Troubleshooting ISDN on Cisco Gateways

• ISDN:

ISDN is a set of communication protocols that enables simultaneous transmission of voice, data, video and other network services over traditional networks like PSTN. The ISDN mainly relies on the following:

  

       Q.921 : Setup of D channel and ensure reliable delivery of Q.931 Signaling.

                    Also known as Link Access Protocol - D channel [LAPD]

       Q.931 : Call Signaling.

Let have a look at the output of a basic troubleshooting command. I have two different scenario outputs shown below. The command is : show isdn status

SCENARIO 1 :

Global ISDN Switchtype = primary-5ess

ISDN Serial1:23 interface
        dsl 1, interface ISDN Switchtype = primary-5ess
    Layer 1 Status:
        ACTIVE
    Layer 2 Status:
        TEI = 0, Ces = 1, SAPI = 0, State = TEI_ASSIGNED
    Layer 3 Status:
        0 Active Layer 3 Call(s)
    Activated dsl 1 CCBs = 0
    The Free Channel Mask:  0x807FFFFF
    Total Allocated ISDN CCBs = 5
The Layer 2 status "TEI_ASSIGNED" indicates that the D channel is not up. 


SCENARIO 2 :

Global ISDN Switchtype = primary-5ess
ISDN Serial0:23 interface
        dsl 0, interface ISDN Switchtype = primary-5ess
    Layer 1 Status:
        ACTIVE
    Layer 2 Status:
        TEI = 0, Ces = 1, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHED
    Layer 3 Status:
        5 Active Layer 3 Call(s)
    Activated dsl 0 CCBs = 5
        CCB:callid=7D5, sapi=0, ces=0, B-chan=9, calltype=DATA
        CCB:callid=7D6, sapi=0, ces=0, B-chan=10, calltype=DATA
        CCB:callid=7DA, sapi=0, ces=0, B-chan=11, calltype=DATA
        CCB:callid=7DE, sapi=0, ces=0, B-chan=1, calltype=DATA
        CCB:callid=7DF, sapi=0, ces=0, B-chan=2, calltype=DATA
    The Free Channel Mask:  0x807FF8FC


The Layer 2 status "MULTIPLE_FRAME_ESTABLISHED" indicates successful set up of 

D channel and the output also shows 5 active calls on Layer 3.

CCB = Call Control Blocks, appears in the message only when there are active calls..

 Displays the Call ID and B Channel it is occupying.

Layer 1 Status:

If not active, check physical layer connectivity.

Also check : show controllers [ t1 | e1 ]

Layer 2 Status:

• TEI_ASSIGNED : D channel is not up.

1) Check switch-type and PRI timeslots.

2) Check network user side configuration. (On the Serial x:23 interface : NO SHUT, ENCAPSULATION, NO LOOPBACK)

Run the debug ISDN q.921 command.

The actual process of setting up of D channel consists of the following messages (in this case User is the gateway and network is the PSTN) :

UI : Unnumbered Information Frame

SAPI : Service Access Point Identifier. It is used to identify the type of frame by the value it carries.

           0 = Q.931

           63 = TEI Assignment

           16 = X.25

SABME : Set Asynchronous Balanced mode Extended

UA : Unnumbered Acknowledge

Router sends an SABME message and receives a UA frame to synchronize with the TELCO switch.

If the UA frame is received, the D Channel is UP.

When D channel is up, we receive Receive Ready packets (Keepalives) every 10 seconds :

ISDN Se0:23: RX <- RRp sapi=0 tei=0 nr=18

If 4 keepalives are missed, D channel is DOWN and setup begins again.

If instead of UA frame, we receive a BAD FRAME message, then there is a problem with the TELCO.

If we receive nothing, it may be a problem with the Hardwire Plug or the Router itself, contact CISCO TAC.

FOR A TROUBLESHOOTING FLOW CHART REFER :

http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a754a.shtml?referring_site=smartnavRD

-Abhinay Mylavarapu.