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
Slide - Public PAD
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
EG,
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.
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
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)
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
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
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
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
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
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
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
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
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
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
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).
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
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
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
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
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
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
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
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
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
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
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 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.
host to pad:
byte byte byte ...
1 2 3
02 parameter value ... parameter value
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
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
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
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
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
___/ |___| 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
_____
___ ___________ [_/ \_]
___/ |___| 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............
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
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)
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
Lots of short transactions, important to minimise cost per call.
uses "basic mode" protocol
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)
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
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
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 !---------------+
! !
!-------------------------!
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
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
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
(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
<--READ PAR
PU2DQH00[ref] PAR INDIC-->
[Val]...
<--SET PAR
<--SET + READ PAR
PU2DQHOO[ref] PAR INDIC-->
[Val]...
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-->
<--INDIC OF BRK
DTE may send INT - check
that data = 00
PU2DQH00[ref]
[Val] PAR INDIC-->
Par indic only if 8 = 1
INVIT TO CLEAR-->
PU2DQH01 <--CLEAR REQUEST
CLEAR CONF-->
<--INVIT TO CLEAR
PU2CLEAR0 0 CLEAR INDIC-->
<--CLEAR CONF
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 ?