CHAPTER 4 HANDOFF MANAGEMENT TECHNIQUES: DTP AND

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CHAPTER 4
HANDOFF MANAGEMENT TECHNIQUES: DTP AND HDMP
In mobile communication handoff is the key matter to provide
continuous service to the mobile nodes without any break. A single cell
does not cover the whole service area. There are two basic reasons for a
handoff such as (i) The mobile node moves out of the range of cell (ii)
The received signal level decreases continuously until it fall below the
minimum requirement for communication. In mobile communication,
the standard OSI model is used for voice and data communication. The
five divisions of the Open Systems Interconnection (OSI) layers are
involved when the handoff takes place. This chapter describes proposed
handoff management techniques such as Double Threshold Protocol
(DTP) and Hybrid Decision Making Protocol (HDMP) to provide QoS.
4.1 DOUBLE THRESHOLD PROTOCOL
When a MN moves into a next cell while a conversation is in
progress, the MSC automatically transfers the call to a new channel
belonging to the new base station. This handoff operation involves
identifying a new base station and allocates the channels associated with
the new base station. The existing handoff techniques take place using
single threshold value which describes the minimum strength of signal to
start handoff process. In some situation the active call is disconnected
due to the handoff delay is high. To avoid this kind of problem, the new
protocol DTP is introduced. In the Figure 4.1, DTP refers the double
fixed threshold values for handoff initialization. When the MN reaches
the first threshold the MN sends an alert-signal to the BSC. The system
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is started to do the basic computation but it will not start the handoff
process. After the mobile node reaching the second threshold value, the
handoff process is initiated. In Figure 4.1, A, B refer the second
threshold and X, Y refer first threshold.
Figure 4.1 Handoff scenario at cell boundary using DTP
4.1.1Double Threshold Protocol Procedure
Figure 4.2 refers DTP procedure, the advertisement comes
from AP regarding its presence and service signal to the MN. Then MN
updation takes place using the ETU. The ETU is already discussed in
detail in Chapter 3. After finding the signal strength, the BSC checks the
whether the mobile is reached the first threshold or not. Suppose it
reached the first threshold, it starts to call the basic computational
function to finish primary computation for handoff process. When the
MN reaches the second threshold, it checks whether the received signal
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from BTS is good or not. The good signal means when the current base
station keeps control of MN, moderate signal strength is necessary to
maintain the good quality of communication between MNs. This is
called good signal.
If the signal is not good, it starts the handoff process and
completes before the active call is disconnected. After the handoff the
MN registers its identity to the new BS. Then the connection is
established and transfers the operational parameters. After that, the
packets are transferred between MNs. The communication follows
continuously as per the DTP procedure.
Figure 4.2 DTP Procedure
4.2 HYBRID DECISION MAKING PROTOCOL
The HDMP is introduced to take handoff decision; the HDMP
performs the functions based on the Better Base Station Selection
Protocol (BBSSP) and IADMP. The Figure 4.3 shows the functional
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block diagram which refers HDMP functions. In this block diagram,
when the MN signal strength decreases continuously, the BBSSP takes
handoff decision using Relative Signal Strength. The IADMP makes
decision from the history of the MN and BBSSP. The IADMP decision
is based on the Apriori algorithm. The BBSSP and IADMP decisions are
processed and the Handoff decision is made. Then it starts the handoff
process, if its decision meets success then MN is registered into the new
base station else the failure is recorded into the IADMP for future use.
Figure 4.3 Functional block diagram of HDMP
Figure 4.4 refers the HDMP procedure. In this procedure, MN
updates into the MSC using ETU. Signal strength is monitored to check
whether the signal is decreased continuously. When the signal strength is
not good then it calls BBSSP, after that it calls IADMP. Handoff
decision is made here for handoff process. Then it starts to register into
the MSC through BSC for new base station. Hence the handoff success
and failure rate are monitored by IADMP for future use.
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Figure 4.4 Hybrid DMP Procedures
4.3 HANDOFF ARCHITECTURE
In mobile computing, the efficiency of the mobile handoffs
depends on the five characteristics (i) Minimum handoff latency (ii) Low
packet loss (iii) Limited handoff failure (iv) Intelligent Agent success
rate (v)Better Base Station Selection. An analytical model has been
developed for evaluating the performance of handoff algorithms based
on Relative Signal Strength (RSS) measurements. Absolute Signal
Strength (ASS) which is the averaged value of the received signal level
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from current serving Base station measured by the mobile unit. The
Measurement reports including the quality of current link are sent to
BSC by the Mobile node about every half second. The BSC receives the
MN signal and take decision, then it sends Handoff-request signal to
MSC. Then MSC will send the signal to the BSC that activate the next
Base station. Then the mobile node is taken over by the activated base
station. The acknowledged signal is sent by mobile node to BSC and
then BSC to MSC.
Figure 4.5 refers the proposed handoff architecture which has
the following blocks such as Neighbor discovery unit, Handoff signal
estimation unit, Speed estimation Unit, Handoff trigger unit, Handoff
execution Unit, RSS measurement unit and IADMP unit. Hence this
architecture supports data services in mobile network. In the Network
layer Mobile IP supports HM in IP based Networks. It uses Tunneling to
forward IP packets when the MN moves away from the Home Network
(HN). Mobile IP is simple to implement but suffers from Triangular
routing, High global signaling load and High handoff latency.
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Figure 4.5 Proposed Handoff Architecture
Triangular routing is eliminated by optimized Mobile IP. The
use of link layer information reduces the handoff requirement detection
delay. The Link Layer information is used to anticipate the possibility of
inter and intra system handoff in advance using IADMP. The Link layer
information are threshold value of RSS to initiate the proposed handoff
process, the minimum value of RSS required for successful
communication between mobile node and BTS, cell size and channel
availability. So the handoff procedures can be carried out successfully
before the MN moves out of the coverage area of the serving BTS. The
use of link layer information significantly reduces the handoff latency
and handoff failure probability of handoff management protocols.
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As shown in Figure 4.5, the data link layer has two units:
Speed Estimation unit and RSS measurement unit. The Network Layer
has Neighbor Discovery unit and the Handoff signal execution unit. The
functions of each unit and the information used by them are as follows.
Information is collected from the Network and Data Link Layer and is
used to carry out the handoff procedures. The functions of these units are
listed below.
Neighbor Discovery Unit: It assists the MN to learn about the
neighboring base stations. The neighbor base stations are found
out based on the signal strength and round-trip time. Some of the
neighboring base station can present in the serving Foreign Agent
(FA), whereas other may belong to different FA. When the MN
moves into the coverage area of a neighboring BTS that belongs
to its serving FA, the resulting handoff is Link Layer Handoff. In
this case the existing handoff algorithms, inter-BSC and interMSC handoff can be used.When the neighboring BTS belongs to
a different FA under the serving system, it is known as the Intra
system handoff. When the neighboring BTS belongs to a
different system other than the serving BTS, the resulting
handoff is an Intersystem handoff. The proposed algorithm is
used in Horizandal system handoff which refers the same
technology.
Handoff Signal Execution unit: It estimates the signalling delay
associated with intra and intersystem handoff.
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Speed Estimation Unit: This unit estimates the mobile speed
using Velocity Estimation.
Handoff trigger unit: It collects information from the handoff
delay estimation unit, speed estimation unit, RSS measurement
unit, IADMP unit and determines the appropriate time to start
handoff procedures.
Handoff Execution Unit: It starts the Handoff registration
process at the handoff initiation time by the handoff trigger unit.
Handoff anticipation: This stage requires information from the
RSS measurement unit. If the RSS of the serving BTS decreases
continuosly it shows a handoff is anticipated.
IADMP: The IADMP is introduced to analyze the handoff
history of the Mobile node and signal strength. At last IADMP
will suggest whether handoff initiation should be done or not.
Due to IADMP uses Apriori algorithm, it uses large item-set
property and it is easy to implement the properties. Though it
requires many database scans, which will not affect the handoff
decision and prediction of the handoff in the Mobile Node.
Because it takes decision before handoff taking place.
Handoff initiation: The Handoff Trigger unit uses the speed and
handoff signalling delay information to estimate the threshold.
The threshold value will be changed based on the requirement.
When the MN RSS comes below the threshold, the active call
might be disconnected. The Handoff Trigger Unit sends a trigger
to the Handoff execution unit to start the handoff procedure.
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The proposed architecture uses Data Link and Network Layer
information to initiate and manage the hanoff process. It depends on the
mobile speed, handoff signalling delay and IADMP decision as the
major information. The operation of the proposed system is
subdividedinto six stages as shown in Figure 4.6.
Figure 4.6 Six Stages in handoff operation
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The flow of the proposed mechanism is shown in Figure 4.7.
Figure 4.7 Flow of handoff operation
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4.3.1 IADM Protocol in Handoff Management
Table 4.1 refers the handoff history of a MN. The IADMP has
been discussed in detail in Chapter 3. The IADMP does the following
works i) Predicts necessity of the updation of the mobile node based on
the MNs movements history and incoming calls history ii) It takes
decision on which BS is better when the handoff takes place based on the
MNs handoff history and signal strength of the MN. In this work Apriori
Algorithm is used for mining process to find frequent sequence-sets. For
easy understanding the table 4.1 refers a, b, c, d, e, f and g are micro
cells, Day refers from Monday to Sunday. Then a-b, b-c, c-d, d-e, e-f,
and f-a are the handoff between micro cells. In addition SS refers signal
strength of the mobile node for communication. The default value of SS
is ‘1’ (the signal strength is below the cut of range), when it exceeds the
handoff threshold, the handoff will be taken place. The handoff event SS
is marked as ‘0’ as shown in Table 4.1.
In the initial process it collects all the details from MSC. The
IADMP does the data mining process and find the optimum handoff
based on the handoff history, timestamp and signal strength of mobile
node using Apriori algorithm.
In Apriori algorithm, the first step, it scans all of the
transactions in order to count the number of sequence-sets of mobile
node. Here number 2 is taken as a minimum support count. The
algorithm counts the each candidate frequent occurrences, the frequency
of the occurrences less than 2 means it will not consider for future use,
otherwise it will be taken into account to take decision. At last the high
frequently occurring pattern is retrieved. Using this pattern, decision is
taken by the IADMP.
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Table 4.1 Handoff History of Mobile Node
The performance analysis of LM and HM has been discussed
in chapter 5.

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