With the CSMA/CD method, if other terminals are transmitting signals, a
terminal must wait for the transmission to complete. It can transmit its own data
immediately if other terminals are not transmitting signals. If the data unfortu-
nately collides with the signals from other terminals, the transmitter waits for a
random length of time and retransmits the data. The reason why random delay is
used is to minimize the probability of another collision. When random delay and
retransmission are repeated, the probability of continued collision becomes
minimized exponentially. This method requires no measurement of the RTD
and if the distance is within several hundred meters, it has very low delay and
high efficiency. However, if the distance reaches several tens of kilometers like in
fiber-access networks, it would take much time to detect if other terminals are
transmitting signals or if there has been a collision. Therefore, there will be a
tendency of frequent collision and retransmission, and adversely this will cause a
significant drop in efficiency and a drastic increase in delay.
Ranging is executed to eliminate the necessity for data retransmission, hence
using the bandwidth efficiently and keeping the maximum delay time to the
minimum by preventing the signals from multiple ONUs from colliding.
5.1.2 Ranging Procedures Overview
The distance between the OLT and ONU normally differs from ONU to
ONU. Unless each RTD is accurately determined, the transmission timing
cannot be established. With that in mind, if we newly connect an ONU, we
must first measure the RTD. By the command of operation system (OPS), the
RTD
2
/2
RTD
1
/2
Time
OLT input
ONU#1 output
ONU#2 output
Time
Time
data#1
data#1
data#1
data#1
data#2
data#2
data#2
data#2
Figure 5.1 Time division multiple access.
216 Ranging and Dynamic Bandwidth Allocation
OLT automatically and regularly makes available the ranging window for delay
measurement (as shown in Fig. 5.2) and specifies an ONU to transmit signals for
delay measurement. The length of the ranging window is set up according to the
distance between the OLT and ONU. If the ONU is located in the range of
5–10 km from the OLT, the length of the ranging window would need to be at
least (10–5 km) 2/(300,000 km/sec/1.5) ¼ 50 msec, according to the time it
takes for light to travel back and forth between the OLT and ONU. In this
expression, 1.5 is the refractive index of the optic fiber.
There are two ways of specifying the ONU for Ranging. One method specifies
only one already-registered ONU and the other method specifies all unregistered
ONUs. In the first method, an ONU with a unique ID number is specified in
advance from the OPS. In the second method, the OLT does not know the
unique ID number of each ONU. In this case, we must also take into account
that several ONUs may transmit signals for delay measurement simultaneously
and that the RTDs are close enough for the signals to overlap. One of the
methods to reduce collision during ranging is to transmit signals for delay
measurement with a random delay, similar to the method used in the Ethernet.
This method is applied to both G-PON [2] and EPON [3]. Even if a collision
unfortunately occurs in the first place, it is possible to execute a delay measure-
ment without collision by repeating the transmission twice or three times, etc.
Since user data transmission is not yet started before ranging is finished, the
latency is not increased. Only the time from connecting to the ONU to actually
starting communication is slightly prolonged. Of course, the random delay used
to avoid contention must not be included in the measured RTD.
OLT input
ONU#1 output
ONU#2 output
Time
Time
data#1
data#1
data#2
data#2
data#1
New ONU output
Time
Time
Ranging window
Signal for delay
measurement
RTD of new ONU
Figure 5.2 Ranging window.
Ranging 217

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