CN104734772B - EPON backbone's optical link protection system and its method for obtaining balanced time delay - Google Patents
EPON backbone's optical link protection system and its method for obtaining balanced time delay Download PDFInfo
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Abstract
The invention discloses a kind of key optical link protection system of EPON and its method for obtaining balanced time delay, it is related to passive optical network technique field, this method includes the L that OLT A set primary optical linkmax‑a、Teqd‑aAnd PLOu is sent to OLT B；OLT B are according to Lmax‑bAnd Lmax‑a, W is setsize；OLT A are by TsendSent with StartTime to OLT B；OLT B are in Teqd‑a+StartTime+RspTimeiAt the time of opening timing device, OLT B position ONUi uplink burst by detecting PLOu and record Trcv, calculate ΔRNG‑B’；OLT B obtain RTDi‑AAnd ΔRNG‑A, and calculate RTDi‑B；OLT B are according to RTDi‑BAnd Teqd‑a, calculate EqD B.The present invention avoids carrying out the ranging of interrupting service service so that the interrupting time reduction that passive optical network is serviced customer service, has fully ensured the data safety of user.
Description
Technical field
It is specifically a kind of key optical link protection system of EPON the present invention relates to passive optical network technique field
System and its method for obtaining balanced time delay.
Background technology
PON (Passive Optical Network, EPON) technology is the broadband optical access skill based on optical fiber
Art, optical network system is by OLT (Optical Line Terminal, optical line terminal), ODN (Optical Distribution
Network, Optical Distribution Network) and ONU (Optical Network Unit, optical network unit) compositions, because PON system has
The many merits such as high bandwidth, high efficiency, large coverage, user interface be abundant, most of operators are realized using the technology and connect
Enter that network service is broadband, synthesization transformation, be laid out extensively, the substantial amounts of Consumer's Experience of acquisition.
The passive optical network applied at present uses point-to-multipoint network structure, and different optical network units use TDMA
(Time Division Multiple Access, time division multiple acess) technology access network.Because remote equipment ONU is distributed in respectively
In user family or in the corridor of cell, each ONU and local side apparatus OLT distance is differed, and each ONU carries the light of data
Signal can produce different time delays after the optical fiber transmission of different length, so also each the time required to reaching local side apparatus OLT
Differ；Therefore, different ONU optical signal reach ODN when, if uncontrolled, it is possible to collide with it is overlapping, make biography
Defeated data can not be parsed.So, needed using the PON system of TDMA to the distance between each ONU and OLT
It is measured, notices the delivery time that each ONU controls upstream data so that each ONU data reaches light point at the time of regulation
Distribution network, so as to avoid occurring signal conflict on whole optical-fiber network.And ONU is used for controlling upstream data delivery time ensureing
The important parameter that whole optical-fiber network does not collide is exactly EqD (Equalization Delay, balanced time delay).
In EPON, key optical link protection system provides double-backbone light chain to provide the reliability of optical link
Road, so when the optical link that works sends failure, can be switched to standby optical link and proceed work.Standby optical link will be just
Often before work, it is necessary to need first to obtain optical link parameter.
And be that measurement each ONU equilibriums time delay EqD, OLT must send downlink frame on optical link, bandwidth is carried out to ONU
Authorize, so as to guide ONU to send uplink burst, the time requirement between OLT and ONU is measured with this.And it is used as key light chain
The spare OLT of road protection system is can not to send downlink frame by optical module, and the ONU in system can not be controlled.Institute
So that when primary optical link breaks down, to obtain the parameter of optical link, former spare OLT turns into primary, its work carried out first
Work is exactly to measure the balanced time delay EqD of each ONU of optical link.Now that is, each ONU is accessed in a new optical-fiber network, to keep away
Exempt from each ONU conflict of data-signal, it is necessary to interrupt the data, services of all users on optical-fiber network, in the case of silence one by one
Each ONU is carried out ranging bandwidth authorizing to measure ONU balanced time delay EqD, this causes passive optical network to user's industry
The interrupting time of business service increases, so as to cause the data safety of user not to be to be fully ensured.
The content of the invention
For defect present in prior art, it is an object of the invention to provide a kind of key optical link of EPON
Protection system and its method for obtaining balanced time delay, the present invention avoid carrying out the ranging of interrupting service service so that passive optical network
The interrupting time reduction of network system of users business service, has fully ensured the data safety of user.
To achieve the above objectives, the present invention is adopted the technical scheme that：The present invention provides a kind of EPON key light
Link protection system, the system includes primary optical line terminal OLT-A, standby optical line terminal OLT-B and multiple optical-fiber network lists
First ONU, each ONU passes through 2：N optical branching devices are connected with OLT-A, OLT-B respectively；OLT-A sets primary optical link most
Big fiber distance difference Lmax-a, in this Lmax-aThe balanced time delay T of the zero distance of lower useeqd-aAnd upstream physical layer expense PLOu hairs
Deliver to OLT-B；The maximum fiber range difference L that OLT-B is set according to standby optical linkmax-bAnd Lmax-a, set detection window big
Small Wsize；OLT-A is by the delivery time T of downlink framesendUplink burst corresponding with any one optical network unit ONU i of mandate
Skew StartTime is sent to OLT-B；OLT-B is in Teqd-a+ StartTime+ONU response times RspTimeiAt the time of open
OLT-B timer, when a length of W of the timersize；In the duration of OLT-B timer, OLT-B is by detecting PLOu
To position ONUi uplink burst：If OLT-B detects PLOu, T at the time of OLT-B records detect PLOurcv, and calculate
Go out ranging time Δs of the ONUi on standby optical linkRNG-B’；OLT-B obtains loop time delays of the ONUi on primary optical link
RTDi-AWith ranging time Δs of the ONUi on primary optical linkRNG-A, and calculate loop time delays of the ONUi on standby optical link
RTDi-B；OLT-B is according to RTDi-BAnd Teqd-a, calculate balanced time delay EqD-Bs of the ONUi on standby optical link.
On the basis of above-mentioned technical proposal, in the duration of OLT-B timer, OLT-B is determined by detecting PLOu
During position ONUi uplink burst, if not detecting PLOu, OLT-A is by the delivery time T of downlink framesendAnd mandate
Any one optical network unit ONU i correspondence uplink burst skew StartTime send to OLT-B；OLT-B is in Teqd-a+
StartTime+ONU response times RspTimeiAt the time of open OLT-B timer, when a length of W of the timersize；
In the duration of OLT-B timer, OLT-B continues to determine whether to detect PLOu, untill OLT-B detects PLOu.
The present invention also provides a kind of EPON method that key optical link protection system obtains balanced time delay, this method
Comprise the following steps：The maximum fiber range difference L that step S1.OLT-A sets primary optical linkmax-a, in this Lmax-aIt is lower to use
The balanced time delay T of zero distanceeqd-aAnd upstream physical layer expense PLOu is sent to OLT-B；Step S2.OLT-B is according to standby light
The maximum fiber range difference L that link is setmax-bAnd Lmax-a, detection window size W is setsize；Step S3.OLT-A will be descending
The delivery time T of framesendThe skew StartTime of uplink burst corresponding with any one optical network unit ONU i of mandate is sent
To OLT-B；OLT-B is in Teqd-a+ StartTime+ONU response times RspTimeiAt the time of opening timing device, the timer
Shi Changwei Wsize；Step S4. is in the duration of OLT-B timer, and OLT-B positions the up prominent of ONUi by detecting PLOu
Hair：If OLT-B detects PLOu, T at the time of OLT-B records detect PLOurcv, and ONUi is calculated in standby optical link
On ranging time ΔRNG-B’；Step S5.OLT-B obtains loop time delay RTDs of the ONUi on primary optical linki-AExist with ONUi
Ranging time Δ on primary optical linkRNG-A, and calculate loop time delay RTDs of the ONUi on standby optical linki-B；Step
S6.OLT-B is according to RTDi-BAnd Teqd-a, calculate balanced time delay EqD-Bs of the ONUi on standby optical link.
On the basis of above-mentioned technical proposal, in step S4, in the duration of OLT-B timer, OLT-B passes through detection
During uplink bursts of the PLOu to position ONUi, if not detecting PLOu, OLT-A is by the delivery time of downlink frame
TsendThe skew StartTime of uplink burst corresponding with any one optical network unit ONU i of mandate is sent to OLT-B；OLT-
B is in Teqd-a+ StartTime+ONU response times RspTimeiAt the time of open OLT-B timer, the timer when it is a length of
Wsize；In the duration of OLT-B timer, OLT-B continues to determine whether to detect PLOu, until OLT-B detects PLOu
Untill.
On the basis of above-mentioned technical proposal, in step S2, the WsizeSetting standard be：If Lmax-aAnd Lmax-bIt is
20KM, then Wsize=250 μ s；Otherwise Wsize=450 μ s.
On the basis of above-mentioned technical proposal, in step S3, the RspTimeiFor 35 μ s.
On the basis of above-mentioned technical proposal, in step S4, the ΔRNG-B’=Trcv-Tsend。
On the basis of above-mentioned technical proposal, in step S5, the RTDi-B=RTDi-A+(1+nds/nus)(ΔRNG-B’-
ΔRNG-A), wherein, ndsFor group velocity refractive index of the light in Optical Distribution Network ODN of down direction, nusFor the light of up direction
Group velocity refractive index in ODN.
On the basis of above-mentioned technical proposal, in gigabit passive optical network GPON and Ethernet passive optical network EPON systems
In system, the wavelength of the light of up direction is 1310 nanometers, and the wavelength of the light of down direction is 1490 nanometers；In 10G GPON and
In 10G EPON systems, the wavelength of the light of up direction is 1270 nanometers, and the wavelength of the light of down direction is 1577 nanometers.
On the basis of above-mentioned technical proposal, in step S6, the EqD-B=Teqd-a-RTDi-B。
The beneficial effects of the present invention are：
1st, the present invention makes full use of the network structure of the key optical link protection system of EPON so that when OLT is in
During standby mode, can without ONU ranging bandwidth authorizing, but the optical signal of the primary optical link of monitoring utilizes optical signal
In the transmission characteristic of optical link, by measurement of the optical signal to primary OLT in key optical link network, using the side of calculating
Method obtains balanced time delay EqDs of the ONU in standby optical link, so that when spare OLT switches to primary OLT, each ONU can
Directly to control the delivery time of optical signal using the balanced time delay EqD, and avoid carrying out the ranging of interrupting service service, make
The interrupting time reduction that passive optical network is serviced customer service is obtained, the data safety of user has fully been ensured.
2nd, the present invention is by calculating the balanced time delay of each ONU on standby optical link in real time, and real-time with optical link
Change, the real-time detection light Link State of spare OLT updates the balanced time delay of each ONU on standby optical link.Cut when occurring optical link
When changing, without interrupting service, each ONU can be directly set to maintain the state of normal work, so that by the business shadow of user
Sound is minimized.
Brief description of the drawings
Fig. 1 is the basic calculation schematic diagram of balanced time delay in the embodiment of the present invention；
Fig. 2 is the key optical link protection system architecture connection figure of EPON in the embodiment of the present invention；
Fig. 3 be the embodiment of the present invention in for the Time Calculation method schematic diagram on double-backbone optical link；
Fig. 4 is the stream of the method for the balanced time delay of the key optical link protection system acquisition of EPON in the embodiment of the present invention
Cheng Tu.
Embodiment
For ease of understanding the present invention, each formula is elaborated below.
EqDi=Teqd-RTDiFormula (1)
It is shown in Figure 1, in EPON, the balanced time delay of measurement, wherein T are calculated by formula (1)eqdBe zero away from
It is deviant of the uplink frame relative to corresponding downlink frame that OLT is received from balanced time delay.Optical link for giving fiber distance
In, TeqdIt is a fixed value, will not be changed in its life cycle.
RTDiIt is loop time delay, it includes descending propagation delay Δ olt, ONU response time RspTimei, upward propagation when
Prolong Δ onuiTotally three parts are constituted：
RTDi=Δ olt+RspTimei+ΔonuiFormula (2)
Wherein, ONU response times RspTimei35 μ s are fixed as, deviation is ± 1 μ s.
So, balanced time delays of the measurement ONU on optical link, when must just measure loops of the ONU on optical link
Prolong RTDi。
For OLT-A, exist：
RTDi-A=ΔRNG-A- StartTime formula (3)
Wherein StartTime is offset being the bandwidth authorizing specified by OLT-A.
The ranging time Δ on specific optical link detected for OLT-BRNG-B’For, there is light propagation delay and exist
The standby optical link upward propagation time delay Δ onu of OLT-Bi-BWith the primary optical link upward propagation time delay Δ onu of OLT-Ai-ADifference
There is following relation：
Δonui-B-Δonui-A=ΔRNG-B’-ΔRNG-AFormula (4)
The derivation of the formula is as follows：
ΔRNG-B’=Δ olt-A+RspTimei+Δonui-B+StartTime；
ΔRNG-A=Δ olt-A+RspTimei+Δonui-A+StartTime；
Subtract each other up and down and can obtain formula (4).
According to the propagation characteristic of light, in same section of optical fiber, descending propagation delay Δ olt and upward propagation time delay Δ onu
Between there is following relation：
Δolt:Δ onu=nds:nusFormula (5)
Wherein, ndsIt is group velocity refractive index of the light for the wavelength that down direction is used in ODN；nusIt is that up direction is adopted
Group velocity refractive index of the light of wavelength in ODN；
For specific passive optical network, according to G.983 standard, G.987 standard and 802.3ah, 802.3av are marked
Standard etc., current GPON (Gigabit-Capable Passive Optical Network, gigabit passive optical network) and EPON
The upstream wavelength that (Ethernet Passive Optical Network, Ethernet passive optical network) system is used is 1310 to receive
Rice, downstream wavelength is 1490 nanometers；The upstream wavelength that 10G GPON and 10G EPON system are used is 1270 nanometers, downstream wavelength
It is 1577 nanometers；So, there is following calculation formula：
In GPON, EPON system, descending propagation delay Δ olt and upward propagation time delay Δ onu ratios are:
Δolt:Δ onu=n1490:n1310
In 10G GPON, 10G EPON systems, descending propagation delay Δ olt and upward propagation time delay Δ onu ratios are:
Δolt:Δ onu=n1577:n1270
It can be obtained according to formula (2) and formula (5)：
RTDi-B=RTDi-A+(1+nds/nus)(Δonui-B-Δonui-A)
Namely：
RTDi-B=RTDi-A+(1+nds/nus)(ΔRNG-B’-ΔRNG-A) formula (6)
According to formula (1), pass through RTDi-BThe balanced time delay of the ONUi when OLT-B turns into primary optical link can be calculated.
The present invention is described in further detail below in conjunction with drawings and Examples.
Shown in Figure 2, the embodiment of the present invention provides a kind of EPON key optical link protection system, the system bag
Primary optical line terminal OLT-A, standby optical line terminal OLT-B and multiple optical network unit ONUs are included, each ONU passes through 2：N
Optical branching device is connected with OLT-A, OLT-B respectively；The maximum fiber range difference L that OLT-A sets primary optical linkmax-a, herein
Lmax-aThe balanced time delay T of the zero distance of lower useeqd-aAnd upstream physical layer expense PLOu is sent to OLT-B.OLT-B is according to standby
The maximum fiber range difference L set with optical linkmax-bAnd Lmax-a, detection window size W is setsize.OLT-A is by downlink frame
Delivery time TsendThe skew StartTime of uplink burst corresponding with any one optical network unit ONU i of mandate send to
OLT-B；OLT-B is in Teqd-a+ StartTime+ONU response times RspTimeiAt the time of open OLT-B timer, this timing
When a length of W of devicesize.In the duration of OLT-B timer, OLT-B positions ONUi uplink burst by detecting PLOu：
If OLT-B detects PLOu, T at the time of OLT-B records detect PLOurcv, and ONUi is calculated on standby optical link
Ranging time ΔRNG-B’；In the duration of OLT-B timer, OLT-B positions ONUi uplink burst by detecting PLOu
During, if not detecting PLOu, OLT-A is by the delivery time T of downlink framesendWith any one optical-fiber network of mandate
The skew StartTime of unit ONU i correspondence uplink bursts is sent to OLT-B；OLT-B is in Teqd-a+ StartTime+ONU is responded
Time RspTimeiAt the time of open OLT-B timer, when a length of W of the timersize；In the duration of OLT-B timer
Interior, OLT-B continues to determine whether to detect PLOu, untill OLT-B detects PLOu.OLT-B obtains ONUi in primary light
Loop time delay RTD on linki-AWith ranging time Δs of the ONUi on primary optical linkRNG-A, and ONUi is calculated in standby light
Loop time delay RTD on linki-B.OLT-B is according to RTDi-BAnd Teqd-a, calculate balanced time delays of the ONUi on standby optical link
EqD-B。
When being switched between primary optical link and standby optical link, for each ONU, its branch road optical link is that do not have
Change, so, each ONU up light is to stablize collisionless when reaching optical branching device, so, optical signal passes through key chain
Road reaches OLT-A along primary optical link respectively, and reaches OLT-B along standby optical link, is also steady during arrival OLT-A and OLT-B
It is fixed collisionless, so provide link basis to carry out ranging simultaneously on two optical links.
When OLT-A carries out ranging to ONUi, OLT-B also monitors up optical link ONU uplink burst (burst) simultaneously
Optical signal.In the protection of key optical link, the transmitter driving circuit of OLT-B optical module does not send light to down direction, still
OLT-B can open the optical signal that drive circuit receives up direction from ODN that receives of optical module, by monitoring up direction
Optical signal is so as to reach the specific loop time delay RTD (Round-Trip Delay) calculated in the network structure purpose.
It should be noted that the specific loop time delay that OLT-B is detected in the above-mentioned network structure is not ONU standby at this
With the loop time delay on optical link.It is shown in Figure 3, the optical link road that the specific loop time delay that OLT-B is monitored is actually passed through
Footpath includes：Key optical link (OLT-A main roads are descending) that OLT-A is connected, ONUi branch road optical link (ONUi branch roads are descending,
ONUi branch roads are up), the key optical links that are connected of OLT-B (OLT-B main roads are up).And OLT-B it needs to be determined that loop when
The optical link path prolonged should be：Key optical link (OLT-B main roads are descending) that OLT-B is connected, ONUi branch road optical link
The key optical link that (ONUi branch roads are descending, ONUi branch roads are up), OLT-B are connected (OLT-B main roads are up).
Shown in Figure 4, the embodiment of the present invention also provides the key optical link protection system of above-mentioned EPON and obtains equal
The method for the time delay that weighs, comprises the following steps：
The maximum fiber range difference L that step S1.OLT-A sets primary optical linkmax-a, in this Lmax-aThe zero of lower use
The balanced time delay T of distanceeqd-aAnd upstream physical layer expense PLOu is sent to OLT-B.
The maximum fiber range difference L that step S2.OLT-B is set according to standby optical linkmax-bAnd Lmax-a, detection is set
Window size Wsize；The WsizeSetting standard be：If Lmax-aAnd Lmax-bBe 20KM, then Wsize=250 μ s, otherwise Wsize
=450 μ s.The size of detection window is determined by unknown loop time delay and burst duration.To ensure that OLT-B can detect ONU's
Uplink burst, detection window size uses the maximum in the quiet window size that OLT-A and OLT-B are individually calculated.It is right
When fiber distance difference is all 20KM in OLT-A and OLT-B, loop transfer Delay Variation is 200 μ s, so detection window is used
250μs.For when at least one link of fiber distance difference is 40KM in OLT-A or OLT-B, loop transfer Delay Variation is
400 μ s, so detection window uses 450 μ s.
Step S3.OLT-A is by the delivery time T of downlink framesendAny one optical network unit ONU i with mandate is corresponding
The skew StartTime of uplink burst is sent to OLT-B；OLT-B is in Teqd-a+ StartTime+ONU response times RspTimei
At the time of opening timing device, when a length of W of the timersize；The RspTimeiFor 35 μ s.
Step S4. is in the duration of OLT-B timer, and OLT-B positions ONUi uplink burst by detecting PLOu,
If OLT-B detects PLOu, T at the time of OLT-B records detect PLOurcv, and ONUi is calculated on standby optical link
Ranging time ΔRNG-B’；Wherein, the ΔRNG-B’=Trcv-Tsend.In the duration of OLT-B timer, OLT-B passes through inspection
During surveying uplink bursts of the PLOu to position ONUi, if not detecting PLOu, OLT-A is by the delivery time of downlink frame
TsendThe skew StartTime of uplink burst corresponding with any one optical network unit ONU i of mandate is sent to OLT-B；OLT-
B is in Teqd-a+ StartTime+ONU response times RspTimeiAt the time of open OLT-B timer, the timer when it is a length of
Wsize；In the duration of OLT-B timer, OLT-B continues to determine whether to detect PLOu, until OLT-B detects PLOu
Untill.
Step S5.OLT-B obtains loop time delay RTDs of the ONUi on primary optical linki-AWith ONUi on primary optical link
Ranging time ΔRNG-A, and calculate loop time delay RTDs of the ONUi on standby optical linki-B；The RTDi-B=RTDi-A+(1
+nds/nus)(ΔRNG-B’-ΔRNG-A), wherein, ndsFor group velocity refractive index of the light in Optical Distribution Network ODN of down direction,
nusFor group velocity refractive index of the light in ODN of up direction.It is preferred that, gigabit passive optical network GPON and Ethernet without
In the optical-fiber network EPON system of source, the wavelength of the light of the up direction used is 1310 nanometer, and the wavelength of the light of down direction is
1490 nanometers；In 10G GPON and 10G EPON systems, the wavelength of the light of the up direction used is 1270 nanometer, descending side
To light wavelength be 1577 nanometers.
Step S6.OLT-B is according to RTDi-BAnd Teqd-a, calculate balanced time delay EqD-Bs of the ONUi on standby optical link.
Wherein, the EqD-B=Teqd-a-RTDi-B。
The present invention is not limited to the above-described embodiments, for those skilled in the art, is not departing from
On the premise of the principle of the invention, some improvements and modifications can also be made, these improvements and modifications are also considered as the protection of the present invention
Within the scope of.The content not being described in detail in this specification belongs to prior art known to professional and technical personnel in the field.
Claims (7)
1. a kind of key optical link protection system of EPON, the system includes primary optical line terminal OLT-A, standby light
Road terminal OLT-B and multiple optical network unit ONUs, each ONU pass through 2：N optical branching devices are connected with OLT-A, OLT-B respectively；
It is characterized in that：
The maximum fiber range difference L that OLT-A sets primary optical linkmax-a, in this Lmax-aThe balanced time delay of the zero distance of lower use
Teqd-aAnd upstream physical layer expense PLOu is sent to OLT-B；
The maximum fiber range difference L that OLT-B is set according to standby optical linkmax-bAnd Lmax-a, detection window size W is setsize；
OLT-A is by the delivery time T of downlink framesendUplink burst corresponding with any one optical network unit ONU i of mandate it is inclined
StartTime is moved to send to OLT-B；OLT-B is in Teqd-a+ StartTime+ONU response times RspTimeiAt the time of open
OLT-B timer, when a length of W of the timersize；
In the duration of OLT-B timer, OLT-B positions ONUi uplink burst by detecting PLOu：If OLT-B is detected
To PLOu, then T at the time of OLT-B records detect PLOurcv, and calculate ranging times of the ONUi on standby optical link
ΔRNG-B’, calculation formula is：ΔRNG-B’=Trcv-Tsend；
OLT-B obtains loop time delay RTDs of the ONUi on primary optical linki-AWith ranging times of the ONUi on primary optical link
ΔRNG-A, and calculate loop time delay RTDs of the ONUi on standby optical linki-B, calculation formula is：RTDi-B=RTDi-A+(1+
nds/nus)(ΔRNG-B’-ΔRNG-A), wherein, ndsFor group velocity refractive index of the light in Optical Distribution Network ODN of down direction,
nusFor group velocity refractive index of the light in ODN of up direction；
OLT-B is according to RTDi-BAnd Teqd-a, balanced time delay EqD-Bs of the ONUi on standby optical link is calculated, calculation formula is：
EqD-B=Teqd-a-RTDi-B。
2. the key optical link protection system of EPON as claimed in claim 1, it is characterised in that：In OLT-B timing
In the duration of device, during OLT-B positions ONUi uplink burst by detecting PLOu, if not detecting PLOu,
OLT-A is by the delivery time T of downlink framesendThe skew of uplink burst corresponding with any one optical network unit ONU i of mandate
StartTime is sent to OLT-B；OLT-B is in Teqd-a+ StartTime+ONU response times RspTimeiAt the time of open OLT-B
Timer, when a length of W of the timersize；In the duration of OLT-B timer, OLT-B continues to determine whether to detect
PLOu, untill OLT-B detects PLOu.
3. the method that the key optical link protection system of EPON described in claim 1 obtains balanced time delay, its feature exists
In this method comprises the following steps：
The maximum fiber range difference L that step S1.OLT-A sets primary optical linkmax-a, in this Lmax-aThe zero distance of lower use is equal
The time delay that weighs Teqd-aAnd upstream physical layer expense PLOu is sent to OLT-B；
The maximum fiber range difference L that step S2.OLT-B is set according to standby optical linkmax-bAnd Lmax-a, set detection window big
Small Wsize；
Step S3.OLT-A is by the delivery time T of downlink framesendIt is corresponding with any one optical network unit ONU i of mandate up prominent
The skew StartTime of hair is sent to OLT-B；OLT-B is in Teqd-a+ StartTime+ONU response times RspTimeiAt the time of
Opening timing device, when a length of W of the timersize；
Step S4. is in the duration of OLT-B timer, and OLT-B positions ONUi uplink burst by detecting PLOu：If
OLT-B detects PLOu, then T at the time of OLT-B records detect PLOurcv, and calculate surveys of the ONUi on standby optical link
Away from time ΔRNG-B’, calculation formula is：ΔRNG-B’=Trcv-Tsend；
Step S5.OLT-B obtains loop time delay RTDs of the ONUi on primary optical linki-AWith surveys of the ONUi on primary optical link
Away from time ΔRNG-A, and calculate loop time delay RTDs of the ONUi on standby optical linki-B, calculation formula is：RTDi-B=RTDi-A
+(1+nds/nus)(ΔRNG-B’-ΔRNG-A), wherein, ndsFor down direction light in Optical Distribution Network ODN group velocity refraction
Rate, nusFor group velocity refractive index of the light in ODN of up direction；
Step S6.OLT-B is according to RTDi-BAnd Teqd-a, balanced time delay EqD-Bs of the ONUi on standby optical link is calculated, is calculated
Formula is：EqD-B=Teqd-a-RTDi-B。
4. the method that the key optical link protection system of EPON as claimed in claim 3 obtains balanced time delay, its feature
It is：In step S4, in the duration of OLT-B timer, OLT-B positions ONUi uplink burst by detecting PLOu
During, if not detecting PLOu, OLT-A is by the delivery time T of downlink framesendWith any one optical-fiber network list of mandate
The skew StartTime of first ONUi correspondences uplink burst is sent to OLT-B；OLT-B is in Teqd-aWhen+StartTime+ONU is responded
Between RspTimeiAt the time of open OLT-B timer, when a length of W of the timersize；In the duration of OLT-B timer
Interior, OLT-B continues to determine whether to detect PLOu, untill OLT-B detects PLOu.
5. the method that the key optical link protection system of EPON as claimed in claim 3 obtains balanced time delay, its feature
It is, in step S2, the WsizeSetting standard be：If Lmax-aAnd Lmax-bBe 20KM, then Wsize=250 μ s；Otherwise
Wsize=450 μ s.
6. the method that the key optical link protection system of EPON as claimed in claim 3 obtains balanced time delay, its feature
It is, in step S3, the RspTimeiFor 35 μ s.
7. the method that the key optical link protection system of EPON as claimed in claim 3 obtains balanced time delay, its feature
It is：
In gigabit passive optical network GPON and Ethernet passive optical network EPON system, the wavelength of the light of up direction is
1310 nanometers, the wavelength of the light of down direction is 1490 nanometers；
In 10G GPON and 10G EPON systems, the wavelength of the light of up direction is 1270 nanometers, the ripple of the light of down direction
A length of 1577 nanometers.
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