This research paper describes a telecommunications standard called Signaling System 7 (SS7). This technology defines a signaling system for control and routing of voice calls between telephone switches and switching locations. SS7 uses out-of-band signaling to place and control calls. It replaces an older system of in-band signaling to control telephone equipment. In-band signaling means the audio channel is used as a control channel for telephone switches. Operators would use tones over the audio channel to connect switches and open paths to the call destination. The use of out-of-band signaling means that control of creating an audio path through telephone switches is performed through a separate data channel that connects the switches together. The caller does not have access to this signaling channel, as they do for in-band signaling. SS7 can also carry data to switching locations about the calls they route. This data can include information for purposes of billing network time back to the call’s originating network and the caller’s account."Signaling System 7 (SS7) is a set of telephony signaling protocols that are used to set up and route a majority of the world’s land line and mobile public switched telephone network (PSTN) telephone calls.” (Ulasien, 2007). SS7 provides more efficiency and reliability for call handling than in-band signaling. SS7 controlled calls can verify that the audio path for a call is ready to initiate, for example, and not create the audio path until the call is answered at the other end. Another example is if the destination phone number returns a busy signal, no audio path needs to be established and the switch directly connected to the caller can generate the busy sound. The strategy of delaying the creation of the audio path until the last moment prevents wasted bandwidth within the switching infrastructure. This scenario would not be possible with in-band signaling, since in-band signaling depends on having an audio path established prior to anyone answering the other end of the call. SS7 allows the creation of innovative customer features and the use of rules-based capabilities for call routing that were previously impossible with in-band signaling technology.Signaling System 7 began development in the 1970s and saw wide deployment beginning in the early 1990s. The technology research and development was sponsored by AT&T and originally named the Common Channel Signaling System (CCSS). AT&T proposed it to the International Telecommunications Union as a standard beginning in 1975. SS7 was issued as a standard in 1980 and has been refined three times since. The ITU Telecommunications Standardization Sector (ITU-TS) develops global SS7 standards. The ITU allows different countries or organizations to make their own refinements and extensions to the global SS7 standard. The American National Standards Institute (ANSI) and Bellcore define a regional SS7 standard for North America and Regional Bell Operating Companies (RBOCs).Before the adoption of Signaling System 7, the only path between telephone switches was the audio channel. Telephone operators would use in-band signaling to set up long distance calls, or route international calls over cable or satellite using touch-tones. Maintenance crews would put telephone switches into special modes using sequences of tones to turn off accounting or allow operations a normal user would not be able to perform. In-band signaling is not just used to control telephone switches. We encounter in-band signaling often through the use of telephone-based services from vendors. Call routing through most of today’s large corporate phone systems require extensive use of the touch-tone keypad. Most voicemail systems require us to enter our personal identification numbers using tones to access messages. Your bank might provide a system to check your balances or transfer money through a phone-based system that uses touch-tones to enter your account information and direct your choices. In-band signaling works well for low-bandwidth situations, such as entering an account code or choosing a menu. Routing instructions to telephone switches can result in a complex series of tones representing access codes and phone numbers. Although it is useful for vendors in providing self-service capabilities to customers, in-band signaling for mission-critical systems such as unprotected telephone switching networks, have been exploited. Exposure of the signaling channel meant that sometimes callers would discover and record the in- band signaling tones used to route calls and control switches. Sometimes the audio signals were discovered completely by accident. During the 1970s and 1980s people such as John Draper (Captain Crunch) were known for their little home-built boxes that could connect to telephone jacks and send sequences of tones to obtain free long distance calls. These were known as black boxes or blue boxes. A whistle that came as a prize in his cereal inspired John Draper’s blue box creation. “The box blasted a 2600-Hz tone after a call had been placed. That emulated the signal the line recognized to mean that it was idle, so it would then wait for routing instructions. The phreaker would put a key pulse (KP) and a start (ST) tone on either end of the number being called; this compromised the routing instructions, and the call could be routed and billed as a toll-free call. Being able to access the special line was the basic equivalent to having root access into Bell Telephone.” (Cross, 2007).Signaling System 7 moves the signaling channel out of the audio channel, and is no longer is accessible to the parties participating in the call. SS7 specifies that telephone switches connect together using a dedicated digital network used only for signaling and managing calls. The signaling network among switches is similar to a traditional computer network. The signaling network can be designed for redundancy and does not need to take the same physical path as the voice data paths. In addition to relocating the signaling channel, the protocol allows for the creation of new and innovative features related to how calls are controlled and routed through the network. The Intelligent Network is a telecommunications industry term and described by Zeichick (1998) as having more reliance on digital technologies, more contextual information about calls in addition to the voice data, and more control provided to the end user for controlling how their telephone experience works. Caller ID works, for example, because the originating caller information is passed from switch to switch through the signaling channels. As mobile phone callers move around, SS7 signaling protocol helps switches find the proper route for calls to this person’s phone. The destination switch for a mobile phone moving in a train or automobile can change quickly. Call routing between switches is optimized with SS7’s definition of shared databases that are accessed through the signaling network. The databases contain rules about how calls should be routed to their destination. Switches on an SS7 network can query shared databases to find out which provider owns a phone number and how to route the call to that number. The databases can also contain feature-specific information. This aspect of the SS7 implementation has been characterized as client-server, meaning the switches act as clients to the shared databases with rules and other information for managing calls. “SS7 links the telephone system with a client-server computer architecture to create a distributed, efficient and easily modified telephone infrastructure. The computers use information from common databases to control call switching and to allow the transfer of messages within the system.” (Krasner, 1997).New technologies are testing the longevity of the Signaling System 7 protocol. Packet switched voice over IP is causing some disruption in SS7 space. However, there is more emphasis on integration and signaling gateways than replacement of existing SS7 infrastructure with something more recent. Session Initiation Protocol (SIP) is a signaling protocol for controlling audio and video connections over Internet Protocol networks. It can be implemented in hardware or software. SIP can be used for voice, video conferencing, and instant messaging and other types of streaming multimedia. H.323 is another streaming multimedia signaling protocol used for audio and video over Internet packet networks. Microsoft’s NetMeeting application uses H.323 as its protocol to connect NetMeeting nodes together in a wide-area conference. H.323 is also a recommendation by the ITU-TS. The business value of SS7 is that it provides opportunities for security, efficiency and optimization of call routing, and it provides the foundation to build innovative features for call handling using contextual information about calls and shared databases. It is a standards-based protocol and has been used throughout the world’s established telecommunications providers for over a decade. The protocol defines the means by which telephone switches exchange call routing and feature information - it does not assume voice data is carried on any particular medium as calls are transferred through the system. This simple abstraction with SS7 allows it to work with new technologies as they arrive in the mainstream. It is possible for SS7 to work within a mixed-technology environment including circuit-switched and packet-switched data networks. Ulasien (2007) says that the extensibility of SS7 allows the incremental migration of an organization from circuit switched to packet switched calls. The voice network is turning into the streaming media network and SS7 will continue to be tested in its role of connection maker and gateway to more recent communication technologies such as VOIP and video conferencing.ReferencesCross, Michael. (2007). Developer’s Guide to Web Application Security. Syngress Publishing 2007. ISBN:9781597490610.Hewett, Jeff. (1996). Signaling System 7: the mystery of instant worldwide telephony is exposed. Electronics Now. 67.n4 (April 1996): 29(7).Krasner, J. L., Hughes, P. & Klapfish, M. (1997). SS7 in transition. Telephony. 233.n14 (October 6, 1997): 54(4).Ulasien, Paul. (2007). Signaling System 7 (SS7) Market Trends. Faulkner Information Services. Document 00011475. July 2007.Zeichick, Alan. (1998). Lesson 125: Signaling System 7. Network Magazine. December 1, 1998: NA.