English narration · English + 中文 subtitles burned in · การบรรยายภาษาอังกฤษ · คำบรรยายภาษาอังกฤษ + 中文 ลอยตัวบนภาพ
2.1
What is data transmission? · การส่งผ่านข้อมูลคืออะไร?
English
Data transmission 数据传输 means moving data from one place to another — for example from your computer to a website, or from a phone to a printer.
Data often travels along a cable. Two common ones are shown below.
Before it is sent, the data is broken down into small, equal-sized pieces called packets 数据包. Each packet travels separately and they are put back together at the other end.
Using packets has benefits:
if one packet is lost or damaged, only that packet is sent again, not the whole file;
packets can take different routes, so a busy or broken route can be avoided;
many users can share the same connection at the same time.
The structure of a packet · โครงสร้างของแพ็กเก็ต
Syllabus · หลักสูตร
English
Candidates should be able to:
Notes and guidance
1 (a) Understand that data is broken down into packets to be transmitted
(b) Describe the structure of a packet
• A packet of data contains a: – packet header – payload – trailer • The packet header includes the: – destination address – packet number – originator’s address
(c) Describe the process of packet switching
• Data is broken down into packets • Each packet could take a different route • A router controls the route a packet takes • Packets may arrive out of order • Once the last packet has arrived, packets are reordered
2 (a) Describe how data is transmitted from one device to another using different methods of data transmission
How data travels in packets · วิธีการเดินทางของข้อมูลในแพ็กเก็ต
Step through packet switching. Splitting a file into addressed packets is why one lost packet only needs resending, and why many users can share the same line. · ขั้นตอนการทำงานของการสลับแพ็กเก็ต การแบ่งไฟล์ออกเป็นแพ็กเก็ตที่มีที่อยู่คือเหตุผลที่หากแพ็กเก็ตหนึ่งหายไปต้องส่งใหม่เพียงครั้งเดียว และเหตุว่าผู้ใช้หลายคนสามารถแชร์สายเดียวกันได้
Methods of data transmission · วิธีการส่งผ่านข้อมูล
English
Serial and parallel
Serial 串行
Parallel 并行
How
one bit at a time, down one wire
several bits at once, down several wires
Distance
good over long distances
good over short distances only
Cost
cheaper (fewer wires)
more expensive (more wires)
Errors
bits stay in order
bits can arrive at slightly different times over long wires
Serial sends one bit after another along a single wire. Parallel sends several bits at the same time along several wires, so it is faster over short distances.
Simplex, half-duplex and full-duplex
These describe the direction data can travel.
Method
Direction
simplex 单工
one direction only (e.g. computer → printer)
half-duplex 半双工
both directions, but only one at a time (e.g. walkie-talkie)
full-duplex 全双工
both directions at the same time (e.g. phone call)
Choosing a method
The choice depends on the distance, the speed needed, the cost, and whether data must travel both ways at once.
ตัวอย่างรูปร่างของขั้วต่อ USB บางส่วน ใช้มาตรฐาน USB เดียวกัน ดังนั้นอุปกรณ์จะถูกตรวจจับอัตโนมัติ
2.2
Why we check for errors · ทำไมเราต้องตรวจสอบความผิดพลาด
Syllabus · หลักสูตร
English
Candidates should be able to:
Notes and guidance
1 Understand the need to check for errors after data transmission and how these errors can occur
• Errors can occur during data transmission due to interference, e.g. data loss, data gain and data change
2 Describe the processes involved in each of the following error detection methods for detecting errors in data after transmission: parity check (odd and even), checksum and echo check
• Including parity byte and parity block check
3 Describe how a check digit is used to detect errors in data entry and identify examples of when a check digit is used, including international standard book numbers (ISBN) and bar codes
4 Describe how an automatic repeat query (ARQ) can be used to establish that data is received without error
• Including the use of: – positive/negative acknowledgements – timeout
2 อธิบายกระบวนการที่เกี่ยวข้องในแต่ละวิธีการตรวจจับข้อผิดพลาดต่อไปนี้用于 detect errors in data after transmission: parity check (odd and even), checksum และ echo check
• รวมถึง parity byte และ parity block check
3 อธิบายการใช้ check digit ในการตรวจจับข้อผิดพลาดในการป้อนข้อมูลและระบุตัวอย่างของการใช้ check digit รวมถึง international standard book numbers (ISBN) และ bar codes
A parity check 奇偶校验 adds one extra bit, the parity bit 奇偶校验位, to each byte. There are two types:
even parity — the total number of 1s in the byte (including the parity bit) must be even;
odd parity — the total number of 1s must be odd.
Example (even parity): the data 1011001 has four 1s, which is already even, so the parity bit is 0:
To find an error: the receiver counts the 1s. If even parity was agreed but a byte arrives with an odd number of 1s, an error has happened during transmission.
A parity check cannot find every error. If two bits flip, the count may still look correct.
Worked example. A single parity bit tells you that an error happened, but not where. A parity block 奇偶校验块 can find the exact bit. Several bytes are sent together, each with a row parity bit, and one extra parity byte 奇偶校验字节 at the bottom holds a parity bit for each column. Even parity is used below, and one bit was flipped during transmission. Find it.
Row byte 3 breaks even parity, and column b3 breaks even parity. The flipped bit is where they cross: byte 3, bit b3. Changing it back from 1 to 0 repairs the data. A single parity bit only says an error happened; a parity block says exactly where, so the receiver can even fix it.
Checksum
A checksum 校验和 is a value worked out from all the data before sending. The receiver works out the checksum again from the data it received. If the two values do not match, an error has occurred and the data is resent.
Echo check
In an echo check 回送校验, the receiver sends the data back to the sender. The sender compares it with the original. If they differ, an error happened. (This needs the data to be sent twice, so it is slow.)
Automatic Repeat reQuest (ARQ)
Automatic Repeat reQuest 自动重传请求 (ARQ) uses messages called acknowledgements 确认.
The receiver sends a positive acknowledgement if a packet arrived correctly, or a negative acknowledgement if it did not.
The sender starts a timeout 超时 timer. If no positive acknowledgement comes back in time, the sender sends the packet again.
Check digit
A check digit 校验码 is an extra digit placed at the end of a number, worked out from the other digits. It is used to find mistakes when a number is typed in.
When the number is entered, the check digit is calculated again and compared. It can catch:
an incorrect digit entered,
a transposition error 换位错误 (two digits swapped, e.g. 21 typed as 12),
a digit left out or an extra digit added,
a phonetic error (a digit that sounds similar, e.g. 13 and 30).
Check digits are used in barcodes 条形码 and ISBNs (book numbers).
• Asymmetric encryption รวมถึงการใช้ public และ private keys
Source: Cambridge International syllabus · แหล่งที่มา: หลักสูตร Cambridge International
English
Why we need encryption
Encryption 加密 scrambles data so that it cannot be understood if it is intercepted 拦截 (caught) by the wrong person. The readable data is called plaintext 明文; after encryption it becomes ciphertext 密文.
Encryption does not stop data from being intercepted. It only stops the data from being understood.
Symmetric encryption
Symmetric encryption 对称加密 uses a single key 密钥 to both encrypt and decrypt the data. The sender and receiver must share this same secret key. The risk is that the key itself could be stolen while being shared.
Asymmetric encryption
Asymmetric encryption 非对称加密 uses two different keys:
a public key 公钥 that anyone can have, used to encrypt;
a private key 私钥 that is kept secret, used to decrypt.
Data encrypted with the public key can only be decrypted with the matching private key, so the key never has to be shared. This is safer than sharing one secret key.
Shift each letter to encrypt a message — the simplest idea of a cipher and its key. · เลื่อนตัวอักษรแต่ละตัวเพื่อเข้ารหัสข้อความ — ความคิดพื้นฐานที่สุดของรหัสลับและ กุญแจ ของมัน
Data is sent in packets (a header with the addresses and packet number, a payload, and a trailer). Packets can take different routes and arrive out of order, then are reordered by their packet numbers.
Match the transmission types: serial (one bit at a time, good over long distances) vs parallel (several bits at once, short distances); simplex / half-duplex / full-duplex describe the direction.
A single parity bit only shows that an error happened; a parity block locates the exact wrong bit — the row and the column that both fail.
Learn the error-detection methods and what each does: parity check, checksum, echo check, ARQ, and the check digit (for typed-in numbers).
Symmetric encryption uses one shared key; asymmetric uses a public key to encrypt and a private key to decrypt, so no secret key is ever shared.
Pick one and the site follows you — notes, papers, videos and practice all open on it. · เลือกหนึ่งตัว และเว็บจะติดตามคุณ — หมายเหตุ, ใบงาน, วิดีโอ และการฝึกฝนจะเปิดอยู่ที่นั้น
Type to search notes, lessons, code, vocabulary and past-paper questions across every subject. · พิมพ์เพื่อค้นหาบันทึก, บทเรียน, โค้ด, คำศัพท์ และคำถามข้อสอบเก่าในทุกวิชา
*แบบ simplex เป็นทางเดียวเท่านั้น; half-duplex เป็นสองทางแต่ทีละทาง; full-duplex เป็นสองทางพร้อมกัน
