X.25, Frame Relay

• X.25 is older, aimed at noisy circuits.
• It provides a description of how the attached device and the network interact.
• The idea of a network is to make it look like the two devices are communicating directly.
• But for packet switching, fragmentation must be performed.
• X.25 covers this.
• A physical level description
  • Describes the physical connection between the machine and the net.
  • Provides reliable transfer of data across the physical link.
  • LAPB - Link Access Protocol Balanced
  • A subset of HDLC
• A packet level description
  • Provides for virtual circuit service.
  • A logical channel is established between two stations and all data follows this channel.
• X.25 error control
  • All packets, data and control are carried over the same circuit.
  • Multiplexing is at layer 3
  • Layer 2 and 3 include flow and error control.
  • Error control occurs at each hop.
    • Exchange data and ack.
• Frame relay was designed to be more efficient when there are fewer network errors.
  • The error control in X.25 requires quite a bit of overhead.
  • It also imposes quite a bit of book keeping at each node in the network. (Due to virtual circuit nature)
• Frame Relay gives us
  • An extra channel for control information.
  • Error control is end to end.
  • Multiplexing is at layer 2.