Collection of Tools & Utilities/Collection of Tools and Utilities.iso/comm/asp_mdms.zip/MNP-9.TXT

Text File | 1989-08-01 | 14KB | 319 lines

                   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.
              ┌───────────────┐
              │               │
              │ Application   │
              ├───────────────┤
              │               │
              │ Presentation  │
              ├───────────────┤
              │               │
              │ Session       │
              ├───────────────┤
              │               │
              │ Transport     │
              ├───────────────┤
              │               │
              │ Network       │
              ├───────────────┤
          ┌─  │               │   ─┐
Modem plus│   │ Data Link     │    │
File Transfer ├───────────────┤    │ MNP Modem Connection
Protocol  │   │               │    │
i.e.Xmodem└─  │ Physical      │   ─┘
              └───────────────┘
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% m