Rejected Outbound ISDN calls

Symptom

Outbound ISDN calls are rejected by the switch due to a missing IE (3.1 Extension Bit/Interface Identifier). This behavior is seen with 4ESS ISDN configuration after upgrading to SR5.1.1 SU125 or higher.

Reason for the problem

According to AT&T Telco Specifications (TR41459) octet 3.1 (Extension Bit/Interface Id) can be omitted when the "interface identifier present" field in octet 3 indicates "interface implicity identified". 

In Service Update 125, corrections was made to the springware firmware, NOT to send octet 3.1information element, if a board was NOT configured in an NFAS group. On earlier SUs, the springware firmware would always set the Interface ID Present and send octet 3.1 even in Non-NFAS configurations. Some switches are configured to require this octet for both configurations and may reject the ISDN call if the setup is missing IE (octet 3.1, Extension Bit/Interface Identifier).

Here's a partial layer 2 trace (D-channel trace log) of the setup message of 2 calls from 2 different SUs.

[Partial Setup from SR5.1.1 SU98. You can see the Interface ID is present and the Extension Bit octet is present by default]

                          1: CHANNEL ID(0x18) 
                          2: IE Length(0x04) 
                          3: 1------- Extension Bit 
                              -1------ Interface ID Present 
                              --1----- Interface Type 
                              ---0---- Spare 
                              ----1--- Preferred/Exclusive 
                              -----0-- D-Channel Indicator 
                              ------01 Info. Channel Sel. 
                       3.1: 1------- Extension Bit 
                           -0000000 Interface Identifier                        3.2: 1------- Extension Bit 
                              -00----- Coding Standard 
                               ---0---- Number Map 
                               ----0011 Channel/Map Element 

[Partial Setup from SR5.1.1 SU125. You can see the setup is compliant according to AT&T's specification, where the Interface ID is not set and extension Bit octet is NOT present by default] 

                       1: CHANNEL ID(0x18)
                       2: IE Length(0x03) 
                       3: 1------- Extension Bit 
                           -0------ Interface ID Present 
                           --1----- Interface Type 
                           ---0---- Spare 
                           ----1--- Preferred/Exclusive 
                           -----0-- D-Channel Indicator 
                           ------01 Info. Channel Sel. 
                    3.2: 1------- Extension Bit 
                           -00----- Coding Standard 
                            ---0---- Number Map 
                          ----0011 Channel/Map Element 

Fix / Solution

Try to get the switch Engineer to make the appropriate changes on the switch.  If the changes cannot made refer to work around solution below.

Workaround

If the switch requires octet 3.1 to be present for non-NFAS configurations, perform the following configuration changes:

Step 1. Modify the nfas.cfg file so that each span is in its own NFAS group. 

#NFAS group 1 
# Board ID                        Interface ID                            Channel board ID                          NFAS group ID 
            1                                    0                                                    1                                                 1 
            2                                    1                                                    2                                                 2 
            3                                    2                                                    3                                                 3 

Step 2. Do not modify parameter 0016 in the isdn parmeter file (4ess.prm) for each span as you normally would within an NFAS environment. Instead, leave the D-channel as enabled: 

;--- 
;--- ENABLE/DISABLE the D channel (Parameter type 16H) 
;--- Used only when the protocol type (Parameter number 13H) is 
PRI ISDN 
;--- for NFAS configuration. 
;--- Possible values for the data are as follows: 
;--- 00H = Undefined. 
;--- 01H = Enable the D channel. 
;--- 02H = Disable the D channel. 
0016 01 

Step 3. Restart the drivers for the changes to take affect.

Product List

• D/240JCT-T1

• D/480JCT-2T1

Glossary of Acronyms / Terms

SU = Service Update