Wireless - Midterm 1

Every 802.11 frame contains 2 flag bits that designate whether the frame is headed to or from which of the following? a) The ESS b) The BSS c) The DS d) The AP

(not AP)

What is multi-user MIMO?

- (802.11ac) supports up to 8 spatial streams - enables simultaneous communication between an AP and multiple client stations - Uses Space Division Multiple Access (SDMA) ~ streams are separated by space as opposed to frequency or time

Explain an Infrastructure WLAN.

- Adds APs to a wired distributions system (connected via Ethernet) - Broadcasts beacon frames

List some physical components of a wireless network (8)

- Client devices - Client radio (wireless NIC) - Access Points ~ Autonomous ~ Controller-based - Wi-Fi router ~ Multifunction device - Antennas - Amplifiers/repeaters/bridges - Switches (backbone) PoE (to power APs)

What is frame aggregation and where is it used?

- Concatenates two or more frames to make data transmission more efficient - used in the 802.11 MAC Layer

Explain a Mesh WLAN

- Mesh nodes connect to each other wirelessly as opposed to via Ethernet - Utilizes MIMO

What is MIMO?

- Multiple Input/Multiple Output - Uses multiple paths to send data

What are some security vulnerabilities to consider when implementing a wireless network?

- Passive monitoring - Unauthorized access - Denial-of-service attacks

Explain an Ad Hoc WLAN.

- Peer-to-peer - Easy to setup/take down - Independent Basic Service Set (IBSS)

What are two implications to consider when implementing a wireless network?

- Security vulnerabilities - Interference

What are the three types of encryption for the 802.11 MAC Layer?

- WEP - TKIP - CCMP (AES)

How is MPDU different than MSDU?

- less efficient because headers are longer

What are the three types of WLANs?

1. Ad Hoc 2. Infrastructure 3. Mesh

Explain how a client connects to an AP?

1. Client scans for networks 2. Client authenticates with AP (Authentication state = Authenticated) 3. AP confirms client is successfully authenticated 4. Client associates with AP (Associated state = Associated) 5. AP confirms the client is authenticated and associated, then the client is actively connected.

What are the main functions of the 802.11 MAC Layer? (4)

1. Data delivery 2. Connectivity 3. Timing synchronization 4. Power management

What are the three ways the 802.11 MAC Layer accesses the medium?

1. Distributed coordination function (DCF) 2. Point coordination function (PCF) 3. Hybrid coordination function (HCF)

Describe the benefits of standardization.

1. Equipments interoperability 2. Easier to use off-the-shelf components 3. Price reductions 4. Stable future migration 5. Fast product development

List some benefits of wireless.

1. Mobility 2. Reduced installation 3. Long term cost savings 4. Increased productivity

What are the two types of standards?

1. Official - controlled by government 2. Public - controlled by private organization

Standardization. The purpose of: 1. Standards 2. Specifications 3. Certification

1. Rules 2. How a standard will be executed 3. Provides assurance that the equipment performs as expected

What are the two optional power saving modes?

1. Spatial multiplexing power save (SMPS) 2. Power save multi-poll (PSMP)

What are the two approaches for retransmitting unsatisfactory block of data using ARQ?

1. Stop-+Wait ARQ 2. Continuous ARQ (often called 'sliding window protocol)

Explain the differences between the three LLC communication services.

1. Unacknowledged connectionless service - no error control or flow control - ideal if higher layers of the OSI model provide reliability and flow control 2. Connection-oriented service - establishes connection before sending data - flow control - error control with ACK 3. Acknowledge connectionless service - error control with ACK (Stop+Wait ARQ) - Flow control (Stop+Wait ARQ) - Does not establish a connection

What are the three services provided by the LLC?

1. Unacknowledged connectionless service 2. Connection-oriented service 3. Acknowledge connectionless service

When a client station is first powered on, waht is the order of frames generated by the client station and AP? ***

1st: !beacon/response 2nd: authentication request/response 3rd: association request/response

Describe the benefits released in 802.11n (4) and 802.11ac (4)

802.11n: 1. MIMO 2. Channel bonding 3. Frame aggregation 4. Beamforming 802.11ac: 1. Extended channel bonding 2. Increased spatial streams 3. Multi-user MIMO 4. Standardized beamforming

What is the purpose of the DTIM interval?

A signal sent as part of a beacon by an AP to a client device in sleep mode.

What is a probe request frame?

A stations ends a probe request frame when it needs to obtain information from another station.

What is passive monitoring (security vulnerability)

An unauthorized user sitting within the signal and intercepting sensitive information. Sol: Encrypt all traffic

Which statement defines frame control operation between two stations? A) A station sends an RTS frame as the first step in a three-way handshake that is required before sending data frames B) After receiving a data frame, the receiving station will send an ACK frame to the sending station if no errors are found C) A stations responds to an RTS frame with an ACK frame, thus providing permission for the requesting station to a data frame D) If the sending station does not receive an ACK frame within a predetermined period of time, the sending station will drop the connection

B) After receiving a data frame, the receiving station will send an ACK frame to the sending station if no errors are found

How are DCF, PCF, and HCF different? Similar?

DCF: 1. Uses carrier sensing access method 2. asynchronous (unpredictable signal) PCF: 1. Uses a polling function 2. synchronous (predictable signal) HCF: Uses both polling and carrier sensing function

What is the listen interval?

During association the client uses the listen interval parameter to indicate to the AP how many beacon intervals it should sleep for before it retrieves the queued frames.

What is required for frame aggregation to apply? (3)

Frames must: - be going to the same destination - have the same QoS level - ready for transmission at the same time

What method of communications is used within an 802.11 WLAN?

Half-duplex communications.

Describe beacon frames.

In an infrastructure network: - access points periodically send beacon that provides synchronization among stations - using 10/second - include a timestamp to sync time via TSF (time synchronization function) - approx. 50 bytes long - Header contain src and dst MAC - Dst MAC will always be all 1s (broadcast)

What is channel bonding?

Joins two channels into one. 802.11n: 20MHz => 40 MHz 802.11ac: 80MHz => 160 MHz

802.11 operates on which physical sublayer?

Logical Link Layer (LLC)

What are the two types of frame aggregation?

MSDU MPDU

When scanning for networks, describe the difference between passive scanning and active scanning?

Passive scanning: - client listens for beacon frames on each radio frequency - usually chooses the station with the strongest signal Active scanning: - client sends a broadcast probe request frame (like beacon) and APs respond

Explain RTS/CTS

RTS: Request to Send CTS: Clear to Send - Provides positive control over a shared medium - minimizes collisions among hidden stations When enabled, the station refrains from sending an 802.11 data frame until the station completes an RTS/CTS handshake with another station.

What is the short guard interval?

Space between symbols being transmitted. Used to ensure that distinct transmissions occur between the successive data symbols transmitted by a device.

What is Stop-+Wait ARQ vs. Continuous ARQ?

Stop-+Wait ARQ - Sending station transmits frame and then waits for some kind of acknowledgement Continuous ARQ - sending station transmits frames continuously until receiving station detects an error

How does the 802.11 MAC Layer handle error recovery?

Through the use of ACK frames.

What is beamforming? What are some pros (2) and cons (1)?

Transmits data in one direction as opposed to radiating errwhere. Pros: - More transmitters than receivers - Increase in SNR and data rate Cons: - Any movement between transmitter and receiver

If three 802.11b access points need to be deployed in close proximity, which three frequency channels should be used? (3) a) 1 b) 3 c) 5 d) 6 e) 8 f) 11 g) 13

a) 1 d) 6 f) 11

Which feature of 802.11n wireless access points allows them to transmit data at faster speeds than previous versions of 802.11 Wi-Fi standards did? a) MIMO b) MITM c) SPS d) WPS

a) MIMO

A student uses a laptop to upload an assignment to a file server. Which wireless frame types did the laptop send to locate and associate with the campus access point? a) management frame b) control frame c) data frame d) beacon frame

a) management frame

What causes of hidden nodes would be more likely to lead to an increase in collisions and retries? (2) a. Obstacles between client stations causing attenuation b. Client stations broadcasting with too much power c. Data frames too large for the physical environment d. Access points broadcasting with too little power e. Large 802.11 cells with physically distributed stations f. Client stations too close in proximity to each other

a. Obstacles between client stations causing attenuation f. Client stations too close in proximity to each other

Which statements about the SSID are true? (2) a. The SSID must be included in an association request frame b. The SSID is an alphanumeric value assigned to device manufacturers by the IEEE c. When configuring a new network, creating an SSID is optional d. The SSID is an alphanumeric value with a maximum length of 32 octets e. The SSID is a security session identifier used in RSNs

a. The SSID must be included in an association request frame d. The SSID is an alphanumeric value with a maximum length of 32 octets

Which three Wi-Fi standards operate in the 2.4GHz range of frequencies? (3) a) 802.11a b) 802.11b c) 802.11g d) 802.11n e) 802.11ac

b) 802.11b c) 802.11g d) 802.11n

AN 802.11 transmitter is configured to send a signal on channel 11. Someone reports a problem receiving the signal, so you investigate and find a second transmitter broadcasting on channel 11. Which one of the following best describes the problem? a) Cross-channel interference b) Co-channel interference c) Path interference d) Adjacent channel interference

b) Co-channel interference

802.11 technology is typically deployed at which fundamental later of network architecture? a) core b) access c) distribution d) network

b) access

When a wireless client attempts to join a network, which one of the following frame types is sent first? a) rate request b) authentication request c) association request d) beacon

b) authentication request

Which two roles are typically performed by a wireless router that is used in a home or small business? (2) a) repeater b) ethernet switch c) access point d) RADIUS authentication server e) WLAN controller

b) ethernet switch c) access point

An ACK frame is an example of which of the following 802.11 frame types? a) Data b) Administration c) Control d) Management

c) Control

Which one of the following specifies the default amount of time between successive 802.11 data frames? a) CCA b) IBSS c) DIFS d) SIFS

c) DIFS

Which type of wireless topology is created when two or more Basic Service Sets are interconnected by Ethernet? a) WiFi Direct b) IBISS c) ESS d) BSS e) ad hoc WLAN

c) ESS

DCF uses what two types of carrier-sense mechanisms to determine whether the medium is busy or idle? (1+) a) Pseudo-random b) Active c) Virtual d) Backoff e) Interframe f) Physical

c) Virtual f) Physical

Which security mode is the least secure chose when a home wireless router is configured? a) WPA2-Personal b) WPA c) WEP d) WPA2

c) WEP

For which discovery mode will an AP generate the most traffic on a WLAN? a) open mode b) mixed mode c) passive mode d) active mode

c) passive mode

Which of the following is typically the most important design goal when designing a warehouse WLAN? a) Throughput b) RF interference c) Capacity d) Coverage

d) Coverage

Which sublayer of the OSI model's Data-Link layer is used for communication between 802.11 radios? a) WPA b) FSK c) LLC d) MAC

d) MAC

What component of the 802.11 standards allows stations to reserve access to the RF medium for a specified period of time? a) Short guard intervals b) Listen interval c) DTIM interval d) RTS or CTS frames e) Probe request frames

d) RTS or CTS frames

Which parameter is commonly used to identify a wireless network name when a home wireless AP is being configured? a) ESS b) BESS c) ad hoc d) SSID

d) SSID

A company has recently implemented an 802.11n wireless network. Some users are complaining that the wireless network is too slow. Which solution is the best to enhance performance? a) Replace the wireless NICs on the computer that are experiencing slow connections b) Disable DHCP on the access point and assign static addresses to the wireless clients c) Upgrade the firmware on the wireless access points d) Split the traffic between the 2.4 GHz and 5 GHz frequency bands

d) Split the traffic between the 2.4 GHz and 5 GHz frequency bands

When a station sends an RTS, the Duration/ID field notifies the other stations that they must set their NAV times to which of the following values? a) 213 microseconds b) The time necessary to transmit DATA and ACK frames c) The time necessary to transmit the CTS frame d) The time necessary to transmit the CTS, DATA, and ACK frames

d) The time necessary to transmit the CTS, DATA, and ACK frames

A wireless client maintains a NAV value that is used for which of the following purposes? a) To navigate the frame through the ESS b) To identify the MAC address of the next client to transmit c) To set the priority of the client's ability to transmit d) To predict when the channel might become free

d) To predict when the channel might become free

During which stage of establishing connectivity between a WLAN client and an AP does the client learn the MAC address of the AP? a. probing b. authentication c. encryption d. discovery e. association

e. Association

Which 802.11 standard defines which medium to be used in a distribution system (DS)? a. 802.3 Ethernet b. 802.15 c. 802.5 token ring d. star-bus topology e. none of the above

e. none of the above

What is the command to get into the AP on a 881?

service-module wlan-ap 0 session