Sunday, August 5, 2012

How To Design A Wide Area Network

WAN Overview:
WANs are usually required for high volume, long-distance data traffic.

To implement WANs, you can use the following transmission media:
  • PSTN (Public switched telephone network)
  • High-speed, high-bandwidth dedicated leased circuits
  • High speed fiber-optic cable
  • Microwave transmission links
  • Satellite links
  • Wireless radiated media (radio frequency)
  • The internet
There are two types of WANs:
  • Enterprise networks: When a network connects a company’s branch offices and divisions, it becomes an enterprise- wide network. For example, a corporation may have sites on every continent, all of which are interconnected to form one wide area network.
  • Global networks: When a network spans several countries and continents and includes many types of organizations and individuals, it can be labeled  global. These networks serve multinational corporations and scientific, academic and military establishments. The internet, often called the “network of networks” fits that definition.
Public Network Services:
    Two of the most popular methods are the PSTN and the internet.

    PSTN:
    •  PSTNs were originally designed exclusively for telephones but have become highly sophisticated, able to handle different kinds of data transmission, including digital data transmission.
    • The PSTN provides a number of options for data transmissions, including services that route packets between different sites.
    Available services and some possible transmission rates include the following:
                 Service                                            Transmission Rate
                  Switched 56                                    56Kbps
                  X.25                                               56Kbps
                  T1 circuits                                       1.544Mbps
                  T3 circuits                                       44.736Mbps
                  Frame relay                                     1.544Mbps
                  SMDS                                            1.544Mbps
                  ISDN                                              1.544Mbps
                  ATM                                              44.736Mbps
    The Internet:
    It is a shared network of government agencies, educational institutions, private organizations and individuals from over a hundred nations.
    No one owns the Internet; anyone can have access to the transmission media.
    SLIP and PPP:
    • Serial Line Internet Protocol (SLIP) and Point-To-Point (PPP) are two very common protocols used to transmit IP packets over serial line and telephone connections, most often as part of a dial-up internet connection.
    • SLIP an PPP are similar protocols. But some differences are there:
    • Basically PPP is a multistage protocol signal transport mechanism. SLIP is designed for maintaining one type of traffic protocol  (TCP/IP traffic) at a moment.
    • With SLIP, you must know both the IP address assigned to you by an ISP and the IP address of the remote system your computer will be dialing into. PPP deals with these problems by negotiating configuration parameters at the beginning of your connection.
    • It also said that PPP can be  negotiate header compression. SLIP does not offer this type of compression.
    • Another things about PPP that it offers IP enhanced security  protection.
    Switching:
    • Switching is an important technique that can determine how connections are made and how data movement is handled on a WAN.
    • Data sent across the PSTN or other internetworks can travel along different paths from sender to receiver.
    Three major switching techniques are:
    • Circuit switching
    • Message switching
    • Packet switching
    Circuit switching:
    • In circuit switching, a dedicated physical connection is established between the sender and the receiver and maintained for the entire conversation.
    • PSTN uses a circuit switching system.
    • Before any two computers can transfer data, a dedicated circuit must be established between the two.
    • The sending machine requests a connection to the destination, after which the destination machine signals that it is ready to accept data.
    • The data is then sent from the source to the destination and the destination sends acknowledgements back to the source.
    • When the conversation is finished, the source sends a signal to the destination, indicating that the connection is no longer needed and disconnect itself.
    • A advantage of circuit switching is that the dedicated transmission channel the machine establish provides a guaranteed data rate.
    • Since the channel is always available, it does not need to be requested again.
    • Disadvantages: It is often an inefficient use of the transmission media.
    • Dedicated channels require more bandwidth than non-dedicated channels, so transmission media can be expensive.
    • Also, this method can be subject to long connection delays; it may take several seconds to establish the connection.
    Message switching:
    • Message switching is unlike circuit switching in that it does not establish a dedicated path between two communicating devices.
    • Instead, each message is treated as an independent unit and includes its own destination and source addresses.
    • Each complete message is then transmitted from device to device through the internetwork.
    • Each intermediate device receives the message, stores it until the next device is ready to receive it and then forwards it to the next device.
    • For this reason, a message switching network is sometimes referred to as a store-and-forward network.
    Advantages:
    • It provides efficient traffic management.
    • It reduces network traffic congestion. The intermediate devices are able to store messages until a communications channel becomes available.
    • Its use of data channels is more efficient than circuit switching.
    • It provides asynchronous communication across time zones.
    Disadvantages:
    • The delay introduced by storing and forwarding complete messages make message switching unsuitable for real-time applications.
    • It can be costly to equip intermediate devices with enough storage capacity to store potentially long message.
    Packet switching:
    • In packet switching, message are broken up into packets, each of which includes a header with source, destination and intermediate node address information.
    • Individual packets don’t always follow the same route; this is called independent routing.
    Independent routing offers two advantages:
    Bandwidth can be managed by splitting data onto different routes in a busy circuit
    If a certain link in the network goes down during the transmission, the remaining packets can be sent through another route.

    Methods of packet switching:
    1. Datagram packet switching
    2. Virtual-circuit packet switching
    Datagram packet switching: In a datagram packet-switched network, a message is divided into a stream of packets.
    Each packet is separately addressed and treated as an independent unit with its own control instructions
    The switching devices route each packet independently through the network, with each intermediate node determining the packet’s next route segment.

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