Telephone call
In a telephone conversation or a telephone call one understands the voice communication between two participants over the telephone network.
Thus a phone call can be performed, a connection between the participants must be established. During their communication, this connection must be sustained and again at the end. A phone call is always temporary. This technical principle is independent of the type of the selected network. A distinction is made between fixed, IP telephony and mobile network.
Technical Process of a telephone conversation
Thus, a subscriber can have a telephone conversation with another participant, a variety of actions in the telephone network are required:
A majority of these actions to place a telephone call are signaling operations. They include either a communication of a subscriber to the telephone network via the terminal ( terminal signaling) or a communication within the telephone network (network signaling).
These actions are in large part within fractions of a second before and after the actual interview participants instead. During the call, the telephone network only a few actions take place, only actions for checking and maintaining the call are then required.
The signaling operations signaling network found in today's telephone networks, instead of in a separate SS7 signaling network, which is constructed separate from the speech channels. In larger telephone systems, the signaling network of signal transfer points (STP) is formed, which communicate with each other over the SS7 protocol. At this signaling network, the network elements of the voice network are connected. The voice network is formed in the fixed network of switching centers ( VST ) in mobile network of Mobile Switching Centers ( MSC).
The terminal signaling is different depending on the terminal equipment. In contrast, ISDN telephones, for example by means DSS1 protocol for analog telephones with tone dialing or the older pulse dialing.
Example: Telephone conversation in the fixed network
Access to the telephone network
In the more densely populated areas in the fixed network mostly for each participant a subscriber line to the local exchange ( local exchange ) exists. When a member with an analog phone to the phone handset, registers this the local exchange, activated the voice channel to the calling party and creates the dial tone. For ISDN phones picking up the telephone receiver over the signaling channel ( D channel) is signaled to the local exchange to which these a voice channel (B channel) is enabled for the calling party. The participant will then hear the dial tone.
In rural areas, the local exchange concentrators are often installed upstream. Here, less subscriber lines are kept as participants exist. When a member of the handset, the concentrator is looking for a free subscriber line OVSt and here about connecting the subscriber to the local exchange. Now the local exchange is ready to receive the dialed digits of the participant. If there is no line to the local exchange longer available, the calling party receives a busy signal alleys.
Dial the destination number
The digits of the dialed number are used either individually or as a block transfer (for so-called block dialing) for local exchange. For analog phones, this is done via tones ( when DTMF dialing ) or pulses ( during pulse dialing ), with ISDN phones via messages on the D channel.
The OVSt analyzed the number signaling and stores the previously dialed digits. If the minimum number of digits received, the local exchange begins with the phone number analysis ( the minimum number is - at least in Germany - usually three, since the shortest occurring numbers consist of three digits, for example " 110 " and " 112 ").
Find the called party
For analysis, each number has a local exchange telephone number list ( routing table ), in which the actions are listed, is to carry out the local exchange with certain numbers ( beginnings ). If the number of digits dialed is not sufficient to determine an action that OVSt waits until enough digits have been received and then performs the analysis continues.
Are there sufficient numbers arrived, the local exchange performs the intended action for these digits. Usually the action is to signal the call information to another exchange, for example, to a remote switching office ( FVST ) on. There is also the previously dialed digits can be analyzed. If the number of digits dialed is not sufficient to decide on the FVST how the call should be treated more digits are requested by the local exchange of the FVST. Are there sufficient numbers arrived, the analysis is continued.
This procedure is repeated when it is forwarded from the FVST the call to further exchanges until finally the destination exchange is found over which the called party can be reached.
If the called subscriber is not in the destination exchange present, which is scheduled for its number range (for example, because he has taken his phone number in the area of another local exchange of the local network while moving, or because he has switched to another phone company ), then it will be found a query on the service Control Point (SCP ), is to be found in what destination exchange or the network operator of the participants. Thither the phone number is signaled.
For special numbers, additional actions are performed when analyzing the number: on dial-up internet access numbers data connections are set to the corresponding ISP.
Shortcuts, such as the emergency numbers 110 and 112 are in the associated long phone number translates ( in the cases of 110 and 112 is the number to the appropriate PSAP ). Then the long number is used instead of the selected speed dial for all further analyzes.
For service numbers ( 0137x, 0180x, 0700x, 0800x, 0900x ) the associated long number is requested and this used instead of the dialed number to the Service Control Point (SCP ). Similarly, the data stored in the SCP tariffs for these numbers are applied to the current telephone call.
Connection possible?
When analyzing the number in the local exchange is checked whether the calling subscriber may choose the number you have dialed at all. Emergency call setup is interrupted ( for example, if the calling party has repeatedly not paid his bill, except for emergency numbers are all numbers blocked)
Along with finding a path from the local exchange of the calling party to the called party's local exchange is also checked whether any free voice channels are available for the desired conversation. Are there any lines free, the necessary lines for the desired call to reserve. If there are no free lines available are listed in the routing tables of the switches of alternative ways through which the destination exchange also can still be achieved. If there is no free line available in the alternate routes, the call setup is canceled with " busy streets ".
From the destination exchange is checked whether the called party can be called. If the line is busy, the calling party from its OVSt a busy tone is played and all the reserved voice channels are released. If the number is not available or the connection even signed off, will the calling party played by his OVSt an appropriate announcement, and all reserved voice channels are released.
If there are no problems, a continuous speech channel is searched for and reserved at the calling party from the destination exchange. If also using alternative routes no free speech channel is found, the call setup is canceled with " busy streets ".
Ringing when the called party
If the called subscriber is found, and a call can be set up to him as a ringing signal is applied to his phone from his local exchange. This is also signaled the local exchange of the calling party, and is now to turn this plays to a ringtone. Ringing signal and ringing are independent of each other, a " messaging " the ringing is only possible under certain conditions ( agreements such as: "When I was well, I let it ring three times " do not work reliably, because if it thrice " toots " when calling subscriber it may have rang four times the called subscriber or only twice).
Switching of voice channels
Takes from the called party, so ringtone and call sign will be terminated and the voice channels for the round trip unlocked: The phone conversation can begin.
At the same time a so-called "Charging - Record" is stored for billing in the local exchange of the calling party. Here, the numbers of the two participants, the start time of the call, as well as the type of connection are stored (for example, voice, fax, data connection).
The called subscriber is located in the network to another phone company, so -called " accounting records " for the billing of the telephone companies are each created in the gateway exchanges. Here, the numbers of the two participants, the start time of the call, as well as the type of connection are also stored.
Performing the subscriber conversation
With the switching of the voice channels ends of the first signaling portion of the call, the call setup. The phone conversation occurs now technically signaling in the so-called " stable " phase one, in which the participants communicate with each other.
While the participants entertained the telephone network performs a few actions. If the calling party is a charge indicator is activated, ( the numbers of the two participants, time, type of connection) charge counter incremented in the local exchange and the results of the billing to be regularly sent to the caller by means of the collective parameters of the call.
The same applies if the caller is calling from a pay phone. The call charges will be deducted from the balance on a regular basis ( coins, phone card ) and communicated to the participants in consumption of the credit that he needs to increase the credit, otherwise the call will be terminated.
Release of the voice channels
Specifies one of the two participants, ends the conversation; the connection will be " triggered ".
At the end of the call be back " Charging Records " in the local exchange of the calling party or " accounting records " stored on the network transitions to the subscriber numbers and the end point of the conversation.
With the end of the conversation, the voice channels are released again and the participants noted as "free". If necessary, special functions are now performed; For example, if a participant "Callback on busy" has activated and in the meantime existed a call request of another user, now try the telephone network, build it up later.
Billing
The billing - known technically as billing - is usually done offline. For this ( phone number of the called subscriber, call start and end, type of call) the charging records from the local exchange will be charged the fees for the participants regularly transmitted to a billing center and then from the interview data.
In certain special functions is not just the fees for the calling party, but also for the called party (for example, 0180 numbers or forwarding). In this case also be created for the called subscriber "Charging records ", which are as well as the charges of the calling subscriber evaluated.
In the same way, the " accounting records " are transmitted to the network transitions to the billing center and determined from the subscriber numbers, call start and end, and the type of call, the origination fees to the telephone companies to each other for forwarding of calls to the other network have to pay.
To unjustified handling billing may occur when individual compounds in cases of False Answer Supervision occurred.
Record phone conversation
The recording of telephone conversations is generally the Penal Code - Special Section (§ § 80-358 ) 15 Section - breach of personal life and secret area ( § § 201-210 ) banned in Germany and is punishable by imprisonment for up to 5 years.
The recording of telephone conversations is allowed in Germany only under very special conditions. Generally, it requires the express consent of all parties to the call.
However, there are special cases in which the record is legally binding. This includes the emergency calls of the authorities and organizations with security tasks ( BOS) 110 and 112 and the internal radio communications, air traffic control, control centers of power utilities ( in-house telephone traffic ) and control centers of the tracks ( in-house telephone and radio communications ). This is used, among other things, the reconstruction of intervention in cases of damage as well as a tool of control center staff to avoid misunderstandings and thus to avoid life-threatening accidents and properly instruct and employ the forces.
In call center voice recording systems are used to phone to document completed contracts or for quality assurance to evaluate employees based on their discussions and training. Some of the records also serve as evidence against the clients for services rendered. The recording is only allowed if the subscriber has expressly agreed (see above).
Other areas of use of voice recording systems are in stock market trading between brokers and threatening calls.
The technical process for recording speech is relatively simple. In general, the recording device is connected in parallel to the connecting line of the terminal or the external line. The audio signal is high impedance tapped and stored. Modern systems, analog signals, ISDN, proprietary digital system telephone connections or to record VoIP.