Handover model

In ellular networks, handoveris the proess ofhanging the radio onnetion(frequeny, radio

basestation, et.) of anongoing all or datasession.

We distinguish between intra- and inter-system handovers. During intra-system handovers,

theterminal movesfromoneradio ellto anothertoobtain abetter radiolink,but stayswithin

the same radio aess system. Inter-system (or inter-tehnology) handovers happen when the

onnetionisbeingtransferredtoadierentaesstehnologyintheproess. Inatypialmobile

system, both intra- and inter-system handovers may happen mostly due to terminal mobility.

Todayintra-systemhandovers aremore ommon,but thetehnology advaneswill leadtomore

inter-systemtypehandovers(e.g., 3GPP-LTE/SAE tehnology [112℄[113℄,whihunies3Gand

non-3G aesses into asingle system). For example,a useronnets to a high-speed LANwhen

in the oe, hands over to a high-speed ellular system in his ar, and nally onnets to a

Wireless LAN aess at home. Today, it is typial that inter-system handovers happen in the

timesale of one handover per a ouple of minutes on average for moving users, and it an be

Table 4.1: Link parameters of radioaesstehnologies

Tehnology Bandwidth[Mb/s℄ RTT [ms℄

3G 0.06-0.4 100-200

3GPP-LTE/SAE 100 10

HSPA 14 20-100

WiMax 20-40 20-50

WLAN 11-54

< 1

During both types of handovers, link harateristis may hange suddenly, as pereived by

theuser and/orappliations.

In ase of inter-system handover, due to the hange in radio tehnologies and also the

in-teronneting terrestrial aessnetworks, the harateristis may beome signiantly dierent

from a TCP onnetionpoint of view. The apaity mayhange several magnitudes, aswell as

theRTT,lossand jitter(see Table4.1 forexamples).

Inaseofintra-systemhandover,theterminalstayswithinthesamesystem,onlythepointof

onnetion, i.e. the radio base station, hanges. Still, signiant BDP hanges may aompany

suheventsespeiallyinaseoftehnologiesthatimplementthesharedhannelonept(WLAN,

HSPA,3GPP-LTE/SAE, WiMax et.). BDPhanges aremostlyaused by population hanges

in a ell beause of mobility, and radio link quality hanges are due to interferene and fading

eets. One ould imagine extreme situations, like a user with an ative TCP session moving

fromanemptyellintoahighlyloadedone,or,ontheontrary,aheavyuserleavingaelland

thus leaving freeapaitybehind. Interferene andfadingmayalsoause abrupthanges, when

theterminal hanges modulation to ompensate for bad radio link. Dueto frequent handovers,

suhapaityhangesareexpetedto bealsofrequent. Also,beauseellsarebeomingsmaller

(miro- and pioells),thevariation ofloadis higher than inxed bakbone networks.

Athirdimpatofhandovers(both intra- andinter-system)isthepossibilityoflink

interrup-tionoroutage. Duringahandover,asequeneofproeduresareexeutedbeforetheterminalmay

hange to the new link,and inmanyases thetehnology allowsfor optimization and performs

a prepared handover(in WCDMA systems soft-handover even enables for seamless handovers).

Nevertheless,inmosttehnologies,evenifhandoverpreparationanbedone,some timeelapses

between the releaseof theoldlinkandthe establishment ofthenewone. Thistimedependson

manyfators; asan example,detailedalulationsfor 3GPPsystemsan be foundinthe3GPP

radio resouremanagement speiation[114℄.

There are also further aspets of handovers worth mentioning. When a handover happens,

some pakets may already be in ight on the wrong route, or waiting in the old queues.

Table4.2: Emulated systemparameters

Aess BW[Mb/s℄ RTT [ms℄

a

Buer[bytes℄

Type-1 10 250 327750

Type-2 100 120 1572840

a

inluding100ms Internet propagation delay

forwardedtothenewradiolink. Thislatterinvolvesmoreompliatedproedures obviously. In

aseofpaketdisard,TCPwillsenseburstyloss;inaseofpaketforwarding,someextradelay

may be experiened. Asa side-eet, we also notethat out-of-sequene paket trains mayalso

beobserveddepending onround-trip time dierenes.

In summary, handovers may generate bursty paket losses, link interruptions (also alled

outage) during whih an ongoing TCP session may time out, and suddenapaity, RTT, jitter

and loss hanges may also our at handovers. Finally, a handover an be a soure of data

mis-sequening beauseof datapaketstraversingdierent paths.

Inour workwe onsiderasimplehandovermodelthatreetstheommon designoftoday's

mobile networks as well as the available information on advanes from future systems (e.g.,

3GPP-LTE/SAE). We usea simple modelfor handovers inour measurementsthat allows us to

evaluate both theeet oflinkinterruption timesduringintra-and inter-systemhandovers,and

the eet of hanging BDP-s. The model is summarized as follows. The data path onsists of

the TCP sender, the emulation node, and the reeiver, with a single bottlenek queue at the

emulation node. At handovertime, paket serviing fromthe queueis stopped for some period

(theinterruption time)to aount for physial linkoutage. During thisperiodnotransmissions

are madebythe emulation node,but end hosts are not notied ofthe event. Inpartiular, the

TCP sender is ontinuing to ll up the bottlenek queue with more data pakets (as long as

it keeps reeiving upstream ACK-s already on their way from the emulation node). After the

interruption time haspassed,we resumepaketserviing, thus resumingtransmission alongthe

whole path. In the ase when link harateristis are dierent before and after the handover

typially at inter-system handovers all other link harateristis (delay, delay variation,

paketlossratio, buersizeset.) are alsohanged at this point.

Many ofthe handover relatedissues desribed inthis setion an beompensated or solved

within the system either using standardized or proprietary solutions (e.g., loss or reordering).

Note, thatwhile we also onsider inter-tehnology handovers (e.g., 3G to LTE), we restritour

In document B daeUiveiyfTe h (Pldal 100-103)