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Worksheet

A12.2 Topologies and wireless networks

Networks and the web · A level · OCR H446 1.3.3, AQA 7517 4.9.2.1, Eduqas A500QS 2.1 · about 25 min

BugBotLab
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What this lesson is about

Physical and logical bus and star; Wi-Fi, SSIDs, WPA2 and CSMA/CA with RTS/CTS.

Questions 6 marks in all

  1. [1 mark]Devices are cabled to a central hub, which repeats every frame out of every port. What is the network's topology?

    1. APhysical star, logical bus
    2. BPhysical bus, logical star
    3. CPhysical star, logical star
    4. DPhysical bus, logical bus

  2. [1 mark]Which of these are advantages of a physical star over a physical bus? Choose all that apply.

    Tick every answer that is true.

    1. AA single cable failure only affects one device
    2. BIt uses less cable
    3. CPerformance holds up better when the network is busy
    4. DThere is no central device that could fail

  3. [1 mark]Why does Wi-Fi use collision avoidance rather than detecting collisions?

    1. AA transmitting wireless device cannot hear other signals over its own, so it cannot detect a collision
    2. BWireless signals never collide
    3. CCollision detection needs a hub
    4. DRTS/CTS is only possible on cables

  4. [1 mark]Put the steps of CSMA/CA with RTS/CTS in order.

    Number the lines 1 to 6 to put them in the right order.

    1. Send a request to send (RTS)
    2. Receive clear to send (CTS) from the access point
    3. Transmit the data frame
    4. If it is busy, wait a random back-off time and listen again
    5. Wait for an acknowledgement (ACK)
    6. Listen to check whether the channel is idle

  5. [1 mark]What problem does RTS/CTS solve?

    1. ATwo devices that cannot hear each other both transmit to the access point at once (the hidden node problem)
    2. BA device forgetting the SSID
    3. CDevices with spoofed MAC addresses
    4. DWeak encryption keys

  6. [1 mark]Why is a MAC address allow list, on its own, weak protection for a wireless network?

    1. AMAC addresses can be read from captured frames and copied (spoofed)
    2. BMAC addresses change every time a device joins
    3. CAccess points cannot read MAC addresses
    4. DIt stops the SSID being broadcast

The task: wait for a quiet channel

The Neighbour is sending frames for the first couple of seconds. The AccessPoint replies CTS bot when it hears RTS, and ACK bot when it hears a message starting DATA. Both replies come about 0.2 s later. Carry out CSMA/CA with RTS/CTS: 1. Call messages() once to clear anything old. Then listen for a window of 0.4 s: wait(0.4), then messages(). 2. If anything arrived in the window, print busy, backing off, wait a random time from 0.1 to 0.5 seconds (random.uniform(0.1, 0.5)), and listen for another window. Repeat until a window is silent. 3. Print channel clear and send RTS. 4. Wait for a message starting CTS, checking messages() every 0.1 s for up to 2 seconds. When it comes, print got CTS and send DATA hello. 5. Wait the same way for a message starting ACK, then print got ACK. The robot does not drive.

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()
import random

send("RTS")
wait(1)
print(messages())

Plan your program here, then type it in and press Run.

QR code
Do it on the robot
www.bugbotlab.com/learn/a12-2-topologies-and-wireless/
The simulator checks it and tells you when it passes. Nothing to install, no account.

Challenges

  1. Count how many times you backed off and print the total. Run it with different values in random.uniform: what changes?
  2. Real Wi-Fi doubles the maximum back-off each time a frame goes unacknowledged. Add that to your retry loop.
  3. Draw a physical star with a switch at the centre. Explain why its logical topology is a star, not a bus.