telecom@ucbvax.UUCP (01/02/86)
In the literature on Microcom's "High Speed Error-Correcting Modems" which are the ones which incorporate Class 6, it says the following: "Additionally MNP Class 6 includes Statistical Duplexing(tm) which allows the modems to allocate line bandwidth and data traffic to achieve maximum throughput in both interactive use and file transfer. ------------------------------------------\ \ Interactive 9600 bps / Communications /------------------------------------------/ / \ 9600 bps \------------------------------------------ _________________________________________ \ 19,200 bps \ File / Transfer _________________________________________/ / 110 bps \----------------------------------------- Statistical Duplexing achieves full duplex performance using half duplex modulation techniques. " It also says that the 19,200 bps performance throughput is reached with 9600 bps transmission plus data compression, etc. (covered in my previous posting) and the minimum packet size shown is 32 characters. Note from the last lines in the quote above that the transmission is half-duplex. My question is how any kind of packets, with half duplex, even with "Fast Training" 9600 bps, can send interactive sessions (e.g., vi) and give good performance? Back of the envelope calculation: 1 data character/packet + 4 other characters/packet, with the packet sent at 9600 bps, and 1 millisecond line turn around time gives 333 echoplexed characters/second. And I think that my assumptions may be too favorable-packet overhead is likely to be >4 characters, and line turnaround is likely to be >1 ms. Maybe this speed is sufficient when a person is typing, and then dynamically switching to longer packets as soon as characters start flowing fast in either direction, e.g., for screen redraw, gives more than adequate performance. Anybody know? --henry schaffer
Earnhardt@MIT-MULTICS.ARPA.UUCP (07/30/86)
Several big problems with MicroCom MNP were not mentioned in the interview. First, while there are N companies who have implemented the protocol, there is no immediate way of knowing if the implementations are tuned. MicroCom does have seminars in implemting MNP, but companies are often interested in reienventing (or perhaps reiemplemting) the wheel. In fact, since MicroCom sells modems itself, it has a vested interest in having its implementation of MNP work "better" than anyone else's. Only Class 1 through Class 3 MNP are free for other modem companies to use. Class 3 only has about a 10% increase in efficiency over the carrier speed. The dramatic throughput increases start appearing in Class 4 and Class 5, which must be licensed. I'd be interested in knowing what percentage of non-MicroCom MNP modems are above and below the Class 3 boundary. There are already modem manufacturers that are using a "superset" of MNP. Some manufacturers are extending capabilities of MNP; some are circumventing the royalties associated with the higher-level implementations. These actions will limit the universality of the high-performance MNP levels. Finally, as has been noted in Telecom already, no modem-to-modem packetization protocol can guarantee reliable end-to-end transmission of information. There must be some additional mechanism to assure that the information is correctly getting transferred. Any reliable byte stream must have its packetization/retransmission facilities exist in the source and destination machines. By its implementation, MNP is not be the answer for error-free communications.