The MUP applies normally the principle that an HLR/MTX initiating a transaction will receive a RESPONSE from the addressed NMT-node receiving an acknowledgement (positive or negative) and possible other updating information.

In cases where the MTXH may be an HLR also, this is specified in the text as "HLR/MTXH". If only the term MTXH or HLR is used, the message in question is only applicable for the indicated network element. The table below is showing the messages handled by an HLR

Table 3.1 HLR related messages

The transaction number (TN) is used as a communication reference to uniquely define the communication i.e. the signalling procedure, as outlined in figure 3.1.

When sending a MESSAGE, the NMT-node will select a transaction identifier included in the TN which is used to identify the RESPONSE. The NMT-node receiving a MESSAGE shall use the received TN when sending RESPONSE.

The transaction identifier is a binary code of 16 bits, which gives a possibility of 65536 different identifications.

When a message is received, the receiving NMT-node must see to it that the response is not delayed so much that a time-out may occur at the originating node.