Chapter Two:
Parameter 1 - PAD Recall Using a Character
2.4 Typical X.25 Scenarios
The examples below provide scenarios of typical activities you might perform and/or experience withX.25 connections. Each one provides the "Activity," a reference to a command description within this document for further discussion, the "Result" of the "Activity," and an "Example Log-on" when applicable. These examples may be helpful as reference when you are writing sequences for users, or when developing software.
Synopsis of a Communication Session with a Packet Switched Network The events in a typical X.25 communication session are detailed below.
Establishing a Network Connection...
X.25 connections begin the same as point-to-point connections - with a modem call to a remote system. Once connected to the network, however, you use the PAD to make one or more virtual connections to other nodes on the network.
Making a Virtual Connection..•
The following procedure explains the steps for the major activities in X.25 communications.
Step ,: Before placing a call to the network, you must configure the modem to make an X.25
configuring connection. First, enter the modem's command state the modem
forX.2S Issue ATIIQ5S44=3S46=6S48=0 <CR>.This command string verifies that the modem is in the error-control mode (IIQ5) and sets the values of three S-Registers that control the modem's feature negotiation. 548=0 controls the negotiation process; 546=6 stipulates an X.25
connection. Now, instead of negotiating aV.42connection, the modem attempts connections using theX.25 protocol.
Step2: Next, dial the number of the network to which you subscribe. This is a standard modem call calling the made with theD command:ATDT9,555-1234 <CR> (for example). If the call is successful, the
network modem will connect and issue a result code sequence. The carrier and connect messages you receive depend on the connection. If you want to see the extended messages, you must issue the W1 command. With this feature enabled, for a2400bps connection, the messages will be CARRIER 2400 PROTOCOL: X.25/LAP-B CONNECT 4800.The PAD automatically enters the command state for channell.
Step 3: To establish a virtual call to a user on the network, you must have the network address of the makinga other node to which you wish to connect and any other information required by the network.
virtual For example, to connect to a service on the network, you would issue:CALL R-3110 20200202 connection to <CR> - CALLis the command for initiating connections with a remote system. The (R)
the remote command permits reverse charging if the user at the other end of the virtual connection system requests it. J 110is a network ID code and20200202 is the destination address. If the call is
accepted by the remote system, the network will return the result code sequence: CON COM:
W2:2,P7:7,X0277 COM.The PAD then enters the data transfer state. If the call is not accepted, the network returns a response such asClR Dn 157 FAC: (followed by network information as alphanumeric characters XOOO...).The PAD remains in the PAD command state.
Step 4: Once connected, you can perform desired communications - file transfers, interactive typing, petfonnlng etc.
on-line activities
Step S: When ready to end the session, issueCTRl-'to return to the channel's command state. Then dearing the issueClR <CII>. This clears the current virtual connection. The network responds with ClR
cal' CONF FAC: (followed by network information as alphanumeric characters XOOO...).
Note: All virtual calls shouldbe cleared before returning to the AT command state.
Step 6: EXIT <CR> - Leaves PAD command state and enters the AT command state.
exiting the
PAD
Step 7: ATHO <CR> - Breaks the physical connection to the network node. This action hangs up the terminating telephone, in the same way other communication sessions are terminated.
the network connection
Establishing a Virtual Connection
The example log-on and results below illustrate a successful attempt to establish a virtual connection.
Activity:
Refer to:
Result:
Example Log-on:
User dials CompuServe™, connects, then issuesCALLcommand to establish a virtual connection.
Call is accepted
CALLcommand description
Call is accepted. Depending on the network requirements, the modem sends back the following:
CON <address> <facility_codes> COM <user_data>
Note: The data in capital letters is sent to you; the data in angle bfackets mayor may notbe sent to you.
User calls Telenet™ and issues CALL command togo on-line with CompuServe:
Enter Result
Enter Result
ATIIQ5 544=3 546=6 548=0 W1 DT 9,5550123 <CII>
CARRIER 2400
PROTOCOL: X.25/LAP-B CONNECT 2400
CALL R-311020200202 <CR>
CON COM: W2:2,P7:7,X 0277 COM
Attempting a Virtual Connection that Is not Accepted
This scenario depicts an unsuccessful attempt at establishing a virtual connection. For various reasons, the connection may notbecompleted. The reason(s) willbeprovided in result codes generated by the PAD.
Activity:
Refer to:
Result:
Example Log-on:
User dials packet-switched network, connects, then issues CALLcommand to establish a virtual connection, but call is not accepted.
CALLcommand description.
Call is not accepted. Depending on the network requirements, the modem returns the following result codes: CLR <clear_cause_codes> <diagnostic_codes> <address> <facility_codes>
User calls Telenet and issues CALL command to go on-line with CompuServe:
Enter Result
Enter Result
ATIIQ5 544=3 546=6 548=0 W1 DT 9,5550123 <CII>
CARRIER 2400
Resetting a Virtual Channel
While on-line to another user on the network, if the escape sequence is used to return to the PAD command state, the channel will be reset.
Activity:
Refer to:
Result:
Example Log-on:
User is connected to an packet-switched network, has established a virtual connection, then escapes to the PAD command state and resets the virtual channel
RESETcommand description
The PAD resets the virtual channel and returns the following information to the user: RESIET
<reset_cause_codes>
n/a
Resetting a Virtual Channel a by Remote PAD
As a result of network activity, the remote PAD may reset a virtual channel. If this occurs, data may be lost. The call must also be re-established.
Activity:
Refer to:
Result:
Example Log-on:
User is connected to a packet-switched network, has established a virtual connection, and the network resets the virtual channel
RESETcommand description
The PAD resets the virtual channel and returns the following information to the user: RESET
<reset_cause_codes> <diagnostic_codes>
n/a
Clearing a Virtual Channel
This scenario is the typical way to end a virtual call following a session. This is also the process to use when other calls are to be maintained.
Activity:
Refer to:
Result:
Example Log-on:
User is connected to a packet-switched network, has established a virtual connection, escapesto PAD command state, and clears the call.
ClRcommand description.
The PAD clears the call, sets all PAD parameters back to their values prior to establishing the virtual connection, and returns the following result codes: ClDl CONF FAC: X 00 00C1 04 00 00 01 03 C2 08 00 00 00 08 80 00 00 04
User calls Telenet and issues CALL command to go on-line with CompuServe:
Enter
AT&Q5 544=3 546=6 548=0 W1 DT 9,5550123 <CR>
CARRIIER 2400
Ctrl-P to return to the PAD command state ClR CONF !FAC:
XOOOOC1 040000001 03C208000000008800000004
Accepting a Request from Remote PAD to Establish Virtual Connection
A request from a remote PAD to establish a virtual connection will be received when another network user is attempting to make a connection with you. Although the standard response is to accept, beaware that accepting the request, you may be accepting certain network charges other than packet transmission fees.
Activity:
Refer to:
Result:
Example Log-on:
User using modem and receives request from remote unit to establish a virtual connection ACC command description
The remotePADsends the following result code to the user IC <facility_codes> COM <user_data>
nfa
Clear Command Failed
This scenario describes a request to clear a channel denied by the PAD. This may occur because the modem is transferring data.
Activity:
Refer to:
Result:
Example Log-on:
User is connected to a packet-switched network, has established a virtual connection, escapes to PADcommand state, but call fails to be cleared
ClR command description
The PADsends the following result code: ClR FAilED nfa
Link Restart Occurs
The example result code below indicates that the virtual links have been dropped. The connections must be re-established.
Activity:
Example:
AIIlink restart" is a major error condition in which all virtual calls are disconnected and the virtual channels return to "start" mode
liNK RESTARTED <restart_cause_codes>
I I I I ' ! I I 1
I I . . I j I