A connection must be established between the participants so that a telephone call can be made. This connection must be maintained during their communication and then disconnected again at the end. A phone call is always limited in time. This technical principle is independent of the type of network chosen. A distinction is made between fixed network , IP telephony and cellular network .
Technical sequence of a telephone call
A large number of actions in the telephone network are necessary for a participant to have a telephone conversation with another participant :
- The calling party ( A-subscriber access to) must telephone network received
- The calling subscriber must - usually by entering a phone number - inform the telephone network with which subscriber he would like to make a telephone call.
- The telephone network must use the telephone number to clarify where the called subscriber ( B subscriber ) can be found and whether a connection can be established to the desired subscriber.
- The telephone network must inform the B-subscriber that he has a call request.
- If this person accepts the call , voice channels must be switched between both participants.
- During the call, the telephone network must, as far as possible, maintain the voice channels between the two participants.
- After the call has ended, these voice channels must be terminated again
- At the latest after the end of the call, the telephone network must have collected all the data for collecting the charges .
Much of these actions for making a phone call are signaling operations . They contain either communication between a subscriber and the telephone network via the terminal ( terminal device signaling ) or communication within the telephone network (network signaling).
For the most part, these actions take place within fractions of a second before and after the actual participant conversation. Only a few actions take place in the telephone network during the call ; only actions to check and maintain the call are then required.
The signaling operations for network signaling take place in today's telephone networks in a separate SS7 signaling network that is set up separately from the voice channels. In larger telephone networks, the signaling network is made up of Signaling Transfer Points (STP) that communicate with each other via the SS7 protocol. The network elements of the voice network are connected to this signaling network. The voice network is formed by exchanges (VST) in the fixed network and by Mobile Switching Centers (MSC) in the cellular network .
The end device signaling takes place differently depending on the end device technology. With ISDN telephones, for example, using the DSS1 protocol, with analog telephones, however, with the multi-frequency dialing method or the older pulse dialing method .
Example: a telephone call in the landline network
Access to the telephone network
In more densely populated areas, there is usually a subscriber line to the local exchange (OVST) for each subscriber in the fixed network .
When a member with an analog phone to the telephone receiver down it registers the local exchange, activates the voice channel to the calling party and sets the dial tone on.
In the case of ISDN telephones , picking up the telephone receiver is signaled to the OVST via the signaling channel ( D channel ), which then activates a voice channel ( B channel ) to the calling subscriber. The subscriber then hears the dial tone .
In rural areas, concentrators are often upstream of the OVST. Fewer subscriber lines are kept available than subscribers exist. If a subscriber picks up the receiver, the concentrator looks for a free subscriber connection line to the OVST and connects the subscriber to the OVST via this. The OVST is now ready to accept the subscriber's digits. If there is no longer a line to the OVST available, the calling subscriber receives a lane busy signal.
Dialing the destination number
The digits of the dialed number are transmitted to the OVST either individually or as a block (with so-called block dialing ). With analog telephones this takes place via tones (with multi-frequency dialing ) or pulses (with pulse dialing ), with ISDN telephones via messages on the D-channel .
The OVST analyzes the number signaling and saves the digits dialed so far. Once the minimum number of digits has been received, the OVST starts analyzing the number (the minimum number - at least in Germany - is usually three, as the shortest possible numbers consist of three digits, for example "110" and "112").
Find the called party
For telephone number analysis, each OVST has a telephone number list ( routing table) in which the actions are recorded that the OVST is to carry out for certain telephone numbers (beginning). If the number of digits dialed is not sufficient to determine an action, the OVST waits until enough digits have arrived and then continues the analysis.
If enough digits have arrived, the OVST takes the action intended for these digits. Often, the most action is the call information to another exchange , for example to a trunk exchange (FVST) to further signal . The digits selected so far are also analyzed there. If the number of digits dialed is not sufficient for the FVST to decide how the call should be handled, the FVST will request additional digits from the OVST. If enough digits have arrived, the analysis is continued.
This procedure is repeated when the FVST forwards the call to other exchanges until the destination exchange through which the called subscriber can be reached is finally found.
If the called party does not exist in the target exchange which provided for its number range (for example, because he the while moving his phone number in the area of another local exchange local network taken has, or because he to another phone company has changed), it is a query at the Service Control Point (SCP) found out in which destination exchange or with which network operator the subscriber can be found. The phone number is then signaled there.
For special numbers, additional actions are carried out when the number is analyzed: In the case of dial-in numbers to the Internet , data connections are established to the relevant Internet service provider .
Abbreviated dialing numbers, such as the emergency numbers 110 and 112, are translated into the associated long number (in the case of 110 and 112, this is the number of the relevant emergency call center). The long number is then 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 at the Service Control Point (SCP) and this is used in place of the dialed number. The tariffs stored in the SCP for these numbers are also adopted for the current telephone call.
Connection establishment possible?
When analyzing the phone number in the OVST, it is checked whether the calling subscriber is allowed to dial the number called. If necessary, the call setup is broken off (for example, if the calling subscriber has not paid his bill several times, all phone numbers are blocked except for emergency numbers).
Along with finding a route from the OVST of the calling subscriber to the OVST of the called subscriber, a check is also made to determine whether there are still free voice channels available for the desired conversation. If there are still lines free, the lines required for the desired call are reserved. If there are no free lines available, alternative routes are listed in the routing tables of the exchanges via which the destination exchange can still be reached. If there is no free line in the alternative routes either, the call setup is aborted with "Lane busy".
The destination exchange checks whether the called subscriber can be called. If the connection is busy, the OVST plays a busy tone to the calling subscriber and all reserved voice channels are released again. If the call number is not available or the connection is canceled, the OVST plays an appropriate announcement to the calling subscriber and all reserved voice channels are released again.
If no problems arise, the destination exchange searches for and reserves a continuous voice channel to the calling subscriber. If no free voice channel is found with the help of alternative routes, the call setup is aborted with "Alley occupied".
The called party rings
If the called subscriber is found and a call can be set up with him, his OVST puts a ringing signal on his telephone. This is also signaled to the OVST of the calling subscriber, so that it in turn plays a ringing tone to this subscriber. The ringing signal and ringing tone are independent of each other, an "exchange of messages" via the ringing is only possible to a limited extent (agreements such as: "If I have arrived safely, I will let it ring three times" do not work reliably, because if the calling party "touts" three times, the called subscriber may have already rung four times or only twice).
Switching the voice channels
If the called subscriber picks up, the ringtone and call sign are ended and the voice channels for the outward and return journey are activated: the telephone conversation can begin.
At the same time, a so-called "charging record" for billing is stored in the OVST of the calling subscriber. The call numbers of the two participants, the start time of the conversation and the type of connection (e.g. voice transmission, fax , data connection ) are stored here.
If the called subscriber is in the network of another telephone company , so-called “accounting records” are created in the network transition exchanges for billing the telephone companies among themselves. The phone numbers of the two participants, the start time of the conversation and the type of connection are also stored here.
Conducting the participant call
When the voice channels are switched through, the first signaling component of the telephone call, the call setup, ends. In terms of signaling, the telephone call now enters the so-called "stable phase" in which the participants communicate with one another.
While the participants are talking, the telephone network only takes a few actions. If the calling subscriber has activated a charge indicator, the tariff parameters of the call (the phone numbers of the two subscribers, time, type of connection) are counted up in the OVST and the results of the charge counting are regularly transmitted to the caller.
The same applies if the caller calls from a telephone booth . The call charges are regularly deducted from the credit (coins, phone card ) and when the credit is used up, the subscriber is informed that he has to top up the credit, otherwise the call is broken off.
Release of the voice channels
If one of the two participants hangs up, the conversation ends; the connection is "released".
At the end of the call, "charging records" are again stored in the OVST of the calling subscriber or "accounting records" at the gateways with the subscriber numbers and the end time of the call.
At the end of the conversation, the voice channels are also released again and the participants are noted as "free". If necessary, special functions are now carried out; For example, if a subscriber has activated "Callback when busy" and another subscriber has requested a call in the meantime, the telephone network will now try to re-establish this call.
Billing - known in professional circles as billing - is usually done offline. For this purpose, the charging records from the OVST are regularly transferred to a billing center and then the fees for the participant are calculated from the call data (phone number of the called participant, start and end of call, type of call).
Certain special functions not only incur charges for the calling subscriber, but also for the called subscriber (for example, 0180 numbers or call diversion ). In this case, “charging records” are also generated for the called subscriber, which are evaluated in the same way as the charges of the calling subscriber.
In the same way, the "accounting records" are transmitted to the billing center at the network gateways and the service charges are determined from the subscriber numbers, start and end of the call and the type of call, which telephone companies allocate to each other for forwarding calls to the other network have to pay.
To unjustified handling billing can occur if, in individual cases of compounds False Answer Supervision occurred.
Record the phone call
Voice recording systems are used in call centers to document contracts concluded by telephone or for quality assurance purposes, to evaluate and train employees based on their conversations. In some cases, the records also serve as evidence for the client for the service provided. Depending on the area, recording is only permitted if the participant has expressly agreed.
Other areas of application of voice recording systems are in stock exchange trading between stockbrokers and threatening call recording.
The technical process for voice recording is relatively simple. As a rule, the recording device is switched in parallel to the connection line of the end device or the exchange line. The audio signal is picked up and stored with high resistance. Modern systems can record analog signals, ISDN, proprietary digital system telephone connections or VoIP.
The recording of telephone calls is generally prohibited in Germany in accordance withParagraph 1, No. 1 of the Criminal Code and can be punished with imprisonment of up to three years or a fine. Recording is only permitted under very special conditions; As a rule, this requires the (express) consent of all call participants.
However, there are also special cases in which recording is legally mandatory. This includes the emergency calls of the authorities and organizations with security tasks (BOS) 110 and 112 as well as internal radio traffic, air traffic control, control centers of the energy providers (internal telephone traffic) and control centers of the railways (internal telephone and radio traffic). This is used, among other things, to reconstruct operations in the event of damage and as an aid for the control center employees to rule out misunderstandings and thus to avoid life-threatening accidents and to instruct and deploy the emergency services correctly.
Some so-called telephone answering machines , which in Austria around 1985/1990 also appeared more widely in private households for landline connections and had a compact cassette as a storage medium, offered the option of activating recording on tape by manually pressing a button for a personally accepted call .
In the USA , the recording of conversations is mostly permitted - but depending on the state - if at least one interlocutor gives their consent, or if at least one person is aware of the recording. The legislator justifies this with the fact that the other person consciously participates in the conversation anyway. In this regard, the states are divided into "one-party" and "two-party consent states". Recordings of conversations are generally prohibited only if the other person is being overheard , i. H. the person recording is not part of the conversation.
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