X.25 TDC 364. What is a PDN? Public Data Network – a packet switched network, not private to one...

X.25

TDC 364

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

PDNs

• Users connect at DTEs

• Network consists of DCEs (nodes) and communication links

• Always sets of redundant links

Timothy S. RamtekeNetworks, Second Edition

Copyright ©2001 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.

FIGURE 17-3 A packet switched network allows terminals at various locations to communicate with each other.

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




FIGURE 17-4 (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.




FIGURE 17-5 A packet switched network provides many alternate paths.

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

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

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.




FIGURE 17-6 ITU’s X.25-related protocols.

DTE

PAD X.3

X.25

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

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

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

Three Modes of Data LinkLayer 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

Other Data LinkLayer Frames

• DTEs and DCEs may also issue:– UA (Unnumbered Acknowledgement)– DISC (Disconnect)– FRMR (Frame reject)– RR (Receive ready)– DM (Disconnect mode)




FIGURE 17-7 Exchange of frames over a DTE-DCE link.

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




FIGURE 17-8 The three layers of X.25.

Four Types ofNetwork 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


• 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


• 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


• 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


• 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

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




FIGURE 17-11 The three-octet packet header.

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)




FIGURE 17-12 Local vs. end-end acknowledgements.




FIGURE 17-13 Establishing a connection using the 3 layers of X.25.




FIGURE 17-17 (a) A typical BSC network. (b) Implementing a BSC network over a packet switched network introduces many new benefits.

Date post:	20-Dec-2015
Category:	Documents
View:	227 times
Download:	1 times

X.25 TDC 364. What is a PDN? Public Data Network – a packet switched network, not private to one...

Documents