EPON - Ethernet Passive Optical Network
IEEE
802.3ah
*** also called EFM (Ethernet in the First
Mile)
*** click Here and Here
for two excellent EPON primers
EPON is a very exciting
technology, because it finally allows Ethernet networks to be directly connected
via fiber, and it supports point-to-multipoint connectivity. The EPON
standard supports EFM technologies such as all of the FTTx standards . . . FTTC
(Fiber To The Curb), FTTP (FTT Premises) FTTB (FTT Building)
There are several primary components of a last-mile PON -
the OLT, the fiber and splitters, and the ONT:
-
OLT (Optical Line Terminal) - located at the CO, the
OLT interfaces with the metropolitan network. It must be high power because
it sends optical signals out, which are immediately broken into several
streams. The main functionality of the OLT is to adapt the incoming
traffic (Voice and Data) from the metropolitan rings into the PON transport
layer.
-
ONT (Optical Network Termination) and ONU (Optical
Network Unit) - ONT and ONU are basically the same device - however, the
ONT is located at the customer premise, and the ONU isd located outside the
home. These devices are the interface between the customer equipment
and the PON. They talk to the OLT via the PON.
Upstream/Downstream EPON Traffic
In an Ethernet PON the process of
transmitting data downstream from the OLT to multiple ONUs is
fundamentally different from transmitting data upstream from
multiple ONUs to the OLT. The different techniques used to
accomplish downstream and upstream transmission in an Ethernet PON
are illustrated in Figures 5 and 6.
Figure 5. Downstream Traffic
Flow in an Ethernet PON
In Figure 5 data is broadcast downstream from the
OLT to multiple ONUs in variable-length packets of up to 1,518 bytes
according to the IEEE 802.3 protocol. Each packet carriers a header
that uniquely identifies it as data intended for ONU-1, ONU-2 or
ONU-3. In addition some packets may be intended for all of the ONUs
(broadcast packets) or a particular group of ONUs (multicast
packets). At the splitter the traffic is divided into three separate
signals, each carrying all of the ONU-specific packets. When the
data reaches the ONU it accepts the packets that are intended for it
and discards the packets that are intended for other ONUs. For
example, in Figure 5 ONU-1 receives packets 1, 2, and 3, however it
only delivers packet 1 to end user 1.
Figure 6 shows how
upstream traffic is managed utilizing time division multiplexing, in
which transmission time slots are dedicated to the ONUs. The time
slots are synchronized so that upstream packets from the ONUs do not
interfere with each other once the data are coupled onto the common
fiber. For example, ONU-1 transmits packet 1 in the first time slot,
ONU-2 transmits packet 2 in a second non-overlapping time slot, and
ONU-3 transmits packet 3 in a third non-overlapping time
slot.
Figure 6. Upstream Traffic Flow
in an Ethernet PON
ONUs transmit data upstream to the OLT in
ONU-specific time slots using time division multiplexing to avoid
transmission collisions.
EPON Frame
Formats
Figure 7 depicts an example of downstream
traffic that is transmitted from the OLT to the ONUs in
variable-length packets. The downstream traffic is segmented into
fixed interval frames, which each carry multiple variable-length
packets. Clocking information, in the form of a synchronization
marker, is included at the beginning of each frame. The
synchronization marker is a 1-byte code that is transmitted every 2
ms in order to synchronize the ONUs with the OLT.
Figure 7. Downstream Frame
Format in an Ethernet PON
Each variable-length packet is addressed to a
specific ONU as indicated by the numbers, 1 through N. The packets
are formatted according to the IEEE 802.3 standard and are
transmitted downstream at 1 Gb/s. The expanded view of one
variable-length packet shows the header, the variable-length
payload, and the error detection field.
Figure 8 depicts an
example of upstream traffic that is time division multiplexed onto a
common optical fiber in order to avoid collisions between the
upstream traffic from each ONU. The upstream traffic is segmented
into frames and each frame is further segmented into ONU-specific
time-slots. The upstream frames are formed by a continuous
transmission interval of 2 ms. A frame header identifies the start
of each upstream frame.
Figure 8. Upstream Frame Format
in an Ethernet PON
The ONU-specific time-slots are transmission
intervals within each upstream frame that are dedicated to the
transmission of variable-length packets from specific ONUs. Each ONU
has a dedicated time-slot within each upstream frame. For example,
referring to Figure 8, each upstream frame is divided into N time
slots, with each time slot corresponding to its respective ONU, 1
through N.
The TDM controller for each ONU, in conjunction
with timing information from the OLT, controls the upstream
transmission timing of the variable-length packets within the
dedicated time-slots. Figure 8 shows an expended view of the ONU-specific time slot (dedicated to ONU-4) that includes two
variable-length packets and some time-slot overhead. The time-slot
overhead includes a guard band, timing indicators and signal power
indicators. When there is no traffic to transmit from the ONU a
time-slot may be filled with an idle signal.
EPON
Optical
System Design Options
Ethernet PONs can be
implemented using either a two-wavelength or three-wavelength
design. The two-wavelength design is suitable for delivering data,
voice, and IP switched digital video (IP-SDV). A three-wavelength
design is required to provide RF video services (CATV) or dense wave
division multiplexing (DWDM).
Figure 9 shows the optical
layout for a two-wavelength Ethernet PON. In this architecture, the
1510 nm wavelength carries data, video, and voice downstream, while
a 1310 nm wavelength is used to carry video on demand/channel change
requests upstream as well as data and voice. Using a 1.25 Gb/s
bi-directional PON the optical loss with this architecture gives the
PON a reach of 20 km over 32 splits.
Figure 9. Optical Design for
Two-Wavelength Ethernet PON
Figure 10 shows the optical layout for a three-wavelength
Ethernet PON. In this architecture, 1510 nm and 1310 nm wavelengths
are used in the downstream and the upstream directions respectively,
while the 1550 nm wavelength is reserved for downstream video. The
video is encoded as MPEG2 and is carried over Quadrature Amplitude
Modulated (QAM) carriers. Using this setup, the PON has an effective
range of 18 km over 32 splits.
Figure 10. Optical Design for
Three-Wavelength Ethernet PON
The three-wavelength design can also be used to provide a DWDM
overlay to an Ethernet PON. This solution uses a single fiber with
1510 nm downstream and 1310 nm upstream. The 1550 nm window (1530 nm
1565 nm) is left unused and the transceivers are designed to
allow DWDM channels to ride on top of the PON transparently. The PON
can then be deployed with no DWDM components, while allowing for
future DWDM upgrades to provide wavelength services, analog video,
increased bandwidth, etc. In this context Ethernet PONs offer an
economical set-up cost, which scales effectively to meet future
demand.