Telecommunications Network - PAD

PAD standards (X3,X28,X29)

X.28 - 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

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

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

X.3 pad parameters

Parameter 1 Pad recall

0 no recall
1 DLE character

following values --- CCITT 1984 only

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.

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.

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:

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 some networks, 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.

Parameter 5 Ancillary device control

0 no use of XON and XOFF
1 use XON and XOFF (data transfer)

following values --- CCITT 1984 only

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.

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

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.

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.

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.

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:

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.

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.

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

Note that 1200/75 lines on some networks are assigned a value 3, to 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.

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.

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.

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 switched off.

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

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.

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.

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:

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.

Parameter 19 Editing PAD service signals

CCITT 84 only

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

Parameter 20 Echo Mask

CCITT 84 only

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

Parameter 21 Parity treatment by the PAD

CCITT 84 only

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

Parameter 22 Page wait

CCITT 84 only

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

pad 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




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