X.25 is a packet-switched network, developed by ITU-T as an interface between data terminal equipment DTE and data circuit-terminating equipment DCE for terminal operation in packet mode on public data network. It is an end-to-end protocol, but actual movement of packet through the network is invisible to the user.The user sees the network as a cloud through which each packet passes on its way to the receiving DTE. It defines how a packet-mode terminal can be connected to a packet network for exchange of data. It describes procedures necessary for establishing, maintaining and terminating connections. It uses virtual network approach to packet switching, SVC and PVC and uses asynchronous TDM to multiplex data...

1. Prof. M
adhumita Tamhane
X.25
Packet Switching

2. Prof. M
adhumita Tamhane
X.25
» Packet-switching wide area network developed by ITU-T.
» X.25 is subscriber network interface (SNI) protocol.
» Defines how data terminal equipment (DTE)
communicates with the network to send packets over it
using data circuit-terminating equipment (DCE).
» It uses virtual circuit approach to packet switching (SVC
and PVC).
» Uses asynchronous TDM to multiplex packets.
» It describes procedure for establishing, maintaining and
terminating connections.

3. Prof. M
adhumita Tamhane
X.25 Layers
» Physical layer, frame layer and
packet layer define function at
physical, data link and network
layers respectively of OSI model.

4. Prof. M
adhumita Tamhane
X.25 Layers
» Physical Layer :
» X.25 specifies a protocol X.21 defined by ITU-T.
» X.25 supports all other physical layer protocols too like
EIA-232 etc as X.21 is similar to them.
» Frame Layer:
» X.25 provides data link controls using a bit-oriented
protocol called Link Access Procedure-Balanced (LAPB).
» LAPB is subset of HDLC, comprising of point-to-point and
asynchronous balance mode.
I-frame: User data
S-frame: Empty
U-frame: Control data

5. Prof. M
adhumita Tamhane
X.25 Layers-Frame Layer
» X.25 being point-to-point and asynchronous balance mode
support only two addresses at frame layer:
» 00000001 for a command issued by a DTE and the
response to this command.
» 00000011 for a command issued by a DCE and the
response to this command.

6. Prof. M
adhumita Tamhane
Frame Layer-Categories of frame
» I-frame: Used to encapsulate packet layer protocol (PLP)
packets from the network layer.
» S-frame:Flow and error control in frame layer.
» U-frame: To set up and disconnect the links between a
DTE and a DCE.
» Most common U-frame packets are Set Asynchronous
Balance Mode (SABM), Un-numbered Acknowledgement
(UA) or Disconnect (DISC).

7. Prof. M
adhumita Tamhane
Frame Layer-Phases
» Link Setup:
» Link between DTE and DCE must be setup before
packets from packet layer must be transferred.
» Either DTE or DCE can setup link by sending an SABM
frame.
» Responding party sends a UA frame to show link setup.
» Transferring Data:
» After link establishment, DTE and DCE can send and
receive network layer packets(data and control) using
I-frame and S-frame.
» Link Disconnect:
» One of the parties can issue a disconnect(DISC) frame
to request disconnect.
» Other party can answer with a UA frame.

8. Prof. M
adhumita Tamhane
Packet Layer
» Network layer in X.25 is called packet layer protocol PLP.
» Responsible for connection establishment, data transfer, and
connection termination.
» Responsible for creating virtual circuit, negotiating network
services between two DTEs.
» Unlike frame layer, packet layer is responsible for end-to-end
connection(DTE to DTE).
» X.25 uses flow and error control at two levels:
» between DTE and DCE by frame layer
» between two DTEs by packet layer.

9. Prof. M
adhumita Tamhane
Virtual Circuits
» X.25 is packet-switched
virtual circuit network.
» Virtual circuit created at
network later.
» Physical layer between
DTE and DCE can carry
many virtual circuits at
network layer.
» Each VC responsible for
carrying either data or
control information
(In-Band-Signalling).

10. Prof. M
adhumita Tamhane
Virtual Circuit Identifiers
» In X.25, Virtual Circuit Identifiers are called Logic Channel
Number.
» VC establishment between two DTEs creates two LCNs:
» One defining VC between a local DTE and a local DCE.
» Other defining VC between remote DTE and remote DCE.

11. Prof. M
adhumita Tamhane
LCNs in X.25
» Two different LCNs helps in making LCN domain local.
» This allows set of LCNs for each local connection to be small
and hence short LCN field.
» Also same LCNs can be reused between two different DTE-DCE
links without any global confusion.
!
» Global LCN would require a larger set of LCNs and hence large
LCN field.
» Moreover LCNs can not be reused as global uniqueness is lost.

12. Prof. M
adhumita Tamhane
PVCs and SVCs in X.25
» X.25 uses both permanent PVC and switched virtual circuits SVC.
» PVCs are similar to telephone leased line and are permanently
assigned and established by X.25 network providers.
» SVCs are established at each session by network layer using a
control packet.
» After connection establishment, both DTE-DCE links are assigned
LCNs.
» After data transfer, VC is disconnected and LCNs are invalid.
» VC establishment and release at network layer are different
from link set-up and disconnect at frame layer.

13. Prof. M
adhumita Tamhane
SVC setup in X.25
» A link is setup between local DTE and DCE and also between
remote DTE and DCE.
» A virtual circuit is established between a local DTE and and the
remote DTE.
» Data are transferred between the two DTEs.
» The virtual circuit is released.
» The link is released.

14. Prof. M
adhumita Tamhane
LCN Assignment
» X.25 allows unto 4096 (212) LCNs.
» One-way LCNs are used for simplex connection.
» Two-way LCNs are used for duplex communications.

15. Prof. M
adhumita Tamhane
PLP Packets
» PLP packet format has 3 or 4 bytes of header and an
optional information frame.

16. Prof. M
adhumita Tamhane
PLP Packets
» General Frame Identifier(GFI): 4 bits
» Bit 1: Q (qualifier) bit - defines source of control
information:
» 0-PLP
» 1- other higher level protocol
» Bit 2: D (delivery) bit - defines which device should
acknowledge the packet:
» 0-local DCE
» 1- remote DTE
» Last two bits: indicates size of sequence number field:
» 01: 3-bit sequence numbering- modulo 8(0 to 7)
» 10: 7-bit sequence numbering- modulo 128(0 to 127)

17. Prof. M
adhumita Tamhane
PLP Packets
» Logical Channel Number(LCN): 12 bits
» Identifies virtual circuit chosen for a given transmission.
» LCN was originally given hierarchal sense by dividing LCN as:
» 4 bits- LGCN logical group channel number
» 8 bits- logical channel number
» Packet Type Identifier(PTI): 8/16 bits
» Defines type of packet.
» Contents of this field depends on packet type.
» Two categories of packet:

18. Prof. M
adhumita Tamhane
PLP-Data Packets
» Transmits user data.
» Header and user data field.
» Packet type identifier PTI consists of packet send P(S) and
packet receive P(R) for flow and error control.
» P(S) indicates sequence number of packet being sent.
» P(R) is sequence number of next packet that the receiver is
expecting to receive.
» This field is also used to piggyback acknowledgement to data
packets when both parties have data to send.
» Two formats for information packets.
» Short format: P(S) and P(R) are 3 bits long.(GFI—QD01)
» Sequence numbers from 0 to 7.(GFI—QD10)
» Long format: P(S) and P(R) are 7 bits long.
» Sequence numbers from 0 to 127.

19. Prof. M
adhumita Tamhane
PLP-Data Packets
» M bit expresses if more packets belonging to same message.
» M=1 —More packets of same message to follow.
» M=0 —Current packet is last packet of a message.
» 0 in LSB of third byte indicates information packet.
Q D 1 0 LCN
LCN
P(R) 0
P(S) M
User Data
Seven bit sequence number
Q D 0 1 LCN
LCN
P(R) M P(S) 0
User Data
Three bit sequence number

20. Prof. M
adhumita Tamhane
Control Packets-RR, RNR, REJ
» Receive Ready (RR), Receive Not Ready (RNR) and Reject (REJ)
packets have headers with 01 in two LSBs of third row.
» Only for flow and error control—
» Carry no data.
» No P(S).
» Packet type:—
Q D 0 1 LCN
LCN
P(R) Packet
Type 0 1
Q D 1 0 LCN
LCN
Packet Type 0 1
P(R) 0
Three bit sequence number Seven bit sequence number

21. Prof. M
adhumita Tamhane
Control Packet Type
» RR (000):— Receive ready means that DTE or DCE is ready to
receive more packets.
» It also acknowledges receipt of all {P(R)-1} data packets sent
before.
» Expecting to receive data packet numbered P(R).
» RNR (001):— Receive not ready means that DTE or DCE is not
ready to receive more packets.
» It also acknowledges receipt of all {P(R)-1} data packets sent
before.
» REJ (010):— Reject means error detected in packet numbered
P(R).
» Other party must resend all packets from P(R) onwards.
» Go-Back-n error control.
» Expecting to receive data packet numbered P(R).

22. Prof. M
adhumita Tamhane
Other Control Packets
» Identified by 11 in two LSBs of third row.
» May carry information only for control purposes and not
data.
» Only one size of header, as no P(R) and P(S) fields.
» Packet type 6 bits long, giving 64 different possible
functions.
» Few functions described are:—
Q D 0 1 LCN
LCN
Packet Type 1 1
Additional Information
Unnumbered Packets

23. Prof. M
adhumita TamhaneDTE to DCE DCE to DTE Type Discription
Local
Call
Request
Remote
Incoming
Call
0 0 0 0 1 0
Request connection establishment between two
DTEs. Facilities are optional, on agreement
between users and networks. Contractual options
like incoming/outgoing calls barred, flow control
parameter negotiations, fast select, reverse
charging, etc..
Remote
Call
accepted
Local
Call
Connected
0 0 0 0 1 1
Indicated acceptance of packets by called system.
Sent in response to call request and incoming call
packets.
Header Address
Length
DTE Address Facility Length Facilities
Header Address
Length
DTE Address Facility Length Facilities
Other Control Packets

24. Prof. M
adhumita TamhaneDTE to DCE DCE to DTE Type Discription
Clear
Request
Clear
Indication
0 0 0 1 0 0
Used at end of an exchange to disconnect(clear)
connection by DTE or DCE. Can also be used by
remote DTE to respond negatively to incoming call
if unable to accept.
Clear
Confirm
Clear
Confirm
0 0 0 1 0 1 Sent in response to clear indication.
Interrupt Interrupt 0 0 1 0 0 0
Under unusual circumstances to break into
exchange to get attention, by DTE/DCE. ALERT.
E.G.user doesn’t get ACK/NACK for long.
Interrupt
Confirm
Interrupt
Confirm
0 0 1 0 0 0 Confirms receipt of interrupt packet. Header sent.
Header Cause Diagnosis
Header
Other Control Packets
Header

25. Prof. M
adhumita TamhaneDTE to DCE DCE to DTE Type Discription
Reset
Request
Reset
Indication
0 0 0 1 1 0
To reset sequence numbers in data exchange over a
particular VC if wants reinitialisation. VC remains
active but transmission re-starts from a
predetermined point. Packets renumbered from 0.
Reset
Confirm
Reset
Confirm
0 0 0 1 1 1 Confirms reset process
Restart
Request
Restart
Indication
1 1 1 1 1 0
Restarts all VCs created by DTE. Different from
reset(which only resets packet numbering over
existing VCs).Establishes new VCs. P(S) from 0.
Restart
Confirm
Restart
Confirm
1 1 1 1 1 1 Confirms restart request. Header sent.
Header Cause Diagnosis
Header
Other Control Packets
Header Cause Diagnisis

26. Prof. M
adhumita TamhaneDTE to DCE DCE to DTE Type Discription
Registration
Request
1 1 1 1 0 0
Allows on-line registration of new user to the
network
Registration
Confirm
1 1 1 1 0 1 Confirms registration
Header Address Length DTE Address Registration Length Registration
Header Address Length DTE Address Registration Length Registration
Other Control Packets

27. Prof. M
adhumita Tamhane
X.25-Call Establishment Packet

28. Prof. M
adhumita Tamhane
X.25-Call Establishment

29. Prof. M
adhumita Tamhane
Data Transfer phase D=0

30. Prof. M
adhumita Tamhane
Data Transfer phase D=1

31. Prof. M
adhumita Tamhane
Acknowledgement

32. Prof. M
adhumita Tamhane
INTERRUPT – Urgent Data after window exhausted

33. Prof. M
adhumita TamhaneCall
Clearing

34. Prof. M
adhumita Tamhane
Grouping of logical channels

35. Prof. M
adhumita Tamhane
X.121 protocol
» X.25 does not define Global addressing for call setup to access
remote DTE before SVC establishment.
» X.25 uses X.121(by ITU-T) to globally address DTEs connected to
public data networks.
» 14 digit address format has —
» First 4 digits Data Network Identification Code (DNIC) defines
specific network with
» 3 digits for country
» 1 digit for network inside country
» Next 10 digits National Terminal Number(NTN) defines DTEs
inside a particular network.
DNIC NTN
Country Code Network
Number
4 digits 10 digits
1 digit3 digits

36. Prof. M
adhumita Tamhane
Tripple X protocol
» X.3, X.28 and X.29 collectively are called Triple X Protocols.
» Used to connect a dumb terminal to X.25 network.
» Dumb terminal can not understand X.25.

37. Prof. M
adhumita Tamhane
Tripple X protocol
» X.3 :-
» Defines packet assembler/disassembler(PAD).
» Needed to connect a character oriented(dumb) terminal to
an X.25 network.
» A PAD buffers dumb-terminal character keys and assembles
them into X.25 packet.
» At receiver, it disassembles the X.25 packet so as to be
displayed on the screen or printed on printer.
» X.3 defines 22 parameters to be used by PAD.
» X.28 :-
» Defines rules for communication between a dumb terminal in
(Asynchronous character mode) and a PAD.
» Defines commands for dumb terminal or PAD for various X.25
activities.

38. Prof. M
adhumita Tamhane
Tripple X protocol
» X.29 :-
» Defines relationship between a PAD and a remote terminal.
» Using X.29, a remote terminal can set some parameters in
packet mode in PAD.
» Defines how the PAD encapsulates characters and control
information in X.25 packets.
» Example: Echo.