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Chapter 12 Wide Area Networks


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Title: Chapter 12 Wide Area Networks

Chapter 12Wide Area Networks
  • Information Technology in Theory
  • By Pelin Aksoy and Laura DeNardis

  • Understand the concept of a wide area network
  • Identify the main technical components of a wide
    area network
  • Distinguish between packet switching and circuit
  • Understand virtual private networks (VPNs)

Objectives (continued)
  • Gain familiarity with the most important
    commercial WAN services
  • Understand WAN access technologies, including
    dedicated lines, xDSL, cable modem access, and
  • Identify important network management functions

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

WAN Background (continued)
  • Wide area networking developed from the way
    businesses first used computer networks to
    exchange information internally, beginning in the
  • 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

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)

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

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

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

Packet Switching (continued)
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
  • Earlier protocols, such as SNA and DECnet, were
    proprietary protocols

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

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

WAN Architectural Components (continued)
  • The router looks up information in a routing
  • 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

WAN Architectural Components (continued)
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)

WAN Architectural Components (continued)
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

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

Private Networks (continued)
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

Internet Services (continued)
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

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

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

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

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

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

WAN Access Alternatives
  • The access mechanism selected depends on several
  • Bandwidth
  • Mobility
  • Security
  • Availability

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

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
  • T-3 link 45 Mbps
  • OC-3 155 Mbps
  • OC-12 622 Mbps
  • OC-48 2.488 Gbps

Leased Private Lines (continued)
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
  • Many types of DSL technology are available
  • Symmetric digital subscriber line (SDSL)
  • Asymmetric digital subscriber line (ADSL)
  • High data rate digital subscriber line (HDSL)

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
  • 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

Digital Subscriber Line (continued)
Digital Subscriber Line (continued)
Digital Subscriber Line (continued)
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

Cable Modem Access (continued)
  • 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

WiMAX (continued)
  • WiMAX (Worldwide Interoperability for Microwave
  • 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)

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

WiMAX (continued)
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

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

Configuration Management (continued)
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

Fault Management
  • One of the most important functions of network
    management is fault managementsolving a network
    outage or performance problem when it occurs

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

  • 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
  • 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

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

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