What problems does PON use to solve?
With the emergence of high-bandwidth services such as video-on-demand, online games and IPTV, it is very urgent for users to increase access bandwidth. the existing broadband access mode based on ADSL is more and more difficult to meet the requirements of high bandwidth, two-way transmission capability and security. Because of its long transmission distance, strong anti-interference ability and large capacity, optical fiber has long been widely used in the backbone network. in recent years, with the reduction of the cost of optical devices, optical fiber has gradually become the first choice for the transmission medium of the access network. Passive Optical Network (PON,Passive Optical Network) is regarded as the ideal solution to solve the "last kilometer" problem because of its relatively low cost and smooth upgrade in the optical fiber access mode, so it is more and more favored by telecom operators.
The main standards of EPON
The standard development of 10G EPON began in 2006 and was formally promulgated in September 2009, and the main technical details have been determined. EPON--Ethernetover PON,IEEE802.3ah protocol standard, that is, transmitting messages in Ethernet format over PON network, can support 1.25Gbps symmetric rate. EPON is based on Ethernet technology, and the protocol is simple and efficient. Compared with APON,GPON, it has obvious advantages in cost.
Key technologies of EPON
EPON system adopts WDM technology to realize two-way transmission of single fiber.
The channel rate is uplink and downlink 1.25Gbps.
Broadcast EPON downlink transmission mode.
EPON uplink transmission mode-- TDMA mode.
Multipoint Control Protocol-MPCP.
Different from the Ethernet P2P structure, PON is the structure of P2MP. ONU has competition for uplink channel resources, so an arbitration mechanism is needed to avoid uplink data conflicts and allocate channel resources reasonably. 802.3ah protocol provides the corresponding control protocol-Multipoint Control Protocol (MPCP,Multi-pointMAC Control Protocol).LMPCP mainly defines the multipoint MAC control sublayer (Multi-pointMAC Control sublayer) to extend and replace the MAC control sublayer (MACControl sublayer) defined by the 802.3 protocol. The priority of the control frame of the MPCP protocol is higher than that of the MACClient data frame.
EPON uplink transmission adopts TDMA mode, and OLT determines the time when ONU sends data. Because each ONU is different from OLT, there will be delay difference. If there is no effective delay compensation mechanism, it will still cause uplink data transmission conflict.
EPON ranging and delay compensation are the key technologies of uplink channel multiplexing. During the DiscoveryProcessing process, OLT measures the distance of the newly registered ONU and calculates the RTT (RoundTrip Time) value of each ONU.
OLT uses RTT to adjust the authorization time for each ONU.
OLT can also start ranging when it receives a MPCPPDU.
Calculation of RTT:
The GATE frame contains a "timestamp" field, which ONU uses to refresh the local time register. OLT can calculate the RTT from the received REPORT frames for experimental compensation.