“…Their popularity and number of applications increase mainly because of the headway in computer science, which is a basic discipline of their existence. Interesting analyses are presented, e.g., by Juneja et al (2012), Naumov et al (2015;, Naumov and Samuilov (2018), , Lisovskaya et al (2017;, Zhernovyi and Kopytko (2016), Matalytsky and Zaj ąc (2019), Cascone et al (2010), Rumyantsev and Morozov (2017), Kerobyan et al (2018), or Nowak et al (2020). It is worth highlighting that, in the cited papers, the service time of a customer is still often treated as independent with regard to his volume so the obtained results can be applied in real systems in a limited range.…”