Signalling system 7 (SS 7) and its network:

Signalling system 7 and its network:

Common channel signaling technique, using a separate network of computer to send and receive message as signals, has been adopted by ITU as CCIS7 and is referred to as SS7.

Prior to SS7:

1.       When a long distance was made , the call was connected  to exchange by exchange through the long distance network.

2.        On calls to a busy telephone, the call  would be established over the network and tie-up numerous circuits in the PSTN just to return a busy signal from the far end exchange.

a.       When a call is placed over the long distance network, the exchange (connected to the calling subscriber) will launch a query over the SS7 network to establish a call setup path and to test the called subscriber.

b.      If the called party is busy, a busy signal is returned to the calling party of the originating exchange.

c.       If the called line is idle, the SS7 now issues signaling message to each exchange that will be used to connect the call.

d.      Each exchange will communicate over SS7 network and identify the free circuits to be used between each exchange for the voice  circuit and trunk circuits are reserved for the call.

e.      A signal is set to the terminating, instructing it to ring the called line.

f.        The caller receives the ring back tone from its originating exchange.

g.       When  the called party answers the call, the reserved trunks are then connected and voice circuit path is completed between the calling and called parties.

h.      SS7 is a network of its own.

This network consists of 3 major parts: STP, SCP and SSP

A)     Signal transfer point(STP):

a)      It is a packed based switch.

b)      It receives and send message signal to proper destination.

c)       It also forms specialized routine function.

B)      Service control point (SCP):

a)      This node acts as a database which provides the information for the advanced call in the network.

C)      Service signaling point(SSP):

a)      This node is the end exchange equipped with SS7 enabled software and having signal lines.

b)      Every  will have SSP to connect with SS7 network.

c)       SSP’s behaves like an exchange for different functions like starting the transmission as well as termination of the line in the network.

d)      SSP’s are connected with other STP’s and SCP‘s in the ss7 network.

There are different steps in SS7 call setup:

1.       Switch A analysis the dialed digits and determines that if it needs to send the call to witch B.

2.       Switch A selects a idle trunk between itself and switch B, and formulates I_{A M}(Initial Address message), the basic message needed to initiate a call. The I_AM addressed to switch B. It identifies the initiating switch (A) , the destination switch (B), the trunk selected, call and calling numbers and other information.

3.       Switch A picks up one of it’s a link (A_w) and transmits the message I_AM  over the selected link to routed to switch(B).

4.       STP -W receives the message, inspects  its routing label and determines if it is to be routed to switch B. it transmits the message to switch over a link(BW).

5.        Switch B receives the message. On analyzing the message, it determines if servers the called number and the called number is idle.

6.       Switch B formulates an ACM(Address complete message) which indicates that the I_AM has received its destination. The message identifies the receipt switch (A), sending Switch(B) and the selected trunk.

7.       Switch B picks on link B-X and transmits ACM over that link for routing to switch A. At the same time, it completes the call path in the backward direction (towards switch A), sending a ringing tone over the trunk towards switch A and rings the line of called subscribers.

8.       STP-X receives the message, inspect its routing label and determines that is is to be routed to A. it transmits the messages to A.

9.       On receiving ACM, switch  A connects the called subscriber to the selected trunk in backward direction (to switch B) so that the caller can hear  the ringing tone sent by switch B.

10.   When the called subscriber picks up the phone, the switch B formulates an ANM (Answer Message) identifying the intended receipt switch(A), the sending switch(B) and the selected trunk.

11.   Switch B select the same link BX to transmit AM and transmits it to STP X. by this time, the trunk also must be connected to the called link in both directions to allow conversation.

12.   STP X recognizes that the ANM is addressed to switch A and forwards it over the same link AX.

13.   Switch A ensures that the calling subscriber is connected to the outgoing trunk and that the conversation can take place/

14.    If the calling subscriber hang-ups first, switch A will generate a message REL (Release message) addressed to  B, identifying the trunk associated with the call. It sends REL to STP-W over the link Aw.

15.   STP W receives REL message determines that it is addressed to switch B and forwards to switch B over BW link.

16.   Switch B receives the REL, disconnects the trunk from the subscriber link, returns the trunk to idle status, generates a release complete message(RLC) addressed back to switch A and transmit it to STP X over link BX.

17.   STP X receives RLC, determines that is addressed to A and forwards to switch A.

18.   On receiving RLC, switch A idles the identified trunk.