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Voice over IP Online Course

Lesson 3 - How Does VoIP Work?

A. Call Control


How does VoIP Work?







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ddresses. Ideally the signaling messages in the VoIP system will be based upon some common format to enable a wide range of terminals, and hence users, to communicate.

SS7


SS7

Signalling System 7 (SS7) is an architecture for performing out-of-band signalling in support of the call-establishment, billing, routing, and information-exchange functions of the public switched telephone network (PSTN). It identifies functions to be performed by a signalling-system network and a protocol to enable their performance.

Whenever you dial a telephone number, you are using SS7. Whether the party you are trying to reach is next door or around the world, you are using SS7.

SS7 is also responsible for the following services:

Out of band

SS7 uses an entirely separate network from the voice network. Its technology is different as well. It uses a packet switching network to connect the different signaling entities.

Out of band just means that the signals used to provide these services do not travel over the same lines as the voice call itself. This provides some benefits.

SS7 provides services

SS7 is connected to databases which allow it to provide extra services. 800/888/900 numbers couldn't exist without these. Caller ID, call forwarding and calling cards are other g>
Voice over IP is a complex operation. It encompasses two phases: call control and voice transmission.

Call Control

Call control is the mechanics of making a telephone call. This is the function of the Public Switched Telephone Network and includes such tasks as setting up a route to the telephone dialed, ringing the bell, billing the customer for long distance charges and hanging up. If VoIP is to provide telephone service, than it too must provide these functions. The call control function is independent of the voice transmission function.

Voice transmission

Voice transmission actually sends the sounds across the network. For VoIP this includes digitizing the voice, compressing and encoding it, placing it in IP packets and routing it to the destination telephone.

As you will shortly see, these two functions require entirely different technologies.


Call Control


Before a call can take place, the first major issue that needs to be addressed is: 'How can the calling user find the called user and establish contact with them?' This requires some form of signaling that is understood by the calling and called users' clients. It also requires a suitable naming and addressing scheme. This scheme will have to deal with traditional telephone numbers and IP addresses. Ideally the signaling messages in the VoIP system will be based upon some common format to enable a wide range of terminals, and hence users, to communicate.

SS7


SS7

Signalling System 7 (SS7) is an architecture for performing out-of-band signalling in support of the call-establishment, billing, routing, and information-exchange functions of the public switched telephone network (PSTN). It identifies functions to be performed by a signalling-system network and a protocol to enable their performance.

Whenever you dial a telephone number, you are using SS7. Whether the party you are trying to reach is next door or around the world, you are using SS7.

SS7 is also responsible for the following services:

Out of band

SS7 uses an entirely separate network from the voice network. Its technology is different as well. It uses a packet switching network to connect the different signaling entities.

Out of band just means that the signals used to provide these services do not travel over the same lines as the voice call itself. This provides some benefits.

SS7 provides services

SS7 is connected to databases which allow it to provide extra services. 800/888/900 numbers couldn't exist without these. Caller ID, call forwarding and calling cards are other good examples of services which need these databases.

SS7
Figure 9: SS7

SS7 entities

Figure 9 illustrates a SS7 system and its relationship with the voice network.

Signal Switching Points (SSP) are hybrid devices that form part of the voice network as well as having an interface to the SS7 network. These are the central office switches which need to do a lookup to route a call. These run SS7 software. They refer a request for call routing to the SCP.

Signal Control Points (SCP) maintain databases of routes, local or long distance, for the voice traffic to take. Therefore when the SSP needs to do a lookup on how to route the voice call, the SCP can provide the answer. In addition, the SCP's databases maintain records of 800/888/900 numbers, name and number pairs for call display and so on.

Signal Transfer Points (STP) are basically the routers used by the SS7 system. They forward requests from the SSPs to the SCPs. STPs make the SS7 network a packet switching network.



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