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836 WIRELESSINTERNETRTP was designed to work in conjunction with an aux-
iliary control protocol called real-time control protocol
(RTCP) (Schulzrinne et al., 1996). In an RTP session, par-
ticipants periodically send RTCP packets to convey feed-
back on quality of data delivery and information of mem-
bership. RFC 1889 defines five RTCP packet types to carry
control information. These packets contain information
regarding number of packets lost, interarrival jitter, and
timestamps. Through these control information packets,
RTCP provides services such as QoS monitoring, conges-
tion control, intermedia synchronization, and calculation
of round-trip delays. RTP is typically run on top of user
datagram protocol (UDP) (Postel, 1980) to make use of its
multiplexing and checksum functions. However, use of
these protocols introduces bandwidth overhead to each
data packet, which is especially important in low-speed
wireless links.Transmission Control Protocol (TCP)
and UDP
Transmission control protocol (TCP) (Postel, 1981b) is the
most commonly used transport protocol on the Internet.
TCP provides a connection-oriented and reliable flow be-
tween two hosts, while UDP provides a connectionless and
unreliable datagram service over the network. UDP was
chosen as the target transport protocol for RTP because of
three reasons. First, since RTP was initially designed for
multicast, it was realized that connection-oriented TCP
does not scale well for a large number of flows and there-
fore is not suitable. Second, for real-time data, 100% relia-
bility is not as important as timely delivery. Since TCP pro-
vides reliability through retransmissions, it is not suitable
for real-time applications, for by the time packet error or
loss is detected and the retransmitted packet is received,
the playback time of the data contained in the packet
would likely have passed. Thus, retransmission only in-
creases the network traffic without benefiting the quality
of the playback. Third, congestion control of TCP does not
match well with the needs of real-time applications, as
real-time applications cannot tolerate packets being held
back even during periods of congestions.Internet Applications: World Wide Web,
E-mail, Instant Messaging
Since the advent of the Internet, a handful of applications
have become the drivers of the growth of the Internet.
Some of these applications include e-mail, file transfer, in-
stant messaging, and the World Wide Web (WWW). File
transfer was one of the original applications envisioned
for the Internet and continues to be one of the most pop-
ular applications. Typically, file transfer protocol (FTP)
(Postel & Reynolds, 1985) is used to send and retrieve
files from a remote computer. E-mail was initially used to
send text messages between individuals or groups of in-
dividuals, but increasingly diverse media such as images
and audio are being transmitted using e-mail. According
to the Nielsen/NetRatings First Quarter 2002 Global In-
ternet Trends report (2002), e-mail was the most domi-
nant online activity in 12 countries over the previous six
months.Three technologies were invented by 1991 to accom-
modate the arrival of the WWW. Hypertext markup lan-
guage (HTML) (Raggett, Le Hors, & Jacobs, 1999) is used
to produce Web documents, hypertext transfer proto-
col (HTTP) (Fielding, Gettys, Mogul, Frystyk, & Berners-
Lee, 1997) is used to transport HTML documents from
the server to the client, and the client Web browser is
used to retrieve, interpret, and display HTML documents.
Perhaps no application has been more instrumental in
piquing the general public’s interest in the possibilities of
the Internet. In March of 1993, WWW traffic measured
mere 0.1% of NSF backbone traffic. By February 1995,
WWW passed FTP as the largest volume Internet appli-
cation. The main attraction of WWW lies in its flexibility
to service documents containing various media including
text, graphics, audio, and video, as well as the ease of ac-
cessing documents using “hotspots.”
Instant messaging (IM) is an application that has been
experiencing a tremendous growth of late. IM enables in-
dividuals to create private “chat” sessions, in which vari-
ous types of messages may be exchanged. It offers all the
capabilities of another immensely popular application, e-
mail, but with near real-time response. The growth of IM
has been strong for both home use and business use. A
November 2001 study from Jupiter Media Metrix showed
that the number of unique business users of the top three
IM applications (AOL, MSN, and Yahoo!) increased from
10 million in September 2000 to 13.4 million in September- During the same time period, the total usage time in-
creased from 2.3 billion minutes per month to 4.9 billion
minutes per month. The growth in home use has been
equally impressive with the number of users reaching
53.8 million and the total usage time reaching 13.6 billion
minutes a month. IM service is very similar to short mes-
saging service (SMS) available in the wireless networks,
and the increasing use of IM over the Internet will only
increase the demand for SMS (Instant messaging, n.d.).
CURRENT STATE OF CELLULAR
SYSTEMS (FOCUS ON 2G)
Cellular Layouts
Most wide area wireless networks today are cellular. The
service area is divided into smaller service areas called
cells. In contrast, the first mobile telephony systems in
operation up to late 1970s did not use the cellular lay-
out. Instead, they relied on a high-power transmitter to
service a large area. Because only a fixed number of fre-
quency channels were assigned to each service area, in-
creasing demand for mobile telephony meant increasing
competition for the available channels, resulting in exces-
sively high call blocking rates. The cellular system was de-
signed to address this issue, to increase capacity through
the employment of frequency reuse. The first systems that
used cells were based on analog technology and are re-
ferred to as first generation (1G) systems. In some places,
such as the United States, the analog network still exists
to serve customers that have not yet transitioned to the
next generation of cellular systems that use digital trans-
mission technology. The digital cellular service is referred
to as the second generation (2G) system. By going over to