Answers

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

What this lesson is about

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

• Bus and star, physically
• Physical star, logical bus
• Wireless networking
• CSMA/CA

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
   Answer: A. It is wired as a star, but every device receives every frame over what is in effect one shared channel, as on a 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
   Answer: A, C. Each device has its own cable, and a switch keeps traffic apart. But a star uses more cable and the central device is a single point of failure.

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
   Answer: A. A device's own transmission drowns out others at its antenna, so it must avoid collisions by sensing the channel and backing off.

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
   Answer:

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

   The device only sends RTS once the channel is idle; the CTS silences hidden nodes before the data is sent, and the ACK confirms it arrived.

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
   Answer: A. Every device in range of the access point hears its CTS, including ones out of range of the sender, and stays quiet.

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
   Answer: A. An attacker can see allowed MAC addresses in traffic and set their own device to use one; strong encryption such as WPA2 is still needed.

The task: wait for a quiet channel

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

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

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

import random

def heard_in(seconds):
    wait(seconds)
    return len(messages()) > 0

def wait_for(word, timeout):
    waited = 0
    while waited < timeout:
        wait(0.1)
        waited = waited + 0.1
        for sender, text in messages():
            if text.startswith(word):
                return True
    return False

messages()
while heard_in(0.4):
    print("busy, backing off")
    wait(random.uniform(0.1, 0.5))
print("channel clear")
send("RTS")
if wait_for("CTS", 2):
    print("got CTS")
    send("DATA hello")
    if wait_for("ACK", 2):
        print("got ACK")