5.2 Messages and Commands

During a handshake session, the DSL modems at each end of the line exchange messages to negotiate the configuration of the DSL communication session. There are fourteen message types defined in G.994.1. The list below contains a summary of ten basic messages; note that some of the messages have different types associated with them, hence the total of fourteen messages overall.

CL ”Capabilities List : The central office unit (xTU-C) sends this message, and it contains a list of possible modes of operation that it can support.
CLR ”Capabilities List and Request : The remote or customer premises unit (XTU-R) sends this message to the central office unit. This message contains a list of the possible modes of operation that the customer premises unit can support and also request that the central office unit sends a CL message in response.
MR ”Mode Request : The customer premises unit sends this message to the central office unit requesting that (XTU-C) send a Mode Select message. Basically the CP unit requests the CO unit to select a particular mode of operation.
MS ”Mode Select : Either the central office or customer premises unit may initiate this command to select a particular mode of operation.
MP ”Mode Proposal : The customer premises unit sends this command to the CO unit proposing a particular mode of operation and requests that the CO unit send a mode select (MS) message in response.
ACK ” Acknowledgment of messages [2 types: ACK(1) and ACK(2)]. Either the central office unit or customer premises unit may send this message.
- ACK(1) acknowledges one of the following:
  - Receipt of a complete CL or an intermediate frame of a segmented CL message and ends a handshake transaction; or
  - Receipt of a complete MS message of intermediate frame of a segmented MS message and initiates the handshake message clear-down procedure.
- ACK(2) acknowledges receipt of an intermediate frame that contains a segmented CL, CLR, MP, or MS message. It also requests transmission of the next frame of the segmented message.
NAK ” Negative acknowledgment of messages (4 types)
- NAK-EF : This is a negative acknowledgment due to reception of an error frame .
- NAK-NR : This message acknowledges complete reception of a MS message or intermediate frame of a segmented MS message, but the negative acknowledgment indicates that the receiving unit is temporarily unable (or not ready ) to invoke the mode requested in the MS message.
- NAK-NS : This message acknowledges complete reception of a MP or MS message (or intermediate frames of the segmented forms of each), but the negative acknowledgment indicates that the receiving unit either doesn't support or has disabled the requested mode sent by the sending unit.
- NAK-CD : This message is a negative acknowledgment indicating that the received message was not understood , and it initiates the clear-down procedure in response.
REQ-MS ”Request MS message : The central office unit sends this message to the customer premises unit in response to an MR message, in that it requests the customer premises unit to send an MS command. In this case, the CO unit does not want to decide on the mode of operation, and it leaves the choice to the CP unit.
REQ-MR ”Request MR message : The central office unit sends this message to the customer premises unit in response to an MS message, in that it requests the customer premises unit to send an MR command. In this case, the CO unit tells the CP unit that it wants to make the choice of operating mode.
REQ-CLR ”Request CLR message : The central office unit sends this message to the customer premises unit in response to reception of either an MR message, an MP message, or an MS message (or intermediate frames of the segmented forms of each) in that it requests the customer premises unit to send a CLR command. In this case, the CO unit wishes to perform a capabilities exchange with the CP unit.

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