Chapter 7: Considerations in Building 802.11 Networks

The successful deployment of an 802.11 system requires design, planning, implementation, operation, and maintenance. This chapter is provided as a very brief overview of what the wireless network planner needs to deploy an 802.11 network.

Design

Many vendors are available with equipment that uses several different 802.11 standards. How does one make sense of it all? The most common protocol in use today is 802.11b. But is it best for my application? Many wireless Internet service providers (WISPs) such as T-mobile have phased out all 802.11 (FH) access points (APs). Much of this gear is available on the surplus market. Is there any merit to using this technology? Two new protocols are currently emerging: 802.11g and 802.11a. Is there a reason to wait for these to become publicly available and fully certified before interworking them? The short answer to this question is that it depends on the application and the requirements for that application.

Some of the questions that must be addressed in selecting an 802.11 solution lead to trade-offs, such as speed versus range. Others have mutually exclusive answers, such as proprietary- versus standards-based extensions. Some of the questions that need to be addressed are: What is the network topology? What kinds of links will be used? What is the environment like? What are the throughput, range, and bit error rate that are needed? Will you need tolerance for multipath? Which frequency band will be used with which protocols? Can the solution be off-the-shelf or surplus standards-based, or will it need to be custom?

Network Topology

One of the goals of network planning is to assure that work gets done within the budget allotted to the project. All nodes should be able to communicate where they need to at any time. Network designs can be redundant at various levels so that if a node fails, no other node should be affected. If security is a concern, trusted parts of the network must be separated from the untrusted parts. One of the major factors that determines throughput, robustness, reliability, security, and cost is the geometric arrangement of the network components, or the topology.

Five major topologies are in use today in wired networks: bus, star, tree, ring, and mesh. In a wireless local area network (WLAN), only the star and mesh have analogues with the wired networks. These topologies can be implemented using the modes of operation supported by the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards, infrastructure and ad hoc, through their service sets the independent basic service set (IBSS), the basic service set (ESS), and the extended service set (ESS). See Figure 7-1.

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Figure 7-1: Service sets available in 802.11 standards

Currently, the most common mode is infrastructure. In this mode, wireless devices can communicate with each other or with a wired network. When an AP is connected to wired network and a set of wireless stations, it is referred to as a BSS. A BSS consists of at least one AP connected to the wired network infrastructure and a set of wireless end stations. Thus, BSS configurations rely on an AP that acts as a switch for a single WLAN cell or channel. An ESS is a set of two or more BSSs, each containing an AP connected together using a distribution system (DS) to form a single subnet. Although the DS could be any type of network, it is often an Ethernet LAN. A mobile user can move between APs and reassociate with the AP providing the best coverage. In this way, seamless coverage is possible within the subnet. Most WLANs operate in infrastructure mode because they require access to the wired LAN for services like file servers, printers, and Internet access.

In ad hoc mode, devices or stations communicate directly with each other, without the use of an AP. Ad hoc mode is also referred to as peer-to-peer mode and uses the IBSS. IBSS configurations are also referred to as an independent configuration or an ad hoc network. Logically, an IBSS configuration is analogous to a peer-to-peer office network in which no single node is required to function as a switch or router. IBSS WLANs include a number of nodes or wireless stations that communicate directly with one another on an ad hoc, peer-to-peer basis. Thus, it contains a set of wireless stations that communicate directly with one another without using an AP or any connection to a wired network. It is useful for quickly and easily setting up a wireless network anywhere a wireless infrastructure does not exist or is not required for services. This could include a hotel room, convention center, or airport, or where access to the wired network is barred (such as for consultants at a client site). Generally, IBSS implementations cover a limited area and aren't connected to any larger network. An example would be two laptops with 802.11b Personal Computer Memory Card International Association (PCMCIA) cards that want to share files.

The star topology, which happens to be in widest use today, is a network in which one central base station or AP is used for communication. Information packets are transmitted by the originating node and are received and routed by the AP to the proper wireless destination node by the AP. See Figure 7-2.

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Figure 7-2: A wireless star network

The mesh topology is a slightly different type of network architecture than the star topology, except that no centralized base station exists. Each node that is in range of another one can communicate freely, as shown in Figure 7-3.

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Figure 7-3: A wireless mesh network

Wireless mesh networks are an exciting new topology for creating low-cost, high-reliability wireless networks indoors, across a campus, or in a metropolitan area. In a mesh network, each wireless node serves as both an AP and a wireless router, creating multiple pathways for the wireless signal. Mesh networks have no single point of failure and thus are self-healing. A mesh network can be designed to route around line-of-sight obstacles that can interfere with other wireless network topologies. However, a wireless mesh currently requires the use of specialized client software that will provide the routing function and put the radio into ad hoc or infrastructure mode as required.

Link Type

802.11 systems can be built using either point-to-point or point-to-multipoint links. The FCC regulations allow both types of links, but they come with power-to-the-antenna implications.

Environment

What is the environment like? Is it indoors, or is it outdoors? Is there a line of sight or are there obstacles in the path?

Throughput, Range, and Bit Error Rate

Throughput has tradeoffs with range and the bit error rate (BER). The best network designs balance these factors by limiting the data rate according to data quantity and latency requirements. Fundamentally, any application will trade off between three factors: range, throughput, and BER. 802.11 was developed as a wireless replacement for an indoor Ethernet. If the primary indoor application is Internet access with the AP less than 100 feet away, then any one of the 802.11 standards will work. Since the throughput is limited by the protocol, and BER has to be reasonably high to get throughput, the only variable left is the range. The available range at a given throughput can be calculated using a link budget.

Multipath Fading Tolerance

Non line of sight communications in the Industrial, Scientific, and Medical (ISM) and Unlicensed National Information Infrastructure (U-NII) bands must allow for significant multipath fading. Multipath is created by reflections canceling the main signal. The choice of frequency band and protocols will in part depend on how much multipath can be tolerated.

Link Budget

A fundamental concept in any communications system is the link budget. The link budget is a summation of all the gains and losses in a communications system. The result of the link budget is the transmit power required to present a signal with a given signal-to-noise ratio (SNR) at the receiver to achieve a target BER.

For wireless fidelity (Wi-Fi), it is sufficient to consider factors such as path loss, noise, receiver sensitivity, and gains and losses from antennas and cable. Before calculating a link budget, factors like the frequency band must be determined.

Frequency Band

802.11 technologies can be deployed on four unlicensed frequency bands in the two ISM and U-NII bands. The 2.4 GHz ISM band has an inherently stronger signal with a longer range and can travel through walls better than the 5 GHz U-NII bands. However, the U-NII band enables more users to be on the same channel simultaneously. The 2.4 GHz ISM band has a maximum of three nonoverlapping 22 MHz channels, while the 5 GHz band has four nonoverlapping 20 MHz channels in each of the U-NII bands.

ISM Band The ISM bands were originally reserved internationally for the noncommercial use of radio frequency (RF) electromagnetic fields for industrial, scientific, and medical purposes. More recently, they have also been used for license-free, error-tolerant communications applications such as cordless phones, Bluetooth, and Wireless WLAN.

U-NII Band The U-NII bands can be used by devices that will provide short-range, high-speed wireless digital communications. These devices, which do not require licensing, will support the creation of WLANs and facilitate access to the Internet. The U-NII spectrum is located at 5.15 to 5.35 GHz and 5.725 to 5.825 GHz.

The 5.15 to 5.25 GHz portion of the U-NII band is intended for indoor, short-range networking devices. The FCC adopted a 200 milliwatt (mW) effective isotropic radiated power (EIRP) limit to enable short-range WLAN applications in this band without causing interference to mobile satellite service (MSS) feeder link operations.

Devices operating between 5.25 to 5.35 GHz are intended to provide communications within and between buildings, such as campus-type networks. U-NII devices in the 5.25 to 5.35 GHz frequency range are subject to a 1-watt EIRP power limit.

The 5.725 to 5.825 GHz portion of the U-NII band is intended for community networking communications devices operating over longer distances. The FCC permits fixed, point-to-point U-NII devices to operate with up to a 200-watt EIRP limit.

FCC Regulations The use of these bands is regulated under Parts 15.247 and 15.407 of the FCC regulations.[1] The following are the relevant passages of Part 15.247 regarding power at the time of writing:

  • (b) The maximum peak output power of the intentional radiator shall not exceed the following:

  • (1) For frequency hopping systems operating in the 2400-2483.5 MHz or 5725-5850 MHz band and for all direct sequence systems: 1 watt.

  • (3) Except as shown in paragraphs (b)(3) (i), (ii) and (iii) of this section, if transmitting antennas of directional gain greater than 6 dBi are used the peak output power from the intentional radiator shall be reduced below the stated values in paragraphs (b)(1) or (b)(2) of this section, as appropriate, by the amount in dB that the directional gain of the antenna exceeds 6 dBi.

  • (i) Systems operating in the 2400-2483.5 MHz band that are used exclusively for fixed, point-to-point operations may employ transmitting antennas with directional gain greater than 6 dBi provided the maximum peak output power of the intentional radiator is reduced by 1 dB for every 3 dB that the directional gain of the antenna exceeds 6 dBi.

  • (ii) Systems operating in the 5725-5850 MHz band that are used exclusively for fixed, point-to-point operations may employ transmitting antennas with directional gain greater than 6 dBi without any corresponding reduction in transmitter peak output power.

Part 15.407 regulates the UNII band and its operation. The following parts are the relevant passages from Part 15.407 for understanding the power limits within the 5.1, 5.2, and 5.8 GHz bands:

  • (a) Power limits:

    1. For the band 5.15–5.25 GHz, the peak transmit power over the frequency band of operation shall not exceed the lesser of 50 mW or 4 dBm + 10logB, where B is the 26-dB emission bandwidth in MHz. In addition, the peak power spectral density shall not exceed 4 dBm in any 1-MHz band. If transmitting antennas of directional gain greater than 6 dBi are used, both the peak transmit power and the peak power spectral density shall be reduced by the amount in dB that the directional gain of the antenna exceeds 6 dBi.

    2. For the band 5.25-5.35 GHz, the peak transmit power over the frequency band of operation shall not exceed the lesser of 250 mW or 11 dBm + 10logB, where B is the 26-dB emission bandwidth in MHz. In addition, the peak power spectral density shall not exceed 11 dBm in any 1-MHz band. If transmitting antennas of directional gain greater than 6 dBi are used, both the peak transmit power and the peak power spectral density shall be reduced by the amount in dB that the directional gain of the antenna exceeds 6 dBi.

    3. For the band 5.725-5.825 GHz, the peak transmit power over the frequency band of operation shall not exceed the lesser of 1 W or 17 dBm + 10logB, where B is the 26-dB emission bandwidth in MHz. In addition, the peak power spectral density shall not exceed 17 dBm in any 1-MHz band. If transmitting antennas of directional gain greater than 6 dBi are used, both the peak transmit power and the peak power spectral density shall be reduced by the amount in dB that the directional gain of the antenna exceeds 6 dBi. However, fixed point-to-point U-NII devices operating in this band may employ transmitting antennas with directional gain up to 23 dBi without any corresponding reduction in the transmitter peak output power or peak power spectral density. For fixed, point-to-point U-NII transmitters that employ a directional antenna gain greater than 23 dBi, a 1 dB reduction in peak transmitter power and peak power spectral density for each 1 dB of antenna gain in excess of 23 dBi would be required. Fixed, point-to-point operations exclude the use of point-to-multipoint systems, omni directional applications, and multiple collocated transmitters transmitting the same information. The operator of the U-NII device, or if the equipment is professionally installed, the installer, is responsible for ensuring that systems employing high-gain directional antennas are used exclusively for fixed, point-to-point operations.

Table 7-1 summarizes the ISM and U-NII unlicensed frequency bands used by Wi-Fi devices. The table also shows their associated power limits.

Table 7-1: Frequency bands and associated power limits

Frequency range (MHz)

Bandwidth (MHz)

Max. power at antenna

Max. EIRP

Notes

2400-2483.5

83.5

1 W (+30 dB above 1 milliwatt [dBm]), 1 W (+30 dBm)

4 W (+36 dBm)

Point to point; point to multipoint following 3:1 rule

5150-5250

100

50 mW

200 mW (+23 dBm)

Indoor use, must have integral antenna

5250-5350

100

250 mW (+24 dBm)

1 W (+30 dBm)

 

5725–5825

100

1 W (+30 dBm)

200 W (+53 dBm)

 

Point to Multipoint Part 15.247(b)(1) limits the maximum power at the antenna to 1 watt. Part 15.247(b)(3) allows antennas that have more than 6 decibels (db) as long as the power to the antenna is reduced by an equal amount in the 2.4 GHz band. This implies that the maximum EIRP is 4 watts or 36 dBm. This limit of 4 watts EIRP no matter the antenna gain is illustrated in Table 7-2.

Table 7-2: Point-to-multipoint operation in the 2.4 GHz ISM band

Power at antenna (mW)

Power at antenna (dBm)

Max. antenna gain (dBi)

EIRP (watts)

EIRP (dBm)

1000

30

6

4

36

500

27

9

4

36

250

24

12

4

36

125

21

16

4

36

63

18

19

4

36

31

15

21

4

36

15

12

24

4

36

8

9

27

4

36

4

6

30

4

36

Point-to-Point Links Point-to-point links have a single transmitting point and a single receiving point. Typically, a point-to-point link is used in a building-to-building application. Part 15.247 (b)(3)(i) allows the EIRP to increase beyond the 4-watt limit for point-to-multipoint links in the 2.4 GHz ISM band. For every additional 3 db gain on the antenna, the transmitter only needs to be cut back by 1 dB. The so-called three-for-one rule for point-to-point links can be observed in Table 7-3.

Table 7-3: Point-to-point operation in the 2.4 GHz ISM band

Power at antenna (mW)

Power at antenna (dBm)

Max. antenna gain (dBi)

EIRP (watts)

EIRP (dBm)

1000

30

6

4

36

794

29

9

6.3

38

631

28

12

10

40

500

27

15

16

42

398

26

18

25

44

316

25

21

39.8

46

250

24

24

63.1

48

200

23

27

100

50

157

22

30

157

52

According to part 15.247(b)(3) (ii), the 5.8 GHz band has no such restriction. However, part 15.407 effectively restricts the EIRP to 53 dBm (see Table 7-4).

Table 7-4: Point-to-point operation in 5.8 GHz U-NII band

Power at antenna (mW)

Power at antenna (dBm)

Antenna gain (dBi)

EIRP (watts)

EIRP (dBm)

1000

30

6

4

36

1000

30

9

8

39

1000

30

12

16

42

1000

30

15

316

45

1000

30

18

63.1

48

1000

30

21

125

51

1000

30

23

250

53

Protocols

Four primary standards-based protocols are currently available: 802.11 802.11b, 802.11a, and 802.11g.

802.11 The 802.11 standard was the first standard to specify the operation of a WLAN. This standard addresses frequency-hopping spread spectrum (FHSS), direct sequence spread sequence (DSSS), and infrared. The data rate is limited to 2 Mbps and 1 Mbps for both FHSS and DSSS.

FHSS handles multipath and narrowband interference as well as a byproduct of its frequency-hopping scheme. If multipath fades one channel, other channels are usually not faded. Thus, packets are passed on those hops where no fading occurs. Operating an FHSS system in a high-multipath or high-noise environment will be seen as an increase in latency. FHSS has 64 hopping patterns that can support up to 15 colocated networks. FHSS systems are limited to 1 Mbps and optionally 2 Mbps. Typically, they have a shorter range than DSSS systems. FHSS is not compatible with today's 802.11b equipment.

DSSS, as implemented in 802.11, occupies 22 MHz of spectrum while providing a maximum over-the-air data rate of 2 Mbps. DSSS is susceptible to multipath and narrowband interference due to the limited amount of spreading that is used (11 bits). DSSS can only support three noninterfering channels and thus does not have nearly as much network capacity as an FHSS system at the same data rate. DSSS is compatible with today's 802.11b equipment.

Surplus 802.11 equipment may work well for some applications where multipath immunity is required, lower data rates can be tolerated, and compatibility with currently available equipment is not desired. Furthermore, be advised that the gear may no longer be covered by warranties and may not have service available for it anymore.

802.11b The most widely used standard protocol, 802.11b, requires DSSS technology, specifying a maximum over-the-air data rate of 11 Mbps and a scheme to reduce the data rate when higher data rates cannot be sustained. This protocol supports 5.5 Mbps, 2 Mbps, and 1 Mbps over-the-air data rates in addition to 11 Mbps using DSSS and complementary code keying (CCK).

IEEE 802.11b standard uses CCK as the modulation scheme to achieve data rates of 5 and 11 Mbps. 802.11 reduced the spreading from 11 bits down to 8 to achieve the higher data rates. The modulation scheme makes up the processing gain lost with the lower spreading by using more forward error correction.

The IEEE 802.11b specification allows for the wireless transmission of approximately 11 Mbps of raw data at indoor distances up to 300 feet and outdoor distances of 20 miles in point-to-point usage of the 2.4 GHz band. The distance depends on impediments, materials, and line of sight.

802.11b is the most commonly deployed standard in public short-range networks, such as those found at airports, coffee shops, hotels, conference centers, restaurants, bookstores, and other locations. Several carriers currently offer pay as you go hourly, per session, or with unlimited monthly access using networks in many locations around the United States and other countries.

802.11a The 802.11a standard operates in the three 5 GHz U-NII bands and thus is not compatible with 802.11b. The bands are designated by application. The 5.1 GHz band is specified for indoor use only, the 5.2 GHz band is designated for indoor/outdoor use, and the 5.7 Ghz band is designated for outdoors only. RF interference is much less likely because of the less crowded 5 GHz bands. The 5 GHz bands each have four separate nonoverlapping channels. They specify orthogonal frequency division multiplexing (OFDM) using 52 sub-carriers for interference and multipath avoidance, they support a maximum data rate of 54 Mbps using 64 quadrature amplitude modulation (QAM), and they mandate support of 6, 12, and 24 Mbps data rates. The protocol specifies the minimum receive sensitivities ranging from -65 dBm for the 54 Mbps rate to -82 dBm for 6 Mb/sec. Equipment designed for the 5.1 GHz band has an integrated antenna and is not easily modified for higher power output and operation on the other two 5 GHz bands.

802.11g 802.11g is an extension to 802.11b and operates in the 2.4 GHz band. 802.11g increases 802.11b's data rates to 54 Mbps using the same OFDM technology used in 802.11a. The range at 54 Mbps is less than the existing 802.11b APs operating at 11 Mbps. As a result, if an 802.11b cell is upgraded to 802.11b, the high data rates will not be available throughout all areas. You'll probably need to add additional APs and replan the RF frequencies to split the existing cells into smaller ones. 802.11g offers higher data rates and more multipath tolerance. Although more interference exists on the 2.4 GHz band, 802.11g may be the protocol of choice for the best range and bandwidth combination. It's also upwardly compatible with 802.11b equipment.

Making the Choice

Which technique is best? It depends on the application and other design considerations. Frequency hopping offers superior reliability in noise and multipath fading environments. Direct sequence can provide higher over-the-air data rates. OFDM offers multipath tolerance and much higher data rates. 802.11 (no letter) is now obsolete, but may offer nonstandard, bargain-basement usable equipment. 802.11b is compatible with most of the public access locations. 802.11a is the best to solve interference cases and has great throughput. 802.11g promises the best range and throughput combination of all the solutions.

Standard or Custom?

Interoperability with other manufacturers' products requires a standard protocol. However, some manufacturers have products that offer features which others don't. For example, Panasonic sells a residential gateway and a client card that use WhiteCap, a technology sold by ShareWave, now part of Cirrus Logic. WhiteCap is an intermediate solution that eventually may upgrade to IEEE 802.11e. To take advantage of the quality of service (QoS) provided by WhiteCap, both the APs and the client adapter cards have to support the extra technology. The chance exists that it may not be possible to buy additional components if the company stops producing the product.

[1]The FCC web site, www.fcc.gov, has a lot of material online. Part 15 in its entirety can be found at www.access.gpo.gov/nara/cfr/waisidx_01/47cfr15_01.html.



Wi-Fi Handbook(c) Building 802.11b Wireless Networks
Wi-Fi Handbook : Building 802.11b Wireless Networks
ISBN: 0071412514
EAN: 2147483647
Year: 2003
Pages: 96

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