The Detailed Specification Work Leading to the GSM Phase 1 Standard used for the Opening of Service (1987–1991) Thomas Haug 1 Mid-1987 was a milestone in the work of GSM. I think everybody in the group felt, despite what they might have felt earlier, that the agreement on the basic parameters in the air interface marked the turning point in the work. Another very important event took place in 1987 after years of preparations, i.e. the Memorandum of Understanding (MoU), signed by – initially – 14