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Differential Manchester Code Frame Delimiters

IP.com Disclosure Number: IPCOM000040206D
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 3 page(s) / 48K

Publishing Venue

IBM

Related People

Bates, RJS: AUTHOR

Abstract

Start and stop delimiters precede and follow data being sent over transmission lines. The Differential Manchester (DM) code frame delimiters defined in the IEEE 802.5 Standard include three adjacent like-encoded bauds. Because of intersymbol interference (ISI) in the analog portion of the physical layer, this pattern is in general more susceptible to incorrect detection than normal data. In a token ring, this results in difficulties in determining the faulty portion of the network. This article describes various frame delimiters that include just two adjacent like-encoded bauds. With typical ISI, these patterns are equally likely to be incorrectly detected as normal data and eliminate the difficulties of fault determination. Figs. 1 and 2 show DM encoded zero and one bits and DM code violation zero (V0) and one (V1) bits.

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Differential Manchester Code Frame Delimiters

Start and stop delimiters precede and follow data being sent over transmission lines. The Differential Manchester (DM) code frame delimiters defined in the IEEE 802.5 Standard include three adjacent like-encoded bauds. Because of intersymbol interference (ISI) in the analog portion of the physical layer, this pattern is in general more susceptible to incorrect detection than normal data. In a token ring, this results in difficulties in determining the faulty portion of the network. This article describes various frame delimiters that include just two adjacent like-encoded bauds. With typical ISI, these patterns are equally likely to be incorrectly detected as normal data and eliminate the difficulties of fault determination. Figs. 1 and 2 show DM encoded zero and one bits and DM code violation zero (V0) and one (V1) bits. The present proposal uses delimiters made up of DM zero violations and DM-coded ones and zeros; with this set of symbols, the maximum run of like-encoded bits is limited to two.

Unlike the delimiters defined in the IEEE Standard, DM one violations are not used.

(Image Omitted)

To be uniquely decodable as delimiters, without imitation by normal data, it is necessary that the proposed patterns be identified even by receivers that, prior to the patterns, are not synchronized to the bit boundaries. For example, two adjacent DM zero violations may be decoded by a receiver as two adjacent ones if the assumed bit boundaries are incorrect. It is necessary that the delimiters have an equal number of one- and zero-encoded bits in order to avoid introducing a DC component into the signal spectrum. With these restrictions, there are an infinite number of possible delimiters that can be generated. The following rules apply: 1. A DM zero violation may be followed either by a DM zero violation or a DM-coded zero. 2. A DM-coded one may be followed either by a DM-coded one or zero. 3. A DM-coded zero may be followed either by a DM zero violation or a DM-coded one or zero. Further, unless a delimiter is known to be preceded or followed by a DM-coded zero, it must not begin or end with a DM zero violation, respectively. The implications of this will depend on details of the frame format in which these delimiters are employed. To illustrate these delimiter-generating rules, without first invoking this last condition we have: Possible five-bit delimiter...........VV011
Possible six-bit delimiters...........V0101V V0110V

V0V011

VV0011 etc., etc. where V = DM zero violation 1 = DM-coded one

0 = DM-coded zero From the symmetry of the generating rules, it may be observed that DM zero violations and DM-coded ones

1

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may be complemented thr...