There are two types of signaling protocols in the telephony ecosystem. Channel Associated Signaling (CAS) is used to describe signaling protocols that use one dedicated signaling channel for each voice channel in a 1 to 1 analogy, according to fixed and pre-determined rules. Most telecommunication protocols to date use CAS to transfer information about call setup and control. The other type of signaling protocols uses Common Channel Signaling (CCS), where the signaling capacity for multiple channels is being integrated in one specific channel, called the signaling channel. The only CCS protocols developed and used so far are the Signaling Systems 6 and 7 (SS6 and SS7) with the latter having seen widespread usage during the past decades.
Over time, SS7 has evolved to address all the limitations of CAS protocols, such as:
- Very fast call setup times.
- Flexibility.
- Capacity to evolve and incorporate new features.
- More cost effective than CAS.
- Vastly superior call control capabilities.
On the other hand, the major drawback of SS7 compared to the traditional CAS systems is that the signaling channel is a single point of failure in the ecosystem.
SS7 is the enabling protocol for most of the telephony services we all use today and take for granted. Among other things, telephony systems based on SS7 offer:
- Toll free numbering that are widely used for marketing campaigns and customer support services.
- Single directory number (a company can have a single incoming number and then redirect the incoming calls to the appropriate extension within its private telephony network, using signaling information).
- Cellular network mobility management and roaming services.
- Local Number portability (which allows us to retain our phone number when switching carrier).
- Custom Local Area Signaling Services (CLASS) which include:
o Call block from pre-specified numbers.
o Distinctive ringing for groups of callers.
o Call completion to busy subscriber.
- SMS.
- EMS (enhanced functionality added to SMS service).
Furthermore, SS7 is a key protocol in the effort for telecommunications and internet convergence. It allows hybrid network services to be deployed such as:
- Internet Call Waiting: displays a message in user’s computer screen when they use the same line for telephony and internet, and an incoming call arrives. The user can then redirect the call to voice mail, accept the call or reject it. This application is extremely useful for dialup connections.
- Click to dial applications (i.e. click a number on a webpage to place a call automatically).
- Unified web and telephony services. This has evolved to an industry in itself, unified communications.
- WLAN hotspot billing.
- Location – based games.
We can see from the above examples that SS7 infrastructures are used to serve everyone in developed countries in their day – to – day activities, even though it is transparent to the end customer. It is also a key enabler of various value added services that provide the telecommunication carriers with additional revenue sources. In a subsequent post we will explore how SS7 works in more detail and examine its layers.
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