What is the exact input signals level for a WDM EDFA?
The main factor in control room while using the WDM technology is that the cable TV signal and internet signal from OLT is combined in WDM EDFA. Then it comes out through a single fiber and splited using the splitters and in customer end, nodes are used to separate the both signals. Internet signal is forwarded to ONU and cable TV signal is forwarded to set of box. This is what happens in a WDM network
As 1550nm is used for cable TV it need to reach every customer end as 1550nm. In WDM EDFA 1550nm is mixed with 1490nm, this 1490 also reaches customer end nodes will make a noise. The unwanted noise will interrupt the cable TV services and may be some channels will miss. So a filter node is essential which will filter 1490 and it only pass 1550. we need to use a filter node in WDM network if the customer no need the internet connection. So it is better to use filter node initially than a non-filter node its only make a 50 rs difference but will help us WDM technology later.
Later if a customer needs internet, we need to change it to WDM node where it gives a RF and Optical output. RF output will be connected to the setoff box and optical output to a ONU for internet. That means within 5 or 10 minutes we can deliver internet to the existing cable TV customers is the main attraction and the additional cost is only the cost of WDM node. So we can see the low cost and time saving is the main attraction of this WDM technology. Comparing with WDM technology a WDM node costs rs 500 but its more economical than designing a new network for internet which costs for fiber, splicing, patch codes, splitters etc.
You need to understand the devices like OLT, WDM EDFA, triple window splitters or PLC splitters , WDM node (customer end) which is necessary for a WDM technology.
Next is how to mix signals? Example in control room it is needed to mix signals in WDM EDFA from OLT . If we simply connect the OLT output to EDFA it won’t work well. The thing we need to keep in mind is how much signal we are planning to give to the customer end or need to work in customer end. Operator use different levels ,but in a professional way, for a signal working in 64com use -6 to -8 for cable TV and -22 to -25 for internet is a good practice. This method contains some reserve signal which will be applicable in some situation if the signal comes down.
Let’s take an example if -6 cable TV and -22 as internet signal their difference is 16 db. Ie the difference between cable and internet should be 16db.
In a fiber optic network almost all wavelength pass but different wavelengths have different losss and almost same coupler loss. So for small networks like 5 – 10km the signals like 1550 and 1490 will be almost same in transmission loss and splitting loss. So 16dbm difference can be maintained in the control room to customer end.
If the EDFA with 16db signal(1550), then the OLT signal to EDFA should be 0 dbm (1490 )ie the mixed output is with 16db difference. Even if some difference is there we have reserve signal but considering some practical matters like mixing insertion loss of some EDFA of some venders (we can find the loss by mixing the OLT after switch of the EDFA) we have to consider that also if there is a considerable loss. Most probably all EDFA’s are withl less insertion loss.
Lets take a 8 port OLT with 6dbm minimum output in every pon module with a c++ or px +++ module, to convert this 6 dbm to 0 dbm we need to use ¼ splitters. That is 8×4=32 outputs, means 32 0dmb outputs will be their. This 32 outputs is a good combination to 32 port 16dbm WDM EDFA. By thus we can get the 32 outputs with internet so if we maintain a -6 cable tv signal at customer end there will be -22 internet signal automatically.
In summary, first we need to take decision in the signal level to customer end. Then keep that difference in control room while mixing signals, is the ratio of WDM network signal. So understanding the concept will make your designing simple. The general suggestion is – 6 for cable and -22 for internet, by maximum keeping this give you many advantages.
Another thing even if a 32 port 16db EDFA with -6 cable signal can give a 2000 cable tv FTTH connections but in terms of internet, a 8 port GPON OLT can deliver 1024 connections . As said earlier an OLT is used to deliver 1000 connections, above 1000 connections we need split OLT output using ½ splitter in place of ¼ and need to place next 8 port OLT and mix in the remaining 16 port which will fulfill 2000 connections. So initially this will be the good method than using a 16 port OLT because technology is changing day by day.