Bluetooth:CarryingVoiceoverACLLinks
RohitKapoor,Ling-JyhChen,Yeng-ZhongLee,MarioGerla
3803H,BoelterHall,UniversityofCalifornia,LosAngeles
Abstract – The Bluetooth technology will enable users to
connect a wide range of electronic devices such as laptops,
headsets,cellularphonesetc.Bluetoothdevicescanconnectto
formapiconet,whichconsistsofamasterandupto7slaves.
Themastercontrolsthemediumaccessinthepiconetusinga
polling scheme. Two types of connections can be established
in a piconet: the Synchronous Connection-Oriented (SCO)
link, andtheAsynchronousConnectionless(ACL) link.SCO
links provide a circuit-oriented service with constant
bandwidth based on a fixed and periodic allocation of slots.
Theyrequireapairofslotsonceeverytwo,fourorsixslots,
depending upon the SCO packet used. ACL connections, on
the other hand, provide a packet-oriented service and span
over1, 3 or 5slots. Themasterofthe piconetusesa polling
mechanism todivide the piconetbandwidth among the ACL
links.SinceSCOlinksrequireaperiodicallocationofapair
of slots, they leave very little of the piconet bandwidth
available to ACL links. Moreover, the controlled access of
BluetoothensuresthatnoACLlinkgetsstarved.Undersuch
an access mechanism, ACL links may be sufficient to carry
high-quality voice and SCO links may not be needed. Our
simulation and hardware experiments show that though the
voicequalityisaffectedslightlybyusingACLinsteadofSCO
linksforvoice,TCPconnectionsperformmuchbetterifSCO
links are not used. This paper, thus, makes a case for using
ACL in place of SCO links for carrying voice. This renders
SCOlinksredundant.
1. INTRODUCTION
Bluetooth [1] is a universal radio interface in the2.4
GHz ISM frequency band, which will enable users to
connect a wide range of small electronic devices such as
notebook computers, cellular phones and other portable
handheld deviceseasilyandquickly,withoutthe needfor
cables.ThekeydistinguishingfeaturesofBluetoothareits
minimal hardware dimensions, low complexity, low price
andlowpowerconsumption[2].
Bluetooth is based on a centralized connection-
oriented approach. Bluetooth devices sharing a wireless
channel form a piconet. One device in a piconet has the
roleofthemasterandcontrolsaccesstothechannel,while
the others are slaves. There may be up to 7 slaves in a
piconet. Bluetooth uses a Time-Division Duplex (TDD)
schemetodividethechannelinto625ustimeslots.Master
and slave units transmit alternately. Each piconet is
characterized by a particular fast frequency-hopping
pattern; the frequency is uniquely determined by the
master’s address and is followed by all the devices
participatinginthepiconet.
There are two types of connections that can be
establishedbetweenamasterandaslave:theSynchronous
Connection-Oriented (SCO) link, and the Asynchronous
Connectionless (ACL) link. SCO links provide a circuit-
orientedservicewithconstantbandwidthbasedonafixed
andperiodicallocationofslots.SCOlinksrequireapairof
slotsonceeverytwo,fourorsixslots,dependinguponthe
SCOpacketchosenforthelink.ACLconnectionsprovide
apacket-orientedserviceandspanover1,3or5slots.For
ACL links, Bluetooth uses a fast acknowledgment and
retransmission scheme to ensure reliable transfer of data.
ThemastercontrolsthetrafficonACLlinksbyemploying
a polling scheme to divide the piconet bandwidth among
the slaves. A slave is only allowed to transmit after the
masterhaspolledit.
SCO links have been designed to support time-
boundedinformationlikevoice.Sincetheselinksrequirea
periodic allocation of a pair ofslotsonce every2,4or6
slots, they leave very little of the piconet bandwidth
availabletoACLlinks.Consider asituationinwhichtwo
TCP connections and two voice connections coexist in a
piconet and the voice connections are carried over SCO
links.Itiseasytoseethateveninthebestcase(iftheSCO
links require a pair of slots every 6 slots), the TCP
connectionsmayeasilygetstarved.
Another point to note is that Bluetooth has a very
controlledchannelaccess.Eachnodeinapiconetisgiven
a chance to transmit by the master: the presence of a
pollingmechanismtodividethepiconetbandwidthamong
the slaves ensures that no ACL link gets starved. Under
suchanaccessmechanism,ACLlinksmaybesufficientto
carry high-quality voice (voice over IP) and SCO links,
maythusberenderedredundant.Thiswouldalsoallowthe
mastertosparemorebandwidthforotherTCPconnections
in the piconet. It is expected that the quality of voice, if
transmittedoverACLlinks,woulddegradeascomparedto
SCO links. It remains to be seen, though, how much the
qualityofvoicesuffers,howlargearethevoicedelays,are
thesedelaysacceptableetc.Moreover,onewouldalsolike
to see how much better TCP connections might be
supportedifvoicewascarriedoverACLlinks.
Inthis paper, wetrytoanswersomeofthequestions
regarding supporting voice over ACL links. We perform
experiments with Ericsson Bluetooth hardware kits
considering SCO and ACL links under various
configurations (varying the number of ACL and SCO
links, varying the packet types etc). Section 2 gives a
discussion of SCO and ACL links. In Section 3, we