CCO Gems: Cisco Aironet Antennas and Accessories Reference Guide

In an attempt to wrap my head around a lot of the Layer 1 details in the Wireless LAN world, I came across the following doc on Cisco’s website under their wireless antennas product info:

Cisco Aironet Antennas and Accessories Reference Guide

One of the most important units of measure to understand when working with RF is the decibel (dB). While this unit is used in many other fields such as sound and audio, this doc presents it in a clear and concise manner as it relates to WLAN’s:

The decibel (dB) scale is a logarithmic scale used to denote the ratio of one power value to another.
For example:
X1`dB = 10 log10 (Power A/Power B)
An increase of 3 dB indicates a doubling (2x) of power. An increase of 6 dB indicates a quadrupling (4x) of power. Conversely, a decrease of 3 dB reduces power by one half, and a decrease of 6 dB results in a one fourth of the power.





0 dB

1 x (same)

0 dB

1 x (same)

1 dB

1.25 x

-1 dB

0.8 x

3 dB

2 x

-3 dB

0.5 x

6 dB

4 x

-6 dB

0.25 x

10 dB

10 x

-10 dB

0.10 x

12 dB

16 x

-12 dB

0.06 x

20 dB

100 x

-20 dB

0.01 x

30 dB

1000 x

-30 dB

0.001 x

40 dB

10,000 x

-40 dB

0.0001 x

If you’re a newbie like me, this makes a lot more sense than some of the other documentation I’ve read on the subject, especially with the table above that clearly shows how dB increases logarithmically.

Now that we know a decibel is a relative value, we need to know what values are being measured and referenced. In the wireless world, those two values are Received Signal Strength Indicator or RSSI, and Signal-To-Noise ratio (SNR).

RSSI is a vendor-specific grading of received signal strength. Because it is vendor-specific and not a standard measure, these values cannot be used to compare between vendors. It is usually measured in decibel milliwatt (dBm), which is basically the ratio of power referenced to 1 milliwatt. For example, 3 dBm is 2mW of power. Again, the Cisco Aironet Antenna reference guide provides a good chart for common dBm values to give you an idea of how dBm scales in reference to wattage.

Table 2. Common mW Values to dBm Values





0 dBm

1 mW

0 dBm

1 mW

1 dBm

1.25 mW

-1 dBm

0.8 mW

3 dBm

2 mW

-3 dBm

0.5 mW

6 dBm

4 mW

-6 dBm

0.25 mW

7 dBm

5 mW

-7 dBm

0.20 mW

10 dBm

10 mW

-10 dBm

0.10 mW

12 dBm

16 mW

-12 dBm

0.06 mW

13 dBm

20 mW

-13 dBm

0.05 mW

15 dBm

32 mW

-15 dBm

0.03 mW

17 dBm

50 mW

-17 dBm

0.02 mw

20 dBm

100 mW

-20 dBm

0.01 mW

30 dBm

1000 mW (1 W)

-30 dBm

0.001 mW

40 dBm

10,000 mW (10 W)

-40 dBm

0.0001 mW

Signal-To-Noise ratio is defined as the power ratio between a signal, such as a WLAN waveform, and the background noise. Again, we’re measuring in decibels since it is the ratio of the RSSI and the surrounding garbage RF noise.

Since I’m coming from a wired world where everything is essentially plug-and-play, I’m working from the ground up at Layer 1 to hone my WLAN skills.
Anyways, the Cisco Aironet document here gives a very succinct overview of various L1 WLAN concepts such as 802.11 modulation techniques, antennas ratings and specs, understanding RF power levels, as well as a slew of other little details. The first 1/3 of the document gives a nice overview of these concepts, while the other 2/3’s list all of Cisco’s Aironet antenna products.

This is one of those gems on Cisco’s site that I’m surely packing the PDF away for future reference (especially for my Cisco wireless studies).


Crossroads: Cisco Wireless Certs Refresh v2.0

A couple weeks ago, Cisco Learning Network released the following announcement:

Cisco has updated its written exams and training courses for the CCNA Wireless and CCNP Wireless certification programs. The changes reflect the addition of more relevant materials that include an update to the current version of software including Autonomous, WLC, and Clients.

CLN Announcement

As with any time Cisco brings down a new exam version and topic refresh, candidates (such as myself for the Wireless track) are faced with a dilemma; do I go for the old version exam, or do I move towards studying for the updated and refreshed exam?

For those who have already been studying the previous version exams, such as the previous CCNA Wireless 640-721, you’re probably better off continuing with the current v1.0 versions. At the time of this writing, we’re still relying on the older Cisco Press study guides (for those doing self-study) so there will be some time spent in “Limbo” where there’s no official material for the newer exams.

It’s always useful as well to compare the exam topics to see how it fares with your knowledge.
Since I’m set to go for CCNA Wireless this year, let’s review the changes for the v1.0 and v2.0 exams (640-721 and 640-722, respectively):

IUWNE v1.0 Topics removed

  • Describe the Cisco Mobility Express Wireless architecture (Smart Business Communication System — SBCS, Cisco Config Agent — CCA, 526WLC, 521AP – stand-alone and controller-based)
  • Configure the basics of a stand-alone access point (no lab) (Express setup, basic security)
  • Describe RRM
  • Install Cisco ADU
  • Describe and configure encryption methods (WPA/WPA2 with TKIP, AES)
  • Install/upgrade WCS and configure basic administration parameters (ports, O/S version, strong passwords, service vs. application)
  • Configure and use maps in the WCS (add campus, building, floor, maps, position AP)
IUWNE v2.0 Topics added

  • Install and configure autonomous access points in the small business environment
  • Describe Radio Resource Management (RRM) fundamentals including ED-RRM.
  • Verify basic wireless network operation
  • Identify basic configuration of common wireless supplicants (Macintosh, Intel Wireless Pro, Windows, iOS, and Android)
  • Implement wireless Guest networking
  • Navigate WCS interface
  • Use preconfigured maps in the WCS (adding/relocating/removing access points, turn on/off heat maps, view client location, and view CleanAir zones of influence)
  • Generate standard WCS reports (inventory, CleanAir, client-related, AP-related, and utilization)
  • Configure authentication and encryption methods on a WLAN (WPA/WPA2 with PSK and 802.1x)

Looking at the v2.0 blueprint, most of the changes are slight reordering of the exam topics and clarification on others. I’m willing to bet that the majority of the changes is the result of the changes in Cisco’s wireless product portfolio (namely, the removal of ADU and MSE specific topics, and Cisco CleanAir features in WCS).

Some of the most important additions/revisions on the v2.0 blueprint is the addition of Implementing wireless Guest networking, 802.1X authentication and configuring common wireless supplicants in Mac, Windows, iOS and Android devices. I’m glad to see some of the more serious topics being added to the new exam since anyone implementing a Cisco WLAN network will need to know the basics of Guest wireless, 802.1X authentication and configuring all the different devices that will be using the WLAN.

So now, the newer candidates such as myself, are at a crossroad. Do we start studying the v1.0 blueprint? Or do we study the updated curriculum that contains all of the recent updates in Cisco’s product portfolio?

Personally, I think I will want to go with the v2.0 exam. While the official course-ware might not be available yet, the fundamentals are the same (with a few additions and updates). The new exam is available as of last week (Jan. 24th, 2012) and for current candidates, the old v1.0 exam will be available until May 11, 2012. What path you choose will depend entirely on you and your studies; either way, keep studying and best of luck!

CCNA Wireless IUWNE v1.0 Exam Blueprints 640-721
CCNA Wireless IUWNE v2.0 Exam Blueprints 640-722