A measuring unit arranged to measure a clean environment is placed on a cell by a retrieval and storage apparatus to measure clean environment of the cell. The filter includes a filter housing presenting a filter inlet, a bypass inlet, and an outlet. Further, the L-shaped connecting pins are easy to assembly. In a second mode of the invention, the first field of the header compression key can be utilized either for distinguishing between the different flows of compressed packets or for distinguishing between different header compression identifiers. The use of additional x-coordinates can be used to reduce computational complexity.