The cause is divided into three fields:

• The most significant bit (bit 8) is always coded as 1.

• The class (bits 5 through 7) indicates the general nature of the event. The value within the class (bits 1 through 4) indicates the event.

• Classes and values are coded as shown in    Table A-1   through Table A-7Some cause elements might also include an optional diagnostic octet.  

  Table A-1 Class 000 and 001 (Normal Events) Cause Codes

  Bit
  7 6 5	4 3 2 1	Hex	Value	Description
  0 0 0	0 0 0 1	81	1	Unallocated (unassigned) number. The ISDN number was presented in a valid format but is not currently assigned to any destination equipment.
  0 0 0	0 0 1 0	82	2	No route to specified transit network. The ISDN exchange was requested to route the call through an unrecognized intermediate network.
  0 0 0	0 0 1 1	83	3	No route to destination. The call was routed through a network that does not serve the destination address.
  0 0 0	0 1 1 0	86	6	Channel unacceptable. The specified channel is unable to accept the connection due to poor line quality.
  0 0 0	0 1 1 1	87	7	Call awarded and delivered in an established channel. The incoming call is being connected to an established channel.
  0 0 1	0 0 0 0	90	16	Normal clearing. No action is required.
  0 0 1	0 0 0 1	91	17	User busy. The called system has acknowledged the connection request but is unable to accept the call because the B channels are in use.
  0 0 1	0 0 1 0	92	18	No user responding. The destination did not respond to the call, and the connection cannot be completed.
  0 0 1	0 0 1 1	93	19	No answer from user (user alerted). The destination has responded to a connection request but timed out before completing the connection. This code indicates a problem at the remote end.
  0 0 1	0 1 0 1	95	21	Call rejected. The destination is capable of accepting the call (the remote end is not busy or incompatible with the local end) but rejected the call for some reason.
  0 0 1	0 1 1 0	96	22	Number changed. The ISDN number used to set up the call is no longer assigned. An alternate address (if one is used) might be displayed in the message.
  0 0 1	1 0 1 0	9A	26	Nonselected user clearing. The destination was capable of accepting the call but rejected it because the call was not awarded to the user.
  0 0 1	1 0 1 1	9B	27	Destination out of order. The destination cannot be reached because the interface and signaling message are malfunctioning. This code indicates that equipment is offline.
  0 0 1	1 1 0 0	9C	28	Invalid number format. The connection cannot be established because the destination address is in an unrecognized format or is incomplete.
  0 0 1	1 1 0 1	9D	29	Facility rejected. The facility requested by the user cannot be provided by the network. This code might indicate a subscription problem.
  0 0 1	1 1 1 0	9E	30	Response to status enquiry. This status message was generated in response to an earlier status enquiry message.
  0 0 1	1 1 1 1	9F	31	Normal, unspecified. No action is required.

  Class 010 (Network Congestion) Cause Codes

  Bit
  7 6 5	4 3 2 1	Hex	Value	Description
  0 1 0	0 0 1 0	A2	34	No circuit/channel available. A connection cannot be established because all appropriate channels are unavailable.
  0 1 0	0 0 1 1	A3	35	Call queued (AT&T only).
  0 1 0	0 1 1 0	A6	38	Network out of order. The network problem is expected to last a relatively long time, and an immediate reconnection attempt is likely to fail.
  0 1 0	1 0 0 1	A9	41	Temporary failure. The network problem is expected to be resolved shortly.
  0 1 0	1 0 1 0	AA	42	Switching equipment congestion. The destination cannot be reached because the switching equipment is overloaded
  0 1 0	1 0 1 1	AB	43	Access information discarded. The network cannot provide the requested access information.
  0 1 0	1 1 0 0	AC	44	Requested circuit or channel not available. The remote equipment cannot provide the requested channel for an unknown reason.
  0 1 0	1 1 1 1	AF	47	Resources unavailable, unspecified. The requested channel or service is unavailable for an unspecified reason.

  Class 011 (Service or Option Not Available) Cause Codes

  Bit
  7 6 5	4 3 2 1	Hex	Value	Description
  0 1 1	0 0 0 1	B1	49	Quality of service unavailable. The requested quality of service cannot be provided by the network. This code indicates a possible subscription problem.
  0 1 1	0 0 1 0	B2	50	Requested facility not subscribed. The requested supplementary service is available, but the requestor is not a subscriber.
  0 1 1	0 1 0 0	B4	52	Outgoing calls barred (AT&T only).
  0 1 1	0 1 1 0	B6	54	Incoming calls barred.
  0 1 1	1 0 0 1	B9	57	Bearer capability not authorized. The user is requesting a bearer capability that he or she is not authorized to use.
  0 1 1	1 0 1 0	BA	58	Bearer capability not presently available. The user is requesting a bearer capability that is normally available, but is not currently available due to either a network or a subscription problem.
  0 1 1	1 1 1 1	BF	63	Service or option not available, unspecified. The network is unable to provide the requested service for an unspecified reason, possibly due to a subscription problem.

   

  Class 100 (Service or Option Not Implemented) Cause Codes

  Bit
  7 6 5	4 3 2 1	Hex	Value	Description
  1 0 0	0 0 0 1	C1	65	Bearer capability not implemented. The requested bearer capability is not available on the network.
  1 0 0	0 0 1 0	C2	66	Channel type not implemented. The requested channel type is not available on the network or the destination equipment.
  1 0 0	0 1 0 1	C5	69	Requested facility not implemented. The requested facility is not available on the remote equipment.
  1 0 0	0 1 1 0	C6	70	Only restricted digital information bearer capability is available. The network cannot provide unrestricted digital information bearer capability.
  1 0 0	1 1 1 1	CF	79	Service or option not implemented, unspecified. The network is unable to provide the requested service for an unspecified reason, possibly due to a subscription problem.

  Class 101 (Invalid Message) Cause Codes

  Bit
  7 6 5	4 3 2 1	Hex	Value	Description
  1 0 1	0 0 0 1	D1	81	Invalid call reference value. The remote equipment has received a call reference that is not being used by the user-network interface.
  1 0 1	0 0 1 0	D2	82	Identified channel does not exist. The equipment receiving the call does not have the requested channel activated.
  1 0 1	0 0 1 1	D3	83	A suspended call exists, but this call identity does not. The network received a call resume request that is already in use for a suspended call.
  1 0 1	0 1 0 0	D4	84	Call identity in use. The network received a call resume request that is already in use for a suspended call.
  1 0 1	0 1 0 1	D5	85	Invalid digit value for number. OR No call suspended. The network received an unnecessary call resume request (no calls were suspended at the time of the request). Retrying the call might clear the problem.
  1 0 1	0 1 1 0	D6	86	Call having the requested call identity has been cleared. The network received a call resume request indicating a suspended call that has been cleared or timed out.
  1 0 1	1 0 0 0	D8	88	Incompatible destination. An attempt was made to connect to a non-ISDN device-for example, an analog telephone line.
  1 0 1	1 0 1 1	DB	91	Invalid transit network selection. The ISDN exchange was requested to route the call through an unrecognized intermediate network.
  1 0 1	1 1 1 1	DF	95	Invalid message, unspecified. This code most likely indicates D channel error. Contact your authorized telephone service provider if this error persists.

  Class 110 (Protocol) Cause Codes

  Bit
  7 6 5	4 3 2 1	Hex	Value	Description
  1 1 0	0 0 0 0	E0	96	Mandatory information element is missing. This code most likely indicates a D channel error. Contact your authorized telephone service provider if this error persists.
  1 1 0	0 0 0 1	E1	97	Message type nonexistent or not implemented. The message type received was either invalid or not implemented on the receiving device. This code indicates a problem with either the D channel or the configuration of the remote device.
  1 1 0	0 0 1 0	E2	98	Message not compatible with call state, or message type nonexistent or not implemented. An invalid message was received and no standard cause applies. This code most likely indicates a D channel error. Contact your authorized telephone service provider if this error persists.
  1 1 0	0 0 1 1	E3	99	Information element does not exist or is not implemented. An invalid message was received containing unrecognized information elements. This code most likely indicates a D channel error. Contact your authorized telephone service provider if this error persists.
  1 1 0	0 1 0 0	E4	100	Invalid information element contents. This code indicates a D channel error. The remote equipment received a message containing an invalid information element.
  1 1 0	0 1 0 1	E5	101	Message not compatible with call state. This code indicates a D channel error. The remote equipment received a message that did not correspond to the current call state.
  1 1 0	0 1 1 0	E6	102	Recovery on timer expiry. A timeout has initiated an error-handling procedure.
  1 1 0	1 1 1 1	EF	111	Protocol error, unspecified. An unspecified D channel error has occurred.

  Class 110 (Protocol) Cause Codes

  Bit
  7 6 5	4 3 2 1	Hex	Value	Description
  1 1 1	1 1 1 1	FF	127	Internetworking, unspecified. This code might indicate an error. A network event has occurred resulting in certain unspecified actions.