Passive Optical Splitters in FTTH Network - Technopediasite


Thursday, June 28, 2018

Passive Optical Splitters in FTTH Network

Posted By: technopediasite

Image result for image of Optical splitter

Splitters are passive power dividers that allow communication between the OLT and their respective ONT who serve. However, not only are dedicated to multiplex or demultiplex signals, but also combine power: they are bidirectional optical distribution devices with one input and multiple outputs: 

• The signal which enters from input port (downlink), it proceeds from the OLT and it is divided among multiple output ports.

• The signals which enter from the exits (uplink), they come from ONT and they are combined at the entrance. 

The fact of being completely passive elements, it allows them to operate without extern power, lowering their cost of deployment, operation and maintenance. They just introduce optical power loss on communication signals, which are inherent in nature. 

There is an inverse mathematical relationship between the losses introduced by the divisor and the number of outputs of the same, being this:

Attenuation splitter= 10log 1/N

Therefore, a splitter with two outputs, in the worst case, it loses 3 dB (half power) at each exit. Graphically, it can be expressed the operation of a splitter with the following Figure:  

Image result for image of graphically  Generic operation of optical splitter

There are various types of splitters, as not all are built from same technology. However, the common divisors are two types:

• For devices with large number of outputs (> 32 outputs) splitters based on planar technology are used.

• For devices with fewer outputs (<32 outputs) splitters based on fused bi-conical couplers are used.

 Description of operation of Passive Optical Network 

Once detailed all the elements that build a PON, it is necessary to know how the global system works and the behavior of the network with all the interconnected elements, from the head OLT towards ONT users, and vice versa.  

The most important thing to note in the generic operation of the network is the existence of two channels, one ascending and one descending. However, both generally work through the same physical cable, so different wavelengths are assigned to each transmission channel and, depending on traffic, coexisting in the same fiber at least 3 different wavelengths: one for video flow in the upstream channel, and two for data flow of uplink and downlink respectively. 

 Downstream channel 

The downstream channel is the direction of information from the OLT operator to the ONT located on the end user. In this network, the PON behaves like a pointmultipoint network. 

The OLT includes plenty of added voice and data frames that go towards PON, through the P-OLT (voice and data) and the V-OLT (video). Frames collected by these teams are transformed to signals which inject in the different branches of the users. These branches are formed by one or two fibers that carry signals bi or unidirectional, and are passively coupled by optical splitters that allow the union of all the ONT in the network, without intermediate regeneration of signals (avoiding active elements). 

These dividers are responsible for receiving information from the OLT and send all the information to all their outputs. Once the information arrives to ONTs, each will only be able to process the traffic it belongs, or for which have the access by the operator, thanks to the AES (Advanced Encryption Standard) security techniques. In this procedure, TDM (time-division multiplexing) broadcast protocol is used, sending information to each user in different times.

The OLT has different pre-set time intervals, each one corresponded to a particular user. So, in function of each temporal segment, the ONT of each user filters the information addressed to him. 

An important aspect to consider is the wavelength (λ) which the OLT transmit information to ONT. These lengths vary according to if a tree branch or ONT connection has a single fiber or two fibers.   

Upstream channel

The upstream channel is the direction of information from the ONT end user to the OLT operator. In this network, the PON behaves like a point to point. 

Each ONT includes the added frames of voice and data (from each user) that are directed toward the OLT. At this point, the ONT performs the same operation as the OLT in the downstream channel, i.e., turn the frames into injecting signals through optical fiber that have been dedicated to the user.  

The splitters of each stage are in charge of collecting information from all corresponding ONTs and multiplex it in a single output fiber towards the operator OLT.

In order to transmit information from different ONT on the same channel, is necessary (as in the downstream channel) the use of TDMA, so that each ONT sends the information in different time intervals controlled by the OLT unit. 

As for the working wavelengths, it is noteworthy that regardless of whether the connection of the ONT to the splitter is two or single fiber, the wavelength of the upstream channel work is always the same. The information sent by the user (voice and/or data), always travel at λD=1310nm.

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