Welcome to the chapter on Wireless networking. This chapter talks about the basics of wireless networking, setting up of a wireless network and security issues related to wireless networks.
A wireless local area network (WLAN) is a data transmission system designed to provide location-independent network access between computing devices by using radio waves rather than a cable infrastructure.
Wireless networks operate at the same OSI layers and use the same protocols as wired networks. What differs is the type of media used and the methods for accessing the media.
5.1 Wireless Basics
Wireless network modes
In its simplest form, a wireless network consists of two or more PCs communicating directly with each other without cabling or intermediary hardware. The more complicated wireless networks use an access point to centralize wireless communication, as well as to bridge wireless network segments to wired network segments. These two different methods, or modes, are called ad hoc mode and infrastructure mode.
Ad-hoc mode wireless network:
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Each wireless node is in direct contact with each of the other wireless nodes in the network. Ad hoc mode does not use an access point; it uses a mesh topology instead.
Two or more wireless nodes communicating in ad hoc mode form an Independent Basic Service Set (IBSS). This is a basic unit of organization in wireless networks. Ad hoc network cannot be used to connect to other networks unless one of the machines is running Internet Connection Sharing (ICS).
Therefore, Ad hoc mode networks are good for small groups of computers that need to transfer files or share printers and as temporary networks to be used as study groups or for business meetings. (Refer figure 5.1)
Infrastructure mode wireless network
Wireless access points (WAPs) are used to connect the wireless network nodes centrally in case of wireless networks running in infrastructure mode. The configuration is similar to the star topology of a wired network.
The infrastructure mode also enables connection of wireless network segments to wired segments in the network. To set up a wireless network for a large number of PCs, infrastructure mode is used. WAPs allow control of wireless networks from a central location. This permits better control and filtration of network traffic.
A Basic Service Set (BSS) is a single WAP servicing a given area. More access points can be added to extend this service area and is called an Extended Service Set (ESS).
Infrastructure mode is used for business networks or networks that need to share dedicated resources such as Internet connection and centralized databases (Refer to figure 5.1)
Figure 5.1: Infrastructure mode
If a small group of computers are being connected into a decentralized workgroup then only wireless adapters are needed but if wireless network segments are to be connected to wired network, the Wireless access point (WAP) is needed. It connects wireless network nodes to wireless or wired networks. A basic WAP functions like a hub and works at OSI Layer 1. Combination devices are also used in wireless networks. These work as a high-speed hub or switch, bridge, and router concurrently. Such devices can function at many different OSI layers.
Wireless Network Card
A wireless network card is necessary for each device on a wireless network. A laptop typically has an expansion (PCMCIA) slot in which the network card fits. A desktop computer needs an internal card, which has an antenna on it. These antennas are optional on most equipment and help to increase the signal on the card.
Figure 5.2: Wireless network card (NIC)
Wireless Access Point
The wired part of the wireless network is connected to the wireless a device in the network with a WAP.WAP allows multiple devices to connect through it so that they can access the network. A WAP can also function as a router so that data transmission can be extended and passed from one access point to another.
Figure 5.3: Wireless access point
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Wireless devices operate on specific broadcasting frequencies to prevent interference from other wireless devices operating on the same wireless band. The original 802.11 standards use the 2.4-GHz frequency. Later standards use either 2.4-GHz or 5.0-GHz frequencies. Knowing these wireless frequency ranges enable troubleshooting interference issues from other devices operating in the same wireless band.
802.11 define three different spread-spectrum broadcasting methods used in implementation of wireless networks these are:
Direct-sequence spread spectrum (DSSS),
Frequency hopping spread spectrum (FHSS)
Orthogonal frequency-division multiplexing (OFDM).
DSSS sends data out on different frequencies at the same time. It uses a bandwidth of 1 MHz. DSSS is capable of greater data throughput but more prone to interference.
FHSS sends data on one frequency at a time, constantly shifting frequencies. It uses a bandwidth of 1 MHz. Throughput lesser than DSSS and less prone to interference as compared to DSSS
OFDM is the latest method. It combines the use of multiple frequencies as in DSSS and the capability of shifting frequencies as in FHSS.
The 802.11 standard defines the security protocols, quality of service and mechanism of wireless networks. All these standards use the Ethernet protocol and the CSMA/CA access method for wireless networks. The standards are used for both infrastructure and an ad-hoc network design.
802.11n is the latest standard that uses two new features Multiple input multiple outputs (MIMO) and Channel bonding. MIMO is the use of multiple antennas to achieve more throughputs as compared to a single antenna. Channel bonding allows 802.11n to transmit data over two channels to attain more throughputs. Given below is a table of all the 802.11 standards.
Up to 600 Mbps
802.11a/b/g 802.11 Collision Avoidance
The 802.11 standard defines two methods of collision avoidance:
Distributed Coordination Function (DCF)
Point Coordination Function (PCF)
Current CSMA/CA devices use the Distributed Coordination Function (DCF) method for collision avoidance.
DCF specifies strict rules for sending data onto the network media.It defines a backoff period above the normal IFS wait period before a wireless network node can try to access the network again when the network is busy. Recieving nodes are required to send an acknowledgement (ACK ) for every packet that they process.
The ACK includes a value that tells other wireless nodes to wait a certain period before trying to access the network media. This period is calculated as the time taken by the data packet to reach its destination and is based on the packet's length and data rate. In case the sending node doesn't receive an ACK, it retransmits the same data packet until it gets a confirmation that the packet has reached its destination.
The channel is a portion of the frequency range available to the wireless network to communicate. Thus, each frequency range is a channel. The 802.11 standard defines 14 channels. These channels have some overlap, hence two nearby WAPs must not use close channels like 6 and 7. Most WAPs use channel 1, 6, or 11 by default this keeps the channels as far apart from each other as possible.
2.3995 GHz - 2.4245 GHz
2.4045 GHz - 2.4295 GHz
2.4095 GHz - 2.4345 GHz
2.4145 GHz - 2.4395 GHz
2.4195 GHz - 2.4445 GHz
2.4245 GHz - 2.4495 GHz
2.4295 GHz - 2.4545 GHz
2.4345 GHz - 2.4595 GHz
2.4395 GHz - 2.4645 GHz
2.4445 GHz - 2.4695 GHz
2.4495 GHz - 2.4745 GHz
2.4545 GHz - 2.4795 GHz
2.4595 GHz - 2.4845 GHz
Note : A wireless network can be fine tuned by moving WAPs to other channels as this avoids overlaps when many wireless networks share the same physical space.
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1.7 Chapter Review Questions
RJ-45 connectors are used with______________________
10Base-2 is also referred to as______________________
Ring topology is an?
The maximum segment length of 100Base-FX at half duplex is.
The maximum transfer speed of 10Base-5 is_____________________
Fibre Distributed Data Interface, shares many of the same features as?
Fibre networks use what kind of connectors?
HierarchicalWhat topology provides each device with a point-to-point connection to every other device in the network?
Ethernet is also known as.
In this chapter, we have learnt to:
Identify the characteristics of a network
Identify types of network topologies
Discuss network media and connectors
Describe access methods
Describe network architectures
Explain network operating systems