1 Introduction
The wireless service's bandwidth requirements for the backhaul network (Wireless Backhaul, the link between the base station and the wireless switching equipment) have rapidly increased with the rapid development of wireless services. In the past, because wireless voice and low-speed wireless data were the main business of wireless, wireless base stations had less bandwidth requirements, and renting E1 / T1 lines could meet the demand. Now, with the advent of the 3G era, the rapid development of wireless data and video services has prompted the continued growth of wireless network requirements for coverage and bandwidth.
However, because the growth rate of bandwidth is much higher than the growth rate of revenue, operators are faced with the contradiction that the increment does not increase revenue. Of the total OPEX costs of operators, the return pass leased line accounts for approximately 45%. For traditional voice services, 2G base stations transmitting 1 to 2 E1 / T1 can basically meet the demand. At this time, the contradiction between bandwidth and revenue is not obvious; but in the 3G era, base stations usually require 4 to 5 E1 interfaces. Base stations that support HSDPA may even require 8 to 16 E1 / T1s. Therefore, if the leased E1 / T1 transmission method is still used, the OPEX of its network will continue to rise due to the increase in bandwidth demand, which forces operators to look for low-cost wireless backhaul solutions with business security and quality assurance.
As one of the network technologies that can meet the needs of the above operators, PON is getting more and more attention from operators and standards organizations. The time synchronization solution of GPON / 10G GPON has been completed and announced in the ITU-T series of related standards, and IEEE is also formulating the relevant standards of the EPON / 10G EPON time synchronization solution and will soon release it.
2 Time synchronization requirements of wireless backhaul network
For WCDMA, because of the use of asynchronous base station technology, there is no need for a wireless backhaul network to provide time synchronization. For cdma2000 and TD-SCDMA, due to the use of synchronous base station technology, the wireless backhaul network is required to provide time synchronization. 4G LTE also tends to use synchronous base station technology. According to the requirements of the 3GPP2 standard (cdma2000 international standard), the accuracy of time synchronization is better than ± 3? S, so the accuracy of time synchronization between the base station and the reference clock is better than ± 1.5? S. According to the requirements of 3GPP standards (TD-SCDMA and WCDMA international standards) for TD-SCDMA, the time synchronization accuracy is better than ± 3? S; Similarly, the accuracy of time synchronization between the base station and the reference clock is better than ± 1.5? S. Therefore, the accuracy requirement of the phase synchronization between the base station and the reference clock is better than ± 1.5? S.
From the above analysis, it can be seen that the time synchronization requirement is the basic requirement for the backhaul network of cdma2000, TD-SCDMA, LTE and other wireless standards. If the PON does not support high-precision time synchronization, it will not be able to serve as a backhaul network for these wireless standards.
3 GPON / 10G GPON time synchronization standard
See Figure 1 for the time synchronization mechanism of GPON / 10G GPON.
Figure 1 GPON / 10G GPON time synchronization mechanism
The specific steps of the method shown in Figure 1 are:
(1) The OLT first completes time synchronization with the upper-level device.
(2) The OLT calculates the ToDx, i (TIme of Day, time) corresponding to the Xth Super Frame when it reaches ONUi.
(3) The OLT informs ONUi through the OMCI message that the corresponding ToDx, i when the Xth Super Frame reaches ONUi.
(4) After receiving the OMCI message, ONUi compensates ToDx, i accordingly to obtain the accurate arrival time of the Xth frame real_ToDx, i.
(5) ONUi sets its local time to Real_ToDx when frame X arrives, i completes time synchronization.
The error of this scheme is mainly introduced by three parts: different upstream and downstream wavelengths, Serdes, and ranging.
The downstream center wavelength of GPON is 1490nm, and the upstream center wavelength is 1310nm. For a typical SMF-28 fiber, the refractive index n1490 = 1.4682 on the downstream and n1310 = 1.4677 on the upstream. The difference between the two 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 can be obtained is about 17ns. This value can be compensated by calculation.
The delay of Serdes serial-to-parallel conversion is uncertain every time the ONU is activated, and the specific error depends on the Serdes bit width. For the currently commonly used 16-bit Serdes, the maximum error introduced during two adjacent activations here can reach ± ​​16 bits. For GPON, the downstream rate is 2.488Gbit / s, then the error is converted into time about ± 6.4ns. The compensation of this error can theoretically be completed by the MAC chip, but the implementation is relatively difficult: this requires the ONU MAC chip to extract the Serdes specific offset after each ONU activation, and then compensate it.
During GPON ranging, the Response TIme of different ONUs is uncertain, and an error of ± 1? S can be introduced, so that the error of the measured one-way delay is ± 0.5? S, which affects the accuracy of time synchronization from extreme conditions. In view of this, this compensation must be done, otherwise it will lead to greater accuracy degradation. However, this compensation is also very easy to implement, because after the chip is completed, this value is relatively fixed, only need to add / subtract the corresponding fixed value.
GPON's EqD error: GPON's DoW alarm trigger threshold is ± 4bits, GPON's upstream rate is 1.244Gbit / s, converted to time ± 3.2ns, then the maximum error introduced in the unidirectional delay is ± 1.6ns. This error cannot be compensated without changing the existing GPON standard.
In summary, the error introduced by different upstream and downstream wavelengths is about 17ns at the limit of 20km. This error can be compensated by simple calculation; for the error introduced by serial-parallel conversion, in the case of using 16-bit Serdes , About ± 6.4ns, this error can also be compensated, but the implementation is more complicated, it is not recommended; for the error introduced by ranging, in the case of implementing response TIme compensation, it is ± 1.6ns, this error does not change the current In the case of a standard, it cannot be compensated: therefore, the theoretical accuracy can reach ± ​​1.6ns when all compensations are implemented; the accuracy is within ± 25ns without any compensation. The error analysis of 10G GPON is similar.
This scheme was first proposed at the ITU-T SG15 Q2 meeting in December 2008 and was approved by the majority of operators and equipment vendors. It is believed that this scheme utilizes the results of GPON's own ranging and GPON's own downlink frame synchronization mechanism. The implementation is relatively simple. Subsequent Q2 conducted in-depth discussions on this program. At the ITU-T SG15 plenary session in October 2009, Q2 and Q13 conducted a joint discussion on this issue, and the plan was approved and officially written into the G.984 standard (GPON's international standard).
For 10G GPON, since the related mechanisms of time synchronization such as ranging and downlink frame synchronization are the same as GPON, the time synchronization scheme of 10G GPON follows the G.984 standard scheme, which has been written into G.987.3 (TC layer of 10G GPON Standard) and G.988 (10G GPON OMCI standard) drafts will be officially released in the near future.
Huawei actively participated in the formulation of GPON and 10G GPON standards, and proposed a large number of valuable technical solutions. The above-mentioned GPON / 10G GPON time synchronization solution was first proposed by Huawei. Huawei ’s expert Frank Effenberger is the drafter of the conference report (Rapporteur) of ITU-T SG15 Q2 (responsible for the formulation of GPON / 10G GPON standard), and also serves as the standard editor (co. -editor), Yuanqiu Luo is the co-editor of G.987.3 (the TC layer part of the 10G GPON international standard), Lin Wei is the co-editor of the G.988 (the OMCI part of the 10G GPON international standard), to promote GPON / 10G The progress of the GPON standard has made important contributions.
4 EPON / 10G EPON time synchronization standard
See Figure 2 for the time synchronization mechanism of EPON / 10G EPON.
Figure 2 EPON / 10G EPON time synchronization mechanism
The specific steps of the method shown in Figure 2 are:
(1) The OLT first completes time synchronization with the upper-level device.
(2) The OLT calculates ToDx, i when ONUi's localTIme is X.
(3) The OLT informs ONUi of ToDx, i when localTime is X through the OSSP message.
(4) After receiving the OSSP message, ONUi compensates ToDx, i accordingly to obtain the real_ToDx, i corresponding to the time when the localTime is X
(5) When the localTime count reaches X, ONUi sets the local time to real_ToDx, i completes time synchronization.
The error of this scheme is mainly introduced by three parts: different upstream and downstream wavelengths, Serdes, and ranging. Similar to GPON.
The errors introduced by different upstream and downstream wavelengths are the same because the upstream and downstream wavelengths of EPON and upstream and downstream wavelengths of GPON are the same, respectively.
The error introduced in the Serdes part is the same, except that the EPON rate is low, and the introduced error is ± 12.8ns.
For the ranging part, EPON's guardThresholdOLT threshold is 12 TIme Quanta, each Time Quanta is 16ns, converted to time is 192ns, then the maximum error introduced in the unidirectional delay is ± 96ns (but the worst case is a small probability event , Usually does not appear) This error cannot be compensated without changing the existing EPON standard.
In summary, the error introduced by different upstream and downstream wavelengths is about 17ns at the limit of 20km. This error can be compensated by simple calculation; for the error introduced by Serdes serial-parallel conversion, when using 16-bit Serdes In the case of ± 12.8ns, this error can also be compensated, but the implementation is more complicated and is not recommended; for the error introduced by ranging, it is ± 96ns, which cannot be compensated without changing the existing standard; so In the case of all compensations, the theoretical accuracy can reach ± ​​96ns; without any compensation, the accuracy is within ± 130ns; the error analysis of 10G EPON is similar.
The scheme was discussed at the IEEE 802.1AS meeting in January 2009, which was approved by the majority of participants. It is believed that the scheme utilizes the EPON / 10G EPON's own ranging results and the EPON / 10G EPON's own synchronization mechanism , The implementation is relatively simple. At present, the 802.1AS standard has been basically finalized, and the plan can also be covered by the official content of the standard text.
5 Conclusion
Huawei actively participated in the formulation of EPON and 10G EPON standards, and proposed a large number of valuable technical solutions. The above-mentioned EPON / 10G EPON time synchronization solution was first proposed by Huawei. Huawei expert Yuanqiu Luo is a standard editor (Clause Editor) of IEEE 802.1AS Clause 13 (the time synchronization part of EPON / 10G EPON), and has made important contributions to the progress of the EPON / 10G EPON standard.
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