Introduction to telephony signaling

Even since the begging of the telephone, signalling has been an integral past of the telephone communications.The first tepehone devices depended on the receiving party standing next to the receiver. Early telephone did not have ringers like today's telephone, and used crude speakre to reject the caller's voice in the the room. If the party being called was not eithin close proximity of the speaker, he or she would have no identication of the incoming call

Later, after the formation of the Bell Telehone Company, Alexander Graham Bell's faithfull assitant Watson invented the telephone ringer.This new signalling method served one purpose:to alert the called party of an incoming call. When the called pasrty lifted the receiver, another form of signalling used DC battery and ground. Although not having an immediate impact, this method became important when the first telephone exchange was created.By lifting the receiver and allowing DC current to flow through hte phone and back through the return of the circuit, a lampt would be lit on the exchange operator's swhichboard. This signalled the operator when someone needed telephone service and was often accompanied by buzzer

Signalling has evolved over decades to include significantly more information then this early methods could.Consider the typical long distance telephone call today. When the caller dials the area code and prefix of the telephoe number, the local exchange must determine how to route the call.In addtion, billing information must be passed to central database.If the caller is using contemporary digital facility (such as T-1 or ISDN), information regardign digitization of the linemust also be provided.

Early signalling methods were limited because they used the same ciscuit for both signalling and voice.They were also analog adn had limitation number of states or values, which could be represented.The ciscuit would be busy from the time the caller started dialling until the caller went on hook.To compoun the probelm the companies were quickly running out of facilities and were in desperate need of additional facilities

Many telephone companies in metropolitan areas sucnh as Los Angeles were facing subtantial invesments to add new facilities to support the million of customers that were creating an enourmous amoutn of traffic.The telephone companies had to find a way to consolidate their facilities, making more economical use of what they had. In addition, they need a service that would vastly improve that network's capability and support the many new services being demanded by subscribers

Europe had already begun the procces of digitazing the network in the early '60s.One of the first step to removing signalling from the voice network adn place the signalling to the network all its own.This way, the call setup and tear down procedures required with every call could be faster than the previous methods and voice and data circuits could be reserved for use when a connection was possible, rather than maintaining the connection even when the destination was busy.Common Channel Signalling (CCS) paved the way for services the early pioneers of signaling never dreamed of.CCS is the technology that makes ISDN adn SS7 possible.

The concept behind SS7 is simple.Rather than use voice trunks for signaling they are used only when a connection is established.For isntance, when a call is placed to a distance party using conventional signaling, the signaling for that call begins from the time the caller lifts the receiver and goes off hook until the caller goes back on hook.After the end office has received the diald digits, an putgoing trunk to the destination end office is seized, based in a routing table entry and the digits dialed

The voice cisrcuits remains busy even if the distant party never answers the call until the calling party hangs up.Meanwhile, other subscribers are trying up other voice circuits by placing call of their own.This is not good utilization of voice circuit and it placed immediate limitations on the networks.But if the signaling could be place over a different network and the voice circuit used only when the called dial party answered, the voice circuit would remain available for a longer period of time.This meant availaibility of voice circuits would be higher and the need for additional circuit would decrease

When a caller is to receive an intercept recording (all circuits are busy), the same trunk used for the voice is also used for the recording.This recording is sent by the distant office.Busy tones and other service tones would be sent over the trunk by the distant office to the caller.With SS7,the caller's local office provides these tones and recordings at the command of the distant office.These commands are received via the signalling network.The voice trunk is left unconnected

The procedure for tearing down a circuit is much faster in Common Channel Siganling (CCS) than in convetional singaling and is not as erroe prone.Even if voice circuits do get donnected with the speed of the siganling network, circuit can be disconnected and wuickly connected again for a new call.While a call is in progress, information regarding the call can be sent through SS7 network (for instance, information from a database requested during an interactive multimedia call)