The keys in understanding computer networks is understanding the protocol stack that an application uses to send information over a network. The protocol stack defines the logic,the service,the steps,the way to successfully send bit's of information between 2 or more points in a network.The other key is to understand the purpose of a stack, what kind of information it was designed to send and the performance criteria.
For example the TCP/IP stack was designed for high availability ,in the case that a part of the network was destroyed to still be able to send to a point ,if an alternate route exists. So information is chopped in to pieces (packets) and every level of the stack attaches the needed information (attaches it to the packet) so that intermediary points know where the packet should go. A packet can have alternative routes through the physical network.
The SS7 stack was desgined for voice. For the telecom industry it is important that you can understand with the other person on the other end of the phone conversation. The TCP/IP stack does not apply very well in this case. The telecom industry needs tiny delays.The SS7 is still a packet switched network,but it establishes virtual "circuits"(virtual physical pathways) and has a different addressing scheme.
The OSI is a generalization of a communication system's logic. It's composed of layers.
The OSI stack has 7 levels as shown below.Every level attaches to the useful information packet instructions and data needed to correctly route the information by intermediary nodes and correctly receive the information by the destination node.
An amazing book on this is :