SS7, the backbone of mobile networks. It is the protocol and makes mobile telephony work. SS7 is the protocol that makes communication between network entities possible. SS7 network structure is very much similar to the TCP/IP network structure. As you can see from the picture SS7 protocol is also derived from OSI model and has got all the 7 different layers.
In TCP/IP you have Ethernet card as your hardware and in SS7 you have SS7 signalling cards. For TCP/IP you have IP addresses that gives a logical structure to the network. In SS7 you have point codes.
All the network elements like HLR, VLR, MSC, SMSC etc are connected to the SS7 network and has got point codes assigned to them. Its almost like having a database server, a web server, an ftp server etc connected to the internet.
You can create your own SMSC, MSC, VLR, HLR or any SS7 network element by using an SS7 signalling card and writing the appropriate software for it. Its like using TCP/IP to create your own web server or FTP server.