Data Communications Networking Devices - 4th Ed.pdf

448 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTFigure 4.51 MNP frame format DLE data link escape, STX start of text, ETX end of text):a) Asynchronous data frame format; b) Synchronous data frame formatFigure 4.51a). If the attached terminal device transmits synchronous data, theframe format is as indicated in Figure 4.51b). For both frame formats the framecheck sequence FCS) characters are generated by a CRC-16 polynomial.Under MNP a special frame called a link request LR) that has a prede®nedformat is used to establish an error-corrected connection between two modems.This frame is also used to negotiate a variety of parameters between two MNPmodems, including the maximum number of frames that can be outstanding andnot acknowledged at a particular point in time, the maximum information ®eldlength and the framing mode to be used. The link request frame is developed byusing a value of one in the header ®eld in the synchronous frame format illustratedin Figure 4.51b). If during negotiation the framing mode is negotiated for thetransmission of asynchronous data the frame format illustrated in Figure 4.51a)will be used.One of the more interesting aspects of MNP is its Class 10, which is designed asan enabling technology architecture. MNP Class 10 can be viewed as residing ontop of different modulation standards, such as V.22 bis, V.32, V.32 bis, V.34, andother standards as well as the de jure and de facto standards for data compressionand error control. This enables an MNP Class 10 modem to provide users withmaximum performance while assuring compatibility with all recognized internationalstandards. Figure 4.52 illustrates the MNP Class 10 architecture.MNP PDUsFrom a technical perspective, an MNP frame's header and information ®eld arecalled a Protocol Data Unit PDU). The PDU provides the information required tode®ne and negotiate facilities, manage communications, and transfer user databetween modems. Eight types of PDUs are currently de®ned. Each has the general