WLAN Q/A

   1) If you have 2 DHCP servers in a N/W, when you connect a STA to AP, from which DHCP server you will get the IP to the STA?

Answer : Its depends on first offer message and dhcp implementation like proprietary device and vendor class 

2) ACK and CTS frames follow which interframe space? 

Answer : SIFS ( Short Interframe Spacing)

3) In which step station will get IP address:*

Answer: Client will send broadcast Discover packet in network

Servers will respond to Discover packet by sending the *DHCP Offer* packet (this offer packet will contain the IP address, DNS, lease time, route information etc)

If client want IP address, then client will send *DHCP Request* packet to perticular DHCP server.

After receiving DHCP request from client, server will assign the IP to client by sending *acknowledge* packet

4) Suppose DHCP server has only 5 ip address and allocated to 5 clients, if 6th client is trying to connect DHCP , How DHCP will provide ip address to that 6th client and how 6th client get ip address?

Answer: create another DHCP server or add more IPs to the existing DHCP server…or give a static IP address to the 6th client,

5) How to debug if an AP and STA not connecting?

Answer:  Client level debugging on the AP or Controller depending on the vendor and platform or take captures and see why is the client not able to connect.

6) How does OFDMA use Resource units to help increase the efficiency of wireless medium?

Answer: OFDMA allows multiple clients to transmit and receive simultaneously where as in OFDM the entire bandwidth is used by a single client at a particular time, if the same client wants to use the only a part of channel bandwith, the rest of the bandwith gets wasted. Using OFDMA the channel bandwith is split into RU's and can be shared with multiple clients simultaneously increasing the efficiency ofdm

7) What is mixed mode how AP-Client communicate in it?

Answer: IEEE802.11n protocol operates in two modes 

HT-Greenfield mode which assumes all the wireless clients are of 802.11n and this mode doesn't support 802.11b/a/g. 2. Mixed mode support both legacy as well as HT.

8) When a wireless client hears another client transmiting a frame. What are the sequence of events it goes through before transmitting frame?

Answer: Random backoff.

9) When you configure an AP in channel 100, will it beacon immediately?*

Answer: No, 100 channel is DFS

7) AP will perform 60sec channel availability check (CAC) if no radar detection on this channel then AP will send beacon on that channel 

Answer : If radar detects then it will move to another channel either DFS and non DFS channel.

8) Consider 4*4 AP and 4*4 STA- How ll you know that STA is 4*4 in wireshark*

Answer: We can check in 802.11 radio information and HT, VHT capabilities 

9) How ll you know PTK in AP has matched r mismatched with STA*

Answer: If EAPOL frame 2 is mismatched it will fail there 

10) what is (802.11k) Radio resource management?

Answer: If radio resource management enabled(802.11k) APs and clients can send neighbor reports, beacon reports, and link measurement reports to each other. This allows the APs and clients to take appropriate connection actions.

11) When AMPDU and AMSDU being used (or) when they comes into picture?

How they help us to improve throughput?

Answer: AMSDU- multiple ethernet frame will go in single 802.11 frame

AMPDU- multiple packets will go with individual mac header but they will all acknowledged with single block ack 

What's Difference between Band steering and Client steering exactly?

Band steering - Suppose you configured and SSID with dual radio (2.4 and 5) , If you enabled band steering then client tries to connect to best frequency band. For example, if the station is near to AP then it tries to connect to 5GHz. On the other hand, if the station is far away from AP then it will select 2.5 GHz band.

Note: Condition for band steering : same SSID and Password for all bands in an AP

Client steering - Suppose you have multiple networks , then client tries to connect to network which is having good signal strength.

Some Important WLAN Interview Questions:

1. CSMA/CA vs CSMA/CD?

2. What is DCF?

3. Explain the working of Inter Frame Spaces.

4. What is Random Backoff Timer?

5. What is the significance of Traffic Indication Map field in a Beacon?

6. Limitations of 802.11b?

7. How many channels are present in 2.4 GHz and 5 GHz?

8. How is 802.11a better than 802.11b?

9. What is mixed mode and AP-Client communication in it?

10. Under which situation does RTS/CTS work.

11. When a wireless client hears another client transmit a frame, what is the sequence of events it goes through before it transmits a frame?

12. How does a 802.11e client contend for the medium?

13. Explain the power saver mechanism for a 802.11a/b/g client and how is it different from a 802.11n client?

14. What is SNR, RSSI, Noise, Interference, Co-channel Interference, Adjacent channel Interference, SSID, BSSID, BSS, IBSS?

15. What is Active and Passive Scanning? Frames exchanged in both scenarios?

Wireless Security:

1. Different methods of wireless authentication?

2. Explain WPA and WPA2.

3. What is WEP and why don’t we use that anymore? ---

4. Difference between Authentication & Association. ---

5. What is EAP? Explain EAP-PEAP, EAP-TLS and EAP-TTLS.

6. What is 4-way handshake? What is the outcome of the 4-way handshake?

7. Why do we use Open-Authentication and not SKA?

8. What is Fast Roaming and how does it happen?

9. Different types of key-caching.

10. How roaming works and what factors are responsible for the client to roam to a new AP?

11. Why is EAP called “Extensible” Authentication Protocol?

12. What is captive portal and explain the flow of events that take place when a client connects to a captive portal SSID?

13. In EAP, what is the use of certificates?

802.11n:

1. What are the limitations of 802.11a/b/g?

2. What are the enhancements in 802.11n protocol?

3. What is channel bonding and is it useful 2.4 GHz?

4. What is spatial multiplexing?

5. Different types of Beamforming used in 802.11n.

6. How does Beamforming work?

7. What is aggregation of frames and how does it work?

8. A-MPDU and A-MSDU. Which one to use?

802.11ac:

1. In which band does 802.11ac work?

2. How is 802.11ac better than 802.11n?

3. How does MU-MIMO work?

4. How does NDP Beamforming work?

5. In a densely deployed network, is 160 MHz channel width a good idea? Why?

6. What is the modulation used in 802.11ac and how is it better than the one used in 802.11n?

802.11ax:

1. Why is 802.11ax called High-Efficiency?

2. Describe OFDMA.

3. Explain MU-OFDM.

4. Explain TWT?

5. What is Overlapping BSS and how does BSS Colouring help?

6. How does OFDMA use Resource Units to help increase the efficiency of the wireless medium?

7. What are UL and DL OFDMA?

WLAN Interview Questions and Answers :

 

1.Suppose DHCP server has 5 IP addresses and 5 clients. If i try to connect 6th client how DHCP will work? And I need all clients to connect.

Ans:  6th client won't get IP address from DHCP server if all 5 clients are connected and DHCP lease time is not expired for those 5 clients.

If you want IP address for 6th client either lease expires/DHCP renewal got failed for existing client during this 6th client may get IP address. Another way is you can configure Static IP address in same network.

 

2. Suppose if the lease time is 24hrs after 24hrs which IP it will give?

Ans: Client should process DHCP renewal at before lease time expires. Suppose 5 clients are connected and one client lease expires first. That client or new client will get same IP address because that IP only free.

 

3.Why is 802.11ax called High efficiency?

Ans : It is due to the preamble what it uses, The information bits in the HT, VHT, or HE preamble is just information to the receiver on how it should process this type frame.

 

4.What is Channel bonding? How is it useful in 2.4/5 Ghz?

Ans : Channel bonding means co-joining of existing 20Mhz channel into 40Mhz, the same way we can reach till 320Mhz in 11be. It basically gives more room to transfer data

 

5.How does OFDMA use Resource units to help increase the efficiency of wireless medium?

Ans: In OFDMA, it divides the Wi-Fi channel into smaller number of frequencies allocations is known as Resource Units (RU).By using this feature AP can communicate with clients simultaneously where smaller frames are transmission

 

6.What is mixed mode how AP-Client communicate in it?

Ans: Mixed Mode Ex:

802.11 b/g/n/ax

802.11 a/n/ac/ax

When AP is configured with mixed Mode, legacy client can connect to the AP. Here protection mechanism will be used.

 

7.When a wireless client hears another client transmiting a frame.What are the sequence of events it goes through before transmitting frame?

Ans: When one client is transmitting. Other clients will check NAV/Duration of the packet of that client.

RTC/CTS packets.

Based on this other clients will set their NAV/Duration value. If still first client occupied medium other client will set IFS+Random back of time.

 

8.Explain the power save mechanism for 802.11 a/b/g client and how is it different from a 802.11n client ?

Ans: In a, b,g PS poll mechanism is used, where that PS poll frame was a control frame.The client will send PS poll frame unless and until more data bit sets to zero in Frame control of MAC header. And in 802.11n UAPSD is used where triggered frames are used and buffered data is sent as TXOP burst.

 

9.What is SNR,RSSI,Noise, Interference,Co-Channel Interference,Adjacent channel interference,SSID,BSSID,BSS,IBSS ?

Ans: SNR is the the value that represents the quality of signal against attenuation.

RSSI is refers to signal strength it is a negative number.

Noise is disturbance in medium

Interference is a problem that will happen due to device which are operated in same frequency.

Because of overlapping of channels co channel interference will happen becz they will share same frequency number.

SSID is name given to wifi network

BSSID is AP's MAC address (it's a Layer 2 Network identifier of an individual BSS)

BSS is AP's topology

IBSS is wifi network having single AP

 

10.What is modulation used in 802.11ac and how is it better than the one used in 802.11n ?

Ans: In ac 256 QAM is used but in n 64 QAM used

In ac MU MIMO used but in n only MIMO

 

11.What is overlapping BSS and how does BSS coloring work?

Ans: If coverage area of 2 APs is overlapping that is called overlapping BSS. If coverage area of 2 APs are overlapping and both are in Same channel then APs will set to different colour(some number or code)

 

12.Describe OFDM,  OFDMA and TWT?

Ans:
OFDM is one multiplexing technology where multiple signals are sent over a link where signals are orthogonal to each other. i.e, When one signal is at peak others will be at null.

OFDMA - Here Resource unit i.e, Sub carriers can serve multiple clients simultaneously.

TWT - is latest power saving mechanism where an AP decides the time depending upon how frequently the clients are using the network.

 

13.What are Uplink and Downlink OFDMA?

Ans: Uplink OFDMA is AP should be able take intake from multiple clients by Resource units simultaneously.

Downlink OFDMA AP should be able to serve multiple clients simultaneously using RUs

 

14.What is random backoff timer?

Ans: If one client is transmitting and other clients has to update their NAV/Duration value.

This Duration is the combination of IFS time + Contention Window.

This IFS time may vary for different frames.

Ex: ACK frames will use SIF time

During this Contention Window clients has to decrease the Backoff timer.

Before every frame transmission, Wi-Fi stations select a random timer value within the contention window range and countdown until the timer expires.

If the medium is idle,then immediately can transfer.

 

> STA will choose the random Backoff timer value during the channel aquisition to avoid wireless collision.

Again we have different Backoff timer range when comes to different access categories in 802.11e protocol for voice, video, BE and BK.

 

15.What is the significance of Traffic Indication Map field in beacon?

Ans: TIM field in beacon helps the associated stations to identify whether they have any buffered data awaiting at the Access point by advertising the AID of those stations.

 

16.Explain the working of Inter frame spaces?

Ans: we have different kind of inter frame spaces which are applicable in wlan namely, DIFS, SIFS, RIFS and AIFS

DIFS will be used during when sta wants to access the channel once after it sense the medium idle by virtual carrier sense and physical carrier sense.

Again in case of QOS feature which added 802.11e DIFS gets replaced by AIFS.

AIFS will be different for different access categories i.e, voice, video, Best effort and background.

RIFS time slice is less than SIFS.

After sending a wireless frame sta will wait for SIFS amount of time to get Ack from the receiver.

Again if all the associated stations in the bss are 802.11n capable then RIFS will come into picture in place of SIFS.

 

17.What is Active scanning and passive scanning? What are the frames exchanged in both scenarios?

Ans: Active Scanning:

In Active Scanning, client will perform scanning on each channel and will send probe request packet.

Once probe request sent, client will start "probe timer countdown" and wait for acknowledgement from AP. Probe timer(30ms) is lesser than the Beacon interval.

Once sta sends the probe request on one channel it has to wait for "Minchannel Timer" if sta didn't received any probe response from AP then there is no AP's on that channel in network. Ten Sta will move to another channel. If STA received probe response from AP with the "Minchannel Timer" then sta will wait upto "MaxChannel Timer" for further probe response from another AP's.

 

In Active Scanning battery life time less.

> In active scanning the client will scans in available channels and starts sending probes. Means Clients initiates the association.

In Passive scanning the AP sends broadcast frames called Beacon frames in Settled Channel. Here AP initiates association

 

In Passive Scanning:

Client will listen beacon frames on each channel and perform client association process.

Passive scan will takes more time than the activitie scan.

In Passive Scanning battery life is more.

 

 

18.How many channels are present in 2.4 GHz,5 GHz,6Ghz ?

Ans : In 2.4GHz - 13

In 5 GHz - 25

In 6 GHz- 59

 

19.CSMA CD vs CSMA CA ?

Ans: CSMA CD is the carrier sense method that is used in Ethernet standard to avoid collision

CSMA CA is medium access method used in Wifi i.e., 802.11 where the client scans whether the channels are free or not. If channel is busy then it wait for some random amount of time. When channel becomes free the client will send RTS (Request To Send - control frame) frame to AP and waits for CTS (Clear To Send - a control frame) . If AP is busy it will not send CTS. Then again client waits for random amount of time. If AP is free then it will send a CTS to client. Then client can send data.

 

20.What is DCF?

Ans: DCF is used for medium access it’s a fixed amount of time that wireless device should wait if medium is free fallows with Random back of timer refer CSMA/CA. DIFS = SIFS+ 2( slot time )

 

21.Under which situation does RTS/CTS work?

Ans: RTS / CTS uses Mainly used for hidden mode and exposed node problems as these are control frames majorly used for medium access only.

 

22.How is 802.11a better than 802.11b ?

Ans: .802.11 a better interm of speed where b is better in terms of coverage. And also protection mechanism is mandatory when we are using b devices

 

23.What are DFS and Non DFS channels? what are they?

Ans: In Dynamic Frequency Selection Channels the AP which is configured with DFS channel will change its channel in DFS range whenever it feels the current channel is congested. There are 16 DFS channel in 5GHz. They are - 52, 56, 60, 64, 100, 104....144.

 

 

24.What is Band steering and Airtime fairness? How does it work?

Ans: Bandsteering is the feature in which the AP tries to select best frequency band. Means Suppose in a AP having 2 radios 2.4 and 5, then Clients will try to connect 5G band if 5G band is not reachable then only clients will connect to 2G.

Airtime fairness is the feature where AP allots

Time to send data for all types of devices. This helps legacy devices to send Data.

 

25.How much max Tx Power (in dBm) we need to set for 2.4Ghz/5Ghz/6Ghz ?

 

Max TX power

2G - 20dBm

5G - 23dBm

6G - 30dBm

 

26.What are the limitations of 802.11a/b/g ?

Ans: Limitations of 802.11a/b/g:

802.11b supports DSSS modulation and support max data rate is 11mbps and.

802.11g supports OFDM and Max data rate is 54mbps

Both are operating on 2.4GHz only. 2.4GHz is more crowded

Due to less non overlap channels throughput will get less.

802.11a operating on 5GHz with OFDM but still data rate supports 54Mbit/s

In 5GHz we have more non overlap channels compared to 2.4GHz and less crowded

 

 

 

27.What are the enhancements in 802.11n protocol ?

Ans: 802.11n Enhancements:

1. Bandwidth - 40MHz

2. Channel Bonding

3. Guard Interval - Short Guard and Long Guard

4. RIFS

5. MCS

6. Aggregation -amsdu and ampdu

7. MIMO

8. Beamforming - implicit & explicit

9. Block Ack

10. HT power management.

11.Number of data tones increased from 48 to 52 in 802.11n

 

28.How does beamforming work and what are different types of beamforming ?

Ans: Beamforming two types: Explicit and Implicit

Explicit beamforming means the beam is only formed if the device on the other end supports beamforming. 

Implicit beamforming will attempt to form the beam even if the endpoint device doesn't support it.

*In 802.11n implicit and explicit beamforming will support

 

*In 802.11ac supports Explicit beamforming

 

If we look into legacy modes 802.11b/g/a signals will travell in Omni direction. In this modes AP will use more power to transmit the signals in all the directional.

 

In 802.11n/ac by using beamforming AP can send the signals in towards to client direction by performing sounding procedure to achieve +3dBm gain signal

 

Before transmitting the data AP and Client has to perform sounding procedures after 4way handshake

 

SU beamforming and MU beamforming

 

 

 

29.What is EAP? In EAP, what is the use of certificates ?

Ans :EAP is extensible authenticate protocol.usage of certificates is to validate is it correct user or not and it uses certificates as keys.

 

 

30.Why is EAP called Extensible Authentication protocol ? Explain EAP-PEAP, EAP-TLS and EAP-TTLS ?

Ans: In EAP process we have several types of authentication types like TLS, PEAP , SIM , MS-CHAP …. Like this we have so many type. Some of them are proprietary protocols also like leap, eap-fast like that …. Each one takes place authentication differently based on certificate , user name and password , based on SIM 

 

31.What is captive portal and explain the flow of events that take place when client connects to captive portal SSID ?

Ans: Captive portal generally we are using in airport , railway station , hotels and public places. How it’s will takes place means initially the HTTP handshakes will be happen req/resp follows with DNS resoluation and basd on it you will able to see login page in your station.

 

32.How roaming works and what factors are responsible for the client to roam to new AP ?

Ans : Roaming occurs when any wireless client device's rssi reached over AP's threshold value. Or say when STA move far away from APs network range and then it connects to another AP having better Signal strength.

 

Factors:

1. Increase in Distance between AP and STA, makes Weak signal strength.

2. Distance is less but antenna does not connected properly which make weak signal strength.

3. Physics Obstacles between AP amd STA such as wall.

4. Wireless Network interface.

 

33.What is fast roaming? How does it happen ?

Ans :  Fast Roaming or IEEE 802.11r or Fast BSS Transition (FT).

In this amendment wireless client device can roam quickly in environments(With WPA2 Enterprise security). client device does not need to re-authenticate to the RADIUS server every time it roams from one access point to another.

 

34.What are different types of key caching ?

Ans: Types of key Caching are:

PMK Caching

OKC

CCKM

 

35.When STA is sending number of MPDU packets to AP, if any packet is corrupted, How the STA will get to know?

Ans : MPDU : MAC address with multiple msdu and FCS

 

Every Wi-Fi frame will have FCS (Frame Check Sequence)

If any packet corrupted that packet won't get ACK

 

Bitmap in Block ACK we can see the wich sequence number got missed. Missing sequence number/frame again it will send.

 

36.            If setup is good and build also good, still getting low throughput. so How debug this issue?

 

Ans:
1. Where we are running the test within chamber or out side of the chamber.

2. Antenna connection

3. If within chamber AP and STA is communicating with same NSS,MCS rates

4. Distance between AP and Client (RSSI,SNR, power levels)

5. If any legacy client is connected

6. Retry

7. Packet drop due to CRC

we have to check first antenna connections, RSSI, power level, MCS and NSS

 

Various reasons:

 

1.Check retry frames are more

2.Check at what mcs rate tx happening

3.Depends up on power leve and antenna proposition

4.Check for aggregation is happened or not .

5.If chip gets more heat also chances for less throughput……….. There are some things also which cause for less throughput

 

Low throughputs in WiFi depends on the following :

1. Distance between AP and STA.

2. Obstacles (such as walls, floors, and ceilings).

3. Electronic interference.

4. Number of users on the network.

 

Reasons :

 

1.check the clock frequency. some times due to lower clock speed of the bus (sdio/pcie) over all throughput might be less.

2.Also, check how many physical antennas are connected and what is the spatial stream being configured in driver conf file. For example with 2x2 enabled in conf file but using only one antenna creates significant throughput drop.

3.Check if BT is enabled. since nowadays, single chip supports both WLAN/BT together, in some devices co-existence does not work properly.

4. And lastly check the AMSDU/AMPDU aggregation function. Lot of time it happens to be the issue with aggregation.

 

Reasons:

Block ack miss , deauths, duplicate frames, retry frames for ack miss, power save type of clients, airtime faireness, Bandwidth, channel load, mcs rates, Co channel interference, inter channel interference, presence of legacy clients, undirected antenna beamforming, background scan interval, beacon interval, fragmentation

 

37.ACK and CTS frames follow which interframe space? 

Ans: SIFS

 

38.Explain about behavior of AP, if it shutdown suddenly due to power loss, One user is using youtube, so how AP will send the data to the particular STA.

Ans:  Connection process will be happen freshly when AP powers off because the user table will be flush out so AP won’t hold any info. Station will connect again and generates new keys

 

39.Consider, If you have 2 DHCP servers in a network, when you connect a STA to AP , from which DHCP server you will get IP to the STA?

Ans: In first DHCP offer packet if some DHCP option is not available (I mean client looking for specific DHCp option) during this time client can take another DHCP offer (if this is having DHCP option). It depends on the offer and DHCP options as well.It’s depends on first offer message and DHCP implementation like proprietary device, vendor class .

40) When AP configured in WPA3 transition security mode, if the client is connected to WPA2 mode and started moving towards the extender in mesh network. The client is not roaming.( It is disconnecting and connecting back)

Note: Extender is configured in WPA2 security mode.

Is this expected behavior.? Or It should roam without any disconnection.

Ans: If your client is connected to the transition network using WPA2 AKM, then it should be able to roam successfully to the extender which is in WPA2, provided both the ciphers are the same (AES, TKIP+AES). If not, it's a bug in client device SW.

But if your client has connected using SAE to the transition network, then ideally it shouldn't roam to WPA2 extender BSSID.