History records that 1876 was the year when Alexander Graham Bell patented the first device able to transform voice into an electrical signal that could travel across a simple wire. As this invention gathered momentum it became quickly obvious that a single home could not be connected to every other home with a dedicated wire (although the first devices were sold in pairs and ranchmen used barbed wire in the US to communicate using the newly invented telephone).
Thus the first switchboards were soon deployed, with human operators physically wiring callers with called people by plugging cords into jacks. Each place with a phone line was given a number to identify it. The line and its associated telephones were connected to a local switchboard. These switchboards were themselves interconnected all over the country. When someone wanted to call another place, a long process started: the caller picked up the phone, triggering an alarm on the local switchboard where an operator plugged a headset to speak with the caller. The caller then requested to be connected with a given phone number. The local operator checked whether the destination number was attached to the local switch or had to be reached through a distant switchboard. In the latter case, she called the destination switchboard, talked to its operator to ask her to make the called phone ring. If the called user picked up the phone, the distant switchboard operator connected a cord between the called line and the incoming line of the calling switchboard. The calling switchboard operator then also connected a cord to the caller’s line, eventually establishing a full voice path between the calling and the called parties…
Growing popularity of the telephone soon made this model reach its limits. Automatic switchboards were then designed to replace human operators. These switches were able to decode the dialed number from electrical pulses generated by new rotary dial phones.
In the ’70s, a new, all digital, telecommunication network started to be deployed. Until then, telephone networks were fairly static: fixed paths were established at the time of call set-up, and a phone number uniquely identified one phone line. Information about how to set up the call path was carried “in band”, i.e. using the same conduit as the voice path. A little bit like railroads before the invention of the telegraph: in the early days, engineers had to step down from their locomotives to manually move the switches at switching stations in order to establish their path along the way. Later on, timetable operation was introduced, but it was only with the advent of telegraphs that switches could be moved ahead of train arrival, greatly improving rail service.
This idea of using a sideline to carry information between switching station is called signaling. The overall telephony signaling infrastructure is called Signaling System #7, or SS7 for short. SS7 is to telephony what the telegraph was to rail service: essential. So essential that networks relying on SS7 were nicknamed intelligent networks.
With the introduction of SS7, it became possible to go back to the early days of switching where the operator could apply human intelligence to routing a call: for instance, in case of medical emergency, a caller could ask to be connected with the local physician. If there was more than one, or if the operator happened to know that one wasn’t working on that particular day, she could make the appropriate decision – and possibly save lives. With SS7, the same type of decisions can be automated: a generic “pseudo” number can be assigned to “medical emergency”; when this number is dialed, the SS7 network calls a special service control system which makes a decision based on where the call initiated, who are the available doctors at this point of the day, and possibly other parameters.
This evolution allowed telecommunication operators to offer lots of new services. For example:
The next huge communication revolution came in the late ’80s with the advent of practical mobile phone networks. Using various radio standards named with barbaric acronyms such as CDMA, AMPS, NMT…, handsets could connect with antennas. A whole set of technology hurdles had to be eliminated to make mobile telephony possible. For instance, it is no longer possible to associate a phone with a given switch, as the phone can freely roam through the country. Fortunately, SS7 was already there to solve that problem and provide the necessary intelligence to perform routing. In particular, very large databases — called HLR for Home Location Register — were connected to the signaling system. Together with mobile switching centers, these HLR’s keep track in real time of where mobile phones are, so that they can always place or receive calls.
When a mobile phone is switched on, it is identified through the radio interface and registered into the HLR which is now informed of its location. While connected, the HLR is informed every time the phone moves to another location. When someone dials this phone number, the signaling system analyzes this number to determine which communication operator is in charge of that phone. This determination can go through some database queries, as in most countries, numbers can be kept by users when they decide to opt for another communication provider (this is called mobile number portability, in short MNP). In the end, this communication operator is identified, and the SS7 network passes the request to call the phone number to the HLR of its “home” operator. As long as this phone is switched on, its HLR knows where it is located and it can thus indicate which switch and associated radio base station must be contacted to reach this phone. The SS7 network takes care of all of that in real time: by design, each node in the signaling system must provide an answer within 150 milliseconds to any request, otherwise it is considered out of service.
In parallel with telecommunication engineers busy coping with the mobile revolution, computer scientists created and evolved a set of de facto data communication standards collectively called the internet (or the Internet). Similar to telephony, the internet relies on various form of signaling, ranging from low-level internet control messages (ICMP) to the high-level session initiation protocol (SIP). However, no dedicated network is used to carry such protocols.
Whereas telephony networks were defined through fairly long and rigorous processes by international institutions from the United Nation under the overall guidance of the International Telecommunication Union (ITU) and their regional representatives such as the American ANSI or European ETSI, the Internet was a cooperative effort where anyone with a good idea could submit a request for comment (RFC) to the internet engineering task force (IETF).
The ITU rigorously defined each and every aspect of telecommunication protocols, thus ensuring great service predictability (phone networks rarely fail). This is a bit like well run railroad systems: trains depart and arrive on time, have a set number of seats, take passengers at predefined stops, providing ubiquitous access to such quality of service is typically expensive.
The IETF allowed much more freedom: experimental protocols could coexist with well established ones. Protocols were defined in the least restrictive way, thus fostering innovative evolutions. This is a bit like automobile transportation: people can drive whenever and wherever they want, but there’s no guarantee that they arrive on time and traffic jams do exist indeed.
After having lived apart from each other, the internet and the mobile telecommunication world are finally starting to blend. In that process, signaling remains a key ingredient to providing intelligence in these converged networks. The IETF Sigtran group has evolved SS7 to make it run over the internet protocols. SIP is at the core of recent telecommunication standards. And the industry is busy stitching these pieces together. Internet penetration now exceeds 42% of the world population while mobile telephony penetration exceeds 100% of humankind.
While internet roads are constantly made larger to avoid traffic jams, the current explosion of content and data threatens to clog our communications. Multimedia content is creating huge pressure on networks, especially on mobile infrastructures. This pressure calls for various forms of policy management and cooperation between network elements, so that vital information can always be delivered. Voice communication poses a specific issue: our human brains cannot tolerate delays nor too much loss of information. Hence, “signaling” is not only here to stay, but its importance will grow along with the penetration rate of the internet.