父母通常導子女坐時要挺直，但英國有醫院進行全球首次以磁力共振測試坐姿的研究後發現，保持輕鬆坐姿原來遠較挺直腰板健康。研究發現人類最佳的坐姿是背脊與大腿保持在一百三十五度，即稍微向後傾斜。傳統的九十度坐姿因為坐得太直，反而對脊骨和附近肌肉造成不必要壓力
。辦公室員工在長年累月工作下，更會引發慢性背痛等疾病。
135°坐姿最舒適自然
阿伯丁伍登德醫院（WoodendHospital）對二十二名健康良好、沒有背痛經歷或曾進行手術的志願人士做研究，找出最適合人類的坐姿。醫院以可移動的磁力共振掃描器，繪製出脊骨在三種角度坐姿下所承受的壓力圖象。
過往病人需要躺下才能進行磁力共振，以這種技術測定坐姿尚屬全球首次。三種坐姿分別是弓起背脊向前傾、九十度坐得筆直，以及雙腳地、腰肢向後挨的一百三十五度輕鬆坐姿。放射線專家在判斷脊骨角度、各骨節的移動和
變化後，發現輕鬆坐姿最接近人類的自然結構。
人體構造不適合長坐
相反，筆直坐姿會令脊骨受壓變形，長期的話更會影響四周肌肉和韌帶，造成背痛、脊骨畸形和其他慢性背部疾病。負責研究的巴希爾醫生表示，人體構造並不適合長時間坐，可惜現代生活卻要全球大部分人口坐工作。
巴希爾又指，在醫學上，脊骨最健康的姿態是平躺下來，但卻難以在辦公室做到。英國約有八成人曾受背痛之苦，政府應致力向員工和學童推廣正確坐姿，並選擇一些能後傾的椅子。
今次發現在美國芝加哥舉行的北美放射學會年會上公布，雖然一百三十五度坐姿最為舒適自然，但似乎不太適合每天面對電腦衝鋒陷陣的「打工仔」。格拉斯哥納菲 爾德醫院整形外科醫生沃德爾更直指研究結果只屬理論。他表示背痛就如頭痛、傷風和感冒一樣非常普遍，也有不少證據顯示坐姿不會對此構成影響。
2006年11月21日星期二
自製1UP磨菇 (*不可食用)
國外網站ThinkGeek開始了可以種植1UP蘑菇的盆栽，連同花盆、種子（胞子）以及營養劑的套裝需要8.99美元。買回家種植在陰暗的角落2、3周邊可以得到這個“永遠生命”的1UP蘑菇。需要注意的是這個蘑菇是不能夠食用的。
記得唔好拎唻食
有盒有說明書
記得唔好拎唻食
有盒有說明書
2006年11月3日星期五
Network Toolkit
The tools in The Network Toolkit cover a wide range of needs, and are conveniently grouped into categories. Below we list the categories along with their respective tools.
Is there a tool that you use every day, and that you would like to see in the next version of The Network Toolkit? Let us know about it!
Network Analyzers
cdpr decoder for the Cisco Discovery Protocol.
Dice decodes network trace files and emits statistics.
WinDump command line network analyzer, the Windows version of the well known tcpdump.
Wireshark formerly known as Ethereal, Wireshark is a powerful network analyzer with support for more than 700 protocols.
Network Monitors
AdapterWatch tool for retrieving statistics about a network adapter.
Analyzer network analysis and monitoring software.
NetMeter bandwidth monitor.
Traffic Generators
D-ITG advanced traffic generator able to generate TCP, UDP, ICMP, DNS, Telnet and VoIP traffic.
SIPp a test tool/traffic generator for the SIP protocol.
ttcp simple TCP and UDP Test utility, designed to test the performance of a network link.
Network Scanners
Angry IP scanner a simple-to-use GUI-based network scanner.
NBTscan command line NETBIOS name scanner.
Nmap utility for network exploration and security auditing.
THC-Amap a scanning tool based on a database of triggers and responses.
UMIT (requires installation) a graphical user interface for NMap.
IDS
AirSnare (requires installation) network monitor useful for tracking hosts that communicate on your network.
SAM Jr: java package that provides real-time analysis of Snort data.
Snort a lightweight network intrusion detection system.
Network Utilities
ArpCacheWatch ARP cache monitor.
CurrPorts displays opened TCP/IP ports and connections.
DnsEye monitors DNS requests.
netcat reads and writes data across network connections, using TCP or UDP protocol.
Network Packet Generator a packet injector that uses WinPcap to send specific packets out of a single or multiple network interfaces.
ngrep grep-like utility to search inside network packets.
Oinkmaster a script to update and manage Snort rules.
OpenSSH server an easy and quick way to launch an OpenSSH server.
Sam Spade a freeware network query tool.
The Metasploit Project a complete environment for writing, testing, and using exploit code.
Winfingerprint an administrative network resource scanner.
WinInterrogate a file system and process enumeration/integrity tool.
Network Clients
Mail
Blat Win32 command line utility that sends e-mail using the SMTP or NNTP protocols.
Secure Clients
PuTTY Telnet and SSH terminal.
WinSCP SFTP client using SSH.
SNMP
FreeSnmp simple tool to perform SNMP queries.
NetSNMP a suite of SNMP manager tools.
Web
wget utility to perform non-interactive downloads of files via HTTP, HTTPS, FTP.
Unix network clients various standard network tools.
Win32Whois simple Whois client.
Auditing Tools
Hydra simple password revealer.
John the Ripper utility to discover weak passwords.
Password Revealers
Asterisk Logger reveals passwords behind asterisks.
AsterWin IE discovers passwords used in Internet Explorer.
Dialupass discovers passwords used in Dial-Up authentications.
Protected Storage PassView discovers passwords used in Dial-Up authentications.
System Tools
DriverView: utility to view details about installed drivers.
RegScanner utility to search inside the registry.
StartupRun shows applications that are scheduled to start during the startup of the OS.
WinUpdatesList displays the operating systems updates, including the involved files.
Supplementary Tools
DOS Command Prompt runs a DOS Command Prompt in the command line tools directory.
Unix Shell Windows version of the Bash shell.
WinPcap 3.1 Installer-version of WinPcap. Can be used with tools not present in this collection.
WinPcap 4.0 beta1 Installer-version of WinPcap. Can be used with tools not present in this collection.
Is there a tool that you use every day, and that you would like to see in the next version of The Network Toolkit? Let us know about it!
Network Analyzers
cdpr decoder for the Cisco Discovery Protocol.
Dice decodes network trace files and emits statistics.
WinDump command line network analyzer, the Windows version of the well known tcpdump.
Wireshark formerly known as Ethereal, Wireshark is a powerful network analyzer with support for more than 700 protocols.
Network Monitors
AdapterWatch tool for retrieving statistics about a network adapter.
Analyzer network analysis and monitoring software.
NetMeter bandwidth monitor.
Traffic Generators
D-ITG advanced traffic generator able to generate TCP, UDP, ICMP, DNS, Telnet and VoIP traffic.
SIPp a test tool/traffic generator for the SIP protocol.
ttcp simple TCP and UDP Test utility, designed to test the performance of a network link.
Network Scanners
Angry IP scanner a simple-to-use GUI-based network scanner.
NBTscan command line NETBIOS name scanner.
Nmap utility for network exploration and security auditing.
THC-Amap a scanning tool based on a database of triggers and responses.
UMIT (requires installation) a graphical user interface for NMap.
IDS
AirSnare (requires installation) network monitor useful for tracking hosts that communicate on your network.
SAM Jr: java package that provides real-time analysis of Snort data.
Snort a lightweight network intrusion detection system.
Network Utilities
ArpCacheWatch ARP cache monitor.
CurrPorts displays opened TCP/IP ports and connections.
DnsEye monitors DNS requests.
netcat reads and writes data across network connections, using TCP or UDP protocol.
Network Packet Generator a packet injector that uses WinPcap to send specific packets out of a single or multiple network interfaces.
ngrep grep-like utility to search inside network packets.
Oinkmaster a script to update and manage Snort rules.
OpenSSH server an easy and quick way to launch an OpenSSH server.
Sam Spade a freeware network query tool.
The Metasploit Project a complete environment for writing, testing, and using exploit code.
Winfingerprint an administrative network resource scanner.
WinInterrogate a file system and process enumeration/integrity tool.
Network Clients
Blat Win32 command line utility that sends e-mail using the SMTP or NNTP protocols.
Secure Clients
PuTTY Telnet and SSH terminal.
WinSCP SFTP client using SSH.
SNMP
FreeSnmp simple tool to perform SNMP queries.
NetSNMP a suite of SNMP manager tools.
Web
wget utility to perform non-interactive downloads of files via HTTP, HTTPS, FTP.
Unix network clients various standard network tools.
Win32Whois simple Whois client.
Auditing Tools
Hydra simple password revealer.
John the Ripper utility to discover weak passwords.
Password Revealers
Asterisk Logger reveals passwords behind asterisks.
AsterWin IE discovers passwords used in Internet Explorer.
Dialupass discovers passwords used in Dial-Up authentications.
Protected Storage PassView discovers passwords used in Dial-Up authentications.
System Tools
DriverView: utility to view details about installed drivers.
RegScanner utility to search inside the registry.
StartupRun shows applications that are scheduled to start during the startup of the OS.
WinUpdatesList displays the operating systems updates, including the involved files.
Supplementary Tools
DOS Command Prompt runs a DOS Command Prompt in the command line tools directory.
Unix Shell Windows version of the Bash shell.
WinPcap 3.1 Installer-version of WinPcap. Can be used with tools not present in this collection.
WinPcap 4.0 beta1 Installer-version of WinPcap. Can be used with tools not present in this collection.
Remove "My Sharing Folders" for MSN Messenger
Those that have installed the latest version of MSN Messenger, or Windows Live Messenger as it is now called, have probably noticed that it installs a link in My Computer called 'My Sharing Folders'. If, like me, you're not interested in this feature of WLM, or you just don't like the way it dumps a shortcut under My Computer, you can remove it by unregistering the Messenger File Sharing Shell Extensions dll.
do this by going to start > run, and typing in:
CODE
regsvr32 -u -s "C:\Program Files\MSN Messenger\fsshext.dll"
<可能個path或者file要自己tune一tune>
do this by going to start > run, and typing in:
CODE
regsvr32 -u -s "C:\Program Files\MSN Messenger\fsshext.dll"
<可能個path或者file要自己tune一tune>
Outlook Express - 點擊郵件上的網址，卻不能開啟網址
點擊郵件中的網址（比如http://www.binbin.net）， 卻沒有任何反應，以前點了網址之後，就會自動開啟IE而連到該網址，要如何恢復這個功能呢？
可以依照以下幾種方法測試。
方法一：將IE設為預設的瀏覽器。
如果你有安裝其他的瀏覽器，將IE設為預設的瀏覽器之後，再測試郵件中的網址是否可以打開。
而將IE設為預設瀏覽器的方法有兩種，請任選一種：
一、開啟Windows XP的控制台，到「新增或移除程式」>>「設定程式存取及預設值」，選「自訂」，將「選擇預設網頁瀏覽器」設為「Internet Explorer」。
二、開啟Windows XP的控制台，到「網際網路選項」，再到「程式集」頁，勾「檢查Internet Excplorer是否為預設的瀏覽器」，按確定。
再去開啟IE，當IE開啟的時候，會詢問你是否要把IE設為預設的瀏覽器，此時請選「是」，IE即定為預設的瀏覽器。
方法二：修改URL的「檔案關聯」。
檢查URL的檔案關聯是否被破壞而導致信中網址無法自動開啟。（安裝 IE 以外的瀏覽器最常碰上的現象）開啟檔案總管，到「檢視」>>「資料夾選項」（或「工具」>>「資料夾選項」），到「檔案類型」這 一頁上。 在登記的檔案類型這裡，你慢慢往下捲動，去找「URL：檔案傳輸通訊協定」。
選了之後，按編輯。
按 OPEN ，按編輯。
在「用來執行動作的應用程式」這一欄裡輸入：（不要忘記引號）
"C:\PROGRAM FILES\INTERNET EXPLORER\iexplore.exe" -nohome
使用DDE勾起來，並檢查以下欄位值。
DDE 訊息："%1",,-1,0,,,,
應用程式：IExplore
DDE應用程式沒有執行：（此處不需要輸入資料。留白）
主題：WWW_OpenURL
按確定。
按確定。
然後，還有檢查以下幾個項目是不是也不正常，如果需要更改的話，就比照上面步驟一一更改過來。
URL:Gopher 傳輸通訊協定
URL:Hypertext 傳輸通訊協定
URL:Hypertext 傳輸通訊協定附加保密
都改好之後，按確定，好關閉資料夾選項。
方法三：重新註冊 urlmon.dll 檔案。
若經歷經上面這些步驟仍然無法解決問題，則重新註冊 urlmon.dll 試試看。
按「開始」，到「執行」，輸入以下指令按確定，注意指令與參數間有一個space空格。
regsvr32 urlmon.dll
可以依照以下幾種方法測試。
方法一：將IE設為預設的瀏覽器。
如果你有安裝其他的瀏覽器，將IE設為預設的瀏覽器之後，再測試郵件中的網址是否可以打開。
而將IE設為預設瀏覽器的方法有兩種，請任選一種：
一、開啟Windows XP的控制台，到「新增或移除程式」>>「設定程式存取及預設值」，選「自訂」，將「選擇預設網頁瀏覽器」設為「Internet Explorer」。
二、開啟Windows XP的控制台，到「網際網路選項」，再到「程式集」頁，勾「檢查Internet Excplorer是否為預設的瀏覽器」，按確定。
再去開啟IE，當IE開啟的時候，會詢問你是否要把IE設為預設的瀏覽器，此時請選「是」，IE即定為預設的瀏覽器。
方法二：修改URL的「檔案關聯」。
檢查URL的檔案關聯是否被破壞而導致信中網址無法自動開啟。（安裝 IE 以外的瀏覽器最常碰上的現象）開啟檔案總管，到「檢視」>>「資料夾選項」（或「工具」>>「資料夾選項」），到「檔案類型」這 一頁上。 在登記的檔案類型這裡，你慢慢往下捲動，去找「URL：檔案傳輸通訊協定」。
選了之後，按編輯。按 OPEN ，按編輯。
在「用來執行動作的應用程式」這一欄裡輸入：（不要忘記引號）
"C:\PROGRAM FILES\INTERNET EXPLORER\iexplore.exe" -nohome
使用DDE勾起來，並檢查以下欄位值。
DDE 訊息："%1",,-1,0,,,,
應用程式：IExplore
DDE應用程式沒有執行：（此處不需要輸入資料。留白）
主題：WWW_OpenURL
按確定。
按確定。
然後，還有檢查以下幾個項目是不是也不正常，如果需要更改的話，就比照上面步驟一一更改過來。
URL:Gopher 傳輸通訊協定
URL:Hypertext 傳輸通訊協定
URL:Hypertext 傳輸通訊協定附加保密
都改好之後，按確定，好關閉資料夾選項。
方法三：重新註冊 urlmon.dll 檔案。
若經歷經上面這些步驟仍然無法解決問題，則重新註冊 urlmon.dll 試試看。
按「開始」，到「執行」，輸入以下指令按確定，注意指令與參數間有一個space空格。
regsvr32 urlmon.dll
2006年11月2日星期四
[www.connect802.com] Receive Sensitivity
A fundamental specification of an 802.11 card is its receive sensitivity. The receive sensitivity is the minimum power level at which a signal can be reliably received. For example, a NIC manufacturer may indicate that their particular card has a receive sensitivity of –96 dBm at 1Mb/sec. If the actual RF energy present at that card were less than –96 dBm, then the card would no longer be able to differentiate between signal and noise. The NIC would not detect the incoming packet at all, and the packet would be lost. But how do vendors measure receive sensitivity and what are the implications of their methods for assessing an 802.11 card's performance?
We asked a major vendor of 802.11 hardware how they measured receive sensitivity in their cards. They told us that to measure receive sensitivity, the WLAN card is placed into an RF-shielded room. This guarantees that the test signal will be the only RF transmission in the room, and no background noise in the environment will interfere with the test. The test receiver is placed on a rotating turntable so that measurements can be taken (and then averaged) for all possible horizontal orientations of the receiving antenna. The vendor then transmits packets at weaker and weaker power levels. As the power level decreases, the bit error rate as measured by the card increases. The receive sensitivity of the card will be the minimum power level at which the bit error rate remained below a certain threshold. Therefore, a lower receive sensitivity value (-93 dBm) is better than a higher one (-85 dBm), since it means that the card was able to “reliably receive” data at lower power levels.
Of course, different data rates, having more and less complex encoding and modulation methods, and being more and less resistant to corruption, will result in different receive sensitivities. As data rate increases, receive sensitivity decreases. To put it another way, the higher the data rate, the stronger the signal strength must be for the packet to be reliably received. This is why 802.11 cards drop to lower data rates when interference is present or when they are at the edges of their coverage range. For example, an 802.11b card might have specifications like this:
Receive sensitivity -95 dBm at 1 Mbps
Receive sensitivity -91 dBm at 2 Mbps
Receive sensitivity -89 dBm at 5.5 Mbps
Receive sensitivity -85 dBm at 11 Mbps
While receive sensitivity might seem like a reliable way of comparing two vendors’ cards, we know of no organization that certifies the veracity of the vendor’s results. Therefore, there is the potential for vendors to manipulate the thresholds of their tests to influence their chipset’s receive sensitivity numbers. For example, a vendor that uses a BER threshold of one error in every 1,000,000,000 bits) will end up with lower receive sensitivities than a vendor that uses a BER threshold of one error in every 100,000,000 even though the second vendor’s card may actually be better at receiving bits. Fortunately, some vendors make their BER threshold available in their card's documentation.
We asked a major vendor of 802.11 hardware how they measured receive sensitivity in their cards. They told us that to measure receive sensitivity, the WLAN card is placed into an RF-shielded room. This guarantees that the test signal will be the only RF transmission in the room, and no background noise in the environment will interfere with the test. The test receiver is placed on a rotating turntable so that measurements can be taken (and then averaged) for all possible horizontal orientations of the receiving antenna. The vendor then transmits packets at weaker and weaker power levels. As the power level decreases, the bit error rate as measured by the card increases. The receive sensitivity of the card will be the minimum power level at which the bit error rate remained below a certain threshold. Therefore, a lower receive sensitivity value (-93 dBm) is better than a higher one (-85 dBm), since it means that the card was able to “reliably receive” data at lower power levels.
Of course, different data rates, having more and less complex encoding and modulation methods, and being more and less resistant to corruption, will result in different receive sensitivities. As data rate increases, receive sensitivity decreases. To put it another way, the higher the data rate, the stronger the signal strength must be for the packet to be reliably received. This is why 802.11 cards drop to lower data rates when interference is present or when they are at the edges of their coverage range. For example, an 802.11b card might have specifications like this:
Receive sensitivity -95 dBm at 1 Mbps
Receive sensitivity -91 dBm at 2 Mbps
Receive sensitivity -89 dBm at 5.5 Mbps
Receive sensitivity -85 dBm at 11 Mbps
While receive sensitivity might seem like a reliable way of comparing two vendors’ cards, we know of no organization that certifies the veracity of the vendor’s results. Therefore, there is the potential for vendors to manipulate the thresholds of their tests to influence their chipset’s receive sensitivity numbers. For example, a vendor that uses a BER threshold of one error in every 1,000,000,000 bits) will end up with lower receive sensitivities than a vendor that uses a BER threshold of one error in every 100,000,000 even though the second vendor’s card may actually be better at receiving bits. Fortunately, some vendors make their BER threshold available in their card's documentation.
[www.connect802.com] RSSI Measurement and dB-Milliwatts (dBm)
Most 802.11 analysis tools and vendors' client management utilities provide a representation of signal strength. Four units of measurement are used to represent RF signal strength in 802.11. These are: mW (milliwatts), dBm ("db"-milliwatts), RSSI (Received Signal Strength Indicator), and a percentage measurement. All of these measurements are related to each other, some more closely than others, and it's possible to convert from one unit to another.
The first two units to consider are the mW and the dBm (pronounced "dee-bee-em" or spoken as "dee-bee milliwatts"). Although these are not the most common units in 802.11, we discuss them first because they are the most basic. Just like a pound is a basic unit for measuring weight, a watt is a basic unit for measuring energy (and, in keeping with metric conventions, a mW is one one-thousandth of a watt). It turns out that measuring RF energy in mW units is not always convenient. This is due, in part, to the fact that signal strength does not fade in a linear manner, but inversely as the square of the distance. This means that if you are a particular distance from an access point and you measure the signal level, then you move twice as far away, the signal will have decreased by a factor of four. This relationship can be characterized as logarithmic, and one can say that "RF power drops off logarithmically."
The "dBm" is a logarithmic measurement of signal strength. Since it is logarithmic, just like the power of the RF signal, as the RF signal's strength changes (logarithmically), the dBm value changes linearly. To put it more generally, if you measure a quantity that changes logarithmically (RF power) with a linear unit (mW), the unit will change logarithmically, which is inconvenient. If you measure a quantity that changes logarithmically with a logarithmic unit (dBm), the unit will change linearly, which is more convenient.
dBm values can be exactly and directly converted to and from mW values. Just like miles and kilometers can be converted directly, so can mW and dBm. The formulas to convert are:
dBm = log(mW) * 10
mW = 10^(dBm/10)
We use dBm because it's much easier to say, and write, "-96dBm" than have to say "0.000 000 000 25 mW". That's a lot of zeroes! You should realize that convenience and ease-of-understanding are two fundamental reasons why the dBm metric is used for RF signal strength, rather than mW.
The IEEE 802.11 standard defines a mechanism by which RF energy is to be measured by the circuitry on a wireless NIC. In 802.11b, g, and a, this numeric value is an integer with an allowable range of 0-255 (a 1-byte value) called the Received Signal Strength Indicator (RSSI). Notice that nothing has been said here about measurement of RF energy in dBm or mW. RSSI is an arbitrary integer value, defined in the 802.11 standard and intended for use internally by the physical and data link layers (the hardware in the card and its drivers). For example, when an adapter wants to transmit a packet it must be able to detect whether or not the channel is clear (i.e.: nobody else is transmitting). If the RSSI is below some very low threshold then the chipset decides that the channel is clear. 802.11 does not require that a particular RSSI value correspond to any particular mW value, so each vendor makes this decision on its own. This means that you probably can't compare "signal strength" values between two vendors' chipsets, because those values are based on the RSSI, and different vendors' chipsets associate different power levels with different RSSI values.
To circumvent the complexities (and potential inaccuracies) of using RSSI as a basis for reporting dBm signal strength, it is common to see signal strength represented as a percentage. The percentage represents the RSSI for a particular packet divided by the maximum RSSI value (multiplied by 100 to derive a percentage). If all vendors used that formula for converting RSSI to signal strength percentage, then percentage for signal strength would provide a reasonable cross-vendor metric for use in network analysis and site survey work. However, if vendors do not consistently use the formula above, then we once again end up in a scenario where it's impossible to compare numbers from different vendors. For example, a vendor might hypothetically use a logarithmic function to map RSSI to signal strength, which would cause the signal strength to stay at high values longer as RSSI decreased, and then to drop off very rapidly as RSSI approached zero. Frankly, we don't know the exact details, on a model-number-by-model-number basis, of how each of the many NIC manufacturers map RSSI to signal strength percentage, so its difficult to draw concrete conclusions on this matter.
The first two units to consider are the mW and the dBm (pronounced "dee-bee-em" or spoken as "dee-bee milliwatts"). Although these are not the most common units in 802.11, we discuss them first because they are the most basic. Just like a pound is a basic unit for measuring weight, a watt is a basic unit for measuring energy (and, in keeping with metric conventions, a mW is one one-thousandth of a watt). It turns out that measuring RF energy in mW units is not always convenient. This is due, in part, to the fact that signal strength does not fade in a linear manner, but inversely as the square of the distance. This means that if you are a particular distance from an access point and you measure the signal level, then you move twice as far away, the signal will have decreased by a factor of four. This relationship can be characterized as logarithmic, and one can say that "RF power drops off logarithmically."
The "dBm" is a logarithmic measurement of signal strength. Since it is logarithmic, just like the power of the RF signal, as the RF signal's strength changes (logarithmically), the dBm value changes linearly. To put it more generally, if you measure a quantity that changes logarithmically (RF power) with a linear unit (mW), the unit will change logarithmically, which is inconvenient. If you measure a quantity that changes logarithmically with a logarithmic unit (dBm), the unit will change linearly, which is more convenient.
dBm values can be exactly and directly converted to and from mW values. Just like miles and kilometers can be converted directly, so can mW and dBm. The formulas to convert are:
dBm = log(mW) * 10
mW = 10^(dBm/10)
We use dBm because it's much easier to say, and write, "-96dBm" than have to say "0.000 000 000 25 mW". That's a lot of zeroes! You should realize that convenience and ease-of-understanding are two fundamental reasons why the dBm metric is used for RF signal strength, rather than mW.
The IEEE 802.11 standard defines a mechanism by which RF energy is to be measured by the circuitry on a wireless NIC. In 802.11b, g, and a, this numeric value is an integer with an allowable range of 0-255 (a 1-byte value) called the Received Signal Strength Indicator (RSSI). Notice that nothing has been said here about measurement of RF energy in dBm or mW. RSSI is an arbitrary integer value, defined in the 802.11 standard and intended for use internally by the physical and data link layers (the hardware in the card and its drivers). For example, when an adapter wants to transmit a packet it must be able to detect whether or not the channel is clear (i.e.: nobody else is transmitting). If the RSSI is below some very low threshold then the chipset decides that the channel is clear. 802.11 does not require that a particular RSSI value correspond to any particular mW value, so each vendor makes this decision on its own. This means that you probably can't compare "signal strength" values between two vendors' chipsets, because those values are based on the RSSI, and different vendors' chipsets associate different power levels with different RSSI values.
To circumvent the complexities (and potential inaccuracies) of using RSSI as a basis for reporting dBm signal strength, it is common to see signal strength represented as a percentage. The percentage represents the RSSI for a particular packet divided by the maximum RSSI value (multiplied by 100 to derive a percentage). If all vendors used that formula for converting RSSI to signal strength percentage, then percentage for signal strength would provide a reasonable cross-vendor metric for use in network analysis and site survey work. However, if vendors do not consistently use the formula above, then we once again end up in a scenario where it's impossible to compare numbers from different vendors. For example, a vendor might hypothetically use a logarithmic function to map RSSI to signal strength, which would cause the signal strength to stay at high values longer as RSSI decreased, and then to drop off very rapidly as RSSI approached zero. Frankly, we don't know the exact details, on a model-number-by-model-number basis, of how each of the many NIC manufacturers map RSSI to signal strength percentage, so its difficult to draw concrete conclusions on this matter.
[www.connect802.com] Reciprocity Theorem
There is a basic principle of antennae that is so unexpected (to the uninitiated student) that some people refuse to believe it's true the first time they hear it. The principle is called the Reciprocity Theorem. Its consequences are that, if we are using the same input and output gain, then regardless of differences in our antenna gain, if one I can hear you, you can hear me. This month, we'll explore this concept and its implications.
Consider the case where an AP has a 12 dBi omni antenna attached and a client has a 2 dBi omni antenna on a PCMCIA card. Both the AP and the client are using 15 dBm of transmit power. It might not surprise you that the AP's high-gain antenna can push a signal a long way out to the client, but you might guess that the client's low-gain antenna couldn't get a signal back to the AP. You'd be wrong. Antenna reciprocity basically means that the exact same qualities that make an antenna good at transmitting a signal also make it good at receiving a signal. To put it another way, the Rayleigh-Helmholtz reciprocity theorem states:
If an electromagnetic force of some particular magnitude is applied to the terminals of antenna "A" and the received current is measured at some other antenna "B" then an equal current (in both amplitude and phase) will be obtained at the terminals of antenna "A" if the same electromagnetic force is applied to the terminals of antenna "B".
As an analogy for an RF antenna, imagine a paddle sticking up out of the smooth surface of a lake. Another paddle is sticking up at the opposite end of the lake. One paddle begins to oscillate back and forth, creating waves that push on the other paddle, causing it to move. In our analogy, the paddles are antennas and the waves are RF waves. To carry the analogy further, imagine that one paddle is much bigger than the other--it represents our high-gain antenna. When the big paddle oscillates, it makes much bigger waves, causing the smaller antenna to move more even though it's got a smaller surface area. When the little paddle oscillates, on the other hand, the big paddle's increased surface area causes it to move more as well! The analogy fails somewhat because, in reality the increased mass of the big paddle would give it enough inertia that it wouldn't really move more, but for the sake of the analogy, the antennas are massless.
Antenna reciprocity arises from a property of physics equations called "time-symmetry". Time symmetry means that it doesn't matter whether time runs forwards or backwards, the physics equations should work out the same. Time symmetry is one of the touchstones of new physics theories. Any theory that violates time symmetry is called into serious question. To understand the significance of time symmetry, consider a pool table with a white ball near one end and a black ball in the center. The white pool ball is accelerated by the force of impact with the cue stick and travels towards the center of the pool table. In the center, the white ball strikes the black ball in a straight, center-to-center impact. The inertia of the white ball is transferred to the black ball and it is now accelerated away from the white ball in a straight line, leaving the white ball stationary at the point of impact. If you were to make a movie of the two balls striking and then played the movie backwards, it would show exactly the same thing except now it would be the black ball that starred in the opening scene of the movie. If the mass, velocity, and other characteristics of the Amazing Pool Ball Adventure movie were represented through mathematical equations, the equations would not be time dependent. Time could run forward or backward and the results would be identical.
To some readers, the reciprocity theorem may be new. The implications of antenna reciprocity are far reaching and, if this is the first time you've encountered the concept, the implications may be too hard to accept without proof. In fact, not only is reciprocity demonstrable in the lab and in real-world installations, but the physicists of the world can provide mathematical proof that the theorem holds true. If the antennae and the space between them are replaced with a network of linear, passive, bilateral impedances, then the current through the network can be calculated in accordance with standard practices in electronics theory. Whether on paper or in practice, given the same input power on both ends, "If you can hear me, then I can hear you!"
Next month, we'll discuss some of the real-world implications of antenna reciprocity
Consider the case where an AP has a 12 dBi omni antenna attached and a client has a 2 dBi omni antenna on a PCMCIA card. Both the AP and the client are using 15 dBm of transmit power. It might not surprise you that the AP's high-gain antenna can push a signal a long way out to the client, but you might guess that the client's low-gain antenna couldn't get a signal back to the AP. You'd be wrong. Antenna reciprocity basically means that the exact same qualities that make an antenna good at transmitting a signal also make it good at receiving a signal. To put it another way, the Rayleigh-Helmholtz reciprocity theorem states:
If an electromagnetic force of some particular magnitude is applied to the terminals of antenna "A" and the received current is measured at some other antenna "B" then an equal current (in both amplitude and phase) will be obtained at the terminals of antenna "A" if the same electromagnetic force is applied to the terminals of antenna "B".
As an analogy for an RF antenna, imagine a paddle sticking up out of the smooth surface of a lake. Another paddle is sticking up at the opposite end of the lake. One paddle begins to oscillate back and forth, creating waves that push on the other paddle, causing it to move. In our analogy, the paddles are antennas and the waves are RF waves. To carry the analogy further, imagine that one paddle is much bigger than the other--it represents our high-gain antenna. When the big paddle oscillates, it makes much bigger waves, causing the smaller antenna to move more even though it's got a smaller surface area. When the little paddle oscillates, on the other hand, the big paddle's increased surface area causes it to move more as well! The analogy fails somewhat because, in reality the increased mass of the big paddle would give it enough inertia that it wouldn't really move more, but for the sake of the analogy, the antennas are massless.
Antenna reciprocity arises from a property of physics equations called "time-symmetry". Time symmetry means that it doesn't matter whether time runs forwards or backwards, the physics equations should work out the same. Time symmetry is one of the touchstones of new physics theories. Any theory that violates time symmetry is called into serious question. To understand the significance of time symmetry, consider a pool table with a white ball near one end and a black ball in the center. The white pool ball is accelerated by the force of impact with the cue stick and travels towards the center of the pool table. In the center, the white ball strikes the black ball in a straight, center-to-center impact. The inertia of the white ball is transferred to the black ball and it is now accelerated away from the white ball in a straight line, leaving the white ball stationary at the point of impact. If you were to make a movie of the two balls striking and then played the movie backwards, it would show exactly the same thing except now it would be the black ball that starred in the opening scene of the movie. If the mass, velocity, and other characteristics of the Amazing Pool Ball Adventure movie were represented through mathematical equations, the equations would not be time dependent. Time could run forward or backward and the results would be identical.
To some readers, the reciprocity theorem may be new. The implications of antenna reciprocity are far reaching and, if this is the first time you've encountered the concept, the implications may be too hard to accept without proof. In fact, not only is reciprocity demonstrable in the lab and in real-world installations, but the physicists of the world can provide mathematical proof that the theorem holds true. If the antennae and the space between them are replaced with a network of linear, passive, bilateral impedances, then the current through the network can be calculated in accordance with standard practices in electronics theory. Whether on paper or in practice, given the same input power on both ends, "If you can hear me, then I can hear you!"
Next month, we'll discuss some of the real-world implications of antenna reciprocity
[www.connect802.com] How To Get More Range: Output Power, Antenna Gain, or Receive Sensitivity?
This month's "Essential Wi-Fi" addresses the issue of increasing range by increasing output power. The specific example given is of switching to higher gain antennas. In that article, we discuss how simply increasing output power may not result in the increase in usable range that you expect. It turns out that the usable range of an AP is dependent on several factors. In this column, we examine those factors and discuss the best way to increase the range of an AP in an indoor environment.
We all know of vendors who sell access points with extremely high power output. For example, one vendor sells an AP that outputs a full watt of power (the FCC maximum) with no external amplifiers. Another vendor sells a PCMCIA card that outputs 250 mW with no external amplification (most PCMCIA cards are around 30 mW). Clients using these devices and expecting dramatic range increases may be disappointed. An 802.11 link is two-way; it doesn't do any good for the one device to be able to blast a signal a long way if the other device can't get a signal back. For example, a 100 mW AP might be able to blast its Beacon packets through several walls. A regular PCMCIA card with 30 mW of output power could receive those Beacons and the user would see the AP in his or her list of available wireless networks. But when the user tried to connect to the AP, the user's card wouldn't have enough power to get back to the AP, and the connection would always fail.
This example points out a fundamental limitation of high-output APs. It might seem at first like a high-powered AP is a great, cost-effective way of increasing the range of your network. An casual site survey, which only measured receive signal strength, would even confirm that the AP's coverage had increased (if all you intend to do is receive packets, then the site survey is right). But as soon as clients actually tried to connect to the network, the flaw would become obvious. In general, there's not much point in having an AP that is much more powerful than the clients that it will serve. All of the AP's extra transmit power will go into pushing signal into areas where the clients can't get a signal back to the AP, and that essentially wastes that coverage, and all the money you spent on a high-powered AP. Typical wireless client cards have an output power between 15 and 30 mW, so we at Connect802 usually design networks with APs that have approximately this output power.
Increased output power is useful if you know that all of the devices in the network will be using the same power output. For example, if you've got two wireless bridges at the end of a point-to-point link, you could increase the usable range of the link by putting an equal amplifier on both of the bridges. This circumstance is difficult to guarantee in the more typical one-AP, many clients scenario, so it's usually better to design that type of network with APs that only put out as much power as the lowest-powered client that you expect to use the network. Another option is to design the network so that clients using maximum power (say, 30 mW) will get maximum data rates (say, 54 Mbps) and clients using less power will still be able to connect, but only at lower data rates.
Increased power output in the AP doesn't result in increased range if clients are not able to get a signal back to the AP, but there's a second parameter in this equation that we have so far neglected: receive sensitivity. Receive sensitivity determines the weakest signal that a device can reliably receive. If an AP combines increased power output with a proportionally better receive sensitivity, then it can not only transmit a more powerful signal to the client, but it can also receive the client's weaker signal. For example, consider a client that transmits at 15 mW and has a receive sensitivity of -72 dBm. Now consider an AP that has a transmit power of 30 mW. The AP is transmitting 3 dB "louder" than the client, so its receive sensitivity must be 3 dB better than the client's (-72 dBm minus 3 dB = -75 dBm) to offset the client's weaker transmit power. If this relationship holds true, then the combination of increased transmit power and increased receive sensitivity will result in better range. To put it simply, just making the AP talk louder doesn't work, but by increasing the AP's receive sensitivity, we've made it both talk louder and listen harder.
Unlike transmit power, the receive sensitivity of an radio is not directly adjustable; it's a fundamental property of the engineering of the radio. But the receive sensitivity of an 802.11 radio differs depending on the data rate that the radio is using, with higher data rates requiring more signal power. Typically, an 802.11 network is designed towards a certain minimum desired data rate, and then radios are purchased with an output power and a receive sensitivity that allows them to achieve that data rate in the required coverage area.
At this point, we have discussed the relationship between usable range, transmit power, and receive sensitivity. Essentially, we are considering two link budgets: one from the client to the AP and one from the AP to the client. These link budgets give a maximum range from the AP to the client and from the client to the AP. The usable range of the AP is limited by the smaller of these two ranges. If the AP-to-client range is already greater than or equal to the client-to-AP range, then increasing the AP's transmit power won't result in more usable range because increasing transmit power only increases the AP-to-client side of the link. Increasing transmit power with a corresponding increase in receive sensitivity will result in more usable range because increasing receive sensitivity also increases the client-to-AP side of the link.
Increasing transmit power can increase usable range in another way that is somewhat independent of receive sensitivity. Each 802.11 data rate requires a certain minimum signal-to-noise ratio. If the signal is too weak to achieve the required signal-to-noise ratio for a given data rate, increased transmit power is the only answer. Increasing receive sensitivity or antenna gain doesn't work because both of those options will amplify the noise as well as the incoming signal, leaving the signal-to-noise ratio the same. In summary, if range is limited by interference, you must at least increase transmit power to the point where the clients receive the minimum signal-to-noise ratio for the desired data rate. At that point, range may still be limited by the AP's receive sensitivity or the client's transmit power or receive sensitivity.
Increasing antenna gain is another way of increasing range. Increasing antenna gain differs from increasing transmit power because of the principle of antenna reciprocity. Put one way, antenna reciprocity states that the same qualities that increase an antenna's gain when it transmits also increase its gain when it receives. This means that an increase in antenna gain on either the client or the AP increases BOTH the client-to-AP side of the link AND the AP-to-client side of the link.
Putting a higher-gain antenna on the access point can be a cheap way of increasing usable range for all clients of that AP, regardless of the clients' transmit powers and receive sensitivities. The tradeoff is that increasing antenna gain changes the antenna's coverage pattern, which may limit the maximum antenna gain that can be achieved. More than about 6-10 dB of antenna gain usually results in coverage area that is small enough to preclude any link except one with fixed endpoints at pre-determined locations.
What conclusions can we draw from this analysis? Putting a higher-gain antenna on the AP increases performance for all clients of the AP, but may not offer enough extra power to significantly increase coverage, especially in indoor environments. Increasing the AP's power output might seem like a cheap way of increasing range, but in reality, a complex link budget relationship exists between transmit power and receive sensitivity on the client and AP that dictates the usable range for each client. Simply dropping a 1000 mW into the center of a building is unlikely to provide the results that the AP's vendor would promise or that naive WLAN administrator might expect. Buying an AP with a very good receive sensitivity (for example, an 802.11g AP with receive sensitivity of -75 dBm or lower at 54 Mbps, -95 dBm or better at 6 Mbps) is a cost-effective way of maximizing the effectiveness of the clients' power output.
We all know of vendors who sell access points with extremely high power output. For example, one vendor sells an AP that outputs a full watt of power (the FCC maximum) with no external amplifiers. Another vendor sells a PCMCIA card that outputs 250 mW with no external amplification (most PCMCIA cards are around 30 mW). Clients using these devices and expecting dramatic range increases may be disappointed. An 802.11 link is two-way; it doesn't do any good for the one device to be able to blast a signal a long way if the other device can't get a signal back. For example, a 100 mW AP might be able to blast its Beacon packets through several walls. A regular PCMCIA card with 30 mW of output power could receive those Beacons and the user would see the AP in his or her list of available wireless networks. But when the user tried to connect to the AP, the user's card wouldn't have enough power to get back to the AP, and the connection would always fail.
This example points out a fundamental limitation of high-output APs. It might seem at first like a high-powered AP is a great, cost-effective way of increasing the range of your network. An casual site survey, which only measured receive signal strength, would even confirm that the AP's coverage had increased (if all you intend to do is receive packets, then the site survey is right). But as soon as clients actually tried to connect to the network, the flaw would become obvious. In general, there's not much point in having an AP that is much more powerful than the clients that it will serve. All of the AP's extra transmit power will go into pushing signal into areas where the clients can't get a signal back to the AP, and that essentially wastes that coverage, and all the money you spent on a high-powered AP. Typical wireless client cards have an output power between 15 and 30 mW, so we at Connect802 usually design networks with APs that have approximately this output power.
Increased output power is useful if you know that all of the devices in the network will be using the same power output. For example, if you've got two wireless bridges at the end of a point-to-point link, you could increase the usable range of the link by putting an equal amplifier on both of the bridges. This circumstance is difficult to guarantee in the more typical one-AP, many clients scenario, so it's usually better to design that type of network with APs that only put out as much power as the lowest-powered client that you expect to use the network. Another option is to design the network so that clients using maximum power (say, 30 mW) will get maximum data rates (say, 54 Mbps) and clients using less power will still be able to connect, but only at lower data rates.
Increased power output in the AP doesn't result in increased range if clients are not able to get a signal back to the AP, but there's a second parameter in this equation that we have so far neglected: receive sensitivity. Receive sensitivity determines the weakest signal that a device can reliably receive. If an AP combines increased power output with a proportionally better receive sensitivity, then it can not only transmit a more powerful signal to the client, but it can also receive the client's weaker signal. For example, consider a client that transmits at 15 mW and has a receive sensitivity of -72 dBm. Now consider an AP that has a transmit power of 30 mW. The AP is transmitting 3 dB "louder" than the client, so its receive sensitivity must be 3 dB better than the client's (-72 dBm minus 3 dB = -75 dBm) to offset the client's weaker transmit power. If this relationship holds true, then the combination of increased transmit power and increased receive sensitivity will result in better range. To put it simply, just making the AP talk louder doesn't work, but by increasing the AP's receive sensitivity, we've made it both talk louder and listen harder.
Unlike transmit power, the receive sensitivity of an radio is not directly adjustable; it's a fundamental property of the engineering of the radio. But the receive sensitivity of an 802.11 radio differs depending on the data rate that the radio is using, with higher data rates requiring more signal power. Typically, an 802.11 network is designed towards a certain minimum desired data rate, and then radios are purchased with an output power and a receive sensitivity that allows them to achieve that data rate in the required coverage area.
At this point, we have discussed the relationship between usable range, transmit power, and receive sensitivity. Essentially, we are considering two link budgets: one from the client to the AP and one from the AP to the client. These link budgets give a maximum range from the AP to the client and from the client to the AP. The usable range of the AP is limited by the smaller of these two ranges. If the AP-to-client range is already greater than or equal to the client-to-AP range, then increasing the AP's transmit power won't result in more usable range because increasing transmit power only increases the AP-to-client side of the link. Increasing transmit power with a corresponding increase in receive sensitivity will result in more usable range because increasing receive sensitivity also increases the client-to-AP side of the link.
Increasing transmit power can increase usable range in another way that is somewhat independent of receive sensitivity. Each 802.11 data rate requires a certain minimum signal-to-noise ratio. If the signal is too weak to achieve the required signal-to-noise ratio for a given data rate, increased transmit power is the only answer. Increasing receive sensitivity or antenna gain doesn't work because both of those options will amplify the noise as well as the incoming signal, leaving the signal-to-noise ratio the same. In summary, if range is limited by interference, you must at least increase transmit power to the point where the clients receive the minimum signal-to-noise ratio for the desired data rate. At that point, range may still be limited by the AP's receive sensitivity or the client's transmit power or receive sensitivity.
Increasing antenna gain is another way of increasing range. Increasing antenna gain differs from increasing transmit power because of the principle of antenna reciprocity. Put one way, antenna reciprocity states that the same qualities that increase an antenna's gain when it transmits also increase its gain when it receives. This means that an increase in antenna gain on either the client or the AP increases BOTH the client-to-AP side of the link AND the AP-to-client side of the link.
Putting a higher-gain antenna on the access point can be a cheap way of increasing usable range for all clients of that AP, regardless of the clients' transmit powers and receive sensitivities. The tradeoff is that increasing antenna gain changes the antenna's coverage pattern, which may limit the maximum antenna gain that can be achieved. More than about 6-10 dB of antenna gain usually results in coverage area that is small enough to preclude any link except one with fixed endpoints at pre-determined locations.
What conclusions can we draw from this analysis? Putting a higher-gain antenna on the AP increases performance for all clients of the AP, but may not offer enough extra power to significantly increase coverage, especially in indoor environments. Increasing the AP's power output might seem like a cheap way of increasing range, but in reality, a complex link budget relationship exists between transmit power and receive sensitivity on the client and AP that dictates the usable range for each client. Simply dropping a 1000 mW into the center of a building is unlikely to provide the results that the AP's vendor would promise or that naive WLAN administrator might expect. Buying an AP with a very good receive sensitivity (for example, an 802.11g AP with receive sensitivity of -75 dBm or lower at 54 Mbps, -95 dBm or better at 6 Mbps) is a cost-effective way of maximizing the effectiveness of the clients' power output.
[2006-11-02] Make your blog, Trace your fault
話咁快又成個月無打blog, 可能依排假期多, 係呀周生無講多, "假期多". 假期多可能個人都唔想做野, 精神消磨, 意志力銳減, 骨質疏鬆..等等. 放假會成日比自己藉口話遲幾日先再寫返, 或者話放假其實都無咩要記得. 到有野真係要記低, 就可能因為太多而唔想記, 不過周生此級覺得養成日日寫Blog係一個好習慣, 真係同自己做返o的record, Make your blog, Trace your fault, 有證有據唔好抵賴. 廢話少講, 講返依排做過o的咩先.
1. Hiteatea - 繼續沉醉係looping o既階段, 周生之前所講o既assumption可能都係錯的, 當然亦有一定o既因素. 不過個場去到一鑊泡o既田地真係唔係幾想再搞落去. 其實係有心想搞的, 但礙於太多人事及人力o既問題, 放軟唻搞都不失為一個上策. 不過諗唔到個cause又真係幾唔憤氣, 話晒都CWNE..XDDD, 會覺得很廢的. 不過周生覺得經一事長一智, 今次o既失敗對將來做返similar o既project會有很大o既幫助. 總括周生study o左咁多outdoor wireless, 天線/amp o既關係, 會有以下結論
- 唔好加AMP, 如加AMP要非常小心, 因為很很容易就會造成UPE (Unbalanced Power Effect), 導致client唔夠power嗌返signal 轉, 周生亦覺得咁會極度影響client o既roaming, 同埋speed selection, 因為佢會覺得隻AP係好很遠, 所以用high data rate但事實並非如此
- 所以唔夠distance最好係用high gain antenna, 用個radio pattern去就返個client, 因為天線有個Reciprocity Theorem, 簡單o的唻講即係client係個個位收到應該都天線都會收到. 如個天線好啦high gain,個client好遠都收到and send到
- 用低gain antenna whenever possible, 因為咁可以predict到個cell size, 唔會好遠都仲收到
- 唔好用Meru..XD
2. CCIE Lab都係仲hea緊, 發覺自己好多CCNP o既東西都未係好熟, 真係要煲熟先再操lab, 如果唔係越做越無心機. 不過好幸運地已經acquire o左CWNE#26了, 很是開心, 多謝各界好友o既支持. Home lab真係砌好o左, 所以要快o的開始周生IE生涯了. 買返CWNE書了, 不過未拎, 希望睇返熟o的野, 睇o下可唔可以向依方面發展.
3. 拎了袍, 紫色周生覺得靚o左, 不過相就唔係影o左好多, 遲o的約大家一齊去影相相, 呵呵
4. 買了N73電話, 發覺Nokia自家出個個Phone Management Software真的不滯, 周生自掏荷包買另外一個, 主要因為搵唔到烈, 有烈就唔駛咁慘了..>.<. 買o左幾舊水, 頂佢個肺佢個outlook import/export話business license先有, 周生再頂佢個肺咪話你個web site無寫明個licensing scheme, 仲要話係cheated by 佢個公司, 好彩, 佢個蘇聯女人可能知周生火都唻埋, 決定upgrade做business license. 依個故事教訓我地做人要努力爭取自己o既權益, 有時係爭取到o的你expect唔到o既野的.正@@
寫住咁多先, 依個都算係一個十月summary啦~ yeah
1. Hiteatea - 繼續沉醉係looping o既階段, 周生之前所講o既assumption可能都係錯的, 當然亦有一定o既因素. 不過個場去到一鑊泡o既田地真係唔係幾想再搞落去. 其實係有心想搞的, 但礙於太多人事及人力o既問題, 放軟唻搞都不失為一個上策. 不過諗唔到個cause又真係幾唔憤氣, 話晒都CWNE..XDDD, 會覺得很廢的. 不過周生覺得經一事長一智, 今次o既失敗對將來做返similar o既project會有很大o既幫助. 總括周生study o左咁多outdoor wireless, 天線/amp o既關係, 會有以下結論
- 唔好加AMP, 如加AMP要非常小心, 因為很很容易就會造成UPE (Unbalanced Power Effect), 導致client唔夠power嗌返signal 轉, 周生亦覺得咁會極度影響client o既roaming, 同埋speed selection, 因為佢會覺得隻AP係好很遠, 所以用high data rate但事實並非如此
- 所以唔夠distance最好係用high gain antenna, 用個radio pattern去就返個client, 因為天線有個Reciprocity Theorem, 簡單o的唻講即係client係個個位收到應該都天線都會收到. 如個天線好啦high gain,個client好遠都收到and send到
- 用低gain antenna whenever possible, 因為咁可以predict到個cell size, 唔會好遠都仲收到
- 唔好用Meru..XD
2. CCIE Lab都係仲hea緊, 發覺自己好多CCNP o既東西都未係好熟, 真係要煲熟先再操lab, 如果唔係越做越無心機. 不過好幸運地已經acquire o左CWNE#26了, 很是開心, 多謝各界好友o既支持. Home lab真係砌好o左, 所以要快o的開始周生IE生涯了. 買返CWNE書了, 不過未拎, 希望睇返熟o的野, 睇o下可唔可以向依方面發展.
3. 拎了袍, 紫色周生覺得靚o左, 不過相就唔係影o左好多, 遲o的約大家一齊去影相相, 呵呵
4. 買了N73電話, 發覺Nokia自家出個個Phone Management Software真的不滯, 周生自掏荷包買另外一個, 主要因為搵唔到烈, 有烈就唔駛咁慘了..>.<. 買o左幾舊水, 頂佢個肺佢個outlook import/export話business license先有, 周生再頂佢個肺咪話你個web site無寫明個licensing scheme, 仲要話係cheated by 佢個公司, 好彩, 佢個蘇聯女人可能知周生火都唻埋, 決定upgrade做business license. 依個故事教訓我地做人要努力爭取自己o既權益, 有時係爭取到o的你expect唔到o既野的.正@@
寫住咁多先, 依個都算係一個十月summary啦~ yeah
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