Computer fundamentals
International Baccalaureate · IB Diploma · Computer Science · SL Topic 1 3:40 English narration · English + 中文 subtitles burned in
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Transcript
A school booking system can calculate correctly and still fail if users lose access or records are disclosed.
学校预订系统即使计算正确，若用户无法访问或记录泄露，仍可能失败。
Success includes the system context.
成功包含系统上下文。
A computer system combines hardware, software, data and people.
计算机系统由硬件、软件、数据和人员组成。
A network enables communication using protocols.
网络通过协议实现通信。
Requirements should distinguish function from constraints such as availability, security and accessibility.
需求应区分功能与约束，例如可用性、安全性和可访问性。
Authentication: Checking an asserted identity.
身份验证：核实所声称的身份。
Authorization: Determining permitted actions.
授权：确定允许执行的操作。
Separate authentication from authorization.
将身份认证与权限分离。
Authentication checks identity, and authorization determines permitted actions.
身份认证用于核实身份，而权限则决定允许执行的操作。
A threat model connects a valuable asset, a possible attack, and an appropriate control.
威胁模型将高价值资产、潜在攻击和适当控制措施联系起来。
In the lab: Use a fictitious school dataset to define users and permissions.
在实验室中，使用虚构的学校数据集来定义用户和权限。
Draw data flows, compare validation and verification, and specify tests for normal, boundary and invalid input.
绘制数据流，比较验证与确认，并指定针对正常、边界及无效输入的用例测试。
Do not use real credentials or student records in exercises.
练习中请勿使用真实凭证或学生记录。
Known: a file contains 12 megabytes, where this example defines one megabyte as one million bytes.
已知：一个文件大小为 12 MB，本例中定义 1 MB 为一百万字节。
Bits equals bytes times 8, which is 12 times 1,000,000 times 8, equaling 96,000,000 bits.
比特数等于字节数乘以 8，即 12 乘以 1,000,000 乘以 8，等于 96,000,000 比特。
At 8,000,000 bits per second, ideal time equals size divided by rate, which is 12 seconds, excluding overhead.
在每秒 8,000,000 比特的传输速率下，理想时间等于大小除以速率，即 12 秒，不包括额外开销。
Your turn: A 40 million bit file transfers at 5 million bits per second.
轮到你了。 一个四千万比特的文件以每秒五百万比特的速度传输。
Find ideal time.
求理想时间。
Bandwidth is not actual end-to-end throughput.
带宽并非实际的端到端吞吐量。
Encryption does not by itself ensure correct authorization or remove every security risk.
加密本身并不能确保授权正确，也无法消除所有安全风险。
Not true: Encryption guarantees that every user has appropriate access permissions.
此说法不正确。 加密并不能保证每个用户都拥有适当的访问权限。
Encryption protects content from being read by others; it does not decide who is allowed to do what.
加密的作用是保护内容不被他人读取，但它并不决定谁被允许做什么操作。
Authorization is a separate check of permissions, and encryption alone leaves it untouched.
授权是对权限的独立检查，而仅靠加密无法处理这一问题。
A computer system combines hardware, software, data and people.
计算机系统由硬件、软件、数据和人员组成。
Separate authentication from authorization.
将身份验证与授权分开。
Watch for bandwidth is not actual end-to-end throughput.
注意：带宽并非实际的端到端吞吐量。
Read the chain from left to right.
从左往右读。
The system first learns who claims to be acting, then checks that claim, then checks whether that person may do this action, and only then records the change.
系统先知道是谁声称在操作，再验证这个身份，再检查此人是否有权执行该操作，最后才记录这次修改。
A 20 MB file uses decimal MB=10⁶ bytes and a 10 Mb/s link.
一个20 MB的文件使用十进制MB，即10的6次方字节，以及一条10 Mb/s的链路。
Find ideal time and explain why observed time can be longer.
求理想时间并解释为何实际观测时间可能更长。
The time is 20 times ten to the six, times 8, divided by 10 times ten to the six, which is 16 seconds.
时间等于 20 × 10⁶ × 8 除以 10 × 10⁶，即 16 秒。
Overhead, contention and delays add time.
开销、竞争和延迟会增加时间。
Distinguish bits from bytes and storage capacity from sustained transfer rate.
要区分比特与字节，以及存储容量与持续传输速率。
Decimal MB means one million bytes; multiply by eight before dividing by bits per second.
十进制MB表示一百万字节；在除以每秒比特数之前，先乘以八。
Zero data has zero ideal transmission duration.
零数据具有零理想传输持续时间。
A nonpositive rate is rejected; actual overhead is outside this arithmetic model.
非正速率被拒绝；实际开销不在该算术模型范围内。
Retrieve the technical meanings, then vary one input in the original transfer and justify the model condition.
请回顾相关技术含义，然后在原始传递中改变一个输入变量，并论证模型条件的合理性。