Abstract:
In a method of recovering timing information over packet networks, a receiver receives a plurality of packet streams over different paths from the same source. The raw delays experienced by the timing packets for each stream are filtered to provide a filtered delay for each stream. The filtered delays are weighted based on the quality of each stream, and the weighted filtered delays are then combined to form an aggregate delay estimate. Frequency adjustments for a local clock at the receiver are derived from the aggregate delay estimate.

Description:
FIELD OF THE INVENTION  
       [0001]    This invention relates to packet networks, and in particular to a method of recovering timing information over such networks. 
       BACKGROUND OF THE INVENTION 
       [0002]    In timing recovery over packet networks dedicated timing packets are time-stamped by the transmitter&#39;s clock and then are sent over a packet network (PSN) to one or multiple receivers. At the receiver side, these timing packets, as they arrive, are time-stamped by the receiver clock. The difference between these two time stamps represents the relative delay between transmitter and receiver clocks, which can be used to synchronize the two clocks.  FIG. 1  is an illustration of this for the case where there is one transmitter and one receiver. One of the main challenges in this synchronization approach is that the timing packets are subjected to packet delay variations (PDV) inherent in any packet networks. As a result, at the receiver side, depending on packet delay variations, the recovered reference clock will have high level of jitter and wander, which will not be acceptable for many applications, especially legacy services that assume a high quality level of synchronization. 
         [0003]    To overcome this issue, it has been proposed to filter timing packets at the receiver such that only those timing packets that are least subjected to packet delay variations are used for clock recovery. Known methods generally rely only on a single stream of timing packets to synchronize the receiver with the transmitter. 
       SUMMARY OF THE INVENTION 
       [0004]    Embodiments of the invention relate to a new synchronization method over packet switched networks, which exploits space diversity in timing packets originating from same clock source but arriving at the receiver though different network paths. In the proposed invention, for each stream, the time delay between the transmitter and receiver is calculated and filtered to minimize the effect of network packet delay variations on timing packets. The filtered time delay estimates for all streams are weighted and combined based on a non-linear function of residual error and sample rate at the output of time delay filter for each stream. Filtered time delays, combined in this fashion are then fed through a loop filter and digital controlled oscillator to recover transmitter clock. The main advantage of this approach is a significant performance improvement and robustness to packet impairment. 
         [0005]    In IEEE 1588 standard relating to precise synchronization over packet networks, in addition to the timing packets sent by the transmitter to the receiver, the receiver also sends timing information (known as delay request) to the transmitter, which then will be relayed back to receiver (delay response). This process of timing packet exchange measures the round trip delay in network for calculating absolute time difference between transmitter and receiver. The timing packets sent by receiver and relayed back by transmitter can also be considered as a second stream of timing packets and can be used to improve single path delay measurement between transmitter and receiver. 
         [0006]    This invention thus provides a method for clock synchronization over packet switch networks using spatial diversity in timing packet streams originating from same clock source and arriving at receiver through different network paths. 
         [0007]    According to the present invention there is provided a method of recovering timing information over packet networks wherein a receiver receives a plurality of packet streams over different paths from the same source, comprising receiving timing packets from each stream; measuring the raw delays experienced by the timing packets for each stream; filtering the measured raw delays to account for network packet delay variations so as to provide a filtered delay for each stream; weighting the filtered delays based on the quality of each stream; combining the weighted filtered delays to form an aggregate delay estimate; and determining frequency adjustments from the aggregate delay estimate for a local clock at the receiver. 
         [0008]    In one embodiment, for each timing packet stream, raw delays between transmitter and receiver are calculated. The raw delays are basically the difference between the transmitter time-stamps (included in timing packets) and the receiver time stamps, which are recorded at arrival time of each timing packet from the transmitter. For each stream the calculated time stamps are filtered to minimize the effect of network packet delay variations. 
         [0009]    For each timing packet stream, a re-sampling circuit (RSC) is used to convert from a non-uniform sample rate (due to irregular arrival of timing packets) to a uniform sample rate. Another reason for using the re-sampling circuit is to adjust for difference in packet rate for each timing stream such that output sample rate for all timing streams are the same. 
         [0010]    The outputs from previous stage are combined using a linear combiner. The coefficients of linear combiner are calculated using a non-linear function of the output residual error and packet rate of step. 
         [0011]    The output of the linear combiner is passed through a loop filter to calculate the update value for a digitally controlled oscillator (DCO) which provides the output clock. Output of DCO is also used to time-stamp arrival time of timing packets. 
         [0012]    According to a second aspect of the invention there is provided a timing recovery unit for recovering timing information over packet networks wherein a receiver receives a plurality of packet streams over different paths from the same source, comprising a block for receiving timing packets from each stream; a block for measuring the raw delays experienced by the timing packets for each stream; a filter for filtering the measured raw delays to account for network packet delay variations so as to provide a filtered delay for each stream; a weighting block for weighting the filtered delays based on the quality of each stream; a combiner for combining the weighted filtered delays to form an aggregate delay estimate; and a phase locked loop for outputting a local clock at the receiver using frequency adjustments determined from the aggregate delay estimate. 
         [0013]    Embodiments of the present invention can provide a better estimate of the delay by exploiting multiple timing packet streams, and an inherent fault tolerance and robustness to packet impairment since if one of network paths fails to supply timing packets (due to network congestion, outage etc.) other timing paths can be used to maintain synchronization 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which: 
           [0015]      FIG. 1  is a high level diagram illustrating timing recovery over packet networks for a single transmitter and single receiver; 
           [0016]      FIG. 2  illustrates an example of two transmitters with the same primary reference clock sending timing packets to one receiver; and 
           [0017]      FIG. 3  is block diagram of an embodiment of the invention for case where there are two timing streams. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    In the following description k is index to packet number, x j (k) represents transmitter time stamp for timing packet k, and timing packet stream j, y i (k) represents receiver time stamp for timing packet k and timing packet stream j, z i (k) is the raw delay estimate between transmitter and receiver for timing packet k and timing packet stream j, and ω i (k) is the filtered raw delay for timing packet k and timing packet stream j. 
         [0019]      FIG. 3  is a block diagram of an embodiment of invention for the special case where there are only two timing packet streams from the same source and travelling over different paths, but it will be understood that the following description is applicable to the more general case of N timing packet streams. 
         [0020]    For each stream j, raw delays (z(k)) are calculated by subtracting corresponding receiver local time stamps (y j (k)) generated by time stamp circuits  5000  from corresponding time-stamps generated by transmitter (x j (k)). The following can be written for the raw delays 
         [0000]        z   j ( k )=φ j ( k )+ξ i ( k )   (1) 
         [0000]    where φ j (k) represents the true delay between the transmitter and receiver for the k_th timing packet and stream j. ξ j (k) represents the error in measurement caused by network packet delay variations for k_th timing packet in stream j. 
         [0021]    The calculated raw delays z j (k) are then filtered by an adaptive time delay filter method  1000 . A suitable method is described in our co-pending application entitled “Timing Recovery Over Packet Networks” and filed on even date herewith, the contents of which are herein incorporated by reference. This filter minimizes error in time delay estimation between transmitter and receiver caused by network packet delay variations. 
         [0022]    The following can be written for the output of block  1000  for timing packet stream j: 
         [0000]      ω j ( k )=υ j ( k )+φ j ( k )   (2) 
         [0000]    where v j (k) represents the remaining residual error at the output of block  1000  for k_th packet and stream j. 
         [0023]    The variance of υ j (k) in combination with the output packet rate of the block  1000  represents the output quality of the block  1000  for stream j. It depends on the nature and magnitude of network packet delay variations and timing packet rate for each stream. Higher time packet rate and/or lower the network packet delay variations will result in better output quality for the block  1000 . The statistical nature of network packet delay variation is also very important in determining output quality of the block  1000 . 
         [0024]    A weighting mechanism based on some non-linear function of residual error and packet rate is applied at the output of the block  1000  to combine the delay estimates from each timing stream to recover the transmitter clock. 
         [0025]    For each stream, the timing packets can arrive at receiver at different rates. For example, for one stream it can be at 32 packets-per-seconds while for other stream it can be at 128 packets-per-second. Even if the average number of packets-per-second for all streams is the same, depending on network induced packet delay variations, the output sample rate for the block  1000  can be different for each stream. Another issue is that packets arrive at non-uniform time intervals and as a result the output of the block  1000  will be updated at non-uniform sample times asynchronously from other streams. 
         [0026]    In order to take this into account, the block  2000  is used to convert non-uniform sample rates at the output of the block  1000  to the same uniform sample rate for all timing streams. The re-sampling circuit  2000  uses a linear interpolation of the output of the block  1000  and re-samples it at uniformly spaced intervals with the same sample rate for all timing streams. 
         [0027]    The block  6000  is a time delay combiner which uses a weighted combination of all outputs of re-sampling circuits to form the best delay estimate between transmitter and receivers. 
         [0028]    R j (n) j=1 . . . N represents the inputs to block  6000  corresponding to N timing streams. The output of block  6000  is calculated based on the following: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    where F(n) is the output of block  6000  and β j (n) are weighting coefficients that are calculated periodically from time t n−1  to time t n  as described below. 
         [0029]    β j (n) are calculated based on residual error {umlaut over (υ)} j (k) at the output of block  1000  for each timing stream j based on following: 
         [0030]    2a) First calculate 
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         [0000]    where Q j (n) represents output quality for timing stream j, M(n) represents number of filtered timing packets at the output of ( 1000 ) for stream j from time t n−1  to t n  and υ j (k) is the residual error at the output of block  1000 , which based on equation (2), is calculated from 
         [0000]      υ j ( k )=ω j ( k )−φ j ( k )   (5) 
         [0031]    In equation (5), since φ j (k) is not known before hand, an estimated value, given by following equation, can be used instead. 
         [0000]      {circumflex over (φ)} j ( k )={circumflex over (φ)} j ( k− 1)+γ j ( k )( x   j )( k )− x   j ( k −1)   (6) 
         [0000]    where γ j (k) is DCO update value at the time k_th timing packet arrived and x j (k) is the transmitter time stamp for k_th timing packet. 
         [0032]    2b) Between Q 1 . . . Q N  find the one has maximum value; i.e., 
         [0000]        Q   j     max   =max( Q   j ) j= 1 , . . . , N    (7) 
         [0000]    if Q j     max   &gt;Q j ×T for all j=1 . . . N where T is the threshold value (e.g. T=2) then set Q j     max   =0 
         [0033]    The above process can be repeated until all remaining Q j  s have values close to each other. Using this process, the timing streams that have a much higher residual output error than the other streams will be eliminated. 
         [0034]    2c) Based on the quality number given by equation (4), β j (n) are calculated from following 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0000]    where N is the total number of timing streams. 
         [0035]    The output of block  6000  is passed through a loop filter  3000  to provide the update value for DCO  4000 . The loop filter design can be similar to the one although other variants of loop filter can be used as well. 
         [0036]    The frequency and phase of the digital controlled oscillator  4000  whose is adjusted based on its input value. 
         [0037]    Block  5000  is the time stamp circuit (TSC) for the receiver and constitutes the feedback loop for the proposed timing recover method. In  FIG. 3 , there are two time stamp circuits corresponding to two timing streams. In general for each timing packet stream j there is one time-stamp circuit which will generate receiver time stamp values based on output value of DCO. 
         [0038]    It should be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the invention. For example, a processor may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term “processor” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, network processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), read only memory (ROM) for storing software, random access memory (RAM), and non volatile storage. Other hardware, conventional and/or custom, may also be included. 
         [0039]    The described embodiment exploits the diversity present in the network and offers improved performance and reliability compared to a method based on single timing packet stream.