address (called the source address), and the binary address of the packet’s destination (called the destination address)


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Packet Switching (continued)

• The path that one packet traverses over a network from source to destination may be different from the next packet’s path, depending on network congestion or other conditions

• In this type of networking approach, known as connectionless packet switching, no dedicated end-to-end physical connection is established for the duration of data transmission


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Packet Switching (continued)• Network devices called routers read the destination

address and determine how to expeditiously route packets through the networks, based on routing algorithms that are designed to minimize latency

• Routers are also designed to minimize hops, the number of times a packet traverses various routers as it is transmitted over a network

• Once all the packets from a given transmission reach their destination, they are reassembled in correct order


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• The packet-switching approach contrasts with the circuit-switching approach of the traditional telephone network

• The circuit-switching approach establishes a physical, dedicated end-to-end path through the network between a caller and receiver, and maintains the path for the entirety of the call


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Network Protocols

• Network protocols are another important technical enabler of WANs

• Unlike older types of networks, such as the SNA and DECnet systems, modern wide area networking has made the transition to more open network protocols

• Earlier protocols, such as SNA and DECnet, were proprietary protocols


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Network Protocols (continued)

• The dominant family of network protocols in modern architectures is TCP/IP

• It is not controlled by a single vendor; it is available for any manufacturer to use, and it enables universal access to the Internet


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WAN Architectural Components

• A router is an intelligent switching device that determines how to direct (or route) a packet across a network, based on the packet’s destination address and network conditions

• An enormous, interconnected web of thousands of routers makes up the backbone of the Internet


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WAN Architectural Components (continued)

• The router “looks up” information in a routing table• These routing tables are constantly changing and

automatically updated as routers probe their network environments, exchange information with other routers, and dynamically update their routing tables


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• Customers who access the Internet or another type of WAN are connected to an edge router

• A device called a CSU/DSU (Channel Service Unit/Data Service Unit) is an important network component between the edge router and the dedicated transmission line

• The equipment at a customer location that serves as the demarcation point between a local network and a service provider’s WAN is often called customer premises equipment (CPE)


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Private Networks

• If a company such as a bank wants a dedicated transmission path between its branches for private digital communication lines, the company has the option of leasing dedicated private lines from a network provider

• A private line is not shared with other customers, so it can guarantee performance and availability to some degree


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Private Networks (continued)

• These dedicated lines may run terrestrially over fiber-optic cables or high-grade twisted pair, via undersea fiber-optic cables, or even via satellite for international communications or remote areas that do not have adequate terrestrial telecommunications

• The primary disadvantage of a private network is cost, because a single enterprise bears the entire cost of the lines


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Private Networks (continued)


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Internet Services

• Many businesses use the infrastructure of the public Internet to communicate internally and with customers and suppliers

• Using the Internet and other shared public WANs is much more cost effective than using dedicated private networks

• Virtual private networks (VPNs) are a response to the performance and security concerns of public networks


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Internet Services (continued)


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Internet Services (continued)• Privacy and security are achieved through the

application of security measures and a technique called tunneling

• VPN services offer quality-of-service (QoS) guarantees to their customers to cover such factors as latency of packets, dropped packets that cannot be routed to their destination, and network availability

• VPN customers usually obtain service level agreements (SLAs) from the VPN provider


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Frame Relay Service

• The frame relay network is owned and operated by the service provider but is used by the customer

• This service uses packets, called frames, with a variable number of bits that are switched throughout the WAN until they reach their destination

• The frame relay standard uses a different format from other WAN standards


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Frame Relay Service (continued)• A software-defined (virtual) path is set between two

devices on the network, and the devices are expected to exchange information– Switched virtual circuits (SVCs)– Permanent virtual circuits (PVCs)

• Historically, frame relay originated as an improvement over an older WAN technology known as X.25


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Asynchronous Transfer Mode

• WAN service providers also offer Asynchronous Transfer Mode (ATM), a network alternative that formats information into fixed-length packets

• These packets are normally called cells in the context of ATM

• ATM cells have a total length of 53 bytes• ATM is a connection-oriented WAN approach• ATM’s fixed transmission delays, virtual circuits,

and fixed cell size are beneficial for low latency applications


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Multiprotocol Label Switching

• A more practical type of WAN service, called multiprotocol label switching (MPLS), is designed to simultaneously support many types of WAN traffic

• MPLS service can handle variable-length packets, in contrast with ATM, which transmits fixed-length cells

• One major difference between MPLS and traditional IP packet switching is that MPLS is connection oriented


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Multiprotocol Label Switching (continued)

• Traditional packet switching is often connectionless, meaning that a dedicated end-to-end connection is not established for the duration of transmission

• Unlike this approach, MPLS routes packets along preconfigured paths


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WAN Access Alternatives

• The access mechanism selected depends on several variables– Bandwidth– Mobility– Security– Availability


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Leased Private Lines

• Businesses and other enterprises that have more than a dozen employees and that need WAN access usually lease a dedicated private line to a commercial WAN service


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Leased Private Lines (continued)

• The line is rented on a monthly basis from a telecommunications service provider, and comes in the following transmission speeds:– 768 Kbps: Sub-T1 link (also called a fractional T1 line)– T-1 link: 1.544 Mbps (also called a dedicated T1 line)– T-3 link: 45 Mbps– OC-3: 155 Mbps– OC-12: 622 Mbps– OC-48: 2.488 Gbps


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Leased Private Lines (continued)


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Digital Subscriber Line

• DSL is a WAN access alternative that connects a user’s DSL modem to the twisted pair cables installed as part of the traditional telephone network

• Many types of DSL technology are available:– Symmetric digital subscriber line (SDSL)– Asymmetric digital subscriber line (ADSL)– High data rate digital subscriber line (HDSL)


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Digital Subscriber Line (continued)

• DSL quality depends on the distance between the user site and the telephone company (the “central office”) that houses the DSL termination equipment

• This equipment is known as the DSLAM, or DSL access multiplexer

• DSL service can offer a transmission rate of up to 7.1 Mbps downstream and approximately 768 Kbps upstream


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Cable Modem Access

• Internet access via cable offers transmission rates in the megabit per second range—approximately 30 to 50 Mbps downstream and 5 Mbps upstream in some areas

• Cable WAN access is geared primarily to residential customers who already subscribe to cable television service and is offered as part of a package with cable television, high-speed Internet access, and Voice over IP service


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Cable Modem Access (continued)


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WiMAX

• High-speed wireless broadband access to the Internet is commonplace over small geographical areas through technologies such as Wi-Fi

• A significant technological requirement is for products and services to provide the same type of wireless WAN access over greater distances, such as across a city using a metropolitan area network


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WiMAX (continued)

• WiMAX (Worldwide Interoperability for Microwave Access)

• Another name for IEEE 802.16• A formal set of networking standards for wireless

metropolitan area networks• Developed by the Institute of Electrical and

Electronics Engineers (IEEE)


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WiMAX (continued)• WiMAX networks would consist of WiMAX

antennas transmitting to residences and businesses with a WiMAX receiver, to laptops with WiMAX cards, or to Wi-Fi hot spots.

• WiMAX provides a theoretical maximum uplink and downlink speed of approximately 70 Mbps, although this rate can be achieved only over short ranges

• Performance varies depending on distance, spectrum allocated, and number of concurrent users sharing bandwidth


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WiMAX (continued)


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Network Management Systems• Businesses, universities, and even individual users require

local and wide area network services that are always available and that perform reliably

• To achieve these requirements, users must provide or outsource network management activities, which fall into four categories:– Configuration management– Performance management– Fault management– Security management


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Configuration Management

• The function of configuration management is to track and manage all the hardware and software associated with the computer network, and to manage any changes that occur to these architectural elements


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Configuration Management (continued)


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Performance Management

• Another function that network managers must provide is performance management, which ensures that the network is performing adequately for the applications and users it supports


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Fault Management

• One of the most important functions of network management is fault management—solving a network outage or performance problem when it occurs


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Security Management

• Network security is a vital management function that is often handled by a dedicated security team

• Some of these security management functions include network access control, user authentication, firewall management, and critical infrastructure protection

• Some security threats to enterprise networks include worms and viruses, spam, unauthorized access attempts, and data interception


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Summary• A WAN is a network that spans a large geographical

distance and can transmit voice, data, and multimedia information

• The Internet is the most prominent example of a WAN• Most WAN services are run over a public network

rather than private lines due to cost concerns• Most WANs, including the Internet, are based on a

network approach known as packet switching, which breaks information into small segments called packets prior to transmission– These packets are sent over a network, possibly routed along

different paths, and reassembled at their destination


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Summary (continued)• A virtual private network (VPN) runs over a public

network like the Internet, but it emulates a private network’s higher performance and security by applying security measures like encryption and by offering quality of service (QoS) guarantees

• WANs require network protocols, the standard rules that allow information to be exchanged over the network

• Important types of WANs are Internet services, frame relay, Asynchronous Transfer Mode, and multiprotocol label switching


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Summary (continued)• Most businesses and large institutions access WANs via

leased lines, while smaller businesses and individual users access WANs via wireless technologies, DSL, and cable modems

• Ensuring adequate WAN performance and reliability requires network management functions, including configuration management, performance management, fault management, and security management


Chapter 12 Wide Area Networks

Documents

theoryinformation technology

internetinformation

callinformation technology

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