Hello everyone!
In the previous article, I explained one of three types of OLAN – FTTO. Today, I will continue to talk about OLANs, now, I will explain the last type of OLAN – Passive optical LANs. Let's started.
Passive optical LAN uses FTTH protocols and architecture, it is different from standard LANs.
Passive optical LANs can use these PON standards:
We have an active part and a passive part of the system. In active segment, there are OLT (Optical Line Terminal) and ONT or ONU (Optical Network Terminal or Optical Network Unit). The passive segment consists optical cables, patch cords, optical patch panels, optical closures, etc.
In computer room or main equipment room, there is OLT. From it to the optical splitters, there are horizontal optical cables (e.g. riser optical cable). These optical splitters divide the optical signal (one PON port of the OLT) to allow up to 32 ONTs. From optical splitters to the ONTs, we use tight buffer optical cables – simplex. Then, on the ONT, we can use standard UTP cables cat 5 for connecting hosts. ONT, usually, has four gigabyte ethernet ports. But with ONT, the system can provide triple-play services – internet, VoIP and video (IPTV or standard CATV – AM CATV signal and/or digital CATV – DVB-S standard). Some types of ONTs have PoE and they can power local devices (video cameras, routers, VoIP phones, etc). We can see this type of OLAN in the next figure.
Figure 1. Passive Optical LAN
System use one optical fiber for bi-direction transmitting, wavelength 1490 nm is for downstream and wavelength 1310 nm for upstream. Passive optical LANs use singlemode (SM) fibers, OS1 or OS2 – low water peak fibers. These types of optical fibers use prepolished/splice optical connectors or factory terminated optical connectors. Typical optical connectors are SC/APC.
Light sources for this system are FB (Febry-Perot) and DFB (Distributed Feedback) lasers. ONTs use an inexpensive FB laser on 1310 nm. On the other side, OLT uses expensive DFB 1490 nm lasers.
The prices of passive optical elements are very low. Security is very big – all data are encrypted downstream and upstream. Passive optical LAN, when the system use GPON, offers 2.5 Gbps downstream and 1.25 Gbps in upstream. There are many advantages.
I will continue to explain this theme in the next article >> OLANs (5): Troubleshooting in OLANs
Thank you!
