The Quality of Service is a measure of the perceived performance of the telex network. The perception of theservice performance varies between that of the calling subscriber, the origin Administration, the destination Administration and the called subscriber. For example, national network difficulties in reaching the origin international exchange will usually be noticed only by the calling subscriber. However, if the origin or destination Administration automatically retries to reach the wanted subscriber following an unsuccessful call attempt, these reattempts are not seen by that part of the network that precedes it. Conversely, a called subscriber is unaware of the number of times that call attempts are being made to his number, if his machine is busy or out of order.