Industrial networks combine elements of distributed control systems. These elements are process stations, operator stations, engineering stations and other elements, which enable the exchange of data between industrial systems. Industrial systems are used to send process variable values and other data exchanged between the stations - elements of a distributed control system and SCADA systems. The fitness of an entire station system depends on the fitness of the communication system. This is why industrial networks are characterized by high reliability. When speaking about the fitness of a data transmission system, we can consider its instantaneous fitness (functional) or interval fitness (task-related). It is not always necessary to maintain instantaneous fitness. Sometimes interval fitness is enough. The paper discusses the issue of a need to maintain one of them, relative to various types of data transmission.