In order to use Internet telephony in a given location, you need to meet a number of conditions that are necessary for its proper operation. In order to install VoIP devices, it is required to provide RJ-45 (Ethernet) sockets with active Internet access for each of the devices. If the devices used to implement Internet telephony have such a possibility, you can use already existing network cables connected to computers, by connecting computers to VoIP devices, and the VoIP devices themselves to the previously disconnected from computers network cables (one-port switch in the VoIP device).
It is also necessary to provide ~230 V power sockets for VoIP gateway devices or VoIP telephones, unless they are powered from the local IT network using PoE[1] mechanism (VoIP devices must have implemented such functionality).
VoIP devices – which is obvious – cannot operate without access to the Internet, and in order to be able to operate properly, it is necessary to provide them with access with proper parameters. The quality of Internet telephony and Internet calls depends to a large extent on the quality of Internet access that the user has (the so-called “last mile”). It is important that the connection to the Internet not only has a proper data transfer speed, but that other parameters – important from the point of view of VoIP telephony – are preserved:
Bit Rate: minimum for one ongoing phone call is 64 kbit/s to the Internet and 64 kbit/s from the Internet (symmetrical line with the speed of 64/64 kbit/s) – with a greater number of simultaneous calls, multiple of these values. In case of a large number of simultaneous connections, e.g. 30 simultaneous connections, it is necessary to ensure a minimum connection being a multiple of the bandwidth as above vs. the number of connections. So in this case it will be 30 pcs. x 64 kbit/s = 1920 kbit/s, so the minimum for such a number of connections is a symmetrical 2Mbit/s to 2Mbit/s link intended only for voice calls in VoIP technology;
minimum for one ongoing telephone call is 64 kbit/s to the Internet and 64 kbit/s from the Internet (symmetrical line with the speed of 64/64 kbit/s) – with a greater number of simultaneous calls, multiple of these values. In case of a large number of simultaneous connections, e.g. 30 simultaneous connections, it is necessary to ensure a minimum connection being a multiple of the bandwidth as above vs. the number of connections. So in this case it will be 30 pcs. x 64 kbit/s = 1920 kbit/s, so the minimum for such a number of connections is a symmetrical 2Mbit/s to 2Mbit/s link intended only for voice calls in the VoIP technology; delay (Delay): if the delays of transmitted packets in any direction are greater than 150 ms, then the quality of VoIP connection is already significantly deteriorating – echo, interruption, bad audibility;
if the delays of transmitted packets in any direction are greater than 150 ms, then the quality of VoIP connection is already significantly deteriorating – echo, interruption, bad audibility; unevenness of packet transmission (Jitter): packets are transmitted with different speed – some faster and some slower – if the jitter is greater than 2 ms, you can hear interrupted words in the handset of the phone;
packets are sent at different speeds – some faster and some slower – if the jitter is greater than 2 ms, you can hear intermittent words in the handset of the phone; packet loss (Packet Loss): it happens that during data transmission some packets are rejected by intermediate devices (for example, the router), most often when there is a data/ traffic congestion phenomenon or are rejected by the VoIP device itself, if the jitter value is high – it is assumed that at a maximum of 5% of the lost packets, the speech is still understandable;
it happens that during data transmission some packets are rejected by middleware devices (e.g. router), most often when there is a phenomenon of data / traffic volume or they are rejected by the VoIP device itself, if the jitter value is high – it is assumed that the value of maximum 5% of lost packets is still understandable; Overbooking: the procedure used by some Internet service providers to sell customers a total higher bandwidth than the actual operator has in its resources – has a direct impact on these phenomena, i.e. delays, jitter and packet loss.
Taking into account the above aspects, it is advisable to use symmetrical lines (the data transfer speed in both directions is the same), the quality of which is guaranteed by a proper SLA agreement.
The second important element is the issue of the LAN itself and the equipment in the network (switches, routers) and modem/router at the interface between the local network and the Internet service provider. This equipment must be very efficient and fast to ensure smooth transmission of voice packets in large cities.
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