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ments. By incorporating an effective DBA algorithm,
and therefore achieving a good network performance,
more subscribers could potentially join the network,
thus decreasing the network operations costs, and
even more standards could be reached on providing
cutting-edge applications to users.
The 10-gigabit-capable passive optical network
(XG-PON) is one of the most promising standards of
the next-generation PONs (NG-PONs). It comes with
several powerful assets such as enhanced cryptog-
raphy, compliance with older standards, higher data
rates for both directions, clear QoS-aware bandwidth
allocation processes, and energy-efficient support.
In this work, an adaptive, learning from experi-
ence, robust resource allocation scheme is proposed
in order to alleviate the impact of time-varying traf-
fic changes in the XG-PON systems. The so-called
hybrid reporting allocation (HYRA) utilizes a com-
bination of different, heterogeneous, yet allowed by
the standard, allocation policies in order to provide a
fully standard-compliant, efficient allocation method.
HYRA exploits the capabilities of traffic monitoring
technique so as to effectively re-distributes the sur-
plus bandwidth gained by isolating the underutilized
ONUs. Demanding users are favored and therefore
more bandwidth is allocated to active users without
overshadowing the network operation. The suggested
scheme seems to be capable of adapting to various
network changes offering thus notable improvements,
in terms of upstream packet delay, as indicated by sev-
eral simulation results based on real multimedia traf-
fic traces.
The remainder of the paper is organized as fol-
lows. Section 2 introduces several features of the un-
derlying allocation policies in order to providea better
understanding of the XG-PON sub-layers. In Section
3 existing research efforts towards resource allocation
in XG-PON are outlined. A detailed description of the
proposed scheme is provided in Section 4. Section 5
illustrates the obtained results, followed by detailed
reports. Finally, conclusions are given in Section 6.
2 BACKGROUND
The XG-PON framework defines a point-to-
multipoint optical access infrastructure providing
(nominal) 10 Gbps data rate in at least one direc-
tion. One of its most determinant layers is the
XG-PON transmission convergence (XGTC) layer,
in which the functional protocols and procedures
including the way of performing resource allocation
and provisioning QoS between the upper layers
and the physical layer, are thoroughly described.
According to the standard specifications, the XGTC
layer is structured in three sub-layers, namely the
service adaptation, the framing, and the physical
(PHY) sub-layer. The service adaptation sub-layer
performs service data unit (SDU) encapsulation and
multiplexing and creates XG-PON encapsulation
method (XGEM) frames. The framing sub-layer
receives the constructed XGEM frame and forms
the downstream XGTC frame. The downstream
frame encloses multiple XGTC payloads which are
distinguished based on their Alloc-ID. The Alloc-ID
field identifies the recipient of the allocation within
the ONU. Lastly, the PHY sub-layer applies bit
error correction algorithms, it performs scrambling
to the content, and it synchronizes the frames. It
is worth mentioning that the XGTC layer holds for
both upstream and downstream directions, hence
the aforementioned procedures reversely hold in the
upstream direction.
In the downstream direction the XGTC layer
is responsible of receiving SDUs from the upper
layers and producing an uninterrupted bitstream at
the nominal interface, which in the downstream di-
rection supports 9.95328 Gbps divided into 125 µsec
downstream frames. The duration of the downstream
frame, in accordance with the given downstream
rate, corresponds to 155520 Bytes. However, this
size includes coding and control information. The
physical synchronization block field comes first
in downstream flow (PSBd), which includes a
synchronization bitstream, the PON identification
number, counters, and other control information.
An important control field, known as BWmap,
which is associated with the bandwidth allocation
process, is enclosed in the XGTC header. It is used
by the OLT to inform the ONUs about the granted
transmission opportunities; it defines the start time
of the transmission opportunity and the grant size
per Alloc-ID for each ONU. In essence, the OLT
continuously broadcasts data to ONUs, including
requested data delivery, messages, and bandwidth
allocation information.
The XG-PON standard implicitly assumes syn-
chronization between downstream and upstream
frames. This means that the i-th downstream frame
is associated to the i-th upstream frame, even though
the i-th upstream frame could reach the OLT late
due to propagation time. Nonetheless, the allocation
information included in the i-th downstream frame
corresponds to the i-th upstream frame. To be syn-
chronized, both frames have the same length, thus the
duration of the upstream frame is 125 µsec. However,
it accounts for 38880 Bytes due to the fact that the
(nominal) upstream data rate is 2.48832 Gbps. The
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