WEB TRAFFIC ACCELERATION OVER CELLULAR
NETWORKS BY USING A COMPRESSING PROXY SERVER
Yair Toaff and Ariel J. Frank
Department of Computer Science, Bar-Ilan University, Israel
Keywords: Internet, cellular, proxy, compression, Performance Enhancing Proxy (PEP), GSM, HSCSD
Abstract: Many Web clients today are connected to the Internet via low speed computer links such as cellular
connections. In order to efficiently use the cellular connection for Web access, the connection must be
accelerated using a Performance Enhancing Proxy (PEP) as a gateway to the Web. In this paper we
investigate the challenges created by the use of PEP. In order to mitigate the performance bottlenecks, we
enhanced the PEP by adding to it a “pre-getting” ability. We tested the enhanced PEP over GSM and
HSCSD cellular networks, as well as using a cellular network software simulator. Our experiments with
enhanced PEP achieved the following results: 1) Average improvement of about 60% over the HSCSD
network throughput can be achieved, 2) The Web page structure has a significant impact on the resulting
performance. In conclusion, using an enhanced PEP can make the experience of browsing the Web over
cellular networks much faster and less frustrating.
1 INTRODUCTION
Cellular access to the Internet began about a decade
ago. The general characteristics of data transmission
over cellular networks are a high bit error rate when
compared to regular phone lines, frequent
disconnects, and a generally slow transfer rate when
compared to other means of data transmission. For
these reasons, effective Internet browsing over
cellular networks requires some means of
acceleration. One such mean is the use of a
compressing proxy server. The compressing proxy
server is the focus of this paper.
We will first review the two types of technical
challenges that we wish to analyze and resolve.
The first challenge is related to Internet browsing
over cellular networks in general. The component
issues are the time required to create a TCP
connection (RFC1072 1988, Stevens 1996), and the
synchronous protocol that the browser implements
(Toaff 2002). Looking at the graph of the bandwidth
used while downloading a Web page over a GSM
(Global System for Mobile Communications)
connection (Scourias 1996), we can see (Figure 1)
that the bandwidth is not fully utilized.
Figure 1: Web page download bandwidth graph
A partial solution for both these challenges is the use
of a Performance Enhancing Proxy (PEP). PEP
servers can usually provide a solution for the low
bandwidth of cellular networks by using
compression. Both client and server compress the
data transferred over the cellular link. The data is
then uncompressed at the target (Liljeberg 1996,
Brown and Singh 1997, Toaff 2002).
The second set of issues is related to the use of PEP.
When using PEP, the average actual bandwidth used
drops significantly. It would be expected that if the
compression ratio is about 1:3 – the resulting time
ratio would be similar. What we have found,
however, is that the average actual bandwidth used
drops but the time ratio obtained is only about 3:4.
This phenomenon is the result of several reasons.
The time needed for a request to reach the server
remains the same but compression times must be
added, causing a delay. There is less data to transfer
40
Yamada H. and Kawaguchi A. (2004).
A METHOD FOR THE PERFORMANCE ANALYSIS OF INTEGRATED APPLICATION SERVICES - Simulating the execution of integrated application
services coordinated by workflow-driven broker-servers.
In Proceedings of the First International Conference on E-Business and Telecommunication Networks, pages 40-45
DOI: 10.5220/0001382900400045
Copyright
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SciTePress
so the periods when data is downloaded are shorter.
In the experiments described in section 5.1, although
the size of the downloaded data is about 30% of the
original data (120KB instead of 379KB), the
download time drops only to about 50% of the
original time (160s instead of 345s). This result still
indicates inefficient use of the available bandwidth.
In the advanced cellular networks, such as HSCSD
(High Speed Circuit Switch Data) (ETSI 1997), the
problem appears to be more severe (data ratio about
1:3, time ratio about 2:3) due to the asymmetric
nature of the network (Toaff 2002).
The paper is organized as follows:
Section 2 reviews related research works.
Section 3 introduces the logical model for the
pre-getting PEP.
Section 4 describes the environment used for
the experiments (both software and
hardware).
Section 5 describes the experiments that
were carried out and the conclusions reached
from them.
Section 6 finally concludes with a discussion
and suggests additional issues that may
require further investigation.
2 RELATED RESEARCH WORKS
In this section we briefly review previous research
works in related topics.
These related topics are:
Data communication over cellular networks.
Internet browsing over cellular networks.
2.1 Data communication over cellular
networks
Several research works (Xylomenos and Polyzos
1999, Brown and Singh 1997, Vardalis 2001,
Balakrishnan 1997, Tsaoussidis and Matta 2002)
discuss the problems with data communication over
cellular networks and suggest several methods of
improving the existing TCP protocol. An important
idea mentioned in these papers is data compression
over the cellular connection but the idea itself was
not implemented and tested, and the challenges
caused by the use of compression were not
discussed.
2.2 Internet browsing over cellular
networks
Article (Liljeberg 1996) suggests a client-server
based proxy that handles the pre-getting of page
embedded objects. This idea is similar to our work;
however, content compression was not implemented.
The paper simply states that the results of
compression are easily predicted. But as implied
above, the results may not be as clear-cut as that
paper predicts.
The main problem we have found with the described
work is that it ignores new asymmetric cellular
networks architectures (such as HSCSD) and
assumes a network supplying stable bandwidth and
equal bit rate for both upstream and downstream
data transfer. In the HSCSD network we used, the
downstream bit rate is 3 times the upstream bit rate.
This influences performance since the time required
for the request to reach the server remains the same,
but the time required for the response to reach the
client decreases by a factor of 3 (in HSCSD
compared to GSM). This means that the full
utilization of the downstream channel will require
that 3 times the number of requests be sent for the
same bandwidth. This network asymmetry causes
the aforesaid problems to become more significant.
Due to these problems, pre-getting is needed to
enhance the utilization of the downstream channel
and compression must be applied to both channels.
3 LOGICAL MODEL FOR
ENHANCED PEP
The main object of our research has been the
enhancement of an existing PEP with the purpose of
resolving the problems previously encountered. We
have found that adding a pre-getting mechanism to
PEP achieves this goal.
In the pre-getting method, no speculative algorithms
are required. Instead, pre-getting evaluates and
modifies the way the browser requests information
from Web servers and how the browser validates
that information.
The pre-getting method works as follows:
1. The proxy client parses the HTML page,
obtains a list of the embedded objects,
compresses it and transmits the list of required
objects to the server.
2. The proxy server builds the HTTP requests
from the list, retrieves and compresses them,
and returns them to the client.
3. The proxy client decompresses the
responses and caches them.
WEB TRAFFIC ACCELERATION OVER CELLULAR NETWORKS BY USING A COMPRESSING PROXY SERVER
41
4. When the browser requests the elements,
they are ready for delivery, stored in the proxy
client’s cache.
The result of using pre-getting is that all the
embedded objects from a certain HTML page are
downloaded without time gaps. The idle download
time previously observed is eliminated. The
observable result is that the browser finishes
downloading the page faster.
The parameter we have used for measuring the
resulting improvement is the time required to
download a Web page. We assume that the user is
willing to “pay” for the reduced download time with
some loss in the quality of the pictures contained in
the page.
As the compression ratio can be assumed to be
constant, therefore the improvement can be
measured by the time required for the Web page to
download. Our goal in enhancing the PEP is for the
achieved time ratio to be similar to that of the
compression ratio.
4 EXPERIMENT ENVIRONMENT
This section describes the environment we have used
for our experiments. The description includes the
software used in the experiments, the various
components, and the hardware infrastructure.
4.1 Software
The general characteristics of the system are:
Use of a client/server system.
The client is connected to the server by a
cellular connection.
Both client and server support compression.
The commercial software used was the Apache
Web Server and MS Internet Explorer browser.
Client
The regular compressing proxy client is
composed of a communications module, a
compression module, and a multiplexor module in
charge of multiplexing the multiple browser
connections into a single server connection.
To the enhanced compressing proxy client we
added an HTML parser module and a cache module.
Server
The regular compressing proxy server is
composed of a communications module, a
compression module, and a multiplexer module. To
the enhanced compressing proxy server we added a
URL handler module for transforming the URLs into
full HTTP requests.
4.2 Cellular network simulator
A software simulation of a cellular link was used
due to the high cost of airtime on cellular networks
and the requirement of the cellular link to supply
constant bandwidth thus enabling the results to be
compared.
4.3 Hardware infrastructure
Two MS Windows based workstations were used for
the experiments. The communication hardware for
the experiments on the real cellular network was a
PCMCIA Nokia card phone. This card phone
supports both GSM and HSCSD network
connections.
For the simulator experiments, a 100Mb/s LAN
was used to connect the computers. The
experimental configuration is described in Figure 2.
Figure 2: Experimental configuration overview
5 EXPERIMENTS AND
CONCLUSIONS
This section describes the experiments. The first set
of experiments analyzes performance over real
cellular networks (GSM and HSCSD). The second
set of experiments used the cellular network
simulator. Following the description of each
experiment, the resulting conclusions are discussed.
5.1 Real cellular network experiments
Our experiments using the pre-getting method
were carried out as follows. A variety of Internet
sites were copied onto a local Web server (to
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42
eliminate delays caused by communication with the
content providers). Each of the chosen sites was
downloaded three times:
Without use of a proxy.
Using the regular PEP.
Using the pre-getting PEP.
For each download, 3 parameters were recorded:
1. The total amount of data.
2. The time required for download.
3. The average transfer rate.
After each download, the browser’s caches were
flushed (so the data and time totals could be
compared).
Conclusion 1. The pre-getting method improves
the compressing proxy bandwidth usage by up to
90% of the measured bandwidth with no
compression used.
As can be seen in Table 1, the pre-getting
method significantly improves the utilization of the
available bandwidth.
Table 1: Cellular downstream average transfer rate (in
bps)
Cellular transfer rate
Methods
GSM HSCSD
Theoretical
9600 43200
No compression 8765 18595
Regular PEP 6062 9830
Pre-getting PEP 7946 17121
On the GSM network, the used bandwidth
dropped by about 30% when using a regular PEP.
The use of the pre-getting PEP lowered the drop to
just under 10%.
On the HSCSD network, the used bandwidth
dropped to about 50% when using a regular PEP.
The use of pre-getting PEP lowered the drop also to
just under 10%.
The improvement in the used bandwidth brings
the resulting time ratio closer to the compression
ratio. The 10% drop in the bandwidth utilization is
due to the data passing more stages and to the
computations requiring additional time (parsing,
(de)compression).
5.2 Cellular network simulator
experiments
The experiments of the pre-getting methods in the
cellular network simulator were done in a similar
manner, except that in this case the simulator
replaces the cellular network
5.2.1 Imitating the cellular network
The first phase of the simulator experiments was to
verify that the simulation closely simulates real
network conditions, enabling reliable use of the
results. This was tested using the parameters of the
HSCSD network.
The parameter that was changed between the
experiments is protocol overhead. The overheads
that were tested included 148 bytes, 248 bytes and
348 bytes per frame.
Conclusion 2. The simulator imitates the
behavior of the real cellular network with good
precision.
It appears that simulated results based on a 23%
loss in the bandwidth utilization (overhead of 348
bytes per frame), result in an effective bit rate that is
almost identical to the HSCSD experiments.
5.2.2 Web page structure influence on the
performance
Analyzing the structure of the Web pages used we
identified that the number and size of embedded
objects in a page affects the download time. In sites
that contain a small number of pictures, (i.e.,
http://www.google.com – 2 small pictures), the time
difference between using a regular PEP and a pre-
getting PEP is very small.
On the other hand, there are sites that contain a
large number of small pictures, (i.e.,
http://www.barnesandnoble.com – when used had 51
pictures, 39 of them with sizes below 1K). On these
sites, the improvements obtained by utilizing a pre-
getting method are very significant. The resulting
download time dropped to about 30% of the
download time using a regular PEP.
The average improvement on all the Web pages
on a local server was about 50%. The bandwidth
utilization ratio on the “small” sites was about 1.3
(i.e., the bandwidth used without PEP was 1.3 times
higher than with pre-getting PEP), while on the
“large” sites the ratio was about 0.7. The following 2
experiments analyzed the relationship between Web
page structure and pre-getting performance, by using
different Web pages built specially for this purpose.
Experiment 1 description
To further analyze the relationship between Web
page structure and performance we added two
additional pages that represent the extreme cases.
The first page contains 6 pictures larger than 100KB
and 8 pictures larger than 50KB. The second page
WEB TRAFFIC ACCELERATION OVER CELLULAR NETWORKS BY USING A COMPRESSING PROXY SERVER
43
contains 24 pictures smaller than 1KB and 28
pictures smaller than 5KB. We will refer to these 2
pages as big and small, respectively.
Conclusion 3. The performance of the pre-
getting method depends on the Web page
structure.
Table 2: Experiment results on “extreme” cases
Big Small
Time (s) 306.6 83.6
Total data 1252 163.3
No
Compression
Average KB/s 32.6 15.6
Time (s) 158.7 64.9
Total data 373.3 73.4
Compressing
proxy
Average KB/s 18.8 9.05
Time (s) 90.8 20.2
Total data 373.3 73.4
Pre-getting
proxy
Average KB/s 32.9 29.1
The experiment results on these pages were as
expected (see Table 2). As can be seen, the
bandwidth utilized while using the pre-getting PEP
remains stable around 30KB/s (on both types of
pages). The bandwidth, while not using PEP at all, is
affected by the page structure and drops by about
50% and while using the PEP it is about 60% of the
bandwidth without a proxy.
Experiment 2 description
The previous experiment tested extreme cases. In
order to analyze cases between the two extremes we
performed an additional experiment. For this
experiment, we collected a series of 8 pictures with
sizes from 43 bytes to 80KB. For each of these
pictures, we built a Web page that contained the
picture 15 times.
The effect of the picture sizes on the bandwidth
utilization can be seen in Figure 3. The basic
conclusion that a large amount of small pictures
degrades bandwidth utilization still holds.
Bandwidth utilization is in direct relationship to the
pictures sizes. The only exception is in the case that
the picture is compressed extremely well (from
30,567B to less than 3,000B). Even in these cases,
the bandwidth utilization obtained by using the pre-
getting PEP is higher than the bandwidth utilization
obtained by using a regular PEP. In normal cases
(compression rate of 50% to 80%), we have found
that the bandwidth utilization is similar to bandwidth
utilization without a PEP and even exceeds it.
Figure 3: Bandwidth utilization in relation to pictures sizes
Only in the most extreme case (the smallest
pictures) a drop in the used bandwidth was
identified. But even with this drop, bandwidth
utilization obtained by using other methods is
exceeded.
Conclusion 4. A pre-getting compressing proxy
gives us the ability to download large Web pages.
When attempting to download a large Web page
using the cellular network a timeout on the browser
was obtained. The browser attempts to download the
page for 5 minutes and if the download is not
completed the browser returns an error message. If
we calculate the page size that will result in a
timeout, we get 1.5MB (43,200/8 * 60 * 5 =
1,620,000) on a HSCSD network.
If we assume a compression ratio of 1:4 and
bandwidth utilization that drops to about half, we
achieve the ability to download Web pages that are
twice as large without getting a timeout. When we
use the pre-getting PEP (so the bandwidth used is
about the same as without a proxy) we can download
Web pages that are four times as large (i.e., up to
6MB in size).
6 DISCUSSION
The work presented here investigates the problems
related to use of a compressing proxy server over
cellular networks. The main problem was the
effectiveness of bandwidth use. We saw the drop in
bandwidth while using the PEP and studied its
sources.
The next step was to offer a solution to this
problem by adding a pre-getting mechanism to the
compressing proxy server. We implemented this
feature into the PEP and tested the effectiveness of
the solution.
We saw that the pre-getting method boosted the
performance of the PEP. With an average
compression ratio of about 65% we obtained an
improvement of about 60% in the download time,
compared to 25% while using the regular PEP.
ICETE 2004 - WIRELESS COMMUNICATION SYSTEMS AND NETWORKS
44
When we reviewed the results, we saw that the
bandwidth ratio while using the pre-getting proxy
server varied from 0.5 to 1.5. In order to identify the
source of this inconsistency in a sterile environment,
we built a cellular network software simulator.
Our experiments indicated that the structure of
the Web page was the root cause of the variance in
bandwidth utilization. We discovered that the best
performance is on pages with a large number of
embedded small pictures and that the worst
performance is with pages holding a small number
of large pictures. We constructed several sample
pages to analyze the relationship between the sizes
of the pictures embedded in the Web page to the pre-
getting performance.
In conclusion, we have identified the problems
which occur while using a regular compressing PEP,
and introduced a mechanism that enhances an
existing PEP in a way that enables improved Web
surfing over a cellular network. Without this
enhancement, Web surfing is slow, there is no direct
relationship between quality loss and improvements
in download time and it is practically impossible to
download large Web pages. With this enhancement,
the ratio between the loss in quality (the
compression ratio) and the gain in time is close. In
other words, if the user is willing to lose 90% of the
data by reducing the quality of the pictures in the
Web page, the page will be gotten 10 times faster.
Future research topics concerning the pre-getting
method are:
Adding compression
for various data types –
in this work, only GIF and JPG graphic files
were explicitly compressed and at a constant
ratio. All other types of files were
compressed with the standard zlib
compression.
Improving the communication protocol
The communication protocol is
implemented over TCP/IP. It can be
improved by using the protocol
improvements referred to in subsection 2.1.
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