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MNP error- correcting modems
MNP Error Correcting Modems
Overview
The Microcom Networking Protocol, MNP, is a communications
protocol that supports interactive and file-transfer
applications. MNP is designed to conform to the
International Organization for Standardization (ISO) Open
System Interconnection (OSI) Network Reference Model, or
simply the OSI model. The OSI model is a network protocol
divided into standardized layers (or modules). The use of
standardized layers assists in the interconnection of
different vendors equipment.
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? Application ?
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? Presentation ?
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? Session ?
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? Transport ?
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? Network ?
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Modem plus? ? Data Link ? ?
File Transfer ????????????????? ? MNP Modem Connection
Protocol ? ? ? ?
i.e.Xmodem?? ? Physical ? ??
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The OSI model allows users to choose how their networking
systems are partitioned and implemented.
The Link Layer of the OSI Network Model is responsible for
provide reliable date transfer. It uses the Physical
Layer to transmit information through the data path. In
dial-up data communications, the data transmission of the
Physical Layer is performed by "traditional" modems using
standards such as Bell 103, Bell 212A and V.22 bis.
Traditional modems cannot provide guaranteed error-free data
communications. The noise and distortion characteristics of
voice-grade telephone circuits are beyond the capabilities of
any signal processing to deliver error-free data. It is the
task of the Link Layer to provide a means of error detection
and error control. Error detection when accessing Bulletin
Boards is provided for file transfers by an error-correcting
protocol (Xmodem for example) but there is no error detection
present when reading ASCII text. That's why garbled
character can sneak thru but you can transfer a file
successfully.
Microcom's MNP error-correcting modems provide the integrity
of data transmission over voice-grade circuits for both file
and text transmission when connected to another MNP equipment
modem. When connected to a "standard" modem there is no
hardware error checking. The user demand for error-free data
communications has made Microcom MNP error-correcting modems
a "standard" in the modem industry implemented by many modem
manufactures.
(There is still a possibility of errors occurring in a MNP-to-
MNP connection if they occur at either end between the serial
port and the modem (in the cable) or in the computer itself.
The probability for error is much, much less here than exists
while the data is being transferred between modems. And, if you
are transfering ARCed files, the CRC checking that occurs when
the file is deARCed is enough to show you that the file transfer
was successful.
While some people run an MNP-to-MNP file transfer with no
additional error checking protocol, there are low-overhead
protocols which transfer large blocks of data between
acknowledgments of successful receipt and these are
particularly well suited to use with the MNP-to-MNP
connections.)
Performance Comparisons of MNP Classes
MNP is designed for easy implementation on many hardware
configurations. Different applications require different
cost and performance mixes. MNP is deliberately structured
to provide different levels of performance without
sacrificing compatibility. Unlike other protocols,
applications that require low-cost solutions can use simpler,
less demanding implementations of MNP and MNP implementations
at all performance levels are compatible with each other. A
small application with a simple implementation of MNP can
communicate with a more powerful system using a high
performance implementation of MNP.
The primary principle of MNP is each implementation
communicates with all other implementations. When an MNP
communications link is being established, the MNP
implementations will negotiate to operate at the highest
mutually supported class of MNP service.
MNP assembles the user data into packets before
retransmission. The use of data protocols by the overhead a
protocol introduces to the communication channel. The
protocol overhead reduces the effective data throughout of
the communications channel.
A description of each MNP performance level follows. The
description shows how MNP offers the user greater throughput
than the basic error-prone communication channel.
Class 1
This is the first level of MNP performance. MNP Class 1 uses
an asynchronous byte-oriented half-duplex method of
exchanging data. MNP Class 1 implementations make minimum
demands on processor speeds and memory storage MNP Class 1
makes it possible for devices with small hardware
configurations to communicate error-free.
The protocol efficiency of a Class 1 implementation is about
70%. A device using MNP Class 1 with a 2400 bps modem will
realize 1690 bps throughput. Modern microprocessors have
become so powerful that implementations of MNP Class 1 are
uncommon in the U.S.
Class 2
MNP Class 2 uses asynchronous byte-oriented full-duplex data
exchange. Almost all microprocessor-based hardware is
capable of supporting MNP Class 2 performance. Common
microprocessor selected for MNP Class 2 implementations are
Z80's and 6800's.
The protocol efficiency of a Class 2 implementation is about
84%. A device using MNP Class 2 with a 2400 bps modem will
realize 2000 bps throughput. Most microprocessor-based
hardware can easily implement MNP Class 2.
Class 3
MNP Class 3 uses synchronous bit-oriented full-duplex
exchange. The synchronous bit-oriented data format is
inherently more efficient than the asynchronous byte-oriented
data format. It takes 10 bits to represent 8 data bits in
the asynchronous data format because of the "start" and
"stop" framing bits. The synchronous data format eliminates
the need for start and stop bits. The user still sends data
asynchronously to the Class 3 modem; meanwhile, the modems
communicate with each other synchronously.
The protocol efficiency of a Class 3 implementation is about
108%. A device using Class 3 with a 2400 bps modem will
realize 2600 bps throughput. At Class 3 performance, the MNP
protocol "rewards" the user for using an error-correcting
modem by producing 8% extra throughput over an ordinary modem
without MNP.
The MultiTech 224E modem implements MNP Class 3.
Class 4
MNP Class 4 introduces two new concepts, Adaptive Packet
Assembly(tm) and Data Phase Optimization(tm), to further
improve the performance of an MNP modem. During data
transfer, MNP monitors the reliability of the transmission
medium. If the data channel is relatively error-free, MNP
assembles larger data packets to increase throughput. If the
data is introducing many errors, then MNP assembles smaller
data packets to transmit. while smaller data packets
increase protocol overhead, they concurrently decrease the
throughput penalty of data retransmissions. The result of
smaller data packets is more data is successfully transmitted
on the first try.
MNP protocol recognizes that during the data transfer phase
of a connection, most of the administrative information in
the data packet never changes. Data Phase Optimization
provides a method for eliminating some of the administrative
information. This procedure further reduces protocol
overhead.
The protocol efficiency of a Class 4 implementation is about
120%. A device using MNP Class 4 with a 2400 bps modem will
realize approximately 2900 bps throughput. With class 4
performance, the MNP protocol produces 20% more throughput
than an ordinary modem without MNP.
Microcom's AX/1200, AX/2400 and PC/2400 support class 4.
Class 5
MNP Class 5 introduces Data Compression as a new feature to
MNP Class 4 service. MNP Data Compression uses a real-time
adaptive algorithm to compress data. The real-time aspects
of the algorithm allow the data compression to operate on
interactive terminal data as well as file-transfer data.
Data compression delivers faster screen updates to the user.
The adaptive nature of the algorithm means data compression
is always optimized for the user's data. The compression
algorithm continuously analyzes the user data and adjusts the
compression parameters to maximize data throughput. Adaptive
compression means users of file-transfers receive maximum
data compression and data transfer.
Data compression algorithms, like sort algorithms, are
sensitive to the data pattern being processed. Most data
being transmitted will benefit from data compression. The
user will see compression performance vary between 1.3 to 1
and 2 to 1 (some files may be compressed at even higher
ratios). The following types of common user files are listed
in order of increasing compressibility:
1) COM or EXE files (ARCed files too)
2) Spreadsheet files
3) Word Processing files
4) Print Files
A realistic estimate of the overall compression factor a user
will experience is 1.6 to 1 or 63%. This is equivalent to
having a net protocol efficiency of 200% for an MNP Class 5
implementation. A device using MNP Class 5 with a 2400 bps
modem will realize 4800 bps throughput. At MNP Class 5
performance, the MNP protocol produces over 100% more
throughput than an ordinary modem without MNP.
Microcom's AX/1200c, AX/2400c and PC/2400c support class 5.
CASE's 4696/VS supports Class 5.
Class 6
MNP Class 6 introduces the new features Universal Link
Negotiation(tm) and Statistical Duplexing(tm) to MNP Class 5
service. Universal Link Negotiation allows MNP to unify non-
compatible modem modulation technology into the same MNP
Error-Correcting Modem. Prior to Class 6, MNP was used to
enhance current modem technology. MNP Class 6 allows Microcom
to create new universal modems.
Most 1200 bps and 2400 bps modems are designed to be
compatible with lower speed modems. Bell 212A type modems
operate at 1200 bps and incorporate the Bell 103 standard for
0-300 bps communications. Likewise, there are V.22 bis modems
that operate as 300 bps 103 modems, 1200 bps 212A modems and
2400 bps modems. However, high speed V.29 and V.32 modems do
not provide compatibility with each other or with the lower
speed modulation techniques found in 212A and V.22 bis
modems. Before the advent of MNP Class 6, it was impossible
for a single modem to operate at a full range of speeds
between 300 and 9600 bps.
Universol Link Negotiation allows MNP modems to begin operations
at a common slower speed and negotiate the use of an alternate
high speed modulation technique. The Microcom AX/9624 is an
example of a modem that uses Universal Link Negotiation.
Universal Link Negotiation uses the 2400 bps V.22bis technology
to negotiate a link. At the end of a successful link
negotiation for Class 6 operation, the modem shifts to operation
using 9600 bps V.29 technology.
In the case where the high-speed carrier technology uses half-
duplex modulation, MNP Class 6 provides Statistical Duplexing.
The Statistical Duplexing algorithm monitors the user data
traffic pattern to dynamically allocate utilization of the half-
duplex modulation to deliver full-duplex service.
An MNP Class 6 modem based on V.29 technology delivers maximum
performance in file transfer applications; up to 19.2 kbps
throughput is possible on dial-up circuits for most
applications. In accordance with the principles of MNP, the
Class 5 Data Compression is fully incorporated in MNP Class 6.
The MNP Class 6 Modem will deliver optimum performance even on
an interactive terminal using character echoplexing. Screen
updates will occur at speeds between 9.6 kbps andf 19.2 kbps.
Most screen updates will take less than a second.
Microcom's AX/9612c, AX/9624c and PC/9624c support Class 6.
Most of the above text was taken from a Microcom Features
Description by Mike Focke 7/7/87
Class 7
MNP Class 7 Enhanced Data Compression, combined with Class 4,
achieves improved throughput with efficiencies up to 300% via
the latest data compression technology. Microcom's enhanced
encoding technique not only ndynamically adjusts to the type of
data being transmitted, but also predicts the probabality of
characters in a data stream. This combined with run length
encoding, which sends repeating characters as a single number
code, results in the superior compression efficiencies supported
in MNP Class 7.
The Microcom QX/12K supports Class 7.
Class 8
Nothing available.
Class 9
MNP Class 9 utilizes Enhanced Data Compression combined with
V.32 technology to deliver maximum throughput up to 300% greater
than ordinary V.32 modems. Class 9 also features Enhanced
Universal Link Negotiation which allows connection to both MNP
and non-MNP modems at the highest performance level.
The Microcom QX/.32c supports Class 9
The above was taken from product description buochures from
Microcom by Mike Focke 8/1/88
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