Decoding the ISDN_Monitor output

The ISDN_Monitor decodes Layer 2 ISDN frames and Layer 3 ISDN messages so that you can find out what is happening to calls on specific channels.

Layer 2

Blueworx Voice Response frames that are sent to the network are on the left-hand half of the screen. Layer 2 frames that are sent to Blueworx Voice Response from the network are shown on the right-hand half of the screen.

Figure 1 shows the trace of a typical startup sequence in which the switch sends a Synchronous Asynchronous Balanced Mode Extended (SABME) frame to start the ISDN Layer 2 link, and Blueworx Voice Response responds with a UA frame to verify that the Layer 2 link is established. If no Layer 3 messages are being sent at this point, there is a handshake protocol on Layer 2 with RR frames (RR means receiver-ready; that is, ready to receive frames). This handshake is at 10 second intervals.

Figure 1. Trace of a typical startup sequence


                                       Q921 NETWORK  12:24:31  TEI=00   Cmd

                                       SAPI=00  U FRAME - SABME

Q921 DT/6000  12:24:31   TEI=00   Rsp

SAPI=00  U FRAME - UA

Q921 DT/6000  12:24:41   TEI=00   Cmd

SAPI=00  S FRAME - RR  NR=000

                                       Q921 NETWORK  12:24:41  TEI=00   Rsp

                                       SAPI=00  S FRAME - RR  NR=000

Trace elements

The elements of the trace are:

Timestamp in hours, minutes and seconds.
Terminal Endpoint Identifier (TEI). Only TEI = 00 is supported on Primary rate. (Broadcast TEI = 127 is not supported.)
Service Access Point Identifier (SAPI). This is 00 for Call primitives. (SAPI 16 for X.25 and 63 for Broadcast messages are not supported.)
Cmd/Rsp flag. This decodes the Layer 2 Command/Response Flag to determine whether the Layer 2 frame is a command or a response. In Figure 1 the SABME was a command and the UA frame was a response.
An RR frame that is sent as a command expects the other end of the connection to respond to the RR command with an RR response as shown in Figure 1.
On the second line of the decode there are U, S, and I frames (Unnumbered, Supervisory, and Information frames).
Unnumbered frames are used to establish and release the Layer 2 link and have no connection to the data that is sent across it.

SABME is sent to try to establish the Layer 2 link.

UA (Unnumbered Acknowledgment) is sent in response to a SABME to verify that the Layer 2 link is established, or sent in response to a DISC when the link is released.

DISC (Disconnect) is sent to release the Layer 2 link.

DM (Disconnected Mode) is sent in response to a SABME if the Layer 2 link cannot start. This is also sent in some error conditions.

Supervisory frames are used to control the flow of the data frames across the Layer 2 link:

RR (Receiver Ready)
RNR (Receiver Not Ready)
REJ (Reject Frame)

These frames have an associated Number of Frames Received (NR) counter; this is displayed on the trace.

Information frames contain the Layer 3 messages for call processing, and for link and channel initialization. These frames are sent with the Number of Frames Received (NR) and also a Number of Frames Sent (NS).

When an ISDN trunk is disabled, Blueworx Voice Response sends a DISC frame to the switch to inform it that the link is released and is unusable. See Figure 2.

Figure 2. Blueworx Voice Response sending a DISC frame to the switch


Q921 DT/6000  12:48:44   TEI=00   Cmd

SAPI=00  U FRAME - DISC

                                       Q921 NETWORK  12:48:44  TEI=00   Rsp

                                       SAPI=00  U FRAME - UA

When the trunk has been enabled, a delay sometimes occurs before the switch starts its Layer 2 protocol. Blueworx Voice Response tries to establish the link if it has a Layer 3 message to send, so it sends up to four SABME frames at one-second intervals (see Figure 3). If the switch responds in this time, the Layer 3 messages are sent; otherwise Blueworx Voice Response waits for the switch to activate the link by sending SABME.

Figure 3. Blueworx Voice Response sending SABME frames at one second intervals


Q921 DT/6000  12:50:01   TEI=00   Cmd

SAPI=00  U FRAME - SABME

Q921 DT/6000  12:50:02   TEI=00   Cmd

SAPI=00  U FRAME - SABME

Q921 DT/6000  12:50:03   TEI=00   Cmd

SAPI=00  U FRAME - SABME

Q921 DT/6000  12:50:04   TEI=00   Cmd

SAPI=00  U FRAME - SABME

Layer 3

Normally, the first Layer 3 messages that are sent between the switch and Blueworx Voice Response are restart messages, which initialize trunks or channels into a predefined state so that they are ready to initiate or receive calls. Figure 4 shows an example of a RESTART message initializing a whole trunk.

Figure 4. A RESTART message initializing a whole trunk


                                       Q921 NETWORK  13:01:44  TEI=00   Cmd

                                       SAPI=00  I FRAME  NS=001 NR=000

 Q931 <---- RESTART Ref=0000 Len=8 08 02 00 00 46

    - 79 01 86

      RESTART INDICATOR

      3 : Single Interface


Q921 DT/6000  13:01:44   TEI=00   Rsp

SAPI=00  I FRAME  NS=001  NR=001

 Q931 ----> RESTART ACKNOWLEDGE Ref=8000 Len=8 08 02 80 00 4E

    - 79 01 86

      RESTART INDICATOR

      3 : Single Interface

Layer 3 messages are displayed on the left hand side of the screen to allow room for textual decodes. The direction of the message is shown by an arrow. The previous Layer 2 frame decode of the I frame also shows the direction of the frame.

The message shown in Figure 4 can be decoded as follows:

Q931 = ISDN Layer 3.
RESTART = Layer 3 message type.
Ref=0000 = ISDN Layer 3 Call Reference. For channel and trunk messages, this value is 0, but it is non-zero for call related messages. The Most Significant Bit (MSB) is used to show which side initiated the event (for RESTART) or which side initiated the call.
If the MSB=0, the call or event was initiated from that side. If it is set to 1, it was initiated at the other side. You can see this by comparing the 'Ref' fields on the RESTART and RESTART ACKNOWLEDGE messages.
Len=8 = Length of ISDN message
08 02 00 00 46
- The first byte is the protocol discriminator. 08 is the one normally used, but different values are used for maintenance control.
- The second byte is the call reference length. This is normally 2, but when RESTART messages are received from some switches, it is 1.
- The third and fourth bytes are the Call Reference value in hexadecimal. If the Call Reference length is only 1, only the third byte is used.
- The fifth byte (or fourth byte if it is not used for the Call Reference value) is used as the Layer 3 message value. The value 46 is RESTART, and 4E is RESTART ACKNOWLEDGE.
After the first line decode, each Information Element (IE) is decoded with the hex information displayed first.
79 01 86
- 79 = RESTART INDICATOR information element
- 01 = length of the Information element
- 86 = Octet 3 which is decoded as 'Single Interface'

The channel ID information element

The default in Blueworx Voice Response is to send RESTART messages to the network whenever a channel is enabled. The Channel ID Information Element tells the network which channel is involved, as shown in Figure 5.

In octet 3 “Interface Implicit” means that the identified B channel is on the same trunk as the signaling channel.

Figure 5. The channel ID information element


Q921 DT/6000  14:00:00   TEI=00   Cmd

SAPI=00  I FRAME  NS=005  NR=003

 Q931 ----> RESTART Ref=0000 Len=13 08 02 80 00 46

    - 18 03 A9 07 83 81

      CHANNEL IDENTIFICATION

      3 : Interface Explicit, Primary, Exclusive, Not D-Chan, Channel in
 next oct

      4 : CCITT standard, Channel number in next octet, B channel units

      5 : Channel 01

    - 79 01 80

      RESTART INDICATOR

      3 : Indicated Channels

Figure 6 shows NFAS on a channel enabled on a trunk with no signaling. The overall length of the message has increased by 1 byte, the interface is defined as 'Explicit' which means that the Trunk Identifier in Octet 3a is defined.

Figure 6. Trace showing NFAS on a channel enabled on a trunk with no signaling


Q921 DT/6000  14:02:23   TEI=00   Cmd

SAPI=00  I FRAME  NS=003  NR=002

 Q931 ----> RESTART Ref=0000 Len=14 08 02 80 00 46

    - 18 03 C9 07 83 81

      CHANNEL IDENTIFICATION

      3 : Interface Explicit, Primary, Exclusive, Not D-Chan, Channel in
 next oct

      3a: Trunk Id 07

      4 : CCITT standard, Channel number in next octet, B channel units

      5 : Channel 01

    - 79 01 80

      RESTART INDICATOR

      3 : Indicated Channels

Outbound calls

Figure 7 shows the flow of call related events for an outbound call from Blueworx Voice Response:

The definition of the messages in Figure 7 is:

SETUP is used to ask for a Service Type in the Bearer IE. The channel to be used is requested in the Channel Id IE. The number to telephone is in the Called Party Number field. Blueworx Voice Response does not send Calling Party Number to the switch, but as a service option the switch may send this IE.
CALL PROCEEDING is used to inform Blueworx Voice Response that the channel can be used.
ALERT is used to inform Blueworx Voice Response that the other party is processing the call, but has not yet accepted it. You can think of this as the RINGING state.
CONNECT means that the other party has accepted the call and a voice path has been established.

This is the Trunk Identifier provided by your service provider. Blueworx Voice Response must be configured to use this trunk identifier or all calls on this trunk will be rejected with the cause code of channel unacceptable.

Figure 7. Flow of call related events for an outbound call from Blueworx Voice Response


Q921 DT/6000  14:22:04   TEI=00   Cmd

SAPI=00  I FRAME  NS=032  NR=032

 Q931 ----> SETUP Ref=0001 Len=21 08 02 00 01 05

    - 04 03 80 90 A3

      BEARER CAPABILITY

      3 : CCITT standard, Speech

      4 : Circuit mode, 64kbit/s

      5 : Layer 1 protocol G.711 A-Law

    - 18 03 A9 83 81

      CHANNEL IDENTIFICATION

      3 : Interface Implicit, Primary, Exclusive, Not D-Chan, Channel in
 next oct

      4 : CCITT standard, Channel number in next octet, B channel units

      5 : Channel 01

    - 70 04 80 31 31 30

      CALLED PARTY NUMBER

      3 : Unknown, Unknown

      4 : Number is 1 1 0

                                       Q921 NETWORK  14:22:04  TEI=00   Rsp

                                       SAPI=00  RR FRAME  NR=032

                                       Q921 NETWORK  14:22:05  TEI=00   Cmd

                                       SAPI=00  I FRAME  NS=033 NR=032

 Q931 <---- CALL PROCEEDING Ref=8001 Len=10 : 08 02 80 01 02

    - 18 03 A9 83 81

      CHANNEL IDENTIFICATION

      3 : Interface Implicit, Primary, Exclusive, Not D-Chan, Channel in
 next oct

      4 : CCITT standard, Channel number in next octet, B channel units

      5 : Channel 01

Q921 DT/6000  14:22:05   TEI=00   Rsp

SAPI=00  S FRAME - RR  NR=033

                                       Q921 NETWORK  14:22:05  TEI=00   Cmd

                                       SAPI=00  I FRAME  NS=034 NR=032

 Q931 <---- ALERTING Ref=8001 Len=5 : 08 02 80 01 01

Q921 DT/6000  14:22:05   TEI=00   Rsp

SAPI=00  S FRAME - RR  NR=034

                                       Q921 NETWORK  14:22:06  TEI=00   Cmd

                                       SAPI=00  I FRAME  NS=035 NR=032

 Q931 <---- CONNECT Ref=8001 Len=5 : 08 02 80 01 07

Q921 DT/6000  14:22:06   TEI=00   Rsp

SAPI=00  S FRAME - RR  NR=035

Unsuccessful outbound calls

Some outbound calls are unsuccessful because channels are busy or telephone numbers are invalid, Figure 8 shows the trace from such a call.

The Cause IE, in the Release Complete message, gives the reason why the call was failed or rejected. These cause codes are mapped onto edges in the MakeCall state table action; in this case a Network Busy edge will be returned.

Figure 8. Outbound call fails


Q921 DT/6000  16:12:04   TEI=00   Cmd

SAPI=00  I FRAME  NS=072  NR=095

 Q931 ----> SETUP Ref=0005 Len=21 08 02 00 01 05

    - 04 03 80 90 A3

      BEARER CAPABILITY

      3 : CCITT standard, Speech

      4 : Circuit mode, 64kbit/s

      5 : Layer 1 protocol G.711 A-Law

    - 18 03 A9 83 81

      CHANNEL IDENTIFICATION

      3 : Interface Implicit, Primary, Exclusive, Not D-Chan, Channel in
  next oct

      4 : CCITT standard, Channel number in next octet, B channel units

      5 : Channel 01

    - 70 04 80 39 39 30

      CALLED PARTY NUMBER

      3 : Unknown, Unknown

      4 : Number is 9 9 0

                                       Q921 NETWORK  16:12:04  TEI=00   Rsp

                                       SAPI=00  RR FRAME  NR=072

                                       Q921 NETWORK  16:12:04  TEI=00   Cmd

                                       SAPI=00  I FRAME  NS=096 NR=072

 Q931 <---- RELEASE COMPLETE Ref=8005 Len=9 : 08 02 80 01 5A

    - 08 02 80 AC

      CAUSE

      3 : CCITT standard, User

      4 : Requested circuit/channel not available

Q921 DT/6000  16:12:05   TEI=00   Rsp

SAPI=00  S FRAME - RR  NR=096

Call clearing

Call clearing messages are defined as follows:

DISCONNECT starts the call clearing process on the specified Call Reference. You don’t have to specify channel id because there is a table in each Layer 3 protocol which links Call Reference to a channel.
RELEASE acknowledges the clearing message so that both sides know that the call is being cleared. When the protocol receives a RELEASE message it sends a RELEASE COMPLETE message to the other end and clears the call fully from that side of the interface.
RELEASE COMPLETE is returned to the side which sent the RELEASE message. The call is now fully cleared. The channel on which the call was allocated is now free for another call to be set up.

Figure 9 shows a trace of a call clearing.

Figure 9. Trace of a call clearing


Q921 NETWORK  16:15:09  TEI=00   Cmd

                                       SAPI=00  I FRAME  NS=103 NR=076

 Q931 <---- DISCONNECT Ref=8009 Len=9 : 08 02 80 01 5A

    - 08 02 80 90

      CAUSE

      3 : CCITT standard, User

      4 : Normal Call Clearing

Q921 DT/6000  16:15:09   TEI=00   Rsp

SAPI=00  S FRAME - RR  NR=103

Q921 DT/6000  16:15:09   TEI=00   Cmd

SAPI=00  I FRAME  NS=077  NR=103

 Q931 ----> RELEASE Ref=0009 Len=5 08 02 00 09 AD

                                       Q921 NETWORK  16:15:10  TEI=00   Rsp

                                       SAPI=00  RR FRAME  NR=077

                                       Q921 NETWORK  16:15:10  TEI=00   Cmd

                                       SAPI=00  I FRAME  NS=104 NR=077

 Q931 <---- RELEASE COMPLETE Ref=8009 Len=5 : 08 02 80 01 5A