A comparison of wi-fi and wimax with case studies - Florida State ...

communications. Taipei is a busy city with many office buildings. It is a “concrete jungle”,
especially in the metropolitan area. This kind of area is very unfavorable to signal transmission
and it will cause serious multipath fading. Thus, the end users may scan the signal, but cannot
establish a connection, or even have no signal at all. The other problem is AP. It includes
numbers of user and locations. Most of APs are located outdoor, such as wire poles and street
lights. Because APs are exposed outside and Taiwan is hot and humid, the failure rate will be
high. Moreover these kinds of AP locations also have big security issues. There are 4600 APs in
Taipei, management and maintenance is a big challenge. And since the AP installation is focus
on population coverage, most suburb residents do not have WiFly signal.
The Q-ware does not limit the numbers of user on an AP. Therefore it may cause congestion
and channel overlapping problems. For channel allocation and reallocation, Linda Yeh, the
Account Manager in Sales & Customer Service Dep. of Q-ware Systems & Services Corp. [148],
said the company has a control platform to manage each AP’s operation channel. In a high
density area, if there is congestion or channel overlapping the network engineers can use this
platform to switch the operation channel for each AP to release the situation. However, the
platform cannot solve the problem completely. In most high usage areas of WiFly, after
switching channels, the problem still exists. She said this is because there are only three channels
that can be used in one area. It is Wi-Fi’s inherent limitation. [148]
The maximum data rate for each user was 512 kbps for both Uplink and Downlink. In
January 2007, Q-ware upgraded to 1Mbps for Downlink and 512 kbps for Uplink. This speed is
not a shearing speed, which means it will not be changed even if the users increased. [148] To
avoid heavy traffic, Linda indicated that they have a flow control scheme to monitor the traffic
flow for APs. The detail of flow control scheme is trade secret, thus she cannot provide more
information for it. Channel overlapping and congestion are two major problems of signal
transmission. There are several methods to solve the congestion issue, such as, hop-by-hop
congestion control and multihop congestion control. [149][150] The main idea of these methods
is to send a congestion feedback packet to slow down the source speed to release the congestion.
The congestion control has been studied for years. Here is some related researche. [151] – [153]
For improving signal stability, end users can install bridge or high power wireless network
cards. These two methods can solve most signal instability. Q-ware also has to install more APs
or use high power APs in high usage areas. Since WiMax licenses have been issued in Taiwan. It
can also be a solution for signal stability and extending geographical coverage. About security
vulnerability, Q-ware said that the wireless connection must be authorized before established and
the user account needs two identifications to apply. However there is no one hundred percent
security scheme. Users still have to protect their account information and PC from spy-programs.
The responsibility is upon both WISP and the users.
The Nortel Wireless Mesh Network solution is a suitable model for WiFly. However, no
matter how perfect the model is, Taipei still has its intrinsic limitations, such as high office
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