X.25

1
X.25

• Most widely used today
• an interface between DTE and DCE for terminal
  operation at the packet mode on public data
  network.
• A packet switching protocol used in a wide area
  network
• use virtual circuit and asynchronous TDM

2
X.25 Layers

• Error detection and correction in both the data
  link and the network layers
• Network layer establishing the connection
• transferring the data
• terminating the connection
• Layers

3
Encapsulation of PLP packet
4
Packet Layer Protocol

• PLP packets Information packets and Control
  packets
• Information Packets (I-packets)
• transmit user data
• consists of a header and a user data field
• short and long packets long packets to support
  facilities with long delays
• The last bit in the header 0 for I-packets

5
Packet Layer Protocol

• I-packet fields
• General format identifier (GFI) field 4 bits
• Q bit not defined in the standard. Users define
  two types of data.
• D bit for packet sequencing.
• Mod the length of the header. (01 the header
  short, 10 long)
• Virtual circuit identification fields
• Logical channel group number (LCGN) 4 bits
• Logical channel number (LCN) 8 bits
• total 12 bits to identify the virtual circuit for
  a transmission
• Control field
• P(S) and P(R) packet sequence numbers for flow
  and error control
• P(S) for packet send, P(R) for packet receive
  sliding window ARQ

6
Packet Layer Protocol

• Control Packets (C-packets)
• flow and error control
• connection, termination and management control
• Category I (for flow and error control)
• RR Receive Ready - the station is ready to
  receive more packets
• RNR Receive Not Ready
• REJ Reject - an error in the packet (go-back-n
  ARQ)

7
Packet Layer Protocol

• Control Packet (C-packet)
• Category II
• for connection, termination and management
  control
• control packet types Table 16.1

8
Control Packets

• Category II
• Figure 16.5
• Call Request/Incoming Call
• request the establishment of a connection between
  two DTEs
• Call Accepted/Call Connected
• indicate the acceptance of the requested
  connection by the called system

9
Control Packets

• Category II (contd)
• Clear Request/Clear Indication
• to disconnect the connection at the end of an
  exchange
• Clear Confirm
• sent in response to the clear indication
• and More...

10
Packet Layer Protocol

• Complete Packet Sequence
• A message size may be too small or too large for
  a network and not compatible with a network.
• X.25 mechanism to break up a long message among
  multiple packets but still keep those packets as
  a single contiguous transmission
• A packets at least one additional packets needed
  to carry the remainder of a message (M1,D0)
• B packets either stand alone or final packet
  (M0, D1)
• M1 final packet in the sequence
• D1 need ACK.

11
Packet Layer Protocol

• Complete Packet Sequence (contd)

12
Packet Layer Protocol

• Virtual Channel ID Numbers
• up to 4096 multiplexed channels to be identified
  between each DTE and DCE
• not permanent, allocated dynamically
• the calling and called hosts use different numbers