Class1 The lowest performance level. Uses an asynchronous byte-oriented half-duplex method of exchanging data. The protocol efficiency of a Class 1 implementation is about 70% (a 2400 bps modem using MNP Class 1 will have a 1690 bps throughput).
Class 2 Uses asynchronous byte-oriented full-duplex data exchange. The protocol efficiency of a Class 2 modem is about 84% (a 2400 bps modem will realize a 2000 bps throughput).
Class 3 Uses synchronous bit-oriented full-duplex data exchange. This approach is more efficient than the asynchronous byte-oriented approach, which takes 10 bits to represent 8 data bits because of the ‘start’ and ‘stop’ framing bits. The synchronous data format eliminates the need for start and stop bits. Users still send data asynchronously to a Class 3 modem but the modems communicate with each other synchronously. The protocol efficiency of a Class 3 implementation is about 108% (a 2400 bps modem will actually run at a 2600 bps throughput).
Class 4 Adds two techniques: Adaptive Packet Assembly and Data Phase Optimization. In the former technique, if the data channel is relatively error-free, MNP assembles larger data packets to increase throughput. If the data channel is introducing many errors, then MNP assembles smaller data packets for transmission. Although smaller data packets increase protocol overhead, they concurrently decrease the throughput penalty of data retransmissions, so more data are successfully transmitted on the first try.
Data Phase Optimization eliminates some of the administrative information in the data packets, which further reduces protocol overhead. The protocol efficiency of a Class 4 implementation is about 120% (a 2400 bps modem will effectively yield a throughput of 2900 bps).
Class 5 This class adds data compression, which uses a real-time adaptive algorithm to compress data. The real-time capabilities of the algorithm allow the data compression to operate on interactive terminal data as well as on file transfer data. The adaptive nature of the algorithm allows it to analyze user data continuously and adjust the compression parameters to maximize data throughput.
The effectiveness of the data compression algorithm depends on the data pattern being processed. Most data patterns will benefit from data compression, with performance advantages typically ranging from 1.3 to 1.0 and 2.0 to 1.0,although some files may be compressed at an even higher ratio. Based on a 1.6 to 1 compression ratio, Microcom gives Class 5 MNP a 200% protocol efficiency, or 4800 bps throughput in a 2400 bps modem installation.
Class 6 This class adds 9600 bps V.29 modulation, universal line negotiation, and statistical duplexing to MNP Class 5 features. Universal link negotiation allows two unlike MNP Class 6 modems to find the highest operating speed (between 300 and 9600 bps) at which both can operate. The modems begin to talk at a common lower speed and automatically negotiate the use of progressively higher speeds.
Statistical duplexing is a technique for simulating full-duplex service over half-duplex, high-speed carriers. Once the modem link has been established using full-duplex V.22 modulation, user data streams move via the carrier’s faster half-duplex mode. However, the modems monitor the data streams and allocate each modem’s use of the line to best approximate a full-duplex exchange. Microcom claims that a 9600 bps V.29 modem using MNP Class 6 (and Class 5 data compression) can achive 19.2 kbps throughput over dial circuits.
Class 7 Uses an advanced form of Huffman encoding called Enhanced Data Compression. Enhanced Data Compression has all the characteristics of Class 5 compression, but in addition predicts the probability of repetitive characters in the data stream. Class 7 compression, on the average, reduces data by 42%.
Class 8 Adds CCITT V.29 Fast-Train modem technology to Class 7 Enhanced Data Compression, enabling half-duplex devices to emulate full-duplex transmission.
Class 9Combines CCITT V.32 modem modulation technology with Class 7 Enhanced Data Compression, resulting in a full-duplex throughput that can exceed that obtainable with a V.32 modem by 300%. Class 9 also employs selective retransmission, in which errors packets are retransmitted, and piggybacking, in which acknowledgment information is added to the data.
Class 10Adds Adverse Channel Enhancement (ACE),which optimizes modem performance in environments with poor or varying line conditions, such as cellular communications, rural telephone service,and some international connections.
Adverse Channel Enhancements fall into five categories:
Negotiated Speed Upshift: modem handshake begins at the lowest possible modulation speed, and when line conditions permit, the modem upshifts to the highest possible speed.
Robust Auto-Reliable Mode: enables MNP10 modems to establish a reliable connection during noisy call set-ups by making multiple attempts to overcome circuit interference. In comparison,other MNP classes make only one call set-up attempt.
Dynamic Speed Shift: causes an MNP10 modem to adjust its operating rate continuously throughout a session in response to current line conditions.
Aggressive Adaptive Packet Assembly: results in packet sizes varying from 8 to 256 bytes in length. Small data packets are used during the establishment of a link, and there is an aggressive increase in the size of packets as conditions permit.
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