Telecommunications - X25 Packet layer

See the following documents for detailed information:

Glossary

Level 3 packet types

Call Setup

Call Clear

Cause and Diagnostic Codes

The packet header

Multiplexing

Routing

Addresses

Datagram v virtual call network

Optional user facilities - Subscription Facilities

Optional user facilities - Per Call parameters

data transfer

Quality of Service (QOS) parameters

OSI Network Layer

common packet layer problems on Public Network


1 - Introduction

The X.25 Packet layer is the highest of the 3 layers that form X25. It performs the following functions:

X.25 Packet layer can form part of the network layer in a system that supports the OSI 7 layer model.


1.1 Relationship between packet layer and link layer


The packet layer defines the packets exchanged between the DTE and the DCE.

Packets are carried in the Information field of frames defined by layer 2 of X.25

In most, but not all, cases the packet is passed through the network with little modification (ie end-to-end), Unlike the frame header/trailer which is regenerated for each link.

Some parts of the packet can be modified as it passes through the network, for instance, LCN, facilities, etc.

1.2 - Trunk failure



1.2 - Link level failure

Effects of bit and link levels on packet level

Ideally the states of levels 1, 2 and 3 should be independent, however it could be very expensive if a failure at the bit or link level caused all calls to hang, so the network will clear all calls if it detects a failure, this will only happen if the network sees a change of status of the physical interface, or if it sees a DISC frame, of if it is unable to deliver a frame to the DTE.


2 - Procedures

2.5 - Example sending a data packet

Customer Switch Switch Customer
--- I frame
RR frame --
--- I frame
RR frame --
--- I frame
RR frame --
--- I frame
RR frame --
I frame ---
---RR frame
I frame ---
--- RR frame
I frame ---
--- RR frame
I frame ---
--- RR frame

The example shows a Data packet being sent between two customers,

Note:

3.2 - Services provided by network layer


4 - Packet headers

All packets contain a 3 byte header and some packets have an optional data field:

<-frame hdr-> GFI LCGN LCN packet type optional data field <-frame trailer ->

5.9.1 Packet Level initialisation

Once the link is established then the packet level is initialised. This can be done by either end,

Restart by network:

customer network

RESTART IND 07FA cause 07 network operational
RESTART CONF diagnostic FA line status change

Restart by customer:

customer network

RESTART REQ 00xx

RESTART CONF

Restart collision:

RESTART REQ

RESTART IND


This ensures level 3 starts in a tidy state with all calls cleared, and keeps level 2 and level 3 independent.

Following a link level failure, calls should not be made or accepted until a Restart exchange has occurred. see section on level 2/3 interface.

Restart - Restart Conf exchanges can be done at any time and will have the effect of clearing all calls on the link.

Valid Restart Causes for the network to send are as follows:

Hex cause
01 local procedure error
03 Network Congestion
07 Network Operational

Note: RESTART and RESTART CONF are the only packet types which affect all calls on the link and so they always use GFI=10, LCN=0, see section on logical channel numbers for details.


7 - Logical Channels

When X.25 was being defined, a basic decision had to be made about whether it should support datagrams or virtual calls:

Datagram network

This is conceptually the simplest type of packet switched network, each packet is self contained and must contain the user data and any information needed by the network to deliver the packet correctly such as the calling address.

Advantages of a datagram network

Disadvantages of a datagram network


8 - Virtual call network

This is similar to a telephone call in that there is a call establishment phase, followed by data transfer, followed by call cleardown. Only the call request packet need contain the calling address and call facilities, this call request packet sets up a route through the network and subsequent packets need only specify a virtual circuit number to identify the call.

Advantages of a virtual call network

Disadvantages of a virtual call network

Conclusion

CCITT chose a protocol that supports virtual call networks, however as a concession to datagram networks X.25 allows 'fast select calls' which are call request packets which may contain upto 128 bytes of data.


20 - Call reconnect on trunk failure

DTE DCE trunk DTE DCE

unique call ID

-- call --? -------------------? -- call --?

?-- acc -- ?------------------- ?-- add ---

?------------------- data transfer ---------------------?

trunk fails

?---CLR F500 * CLR F500 ---?

------- re-est over diff --?

route

same unique call ID

-------- REJ 0 ------------?

?------- REJ 0 -------------

recover any data that may

have been lost on failed

trunk

---------DATA 0 -----------?

?--------------------- data transfer ---------------------?

=================================================================

If the call cannot be re-established __ ___ ____ ______ __ ______________

DTE DCE trunk DTE

DCE

unique call ID

-- call --? -------------------? -- call --?

?-- acc -- ?------------------- ?-- add ---

?------------------- data transfer ---------------------?

trunk fails

?---CLR F500 * CLR F500 ---?

------- re-est over diff --?

route

same unique call ID

?-- CLR NC ---- --- CLR NC --?

21 - Module 3 / Practical 1 - Decoding packet headers using

Halcyon Tester

Procedure

1 - tee in the Halcyon tester to the circuit (at the exchange end) as follows:


2 - Connect and switch on mains power to all equipment.

3 - Start trace:

Set up Halcyon for: HDLC

ASCII 8

Press RUN on Halcyon

4 - Make a call from one of the terminals and send some data.

Clear the call from any end by typing <CNTL-P>CLR

You should see the hex data on the trace and if you press

the CODE/HEX key you should see the data in ASCII as you

typed

it on the terminal.

The format of the packet is as follows:

'7E' in half tone to indicate the start of the frame.

then 2 byte frame header

then 3 byte packet header

then the user data (CODE/HEX) key toggles between ASCII and

HEX

try decoding the 3 byte packet header.

5 - Stop the trace by pressing the CAPTURE key on the Halcyon.

6 - To decode the information into a more useful form,

press the ANALYSE key (FORMAT key on Mk2) on the Halcyon

press the ENTER key to step through the trace 10 frames at a

time.

7 - Press SHIFT then CAPTURE - this sequence tells the Halcyon to

display

the text in the data packets.

8 - Keep pressing ENTER until you get some packets.

(packets are carried in I frames and are displayed in inverse

video - black characters on a white background)

(ENTER steps through 10 frames

^ next frame

v last frame

to see the current frame in HEX press the CODE/HEX key

each time you press the key you will get the whole frame

eg,

7E <2 byte frame header> <3 byte packet header> <user data>

note no FCS is displayed if correct.

9 - Write down the sequence of packets on the line and deduce

from

the trace,

what was typed on the terminals,

What logical group and channel was the call on.

Module 3 / Practical 2 - Decoding packet headers using Tekelec

simulator

Procedure

1 - connect up the equipment as shown below:

/---\ /-------------------------\

/-----------\ /---\

/--/ \---/ \-----------------/

port\---/ \---\

\------/---| |-----------------|trunk

|---\-------/

Texas /--/ | |

terminal

Printer |/-/ | |

|

|| \-------------------------/

\-----------/

|| Tekelec TE92 or Chameleon PAD(DTE)

||

|| /-------------------\

|\-/ |

\--/ Keyboard |

\-------------------/

2 - Connect and switch on mains power to all equipment.

3 - Set up Tekelec as simulator

and bring the link up (SLON,SLG4,SPOF)

4 - Use the Tekelec to make a call and send some data, you will

have

to work out the packets and send them in hexadecimal, when

you have

worked out what to send then type PH followed by pairs of

hexadecimal

digits with no spaces in between -

eg, PH14060B0E2342192010064000

5 - Try sending all the other types of packet.



Slide Net 10 ................ Common packet level problems

24 Typical Faults

* Cannot make call - Check clear cause and diagnostic

* Call statistics: common problem calls not clearing

* Multiline: Difficult to find faults

on problems get circuit number

25 - Questions on network layer

1 - Which of these is an advantage of a virtual call network over

a datagram network,

a) it is more flexible

b) packets always arrive in the correct order

c) it is easier to design.

d) it is an international standard.

2) When a call packet arrives at a node (PSE) in the network,

what determines where it will be switched to.

a) The called address

b) The calling address

c) The logical channel and LCGN

d) The facility field.

3) When a data packet arrives at a node (PSE) in the network,

what determines where it will be switched to.

a) The called address

b) The calling address

c) The logical channel and LCGN

d) The facility field.

4) These packets are taken from a trace in hex, what types of

packet are they,

4.1) 14 06 0B 0E 23 42 19 20 10 06 40 00

a) a call packet

b) a data packet

c) a restart packet

d) an interrupt packet

4.2) 14 06 22 48 45 4C 4C 4F

a) a call packet

b) a data packet

c) a restart packet

d) an interrupt packet

5 - A Packet terminal sends and receives the following packets -

Subscriber -? Exchange 14 06 0B 0E 23 45 67 89 0A BC 00

Exchange -? Subscriber 14 06 13 03 79

What is happening,

a) a successful call has been made

b) an attempted call has been cleared by the remote end due

to the remote end

c) an attempted call has been cleared by the network due to

network congestion

d) an attempted call has been cleared by the network due to

invalid facility request

26 - Example

This is an example from a HP tester:

s = start flag e = end flag a = abort b = bad fcs g = good fcs

f f a b g

Find the call request and call accept.

Hewlett - Packard 4953A Protocol Analyzer

Char = A Hex = A1 Binary = 10100001 Type = DTE Character

Agges0A100E242402024001000004004000000gge s0 A 1165gge

1ggff3622BE327013032000508113222771000ggf f3 8 2063ggf

s0 8 gge s0Agges0A1165gges0A11

f3 1 ggf f31ggff1A2083ggff1C20

s0A11Agge s0 E gges0E1206gge s0 E124gge

f3A2A1ggf f1 1 ggff3C2921ggf f3 E221ggf

8gge s0 C gge s0C100gges0E gge s0E124gges0E9000000

1ggf f3 1 ggf f1E22Fggff31 ggf f10291ggff122202204

s0 0 gge s02 gge

f1 1 ggf f11 ggf

BLOCK NUMBER = 41


Hewlett - Packard 4953A Protocol Analyzer

DTE DCE

QD MOD LCN TYPE P(S) M P(R) DATA QD MOD LCN TYPE P(S) M P(R) DATA

00 8 207 RR 7

00 8 207 Data 0 0 1 11000

E49AA

00 8 202 Call E2424

E3270

00 8 210 Data 4 0 3 5

3

00 8 210 RR 4

00 8 21A RR 5

00 8 229 Data 1 0 0 6

1

00 8 202 Call Acc

00 8 222 RR 2

00 8 229 RR 2

10 8 202 Data 0 0 0 00000

22041

10 8 202 Data 1 0 0 00000

61020

00 8 222 Data 3 0 6 22222

00000

00 8 219 RR 5

00 8 219 Data 6 0 2 54332

5C870

00 8 214 RR 4

00 8 22B Data 4 0 7 5

F

BLOCK NUMBER = 41



 


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