Wireless Location Services

Question 1

What is roaming

Answer 1

When a wireless client device moves between AP’s.

Question 2

What are the different types of roaming

Answer 2

Autonomous AP roaming
Intra-controller roaming
Inter-controller roaming

Question 3

Wireless devices are always probing on every channel available to them to check for other AP’s to join. If a signal from one AP degrades enough it will join the other.

True or False

Answer 3

True

Question 4

When a client roams between 2 autonomous AP’s, which of the following statements are True and/or False

1. Once a client devices moves to a different AP any remaining traffic for the client will be sent to the new AP using the MAC address of the client.
2. The AP associations table will show the client in both AP association tables.
3. the client handles the roaming process

Answer 4

1. True
2. False - Only the AP that the client is connected to will have that client in its association table.
3. False - the AP handles the roaming process

Question 5

What is intra-controller roaming

Answer 5

When a client moves between 2 AP’s that are connected to the same WLC.

Question 6

The WLC handles the roaming process and updates the client association table.

True or False

Answer 6

True

Question 7

What other process’ might slow down the roaming process

Answer 7

DHCP - When connecting to anew AP/WLC a new lease or IP address may be required.

Authentication - WLC may be configured to use 802.1x for each client.

Question 8

What are the 3 methods of improving the efficiency of wireless authentication

Answer 8

1. Cisco Centralised Key Management (CCKM) - 1 controller maintains a database of all clients and keys on behalf of the AP. this can be provided to other WLC and AP’s
2. Key Caching - Each client can contain a list of prior key associations (Maximum 8 keys)
3. 802.11r - Clients can cache a portion of the authentication servers key to present in future (increase speed)

Question 9

What is Inter-controller roaming and what are the 2 types

Answer 9

When a client moves between 2 AP’s that are connected to different WLCs.

Layer 2 roam - when the roam is between 2 AP/WLC that are using the same VLAN and subnet (very efficient)

Layer 3 roam - When client moves between different AP/WLC with a different VLAN and subnet.

Question 10

How does the client maintain its IP address in a layer 3 roam

Answer 10

The client will move from an anchor AP to the new AP. the 2 WLCs will form a CAPWAP tunnel between them to encapsulate the clients IP back to the original WLC/AP.

Question 11

What is a mobility group and what is the maximum amount of controllers

Answer 11

When 1 or more WLCs agree to be part of a group they can share credentials between them making roaming very efficient.

Max 24 entries per group

Question 12

What is a mobility list

Answer 12

a group of controllers that can easily roam between

max 72
e.g. 3 groups of 24 members
2 groups of 36 members
etc…

Question 13

What value can be used to measure the distance from an AP

Answer 13

Received Signal Strength (RSS)

Question 14

What must be considered when analysing the RSS

Answer 14

the free space path loss (FSPL)
If there is any objects/walls/windows/desks/etc.. between AP and client

Question 15

What is RF fingerprinting used for

Answer 15

To map out the RF readings on to a map of the location showing walls/desks/APs/clients.

Question 16

How does the location of wireless devices attained when the device can be on different channels/networks

Answer 16

Each client device sends 802.11 probe requests on every channel available to them to every AP. this allows a picture to be built from all the probes.