Difference between SS7 and ISDN PRI

Both are similar in many ways, but SS7 is more powerful and core to the network. PRI is a subset of SS7.

ISDN PRI link must directly be connected to switch. It has no capability to route the call intelligently. Using SS7 every node in the network can be accessed. Nodes need not be connected directly, SS7 has got built in intelligence to redirect packets to the correct destination.

ISDN PRI is divided into Transport (B) channels and signaling (D) channels. Voice/Data is transmitted through the B channels and the associated signaling information is transmitted through the D channel. T1 has 23B and 1D channels. E1 has 30B and 2D channels.

SS7 allows to access the telecom companies databases, like the subscriber data, and other configuration information. Now a days most of the call routing is based on database information about the customer and the destination number.

An example for this would be, the credit limit approaching message that we hear when our credit limit is about to expire. Whenever a call is placed, subsriber information is looked up from the database the appropriate message is played or routed to the destination.

SS7 is used to interconnect the switches. Its not a consumer end protocol. Modern day switches convert SS7 to PRI before giving it to a vendor or a consumer.

The difference between PRI and SS7 is the kind of signaling that occurs. SS7 is the signaling network for telcos. Like ISDN, inter-telco signaling is OOB. However, SS7 links carry only signaling. And signaling is not solely related to call passing. LNP (Line Number Portability) transactions occur on this kind of link. You may think of ISDN as an IGP and SS7 as an EGP. ISDN is for processing calls between the telco and the customer. SS7 is for processing signaling, generally between telcos.