PSS PAD

After this module you should know about the following:

Martin Baker PDN/SO/TS 01-250 9061

GOLD 85: TJT106

ISSUE 1 May 1987











Copyright MJB 1987 _________ ___ ____

INDEX


____________________

| 1 - |___________

|_______________________________________________| |

____________ ____________ _____________ _____________

|2| | |7| | |12| | |18| |

| | | | | | | |

|____________| |____________| |_____________| | |

____________ ____________ _____________ |_____________|

|3| | |8| | |13| | |

| | | | | | |19| |

| | |____________| | | | |

|____________| ____________ _____________ | |

____________ |9| | |14| | |_____________|

|4| | | | | | _____________

| | |____________| | | |20| |

|____________| ____________ |_____________| |_____________|

____________ |10| | _____________ |

|5| | | | |15| | _

| | | | | |

|____________| |____________| |_____________|

____________ ____________ _____________ |

|6| | |11| | |16| | _

| | | | | |

|____________| |____________| |_____________|

|

|17| |

PAD ___

Public PAD ............... Public PAD

Private PAD .............. Private PAD

X3a ...................... X3 parameters 1-5

X3b ...................... X3 parameters 6-10

X3c ...................... X3 parameters 11-18

X3d ...................... X3 parameters 19-22

Pad problems ............. PAD problems

Telenet ITI parameters


Slide - Public PAD


Public PAD (Packet assembler disassembler)

It would be nice if all devices on the network could be connected by packet lines, however a full X25 protocol is expensive in hardware and software. To allow existing mass produced terminals to be used a cheap way of connecting to PSS is required.

A PAD is a device which has an X25 line on one side and one or more non-X25 ports on the other side.

The network has PADs for asychronous terminals

mark _______ ____ ____ _________ _______________

| | | | | | | |

| | | | | | | |

|____| |__| |__| |___|

space

S S

t t

a o

r p

t

p

p u

u l

l s

s e

e


Asynchronous Termals

X.25 is an good protocol for linking computers into the network because it provides facilities such as error correction, multiple calls, etc.

Therefore this protocol forms the core of the network.

There are a large number of Asynchronous terminals in use in the computer world, a need exists for such simple and relativly cheap terminals to be connected into the network.

This need is met by a device known as a PAD (Packet

Assembler/Dissasembler).

Slide - private PAD

A number of PADs are availible the market which can be sited at remote premises and could in theory work with any of the following types of devices.


__________________

| | VDU

| PAD | Printing terminal

| | Clever VDU

| | Tape reader

| | Prestel / Viewdata terminal

| | Graphics terminal

| | Block mode terminal

| | Transaction Telephone

| | Error correcting protocol

X25 network | | Telex (TNA)

-------------------| | Facimile

| | Voice

| | ISDN network

| | Circuit switching network

| | Slow scan TV

| | Machine control

| | Alarm (burglar alarm,machine status etc.)

| | micro-computers

| | local area networks

| | other protocols

| | concentrators __________________





The PAD _______


PAD stands for Packet Assembler/Disassembler.

It is an interface between simple Asynchronous Character (Stop-start) terminals and an X.25 network .

From the start, the functions of the PAD were built into the PSS network.

An alternative way to provide this function is for a customer to have a stand-alone PAD at the customers premises. This would allow a remote site with many terminals to feed all these terminals into the network by 1 or 2 high speed X.25 lines.

A private PAD may also provide additional facilities or allow more unusual type of terminal.

Slide PAD 1 (shows transmit and receive buffers)

How does the PAD Work ? _______________________

The PAD is an interface between Asynchronous terminals and X.25, as its

name suggests it Assembles and Dissasembles packets.

When the Asynchronous terminal user presses a key, a single character is coded (usually in ASCII) and with its start and stop bits it is transmitted to the PAD.

The PAD stores the character in its Transmit buffer, subsequent characters build up in the buffer until a forwarding character is received (definable by the user - often a carrage return). When this happens the complete buffer, including the forwarding character is 'wrapped up' in a packet and a frame and sent into the network using the X.25 protocols. Other conditions for forwarding a buffer exist, for example, a timer or a buffer full condition.

In the other direction frames from the X.25 side are 'unwrapped' and sent to the terminal as individual stop-start characters.

In addition there is the ability to communicate with the PAD to control the setting up of calls and setting of PAD parameters.


Slide - PAD 2 _____ _ ___ _

X3

X28

X29

PAD Standards ___ _________

The operation of public PADs is defined by the CCITT in three

recommendations known as, X3 X28 and X29, these are somtimes refered to as

triple-X.

______________________________________

| X29 remote setting of PAD parameters |

| |

____________________ _________________________________ ___________

| | | | | | |

| Character | X28 | PAD | X25 network | X25 | Packet |

| terminal |--------| X3 | |-----| terminal|

| | | | | | (host |

| DTE-C | | | | |computer)| ____________________ ________________________________ _________

DTE-P

X.3 ___

This defines a number of PAD parameters, which are set in the PAD to match the characteristics of the terminal to the PAD and the remote end.

X.28 ____

This defines the procedures for the Character terminal to communicate with and control the PAD.

X.29 ____

This defines the procedures for a remote Packet terminal to communicate with and control the PAD. The Standards do not make it clear - should the character terminal or the remote packet terminal have main responsibility for setting the PAD parameters, (either end can alter them at any time during the call.


blank

need for PAD parameters ____ ___ ___ __________

eg, asychronous

ASCII coded

  1. line speed
  2. is line feed is inserted after carrage return
  3. are characters typed echoed back to screen

EG,

  1. should PAD parameters be set up from the terminal or the host.
  2. should line feeds be included in packets.
  3. should echoing of packets be done at the terminal, the PAD or the host.

PAD commands and service signals

Dial up PAD

After dialing the PAD the terminal user has 30 seconds to initialise the PAD otherwise the call will time out.

The user must type:

<cr><cr>[profile code]<cr>

where <cr> is the carrage return key

[profile code] is one of the following

two digit codes:

A1 A2 A3 A4 A5 A6 A7 A8 A9 B1 B2 D1 D2

V1 V2 V3 V4 V5 SP TP

the first <cr>s are used to detect the line speed rate and to set parameter

11, the [profile code] upper case or lower case, this sets the initial

profile.

Pad Identification:

LON\A001-1920040302 Acknowlages receipt by the PAD of the Service Request PAD command signal and indicates that the PAD waiting state has been entered.

to make a call

N(nui),R,G(cug)-A(address)*D(data)

or *P(data)

where N(nui) = callers NUI

R, = reverse charge request, optional

G(cug) = closed user group local number, if any

- = NUI/NUA field separator

A(address) = address of DTE-P and optionally the sub-address

*D(data) = call data - echoed to terminal (optional)

*P(data) = call data - not echoed to terminal (optional)

Call connected:

[address]+[facilities]COM

where:

[address] is the full called DTE-P address including

the sub-address, if known.

[facilities] if present,

R for reverse charge

G[CUG code]for closed user group


Slide - clear causes _____ _ _____ ______

Incoming Call ________ ____

(can only be received on a dataline DTE-C)

[address]+[facilities]COM {was +INC}

[call user data]

where:

[address] is the full called DTE-P address including

the sub-address, if known.

[facilities] if present,

R for reverse charge

G[CUG code]for closed user group

F for fast select

separated by commas and ending with '-'

[call user data] if present upto 124 octets of data


Call clear

This indicates to the terminal that the call has been cleared:

CLR [call progress signal] [diagnostic] [call statistics]

where:

[call progress signal] is a 3-character code to represent the cause

[diagnostic] is the hex value of the diagnostic field

of the clear packet, if present.

[call statistics] info on duration of call and number of segments

transmitted and received.

typical format:

CLR DTE 01 00:00:04:05 15 11

| | | | | | |____ tx segments

| | | | | |_______ rx segments

| | | | |__________ seconds

| | | |_____________ min

| | |________________ hours

| |___________________ days

|_______________________ diagnostic (hex)


reset indication

indicates a RESET has occured in data transfer state.

may be in one of the following formats:

RESET DTE

RESET ERR

RESET NC

blank

The following commands can be used while a call is in progress (provided parameter 1 = 1). If they are used during a call they must be prefixed with the control-P key, if a call is not in progress then control-P must not be used.

PAR? Read back all parameters

PAR? 1,3,6 Read back parameters 1,3,6

typical reply PAR 1:1,3:126,8:0

SET 1:1,3:4 Set parameter 1 to 1 and 3 to 4

SET? Set and read all parameters to initial values

(or last PROF command)

SET? 1:1,3:4 Set and read parameter 1 to 1 and 3 to 4

PROF A2 Set pad profile to A2

CLR Clear request

STAT Requests status information from the PAD

INT Requests the pad to transmit an interrupt packet

RESET Requests the pad to reset the virtual call

non-X.28 - Pad testing

extra commands for test patern

TEST E echo character

TEST C character

TEST T display triangle

TAPE profile for paper tape reader

INTD interrupt + par 8:0

slide


Parameter 1 Pad recall

0 no recall

1 DLE character

following values --- CCITT 1984 only --- not PSS yet ---

32-126 using one graphic character defined by user







Setting PAD parameter 1 to zero ensures that all characters transmitted by

the DTE-C in the Data Transfer state will be regarded by the PAD as user

data and none will therefore cause the PAD to escape to the Waiting for

command state.

If this character transparency in Data Transfer state is required together

with the ability to escape to the Waiting-for-Command state then PAD

parameter 7 must be set to value 8. This will allow break to be used by the

DTE-C as the escape signal.

slide


Parameter 2 Echo

0 no echo by the PAD

1 echo by the PAD

The user of a character terminal requires the characters typed on the

keyboard be displayed on the screen, there are 3 ways this could be done

* done internally by the character terminal.

* echo characters back from the PAD

* echo back from the remote host.

The third option is usually too slow and expensive, but termainals do not

always provide their own echo, so this parameter allows the PAD to echo

back to the character terminal.

PAD parameter 2 is effective in all states including the waiting-for-

connection state. The following exceptions apply:

* No characters echoed in Connection in Progress or Service Signal States

* The following parts of the Selection command signal are not echoed:

last 6 characters of NUI

Call data if proceded by P (instead of D)

* In the Data Transfer state when due to buffer space limitations a

character must be discarded by the PAD the character will not be echoed,

instead the PAD will return the IA5 BEL character

* Control characters, escape terminators and editing characters are never

echoed.

slide


Parameter 3 Selection of data forwarding characters

0 no forwarding

1 A-Z,a-z,0-9

2 character CR

4 ESC,BEL,ENQ,ACK

8 DEL,CAN,DC2

16 ETX,EOT

32 HT,LF,VT,FF

64 all characters between NUL and US of IA5 not included above

128 Graphic Characters

As characters are received, from the character terminal by the PAD, they

are stored in a buffer in the PAD, one of the conditions for 'wrapping'

this buffer up in a packet and sending it to the remote end is the data

forwarding characters.

the above values can be added together to give different combinations of

forwarding characters.

Regardless of any setting of parameter 3 and flow control permitting, the

PAD will forward 128 characters to the DTE-P in a DATA packet with the M-

bit set to 1 on receipt of the 129th character from the DTE-C.

The PAD may also forward data to the DTE-P under the following

circumstances:

slide


Parameter 4 Selection of idle timer delay

value of idle timer in 1/20 s units

The data forwarding timer is reset and restarted on receipt of any character from the DTE-C in the Data Transfer state. On expiry of the timeout all data from the DTE-C held by the PAD will be forwarded to the DTE-P subject to flow control. Thus on expiry of the data fowarding timer a data packet containing from 1 to 128 characters is transmitted from the PAD to the DTE-P.

Reducing the value of parameter 4 has the effect of, for a given rate of data transfer, increasing the number of packets transmitted by the PAD and consequently reducing their fill. Users are advised that because segments rather than characters are charged for in PSS, operating with a low values of parameter 4 can dramatically increase costs. eg In a single DATA packet

0-64 characters are charged 1 segment

65-128 characters charged 2 segments.

slide


Parameter 5 Ancillary device control

0 no use of XON and XOFF

1 use XON and XOFF (data transfer)

following values --- CCITT 1984 only --- not PSS yet ---

2 use of XON and XOFF (data transfer and command)



This parameter is for flow control of the DTE-C by the PAD

To do this the PAD transmits ASCII X-OFF to the DTE-C to stop the DTE-C sending and ASCII X-ON to resume transmission again.

The PAD will transmit X-OFF to the DTE-C whenever:

the PAD leaves the data transfer state and

in the data transfer state the PAD is unable to accept no additional characters.

The PAD will transmit X-ON to the DTE-C whenever:

the PAD enters or re-enters the Data Transfer state and;

in the Data Transfer state the PAD after transmission of X-OFF is able once again to accept additional characters.

slide


Parameter 6 Control of PAD service signals

0 no PAD service signals

1 PAD service signals transmitted in the standard format

following values --- CCITT 1984 only --- not PSS yet ---

5 PAD service signals and the prompt PAD service

signal are transmitted in the standard format

8-15 PAD service signals transmitted in a network dependent format

When set this parameter stops the pad from generating any messages for the terminal, therefore the only data received by the terminal will be the data sent by the distant end. This allows 'transparent' working but has the danger that the terminal user will not know about any resets, or know the reason for any clears.

slide


Parameter 7 Action on break

0 Nothing

1 send Interrupt

2 send Reset

5 send Interrupt and indication of break

8 escape from data transfer state

21 discard output (par 8=1),send interrupt and indication of break

Receipt of the Break signal by the PAD from the DTE-C is a data forwarding

condition for all non-zero values of parameter 7.

Parameter 7 controls action by the PAD on receipt of the break signal from

the DTE-C in data transfer state only.

Setting the value of PAD parameter 8 to 1 prevents further data delivery to

the DTE-C. The DTE-P will have to reset the value of parameter 8 to 0 - by

means of a PAD message or RESET packet - before normal data delivery can be

resumed.

Note that the procedure is used to reinitialise the circuit between the

DTE-P and the DTE-C so that all data en route is likely to be lost.

slide


Parameter 8 Discard output

0 normal data delivery

1 discard output

Parameter 8 allows supression of data delivery to the DTE-C.

Receipt of a RESET INDICATION or RESET CONFIRMATION packet or a "reset PAD" command by the PAD will set parameter 8 to 0.

If parameter 7 is set to value 21 then receipt of the break signal from the DTE-C by the PAD will cause parameter 8 to be set to 1. The PAD wil inform the DTE-P that it is discarding all data by transmitting the "indication of break" PAD message.

slide


Parameter 9 Padding after carriage return

0 no padding after CR

1-7 number of padding characters

This parameter causes the PAD to insert a delay after transmission of a CR. This is used for mechanical terminals to allow the print head time to return to the beginning of the line.

The setting of parameter 9 determines the number of padding characters inserted after the CR in the character string:

a. from the DTE-C to the DTE-P

b. echoed to the DTE-C

The setting of parameter 9 also determines the number of padding characters inserted in the character string to the DTE-C as part of a PAD-generated format effecter as follows:

value zero: terminal speed 110 bits/sec - 2 padding characters

terminal speed 300 bits/sec - 4 padding characters

values 1-7: 1-7 padding characters

A padding character is a time interval 1 character in length ie. the time that it would take for 10 contiguous bits to pass a fixed point in space.

slide

Parameter 10 Line folding

0 no line folding

1-255 number of graphic characters per line

This parameter controls the maximum number of printable characters on one line.

Provided the setting of parameter 10 is not zero or greater than 255 the PAD increments a counter for every graphic character transmitted or echoed on the DTE-C. When trhe count reaches a value of parameter 10 the PAD will transmit a format effector to the DTE-C and reset the counter to zero.

The counter is reset to zero whenever a format effector is transmitted to the DTE-C from any source and also when the PAD receives a buffer delete character from the DTE-C.

slide

Parameter 11 Binary speed

This parameter is set by the PAD to one of the values below, according to

the transmission speed of the DTE-C line:

0 110 b/s

1 134.5 b/s

2 300 b/s

3 1200 b/s

4 600 b/s

5 75 b/s

6 150 b/s

7 1800 b/s

8 200 b/s

9 100 b/s

10 50 b/s

11 75/1200 b/s

12 2400 b/s

13 4800 b/s

14 9600 b/s

15 19200 b/s

16 48Kb/s

17 56Kb/s

18 64Kb/s

The only values offered by PSS are 0, 2 and 3.

Note that 1200/75 lines on PSS are assigned a value 3, because we fudge the signal.

Parameter 11 may be read but not set by either the DTE-C or the DTE-P. It is set by the PAD on detection of the line rate (for dial-up operation) or at subscription time (for dataline operation).

Parameter 12 Flow control of the PAD

0 no use of X-ON (DC1) and X-OFF (DC3) for flow control

1 use of X-ON (DC1) and X-OFF (DC3) for flow control

This parameter allows the DTE-C to flow control the PAD.

This is used when the terminal cannot accept the information at the full line speed.

When Parameter 12 has a value of 1 and the interface is in the Data Transfer state the DTE-C may indicate a temporary inability to accept characters by transmitting X-off to the PAD.

The X-off condition is cancelled by either:

a. receipt of X-on by the PAD from the DTE-C or

b. The PAD escaping from, and subsequently re-entering, the Data Transfer state.

The X-OFF condition only exists in the Data Transfer state.

slide

Parameter 13 Linefeed insertion after CR

0 no linefeed insertion

1 insert linefeed after transmission of CR to the

start-stop mode DTE

2 insert linefeed after CR from the start-stop DTE

4 insert linefeed after echo of CR to start-stop mode DTE

these values can be added together, only valid are value, 0,1,4,5,6,7

setting this parameter allows a linefeed to be generated by the PAD following receipt of a carrage return.

PAD inserts linfeed characters under the control of parameter 13 in the Data Transfer state only.

slide

Parameter 14 Padding after linefeed

0 no padding after linefeed

1-7 number of padding characters inserted after linefeed

This parameter causes the PAD to insert a delay after transmission of a LF.

This is used for mechanical terminals to allow the paper time to move up one line.

Parameter 14 is effective during the data transfer state only.

The padding inserted into the character stream transmitted or echoed to the DTE-C as part of the line-folding activity of the PAD is controlled by PAD parameter 9.

A padding character is a time interval 1 character in length ie. the time that it would take for 10 contiguous bits to pass a fixed point in space.

slide

Parameter 15 Editing enable

0 no use of editing in the data transfer state

1 use of editing in the data transfer state

setting this parameter to 1 enabled parameters 16,17 and 18 to allow character delete, buffer delete and buffer display.

Because editing is performed on characters read by the PAD awaiting packet forwarding, the packet forwarding timeout (parameter 4) is automatically swiched off.

The editing facility available in the PAD waiting and waiting for command states is permanently enabled.

slide

Parameter 16 Character delete

0 no character delete

1-127 one character from IA5

this parameter defines a character which is used to delete the previous character.

value 8 selects BS to be the Character Delete character.

Successive use of the Character Delete character will delete all characters entered by the DTE-C since the last packet was forwarded to the DTE-P. Characters will be deleted one at a time and in reverse order of entry. The deleted character will be printed but the character delete character will not itself be printed.

Any character except NUL may be used as the Character Delete character and is chosen by setting the parameter value to the decimal code of the character required. However using CR, DLE or any character that is used as any other editing or forwarding character, or a character used in a PAD command should be avoided.

Parameter 15 must be set to 1 before Character Delete can be used.

slide

Parameter 17 Line delete

0 no line delete

1-127 one character from IA5

this parameter defines a character which is used to delete the buffer.

value 24 selects CAN to be the Buffer Delete character.

A single use of the Buffer Delete character will delete all characters entered by the DTE-C since the last packet was fowarded to the DTE-P and cause the PAD to transmit the characters XXX preceded by a space and followed by a format effector, the buffer delete character will not itself

be printed.

Any character except NUL may be used as the Buffer Delete character and is chosen by setting the parameter value to the decimal code of the character required. However using CR, DLE or any character that is used as any other editing or forwarding character, or a character used in a PAD command should be avoided.

Parameter 15 must be set to 1 before Buffer Delete can be used.

slide

Parameter 18 Line display

0 no buffer display

1-127 one character from IA5

Use of the Buffer display character causes the PAD to transmit to the DTE-C a format effector followed by all characters input by the DTE-C and not subsequently deleted since:

a) the last DATA packet was forwarded to the DTE-P (data transfer state)

b) escaping from the data transfer state (waiting for command state) or

c) the last PAD Service signal was received (PAD Waiting state)

The buffer display character will not itself be printed.

Any character except NUL may be used as the buffer display character and is chosen by setting the parameter value to the decimal code of the character required. However using CR, DLE or any character that is used as any other editing or forwarding character, or a character used in a PAD command should be avoided.

Parameter 15 must be set to 1 before Buffer Display can be used.

slide

Parameter 19 Editing PAD service signals

CCITT 84 only - not implemented on PSS

This controls the format of the editing service signals sent to the DTE-C by the PAD depending on the type of terminals, namely a printer or visual display unit (VDU).

0 no editing PAD service signals

1 editing PAD service signals for printing terminals

2 editing PAD service signals for display terminals

8 editing PAD service signals using one character

32-126 from the range of IA5

slide

Parameter 20 Echo Mask

CCITT 84 only - not implemented on PSS

If the echo function is enabled, then received characters will be echoed to the PAD. Parameter 20 permits the terminal user to control which group of characters are echoed back to the PAD.

0 no echo mask (all characters echoed)

1 no echo of CR

2 no echo of LF

4 no echo VT,HT,FF

8 no echo of BEL,BS

16 no echo of ESC,ENQ

32 no echo of ACK,NAK,STX,SOH,EOT,ETB,ETX

64 no echo of characters as designated by parameters 16,17,18

128 no echo of all other characters in columns 0 and 1 not mentioned above and DEL

slide

Parameter 21 Parity treatment by the PAD

CCITT 84 only - not implemented on PSS

The PAD can now check the parity of the data received from the terminal and/or generate parity of the data transmitted to the terminal. The type of parity (even,odd,mark,space) is indicated by the eighth bit of the character received by the PAD. If there is a parity error, the PAD returns an error-service signal and does not echo the character in error.

0 no parity detection or generation

1 parity checking

2 parity generation

3 parity checking and parity generation

slide

Parameter 22 Page wait

CCITT 84 only - not implemented on PSS

The PAD can be directed by the DTE-C to suspend the transmission of data to it after a specified number of lines is transmitted. The PAD can resume the transmission of data when, for example, it receives an X-on character from the DTE-C. This type of operation is intended for VDU terminals that might be receiving a large volume of data.

0 page wait disabled

1-255 number of line feed characters considered by the pad

for the PAD for the page wait function


Profile Table

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

_______________________________________________________________

A1 |1 |1 |126 |160 |0 |1 |21 |0 |0 |72 |* |0 |1 |1 |0 |127 |24 |18 |

A2 |1 |1 |126 |160 |0 |1 |21 |0 |6 |80 |* |0 |1 |2 |0 |127 |24 |18 |

A3 |1 |1 |126 |160 |0 |1 |21 |0 |0 |120|* |0 |1 |0 |0 |127 |24 |18 |

A4 |1 |1 |126 |160 |0 |1 |21 |0 |0 |120|* |0 |1 |6 |0 |127 |24 |18 |

A5 |1 |1 |126 |160 |0 |1 |21 |0 |8 |120|* |0 |1 |0 |0 |127 |24 |18 |

A6 |1 |1 |126 |160 |0 |1 |21 |0 |1 |80 |* |0 |1 |0 |0 |127 |24 |18 |

A7 |1 |1 |126 |160 |0 |1 |21 |0 |0 |80 |* |0 |4 |4 |0 |127 |24 |18 |

A8 |1 |1 |126 |160 |0 |1 |21 |0 |1 |132|* |0 |1 |0 |0 |127 |24 |18 |

A9 |1 |1 |126 |160 |0 |1 |21 |0 |16|132|* |0 |1 |8 |0 |127 |24 |18 |

B1 |1 |1 |126 |160 |0 |1 |21 |0 |2 |132|* |0 |1 |0 |0 |127 |24 |18 |

B2 |1 |1 |126 |160 |0 |1 |21 |0 |6 |132|* |0 |1 |2 |0 |127 |24 |18 |

D1 |1 |1 |126 |160 |0 |1 |21 |0 |0 |80 |* |0 |1 |0 |0 |127 |24 |18 |

D2 |1 |1 |126 |160 |0 |1 |21 |0 |0 |72 |* |0 |1 |0 |0 |127 |24 |18 |

V1 |1 |1 | 48 | 0 |0 |1 | 5 |0 |0 |72 |* |0 |6 |1 |1 |127 |24 |18 |

V2 |1 |1 | 48 | 0 |0 |1 | 5 |0 |0 |120|* |0 |6 |0 |1 |127 |24 |18 |

V3 |1 |1 | 48 | 0 |0 |1 | 5 |0 |1 |80 |* |0 |6 |0 |1 |127 |24 |18 |

V4 |1 |1 | 48 | 0 |0 |1 | 5 |0 |2 |132|* |0 |6 |0 |1 |127 |24 |18 |

V5 |1 |1 | 48 | 0 |0 |1 | 5 |0 |0 |80 |* |0 |6 |0 |1 |127 |24 |18 |

SP |1 |1 |126 | 0 |1 |1 | 2 |0 |0 | 0 |* |1 |0 |0 |0 |127 |24 |18 |

TP |0 |0 | 0 | 20 |0 |0 | 2 |0 |0 | 0 |* |0 |0 |0 |0 |127 |24 |18 |

|__|__|____|____|__|__|___|__|__|___|__|__|__|__|__|____|___|___|

X.29

X.29 defines the procedures used to allow the remote packet terminal to control the PAD.

The following messages can be sent in the user data field of a DATA packet

with the Q-bit set to 1.

SET parameters

host to pad:

byte byte byte ...

1 2 3

02 parameter value ... parameter value

SET and READ parameters

host to pad:

byte byte byte ...

1 2 3

06 parameter value ... parameter value

pad to host:

byte byte byte ...

1 2 3

00 parameter value ... parameter value

invalid values are indicated by setting the MSB

to 1

READ parameters ____ __________

host to pad:

byte byte byte ...

1 2 3

04 parameter 00 ... parameter 00 ...

pad to host:

byte byte byte ...

1 2 3

00 parameter value ... parameter value


Invitation to clear

host to PAD

byte 1

01

If the packet terminal sends data, and then wishes to clear the call, it is possible that the CLEAR packet could overtake and wipe out some of the data.

to avoid this problem the packet terminal can send an invitation to clear at the end of the data. This is carried in a data packet (with the q-bit set to one) and so will not overtake the data, the PAD will then clear the X.25 call.

term PAD packet term ____ ___ ___________

<--- text --- <----- DATA ------------

<--- text --- <----- DATA ------------

<----- inv to clear ----

wait until

all data sent to

terminal

----- CLEAR REQ ------>

cause=00

Indication of break

Many terminals have a 'break key', this sends a space to line, and is not therefore coded as an ASCII character.

therefore an indication of break can be sent across the X.25 part of the link to represent the break key.

An indication of break can be sent in either direction as follows:

PAD to HOST:

03 param value

where: param is parameter 8

value is 0 if param 7 set to 5

value is 1 if param 7 set to 21

or HOST to PAD

03

Error indication

this allows error messages to be sent from the PAD to the packet terminal.

05 00 received PAD message contained less than 8 bits

05 01 0x Received PAD message contained unrecognised message code 0x.

05 02 0x Received PAD message was incorrect or incompatible with message code 0x.

05 03 0x Received message did not contain an integral number of octets


Module 7 / Test 1 - The PAD

1 - Where does the PAD fit into the ISO 7 layer model.

a) Above the packet layer

b) in the packet layer

c) below the packet layer

d) does not fit into the 7 layer model

2 - X3 defines,

a) The PAD parameters

b) The terminal-PAD interface

c) The PAD-remote computer interface

d) The DTE-DCE interface

3 - X28 defines,

a) The PAD parameters

b) The terminal-PAD interface

c) The PAD-remote computer interface

d) The DTE-DCE interface

4 - X29 defines,

a) The PAD parameters

b) The terminal-PAD interface

c) The PAD-remote computer interface

d) The DTE-DCE interface

5 - The PAD is necessary because,

a) It is more friendly for humans to use.

b) It assembles packets using ASCII

c) It provides a means for simple, low cost terminals to access the network

d) It allows SNA terminals to connect to OSI terminals.

6 - When a user types a carrage return on a terminal connected to the PAD

the cursor moves to the start of the current line but does not move on

to the next line. The user should,

a) Alter parameter 3 from 2 to 32

b) Alter parameter 13 from 1 to 5

c) Alter parameter 5 from 0 to 1

d) Alter parameter 10 from 0 to 80

7 - In the middle of a call using the PAD the user gets the message:

CLR DTE (43)

This is due to:

a) Network congestion

b) The remote host clearing the call

c) Unauthorised Interrupt conf

d) Invalid calling address


Practical - Assembling and disassembling Packets


___/ |___| PAD |_________| PAD |______| \___

|_______| . |___________| . |_____________| . |_______|

. . .

. . . _________ _________

..................................................| Halcyon |____| Monitor |

|_________| |_________|

1 - Connect up the equipment as shown above

2 - Switch on

3 - Type <cntl-P>PAR?<cr>

to see what the PAD parameters are set to

what profile is this ?

4 - Type <cntl-P>SET?<param no>:<value>

to alter the PAD parameter

5 - Alter each pad parameter in turn and test its effect

The Public Pad - Parameters and profiles

_____

___ ___________ [_/ \_]

___/ |___| Modem |_____/ O \

|_______| . |___________| /_____\

. _________

...| Monitor |

|_________|

Call 928 9111 or

wait for modem tone

ype <CR> <CR> (note <CR> = carrage return)

type profile (say A1 or A2 or D1 or D2)

type <CR>

type N............


Protocol Identifier Field

Information about profile sent in call user data field of call packet.

Byte Data ____ ____

1 01

2 Pad profile

3 Parameter 11 (speed)

4 00

profile codes:

profile byte 2 value

a1 2D

A2 2E

A3 2F

A4 30

A5 31

A6 32

A7 33

A8 34

A9 35

B1 38

B2 39

D1 42

D2 43

SP 58

TP 59

V1 5A

V2 5B

V3 5C

V4 5D

V5 5E


Other types of terminal

1) Block mode - intelligent VDU editing done within terminal

then whole page (screen full) send to line at once

when enter key is pressed

also fields can be defined for form filling applications

(TAB moves cursor to next entry area)

only the field which are changed are sent to line.

The control sequences normally start with an ESC character.

so important factors are,

flow control(both directions)

forwarding characters

main types are,

IBM 3270,3278,3279

interfaces: BSC

SDLC/SNA

monochrome or colour

keyboard: EBCIDIC

cost: 1455 pounds

IBM PO box 41, North harbour,Portsmouth, 0705 694941

DEC VT100

Interfaces: RS232C

line graphics

optional 20mA interface

VT131 has local echo,half and full duplex,modem controls

cost: 1460 pounds

Digital Equipment

Digital Park,PO box 110,Imperial way

Reading,RG2 OTR (0734) 868711

ADM-3a (regent 25)

Netmux Profiles

M1 M2

1 1 1

2 0 0

3 2 16

4 40 40

5 1 1

6 1 1

7 21 21

8 0 0

9 0 0

10 0 0

11 - -

12 1 1

13 0 0

14 0 0

15 0 0

16 127 127

17 24 24

18 18 18

Transaction Telephones /EFT

Lots of short transactions, important to minimise cost per call.

uses "basic mode" protocol

Error Correcting Protocol (EPAD)

uses "basic mode" protocol

The follnowing methods can be used for error correcting,

1 - Some modems do error correction.

2 - X28 optional parity bit

no error correction

uses existing low cost terminals.

3 - X25 level 2 - error detection

- error correction

- high throughput

- flow control

- high cost

4 - "basic mode" - error detection

error correction

- possibility of data duplication

- cheaper than X25 L2 but still requires special terminals

4 - Telex _________

50 baud start-stop,5 bit code (IA2), uses +80v,-80v instead of modem

about 96,000 teleprinters on network

most countries of the world connected

not very compatable with new technology

(modernisation ofee Telex system network with low voltage single

channel voice frequency (scvf) transmission system (CCITT R20)

Viewdata/prestel

popular with the "travel industry".

Prestel gateway uses special high level protocol (on PVC's), other

viewdata hosts will only work if they generate and accept

even parity

Prestel can also act as a Host computer on 234219201025

speed: 75bps from keyboard

1200 bps to display

keyboard:

minimum 0 to 9

star *

hash #

or full keyboard

Display: 24 by 40 characters

ASCII,control (colour change,double hight,etc.)

or "chunky graphics"

carrage return does not clear the rest of the line, so screen must be

cleared at beginning or full line must be sent.

No scrolling.

general _______

No internal echo (full duplex)

even parity required.

The following PAD parameters have been found to give the best results,

Pad Value

1 1 escape from data transfer on

2 1 echo on (full duplex)

3 126 forward on non-alphanumeric

4 40 timeout of 40/20 seconds

5 0 flow control (send) off

6 1 no suppression of service signals

7 21 PAD send Interrupt and indication of break

8 0 no suppression of data

9 1 one character padding after CR

10 40 line folding after 40 characters

11 - terminal speed 1200/75

12 0 no flow control

13 0 no line feed insertion

14 0 no padding after line feed

15 0 editing off

16 08 delete character=backspace

17 27 buffer delete character

18 18 buffer display character


Graphics

Methods raster scan

vector graphics

storage tube

Standards _________

Tektronix 4010 - storage tube now obsolete

but still used as industry standard,

Cifer and others are raster scan which emulate 4010

4010 ____

1024 by 1024 points

first address gives starting position, each subsequent address plots

a line,

Addresses sent as,

high Y (top 5 bits)

low Y (low 5 bits)

High X (top 5 bits)

low X (low 5 bits)

7 6 5 4 3 2 1 0 ______________________

0 ! ! (five address bits)

! !

0 1 high X or Y

1 0 low X

1 1 low Y

CR or US to leave graph plot mode

Flow control may be a problem as graphics requires a large

ammount of data to be sent quickly and flow control characters

(XON,XOFF) may be interpreted as graphics data.

Characters not used as ASCII characters ,therefore any code may

be sent

Best results given by,

Par Val

--- ---

1 0 Escape from data transfer off

2 0 no echo

3 0 forwarding on timeout/buffer full

4 5 forwarding after 5/2ec

5 0 flow control off

5 1 flow control on

6 1 no suppression of pad service signals

7 8 escape from data transfer state on break

8 0 no suppression of data

9 0 no padding

10 0 no line folding

11 - terminal speed

12 1 flow contr{l to by DTE on

13 0 no line feed insertion

14 0 no padding after line feed

15 0 editing off

16 127 editing not used

17 24 editing not used

18 18 editing not used


Teletex

Terminals work directly to each other, either using PSTN (1200 or 2400 bps) or PSS (2400 bps).

Terminals may range from memory typewriters to a word-processor or a message switchig system.

All terminals must have a electronic memory (32,000 characters about 20 pages of text is recommended).

contains: Transport layer

Session layer

Document layer

There is a requirement on PSS for a Teletex directory service.

!---------------------! !------------------------!

! ! ! !

! Telex ! -------TNA---------------! PSS !

! ! ! !

!---------------------! !------------------------!

! !

????? PNA

! --------------------------- !

! ! ! !

+----------------! PSTN !----------+

! !

!-------------------------!

!

! dial up

!

! !

!-------------------------! !

! ! X25 !

! Teletex terminal !---------------+

! !

!-------------------------!


New PAD Facilities

The following are ways the PAD could theoretically be expanded.

(not in CCITT standards)

1 - Inform user of remote changes to parameters

-----------------------------------------------

2 - Help Facility

-----------------

Useful but would require large memory

3 - Local Editing

-----------------

Editing of a full screen (or page) of text could be done within the

PAD, this would allow a simple terminal to be used in the same way

as a block mode terminal.



Broadcast

---------

If the Pad had local editing then other facilities such as broadcasting

to multiple destinations could be done.

Page separation

---------------

Extra PAD parameters could be added to allow say, a page throw after

every 60 lines (for a line printer) or for a "press any key to continue"

message every 20 lines for a VDU

Outgoing calls to PSTN from PAD

-------------------------------

Addition of autodialler to public PAD

Code or Protocol conversion

---------------------------

ie ASCII to EBCIDIC


101 Pad problems

1 - problem: all output on one line

reason: parameter 13 set incorrectly

2 - problem: all output double spaced

reason: parameter 13 set incorrectly

3 - problem: about half of the characters not echoed or

echoed back incorrectly.

reason: parity problem

4 - problem: output garbled

reason: speed problem

5 - problem: CLR DTE(00) from host

reason: host not accepting calls

6 - problem: backspace/delete key does not work

reason: par 15 to 17 not set

7 - problem: loss of data (block mode terminals or intellegent terminals)

reason: flow control

8 - problem: unable to make international call

reason: remote host does not understand PIF

9 - problem: Half duplex v Full duplex

New requirements for PSS PAD profiles (P.Hall & J.Roche)

1 - allow the configuration of "bespoke" profiles for DTC-C dataline

customers.

2 - Alteration of all parameter profile codes with par 13:1 to par 13:4

3 - Addition of profiles C1,C2,M1,M2,G1,P1 with parameter settings as below

4 - Removal of profiles A6,V1,V4,V5,A3,B1 Note profile A7 is needed par7=4.

C1 C2 M1 M2 G1 P1

1 1 1 1 1 0 1

2 1 1 0 0 0 1

3 2 2 2 16 0 126

4 160 160 40 40 5 40

5 1 1 1 1 1 0

6 1 1 1 1 1 1

7 21 21 21 21 8 21

8 0 0 0 0 0 0

9 0 6 0 0 0 1

10 0 0 0 0 0 40

11 - - - - - :

12 1 1 1 1 1 0

13 0 0 0 0 0 0

14 0 2 0 0 0 0

15 0 0 0 0 0 0

16 127 127 127 127 127 08

17 24 24 24 24 24 27

18 18 18 18 18 18 18

C1 CLEVER VDU - ancillary device control ON

- no line folding by PAD

- flow control by DTE-C on

- NO line feed insertion by PAD

- PAD editing not enabled

C2 clever KSR/printer - padding after carriage return

M1 blck mode - forwarding on carriage return or

2-second timeout.

M2 block mode - forwarding on EOT/ETX or 2 second timeout.

G1 graphics mode - no escape from data transfer mode

- no echo

- no data forwarding character

- forwarding on 0.25 second timeout

- escape from data transfer on break

- mno line folding

P1 viewdata - forwarding on all characters between NUL

and US of IA5 (not HASH) and forwarding on

2 second timeout.

- line folding on 40 characters.

(NB the PSS PAD will not perform right-hand justification

nor clear-page functions. These are prerequisites to

viewdata terminal operation)

Sample session on the public PAD

&+++ other tandata commands

90212146191 V23T

&DLG,90212146191 ^A

&WAT sp1

&CTL

<cr><cr>A2<cr>

BIR\A002-2122240102

N591LUJ

ADD?

A21920100485

23421920100485+COM

Welcome to Telecom Gold's System 85

Please Sign On

>ID MJB

Password:

Password:

Illegal Sign-On

CLR PAD (00) 00:00:00:32 13 17

NUI?

N591LUJ

ADD?

P

PAR1:1,2:1,3:126,4:160,5:0,6:1,7:21,8:0,9:6,10:80,11:3,12:0,13:1,14:2,15:0,16:12

7,17:24,18:18

23421920100485+COM

Welcome to Telecom Gold's System 85

Please Sign On

>ID TJT106

Password:

TELECOM GOLD Automated Office Services 19.4I.122(85)

On At 19:17 11/06/87 BST

Last On At 18:58 11/06/87 BST

BT's own professional Technical Publications Service - see >BTNEWS.

Datasolve:- New Service (McCarthy Online), files and menu - See >INFO DB.NEWS

Mail call (25 Read, 5 Unread, 1 Read express, Total 31)

************************************************************************

YOUR SYSTEMS MANAGER IS SUE ELLIS, MY TELEPHONE NO IS 01-250 8623,

MY ADDRESS IS 3RD FLOOR TENTER HOUSE 45 MOORFIELDS LONDON EC2Y 9TH.

*************************************************************************

>SET? 10:3

PAR

10:

3

HEL

LO

Ill

ega

l C

omm

and

>SE

T 1

0:0

HELLO

Illegal Command.

>PAR?

PAR1:1,2:1,3:10,4:20,5:1,6:1,7:21,8:0,9:6,10:0,11:3,12:1,13:4,14:2,15:0,16:127,17:24,18:18

LO

Off At 19:19 11/06/87 BST

Time used: 00h 01m connect, 00m 01s CPU, 00m 00s I/O.

Mail call (25 Read, 5 Unread, 1 Read express, Total 31)

CLR PAD (00) 00:00:02:15 30 14

X.29 Protocol

(optional)

es equipment generate X.29

work to PSS PAD y/n

does equipment accept X.29 y/n

what action is taken on X.29 packets

we cannot enforce this

example PSS mail





7.1 PAD incoming call _____________________

INCOMING CALL-->

Check that PIF is not

interpreted as data

PU2CALL[a1},

[a2],FO1 00 43 02

02 42 07 07,DH01 <--CALL ACCEPTED

43 03 00

[12 octets of user data]

7.2 DTE Parameters (set, read, set and read) ____________________________________________

READ PAR-->

Check that PAR 2 = 1 (echo on)

PU2DQH04 <--PAR INDIC

PU2DQH02 02 00 SET OAR 2:0-->

Set PAR 2 = 0 (echo off)

<--DATA

DTE sends data (not echoed

echoed at DTE)

PU2DQH04 02 00 READ PAR 2-->

Read PAR 2 = 0

<--PAR INDIC 2:0

PU2DHQ06 02 01 SET + READ 2:1-->

Set and read PAR 2 = 1

(echo on)

<--PAR INDIC 2:1

7.3 Invalid Parameters ______________________

SET B = 2-->

Set PAT 11 = 2 (300 bit/s)

PU2DQH02 OB 00

Check that DTE indicates invalid

parameter field

<--PAR INDIC 8B = 00

PU2DQHOA INVALID CODE-->

Send invalid message code

Check ERROR coded 05 01 DA

<--ERROR

PSS PAD Parameters(Set, Read, Set and Read) _______________________________________________

<--READ PAR

PU2DQH00[ref] PAR INDIC-->

[Val]...

<--SET PAR

<--SET + READ PAR

PU2DQHOO[ref] PAR INDIC-->

[Val]...







Indication of Break to DTE

DCE INT-->

PU2INT00

<--DTE INT CONF

PU2DQH03 08 01 INDIC OF BRK(8=1)-->

Check DTE resets 8 = 0

<--SET OR SET + READ

<--PAR INDIC

Par Indic only if set +

read

PU2DQH03 INCIC OF BRK-->

Indication of Break to PSS PAD

<--INDIC OF BRK

DTE may send INT - check

that data = 00

PU2DQH00[ref]

[Val] PAR INDIC-->

Par indic only if 8 = 1

DCE Invitation to Clear

INVIT TO CLEAR-->

PU2DQH01 <--CLEAR REQUEST

CLEAR CONF-->

DTE invitation to clear

<--INVIT TO CLEAR

PU2CLEAR0 0 CLEAR INDIC-->

<--CLEAR CONF

Typical X.29 Fault

X.25 cust PSS X.25 Cust

--------call request ------>

<-------call connect -------

<---- 7E 01 02 92 00 00 02 07 05 0A 84 0D 02 7E

----- 7E 03 02 92 00 20 00 87 00 7E ------>

note: 7E at beginning and end is flag.

What is happening here ?