Qualitative vs Quantitative

• Qualitative
  • Develop understanding of human experience
  • Emphasis on preserving the richness of the data
  • Better external validity
• Quantitative
  • Objectively measure human performance
  • More narrowly focused
  • Better internal validity

Qualitative approach

Qualitative approaches focus on high-level effects!

• Identifies task flow problems
• Highlights ambiguities in task description
• Revel contextual and situational insights e.g. conflict with social norms or expectations, usability mismatches

Transport-layer security (TLS)

Widely deployed security protocol above the transport layer!
It is supported by almost all browsers, and web servers.

TLS provides:

• Confidentiality via symmetric encryption
• Integrity via cryptographic hashing
• Authentication via public key cryptography

TLS protocol

Radiometry

Radiometry is a set of techniques for measuring electromagnetic radiation, including visible light.

• Many physcial processes convert energy into photons. (e.g. incandescent lightbulb turns heat into light)
• Each photon carries a small amount of energy.
• Brightness is determined by energy of photons hitting an object. We need this information to make accurate images!

Solid angle

Network security

• Confidentiality: only sender, intended receiver should understand message contents
  • Sender encrypts message
  • Receiver decrypts message
• Authentication: sender, receiver want to confirm identity of each other
• Message integrity: sender, receiver want to ensure message not altered (in transit, or afterwards) without detection
• Access and availability: services must be accessible and available to users

Attacks on internet security

Bob and Alice want to communicate securely each other.
Trudy (intruder) may interrupt!

• Eavesdrop: intercept messages
• Actively insert messages into connection
• Impersonation: can fake (spoof) source address in packet (or any field in packet)
• Hijacking: take over ongoing connection by removing sender or receiver, then inserting himself in place
• Denial of Service: prevent service from being used by others (e.g. by overloading resources)

User Testing

Test the interface with real users!
Problem: Users are human beings with feelings and rights...
The ethics and responsibility of evaluators are necessary.

• Qualitative/Naturalistic
• Quantitative/Experimental
• Field Study (usually long term and qulitative)

Milgram's Obedience Experiment

Measured the willingness of study participants to obey an authority figure who instructed them to perform acts that conflicted with their personal conscience.
e.g. Martial law by president Yoon?

VLAN (Virtual Local Area Network)

Motivation

If user change point of attachment, All layer-2 broadcast traffic must cross entire LAN. (e.g. ARP, DHCP, unknown MAC)
Also, user might wants to remain logically attached to certain switch even when it moves.

Port-based VLANs

Switch ports are grouped by switch management software.
Single physical switch can operate as multiple virtual switches!

Problem

Ray tracing is too slow. We need to accelerate it!
To make ray tracing faster, we need to solve this problem:

Given a scene defined by a set of $N$ primitives and a ray $\mathbf{r}(t)$, find the closest point $\mathbf{p}$ of intersection of $\mathbf{r}(t)$ with the scene.

Bounding Volume

Idea: Precompute conservative but smallest bounding volume that includes all primitives, then early reject if ray does not hit the box.
Problem: In the worst case, we still need to examine all primitives.

Memory Address Space

User process uses virtual address space, while hardware such as memory uses physical address space.

Device has its own MMU (memory management unit) for virtual to physical address translation.

Virtual address space

MAC address

IP address is used for layer 3, MAC address is used for layer 2!

MAC (or LAN or physical or Ethernet) address is used locally to get frame from one interface to another physically-connected interface.
48-bit MAC address is burned in NIC ROM, but it is software settable nowdays.
e.g. 1A-2F-BB-76-09-AD

Q. How do I get a unique MAC address?
A. There are few network card manufacturers, and each network card manufacturers have unique prefix of MAC address.

MAC address is portable; We can move interface from one LAN to another.
c.f. IP address is not portable! It depends on IP subnet to which node is attached.

Design Heuristics

Goal: Evaluate the evolving design when no users are present.
Why? Using users is the best, but it's not always possible.

• Cognitive Walkthroughs: Path through interface with pre-determined tasks and actions
• Action Analysis: Predict the time for an exper to perform a task
• Heuristic Analysis: Interface oriented, Path through interface (without pre-determined tasks)

Cognitive Walkthrough

Cognitive walkthrough is a formalized way of imagining people's thoughts and actions when they use an interface for the first time.