5. 2. PSTN-1 reserves an idle trunk (circuit) & transmits ISUP IAM on an idle trunk circuit. The
IAM includes OPC, DPC, CIC, dialed digits, Calling party number.
3. IAM is routed via STP to the destination SSP. (STP is optional)
4. PSTN-2 checks the dialed number against its routing table and confirms that the called party
is idle. It alerts the Called party & it starts ringing.
5. Destination Switch transmits ACM to the originating Switch to confirm that the particular
circuit has been reserved.
6. The STP routes the ACM to the originating SSP which connects the calling party’s line to the
trunk to complete the voice circuit. The calling party hears ringback tone.
7. Called party goes off-hook. Destination switch terminates ringing tone and transmits an ISUP
answer message (ANM) to the originating switch via its home STP.
8. STP routes ANM to originating switch which verifies that the calling party is connected to
the reserved trunk. Billing is initiated.
9. If the calling party hangs up first, the originating switch sends an ISUP release message
(REL) to release the trunk between the two switches. If the called party releases first, the
destination switch sends an REL message to the originating switch to release the circuit.
10. When the destination switch receives the REL, it disconnects and idles the trunk, and
transmits an ISUP release complete message (RLC) to the originating switch to acknowledge
the release of the remote end of the circuit.
11. When the originating switch receives or sends an RLC, the billing cycle ends and the trunk
state is returned to idle.
6.
7. ISUP Message Structure:
Early ACM:
When a PSTN exchange sends IAM to the MSC, it starts timer T7 & waits for ACM.
Normally MSC sends ACM after checking the called party’s status (User Busy, Not Reachable or Idle). If
the called Party is a prepaid subscriber, then MSC will need to interact with SCP. This SCP interaction will
involve typically more than 4 messages.
After the SCP interaction, if the called party has sufficient balance & the called party is free, then the
MSC will send ACM. But the PSTN side will timeout before that & release the call.
8. IN
IAM
REL
IN-Prepaid
interaction
IN-Resp
RLC
PSTN
MSC
Early ACM
T7 timer
Waiting for
ACM
. . .
Timer
expiry
Typically more than 4
messages
In order to avoid this, MSC will send ACM immediately after receiving the IAM (without checking the
called party status). In the ACM message, it will send Called_Party_Status_Indicator = No Indication.
This is called as “Early ACM”
After sending Early ACM, MSC will start SCP interaction & if he has sufficient balance, then it will check
the called party status. If he is idle, then the called Party will start Ringing & at the same time, MSC will
send CPG with event as “Alerting”
9. IN
IAM
IN-Prepaid
interaction
IN-Resp
PSTN MSC
Early ACM
. . .
User has sufficient
balance
ACM
[Called Party status = No Ind]
CPG [Alerting]
ANM
In the ACM message, if the Called_Party_Status_Indicator = No Indication, then it is called as “Early
ACM” Method.
Connect ACM + ANM
SUSPEND – RESUME:
10. IAM
ACM
ANM
SUS
RES
PSTN-1 PSTN-2
SUSPEND-RESUME
Called Party Hangs
the call
Call Conversation
Called Party lifts the
handset again before
timer expiry
Call Conversation
REL
Calling Party
Hangs the call
Call released
immediately
RLC
Enbloc & Overlap method:
11. In some applications the length of the dialed digits are huge such that it cannot be accommodated in the
IAM message. Example of such application is calling card.
What is Calling Card?
Calling card is a future in which the user can make any local or ISD call from a public telephone booth. In
this case the user has to purchase the calling card from shops which will have secret PIN. The user has to
input this secret key when he tries to make a call from a telephone booth. This secret PIN will be
normally of 15 to 20 digits.
In this case the user will dial the calling card service number + Secret PIN + Called party number. All
these digits cannot be accommodated in IAM message.
IN
IAM
SAM [Subsequent number]
ACM
IN-Query
IN-Resp
ANM
REL
RLC
PSTN-1
(LE)
PSTN-2
(TE)
Overlap method (SAM)
Wait for
SAM
So first the PSTN-1 will send partial dialed digits in IAM . Then the remaining digits will be sent on SAM
[Subsequent Address Message]. Based on the size of the dialed digits there can be even multiple SAM
messages.
On receiving the SAM message, PSTN-2 will concatenate all the digits & send it in the IN query. The IN
box will check for validity of secret PIN & the available balance. This is called as Overlap method
If all dialed digits are sent in IAM then En-bloc Method
Else if SAM message is sent then overlap method
12. Continuity Check procedures:
Typically in ISUP world, Continuity Check procedures are done to ensure that the circuit is functioning
correctly. The originating exchange will send IAM with “Nature of Connection Indication” is set to
“Continuity Check required on this circuit”. This indicates the remote exchange that the voice circuit
should be tested for continuity. The remote exchange performs the continuity test by using a loopback
test.
If this test is passed, then the remote end sends COT [continuity check passed] after which the caller is
alerted and ACM is received.
If this test is failed, then the remote end sends COT [continuity check failed] after which the call is
released and the originating exchange sends IAM on a different circuit.
If no continuity test is required then the NCI in IAM is set to “continuity check not required”
IAM [ NCI = Continuity
Check Req]
ACM
COT [continuity check passed]
PSTN-1 PSTN-2
Continuity Check Procedure
PSTN-2 initiates Loop Back test
ANM
Call Conversation
13. Dual seizure or glare:
When two exchanges select the same CIC for a call, then both the exchanges will send IAM with same
CIC at the same time in opposite direction. This condition is called Glare.
This can be avoided by using Circuit selection method
First to Last
Last to First
Select odd only
Select even only
Call Forward
CLIR
Circuit Management Messages:
RSC [Reset Circuit]
RSC can be sent by the MSC by itself or it can be triggered by the maintenance persons.
The purpose of this message is to reset a circuit to the idle state.
MSC will send this message by itself if there is a memory error occurs at the MSC & it does not know the
state of the circuit.
If the destination exchange receives RSC while
Call in progress, then the call will be released
Blocked state, then the remote exchange will send Block message again
GRS [Group Reset]
RSC is used to reset one circuit whereas GRS (Group Reset) is used to reset a group of voice circuits
14. BLO [Block]
Types of Block:
1. Maintenance Blocking
2. Hardware Blocking
When MSC receives Hardware BLOCK during call conversation, then the call will be released
immediately by MSC & will send REL.
When MSC receives Maintenance BLOCK during call conversation, then the call will NOT be released &
the Block will be applied only after the end of the call.
ISUP messages:
CPG:
The call progress message is used to notify the call-related event to the remote exchange.
Events:
Alerting
CFU
CFB
CFNA
15. Mandatory Parameters:
Message Mandatory
IAM Called party number
Nature of connection indicators
Transmission medium requirement
Forward call indicators
Calling party's category
ACM Backward call indicators
ANM
REL Cause indicators
RLC
SAM Subsequent number
CPG Event information
COT Continuity indicators (Pass/Fail)
CGB/ CGBU Circuit group supervision message
type (Hardware failure
/Maintenance)
Range and status (Block)
16. Parameter Sub parameters
Nature of Connection Indicator
Satellite Indicator
Continuity Check Indicator
Echo Control Device Indicator
Forward call Indicator Nat/Inter Indicator
Inter-working Indicator
ISUP Preference
Backward call Indicator Called party's status indicator
Called party's category indicator
Inter-working indicator
Echo control device indicator
Calling Party Category = Ordinary, Pay phone, Test call etc.
ISUP-FAQ
What are the mandatory parameters of IAM?
What is Early ACM?
What are the different states of a circuit? [Idle, Block, Busy]
What is En-bloc & Overlap method?
What is CON message?
What is use of SAM?
When SAM will be received in a PSTN network?
What is the use of CPG?
What is the use of COT message?
How connectivity test is performed in PSTN network?
When SUSPEND & RESUME messages are expected?
17. What is the use of RSC?
What is the difference between BLO & RSC message?
Will ISUP use SCCP?
What are the different ISUP variants?
How is echo control performed?
How to avoid echo?
How do you troubleshoot one way speech problem?
What is the maximum number of CICs?
CIC is 12 bit. First 7 bits are used to identify E1.
So we can have 27
=128 E1 and the remaining 5 bits are meant for the identifying the Time slots.
(25
=32).
So you can have max =128*32 =4096 trunks.
What is the maximum number of Signaling Links?
SLC is 4 bit. So 24
= 16
So in total 16 Links are possible between two exchanges
What is the use of CRG?
What are the different types of CDR?
How do you correlate different ISUP messages of a same call?
![ISUP
Training course
[Draft_0.3]
2012
Shiv
reachshivablr@gmail.com
2/03/2012](https://image.slidesharecdn.com/isup-140507015515-phpapp01/85/isup-1-320.jpg)
![Contents
ISUP Call Sequence........................................................................................................................................4
ISUP Message Structure:...........................................................................................................................7
Early ACM:.................................................................................................................................................7
SUSPEND – RESUME: ................................................................................................................................9
Enbloc & Overlap method:......................................................................................................................10
Continuity Check procedures:.................................................................................................................12
Dual seizure or glare: ..............................................................................................................................13
Circuit Management Messages:..............................................................................................................13
RSC [Reset Circuit]...............................................................................................................................13
GRS [Group Reset]...............................................................................................................................13
BLO [Block]..........................................................................................................................................14
ISUP messages: .......................................................................................................................................14
CPG:.....................................................................................................................................................14
Mandatory Parameters:..........................................................................................................................15
ISUP-FAQ.....................................................................................................................................................16](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)