How to translate EPON? What is the principle?

EPON is known as Ethernet Passive Optical Network, that is, Ethernet-based passive optical network. It is one of the many optical access technology, its characteristics are reflected in, one is the use of Ethernet frame structure, one is the use of one-to-many PON network topology

PON Passive Optical Network, Passive Optical Network abbreviation, a technology based on the P2MP topology, the so-called passive refers to the optical distribution network ODN does not contain any electronics and electronic power supply, ODN all by the optical splitter Splitter and other passive optical networks, the ODN all by the optical splitter Splitter. ODN is composed of passive devices such as optical splitter Splitter, and does not require expensive active electronic equipment.

Optical fiber

Optical fiber is an extremely important transmission medium. Fiber is resistant to electromagnetic interference and does not generate electromagnetic interference, allowing optical signals to be transmitted over very long distances with little or no distortion. The bandwidth of single-mode fiber can be as high as 50 THz

Optical splitter/combiner

PON uses a passive device that does not require any power to split optical signals from one fiber to many fibers; in the opposite direction, the optical signals from many fibers drink from one fiber. This device is an optocoupler. The simplest form of optocoupler is to have two optical fibers fused together.

PON topology

Point-to-multipoint (P2MP) network

Fast protection inversion

WDMA PON vs. TDMA PON

In the downstream direction, PON is a point-to-multipoint network, where the OLTs always have the entire downstream bandwidth, and in the upstream direction, pon is a point-to-point network, where multiple ONUs are sending data to a single OLT. one OLT to send data. The optical splitter/combiner ensures that signals sent by one ONU are not detected by other ONUs. However signals sent by different ONUs at the same time may collide. In this way, the uplink direction PON should be able to avoid data collision by using channel splitting mechanism. And there can be fair *** enjoyment of the backbone fiber and resources.

Burst Mode Transceiver

1. Light Transmission in Optical Fiber

Optical fiber is a kind of optical waveguide, and in layman's terms optical fiber is a very fine glass filament. The properties of glass are characterized by the refractive index n, which is the ratio of the speed of light in a vacuum to the speed of light in glass (n = Cvaccum/Cglass). An optical fiber consists of two layers of glass, the core on the inside and the cladding on the outside. These two layers of glass have different refractive indices, with the core having a higher refractive index than the cladding (Ncore>Nclad). Such an optical fiber is called a step fiber. Optical fibers can also be manufactured into a gradient fiber, gradient fiber core refractive index from the core to the cladding gradually decreases.

2. Single-mode and multimode fibers

While total internal reflection occurs whenever Θcore is greater than a critical angle, not all light greater than a critical angle propagates through an optical fiber due to phase cancellation interference between incident and reflected light. The specific angle of incidence in an optical fiber that supports propagation is called the "mode" of the fiber. There are two types of optical fibers, single-mode and multimode. Multimode fibers support the propagation of light in multiple modes (multiple angles), while single-mode fibers support only one mode called the fundamental mode. The core of a single-mode fiber is much smaller than the core of a multimode fiber. The number of modes m supported by a multimode fiber is related to the core d and the wavelength.

If the fiber diameter is known, the cutoff wavelength can be calculated. When the wavelength is greater than the cutoff wavelength, light is transmitted in the fiber single mode, that is to say, for such a wavelength signal fiber for single mode fiber. There are two working wavelength region of the fiber, respectively, 1270 ~ 1370nm wavelength region and 1430 ~ 1610nm wavelength region. The cutoff wavelength of the single-mode fiber is slightly less than the lower limit of the operating wavelength, around 1260nm.

3 . Mode dispersion

Multimode fiber was first commercialized. Its large core diameter facilitates coupling to low-cost large-emitting surface light sources and facilitates connector fabrication. However, signal transmission in multimode fibers is impaired by mode dispersion. The so-called mode dispersion is the pulse broadening due to the different transmission speeds of different modes. The refractive index difference between the core and cladding of a multimode fiber is larger than that of a single-mode fiber, with a typical value of 1.5%, and an effective refractive index of 1.48. The maximum transmission distance at each rate can be calculated. For example, a 1Gbit/S Ethernet link (actual line rate of 1.25gbit/s) will have a distance of no more than 5.5m, and a 1Gbit/s Ethernet link using gradient fiber will have a transmission distance of no more than 2.9km.

4. Chromaticity Dispersion

Another reason for the broadening of pulses is that the different spectral components of the signal propagate at different speeds. In other words, the refractive index of glass (silica) is frequency dependent. This dispersion is called material dispersion. Material dispersion depends on the spectral width of the signal.

The other part of chromatic dispersion is known as waveguide dispersion. Waveguide dispersion is caused by light propagating partly in the core and partly in the cladding. The structure of an optical fiber determines the proportion of light signal energy distributed between the core and the cladding. Since the refractive index of the core is greater than the refractive index of the cladding, the signal component propagating through the cladding reaches the receiving end faster than the signal component propagating through the core, which results in pulse broadening.

The optical power received at one input port is split between two output ports. The splitting ratio can be controlled by varying the length of the fusion zone, determined at the desired value.N:N couplers are constructed from multiple 2:2 couplers in cascade or are fabricated using flat plate waveguide technology.

The characteristic parameters of an optocoupler are as follows:

(1) Split Loss: Split loss (dB) is the output power minus the input power identified in dBm. For the ideal 2:2 coupler, the split loss is 3dB. 8:8 coupler consisting of multiple 2:2 couplers, if the 4-stage topology, only 1/16 of the input optical power is divided into each output, and "multi-stage interconnection network" structure is more effective, each output to get 1/8 of the input The "multilevel interconnection network" structure is more efficient, with each output receiving 1/8 of the input optical power.

(2) Additional Loss: Power loss due to the production process, generally in the range of 0.1~1dB.

(3) Directionality: Directionality (dB) is used to measure how much of the input optical power "cascades" into other input ports, which is specifically expressed in dBm. "String" into another input port optical power minus the input port input optical power. Generally, the coupler is symmetrical, and the directivity parameter is generally -40 to -50 dB.

Usually, the coupler has only one input port or only one output port. A coupler with only one input port is called a splitter, and a coupler with only one output port is called a combiner. Sometimes 2:2 couplers need to be made non-homogeneous, such as for splitting out a small portion of the power for monitoring, with a split ratio of 5:95 or 10:90, and such a coupler is called a tap coupler.

WDMA PON

One way to differentiate the uplink channel of an ONU is wavelength division muliple access. the individual ONUs of a WDMA pon operate on different wavelengths. In theory this is a simple solution, but in practice it is not feasible due to cost issues.The WDMA solution requires the OLT to receive multiple wavelength channels, either with tunable optical receivers or with an array of receivers.

A more serious problem is that WDMA PONs require the maintenance of multiple ONUs at different wavelengths, each of which has to use narrow-spectrum lasers, which raises costs. Also when replacing a faulty ONU with one of the wrong wavelength, it can interfere with other ONUs due to the wrong wavelength.ONUs with tunable lasers can solve the problem of multiple types of ONUs, but the cost is too high in the current situation.

WDMA PON has several other implementations:

Wavelength routed PON (WRPON), which uses waveguide array grating WDM instead of wavelength-independent optical splitter/combiners.

There is another approach, where the same wavelength is used for the upstream and downstream channels, and the ONU uses an external modulator to modulate the signal it receives from the OLT (as an upstream carrier signal). This approach is very costly, preventing optical amplifiers close to the ONU to compensate for signal attenuation, and requires more expensive optical devices to minimize reflections; also, for independent transmission from N ONUs, the OLT must have N receivers.

TDMA PON

TDMA pon in which multiple ONUs send signals may arrive at the optical collider and collide. To avoid collisions, each ONU must send data only in its own transmit window (time slot). one of the main advantages of TDMA PON is that all ONUs operate at the same wavelength, use the same devices, and the OLT requires only one receiver. the ONU transceivers must operate at line rate, even if the ONUs get bandwidth that is lower than the line rate bandwidth. However, TDMA PON can effectively change the bandwidth allocated to the ONU by varying the time slot length, using statistical multiplexing to fully utilize the PON channel capacity.

In an access network, the vast majority of traffic is typically downstream traffic from the network to the subscriber and upstream traffic from the subscriber to the network, rather than peer to peer (peer to peer). Therefore, it is reasonable to separate the upstream and downstream channels. A simple separation method is space division multiplexing, where the upstream and downstream signals are transmitted in two separate fibers. To save and reduce maintenance costs, one fiber is used for bidirectional transmission. In this case, two wavelengths are used. 1490 nm for downlink transmission and 1310 nm for uplink transmission. Each wavelength channel capacity can be flexibly allocated among ONUs through time slot **** enjoyment. Since only one uplink wavelength and one OLT optical transceiver are required, timeslot ***sharing is a low-cost method for optical channel ***sharing in access networks.

Due to the varying distances between the OLT and individual ONUs, the attenuation of the optical signals may be different for individual ONUs. the power level received by the OLT is different for each time slot. It is possible that due to longer distance, the signal level of one ONU is lower, and if the OLT receiver is adjusted to receive the high level signal of the closer ONU, then it may mistake the "1" of the farther ONU as "0", and on the contrary if the OLT receiver is adjusted to receive the weaker ONU, then the "1" of the farther ONU may be mistaken as "0". On the other hand, if the OLT receiver is adjusted to receive weaker signals, it may mistake the "0" of a stronger signal for a "1".

AGC

In order to receive the bit stream correctly, the OLT receiver must adjust the judgment level at the beginning of each burst time slot, a mechanism called automatic gain control (AGC). Receiving burst time slots with variations in power levels is known as burst mode reception.

The requirement for the dynamic range of the OLT receiver's AGC is relaxed by adjusting the ONU's transmit power to be approximately equal to the power level of each ONU time slot received by the OLT. This makes the ONU hardware more complex, requires associated control protocols between the OLT and the ONUs, and "degrades" all the ONUs to match the furthest ONU. So equipment vendors don't like this approach.

Due to the varying distances between the OLT and individual ONUs, the attenuation of the optical signal may be different for each ONU, and the power level received by the OLT is different for each time slot. It is possible that due to the longer distance, the signal level of one ONU is lower, and if the OLT receiver is adjusted to receive the high level signal of the closer ONU, then it may mistake the "1" of the farther ONU for a "0", and conversely, if the OLT receiver is adjusted to receive the weaker ONU, then the "1" of the farther ONU may be mistaken for a "0". On the other hand, if the OLT receiver is adjusted to receive weaker signals, it may mistake the "0" of a stronger signal for a "1".

AGC

In order to receive the bit stream correctly, the OLT receiver must adjust the judgment level at the beginning of each burst time slot, a mechanism called automatic gain control (AGC). Receiving burst time slots with variations in power levels is known as burst mode reception.

The requirement for the dynamic range of the OLT receiver's AGC is relaxed by adjusting the ONU's transmit power to be approximately equal to the power level of each ONU time slot received by the OLT. This makes the ONU hardware more complex, requires associated control protocols between the OLT and the ONUs, and "degrades" all the ONUs to match the furthest ONU. So equipment vendors don't like this approach.

Relatively low cost, simple maintenance, easy to expand, easy to upgrade the PON structure in the transmission of no power, no electronic components, so easy to lay, basically no maintenance, long-term operating costs and management costs of the savings of a large;

Passive optical network is a pure media network, completely avoid electromagnetic interference and lightning effects, very suitable for use in the natural conditions of the harsh areas.

The passive optical network is a purely dielectric network that completely avoids electromagnetic interference and lightning effects, making it extremely suitable for use in areas with harsh natural conditions.

The PON system occupies very few resources on the local office, the initial investment of the system is low, the expansion is easy, and the return on investment is high;