Good afternoon everyone. I am glad to give you my presentation, about 3G wireless communication system.

You may hear about CDMA 2000 of China Unicom, it is nearly a 3G system but not yet. Then what is 3G exactly?

3G is one step of personal communication system. The ultimate goal of personal communication system is to provide communication service anytime, anywhere, between any pair, in any required form.

To achieve this goal, two aspects should be ready. First, to provide service for every one, the whole system should have a high capacity, so the first problem is the bandwidth requirement. Officially, we define a connection over 384kbps to be a Broadband connection. Maybe it is a little boring for unprofessional one, so, let me show an example. When you connect to USTC Video Server to watch TV program online, the bandwidth you take up is about 300kbps, which is quite enough for a high quality VOD service. The common Internet cable access in dormitory is 100Mbps to desktop.

However, conditions in wireless environment are more complex. A direct consequence is that it is more difficult to achieve as broad bandwidth as under cable environment. So, in 3G systems, we apply some key technology to improve bandwidth condition. A typical one is CDMA- Code Division Multiple Access.

Secondly, providing service in any required form calls for multiformity of communication system. As we know, we can gain every kind of communication through Internet. In another word, service based on Internet has maturely developed. It is easy to develop wireless service based on Internet. In another aspects, Internet is most popular Communication network now, joining with Internet will bring about huge advantages.

To Provide service everywhere and every when means wireless signal should cover as more areas and base stations in system should power on as long as possible, which is not going to be focused on today.

In current system, when you want to send or receive emails, you need a PC connected to the internet; when you want to watch some TV programs you need a TV set; when you want to talk to a friend far away form you, you need a phone, even a cell phone. However, all of these will change after 3G, all you need is just a single Mobile Station providing all kinds of service. 3G, makes life more easily.

Thanks for your listening and cooperation.

An Open Letter to …… PH.D Class IV

it is with heavy heart that this letter is being written to this class of students. the events that transpired during the week of april 13th was substandard and not befitting a class of this caliber or level of studies.

when one student leaves during class time, without permission to pick up a fedex parcel, is met by me in the hallway and when asked whether permission was granted, the answer was……”My phone did not ring!” how did the student know that there was a parcel waiting? i believe more attention is paid to look at a cell phone than what is going on in the classroom. that indicates to me where the priorites are!

each student was notified and wared for 4 weeks that personal presentation would start and what was the result? one student was laughing through most of his presentation and others presented such a weak one that there is strong evidence that no time was spent on preparation. the proof lies with the manner of substance contained in the presentation itself and its delivery. yes, i am aware of the pressure placed on you for maintaining a high academic standard in your studies in program of your major. however, this does not invite you to be slack concerning this perquisite course of oral english. you require a specific mark in order to pass and obtain your degree from this course. i would conclude from your actions that you would be high school students rather than PH.D. students solely based on events for the past two weeks.

coming from canada to teach in this school was priviledge and anhonor especially being recommended by previous teachers, i deem it that each student is not satisfied with the curriculum of this class and its teacher, hence these events and troubled classroom atmosphere. thus i am willing to remove myself and norify the english department accordingly and you may continue your studies in oral english by yourself and user whatever means to improve your level and competency.

on the other hand, of the students of this class are willing to do their homework, do their presentation in a orderly manner which indicates that serious effort has taken place, i am more than willing to continue in this class with the format laid out in the syllabus. every effort will be extended by me to correct, instruct and introduce new vocabulary, and pronunciation in an oral setting. the choice rests with each student in this class. when you are serious… email me at [email protected] and i will return to this class. your time as well as mine is valuable and it is too precious to waste… …thank you!

问题：PC A，PC B，8541 C，8541，D

A和C以及B和D分别连接为局域网，C和D之间是空中借口，现在要在A-B之间进行网络通信(例如ftp)。之前用HOOK抓包再送到以太网卡上遇到的问题是因为A B的网络配置把彼此的IP地址配置到了同一个子网里面，因此会有arp查询包，需要在8541上做arp欺骗。只要把它们的网络设置为不同的网段，例如

A:192.168.0.1/255.255.255.0

B:192.168.1.1/255.255.255.0

C:192.168.0.2/255.255.255.0

D:192.168.1.2/255.255.255.0

并把C与D设置为网关，这样一来由A发往B的IP包会发到网关C上，在C上做Hook抓到，通过空中借口原样发到D上，D以网关的身份发给B。不需要做中继欺骗，相当于我们用空中借口实现了一个隧道。而且这样一来可以在一个片子上接多个MT，只要IP地址不冲突。

由此推想，通过VPN接入专有网络的机器的IP地址应该是不同于真实专有网络的IP网段，否则路有肯定会出错。