As an effective communication method that is often used. EPON is used by users to connect to the access network.In this paper, the key technology of EPON is briefly described, and the application of EPON in optical communication is introduced in detail, and its technical principle is analyzed.
1.The introduction of EPON
PON is a contraction of Passive Optical Network, which is an optical access technology developed to support point-to-multipoint applications.PON consists of Optical Line Terminal (OLT), Optical Network Unit (ONU) and Optical Distribution Network (ODN).Its essential feature is that ODN is all composed of passive devices, and the signal is dispersed from a single Shared optical fiber to each individual user through a splitter.This system is called a Passive Optical Network because it is different from the traditional connection between the central office and the client, and the source electronic devices are between this access network.In addition to the advantages of saving fiber resources, PON can greatly simplify the operation and maintenance of network system,which is very effective in reducing construction and operation costs.Moreover, the structure of pure optical media and transparent optical fiber broadband network ensure the technical security of future business expansion.
EPON technology combines the Ethernet technology with PON technology to realize point-to-multipoint high-speed Ethernet fiber access in a simple way.The point-to-multipoint topology is the structural mode adopted by EPON, while the broadcast mode is used for downlink and TDMA mode is used for upline, which can realize two-way data transmission.
2.Composition of EPON
As a point-to-multipoint fiber access technology, Passive Optical Network (PON) consists of local Optical Line Terminal (OLT), user-side Optical Network Unit (ONU) and Optical Distribution Network (ODN).
2.1 OLT
Most of the time, OLT is placed in the central machine room. It provides optical fiber excuse for passive optical network in the downward direction, GE, 10baes-t, 100base-t, 10gbase-x and other interfaces in the upward direction, and OLT supports EI interface to realize TDM voice access.
2.2 ONU/ONT
ONU/ONT is placed in the user end, mainly using Ethernet protocol to realize transparent transfer of user data. Data can be forwarded between OLT and ONU.
2.3 ODN
As a passive fiber branch, ODN connects the passive equipment of OLT and ONU. The main function of ODN is to distribute downlink data and centralize uplink data.Because it is a passive operation, passive splitter deployment is very flexible and suitable for many environments.In common sense, each POS has a split rate of 8, 16, 32 or 64, and can be connected at multiple levels.
3.Introduction of key technologies of EPON
3.1Dbas for dynamic bandwidth allocation
Real-time (ms/us magnitude) changes the uplinking bandwidth mechanism of each OUN on EPON, known as dynamic bandwidth allocation algorithm.In EPON, if the bandwidth is allocated statically, then the transmission rate service for data communication is very inappropriate.If bandwidth is allocated statically at peak speed, the entire system bandwidth will be exhausted in a short time.W rate of bandwidth is not high, on the other hand, dynamic bandwidth allocation will improve the bandwidth utilization of the system.Sudden service requirements of ONU can be realized by DBA. Dynamic bandwidth adjustment between ONU can improve the efficiency of PON upline bandwidth.Due to the improvement of bandwidth utilization efficiency,more W users can be added on an existing PON, and the bandwidth peak value that W users can reach can be comparable to or even exceed the bandwidth of the traditional uniform allocation method.
Centralized control is a way of dynamic bandwidth allocation.This way is for all the ONU uplink messages, are applied to OLT for bandwidth, then OLT according to the request of the ONU authorization in accordance with the relevant algorithm for broadband accounted for W.The basic idea of the allocation criteria algorithm is that each ONU lee uplink can segment the time distribution of cell arrival and request bandwidth.According to the request of each ONU, OLT allocates bandwidth fairly and reasonably, and handles processing overload, information error code, cell loss, etc.
3.2 Reuse technology of uplink channel
At present, the main implementation is time division multiple access multiplexing (TDMA), which can be used at the same time slot time division multiplexing, statistical time division multiple access multiplexing, random access and so on.However, M – time – slot time – division multiplexing has some shortcomings.For example, when some of the time slots are not used, it occupies a certain bandwidth,so that the high burst rate service adaptability is not strong enough.The ONU needs synchronization and other random access methods without a certain access time.Therefore, statistical time division multiple access multiplexing is generally used after comparing the shortage of the two.When the uplink signal is transmitted, the Ethernet frame is sent in the time slot to which the ONU is allocated, and the size of the data provided by the statistical multiplexing is used to change the size of the time slot.
3.3 OLT’s ranging and delay compensation technology and ONU plug-and-play technology
Because the upstream channel of EPON USES TDMA, multi-point access makes the data frame delay of each ONU different, so the ranging and delay compensation technology is introduced to prevent the collision of data in time domain.In order to avoid the collision of time domain data, distance measurement and time delay compensation technology should be used to synchronize the whole network time gap.In this way, packets arrive at a defined time slot according to the DBA algorithm and support plug and play for ONU.Measuring the distance from each ONU to OLT accurately and adjusting the transmission delay of ONU precisely can reduce the interval between the sending Windows of ONU, improve the utilization of uplink channel and reduce the delay.The EPON ranging is initiated and completed at the same time that the OLT passes, marking the same time that the plug and play of the ONU is detected.
3.4Sending and receiving of burst signals
Since the burst signal of each ONU is received by OLT, OLT needs to realize phase synchronization for a period of time and then receive data.This requires the use of optical devices capable of supporting burst signals in ONU and OLT.Most optical devices cannot meet this requirement, and a small number of burst mode optical devices have a working speed of about 155M, which is relatively high in price.Therefore, in order to realize burst mode more effectively, special techniques are used for the receiving end. The optical burst transmission circuit needs to be able to close and open very quickly and establish signals quickly.Therefore, the traditional electro-optical conversion module using automatic power control with feedback is no longer suitable for use, but requires lasers with faster response.The receiving end receives the signal light power of each user is different and even more variable. Therefore, in the burst receiving circuit, the receiving level (threshold) needs to be adjusted every time a new signal is received.
4.Application of fiber optic communication in cell
The ONU can be set on the client side (FTTH) or on the corridor (FTTB), but this is in the case of access cells.In FTTH mode, the number of users is uncertain. In this case, in order to improve the utilization rate of equipment, reduce costs and facilitate maintenance.The setting of optical divider is relatively concentrated, and the use of a level of light distribution, the setting of the place of many things in the computer room of the community or the community inside the light handover box. After the construction in such a way, no matter the number of users increases or decreases, the use of equipment can be maximized. However, when the number of users is large, the need for access to optical fiber will also increase a lot.While in FTTB mode, OMU is set in the corridor, and the optical splitter is set in the same way as FTTH. This mode of access is generally carried out in the corridor switch.
Conclusion
EPON technology has many advantages, such as wide coverage of users, high speed of upstream and downstream, efficient optical transmission characteristics, saving fiber resources from point to multi-point networking and so on.For voice data, video multi-service bearing and carrier-level operation designated technical architecture, but also has passive, no electromagnetic radiation energy saving and environmental protection characteristics.As an optical communication technology, EPON technology is of great significance.As one of the mainstream technologies in the future, EPON technology has the characteristics of strong adaptability to the deployment environment, high reliability and maintenance-free, becoming the best choice for the construction of the next generation broadband access network.