X3X28 PAD Answer Codes

From time to time, X.25 networks can transmit signals

• As a reply to a command (X.3 PAD parameters change, reading PAD parameters, etc.)

• On their own initiative

• As a consequence of an action from the remote DTE

Following this logic, you could receive four different types of signal codes (see Table 6-2):

• Error signals

• Disconnection signals

• Reset signals

• PAD editing signals (not covered in this section)

Error Code

Error

Description

COM

Call connected

The X.25 call has been established.

NP

NUA not present

The called X.25 address does not exist.

DER

Out of order

The called remote DTE is out of order.

OCC

Busy

The called remote DTE does not have any available virtual channels (VCs) at the moment.

DTE

Dropped by remote DTE

The called remote DTE canceled your X.25 call. This can mean that the remote DTE requires a subaddress specification (1 to 2 digits, such as: 0->9 or 00 > 99).

ACLs could avoid this to establish a session with the remote DTE. In this case, an X.25 spoofing attack could help a lot.

RPE

Remote procedure error

Called DTE is waiting for additional information (called "optional information") in the X.25 packet. This information could be represented by subaddresses under a numerical format (generally three digits are required even if the address has only three digits total or alphanumeric characters). In some X.3 PADs, this extension must be preceded by the letter D or P. Using D before the User Field displays the additional information, whereas using P applies for a "no echo" on the X.3 PAD.

RNA

Reverse not allowed

The called remote DTE does not accept reverse charge X.25 calls.

NA

Access barred

The called remote DTE does not accept the X.25 call from the calling DTE. It only accepts X.25 calls from authorized X.25 addresses. This case is very different from the previously mentioned DTE error: The customer is not defining the ACL. In this specific case, X.25 carrier enables this filtering service and authorizes the remote DTEs at a network level.

Table 6-1

Basic Answer and Error Codes

Signal Type

Description

Error Signals

ERR CAN

The command is correct from a syntax point of view, but it's not allowed in this state.

ERR ILL

The command is not correct from the syntax point of view and is not recognized.

ERR EXP

A timeout has been reached and the command hasn't been completed.

ERR PNA

X.3 PAD profile has not been assigned.

Disconnection

Signals

CLR OCC

The called remote DTE does not have any VCs available at the moment.

CLR NC

Network congestion conditions or a temporary fault in the network itself does not allow new virtual calls to be established.

CLR INV

The request is not valid.

CLR NA

The called remote DTE does not accept X.25 calls from the calling DTE. It only accepts X.25 calls from authorized X.25 addresses. This also means that the Closed User Group (CUG) is not compatible.

CLR ERR

The requested call is canceled due to a local procedure error.

CLR RPE

The requested call is canceled due to a remote DTE procedure error.

CLR NP

The called NUA is not assigned.

CLR DER

The called DTE is out of order.

CLR PAD

PAD canceled the X.25 call, following a "clear call" invitation from the remote DTE.

CLR DTE

Remote DTE canceled the X.25 call.

CLR RNA

Remote DTE does not accept reverse charge X.25/X.28 calls.

CLR ID

The requested X.29 protocol application modalities between the X.25 network PAD and the remote X.25 DTE are not correct.

Table 6-2 X.25 Signal Codes

Signal Type

Description

Reset Signals

RESET DTE

Remote DTE put the Virtual Call in reset mode.

RESET RPE

The call has been put in reset mode due to a remote DTE

procedure error.

RESET ERR

The call has been put in reset mode due to a local procedure error.

RESET NC

The call has been put in reset mode due to a remote DTE network

congestion state.

RESET DER

The call has been put in reset mode due to a remote DTE out-of-

service state.

RESET NOP

The call has been put in reset mode because the network is

restarting its service.

RESET DOP

The call has been put in reset mode because remote DTE is

restarting the service.

Table 6-2 X.25 Signal Codes (continued)

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