CWNA

RF Tech

100

You want to increase the EIRP transmitted through an antenna. What kind of device can increase the EIRP?

What is a directional antenna can be used to increase the EIRP (Equivalent Isotropically Radiated Power), which is the power output at the antenna in the intended direction of propagation.

200

Polar antenna charts are graphical representations of an antenna's coverage pattern. Describe how polar charts are used and interpreted?

What is an antenna’s beamwidth is calculated by measuring how many degrees off the center axis is the strength ½ (-3dB) of the zero degree position (peak amplitude).The vertical beamwidth is measured in degrees and it is defined perpendicular to the Earth Surface. The horizontal beamwidth is also measured in degrees but is defined parallel to the Earth Surface. Azimuth and elevation charts are a standard way to represent an antenna’s coverage pattern. The azimuth chart shows a top-down view of the antenna’s coverage and the elevation chart shows a side-view of the antenna’s coverage.

300

Name three valid power management modes for 802.11-2012 WLAN devices?

What is Active mode, Power Save mode, WMM-PS mode. In Active Mode stations are always powered up. Each station that associates with an AP is given an Association ID number (AID). The AP can keep track of which STAs are associated with it at any point in time. At specified intervals, beacon frames are transmitted. These beacons contain a TIM (Traffic Indication Map) which tells a particular STA if there is unicast data buffered for it. The AID values are set in part of the TIM element called the Partial Virtual Bitmap. Thus, if a station sees its AID set in the Partial Virtual Bitmap, it knows there is data buffered for it. For multicast/broadcast delivery, certain beacon frames are designated as DTIM (Delivery Traffic Indication Message) frames. The DTIM Count and DTIM period fields in the TIM Information Element let the STA know when multicast/broadcast delivery will commence. A special AID (AID 0) is used to indicate the presence of multicast/broadcast traffic when set to “1”. Some texts refer to the DTIM beacon as the “Data beacon”. When in Power Save mode, stations will “sleep” or “doze’ (in a low power state) and will “wake up” to check TIM and DTIM settings at the appropriate time. Stations will doze when there are no frames buffered for them at the access point. PS-Poll frames are used by client stations to retrieve their queued data from the access point when they return to a full power state. WMM-PS (Wi-Fi Multimedia Power Save) improves upon legacy power-save methods in several ways. In legacy PS modes, STAs have to wait until a beacon containing TIM information is sent before they can check whether there are buffered frames stored at the AP. WMM-PS STAs can check for buffered frames at any time by using a trigger frame to “poll” the AP. This has important ramifications for delay sensitive applications such as VOIP. In legacy PS modes, significant amounts of overhead may be added with the use of PS-Poll frames for each data frame sent from the AP’s buffer. WMM-PS utilizes QoS data frames to request data from the AP’s buffer, which is more efficient than legacy power save.

400

When calculating the data rates for 802.11n and 802.11ac links, the guard interval (GI) impacts the final calculation. What is the length of the short GI in nanoseconds?

What is 802.11n and 802.11ac both support a short guard interval (GI) of 400 ns. The standard or long GI is 800 ns. When using the short GI, an increase in throughput of up to 10% may be achieved; however, it is remotely possible that the short GI may result in intersymbol interference in high multipath environments - though this has not been a common problem in implementations. The GI is used between symbols and not between frames. Remember, a symbol is used to transmit a bit or series of bits depending on the modulation. For example, with 256-QAM, each symbol transmits 8 bits and with 64-QAM, each symbol transmits 6 bits. The space between frames are IFS or interframe spaces.

500

You have an AP that transmits at 30 mW. It is connected to an antenna that provides 11 dBi of gain; however, a cable is used to connect the antenna and it causes 5 dB of loss. What is the output power at the antenna in the intended direction of propagation?

What is 120 mW. Here is the math: 11 dBi gain + 5 dB loss = 6 dB gain 30 mW + 6 dB gain = 30 mW * 2 * 2 30 mW * 2 * 2 = 120 mW

Reg's and Standards

100

You plan to use PoE to power a wireless bridge based on the 802.11-2012 standard and the 802.3 clause 33 standard. The device requires 20 watts of power at the wireless bridge. What amendment provided for this level of power and greater?

What is PoE was defined in 802.3af and 802.3at and is part of the 802.3 standard as clause 33. 802.3af only provides up to 12.95 watts at the powered device, but 802.at provides up to 25.5 watts at the powered device.

200

The 802.11-2012 standard specified two authentication mechanisms. What are these mechanisms?

What is the use of 802.1X/EAP and preshared keys.

300

Wired network devices can detect collisions and use CSMA/CD for communications. 802.11 wireless devices cannot detect collisions, but through a defined mechanism they can assume that corruption has occurred and that a frame should be re-transmitted. What is this mechanism that allows for the detection of a likely corrupted frame?

What is all the data frames regardless of it they are fragmented are acknowledged by the receiver by sending back an acknowledgement frame. The reason that radios have to do this is because they are half-duplex (meaning they can either transmit or receive, but can’t do both at the same time) and cannot hear collisions with the frame while it is transmitting. If an ACK frame is not received in a specific time period, the transmitting station assumes that there was a collisions and then retransmit the frame again. Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is a method used to help avoid collisions, but it is not used to determine if a collision occurred.

400

When operating in the 2.4 GHz frequency band at a data rate of 5.5 Mbps, what coding method is used?

What is Complimentary Code Keying (CCK) is used at 5.5 and 11 Mbps with HR/DSSS, which is supported by ERP-OFDM (802.11g) and HT (802.11n) devices operating in the 2.4 GHz frequency band. Barker codes are used at 1 and 2 Mbps. Convolution coding is used at higher data rates. 256-QAM is a modulation technique and should not be confused with coding. The data to be transmitted is first encoded and then it is modulated onto the RF signals.

500

You are considering an upgrade from an existing ERP-OFDM WLAN to an HT WLAN in the same frequency domain. What is the maximum data rate of the current ERP-OFDM WLAN?

What is ERP-OFDM is modulation method is used with 802.11g technology. ERP-OFDM operates in the 2.4 GHz band but matches the capabilities of 802.11a OFDM in the 5GHz band. 802.11g also supports DSSS (1 & 2 Mbps, ERP-DSSS) and HR-DSSS (5.5, 11 Mbps, ERP-CCK) data rates. ERP-OFDM supports 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. Only 6, 12, and 24 Mbps are required by IEEE.

Protocol and Devices

100

You are the networking consultant of small and medium businesses. You are asked about the difference between the more expensive enterprise APs and Wi-Fi routers for small offices and home offices (SOHO). (Name Three)

What is Enterprise WLAN APs support centralized management, providing a single point of administration for all APs in the network. SOHO Wi-Fi routers typically do not. Enterprise WLAN APs support advanced features like customized QoS settings, airtime fairness, multicast optimization, and spectrum analysis. SOHO Wi-Fi routers typically do not. Enterprise WLAN APs incorporate detailed statistics reporting, security policy monitoring, and client troubleshooting functionality. SOHO Wi-Fi routers typically do not.

200

You are asked to explain the benefits of WLAN controllers. Name three features that are usually supported by WLAN controllers?

What is WLAN controllers are layer 2/3 devices. Rogue detection and mitigation are available on most WLAN controllers and controller-based AP. HTTP, HTTPS, SNMP, Telnet, and SSH1/2 protocols are usually used to manage WLAN controllers. Many vendors incorporate features and design practices for 802.11n and 802.11ac throughput rates, they offer the option to tunnel data centrally (through the controller) or to perform local bridging at the AP.

300

You are tasked with selecting to use the built-in client utilities in the operating system or a third-party client utility. Name 3 common features or functions?

What is WLAN client utilities often have features to allow channel selection for an Ad Hoc network. Scanning features are used to locate APs and Ad Hoc networks in most client utilities. Client utilities work hand in hand with the driver that communicates with the 802.11 hardware.

400

A client station is experiencing excessive frame collisions and corruption. Name three conditions that can increase the rate of CRC errors in a WLAN connection?

What is if a unicast frame is corrupted, then the cyclic redundancy check (CRC) will fail and the receiving radio will not return an ACK frame to the transmitting radio. Since the transmitting radio did not receive an ACK frame, the unicast frame is not acknowledged and is then retransmitted. High amounts of multipath, RF interference, low SNR, hidden nodes, mismatched power settings, near/far problems and excess co-channel interference all may cause layer 2 retransmissions.

500

An 802.11ac client station transmit an aggregate data frame (A-MPDU) with a 300 byte MPDU to an 802.11n AP. The destination of the frame is a file server located on the Ethernet network. The AP does not support jumbo (giant) frames on its Ethernet port. What will the AP do with the aggregated data frame?

What is 802.11n introduced the frame aggregation concept to 802.11 WLAN. The aggregations can be either A-MSDU or A-MPDU. When the AP receives an aggregated frame, it de-aggregates it before it is sent onto the wired (often Ethernet) medium. Frame aggregation improves wireless production by reducing overhead. Because there are limitation on the size of Ethernet frames (1522 bytes, including layer 2), aggregated frames must be de-aggregated before frame translations by the AP. In a controller-based WLAN, the AP is in charge of the aggregation and de-aggregation.

Implementation

100

It is important to implement effective security solutions based on long-established best practices. One best practice is ensuring CIA in a system. What does the security acronym CIA reference?

What is Confidentiality, Integrity , Availability.

200

You are planning a new 802.11ac WLAN. The implementation will function with all WLAN intelligence, such as the DS, IS, encryption, decryption and other features in the AP. What type of WLAN architecture have you selected?

What is the autonomous AP or autonomous architecture includes all WLAN functionality in the AP.

300

You are implementing an 802.11ac WLAN with a primary focus on capacity because it is assumed that coverage must be achieved. What (3) design decisions can assist in increasing capacity?

What is band steering so that clients are balanced between 2.4 GHz and 5 GHz radios as much as possible. Implementing dual-radio APs can help. Using lower output power settings allows you to create smaller cells and therefore have fewer client stations (STAs) per AP. This would also increase capacity. Semi-directional antennas (such as patch or panel) may also be used to that the cell is focused in a direction

400

You are implementing an 802.11ac dual-band AP that uses 802.11n in the 2.4 GHz band. You notice, during the site survey, that the next-door facility is using a 40 MHz configuration in the 2.4 GHz band. What can you enable on your AP to force the next door facility to play nice and use 20 MHz channels?

What is set the 40 MHz intolerant bit to 1, the 802.11 standard 802.11n AP must stop using 40 MHz channels and use a 20 MHz channel instead.

500

You are running an HT network in the 2.4 GHz frequency band. You use the HT-Greenfield PLCP format only on the AP. What device types can communicate with this AP?

What is when using HT-Greenfield mode, only HT clients may communicate with the AP. These are 802.11n clients. 802.11g clients are not supported and neither are 802.11b clients because they do not understand the HT-Greenfield PLCP. 802.11a clients are not supported because the operate only in the 5 GHz band.

Site Survey

100

What documents might be needed prior to performing an indoor site survey for a new wireless LAN? Name two.

What is Blueprints will be needed for the site survey interview to discuss coverage ad capacity needs. A network topology map will be useful to assist in the design of integrating the wireless network into current wired infrastructure.

200

What are some additional recommendations that can be made along with the final site survey report? Name three.

What is training, security, and choice of vendor are consider extra recommendations. The site survey should already report coverage, capacity, and roaming requirements.

300

You are planning for data rates in a VHT implementation in the 5 GHz frequency band. Which of the following factors impact the available data rate for VHT STAs linked to a VHT AP? Name at least two.

What is he number of spatial streams impacts the available data rate. More spatial streams equals higher data rates. Modulation also impacts data rates. For example, 64-QAM modulation results in lower data rates than 256 QAM modulation, but about 33%. Coding also impacts data rates. Coding is represented as a ratio of data bits to coded bits, for example 5/6. Additionally, the guard interval, which may be long (800 ns) or short (400 ns) impacts the data rate.

400

ABC company wants to implement an 802.11ac WLAN to support low-bandwidth, latency-sensitive application. As the network administrator, your task is to assure that clients can connect to the network when needed. When doing a site survey, which measurement best indicates the clients ability to maintain connection with the WLAN?

What is the signal-to-noise ratio (SNR) is a measurement of the strength of the APs RF signal compared with the background noise. SNR is usually the best measurement to indicate capability to maintain a connection between the AP and the client because it reports the difference between noise floor and signal and is not just a measurement of signal strength.

500

One client station is operating at a high power and is located close to the AP. Because of this client, the AP cannot decode the frames from a client station located near the edge of its service area. What is the term that defines this situation?

What is Near/Far is a type of hidden node problem where a nearby AP operating at high power increases the noise floor at the AP causing the low level signals from far away client stations to become corrupted.