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X'25 and Frame Relay

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Hence unlike a circuit switched network certain cooperation ... Example: Station D keep tracks of thee message streams depending on their virtual circuit umber. ... – PowerPoint PPT presentation

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Title: X'25 and Frame Relay


1
X.25 and Frame Relay
  • Dr. E.C. Kulasekere

2
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

3
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.

4
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

5
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

6
X.25 use of Virtual Circuits
7
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.

8
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

9
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)

10
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.

11
Frame Relay
  • Designed to to provide a more efficient
    transmission scheme than X.25
  • Frame relay is a precursor to ATM
  • Purely connectionless approach

12
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

13
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

14
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.

15
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.

16
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.
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