Khoa hoc moi cua chung ta se khai giang vao ngay THU BA 8 THANG 9 NAM 2009.  (Tuesday September 8,2009.)  Moi buoi hoc rut ngan lai tu 6:30 den 9:00 toi.

Mong gap mat lai tat ca anh chi cua khoa cu.  Nho cac anh chi thong bao den cac ban moi muon trao doi them kien thuc ve nganh dien toan cung tham du.

BO CHUA (STORAGE)

Bo chua giup chung ta luu tru tin hoc vinh vien ma khong can phai cung cap dien nang cho no. Bo chua co rat nhieu dang, nhung thong dung nhut hien nay la o dia cung (HardDrive) va the nho Flash (Solid-State Memory, Flash Drive, hoac Memory Card).

De mo rong thi truong va canh tranh manh voi o dia cung, gan day mot so hang tung ra thi truong loai dia nho Flash, con goi la SSD (Solid-State Drive), chu yeu nham vao may xach tay nhe.

Cac dac tinh uu dai cua SSD bao gom:

Dac tinh uu viet cua SSD la do ben vung (durability),  yen tinh (quiet) va nguoi (low heat).

Ke den la kha nang doc nhanh bao gom thoi gian doc ngau nhien (random access times) va tai luong tin hoc (Read throughput). Thoi gian doc ngau nhien cua MLC vao khoang 3% va SLC vao khoang 1% so voi thoi gian doc cua o dia cung trung binh la 12ms.

Tuoi tho cao, co the hoat dong toi 60 nam.

Nguoc lai, SSD cung co nhuoc diem nhu sau:

Gia thanh cao, 1 dia Flash chi chua 64GB voi gia thanh tu $300 cho toi $1,500.

Suc chua thap, hien nay tren thi truong chi moi pho thong loai 64GB va 128GB. Dis co suc chua 256GB se gia nhap gia dingh SSD trong thoi gian gan.

Khong tiet kiem dien nang hon o dia cung, co khi dia SSD con tieu nhieu dien hon ca o dia cung.

Viet cham hon o dia cung trong ca hai ung dung viet ngau nhien va viet lien tuc, (Random write & sequential writes), voi ca hai nhom MLC va SLC. MLC viet rat cham vao khoang 25% van toc viet cua o dia cung, trong khi SLC thi ngang ngua voi o dia cung.

Ngan chua co the bi chung quen dan voi ket qua la tin hoc se bien mat sau mot so lan dung, hien nay vao khoang 10,000 tai dung cho the nho va 50,000 cho dia nho.

BO NHO (MEMORY)

Trong may dien toan co ba bo nho quan trong khac nhau:

Bo nho vinh cuu (Non-Volatile Memory hay Firmware hay Flash BIOS) dung de tru tin hoc chu chot cho may ma neu hu hong hay thieu sot se lam cho may khong the khoi dong duoc. Bo nho nay co ten goi la BIOS, bao gom nhung huan lenh can ban de khoi dong may khi ta mo nut dien (Power Switch, Power Button) vao may. Bo nho nay khong the thay doi hay xoa di du may hoan toan khong co dien nang.

Hien nay IE van chiem khoang 72% thi truong, trong khi Firefox kiem soat gan 20%, phan con lai chia cho Apple Safari, Opera 9.5 va nhieu hang nho khac.

Moi nhin thi tuong la Chrome se doi dau voi IE, nhung thuc ra Fire fox moi la doi thu chinh cua Chrome, vi ca hai cung thai nghen tu phong trao thao chuong mo rong (open source movement).

Viec tung Chrome ra thi truong nhan ngay sinh nhat thu 10 cua Goolgle khong phai la mot viec ngau nhien, nhung nam trong chien thuat kinh doanh cua Google nham thong linh thi truong truy cap tiep thi nham danh do vai tro ba quyen cua Microsoft tren lanh vuc may dien toan ca nhan.

Chrome duoc thiet ke de van hanh don gian vi khong co menu bar, cac chon lua thay doi phai nho vao hai icons nam gan location bar. Chrome chi co address bar ma khong co search bar, nen nguoi dung co the truy cap thong tin hoac trang web voi cung mot phuong phap. Thi du, de tim tai tu cine, ban co the go vao IMDB, xong bam Tab key, roi ten tai tu. IMDB la chu viet tat cua Internet Movie Database. Chrome cung khong co progress bar nen ta khong biet duoc trang Web dang van trien ra sao. Click tren Tab moi se xo danh sach cac web site ua chuong va cac dich chuyen (links) lien he.

Bu lai, Chrome chep trang nhanh nhu Firefox va thuong khong bi nghet tai (hung up, choking) nhu Firefox vi Chrome dung “isolated” tabs de quan tri trang nhu windows task Manager nen ta co the tam thoi dong lai trang nao xu dung qua nhieu tai nguyen cua may . Tab cua trang bi co lap trong khung rieng, (isolated sandbox), va cac phu nhu (plug-ins) cung duoc ban cho tab rieng de khi nguoi dung dong tab thi ca van trinh (process) se bi tieu dut. Ngoai ra, khi mot web site khong the tu khoi (launch) duoc, Google co the mo trang Web tu bo tru tam cua no.

Chrome da dung cac co phan tu Apple’s WebKit and Mozilla’s Firefox, voi Webkit de dieu hanh bo nho them huu hieu va cung giup cho cac nha thao chuong de trao doi va dong gop vao viec phat huy cac trang phuc tap. Phan lon dac tinh cua Tab lay tu cao-toc ket-noi (speed dial) cua Opera.

Giong nhu IE8, Chrome co mot khung ca biet (private) goi la incognito mode, tuong trung voi hinh mot con nhen tren goc trai cua browser. Dung mode nay, cac giao chuyen voi trang Web se khong bi ghi nho voi cookies hay cac dau truy tim khac.

Mot dac tinh khac la Google hoa nhap (integrate) Google Gears, hay con goi la applications Shorcuts, giup nguoi dung cat giu cac web apps nhu la cac ung dung cua desktop vay.

Cac dien bien lien he toi Browsers:

Tuan qua, cuoi thang 8, 2008, Microsoft tung ra Explorer 8 beta 2, voi cai tien ve an ninh va tro luc cho truy cap.

Fire Fox the he thu 3 vua moi chao doi, voi 8 trieu ha tai (down load) trong 24 tieng.

Hop dia chi (address bar) co the chuyen dang thanh hop het xay (awsome bar).

Mercury News co mot bai viet ngay thua hai 6/23/2008 voi ket luan nhu sau:

“If you’re already using Firefox, version 3 is worth the upgrade. If you aren’t yet using Firefox, you should definitely check out the latest version. Doing so is easy: The software is free, takes only a few minutes to download and install and doesn’t require you to reboot your computer after you do so.”

Toan bai viet co the doc tai: http://www.mercurynews.com/businessheadlines/ci_9671716?nclick_check=1

LCDs khong dung he tu phat quang ma phai le thuoc vao nen tro quang (back Lighting) de hinh anh hien thi (visible). Co nhieu phuong phap khac nhau de roi sang nen tro quang, nhung hai phuong phap sau day lduoc xem la pho thong nhat.

Cold Cathode Fluorescent Lamp (CCFL) Backlighting

Cold Cathode Fluorescent Lamp, or CCFL, da duoc phan lon cac nha san xuat xu dung trong suot nhung nam qua. CCFL co dac diem la tieu thu it dien nang nhung cung ung duoc anh sang trang xoa (bright white light) phan deu den mat anh. CCFL duoc dung doc theo cac canh cua man anh, chieu xuyen qua bo phan quang (diffuser), nho vao dien the tu 270 den 300VAC duoi tan so 35KHz nhan tu may chuyen the inverter.

CCFL thuong co tuoi tho tu 10,000 den 20,000 gio nhung rat nhay ung voi giao dong (vibration) vi tuoi tho co the giam den 50%. Ca hai CCFL va bo chuyen the deu khong thich nhiet do thap, co the anh huong toi 60% kha nang phat quang cua CCFL, dong thoi cuong do phat sang khong the thay doi duoc, hoac sang hoac toi chu khong the mo hay lu duoc.

LED Backlighting

Light Emitting Diode, or LED, backlight dang duoc tung ra thi truong nhu la ky thuat cao cua LCD nham canh trnah voi ky thuat plasma. LED nen quang co doi song lau toi 50,000 tieng, tuong duong voi 15 nam dung suot 8 tieng moi ngay, khong chiu anh huong cua giao dong, dien the thap va co the kiem soat duoc cuong do phat quang , tu lu mo den sang choi.

Nhung kho noi LED nen quang tieu thu dien nang nhieu, du dien the van hanh rat thap o 5VDC va co the dung ca 12VDC.

]]> http://dientoanbacai.wordpress.com/2008/07/29/ky-thuat-chieu-sang-mang-anh-mong-lcd-display/feed/ 2 dientoanbacai LCD Backlighting - CCFL CCFL Backlighting - Temperature Preformance & Module Cross-section LCD Backlighting - LED LED Backlighting - Module cross-sections http://dientoanbacai.wordpress.com/2008/07/22/gps-he-dinh-vi-toan-cau-global-positioning-system/ http://dientoanbacai.wordpress.com/2008/07/22/gps-he-dinh-vi-toan-cau-global-positioning-system/#comments Tue, 22 Jul 2008 00:23:08 +0000 dientoanbacai http://dientoanbacai.wordpress.com/?p=17 Continue reading →]]> Neu ban biet duoc khoang cach tu vi tri cua ban den ba diem khac nhau co toa do xac dinh, ban co the dung toan luong giac de cham toa do cua ban tren ban do the gioi mot cach chinh xac. Nganh ky nghe dinh vi toan cau dung 24 ve tinh treo trong khong gian o cao do 12,645 dam trong he thong NAVSTAR (Navigation Satellite Timing And Ranging) de lam ban dong hanh cung ban ngao du khap hang cung ngo hep tren nam chau bon bien voi mot may GPS ma van biet ngo ve. Hien nay NAVSTART co them 5 ve tinh phong ho dung trong truong hop khan cap.

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Moi ve tinh nang khoang tu 2000 den 4000 lbs voi tam tay vuon dai 17 feet ngao du vao khoang 7000 dam mot gio, nen co the hoan tat mot vong dia cau trong khoang nua ngay duong, duoi theo tren cung hanh trinh nhung anh em ve tinh khac cung chia xe 6 qui dao voi nhau.

Neu ban nguoc mat nhin mong lung den duong chan troi, ban co the kham pha ra it nhat la 4 ve tinh dang vui dua ben tren vong dai 15 do va ben len ngam nhin sinh hoat cua ban. Cac ve tinh ca hat suot ngay voi ba diep khuc:

a pseudorandom code, satellite ephemeris, and almanac data. Ve tinh dung nhac dieu chao anh “pseudorandom code” de gioi thieu minh voi so ly lich “satellites’ PRN (pseudorandom number) tu 1 toi 32.

Diep khuc “than phan” (ephemeris data} thong bao tinh trang van hanh cua ve tinh voi gio giac duoc cap nhat bang dong ho dien tu, atomic clocks.

Sau cung voi nhac dieu “Em o day” (almanac data) dung bao cho bo nhan GPS (receivers) vi tri cua toan bo gia dinh ve tinh.

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Cong thuc dung de xac dinh vi tri cua mot diem thuong goi la triangulation dua tren goc do va it nhat mot khoang cach da biet. Voi GPS, ky thuat dung hoi khac mot ti va goi la tam-giac-nhi-so (trilateration) with it nhat hai khoang cach da biet tu ba toa do. Thuc ra phai goi ky thuat GPS la tam-giac-da-so moi dien ta dung dan cong thuc dung de xac dinh vi tri 3 chieu. Voi ky thuat nay, it nhat la 3 khoang cach phai biet truoc tu 4 toa do, de xac dinh mot diem voi 3 thong so: vi do, kinh do va cao do. Voi du kien duoc cap nhat lien tuc, van toc (ground speed) va huong di chuyen (ground track, direction of travel) cung co the tinh tu may nhan GPS.

Cac GPS cu chi co mot mach phan giai so, trong khi may moi thuong duoc tranng bi tu 5 den 12 bang va hoat dong dong hanh ( parallel). Du ve tinh lo lung ngay tren dinh dau cua GPS, cung phai mat it nhat la 0.06 giay de tin hieu di chuyen tu ve tinh den may nhan.

Mot milisecond co the di chuyen vi tri ra xa 300 met sai trat.

Mot ten tuoi quen thuoc trong nganh dien thoai luoi la Skype, voi hon 300 trieu lan ha tai (download) nhu lieu Skype can thiet cho viec ket noi mot cu dien thoai luoi. Hom 8 trieu nguoi dung nhu lieu Skype nay cung mot luc (concurrent users) ghi lai mot ky luc moi trong ky thuat dien thoai luoi vo phi (free).

Vonage cung la mot ten tuoi lung danh nhu Skype, nhung la mot dich vu VOIP re tien chu khong vo-phi nhu Skype, cung da dat duoc 2 trieu khach hang vao thang 9 nam 2006.

POTS dung mach hai chieu truc dung (two-way circuit switching network) de hoan tat mot cu dien thoai, voi nguoi nghe va nguoi noi chiem dung nguyen mach tu dung (private data highway for the duration of the conversation) trong suot cuoc dien dam.

Nguoc lai, VOIP dong goi tin thoai (audio content) trong goi nho voi thoi luong 100ms (packets) va chuyen vao lien luoi (internet).  VOIP co khuyet diem la le thuoc vao cac cong ty dich vu lien luoi va dien ich de duy tri cac cuoc dien dam, trong khi cac mach tu dung bao dam luon luon hien dung du co mat dien nang di nua.

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Ky thuat Ethernet ra doi tai trung tam khao cuu Xerox (Xerox’s Palo Alto Research Center) giua thap nien 1970s, cho phep cac may dien toan chuyen tin voi nhau bang cach dung dia chi MAC (Media Access Control), mot thong so cung gom 6 byte duoi dang HEX. Thi du may goi co MAC la 80 00 20 7A 3F 3E va may nhan co MAC la 80 00 20 20 3A AE.

MAC duoc chon tu danh sach cua nha san xuat NIC (Network Interface Card) da duoc thoa thuan toan cau va khong duoc tai dung.

Cac may phai lang nghe tin hieu trong cua he luoi truoc khi chuyen tin de tranh tin nan (collision) voi ky thuat CSMA/CD (Carrier Sense Multiple Access with Collision Detect).

The he dau tien cua he luoi rat gioi han va duoc nang cap khi hop dum (hub) ra doi.  Sau do hop chuyen (switch) thuc su mang lai mot cai thien quan trong cho he luoi.  Moi cua noi (port) cua switch co the nhan tin hieu truc tiep tu he luoi ma cac cua noi khac khong can du toi.

Co nhieu loai day cap khac nhau: BNC, Unshielded Twisted Cable, Cat5 , Cat 5e, Cat6.

Wi-Fi cung dung mot he thong chuyen tin nhu Ethernet nhung khong xai day.

Thong thuong Microsoft dat ten Windows khong theo mot qui uoc nao ca. Ngay xua Windows duoc dat ten theo the he so tu 1 den 3. Dot nhien Microsoft doi ten the he thu tu cua OS, dua tren nam chao doi, nen chung ta co Windows 9x bao gom 95, 98, 98SE va 98 ME, that ra cung la Windows 4 ca.

Den the he thu 5, Microsoft lai dung mot cach dat ten moi XP, vua bi mat vua don gian, de nhan manh kha nang hanh xu doi dau voi he van hanh cua Apple. The he thu 6 ra doi voi ten Vista.

Bay gio thi Microsoft lai muon tro lai voi ten so vi Apple chi goi OS cua ho voi so La Ma la Mac OS X ma thoi ,nhung he nay da qua mat Vista rat xa ve kha nang xu ly thong tin.

The he 7 Windows se chao doi vao cuoi nam 2009 hoac dau nam 2010 voi haitrung tac (interface) moi: dien xuc (surface touch) va da xuc kiem don (multi touch control menu).