Chapter 12 Wide Area Networks

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Chapter 12Wide Area Networks

• Information Technology in Theory
• By 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)

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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 senders binary address
  (called the source address), and the binary
  address of the packets 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 packets 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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Packet Switching (continued)

• 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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Packet Switching (continued)
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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 packets
  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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WAN Architectural Components (continued)
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WAN Architectural Components (continued)

• 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 providers WAN is often called
  customer premises equipment (CPE)

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WAN Architectural Components (continued)
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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
• ATMs 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
  users 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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Digital Subscriber Line (continued)
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Digital Subscriber Line (continued)
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Digital Subscriber Line (continued)
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Cable Modem Access

• Internet access via cable offers transmission
  rates in the megabit per second
  rangeapproximately 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 managementsolving 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 networks 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