7 — EPON ONT overview
7-10 November 2013 Alcatel-Lucent 7302 ISAM | 7330 ISAM FTTN | 7360 ISAM FX ONT R04.06.02
Edition 01 3FE 55873 AAAA TCZZA
ONT Product Information Guide
The OLT and the ONT have 32-bit counters that increment every 16 ns. These
counters provide a local time stamp. When either device transmits an MPCPDU, it
records its counter value into the timestamp field. To set the timestamp value, the
time of transmission of the first octet of the MPCPDU frame from the MAC control
to the MAC is used as the reference time.
When the ONT receives MPCPDUs, the ONT sets its counter according to the value
in the timestamp field in the received MPCPDU. When the OLT receives
MPCPDUs, the OLT uses the received timestamp value to calculate or verify a
Round Trip Time (RTT) between the OLT and the ONT. The RTT is equal to the
difference between the time value and the value in the timestamp field. The MAC
client then uses the calculated RTT for ranging.
7.6 EPON ONT discovery and activation
Discovery is the process by which offline or newly-connected ONTs are provided
access to the PON. The OLT drives the process. On a periodic basis, the OLT makes
available Discovery Time Windows. At these times, the offline ONTs can make
themselves known to the OLT.
The OLT broadcasts a discovery gate message to advertise a discovery period. The
message includes the starting time and length of the discovery window. After the
ONT receives this message, offline ONTs wait until the period begins to send a
Register Request message to the OLT.
Discovery windows are the only times when multiple ONTs can simultaneously
access the PON. Therefore, transmission overlaps may occur. To minimize
overlapping transmissions, online ONTs temporarily stop transmitting to the PON,
and offline ONTs wait a random period of time that is less than the length of the
discovery time window before sending their Register Request message.
After the OLT receives a valid Register Request message from an ONT, the
following events occur.
1 The OLT registers the ONT, assigns a port identifier (LLID) to the ONT, and
relates the MAC address of the ONT to the LLID.
2 The OLT sends the LLID and the required synchronization time to the ONT.
3 The OLT echoes the maximum number of pending grants.
4 The OLT schedules the ONT to the PON.
5 The OLT sends a standard gate message to the ONT.
6 The ONT acknowledges the registration by sending a Register ACK.
The discovery process is complete when the OLT receives the Register ACK from
the ONT. The ONT is registered and the OLT and the ONT can exchange normal
messages.
The OLT monitors report messages that are received from online ONTs for
transmission requests. In return, the OLT sends gate messages to the ONTs to report
their allocated DBA grants. To maintain the watchdog timer at the ONT, the OLT
periodically generates DBA grants when empty gate messages are sent.
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