CCNA Exploration: LAN Switching And Wireless

CCNA Exploration: LAN Switching and Wireless

Chapter 7 Case Study

© 2009 Cisco Learning Institute

Intro:

Red Enterprise is having trouble with the wireless section of their network. While the wired portion works

fine, the wireless traffic is too slow.

The Scenario:

Red network can be divided in 2 sections: wired and wireless. The 2 sections are connected by Cisco

1811 wireless router (R1) which is also used to route packets to/from The Internet (devices located

outside Red’s Network). The wireless traffic is being transmitted clear (no encryption used). The topology

is shown below:

Topology:

CCNA Exploration: LAN Switching and Wireless

Chapter 7 Case Study

© 2009 Cisco Learning Institute

Step 1 – High Delays and Packet Loss

You get to Red’s office and connect to their wireless network using your laptop’s wireless card. The

association process (between your laptop and Red’s Access Point – R1) takes longer than usual and you

decide to run a few tests.

Once your laptop is associated with Red’s Wireless Access Point (Cisco 1811) you issue a few pings from

it. Pings issued from your laptop to any address (inside Red’s network or internet addresses) have high

response time and high loss rate.

You suspect the walls are weakening the signal and decide to move closer to the AP. From a closer spot

you issue the pings again and although the shorter physical distance, the loss rate and delay are still very

high.

Question 1:

Is physical distance relevant when it comes to wireless traffic throughput?

Answer: Yes. Because wireless signals are essentially RF waves, the further a client is from the AP, the

weaker is the signal. Weak signal means lost frames which leads to throughput drops.

Sitting with your laptop just by the AP and still facing high delays and packet loss, you conclude physical

distance is not the main problem.

You disable the wireless card installed in your laptop and, using your laptop’s wired card, you connect it

to the wired portion of Red’s network. In this situation everything works fine.

In order to have a better understanding of the problem, you need a wireless tool. From your laptop you

run a wireless diagnose tool.

Note: There are a number of free and non-free wireless diagnose tools available on the Internet,

designed to different platforms and different type of Operating Systems.

A wireless diagnose tool is able to scan the medium and provide information about the wireless network.

Wireless diagnose tools usually provide information like a list of open APs, their MAC addresses, SSIDs

and their channels of operation.

After running a wireless diagnose tool in your laptop you learn that there are 10 other wireless Access

Points running in the same building. Because Red Enterprise’s office is located in a commercial building

and other companies also have a wireless structure installed, the proximity of their APs allows your laptop

to catch their signal too.

Just for the record, you name the APs you found as AP1 through to 10 and since your wireless diagnose

tool gave out the channels of operation of all APs it has found, you compiled the table shown below:

CCNA Exploration: LAN Switching and Wireless

Chapter 7 Case Study

© 2009 Cisco Learning Institute

SSID Channel

REDs 11

AP1 11

AP2 9

AP3 11

AP4 11

AP5 9

AP6 1

AP7 11

AP8 9

AP9 11

AP10 11

As you can see on the table above, many APs are using channel 11 to communicate, including Red’s.

Because so many APs are using the same channel in the same physical area, the channel 11 is

congested. Although most wireless Access Points are able to scan and select the best channel

automatically, others either don’t have this feature or it is disabled.

Because the way wireless access points channels of operation were split (overlapping frequencies

between consecutive APs) it is considered good practice not use consecutive channels when dealing with

neighbor wireless APs.

Question 2:

Give an example of a good channel to be assigned to Red Enterprise’s AP.

Answer: Channel 6

CCNA Exploration: LAN Switching and Wireless

Chapter 7 Case Study

© 2009 Cisco Learning Institute

Question 3:

Even though is possible to find a good channel to be statically assigned to Red’s AP, why is this not a

good idea?

Answer: With so many APs in the same area, to statically specify a channel might cause the same

problem in the case of all the neighbors APs are configured to that same channel once more.

You connect to the 1811’s console port and check its configuration. The relevant portion (the wireless

section) is shown below:

interface Dot11Radio0/0/0

ip address 192.168.1.1 255.255.255.0

ip nat inside

ip virtual-reassembly

!

!

ssid REDs

authentication open

!

speed basic-1.0 basic-2.0

...