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Wireless Questions 3

January 30th, 2021 in ENCOR 350-401 Go to comments

Question 1

Explanation

Output power is measured in mW (milliwatts). A milliwatt is equal to one thousandth (10−3) of a watt.

Question 2

Question 3

Explanation

+ From the output of WLC “show interface summary”, we learned that the WLC has four VLANs: 999, 14, 15 and 16.
+ From the “show ap config general FlexAP1” output, we learned that FlexConnect AP has four VLANs: 10, 11, 12 and 13. Also the WLAN of FlexConnect AP is mapped to VLAN 10 (from the line “WLAN 1: …… 10 (AP-Specific)).

From the reference at: https://www.cisco.com/c/en/us/td/docs/wireless/controller/8-1/Enterprise-Mobility-8-1-Design-Guide/Enterprise_Mobility_8-1_Deployment_Guide/ch7_HREA.html

FlexConnect VLAN Central Switching Summary
Traffic flow on WLANs configured for Local Switching when FlexConnect APs are in connected mode are as follows:

+ If the VLAN is returned as one of the AAA attributes and that VLAN is not present in the FlexConnect AP database, traffic will switch centrally and the client is assigned this VLAN/Interface returned from the AAA server provided that the VLAN exists on the WLC. (-> as VLAN 15 exists on the WLC so the client in connected mode would be assigned this VLAN -> Answer G is correct)
+ If the VLAN is returned as one of the AAA attributes and that VLAN is not present in the FlexConnect AP database, traffic will switch centrally. If that VLAN is also not present on the WLC, the client will be assigned a VLAN/Interface mapped to a WLAN on the WLC.
+ If the VLAN is returned as one of the AAA attributes and that VLAN is present in the FlexConnect AP database, traffic will switch locally.
+ If the VLAN is not returned from the AAA server, the client is assigned a WLAN mapped VLAN on that FlexConnect AP and traffic is switched locally.

Traffic flow on WLANs configured for Local Switching when FlexConnect APs are in standalone mode are as follows:

+ If the VLAN returned by the AAA server is not present in the FlexConnect AP database, the client will be put on a default VLAN (that is, a WLAN mapped VLAN on a FlexConnect AP) (-> Therefore answer B is correct). When the AP connects back, this client is de-authenticated (-> Therefore answer C is correct) and will switch traffic centrally.

Question 4

Question 5

Explanation

Once you know the complete combination of transmitter power level, the length of cable, and the antenna gain, you can figure out the actual power level that will be radiated from the antenna. This is known as the effective isotropic radiated power (EIRP), measured in dBm.

EIRP is a very important parameter because it is regulated by governmental agencies in most countries. In those cases, a system cannot radiate signals higher than a maximum allowable EIRP. To find the EIRP of a system, simply add the transmitter power level to the antenna gain and subtract the cable loss.

EIRP_wireless.jpg

EIRP = Tx Power – Tx Cable + Tx Antenna

Suppose a transmitter is configured for a power level of 10 dBm (10 mW). A cable with 5-dB loss connects the transmitter to an antenna with an 8-dBi gain. The resulting EIRP of the system is 10 dBm – 5 dB + 8 dBi, or 13 dBm.

You might notice that the EIRP is made up of decibel-milliwatt (dBm), dB relative to an isotropic antenna (dBi), and decibel (dB) values. Even though the units appear to be different, you can safely combine them because they are all in the dB “domain”.

Reference: CCNA Wireless 640-722 Official Cert Guide

Question 6

Explanation

This is called Inter Controller-L2 Roaming. Inter-Controller (normally layer 2) roaming occurs when a client roam between two APs registered to two different controllers, where each controller has an interface in the client subnet. In this instance, controllers exchange mobility control messages (over UDP port 16666) and the client database entry is moved from the original controller to the new controller.

Question 7

Explanation

Windows can actually block your WiFi signal. How? Because the signals will be reflected by the glass.

Some new windows have transparent films that can block certain wave types, and this can make it harder for your WiFi signal to pass through.

Tinted glass is another problem for the same reasons. They sometimes contain metallic films that can completely block out your signal.
Mirrors, like windows, can reflect your signal. They’re also a source of electromagnetic interference because of their metal backings.

Reference: https://dis-dot-dat.net/what-materials-can-block-a-wifi-signal/

An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. WiFi operates in the gigahertz microwave band. The FCC has strict regulations on RFI (radio frequency interference) from all sorts of things, including light bulbs -> Incandesent lights do not interfere Wi-Fi networks.

Note:

+ Many baby monitors operate at 900MHz and won’t interfere with Wi-Fi, which uses the 2.4GHz band.
+ DECT cordless phone 6.0 is designed to eliminate wifi interference by operating on a different frequency. There is essentially no such thing as DECT wifi interference.

Question 8

Explanation

When the primary controller (WLC-1) goes down, the APs automatically get registered with the secondary controller (WLC-2). The APs register back to the primary controller when the primary controller comes back on line.

Reference: https://www.cisco.com/c/en/us/support/docs/wireless-mobility/wireless-lan-wlan/69639-wlc-failover.html

Question 9

Question 10

Explanation

Directional antennas
Directional antennas come in many different styles and shapes. An antenna does not offer any added power to the signal; it simply redirects the energy it receives from the transmitter. By redirecting this energy, it has the effect of providing more energy in one direction and less energy in all other directions. As the gain of a directional antenna increases, the angle of radiation usually decreases, providing a greater coverage distance but with a reduced coverage angle. Directional antennas include patch antennas and parabolic dishes. Parabolic dishes have a very narrow RF energy path, and the installer must be accurate in aiming these types of antennas at each other.

directionl_patch_antenna.jpgDirectional patch antenna

Reference: https://www.cisco.com/c/en/us/products/collateral/wireless/aironet-antennas-accessories/product_data_sheet09186a008008883b.html

Omnidirectional antennas

An omnidirectional antenna is designed to provide a 360-degree radiation pattern. This type of antenna is used when coverage in all directions from the antenna is required. The standard 2.14-dBi “rubber duck” is one style of omnidirectional antenna.

ominidirectionl_antenna_direction.jpgOmnidirectional antenna

-> Therefore Omnidirectional antenna is best suited for a high-density wireless network in a lecture hall.

Question 11

Explanation

172.16.50.5 in hex is

We will have the answer from this paragraph:

“TLV values for the Option 43 suboption: Type + Length + Value. Type is always the suboption code 0xf1. Length is the number of controller management IP addresses times 4 in hex. Value is the IP address of the controller listed sequentially in hex. For example, suppose there are two controllers with management interface IP addresses, 192.168.10.5 and 192.168.10.20. The type is 0xf1. The length is 2 * 4 = 8 = 0x08. The IP addresses translates to c0a80a05 (192.168.10.5) and c0a80a14 (192.168.10.20). When the string is assembled, it yields f108c0a80a05c0a80a14. The Cisco IOS command that is added to the DHCP scope is option 43 hex f108c0a80a05c0a80a14.”

Reference: https://www.cisco.com/c/en/us/support/docs/wireless-mobility/wireless-lan-wlan/97066-dhcp-option-43-00.html

Therefore in this question the option 43 in hex should be “F104.AC10.3205 (the management IP address of 172.16.50.5 in hex is AC.10.32.05).

Question 12

Comments
  1. BonezUK
    February 14th, 2021

    For Question 3:

    Can someone explain, below question:

    G. When the AP is in connected mode, the client will be placed in VLAN 15.

    ■If the VLAN is returned as one of the AAA attributes and that VLAN is not present in the Flex AP database, traffic will switch centrally. If that VLAN is also not present on the WLC, the client will be assigned a VLAN/Interface mapped to a WLAN on the WLC.

    Where are we getting VLAN15, as I do not see it mapped under “Config General FlexAP1”

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