CCNP ENCOR v8 Chapters 17 – 19: Wireless Essentials Test Online

Hint

According to the dB laws: Law of 0: If two power levels are the same, one is 0 dB greater than the other. Law of 3s: A value of 3 dB means that the power value of interest is double the reference value. A value of -3 dB means that the power value of interest is half the reference value. Law of 10s: A value of 10 dB means that the power level of interest is 10 times the reference value. A value of -10 dB means that the power level of interest is 1/10th of the reference value.

Hint

Noise is any other signal that is received on the same frequency as a signal of interest and noise floor is the average signal strength of that received noise.

Hint

Source B is double the value of source A. Each time you double a value, simply add 3 dB. Because 10 mW is double 5 mW, source B is 3 dB greater than the strength of source A.

Hint

Increasing the antenna gain or increasing the transmitter output power can be simple ways to overcome free space path loss.

Hint

A MIMO device can distribute spatial streams across multiple radio chains and does not have to assign a unique spatial stream to an individual transmitter/receiver. The number of possible spatial streams depends on the processing capacity and the transmitter feature set of the wireless device.

Hint

Spatial multiplexing allows faster data throughput by multiplexing or distributing data across two or more radio chains operating on the same channel but separated through spatial diversity.

Hint

Whenever an RF signal is transmitted from an antenna, the signal amplitude decreases as it travels through free space. Even if there are no obstacles in the path between the transmitter and receiver, the signal strength will weaken. This is known as free space path loss.

Hint

In comparing 802.11ax (Wi-Fi 6) to previous 802.11 implementations, one of the key differences is that 802.11ax can use OFDMA to allocate air time for transmission by multiple devices simultaneously.

Hint

The 802.11n standard is backwards compatible with both the 802.11a and 802.11b standards and therefore operates at both 5 GHz and 2.4 GHz.

Hint

In measuring an RF signal, there is a correlation between frequency and wavelength. As the frequency increases, the wavelength decreases. When there are fewer cycles per second, the wavelength will cover a greater distance.

Hint

Signals designated as out of phase do not have matching cycles and are not identical. These signals may cancel each other out as they are received.

Hint

The embedded wireless network topology employs an embedded WLC within each access layer switch. Each AP would be connected to the access switch via twisted-pair cable and is subject to the 100 meter distance limitation.

Hint

A centralized or unified wireless LAN topology utilizes a centralized WLC location that provides easy enforcement of security policies and the ability to maximize the number of APs that can join to it. The centralized topology also supports user mobility because all of the APs associate to the same WLC no matter how many APs the user device associates with while roaming.

Hint

Autonomous APs are standalone devices that are configured via Cisco CLI or a GUI. Controller-based APs are server-dependent devices that require no initial configuration to operate, because they are automatically configured and managed by a WLAN controller. Most autonomous and controller-based APs support PoE.

Hint

When attempting to discover a WLC, an access point can send a unicast CAPWAP Discovery Request to the IP address of a controller over UDP port 5246 or can send a broadcast over the local subnet.

Hint

A Mobility Express topology uses a fully functional Cisco AP with software acting as a WLC instead of investing in a dedicated WLC device.

Hint

The AP will have a single CAPWAP tunnel to the active controller. The active unit keeps CAPWAP tunnels, AP states, client states, configurations, and image files all in sync with the hot standby unit. The active controller also synchronizes the state of each associated client that is in the RUN state with the hot standby controller.

Hint

A WLC will deny an AP from joining it if the maximum number of lightweight APs already joined to it has already been reached.

Hint

Omnidirectional antennas send the radio signals in a 360 degree pattern around the antenna. This provides coverage to devices situated anywhere around the access point. Dishes, directional, and Yagi antennas focus the radio signals in a single direction, making them less suitable for covering large, open areas.

Hint

Once a special mode is activated on a lightweight AP, local mode and the corresponding BSSs will be disabled. A lightweight AP acting as a sniffer will dedicate all of the radios to receiving 802.11 traffic from other sources and forwarding that traffic to a PC that has network analyzer software installed.

Hint

Beamwidth is viewed as a measure of focus for a particular antenna. Beamwidth is determined by finding the strongest point on the plot and following it in either direction until the signal is one-half of the strongest power or 3 dB less.

Hint

In order for two wireless antennas to not suffer from signal degradation, the polarization of the transmitting antenna should match the polarization of the receiving antenna.

Hint

When a client wants to join the BSS of an AP, it actively scans channels and sends probe requests to discover candidate APs. The client then selects one and tries to associate with it. A wireless client can send association request and reassociation request frames to the AP selected. Association requests are used to form a new association, while reassociation requests are used to roam from one AP to another, preserving the original association status of the client.

Hint

When a wireless client is roaming among APs that are bound to the same WLC controller, the roam occurs entirely within the controller. This is known as intracontroller roaming. Intercontroller roaming occurs when a client is roaming among APs that are bound to different WLC controllers. Layer 2 roaming is a type of the intercontroller roaming.

Hint

In a Cisco wireless network, when a client initiates an intercontroller roam, the two controllers involved can compare the VLAN numbers that are assigned to the respective WLAN interfaces. If the VLAN IDs are the same, the client undergoes a Layer 2 intercontroller roam (also called local-to-local roam) and can continue to use the original IP address on the new controller. If the two VLAN IDs differ, the controllers arrange a Layer 3 roam that will allow the client to keep using the same IP address.

Hint

In Layer 3 roaming, anchor and foreign controllers are normally determined automatically. When a client first associates with an AP and a controller, that controller becomes its anchor controller. When the client roams to a different controller, that controller can take on the foreign role. However, sometimes it is desirable to control which WLC to be the anchor controller. For example, guest users should not be allowed to associate with just any controller in the network. Guests might be forced onto a specific controller that is situated behind a firewall or contained in a protected environment. This objective can be achieved by configuring one controller to be a static anchor for a WLAN so that other controllers will direct clients toward it through Layer 3 roaming tunnels.

Hint

To improve the accuracy of locating a device, an AP can use the received signal strength (RSS) of a client device as a measure of the distance between the two. However, if the distance is measured from a single AP only, it is difficult to determine where the client is situated in relation to the AP. In the case of an indoor AP with an omnidirectional antenna, the client could be located anywhere along a circular path of fixed distance because the received signal strength would be consistent at all points on the circle.

Hint

Cisco APs and WLCs can integrate with management platforms like Cisco Prime Infrastructure or DNA Center, along with location servers like Cisco Mobility Services Engine (MSE), Cisco Connected Mobile Experiences (CMX), or Cisco DNA Spaces to gather location information in real time and present that information in a relevant way.

Hint

The Cisco approach to improve the results of wireless device location based on RSS values received by multiple APs is to implement RF fingerprinting, where each mapped area is influenced by an RF calibration template that more closely resembles the actual signal attenuation experienced by the APs and clients.

Hint

In a lightweight AP topology, APs are bound to wireless LAN controllers through CAPWAP tunnels.

Hint

Cisco controllers offer three techniques to minimize the time and effort spent on key exchanges during roams:

• Cisco Centralized Key Management ( CCKM ): One controller maintains a database of clients and keys on behalf of bound APs and provides the clients and keys to other controllers and bound APs as needed during client roams.
• Key caching : Each client maintains a list of keys used with prior AP associations and presents the keys as it roams.
• 802.11r : This 802.11 amendment addresses fast roaming or fast BSS transition. A client can cache a portion of the key of the authentication server and present that to future APs as it roams.

Hint

To locate a device more accurately, an AP can use the received signal strength (RSS) of a client device as a measure of the distance between the two. However, if the distance is measured from a single AP only, it is difficult to determine where the client is situated in relation to the AP. A better solution is to obtain the same measurement from three or more APs, then correlate the results and determine where they intersect.

Hint

A Layer 3 intercontroller roam consists of an extra CAPWAP tunnel that is built between the original controller with which the client associated (called the anchor controller) and the controller it has roamed to (called the foreign controller). The tunnel carries data to and from the client as if it is still associated with the original controller and IP subnet.