Abstract: A digital cellular communications system has a bidirectional transmission link for conveying encoded speech and control data in the form of blocks, a switching unit coupled to a first end of the transmission link, and a base station coupled to a second end of the transmission link. The switching unit includes a speech encoder for encoding speech signals, an error detection code generator for generating an error detection code from the encoded speech signal, and a formatter, coupled to the error detection code generator, for producing a formatted block to be transmitted on the transmission link, the formatted block having a header, the encoded speech data, and the error detection code. The base station has a transmission link error detector for signalling the existence of an error in a received block in response to the received block containing an error.