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Flashcards · AP® Computer Science Principles · Unit 4

Unit 4: Computer Systems and Networks (CSN) flashcards

18 cards on topics 4.1 to 4.3. Pick a topic or study the whole unit.

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4.1 The Internet

What are a computer network, a path and routing?
A computer network is interconnected devices that send or receive data. A path is a sequence of directly connected devices from sender to receiver. Routing is finding a path.
What is bandwidth, and how do you calculate transfer time?
Bandwidth is the maximum data per fixed time, in bits per second. Time = file size in bits ÷ bandwidth. A 12 MB file on a 24 Mbps link is 12 × 8 = 96 megabits, so 96 ÷ 24 = 4 seconds.
What are packets?
Data travel as a stream of packets: a chunk of data plus metadata for routing and reassembly. Packets may arrive in order, out of order or not at all, so the receiver uses the packet numbers.
What is the difference between the Internet and the World Wide Web?
The Internet is interconnected networks using standard, open protocols. The Web is a system of linked pages, programs and files that uses the Internet and HTTP.
What is a protocol?
An agreed-upon set of rules. IP, TCP and UDP are common Internet protocols, and they are open and standardised rather than private to one company.
Why does the Internet scale and use dynamic routing?
It is built from interconnected networks using open protocols, so it can grow. Routing is usually dynamic: a different route can be chosen if one is busy or fails.

4.2 Fault Tolerance

What is a fault-tolerant system?
A system that can support failures and still continue to function. It does not mean the system never fails.
What is redundancy?
Extra components that can take over if others fail. In networks it means more than one path between two devices.
What is a single point of failure?
A device or link that, if it fails, breaks the system on its own. Look for a component shared by every path.
How do you test a network design for fault tolerance?
Remove one device or link at a time and check whether a path still exists. Check pairs of failures too if the question asks.
What are the benefit and the cost of redundancy?
Benefit: reliability, and it helps the system scale. Cost: additional links, devices and maintenance. State both in an evaluation.
How many links does a full mesh of n devices have?
n × (n − 1) ÷ 2. For 5 devices that is 5 × 4 ÷ 2 = 10 links.

4.3 Parallel and Distributed Computing

What are sequential, parallel and distributed computing?
Sequential: operations run in order, one at a time. Parallel: the program is split so some parts run at the same time. Distributed: multiple devices linked by a network run a program.
How do you calculate the time of a parallel solution?
Add the sequential tasks, plus the longest task in each group that runs in parallel. Tasks of 15 s, then 25 s and 40 s together, then 10 s take 15 + 40 + 10 = 65 s.
How is speedup calculated?
Speedup = sequential time ÷ parallel time, a number rather than a time. For 90 s sequential and 65 s parallel it is about 1.38.
What limits the speedup from parallel computing?
The sequential portion. A 60 s program with 12 s that must run alone can never have a speedup above 60 ÷ 12 = 5, however many processors are used.
How long does a divisible portion take on p equal processors?
Time = sequential portion + parallel portion ÷ p. With 12 s alone and 48 s shared among 4 processors: 12 + 48 ÷ 4 = 24 s.
What are the benefits and challenges of parallel and distributed computing?
Benefits: scales well, solves problems too large for one computer and does much larger problems faster. Challenges: the sequential limit, coordination, communication and failures.

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