Download CCNA Wireless 200-355.pdf PDF

TitleCCNA Wireless 200-355.pdf
File Size1.6 MB
Total Pages95
Table of Contents
Chapter 13 Understanding RRM
	“Do I Know This Already?” Quiz
	Foundation Topics
	Configuring 802.11 Support
		Configuring Data Rates
		Configuring 802.11n and 802.11ac Support
	Understanding RRM
		RF Groups
		Coverage Hole Detection Mitigation
		Manual RF Configuration
		Verifying RRM Results
	Exam Preparation Tasks
	Review All Key Topics
	Define Key Terms
Document Text Contents
Page 1 download a free sample chapter

Page 2

Cisco Press
800 East 96th Street

Indianapolis, IN 46240

Official Cert Guide


Page 47

320 CCNA Wireless 200-355 Official Cert Guide

The bottom portion of the web page contains a list of channels that DCA can use as it
assigns channels to APs in the respective band. This list is populated with channel numbers
by default, but you can edit the list as needed. You can also enable or disable individual
channel use by using the list of Select check boxes.

The DCA algorithm normally runs on an automatic schedule or manually on demand. Event-
Driven RRM (ED-RRM) takes this a step further; DCA can be triggered based on RF events
that occur in real time. The CleanAir feature, covered in more detail in Chapter 19, “Dealing
with Wireless Interference,” provides the triggers for ED-RRM. By default, ED-RRM is dis-
abled. You can enable it with the EDRRM check box at the very bottom of the web page.

Figure 13-12 Adjusting the RRM DCA Algorithm Parameters

Coverage Hole Detection Mitigation
The TPC algorithm normally reduces AP transmit power levels to make cell sizes appropri-
ate. Sometimes you might find that your best intentions at providing RF coverage with a
good AP layout still come up short. For example, you might discover that signals are weak
in some small area of a building due to the building construction or surrounding obstacles.
You might also have an AP radio that happens to fail, causing a larger coverage hole. How
would you discover such a condition? You could make a habit of surveying the RF cover-
age often. More likely, your wireless users will discover a weakness or hole in the coverage
and complain to you about it.

Page 48


Chapter 13: Understanding RRM 321

A Cisco controller-based wireless network offers an additional RRM algorithm that can
detect coverage holes and take action to address them. Coverage hole detection mitigation
(CHDM) can alert you to a hole that it has discovered and it can increase an AP’s transmit
power level to compensate for the hole.

CHDM is useful in two cases:

■ Extending coverage in a weak area

■ Rapidly healing a coverage hole caused by an AP or radio failure, sooner than the TPC
algorithm can detect and correct

The algorithm does not run at regular intervals like TPC and DCA do. Instead, it monitors
the RF conditions of wireless clients and decides when to take action. In effect, the algo-
rithm leverages your wireless users who are out in the field and tries to notice a problem
before they do.

Every controller maintains a database of associated clients and their RSSI and signal-to-noise
ratio (SNR) values. It might seem logical to think that a low RSSI or SNR would mean a cli-
ent is experiencing a hole in coverage. Assuming the client and its AP are using the same
transmit power levels, if the AP is receiving the client at a low level, the client must also be
receiving the AP at a low level. This might not be true at all; the client might just be exiting
the building and getting too far away from the AP. The client might also have a “sticky”
roaming behavior, where it maintains an association with one AP until the RSSI falls to a
very low level before reassociating elsewhere.

CHDM tries to rule out conditions that are experienced by small numbers of clients and
signal conditions due to client roaming behavior. A valid coverage hole is detected when
some number of clients, all associated to the same AP, have RSSI values that fall below a
threshold. In addition, the coverage hole condition must exist longer than a threshold of
time without the client roaming to a different AP.

By default, the following conditions must all be met for a coverage hole to be detected:

■ Client RSSI at the AP is at or below –80 dBm.

■ The low RSSI condition must last at least 60 seconds over the past 180 seconds.

■ The condition must affect at least three clients or more than 25 percent of the clients on
a single AP.

Be aware that CHDM runs on a per-band basis. Unlike TPC and DCA, which operate on the
entire RF group of controllers, CHDM runs on each controller independently, on a per-AP
radio basis.

You can display and configure the CHDM thresholds by selecting Wireless > 802.11a/n/ac
or 802.11b/g/n > RRM > Coverage. Figure 13-13 shows the threshold parameters for the
5-GHz 802.11a band.

Page 94

WLC (wireless LAN controller) 569

honeycomb pattern, 167

three dimensions, 168


session timeouts, 365

settings, verifying, 462-463

controller configured, displaying, 366

converged WLCs, configuring, 252

coverage/performance verification

active site surveys, 176-178

AP deployment phases, 178-179

AP-on-a-stick surveys, 175

device/application requirements,

location services, 170

passive site surveys, 174-175

planning surveys, 172-173

site surveys, 171-172

defining, 355

dynamic interface, creating, 358-360

Express Setup, 254-256


dynamic interface, 377

interface, assigning, 378

IP address information, 377

isolation, 375

Layer 3 roaming, 376

mobility anchors, 376, 380-381

open authentication, 379

scaling, 375-376

SSID, 378

web authentication, 375, 379

limiting, 356

listing of, displaying, 360

Local EAP, 349-350

management access, allowing, 367

new, creating

broadcasting SSIDs, 362

controller interfaces, 362

enabling/disabling, 362

general parameters, 361

names/ID numbers, 361

radio selection, 362

WLAN list, displaying, 360

open authentication, 336

QoS, 364-365

RADIUS server, configuring, 356-357

security, 362-364

authentication, 363

centralized controllers, 364

client exclusion policies, 365

configuring, 344

converged controllers, 364

Layer 2 types, 362-363

too many, creating, 355


BSSs, 116-118

distribution systems, 118-120

ESSs, 120-121

IBSSs, 122

user segregation into logical networks,

WPA2, configuring

enterprise mode, 346-348

Local EAP, 348-350

personal mode, 344-345

WLC (wireless LAN controller), 193

activities, 196

centralized, 200

location, 197

traffic paths, 198-199

user mobility, 198

client states, 455

Page 95

570 WLC (wireless LAN controller)

Configuration wizard

802.11, 252

administrative users, creating,

clock, 253

management ports, 250

mobility, 251

RF mobility, 251

SNMP parameters, 249

starting, 248

verifying, 253

wireless management, 250

WLAN, 252

converged location, 201

discovery, 270-271

LAPs, linking, 193

LAPs to central WLC, connecting, 195

platforms/capabilities, 205-206

selecting, 271

WMAN (wireless metropolitan-area
network), 114

WMM (Wi-Fi Multimedia), 149

working groups, 45

Worldwide interoperability for Micro-
wave Access (WiMAX) interference,

WPA (Wi-Fi Protected Access),

WPA2 (WPA Version 2), 342-343

enterprise mode, 346-348

Local EAP, 348-350

personal mode, 344-345

WPAN (wireless personal-area net-
work), 114

WWAN (wireless wide-area network),

X – Z
Xbox interference, 435

Yagi antennas, 104-105

ZigBee Alliance, 434

ZigBee interference, 433

Similer Documents