Handover and Call Control

Università degli studi di Roma ““Tor Vergata””Corso di Laurea Magistrale in Internet Engineering

Handover and Call ControlApplication

Presentation Application

Session

Transport Transport

Network Internet

Data Link Network Interface

Physical Layer Physical

Handover generalities• In the case of relative motion between satellites and users the

link geometry variation imposes the dynamic modification of active connections, i.e. handover.

• Different types of handover:– Handover between satellites– Handover between on-board spots– Handover between GESs– Handover between segments (chapter 17)

Internet via Satellite (AA2017/18) P8/2

21485Globalstar

10278Iridium

Minimum (s)Mean (s)

Time between handoversHandover between satellites

Handover Procedure Schemes

Backward Handover

Forward Handover

Signalling Diversity

Hard Handover

Soft Handover

Execution Phase

Decision Phase

HANDOVER STRATEGIES

Handover strategies

Controlling schemes

NCHO NAHO MCHO MAHO

Connection establishment

scheme

BWHO FWHO

Connection transference

scheme

SWHO CBHO

HDHO DVHO

SWHO: SWITCHED HOCBHO: COMBINED HO

SWHO: SOFT HOHDHO: HARD HODVHO: DIVERSITY HO

BWHO: BACKWARD HOFWHO: FORWARD HOMAHO: MOBILE ASSISTED HO

MCHO: MOBILE CONTROLLED HONAHO: NETWORK ASSISTED HONCHO: NETWORK CONTROLLED HO

Characteristics of handover controlling schemes

MCHO NCHO MAHO NAHOHandover process Centralized Centralized Decentralized DecentralizedTerminal complexity High Low Moderate HighSpeed of handover Fast Slow SlowSignaling load Low Low High HighReliability Moderate Moderate High High

Hard handover

Soft Handover- Switched diversity HO

Before handover

Tr ansition stage – preparation of target links

Soft Handover- combined diversity HO

Before handoverTransition stage – preparation of target links

After handover

Signalling Diversity

Before Handover

traffik link signaling link

transition stage – preparation of target links

traffic link signaling link

After Handover

traffic link signaling link

HO and constellationsGEO HEO MEO LEO

User Typology Mobile Fixed Mobile Fixed Mobile Fixed Mobile Fixed

Satellite HO Not critical - Not critical Not critical Critical Critical Very critical Critical

Spot HO Not critical - Not critical - Critical Critical Very critical Very critical

Gateway HO Not critical - - - Not critical Not critical Critical -

Low altitudes and/or high number of spots:

Moving and multiple spots ⇒ All types of handoverFixed or single spot ⇒ No spot handover

Handover and types of local area coverage

• increased probability of spot handover• reduced available time to end the procedure

Call Control and Handoff Control procedures for UMTS satellite access link

Scenario

ISDN PSTN IP LAN

NetworkGW Terminal

Characteristicsü LEO satellite constellation

ü CDMA access system

ü Satellite diversity

ü GW controlled system

Sat#1Sat#2

Forward Link

• DPDCH (Dedicated Physical Data Channel) carries the following logical channels:

– DTCH (Dedicated traffic Channel) supports user traffic.

– DCCH (Dedicated Control Channel) supports in band signalling for control procedures.

• CCPCH (Common Control Physical Channel) carries the following logical channel:

– FACH (Forward Access Channel) supports out of band signalling for control procedures.

• DPDCH (Dedicated Physical Data Channel) carries the following logicalchannels:

– DTCH (Dedicated traffic Channel) supports user traffic.

– DCCH (Dedicated Control Channel) supports in band signalling for control procedures.

• Il PRACH (Physical Random Access Channel) carries the following logicalchannel:

– RACH (Random Access Channel) supports out of band signalling for control procedures.

MT GWDPDCH

Return Link

Logical channels over dedicated physical channel

Slot #1 Slot #2 SSlloott ## ii Slot #Ns

PilotNpilot bits

TPCNtpc bits

RINri bits

DataNdata Bits

DDPPCCCCHH DDPPDDCCHH

TTff ==1100 mmss ((TTBBCC))

Frame #1 Frame #2 FFrraammee ## ii Frame #Nf

TTssuuppeerr == 772200 mmss

TTss== 00..662255mmss ((TTBBCC)),, 2200**22 bbiittss ((kk==00 ......66))kk

DPDCH = Dedicated Phisical Data Channel = DCCH + DTCHDCCH = Dedicated Control ChannelDTCH = Dedicated Traffic Channel

Call Control Procedures • Include all the functionality to set up, maintain and tear down the dedicated

physical channel necessary to transfer both user traffic and in band signalling(Dedicated Physical Data Channel)

Ø Registration Ø Handoff Control

• Involves MT and GW; initiated by MT• Allows GW to know of the presence of

MT in its coverage area• Such info is used by GW for paging

function in case of Call Setup initiatedby remote users (or by GW)

• Localization imposed by:Ø Terminal mobilityØ Satellite mobility (LEO, MEO)

• Control messages are exchanged usingRACH and FACH channels for out of band signalling (dedicated channelDCCH is not available)

• Both on MT and GW side• It can be initiated by both MT and GW• Allows, on demand, to set up a DTCH

for user traffic between MT and terrestrial network and viceversa

• If initiated by GW, it is based on pagingfunction

• The channels utilized to exchangecontrol messages are:Ø RACH and FACH for out of band

signalling,Ø DCCH, when DPDCH is available

• Call Setup procedure ends whenDPDCH is succesfully set up or not

Registration Call SetupØ Call Setup Ø Call Teardown

Handover

• Involves both MT and GW• Initiated by GW on the basis of

measurements performed by MT (MAHO) (design option)

• Due to– Terminal mobility– Satellite mobility (LEO)

• Diversity is supported• Soft handover• Control messages are

exchanged through DCCH• Handover procedure ends on

demand (Teardown) or in case of loss of coverage

• Both on MT and on GW side, can be initiated both by MT and by GW

• Can be initiated both explicitly by the terminal and by HO procedure

• DPDCH is released• Control messages are

exchanged through DCCH• Ends always successfully

Teardown

Inter-satellite and inter-beam handovers

Sat#2Sat#1

State transition diagram (MT)

REGISTRATION HANDOVER

MT_CALLSETUP MT_TEARDOWN

GW_TEARDOWNGW_CALLSETUP

end (positive outcome)

end (negative outcome)

Callteardown requestfrom remote system

callteardown request

from local user

Callsetup requestfrom remote system

Callsetup requestfrom local user

Sistem start

Mobile Terminal:

end (negative outcome)

end (positive outcome)

Interfaces - control plane

MT-CCU

MT-PIU

GW-CCU

GW-PIU

RACHDCCH

user interface

call control unit

physical layercontrol interface unit

Registration

Gateway:

Mobile Terminal:

MT_NFSE(MTCCU)

GW_NFSE(GWCCU)

MT.REG.REQ

GW.REG.ACK

MT.COV.REQMT.COV.RESP

MT.REG.REQ

GW.REG.ACK

ETH ETHHW-SAT.SIM

START START

WAIT REGIST

STAND-BY STAND-BY

Call Setup initiated by MT

Gateway:

Mobile Terminal:

MT.CODE.ACK

GW.CODE.SETFACH

MT.CODE.SET.REQMT.CODE.SET.RESP

MT.FL.ACKRACH

GW.CODE.SET.REQGW.CODE.SET.RESP

GW.FL.DUMMY

FL-DPDCH

MT.RL.DUMMY

RL-DPDCH

GW.RL.ACKFL-DPDCH

MT_NFSE

(MTCCU)

GW_NFSE(GWCCU)

MT_PIU GW_PIUETH ETHHW-SAT.SIM

MT.CALL.REQRACH

START STAND-BY

WAIT CALLPROC

Call Setup initiated by GW

Gateway:

Mobile Terminal:

STAND-BY CALL PROC SETUPWAIT

GW.CALL.REQ MT.CALL.ACK

GW.CODE.SETUP

TEAR DOWN

ERRORSYNC

MT.CODE.ACK

TIME-OUT

ERROR SYNC.ACK

START WAIT CALL ACK

WAIT CODE ACK

CODE DEF

GW.CALL.REQ MT.CALL.ACK

TIME-OUT

GW.CODE.SETUP

TEAR DOWN

TIME-OUTSYNC

MT.CODE.ACK

ERROR SYNC.ACK

MT.CODE.ACK

GW.CODE.SETFACH

MT.CODE.SET.REQMT.CODE.SET.RESP

MT.FL.ACKRACH

GW.CODE.SET.REQGW.CODE.SET.RESP

GW.FL.DUMMY

FL-DPDCH

MT.RL.DUMMY

RL-DPDCH

GW.RL.ACKFL-DPDCH

MT_NFSE

(MTCCU)

GW_NFSE(GWCCU)

MT_PIU GW_PIUETH ETHHW-SAT.SIM

MT.CALL.ACKRACH

GW.CALL.REQFACH

Handover

Gateway:

Mobile Terminal:

MT_NFSE(MTCCU)

GW_NFSE(GWCCU)

MT.COV.REFR

GW.COV.ACK

MT_PIU GW_PIU

RL-DPDCH

FL-DPDCH

HW-SAT.SIM.

ETHETH

MT.COV.REFR

GW.COV.ACKRL-DPDCH

FL-DPDCH

MT_NFSE(MTCCU) GW_NFSE

(GWCCU)MT_PIU GW_PIU

GW.CODE.ADD.REQ

GW.CODE.ADD.RESP

MT.CODE.ADD.REQMT.CODE.ADD.RESP

MT.ADD.ACK

GW.SYNC.REQ

MT.CODE.SYNC.REQMT.CODE.SYNC.RESP

RL-DPDCH

FL-DPDCH

HW-SAT.SIM.

ETHETH

MT.DROP.ACKRL-DPDCH

MT.CODE.DROP.REQMT.CODE.DROP.RESP

MT.COV.REFR

GW.HO.REQ

RL-DPDCH

FL-DPDCH

Handover and Call Control

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