[mod.telecom] MNP

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.