MNP: Microcom Networking Protocol Standards

Microcom Networking Protocol is an entire hierarchy of standards, starting with MNP Class 1, an out of date error correction protocol to MNP Class 10, Adverse Channel Enhancements, which is designed to eke the most data transfer performance from poor connections. MNP does not stand alone but works with modems that may conform to other standards. The MNP standards specify technologies rather than speeds. MNP Classes 2 through 4 deal with error control and are in the public domain. Classes 5 through 10 are licensed by Microcom and deal with a number of modem operating parameters.

MNP Class 1 uses an asynchronous byte-oriented half duplex method of exchanging data designed to make a minimum demand on the processor in the PC managing the modem. It was originally designed to enable error free communications with first generation PCs that had little speed and less storage. Using MNP Class 1 steals about 30 percent of the throughput of a modem, so a 2400-bits-per-second modem using MNP Class 1 achieves an actual throughput of about 1690 bps.

MNP Class 2 takes advantage of full duplex data exchange. As with MNP Class 1, it is designed for asynchronous operation at the byte level. MNP Class 2 achieves somewhat higher efficiency and takes only about a 16 percent toll on throughput.

MNP Class 3 improves on MNP2 by working synchronously instead of asynchronously. Consequently, no start and stop bits are required for each byte, trimming the data transfer overhead by 25 percent or more. Although MNP3 modems exchange data between themselves synchronously, they connect to PCs using asynchronous data links, which means they plug right into RS-232 serial ports.

MNP Class 4 is basically an error correcting protocol but also yields a bit of data compression. It incorporates two innovations. Adaptive Packet Assembly allows the modem to package data in blocks or packets that are sent and error checked as a unit. The protocol is adaptive because it varies the size of each packet according to the quality of the connection. Data Phase Optimization eliminates repetitive control bits from the data traveling across the connection to streamline transmissions. Together these techniques can increase the throughput of a modem by 120 percent at a given bit rate. In other words, using MNP4, a 1200-bits-per-second modem could achieve a 1450-bits-per-second throughput. Many modems have MNP4 capabilities.

MNP Class 5 is purely a data compression protocol that squeezes some kinds of data into a form that takes less time to transmit. MNP5 can compress some data by a factor up to two, effectively doubling the speed of data transmissions. On some forms of data, such as files that have been already compressed, however, MNP5 may actually increase the time required for transmission.

MNP Class 6 is designed to help modems get the most out of telephone connections independent of data compression. Using a technique called Universal Link Negotiation, modems can start communicating at a low speed, and then, after evaluating the capabilities of the telephone line and each modem, switch to a higher speed. MNP6 also includes Statistical Duplexing, which allows a half duplex modem to simulate full duplex operation.

MNP Class 7 is a more efficient data compression algorithm (Huffman encoding) than MNP5, which permits increases in data throughput by factors as high as three with some data.

MNP Class 9 (there is no MNP Class 8) is designed to reduce the transmission overhead required by certain common modem operations. The acknowledgment of each data packet is streamlined by combining the acknowledgment with the next data packet instead of sending a separate confirmation byte. In addition, MNP9 minimizes the amount of information that must be retransmitted when an error is detected by indicating where the error occurred. Although some other error correction schemes require all information transmitted after an error to be resent, an MNP9 modem needs only the data that was in error to be sent again.

MNP Class 10 is a set of Adverse Channel Enhancements that help modems work better when faced with poor telephone connections. Modems with MNP10 will make multiple attempts to set up a transmission link, adjust the size of data packets they transmit according to what works best over the connection, and adjust the speed at which they operate to the highest rate that can be reliably maintained. One use envisioned for this standard is cellular modem communications (the car phone).