Digital telephony is a fairly general term for telephone traffic that is transported in a digital way. The original analog telephone network has been digitized since the 1980s.
Circuit switched analog telephone network
In the original analog telephone network, copper wire connections were connected to a telephone exchange. The telephone exchange was responsible for making the connection and was previously connected to other exchanges. Several conversations could be conducted over one copper wire by means of frequency multiplexing, that is to say, each call received part of the available frequency space available on the wire.
The analog telephone network was a circuit switched network, which means that a fixed communication channel is built up from the caller to the called party.
Digital networks have advantages over an analog network, but the biggest advantage of a digital network is the sound quality; with an analog telephone network, the sound quality decreases the further away the interlocutor is, with a digital network it is constant.
The analog telephone network has been abolished.
Time switched digital telephone network
In a time-switched digital telephone network, the speech signal is digitized, creating a digital data stream. These data flows are divided into data packets, and these packets are in turn neatly passed over the connections between the telephone exchanges. There is a soft guarantee that the packages are not lost and a hard guarantee that they will be delivered in the correct order to the destination.
Most telephone networks are time-switched today.
Packet switched digital telephone network
In a packet switched telephone network, the signal is again digitized and divided into packets. These packets are then sent in a packet-switched network. A packet switched network delivers individual packets at the destination. No effort is made to distribute the available capacity fairly, to ensure that the packages arrive or to ensure that they arrive in the correct order. If parcels arrive delayed, are lost or arrive in the wrong order, the phone will temporarily disturb.
Packet switched networks are not very suitable for telephony due to their properties. Nevertheless, they are very popular because the internet is a packet-switched network, which allows cheap communication over long distances.
Digitization of the customer connection
Initially, the digitization of the telephone network took place solely in the connections between telephone exchanges. Subsequently, work was done to digitize the connection from the telephone to the exchange.
ISDN is a technique that normally connects over a time-switched digital telephone network. With ISDN, multiple telephone lines can be implemented over one copper wire. The commonly used ISDN-2 connection implements two lines, plus a control channel over the telephone line. ISDN lines usually have good call quality, good reliability and a lot of functionality. They are usually combined with a small telephone exchange, which allows internal calls and transfers.
VoIP connects directly to a packet-switched network, there is no longer a telephone exchange. A VoIP phone generates direct internet traffic that is sent. Because packet-switched networks are not made for telephony applications, a relatively high-quality internet connection with sufficient spare capacity must be available, which minimizes the chance that packets will not arrive or are delayed. VoIP is characterized by low call costs.
With VoDSL, a telephone connection is implemented over an ADSL connection in addition to an Internet connection. It is a time switch system based on ATM. The internet connection is transferred to the packet-switched internet at the internet provider, while the telephone connection is connected to the time-switched telephone network. The telephone connection and internet connection cannot influence each other. VoDSL is a technology where, as with VoIP, telephony and internet can be offered over one infrastructure.
Cable telephony is done using EuroDOCSIS. An analog telephone is connected to the network via a cable modem. EuroDOCSIS is a packet-switched network where internet and telephone traffic can influence each other. To overcome the disadvantages of packet-switched networks, EuroDOCSIS has built-in facilities to reduce the chance that telephone traffic will be disrupted by other internet traffic.
Here we also speak about VoIP. The most commonly used VoIP protocols in a cable network are SIP (Session Initiation Protocol) and MGCP (Meadia Gateway Control Protocol).