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Computer Systems and Networks

AP Computer Science Principles · Topic 4

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Computer Systems and Networks

People call it the cloud. It is not a cloud. This box sits in somebody's home: a router, taking data from your phone and passing it on. This plug is the other…

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4.1

The Internet
Syllabus
Enduring UnderstandingLearning ObjectiveEssential Knowledge

CSN-1
Computer systems and networks facilitate the transfer of data.

CSN-1.A
Explain how computing devices work together in a network. [Skill 5.A]

  • CSN-1.A.1 A computing device is a physical artifact that can run a program. Some examples include computers, tablets, servers, routers, and smart sensors.
  • CSN-1.A.2 A computing system is a group of computing devices and programs working together for a common purpose.
  • CSN-1.A.3 A computer network is a group of interconnected computing devices capable of sending or receiving data.
  • CSN-1.A.4 A computer network is a type of computing system.
  • CSN-1.A.5 A path between two computing devices on a computer network (a sender and a receiver) is a sequence of directly connected computing devices that begins at the sender and ends at the receiver.
  • CSN-1.A.6 Routing is the process of finding a path from sender to receiver.
  • CSN-1.A.7 The bandwidth of a computer network is the maximum amount of data that can be sent in a fixed amount of time.
  • CSN-1.A.8 Bandwidth is usually measured in bits per second.

CSN-1.B
Explain how the Internet works. [Skill 5.A]

  • CSN-1.B.1 The Internet is a computer network consisting of interconnected networks that use standardized, open (nonproprietary) communication protocols.
  • CSN-1.B.2 Access to the Internet depends on the ability to connect a computing device to an Internet-connected device.
  • CSN-1.B.3 A protocol is an agreed-upon set of rules that specify the behavior of a system.
  • CSN-1.B.4 The protocols used in the Internet are open, which allows users to easily connect additional computing devices to the Internet.
  • CSN-1.B.5 Routing on the Internet is usually dynamic; it is not specified in advance.
  • CSN-1.B.6 The scalability of a system is the capacity for the system to change in size and scale to meet new demands.
  • CSN-1.B.7 The Internet was designed to be scalable.

CSN-1.C
Explain how data are sent through the Internet via packets. [Skill 5.A]

  • CSN-1.C.1 Information is passed through the Internet as a data stream. Data streams contain chunks of data, which are encapsulated in packets.
  • CSN-1.C.2 Packets contain a chunk of data and metadata used for routing the packet between the origin and the destination on the Internet, as well as for data reassembly.
  • CSN-1.C.3 Packets may arrive at the destination in order, out of order, or not at all.
  • CSN-1.C.4 IP, TCP, and UDP are common protocols used on the Internet.

CSN-1.D
Describe the differences between the Internet and the World Wide Web. [Skill 5.A]

  • CSN-1.D.1 The World Wide Web is a system of linked pages, programs, and files.
  • CSN-1.D.2 HTTP is a protocol used by the World Wide Web.
  • CSN-1.D.3 The World Wide Web uses the Internet.

Source: College Board AP Course and Exam Description

An RJ45 Ethernet connector — physical links carry packets across the Internet
An RJ45 Ethernet connector — physical links carry packets across the Internet

The Internet 互联网 is a global network of networks. Data travels as packets 数据包 – small chunks that are sent separately and reassembled at the destination. Two ideas make it work at scale:

The internet is the worldwide network; the web is one service running on it
The internet is the worldwide network; the web is one service running on it
  • Protocols 协议 are agreed rules for communication. IP (Internet Protocol) addresses and routes packets; TCP reassembles them in order and re-requests lost ones; HTTP carries web pages; DNS translates a name like example.com into an IP address.
  • Redundancy 冗余 and routing: there are many possible paths between two points, so if one path fails, packets take another. This makes the Internet fault-tolerant 容错.

The Internet is designed to be open and scalable – built on standards anyone can use, so it keeps working as billions of devices join. Bandwidth 带宽 is the amount of data a connection can carry per second.

Explore

How data crosses the Internet in packets

The Internet breaks a message into numbered packets, each addressed and routed independently (often by different paths), then reassembled at the destination. That design makes it fault-tolerant — if one route fails, packets take another.

Vocabulary Train
English
Internet/ˈɪntənet/
packets/ˈpækɪts/
Protocols/ˈprəʊtəkɒlz/
Redundancy/rɪˈdʌndənsi/
fault-tolerant/fɒlt ˈtɒlərənt/
Bandwidth/ˈbændwɪdθ/
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4.2

Fault Tolerance
Syllabus
Enduring UnderstandingLearning ObjectiveEssential Knowledge

CSN-1
Computer systems and networks facilitate the transfer of data.

CSN-1.E
For fault-tolerant systems, like the Internet:
a. Describe the benefits of fault tolerance. [Skill 1.D]
b. Explain how a given system is fault-tolerant. [Skill 5.A]
c. Identify vulnerabilities to failure in a system. [Skill 1.D]

  • CSN-1.E.1 The Internet has been engineered to be fault-tolerant, with abstractions for routing and transmitting data.
  • CSN-1.E.2 Redundancy is the inclusion of extra components that can be used to mitigate failure of a system if other components fail.
  • CSN-1.E.3 One way to accomplish network redundancy is by having more than one path between any two connected devices.
  • CSN-1.E.4 If a particular device or connection on the Internet fails, subsequent data will be sent via a different route, if possible.
  • CSN-1.E.5 When a system can support failures and still continue to function, it is called fault-tolerant. This is important because elements of complex systems fail at unexpected times, often in groups, and fault tolerance allows users to continue to use the network.
  • CSN-1.E.6 Redundancy within a system often requires additional resources but can provide the benefit of fault tolerance.
  • CSN-1.E.7 The redundancy of routing options between two points increases the reliability of the Internet and helps it scale to more devices and more people.

Source: College Board AP Course and Exam Description

A Wi-Fi router: the local gateway that forwards packets toward the wider Internet
A Wi-Fi router: the local gateway that forwards packets toward the wider Internet

A system is fault-tolerant if it keeps working even when part of it fails. The Internet achieves this through redundant connections: because packets can be routed along multiple paths, the failure of one router or cable does not stop communication – traffic simply reroutes. Fault tolerance costs extra resources (the redundant paths) but greatly improves reliability 可靠性. A single path with no backup is not fault-tolerant.

Packet switching sends packets by independent routes, so the network tolerates a failed link
Packet switching sends packets by independent routes, so the network tolerates a failed link
Vocabulary Train
English
reliability/rɪˌlaɪəˈbɪlɪti/
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4.3

Parallel and Distributed Computing
Syllabus
Enduring UnderstandingLearning ObjectiveEssential Knowledge

CSN-2
Parallel and distributed computing leverage multiple computers to more quickly solve complex problems or process large data sets.

CSN-2.A
For sequential, parallel, and distributed computing:
a. Compare problem solutions. [Skill 1.D]
b. Determine the efficiency of solutions. [Skill 1.D]

  • CSN-2.A.1 Sequential computing is a computational model in which operations are performed in order one at a time.
  • CSN-2.A.2 Parallel computing is a computational model where the program is broken into multiple smaller sequential computing operations, some of which are performed simultaneously.
  • CSN-2.A.3 Distributed computing is a computational model in which multiple devices are used to run a program.
  • CSN-2.A.4 Comparing efficiency of solutions can be done by comparing the time it takes them to perform the same task.
  • CSN-2.A.5 A sequential solution takes as long as the sum of all of its steps.
  • CSN-2.A.6 A parallel computing solution takes as long as its sequential tasks plus the longest of its parallel tasks.
  • CSN-2.A.7 The "speedup" of a parallel solution is measured in the time it took to complete the task sequentially divided by the time it took to complete the task when done in parallel.

CSN-2.B
Describe benefits and challenges of parallel and distributed computing. [Skill 1.D]

  • CSN-2.B.1 Parallel computing consists of a parallel portion and a sequential portion.
  • CSN-2.B.2 Solutions that use parallel computing can scale more effectively than solutions that use sequential computing.
  • CSN-2.B.3 Distributed computing allows problems to be solved that could not be solved on a single computer because of either the processing time or storage needs involved.
  • CSN-2.B.4 Distributed computing allows much larger problems to be solved quicker than they could be solved using a single computer.
  • CSN-2.B.5 When increasing the use of parallel computing in a solution, the efficiency of the solution is still limited by the sequential portion. This means that at some point, adding parallel portions will no longer meaningfully increase efficiency.

Source: College Board AP Course and Exam Description

Data-centre server racks: distributed computing spreads work across many machines
Data-centre server racks: distributed computing spreads work across many machines
  • Sequential computing 顺序计算 runs one operation at a time.
  • Parallel computing 并行计算 splits a task into parts that run at the same time on multiple processors, finishing faster.
  • Distributed computing 分布式计算 uses many computers connected by a network to work on one problem – essential for problems too big for a single machine.
Parallel computing: many processors work at the same time
Parallel computing: many processors work at the same time

A parallel solution's speedup 加速比 is the sequential time divided by the parallel time. Speedup is limited: parts that must run in sequence cannot be sped up by adding processors, so doubling the processors rarely doubles the speed.

Worked example. A task has a part that must run sequentially, taking $40$ seconds, plus a parallelizable part that takes $60$ seconds on one processor – so on a single processor the whole task takes $40+60=100$ seconds. Spread the parallel part across $3$ processors and it takes $\tfrac{60}{3}=20$ seconds, so the total parallel time is $40+20=60$ seconds and the speedup is $\tfrac{100}{60}\approx1.67$. The $40$-second sequential part is a floor: even with infinitely many processors the task can never finish in under $40$ seconds.

Exam skill: given the times for the sequential and parallel portions of a task, be able to calculate the total parallel time and the speedup.

Vocabulary Train
English
Sequential computing/siːˈkwenʃl kəmˈpjuːtɪŋ/
Parallel computing/ˈpærəlel kəmˈpjuːtɪŋ/
Distributed computing/ˈdɪstrɪbjuːtɪd kəmˈpjuːtɪŋ/
speedup/ˈspiːdʌp/
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4.3

Exam tips
  • Describe how data travels in packets over a redundant, fault-tolerant network of independent routers.
  • Know that protocols (IP, TCP, HTTP) are agreed rules, and that open standards let different systems interoperate.
  • Explain scalability and how the Internet grows without central control.
  • Contrast bandwidth (rate) with latency (delay), and describe the DNS name-to-address lookup.
  • Discuss the digital divide and security basics (encryption, authentication) in plain terms.

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