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Configuring X.25

This chapter covers these topics:
Introduction to Ascend X.25 implementation
Configuring the logical link to a X.25 switch
Configuring X.25 IP connections
Configuring X.25 PAD connections
Setting up X.25 PAD sessions
Monitoring X.25 and PAD service
Setting up ISDN D-channel X.25 support

Introduction to Ascend X.25 implementation

This chapter describes X.25 support on the MAX. The CCITT Blue Book Recommendation X series 1988 has full technical specifications for X.25, X.3, X.28, X.29, and LAPB (Link Access Protocol-Balanced). IETF RFC 1356 has technical specification for IP over X.25 (X25/IP).

X.25 is a connection oriented (virtual circuits) protocol, providing services such as multiplexing, in-sequence delivery, transfer of addressing information, segmenting and reassembly, flow control, transfer of expedited data, error control, reset and restart. Allocation of logical channels can be either static (PVC) or dynamic (SVC).

Configuring the MAX to communicate with an X.25 switch involves the following elements:

Configuring the logical link to a X.25 switch

A X.25 profile defines the logical data link between the MAX and a remote X.25 switch. The Ethernet menu contains X.25 profiles, with following parameters:

For more information about each of these parameters, see the MAX Reference Guide.

Understanding the X.25 parameters

This section provides some background information about the X.25 parameters.

Profile name and activation

User connections link up with the X.25 connection specified in this profile by specifying the profile name. The name must be unique and cannot exceed 15 characters.

Set the Active parameter to Yes to make this profile available for use.

Physical connection type

The call type may be nailed or switched (X.25 PAD requires nailed). If it is a nailed connection, specify the Nailed Grp number. If it is a switched call, specify the Dial # and telco options.

LAPB and reliable data transfer

The X.25 frame layer implements LAPB (Link Access Protocol-Balanced), an HDLC-like protocol that facilitates the exchange of information packets.

X.25 packet handling

 The X.25 packet layer defines the packet format as well as the procedures for the exchange of packets containing control information and user data.

X.25 PVC and SVC numbers

X.25 diagnostic fields in packet types

X.25 options

 X.25 options can be set to None (no options) or NPWS (specifying that the MAX negotiates packet and window size). None is the default.

X.25 reverse charge accept

This parameter specifies whether the MAX accepts call packets with "0101" in the facility field to request reverse charge. The default is No.

X.25 network type

Currently, the MAX supports only the CCITT network type.

Controlling Restart-Requests

X.25 T20 sets the duration of the Restart timer (the number of ten-second ticks the MAX waits before retransmitting a Restart-Request packet) and the corresponding X.25 R20 parameter specifies the number of Restart-Request retransmits the MAX sends before waiting indefinitely for a response.

Controlling Call-Requests

X.25 T21 sets the duration of the Call-Request timer (the number of ten-second ticks the MAX waits before clearing an unacceptable outgoing call).

Controlling Reset-Requests

X.25 T22 sets the duration of the Reset-Request timer (the number of ten-second ticks the MAX waits before retransmitting a Reset-Request packet) and the corresponding R22 parameter specifies the number of times the MAX retransmits a Reset-Request packet before clearing a call.

Controlling Clear-Requests

X.25 T23 sets the duration of the Clear-Request timer (the number of ten-second ticks the MAX waits before retransmitting a Clear-Request packet) and the corresponding R23 parameter specifies the number of Clear-Request retransmits the MAX sends before waiting indefinitely for a response.

X.121 source address is MAX source address for logical links using this profile.

An X.121 address contains between 1 and 15 decimal digits, such as 031344159782738.

Setting the VCE (Virtual Call Establishment) timer value

Virtual Call Establishment timer interval specifies the number of seconds to maintain a connection to a character-oriented device (such as a terminal server) that has not established a virtual call. This timer value is link-wide. Each X.25 PAD connection has a parameter to enable or disable this timer on a per-connection basis. A value of 0 disables this timer system-wide regardless of the value of the VC timer enable flag per connection. The default is 300 seconds.

Example X.25 profile configuration

This example configuration shows an example X.25 profile that establishes the logical link to an X.25 switch. It does not show how to configure the nailed channels used for the physical connection to the switch. For details on configuring physical nailed connections, see Chapter 2, Configuring the MAX for WAN Access.

Note: You must obtain a copy of the telco's subscription form containing the values provisioned in the switch and configure the MAX X.25 profile to comply with those values.

Table 6-1 shows a sample telco subscription form for X.25 service:

Table 6-1. Sample Telco subscription form

Subscription Form

Value

X.25 Profile
Maximum seconds the transmitter waits for acknowledgment before starting recovery procedure (T1):

 5

 LAPB T1=5

Maximum times to resend a frame after the T1 timer expires (N2): .

 10

 LAPB N2=10

Maximum sequentially numbered frames that a given DTE/DCE link may have unacknowledged at any given time (k):.

 7

 LAPB k=7

Is the X.25 node a DTE or DCE?:

DTE

 X.25 Node Type=DTE

Is the link SVC or PVC?:

 SVC

 X.25 Link Setup Mode=Active
X.25 lowest PVC=1
X.25 highest PVC=8

Maximum packet size:

 2048

 X.25 Max pkt size=2048

Maximum number of data packets that can be outstanding between a DTE and a DCE before acknowledgment is required (W):

2

 X.25 window size=2

Number of PVCs:

 0

 X.25 highest PVC=0

Highest PVC channel number:

 0

 X.25 highest PVC=0

Default packet size:

256

 X.25 pkt size=256

Minimum packet size:

64

 X.25 Min pkt size=64

Maximum packet size:

2048

 X.25 Max pkt size=2048

To configure the X.25 profile to comply with this subscription form:

  1. Open the X.25 profile, assign the profile a name, and activate it.

  2. Set the Call Type to Nailed and specify the nailed group number.

  3. Set the LAPB parameters to comply with the settings in the subscription form.

  4. Set the X.25 node type to DTE, as specified in the subscription form.

  5. Configure the profile to support up to 8 switched virtual circuits.

  6. Configure packet sizes and flow control.

  7. Specify the X.121 source address to use on this link.

  8. Close the X.25 profile.

Configuring X.25 IP connections

This section describes how to configure the MAX to exchange IP datagrams over the X.25 network connection specified in an X.25 profile. X.25 IP connections must be routed, they cannot be bridged. These are the related parameters:

For more information about each parameter, see the MAX Reference Guide.

Understanding the X.25 IP connection parameters

This section provides some background information about the X.25 IP connection parameters and the required IP configuration for this type of connection.

X.25 profile name

This 15-character text field contains the name of an X.25 profile that the MAX uses for this logical connection. If the matching X.25 profile cannot be found, the MAX does not start a session for this Connection profile. To guard against this misconfiguration, an active Connection profile specifying X.25 encapsulation can not be saved unless you define the named X.25 profile and make it active.

LCN (logical channel number) number

The LCN specifies the logical channel number to use in the case of a PVC. The default zero means the MAX does not provide LCN, so the connection is not a PVC.

Encapsulation type

The encapsulation type may be RFC877 for backward compatibility, SNAP, or NULL (multiplexing) encapsulation. This fields specifies which encapsulation to use when calling the remote site. When receiving a call, the MAX accepts any of the three types of encapsulation. The default is RFC877.

X.25 reverse charge

This parameter specifies whether the X.25 facility field indicates reverse charge request when the X.25 user calls a host. The default is No.

RPOA

This parameter specifies the set of RPOA (Recognized Private Operating Agency) user facilities to use in the next call request. The RPOA facilities provide the data network identification code for the requested initial RPOA transit network. You can specify up to 4 digits. The default is null.

CUG Index

This parameter specifies the Closed User Group (CUG) index/selection facility to use in the next call request. The closed user group selection/index facility specifies to the called switch the closed user group selected for a virtual call. You can specify up to two digits. The default is null.

NUI

This parameter specifies the set of Network User Identification (NUI) related facilities to use in next call request. NUI provides information to the network for billing, security, network management purposes, and for activating subscribed facilities. You can specify the NUI to use in the next call request. You can specify up to six digits. The default is null.

Maximum number of unsuccessful calls

You can specify the maximum number of unsuccessful X.25 calls the MAX tries to place before dropping the modem connection. The default zero means an unlimited number.

Inactivity timer

The inactivity timer specifies the number of seconds to allow a connection to remain inactive before dropping the virtual circuit.

MRU

This parameter specifies the maximum number of bytes the MAX can receive in a single IP packet on the X.25 link. The IP packet is further fragmented/reassembled to fit the maximum X.25 packet size, if the MRU is larger than the X.25 packet size. The default is 1500 bytes.

Call mode

The call mode specifies whether the MAX can initiate a call request on this connection.

Answer X.121 address

 This specifies the X.121 address of the remote X.25 host to which this profile connects. The remote host also supports RFC1356 encapsulation of IP packets. This field cannot be left empty if you set Call Mode to Both or Incoming.

Remote X.121 address

This specifies the X.121 address of the remote X.25 host to which this profile connects. The remote host also supports RFC1356 encapsulation of IP packets. This field cannot be left empty if you set Call Mode to Both or Outgoing.

IP configuration parameters

The IP configuration for an X.25 IP connection is identical to an IP routing connection using PPP encapsulation. You must specify the address of the remote Ascend unit in the LAN Adrs parameter. If you are using numbered interfaces, you can also specify local IF Adrs and a remote WAN Alias value. For details on IP routing configurations, see Chapter 10, Configuring IP Routing.

Example X.25 IP configuration

This section shows an example configuration enabling two IP networks to connect through a Public or Private Packet Switched Network (PSPDN).

Figure 6-1. Example X.25 IP connection

To configure this example connection:

  1. Open the Answer profile and enable X.25 IP encapsulation.

  2. Open a Connection profile, name it, and activate the profile.

  3. Enable IP routing and specify the IP address of the answering unit.

  4. Enable X.25/IP encapsulation and then open the Encaps Options subprofile.

  5. Specify the name of the X.25 profile that carries this connection.

  6. Set the inactivity timer. For example, set it to 30 seconds.

  7. Set the call mode and the local and remote X.121 addresses.

  8. Close the Connection profile.

Configuring X.25 PAD connections

An X.25 PAD (Packet Assembler/Disassembler) is an asynchronous terminal concentrator that enables several terminals to share a single network line. It has its own command interface and uses an X.3 profile to fine-tune its parameters.

When a user calls X.25 PAD through a modem, a digital modem processes and forwards the call to the terminal server. The terminal server authenticates the call using the password specified in the caller's Connection profile and establishes the session. If the MAX does not authenticate the session, either because an unauthenticated user types PAD at the terminal-server prompt or because you use the terminal server's immediate X25/PAD services, the MAX uses X.25 parameters specified in the Answer Profile.

When the MAX establishes the session, the caller may see the terminal-server command line or be directed immediately to an X.121 host. If the connection auto-calls an X.121 host, the initial session display looks like this:

If the MAX directs the user to the terminal-server command line, the user sees the terminal-server login banner instead. The user can then establish a PAD session by using the Pad command, for example:

(The asterisk is the PAD prompt for input.) The user can then place a call, for example:

See X.25 PAD commands for more details. This section describes how to configure these X.25 PAD connections. These are the related parameters:

For more information about each parameter, see the MAX Reference Guide.

Understanding the X.25 PAD connection parameters

This section provides some background information about the X.25 PAD connection parameters.

X.25 profile name

This 15-character text field contains the name of an X.25 profile that the MAX uses for this logical connection. If the matching X.25 profile cannot be found, the MAX does not start a session for this Connection profile. To guard against this misconfiguration, an active Connection profile specifying X.25 encapsulation cannot be saved unless you name the X.25 profile and make it active.

Receive password

This specifies a case-sensitive password to use to authenticate the caller.

LCN (logical channel number) number

The LCN specifies the logical channel number to use in the case of a PVC. The default zero means the MAX provides no LCN, so the connection is not a PVC.

X.3 parameter profile

Table 6-3 on page 17 lists supported X.3 parameter profile. You can specify a profile using a PAD command, and you can specify a connection default profile in the X.3 Param Prof parameter. A profile specified on the command line overrides this default for the length of the current session.

Maximum number of unsuccessful calls

You can specify the maximum number of unsuccessful X.25 calls the MAX tries to place before dropping the modem connection. The default zero means an unlimited number.

VC (Virtual Call Establishment) timer enabled

You can enable or disable use of the VCE timer on a per-user basis. The VCE timer specifies the number of seconds to maintain a connection to a character-oriented device (such as the terminal server) that has not established a virtual call. If the X.25 profile disables this parameter, it has no effect in a Connection profile.

Auto-call to an X.121 address

The Auto-Call X.121 Addr specifies an X.25 host to call immediately when the MAX establishes an X.25/PAD session via this Connection profile. If this parameter specifies an address, the PAD session can begin automatically; otherwise, the MAX displays the terminal-server prompt, where the user can issue the pad command to begin a session.

X.25 reverse charge

This parameter specifies whether the X.25 facility field indicates reverse charge request when the X.25 user calls a host. The default is No.

X.3 Custom

Ascend's X.25/PAD implementation contains ten permanent X.3 parameter profiles that contain settings for a range of devices, such as terminals and printers. However, these profiles do not cover all devices. You can define a new profile that describes a device not specified by the permanent profiles.

Example X.25 PAD configuration

This section shows an example configuration in which the MAX directs the X.25 modem caller immediately to a PAD interface on the host whose X.121 address appears in Figure 6-2.

Figure 6-2. Example X.25 PAD connection

To configure this example X.25 PAD connection:

  1. Open the Answer profile and enable X.25/PAD encapsulation.

  2. Open a Connection profile, name it, and activate the profile.

  3. Enable X.25/PAD encapsulation and then open the Encaps Options subprofile.

  4. Specify the name of the X.25 profile that carries this connection.

  5. Specify the password that authenticates the user connection.

  6. Specify a default X.3 parameter profile for this connection.

  7. Specify the X.121 address and password to auto-call.

  8. Close the Connection profile

Setting up X.25 PAD sessions

This section describes some of the PAD commands and X.3 parameter profiles that can affect how users' terminal sessions operate.

X.3 parameters and profiles

The user's terminal or host DTE can modify operations the PAD performs by setting one or more X.3 parameters or by applying an X.3 profile. This section lists the X.3 parameters and profiles and then describes how to set them from the PAD. These are the X.3 parameters, numbered 1 through 22.

Table 6-2. X.3 parameters

Parameter

Description

Possible values
1r

 PAD recall

 0-Escape not allowed
1-Escape allowed (the default)

2

 Echo

 0-No echo
1-Echo (the default)

3

 Data forwarding characters

 0-None (full packet)
1-Alphanumeric
2-Carriage return (the default)
4-ESC, BEL, ENQ, ACK
8-DEL, CAN, DC2
16-ETX, EOT
32-HT, LT, VT, FF
64-All other characters in columns 0 and 1 of International Alphabet #5

4

 Idle timer delay

 0-No timer
1-255-Delay value in twentieths of a second

5

 Ancillary device control

 0-Not operational
1-Use X-ON (DC1 of International Alphabet #5) and X-OFF (DC3 of International Alphabet #5)

6

 PAD service and command signals

 0-Do not transmit service signals 1-Transmit service signals

7

 PAD operation on receipt of break signal from the start-stop mode DTE

 0-No action
1-Transmit Interrupt packet
2-Reset
4-Indication of break (PAD message)
8-Escape from data transfer
16-Discard output to DTE-C
21-Combine actions 1, 4, and 16

8

 Discard output

 0-Normal data delivery (the default)
1-Discard output to the DTE-C

9

 Padding after carriage return

 0-No padding
1-7-Number of padding characters inserted after the carriage return

10

 Line folding

 0-No line folding (the default)
1-255-Number of characters per line

11

 Terminal server access speed

 10-50 bps
5-75 bps
9-100 bps
0-110 bps
1-134.5 bps
6-150 bps
8-200 bps
2-300 bps

...

11 (continued)

 Terminal server access speed

 The following values are dependent on the PAD type:

4-600 bps
3-1200 bps
7-1800 bps
11-75 bps from, 1200 bps to DTE-C.
12-2400 bps
13-4800 bps
14-9600 bps
15-19200 bps
16-48000 bps
17-56000 bps
18-64000 bps

12

 Flow control of the PAD by the start-stop mode DTE

 0-Not operational
1-Use X-ON and X-OFF (DC1 and DC3 of International Alphabet #5)

13

 Linefeed insertion after carriage return

 0-Option not selected
1-Linefeed insertion after a carriage return in data the PAD sends to the DTE-C
2-Linefeed insertion after a carriage return in data the PAD receives from the DTE-C
4-Linefeed insertion after echo of each carriage return to the DTE-C

14

 Linefeed padding

 0-No padding
1-7-Number of padding characters inserted after the linefeed

15

 Editing

 0-No editing in data transfer
1-Editing in data transfer

16

 Character delete

 0-127 (a character from the International Alphabet #5)

17

 Line delete

 0-127 (a character from the International Alphabet #5)

18

 Line display

 0-127 (a character from the International Alphabet #5)

19

 Editing PAD service signals

 0-No editing PAD service signals
1-Editing PAD service signals

20

 Echo mask

 0-None (full packet)
1-Alphanumeric
2-Carriage return (the default)
4-ESC, BEL, ENQ, ACK
8-DEL, CAN, DC2
16-ETX, EOT
32-HT, LT, VT, FF
64-All other characters in columns 0 and 1 of International Alphabet #5

21

 Parity treatment

 0-No parity checking or generation
1-Parity checking
2-Parity generation

22

 Page wait

 0-No page wait
1-255-The number of linefeed characters sent by the PAD before page wait condition

Table 6-3 lists the supported X.3 profiles, shown with the profile name and the settings of each X.3 parameter in that profile.

Table 6-3. X.3 profiles

X.3 profile

Contents
CRT

 1:64, 2:1, 3:2, 4:0, 5:0, 6:5, 7:2, 8:0, 9:0, 10:0, 11:0, 12:1, 13:4, 14:0, 15:1, 16:8, 17:24, 18:18, 19:2, 20:0, 21:3, 22:0

INFONET

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

SCEN

 1:64, 2:1, 3:2, 4:0, 5:1, 6:5, 7:21, 8:0, 9:0, 10:0, 12:1, 13:4, 14:0, 15:1, 16:127, 17:24, 18:18, 19:1, 20:0, 21:0, 22:0

CC_SSP

 1:1, 2:1, 3:126, 4:0, 5:1, 6:1, 7:2, 8:0, 9:0, 10:0, 12:1, 13:0, 14:0, 15:0, 16:127, 17:24, 18:18, 19:1, 20:0, 21:0, 22:0

CC_TSP

 1:0, 2:0, 3:0, 4:20, 5:0, 6:0, 7:2, 8:0, 9:0, 10:0, 12:0, 13:0, 14:0, 15:0, 16:127, 17:24, 18:18, 19:1, 20:0, 21:0, 22:0

HARDCOPY

 1:64, 2:1, 3:2, 4:0, 5:2, 6:5, 7:21, 8:0, 9:5, 10:80, 12:1, 13:4, 14:5, 15:1, 16:8, 17:24, 18:18, 19:1, 20:0, 21:3, 22:0

HDX

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

SHARK

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

DEFAULT (MINIMAL)

 1:64, 2:1, 3:2, 4:0, 5:2, 6:5, 7:2, 8:0, 9:25, 10:72, 12:1, 13:5, 14:25, 15:1, 16:8, 17:24, 18:18, 19:1, 20:0, 21:0, 22:0

NULL

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

X.25 PAD commands

This section describes the X.25 PAD user commands in two categories: those that manage calls from the PAD and those that affect X.3 profile and parameter settings for the local or remote PAD.

To display a list of all X.25 PAD commands and syntax, use the Help command. Underlined letters in a command indicate the minimum string you have to type to execute the command. For example:

Commands for working with X.3 parameters and profiles

 These are the commands you can enter at the PAD prompt (*) to change an X.3 parameter setting or profile:

There are similar commands for changing X.3 settings on the remote PAD:

X.25 PAD commands for managing calls

 These are the commands you can enter at the X.25 PAD prompt to generate calls, specify a matching pattern for incoming calls, and perform related functions:

PAD service signals

 The PAD transmits PAD service signals to the terminal server in order to acknowledge PAD commands and to inform the user about the internal state of the PAD. The terminal server user can suppress the reception of PAD service signals by setting PAD parameter #6 to 0 (zero). The following table lists the PAD service signals.

Table 6-4. PAD service signals

Service signal

Description
RESET DTE

 The remote DTE has reset the virtual circuit.

RESET ERR

 A reset has occurred because of a local procedure error.

RESET NC

 A reset has occurred because of network congestion.

COM

 A call has been connected.

PAD ID

 Precedes a string that identifies the PAD.

ERROR

 The terminal server user entered an X.25/PAD command using faulty syntax.

CLR

 A virtual circuit has been cleared.

ENGAGED

 In response to the STAT command, this signal indicates that a virtual call is up.

FREE

 In response to the STAT command, this signal indicates that a virtual call is cleared.

PAR with X.3 parameter reference numbers and their current values

 This string is a response to the SET? command.

X.25 clear cause codes

Table 6-5 shows hexadecimal X.25 clear cause codes:

Table 6-5. Clear cause codes

Hex value

Cause code
01

 Number busy

03

 Invalid facility request

05

 Network congestion

09

 Out of order

0B

 Access barred

0D

 Not obtainable

11

 Remote procedure error

13

 Local procedure error

15

 RPOA out of order

19

 Reverse charging acceptance not subscribed

21

 Incompatible destination

29

 Fast select acceptance not subscribed

39

 Ship absent

C1

 Gateway-detected procedure error

C3

 Gateway congestion

X.25 diagnostic field values

Table 6-6 shows X.25 diagnostics:

Table 6-6. X.25 diagnostic field values

Hex value

Dec value

Diagnostic
0

 0

 No additional information

1

 1

 Invalid P(S)

2

 2

 Invalid P(R)

10

 16

 Packet type invalid

11

 17

 for state r1

12

 18

 for state r2

13

 19

 for state r3

14

 20

 for state p1

15

 21

 for state p2

16

 22

 for state p3

17

 23

 for state p4

18

 24

 for state p5

19

 25

 for state p6

1A

 26

 for state p7

1B

 27

 for state d1

1C

 28

 for state d2

1D

 29

 for state d3

20

 32

 Packet not allowed

21

 33

 unidentifiable packet

22

 34

 call on one-way LC

23

 35

 invalid packet type on a PVC

25

 37

 Reject not subscribed to

26

 38

 packet too short

27

 39

 packet too long

29

 41

 Restart packet with non-zero LC

2B

 43

 unauthorized interrupt confirmation

2C

 44

 unauthorized interrupt

2D

 45

 unauthorized reject

30

 48

 Timer expired

31

 49

 for incoming call (or for DTE timer expired for all request)

32

 50

 for clear indication (or for DTE timer expired or retransmission count surpassed for clear request)

33

 51

 for reset indication (or for DTE timer expired or retransmission count surpassed for reset request)

34

 52

 for restart indication (or for DTE timer expired or retransmission count surpassed for restart request)

40

 64

 Call setup, call clearing, or registration problem

41

 65

 Facility/registration code not allowed

42

 66

 Facility parameter not allowed

43

 67

 Invalid called address

44

 68

 Invalid calling address

45

 69

 Invalid facility/registration length

46

 70

 Incoming call barred

47

 71

 No logical channel available

48

 72

 Call collision

49

 73

 Duplicate facility requested

4A

 74

 Nonzero address length

4B

 75

 Nonzero facility length

4C

 76

 Facility not provided when expected

Monitoring X.25 and PAD service

The terminal server supports two commands for obtaining information about X.25 and PAD service. To invoke the terminal server, select System > Sys Diag > Term Serv and press Enter.

Displaying information about PAD sessions

To display information about PAD sessions:

The output includes the following fields:

Displaying information about X.25

 To view information about X.25 frame and packet layers:

The output includes these fields:

Setting up ISDN D-channel X.25 support

In addition to supporting X.25 over ISDN B-channels, the MAX can also support X.25 over the signaling D-channel.

Configuring ISDN D-channel X.25 support

To configure the MAX to support X.25 over the signaling D-channel:

  1. Open Ethernet > X25 > Any X25 profile.

  2. Set TEI to the value specified by your X.25 carrier.

    You can set TEI to any value from 0 to 63. The default value is 23. If you set TEI to 0, the Ascend unit requests a TEI assignment from the network.

  3. Set Call Type to D-Channel.

  4. Exit and save the settings.

Customized X.25 T3POS support

 MAX units with X.25 support the T3POS protocol, which can be used to send point of sale (POS) data over the ISDN D channel.

The MAX provides X25 Transaction Processing Protocol for Point-of-Service (T3POS) support over the existing Ascend X.25 stack. T3POS is a character-oriented, frame-formatted protocol designed for point-of-service (POS) transactions through an X.25-based packet switched network. T3POS allows you to send data over the ISDN D channel while continuing to send traffic over both B channels. The T3POS protocol involves three parties: the T3POS DTE (DTE), the T3POS PAD (PAD) and the T3POS Host (host). See Figure 6-3.

Figure 6-3. T3POS set up

A typical use of T3POS performs credit card authorization over the D channel while using the B channels to transmit inventory control data and other traffic. Figure 6-4 is an example of T3POS setup.

Figure 6-4. Example T3POS configuration

The Ascend T3POS implementation supports the following T3POS features:

Protocol summary

 This section provides a brief summary of the T3POS protocol. For complete details on the protocol and the MAX X.25 PAD, refer to the documents listed in References.

The T3POS protocol provides reliable and efficient data interchange (transactions) between a host (usually a transaction server) and a DTE (usually a client). The T3POS DTE is usually a client device communicating through an asynchronous port, while the T3POS host is a mainframe or server communicating through an X.25 packet network. The T3POS PAD (the MAX) converts data arriving from a T3POS DTE to a format that is capable of being transmitted over a packet network. It also ensures reliability and efficiency as described in the protocol.

Note that the T3POS PAD does not alter, check or convert the parity of characters it receives from or sends to the X.25 network or the T3POS DTE. T3POS essentially uses a data format of 8 bit no parity, or more accurately 7 bits, 1 parity, but the MAX ignores the parity bit.

T3POS frame types
Depending on the current state of a transaction or call, and the mode of operation selected, T3POS uses different data formats and frame structures. The MAX supports four modes of operation: Local, Binary-local, Transparent and Blind.

General frames
A general frame (or data frame) is any sequence of octets received from or sent to the DTE within the period specified by the T1 timer (this timer is known as the char-to-char timer). Furthermore, in Local and Binary-local modes and in opening frames, general frames are encapsulated in the format:

<STX [data] ETX XRC>

where:

Control frames
 The MAX uses control frames only when the MAX establishes a call and not during data transfer. You can configure the T3POS modes and most of the T3POS parameters for the T3POS PAD using the VT-100 interface in the MAX. However, the operating mode as well as called number and call user data and some user facilities can be overridden by using a control frame. A control frame is a supervisory frame of the format:

<SOH MSS CUD STX [data] ETX XRC>

where:

T3POS Timers
 The T3POS protocol defines six timers:

DTE-initiated calls
 If the first T3POS frame (which can be either a general frame or a control frame) the MAX receives is from the DTE, the session is qualified as DTE-initiated. When the MAX receives a general frame from the DTE, it triggers a call to the host using the settings in the Answer profile (or the Connection profile). When the MAX receives a control frame from the DTE, it also triggers a call to the host. In this case, however, the MAX uses the mode and called address specified in the control frame (if any) for the call, overriding any setting configured in the MAX.

Host-initiated calls
This implementation does not directly support incoming calls to the DTE. Instead, the DTE answers any calls initiated by the host connecting to the T3POS PAD and listening for host-initiated calls. The host must send a called address matching the pattern the DTE is listening for. This pattern does not need to be a complete X.121 address, but could be a sub-pattern (including wildcard characters).You configure the listening pattern using the Listen X.121 Addr parameter described in the MAX Reference Guide.

Flow control
Flow control should not be an issue for the X25 T3POS implementation. This is because the T3POS protocol has an effective window size of one (that is, every frame must be acknowledged before another frame is sent) and because the MAX buffers all the frames before forwarding them to the DTE or the host. However, you should chose the T2, T3 and T4 timers carefully to account for the fact that the MAX buffers the data before forwarding it. Note that the current Ascend modem code does RTS/CTS flow control all the time and this cannot be disabled.

References
The T3POS protocols derived from several documents that have become de facto standards:

Refer to the MAX 4.6C and 5.0A addenda for information on the MAX X25/PAD.

Configuring a T3POS connection

You can configure a T3POS connection using either the Connection profile (for authenticated users) or an Answer profile (for unauthenticated users).

Note: For more complete information about each of the T3POS parameters, see the MAX Reference Guide.

Configuring a T3POS connection consists of these general steps:

Note: The settings in the Connection or Answer profile can be overridden by the settings sent in control frames.

To configure a T3POS Connection profile:

  1. From the main Edit menu select Ethernet > Connections > any Connection profile.

  2. Set Active to Yes.

  3. Set Encaps to X25/T3POS.

  4. Open the Encaps Options submenu.

  5. Set X.25 Prof to the name of the X.25 that is to be used for this T3POS connection.

    The X.25 profile must exist and be active before you can save this Connection profile.

  6. Specify the Recv PW used to authenticate the caller.

  7. Specify the parameters used for the T3POS connection.

  8. Exit and save the Connection profile.

To configure a T3POS Answer profile:

  1. From the main Edit menu select Ethernet > Answer > Encaps.

  2. Set X25/PAD to Yes and X25/T3POS to Yes.

  3. Exit the Encaps submenu.

  4. Select T3POS Options.

  5. Set X.25 Prof to the name of the X.25 that is to be used for this T3POS connection.

    The X.25 profile must exist and be active before you can save the Answer profile.

  6. Specify the parameters used for the T3POS connection.

  7. Exit and save the Answer profile.

Accessing the T3POS

 User can access the T3POS in any of the following ways:

Accessing the T3POS from a dial-in connection
 This following example describes how to access the X.25/T3POS from a modem. The X.25 data link is already up because it is a nailed physical connection. This scenario also applies to Telnet users connecting the port 150 of the MAX.

Note: Telnet client programs should use 8 bit mode to connect to the MAX.

  1. Dial in through a modem or through Telnet.

  2. The user is authenticated against a Connection profile. If no Connection profile exists for the user, the Answer profile is used (if configured).

  3. Both the Connection and Answer profiles specify that the user is an X.25 user (that is, Encaps is set to X25/T3POS) as well as an X.25 profile that specifies the physical interface where the X.25 call is to be established.

    The X.25 profile determines the settings for the LAPB (or LAPD) and packet level, including timers, window size, and so on. For LAPB, the X.25 profile also specifies the nailed group to use for the logical call.

  4. The connection is then established using the settings in both the Connection profile (or Answer profile) and the X.25 profile and the call is directed to the T3POS.

  5. The user then must use the normal X.25/PAD commands as explained in the MAX 4.6C and 5.0A addenda.

Accessing the T3POS from the MAX terminal server interface
 This following example describes how to access the X.25/T3POS from the MAX terminal server interface or through Telnet.

  1. From the terminal server prompt, the user enters the T3POS command. For example:

    ascend% t3pos

  2. The user is then directed to the T3POS PAD and T3POS traffic can now be transmitted.

Accessing the T3POS through immediate mode
 To allow access the T3POS PAD immediately upon connecting, set Immediate Service to X25/T3POS in the Ethernet > Mod Config > TServ options submenu. This is typically how users connect to the T3POS PAD.

We recommend that when use immediate service, you suppress the terminal server banner (using the Banner parameter) as well as reducing the PPP delay parameter to its minimum. Both these parameters are in the Ethernet > Mod Config > TServ options submenu.


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