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Business Data Communications. Chapter Seven Wide Area Networking Fundamentals. Primary Learning Objectives. Understand the need for varying WAN connectivity options Define the term switching network Describe circuit switching networks Describe packet switching networks

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Business data communications

Business Data Communications

Chapter Seven

Wide Area Networking Fundamentals

Primary learning objectives

Primary Learning Objectives

  • Understand the need for varying WAN connectivity options

  • Define the term switching network

  • Describe circuit switching networks

  • Describe packet switching networks

  • Differentiate X.25, Frame Relay, and ATM

  • Explain Point-to-point protocol

  • Identify common trunk carrier services

  • Distinguish between symmetric and asymmetric Digital Subscriber Line services

Wan connectivity options

WAN Connectivity Options

  • There are several ways in which WAN connections can be established

  • Many connection options are provided by common carrier infrastructures (AT&T, MCI, Sprint, for example)

  • WAN connectivity options include:

    • Circuit switching

    • Packet switching: X.25, Frame Relay, ATM

    • Point-to-point protocols

    • Trunk carrier services

    • Digital subscriber lines

Wan connectivity options1

WAN Connectivity Options

Switching networks

Switching Networks

  • Composing a switching network are switches, which:

    • Are a series of interlinked devices

      • Interlinks between two communicating devices are temporary

    • Can be hardware, software, or a combination of both

    • Can be connected to multiple links

  • The switching network infrastructure is referred to, and graphically illustrated as, a “cloud”

    • Edge switches and edge routers connect the enterprise to the cloud

Switching network cloud

Switching NetworkCloud

Circuit switching networks

Circuit Switching Networks

  • Are well-suited for voice, but not data, communications

    • Voice communications are continuous; data communications are bursty

  • Create a direct connection between two communicating devices:

    • This direct connection is referred to as a “path”

    • While established, no two other devices can use the circuit path

    • The circuit path uses a constant rate determined by the slower of the two devices

  • Circuit-switched paths can be temporary or permanent

Circuit switching networks1

Circuit Switching Networks

  • Temporary paths:

    • Require a call setup

    • Are less expensive than permanent paths

    • Do not guarantee that a circuit will always be available

  • Permanent paths:

    • Do not require a call setup

    • Are more expensive than temporary paths

    • Guarantee that a circuit will always be available

  • Neither temporary nor permanent circuit switching prioritizes communications

Packet switching networks

Packet Switching Networks

  • Break data into units called “packets”:

    • Packets traverse the packet switching cloud through a series of links

    • Links are connected by packet switching nodes

  • Improve line efficiently significantly because many packet streams, from differing communications, can use the same links between packet switching nodes

  • Deliver packets even when the network is busy, although the packets may be delayed

  • Provide for data-rate link conversion between packet switching nodes

Data rate link conversion

Data-Rate Link Conversion

Packet switching networks1

Packet Switching Networks

  • Use the network implementation to determine packet size

    • Packets contain not only core data, but control information as well

  • Temporarily store and then forward packets as they pass through the packet switching cloud

  • Allow for prioritization of communications

  • May be based on datagram or virtual circuit delivery

Packet switching networks2

Packet Switching Networks

  • Datagram delivery:

    • Is best for small messages

    • Treats each packet independently

    • Does not guarantee packet delivery

    • Does not sequence packets

    • Permits packets to be dynamically rerouted

    • Does not require a setup procedure between sender and receiver

    • Requires little overhead

Packet switching networks3

Packet Switching Networks

Packet switching networks4

Packet Switching Networks

  • Virtual circuit delivery:

    • Is best for data-sensitive messages

    • Requires a set-up procedure between sender and receiver

    • Requires all packets in a message to take the same route

    • Guarantees packet delivery

    • Sequences packets

    • Cannot dynamically reroute packets

    • Has significant overhead

    • Takes one of two forms:

      • Switched virtual circuit

      • Permanent virtual circuit

Packet switching networks5

Packet Switching Networks

  • Switched virtual circuits:

    • Are similar in concept to a dial-up circuit switched connection

    • Temporarily set up the route of links a circuit takes, meaning only for the duration of the communication

    • Require a setup procedure for each communication

Packet switching networks6

Packet Switching Networks

  • Permanent virtual circuits:

    • Are similar in concept to a leased circuit switched connection

    • Once defined, require no additional set-up procedures, meaning that the route of links does not have to be recreated

    • Are always available as they are permanent

    • Require virtual circuit identifiers for each communication

Packet switching networks7

Packet Switching Networks

Packet switching networks8

Packet Switching Networks

  • Three common forms of packet switching network are:

    • X.25 – The oldest

    • Frame Relay

    • ATM – The newest

Business data communications


  • Uses data terminal equipment and data-circuit terminating equipment

  • Uses virtual circuits and statistical time division multiplexing

  • Supports variable length packets

  • Has a three-layer design: physical, frame, and packet

  • Uses Link Access Procedure-Balanced protocol (based on HDLC) at the frame layer

  • Provides for significant error checking, a drawback given today’s much improved transmission media

  • Was not designed with multimedia data in mind

Frame relay

Frame Relay

  • Utilizes two layers: physical and data link

  • Supports variable length packets

  • Requires Frame Relay Access Devices (FRADs) to disassemble and reassemble packets

  • Requires that subscribers negotiate a committed information rate with a common carrier

    • A committed burst rate can also be negotiated

  • Uses the Link Access Procedure-Function protocol (based on HDLC)

  • Does not provide for flow and error control

    • These are left to higher-level services

Atm asynchronous transfer mode

ATM – Asynchronous Transfer Mode

  • Is a universal integrated carrier of voice, data, audio, and video

  • Has a significant implementation cost and high degree of configuration complexity

  • Utilizes two layers: physical and data link

  • Is a point-to-point solution

  • Uses virtual path and virtual channel identifiers

  • Requires fixed-length packets, called cells, of 53 bytes

    • 5 bytes of header, 48 bytes of data

Atm asynchronous transfer mode1

ATM – Asynchronous Transfer Mode

Point to point protocol

Point-to-Point Protocol

  • Was developed by the IETF

  • Creates a physical serial link between two devices

  • Is inexpensive, but provides limited bandwidth capacity

  • Is the most common protocol used by dial-up modems

  • Replaced Serial Link Internet Protocol (SLIP)

  • Requires no media access control

  • Uses protocols such as PAP and CHAP for authentication

  • Has a five-phase sequence

Point to point protocol five phases

Point-to-Point Protocol – Five Phases

Trunk carrier services

Trunk Carrier Services

  • Most commonly used forms include: T-1, T-3, and Fractional T-1; the “T” stands for trunk services

  • The higher the “T” level designation, the greater the possible number of communication channels

  • Significantly more costly than point-to-point protocol, but offers much higher bandwidths

  • Digital leased lines have a range of bandwidths referred to as digital signal (DS) speeds

  • Subscribers negotiate with common carriers the level of service quality, for a cost

Trunk carrier services1

Trunk Carrier Services

Trunk carrier services2

Trunk Carrier Services

  • Usually used in conjunction with a:

    • Channel service unit (CSU):

      • The end-point of the digital link

      • Keeps the link open and active

    • Data service unit (DSU):

      • Converts signals from a connecting device into the type of signal required by the leased line

      • Connecting devices may be switches, routers, or Private Branch Exchanges

Trunk carrier services3

Trunk Carrier Services

  • Usually used in conjunction with a:

    • Private Branch Exchange (PBX):

      • A common connecting device used with a CSU and a DSU

      • An on-site switching facility used to interconnect telephones to a Public Switched Telephone Network

Trunk carrier services4

Trunk Carrier Services

Dsl digital subscriber line

DSL – Digital Subscriber Line

  • Supports simultaneous voice and data communications

  • Has a relatively low cost, with high transmission speeds

  • Is technically referred to as “xDSL”, whereby the “x” indicates the type of DSL service provided, for example:

    • ADSL

    • HDSL

    • SDSL

    • IDSL

    • VDSL

Dsl digital subscriber line1

DSL – Digital Subscriber Line

  • Two terms associated with DSL are Downstream and Upstream:

    • Downstream describes the transmitting of data from a remote location to a local device

    • Upstream describes the transmitting of data from a local device to a remote location

    • Most users of DSL primarily utilize downstream transmission

    • Downstream and upstream transmission rates can vary or be the same, depending on the DSL service

      • Asymmetric versus Symmetric DSL

Dsl digital subscriber line2

DSL – Digital Subscriber Line

  • The most common form of DSL is ADSL, or Asymmetric Digital Subscriber Line

  • With ADSL, downstream transmissions are faster than upstream transmission, for example:

    • Up to 1 Mbps downstream

    • Up to 512 Kbps upstream

  • Users located less than 18,000 feet from a local provider’s point-of-presence are good candidates for ADSL

  • DSL users also need to select between static and dynamic IP addresses

Dsl digital subscriber line3

DSL – Digital Subscriber Line

Dsl digital subscriber line4

DSL – Digital Subscriber Line

In summary

In Summary

  • There are a variety of WAN connectivity solutions

  • Circuit switched networks are most appropriate for voice

  • Packet switched networks are most appropriate for data

  • Common packet switched networks include X.25, Frame Relay, and ATM

  • Point-to-Point Protocol, while inexpensive, offers low bandwidth

  • Trunk services, particularly T-1, T-3, and Fractional T-1, are popular business WAN solutions

  • Forms of Digital Subscriber Line are becoming increasingly implemented as home and business solutions

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