given voice call. 6.1.2 Discussion on Telecom Operator vs. OTT Voice Service Quality As already noted, VoIP is used in two main types, operator VoIP and OTT VoIP. How- ever, all VoIP types have differences compared with PSTN, which may influence the QoS. In general, the differences between VoIP and PSTN may be summarized as follows: Bit rates. PSTN provides digital telephony in each direction with a bitrate of 64 Kbit/s by using ITU-T G.711 codec. VoIP can support various other codecs (G.723, G.729, etc.) with different bitrates, which may provide higher utilization of resources due to statistical multiplexing (i.e. there is no more channel with a given bitrate dedicated to a single voice connection in each direction between caller A and called party B). Packet delay (i.e. latency). PSTN has constant packet delay end-to-end which is below 150 ms in each direction (due to reservation of a channel end-to-end, and due to no need for buffering in telephone exchanges, i.e. network nodes in PSTN), while packet delay is higher in IP-based networks due to different causes that increase the end-to-end delay budget, including (but not limited to) voice packetization (creating IP packets from digitalized voice on the end-user's side), statistical multiplexing (mix- ing different IP packets with the same priority over the same IP network paths), and higher processing time of IP packets in network nodes (e.g. buffering of packets, traf- fic shaping by delaying packets at peak traffic periods, scheduling at different network nodes on the path of IP packets). Jitter (i.e. packet-delay variation). PSTN has very low jitter (again, due to the reserva- tion of channels with a given bitrate of 64 Kbit/s in each of the two directions between 6.2 QoS-enabled Video and IPTV Services the end-users), while VoIP packets accumulate more jitter due to statistical multiplex- ing, traffic shaping, packet scheduling, etc., which introduces variable delay to VoIP packets. Therefore, voice playout buffers are used in receiving terminals, which m