1. Keywords— WiMAX, Handover Delay, DTS, MT speed .
Abstract— Throughout history of the communication and
broadband networks particularly a major challenge and the battle still
to speed up the development of quality of services (QoS) and to pay
more attentions to reduce the delay of delivering signals and data
over the entire network. WiMAX (Worldwide Interoperability for
Microwave Access) network has one of these challenge , it's the
handover delay (HD) during crossing borders with different speeds ,
the researches among many schemes and models presented succeeds
to reduces the delay, in most cases used different facts , factors and
principles which have an influence over delay, Mobile Terminal
Speed (MT), Distances between the BSs and MT, Radio Strength
Signal (RSS), Locations Management Area (LMA) and Cell size..etc,
all these parameters have a significant effect on the handover delay.
In this paper the modified scheme used Duration Time Session (DTS)
, MTs speeds and Number of MTs in location area and zones,
targeting the handover delay, the comparison results showed the
effect of all these factors and parameters over the handover delay
and at which certain levels of MT speed and DTS the handover delay
has been reduced.
I. INTRODUCTION
owards to achieve the high data rate in wireless services
such as VoIP and IPTV, the Mobile WiMAX (Worldwide
Interoperability for Microwave Access) based on the IEEE
802.16e [1] standard is developed as broadband wireless
solution to the wired backhaul. Fixed WiMAX, based on IEEE
802.16 standard is a cost effective fixed wireless alternative to
cable, and DSL services. IEEE 802.16e is the new version to
the Fixed WiMAX [2] with the mobility features WiMAX
provides high data rate mobile wireless services for
metropolitan areas, and coverage range is up to thirty mile
radius and data rates between 1.5 Mbps To 75 Mbps
theoretically. Handover (HO) is one of the key requirements
to sit mobility and quality of service( QoS) for the subscribers
in IEEE 806.16e. Handover refers to the process in which an
MT transition from the air interface provided by one BS to an
air interface provided by other BS, the IEEE 802.16 defines
three types of handover [3]: hard handover (HHO), Marco
Diversity Handover ( MDHO) and Fast Base Station
Switching (FBSS). MDHO and FBSS are soft handover and
adopt the make- before- break scheme , HHO is imperative in
Wimax system but MDHO and FBSS are elective. In HHO the
MS is connected to only one BS at a time , if the MS decides
to connect to the new target BS, its selects only one target BS
from a group of BSs and MTs stops its radio link with the
serving BSs before establishing its radio link with the target
BS, HHO is causes long handover delay when velocity of
MS is high. in MDHO or FBSS scheme, a MS is registered to
several BSs at the same time for MDHO a MS communicate
with two or more BSs in a given interact while FBSS
services flows are set up with multiple BSs.
few published papers are proposed several models in this area
some researchers have been used soft handovers and the
others used hard handover models. In this paper the handover
delay modified scheme has been introduced with different
factors such as Duration Time Session (DTS) with different
MSs speeds and number of MSs in location areas which play
important keys , the results based on the comparison showed
how these factors have significant influence on the handover
delay, and how these factors in certain level of speed and
DTS reduce the handover delay. This paper is organized as
following, in section 2 describes the architecture of Wimax ,
the WiMAX reference model and handover process, section 3
the modified model is presented, section 4 the analytical
comparison results, finally the conclusion.
. WiMAX NETWORK ARCHITECURE
With IEEE 802.16 limiting itself to PHY and MAC layer the
WiMAX forum is developing end to end network architecture
[4], specifying the access and the core systems and its
functionalities ,it contains procedures and protocols for how
the network will support the mobility and security and
interworking and authentication to a WiMAx subscribers
stations.
A depiction of the network architecture is presented in the
network reference models figure (1) its contains entities such
as Mobile subscriber station or Mobile terminal (MT), Access
Service Network (ASN)and connectivity services networks
(CSN). This reference model contains interface between the
different entities these interface defined procedures and
protocols, rather than physical links across the entities The
ASN consist of one of several ASN Getaways ,the base
stations supplying WiMAX radio coverage to geographical
area. ASN mange MAC access functionalities such as Paging,
location, Radio Resources Management (RRM) and mobility
between base station. The ASN thus services as management
Impact Duration Time Session over The
Handover Delay in Broadband Network
E. Elgembari , K. Seman
T
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2. of the WiMAX radio links only, leaving much of the high level
management to the CSN. The ASN is deployed by the business
entity called Network Access Provider (NAP) which provides
a SS/MSS with L2 connectivity to a WiMAX radio network
and connect MT to Network Services Providers (NSP)
management. The ASN Gateway serves as the
interconnections between ASN and CSN, this logical partition
of the access network from the services network enables
individual access networks to be deployed.
Fig.1 Network Reference Model
III. Network Topology Acquisition (NTA)
.
Its consist of Network Topology Advertisement , MT scanning
of neighbor BSs and association process before HO procedure
the BS periodically send an advertisement message
MOB_NBR_ADV, which includes channel information of the
neighboring BSs such as BS ID (identifier), radiation power,
frequency assignment , mobility and handover support, and
their UCD (Uplink channel descriptor),and DCD( Down
channel descriptor) information. The information contained in
MOB_NBR_ADV message process faster handover to one BS
out of all neighboring BSs. After synchronizing with
neighboring BSs scanning of neighboring BSs can be started.
The MOB_SCAN_RSP message contains the information
about scanning procedure n procedure with the serving BS.
All down link and uplink transmission are paused during
scanning process and the next step, the association procedure
optionally, perform between MS and neighboring BSs. Initial
ranging process performed during MT's scanning interval is
optional . the association procedure is to enable the MT to set
and record ranging parameters and other parameters and other
information by reusing those information MT is able for future
HO to a target BS. If MT decided to skip the association
process . it must perform an initial ranging procedure with the
target BS.
To shorten the HO delay and to reduce wireless channel
resources waste a good handover scheme should be present.
But in spite of that, some factor degrading the system
performance also exist in the HO process. Suppose that a MT
moves with low velocity, the network topology architecture
may maintain the same in a long time. Thus in the cell
reselection stage, the MT can use the same network topology
information and skip this stage since the channel information
of the neighboring BSs do not change . MT could synchronize
to target BS downlink by performing non contention ranging
and therefore the HO delay minimizes , on the other hand
when the MT moves with high velocity the rapid change of
channel condition makes per obtained information useless so
during actual HO procedure the neighboring BSs scanning
and contention based ranging operation must be performed
which generates long handover delay and wireless channel
resources waste at higher speed of the MT.
the user spatial disruption is follows a two dimensional
Poisson disruptions with net rate which defined the average
number of users per unite area. The average number of user
per unite area for i-th popular session is :
(1)
Total number of MBS session duration time S is based on the
exponential distribution with means of and is the
condition probability that the arriving signaling request are
made for all the rank sessions [6], is given by
= ,
where , 0˂α ˂ 1 (2)
The residence time in cell and LMA and MBS are based on
the exponential distribution with means , and
, respectively.
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3. Assuming Z, L and C be the random variables for the number
of MBS zones crossings per session , if Z3 the number of intra
MBS zone handovers [7] then E[Z] = E[Z1]+E[Z2] and
E[C]=E[Z]+E[Z3]. .
Where D is the handover delay , the average services
disruption time for an i-th session in MZ can be;
TM Z,i = E[Z1,i] . DZ1 + E[Z2,i] . DZ2 + E[Z3,i] . DZ3 (3)
Assuming the random variable for session duration time is ts, tz
and tc so probability crossing MBS zones and cells are given
in [8] as a following:
(4)PZ = Pr (ts >tz) =
PC = Pr (ts >tz) = (5)
Assuming p(k,i,A) denote the probability that there are k
users subscribing the i-th popular session in an area A
P(k,i ,A) = (6)
average number of inter MBS zone handover to active MBS
zones can be denoted as
(7)
On the other hands, the average number of cell crossing is
given by ,
(8)
average number of cell crossing is given by ,
(9)E[Z3,i]=
So the disruption time in MZ and LMA can be given by
TMZ,i = [
(10)
D LMA,I = [
(11)
Suppose that MBS zones LMAs and cells are square shaped
where there are NZ cells in a MBS zones and NL cells in an
LMAs then by the fluid flow mobility model in [9] the cell
boundary crossing rate can be expressed as = (16v)/(πl) ,
where v is the average velocity of MTs and l is the perimeter
of a cell . in addition the average duration session time is
and S is total number of MBS session.
. THE RESULTS
In Figure. 2, the effect of the duration session time on the
handover delay is clear ,the results showed that, when the
duration session time is 60, with different speeds of mobile
terminal the handover delay is becomes very high in the
lower Mobile terminal speed . in the other hand the handover
delay is decrease when the mobile terminal speed is becomes
higher , but comparing the graduating time of the decreasing
of delay within different mobile speeds its clear in speed of 30
(km/h)up to 60 (km/h) it has a high drop delay comparing to
speed from 60 to 100, the point is the handover channels as a
bandwidth cannot handle all the signaling traffics within lower
mobility and speed with 60 duration session time ,however the
high speed it will be more reliable for the handover processing
to handle it with less the effect .
0
1
2
3
4
5
6
7
6065707580859095100110120130140150
HandoverDelayUnit
MT Speed (Km/h)
Fig 2. Handover delay using different MT speed with fixing the
duration session time
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4. 0
1
2
3
4
5
6
050100150
HandoverDelayUnit
Duration Session Time (ms)
Fig. 3 Handover delay within different Duration Session Time
In Figure. 3, The results shown that the handover delay has
inverse proportional with the duration session time so when
the session time is high the delay goes down and vice versa
with consider fixed mobile terminal speed ,this scenario
applies to figure 4, 5 and 6, although the number of mobile
terminals is changed from 100 to 150 and 200 respectively.
1.04
1.05
1.06
1.07
1.08
1.09
1.1
1.11
1.12
1.13
050100
HandoverDelayUnit
MTs
Fig. 4 Effect Duration Session Time on handover delay based on
100 mobile terminals
1.02
1.03
1.04
1.05
1.06
1.07
1.08
1.09
020406080100
Handverdelayunit
Duration Session Time
Fig. 5 Effect Duration Session Time on Handover Delay Based On
150 Mobile Terminal
1.02
1.025
1.03
1.035
1.04
1.045
1.05
1.055
1.06
1.065
020406080100
HandoverDelayUnit
Duration Session Time
Fig. 6 Effect Duration Session time on Handover Delay Based on
200 Mobile Terminal
In Fig. 7, comparison the handover delay based on different
duration session time and different numbers of mobile
terminals is shown that the delay has inverse proportion
relation with the duration session time under the certain
criteria such as the fix speeds and locations areas. Obviously
this relation has real impact on the bandwidth channels where
the connections between the session duration and the
bandwidth should be on the same tone when the processing
itself deal with the handover delay, in this figure result shown
that delay comes lower and closer between 60 to 80 duration
session time to all different number of mobile terminals.
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5. Indicated that in such this case its good scenario of lower delay
if the duration session could be in this range of time but still
the higher number of terminals has an acceptable lower
handover delay than the others.
0.96
0.98
1
1.02
1.04
1.06
1.08
1.1
1.12
1.14
304050607080
HandoverDelayUnit
Duration Session Time
Dly,M
T=100
Dly,M
T=150
Dly,
MTs=
200
Fig.7 Handover Delay based on Mobile Terminals and Different
Duration Session Time
VI. CONCLUSION
The influence of the session duration time , MTs numbers and
speed has an obvious impact to the handover delay, as far as
the session duration time is low the network performance shall
record a positive quality of service and low handover delay.
with high speed and low session duration time the handover
has lower delay, its necessary not to be neglect the importance
of bandwidth channels which has an additional positive value
to all overcome the network performance, particularly when
the reduction the handover delay is the main target to achieve.
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