X'25 and Frame Relay

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X.25 and Frame Relay

• Dr. E.C. Kulasekere

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Section Objectives

• The need for X.25 Protocol as the interface
  between an end system and a packet switching
  network.
• Frame relay system
• Frame relay as an improvement to X.25

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What is X.25?

• In a packet switched network the data travels in
  packets. Hence unlike a circuit switched network
  certain cooperation is necessary between DTEs.
• This cooperation strategy is embodied in the X.25
  interface standard.
• This is almost universally used to interface
  packet switched networks.

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X.25

• Interface between host and packet switched
  network
• Almost universal on packet switched networks and
  packet switching in ISDN
• Defines three layers
• Physical
• Link
• Packet

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X.25

• Physical level
• This deals with the physical interface between
  the attached station (computer, terminal).
  Version of X.21 standard is used.
• Link level
• The link level is responsible for reliable
  transmission of information using the physical
  link.
• Packet level
• Establishment of the logic channel or virtual
  circuit

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X.25 use of Virtual Circuits
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X.25 use of Virtual Circuits

• Virtual Call
• Dynamically established
• Permanent virtual circuit
• Fixed network assigned virtual circuit
• Not a one to one link as in circuit switching but
  a virtual/logical link that can connect multiple
  DTEs.
• Example Station D keep tracks of thee message
  streams depending on their virtual circuit umber.

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X.25 Multiplexing

• DTE can establish 4095 simultaneous virtual
  circuits with other DTEs over a single DTC-DCE
  link
• Packets contain 12 bit virtual circuit number

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X.25 Flow and Error Control

• Each packet is uniquely numbered so duplicates
  can be discarded
• Nodes can remember packets already forwarded to
  keep network load in bounds
• Can include a hop count in packets
• X.25 involve extensive flow control and error
  control (hop-to-hop)

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X.25 Characteristics

• Call control packets, in band signaling
• Multiplexing of virtual circuits at layer 3
• Layer 2 and 3 include flow and error control
• Considerable overhead
• Not appropriate for modern digital systems with
  high reliability. Since with the new technology
  the overheads used for X.25 cannot be justified.

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Frame Relay

• Designed to to provide a more efficient
  transmission scheme than X.25
• Frame relay is a precursor to ATM
• Purely connectionless approach

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Frame Relay Characteristics

• Call control carried in separate logical
  connection
• Multiplexing and switching at layer 2
• Eliminates one layer of processing
• No hop by hop error or flow control
• End to end flow and error control (if used) are
  done by higher layer
• Single user data frame sent from source to
  destination and ACK (from higher layer) sent back

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FR Advantages and Disadvantages

• Lost link by link error and flow control
• Increased reliability makes this less of a
  problem
• Streamlined communications process
• Lower delay
• Higher throughput
• ITU-T recommend frame relay above 2Mbps

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How does Frame relay differ from X.25?

• Frame relay is much simplified, compared to X.25.
  
• Frame relay uses out-of-channel signaling while
  X.25 uses all in-channel control.
• In frame relay there is no "hop-by-hop" flow
  control or error control as there is in X.25. If
  a frame error is detected it is just dropped
  rather than being retransmitted.

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How does Frame relay differ from X.25?

• Similarly, in frame relay, on an end-to-end
  basis, there is no error control or flow control
  except what is provided by higher level protocols
  outside of frame relay.
• Frame relay is a two level (physical and link)
  layer protocol and the multiplexing of logical
  channels takes place at Level 2 rather than in
  the Level 3 Packet Layer as in X.25.

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Problems.

• What are the relative advantages and
  disadvantages of frame relay compared to X.25?
• Because frame relay is so much simpler than X.25,
  the processing to support switching can be
  reduced and higher data rates than for X.25, up
  to several megabits per second, can be supported.
  On the other hand, because of the lack of
  hop-by-hop flow control, the user of frame relay
  has fewer tools to manage network congestion.
  The effective use of frame relay also depends on
  the channels being relatively error free. For
  example, this is true for fiber optics, but
  probably not for most forms of broadcast,
  wireless transmission.