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					Switching

 Chapter 7
Switching Techniques
            1. Circuit Switching
              •   Dedicated resource
            2. Packet Switching
              • Shared resource
              1. Virtual circuits
              2. Datagram
                   Circuit Switching
• A complete circuit (route or path) between source and destination nodes
  is established before the data can be transmitted.
• The circuit between the source and destination can be established on
  any communication link/transmission medium (telephone lines, coaxial
  cable, satellite link, microwave link, etc.).
• The following three steps are required to establish the connections:
1. Connection Setup. This requires a subscriber’s request for service,
    identifies the terminal, searches and grabs a circuit, informs the
    required terminal and then receives its response.
2. Data Interchange. The established link is held during the transmission
    of data between source and destination and sends out the billing
    information (depending on the distance and duration of the
    connection) to the subscribers.
3. Connection Termination. After the communication is completed, the
    link channels and shared devices are released.
               Circuit Switching
• Problems
1. This allocated capacity is idle when the session has
   nothing to send. Thus not efficient.
2. Circuit switching usually uses a fixed data rate
   (e.g., 64 kbps)
Packet Switching
               Packet Switching
• Packets are parts of messages and include control bits
  (for detecting transmission errors).
• Networks break the message into blocks (or packets).
• The message is divided into blocks (or packets) of fixed
  size with its own control information regarding the
  routing, etc., across the network.
• Route chosen on packet-by-packet basis.
• Different packets may follow different route, results in
  out-of-sequence arrival at destination.
• The receiver, after receiving the packets out of
  sequence, has to arrange the packets in the same order
  as they were transmitted from the source.
                 Packet Switching
• If any node receives a garbled packet, it will request the
  sending node to transmit the same packet again.
• The acknowledgement will be sent upon receiving the last
  packet.
• If the destination node does not receive all the packets within
  the specified time, it sends a request for the data (instead of
  acknowledgement) to the source about the missing packets.
• The packet switching technique allows the switching nodes to
  transmit the packets without waiting for a complete message
  and also allows them to adjust the traffic they have, thus
  minimizing the resource requirements of the nodes.
  If any particular node is already heavily loaded, it will reject
  the packets until its load becomes moderate.
                Packet Switching
1. Datagram Packet Switching (Connectionless service) :
I. No connection setup is required.
II. Packets are routed from entry to exit node independently of
     each other.
III. Each packet may follow a different path through the
     network. Packets may arrive at the exit node out of
     sequence.
IV. Packets transmitted by an entry node may never reach the
     exit node.
V. No guarantee of sequenced or reliable delivery is made.
VI. Can provide connection-oriented service if the exit node is
     capable of re-sequencing out-of-order packets and
     requesting any missing packets from the entry node.
              Packet Switching
2. Virtual Circuit Packet Switching (Connection-
     oriented service) :
I. A connection needs to be established and torn
     down between the source and destination.
II. In the connection setup procedure, a path through
     the network is selected.
III. All packets travel from the entry node to the exit
     node through this path.
IV. Reliable and Sequenced delivery of data packets to
     the destination host are guaranteed.
                           X.25
• In the case of packet-switching networks, the attached
  stations must organize their data into packets for
  transmission. This requires a certain level of cooperation
  between the network and the attached stations.
• This cooperation is embodied in an interface standard. The
  standard used for traditional packet-switching networks is
  called X.25.
• ITU-T standard for interface between host and packet
  switched network.
• Defines three layers
   – Physical
   – Link
   – Packet
                   X.25 Physical
• The physical level deals with the physical interface
  between an attached station computer, terminal and
  the link that attaches that station to the packet-
  switching node. Two ends are distinct
   – Data Terminal Equipment DTE (user equipment)
   – Data Circuit-terminating Equipment DCE (node)
• It makes use of the physical-level specification in a
  standard known as X.21
                     X.25 Link
• The link level standard is referred to as LAPB (Link
  Access Protocol - Balanced).
• The link level provides for the reliable transfer of
  data across the physical link, by transmitting the
  data as a sequence of frames.
                    X.25 Packet
• The packet level provides a virtual circuit service. This
  service enables any subscriber to the network to set up
  logical connections, called virtual circuits, to other
  subscribers.
• All data in this connection form a single stream between
  the end stations.
• Established on demand.
• What is important for an external virtual circuit is that
  there is a logical relationship, or logical channel,
  established between two stations, and all of the data
  associated with that logical channel are considered as
  part of a single stream of data between the two stations.
                          X.25 Packet
• X.25 virtual circuits is shown in
  Figure.
• In this example, station A has a
  virtual circuit connection to C;
  station B has two virtual circuits
  established, one to C and one to
  D; and stations E and F each have
  a virtual circuit connection to D.
• As an example of how these
  external virtual circuits are used,
  station D keeps track of data
  packets arriving from three
  different workstations (B, E, F) on
  the basis of the virtual circuit
  number associated with each
  incoming packet.
                                  X.25 Packet
•   User data are passed down to X.25
    level 3, which appends control
    information as a header, creating a
    packet. This control information
    serves several purposes, including
    identifying by number a particular
    virtual circuit with its associated data,
    and providing sequence numbers that
    can be used for flow and error control
    on a virtual circuit basis.
•   The entire X.25 packet is then passed
    down to the LAPB entity, which
    appends control information at the
    front and back of the packet, forming
    a LAPB frame (Link Access Protocol–
    Balanced). Again, the control
    information in the frame is needed for
    the operation of the LAPB protocol.
                 Issues with X.25
• key features include:
   – multiplexing of virtual circuits at layer 3.
   – layers 2 and 3 include flow and error control.
• hence have considerable overhead.
• not appropriate for modern digital systems with high
  reliability.
                              Frame Relay
•    Today's networks employ reliable digital transmission technology over high-
     quality, reliable transmission links, many of which are optical fiber. In addition,
     with the use of optical fiber and digital transmission, high data rates can be
     achieved. In this environment, the overhead of X.25 is not only unnecessary but
     degrades the effective utilization of the available high data rates.
•    Frame relay is designed to eliminate much of the overhead that X.25 imposes on
     end user systems and on the packet-switching network. The key differences
     between frame relay and a conventional X.25 packet-switching service are:
1.     Call control signaling, which is information needed to set up and manage a
       connection, is carried on a separate logical connection from user data. Thus,
       intermediate nodes need not maintain state tables or process messages relating to
       call control on an individual per-connection basis.
2.     Multiplexing and switching of logical connections takes place at layer 2 instead
       of layer 3, eliminating one entire layer of processing.
3.     There is no hop-by-hop flow control and error control. End-to-end flow control
       and error control are the responsibility of a higher layer, if they are employed at
       all.
4.     Thus, with frame relay, a single user data frame is sent from source to
       destination, and an acknowledgment, generated at a higher layer, may be carried
       back in a frame. There are no hop-by-hop exchanges of data frames and
       acknowledgments.

				
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