1. X.25
TDC 364

2. What is a PDN?
Public Data Network – a packet switched network, not private to one company
One company may own it, but they “lease” time
Many PDNs in the world!
The big question – How do you connect (interface) to a PDN? X.25

3. PDNs Users connect at DTEs
Network consists of DCEs (nodes) and communication links
Always sets of redundant links

4. FIGURE A packet switched network allows terminals at various locations to communicate with each other.
Timothy S. Ramteke Networks, Second Edition
Copyright ©2001 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

5. Statmux vs. Packet Switch
Statmux collects data from multiple devices and delivers to multiple devices
Packet switch is a switch – can forward traffic to the appropriate device
Packet switches provide more flexibility

6. FIGURE (a) In this statistical multiplexing connection, x1 may only link up with y1. (b) In this packet switched connection, x1 may connect with any of the other terminals.
Timothy S. Ramteke Networks, Second Edition
Copyright ©2001 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

7. FIGURE 17-5 A packet switched network provides many alternate paths.
Timothy S. Ramteke Networks, Second Edition
Copyright ©2001 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

8. X.25 is not alone X.25 is only one set of protocols
X.25 is a three-layer protocol (physical, data link, and network/packet) similar but not exactly the same as the OSI model
X.25 is the protocol that connects a DTE to a DCE, but only if the DTE is X.25 compatible

9. The PAD
If the DTE is a un-intelligent terminal (non-X.25 compatible), then you need a PAD (packet assembler/disassembler)
The non-compatible DTE connects first to the PAD (using X.28)
The PAD follows the X.3 protocol

10. The PAD
The PAD can be on the user premises. If so, then the PAD connects to the PDN via X.25
The PAD could also be provided by the network provider and reside at the PDN.

11. FIGURE 17-6 ITU’s X.25-related protocols.
DTE
PAD
X.3
X.25
Timothy S. Ramteke Networks, Second Edition
Copyright ©2001 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

12. Other Protocols X.75 – The protocol that interconnects multiple PDNs
X.121 – The protocol that provides the proper way of addressing networks via country codes, network codes, and terminal numbers

13. The Physical Layer of X.25
The physical layer of X.25 is defined by X.21
Since X.21 is rarely used, most installations use EIA-232 or V.35 or V.24/V.28 standard

14. The Data Link Layer
ITU-T recommends the LAP-B protocol for the data link layer
Recall: LAP-B is similar to HDLC or SDLC
Thus, LAP-B defines 3 types of frames:
Information
Supervisory
Unnumbered

15. Three Modes of Data Link Layer Operation
SNRM (Set Normal Response Mode) – a multipoint protocol is used to perform polling and selecting
SARM (Set Asynchronous Response Mode) – used for half-duplex point-to-point connections
SABM (Set Asynchronous Balanced Mode) – Full-duplex and point-to-point connection

16. Other Data Link Layer Frames
DTEs and DCEs may also issue:
UA (Unnumbered Acknowledgement)
DISC (Disconnect)
FRMR (Frame reject)
RR (Receive ready)
DM (Disconnect mode)

17. FIGURE 17-7 Exchange of frames over a DTE-DCE link.
Timothy S. Ramteke Networks, Second Edition
Copyright ©2001 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

18. The X.25 Network Layer
The third layer of X.25 is responsible for getting data through the PDN and making a connection with the two endpoints.
The data is encapsulated into the various layers as shown in the next figure

19. FIGURE 17-8 The three layers of X.25.
Timothy S. Ramteke Networks, Second Edition
Copyright ©2001 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

20. Four Types of Network Connections
Permanent virtual circuit – similar to a leased line in PSTN.
Both ends reach an agreement on the connection and a logical connection number is assigned.
All subsequent packets use this connection number

21. Four Types of Network Connections
Virtual call – similar to a POTS connection
First side issues a call request. If accepted, a call accept packet is returned.
A logical connection number is assigned and all subsequent packets use this ID
When call is complete, a clear request is issued
Temporary connection

22. Four Types of Network Connections
Fast select – allows one DTE to transfer data to another DTE without the call establishment and termination procedures
When a fast select is issued, up to 128 bytes of data may accompany the command
Receiver may respond with clear request, which means I accept your data, thanks, see you later

23. Four Types of Network Connections
Receiver may also respond with call accepted, which means I accept your data and let’s establish a connection for further data transfers
In both cases, receiver may also attach 128 bytes of immediate data

24. Four Types of Network Connections
Fast select with immediate clear – same as fast select but receiver has only one choice when it receives data – thanks for the data and see you later

25. Network Layer Packet Format
Four basic packet formats:
Data packet with 3-bit S and R counts
Data packet with 7-bit S and R counts
Interrupt packet
Control packet
Let’s examine the data packet with 3-bit S and R counts

26. FIGURE 17-11 The three-octet packet header.
Timothy S. Ramteke Networks, Second Edition
Copyright ©2001 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

27. Network Layer Packet
Note that you can have up to 12 bits for a logical channel identifier
4-bit logical group number
8-bit logical channel number
This gives 4095 possible connections between a DTE and a DCE (channel 0 is reserved for network diagnostics)

28. FIGURE 17-12 Local vs. end-end acknowledgements.
Timothy S. Ramteke Networks, Second Edition
Copyright ©2001 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

29. FIGURE 17-13 Establishing a connection using the 3 layers of X.25.
Timothy S. Ramteke Networks, Second Edition
Copyright ©2001 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

30. FIGURE 17-17 (a) A typical BSC network
FIGURE (a) A typical BSC network. (b) Implementing a BSC network over a packet switched network introduces many new benefits.
Timothy S. Ramteke Networks, Second Edition
Copyright ©2001 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.