EPON architecture and key technologies
Based on the “draft EPON” which IEEE EFM (Ethernet in the First Mile Study) Working Group developed in July 2002, EPON structure and protocol layering shown in Figure following.
OLT, ODN and ONU
EPON is mainly composed of three parts:
- An optical line terminal (OLT),
- The Optical Distribution Network (ODN)
- And an optical network unit/optical network terminal (ONU / ONT).
As shown in the picture following, the OLT unit is located in the central office, and ONU / ONT is directly located in the user terminal side;
ODN network includes passive optical device (like 1:N PLC Splitters) and the optical fiber cables.
As shown in EPON/GPON protocol structure layering diagram: the main complex functions focus on OLT part, and ONU/ONT should be as simple and inexpensive as possible.
EFM defines the spectral capacity of a passive optical splitter between 116 and 1128. The direction from the OLT to the ONU is called the downstream/downlink direction, otherwise known as the upstream/uplink direction .
Uplink and downlink’s two different wavelengths occupies C-band optical signal (1550 nm window ), rates are 1 Gbit / s, transmission distance up to 20 km.
On the downlink, OLT send Ethernet data frame via broadcast way. Through 1:N passive optical splitter to ONU. The ONU will determine to receive the data or not by checking the received data frames,like the media access control(MAC) and frame type(eg. Broadcast/OAM etc).
On the uplink, ONU’s data frames, in burst-mode, transfer to OLT through common passive coax distribution network.So there must be a multiple access method that make sure each activated ONU occupy certain uplink bandwidth.
Key technologies: TDMA
Similar to the Multiple Access solution in satellite, so do Fiber PON network application:
Wavelength division multiple access (WDMA), time division multiple access (TDMA), code division multiple access (CDMA) and random access (CSMA/CD).
WDMA is the most ideal access solution: each ONU occupy a wavelength, OLT distinguishes ONU according to different wavelengths.
However, the very high cost of turnable optical transceiver, and the complex management of WDMA access cause EPON/GPON away from it currently.
Protocol Restrictions: Random access CSMA/CD protocol bandwidth utilization is too low,not support QoS.
Existing problem in multiple access CDMA: interference, optical orthogonal code spread spectrum bandwidth, which are not suitable for home users…
The target clients of EPON are home users and small businesses, taking into account the asymmetry of the business and low cost ONU, EFM Working Group decided to use TDMA manner in the upstream.
Next post will display more information of EPON’s key technologies.
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