國立臺灣大學資訊工程學研究所碩士論文
指導教授：周承復博士
一種在異質網路中
對於垂直無縫交遞的聰明決策模組
An Intelligent Decision Model for Seamless Vertical
Handoff across Heterogeneous Networks
研究生：蕭偉成 撰
學號：R93922022
中華民國九十五年七月
Abstract
In recent years, network technologies such as WLAN, GPRS, and 3G are getting
more and more popular.
There is high probability that signal coverage of these
coexisting networks overlaps with each other.
For a user roaming among these
networks, it is desirable that his or her mobile device can have anytime connectivity, and
therefore vertical handoff becomes an important issue.
To achieve high performance, a user may want to choose the best network to
connect to. However, properties such as signal coverage area, maximum data rate,
monetary cost, and power consumption on end devices vary largely among these
networks. Moreover, different types of services require various combinations of these
properties.
While most vertical handoff schemes are based on single optimization
policy in which service difference is not taken into consideration, we propose a
service-based handoff decision model.
Through this decision model, weight of the
parameters in the cost function is adjustable based on the type of service in use, and
therefore we can make a more appropriate decision on network selecting.
In recent research, mobile IP is a generally used handoff scheme. However, to
facilitate vertical handoff, help from a home agent and a foreign agent is required in the
mobile IP scheme, which induces overloading on the agents themselves. In our handoff
scheme, only with mobile host itself can handoff be realized, hence the overall handoff
scheme is simplified.
Moreover, through our implementation, handoff latency can
probably be even shorter than that in the mobile IP scheme.
1
中文摘要
近年來，如 WLAN、GPRS、3G 等的網路技術越來越普及。這些網路的信號範圍彼
此互相覆蓋的機率相當高。對於漫遊在這些網路之間的用戶而言，他們希望不管漫
遊到哪個網路中，都能夠隨時隨地連接上線。因此，垂直無縫交遞成為一個重要的
問題。
為實現高性能，使用者會希望能選擇最好的網路來連接。 然而，如信號複蓋範
圍，最大資料傳輸率，價格，耗電量等特性，在不同的網路之間，會有很大的差異。
此外，各種不同類型的服務，需要這些特性的不同組合。 雖然大多數對於垂直交遞
是基於單一政策來優化服務，並未考慮不同服務間的差異。因此，我們提出了一個
以服務差異為主的交遞決策模式。通過這樣的決策模式，使用者可以基於所使用服
務的不同來調整估價函數中各個參數的比重。因此我們可以更恰當的方式來選擇所
要用的網路。
在最近的研究中，大部分都是用 mobile-IP 來實現垂直交遞。然而，在 mobile-IP
裡，需要透過自家網路的代理伺服器與外界網路代理伺服器的幫助，才能實現垂直
交遞，而這樣的模式會造成代理伺服器負荷過重的情況。而在我們的垂直交遞計畫
裡，只需要靠使用者自己的移動裝置，就能實現垂直交遞，因此簡化了整個垂直交
遞所需的步驟。此外，我們透過實作讓我們的垂直交遞能在比 mobile-IP 更短的時
間內完成。
2
致謝
能完成論文，順利畢業，首先我要感謝我的指導教授－周承復老師。在這兩年
間，在我的研究題目上，老師提供了不少寶貴的意見。另外，老師也提供了自己的
筆記型電腦來幫助我進行論文實驗，特此感謝。除了周承復老師外，我還要感謝中
研院資訊所的陳伶志博士，它提供了這次研究的主要構想，屢次幫助我解決研究上
所遇到的瓶頸，而且幫忙我準備了實驗所需的器材，著實給了我相當大的幫助。對
於實驗所需器材的籌措，我還要感謝靖茹學姊，幫我採購了無線網路基地台及無線
網路卡，讓我能更順利地進行實驗。
在我論文的前半段裡，有一半的概念以及實作，是靠著我同學－唐士軒的幫忙
才能完成，如果沒有他，我沒辦法順利完成論文的前半段，更遑論繼續後半段的研
究，在此感謝唐士軒同學。
我還要感謝佳蓉，文惠以及其他組上的成員，謝謝你們在這段期間幫了我許多
大大小小的忙，聽我抱怨，幫我舒解壓力，陪我走過這兩年的碩士生涯，謝謝你們。
最後，我要特別感謝我的朋友－劉恩菡。感謝妳提供了在英文寫作上的專業能
力，讓我的論文寫作可以更順利。謝謝妳！
蕭偉成
謹識
中華民國九十五年 七月
3
TABLE OF CONTENTS
Page
ABSTRACT....................................................................................................................... 1
TABLE OF CONTENTS ................................................................................................. 4
LIST OF FIGURES .......................................................................................................... 6
CHAPTER 1
INTRODUCTION................................................................................. 7
1.1
VERTICAL HANDOFF ................................................................................................ 7
1.2
RESEARCH ISSUES .................................................................................................... 8
1.3
OUR APPROACH ....................................................................................................... 9
1.4
THESIS ORGANIZATION .......................................................................................... 10
CHAPTER 2
2.1
RELATED WORK ............................................................................. 11
RELATED WORK OF VERTICAL HANDOFF .............................................................. 11
2.1.1
Network Layer Solutions........................................................................... 11
2.1.2
2.1.3
Transport Layer Solutions ........................................................................ 12
Upper Layer Solutions .............................................................................. 12
2.2
RELATED WORK OF HANDOFF DECISION ............................................................... 13
2.3
RELATED WORK OF NETWORK MONITORING ........................................................ 15
CHAPTER 3
SEAMLESS VERTICAL HANDOFF .............................................. 16
3.1
BASIC CONCEPT ..................................................................................................... 16
3.2
IMPLEMENTATION DETAIL ..................................................................................... 18
CHAPTER 4
4.1
INTELLIGENT DECISION MODEL.............................................. 21
COST FUNCTION ..................................................................................................... 21
4
4.1.1
4.2
VERTICAL HANDOFF POLICY ................................................................................. 25
4.2.1
CHAPTER 5
5.1
5.2
Throughput Estimation ............................................................................. 24
Stability Period and Emergency Handoff ................................................. 27
PERFORMANCE EVALUATION................................................... 28
HANDOFF LATENCY ............................................................................................... 28
5.1.1
Experimental Environment ....................................................................... 28
5.1.2
Experimental Result .................................................................................. 31
AVERAGE THROUGHPUT ........................................................................................ 32
5.2.1
Experimental Environment ....................................................................... 34
5.2.2
Experimental Result .................................................................................. 34
CHAPTER 6
CONCLUSIONS AND FUTURE WORK ........................................ 37
6.1
CONCLUSIONS ........................................................................................................ 37
6.2
FUTURE WORK ...................................................................................................... 38
BIBLIOGRAPHY ........................................................................................................... 39
5
LIST OF FIGURES
Figure 1. Basic concept of the system design in the OSI network architecture................ 17
Figure 2. When handoff occurs, an ARP message is sent out to update the ARP table on
the gateway. .............................................................................................................. 19
Figure 3. The scenario of our vertical handoff policy ...................................................... 26
Figure 4. The changes of FTP throughput and sequence number before and after handoff
in the first experiment. .............................................................................................. 29
Figure 5. The changes of FTP throughput and sequence number before and after handoff
in the second experiment. ......................................................................................... 30
Figure 6. (a) The experimental environment (b) Scenario of the background traffic....... 33
Figure 7. The average throughput of different decision policies under different
background traffic..................................................................................................... 35
6
Chapter 1
Introduction
With the popularity of many types of networks, issues of vertical handoff are getting
more and more important. In this thesis, we present an intelligent decision model for
vertical handoff.
Different from most decision policies, differences of service type are
taken into consideration to optimize overall performance in a more precise fashion. In
this chapter, we first give an introduction of the vertical handoff.
Second, problems to
be solved are discussed, and then we will give a brief view of our solution in next section.
1.1 Vertical Handoff
Nowadays and in the future, portable devices tend to be equipped with multiple
network interfaces. On most laptops, wired and wireless LAN NICs have become basic
components. In addition, there are also some popular techniques such as Bluetooth and
general packet radio service (GPRS) supported by some mobile devices.
If these
devices are to connect to various networks and to perform a continuous data transmission
whatever location they roam to, technique of vertical handoff is needed.
That is,
different from horizontal handoff in which handoff occurs among networks of the same
type, in vertical handoff, handoff can occur among networks with different types. As to
vertical handoff, two important issues – seamlessness and handoff decision – are
7
discussed in most papers. “Seamlessness” is achieved in the sense that handoff is
unnoticeable. The handoff decision is to decide which network(s) to connect for a
mobile device in a service region of multiple networks.
Mobile-IP is a general solution for vertical handoff. With the help of agents or
handoff servers, a mobile host is able to roam among networks with different link-layer
schemes. All data that are sent or to be received by the mobile host are relayed by these
agents. However, agents or handoff servers are essential for the mobile-IP scheme. In a
huge network, these agents may suffer from overloading. In addition, to achieve
seamlessness, some buffering mechanisms are required, and more traffic will then be
introduced into the local network.
1.2 Research Issues
When talking about decision making in the field of vertical handoff, the most
important issue is: how to define a handoff policy that can lead to the best overall
performance?
It is a simple policy that a mobile host just selects a network in which the
highest throughput can be provided.
Unfortunately, this policy ignores many other
metrics that contribute the same extent of influence as throughput. These metrics include:
(1) the power consumption of using a network device, (2) the monetary cost we have to
pay in connection with a network, (3) the delay of packets passing a network, (4) the
handoff latency, (5) the maximum achievable throughput, and (6) the connection stability
and reliability of a network. In the policy design, it is necessary to take these metrics into
consideration. Another important viewpoint is that different types of services may put
8
different levels of emphasis on these metrics, and therefore only one fixed policy may not
be able to well meet the diversity of resource requirement of a variety of applications. A
more flexible handoff policy is needed.
1.3 Our Approach
We propose a data link layer solution to vertical handoff. In contrast with the
disadvantages of the mobile-IP scheme, our solution is more elegant. Our system can be
applied to current IP network without any modification of the network layer. In our
system, moreover, only with a mobile host can vertical handoff be realized; that is, help
of agents or handoff servers is not required. All the handoff processes are accomplished
by our modified Linux kernel. Moreover, because all processes are run in the kernel level,
the handoff latency can be significantly short, i.e. less than 9ms. Even if without the
mechanism of buffering, our system can achieve the “seamlessness” property in a very
simple fashion.
Another important issue is the handoff policy. In our work, the metrics involved in the
proposed decision model include: (1) the power consumption of using a network device,
(2) the monetary cost we have to pay in connection with a network, (3) the maximum
achievable throughput, and (4) the connection stability and reliability of a network. We
don’t take the metric of handoff latency into consideration because we build the function
of vertical handoff into the Linux kernel, and therefore the handoff latency will always be
very short. Moreover, we add the concept of service diversity into our handoff decision
model in which weights in the proposed cost function are adjustable for various resource
9
requirements, and then the overall performance can be optimized.
1.4 Thesis Organization
The rest of the thesis is organized as follows. In chapter 2, we give related works of
the vertical handoff system and the decision policies. The implementation of our seamless
vertical handoff system is presented in chapter 3. Then, we discuss our intelligent
decision policy in chapter 4. In chapter 5, we show results of our experiments on the
seamlessness and intelligence of the system and policies. Finally, we conclude our thesis
in chapter 6.
10
Chapter 2
Related Work
2.1 Related Work of Vertical Handoff
Recently, research of vertical handoff is getting more and more popular, especially
on the topics of interworking between WLAN and cellular networks – GPRS or the
CDMA-series [1][3][4][5][6][7][8][9][11]. Through the interworking between WLAN
and cellular networks, people having mobile devices can connect to the Internet via
WLAN when they are near 802.11 hotspots and via cellular network in most areas. There
are still some papers [1][7] talking about other types of network such as Bluetooth,
Infrared in-room LAN, in-building WaveLAN, Metricom Ricochet, EDGE, and GSM
cellular network.
2.1.1 Network Layer Solutions
The handoff problem in IP networks can be considered as a special issue of the
broader mobility management problem. Among network layer solutions, mobile IP is the
most general solution [1][3][4][7][8]. In mobile IP, there are home and foreign agents
running on different networks. Every mobile host roaming among these networks has it
own home address and care-of-address. The former address is to identify which network
11
the mobile host belongs to while the latter is related to the network in which it is
currently located. When a mobile host is roaming to a network with physical link
property different from current network, vertical handoff occurs. Then, the home agent
relays all the data packets intended for the mobile host to the foreign agent through an IP
tunnel (IP-in-IP encapsulation) established between these two agents. It is an intuitive and
simple way to migrate the idea of mobile IP to vertical handoff. However, it needs help of
the agents to keep the data transmission, and this may cause overloading on the agents.
2.1.2 Transport Layer Solutions
Besides network layer solutions, there are also several handoff solutions using
transport layer approaches [19][20]. [19], [20] use multi-homing technique to support
vertical handoff. Through the multi-homing technique, a mobile host may connect to
multiple networks simultaneously, and then communicates with a remote host via these
networks. However, deploying these systems requires upgrading both transport layers and
applications on both mobile hosts and Internet servers. Therefore, the deployment cost is
too high to be feasible.
2.1.3 Upper Layer Solutions
There are still some solutions that introduce proxy-like servers to realized vertical
handoff. In USHA [11], all mobile hosts connect to the Internet with the help of a handoff
server, which is equipped with multiple network interfaces with heterogeneous physical
properties. Hence, mobile hosts can communicate with the handoff server in various
12
physical connections. The IP tunneling technique (IP encapsulation) is used in USHA
with the handoff server functioning as one end and the mobile host as the other. Upper
layer communications are bounded to a virtual interface – the tunnel interface, instead of
physical interfaces. All data packets are transmitted through this IP tunnel. When the
handoff event occurs, the underlying physical connection of the virtual tunnel is
automatically switched to the new physical interface. In the meantime, the mobile host
also notifies the handoff server of its change in physical connection. However, USHA
tries to keep the upper layer TCP connection during the vertical handoff, and then the
data packets can only be transmitted using UDP connections between mobile hosts and
the handoff server. This causes new problems. In addition, similar to the mobile IP
scheme, the handoff server may be overloaded as the network scale is getting large.
2.2 Related Work of Handoff Decision
It is a general fashion to give a decision algorithm or policy on the topic of vertical
handoff decision making. In handoff decision making, many metrics are considered and
even quantified to give an indication of which network to handoff to and whether or not a
handoff is needed. The metrics suggested in previous research [1][3][6][7][9][10]
include:
z
Link Capacity. The link capacity directly reflects the theoretical upper bound of
throughput that can be reached in the network.
z
Link Quality. The overall link quality may be based on the channel modulation
scheme, the level of contention or interference, signal-to-noise ratio (SNR), the
13
bit error rate (BER), path loss, etc. The higher link quality, the lower difference
between the achievable throughput and the link capacity.
z
Network Delay. For real-time application, the network delay is an important
issue.
z
Handoff Latency. The handoff latency can also include the time to bring up the
new network interface, if needed. Lower handoff latency induces fewer packet
drops during the handoff process.
z
Monetary Cost. This is a major consideration to users. Different networks may
employ different billing strategies, which may affect the user’s choice to
handoff.
z
Energy Consumption. Life time of a mobile device is an important issue.
Limited battery power of mobile devices requires deliberated selection of the
communication channel.
z
User Preference. User preference reflects special requests for users that prefer
one type of network to another.
As to the policy specification model – the cost function – in each of the previous
research, a linear combination of the quantified metrics and the weights of these metrics
is defined to represent the total cost of using a network. These weights can be adjusted
based on the user’s requirements and the importance diversity of the metrics.
The work in [1] takes logarithm on each parameter in its cost function. This
mathematical technique prevents the unit diversity among various parameters and
preserves the order of resulting cost values.
The work in [6] adds the factor of service type into its policy model. It checks
whether the constraints of running a service can be satisfied when calculating the cost
14
function. If a network cannot meet the required constraints, it will not be chosen to be the
new network when vertical handoff is being performed. This idea of considering service
diversity is also adopted in our work.
2.3 Related Work of Network Monitoring
When talking about decision making, the metric of maximum available bandwidth is
mostly considered in previous works. However, it is difficult to accurately measure the
maximum available bandwidth, especially in a wireless environment. Traditional
bandwidth measuring tools send probing packets to the network to estimate the
achievable throughput. However, this method is intrusive to the network since they
introduce overhead traffic. Moreover, it also consumes battery energy, which is precious
to most mobile devices. On the other hand, some research [13][15] proposed
non-intrusive methods to estimate available bandwidth of a 802.11 wireless channel. The
bandwidth value is derived from the measured SNR, the channel modulation scheme, and
some network properties. In our work, a simpler method is applied to monitor the
network because all we have to do is to choose a better network, not necessarily to get the
precise value. We will discuss the details later.
15
Chapter 3
Seamless Vertical Handoff
In this section we describe the basic concepts and the implementation details of our
vertical handoff system.
3.1 Basic Concept
For a user who wants to move across heterogeneous networks and still maintain all
active connections, an auto handoff mechanism is needed to change the communication
path and the physical interface in use. This mechanism also has to address the issue that
handoff can be achieved without changing the IP address of the mobile host; that is, all
handoff processes can be completed below the network layer in the OSI network
architecture. Then, under an infrastructure network, the gateway should be notified of the
handoff events so that it can adjust the communication path for incoming packets.
Moreover, the handoff mechanism should be user unaware, i.e., seamless, so that the
handoff process can be completed in an elegant fashion.
The basic concept of our system design is shown in Fig. 1. We introduce a virtual
interface layer between the network layer and the data link layer and integrate this layer
into the Linux kernel. The virtual interface layer includes two components – the virtual
interface and the kernel interface selector. The virtual interface is assigned a unique and
16
static IP address. Additionally, it’s the only network interface that upper layer applications
can see. All kernel routing rules are associated with the virtual interface. The kernel
interface
selector
is
responsible
for
deciding
which
Application Layer
···
Network Layer
Virtual
Interface
Layer
Virtual Interface
Kernel Interface Selector
Data Link Layer
Ethernet
Bluetooth
802.11
···
Figure 1. Basic concept of the system design in the OSI network architecture.
physical interface that should be used in reality to send and receive data packets. In
packet transmission, all packets generated from the applications are directed to the virtual
interface and encapsulated with an IP header, which contains the IP address of the virtual
interface. Then, these packets are forwarded to one of the physical interfaces, according
to the selection result of the kernel interface selector. Finally, they are sent out via the
selected physical interface. In packets reception, packets received from a physical
interface are forwarded to the virtual interface, decapsulated the IP headers, and then
passed to upper layers. Actually, all the kernel interface selector does is to get the result
from the intelligent decision model, which will be introduced in section 4, and to change
17
the way of packet forwarding between the physical and virtual interfaces.
We note that our work is a home network solution; that is, all the networks are
located at the same home or building, and these networks connect to the Internet via the
same gateway. Extending our work to an Internet solution is the future work.
3.2 Implementation Detail
In this section, we describe the implementation details in vertical handoff. Let’s
introduce an example to describe the details. Fig. 2 illustrates an example of a mobile
host (sender) communicating with a receiver on the Internet via a gateway. The mobile
host is equipped with two physical interfaces, which have their MAC address MACold
and MACnew. Additionally, as mentioned in the previous section, a virtual interface is
created on the mobile host, and an IP address – IPs – is assigned to the virtual interface. In
the beginning, the mobile host uses the old physical interface to communicate with the
receiver. All packets sent out contain the MAC address MACold and the IP address IPs in
their headers, which are identifiers for the gateway and the receiver respectively. At this
time, the ARP table maintained on the gateway contains an entry with IPs mapped to
MACold. Therefore, after receiving ACKs from the receiver, the gateway can relay the
ACKs back to the correct mobile host. When the decision model decides to perform a
handoff, the device name (ex. eth0 in Linux) of the new selected interface is then written
to a file in the /proc filesystem, which is the medium user processes communicate with
kernel processes. Then, the action of writing the /proc filesystem triggers the event of
changing the default physical interface in kernel. The kernel then immediately uses the
18
IPs
MACold
MACnew
Sender
Gateway
Data P
Receiver
ac
ket
( source
.IP = IP
s,
source.M
AC = M
ACold )
Data P
acket
ACK
IPs,
)
t.IP =
MACold
( t a rg e
=
C
A
M
target.
ACK
t.IP =
(Targe
IP s )
handoff occurs
ARP
(source update
.IP
source.M = IPs,
AC = M
ACn
ew)
Data
(source.I Packet
P=
source.M IPs,
AC = M A
Cnew)
ACK P s,
IP = I
)
t.
MACnew
( t a rg e
=
C
A
M
target.
Data Pac
ke t
ACK
t.IP
(Targe
= IP s )
Figure 2. When handoff occurs, an ARP message is sent out to update the ARP table
on the gateway.
19
new physical interface to broadcast an ARP-update message to all the first-hop hosts,
including the gateway. As soon as receiving an ARP-update message, the gateway
updates its ARP table with IPs mapped to MACnew. After that, all packets on the gateway
to be received by the mobile host are encapsulated a link-layer header with MAC address
MACnew, and then these packets can be correctly received.
According to our real-world experiment, the handoff latency – timestamp difference
between the two consecutive packets in which the MAC address is MACold in one packet
and is MACnew in the other – is very short, i.e., less than 9ms.
20
Chapter 4
Intelligent Decision Model
4.1 Cost Function
The vertical handoff cost function is a measurement of the benefit obtained by
handing off to a particular network. The network choice that results in the lowest cost
value provides the most benefit to the user.
The cost of using a network n at a certain time is a function of several parameters:
the energy consumption of using the network device (En), the monetary cost of using this
network (Pn), the achievable throughput in this network (Bn), and the stability of the
network (Sn). In addition, the cost function should be able to reflect differences of
services because different types of services require various combinations of the
parameters above.
Costn = ∑ f v ( En , Pn , Bn , S n )
v
(1)
Energy consumption (En) and monetary cost (Pn) are parameters with fix budgets.
They directly reflect the battery life of mobile devices, and money a user is willing to pay
for his or her network usage. The value En of wireless devices can be obtained from the
iwconfig command in Linux, and unit of this value is dBm. For wired Fast Ethernet
21
devices, we can assign En a very small value. The parameter Bn estimates the maximum
throughput that can be achieved currently in network n. This is also what a majority of
users are most concerned about. It is important and also difficult to estimate the value Bn
correctly. We will discuss this problem in section 4.1.1. Parameter Sn represents the
stability of a network, including the variation of signal strength in wireless networks and
the reliability of using the network. In our work, for 802.11 wireless networks, we take
the standard deviation of SNR (signal-to-noise) for the value of Sn. For general networks,
Sn is the percentage of the time the network is unavailable in a constant period t.
The cost function of our decision model is listed in Equation 1, 2, and 3. v is the
index representing the user-requested services. As the index of service is v, we,v, wp,v, wb,v,
and ws,v are weights of each parameter, which sum to 1. uv is the coefficient of Cv,n, and it
represents the user-specified priority and the importance of service v among all service
types. All uv in different v also sum to 1. For parameters that are not of concern to the user,
he or she can set those weights to 0. Furthermore, these weights, u or w, can be adjusted
by the user or the system at run-time. For example, when the battery of the user’s mobile
device is running out or when the expenditure is approaching the limit, weight of energy
consumption and monetary cost (we and wp) should be increased to reflect these
conditions. In addition, for a user often run video streaming applications, throughput and
stability become the major concerns, and then he or she should raise the value of wb and
ws, and the weight u of the real-time streaming service.
f v = u v Cv , n
(2)
22
1
⎧
C
w
ln
E
w
ln
P
w
ln
=
⋅
+
⋅
+
⋅
+ ws ,v ⋅ ln S n
e ,v
n
p ,v
n
b ,v
⎪ v ,n
Bn
⎪⎪
⎨we,v + w p ,v + wb ,v + ws ,v = 1,∀v
⎪
⎪∑ uv = 1
⎪⎩ v
(3)
As the units of the parameters are different and their values may vary a lot, normalization
is then needed to ensure that the sum of the weighted values is meaningful. Here we use
logarithm to eliminate the unit differences among these parameters and to achieve
normalization at the same time. We show this as follows:
Cv ,1 = we ,v ⋅ ln E1 + w p ,v ⋅ ln P1 + wb ,v ⋅ ln
1
+ ws ,v ⋅ ln S1
B1
Cv , 2 = we,v ⋅ ln E2 + w p ,v ⋅ ln P2 + wb ,v ⋅ ln
Cv ,1 − Cv , 2 = we,v ⋅ ln
1
+ ws ,v ⋅ ln S 2
B2
E1
P
B
S
+ w p ,v ⋅ ln 1 + wb ,v ⋅ ln 2 + ws ,v ⋅ ln 1
E2
P2
B1
S2
(4)
When we compare the Cv value of two networks, subtraction of values in logarithm of
these parameters are then transformed into division, which helps eliminate the units of
these parameters and leave the ratio only.
To ensure the value of each parameter being larger than 0, we can add a small
constant to the original value and do not affect the result a lot.
23
4.1.1 Throughput Estimation
It’s not easy to measure or to estimate the maximum available bandwidth of a
network link, especially in a wireless environment. Instead of directly measuring the
maximum available bandwidth of each network, we use a simpler method to evaluate the
parameter Bn. We periodically trigger the vertical handoff procedure, hand over all
current traffic onto each network device one by one, and then fetch current throughput
value of each network device. We assume that the latency of performing handoff on a
round of all network devices is so short that the transient throughput does not vary a lot.
In Linux, the current throughput value of each network device can easily be gotten
through the ifstat command.
Although the throughput value from the ifstat command may not equal to the actual
available bandwidth – it is always less than or equal to the value of available bandwidth,
it can represents the upper bound or lower bound of the available bandwidth of each
network. The highest throughput value is the lower bound of available bandwidth of the
networks which get this value, and it is also the current bandwidth usage of the mobile
device. In our work, we take the Bn value of these networks the same. Other networks
which get their throughput less than this value reach their bandwidth upper bound; that is,
the value they get is exactly the available bandwidth of them.
Unlike other bandwidth measurement methods, injecting probing packets into the
network is not needed in our work. Only some ARP update messages are generated as
vertical handoff occurs in the periodic throughput measurement. Therefore, our system
induces much fewer overhead than other system does. However, frequent throughput
measurement can also induces some overhead and probably cuts down the overall
24
performance because we also have to handoff to networks with lower available
bandwidth and measure their throughput. To solve this problem, we propose a mechanism
to dynamically adjust the measuring period.
In fact, for wireless networks, there is high correlation between the SNR value and
the achievable throughput. Therefore, we can change the period dynamically based on the
extent the SNR value varies. If the SNR value stays almost static, we can prolong the
measuring period. On the other hand, if the variance of the SNR value is high, it probably
means that the mobile device is moving, and we should measure the throughput more
frequently. Under this condition, frequent measurement is not avoidable.
As to wire networks, the link quality is much more stable than wireless networks, so
the period of measurement can be long.
4.2 Vertical Handoff Policy
Fig. 3 shows the scenario of our vertical handoff policy. As mentioned in previous
sections, the value of Bn is calculated every measurement period. Every time after
throughput measurement, we gather and calculate all the values of the parameters,
evaluate the cost function of all network interfaces, and then decide whether the vertical
handoff is needed and which interface to handoff to based on the results of cost function.
However, there is still one factor that influences the handoff decision – the stability
period.
25
update information of
all interfaces
evaluate cost function of
all interfaces
Yes
emergency
handoff ?
No
any better
networks found?
No
Yes
++stability_count
stability_count reaches
the threshold ?
--stability_count
if stability_count > 0
No
Yes
do vertical handoff,
set stability_count to 0
wait for a measurement period
Figure 3. The scenario of our vertical handoff policy
26
4.2.1 Stability Period and Emergency Handoff
In our system, the vertical handoff occurs not when a network is better than the
current one in use, but when a network is “consistently” better than the current one in use.
If a mobile user only transiently handoff to a better network, the gain from using that
network may be diminished by the handoff overhead and the short usage duration.
Therefore, we define a stability period to avoid this condition. Only if a network is
consistently better than the current one for a stability period does the mobile host perform
vertical handoff. In our work, the stability period is defined as a stability threshold times
the measurement period. As shown in Fig. 3, a variable – stability count is defined. Once
a network is better than the current one, the stability count is increased by 1, otherwise it
is decreased by 1. When the stability count reaches the stability threshold, vertical
handoff is performed, and the stability count is set back to 0. However, there is an
exception. If current network in use suddenly becomes unavailable or if the signal
strength is very weak, handoff should be performed immediately. In this condition, we
choose the network with the lowest cost value to handoff to. We name this emergency
handoff.
27
Chapter 5
Performance Evaluation
5.1 Handoff Latency
In this section, we use two experiments to illustrate how our virtual-interface
scheme (mentioned in section 3) helps TCP connections smoothly and seamlessly
handoff from one network to another. Although we test TCP connections only, other types
of connection can still benefit from our virtual-interface scheme.
5.1.1 Experimental Environment
We run the experiments on a laptop with Intel Pentium M 1.73 GHz CPU and
512MB Memory. The operating system of the laptop is Debian Linux with kernel version
2.6.12.6. The laptop is equipped with a PCI Gigabit Ethernet NIC, a D-Link
DWL-AG660 802.11 a/b/g Wireless NIC, and an Intel PRO/Wireless 2200BG NIC. We
also setup two 802.11 access points – one is Linksys WAP55AG 802.11a AP and the
other is Buffalo 802.11b/g AP.
In the following two experiments, we build a FTP connection between the laptop
and a FTP server, which is located in the same LAN as the laptop. Then, we download a
file via the FTP session and use ethereal to observe the changes of transient throughput,
28
(a) TCP Sequence Number
(b) FTP Throughput
Figure 4. The changes of FTP throughput and sequence number before and after handoff
in the first experiment.
29
(a) TCP Sequence Number
(b) FTP Throughput
Figure 5. The changes of FTP throughput and sequence number before and after handoff
in the second experiment.
30
the fashion TCP sequence number increases, and the handoff latency. In the first
experiment we test the vertical handoff from the Intel 2200BG wireless device to the
gigabit NIC wire device while in the second experiment the vertical handoff is performed
from the Intel 802.11g wireless device to the D-Link 802.11a wireless device.
5.1.2 Experimental Result
The experimental results are shown in Fig. 4 and 5. In the first experiment, the
vertical handoff occurs at around 5.0s. The average throughput is around 3000KB/s
before handoff while it becomes 11,000KB/s after handoff. This is because the interface
in use after handoff has changed to the gigabit NIC. As the vertical handoff occurs,
moreover, the transient throughout does not drop, and the curve of sequence number does
not stop increasing either. Therefore, we can see from Fig. 4 that the TCP connection is
not broken and that the data transmission continues after handoff.
The results of the second experiment are similar to the first one. In this experiment,
the handoff occurs at around 7.3s. Because the distance between the laptop and the AP is
longer than that in the first experiment, the throughput varies more. Due to the inherent
hardware property of the two APs and the protection mode of the 802.11g AP, the average
throughput of the 802.11a and the 802.11g network has difference about 1000KB/s.
From these two experiments, we can see that the handoff latency is very short either
handoff is performed between one wired device and one wireless device or between two
wireless devices. According to the dumped information, the handoff latency is less than
9ms, which can be neglected.
31
5.2 Average Throughput
In this section, we design an experiment to estimate the average throughput when
the FTP session handoff from an 802.11a network to an 802.11g network. We compare
our intelligent decision policy with other three policies and show that our intelligent
decision policy performs the best. The three policies for comparison include:
A.
Handoff by SNR – Instead of calculating the cost function, the SNR value is
used directly in this policy to decide which interface to handoff to. The policy
always chooses the network with larger SNR value to connect to. The SNR
value can be gotten from the iwconfig command in Linux.
B.
Handoff by Link Quality – Link quality is the overall quality of the link. It is
based on the level of contention or interference, the BER (bit error rate) or FER
(frame error rate), the RSS (received signal strength), some timing
synchronization, or other hardware metric. This is an aggregate value, and
depends totally on the driver and hardware. This value ranges from 0 to 100,
and we can get this value from the iwconfig command too. Similar to A., we use
the value of link quality only to decide when the handoff should be performed.
This policy chooses the network with better link quality.
C.
No Handoff – In this policy, we do not trigger handoff. The laptop always
connects to the 802.11a network until the data transmission ends. This policy is
taken as the baseline only. It introduces the lower bound of average throughput
for comparison.
Due to limits of the experimental environment, the metric we can use on
32
AP - 802.11g
A
B
9m
AP - 802.11a
36 m
(a) The experimental environment
FTP Server
Mobile Host
LAN
Switch
UDP
Background
Traffic
UDP Traffic
Generator
802.11g AP
802.11a AP
UDP Traffic Receiver
(b) Scenario of the background traffic
Figure 6. (a) The experimental environment (b) Scenario of the background traffic
33
performance comparison is the average throughput only. However, we believe that our
decision policy can outperform others not only on average throughput but on other
metrics.
5.2.1 Experimental Environment
Fig. 6 shows the environment of the experiment. We setup the two APs mentioned in
section 5.1.1 in two separate rooms. The laptop in 5.1.1 is also used in this experiment.
We test the average throughput of the four policies under different background traffic – 0,
400, 800, 1200, and 1600 KByte/s. The background traffic is generated by sending
constant bit-rate UDP packets using the D-ITG [17] traffic generator. For each setting of
background traffic in each policy, we run the tests for 5 times to estimate the average
throughput, and then calculate the mean. In each test, we take the laptop from point A to
point B (as shown in Fig. 6) at a constant moving speed and download a file with size
about 230 MBytes from the FTP server simultaneously. As the policy is No Handoff,
however, the moving area is restricted in the signal range of the 802.11a AP.
5.2.2 Experimental Result
The experimental result is shown in Fig. 7. From the char we can see that our
intelligent decision policy (Handoff-Cost) outperforms the other three ones because our
policy always chooses the best network to connect to.
As expected, the policy – No Handoff performs worst because it does not select a
better network when the signal strength is getting weaker.
34
As to the Handoff by SNR policy, the SNR value cannot perpetually reflect the
relative goodness of the throughput, especially when the SNR values of the two networks
are getting close. In fact, through the real-world test, the 802.11a network can bring
higher throughput than the 802.11g network when their SNR value are equal. Moreover,
if background traffic exists, the SNR value drops only a little, while the actual throughput
drops more. In other words, the throughput decline resulting from the background traffic
cannot completely reflect to the drop of the SNR value.
The performance of the policy Handoff by Link Quality is not good because the link
quality value of the 802.11a NIC is always much smaller than that of the 802.11g NIC
when their actual qualities are equivalent. Note that we have mentioned previously that
Figure 7. The average throughput of different decision policies under different
background traffic
35
the value of link quality depends on the driver and the hardware. As the background
traffic is getting larger, however, there are only few declines on the average throughput.
This is because the link quality value of the 802.11a network becomes smaller than the
802.11g network before long after the laptop starts moving, and then the occurrence of
the vertical handoff is early. Therefore, the average throughput is almost not influenced
by the background traffic.
36
Chapter 6
Conclusions and Future Work
6.1 Conclusions
In this thesis, we have presented a seamless vertical handoff system across
heterogeneous networks. We also implemented the system in current Linux operating
system. In our system, the handoff latency is much shorter than other current systems
focusing on the issue of vertical handoff. Different from other vertical handoff systems,
handoff agents or servers are not required in our system. Only with the mobile host itself
can the vertical handoff be realized. Moreover, the overhead during handoff is also quite
low. Besides, we also presented an intelligent decision policy, in which the user can
adjust the weights of the cost functions to optimize the overall performance based on
their preference and requirements. In addition, the cost functions are designed in
consideration of the service diversity and are able to meet the requirements of different
users. We also addressed the issue of system stability: we used a stability period to ensure
that a handoff is meaningful. We evaluated our system through a series of experiments.
The experimental results demonstrated the transience in handoff latency and significant
gains in the metric of average throughput.
37
6.2 Future Work
We have tested our system in the real-world analyses. However, we only evaluated
the average throughput currently, not other metrics, namely power consumption and
monetary cost, which are important as well. We hope we can do the experiments under
more different environments and evaluate our system in more metrics. Moreover, our
work is a home network solution only, and we will extend it to an Internet solution in the
future.
38
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