Ch. 15 Wireless Networks

7. What is a distribution system in wireless LANs?

A distribution system is the technology that is used to connect basic service sets into a larger campus-wide network. Thus, a campus-wide wireless LAN consists of many basic service sets connected to each other through a distribution system.

What is an access point? What are some reasons why you would prefer access points to wireless routers to create a wireless network in your organization?

A wireless access point is a device that allows wireless hosts to connect to a wired network using wireless LAN technologies such as Wi-Fi. The advantage of using access points (basic service areas) over routers is that it facilitates mobility. The entire wireless network appears to end users as one large LAN. Users can move anywhere within an extended service set and still retain the same IP address and subnet membership. If each access point served as a router, it would create an independent subnet and each time a user moved from one access point to the next, he would connect to different subnet. This would potentially give him a different IP address and gateway router address. This address reallocation would stop any ongoing transfers and could also potentially disturb the network connectivity of some applications. With the concept of an extended service set, when users move from one access point to another, there is no change to any network setting, and ongoing network transfers can continue without interruption.

15. What are wireless MANs? What are their primary uses?

A wireless metropolitan area network (MAN) is a moderately high-speed computer network that usually spans a city or campus. It is used to interconnect local area networks (LANs) within its coverage area to each other and to the Internet.

8. What is an extended service set?

An extended service set is the collection of all basic service sets that compose the campus-wide wireless network

13. What are some important characteristics of Bluetooth?

Bluetooth operates in the 2.4 GHz ISM band, same as 802.11 b/g wireless LANs. Bluetooth is designed to offer data rates of up to 1 Mbps. This is much slower than the 11/ 54 Mbps offered by 802.11 LANs. But 1 Mbps is adequate for applications such as keyboards and headsets that use Bluetooth. Bluetooth uses frequency hopping spread spectrum (FHSS) modulation for signal transmission

3. What are ISM frequency bands? Why are they useful?

ISM frequency bands are the signal frequencies that enable wireless networking. These signal frequencies are available for free use for Industrial, Scientific, and Medical applications, hence the name ISM bands. The bands include 915 MHz ± 13 MHz, 2.45 GHz ± 50 MHz and 5.8 GHz ± 75 GHz. ISM bands are very useful because the terms industrial and scientific are interpreted very broadly, and ISM frequencies may be put to almost any use by anybody without permission from anyone or payments of license fees to anyone. These frequencies are therefore also called unregulated frequencies. Cordless phones, remote controlled cars, microwave ovens, wireless keyboards, and mice are other applications that use ISM frequencies. For the purposes of this chapter, these frequencies are used for free to create wireless LANs.

17. What are the differences between wireless LANs and wireless MANs?

One difference between wireless LANs and MANs is that whereas LAN access points cover only a radius of about 300 feet, a single MAN base station can cover large parts of a metro area. Another difference is that all communication in wireless MANs is required to pass through the base station, but LANs allow ad hoc networking where stations can directly communicate with each other. A major commercial difference between the two categories of networks is that MANs allow operators to use licensed frequencies if desired, to improve network performance, in return for a subscription fee

12. What are personal area networks? How are they different from LANs?

Personal area networks like Bluetooth provide communication over short distances, usually within 30 feet. This distance is sometimes called the personal operating space because people and devices within this range can usually see each other. There are some important differences between personal area networks and LANs. Wireless LANs are largely used by computing devices such as laptops, with large batteries or direct power connections. Bluetooth is designed to be used by any electronic device to communicate with any other Bluetooth-capable electronic device. Unlike wireless LANs, Bluetooth is typically only used to connect to other nearby devices, for example a keyboard to a desktop, or a headset to a cell phone. As a result, while high data rate is a very important requirement for wireless LANs, it is less important for Bluetooth. Also, unlike wireless LANs, Bluetooth requires no infrastructure equipment such as access points. Finally, Bluetooth is considered an electronics technology more than a computer technology. Bluetooth is seen in devices such as microphones for cell phones and GPS devices in cars.

1. What are wireless networks? Why are they useful?

Wireless networks are computer networks that use the ISM wireless frequency bands for signal transmission. The reasons for the popularity of wireless networks include their convenience and ease of deployment. On battery-powered laptops, wireless networking allows users to compute and communicate without any power or network cords. Businesses like wireless networking because setting up a basic wireless network in a small office requires nothing more than an inexpensive wireless router. By comparison, wired networking requires cables to be drawn through ceilings, floors, and walls. Wireless networking is becoming so popular that many organizations are finding that more than half the Ethernet ports in the organization are unused because users prefer wireless networks over wired networks. Wireless networking may be one of those rare services loved by both businesses and employees

What are the common wireless LAN categories? What are the important differences between them?

The common wireless LAN categories are a, b, g and n. The important differences between the categories are in speed and frequency of operation. 802.11a operates in the 5.5 GHz ISM band and offers data rates of up to 54 Mbps. 802.11b operates in the 2.4 GHz band and offers data rates up to 11 Mbps. 802.11g also operates in the 2.4 Ghz band, but offers data rates up to 54 Mbps. 802.11n offers data rates up to 6000 Mbps and also allows a single base station to cover a wider geographical area than any of the other wireless LAN technologies.

16. What data rates and ranges are likely to be available on wireless MANs?

The data rates likely to be available on wireless MANs are 96 - 134 Mbps, 64 - 90 Mbps and 32 - 45 Mbps. The highest data rates are available close to the base station and the slower data rate is expected to be available up to 10 miles away from the base station.

14. What are master and slave devices in a piconet?

The device in a Bluetooth piconet that performs the management functions of the piconet is called the master. All other devices in the piconet are called slaves. The master provides a synchronization clock that helps all other devices in the piconet remain in sync with each other. Whereas a device may be a slave on multiple piconets at the same time, it can only be a master on one piconet at a time

What are some differences between wired and wireless LANs? How do they impact the design of the wireless LAN header?

The most important difference between wireless and wired LANs is that wireless LANs have no defined boundaries. An Ethernet wall jack is hardwired to a specific port on a specific switch. When you connect to the network through a wall jack, you become part of a well-defined network. On the other hand, wireless networks can overlap and they often do. At home, if you open up your "connect to network" dialog (right-click on the wireless icon in your system tray → Connect to a network), you are likely to see wireless networks from many of your neighbors. If any of these wireless networks is not security enabled, you can use it to connect to the Internet. Therefore, whereas geographical location uniquely defines network membership in Ethernet, it does not define network membership in wireless LANs. The technical implication is that, the network performance of a wireless connection depends upon the distance of the host from the access point. A user who is very far from an access point will get very weak signals. Wireless networks are also affected by adverse weather, humidity, temperature, and other environmental conditions. Another difference is that stations at two opposite ends of a wireless LAN may not be able to hear each other. As a result, collision detection may be unsuccessful in wireless LANs. Wireless LANs therefore do not use CSMA/ CD for medium access control. Instead, wireless LANs use collision avoidance, and the medium access control (MAC) mechanism used in wireless networks is called carrier sense multiple access with collision avoidance (CSMA/ CA). Yet another difference concerns multiplexing. Ethernet does not use multiplexing because it uses all the available bandwidth in the medium to transmit signals. This is possible because Ethernet cables are not used for other applications. But wireless LANs share the bandwidth in the air with other users and have to send signals in specified signal bands. Therefore, wireless LANs use multiplexing. To use the available bandwidth efficiently, multiple channels have been defined within the 2.4 and 5.8 GHz bands. The implication of these differences between wireless LANs and Ethernet is that wireless LANs require far greater error-detection capabilities than Ethernet. This is manifested in the physical layer of wireless LANs which adds error protection over and above the CRC error detection introduced in Ethernet.

10. What are some differences between the physical layers in wireless and wired LANs?

The physical layer in wired LANs adds no additional information to the frame. However, the wireless physical layer adds a header-error-check field to the frame. This field provides the necessary error-detection to deal with the unreliability of the wireless medium. Another minor difference is that the sync and start-of-frame delimiter fields are considered parts of the physical layer header instead of the data link layer header.

9. What is a portal in a wireless LAN?

The portal is the connection point where the entire wireless LAN is connected to the rest of the wired Internet

2. What are some of the concerns with using wireless networks?

The primary concerns with wireless networks are associated with security and speed. Wired networks have wall outlets in specific locations that can only be reached by users with access to the building. By contrast, wireless signals spread out in all directions and can easily bleed outside the organization's boundaries. Without adequate security, malicious users can easily access the organization's computer network through an improperly secured wireless access point. An example is the incident with T. J. Maxx in 2006, where intruders were able to exploit weaknesses in the store wireless networks to steal over 45 million credit-card records. Wireless networks are also generally slower and less reliable than wired networks. Connection drop-offs are common with wireless networks. While this is not a major concern for browsing, email, and other light applications; when continuity or speed of the connection is essential, wired networks are still greatly preferable to wireless networks. Another potentially important issue is related to health. Though the signals generated by wireless access points have very low energy and there is no evidence yet of health hazards from these signals, our knowledge of the health hazards of exposure to wireless signals is limited. Wireless LANs have only been around for about a decade. It is possible that exposure to wireless signals over longer durations could have adverse health effects

5. What is a basic service set? A basic service area?

The smallest component unit of a wireless LAN is the area covered by a single access point. This area is called a basic service area (BSA). The basic service area and the access point covering that area together are called a basic service set (BSS).