chapter 6 Data and communications

Distinguish between 802.3 standards and 802.11 standards. b) What is the actual difference between 802.11 and Wi-Fi? c) Do we use the two terms interchangeably?

802.11 global standards developed by IEEE organization it is a set of physical for implementing wireless LAN IEEE 802.11 working group created the wireless LAN standards Wi-Fi stands for wireless fidelity and it is a wireless local area networking technology it is designed to provide broadband coverage Wi-Fi stems from Wi-Fi alliance, a nonprofit organization the promotes Wi-Fi technology

Broadband Channels

Broadband means wide channel bandwidth and therefore high speed. Today, "broadband" has come to mean "fast" whether or not radio transmission in channels is used.

Channel Bandwidth

Channel bandwidth is the highest frequency in a channel minus the lowest frequency. An 87.5 MHz to 88.1 MHz channel has a bandwidth of 0.2 MHz (200 kHz).

The Tests group performs three important tests.

Connection Test Speed Test Quality Test

In the upper right, there are instructions to "Rightclick on SSID nan1e to Locate." In the section on the radar window, we save that the window does not give the physical locations of access points. The Locate function under networks addresses this lack of physical location in a limited but interesting way .If you right-click on an SSID name such as Nalu24, your computer begins beeping. If you are far away, it will beep slowly. As you approach it, the beeping speed will be increased. Essentially, you are using the network analysis version of a Geiger counter.

Essentially, you are using the network analysis version of a Geiger counter.

The connection test shows how well you are connected to the outside world and to critical internal devices.

Figure 6a-6 shows the results of a connection test. It show's that Wi-Fi Inspector uses ping to test latency to your DNS server, default gateway (router), and a host on the Internet (Internet Reachable). It also does a DNS lookup, in this case for www.google.com The test shows that the user has low latency for the default router and an Internet host. It also shows that the DNS lookup was successful. In color, these are shown in green, with the word Pass. However, there is relatively high latency to the user's DNS server (152 ms). This is indicated by a yellow bar with the text Warning: high latency. However, latency is not very high. This connection looks good.

The speed test takes the user to speediest.net.

Figure 6a-7 shows a test in which there was a download speed of 14 Mbps and an upload speed of just under 1 Mbps. These are reasonable numbers.

licensed service bands

If two nearby radio hosts transmit in the same channel, their signals will interfere. Most service bands are licensed bands, in which hosts need a license to transmit The government limits licenses to reduce interference. Television bands, AM service bands, etc. are licensed. In cellular telephone bands, which are licensed, only the central antennas are licensed, not the mobile phones.

orthogonal frequency division multiplexing (OFDM)

In OFDM, each broadband channel is divided into many smaller subchannels called subcarriers. OFDM tra nsn1its part of a frame in each subcarrier. OFDM sends data redundantly across t11e subcarriers, so if there is impairment in one or even a few subcarriers, all of th e frame will usually still get through. Why Use Subcarriers instead of simply spreading thesignalovertheentirechan- nel? The problem is t11at sending d ata over a very wide channel reliably is technically difficult. It is much easier to send many slow signals in n1any small subcarriers.

five common wireless problems

Inverse Square Law Attenuation, Absorptive Attenuation, Dead Zones, Multipath Interference, electromagnetic interference EMI Frequency-Dependent Propagation Problems

Relative Direction (Meaningless)

It appears that the radar window shows the relative directions of the access points, much as an air traffic radar display showS the directions of nearby aircraft. Actually, it does not. The access points are merely spread out for readability. Direction is meaningless. In this sense, the radar window is n1is leading. However, it looks cool.

Distance From the Center {Signal Strength)

It looks like it means physical distance, as it would on a physical radar screen. Rather, it means signal strength. Access points that are shown closest to the center are the strongest, and access points that are the farthest from the center are the weakest .

connection window

It showS the SSID (the network name, in this case, Nalu24), the BSSID access point's MAC address, in this case, Cisco-Linksys:73:22:511), the channel (6), the signal strength (--65 dBn1), and the network mode (802.l l n). In the middle is information about the user's PC. It shows the user's MAC address and configuration information, including the user's IP address, the IP address of the destination server, the IP address of the default gateway (router), and the network's external IP address given to it by the ISP. (This is a home network.) This information does not tell the user about nearby access points, but it can be very useful in assessing connection problems. On the right is a Connect/Disconnect button. Clicking this button shows a list of potential networks and allows the user's computer to disconnect from the current access point and pick another to connect to. The user can also turn off the computer's wireless adapter.

The Signal History window shows these graphs. The graphs in Figure 6a-5 show that the signal strength for Nalu24 is uniformly excellent and that the signal strength for Belkin in is uniformly poor.

Major fluctuations would indicate serious problems.

Wi-Fi analysis programs for mobile devices. Many have "stun1- bler" in their names in homage to one of the first examples,

NetStumbler.

Unlicensed Service Bands

Some bands are set aside as unlicensed bands. Hosts do not need to be licensed to be turned on or moved. 802.11 Wi-Fi operates in unlicensed service bands. This allows access points and hosts to be moved freely. However, there is no legal recourse against interference from other nearby users. Your only recourse is to negotiate. At the same time, you may not cause unreasonable interference by transmitting at illegally high power.

windows in Wi-Fi Inspector provide information visually.

The Tests group allows the user to conduct more detailed tests. These tests are good for troubleshooting.

How wde n1ust the channel bandwidth be?

The channel bandwidth n1ust be wide enough for a signal's bandwidth. Claude Shannon discovered a remarkable thing about signal transmission. A signal carrying X bits per second only needs half the bandwidth of a signal carrying 2X bits per second.2 Looked at the other way, if you want to transmit twice as many bits per second, you need to double your bandwidth. More generally, if you want to be able to transmit N times as fast, you need N times as much channel bandwidth. High bandwidth brings high radio transmission speed.

Multipath Interference

radio waves tend to bounce off walls, floors, ceilings, and other objects. that a receiver will receive two or more signals- a direct signal and one or more reflected signals The direct and reflected signals will travel different distances and so n1ay be out of phase when they reach the receiver. For example, one may be at its highest amplitude and the other at its lowest giving an average of zero. they will cancel out. in real life they will interfere Multipath interference may cause the signal to range from strong to nonexistent within a few centimeters. If the difference in time between the direct and reflected signal is large, some reflected signals may even interfere with the next direct signal. We will see later that this is controlled by spread spectrum transmission, which spreads the signal over a wide range of frequencies so that multipath interference effects average out to zero. Multipath interference is the most serious propagation problem at WLAN frequencies.

Signal Bandwidth

shows a wave operating at a single frequency.However, most signals are spread over a range of frequencies. The range between the highest and lowest frequencies is the signal's bandwidth. As transmission speed increases, the signal bandwidth increases.

How can you sort the networks window?

sorting process network option for each column when we connect to a network it is highlighted with orange color. we have a small arrow beside signal level. we can sort the signal strength by ascending or descending order based on their signal strength

What information does the Speed Test give you?

speed test is link to seeptest.net a website that checks your connectivity to you ISP and test your upload and download speeds the information from this test gives you the throughput of your connection

Wi-Fi analysis programs listen to nearby access points (and sometimes wireless hosts) to determine

such things as how strong their signals are,what types of security they use, what their SSIDs and BSSIDs are, and sometimes the directions of the individual access points.

What information does the Connection Test give you?

the connection test shows how well you are connected to the outside world. this is done by using ping to test the latency of the DNS sever, default gateway public IP address and host on the internet the test returns pass or warning i.e high latency

How would you locate an access point despite the limitations of the radar window? This will take one to four paragraphs.

the radar shows signal strength access points that are shown closet to the center are strongest, so you could move around your laptop to increase the signal to the access point we enter into locate mode by right clicking on the network name and then click on locate a specific wi-fi network. operates similar to a geiger counter using sound and visual information to indicate proximity

What information does the Quality Test give you?

this test gives you the information on latency, jitter, the number of packets lost and the MOS score it is basically a link to another website i.e pingtest.net

In licensed service bands,

transceivers must have a government license to operate. They also need a license change if they move. Commercial television bands are licensed ban ds, as are AM and FM radio bands. Government agencies control who may have licenses in these bands. By doing so, the government limits interference to an acceptable level. In some licensed service bands, the rules allow mobile hosts to move about but cen tral transceivers are regulated. This is the case for mobile telephones.

Wireless LANs (WLANs)

use radio for physical layer transmission on the customer premises.

802.11 Wireless LANs (WLANs)

use radio for physical layer transmission on the customer premises. In the last chapter, we saw that the 802.3 Working Group of the IEEE's 802 LAN/MAN Standards Committee creates Ethernet standards. Other working groups create other standards. The dominant WLAN standards today are the 802.11 standards, which are created by the IEEE 802.11 Working Group.

the quality test, which takes you to pingtest.net. The results give the user's quality level a B. However, the box on the left notes that the connection should be fine for anything but gan1ing.

• The ping (latency) averaged 84 ms, which is a little high for games. The server is less than 50 miles away. Connecting to a more distant server would increase latency. • Jitter, which is variation in latency from packet to packet is 24 ms. This can affect voice and video, for which jitter can result in jittery voice or video. Again, the number is fairly good. • There was zero packet loss. The connection appears to be reliable. • There is a MOS score of 4.33. This is a traditional subjective indicator of voice call quality. A MOS score of 5 indicates toll-call quality on the telephone system. A MOS of 4.33 is quite good. One caveat is that pingtest.net is a bit "grabby." It tries to sell you its tools and is slightly aggressive. In addition, the site uses Java, which you n1ay have to download. You may also have to give a firewall exception to this Java program.

Figure 6-12 illustrates two important points about how Wi-Fi uses its channels.

The first is that an access point normally uses a single channel (although some can operate on more than one). Access Point A is transmitting on "Channel l," and so is adjacent Access Point B. They will interfere. This is called co-channel interference because they are using the same channel.What about Access Point A and Access Point D? They are adjacent, but they are operating on different channels (1 and 6). Therefore, they will not interfere with one another. When co-channel interference occurs, it does not stop transmissions, but it does slow them down . One hotel decided to be "consistent" and put all access points on the san1e channel. Service was terrible. To reduce co-channel interference, network administrators try to set adjacent access points on different channels. However, there are a limited number of channels in the service bands that Wi-Fi uses, so if there are n,any nearby access points, some of then, will inevitably suffer co-channel interference. Reducing Co-channel interference is an important goal in design .

Channel Bandwidth and Propagation Speeds

The maximum possible transmission speed increases with bandwidth. Doubling the bandwidth doubles the maximum possible transmission speed. Multiplying the bandwidth by X multiplies the maximum possible speed by X. Higher-speed signals need wider channel bandwidths.Channel bandwidth must be sufficient for the signal's bandwidth.

The Radar Window (Read the Fine Print)

The most obvious window, is the radar window, which shows all access points in the vicinity. The access points are spread out across the two-dimensional picture.

The Networks Window

The networks window shows detailed information about each of the nearby access points. This is what the user goes to when he or she wants detailed information. The row for the access point to whim the user is currently connected is shown in orange highlighted. Wi-Fi Inspector updates the information in the networks window fre- quently. As Figure 6a-3 shows, the information in this window is detailed. • SSTD.The network name. • Signal level in either dBm or percentage. Remember that smaller negative dBn1 numbers indicate higher strength. Next to the number is a colored bar. Green is for signals of-70 dBm and above (--60 dBm, etc.). Yellow is for signals between -71 dBm and -80 dBm. Orange is for signals between -81 dBn1 and -90 dBn1. Red is for -91 dBm and below Network Mode. 802.llg, 802.lln, etc. • Default Encryption. None, WEP, TKIP (in WPA), or AES (802.lli). Default Authentication. Open (none), WP A/PSK, WPA2/ PSK, WP A/802.lX, or WPA2/802.1X. Vendor. The nan1e of the device manufacturer. BSSID. The access point's MAC address. Channel. The channel number. Frequency. The center frequency of the channel. Network Type. Access point or ad hoc (no access point). Graph. This is a checkbox that tells Wi-Fi Inspector to graph the signal level over time (checked) or not to do so (unchecked). In the figure, both are checked, so both will be graphed.

Expanding the Radar Window

The radar window in its normal small form can only display four access points. Under the Layout section of the menu, selecting Radar in the Layout Group will maximize the radar window. This allows up to ten access point names to be seen. By the way, "network" and "SSID" are synonyms. Figure 6a-2 showS the expanded radar window There are only two nearby access points, so there is no need for a large radar window. However, it certainly is easier to read the relative indicated signal strength.

Dead Zones

To some extent, radio signals can go through and bend around objects. However, if there is a dense object (e.g., a thick wall) blocking the direct path between the sender and the receiver, the receiver may be in a dead zone, also called a shadow zone. In these zones, the receiver cannot get the signal. If you have a mobile phone and often try to use it inside buildings, you may be familiar with this problem.

The Help group provides

a user's gmde to explain the program's

At what layers do wireless LANs operate? b) Do wireless LAN standards

a) the OSI model designed for the computer networks has seven layers Physical layer(physical connections ) and data link layer, DLL which provides transmission of data between hosts. OSI standards must be used by all types of computer networks the layers at which the wireless network operated are physical layer and data link layer b) wireless LAN standards come from OSI because it provides the step by step ways of creating connection between hosts and sharing devices like printer 1st facilitated to install and connect working devices for establish connection between two hosts physical layer then it facilitated for transmitting messages between hosts with fault and error control mechanism through data link layer at the dLL addressing system is found to attach the inter IP address with the message conversion mechanism also works here for covering the internal ip address into ip address provided by the inter service provider

The Settings group allows the user to adjust many settings, for example,

expressing RSSI in percentage terms instead of in terms of dBm

There is a value of -44 dBm for signal strength. How good is this?

for signal strength smaller negative number are better. so -44 dBm is very strong signal less the value more the strength of the signal

Measuring Signal Strength Signal strength gives the RSSI (relative signal strength indicator)

for the access point. Smaller negative numbers are better. For example, -60 dBm is a very strong signal, while -87 dBm is a very weak signal. In Figure 6a-1, Nalu24 has a signal strength of --65, which is quite good. Belkin has a signal strength of-89, which is terrible. For signal strength, smaller negative numbers are better. (It's a double negative.)

Why is then radar window's in 1arge of a radar scope misleading?

it doesn't show the direction of the access points, the access points spread out for readability and hence the direction is meaningless but we see the image it is like showing on which direction access point is

The 2.4 GHz unlicensed service band is the same in ,most countries in the world. Unfortunately, it only has 83.5 MHz of

l service band bandwidth. Traditionally, each 802.11 channel was 20 MHz wide, although 40 MHz bandwidth channels were introduced in 802.lln. Due to the way channels are allocated, there are only three possible nonoverlapping 20 MHz 802.11 channels. These are centered at Channels 1, 6, and 11.3 In addition, there can only be a single 40 MHz channel, and if an 802.lln station finds itself in a crowded area, it will drop back from 40 MHz to 20 MHz to reduce interference. This will, of course, cut transmission speed in half.

The Layout group on the ribbon is the

most-used feature of the Xirrus Wi-Fi Inspector.

The 2.4 GHz Unlicensed Service Band

2.400 GHz to 2.835 GHz for the entire unlicensed service band. This is small total bandwidth (435 MHz).There can only be three non overlapping 20 MHz channels. Difficult to put nearby access points on different channels.If not, there will be co-channel interference.