The microwaves travel in straight lines and can therefore be narrowly focused. Concentrating all the energy into a small beam using a parabolic antenna gives a much higher signal to noise ratio, but transmitting and receiving antenna must be accurately aligned with each other.

It is also necessary to re-examine various transaction processing issues in mobile computing systems to account for characteristic of wireless client/server computing. As a mobile host interactively submits the operations of a transaction to its co-ordinator, the latter subsequently forwards these operations to the distributed database servers. The characteristics of a mobile transaction can be described as follows:

• A mobile transaction tends to be long-lived because of high latency of wireless communications and long disconnection of users.
• A mobile transaction tends to be error-prone because mobile hosts are more prone to accidents than fixed hosts.
• A mobile transaction may access a distributed and heterogeneous database because of the mobility of the transaction hosts.

The top level of a wireless information networks is shown in Figure 1. The public network (Internet and Phone) and the private network such as the one modelled as a university are usually not secure, The private networks modelled as an industry, a wireless service provider, and a private local area network are usually secure. Figure 1 also illustrates security firewalls for the secure private networks. A scenario of corporate firewalls illustrates concepts of security management correctness, sufficiency, and completeness.

The present technology that implements the wireless channels is not secure except for some data encryption, authentication, and spread-spectrum implementations that provide limited protection to elementary attempts at jamming, spoofing, and interception.

General security solutions try to establish perimeters or layers of protection to filter what data passes in or out. Multiple layers and access points make robust network security systems a nature example of distributed operations in both implementation and management aspects. The level of threat to the resources and data within a system makes active management of security capabilities an important distributed operations mission. These solutions are particularly applicable for solving the computer security problem.

In general, a wireless channel security system (see ?Channel Security? in Figure 1) should be exist and used to process and filter signals after the transmitter sent these signals and before the receiver will receive them over a wireless network. Like firewalls in wired network, the channel security system will check the status of signals or transmitters and then take a suited action, such as correcting some errors, enhancing the signal power, and rejecting the entrance of unauthorised users. The most important point is to prevent unauthorised wireless users from illegally entering into this wireless network. To some extent, it is hard to make this channel security system.

Some wireless systems have given more implications of solving the wireless and mobile network security problem. In these systems, while security transmitters send signals to a network of wireless modem or receivers, this information is gathered and relayed to a security control panel (on a wireless channel security system). All the information received is then filtered and processed by the system in order to ensure the wireless security by checking the status of wireless modem or receivers. If a wireless receiver fails to respond to normal interrogations or if power is lost to a wireless receiver, an alarm is generated and personnel are dispatched to correct the problem.