P1: C-167
Guizani WL040/Bidgolio-Vol I WL040-Sample.cls July 16, 2003 10:7 Char Count= 0
820 WIRELESSCOMMUNICATIONSAPPLICATIONSTable 1continueWireless network Wireline network
Web application
Protocol stack WAP—based on existing Internet standard,
such as IP, XML, HTML, HTTPHTTP/TCP/IP—Application layer
Wireless application environment (WAE) —Session layer (like HTTP)
Wireless session protocol (WSP) —Transport layer (TCP)
Wireless transaction protocol (WTP) —Like Netscape SSL
Wireless transport layer security (WTLS) —UDP
Wireless datagram protocol (WDP) —Physical layer
Bearer layer (GSM, CDMA, D-AMPS, GPRS)
Makeup language WML, C-HTML, HDML, XML, WMLScript HTML, XML, JavaScript
ATM network
Cell size 24-byte payload; header can be compressed
and expanded to standard ATM48-byte payload, 5-byte
header
Media access control (MAC) Problem: find suitable channel sharing/media
access control technique at data link layerCDMAData link layer Needs to provide an error control, needs
fragmentation, and reassemblyDHLCCELLULAR NETWORKS
Cellular networks use many base stations, allowing them
to cover a much wider area. The base stations are usually
connected to each other using fiber optic cables to wireless
links, then to external networks such as phone systems
and the Internet.
A cell is the coverage area of a single base station. As
a mobile phone moves through a network, it accesses ser-
vices via the base station of whichever cell it is in. Radio
waves radiate out from a transceiver. The further the mo-
bile user is from the base station, the weaker the signal
gets. The cell boundary is the limit where the mobile ter-
minal can no longer send and receive reliably.
The overlap between cells also must be taken into ac-
count, as in some parts a user may be able to commu-
nicate with two or three base stations. One of the most
important features of a mobile network is the ability for
a user to move from one cell to another, originally known
as roaming.
The process of switching a user from one cell to another
while a call is in progress is called a handoff. Handoffs are
very complex procedures because the base stations must
calculate exactly when a user is crossing the cell boundary.
This can take several seconds, so if users move too fast,
their calls may be dropped.
The speed limit for analog systems is usually no more
than 100 kph. Some digital systems can function at speed
above 300 kph (on a high-speed train). No system can
complete a handoff at the cruising speed of an airliner, one
reason for not using a mobile phone on a plane (Collins,
2001; Dorman, 2001;Evolution of a mobile market, 2001).Types of Handoff
There are three types of handoff:Soft Handoff
Soft handoff ensures that a link is set up to the base station
in the new cell before the old one is torn down. This systemis very reliable. It should result in dropped calls only if the
user is moving extremely fast or actually passes outside
the cellular network. One problem is that a connection
with two different base stations is very difficult to achieve.
In most types of networks, adjacent base stations need to
use different frequencies, while a phone can be tuned to
only one frequency at a time.Hard Handoff
Hard handoff requires that a phone break its connection
with one base station before connecting to another. It is
less reliable than soft handoff because a phone is not al-
ways able to establish a new connection. The new cell
could already be full, or there may not be another cell
available. A base station sometimes decides to make a
handoff based on how far away a phone is, without consid-
ering whether another phone can pick it up. Hard handoff
causes a noticeable break in conversation, which can be
annoying for a user moving rapidly between small cells. To
fix this problem, networks with microcell overlays tries to
detect which users are moving and connects them to the
main larger cell.No Handoff
No handoff is very simple and relies on the mobile termi-
nal actually making a new call once it has moved out of
range on one transmitter. It is rare in traditional cellular
networks because many mobile phone systems can take
up to 30 s to set up a new call.Effects of Frequency
The frequencies used by cell phones all lie within the UHF
microwave band (same as used by TV transmission) rang-
ing from 400 to 2000 MHz. Higher frequencies are more
easily blocked by droplets of cloud in the atmosphere,
which result in a shorter range. Networks based on these
higher frequencies require smaller cells and more base
stations. Lower frequencies initially seemed preferable