Ethernet Networks: Design, Implementation, Operation, Management 4th phần 8 docx

408 chapter eight ISO OSI Reference Model IEEE 802.11 layers Data link layer Logical link control Media access control (CSMA/CA) Physical layer FHSS DSSS IR Legend: FHSS DSSS IR Frequency Hopping Spread Spectrum Direct Sequence Spread Spectrum Infrared Figure 8.1 IEEE 802.11 architecture. several additions to the IEEE standard. One addition was the IEEE 802.11b specification, which extended the operating rate of DSSS to 5.5 Mbps and 11 Mbps and which represented the most popular type of wireless LAN when this book revision occurred. Both the basic 802.11 and the 802.11b specifications operate in the 2.4 GHz unlicensed Industrial Scientific and Medical (ISM) band. While the Federal Communications Commission (FCC) in the U.S. regulates the maximum power and transmission method, the fact that the ISM band is unlicensed means that a user does not have to obtain a license to use equipment in that frequency band. A second addendum to the IEEE 802.11 standard is the 802.11a specification. This specification defines the use of a multi-carrier frequency transmission method in the 5 GHz ISM band. The multi-carrier frequency method is referred to as orthogonal frequency division multiplexing (OFDM), which results in a large number of carriers being used, each of which operates at a low data rate, but cumulatively they support a high data rate up to 54 Mbps. Because higher frequencies attenuate more rapidly than lower frequencies, the range of 802.11a-compatible devices is significantly less than that of 802.11b devices. This results in a requirement to install additional access points to obtain the same area of wireless LAN coverage and increases the cost of a very high speed wireless LAN. Because many network operators require more speed than that wireless ethernet 409 provided by the 802.11b specification but a higher range than that supported by the 802.11a specification, the IEEE has been working on a new standard, referred to as 802.11g, which doubles the data rate of 802.11b networks to 22 Mbps in the 2.4 GHz frequency band. Network Topology The IEEE 802.11 wireless LAN standards support two types of network topology, referred to as ad hoc and infrastructure. Figure 8.2 illustrates an example of an ad hoc network. An ad hoc network consists of two or more wireless nodes or stations that recognize one another and communicate on a peer-to-peer basis within their area of RF or IR coverage. The term ‘‘ad hoc’’ is assigned as this type of network environment is commonly formed when two wireless devices come into the range of one another and communicate on a temporary basis until one or more devices depart the area. A second type of wireless LAN topology is known as a network infrastruc- ture. In its most basic form a wireless network infrastructure consists of an access point (AP) connected to a wired LAN and one or more client stations. Figure 8.3 illustrates an example of a wireless network infrastructure. In this example an access point is shown connected to a hub on a wired LAN. The access point can be considered to represent a bridge between the wired and wireless LANs. However, in addition to providing bridging between the wired and wireless networks, an access point also interconnects wireless clients. That is, when an access point is present, client stations communicate with one another through the AP and not on a peer-to-peer basis. Station Station Station Figure 8.2 A wireless ad hoc net- work infrastructure. 410 chapter eight Client station Access point Basic service area Wired hub/switch Figure 8.3 A wireless network infrastructure contains at least one access point and one wireless station, referred to as a Basic Service Set. When two or more mobile nodes come together to communicate or if one mobile client comes into close proximity to an access point, this action results in the formation of a Basic Service Set (BSS). Each BSS has an identification that typically corresponds to the 48-bit MAC address of the wireless network adapter card. That identification is referred to as a Basic Service Set Identifi- cation (BSSID) and the area of coverage within which members of a BSS can communicate is referred to as a Basic Service Area (BSA). When wiring an office, college campus or government agency, you will more than likely need to install multiple access points. When this is done, the basic service areas of coverage from multiple Basic Service Sets form what is referred to as an Extended Service Set (ESS). The wired LAN infrastructure functions as a distribution system, which enables clients to roam and be serviced by different APs. Figure 8.4 illustrates an Extended Service Set formed by the use of two access points interconnected by a wired LAN used as a Distribution System (DS). Each BSS within a DS is said to be operating in an infrastructure mode. In examining Figure 8.4 it should be noted that the Basic Service Sets may or may not overlap. In addition, each station associates itself with a particular access point based upon selecting the one with the greatest received signal strength. Each access point in the Extended Service Set will have an ESSID (Extended Service Set Identifier) programmed into it. The ESSID can be considered to represent the subnet the access point is connected to. You can wireless ethernet 411 Access point Access point Hub Hub Hub Client BSS-1 BSS-2 Client Client ESS Server Distribution system Legend: BSS Basic Service Set ESS Extended Service Set Figure 8.4 An extended service set consists of one or more basic service sets connected via a distribution system. also program the ESSID into a station, which then requires it to connect to a like programmed access point. When creating an extended service set it is also important to consider the frequency of operation of each access point. This is due to the need to minimize the overlapping of frequency use by adjacent access points. Because FHSS and DSSS operating access points have different restrictions concerning frequency overlap, you must also consider the transmission scheme used when you design a large wireless infrastructure. Roaming In examining Figure 8.4 note that the movement of a client from BSS-1 to BSS- 2 or vice versa represents a roaming action. Although IEEE 802.11 wireless LANs support roaming, a wireless operational LAN environment is commonly a fixed-location environment in comparison to cellular telephones, which are used anywhere from a reception area, to the office, and even in the powder room. Thus, while 802.11 wireless LANs support roaming, the actual degree of this activity is limited in comparison to a cellular telephone. 412 chapter eight As a mobile client moves from one access-point service area to another, a mechanism is required for one AP to drop the user while the other begins servicing the user. A mobile client will typically monitor the signal-to-noise ratio (SNR) as it moves and, if required, scan for available access points and connect to a desired AP. APs periodically transmit a beacon frame that enables clients to note the presence of one or more APs and select the one with the best SNR. However, the actual method used depends upon a vendor’s implementation method. For example, in a Cisco wireless LAN roaming environment a client will become associated with a new access point when three conditions occur. First, the signal strength of the new access point must be at least 50 percent. Second, the percentage of time the client’s transmitter is active is less than 20 percent of the present access point. The third condition requires the number of users on the new access point to be four fewer than on the present access point. If the first two conditions are not met, then the client will not change access points regardless of the number of users associated with the AP. Physical Layer Operations As discussed earlier in this chapter, the original IEEE 802.11 wireless LAN standard supports a choice of three physical layers — infrared and two radio- frequency layers. The infrared physical layer is based upon the use of pulse position modulation (PPM) at peak data rates of 1 Mbps, with an optional 2 Mbps rate. Because infrared is limited to use within a single room without barriers, its use is severely limited. In fact, this author is not aware of any infrared-based 802.11 LANs. Because of this, in this section we will focus our attention upon the RF physical layers. Both Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum operate in the 2.4 GHz ISM band, which represents a worldwide-recognized unlicensed band. However, it should be noted that the actual frequencies for the 2.4 GHz band can vary from country to country, as noted in Table 8.1. FHSS Under Frequency Hopping Spread Spectrum data is transmitted for a short duration, referred to as dwell time at a single frequency. At the end of that time duration the transmitter shifts to a new frequency and resumes transmission. Thus, a FHSS system uses narrow-band data transmission but changes its frequency periodically to create a wide-band transmission system. Figure 8.5 illustrates an example of how an FHSS system hops for predefined time intervals using different center frequencies based upon a predefined wireless ethernet 413 TABLE 8.1 2.4 GHz ISM Frequency Allocation Region Allocated Frequency United States 2.400–2.4835 Europe (except France/Spain) 2.400–2.4835 Japan 2.4710–2.4970 France 2.4465–2.4835 Spain 2.4450–2.4750 Time f 5 f 3 f 2 f 1 f 0 f 4 Frequency Figure 8.5 A frequency hopping spread spectrum system hops at a fixed time interval, known as the dwell time, around a wide band using different center frequencies in a predefined sequence. algorithm. By only dwelling at one frequency for a short time duration, an FHSS system can alleviate the effect of narrow-band noise occurring in portions of the transmission band. Although a military system based upon FHSS keeps the algorithm used for hopping a secret, in the wonderful world of wireless LANs the hopping sequence is well known. In fact, both the frequencies at which hopping occurs as well as the number of hops within an assigned ISM band are commonly 414 chapter eight regulated to prevent a wireless LAN from interfering with other electronic equipment. In the United States FHSS uses 79 channels, each 1 MHz wide. In Japan, the number of channels is reduced to 23. For both locations channels are selected according to a pseudo-random selection algorithm that requires a dwell time of 20 ms per channel and all channels to be used prior to being able to reuse a channel. Under the IEEE 802.11 standard 78 different hopping sequences are defined. Each hopping sequence is referred to as a channel, which can cause a degree of confusion if you scan the standard without noting this relationship. At the physical layer FHSS uses two- or four-level Gaussian Frequency Shift Keying (GFSK) modulation. Under two-level FSK modulation each bit is encoded by the transmission of a distinct frequency from two available frequencies. Thus, the bit rate is the same as the baud or signaling rate and the 1 MHz bandwidth used for each short transmission supports a data rate of 1 Mbps. When four-level GFSK is used, each pair (dibit) of bits is encoded into one of four frequencies. Thus, the bit rate is twice the baud rate, resulting in a data rate of 2 Mbps. The term ‘‘Gaussian’’ prefixes FSK because the wave form is Gaussian filtered. Now that we have an appreciation for FHSS let us turn our attention to how DSSS operates. DSSS Under Direct Sequence Spread Spectrum (DSSS) a spreading code is used to spread each bit to be transmitted such that a number of bits representing each bit are actually transmitted. The spreading code used under the 802.11 standard is referred to as a Barker code and its use results in each bit being replaced by 11 bits. At 1 Mbps Differential Binary Phase Shift Keying (DBPSK) is used for modulation, resulting in each bit being represented by one of two possible phase changes. Because 11 bits replace each data bit, the resulting signal is spread over 11 MHz. At 2 Mbps Differential Quadrature Phase Shift Keying (DQPSK) is employed as the modulation method, which results in two bits being encoded into one phase change. When this modulation method is used, the bit rate becomes twice the baud rate, which results in a 2 Mbps data rate. Table 8.2 gives an example of DSSS coding using a five-bit sequences from a pseudo-random bit generator. Note that data for transmission is simply logically modulo-2 added to obtain the data stream to be modulated. Upon demodulation the same pseudo-random bit sequence is modulo-2 subtracted to obtain the original setting of the bit that was spread. If a transmission error occurs, the receiver simply selects the most popular bit wireless ethernet 415 TABLE 8.2 DSSS Bit Spreading Example using a Five-bit Spreading Code Data bits 1 0 Five-bit spreading code 10110 01001 Modulo-2 addition (data to be modulated) 01001 01001 Demodulated data 01001 01001 Five-bit spreading code 10110 01001 Modulo-2 subtraction 11111 00000 setting. That is, a if a five-bit spreading code was used and as a result of the modulo-2 subtraction process at the receiver, three bits were set to a value of 1 while two were set to a value of 0, the bit setting would be assumed to be 1. Similar to FHSS, the use of DSSS can vary by location. Under the IEEE 802.11 standard the use of 13 DSSS channels is defined for transporting an 11-bit Barker-coded 22 MHz signal. For operation in the United States the 802.11 standard defines the use of 11 independent channels. Although Europe and many Asian countries permit the use of 13 channels, in Japan the small amount of available bandwidth (see Table 8.1) results in the support of a single channel. Table 8.3 lists the carrier-frequency channel assignments. As previously noted, depending upon the physical location of a DSSS system a subset of available channels may be required to be used. In the United States and Europe DSSS channel definitions permit three frequency-isolated channels available for co-location. An example of chan- nel co-location is illustrated in Figure 8.6. This frequency isolation enables organizations to operate up to three DSSS functioning access points within close proximity to one another without one access point interfering with another. High-Speed Wireless LANs There are two extensions to the basic IEEE 802.11 standard for which equip- ment had reached the market when this book revision was performed. Those extensions are the IEEE 802.11b specification, for which equipment conform- ing to that standard dominates the market, and the IEEE 802.11a specification. Although the modulation methods differ for each method, they use the same 416 chapter eight TABLE 8.3 2.4 GHz DSSS Channels Channel Frequency (MHz) 1 2412 2 2416 3 2422 4 2427 5 2432 6 2437 7 2442 8 2447 9 2452 10 2457 11 2462 12 2467 13 2473 2.4 GHz Frequency 2.4835 GHz Figure 8.6 DSSS supports up to three non-overlapping channels in the 2.4 GHz band. access protocol, a topic we will focus our attention upon once we obtain an appreciation of the two extensions to the basic IEEE 802.11 standard. 802.11b Under the 802.11b extension to the IEEE 802.11 standard the data rate was increased to 5.5 Mbps and 11 Mbps under DSSS operations. At the higher data wireless ethernet 417 rates of 5.5 Mbps and 11 Mbps DSSS transmitters and receivers use different pseudo-random codes. Collectively, the higher modulation rates are referred to as Complementary Code Keying (CCK). 802.11a Under the 802.11a extension to the IEEE 802.11 standard orthogonal frequency division modulation (OFDM) is employed in the 5 GHz frequency band. Under OFDM multiple-modulated carriers are used instead of a single carrier, as illustrated in Figure 8.7. Here each modulated signal is orthogonal to the other modulated signals. The term orthogonal describes the axis of the signals and the fact that they do not interfere with one another. Because multiple signals are transmitted by a single user, the carriers can be said to be multiplexed. Thus, the transmission of multiple carriers at 90 degree angles to one another was given the term OFDM. However, if you are familiar with the operation of DSL modems or one of the first 9600 BPS analog dial modems, you are also probably aware of the term ‘‘multitone’’ used to denote the use of multiple carriers. Thus, OFDM can be considered to represent a multitone transmission scheme. Under the 802.11a standard 48 data and four pilot carriers or a total of 52 carriers are transmitted within a 20 MHz channel. This action makes use of the three blocks or bands of frequency allocated by the FCC for unlicensed operations in the 5 GHz band. A 200 MHz band from 5.15 GHz to 5.35 MHz has two sub-bands. The first 100 MHz in the lower section is restricted to a maximum power output of 50 mW, while the second 100 MHz has a more generous 250 mW maximum power output. A third band at 5.725 MHz to 5.825 MHz is designed for outdoor applications and supports a maximum of 1 W of power output. Because the 5 GHz band has almost four times the bandwidth of the ISM band, the developers of the 802.11a specification turned to OFDM to make Frequency Power Figure 8.7 Orthogonal frequency division multiplexing results in the trans- mission of multiple carriers, each modulating a small amount of data. [...]... 0 wireless ethernet TABLE 8. 4 Type and Subtype Values Type Value b3 b2 Type Description 00 Management 0000 Association Request 00 Management 0001 Association Response 00 Management 0010 Association Request 00 Management 0011 Association Response 00 Management 0100 Probe Request 00 Management 0101 Probe Response 00 Management 0110–0111 00 Management 1000 Beacon 00 Management 1001 ATM 00 Management 1010... more rapidly than lower frequencies As a result of this, the range of 80 2.11a equipment is probably half that of 80 2.11b products, which means the radius of coverage of an 80 2.11a access point will be one-fourth that of an 80 2.11b access point Access Method Unlike wired Ethernet, which uses the CSMA/CD access protocol, wireless Ethernet LANs use what is referred to as a distributed coordination function... microseconds required to transmit the ACK frame and its SIFS interval 430 chapter eight Management Frames As noted in Table 8. 4, there are 10 defined management frames Two of the more popular types of management frames are Beacon and Probe frames, both of which we will examine in this section The Beacon Frame Figure 8. 12 illustrates the basic format of the body of a Beacon and Probe frame as well as the... parameter set IBSS wireless ethernet 431 432 chapter eight as indicated in the lower portion of Figure 8. 12 The function of the capability information field is to indicate requested or advertised capabilities Under the current Draft 8 version of changes to the 80 2.11 standard the second byte remains to be defined Physical Protocol Data Units The transfer of information in an IEEE 80 2.11 environment occurs... in Figure 8. 14, uses a 144-bit Physical Layer Convergence Procedure (PLCP) preamble divided into a 1 28- bit sequence used for signal detection and a Start of Frame Delimiter (SFD) The PLCP header consists of four fields Those fields include an eight-bit Signal field, which indicates the data rate Currently this field supports four 1 28 bits SFD 16 bits PLCP preamble 144 bits Signal 8 bits SERVICE 8 bits PLCP... Depending upon the manufacturer of the adapter card, you may be able to select an ad hoc or infrastructure mode of operation, enable or disable a power-saving mode of operation, select one of 13 RF channels for DSSS operation, and enable or disable WEP Typically, wireless ethernet 435 Figure 8. 15 The SMC networks EZ wireless PC card is designed for insertion into a Type II PC slot in a notebook or laptop... modem Wireless router access point Wired LAN hub ISP assigned IP address RFC 19 18 translated addresses Client Figure 8. 18 Wired stations Client Using a wireless router/access point wireless ethernet 439 wired clients on the existing wired LAN Because most wireless router/access point devices perform NAT using a single RFC 19 18 network address, you are limited to supporting a total of 253 stations for... 80 2.11 standard supports three physical layers, as you might expect there are three PPDU frame formats Because practical wireless LANs are restricted to RF communications, we will focus our attention upon the protocol frames for FHSS and DSSS FHSS Figure 8. 13 illustrates the frame format for the FHSS physical layer This frame consists of an 80 -bit preamble of synchronization bits in the repeating 80 ... modulation method is supported 64 QAM can operate encoding either 8 or 10 bits per signal change, permitting a maximum data rate of 1.125 Mbps per 300 Hz channel Because 48 data subchannels are supported per channel, this results in a maximum data rate of 54 Mbps Although the 80 2.11a specification supports a much higher data rate than the 80 2.11b specification, it is important to remember that higher frequencies... access points wireless ethernet 421 8. 2 Frame Formats Similar to wired Ethernet, where there is one basic frame format, wireless LANs also have a basic data frame format However, wireless LANs also support two additional types of frames One type, referred to as control frames, was briefly mentioned when we discussed the hidden node The third type of frame supported by wireless LANs is management frames, . Response 00 Management 0010 Association Request 00 Management 0011 Association Response 00 Management 0100 Probe Request 00 Management 0101 Probe Response 00 Management 0110–0111 Reserved 00 Management. IEEE 80 2.11 standard. 80 2.11b Under the 80 2.11b extension to the IEEE 80 2.11 standard the data rate was increased to 5.5 Mbps and 11 Mbps under DSSS operations. At the higher data wireless ethernet. Reserved 00 Management 1000 Beacon 00 Management 1001 ATM 00 Management 1010 Disassociation 00 Management 1011 Authentication 00 Management 1100 De-authentication 00 Management 1101–1111 Reserved 01