Analysis of GPON and EPON Time Synchronization Technology (Part 2)

- Aug 16, 2019-

GPON/10G GPON Time Synchronization Standard

The time synchronization mechanism of GPON/10G GPON is shown in Figure 1.


Figure 1 GPON/10G GPON time synchronization mechanism

The specific steps of the method shown in Figure 1 are as follows:

(1) OLT first synchronizes with the completion time of the superior equipment.

(2) OLT calculates the corresponding ToDx, I (TIme of Day) when the X Super Frame arrives at ONUi.

(3) OLT informs ONUi through OMCI messages that when the X Super Frame arrives at ONUi, the corresponding ToDx, I.

(4) When ONUi receives the OMCI message, it compensates ToDx and I to get the exact arrival time of frame X, real_ToDx, I.

(5) ONUi sets its local time to Real_ToDx when the X frame arrives, I is the completion of time synchronization.

The error of this scheme is mainly introduced by three parts: different up/down wavelength, Serdes, ranging.

The downlink central wavelength of GPON is 1490 nm and the upstream central wavelength is 1310 nm. For typical SMF-28 fibers, the downlink index n1490 = 1.4682 and the upstream index n1310 = 1.4677, the difference between them is 0.0005, so the correction factor is obtained. If 0.5 is used as an approximation, the error introduced is about 170 ppm, which is about 17 ns at the maximum physical distance of 20 km. Because the one-way transmission delay of 20km is about 100? S, multiplying this 170ppm, the error is about 17ns. This value can be compensated by calculation.

The delay of series-to-parallel conversion of Serdes is uncertain at each activation of ONU, and the specific error depends on the bit width of Serdes. For the commonly used 16-bit wide Serdes, the maximum error introduced here can reach (+16 bits) when two adjacent activations occur. For GPON, the downlink rate is 2.488 Gbit/s, and the conversion time of the error is about +6.4 ns. In theory, this error compensation can also be accomplished by MAC chip, but it is relatively difficult to achieve: this requires that the MAC chip of ONU extract the specific offset of Serdes after each ONU is activated online, and then compensate for it.

In GPON ranging, the Reponse TIme of different ONU s is uncertain, which can introduce an error of (+1?) s, so that the error of one-way delay is (+0.5?) s. In view of the effect of extreme bad conditions on the accuracy of time synchronization, this compensation must be done, otherwise it will lead to a greater deterioration of accuracy. However, this compensation is also easy to achieve, because after the chip is completed, the value is relatively fixed, only need to add/subtract the corresponding fixed value.

EqD error of GPON: The threshold of DOW alarm trigger of GPON is +4 bits, the upstream speed of GPON is 1.244 Gbit/s, converted to time +3.2ns, and the maximum error introduced in one-way delay is +1.6ns. This error cannot be compensated without changing the existing GPON standards.

In summary, for the errors introduced by different upstream and downstream wavelengths, the error can be compensated by simple calculation at the limit of 20 km. For the errors introduced by series-to-parallel conversion, the error can also be compensated by using 16-bit wide Serdes, but the implementation is more complex and not complicated. For the errors introduced in ranging, it is recommended to do; under the condition of realizing response time compensation, it is (+1.6ns), which can not be compensated without changing the existing standards: therefore, under the condition of all compensation, the theoretical accuracy can reach (+1.6ns); without any compensation, the accuracy is within (+25ns). The error analysis of 10G GPON is similar.

The scheme was first proposed at ITU-T SG15 Q2 conference in December 2008, which was approved by operators and equipment providers. It is believed that the scheme uses GPON's own ranging results and the downlink frame synchronization mechanism of GPON itself, and is relatively simple to implement. Follow-up Q2 discussed the scheme in depth. At the ITU-T SG15 plenary meeting in October 2009, Q2 and Q13 jointly discussed this issue. The scheme was approved and formally incorporated into the G.984 standard (the international standard of GPON).

For 10G GPON, because the time synchronization mechanism of ranging and downlink frame synchronization is the same as that of GPON, the time synchronization scheme of 10G GPON follows the scheme of G.984 standard, which has been written into the draft of G.987.3 (TC layer standard of 10G GPON) and G.988 (OMCI standard of 10G GPON), and will be officially released in the near future.

Huawei actively participated in the formulation of GPON and 10G GPON standards, and put forward a large number of valuable technical solutions. The above GPON/10G GPON time synchronization scheme was first proposed by Huawei. Huawei's expert Frank Effenberger is the drafter of ITU-T SG15 Q2 (responsible for the formulation of GPON/10G GPON standards), co-editor of G.984.3 (TC layer part of GPON international standards), co-editor of G.987.3 (TC layer part of 10G GPON international standards), co-editor of Yuanqiu Luo, G.988 (10G GPO layer part of TC international standards), and Lin Wei, G.988 (10G GPO layer part of TC layer of 10G GPON international standards). The co-editor of OMCI part of N International Standard has made an important contribution to the progress of GPON/10G GPON standard.

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