Wireless Flashcards
When looking at a radio wave - what is the relationship of frequency to distance?
The higher the frequency of the radio wave, the shorter the distance it will be able to travel.
What is the frequency of a radio wave?
The number of oscillations or pulses in a given time period - expressed in Hertz (Hz)
What is the wavelength of a radio wave?
The distance between two peaks or valleys of a radio wave.
What is the amplitude of a radio wave?
The “height” of a radio wave, which is indicative of it’s power/energy.
In wireless, what are the two purposes to manipulate the amplitude of a wave?
- Increase broadcast distance
- Convey information or data (using small variations in amplitude)
In wireless, what is the unit of measure for the power of a radio wave’s amplitude?
Watts - most wireless signals are expresses in milliwatts, though (1/1,000 of a Watt)
In wireless, what is the function of a Decibel (dB)?
To compare two levels of absolute power to each other: the source and the reference
What is EIRP and how is it measured?
Effective Isotropic Radiated Power - measured in Decibels (dB), an expression of how powerful a signal is being broadcast:
Transmitter (dB) - Cable Loss (dB) + Antenna Gain (dB)
What is RSSI and how is it measured?
Received Signal Strength Indication - expressed as a negative Decimal number relative to 1 milliwatt (-dBm). The lower the negative number, the better the quality of signal. (-20 great, -90 not usable)
What is SNR and how it is measured?
Signal to Noise Ratio - in Decibels (dB) its an expression of the difference of our wireless signal over the noise floor. A good number is between 25 - 40 dB.
What are the three most prevalent wireless channels used in the 2.4 Ghz range and why?
Channels 1, 6, and 11 - because they are spaced out far enough in the 2.5 Ghz frequency range (2400 Mhz - 2483 Mhz) there is no frequency overlap.
What two characteristics make 802.11ax unique among wireless standards?
- ability to use 1Ghz through 6Ghz frequency
- Bi-Directional MUMINO (Multi-User Multi-In/Multi-Out)
What is one of the biggest drawbacks of the 802.11ac standard?
160Mhz channels has a lot of potential for channel overlap and interference.
When considering any wireless standard that leverages MIMO, what client-side characteristic should be considered?
The number of wireless antennas the device has. This will affect multi-path capabilities and throughput.
What wireless standards support some form of MIMO?
- 802.11n
- 802.11ac
- 802.11ax
What is the biggest drawback to wireless being a Half-Duplex technology?
The slowest client in the “chain” of communication can cause delay for all the others communicating with a given access point.
What is an Isotopic radiation pattern?
A theoretically “perfect” radiation pattern where the signal is broadcast evenly in every direction. This is not possible in actuality but used to describe a “perfect scenario” in wireless terms.
What is inefficient about the way a dipole antenna broadcasts a signal?
The broadcast signal is weaker above and below the antenna (via the E-Plane)
What is the difference between an Autonomous Access Point and a Light-Weight Access Point?
Autonomous: All of the management of the wireless traffic is done on the AP (home router)
Light-Weight: Management and routing decisions are made by Controllers the APs are attached to on the back-end.
What is a WLC?
Wireless LAN Controller - a device that manages wireless access-points.
What is a BSS and an SSID?
- Basic Service Set
- Service Set Identifier - The name of the wireless network
For an access point operating in Autonomous mode, what is the name of an IP address designated to a given VLAN?
BVI - Bridged Virtual Interface
What type of layer 2 connection is required to connect to an access point operating in Autonomous mode?
Trunk - because multiple VLANs may need to be carried up from the access point.
What is the biggest drawback to running APs in autonomous mode?
Wireless clients cannot roam between APs because it requires an IP address to change.
How does an AP running in Lightweight mode connect back to a Wireless Lan Controller (WLC)?
Over a layer 2 CAPWAP tunnel.
What are the benefits of using an AP in Lightweight mode
- Lightweight APs can extend VLANs across a WLC
- Wireless clients can roam between APs without needing an IP address change
- Management of APs can be centralized to a WLC
- WLCs can “self-heal” wireless networks
What are the drawbacks of using APs in Lightweight mode?
- More complex configuration
- Clients connecting back to WLCs inside a data center could pose security risks (close virtual proximity to sensitive applications)
- CAPWAP tunnels connections could bypass a firewall
What are the 4 main WLC topology deployment designs?
- Centralized
- Distributed
- Branch (Flex Connect)
- Cloud
NOTE: Fifth option is an SD-Access solution
What are the characteristics of a Centralized WLC deployment design?
- There is one (or very few) WLC located in or near the data center
- WLC controls all lightweight APs
- Clients connect via CAPWAP tunnel back into a data center subnet
- Allows for the highest level of Layer 2 roaming for wireless clients
What are the characteristics of a Distributed WLC deployment design?
- Multiple WLCs spread across the distribution layer of the network
- Roaming is limited to APs connected to the same WLC
What are the characteristics of a Branch/Flex-Connect WLC deployment design?
- Only Control Plane traffic is sent over the CAPWAP tunnel from the AP
- Data Plane traffic will be placed on the regular network
- Guest traffic could still be sent across the CAPWAP tunnel, optionally
- No Layer 2 roaming for wireless clients
What are the characteristics of a Cloud WLC deployment design?
- APs are managed via a Cloud Service (Meraki)
- the WLC lives in the Cloud
What are the two C’s of wireless design?
- Coverage - signal strength and reach
- Capacity - number of clients that can connect
What is the industry standard for wireless coverage overlap of a usable wireless signal between two or more APs?
20% traditional overlap
35% coverage overlap for voice and video
What is considered the minimum level for a usable wireless signal (dBm)?
(-67) to (-70) dBm
NOTE: -67 dBm to support voice and video
Which four pieces of Cisco Software can leverage Wireless Real Time Location Service (RTLS)?
- Prime Infrastructure
- Cisco Mobility Services Engine (MSE)
- Cisco Connected Mobile Experiences (CMX)
- DNA Spaces
What two methods are used by Cisco devices to improve location based service accuracy?
- Triangulation
- RF Fingerprinting
What are the characteristics of a Lightweight Wireless AP configured for “Local” Mode?
- The AP is “local” to the WLC (same subnet or physical location)
- The default Lightweight mode that offers more BSSs ()
- The AP will scan other channels, measure noise and interference, detect rogue devices, and match IDS events
What are the characteristics of a Lightweight Wireless AP configured for “Bridge” Mode?
- Two APs connect to each other to link to separate locations
- Multiple APs can be bridged to create a mesh network
What are the characteristics of a Lightweight Wireless AP configured for “Flex-Connect” Mode?
- The AP is remote from the WLC
- The AP can switch traffic locally between an SSID and a VLAN if the CAPWAP tunnel to the WLC is down
What are the characteristics of a Lightweight Wireless AP configured for “Rogue detector” Mode?
- the AP is dedicated to detecting rogue devices.
Rogue Devices - a device connected to the wireless and wired networks.
What are the characteristics of a Lightweight Wireless AP configured for “Sniffer” Mode?
- The AP only receives traffic for the purpose of sniffing or capturing traffic (i.e. PCAP)
What are the characteristics of a Lightweight Wireless AP configured for “Monitor” Mode?
- The AP does not transmit
- Acts as a sensor for IDS, rogue access points, location-based services
What are the characteristics of a Lightweight Wireless AP configured for “SE-Connect” Mode?
- The AP is dedicated to spectrum analysis
- You can connect to the AP using software like Cisco Spectrum Expert to analyze the data and look for interference.
What are the three types of Wireless Ethernet frames?
- Data Frames: Source/Dest MAC headers and CRC
- Management Frames: Beacon, Probe, Auth, and Association
- Control Frames: Request to Send (RTS), Clear to Send (CTS), ACK
What types of information may be contained in a Beacon Frame?
- SSID
- Channel
- Rules: data rates, encryption, power saving
What type of request is sent by a client that is actively scanning?
Probe Request - searching for an AP that is hosting a known SSID
What are the four basic phases a client goes through when trying to join an SSID?
1.) Authentication Request (client to AP)
2.) Authentication Response (AP to client) - negotiation of authentication method
3.) Association Request (client to AP)
4.) Association Response (AP to client)
What are the phases of the “State Machine” a LWAP goes through?
- Boot
- WLC Discovery
- Build CAPWAP Tunnel to WLC
- WLC Join
- Download Image
- Download Config
- Run State
- Reset
What are three methods a LWAP uses to discover a WLC?
- Prior Knowledge of WLCs
- DHCP and DNS information
- Broadcast on the local subnet
What are the steps a LWAP goes through during WLC discovery?
1.) Broadcast CAPWAP Discovery on local wired subnet
2.) Looks for pre-configured (up to 3) or previously known (up to 8) WLC IP addresses
3.) DHCP option 43 (if configured)
4.) Attempts to resolve “CISCO-CAPWAP-CONTROLLER.local” (local is local domain)
5.) Reset and start from #1 again
What port does a LWAP use when trying to discover a WLC using a Unicast connection?
UDP 5246
What is the key characteristic of an AP operating in Flex Connect mode?
If the CAPWAP tunnel to the WLC goes down, it can switch traffic locally between the SSID and a local VLAN (if configured)
When a client roams between two APs connected to the same controller, what kind of roam is this?
Intra-controller Roam
When a roam occurs, what two processes MAY occur while switching between APs?
- a new DHCP request by the client
- Client Authentication (802.1x)
When a client roams between two APs connected to the different controllers (WLC) on the same VLAN, what kind if roam is this?
- Inter-controller Layer 2 Roam
When a client roams between two APs connected to the different controllers (WLC) on different VLANs/subnets, what kind if roam is this?
- Inter-controller Layer 3 Roam
aka local-to-foreign roam
In a Layer 3 Roam, what roles do the two WLCs assume in respect to the client?
Anchor Controller - the controller where the client was originally bound
Foreign Controller - the controller where the client moved to
NOTE a CAPWAP tunnel is built from the anchor to the foreign controller.
What feature does 802.11r provide?
Fast Roaming (credential caching)
What function do Mobility Groups perform?
Group WLCs into groups of trusted controllers to facilitate faster Layer 3 roaming.
How do Mobility Groups expedite roaming?
By use of a shared Virtual IP Address (per group)
