Multiplexing traffic into structure blocks in ATM cells

Telecommunications traffic is transported between first and second narrow band networks across an ATM network. Traffic from users of the first narrow band network is packetised into structured blocks within ATM cells. Each cell is provided with a respective control information field and with a header for routing the cells across the ATM network to the second narrow band network. The control information field incorporates a length indicator indicative of the size of the structured blocks within that cell whereby to control recovery of the user traffic from the cell.