6.4 - Wireless Networking, CSMA &SSID

Question 1

What is Wi-Fi?

Answer 1

A wireless networking technology providing high-speed Internet and network connections

Question 2

How do devices connect to the internet?

Answer 2

Via a WAP (wireless access point)

Question 3

What is needed to a wireless network?

Answer 3

ISP (Internet Service Provider)
Modem and wireless router (often combined)
Device with a NIC

Question 4

What is a Service Set IDentifier? (SSID)

Answer 4

Identifies each network by a unique
- must be used by all devices on that network
- can be set manually or automatically
- can be hidden to make it hatreer to detect
- can be set to broadcast to wireless devices in range of the wireless access point

Question 5

Why are wireless networks less secure?

Answer 5

• unauthorised users can be hard to spot
• transmitted data can be easily intercepted

Question 6

How can the wireless networks be made more secure?

Answer 6

Have protocols:
WPA
WPA2

Question 7

What does WPA/WPA2 do?

Answer 7

Generates a 128-bit key for every packet of data sent
- very strong encryption
- usually pre-installed on wireless NIC’s

Question 8

What happens if two devices transmit data simultaneously via a single WAP?

Answer 8

Data collisions can occur

Question 9

How can you avoid data collisions?

Answer 9

Have a protocol
CMSA/CA

Question 10

How does the CMSA/CA (carrier sense multiple access with collision avoidance) protocol work?

Answer 10

• a frame is assembled
  If the channel is idle:
• data is transmitted
  If not:
• wait a random time until the channel is idle

Question 11

What does the carrier sense part of CMSA mean?

Answer 11

Each node continually listens to/tests the network frequency for the presence of a signal prior to transmitting

Question 12

What does the multiple access part of CMSA mean?

Answer 12

Many computers attempting to transmit and complete for opportunity to send data

Question 13

How does CMSA/CA work? (a lot)

Answer 13

• sending computer listens
• if there is already traffic on the network it waits
• when no traffic, sender issues RTS (request to send)
• WAP responds with CTS (clear to send) if it ok to send the RTS/CTS signals block any other transmissions
• if CTS not received the user must wait
• once received has received the data it sends ACK
• sender waits for ACK so it knows data is received
• if no ACK received, sender will retransmit after a reasonable time
• ACK indicates to other devices that they can transmit

Question 14

Draw the flowchart for CMSA/CA using RTS/CTS?

Answer 14

Start (oval)
Assemble a frame (rectangle)
Channel idle (diamond) -> no -> wait a random time (rectangle) -> back to channel idle
If channel idle == yes:
Transmit RTS (request to send)(rectangle)
CTS (clear to send) received? -> no -> wait a random time
If CTS == yes:
Transmit data
Stop