AQA A-Level Computer Science AS Paper 2, June 2025: Question 13

✅ Acceptable Answers (1 Mark)

Any one of the following clear definitions:

• Data is sent along a single wire / cable / channel / path.
• Bits are sent one after another (sequentially / one bit at a time).

💡 Key Knowledge

• Serial vs. Parallel: Parallel sends multiple bits simultaneously across multiple lines. Serial sends bits sequentially over a single physical channel.
• Serial avoids skew (bits arriving out of sync) and crosstalk (interference between adjacent wires).

❌ Common Errors

• Vaguely stating "data is sent in a line" without mentioning single wire or one bit at a time.
• Confusing serial transmission with simplex communication (direction of flow).

🧠 Exam Technique

Always use precise standard definitions: either specify the physical medium ( single wire ) or the temporal behaviour ( one bit at a time ).

✅ Acceptable Answers (1 Mark)

• By sending more than one bit per signal change (or per state change / baud / symbol).

📐 Mathematical Relationship

Formula:

Bit Rate = Baud Rate × Number of Bits per Signal Change

• If a signal has 4 voltage levels, each signal state represents 2 bits (since 2² = 4).
• If Baud = 1000 symbols/sec, Bit Rate = 1000 × 2 = 2000 bps (Bit Rate > Baud Rate).

💡 Definitions

• Baud Rate: The number of signal changes (state transitions) per second.
• Bit Rate: The number of bits transmitted per second (bps).

❌ Common Misconceptions

• Assuming that 1 baud always equals 1 bit per second (only true if there are just 2 signal states).
• Writing "by using multiple wires" — the question specifically stipulates a serial cable!

✅ Marking Guidance (Max 2 Marks)

Need two distinct points explaining protocol or central device behaviour:

• Point 1 (Protocol): The network uses a bus transmission protocol (e.g. CSMA/CD).
• Point 2 (Hardware/Transmission):
  • A central hub is used which transmits / broadcasts incoming data packets to all connected devices/ports.
  • OR: The network uses appropriate switching that creates temporary buses between two nodes.

💡 Physical vs. Logical Topologies

• Physical Topology: How cables and devices are physically arranged (each computer has its own dedicated cable connected to a central device = physical star).
• Logical Topology: The path data actually travels across the network. If a hub repeats data to all ports simultaneously, data behaves as if it's on a shared central backbone wire = logical bus.

🧠 Exam Technique

To secure both marks, contrast how data is handled with what protocol coordinates it: naming CSMA/CD satisfies the protocol criterion, while identifying a hub repeating signals to all nodes satisfies the operational criterion.

❌ Common Errors

• Confusing a switch with a hub: a standard switch directs frames only to the intended MAC destination (logical star), whereas a hub broadcasts to all devices (logical bus).
• Simply stating "it acts like a bus" without explaining why (e.g. broadcast nature or collision management protocols).

✅ Marking Guidance (Max 2 Marks)

Any two points from:

• The wireless access point (WAP) stops broadcasting/sending out the SSID / the network name will not appear in the list of available networks (hidden from scans).
• Users must manually enter / already know the exact network name to connect (stops accidental or casual connection attempts).
• Casual attackers / unauthorized users won't know the network exists or its name unless they use specialized software (packet sniffing / packet analysis).

💡 What is an SSID?

• Service Set Identifier (SSID): A unique alphanumeric name (up to 32 characters) identifying a Wi-Fi network.
• Normally, beacon frames are broadcast by the WAP so client devices can display nearby networks. Disabling this hides the network name from regular device discovery.

🧠 Examiner Insight

Notice the condition: "unless they packet sniff". Disabling SSID broadcast is security-by-obscurity. Strong answers recognize that it deters casual users and automatic connection attempts, but does not encrypt traffic or stop a dedicated attacker with a packet sniffer.

❌ Common Errors

• Claiming that disabling SSID broadcast encrypts the network (it does NOT encrypt data).
• Claiming that it is 100% impossible for an attacker to find the network (packet sniffers easily capture the SSID when an authorized user authenticates).