Describe the basic components of a wireless network

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Assignment 07

Part 1

  1. Describe the basic components of a wireless network.

The NIC is a small radio transmitter/receiver that enables a computer to transmit to receive from the access point. The access can have a directional or omnidirectional antenna and is usually wired into a traditional wired network. Most WLANs operate in the 2.4GHZ and 5GHZ frequency ranges and transmit 100-500 feet.

  1. How do the NIC and AP work together to transmit message in an 802.11b WLAN?

The AP acts as a repeater to ensure that all computers within range of the AP can hear the signals of all computers in the WLAN. All NICs in the WLAN transmit their packet to the AP and then the retransmits the packet over the wireless network to its destination-or retransmits the packet over the wired network to its destination. Wireless NICs never communicate with each other directly; they always transmit through the AP.

  1. Compare and contrast the two types of antennas.

The two types of antennas that can be used on APs are omnidirectional antennas and directional antennas. Most WLANs are installed using APs have omnidirectional antennas, which means that the antenna transmits in all directions simultaneously. One common omnidirectional antenna is the dipole antenna. Directional antenna projects a signal only in one direction. Because the signal is concentrated in a narrower, focused area, the signal is stronger and therefore will carry farther than the signal from an AP using an omnidirectional antenna.

  1. What are the two ways in which omnidirectional antennas differ from a directional antenna?

Omnidirectional antennas transmit in all directions, both horizontally and vertically. The signal goes in all directions, as well as up and down, although there is often a small dead spot with no signal that is a very small area directly above the antenna. The directional antenna projects a signal only in one direction. Directional antennas are most often used on the inside of and exterior wall of a building, pointing to the inside of the building. The signal inside the building and also has the benefit of increasing the range of the AP. Directional antennas, because the signals is concentrated in a narrower, focused area, the signal is stronger and therefore will carry farther than the signal from an AP using an omnidirectional antenna.

  1. How does Wi-Fi perform media access control?

Media access control in Wi-Fi is Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), which is similar to the contention-based CSMA/CD approach used by traditional Ethernet. With CSMA/CA, computers listen before they transmit and if no one else is transmitting, they proceed with transmission. Detection collisions is more difficult in radio transmission than in transmission over wired networks, so Wi-Fi attempts to avoid collision to a greater extent than traditional Ethernet.

7. How does 802.11g differ from 802.11b and 802.11a?

802.11a provides data rates up to 54Mbps over short distances, while 802.11b provides data rates up to 11 Mbps over longer distances (up to 500 feet). 802.11g is designed to replace both of these by providing 54 Mbps over long distances.

9. How does Wi-Fi differ from shared Ethernet in terms of topology, media access control, and error control?

The logical and physical topologies of Wi-Fi are the same as those of shared Ethernet. Detecting collisions is more difficult in radio transmission than in transmission over wired networks, so Wi-Fi attempts to avoid collision to a greater extent than the traditional Ethernet.

11. Explain how CSMA/CA DCF works.

The first media access control method is the distributed coordination function (DCF) also called physical carrier sense method because it relies on the ability of computers to physically listen before they transmit. When a node wants to send a message, first listens to make sure that the transmitting node has finished, then waits a period of time longer. Each frame is sent using stop-and –wait ARQ, by waiting, the listening node can detect that the sending node has finished and can then begin sending its transmission. ACK/NAK sent a short time after a frame is received. Message frames are sent a somewhat longer time after (ensuring that no collision will occur)

12. Explain how CSMA/CA PCF works.

The second media access control technique is called the point coordination function (PCF) is also called virtual carrier sense method. When hidden node problem exists, the AP is the only device guaranteed to be able to communicate with all computers on the WLAN. Therefore, the AP must manage the shared circuit using controlled-access techniques, with this approach, any computer wishing to transmit first sends a request to transmit (RTS) to the AP, which may or may not be heard by all computers. The RTS requests permission to transmit and to reserve the circuit for the sole use of the requesting computer for a specified time period. If no other computer is transmitting, the AP responds with a clear to transmit (CTS), specifying the amount of time for which the circuit is reserved for the requesting computer. All computers hear the CTS and remain silent for the specified time period.

15. Compare and contrast the two types of WiMAX.

There are two types of WiMAX. 802.16d and 802.16e. 802.16d is fixed wireless WiMAX connecting multiple buildings to one center access point, while 801.16e is designed to provide access for mobile users. A growing use for 802.16e is to connect multiple Wi-Fi public access points to a central switch, so they can connect into the Internet. This eliminates the need to put wires and enables a quick rollout of new technology.

16. Is WiMAX a competitor to Wi-Fi? Explain.

802.16e is a direct competitor to public access Wi-Fi and current cell phone technologies but is incompatible with both. Manufactures will have to build in separate 802.16e chips and antennas into phones and laptops or users will need to purchase add-on 802.16e NICs. WiMAX is designed to replace outdoor public access Wi-Fi, but it is unclear which technology will win the battle.

19. Explain the topology and media access control of Bluetooth.

Topology: Uses the term “piconet” to a Bluetooth network. I t consists of eight devices. A “master” device controlling other devices, “slaves”. It acts like an AP. Selects frequencies and controls access. All devices in a piconet share the same frequency range.

Media Access Control uses Frequency Hopping Spread Spectrum (FHSS). Available frequency range (2.4000-2.4835) divided into 79 separate 1-MHz channels. A data burst transmitted using one channel; next data burst uses the next channel, and so on. Channels changed based on a sequence and established by the slave and the master prior to the data transfers. 1,600 hops or channel changes per second. It is also used to minimize interference. It is not compatible with 802.11b.

25. What are wardriving and warchalking?

Wardriving: The act of finding unsecured WLAN access points (usually by driving around).

Warchalking: marking (usually using chalk on pavement) the location and information for unsecured WLANs.

26. Explain how SSID works

The most basic security applied to WLANs is to require all client computers wanting to access an AP to include a Service Identifier (SSID) in all packets. Any packet with the incorrect SSID is not processed by the AP. This provides very basic security but is easy to break. The SSID is included in all packets in plain text, so any device within range of the AP that has the right software can listen to packets and easily read the SSID they contain. Simply put, using SSID does not provide security.

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