Wifi Networking Final Dcs

Wireless Networking

Network Topology IEEE 802.11 is limited in scope to the Physical (PHY) layer and Medium Access Control (MAC) sublayer, with MAC origins to IEEE802.3 Ethernet standard. The basic topology of an 802.11 network an ad hoc network, or Independent Basic Service Set (IBSS) is shown in Figure.

Network Topology

The basic topology of an 802.11 network an ad hoc network, or Independent Basic Service Set (IBSS)

Network Topology In most instances, the BSS contains an Access Point (AP). The main function of an AP is to form a bridge between wireless and wired LANs. When an AP is present, stations do not communicate on a peer-to-peer basis. All communications between stations or between a station and a wired network client go through the AP.

Infrastructure versus Ad Hoc

Network Topology The Extended Service Set (ESS) shown in figure 2 consists of a series of overlapping BSSs (each containing an AP) connected together by means of a Distribution System (DS). Although the DS could be any type of network, it is almost invariably an Ethernet LAN. Mobile nodes can roam between APs and seamless campuswide coverage is possible.

Network Topology

Radio Technology IEEE 802.11 provides for two variations of the PHY. These include two RF technologies namely Direct Sequence Spread Spectrum (DSSS), and Frequency Hopped Spread Spectrum (FHSS). Both FHSS and DSSS PHYs currently support 1 and 2 Mbps. However, all 11 Mbps radios are DSSS.

Radio Technology Region US Europe Japan France Spain

Allocated Spectrum 2.4000 - 2.4835 GHz 2.4000 - 2.4835 GHz 2.471 - 2.497 GHz 2.4465 - 2.4835 GHz 2.445 - 2.475 GHz

Radio Technology

Regardless of whether the data rate is 1, 2, 5.5, or 11 Mbps, the channel bandwidth is about 20 MHz for DSSS systems. Therefore, the ISM band will accommodate up to three non-overlapping channels

WiFi’s Radio Technology (Cont’d) 

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WiFi radios that work with the 802.11b and 802.11g standards transmit at 2.4 GHz, while those that comply with the 802.11a standard transmit at 5 GHz. Normal walkie-talkies normally operate at 49 MHz. The higher frequency allows higher data rates. WiFi radios use much more efficient coding techniques (process of converting 0’s and 1’s into efficient radio signals) that also contribute to the much higher data rates.

WiFi’s Radio Technology (Cont’d)  

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The radios used for WiFi have the ability to change frequencies. For example, 802.11b cards can transmit directly on any of three bands, or they can split the available radio bandwidth into dozens of channels and frequency hop rapidly between them. The advantage of frequency hopping is that it is much more immune to interference and can allow dozens of WiFi cards to talk simultaneously without interfering with each other.

802.11b, 802.11a, and 802.11g 

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802.11b was first to reach the marketplace. It is the slowest and least expensive of the three. 802.11b transmits at 2.4 GHz and go up to 11 Mbps. 802.11a was next. It operates at 5 GHz and can handle up to 54 Mbps. 802.11g is a mix of both worlds. It operates at 2.4Ghz (giving it the cost advantage of 802.11b) but it has the 54 megabits per second speed of 802.11a. It is also backward compatible to 802.11b. Most WiFi cards nowadays are capable of all three of these radio technologies.

IEEE Wireless Standards Wireless Standard

802.11b

802.11g

Frequency Range

2.4 – 2.4835 2.4 – 2.4835 5.725 GHz GHz 5.850 GHz

Max Speed

11 MBPS

Max Encryption

128 bit WEP 128 bit WEP 152 bit WEP 256 bit AES

Discrete Channels

3

3

8

Natively Compatible

802.11b, 802.11g

802.11b, 802.11g

802.11a

Potential user

Entry level and home networks

Larger networks, small business

Large business concerned with

54 MBPS

802.11a

54 MBPS

CHANNELS 802.11b & 802.11g Both 802.11b and 802.11g operate in the 2.4 GHz frequency band, specifically between 2.400 GHz (2400 MHz), and 2.484 GHz (2484 MHz). Each Channel is of 22 Mhz. The picture below shows the number of channels in the frequency band. As can be seen adjacent channels over lap and there are 3 over lapping channels, viz. 1, 6 and 11.

802.11b Spectrum Coverage

CHANNELS 802.11b & 802.11g As shown below, Channels 1, 6, and 11 are "non-overlapping," meaning they can all be used in the same area without causing "cochannel interference" (CCI). In this way, users can be load balanced across three channels, each providing up to 11Mbps of throughput, thereby effectively providing up to 33 Mbps of aggregate bandwidth. Therefore, larger scale WLAN deployments utilize these three channels in a "geographic space" overlapping fashion to maximize coverage area while prnting channel interference

CHANNELS 802.11b & 802.11g Non-overlap Channel Placement

Multiple Access The basic access method for 802.11 is the Distributed Coordination Function (DCF) which uses Carrier Sense Multiple Access / Collision Avoidance (CSMA / CA) This requires each station to listen for other users. If the channel is idle, the station may transmit. However if it is busy, each station waits until transmission stops & then enters into a random back off procedure. This prevents multiple stations from seizing the medium immediately after completion of preceding transmission

Multiple Access The period between completion of packet transmission and start of the ACK frame is one Short Inter Frame Space (SIFS) ACK frames have a higher priority than other traffic Fast acknowledgement is one of the salient features of the 802.11 standard, because it requires ACKs to be handled at the MAC sublayer

Multiple Access Transmissions other than ACKs must wait at least one DCF inter frame space (DIFS) before transmitting data. If a transmitter senses a busy medium, it determines a random back-off period by setting an internal timer to an integer number of slot times. Upon expiration of a DIFS, the timer begins to decrement. If the timer reaches zero, the station may begin transmission. However, if the channel is seized by another station before the timer reaches zero, the timer setting is retained at the decremented value for subsequent

Multiple Access

Multiple Access The method described above relies on the Physical Carrier Sense The underlying assumption is that every station can "hear" all other stations. This is not always the case. Referring to Figure 8, the AP is within range of the STA-A, but STA-B is out of range. STA-B would not be able to detect transmissions from STA-A, and the probability of collision is greatly increased. This is known as the Hidden Node.

Multiple Access

Multiple Access To combat this problem, a second carrier sense mechanism is available. Virtual Carrier Sense enables a station to reserve the medium for a specified period of time through the use of RTS/CTS frames. (Request / Clear - to send) In the case described above, STA-A sends an RTS frame to the AP. The RTS will not be heard by STA-B. The RTS frame contains a duration/ID field which specifies the period of time for which the medium is reserved for a subsequent transmission. The reservation information is stored in the Network Allocation Vector (NAV) of all stations detecting the RTS frame.

Multiple Access Upon receipt of the RTS, the AP responds with a CTS frame, which also contains a duration/ID field specifying the period of time for which the medium is reserved. While STA-B did not detect the RTS, it will detect the CTS and update its NAV accordingly. Thus, collision is avoided even though some nodes are hidden from other stations. As mentioned above, DCF is the basic media access control method for 802.11 and it is mandatory for all stations

Multiple Access The collision will result in an undecipherable message to the intended receivers (listeners). What we need is a polite contention method to get access to the medium; this is the collision avoidance part of CSMA/CA. 802.11 has come up with two ways to deal with this kind of collision. One uses a two-way handshake when initiating a transmission. The other uses a four-way handshake.

2 Way Handshake Node with packet to send monitors channel. If channel idle for specified time interval called DIFS, then node transmits. If channel busy, then node continues to monitor until channel idle for DIFS. At this point, terminal backs-off for random time (collision avoidance) and attempts transmitting after waiting this random amount of time.

2 Way Handshake If the node does not back-off the random time, then it will definitely collide with another node that has something to send. Reason for random back-off time is that if I choose a random time and you choose a random time, the probability that we choose the same random time is slim. This way we both back-off transmitting and will therefore will probably not interfere with each other when we are ready to transmit.

2 Way Handshake (Cont’d) 

First way of the 2 way handshake was for the transmitter to send its information packet to the destination node, after following the collision avoidance method described above.

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If the packet reaches the destination without problems, the destination sends a short packet over the wireless medium acknowledging the correct reception.

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This packet is typically called an ACK packet. ACK is the second way of the 2 way handshake.

4 Way Handshake 

“Listen before you talk”

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If medium is busy, node backs-off for a random amount of time after waiting DIFS, just as before.

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But now, instead of packet, sends a short message: Ready to Send (RTS). This message is basically attempting to inform others that “I have something to send.”

4 Way Handshake (Cont’d) 

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RTS contains destination address and duration of message. RTS tells everyone else to back-off for the duration. If RTS reaches the destination okay (no one else collides with this message), the destination sends a Clear to Send (CTS) message after waiting a prescribed amount of time, called SIFS.

4 Way Handshake (Cont’d) 

After getting the CTS, the original transmitter sends the information packet to its destination.

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In these systems, the transmitter cannot detect collisions. The receiver uses the CRC to determine if the packet reached correctly. If it does then, it sends out an ACK packet.

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If the information packet not ACKed, then the source starts again and tries to

4 Way Handshake (Cont’d) Access Point

Laptop RTS CTS Data ACK

Components required for WiFi A PC, laptop or PDA, running Windows 98 or above.  A wireless PCMCIA card, or a wireless adapter. Now a days all the latest LAPTOPS are coming with in built CENTRINO 

Components required for WiFi 

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A Network Interface Card (optional) - Only if a LAN connection is required) An Access Point - essentially a compact radio transmitter with an antenna that connects to a wired connection, such as an Ethernet, DSL, or Cable Network. A valid internet connection (Usually a Broad Band).

Components required for a larger environment 

Multiple Access Points - To provide overlapping coverage throughout a site. Access points can be installed almost anywhere.

Components required for a larger environment 

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Network switch - A device that joins multiple computers together. A set of Access Points can be connected to a single network switch. Wireless LAN bridge (optional) – A wireless LAN workgroup bridge enables connection between two different hotspot networks.

Components required for a larger environment Wireless Router - It integrates an Access point, IP Router & Ethernet switch. The Switch connects the integrated Access point & the Router internally & allows for external wired ethernet LAN devices as well as single WAN devices such as DSL modem or Cable modem. It allows to configure all devices through central configuration utilities (An integrated web server).

Components required for a larger environment Authentication and Billing Server – Enables control access to the Wi-Fi hotspot network by conducting authentication checks similar to credit card or member ID authentication. Also tracks wireless usage for billing purposes and provides payment transaction services.

Components required for a larger environment Wireless Access Gateway – A device which connects wireless subscribers to the wired network. It employs one Ethernet port to interface with the router and one wireless subscriber port that supports IEEE 802.11b/g standard .

PCMCIA wireless card Used For Portables:

PCI and USB adapters Used For Desktops: PCI adapter (inside)

USB adapter (outside)

Important Specifications of Access Points        

Transmit Power [30 mw, 100 mw, 200 mw, 500 mw, 1 W] Automatic/Manual Power Control Provision for Connecting External Antenna Multiple SSIDs/VLANs Security WPA/WPA-2[802.11i] and Security Configuration per SSID QOS support Rogue Access Point detection Manageability [SNMP]

Important Specifications of Access Points   

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WDS [Wireless Distribution System] Single Radio/Dual Radio Mesh Networking Client Isolation or Intra Cell Blocking POE [Power Over Ethernet] Radius Authentication and Accounting Support Automatic Configuration Auto Channel Selection

1 Mbps DSSS 2 Mbps DSSS

5.5 Mbps DSSS

11 Mbps DSSS

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How much distance can be supported by one AP (802.11b) 50M  80M  120M  150M 

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11 Mbps 5.5 Mbps 2 Mbps 1 Mbps

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How much distance can be supported by one AP (802.11g) 20M  40M  80M  100M 

: : : :

54 Mbps 27 Mbps 6 Mbps 1 Mbps

Microsense

Microsense

Microsense

Antenna Basics •

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An antenna propagates and receives RF signals from the air and makes them available to the receiver Frequency – Antennas should be tuned to either 2.4 GHz (802.11 b/g) or 5 GHz (802.11a) Power - Antennas can handle specific amounts of power put out by the transmitter. Antennas are generally rated >1W Radiation pattern – Defines the radio wave propagation of the antenna. An isotropic pattern means the AP transmits radio waves in all directions equally (beach ball pattern)

Antenna Basics •

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Gain – Represents how well the antenna increases effective signal power, with decibels as unit of measure. For instance, AP transmitting at 100mw with 3 dB gain produces 200mw effective output. dBi is the gain relative to an isotropic source. SNR (Signal to Noise Ratio) – Ratio of amplitude of radio signal to amplitude of noise in a transmission channel. The greater the ratio, the better the transmission. Receiver sensitivity – A measurement of the weakest signal a receiver can receive and still correctly translate it into data.

Antenna Basics •

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Omni-directional antennas – Propagate RF signals in all directions(360-degree) equally in the horizontal plane, but limit range on the vertical plane. Radiation pattern resembles a doughnut with the antenna at the center of the hole. Directional antenna - Transmits and receives RF energy more in one direction than others. Radiation pattern is similar to a flashlight or spotlight. The higher gain antennas have a narrower beam width, which limits coverage on the sides of the antennas.

Connecting to a Hotspot 

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Access the software for the 802.11 card -- normally there is an icon for the card down in the system tray at the bottom right of the screen. Click the "Search button" in the software. The card will search for all of the available hotspots in the area and show you a list. Double-click on one of the hotspots to connect to it.

Connecting to a Hotspot (Cont’d) 

On most notebook models, you will see some sort of signal icon on the bottom right hand corner of your screen or a lit indicator on the notebook itself, which will give you feedback for "On" and signal strength (a red screen means your radio is Off; a green screen indicates it is On). OFF

ON

Connecting to a Hotspot (Cont’d) 

You can also see the quality of the signal by clicking on the radio icon (may vary by system):

Connecting to a Hotspot (Cont’d) 

Simply follow the instructions to sign up for the service, or enter your user name and password if you are already a customer.

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Once you successfully log on, you should see the following icon in your tool bar, indicating the connection has been made:

Setting up a Hotspot at Home 

If you already have several computers hooked together on an Ethernet network and want to add a wireless hotspot to the mix, you can purchase a Wireless Access Point and plug it into the Ethernet network.

Wireless Access Point

Setup #1

Alternate Setup using a Wireless Router 

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If you are setting up a network in your home for the first time, or if you are upgrading, you can buy a Wireless Access Point Router. This is a single box that contains:  1) a port to connect to your cable modem or DSL modem,  2) a router,  3) an Ethernet hub,  4) a firewall and  5) a wireless access point. You can connect the computers in your home to this box either with traditional Ethernet cables or with wireless cards.

Alternate Setup (Cont’d)

Typical Wi-Fi Setup

WiFi Range 

Regardless of which setup you use, once you turn your Wireless Access Point on, you will have a WiFi hotspot in your house.

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In a typical home, this hotspot will provide coverage for about 100 feet (30.5 meters) in all directions, although walls and floors do cut down on the range.

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Even so, you should get good coverage throughout a typical home. For a large home, you can buy inexpensive signal boosters to increase the range of the Hotspot.

Way to Amplify WiFi Signals

Wireless Access Point

A WiFi repeater is installed to extend coverage.

Things to Configure in a Hotspot 

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The SSID -- Service Set IDentifier is a sequence of characters that uniquely names a WLAN.  It will normally default to the manufacturer's name (e.g. “Nomadix").  You can set it to any word or phrase you like. The channel – the radio link used by access point/router to communicate to wireless devices.  Normally it will default to channel 6.  However, if a nearby neighbor is also using an access point and it is set to

Things to Configure (Cont’d) 

The WEP key -- The default is to disable WEP.  If you want to turn it on, you have to enter a WEP key and turn on 128-bit encryption.  WEP can be in text format. Access points come with simple instructions for changing these three values. Normally you do it with a Web browser. Once it is configured properly, you can use your new hotspot to access the Internet from anywhere in your home.

Infrastructure versus Ad Hoc 

All the connections that we have talked about today require a connection from a device equipped with a wireless network interface card (NIC) to a wireless access point.

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Generally, all such connections are operating in what is known as the infrastructure mode. Here the wireless network resembles a cellular architecture.

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Wireless devices can also communicate directly with each other, i.e., it is not required that they communicate with an access point first.

Infrastructure versus Ad Hoc 

When devices with NIC cards communicate directly with each other, the wireless network operates in ad hoc mode.

Ad Hoc Mode 

Ad Hoc connections can be used to share information directly between devices. This mode is also useful for establishing a network where wireless infrastructure does not exist.

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Some uses,  Synchronize data between devices.  Retrieve multimedia files from one device and “play” them on another device.  Print from a computer to a printer

WiFi Security 

WiFi hotspots can be open or secure.

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If a hotspot is open, then anyone with a WiFi card can access the hotspot.

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If it is secure, then the user needs to know a WEP key to connect.

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WEP stands for Wired Equivalent Privacy

WiFi Security (Cont’d) 

WEP is an encryption system for the data that 802.11 sends through the air.

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Encryption system prevents any nonauthorized party from reading or changing data.

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Specifically, it is the process of encoding bit stream in such a way that only the person (or computer) with the key (a digital sequence) can decode it.

WEP 

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WEP has two variations: 64-bit encryption (really 40-bit) and 128-bit encryption (really 104-bit). 40-bit encryption was the original standard but was found to be easily broken. 128-bit encryption is more secure and is what most people use if they enable WEP. For a casual user, any hotspot that is using WEP is inaccessible unless you know this WEP key.

WEP (Cont’d) 

If you are setting up a hotspot in your home, you may want to create and use a 128-bit WEP key to prevent the neighbors from casually eavesdropping on your network.

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Whether at home or on the road, you need to know the WEP key, and then enter it into the WiFi card's software, to gain access to the network.

Infrastructure Required for Wi-Fi Security

Client

Access Point

Radius Server

Certificate Server

Considerations for Enterprise Wi-Fi  

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Access Point must support multiple SSIDs/VLANs Must support 802.1x and WPA, WPA-2 to provide security to internal users and internal networks Must support Rogue Access Point Detection QOS support for future applications such as VOWIFI and Video applications Manageability [SNMP] Convenient Mechanism to provide Internet Access to Visitors and Guests Plug and play service. The user should not have the need to change IP address settings or Proxy settings

Considerations for Public Hot Spots 

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Simple and easy to use login mechanism to the guest, which includes authentication and billing. This would include Radius based authentication as well as PMS [Property Management Systems] integration in the case of hotels Seamless switch over from wired to wi-fi networks and vice versa. For example a guest who has purchased a 24 hour account by connecting to the wired port in the room should be able to use it from the Wi-Fi network as well, say in the Coffee Shop or Lobby or elsewhere, automatically

Considerations for Public Hot Spots    

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Complete support for VPN applications Provision for Bandwidth on Demand Provision for Public IPs for User Laptop Support for both Open/Unsecure Connections and Secure Connections Roaming between hotspots and between different service Providers Branding on Portal Pages

Considerations for Public Hot Spots 

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Network Management system to provide Performance and Configuration Management as well as Fault Monitoring and Notifications There should be provision for both historical reports as well as online live reports, guest wise, date wise, period wise, Plan wise, fault wise, usage wise and so on Automatic redirection to welcome page when user tries to access the Internet