The signalling importance could be understood with the fact that you may connect 1000 E1s between nodes but if there are no signalling links in between them the calls won’t land on this connectivity. But if you have only 1 signalling link you can manage to have calls on any number of connectivity until it gets congested.
Signalling link is made on a single time slot of a connected E1 between two nodes. It is a single time slot out of 31 time slots of an E1 so still the rest 30 time slots can be used for voice traffic. Signalling carries following types of messages between two nodes.
1. SMS message and related pre delivery, confirmation post delivery and the message text itself sent by node A node B acknowledgement, sms text delivered via signalling by breaking the ASCII characters converted to binary characters into packets and delivering it to destination node.
2. Also the synch messages of the two nodes also called heart beats are sent on this connectivity. Because if this link is down the complete connectivity will go down. Synch is important at MTP2 and MTP3 because some times the base level MTP2 is down and MTP3 is working but still the connectivity is down as MTP2 messages are not getting communicated.
These are the types of signalling links:-
1. HSL:- High Speed Link
2. LSL:- Low Speed Link
The HSL connectivities are usually connectivity between BSC-TRC, MSC-STP-IN-HLR. These are critical and high utilized connectivity where overall traffic is high and service affecting.
Whereas LSL connectivity are not critical and highly utilized and network service affecting. Yes they are affecting the connectivity between two nodes but are not an huge impact on overall network service. Also the type of hardware used in the connectivity are also appropriately available. This connectivity is MSC-GMSC-BSC-BTS connectivity.
One more type of signalling connectivity called as Nailed-up connection is also useful. As this connection can ensure efficient use of highly critical HSL links. To explain the concept I sight an example here. Node A-STP connected directly via direct E1 connecting locally on which traffic is flowing to make Node B connect to STP a nailed up connection is made between Node A-Node B. Hence forth this nailed up connection once defined makes Node B to send all messages to STP via this nailed up connection with Node A. This exercise saves an important HSL hardware at both STP and Node B and also makes efficient use of HSL connectivity between Node A and STP. Also incase of fibre cut the chances of HSL connectivity getting down is low. As these nailed up connections are made on multiple E1s and are between Node A and Node B hence secured.
This is all about signalling links many more topics in this discussion are intentionally left for subsequent posts to take shape.