DD289188A7 - CIRCUIT ARRANGEMENT FOR DETECTING COLLISIONS IN LOCAL COMPUTER NETWORKS - Google Patents

Abstract

The circuitry for detecting collisions in local computer networks allows, in applications such as office communication CAD / CAM u. a. drastically reduce the cost of the LAN main cable or increase the permissible transmission rate or data transmission speed. In doing so, use is made of the property of different codes and transmission methods of having phases of one's own transmission activity within one bit period. In these phases signals from other stations can be almost unaffected, i. H. be received with high security. According to the drawing, an evaluation circuit with permission input for checking the coded and possibly corrupted reception information as well as a circuit for blanking short interference pulses are necessary for exact evaluation. Fig. 1 {collision detection; Computer network, local; LAN; CSMA / CD; Buerokommunikation; CAD / CAM; LAN main cable; UEbertragungslaenge; Datenuebertragungsgeschwindigkeit; code; bit period; Datenuebertragungssicherheit; glitches; Immunity}

Description

For this 1 page drawings

Field of application of the invention

The invention can be used in all areas of computing, automation and control technology when it comes to connecting computers in a limited area with each other. Typical individual cases are office communication and such areas as computer aided design (CAD), computer aided manufacturing (CAM), computer aided design (CAP), integrated manufacturing (CIM) and teaching.

As a channel access method, the known CSMA / CD, but also other channel access methods such as the well-known token bus (collisions occur here in the Iniiialisierungsphase) can be selected.

Characteristic of the known technical solutions

In local computer networks, situations may occur in which several computers simultaneously send information to a common transmission medium. In the case of the simultaneous sending of multiple computers, information corruption occurs on the transmission medium. This phenomenon is called a collision. In order to be able to recognize and eliminate such unwanted states as quickly as possible, various paths have been taken in the past, which can be classified in principle in four cases:

A) Averaging the currents impressed by the individual transmitters and interrupting the transmission flow if the average significantly exceeds the current yield of a transmitter.

This solution is the world's most common, used, such. Nonetheless, it has principal shortcomings, for example, in the quasi-standard model ETHERNET according to US Pat. No. 4,063,220. These include: The relatively large detection period due to the averaging and, for larger systems, the relatively limited collision detection reliability. The reason for this lies in the different signal levels of near and distant transmitters due to the cable attenuation.

B) Comparison of the transmitted, unadulterated and received, possibly corrupted information on the transmitter.

In this solution, it is assumed that overlap the information from different transmitters on the cable and thus in the event of collisions of its own transmitter no longer receives the information originally intended to be broadcast.

Accordingly, DE-OS3327489 (analogous to JP57134196) describes a data transmission system with collision monitoring. The system consists of a control circuit with a downstream logic circuit (AND and OR gate) and a transmission amplifier (driver), an EXOR gate (Exclusive OR), which the sent with the received Signa! connected. Via a circuit for forming the collision signal, the output signal of the EXOR gate is applied to the OR gate of the o.g. logical circuit. However, this patent does not relate to coded signals and primarily considers the opening of data transmission.

The solution to Case B) can be very fast at small distances to the coaxial trunk (less than or equal to one bit period). However, larger supply line distances require complex buffering and a time-delayed comparison of the transmitted and received data streams.

C) filtering outsourcing of low-frequency beat components of the heterodyne signals. In this case, it is assumed that the distorted signals produce low-frequency beat amplitudes during collisions. Advantageously, it has the effect that significant attenuations can be allowed, since beats due to their significantly different frequency position are relatively low detectable from signals with significantly different amplitudes. The drawback is that the filter effort is not insignificant and the detection time is significantly greater than in solutions according to point B.

O) emission of a so-called Jam sequence in collision detection.

In this case, the collision detection security is increased (amplified) by artificially increasing the time of collisions so that as many stations as possible get a chance to detect the collisions

For example, United States patent no. CL ² .H04Q9 / 00, RM Metcalfe's Multipoint Data Communication System with Collision Detection Nos. 4,063,220 (31.03.1975), OR Boggs, CP Thacker and BW Lampson, now use the jam sequence Worldwide access method "CSMA / CD".

This approach is at the expense of additional time required to improve all listed under A) to C) ways, but he fails as soon as collisions mix the known methods are no longer detectable.

Object of the invention

The aim of the invention is to be able to transmit much further with relatively thin and inexpensive cable, as was usual. The economic savings result not only from the saving of cable costs, but also from the possibility to open up larger transmission distances or to be able to work in the range of higher attenuation with a larger transmission speed.

In conjunction with the higher recognition reliability and the faster detection probability, there are qualitative improvements.

Essence of the invention

With the invention, the problem is solved, safer to recognize collisions quickly and under unfavorable boundary conditions (for example, interference).

High detection speed and security is achieved by making a digital decision on the presence of signal components of other stations within each bit period. The essence of the invention is that the characteristics of particular transmit levels, to be active in only one level and the property of certain coding rules to produce a fixed duty cycle per bit period, are used to convert the signal of others in the phases of its own transmit inactivity occurring per bit period Stations to receive virtually unadulterated

 For this purpose, a circuit arrangement is used in which the data to be transmitted are fed into the computer via a coder and transmission amplifier and the received data of the station are available via an input amplifier and a decoder. The coded data are at the same time at the transmit amplifier and at the prohibit input of an inhibit function. The inhibit function can be followed by a circuit for blanking glitches and a retriggerable monostable multivibrator.

In this way even signals from distant stations can be evaluated almost unaffected. The maximum transmission length is limited in the proposed solution primarily by the sum of the residual currents of all connected stations. As further limitations of the maximum transmission length act when using the circuit arrangement according to the invention only secondary-order effects such. B. Backscatter by inhomogeneities of the cable and delayed release of charge through the dielectric of the cable.

embodiment In the drawings show Fig. 1: the circuit arrangement according to the invention and Fig. 2: a timing diagram of the information to be sent in the Manchester code.

The information to be sent (TxD) passes through a Manchester encoder 1 and then a transmission amplifier 2 before it reaches the LAN main cable of the local computer network 3. In parallel, the signal of the main cable is tapped via a high-impedance input amplifier 4 and fed to an inhibit function 6. The evaluation of element 6 is permitted by supplying the encoded transmit signal at the prohibit input only during the inactive time phases formed by the Manchester encoder. The connected computers receive the reception information (RxD) after Manchester decoding 5. Short cycle times of elements 2 and 4, as well as short glitches are hidden by a conventionally designed circuit for suppressing short pulses 7 (RC element). The signal on the subject of collisions forms in a retriggerable monostable multivibrator 8 and makes available to the connected computers as an interrupt signal "collision." The zooming also shows the terminating resistances of the main LAN cable 9, 10 as well as other stations 11, 12, 13 ,

Claims (2)

1. Circuit arrangement for detecting collisions in local computer networks, in which the data to be transmitted via an encoder, which operates at a fixed duty cycle per bit period, and a transmission amplifier, which is active only in a level position, are fed into the computer network and via a Input amplifier and a decoder, the received data of the station are available, characterized in that the encoded in the encoder (1) own transmission data (TxD _M ) at the same time at the prohibit input of an inhibit function (6) and the transmission amplifier (2) abut while the possibly by other transmitter (11-13) falsified coded input information (RxD _M ) both the second input of the inhibit function (6) and the decoder (5) are supplied. 2. A circuit arrangement according to claim 1, characterized in that the inhibit function (6) is followed by a circuit for blanking glitches (7) and a retriggerable multivibrator.