802.16 MAC Layer Scheduling and its Effect on
TCPerfor!anceProfessor Thong NguyenUniversity of Technology Sydney
(UTS)AUSTRALIA1. 802.16 MAC Layer Ser"ice #ata $nit %S#$&' Data
units exchanged et!een ad"acent #rotocol layersProtocol #ata $nit
%P#$&' Data units exchanged et!een #eer entities$ e$g$
%A&PDU is the data unit exchanged et!een the %A& layers of
a 'S and its SSs$(igure 1.1' (i%A) Protocol ArchitectureSender
)ecei"erSDU&S SAP SDUPDU PDU%A& &o**on Part Sulayer P+,
SAP P+, LA,-RPDU PDUAirlin.Service/S#ecific &onvergence
Sulayers%A& SAP Privacy SulayerMACP*+Pac.et
Pac.et1.1Ser"ice,Secific%Protocol&
Con"ergenceSu-layers%CS&%ATMCS. Pac/etCS&The (i%A) %A&
#rotocol su##orts a variety of ac.haul re0uire*ents includingoth
cell/ased AT% and #ac.et/ased IP #rotocols (including
123$45-thernet)$ The%A& &onvergence Sulayers *a# the
trans#ort/layer/s#ecific traffic to a %A& that isflexile enough
to efficiently carry any traffic ty#e$Inthefollo!ing6 !edescrie
onlythePac.et &S$ This sulayer #erfor*s thefollo!ing functions
relying on the service of the %A& &PS7/ &lassification
of the higher/layer PDU(fro*'S host) into a##ro#riate(i%A)
connection6/ Su##ression of #ayload header infor*ation (o#tional)6/
Delivery of the resulting &S PDU to the %A& SAP associated
!ith the serviceflo! for trans#ort to peer %A& SAP at the other
end (e$g$ SS end)6 / Recei#t of &S PDU fro* the peer %A&
SAP (only)6/ Reuilding of any su##ressed #ayload header infor*ation
(o#tional)An IP Pac.et PDU arrives at the (i%A)8s %A& layer is
called a %A& SDU$Classification &lassificationis a #rocess
y!hicha %A&SDUis *a##ed ontoa #articularconnection for
trans*ission et!een %A& peers$ The *a##ing #rocess associates
a%A& SDU to a connection (!ith a &ID) and to the service
flo! characteristics (i$e$ alist of 9oS #ara*eters) of that
connection !hich #rovide the 9oS for the #ac.et$A classifierisaset
of*atchingcriteriaa##liedtoeach#ac.et entering(i%A)net!or.$ It
consists of so*e #rotocol/s#ecific #ac.et *atching criteria
(e$g$destination IP address)6 a #riority6 and a reference to a
&ID$If *atched6 the #ac.et isthendeliveredtotheSAPfor
deliverytotheconnectionidentifiedythe&ID$I*#le*entation of each
s#ecific classification ca#aility (e$g$ IPv:/asedclassification) is
o#tional$Several classifiers *ay refer to the sa*e service
flo!$Inthiscasetheclassifier #riorityisusedtoorder thea##licationof
classifierstoa#ac.et$Do!nlin. classifiers are a##lied y the 'S to
#ac.ets it is trans*itting6 andu#lin. classifiers are a##lied at
the SS$ Priority ofservice flo!&lassification
Para*eters(encoded using ty#e5length5value for*atting
sche*e)&ID andS;ID123$4 123$3TL>4TL>:3 and :bytes(igure
1.2' Classifier for!atThe 3/yte &ID *a#s each :/yte S;ID to a
connection$The TL> sche*e adds a tagto each trans*itted value
!hich contains the ty#e and length of the encoded#ara*eter$The
relevant #ara*eters for &S &lassifiers are7TL><
(-thernet)7 destination and source addresses6 -therty#e5SAPTL>3
(123$LAN IDTL>4(IP)7 ToS Range5%as.6 Protocol6 Source
address5*as.6 Destinationaddress5*as.TL>: (T&P5UDP)7
T&P5UDP source #ort start6 T&P5UDP source #ort end6
T&P5UDPdestination #ort start6 T&P5UDP destination #ort
end$(igure 1.0a' Classification and C1# !aing %2S to SS&(igure
1.0-' Classification and C1# !aing %SS to 2S&Packet Header
Suppression %P*S&Since(i%A)isaconnection/orientedservice6 *ost
ofroutinginfor*ationina#ac.et eco*es redundant (i$e$ re#etitive)
once the connection is set u#6 therefore thePac.et &onvergence
Sulayer also#rovides apacket header suppression(P+S)*echanis* in
order to avoid the trans*ission of redundant infor*ation in the
headersof the%A&SDUs$ Thesu##ressedinfor*ationisthenrestoredat
thereceiver$I*#le*entation of theP+Sca#ailityiso#tional$If P+S is
enaled atthe%A&connection6 each %A& SDU is #refixed !ith a
P+SI !hich references the Payload+eader Su##ression;ield(P+S;)$
Theclassifier uni0uely*a#s a#ac.et toitsassociated P+S
rule$Therefore !hen a classifier is deleted6 any associated P+S
ruleshall also e deleted$ The receiver uses the &ID and the
P+SI to restore the P+S;$The %A& SDU (i$e$ the Pac.et PDU) can
e #rocessed as is (i$e$ !ith header) if the1/it P+SI index ?26 or
it can e o#tionally added a P+SI @ 2 (i$e$ !ithout header)$ P+S has
a Payload +eader Su##ression >alid (P+S>) o#tion for use to
verify or notverifythe#ayloadheader eforesu##ressingit$
P+SalsohasaPayload+eaderSu##ression %as. (P+S%) o#tion to allo!
select ytes not to e su##ressed$ TheP+S% facilitates su##ression of
header fields that re*ain static !ithin a higher/layersession(e$g$
IP addresses)6 !hileenalingtrans*issionoffieldsthatchangefro*#ac.et
to#ac.et (e$g$ IP Total Length)$The 'S shall assign all &ID
values and all P+SI values$-ither sending or receivingentity shall
s#ecify the P+S; and the Payload +eader Su##ression SiAe
(P+SS)$This#rovision allo!s for #reconfigured headers or for higher
level #rotocols outside thesco#e of (i%A) standards to estalish
cache entries$ SS to BS oerationA #ac.et is su*itted to the #ac.et
&S$The SS a##lies its list of &lassifiers rules$A*ach of
the rule shall result in an U#lin. Service ;lo!6 &ID6 and a P+S
Rule$ TheP+S Rule #rovides P+S;6 P+SI6 P+S%6 P+SS6 and P+S>$ If
P+S> is set or not#resent6the SS shall co*#are the
ytesinthe#ac.et header!ith theytes in theP+S; that are to e
su##ressed as indicated y the P+S%$The SS shall then #refixthe PDU
!ith a P+SI to turn it into a %A& SDU to e #resented to the
%A& SAP fortrans#ort on the U#lin.$At the 'S6 !hen the %A&
PDU is received and the 'S %A& layer shall deter*inethe
associated &ID y exa*ining the generic %A& +eader$The 'S
%A& layer sendsthe PDUA& SAP associated !ith that &ID$
The 'S (receiver) #ac.et &S uses the&ID and the
PS+Itoloo.u# P+S;6P+S%6 andP+SS$ The'Sreasse*les the#ac.et$If
P+S> !as not enaled6 then there is no guarantee that the
restored P+S;ytes *atch the original header ytes$ A si*ilar
o#eration can e descried for 'S to SS do!nlin. o#eration$oIP !ith
silence$Sincethe siAe of arriving #ac.ets is not fixed in rtPS6 'S
has to #oll the connection of thisclass #eriodically (unicast
#olling) to as. ho! *uch and!idth is needed$Traffic 9oS
s#ecification #ara*eters are7 *ini*u* reserved traffic rate6
*axi*u*sustained traffic rate6 and *axi*u* delay5latency$$on
real"ti#e Polling Serice(nrtPS) 6 for traffic !ithout any s#ecific
delayre0uire*ent6 e$g$ ;TP traffic6 has higher #riority than the
lo!est #riority '- class$Itis the sa*e as '- class exce#t it *ay
have additional and!idth allocated throughnon #eriodic
#olling$nrtPS therefore6 is not li.ely to e NstarvedO$Traffic
s#ecification #ara*eters are7 *ini*u*reserved traffic rate6
*axi*u*sustained traffic rate6 and traffic #riority$Best
%ffort('-)classisusedtosu##ort traffic!ithno9oSsuchas+TTP
andT-LN-Ttraffic$ This class is allo!edtouse
onlycontention/asedand!idthre0uest$There is a fixed nu*er of *ini
slots in the UL sufra*e that all SSs have toshare to #lace their
re0uests (;igure :$3)$An increase in the nu*er of slots can onlye
*ade at a decrease in the and!idth availale for the trans*ission of
the actualdata$ There is #ossiilitythat '-traffic is starvedythe
lac.of and!idth$&ollisions of re0uests cancorru#t so*e slots
andcontentionac./off cancauseserious delay to so*e SSs$Traffic
s#ecification #ara*eters are7 *axi*u*sustained traffic rate6 and
traffic#riority$Ta-le 3.1' Characteristics of Ser"ice (lo5of
Classes of $Lser"ice andcorresonding ;oS ro"ided -y Mo-ile oice
!ith activitydetection (>oIP) %in reserved rate %ax sustained
rate %ax latency tolerance Pitter tolerance Traffic
#rioritynrtPSNon/real/ti*e PollingService(contention
re0uestsallo!ed6 #olled lessfre0uently);or nrtS;s !ith etter
thanest/effort service6 e$g$and!idth/intensive filetransfer %in
reserved rate %ax sustained rate Traffic #riority2E'est/-ffort
Service(contention re0uestsallo!ed6 9oS notguaranteed);or '-
traffic6 e$g$ generictraffic6 data transfer6 !ero!sing6 etc$ %ax
sustained rate Traffic #riority3.3 Poular Algorith!s for 802.16 MAC
Layer Schedulers)ound )o-in %))& 829As the na*e descries6 the
RR algorith* allocates the resource to the SSs in a cyclicorder in
a fair and deter*inistic ti*e *anner$Therefore it ignores the
current channelconditions$ Theasenceof
theschedulingada#tationtotheshort/ter*channelvariations goes
against the ada#tive *odulation and coding nature of (i%A)resulting
in lo! net!or. through#ut$Its advantage is its si*#licity$(air
;ueuing %(;& 819'and!idth and uffer s#ace are the t!o ovious
sharing resources$;air allocation ofuffer
s#aceissi*#leydro##ing#ac.ets inthelong0ueueif necessary$
;airallocation of and!idth is not si*#le$ ;I;G or ;&;S 0ueuing
disci#line is not fairecause it delegates all congestion control to
the traffic sources and therefore
favoursNaggressiveOsourceshavinglong#ac.etsandcontinuoussuchas;TP$PureRRsche*e
also favours long #ac.ets and continuous traffic such as ;TP !hich
is al!ays#resent at the0ueueandnever*issestheo##ortunity$
Pac.etsfro*inter*ittenttraffic such as e*ail is 0uite often asent
at the 0ueue6 thus *issing the RR #olling$Su##ose the 'S allocates
a total a*ount of resource Qtotal toSSs each re0uesting aa*ount Ri$
Under a certain allocation sche*e each SS is allocated an a*ount
Qi6 sothat==
ii total< $ Su##osethereisafairsharea*ount Qfairthat all
SSsshouldreceive$ The *ax/*in fairness criterion states that an
allocation is fair if7 (i)no user receives *ore than its re0uest6
i$e$ ) 6 *in(i fair i =6(ii)
nootherallocationsche*esatisfyingcondition(i)hasahigher*ini*u*allocation6
and(iii)&ondition (3) re*ains recursively true as !e re*ove the
*ini*al userand reduce the total resource accordingly ) (*in total
totalIn K
