802.11 Flashcards
What are the commonly used wi-fi bands
2.4 Ghz, 5.7 Ghz
True/False - In a wireless environment a wireless unit cannot tx/rx at the same time
True
Describe the hidden terminal problem
When a two wireless terminals are unaware of each other and so they xmit causing collision
True/False - Every port of a router constitutes a separate network
True
True/False - Even though a L2 switch and an Access Point are layer 2, the AP is not transparent. The wireless client needs to know the ID of the AP
True
What wireless channels do not overlap
1, 6, 11
Describe the AP passive scanning process
- Periodically the AP broadcasts a beacon frame
- A host sends a association request frame to AP
- AP sends a association response frame to host
- Authentication and DHCP may be performed
Describe the AP active scanning process
- The host broadcasts a probe request frame
- AP sends a probe response frame
- Host sends association request frame to selected AP
- AP sends an association response frame to Host
- Authentication and DHCP may be performed
True/False - In ethernet no ack is used. For wi-fi an ack is used.
True
True/False - In wifi back to back frames are not allowed, there must be interframe spacing (IFS)
True
Describe 802.11 CSMA/CA
- Station has a frame to send, start sensing the medium
- If medium free for DIFS, start sending
- If medium is busy wait for DIFS + random back-off
- If medium becomes busy during back-off pause the back off timer, resume backoff when medium idle
- Start sending
- When rx gets frame, wait sifs then send ack
Describe 802.11 CSMA/CA w/ RTS/CTS
- Station has a frame to send, start sensing the medium
- If medium is free for DIFS, send RTS with reservation for amount of time needed
- When Rx gets RTS, wait SIFS and send CTS
- Station waits SIFS and starts sending data
- After getting data Rx waits SIFS and sends ACK
- Other stations go to sleep when they hear the CTS
- After other stations wake up they sense the medium, if idle for DIFS + contention window they start the process
- If a station cant hear RTS (hidden terminal) and sends RTS concurrently there will be collision. The stations will know collision occured bc they never got a CTS
True/False - Physical sensing is the process of the radio actively measuring the strength of signals on the channel
True
True/False - In virtual sending the xmiter doesnt have a frame to send but they are hearing others xmitting.
True
Describe the exposed terminal problem
When a terminal goes to sleep erroneously after hearing a RTS from another station but could have transmitted to a different AP in a non-interfering physical area.