Data user part (facility registration/cancellation messages)
Signalling Information Field (SIF). SIF may consist of upto 272 octets and contains the information to be transmitted. The SIF in an MSU contains the routing label and signalling information. LSSUs and FISUs contain neither a routing label nor SIO as they are sent between two directly connected signalling points.
MTP level 3 routes messages based on the routing label in SIF of MSU. The routing label is comprised of the destination point code (DPC), originating point code (OPC) and signalling link selection (SLS) field. Point codes are numeric addresses which uniquely identify each signalling point in the SS7 network. When the DPC in a message indicates the receiving signalling point, the message is distributed to the appropriate user part indicated by the service indicator in SIO.
Messages destined for other signalling points are transferred provided that the receiving signalling point has message transfer capabilities (like an STP). The selection of outgoing link is based on the information in the DPC and SLS.
7.5.6. Basic Call Setup with ISUP
Fig. 7.16 shows the example where a subscriber on switch A places a call to a subscriber on switch B.
1. When calling subscriber calls a subscriber, the originating SSP transmits an ISUP initial address message (IAM) to reserve an idle trunk from switch A to B. The IAM includes the originating point code, destination point code, calling and called numbers.
2. The IAM is routed via the home STP of the originating switch to the destination switch. Same signalling links are used for the full duration of a call unless a link failure forces to change link.
3. On reception of IAM by called subscriber (if idle), the destination switch transmits an ISUP address complete message (ACM) to its home STP.
4. The destination switch rings the called subscriber and send a ringing tone over the trunk to the originating switch.
5. When the called panty picks the phone, the destination switch terminates the ringing tone and transmitts an ISUP answer message (ANM) to the originating switch via its home STP.
6. The STP routes the ANM to the originating switch and initiates billing.
7. If the calling subscriber hangs up first, SSP (A) sends an ISUP release message (REL) to release the trunk circuit between the switches.
8. The STP routes REL to the destination switch.
9. If the called subscriber hangs up first, the destination switch sends an REL to the originating switch (not shown).
10. Upon receiving REL, the destination switch disconnects the trunk from the called party's line. Then, it transmits an ISUP release complete message (RLC) to the originating switch to acknowledge the release of the trunk circuit.
11. When originating switch receives RLC, it terminates billing cycle and sets the trunk state to idle.
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