Introduction to Networking · 6/8/2010  · Management, 2. Control and 3. Data Only the data frame type is similar to 802.3 frames. The payload of wireless and 802.3 frames is 1500

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Provides all the features and benefits of traditional LAN technologies such as Ethernet without the limitations of wires or cables

A WLAN, just like a LAN, requires a physical medium through which transmission signals pass. Instead of using twisted-pair or fiber-optic cable, WLANs use infrared light (IR) or radio frequencies (RFs).

High availability;

Scalability;

Manageability; and

Open architecture.

Modern WLANs offer many benefits to networking:◦ Mobility

◦ Scalability

◦ Flexibility

◦ Short and long term cost savings

◦ Installation advantages

◦ Reliability in harsh environments

◦ Reduced installation time

IT professionals or business executives who want mobility within the enterprise, perhaps in addition to a traditional wired network.

Business owners or IT directors who need flexibility for frequent LAN wiring changes, either throughout the site or in selected areas.

Any company whose site is not conductive to LAN wiring because of building or budget limitations, such as older buildings, leased space, or temporary sites.

Any company that needs the flexibility and cost savings offered by a line-of-sight, building-to-building bridge, that avoids expensive trenches, leased lines, and right-of-way issues

IEEE is the prime issuer of standards for wireless networks.

The standards have been created within the framework of the regulations created by the Federal Communications Commission (FCC)

802.11a 802.11b 802.11g 802.11n

Frequency band 5GHz 2.4GHz 2.4GHz 2.4, 5GHz

Max. data rate 54Mpbs 11Mbps 54Mpbs 248Mbps

1. Management, 2. Control and 3. Data

Only the data frame type is similar to 802.3 frames.

The payload of wireless and 802.3 frames is 1500 bytes; however, an Ethernet frame may not exceed 1518 bytes whereas a wireless frame could be as large as 2346 bytes. Usually the WLAN frame size will be limited to 1518 bytes as it is most commonly connected to a wired Ethernet network.

Since radio frequency (RF) is a shared medium, collisions can occur just as they do on wired shared medium. The major difference is that there is no method by which the source node is able to detect that a collision occurred. For that reason WLANs use Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). This is somewhat like Ethernet CSMA/CD.

CSMA/Collision Detection ◦ In CSMA/Collision Detection (CSMA/CD), the device

monitors the media for the presence of a data signal. If a data signal is absent, indicating that the media is free, the device transmits the data. If signals are then detected that show another device was transmitting at the same time, all devices stop sending and try again later. Traditional forms of Ethernet use this method.

CSMA/Collision Avoidance ◦ In CSMA/Collision Avoidance (CSMA/CA), the device

examines the media for the presence of a data signal. If the media is free, the device sends a notification across the media of its intent to use it. The device then sends the data. This method is used by 802.11 wireless networking technologies.

Only Star and Mesh topologies make sense in a wireless environment

Star Topology Mesh Topology

Wireless LANs usually have two types of realization: ◦Infrastructured

◦ad-hoc

Ad-hoc Network In the ad-hoc network, computers are brought together to form a network "on the fly." There is no structure to the network, there are no fixed points and usually every node is able to communicate with every other node

Infrastructure Network Uses fixed network access points with which mobile nodes can communicate. Higher-speed wired (or wireless) backbone. Network traffic divided into two directions:

-uplink (into the backbone) and

-downlink (from the backbone) The contact points to the backbone are called Access Points (AP)

WLAN authentication occurs at Layer 2. It is the process of authenticating the device not the user.

Authentication may be a null process, as in the case of a new AP and NIC with default configurations in place. The client will send an authentication request frame to the AP and the frame will be accepted or rejected by the AP. The client is notified of the response via an authentication response frame. The AP may also be configured to hand off the authentication task to an authentication server, which would perform a more thorough credentialing process.

Association, performed after authentication, is the state that permits a client to use the services of the AP to transfer data.

Unauthenticated and Unassociated◦ The node is disconnected from the network and not

associated to an access point.

Authenticated and Unassociated◦ The node has been authenticated on the network

but has not yet associated with the access point.

Authenticated and Associated◦ The node is connected to the network and able to

transmit and receive data through the access point.

1. First one is an open connectivity standard in which only the SSID must match. This may be used in a secure or non-secure environment even though the ability of low level networks sniffers to discover the SSID of the WLAN is high. SSID shorts for Service Set Identifier, a 32-character unique identifier attached to the header of packets sent over a WLAN that acts as a password when a mobile device tries to connect to the Wireless Access Point (WAP).

2. The second process is the shared key. This process requires the use of Wired Equivalent Privacy (WEP) encryption. WEP is a fairly simple algorithm using 64,128 and 256 bit keys. The AP is configured with an encrypted key and nodes attempting to access the network through the AP must have a matching key.

Computers send data signals electronically. Radio transmitters convert these electrical signals to radio waves. Changing electric currents in the antenna of a transmitter generates the radio waves.

Radio waves attenuate as they move out from the transmitting antenna. In a WLAN, a radio signal measured at a distance of just 10 meters (30 feet) from the transmitting antenna would be only 1/100th of its original strength. Like light, radio waves can be absorbed by some materials and reflected by others.

Wireless signals are electromagnetic waves which can travel through space.

802.11g Radio Frequency Channels

Channel Center Frequency Frequency Spread

1 2412 MHz 2399.5 MHz - 2424.5 MHz

2 2417 MHz 2404.5 MHz - 2429.5 MHz

3 2422 MHz 2409.5 MHz - 2434.5 MHz

4 2427 MHz 2414.5 MHz - 2439.5 MHz

5 2432 MHz 2419.5 MHz - 2444.5 MHz

6 2437 MHz 2424.5 MHz - 2449.5 MHz

7 2442 MHz 2429.5 MHz - 2454.5 MHz

8 2447 MHz 2434.5 MHz - 2459.5 MHz

9 2452 MHz 2439.5 MHz - 2464.5 MHz

10 2457 MHz 2444.5 MHz - 2469.5 MHz

11 2462 MHz 2449.5 MHz - 2474.5 MHz

12 2467 MHz 2454.5 MHz - 2479.5 MHz

13 2472 MHz 2459.5 MHz - 2484.5 MHz

No license necessary

Many devices use the same RF

Examples: Cordless phones, Bluetooth

devices, other WLANs and microwave ovens.

Degrade user connectivity in terms of

throughput, connection quality and range

Causes 802.11 stations to wait indefinite

periods of time before attempting to access

medium

What happens when an interfering signal

starts while an 802.11 station is transmitting?

Analyze the potential for RF interference

Prevent the interfering sources from

operating

Provide adequate WLAN coverage

Set configuration parameters properly

Deploy 5GHz wireless LANs

When using RF technology many kinds of interference must be taken into consideration.

Narrowband does not affect the entire frequency spectrum of the wireless signal. One solution to a narrowband interference problem could be simply changing the channel that the AP is using.

All band interference affects the entire spectrum range. Bluetooth™ technologies hops across the entire 2.4 GHz many times per second and can cause significant interference on an 802.11b network. It is not uncommon to see signs in facilities that use wireless networks requesting that all Bluetooth™ devices be shut down before entering.

In homes and offices, a device that is often overlooked as causing interference is the standard microwave oven. Leakage from a microwave of as little as one watt into the RF spectrum can cause major network disruption. Wireless phones operating in the 2.4GHZ spectrum can also cause network disorder.

Generally, the RF signal will not be affected by even the most extreme weather conditions. However, fog or very high moisture conditions can and do affect wireless networks. Lightning can also charge the atmosphere and alter the path of a transmitted signal.

A number of new security solutions and protocols, such as Virtual Private Networking (VPN) and Extensible Authentication Protocol (EAP) are emerging. With EAP, the access point does not provide authentication to the client, but passes the duties to a more sophisticated device, possibly a dedicated server, designed for that purpose.

EAP-MD5 Challenge◦ Extensible Authentication Protocol is the earliest

authentication type, which is very similar to CHAP password protection on a wired network.

LEAP (Cisco)◦ Lightweight Extensible Authentication Protocol is the

type primarily used on Cisco WLAN access points. LEAP provides security during credential exchange, encrypts using dynamic WEP keys, and supports mutual authentication.

User authentication◦ Allows only authorised users to connect, send and

receive data over the wireless network.

Encryption◦ Provides encryption services further protecting the data

from intruders.

Data authentication◦ Ensures the integrity of the data, authenticating source

and destination devices.