ISUP (ISDN User Part)

Questionnaire on ISUP:

1.) Define the call flow for ISUP call.
2.) What is terminating party on-hook the calls. Is the call dropped?
3.) What is CIC. Is it necessary to have same CIC value at both end, why?
4.) Define the type of Address Signaling.
5.) What is circuit glare condition. How to remove and avoid the glare condition.
6.) What is continuity test (COT)?
7.) define the Message structure of ISUP.
8.) Provide the message element of IAM, ACM, ANM, REL, RLC, COT...

Answers:

1.) ISUP allows the call control signaling to be separated from the circuit that carries the voice stream over interoffice trunks.
ISUP call scenario:

A-Party                               B-Party

IAM--------------------------------->

COT--------------------------------->

      <------------------------------------ACM

     <------------------------------------ANM

REL---------------------------------->

  <------------------------------------RLC

2.) If the terminating party goes on-hook first, the call might be suspended instead of being released. Suspending a call maintains the bearer connection for a period of time, even though the terminator has disconnected. The terminator can go off-hook to resume the call, providing that he does so before the expiration of the disconnect timer or a disconnect by the originating party.

3.) Circuit Identification Codes (CIC): One of the effects of moving call signaling from CAS to Common Channel Signaling (CCS) is that the signaling and voice are now traveling on two separate paths through the network.ISUP uses a Circuit Identification Code (CIC) to identify each voice circuit.Because the CIC identifies a bearer circuit between two nodes, the node at each end of the trunk must define the same CIC for the same physical voice channel.

4.) Address Signaling in ISUP are two types:
- Enblock Signaling: The enbloc signaling method transmits the called part number as a complete entity in a single message. When using enbloc signaling, the complete number is sent in the IAM to set up a call.
-Overlap Signaling: Overlap signaling sends portions of the number in separate messages as digits are collectedfrom the originator.When using the overlap method, the IAM contains the first set of digits. The Subsequent Address Message (SAM) is used to transport the remaining digits.

5.) Circuit glare (also known as dual-seizure) occurs when the node at each end of a two-way trunk attempts to set up a call over the same bearer at the same time. Using ISUP signaling, this occurs when an IAM for the same CIC is simultaneously sent from each end. Each end sends an IAM to set up a call before it receives the IAM from the other end.
To remove the Glare Condition, one node must back down and give control to the other end.
For avoiding the glare condition, common method is to perform trunk selection in ascending order of the trunk member number at one end of the trunk group, and in descending order at the other end. Another method is to have one end use the “Most Idle” trunk selection while the other end uses the “Least Idle” selection.

6.) Continuity Test: Continuity testing verifies the physical bearer facility between two SSPs. When CAS signaling is used, a call setup fails if the voice path is faulty. Using ISUP signaling, it is possible to set up a call using the signaling network without knowing that the bearer connection is impaired or completely broken. originating exchange determines whether a continuity test should be performed.

7.) ISUP message same as SCCP with CIC associated with it.
- CIC
- Message Type
- mandatory Fixed Part  (only content)
- Mandatory Variable Part ( have length and value)
- Optianal Part (have name, length and content)