Clock, data and time recovery using bit-resolved timing registers

A clock recovery method is disclosed wherein the FIFO delay of data words and the phase difference between a data word and a receiver clock are used to time data transmissions from a transmitter. The phase difference between the data word and the receiver clock is determined by the offset of a word relative to a desired position in a storage buffer. The FIFO delay is determined either by measuring the difference between a read pointer and a write pointer in the FIFO or, alternatively, by calculating the difference between a timestamp of the time a data word entered the FIFO and the current time as the data word is read from the FIFO.

FIELD OF THE INVENTION

The present invention relates to digital transmission networks and, more specifically, to enhancing the efficiency of data transmission in such networks.

BACKGROUND OF THE INVENTION

There are numerous manners in which to transfer data from a transmitter to a receiver. In a typical system for transferring data, a transmitter has clock circuitry that controls the speed at which data are transferred via a communications medium. A receiver in such a system also typically has clock circuitry that controls the speed at which the data that are received from the communications medium is processed.

Ideally, the receiver's clock and the transmitter's clock will operate at exactly the same frequency and will be appropriately aligned in phase. However, the clock used by the transmitter is typically different in phase and slightly different in frequency as compared to that of the receiver. Further, the data may be variably delayed through the transmission medium as well as through the receiver circuitry. For the case of transmission systems in which several transmitters are transmitting to one or more receivers (for instance, over a time-division multiplexed network), the receivers must recover each transmitter's clock and data, and therefore the receiver circuitry must be able to respond to any number of different phases and perhaps slightly different frequencies within a specified tolerance. Such a system in which data traveling along the transmission medium contains time-division-multiplexed “bursts” of data originating from transmitters with nearly the same clock frequency and no phase alignment is henceforth referred to as a “burst-mode” transmission system.

The efficiency of burst-mode systems is characterized by the ratio of a) the time in the data stream occupied by the readable component of the data bursts, to b) the “unused” time in the transmission line comprised of inter-burst time gaps. To increase the efficiency of any burst-mode system, one seeks to reduce the time overhead introduced by the receiver circuitry and to minimize these inter-burst time gaps. In order to achieve the latter, all sources of delays in the transmission system must be accurately characterized and controlled. For instance, the use of elastic buffers in burst-mode clock and data recovery circuits, such as first-in-first-out buffers (FIFOs), introduces timing delays that prevent the efficient control and minimization of inter-burst time gaps. One method of addressing these efficiency and gap minimization problems is to measure the time delay and phase differences between the clock at the transmitter and the clock at the receiver. However, in order to measure this timing delay, it is necessary to know the characteristics of the clock at the transmitter, which is usually remote from the receiver. Therefore, some method of recovering the transmitter clock characteristics and associating those characteristics with the receiver clock and a particular data transmission is required.

Generally, in non-burst mode transmission systems such recovery, known as clock and data recovery (CDR), can easily be achieved by either standard open-loop or closed-loop clock recovery systems. Examples of these systems, which are well known in the art, are described in I. Dorros et al., An Experimental 224 Mb/s Digital Repeated Line, The Bell System Technical Journal, Vol. 45, No. 7, pp. 993–1043 (September 1966) and R. R. Cordell et al. in A 50 Mhz Phase- and Frequency-Locked Loop, IEEE Journal of Solid State Circuits, Vol. SC-14, No. 6, pp 1003–1010 (December 1979), respectively. Open-loop systems are characterized by a narrow bandpass filter (e.g., a SAW filter), while closed-loop systems typically contain a simple phase-locked loop, which attempts to lock onto the phase of the incoming signal. While such methods sufficiently recover clock and data for continuous or pseudo-bursty data, they are ineffective at CDR in burst mode systems. More recently, cost-effective methods and apparatus for recovering the phase of a signal in a burst-mode transmission system have been developed that avoid many of the deficiencies associated with prior apparatus and methods. One such method suited for use with burst mode signals generates a recovered clock more quickly than other methods in the prior art. This method is the subject of U.S. Pat. No. 5,237,290, issued on Aug. 17, 1993 to Mihai Banu et. al., which is hereby incorporated by reference herein in its entirety. Specifically, according to the '290 patent, the transmitter clock is recovered with a bounded phase relationship with respect to the incoming data signal. Thus, the recovered transmitter clock and the incoming data signal will have the same frequency and their relative phase will remain within a given range.

In another prior attempt, described in U.S. Pat. No. 5,757,872, issued on May 26, 1998 also to Mihai Banu et. al., a FIFO buffer was incorporated into the method of the '290 patent in order to prevent the loss of data that could result from the potential lack of synchronization between the local receiver clock and the recovered transmitter clock.

However, while the prior methods such as those disclosed in the '290 patent and the '872 patent provide a method of accurately recovering the transmitted clock and data, the clock information was only used to assist the receiver in capturing and correlating the incoming data for further transmission within the network.

SUMMARY OF THE INVENTION

We have discovered that, in addition to using the recovered transmitter clock to capture and align the words of an incoming data stream, it is advantageous to accurately measure and capture the phase difference between an incoming data word from an individual transmitter and the local receiver clock. The difference is measured in the word alignment circuitry by determining the offset of the data word as positioned in at least a first register of the word circuitry. This offset is measured by determining the difference between a) the location of a first available position within a first data register and b) that position of a selected bit of the data word within that data register or within an adjacent data register. Since each bit position within the register is filled sequentially at a known fraction of a clock frequency, offset within the register is equal to the phase difference between the recovered transmitter clock associated with a data word and the receiver clock.

It is also advantageous to measure the time delay in an elastic storage buffer experienced by a particular data word from an individual transmitter. In a first embodiment, the delay within an elastic storage buffer is determined by measuring the difference in the position of two memory pointers, one of which is associated with the first data word written to the FIFO, and the other pointing to the first available location in the FIFO. The difference in the location of the pointers is determined by a subtractor circuit. In a second embodiment, a timestamp on each data word may be used to determine the difference between the time when a word is written to the FIFO and the time when a word is read from the FIFO.

Once measured, the aforementioned phase difference and time delay are used to time the transmission of data from individual transmitters in the network with a maximum efficiency while, at the same time, preventing conflicts between successive data words transmitted by different transmitters.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1Ashows an illustrative communications network having a transmitter101, a communication medium102, and a receiver103, connected as shown in theFIG. 1. Data104are outputted by the transmitter101into the communication medium102and is carried by medium102to receiver103as input data stream105. The receiver103decodes input data stream105to produce output108, which comprises an output data word stream supplemented with timing information.

FIG. 1Bshows more specific illustrative communications network, namely a time-division multiple access (TDMA) passive optical communication network (PON) having multiple user transmitter nodes110–113, within a group of transmitters101, transmitting and receiving data from a central office node103. While a TDMA PON is one representative example, one skilled in the art will recognize that the principles of the present invention may be applied to any network with one or more transmitter nodes that requires accurate timing information for use in scheduling or for other purposes. Referring toFIG. 1B, transmitter nodes110–113are connected by optical fiber link102to central office node103through illustrative optical power splitter106. It will be apparent to one skilled in the art that other suitable communications mediums, such as electrical wires, are equally advantageous.

The equipment in the central office103is comprised of, in part, a scheduler and timing circuitry for scheduling transmission from the transmitters110–113, an optical receiver with a burst mode clock data recovery circuit for receiving incoming optical signals and recovering the transmitted clock and data, and an optical transmitter for transmitting optical signals to the transmitters110–113and other network nodes. The timing circuitry passes timing data to the scheduler that, in turn, assigns timeslots to the transmitters110–113such that the different blocks of user data are separated in time by, for example, guard band109to avoid interference when they reach the optical receiver at the central office103. This ensures, for example, that the end of the transmission A from transmitter110does not interfere with the beginning of the transmission B from transmitter111. Maximum efficiency is achieved when the guard band is as narrow as possible. It follows that, when the timing is more accurately known, the guard band can be decreased.

FIG. 2shows an expanded view of the receiver103ofFIG. 1and the functional circuits contained therein. This circuitry is, in part, comprised of a preprocessing circuit201, a clock and data recovery circuit202, a word alignment circuit203, an elastic buffer circuit204and a post-processing circuit205. The preprocessing circuit201accepts the input data stream105from the transmission medium and produces data signal206. The data signal206is input into the clock and data recovery circuit202that produces the recovered transmitted clock signal215and the recovered data signal207. An illustrative example of the clock and data recovery circuit202is described in U.S. Pat. No. 5,757,872, which is incorporated by reference in its entirety as previously set forth hereinabove. As discussed above, and as is well known in the art, the clock and data recovery circuit202recovers the transmitter clock with a bounded phase relationship with respect to the incoming data signal. Thus, the recovered transmitter clock and the incoming data signal will have the same frequency and their relative phase will remain within a given range.

Once the transmitted clock is recovered, the serial data signal207and the recovered clock signal215are transmitted to the word alignment circuit203. The word alignment circuit203uses a detection method to reconstruct the incoming words of the incoming data stream207.FIG. 3shows one illustrative embodiment of a word alignment circuit203wherein a particular number of bits in a bit serial data stream207, corresponding to the known word length used in the data stream, is transferred into a shift register301at the clock rate of the recovered transmitter clock. Illustratively, the word length is 10 bits and the local receiver clock rate is 1.25 GHz. One skilled in the art will recognize that many different word lengths and clock rates are equally advantageous. The data entered into the shift register301may consist of the bits of a single word or, alternatively, may consist of bits from two different words. Once these data are entered into the shift register301, it is then transferred at a lower clock rate into a first storage register302at the same time the data in the first storage register301is copied into a second storage register303. This clock rate is, advantageously, the clock rate determined by the following relationship:

fword=fbitsW(Equation⁢⁢1)
where fwordis the frequency at which the data are shifted from the shift register301into the first storage register302, fbitsis the frequency at which the data are entered into the shift register301, and W is the data word length. Thus, according to the relationship in Equation 1, assuming the illustrative fbitsof 1.25 GHz and a data word length of 10 bits, fwordwill be 125 MHz. It follows that, if the receiver clock is aligned in phase with the incoming data word, as soon as one complete 10-bit word is entered into the shift register301(10 clock cycles at 1.25 GHz), that word will be transferred into the first storage register302.

Thus two word-length sets of data are stored, one set in the second storage register303and one set in the first storage register302. If the receiver clock is aligned in phase with the incoming data word, the first storage register302and the second storage register303will each hold a single complete word of, in this illustrative example, 10 bits. However, if there is a phase difference between the receiver clock and the incoming data word, data in the shift register301will not be transferred into the first register302as a complete word. Instead, the first and second storage registers will each contain portions of two different words. For example,FIG. 5shows the two storage registers,302and303ofFIG. 3, in an illustrative example showing how portions of words can be contained within those registers when the clocks are out of phase with respect to one another. Specifically, in this illustrative example, the data from the shift register,301inFIG. 3are transferred in an out-of-phase manner into the first storage register302. It follows that, after two successive transfers of incoming word-length sets, register303will contain a portion of word503and the first serial portion of word502. Similarly, register302will contain the second portion of word502and the first portion of word501.

Thus, in order to route the complete incoming words to their intended destination when the transmitter clock and receiver clock are out of phase, it is necessary to reconstruct, or align, the bits of the complete words such as, for example, word502inFIG. 5. Referring once again toFIG. 3, this reconstruction (or word alignment) function is accomplished by shifter304which is, illustratively a well-known barrel shifter. This barrel shifter304utilizes a well-known word detection scheme to reposition the bits of a word within storage registers302and303. One method of detecting this position is to detect a known calibration pattern that is transmitted in the data stream for this purpose. One skilled in the art will recognize that there are advantageous methods used in other implementations for detecting the position of the bits of words relative to the storage registers and reconstructing those words.

Illustratively, referring once again toFIG. 5, comparator circuits507–516within the word detection circuitry306ofFIG. 3, are used for this purpose. Specifically, each of comparator circuits507–516is connected to a different sequence of bit locations in registers302and303. Specifically, a sufficient number of comparator circuits507–516are used such that each possible word location corresponds to an individual comparator circuit. Each comparator circuit detects the bits in the bit locations within the two registers504and505to which it is connected. When word alignment is required and the calibration pattern is detected, the offset between the actual position of the bits of the calibration pattern relative to the desired position in the storage register301inFIG. 3is measured.

The desired position of word502is, illustratively, when the first bit of word502(shown in position4of register303) is in position10in register302. Thus, the last bit of that word (shown in position5of register302) would be in position1of register302. In the example ofFIG. 5, however, word502(which is, for example, the calibration word) is not in this location. Specifically, in this case, comparator circuit507, which is connected to positions5–10of register4and to positions1–4of register505, would detect the calibration pattern. Thus, as previously discussed, the bits of the data word502were entered into the shift register serially at a 1.25 GHz clock rate, but the 125 MHz clock governing the register302inFIG. 3was out of phase relative to the incoming data word by the equivalent of five cycles of that higher rate clock. This phase difference, representative of the positional offset of the word from its desired location, is measured and output in signal210to an elastic storage unit, such as the FIFO ofFIG. 4, as described herein below. This phase difference is then transmitted to the aforementioned timing circuitry associated with the scheduler of the central office103inFIG. 1B. The scheduler then compensates for this phase difference by adjusting future scheduled transmissions from the individual transmitters110–113.

Once this phase difference/offset has been measured, barrel shifter304inFIG. 3which is, once again, well known in the art, reconstructs the complete word from its bit-positions in registers302and303and stores the word in a third register305. This word is then transmitted via signal209, in accordance with the timing of the FIFO clock signal210, to the aforementioned FIFO.

FIG. 4shows an elastic storage unit in accordance with the principles of one embodiment of the present invention, more specifically a FIFO204. FIFOs such as FIFO204, are generally well-known in the art. However, the FIFO ofFIG. 4is unique in data networking in that, unlike any such prior FIFOs, FIFO delay and word alignment delay for each data word are stored in registers within the FIFO that correspond to the data word. FIFO204is comprised of a number of equal registers, such as register401. Each of the registers has, illustratively, three fields. The first field411in register401stores the data word transmitted in signal209from the word alignment unit ofFIG. 3; the second field412stores the FIFO delay corresponding to the data word; and the third field413stores the word alignment delay measured by the word alignment unit ofFIG. 3and transmitted via signal210. One skilled in the art will recognize that, in some systems wherein word alignment delay and/or FIFO delay is relatively constant, an alternative embodiment of the FIFO that includes only a single register for storage of that word alignment delay and/or FIFO delay may be equally advantageous.

Read pointer403and the write pointer405indicate the register into or from which data are being read from or written to, respectively, at that particular point in time. Pointers403and405move vertically relative to the registers, including registers401,402and410, in order to point to that register where data are being read from or written to, respectively. Arithmetic subtractor circuit404determines the difference between the relative positions of the write pointer and the read pointer. The result corresponds to the latency of a data word in the FIFO204prior to being read. Thus, this difference is a direct measurement of the delay experienced as a result of using the FIFO204.

Alternatively, to calculate the latency of a data word in the FIFO204, the subtractor circuit or an external circuit could be used to determine the difference between a timestamp corresponding to each data word as it is read and the current time. The timestamp is a copy of the status of a reference counter which is assigned to a data word via input218at the time this word is written to the FIFO204.

The writing of logic values of inputs209,218and210to one of the FIFO registers, such as register401is triggered by the write clock input signal208. The register to be written to, in this case register401, is selected by the write pointer405generating the write enable signal409to register401. Simultaneously with writing values of inputs209,218and210to register401, the write pointer405updates its value. The write pointer405keeps the same value when no registers in the FIFO204are being written to. When the value of the write pointer405is to be updated and the current value of the write pointer405corresponds to any register other than the last register410, the new write pointer value is obtained by increasing its current value by one. When the current value of the write pointer405corresponds to the last register410, the new value of the updated write pointer405is the one that points to the first register402.

Reading logic values from, for example, register402is triggered by the read clock signal216. The register to be read from (402in this example) is selected by the read pointer403generating the read enable signal408to register402. Simultaneously with making the contents of register402available at outputs211,212and213for transmission to post processing circuit205inFIG. 5, the read pointer403updates its value. The read pointer403keeps the same value when no registers in the FIFO204are read from. When the value of the read pointer403is to be updated and the current value of the read pointer403selects any register other than the last register410, the new value is obtained by increasing its current value by one. When the value of the read pointer403is to be updated and the current value of the read pointer403selects the last register410, the new value is the one that points to the first register402.

As discussed above, in one illustrative embodiment, the subtractor circuit404performs arithmetic subtraction of the value of the write pointer405from the value of the read pointer403. If the result is positive or zero it is presented to the output214. If the result is negative, the subtractor circuit404makes a correction of this result by adding the total number of registers to the result and presents the new value to the output214. The value output at output214is representative of the total delay of a data word attributable to FIFO204processing.

As previously discussed, FIFO204outputs to the illustrative post-processing circuit205inFIG. 2the data word211, the word alignment delay213, the FIFO latency214calculated from subtractor circuit404and, optionally, the FIFO delay212calculated as the difference between the timestamps on the word being read out from the FIFO204and the current time. One skilled in the art will recognize that outputs212and214are somewhat redundant and, therefore, a particular embodiment may illustratively only use one of the outputs212or214.

The FIFO delay, calculated as described above, is transmitted to the aforementioned timing circuitry associated with the scheduler in the central office103inFIG. 1B. The scheduler then uses this delay information to schedule future transmissions from the individual transmitters110–113.

The foregoing describes an illustrative embodiment wherein accurate time information is used by a scheduler in a communication network to increase the efficiency of network transmissions. Another type of system in which accurate time information is required is illustrative shown inFIG. 6. Specifically,FIG. 6shows a system that relies on synchronous circuit emulation over asynchronous packet-switched networks. Two synchronous networks602and606, for example public switched telephone networks (PSTN), are interconnected over an asynchronous transport network, for example an Ethernet network604using adapter systems603and605. In synchronous networks, all equipment that constitutes such a network must operate clocks that are synchronized in frequency. Typically, a synchronous network comprises one or a small number of highly accurate primary clock sources601, such as atomic clocks and all other equipment synchronizes to its local clock to said primary reference source using a hierarchical distribution path. If a segment of a synchronous network is connected to the rest of a network though an asynchronous network only, such as when segment606is connected to segment602though network604, there are no means for directly transporting clock indications from one network to the other. In such cases, special methods and systems must be developed to synchronize said two segments of the synchronous networks. Adapter systems603and605implement this function.

Using these adapters, the aforementioned synchronization is achieved by using packet arrival events at the adapter605as clock indications. Since packet departures at adapter system603are deterministic and synchronous to the clock in network602, processing the packet arrival events at adapter605, so that the effects of the jitter introduced by asynchronous network604is removed can be used to implement the synchronization method. Better accuracy and faster convergence will result if the packet arrival events are measured with higher precision.

The highest possible measurement precision of packet arrival times is within the accuracy of one bit time. Using the accurate timing method of the present invention simplifies the implementation of a system that measures the packet arrival times with bit granularity by minimizing the amount of circuitry that runs at the high-speed bit-clock. The circuit that implements the present invention allows for using the parallel interface of the receiver103to detect packet arrivals followed by using the delta-alignment delay signal and the signal representing the number of entries in the FIFO to reconstruct the original arrival time at the serial side of the receiver. Said reconstruction circuitry operates at the frequency which is W times lower than the bit rate of the receiver, where W is the word length at the parallel interface of the receiver.

The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are within its spirit and scope. Furthermore, all examples and conditional language recited herein are intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting aspects and embodiments of the invention, as well as specific examples thereof, are intended to encompass functional equivalents thereof.