Milosz,
They must be grenadiers. I can see the number "9" on some shoulder straps and Telegraphen troops would have shakos. I think that by this date infantry units had their own internal communications squad. I do find it interesting that these troops are still being issued the early war Ersatz overcoats in 1917. You can tell by the Pickelhauben and the Kaserne in the background that they are still in training.
Chip