High capacity wireless access for mobile users is increasing rapidly.
Rapid progress in the research and development of wireless networking
and communication technologies has created several wireless communication
systems such as GSM (Global System for Communication), GPRS (General Packet
Radio Services), UMTS (Universal Mobile Telecommunication Services) and
Wireless LAN (WLAN), Bluetooth for personal area and Satellite networks
for Global networking. These networks complementary to each other and
hence their integration can realize unified Next-Generation Wireless Systems
(NGWS). In the integrated NGWS, users are always connected to the best
available networks and switch between different networks based on their
service needs (Akyyildiz et al., 2005). A challenging issue in
NGWS is to support seamless mobility management. Mobility Management consists
of two components as shown in Fig. 1 . Location Management
enables the system to keep track of the location of mobile users between
consecutive communications. Handoff management is to keep the user connection
active when they move from one Base Station to other. There are several
solutions for Location Management, but the solution for handoff management
suffers from handoff delay. The Handoff types in NGWS are shown in Fig.
Horizontal handoff: Handoff between two Base Station of the same
system. It is further classified into
|| Mobility management
|| Type of handoff
||Link layer handoff: Handoff between two BSs that are under
the same foreign agent (FA)
||Intrasystem handoff: Handoff between two BSs that belong
to two different FAs and both the FAs belong to the same system.
Vertical handoff: Handoff between two Base Station that belong
to two different systems.
In the literature there are several algorithms to support seamless link-layer
handoff (Perkins et al., 2002). The solution for Intra system and
intersystem handoff is still a research issue. The efficient intra and
inter system handoff depends on the following characteristics for seamless
handoff and are:
|| Handoff management protocols in TCP/IP stack
||Minimum handoff latency
||Low packet loss
||Limited handoff failure
There exist many solutions for Handoff management in different layers
of Internet Protocol. The different layers and the techniques used are
shown in Fig. 3 . The Application Layer handoff is initiated
using the Session Initiation Protocol (SIP) and it doesn`t require any
changes to the TCP/IP Protocol Stack (Wedlund and Schulzrinne, 1999).
In the Transport Layer TCP-Migrate is used to support end-to-end Transport
Layer Handoff Management (Snoeren and Balakrishnan, 2000). An architecture
called MSOCKS is proposed (Matz and Bhagwat, 1998) for handoff using split-connection
proxy architecture. In the Network Layer Mobile IP (Perkins et al.,
2002) is proposed to support mobility management in IP based Networks.
It uses Tunneling to forward IP packets when the MS moves away from the
Home Network (HN).
Mobile IP is simple to implement but suffers from:
||High global signaling load
||High handoff latency
Triangular routing is eliminated by optimized Mobile IP, the handoff latency
is not addressed by it. At present the use of link layer information to reduce
the handoff requirement detection delay has gained attention in (Akyyildiz et
al., 2004; Akyyildiz and Wang, 2004). The Link Layer information is used
to anticipate the possibility of inter and intra system handoff in advance so
that the handoff procedures can be carried successfully before the MT moves
out of the coverage area of the serving Base Station (BS). The use of link layer
information significantly reduces the handoff latency and handoff failure probability
of handoff management protocols (Akyyildiz et al., 2004).
Handoff architecture: In the proposed system we have used architecture
as shown in Fig. 4 for Physical and data link layer.
In the physical layer it has two units: Speed Estimation unit and RSS
measurement unit. In the Data Link Layer it has Neighbor Discovery unit
and the Handoff signaling delay estimation unit.
The functions of each unit and the information used by them are as follows.
Information is collected from the Physical and Data Link Layer and is
used to carry out the handoff procedures, handoff trigger unit and handoff
execution unit. The functions of these units are listed below.
||Neighbor discovery unit: It assists the Mobile Node to learn
about the neighboring nodes. It uses Candidate Access Route Discovery
protocol (Liebsch et al., 2004) to identify the neighboring
||Handoff signaling delay estimation unit: It estimates the
delay associated with intra and intersystem handoff.
||Speed estimation unit: This unit estimates the mobile`s speed
using VEPSD (Velocity estimation using the power spectral density
of the received signal envelope) as in (Zhang and Holtzman, 1996).
Here Doppler frequency fm is related to speed of a mobile
user, speed of light in free space and carrier frequency of the received
where, v is the speed of the mobile device, c is the speed of light in
free space, fc is the carrier frequency of the received signal, fm is
the Doppler frequency related to the speed of the mobile user.
||Handoff trigger unit: It collects information from the handoff
signaling delay estimation unit, speed estimation unit, RSS measurement
unit and determines the appropriate time to start handoff procedures.
||Handoff execution unit: It starts the Handoff registration
process at the handoff initiation time calculated by the handoff trigger
Operation: The proposed architecture uses Data Link and Network
Layer information to initiate and manage the handoff process. It depends
on the mobile`s speed and handoff signaling delay as the major information.
The operation of the proposed system is subdivided into five stages as shown in Fig. 5.
|| Modules in handoff management architecture
|| Stages in handoff operation
Each stage and the happening
are explained below. The flow of the proposed mechanism is shown in Fig.
Neighborhood discovery: In a Wireless system, Mobile Station (MS)
is served by a Base Station (BS) and it learns about the neighboring BSs
(BSs that are the immediate neighbors of the serving BS) using the Neighbor
Discovery unit. Some of the neighboring BSs can present in the serving
FA, whereas other may belong to different FA. When the MS moves into the
coverage of a neighboring BS that belongs to its serving FA, the resulting
handoff is Link Layer Handoff. In this case the existing handoff algorithms
(Zhang and Holtzman, 1996) can be used without the proposed algorithm.
When the neighboring BS belongs to a different FA under the serving system,
it is known as the Intra system handoff. When the neighboring BS belongs
to a different system other than the serving BS, the resulting handoff
is an Intersystem handoff. The proposed algorithm is used in Inter and
Intra system handoff.
|| Flow of handoff operation
Handoff signaling delay estimation: During change in location,
it is difficult to predict to which particular BS the MS will move. The
main objective of the proposed system is to estimate the handoff signaling
delay in advance without knowing to which particular BS the MS will move.
In the proposed algorithm, we estimate the handoff signaling delay of
a possible handoff to a particular neighboring BS in advance we use the
following steps. We have used Invalid authentication extensions to just
learn the handoff signaling delay without changing the mobility binding
at GFA or HA.
||For Intrasystem Handoff, MS sends HMIP registration message to the
GFA with an invalid Mobile-GFA Authentication extension.
||For Intersystem Handoff, MS sends HMIP registration message to the
HA with an invalid Mobile-Home Authentication extension.
||When GFA or HA receives the HMIP registration messages and learns
the presence of invalid Authentication Extensions, they return the
HMIP Registration Reply with the appropriate code that signifies the
MS has failed authentication.
The handoff signaling delay is estimated based on the difference between
the transmission time of the HMIP registration request message and the
reception time of the Handoff registration reply message. Using the above
steps the MS learns (i) handoff signaling delay in the event of movement
to the BS, (ii) signaling delay of the associated handoffs to other neighboring
BSs. From the above study it shows that for intrasystem handoff, as few
messages are exchanged the handoff delay is lower compared to intersystem
Handoff anticipation: This stage requires information from the
RSS measurement unit. If the RSS of the serving BS decreases continuously
it shows a handoff is anticipated. The Handoff Trigger learns the signaling
delay for that particular BS from the handoff signaling delay estimation
Handoff initiation: Once the MS learns the BS that it is going
to move, it estimates the right time to start the HMIP registration. The
Handoff Trigger unit uses the speed and handoff signaling delay information
to estimate the threshold T1. When the RSS of the serving BS drops below
T1, the Handoff Trigger Unit sends a trigger to the Handoff execution
unit to start the HMIP handoff procedure.
Handoff execution: The Handoff execution unit receives the handoff
trigger from the handoff trigger unit it starts the handoff registration.
Once the handoff registration is competed, the MT is switched to the new
BS. The MS keeps its registration with the old BS for a specified time
period to avoid the ping-pong effects during handoff using the binding
method. The MS binds the CoA of the old FA and new FA at the GFA in intrasystem
handoff and at HA in intersystem handoff. Thus GFA and HA forwards packets
destined for the MS to both CoAs during this time interval (Zhang and
The operation of Handoff management is explained with the diagram shown
in Fig. 6. The Mobile Station (MS) learns about the
neighbor BSs using the neighbor discovery protocol and determines the
type of handover in the movement to the new Base Station (BS). Once the
neighboring BS is learnt, the Handoff Signaling Delay Estimation Unit
estimates the signaling delay associated with the neighboring BS. The
RSS monitoring unit starts to monitor the RSS of the serving BS and anticipates
a handoff when the RSS decreases continuously and in the selection of
the next BS. The different types of handover are as follows.
||Link layer handoff: If the associated handoff to the next
BS is link layer handoff, the existing algorithm by Zhang and Holtzman
(1996) is used.
||Intrasystem handoff: If the associated handoff to the next
BS is an intrasystem handoff, the handoff trigger unit estimates the
dynamic RSS threshold t1.When the RSS of the current BS
drops below t1, the MS starts the proposed mechanism with
the next BS.
||Intersystem handoff: If the associated handoff to the next
BS is an intersystem handoff, the handoff trigger unit estimates the
dynamic RSS threshold t2. When the RSS of the current BS
drops below t2, the MS starts with the intersystem handoff
procedure that we have proposed.
RESULTS AND DISCUSSION
The typical handoff scenario in the next generation wireless networks
is shown in Fig. 7. We have considered an integrated
architecture in which two different wireless systems namely System A (Cells
shown in straight lines) and System B (Cells shown in dotted lines) are
connected to the Internet using Gateway foreign Agents GFA1, GFA2, respectively.
The Base Stations BS10 and BS12 of System A are connected to Foreign Agents
FA10, Base Station BS11 is connected to Foreign Agent FA11 and then connected
to the Internet Backbone using Gateway Foreign Agent GFA1. The Base Stations
BS20 and BS21 of System B are connected to Foreign Agent FA20 and then
connected to the Internet Backbone using Gateway Foreign Agent GFA2.
To evaluate the performance of TCP during handoff in such integrated
next generation wireless system (NGWS), we have used the handoff scenario
as shown in Fig. 8. Handoff takes from the current BS
referred as old (OBS), to the future BS, referred as (NBS). We have used
the following parameters,
||: The threshold value of the RSS(Received Signal Strength) to initiate
the proposed handoff process. When the RSS of OBS drops below the
proposed registration procedure is initiated for MS`s handover to
the new BS(NBS).
||: The minimum value of RSS required for successful communication
between an MS and OBS.
||: Cell size and we have taken hexagonal cell shape.
In the simulation, we have considered that the MS is served by the OBS
and is moving with a speed v. The speed v is uniformly distributed in
[vmin,vmax]. The probability density function (pdf)
of v is given by
During the movement of the mobile station (MS) it is to move into the
coverage area of the NBS and the proposed registration procedure is needed
to register with the FA of the serving NBS known as (NFA). The MS can
learn about the possibility of moving into another cell when the RSS of
OBS decreases continuously.
|| Architecture of next generation wireless system
|| Handoff scenario
Once the MS knows that it can move into the
coverage area of the NBS, the next step is to determine the right time
to decide registration to the NFA. For our simulation, we consider a microcellular
system with a cell size of a = 1 km, macro cell reference distance d0
= 100 m, a standard deviation of shadow fading parameter e = 8 dB, path-loss
co-efficient = 4, handoff failure probability pf = 0.02 and
maximum value of mobile speed as 14 km h-1. The minimum RSS
required for successful communication between MS and BS as -64 dBm. In
the simulation environment we assumed that the MS moves from the old base
station (OBS) to a new base station (NBS). During data transmission between
the MS and BS we have used the TCP/IP as the protocol and the behavior
of the proposed mechanism is analyzed with the following parameters and
Simulation of the proposed handoff Management mechanism is carried out
in ns-2 simulator and we obtained the results for various factors and
are discussed as follows.
||Relationship between RSS and speed of mobile device
Relationship between RSS Threshold Tavg and speed: For
different values of handoff signaling delay, we analyzed the relationship
between Tavg and MS speed (v). Taking different values of speed,
we calculated the required value of d and using all we found the required
value of Tavg. The relationship Tavg between and
speed in microcellular system is shown in Fig. 9. The
graph implies that, for an MT moving at high speed the handoff should
be initiated earlier as compared to a slow-moving MT to guarantee the
desired handoff failure probability independent of MTs speed. When τ
is large, the handoff must start earlier compared to when τ is small.
The small and large values of τ correspond to intra and inter handoffs
respectively and so calculation of Tavg is adaptive v and τ.
Relationship between handoff failure probability and speed: In
the simulation environment, we analyzed the handoff failure probability
for different types of inter and intrasystem handoffs and compared that
with existing fixed RSS-threshold-based handoff protocols. We calculated
Tavg using speed and handoff signaling delay information and
20% of error is introduced in speed estimation. Then Tavg is
used to initiate the handoff and determine handoff failure probability.
The relationship is shown in Fig. 10. From the graph
it shows that with the existing fixed RSS-threshold based algorithms in
the proposed mechanism error is reduced 70-80% and shows that handoff
mechanisms should be adaptive to the type of handoff.
Relationship between handoff failure probability and handoff signaling
delay: We analyzed the handoff failure probability for different
values of handoff signaling delay, τ. The analysis showed that unlike
the fixed RSS-based handoff protocols, pf remains independent
||Relationship between handoff failure probability and
speed of the mobile device
||Relationship between handoff failure probability and
handoff signaling delay
||Performance of TCP/IP with and without the proposed
The proposed mechanism estimates τ and uses it for the calculation of dynamic RSS threshold and it enables 70-80% reduction
in pf compared to the fixed RSS-based handoff protocols. The
lower value corresponds to intersystem handoff and higher value corresponds
to intersystem handoff. The Relationship between Handoff Failure Probability
and Handoff Signaling Delay is shown in Fig. 11.
Performance of TCP/IP
Performance of TCP/IP protocol in wireless: Environment is studied
and the results are shown in Fig. 12. When TCP/IP protocol
is used without the proposed mechanism the throughput decreases during
the event of handover with the speed of the mobile device. If the proposed
mechanism is used the throughput increases compared to the fixed RSS-based
handoff. This is because we are following five stages in the operation
of the proposed mechanism.
In this study, we have studied the different types of handoff mechanism
in wireless environment. The different handoff mechanism in the TCP/IP
protocol stack is listed. The proposed mechanism with five different stages
and the operation of the proposed mechanism is explained as next. In the
last section we have shown the performance of the proposed mechanism,
with the data link layer and network layer information which gave enhanced
performance compared to the fixed RSS based mechanism. At last we studied
the performance of TCP/IP protocol if the proposed mechanism is used during
handoff. The proposed mechanism gives improvised throughput and goodput
in wireless environment.