ISDN Messages
ISDN signaling is carried out by messages that are sent between endpoints on the D channel. The Message Type is a single byte (octet) that indicates what type of message is being sent or received. Four general categories of messages might be present: Call Establishment, Call Information, Call Clearing, and Miscellaneous. Generally, the most useful messages to understand are the Call Establishment and Call Clearing messages. The most common messages are listed in Table 4-5.
Table 4-5 ISDN Messages
|
Message Type |
Binary Value |
Message Name |
|
000 Call Establishment |
00001 |
ALERTing |
|
00010 |
CALL PROCeeding |
|
|
00011 |
PROGress |
|
|
00101 |
SETUP |
|
|
00111 |
CONNect |
|
|
01101 |
SETUP ACKnowledge |
|
|
01111 |
CONNect ACKnowledge |
|
|
001 Call Information |
00000 |
USER INFOrmation |
|
00001 |
SUSPend REJect |
|
|
00010 |
RESume REJect |
|
|
00101 |
SUSPend |
|
|
00110 |
RESume |
|
|
01101 |
SUSPend ACKnowledge |
|
|
01110 |
RESume ACKnowledge |
|
|
010 Call Clearing |
00101 |
DISConnect |
|
00110 |
Restart |
|
|
01101 |
RELease |
|
|
01110 |
Restart ACKnowledge |
|
|
11010 |
RELease COMplete |
Table 4-5 ISDN Messages
|
Message Type |
Binary Value |
Message Name |
|
011 Miscellaneous |
00000 |
SEGment |
|
00010 |
FACility |
|
|
01110 |
NOTIFY |
|
|
10101 |
STATUS ENQuiry |
|
|
11001 |
Congestion Control |
|
|
11011 |
INFOrmation |
|
|
11101 |
STATUS |
Table 4-6 provides a list of message types and the IEs that can be associated with each message.
Table 4-6 Most Common Message Types and Associated IEs
|
Message Type |
IEs Associated with Message |
|
ALERTing |
Bearer capability, CID, Progress indicator, Display, Signal, Higher layer compatibility |
|
CALL PROCeeding |
Bearer capability, CID, Progress indicator, Display, Higher layer compatibility |
|
SETUP |
Sending complete, Repeat indicator, Bearer capability, CID, Progress indicator, Network specific facilities, Display, Keypad facility, Signal, Calling party number, Calling party sub address, Called party number, Called party sub address, Transit network selection, Repeat indicator, Lower layer compatibility, Higher layer compatibility |
|
CONNect |
Bearer capability, CID, Progress indicator, Display, Date/time, Signal, Lower layer compatibility, Higher layer compatibility |
|
SETUP ACKnowledge |
CID, Progress indicator, Display, Signal |
|
CONNect ACKnowledge |
Display, Signal |
|
DISConnect |
Cause, Progress indicator, Display, Signal |
|
RELease |
Cause, Display, Signal |
|
RELease COMplete |
Cause, Display, Signal |
|
STATUS ENQuiry |
Display |
|
STATUS |
Cause, Call state, Display |
ISDN Information Elements
Each type of message has Mandatory and Optional IEs associated with it. The Information Element is identified with a single octet. Although a few single octet IEs exist, most have multiple octets associated with them.
A study of all available IEs is beyond the scope of this topic. However, commonly used IEs are listed in Table 4-7. For further details, many public references are available on the Internet. For this section, the cause, facility, progress, and display IEs are reviewed. They represent the information most in demand in telephony systems and, therefore, most important in the communications between the voice gateway and PBX or PSTN. Figure 4-9 illustrates the structure of common information elements.
Figure 4-9 Common Information Elements Cause IE
The cause IE provides one or more octets that might help in diagnosing network or customer premises equipment (CPE) problems. When a call is terminated, a cause ID indicates the reason for the termination. Both sides can generate a cause ID, and cause IDs are created for every call. When an ISDN problem occurs in the network, the cause value, shown in octet 4 in Figure 4-9, represents useful debug information in the ISDN protocol log. The telephone company equipment translates these values to associated phrases. Cause messages are classified as normal events, resource or service availability, message validity, protocol error, or interworking. The most common phrases are listed in Table 4-8.
Facility IE
Supplemental services are invoked by sending facility IEs in a facility message to an ISDN switching device such as a PBX.
Supplemental services are widely used by PBXs and in the PSTN. IP telephony systems that are connected to these types of switches must be able to send and receive these messages. The supplemental service and associated parameters that are invoked are PBX-specific and should be provided by the PBX manufacturer.
Progress IE
Progress tones such as ring back and busy tones, and announcements, such as "The number you have dialed is no longer in service," are required to successfully signal voice calls. Progress tones can be generated by the originating, terminating, or intermediate devices.
The indication of in-band tones and announcements is controlled by the progress IE in ISDN and H.323 networks. The progress IE signals those interworking situations where in-band tones and announcements must be used.
The indication that tones and announcements are available is signaled by an alerting, call proceeding, progress, connect, setup acknowledge, or disconnect message containing a progress indicator (PI) of PI = 1 or 8, which would be sent in the progress description field in octet 4. A SETUP message of PI = 3 means the switch is indicating to the originating gateway that in-band messages are expected.
Table 4-7 describes the meanings for various Progress Description field values.
Table 4-7 ISDN Progress Description Field Values
|
Hex Value |
Decimal |
Binary |
Description |
|
0×01 |
1 |
000 0001 |
Call is not end-to-end ISDN. |
|
0×02 |
2 |
000 0010 |
Destination address is non-ISDN. |
|
0×03 |
3 |
000 0011 |
Origination address is non-ISDN. |
|
0×04 |
4 |
000 0100 |
Call has returned to the ISDN. |
|
0×08 |
8 |
000 1000 |
In-band information or the appropriate pattern is now available. |
|
0x0A |
10 |
000 1010 |
Delay in response at the destination interface. |
Display IE
The display IE sends text to do such things as provide output for an LCD display. This IE is commonly used to pass caller name information over a PRI, although PBXs and telecommunications service providers with NI3-type ISDN switches exist that only pass calling name information with the facility IE in QSIG. The display and facility IEs are used by Cisco Unified Communications Manager (UCM) to support caller name and number identification presentation. These services are based on the device control protocols that handle the call. Not all device protocols provide caller number and name information in the protocol messages.
Table 4-8 details common information elements.
Table 4-8 Common IEs
|
Value |
Name |
Description |
|
|
0×04 |
Bearer |
Specifies packet or circuit mode, data rate, and type of information content (voice). |
|
|
0×08 |
Cause |
Provides the reason a call was rejected or disconnected. Following is a sample of possible causes: |
|
|
0×01 |
Unassigned number |
||
|
0×03 |
No route to destination |
||
|
0×06 |
Channel unacceptable |
||
|
0×10 |
Normal call clearing |
||
|
0×11 |
User busy |
||
|
0×12 |
User not responding |
||
|
0×13 |
User alerting; no answer |
||
|
0x1B |
Destination out of order |
||
|
0x1C |
Invalid number format |
||
|
0×22 |
No circuit or channel available |
||
|
0x2A |
Switching equipment congestion |
||
|
0×14 |
Call State |
Current status of a call in terms of the standard Q.931 state machine. |
|
|
0×18 |
CID |
Defines the B channel being used. |
|
Table 4-8 Common IEs
|
Value |
Name |
Description |
||
|
0x1C |
Facility |
Indicates the invocation and operation of supplemental services, identified by the corresponding operation value within the facility IE. Following are some examples of supplemental services: |
||
|
Called or calling party identification |
||||
|
Sub addressing |
||||
|
Hold or retrieve |
||||
|
Call transfer |
||||
|
Message waiting |
||||
|
0x1E |
Progress Indication |
Provides information about the call in progress. Following are some examples of progress indication: |
||
|
0×01 |
Call is not end-to-end ISDN. |
|||
|
0×02 |
Destination address is non-ISDN. |
|||
|
0×03 |
Origination address is non-ISDN. |
|||
|
0×04 |
Call has returned to the ISDN. |
|||
|
0×08 |
In-band information or the appropriate pattern is now available. |
|||
|
0x0A |
Delay in response at the destination interface. |
|||
|
0×28 |
Display |
Provides human-readable text that can be specified with almost any message (for example, to provide text for an LCD display). |
||
|
0x2C |
Keypad |
Dialed digits. |
||
|
0×34 |
Signal |
Provides call status tones according to the following chart: |
||
|
0×00 |
Dial tone |
350 Hz + 440 Hz; continuous |
||
|
0×01 |
Ringing |
440 Hz + 480 Hz; 2 sec on/4 sec off |
||
|
0×02 |
Intercept |
Alternating 440 Hz and 620 Hz; 250 ms |
||
|
0×03 |
Network congestion (fast busy) |
480 Hz + 620 Hz; 250 ms on/250 ms off |
||
|
0×04 |
Busy |
480 Hz + 620 Hz; 500 ms on/500 ms off |
||
|
0×05 |
Confirm |
350 Hz + 440 Hz; repeated three times: 100 ms on/100 ms off |
||
|
0×06 |
Answer |
Not used |
||
|
0×07 |
Call waiting |
440 Hz; 300 ms burst |
||
|
0×08 |
Off-hook warning |
1400 Hz + 2060 Hz + 2450 Hz + 2600 Hz; 100 ms on/100 ms off |
||
|
0x3F |
Tones |
Off |
||
Table 4-8 Common IEs
|
Value |
Name |
Description |
|
0x3A |
SPID |
Contains a service profile identifier (SPID). |
|
0x4C |
Connected Number |
Indicates the remaining caller if a disconnect occurs during CONFERENCE. |
|
0x6C |
Calling Party Number |
Origin phone number. |
|
0×70 |
Called Party Number |
Phone number being dialed. |
|
0x7C |
LLC |
Lower layer compatibility. |
|
0x7D |
HLC |
Higher layer compatibility. |
|
0x7E |
User-User |
User-user information. |
The debug isdn q931 command can be used to view detailed Layer 3 signaling information (that is, Q.931 information). Example 4-1 provides sample output from a debug isdn q931 command.
Example 4-1 debug isdn q931 Command Output
Table 4-9, taken from the Debug Command Reference, provides an example of how to read the hexadecimal values with the ISDN bearer capability values.
Table 4-9 ISDN Bearer Capability Values
|
Field |
Value Description |
|
0x |
Indication that the values that follow are in hexadecimal |
|
88 |
ITU-T coding standard; unrestricted digital information |
|
90 |
Circuit mode, 64 kbps |
|
21 |
Layer 1, V.110/X.30 |
|
8F |
Synchronous, no in-band negotiation, 56 kbps |
|
0x8090A2 |
Voice call (mu-law) |
|
0x9090A2 |
Voice call (mu-law), 3.1 kHz Audio |
|
0x8090A3 |
Voice call (a-law) |
|
0x9090A3 |
Voice call (a-law), 3.1 kHz Audio |
0×8890 is for 64 kbps or 0x218F is for 56 kbps. In Example 4-1, the SETUP message in the example configuration indicates Bearer Capability i = 0×8090. Therefore, you know that you have a 64-kbps bearer stream.
ISDN cause codes use the following format: i = 0x y1 y2 z1 z2 [a1 a2], as detailed in Table 4-10.
Table 4-10 ISDN Cause Code Fields_
|
Field |
Value—Description |
|
0x |
The values that follow are in hexadecimal. |
|
y1 |
8—ITU-T standard coding. |
|
y2 |
0—User |
|
1—Private network serving local user |
|
|
2—Public network serving local user |
|
|
3—Transit network |
|
|
4—Public network serving remote user |
|
|
5—Private network serving remote user |
|
|
7—International network |
|
|
A—Network beyond internetworking point |
|
|
z1 |
Class (the more significant hexadecimal number) of cause value. |
|
z2 |
Value (the less significant hexadecimal number) of cause value. |
|
a1 |
(Optional) Diagnostic field that is always 8. |
|
a2 |
(Optional) Diagnostic field that is one of the following values: |
|
0—Unknown |
|
|
1—Permanent |
|
|
2—Transient |
Table 4-11 lists some of the cause value fields of the cause information element.
Table 4-11 ISDN Cause Values
|
Decimal |
Hexadecimal |
Cause |
Explanation |
|
30 |
1E |
Response to STATUS ENQUIRY |
The status message was generated in direct response to the prior receipt of a status enquiry message. |
|
31 |
1F |
Normal, unspecified |
Reports the occurrence of a normal event when no standard cause applies. No action required. |
|
34 |
22 |
No circuit/channel available |
The connection cannot be established because no appropriate channel is available to take the call. |
