Hello, everyone!
Today, I want to continue to explain the XG(S)-PON technology. This is the second article about this topic. Now, I will explain the basic concepts of XG(S)-PON.
XG(S)-PON is defined with the next standards and protocols:
ITU-T G.987: This standard explains basic terms such as definitions, abbreviations and acronyms,
ITU-T G.987.1: It describes general XG(S)-PON requirements,
ITU-T G.987.2: This standard explains PMD (Physical Media Dependent) layer specification,
ITU-T G.987.3: In this standard, XG-PON TC Layer Specification is explained,
ITU-T G.988: It describes OMCI (ONU management and control interface) specification, and
ITU-T G.9807.1: This standard shows XGS-PON (10-Gigabit symmetric passive optical network).
XG(S)-PON system uses two wavelengths for communication between OLT and ONUs, 1577 nm for downstream and 1250 nm for upstream. The system uses one optical fiber with WDM (Wavelength Division Multiplexing). XG(S)-PON technology use broadcast mode for downstream and TDMA (Time Division Multiple Access) modes for upstream.
In broadcast mode, OLT sends all downstream frames to all ONUs. ONUs devices receive all downstream frames. ONUs have an XGEM (XGPON Encapsulation Mode) port ID filter, which extracts their frame. The length of the downstream frame is fixed and is 125 us. In the next figure, we can see broadcast mode.
Figure 1. Broadcast mode
In TDMA mode, each ONU on the PON port has its own time slot when broadcasting data to the OLT. Only in that time interval is it allowed to broadcast that ONU. These time slots are allocated by the OLT. This information is located in a special field within the downstream frame. This field is upstream bandwidth. If the ONU were broadcast outside the assigned time slot, there would be data collision on the optical splitters. There is a problem with the ONU that is constantly broadcasting or randomly broadcasting the data - rogue ONU. But I will write about it separately. Figure 2. shows TDMA mode.
Figure 2. TDMA mode
This is the end of this article. I will continue to write about this topic in the next article >> XG(S)-PON (3): Basic concept.
Thank you!
