This topic is based on the IoT network Protocol which is based on the X.25 network protocol.In this topic we discussed about X.25 introduction,basic features and work performance of the X.25..

1. X.25
GUIDED BY:- NAME-RAJKUMAR MONDAL
MS.KANIKA THAKUR COURSE-BSCIT
E.NO-A35404916005

2. OUTLINE
 1.BASIC FEATURE OF X.25
 2.THREE LAYERS OF X.25
 3.FRAME AND PACKETS FORMATS OF
X.25
 4.VIRTUAL CIRCUIT OF X.25
 5.MULTIPLEXING IN X.25

3. INTRODUCTION-X.25
 X.25 is a packet-switched network developed by ITU-T in 1976.
 It defines how a packet mode terminal can be interfaced to a packet
network for data communication. User machine is termed as DTE. Packet
switching mode to which a DTE is attached as DCE.


4. X.25 LAYERS
 X.25 has three layers:
 1.PHYSICAL LAYER
 2.FRAME LAYER
 3.PACKET LAYER

5. X.25 LAYERS

6. X.25 LAYERS
 Physical Layers:- Deals with the physical interface b/w an attached station the link
that attaches that station to the packet-switching node. X.25 is the most commonly
used physical layer standard.
 Frame Layer:-Facilities reliable transfer of data across the physical link by
transmitting the data s a sequence of frames.Uses a subset of HDLC known as link
access protocol balanced(PALB) bit oriented protocol.
 Packet Layer:-Responsible for end-to-end connection b/w two DTEs.
Functions performed are:
1.Establishing Connection 3.Terminating a connection
2.Transferring Data 4.Error and flow control
With the help of x.25 packet layer data are transmitted in packets over external virtual
circuit.

7. X.25 Interface

8. X.25 FRAME FORMAT
User data
Layer 3 Header
LAPB Header LAPB Header

9. X.25 Frame Format

10. PACKET LAYER

11. VIRTUAL CIRCUIT
 Virtual circuit are created by the packet layer.
 Virtual circuit identifier is to knowns as LCN. Several virtual
circuits through the same link using in band signaling.

12. TYPES OF VIRTUAL CIRCUITS
 1.PVC:-It is a fixed, similar to lased line, established by the network provider.
 Data transfer occurs as with virtual calls. No call setup or termination is required.
 2.svc:- Svc are established in each session. Sequence to be followed.
(a)Link are set up b/w the local DTE-DCE and also the remote DTE-DCE
(b)A virtual circuit is set between the local and remote DTEs
(c)Data transfer are performed between the DTEs and virtual circuit are released
(d)The link is disconnected

13. PLP PACKETS
 PLP packets are of two Types
 1) Data packets 2) Control packet
 Control packets are two types:-
 1) RR,RNR,REJ Packets 2) Other packets

14. X.25 PACKET FORMAT
 User data are broken into blocks of some maximum size ,
and a 24-bit or 32-bit header is appented to each block to
form a data packet.
 Uses sliding-window protocol with piggybacking for flow
control
 Go-back-N for error control.

15. X.25 PACKET FORMAT
 X.25 is also transmits control packet related to the
establishing, maintenance and termination of virtual circuits.
Each control packet includes:
(a)The virtual circuit number.
(b)The packet type(call request, call accepted, Call confirm,
intteruppt, reset, etc).
(c)Additional control information specific to the type of packet.

16. X.25 Packet Format

17. X.25 MULTIPLEXING
 One of the most important services provided by X.25.
 A DTE is allowed to establish up to 4095 simultaneous
virtual circuits with other DTEs over a single DTE-DCE link.
 Each packet contains a 12-bit virtual circuit number .
-----Expressed as a 4-bit logical group number plus an 8-bit
logical channel number .
 The DTE-DCE link provides full duplex multiplexing..