EPON ANALYSIS OF TECHNOLOGY BASED ON EPON OPTICAL COMMUNICATION

As an effective communication method that is often used, EPON is used by users to connect to the access network, this paper briefly describes the key technologies of EPON, and introduces the application of EPON in optical communication in detail, and analyzes its technical principles
.

1 Introduction to EPON

PON is the abbreviation of passive optical network, which is an optical access technology
that supports the development of point-to-multipoint applications.
Optical line terminal (OLT), optical network unit (ONU) and optical distribution network (ODN) three three constitute passive optical network (PON), its essential feature is that ODN is composed of passive components, and the signal is dispersed from a single shared optical fiber to each independent user
through the optical splitter.
Because this system is different from the traditional central office and client connection, and the source electronic device device is between this access network, it is called a passive optical network
.
In addition to saving fiber resources, PON can also greatly simplify the operation and maintenance of network systems, which is very effective
in reducing construction and operating costs.
Moreover, the structure of pure optical media and transparent optical fiber broadband network greatly guarantee
the technical security of future service expansion.

EPON technology combines Ethernet technology with PON technology to achieve point-to-multipoint high-speed Ethernet fiber access in a simple way
.
The topology of point-to-multipoint is the structure adopted by EPON, the downstream uses the broadcast mode, and the upstream uses the TDMA mode, which can realize two-way data transmission
.

2 Composition of EPON

As a point-to-multipoint optical fiber access technology, passive optical network (PON) consists of optical line terminal (OLT) on the central office, optical network unit (ONU) on the user side, and optical distribution network (ODN).

2.
1 OLT

Many times, OLT is placed in the central computer room, which provides optical fiber excuses for passive optical fiber networks in the downstream direction, GE, 10Baes-T, 100Base-T, 10GBase-X and other interfaces in the upstream direction, and OLT supports EI interface to achieve access
to TDM voice.

2.
2 ONU/ONT

ONU/ONT is placed on the user side, mainly using the Ethernet protocol to achieve transparent transmission
of user data.
Data forwarding
can be performed between OLT and ONU.

2.
3 ODN

ODN acts as a passive fiber splitter, connecting the passive equipment
of OLT and ONU.
The function of ODN is mainly to distribute downstream data and centralize upstream data
.
Because it is passive operation, passive splitter deployments are flexible and suitable for many environments
.
In common sense, the split rate of each POS is 8, 16, 32 or 64, and multi-level connections
can be made.

3 Introduction to EPON's key technologies

3.
1 DBA for dynamic bandwidth allocation

The mechanism of changing the upstream bandwidth of each OUN of EPON in real time (on the order of ms/us) is called the dynamic bandwidth allocation algorithm
.
If the bandwidth is statically allocated in EPON, then it is very inappropriate for the variable rate business of data communication, if the bandwidth is statically allocated according to the peak speed, then the entire system bandwidth will be exhausted in a short time, the bandwidth profit W rate is not high, on the other hand, dynamic bandwidth allocation will make the system bandwidth utilization increase a lot
.
The requirements of ONU burst services can be realized by DBA, and dynamic bandwidth adjustment between ONUs can improve the upstream bandwidth efficiency of PON
.
Because the efficiency of bandwidth usage is improved, more W users can be added to the implementation of an existing PON, and W users can reach a peak bandwidth that is comparable to or even exceeds the bandwidth of the traditional homogeneous allocation method
.

Centralized control is a method
of dynamic bandwidth allocation.
This way is that all ONU upstream information sending, all apply to OLT for bandwidth, and then OLT according to the ONU request according to the relevant algorithm to account for W authorization, the basic idea of the allocation criterion algorithm is that each ONU Lizhou upstream can divide the time slot to reflect the time distribution of the arrival of cells, and request bandwidth, OLT according to each ONU request, fair and reasonable allocation of bandwidth, while processing overload, information errors, cell loss and other situations are processed
.

3.
2 Multiplexing technology of upstream channel

At present, the main implementation method is the time division multiple access multiplexing (TDMA) mode, which can use some methods
such as time division multiple access multiplexing with the same time slot, statistical time division multiple access multiplexing, random access and so on.
However, the M-time slot time division multiple access multiplexing method has shortcomings
.
For example, when some time slots are not used, a certain amount of bandwidth is occupied, making the high burst rate service not strong enough adaptability.
In the ONU, random access methods such as synchronization are required, and the access time
is not determined.
Therefore, after comparing the shortcomings of the two, statistical time division multiple access multiplexing
is generally used.
When the upstream signal is transmitted, Ethernet frames are sent in the time slot allocated by the ONU, and the size of the time slot is changed by providing the amount of data through statistical multiplexing
.

3.
3 OLT ranging and delay compensation technology and ONU plug and play technology

Because the uplink channel of EPON adopts TDMA mode, multi-point access makes the data frame delay of each ONU different, so ranging and delay compensation technology should be introduced to prevent collisions in the data time domain
.
And because to avoid collisions in this data time domain, it is also necessary to use ranging and delay compensation technology to synchronize the time slots of the whole network, so that the packet arrives according to the determined time slot of the DBA algorithm and can support the plug and play of the ONU
.
Accurately measuring the distance from each ONU to the OLT and precisely adjusting the transmit delay of the ONU can reduce the interval between the ONU transmit windows, improve the utilization of the upstream channel, and reduce the delay
.
EPON ranging is initiated and completed
at the same time as the OLT pass time, marking the insertion and play of the detection ONU.

3.
4 Sending and receiving burst signals

Because the burst signal of each ONU is received by the OLT, the OLT synchronizes the phases over a period of time and then accepts the data
.
This requires the use of optical components
in ONUs and OLTs that can support burst signals.
Most optical devices can not meet this requirement, and a small number of burst mode optical devices work at a rate of around 155M, which is relatively expensive
.
Therefore, in order to better implement burst mode, special technologies
are used for the transceiver side.
The optical burst transmission circuit needs to be able to close and open very quickly and quickly establish the signal, so the traditional electro-optical conversion module uses the plus feedback automatic power control is no longer suitable for use, but needs a faster response laser
.
The optical power of the signal received by each user at the receiving end is different and even more variable, so in the burst receiving circuit, the receive level (threshold)
needs to be adjusted each time a new signal is received.

4 Application in cell optical fiber communication

The ONU can be set on the user side (FTTH) or corridor (FTTB), of course, this is when accessing the cell
.
In FTTH mode, the number of users is uncertain, at this time, in order to improve the utilization rate of the equipment, reduce costs and facilitate maintenance, the setting of the optical splitter is relatively centralized, and the use of a level of spectroscopy, the setting place is eventful in the computer room of the community or the optical handover box
in the community.
After using this method of construction, regardless of the increase or decrease in the number of users, the use of equipment can be maximized, but when the number of users is large, the demand for access to optical fiber will also increase a lot
.
In FTTB mode, the OMU is set in the corridor, and the optical splitter is set in the same
way as the FTTH method.
The access of this mode is generally carried out
in the corridor switch.

epilogue

EPON technology has many advantages, such as wide user coverage, high uplink and downlink speed, efficient optical transmission characteristics, and point-to-multipoint networking that saves fiber resources
.
For voice data, video multi-service bearer and carrier-class operation specified technical architecture, it also has the characteristics of passive, no electromagnetic radiation, energy saving and environmental protection, EPON technology as an optical communication technology, is of great significance
.
As one of the mainstream technologies in the future, EPON technology has strong adaptability to the deployment environment, high reliability and maintenance-free characteristics, and has become the best choice
for the construction of next-generation broadband access networks.