X.25/PAD in D and B Channels

X.25/PAD in D and B Channels

5. X.25/PAD in D and B Channels

Pace ISDN TA supports X.25/PAD in D and B channels for access to remote host or terminal through an X.25 Packets Switched Data Networks (PSDN). Pace ISDN TA supports:

2 PAD access through D Channel (mode 3),

1 PAD access through one B channel.

For establishing a PAD session Pace ISDN TA supports extended ATD command able to establish two sessions using one ATD command and two addresses :

The first address is used for routing the ISDN call to the ISDN B channel gateway, This first address is empty if a D channel is used.

The second address is used for routing the X.25 Virtual Circuit to the remote X.25 end point.

D channel connection to the gateway is established without ISDN signalling or using ISDN signalling for establishing a B channel connection to the X.25 PSDN and than to use AT&O command for starting the PAD. X.25 Virtual Connection is then established using PAD standard commands. PAD is supported through ASCII and also VIDEOTEX (Minitel) terminals.

PAD commands are not listed here. They are stricly conformant to CCITT Recommendations X.3 and X.28.

All the standard PAD parameters are supported as well as the following PAD profiles:

prof BI: B-channel, interactive, profile suited to interactive dialog on a B channel. This is the default PAD profile associated with DTE channel 11.

prof BT: B-channel, asynchronous transparent, profile suited to asynchonous transparent service using PAD to PAD rate adaptation on a B channel. This is the default PAD profile associated with DTE channel 1.

prof DI: D-channel, interactive, profile suited to interactive dialog on the D channel. This is the default PAD profile associated with DTE channel 12.

prof DT: D-channel, asynchronous transparent, profile suited to asynchonous transparent service using PAD to PAD rate adaptation on the D channel.

prof VPI: B or D-channel, interactive videoPAD, profile suited to interactive videotex terminal (Minitel) on B or the D channel. This is the default PAD profile associated with DTE channel 14 (B channel) or 15 (D channel).

In addition to those special profiles, the following standard PAD profiles are supported:

CCITT profiles 90 and 91,

France Telecom Transpac PAD profiles: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 20, 21, 22, 23, 90, 91.

In order to emulate a full Transpac connection the S register is used to display Transpac home message at the starting of the PAD session.

In order to emulate a Leased Line access to an X.25 PSDN through D or B channel, an ‘AT’ register &Z00 could store an ‘AT’ command automatically executed when the DTE raised the signal DTR. This command is loaded and store in the non-volatile memory through and AT&Z00=‘ATstring to execute’. Raising/falling the DTR signal is delayed for correct operations.

5.1 PAD Through AT Commands

5.1.1 Pace ISDN TA Modes

Pace ISDN TA X.25/PAD can be used in two major modes :

DTE to DCE: Pace ISDN TA is connected to a PSDN through :

Switched ISDN B channel or Permanent ISDN D channel.

Pace ISDN TA is used as a standard CCITT PAD (X.3, X.28, X.29) and the DTE is a PAD CCITT N° 5 terminal or a Minitel videotex terminal (VideoPAD).

DTE to DTE (DXE mode): Pace ISDN TA is connected to another ISDN terminal though :

switched B channel.

This mode, based on a special profile, is used as an asynchronous rate adaptation quite similar to V.120 except that the stack of protocol is X.25 PLP/LAPB instead of LAP V.120. The flow control is better supported by the X.25 windowing mechanism than by the Data Link RNR mechanism. PAD to PAD due to its high performance flow control is able to support very dissimilar end rates.

5.1.2 PAD ‘AT’ Commands

Regular « AT » commands are used for driving PAD, only extensions have been done for a better support of PAD :

ATO: Enter the PAD command mode (ATS112 bit 5 shall be correctly set-up),

ATD: Dial a PSDN gateway number and then go to PAD command mode or used a supplementary X.25 number to establish X.25 Virtual Circuit and go to PAD data mode.

ATA: Answer to a PAD to PAD incoming call

ATH: Clear a PAD call

AT&D: Select DTR action upon PAD

AT&K: Select flow control option for PAD

ATS112 bit 5: Control access to PAD command (0) or to data transfer mode (1) after Virtual Circuit establishment through an ATD command.

AT#BANNER: Configuration parameter is set to 0 (OFF) or 1 (ON). If 1 Transpac home message is displayed « TRANSPAC 000000 ».

PAD escape: Character (‘^P’)allows a return from PAD data mode to PAD command mode.

+++ sequence: Allows a return to AT command mode.

5.2.3 V.24 Signal in PAD Mode

The supported DTE interfaces across the V.24 signals are:

DTR: Action of DTR drop is according to &D parameter (similar to a non-PAD mode) :

&D0: DTR drop has no action.

&D1: DTR drop has the same effect as if a ‘PAD recall character’ ( ‘^P’) has been typed allowing to change from ‘data transfer’ state to ‘PAD command state’.

&D2: DTR drop clears the call (default).

&D3: DTR drop rests the equipment.

RTS: Used for hardware flow control if &K3 is selected.

DSR: Set to ON when entering PAD mode set to OFF when leaving PAD mode.

CTS: Used for hardware flow control if &K3 is selected, Always to ON in PAD command mode.

DCD: Set to ON when X.25 end to end communication is established (X.25 call packet has been successfully confirmed), and when PAD is in ‘data mode’. Set to OFF while PAD in ‘command mode’

5.1.4 Flow Control

Flow control at DTE-DCE interface can be selected using &K parameters:

&K0 : no flow control,

&K2 : flow control through XON/XOFF (DC1/DC3) characters,

&K3 : Hardware flow control through RTS/CTS V.24 signals.

This allows DTE communication to operate at a rate different from the Virtual Circuit rate.

5.1.5 Break Management

A BREAK (long sequence of zeros ‘0’) sent by DTE to PAD is processed according to PAD parameter 7. Possible actions include:

sending of a reset packet,

sending of an interrupt packet,

transition to PAD command mode.

5.1.6 PAD Parameters and Commands

PAD parameters are in conformance with CCITT (ITU) X.3/X.28 and supports followings (see complete description in CCITT X.28 table 1/X.28):

1 : PAD recall using a character (escape character)

2 : Echo

3 : Selection of data forwarding signal (‘End Of Line character)

4 : Selection of idle timer delay

5 : Flow control of the DTE by PAD through XON/XOFF

6 : Prompt character

7 : Break processing

8 : Discard output

9 : Padding after carriage return (CR)

10 : Line folding

11 : Binary speed of DTE junction (read only)

12 : Flow control of PAD by the DTE through XON/XOFF

13 : Linefeed (LF) insertion after Carriage Return (CR)

14 : Linefeed (LF) padding

15 : Editing

16 : Character delete (Backspace)

17 : Line delete

18 : Line display

19 : Editing PAD service signals

20 : Echo mask

21 : Parity Treatment

22 : Page wait

PAD parameters (1 to 22) can also be set and display using « AT » S registers S201 (PAD parameter 1) to S222 (PAD parameter 22). In addition S223 register specifies the X.25 M-bit management.

Command

Range

Default

Description

S201

0..126

PAD recall using a character

S202

0..1

PAD echo

S203

0..126

PAD selection of data forwarding character

S204

0..255

PAD idle timer delay

S205

0..1

PAD ancillary device control

S206

0..48

PAD service and command signals control

S207

0..21

PAD break processing

S208

0..1

PAD discard output

S209

0..255

PAD padding after carriage return

S210

0..255

PAD line folding

S211

0..18

PAD binary speed

S212

0..1

PAD flow control

S213

0..7

PAD linefeed insertion after carriage return

S214

0..255

PAD padding after linefeed

S215

0..1

PAD editing

S216

0..127

PAD character delete

S217

0..127

PAD line delete

S218

0..127

PAD line display

S219

0..127

PAD service signals editing

S220

0..128

PAD echo mask

S221

0..3

PAD parity treatment

S222

0..255

PAD page wait

S223

0..1

0

X.25 M-bit management
0 : no X.25 packet with the M-bit is generated by the PAD
1 : characters received continuously (before idle timer expiration) from DTE by the PAD are sent in the same X.25 message. The M-bit is set in all packets but the last.

Examples

at&o11

at&v

DTE channel 11 (PAD, B-channel) parameters:

E1 Q0 V1 W0 X4 \Q3 %C0

&C1 &D2 &K3 &M0 &O11 &Q0 &S0 &Y0

S000:000 S001:000 S002:043 S003:013 S004:010 S005:008 S007:060

S012:050 S025:002 S039:003 S095:000

S110:011 S111:007 S112:032 S113:001 S114:057 S117:005

S121:000 S122:254 S123:000 S161:000

S201:001 S202:001 S203:002 S204:002 S205:000 S206:005 S207:000 S208:000

S209:000 S210:000 S211:017 S212:000 S213:000 S214:000 S215:000 S216:008

S217:024 S218:018 S219:001 S220:000 S221:000 S222:000 S223:000

&Z00=

at&o12

at&v

DTE channel 12 (PAD, D-channel, 1) parameters:

E1 Q0 V1 W0 X4 \Q3 %C0

&C1 &D2 &K3 &M0 &O12 &Q0 &S0 &Y0

S000:000 S001:000 S002:043 S003:013 S004:010 S005:008 S007:060

S012:050 S025:002 S039:003 S095:000

S110:012 S111:007 S112:032 S113:001 S114:057 S117:000

S121:000 S122:254 S123:000 S161:000

S201:001 S202:001 S203:002 S204:002 S205:000 S206:005 S207:000 S208:000

S209:000 S210:000 S211:017 S212:000 S213:000 S214:000 S215:000 S216:008

S217:024 S218:018 S219:001 S220:000 S221:000 S222:000 S223:000

&Z00=

PAD commands are available using the two CCITT-specified keyword series (basic and extended) :

None (CALL) : call X.25/X.29 host equipment. All options are supported including user data.

CLR or LIB (CLEAR) : clear request (clearing of an established Virtual Circuit).

INT (INTERRUPT) : interrupt.

PAR? (PAR, PARAMETER, READ) : read PAD parameters.

RESET : reset PAD.

SET : set PAD parameters.

SET? (SETREAD) : set and read PAD parameters.

STAT (STATUS) : status.

(BREAK) : break.

(HELP) : help.

(LAN, LANGUAGE) : language.RPAR? (RREAD) : remote read.

ICLR (ICLEAR) : invitation to clear.

RSET? (RSETREAD) : remote set and read.

Some extensions to these CCITT commands have been added to DTE to DTE communication (PAD to PAD). Extension to the ‘CALL’ command allowing arbitrary data in hexadecimal format and to suppress the 4-byte reserved field in the user data.

RSET? : remote set and read parameters.

RPROF : set remote parameters with the values of the parameters of a local PAD profile.

5.1.7 Examples

Some examples of PAD dialogs are given below

5.1.7.1 PAD in a switched B channel (PSDN access through a gateway)

ats112=0OK

atd8836086464CONNECT 57600

*statFREE

*libLIB ERR

*par?PAR 1:1, 2:1, 3:2, 4:2, 5:0, 6:5, 7:0, 8:0, 9:0, 10:0, 11:17, 12:0, 13:0, 14:0, 15:0, 16:8, 17:24, 18:18, 19:1, 20:0, 21:0, 22:0

*19628030COM

Host Name:

Please contact Customer Assistance

LIB OCC 000

NO CARRIER

atd:901002:12

CONNECT 57600

THE BROWN FOX JUMPED OVER THE LAZY DOG 0123456789 0123456789 00000

DSR DOWN

at&o12OK

atd:at&o12OK

atd:901002

CONNECT 57600

THE BROWN FOX JUMPED OVER THE LAZY DOG 0123456789 0123456789 00000

5.1.7.2 PAD IN D channel (PABX X.25/X.29 Responder)

at&o12OK

ato*901002

COM

THE BROWN FOX JUMPED OVER THE LAZY DOG 0123456789 0123456789 00000

*lib

THE BROWN FOX JUMPED OVER THE LAZY DOG 0123456789 0123456789 00000

LIB CONF

*^p

atd8836086464:19628030:11CONNECT 57600

Host Name: Host Name: +++

at&o11OK

atd/88136086464:19628030CONNECT 57600

Host Name: +++

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