Browse Prior Art Database

False Preamble Mechanism for a Head-End Remodulator

IP.com Disclosure Number: IPCOM000034788D
Original Publication Date: 1989-Apr-01
Included in the Prior Art Database: 2005-Jan-27
Document File: 4 page(s) / 54K

Publishing Venue

IBM

Related People

Ott, WB: AUTHOR [+2]

Abstract

This article describes a technique for use in a head-end remodulator or regenerative translator (RGT) which eliminates the propagation delay introduced by the RGT by immediately transmitting the preamble upon RF signal detection on the inbound path and restores the preamble to at least its original length. A broadband local area network (LAN) allows personal computers (PCs) to communicate with each other over a single coaxial cable. By broadband it is meant that the information on the cable is frequency division multiplexed. The frequency spectrum on the cable is divided up into 6 MHz wide channels just like cable TV. A typical broadband LAN would occupy two of those channels; one for the transmit channel, and the other for the receive channel.

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False Preamble Mechanism for a Head-End Remodulator

This article describes a technique for use in a head-end remodulator or regenerative translator (RGT) which eliminates the propagation delay introduced by the RGT by immediately transmitting the preamble upon RF signal detection on the inbound path and restores the preamble to at least its original length. A broadband local area network (LAN) allows personal computers (PCs) to communicate with each other over a single coaxial cable. By broadband it is meant that the information on the cable is frequency division multiplexed. The frequency spectrum on the cable is divided up into 6 MHz wide channels just like cable TV. A typical broadband LAN would occupy two of those channels; one for the transmit channel, and the other for the receive channel. An RF modem's transmitter on a computer's communications adapter connected to the coaxial cable converts a serial bit stream from the link level protocol of the adapter into a RF signal suitable for transmission over the LAN. Conversely, receiving modems convert the received RF signal back into the serial

(Image Omitted)

bit stream. All of the modems transmit at the same frequency over the cable up to the head end. This is the inbound path. At the head end, a frequency translator converts the incoming RF signal for transmission out to the modem receivers. This is the outbound path. Fig. 1 is a block diagram of a typical single cable broadband LAN. An overview of the sequence of events in the operation of the broadband network is as follows: 1. The originating communications adapter begins to send a message to that adapter's RF modem in the form of a serial bit

stream. This message is composed of preamble, start flag,

address information, data, cycle redundancy check (CRC) and

stop flag. 2. The RF modem's transmitter converts the serial bit stream to a RF signal for transmission to the head end. 3. The head end converts the inbound path frequencies to the outbound path frequencies. The message is never demodulated

to baseband. 4. The RF signal now travels down the outbound path to all of the RF modem's receivers. 5. The RF modem receiver converts the RF signal back into the serial bit stream for reception by the communications adapter. One type of translator is known as a limited function translator (LFT). This type of translator simply translates the incoming RF signal at the inbound channel frequency to the outgoing RF signal at the outbound channel frequency. The RF signal is never demodulated. It is always at radio frequencies. While an adequate form of translation for some applications, it has the disadvantage of passing along any RF signal degradation and accumulated signal path

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tolerances from the inbound path to the outbound path. The other type of translator is known as a head-end remodulator or regenerative translator (RGT). Fig. 2 is a block diagram of a typical RGT. The sequence of events is the same as described a...