1. A method of updating a lookup table associated with a TDM transmit unit which is coupled to a packet network to receive data therefrom, the lookup table mapping packet network contexts to TDM channels, the method comprising: priming a controller with data for updating the lookup table in advance of the receipt of packets containing data to which the update relates; incorporating a flag in each packet sent over the packet network to the TDM transmit unit, the flag being set to indicate whether or not a packet is the first packet to which the update relates; and detecting the setting of said flag in a first packet and in response updating the lookup table with the update data.