VERTICAL HANDOFF AND TCP PERFORMANCE OPTIMIZATION

USING CROSS LAYER APPROACH

Department of Electronics & Electrical Communication Engineering

Indian Institute of Technology, KharagpurMay 2012

byAnurag Mondal

Roll No : 10EC64R04

Under the guidance ofProf. Rajashri Roy

Outline Of Presentation• Problem Statement & Motivation• Introduction• Different Approaches of Vertical Handoff• Proposed Algorithm• Simulation Environment, Characteristics & Results• Conclusion & Future Work• Related Work• Reference

Problem Statement This thesis focuses on finding a solution to the vertical mobility problem together with data transmission over TCP.

The basic objective is how to utilize the asymmetric data rates in overlapping heterogeneous wireless networks to the fullest. The objective is to find a method that will give better performance than a traditional handoff algorithm based on a received signal strength threshold.

Motivation The motivation for addressing the given problem areas is to develop wireless packet switched communications by facilitating a holistic approach for handoff and mobility management in heterogeneous wireless networks.

The cross-layer approach involves taking into consideration on handoff technique that are employed in the physical layer solutions in cellular networks and on the other hand mobility management solutions that are employed in the Transport layer.

Introduction

Mobility is the most important feature of today's wireless networking system.

Mobility can be attained by handoff mechanisms in wireless networks.

Handoff is the process of changing the channel (frequency, time slot, spreading code, or combination of them) associated with the current connection while a call is in progress.

Types Of Handoffs

Horizontal Handoff Handoff between two base stations (BSs) of the

same system is called Horizontal handoff. Horizontal handoff involves a mobile device to

change cells within the same type of network.Vertical Handoff (VHO)

Vertical handoff refers to a mobile node changing the type of connectivity , usually to support node mobility.

For example switching connection from a cellular network to wireless local area network.

Steps Required For Handoff Available network domain detection Selecting the best network among the available Deciding the handover policy Handover execution based on decision policy

Requirements Of Handoff Mechanism Bandwidth Handoff Latency Power Consumption Network Cost Received Signal strength (RSS)

Heterogeneous Network Architecture

Fig1: Heterogeneous network architecture integrating with Wi-Fi, and UMTS/B3G

Different Approaches of Vertical Handoff Vertical Handoff based on Hysteresis Technique and Predictive RSS

Vertical Handoff based on Polynomial Regression-Based Prediction

Cross Layer-Based Technique for Increasing TCP Throughput in Vertical Handover

Hysteresis Technique And Predictive RSS Handoff

The RSS-based scheme with hysteresis is adopted to avoid unnecessary handoff due to the ping-pong effect.

Fig 2:Handoff points in different handoff approaches.[6]

The mobile node MN moving from network 1 to network 2. In the RSS-based handoff approach, the handoff will

occur at point 2, 3, or 4. This causes a serious ping-pong effect . In the RSS with hysteresis approach, the mobile node

performs handoff at point 4. The predictive RSS-based handoff approach to perform

handoff at point 2 when the mobile node predicts that it is moving toward network 2.

Hysteresis Technique And Predictive RSS Handoff

Vertical Handoff Based On Polynomial Regression-based Prediction

• Based on the hysteresis mechanism, the paper[6] proposes a polynomial regression predictive RSS handoff approach that consists of two steps, i.e.,

The Preprocess step The RSS prediction step.

• The Reprocess Step: In prediction, some previous RSSs are important for determining the next predictive RSS. To intensify the polynomial regression-based curve fitting, the preprocess of accumulated generating operation, which is based on, is adopted to achieve the accuracy of the prediction results.

•RSS prediction step: After the preprocess step, the new data sequence is used as the input data for curve fitting in the RSS prediction step.

This process uses polynomial function to determine the predicted RSS from previous samples of RSS values.

Handoff Detection Algorithm based on Polynomial Regression-Based Prediction

Scenario: Considering two kinds of wireless networks in the system B3G & WLAN . Mobile node is currently in a WLAN network.

Fig3: Vertical Handoff based on RSS prediction

Cross Layer-based Approach For Increasing TCP Goodput

• Cross-layer-based mechanism between layers 2(MAC layer) and 4(TCP Layer) to increase the TCP goodput .

• TCP protocols adopt automatic repeat request (ARQ) [6] to determine cwnd, it causes inaccurate window control and low utilization in a wireless network.

• The key idea is to notify the layer 4 TCP protocol by the layer 2 MAC protocol at the TCP receiver when the network types of these two wireless networks are different. The TCP receiver then replies to the TCP sender with the MAC type of the target network by using the optional field of the TCP ACK segment.

Fig 4: Message flow in the cross-layer mechanism [6]

• The mobile node sends the network type of the target network to the TCP sender, the TCP sender computes the predictive available bandwidth based on the RTT-based bandwidth prediction by

where BWnew_max is the maximum capacity of the target network, and the factor of (RTTmin/RTT) is called the shrink factor. Since the predictive bandwidth BWpredictive can be expressed as

• the optimal cwnd (i.e., cwndopt) for the target wireless network is

determined by

RTTRTTminnew_max

predictive

.BW BW

RTTcnwd

BWpredictive

RTTRTTmin

opt cnwd cnwd

The Proposed Algorithm• A vertical handoff algorithm is proposed which uses

the basic idea of cross layer optimization. • Similar to the above technique, a cross- layer approach

using MAC layer and TCP layer is can be used for the modification of cnwd for TCP.

• In the above algorithm, server increases or decreases the Congestion Window and Slow Start Threshold depending on type of the network receiver is in.

• For example if the receiver moves from the lower data rate network to higher one, the congestion window is gradually increasing.

• But, the above algorithm is not considering the situation of packet loss at cell boundary ,since the RSS is gradually decreasing in current network and also due to ping-pong effect at the cell boundary.

• So, solution to the problem proposed is to stops TCP transmission and set the congestion window at a low constant value at cell boundary until connection is established to a new network and RSS reaches a threshold .By this method, TCP packets loss can be reduced as a result TCP throughput increases.

Simulation Environment Let a MT (Mobile Terminal) is moving and after a certain instance of time

makes a hand off.

The handoff is RSS based with Hysteresis technique, means it makes

sure it remains connected to the best network available based on handoff.

The Mobile Terminal is receiving data in the form of TCP segment. The

Mobile Terminal is both WLAN and B3G enabled.

When the Mobile Terminal come near the cell boundary and it changes

the MAC type in the TCP ACK. When the server the receives the ACK, the

server understands that MT is trying to make a vertical handoff. Server

suspends the packet transmission for a predetermined period of time until

MT the mobile terminal connects to a new network.

Simulation Parameters• Number of mobile Node: 1• Number of WLANs hotspots: 10• Number of B3G / UMTS BTS :1• Transmission range of Cellular Network (B3G/UMTS) = 3000m• Transmission range of WLAN = 200m• Transmit Power for Cellular Network (B3G/UMTS) = -40dBm• Transmit Power for WLAN=-50dBm• Path loss constant ,S= 5• Path Loss exponent ,n=3.5• RSS_factor=2.8 (Signal Delay Factor for WLAN)• Data rate for Cellular Network (B3G/UMTS) = 384kbps• Data rate for WLAN = 2Mbps• Mobilty Model: Random walk • Velocity of the Mobile Terminal: 1~25m/s• TCP segment size = 1kbytes• Maximum Congestion Window Size= 64kbytes

Simulation Characteristics & Results

• In a simulation, we need to take many considerations such that we can generate an environment which resembles real life environment keeping in mind the limitations of the simulation platform.

• The RSS received by a mobile node is different when it uses different wireless networks. In WLAN, the RSS is computed based on

• In the case of B3G/UMTS Network ,RSS is computed as

• where, Ptrans is the transmit power, and PL(d) is the path loss at distance d. The path loss at distance d is defined as

factorRSSRSS _10 )9.37distance.3(100.log.10

)()( dPLPdRSS trans

)10log(..10)( nSdPL

Fig 5: Signal strength received by the Mobile Terminal

For simulating the motion of the mobile terminal, Random walk mobility model is used. In this model, the mobile terminal is considered to be moving randomly with a certain probability in forward, backward direction. Also, the mobile node is considered to stay a certain position (no movement).

Fig 6: Random Movement of the Mobile Terminal

Now, using the above simulation characteristic, the number of vertical handover is calculated over different velocity (1-25m/s).

Fig 7: No. of Vertical Handoff against velocity of the Mobile Terminal

Now, the mobile terminal is considered to moving in the wireless environment, and the situation of data transmission is considered. It

is considered that mobile terminal receiving data throughout the simulation. Data received is considered as TCP packets.

Fig 8: Data received by the Mobile Terminal

Next, a comparison between the throughput of the TCP (proposed by [6]) and throughput of TCP proposed by thesis is plotted against the no. of Vertical Handoff.

Fig 9: Comparison of the Data rate of TCP [6] and Modified TCP proposed with no. Vertical Handoff

The plots below show a comparison between the congestion windows during vertical handoff.

Fig 10: Comparison of the Congestion Window of TCP [6] and Modified TCP proposed during Vertical Handoff

Now a comparison between the number of packet drops for both the algorithm is calculated. We can see that the no. of packet drop

is comparatively lower in proposed algorithm.

Fig 11 :Comparison of the Packet Drop of TCP [6] and Modified TCP proposed

Similarly, a plot for comparison of Efficiency of TCP for both the algorithm shows a better result in the proposed algorithm.

Fig 12: Comparison of the Efficiency of TCP [6] and Modified TCP proposed

Conclusion• Vertical handover and different decision algorithms are being

studied. Knowledge from the prior analysis is used to understand the architecture of Wireless Heterogeneous Network and is also helpful in developing a simulation environment for vertical Handoff. Basics of TCP and Characteristics of TCP over a wireless environment are being analyzed.

• The problem regarding the TCP data transmission over a vertical handover situation is being acknowledged and a suitable solution to this problem based on cross-layer optimization is being proposed.

• The proposed algorithm shows an improvement on TCP performance on a wireless network while performing a vertical handover.

Future Work

In this thesis, cross layer optimization is being to acknowledge the problem of vertical handover and TCP performance. Here ,cross-layer optimization is done between MAC-Transport Layer. Such optimizations can be done using other layers of protocol stack. Network Layer can be a valid candidate for such optimizations. Mobile IP can be also be included for optimization as a part of future work.

RELATED WORK• In the first article [1] the requirements of a vertical handoff from the

literature surveyed. The evaluation of the existing work is also being done on the basis of required parameters for vertical handoff.

• Handoff Decision algorithm are described in article [2],[3].• The fourth article [4] does a comparative study between different

decisions algorithms .• Predictive algorithms are described in article [5],[6].• The paper [6] describes a “RSS based predictive algorithm” which

is used to take handoff decision depending upon the received signal strength.

• TCP performance in a Wireless environment are being described in the Article [10], 11], [12].

• Cross layer optimization is a vital technique used in this thesis and it is described in Article [15]. In Article [6], a cross layer approach for the TCP performance improvement is described

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