Op imizing mul icas se ices in Flexible-Ra e Passi e Op ical
Ne wo ks
Xiang Lu, Luis Velasco, and Ma c Ruiz
Op ical Communica ions G oup (GCO), Uni e si a Poli ècnica de Ca alunya (UPC), Ba celona, Spain;
*e-mail: [email protected]
ABSTRACT1
Flexible- a e passi e op ical ne wo ks (PON) allow op ical ne wo k uni s (ONU) o be g ouped acco ding o hei
channel condi ions, he eby enhancing he sys em h oughpu . In his pape , we show ha in he case o mul icas
se ices, scheduling downs eam wi h he peak- a e ONU g ouping scheme migh esul in da a edundancy, which
would deg ade e ec i e h oughpu . The oppo uni ies o enhance he e ec i e h oughpu by op imizing he ONU
g ouping scheme a e highligh ed.
Keywo ds: Flexible- a e passi e op ical ne wo k, Mul icas se ices, ONU g ouping
1. INTRODUCTION
The apid g ow h o high bandwid h ideo applica ions, such as li e s eaming and ul a-high-de ini ion ideo-
on-demand se ices, has posed signi ican challenges o he capaci y o op ical access ne wo ks [1,2]. To mee his
ising demand, lexible- a e passi e op ical ne wo ks (PON) ha e eme ged as a p omising app oach o inc easing
sys em capaci y [3].
In a lexible- a e PON, op ical ne wo k uni s (ONU) wi h simila channel condi ions can be g ouped o sha e he
same ansmission pa ame e s, e.g., modula ion o ma s and o wa d e o co ec ion (FEC) coding, enabling hem
o ope a e a a speci ic peak da a a e. These pa ame e s can be uned wi hin he powe budge o adap he da a
a e as needed [3-5]. In he downs eam di ec ion, since ONUs may employ di e en ansmission pa ame e s, he
adi ional b oadcas -based downs eam scheduling p o ocol in he ixed- a e PON becomes ine ec i e. To
add ess his, he downs eam PHY ame is di ided in o mul iple sub ames, each occupying mul iple speci ic ime
slo s [6]. The sub ame di e s in modula ion and coding pa ame e s, which a e assigned o a g oup o ONUs wi h
simila channel condi ions and da a a e equi emen s. Di e en om ixed- a e PONs, whe e ONUs need o
p ocess he whole downs eam ame, ONUs in lexible- a e PON only ecei e and p ocess hei own downs eam
sub ame o educe compu ing complexi y and ene gy consump ion [6]. While his app oach add esses he
scheduling issue caused by ONU g ouping, i also in oduces edundancy o mul icas se ices.
In his pape , we compa e he downs eam scheduling p o ocols used in ixed‑ a e and lexible‑ a e PONs o
mul icas se ices. Building on his, we e alua e e ec i e h oughpu unde a ious ONU g ouping schemes o
e eal how g ouping decisions in luence sys em pe o mance.
2. FLEXIBLE-RATE PASSIVE OPTICAL NETWORK
In his sec ion, we in oduce he concep o lexible- a e PON. Fig. 1 illus a es he high-le el a chi ec u e o a
lexible- a e PON, which consis s o an OLT and mul iple ONUs. The ONUs adop di e en ansmission
pa ame e s, e.g., modula ion o ma , acco ding o hei OPL, which is in luenced by ac o s such as physical ibe
dis ance and he numbe o op ical spli e s along he ODN. Fo example, ONU1 in Fig. 1, is connec ed o he OLT
ia a single op ical spli e , hus expe iencing a low OPL, which enables u ilizing he 4-le el pulse ampli ude
modula ion (PAM4) o ma o achie e highe da a a es. In con as , ONU2 and ONU3 a e connec ed ia wo
op ical spli e s, hence expe iencing a highe OPL. The e o e, hey employ he non- e u n- o-ze o (NRZ)
modula ion o ma o main ain eliable pe o mance unde mo e challenging channel condi ions. Based on hei
OPL, hese ONUs a e g ouped in o wo g oups: ONU1 belongs o G oup 1, while ONU2 and ONU3 belong o
G oup 2. To suppo his, digi al signal p ocessing (DSP) modules a e in oduced in bo h he OLT and he ONUs,
enabling modula ion o ma adap a ion du ing ansmission and ecep ion. The ope a ion o he DSP modules is
managed by he PON ansmission con e gence (TC) laye , which consis s o he se ice adap a ion sublaye ,
aming sublaye , and PHY adap a ion sublaye . These sublaye s handle aming, scheduling, and g oup
managemen , among o he s.
In he downs eam ansmission, he OLT di ides he ame in o mul iple codewo ds cus omized o di e en
ONU g oups. Fo ins ance, codewo ds modula ed wi h PAM4 a e designa ed o G oup 1, while hose modula ed
wi h NRZ a e di ec ed o G oup 2. Upon ecei ing he downs eam ame, each ONU iden i ies he codewo ds
co esponding o i s modula ion o ma and econs uc s i s assigned sub ame. Fo example, G oup 1 only
p ocesses he PAM4-modula ed codewo ds (yellow blocks), while G oup 2 p ocesses he NRZ-modula ed
1 The esea ch leading o hese esul s has ecei ed unding om he Eu opean Commission MSCA-DN NESTOR (G.A.101119983) and he
Ho izon Eu ope SEASON (G.A. 101096120); and om he ICREA Ins i u ion.
979-8-3315-9777-1/25/$31.00 ©2025 IEEE
2025 25 h Anni e sa y In e na ional Con e ence on T anspa en Op ical Ne wo ks (ICTON) | 979-8-3315-9777-1/25/$31.00 ©2025 IEEE | DOI: 10.1109/ICTON67126.2025.11125100
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codewo ds (g een blocks). This ensu es ha each ONU g oup handles only i s designa ed da a, he eby educing
DSP complexi y. The lexible- a e PON a chi ec u e de e mines a special downs eam scheduling scheme ha is
dis inc om ha o con en ional ixed- a e PONs. Downs eam scheduling p o ocol plays a c i ical ole in
implemen ing dynamic ONU g ouping, especially in mul icas scena ios.
Tx(
)
OLT: Op ical line e minal ONU: Op ical ne wo k uni Tx: T ansmi e Rx: Recei e
DSP: Digi al signal p ocessing TC: T ansmission con e gence
Rx(
)
Rx(
)
ComTC
F aming
sublaye
PHY adap a ion
sublaye
Se ice adap a ion
sublaye
Tx
4:1 Spli e
Rx
PON TC
F aming
sublaye
PHY adap a ion
sublaye
Se ice adap a ion
sublaye
Fil e
DSP
OLT
4:1 Spli e
ime
a e
NRZ NRZ
ime
a e
PAM4
PAM4
ime
a e
NRZ NRZ
PAM4
PAM4
Downs eam PHY ame
Rx
Tx
ONU
1
PON
TC
DSP
(PAM4)
ONU
2
ONU
3
Tx
Rx
PON
TC
DSP
(NRZ)
G oup 1
G oup 2
Fig. 1 High-le el a chi ec u e o lexible- a e PON.
3. DOWNSTREAM SCHEDULING PROTOCOL
In his sec ion, we compa e he downs eam scheduling p o ocols o ixed‑ a e and lexible‑ a e PONs, wi h a
ocus on mul icas scheduling. Fig. 2(a) illus a es he downs eam scheduling p o ocol based on he ITU-T 50G-
PON Common TC laye speci ica ion [7]. In he OLT, se ice da a uni s (SDU), such as E he ne ames, a e i s
classi ied and added o di e en queues based on hei se ice ypes (s ep 1). Then, in he se ice adap a ion
sublaye , SDUs a e encapsula ed in o XGEM ames using an XG-PON encapsula ion me hod and assigned a Po -
ID, which is included in he heade (s ep 2). The XGEM Po -ID iden i ies i s co esponding se ice ype and
b oadcas mode. The XGEM Po -ID o a unicas XGEM ame co esponds o a single ONU, whe eas he XGEM
Po -ID o a mul icas XGEM ame can be ecognized by mul iple ONUs. Fo illus a i e pu poses, Fig. 2(a)
shows wo mul icas (M1 and M2) and wo unicas (U1 and U2) a ic lows belonging o di e en se ices.
Mul icas XGEM ames a e ma ked wi h dashed-line blocks, while he unicas XGEM ames a e ep esen ed by
solid-line blocks. As a esul , all mul icas XGEM ames need o be ansmi ed only once, and he e o e, each
se ice equi es one single dedica ed ime slo .
In he aming sublaye , a bandwid h alloca ion module alloca es downs eam bandwid h o each a ic,
de e mining he XGEM ames ha need o be scheduled (s ep 3). As pe he bandwid h alloca ion in o ma ion,
he a ic schedule agg ega es hese XGEM ames and encapsula es hem in o an FS ame by adding an FS
heade (s ep 4). The FS heade con ains he ups eam bandwid h alloca ion in o ma ion o ONUs (i.e., he BWmap
ield). Since a mul icas XGEM ame can be ecognized by mul iple ONUs, each mul icas a ic (M1 and M2) is
alloca ed one single dedica ed ime slo in an FS ame. Subsequen ly, in he PHY adap a ion sublaye , his FS
ame is u he encapsula ed in o a 125-μs downs eam PHY ame a e FEC encoding, sc ambling, and bi
in e lea ing (s ep 5). A e adding a downs eam physical synch oniza ion block (PSBd), he downs eam PHY
ame is b oadcas o all ONUs in he ne wo k. Upon ecei ing he PHY ame, each ONU synch onizes wi h he
PSBd ield and ex ac s he FS ame (s ep 6). The FS ame is hen u he de-encapsula ed in o XGEM ames a
he aming sublaye , whe e he BWmap ield is ex ac ed om he FS heade and u ilized by he ONU o
ups eam scheduling (s ep 7). A a ic classi ie hen ca ego izes he da a ames and selec s i s co esponding
XGEM ames based on he XGEM Po -IDs, es o ing hem in o SDUs in he se ice adap a ion sublaye (s ep
8). No ably, in his p o ocol, e e y ONU mus ecei e and p ocess he en i e downs eam PHY ame.
In lexible- a e PONs, da a o di e en ONU g oups mus be independen ly encapsula ed due o a ying
ansmission pa ame e s among he g oups. To achie e his, a e p ocessing in he se ice adap a ion sublaye ,
XGEM ames belonging o he same ONU g oup a e agg ega ed and encapsula ed in o an independen FS ame.
Fo mul icas a ic, i all i s ONU membe s belong o he same g oup, he co esponding FS ame can di ec ly
ca y i s mul icas XGEM ames wi hou duplica ion. O he wise, he XGEM ames mus be duplica ed and
encapsula ed in o n FS ames, whe e n ep esen s he numbe o ONU g oups spanned by he mul icas se ice.
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(b) Flexible- a e PON OLT TC laye ONU TC laye (G oup 1: ONU 1)
SA sublaye PHY sublaye SA sublaye
FS sublaye PHY sublaye
XGEM ames
FS ame 1
FS ame 2
DS PHY ame
FS ame 2
(a) Fixed- a e PON OLT TC laye ONU TC laye
SA sublaye FS sublaye PHY sublaye SA sublaye
FS sublaye PHY sublaye
XGEM ames
FS ame DS PHY ame FS ame
U1
M2
U2
FS Heade
(BWmap)
H M1U1
M2U2
FS sublaye
DS bandwid h alloca ion
DS bandwid h alloca ion
T a ic
schedule
T a ic
schedule
DS PHY ame
T a ic
classi ie
T a ic
classi ie
H M1U1
M2U2
P
HM1
U
1
HM1
M
2
U
2
HH M1
U
1
U
2
PM1
M
2
HM1U1
PH M1U1
PSBd
HM1
U
1
M
2
U
2
PHM1
U
1
M
2
U
2
M1
U1
DS PHY ame 1
HU2
PM1M2
DS PHY ame 2
HU2
M1M2
T a ic
classi ie
M1
U2
FS ame 1
ONU TC laye (G oup 2: ONU 2 and 3)
M1
U1
M2
U2
M2
MT 2 (ONU
2&3
)
UT 2 (ONU
2
)
MT 1(ONU
1&2
)
UT 1 (ONU
1
)
Ups eam scheduling
OLT: Op ical line e minal ONU: Op ical ne wo k uni TC: T ansmission con e gence MT: Mul icas a ic
UT: Unicas a ic SA: Se ice adap a ion FS: F aming PHY: Physical in e ace adap a ion
DS: Downs eam BWmap: Bandwid h map PSBd: Physical synch oniza ion block downs eam
PLOAMd: Physical laye ope a ion, adminis a ion and main enance downs eam
1
3
2
3
5
46
7
8
5
4
67
8
M1
U1
M2
U2
MT 2 (ONU
2&3
)
UT 2 (ONU
2
)
MT 1(ONU
1&2
)
UT 1 (ONU
1
)
M1
1
2
Ups eam scheduling
Ups eam scheduling
Fig. 2 Compa ison o downs eam scheduling p o ocols: (a) ixed‑ a e PON and (b) lexible‑ a e PON.
This implies ha a mul icas se ice will be assigned a di e en ime slo in mul iple dis inc FS ames. Fig. 2(b)
illus a es he downs eam scheduling p o ocol o lexible- a e PONs ha inco po a es ONU g ouping [6]. Assume
ha ONU1 belongs o G oup 1, ONU2, and ONU3 belong o ano he g oup. The mul icas XGEM ame M1 o
mul icas a ic 1 is in ended o ONU1 and ONU2. The e o e, M1 is duplica ed and encapsula ed in o bo h FS
F ame 1 and FS F ame 2 (s ep 4). In con as , mul icas a ic 2 has ONU membe s ONU2 and ONU3 in he same
g oup, and hus M2 is only encapsula ed in o FS F ame 2. No ed ha he duplica ion o XGEM ames occu s only
du ing he encapsula ion in o FS ames. Addi ionally, each FS ame includes he ansmission pa ame e
in o ma ion wi hin he downs eam physical laye ope a ion, adminis a ion, and main enance (PLOAMd) ield
o he ONUs o he g oup. This in o ma ion is gene a ed by a bandwid h alloca ion module and copied o all FS
ame heade s by he a ic schedule (s ep 3). The FS ames om di e en g oups a e subsequen ly agg ega ed
in he PHY adap a ion sublaye and encapsula ed in o a comple e downs eam PHY ame (s ep 5). Du ing his
p ocess, a codewo d in e lea ing me hod be ween FS ames can be pe o med o educe DSP complexi y [6].
Upon ecei ing he downs eam PHY ame, he ONU econs uc s he FS ame o i s g oup (s ep 6).
Subsequen ly, he FS ame is de-encapsula ed a he aming sublaye (s ep 7). The ex ac ed XGEM ames a e
hen p ocessed a he se ice adap a ion sublaye (s ep 8), whe e he a ic classi ie de e mines whe he he ames
a e in ended o he ecei ing ONU.
Compa ed wi h he ixed‑ a e PON, he lexible‑ a e p o ocol exhibi s a po en ial d awback. When a mul icas
se ice spans mul iple ONU g oups, he schedule eplica es he same mul icas XGEM ame in o e e y g oup’s
FS ame (s ep 4). This eplica ion p oduces edundan a ic in he downs eam PHY ame, which can deg ade
mul icas e iciency and indica es ha he bo leneck esides in he p esen ONU‑g ouping me hod.
4. EFFECTIVE THROUGHPUT EVALUATION
To in es iga e he po en ial impac o ONU g ouping on mul icas e iciency, we u he e alua e he downs eam
e ec i e h oughpu unde h ee scena ios: a) a ixed- a e PON wi hou ONU g ouping; b) a lexible- a e PON
wi h peak- a e ONU g ouping; and c) a lexible- a e PON wi h op imized ONU g ouping.
Fig. 3(a) illus a es an example o a downs eam PHY ame in a ixed- a e PON wi hou ONU g ouping. Fo
simplici y, he o e head is omi ed. The colo ed blocks ep esen he bandwid h alloca ed o di e en se ices,
which is de e mined by a ixed da a a e 0 and ame du a ion 0. Uji and Mji indica e he j h unicas and mul icas
se ice a ge ing ONU i, espec i ely. Suppose ha he e a e six ONUs and i e a ic lows ( om ou unicas
se ices and one mul icas se ice) ha equi e ansmission. Since bo h unicas and mul icas da a a e ansmi ed
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(b)
(a)
,
,
(c)
,
G1(ONU
1&2
)
G2(ONU
3&4
)
G3(ONU
5&6
)
G1(ONU
1&2
)
G2(ONU
3
)
G3(ONU
4&5&6
)
U3
Fig. 3 Examples o a downs eam PHY ame in (a) a ixed- a e PON, (b) a lexible- a e PON, and (c) a lexible- a e PON
wi h ONU g ouping op imiza ion.
only once, no edundancy is gene a ed. In his scena io, he downs eam e ec i e h oughpu equals he sum o
bandwid h alloca ed o each se ice.
Fig. 3(b) p esen s an example o a downs eam PHY ame in a lexible- a e PON wi h a ixed ONU g ouping
me hod. In his case, he sys em achie es i s maximum sys em capaci y. Six ONUs a e di ided in o h ee g oups:
G oup 1, G oup 2, and G oup 3. ONU1 and ONU2 in G oup 1 achie e he highes da a a e 1 due o a o able
channel condi ions, while ONU3 and ONU4 in G oup 2 ope a e a a lowe peak da a a e 2. The ONU5 and ONU6
in G oup 3 expe ience he poo es channel condi ions and ope a e a he lowes peak da a a e 3= 0. Assuming he
FS ame o each g oup is alloca ed an equal ime slo du a ion (i.e., 1= 2= 3), he lexible- a e PON can ansmi
he same da a in less ime (indica ed by he ed dashed blocks), he eby inc easing he maximum sys em h oughpu .
Howe e , he mul icas da a M1 di ec ed o ONUs in h ee di e en g oups (i.e., ONU1, ONU4, and ONU6) mus
be ansmi ed sepa a ely o each g oup, occupying up o h ee ime slo s (M11, M14, and M16), which in oduces
unnecessa y edundancy and signi ican ly educes he e ec i e h oughpu .
Fig. 3(c) shows an example a e op imizing he g ouping by mo ing ONU4 om G oup 2 o G oup 3 (i.e.,
lowe ing i s da a a e o 3), so he edundan ansmission M14 can be elimina ed. Compa ed o he solu ion in Fig.
3(b), his adjus men u he imp o es he e ec i e h oughpu . Howe e , mo ing ONU1 o G oup 3 o u he
educe edundancy is no ad isable, as lowe ing i s da a a e would inc ease he ime slo equi ed o he unicas
se ice U11, hus dec easing he o e all e ec i e h oughpu . The e o e, op imizing ONU g ouping in ol es a
ade-o be ween minimizing edundancy and main aining high ONU da a a es.
5. CONCLUSION
To op imize mul icas se ices in lexible- a e PONs, we compa ed he downs eam scheduling p o ocols o
ixed- a e and lexible- a e PONs and ound ha he peak- a e ONU g ouping scheme in lexible- a e PONs causes
no able mul icas ine iciency. Using illus a i e examples, we compa ed he e ec i e h oughpu o di e en
ONU g ouping schemes and highligh ed he main ac o s ha in luence e ec i e h oughpu . These indings lay a
ounda ion o u u e wo k on downs eam ONU g ouping and bandwid h alloca ion schemes.
REFERENCES
[1] H. Li e al., "B oadcas ing and 6G con e ged ne wo k a chi ec u e," IEEE T ans. B oadcas 70, 971-979
(2024), doi: 10.1109/TBC.2024.3407482.
[2] "Pe o mance me ics o end- o-end IPTV ideo quali y," ITU-T Technical Pape GSTP-IPTV-QoS (2020).
[3] R. Bo kowski e al., "FLCS-PON – a 100 Gbi /s lexible passi e op ical ne wo k: concep s and ield ial,"
J. Ligh wa e Technol. 39, 5314-5324 (2021), doi: 10.1109/JLT.2021.3102383.
[4] V. Hou sma e al., "In es iga ion o modula ion schemes o lexible line- a e high-speed TDM-PON," J.
Ligh wa e Technol. 38, 3261-3267 (2020), doi: 10.1109/JLT.2020.2976959.
[5] J. Zhou e al., "100G ine-g anula i y lexible- a e passi e op ical ne wo ks based on disc e e mul i- one wi h
PAPR op imiza ion," J. Op . Commun. Ne w. 14, 944-950 (2022), doi: 10.1364/JOCN.463671.
[6] W. an Hoo e al., "G.hsp: educing ONU complexi y h ough FEC codewo d in e lea ing," in P oc. ITU-
TQ2/15, con ibu ion SG15-C1914, Gene a, 2020.
[7] "Highe speed passi e op ical ne wo ks – common ansmission con e gence laye speci ica ion," ITU-T
Recommenda ion G.9804.2 (2021).
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