System for regenerating a data word on a communications ring

A system for regenerating an n-bit data word on a token mediated communications ring includes a decoder for receiving the data signal from the ring and deriving the n-bit data word and an associated clock signal from the received data signal. A re-transmit clock generator generates a transmit clock signal incorporating the i.sup.th through the n.sup.th cycles from the derived clock signal, followed by i cycles at the nominal system clock rate. A delay network delays the derived data word by a period approximately equal to the period of the nominal system clock. An encoding network regenerates the n-bit data word for re-transmission on the ring by encoding the delayed derived data word with the transmit clock signal.

FIELD OF THE INVENTION 
The present invention is in the field of data communications, and more 
particularly relates to closed loop communications networks. 
BACKGROUND OF THE DISCLOSURE 
There are many known communications systems which use encoded data from 
which timing and data signals may be derived. In one form, a data 
communication network may consist of a ring-like signal path including one 
or more active repeaters and use a continuously circulating token to 
mediate access. For example, see the network described in Clark, Pogran 
and Reed, Proc. IEEE 66, pp. 1497-1516. In such networks, a plurality of 
user terminals are coupled to a ring. The ring is normally quiet (in the 
absence of use) while a digital "token" circulates around the ring. Upon 
detection of the token at his terminal, a user may "grab" the token and 
thereby gain access to the entire bandwidth of the ring. The user then 
transmits a message followed by re-insertion of a token onto the ring. Any 
other user may then gain access to the ring when he detects the token at 
his terminal. 
In practice, the token may recirculate many times before a user may desire 
access to the ring. However, as the token recirculates, low level noise 
adds to that signal. The resultant token-plus-noise appears to a detector 
(for example, at a user terminal) as a digital signal with phase jitter, 
i.e. the transitions between digital states appear delayed or advanced in 
time from their nominal positions. Particularly for relatively short 
tokens it is necessary to correct for the corrupting influence of this 
random noise on the timing of the pules that constitute the digital 
signals. To this end, rings in such a digital communication networks 
include active repeaters for restoring the digital signal on the ring to 
nominally correct timing. 
For a token-mediated ring, there are two modes of operation, originating a 
message and circulating a token. When originating a message, one user 
terminal breaks the ring (after capturing the token), transmits its 
message into the ring followed by the token, and awaits return of the 
message on the receive side, and then drains the message from the ring. In 
this mode, each transmitted bit is repeated once by each station of the 
ring, and one can calculate the expected accumulation of timing noise as 
the bits progress around the ring. The designer can then choose parameters 
of signal levels so as to assure that every station will be able to 
receive the message with high probability. Thus, in this mode, since the 
message only goes around the ring once and the phase jitter may be 
accommodated, no timing restoration need be done by the repeaters. 
When circulating a token, however, the token bit pattern, once introduced 
to the ring, circulates around and around the ring until such time as some 
user terminal decides to originate a message; the token bit pattern may go 
through many millions of cycles of detection and retransmission. In this 
case, no choice of signal-to-noise ratio can prevent the token timing from 
eventually being degraded to unrecognizability, and some timing 
restoration measure is necessary. 
In the prior art, timing regeneration may be accomplished by introducing 
new, corrected timing on every repeated bit. In that scheme, every 
repeater has its own independent clock that is used for transmission of 
the repeated signal. As successive repeaters will have clocks that operate 
at slightly different frequencies, it is necessary to introduce occasional 
time wedges in some bits to maintain approximate phase match between the 
received and the regenerated signal. That system uses a timing clock that 
is some multiple of the data transmission rate (e.g. six times) and the 
speed of the circuitry using that clock limits the maximum frequency of 
transmission. In addition, the cumulative effect of timing wedges applied 
to a continuously circulating token must be somehow controlled. 
A second alternative approach requires the synchronization of the clocks of 
the ring of repeaters, using a phase-locked-loop and voltage-controlled 
oscillator for the timing clock at each repeater. This approach requires 
continuous transmission of timing bits to maintain loop lock which in turn 
requires a closed loop circulation delay of an integral number of bit 
times. It also requires careful analysis and design to assure stability of 
the ring of phase-locked-loops and phase-delay compensators. The analog 
circuitry required to obtain frequency lock and correct phase delay is 
relatively complex, and is generally concentrated in a special station, as 
is done for example in The Cambridge University ring (Wilkes and Wheeler, 
Proc. Local Area Comm. Network Symp., pp. 47-60.) and the Century Data Bus 
(Okuda, Kunikyo, and Kaji, Proc. Fourth Int. Conf. on Computer Comm., pp. 
161-166). 
It is an object of the present invention to provide a ring communications 
network having improved data regeneration. 
SUMMARY OF THE INVENTION 
Briefly, the present invention is a system for regenerating a n-bit data 
word on a communications ring signal path, where that system has a nominal 
system clock rate. The system of the invention is adapted for use in an 
active repeater (which may be coupled to the ring at a terminal, or may be 
coupled separately to the ring). In various forms of the invention, data 
on the ring may be baseband, or modulated carrier, for example. 
The system includes an input coupled to the ring for receiving a data word, 
such as a token, propagating on the ring, where the word may be in a 
conventional format. A decoder is responsive to the received data word to 
derive an n-cycle derived clock signal and an n-bit derived data word. 
A delay network generates a local data word corresponding to the derived 
data word but delayed by i periods, where each period is approximately 
equal to the period associated with the nominal system clock rate. In 
various forms of the invention, the duration of the i periods may equal 
the period associated with the nominal system clock rate plus or minus 
tolerance values b or a, respectively, where a and b are values related to 
the system noise value. By way of example, the periods may equal the 
period of the first clock cycle of the derived clock signal, or may equal 
just the period associated with the nominal system clock rate. 
A re-transmit clock generator generates an n-cycle transmit clock signal 
which includes n-i cycles at the same rate as the (i+1).sup.th through the 
n.sup.th cycles of the derived clock signal followed by i cycles at the 
system clock rate. Thus, the transmit clock signal is a train of clock 
pulses, the first group of which are identical to the corresponding cycles 
in the derived clock signal and the last group of which are "correctly" 
timed at the nominal system clock rate. An encoder generates an n-bit 
transmit data word by encoding the local data word, clocking that encoded 
data with the transmit clock signal. The resultant transmit data word 
corresponds to the re-clocked, or regenerated, n-bit data word. 
In one form of the invention, the transmit clock signal includes cycles 
corresponding to the derived clock signal in all but the last cycle, which 
is at the nominal system clock rate. Using this configuration, the 
re-clocked data is generated using input derived clocks (and thus noisy 
clock cycles for all but the last bit of the token), and a new correctly 
timed clock signal is used for the last bit. In the next repeater, this 
last correctly timed pulse is used with one repeater's noise added to 
transmit the next-to-last token bit. After passing through the number of 
repeaters required to replace each of the timing bits, the first corrected 
timing pulse will be eliminated and the next repeater will generate a 
replacement. Thus, for example, in a system utilizing a five-bit token, 
any given timing pulse will accumulate the noise of no more than four 
repetitions before it is discarded. As long as the noise level is low 
enough to allow digital data to be reliably detected after five 
repetitions, the present invention will allow a token to be repeated 
indefinitely without accumulating substantial noise.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a communications system 10 including an embodiment of the 
present invention. The system 10 includes a "ring" unidirectional signal 
path (or ring) 12. A plurality of parallel terminal and repeater pairs 
(exemplified by repeaters R.sub.1, R.sub.2 and R.sub.3 and terminals 
T.sub.1, T.sub.2 and T.sub.3) and associated switches (exemplified by 
switches S.sub.1, S.sub.2, and S.sub.3) are coupled along that ring signal 
path. The ring 12 may be formed, for example, with twisted wire pairs for 
a baseband system or a conventional r.f. cable configuration for a 
modulated r.f. system, or optical fibers for an optical system. Each of 
the terminals and repeaters includes an input port for receiving a signal 
from the ring. Each of switches S.sub.1, S.sub.2 and S.sub.3 includes an 
output port for transmitting a signal (either from a terminal or a 
repeater) onto the ring. In alternate embodiments, there may be one or 
more repeaters present without associated terminals. 
In the present embodiment, each of the terminals T.sub.1, T.sub.2 and 
T.sub.3 may be controlled to transmit a digital token signal, or token, 
from the ring 12. The token is in the form of a self-timed code, for 
example, a biphase code in which every pulse is a standard nominal width 
(m nanoseconds) for each half of the biphase signal. 
In operation, initially the token is transmitted onto the ring 12. While no 
terminals are transmitting on the ring, the switches are controlled so 
that the repeaters sequentially receive and re-transmit the token as it 
continuously propagates around the ring. Each of terminals T.sub.1, 
T.sub.2 and T.sub.3 may be requested by externally connected devices (not 
shown) to gain access to the ring 12 and transmit a digital message 
addressed (or not) to any, some, or all of the other terminals coupled to 
the ring 12. 
Upon such a request to a terminal (such as T.sub.1) that terminal monitors 
the ring 12 to detect the receipt of the token from the ring. When the 
token is received by T1.sub., T.sub.1 first controls switch S.sub.1 to 
prevent immediate re-transmission of the token received by repeater 
R.sub.1 on the ring and then transmits (by way of switch S.sub.1) its 
message followed by a new token. This message, followed by the token, 
propagates around the ring by way of the repeaters. Since no other 
terminal can transmit a message until it receives the token, the message 
transmitted by termials T.sub.1 is assured of reaching the desired 
terminal without being garbled by another transmission. When the 
transmitted message returns to terminal T.sub.1, terminal T.sub.1 removes 
the message from the ring, leaving just the token on the ring. 
In order to maintain the integrity of the token as it propagates around the 
ring, one or more of the repeaters (as many as necessary, depending in 
part upon the length of the propagation path around the ring) include a 
timing regenerator network adapted to derive a timing signal from a 
received token, which timing signal is used to re-transmit the received 
token on the ring. 
FIG. 2 shows the preferred form for the repeater R.sub.1. That repeater 
includes a decoder 40, delay network 42, re-transmit clock generator 44 
and encoder 46. 
FIGS. 3-10 show waveforms which are illustrative of the operation of the 
repeater of FIG. 2 using the bi-phase code of FIG. 3 for the exemplary 
five bit token 01111. In operation, the encoded token is initially 
transmitted onto the ring in the form shown in FIG. 4. As that token 
propagates around the ring, it is degraded until, for this example, it is 
received at repeater R.sub.1 in the form shown in FIG. 5, where the 
nominal (in the absence of noise) 0-1 and 1-0 transition points are 
indicated by the broken vertical lines. 
FIGS. 6 and 7 show the clock signal and data signals, respectively, which 
are conventionally derived by the decoder 40 from the received token shown 
in FIG. 5. For illustrative purposes, those clock and data signals are 
shown as three-level. In practice, those signals might be two-level with a 
third line being used to indicate signal presence. 
To regenerate the data for re-transmission, network 42 delays the derived 
data by a period approximately equal to the duration of the period 
associated with the nominal system clock (i.e. T.sub.o, as shown in FIG. 
6). In various embodiments, the precise delay may differ from T.sub.o (by 
as much as T.sub.o /4, for example) depending on the noise level of the 
system. 
FIG. 8 shows the delayed derived data signal for the present example. 
Network 44 includes a switch coupled to the derived clock signal and a 
local oscillator (at the nominal system clock rate). The switch is driven 
to generate a re-transmit clock signal (shown in FIG. 9) from the derived 
clock by dropping the first clock cycle in that signal and utilizing the 
remaining four cycles followed by an added cycle having two m nanosecond 
duration portions. Thus, the last cycle of the transmit clock is precisely 
maintained at the system nominal clock rate. 
Encoder 46 then clocks the delayed derived data (as shown in FIG. 8) with 
the transmit clock signal to generate the re-clocked transmit data, as 
shown in FIG. 10, for transmission onto the ring. 
With this configuration, the token is repeated using input-derived, and 
thus noisy clock cycles for the first four bits of the token and a new, 
correctly timed clock cycle for the last bit. In the next repeater, this 
last, correctly timed pulse is used, with one repeater's noise added, to 
transmit the next-to-last token bit. After passing through four such 
repeaters, this timing pulse is used to transmit the first bit of the 
token, and then the next repeater will discard it. Any given timing pulse 
will thus accumulate the noise of no more than four repetitions before it 
is discarded. 
As long as the noise level is low enough to allow digital data to be 
reliably detected after five repetitions, this system permits a token to 
be repeated indefinitely without accumulating noise. The principle is 
extrapolatable to any token length and bit pattern. 
The invention may be embodied in other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiments are therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description, and all changes 
which come within the meaning and range of equivalency of the claims are 
therefore intended to be embraced therein.