Cnna case study chapter 7

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CCNA Exploration: LAN Switching and Wireless  Chapter 7 Case Study
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 routepackets 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:


© 2009 Cisco Learning Institute


CCNA Exploration: LAN Switching and Wireless  Chapter 7 Case Study
Step 1 – High Delays and Packet Loss You get to Red’s office and connect to their wireless network using yourlaptop’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 fromthe 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 thissituation 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 provideinformation 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 othercompanies 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:

© 2009 Cisco Learning Institute

    CCNA Exploration: LAN Switching and Wireless  Chapter 7 Case Study
SSID REDs AP1 AP2 AP3 AP4 AP5 AP6 AP7 AP8 AP9 AP10 Channel 11 11 9 11 11 9 1 11 9 11 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 bestchannel 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....
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