Data Communications Networking Devices - 4th Ed.pdf

4.3 INTELLIGENT MODEMS ________________________________________________________ 449Figure 4.52MNP Class 10 architectureformat:LEN TYPE PARAMETERS...where LEN represents the length of the PDU in bytes, TYPE is a numeric code of1 through 9 that indicates the type of PDU, while PARAMETERS represent astring of octets that have special meaning. Both the position of the parameter in thePDU and the code in the TYPE octet are used to de®ne the role of the parameter.With the ability to include up to 256 parameters in a PDU and de®ne up to 256different PDUs, MNP can be considerably expanded. In fact, it is very easy toenhance MNP by adding new parameters to PDUs or adding new PDUs sincecompatibility with existing MNP implementations is maintained as the use of newenhancements must be mutually agreed on.Figure 4.53 illustrates the basic formats of the eight MNP PDUs which aretransported within an asynchronous or synchronous frame. Both the length andtype ®elds in each PDU are one octet in length. Up to 256 different parameters canbe carried within a PDU and user data is attached to the LT PDU as a variablelength ®eld up to 256 bytes in length.The Link Request PDU is used to initiate or acknowledge a connection request.This PDU can transport such parameters as the MNP service class, the number ofmessages that must be acknowledged, and the maximum amount of user data thatwill be carried in a message.The Link Disconnect PDU is used to request that the link should be disconnected.Information in the parameters ®eld indicates the reason for thedisconnection request.The Link Transfer PDU is used to move user data across an establishedconnection. User data is attached to this PDU as a variable length ®eld of up to 256bytes. The only parameter that is included in this PDU is the Send SequenceNumber SSN) which identi®es the location of the current PDU within a stream ofPDUs being transmitted.The Link Acknowledgement PDU is used to acknowledge the receipt of amessage. Parameters within the PDU indicate how many additional Link Transfer