Signaling System 7

Introduction
SS7 is a critical component of modern telecommunications systems. SS7 is a communications protocol that provides signaling and control for various network services and capabilities. While the Internet, wireless data, and related technology have captured the attention of millions, many forget or don't realize the importance of SS7. Every call in every network is dependent on SS7. Likewise, every mobile phone user is dependent on SS7 to allow inter-network roaming. SS7 is also the "glue" that sticks together circuit switched (traditional) networks with Internet protocol based networks.
SS7 Technology
SS7 signaling is a form of packet switching. Unlike circuit switching, which utilizes dedicated data "pipes" for transmission of information, packet switching dynamically assigns "routes" based on availability and "least cost" algorithms. Another example of packet switching is TCP/IP, the protocol used for routing messages over the Internet. Unlike the Internet, which utilizes a vast public "web" of interconnecting facilities and routing equipment, SS7 networks are private and logically self-contained. The private nature of SS7 networks is critical for security and reliability.
SS7 involves two different types of signaling: connection oriented signaling and connectionless oriented signaling. Connection oriented signaling refers to the establishment of switch-to-switch facilities call inter-office trunks. These trunks carrier voice communications. The ISDN User Part (ISUP) part of the SS7 protocol is utilized to establish trunks between switches. In contrast, the Transaction Capability Application Part (TCAP) is utilized for connectionless signaling which typically entails switch-to-database or database-to-database communications. An example of connectionless signaling is TCAP signaling of HLR to VLR communications discussed in the mobile networking article.
SS7 Networks
SS7 is comprised of a series of interconnected network elements such as switches, databases, and routing nodes. Each of these elements is interconnected with links, each of which has a specific purpose. The routing nodes are the heart of the SS7 network and are called a Signal Transfer Point (STP). STPs are connected to Service Switching Points (SSP) that are switches equipped with SS7 control logic. SSP switches are connected to the STPs via Access links (A links). STPs also connect to databases called Service Control Points (SCP) via A links. The SCP is the network element that contains service control logic such as instructions for converting a 8XX (toll-free) number into a routable number.
STPs are always deployed in pairs, allowing a spare should one of the STPs have a problem. Each STP of a "mated pair" are connected to each other via Cross links (C links). STP pairs connect to other STP pairs via Bridge or Diagonal links (B or D links). B links connect STP pairs that are at the same level of hierarchy while D links connect STP pairs that are different hierarchial levels. An example would be STPs in a local network connecting with STPs of a long distance network. Being at different hierarchies, the local-to-long distance links would be considered D links.
Links used for SS7 communication directly between SSPs (no STP involved) are called Fully associated links (F links). An example of these links are those that are used in combination with voice trunks between two mobile network SSPs. The F link is used to signal a hand-off message from one SSP to the other, allowing the mobile phone user to travel from one area (served by one switch) to another area (served by another switch).
Extended links (E links) are used to connect an SSP to an alternative STP pair. In the event that the primary STP pair is inoperable, the alternative pair establishes operations with the SSP over the E links.
Business Issues
In today's modern telecommunications networks, SS7 is used for virtually every call to establish a voice connection between the calling and called party locations. SS7 is also the medium for advanced capabilities and applications including mobile networking and services as well as wireline applications such as toll-free calling and automatic calling card identification.

Source : www.mobilein.com