The demands placed on the telecommunications signaling system continue to change. Signaling is a very vital part of telecommunication and has a number of characteristics that it must exhibit. A signaling system must be able to handle a large number of calls or requests at once without becoming noticeably slower. At the same time, the cost of running the signaling system should not detract from an operator’s ability to make money.
The system must be reliable and always available for use. Since individual components or messages can fail, the system as a whole must contain devices and procedures to recover from problem areas.
One shortcoming of early tone-based signaling systems was that they were very difficult to adapt to new requirements. This problem was encountered with Signaling System #6, which was also a computer protocol but designed for absolute bandwidth efficiency; it was not easy to spread. This is how Signaling System #7 was born.
Among PABX, we often use DPNSS which stands for Digital Private Network Signaling System. This protocol is used in digital trunk lines to connect two PABXs. It supports a limited set of networking facilities based on the ISDN protocol. This protocol has been defined by BT
A subscriber and a telephone company notify each other of the status of the call with audible tones and an exchange of electrical current. This exchange of information is called supervisory signalling. There are three different types of supervisory signaling:
On-hook: When the handset rests on the base, the circuit is on-hook. The switch prevents current from flowing through the phone. Regardless of the type of signaling, a circuit is hung up when the handset is placed on the telephone base and the switch is toggled to an open state. This prevents current from flowing through the phone. Only the ringer is active when the phone is in this position.
On Hook: When the handset is removed from the telephone base, the circuit is off hook. The switch hook changes to a closed state, causing circuit current to flow through the electrical circuit. The current notifies the phone company team that someone is requesting to make a phone call. When the telephone network detects the connection being taken off-hook by the current flow, it provides a signal in the form of a dial tone to indicate that it is ready.
Ringer: When a subscriber makes a call, the phone sends voltage to the ringer to notify the other subscriber of an incoming call. The telephone company also sends a ringback tone to the caller to alert the caller that it is sending ring voltage to the recipient phone. Although the ringback tone sounds similar to ringing, it is a call progress tone and is not part of supervisory signalling.
Digital Subscriber Signaling System No.1 (DSS1) is a set of protocols that provides the means for users to invoke the full range of services and capabilities available from the Integrated Services Digital Network (ISDN).
The Q.931 standard defines a protocol that deals with these services. This protocol is located in the third, that is, in the network layer of the OSI protocol stack. All of the signaling mentioned above is accomplished through the D-channel defined by the ISDN standard. Q.931 will use this channel to establish the desired connection. As indicated in the OSI Reference Model, the upper protocol layers depend on the services provided by the lower layers. Therefore, Q.931 requests data link layer services that reside at layer two. The Q.921 standard defines a protocol that can be called a Data Link Service Provider (DLSP). Q.931 is therefore the Data Link Service User (DLSU). Q.921 is also better known as LAPD, as in the D-channel of the Link Access Protocol. Service requests coming from the above layers are packaged in LAPD frames and sent over the network. All Q.931 messages are made up of a certain message body.
Signaling System Number 7 (SS7) is a suite of signaling protocols developed for telephony in 1975, used to establish, maintain, and terminate telephone calls. It is also used for a number of other functions and features, such as:
Local number portability
Translation of numbers
SMS (short message service)
Prepaid billing
It is referred to by a series of abbreviations such as CCSS7 (Common Channel Signaling System 7) and C7 (CCITT number 7), the International Telegraph and Telephone Consultative Committee.
The SS#7 model uses the concept of layers to separate the different functions, however, the structure predates the adoption of the OSI model and there are only 4 layers. Circuit and non-circuit components are shown, including the overlay where ISUP uses both. There are other major non-circuit related protocols such as MAP (Mobile Application Part). Being open, SS#7 can be extended for new requirements.