After discussing SS7 basic concept in signalling network, we continue about Signalling Point ( SP ) and further about Signalling Link, Signalling Linkset and Signalling Route in SS7 Network.
Just to the point, Signaling Point refers to the communication network node equipped with the SS7 system.
The SP is identified via the signaling point code (SPC). The SP generating signaling messages is called the originating signaling point (OPC), and the SP to which the signaling messages are sent is called the destination signaling point (DPC).
DSP is any SP visible to the OPC in the entire signaling networks where the OPC is located. The DSP with direct signaling link is called adjacent DSP, while the DSP without direct signaling link is called non-adjacent DSP.
Then, we will know about Signalling Transfer Point ( STP ) that is The SP that is neither the message originating SP nor message DSP. It only transfers messages from one node to another, doing function in MTP Layer 2. The STP with the SP function is the integrated STP, while the STP without SP function is the independent STP.
The other thing, we know about Signalling Unit ( SU ). The signaling unit is the minimum unit for signaling message transmission between the SPs, which is formed by numerical coding.
The length of the signaling unit is variable. Generally, the signaling unit consists of 8 bits. Such a long string of codes is segmented to multiple 8 bits inserted in a certain time slot in the Pulse-Code Modulation ( PCM) system for transmission.
Signalling Transfer Mode
Now, I just introduce about Signalling Transfer Mode which is related with above concepts. This is the combination mode between the signaling message transfer path and the signaling relationship the message belongs to. It is the way the message passes from the original point to the destination point. Signalling Transfer Mode has 2 mode : Direct-connect mode and Quasi direct-connect mode.
Direct-connect mode
In this mode, the message is transmitted via the direct signaling link between two SPs where the speech channel is parallel to the signaling link like this picture :
Direct-connect mode-Signalling Transfer Mode
Quasi direct-connect mode
In this mode, the signaling message between two switching offices is transmitted via two or more concatenated sections of signaling links. The message can only pass the preset path and STPs, you can see in this picture :
Quasi direct-connect mode-SS7 transfer mode
In the SS7 system, it will be only direct-connect mode and quasi direct-connect mode can be used. The direct-connect mode is mainly used for the STPs. To fully utilize the capacity of the signaling link, the condition for adoption of the direct-connect mode between two non-STP switching offices is that there is trunk circuit group of enough size between the two offiices.
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Slide 26
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