Communication Networks/Wireless Internet
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Introduction
editThe ability to communicate with the rest of the world instantaneously has been the ultimate goal for the design of network communication system. For such a large coverage, it seems only realistic and achievable through wireless networks. This becomes the driving force of all the wireless network research done all over the world.
After the huge success of Internet, IEEE came up with the protocols for Wireless Networks. In this chapter, we will study the IEEE 802.11 standards, and the different types of wireless networks.
There are 2 different protocols that are prominent in the field of wireless internet: WiFi and WiMAX.
Basics in Wireless WiFi
editA wireless LAN (WLAN) is a set of network components connected by electromagnetic (radio) waves instead of wires. WLANs are used in combination with or as a substitute to wired computer networks, adding flexibility and freedom of movement within the workplace. Wireless LAN clients enjoy great mobility and can access information on the company network or even the Internet from the store, boardroom or throughout the campus without relying on the availability of wired cables and connections.
The proposed standard 802.11 works in two modes:
1. In the presence of base station.
2. In the absence of base station
In first case all communication goes through the base station known as the access point in 802.11 terminologies. This is known as infrastructure mode. In latter case, the computers just communicate with each other directly this mode is called as ad hoc networking.
IEEE 802.11 denotes set of wireless LAN/WLAN standards developed by IEEE standards working committee (IEEE 802). Some of the many challenges that had to be met where :finding a suitable frequency band that was available, preferably worldwide; dealing with the fact that radio signals have a finite range; ensuring users privacy and security; worrying about human safety; and finally, building a system with enough bandwidth to be economically feasible.
At the time of standardization process it was decided that 802.11 be made compatible with Ethernet above data link layer. But several inherent differences exist and had to be dealt with by the standard.
First, a computer on Ethernet always listens to the ether before transmitting. In case of wireless LANs this is not possible. It may happen that the range of a station may not be able to detect the transmission taking place between other two stations resulting in collision.
The second problem that had to be solved is that radio signals can be reflected off the solid objects, so it may be received multiple times. This interference results in Multipath fading.
The third problem is that if a notebook computer is moved away from base station to another there must be some way of handing it off.
After some work the committee came up with a standard that addressed these and other concerns. The most popular amendments are 802.11a, 802.11b and 802.11g to original standard. The security was also enhanced by amendment 802.11i.The other specifications from (c-f, h, j) are service enhancements and extensions
The Electromagnetic Spectrum
editThe industrial, scientific and medical (ISM) radio bands were originally reserved internationally for non-commercial use of RF electromagnetic fields for industrial, scientific and medical purposes
Figure 3.1 The Electromagnetic Spectrum
As the figure shows the ISM band is shared by license free communication application such as wireless LANs and Bluetooth. IEEE 802.11 b/g wireless Ethernet operates on 2.4 GHz band. Although these devices share ISM band they are not part of ISM devices. Due to the ISM Band which includes Bluetooth, microwave oven and cordless telephones the 802.11b and 802.11g equipment have to sustain interference. This is not the case with 802.11a since it uses 5 GHz band
Comparison between three unlicensed bands:
IEEE 802.11 Standards / WiFi
editWifi simply stands for Wireless Fidelity.
The services and protocols of 802.11 maps to lower two layers of OSI reference model. The protocols used by all 802 variants have a certain commonality of structure. The partial view of protocol stack is shown in figure 3.2. The data link layer is split into two sub layers. The MAC (Medium access control) sub layer is responsible for allocation of channels and also determines who transmits next. The function of Logic Link Layer is to hide the differences between different 802 variants.
Figure 3.2 Protocol Stack 802.11
The 802.11 standard initially specified three transmission techniques. The infrared method which uses the same technology as television remote controls do. The other two methods use short radio and are called as FHSS and DSSS. Both of these don't require licensing. In 1999 two new techniques were introduced to achieve higher bandwidth. These are called as OFDM and HR-DSSS. They operate at up to 54 Mbit/s and 11 Mbit/s respectively.
Each of the five permitted transmission techniques makes it possible to send a MAC frame from one station to another. They differ in technology used and speed achievable. Let’s have a look at them one by one:
Infrared option used diffused transmission at .85 or .95 microns. Two speeds are permitted: 1 Mbit/s and 2 Mbit/s. A technique called as Gray encoding is used for 1 Mbit/s. In this scheme a group of 4 bits is encoded as a 16 bit codeword containing 15 zeros and single 1. At 2 Mbit/s the encoding takes 2 bits and produces 4 bit codeword. Infrared cannot penetrate from walls hence two cells are well isolated from each other. Nevertheless due to low bandwidth this is not the popular option.
FHSS (Frequency Hopping Spread Spectrum) uses 79 channels each 1 MHz wide starting at the low end of 2.4-GHz ISM band. A pseudorandom number generator is used to produce sequence of frequencies hopped to. The only condition is the seed to random number must be known by both and synchronization must be maintained. The amount of time spend at each frequency is known as Dwell Time and must be less than 400 ms. The two main advantages of FHSS are Security offered due to hopping sequence and resistance to Multipath fading. The main disadvantage is low bandwidth.
Figure 3.3 Frequency Hopping Spread Spectrum
Direct Sequence Spread Spectrum (DSSS) is also restricted to 2 Mbit/s. In this method a bit is transmitted as 11 chips using Barker sequence. It uses Phase shift Modulation at 1 Mbaud transmitting 1 bit per baud when transmitting at 1 Mbit/s and 2 Mbaud when transmitting at 2 Mbit/s.
Orthogonal Frequency Division Multiplexing (ODFM), used by 802.11a, is the first of the sequence of high speed wireless LANs. It delivers the speed of up to 54 Mbit/s operating at 5 GHz ISM. As the term suggests different frequencies are used, in all 52, 48 for data and 4 for synchronization. Phase shift modulation is used for speed up to 18 Mbit/s and QAM is used after that.
High Rate Direct Sequence Spread Spectrum (HR-DSSS), 802.11b, is another spread spectrum technique, which used 11 million chips per second to achieve 11 Mbit/s in the 1.4 GHz Band. The data rates supported by 802.11 are 1, 2, 5.5, and 11 Mbit/s. The two slow rates run at 1 Mbaud, with 1 and 2 bits per Baud, respectively using Phase shift modulation. The two faster rates run at 1.375 Mbaud, with 4 and 8 bits per Baud respectively, using Walsh/Hadamard codes. In practice operating speed of 802.11b is nearly always 11 Mbit/s. Although 802.11b is slower than 802.11a the range is about 7 times that of 802.11a, which is considered more significant in many situations.
An enhanced version of 802.11b, 802.11g, uses OFDM modulation method of 802.11a but operates in the narrow 2.4 GHz ISM band along with 802.11b. It operates up to speed of 54 Mbit/s. To conclude the 802.11 committee has produced three different high speed wireless LANs (802.11a, 802.11b, 802.11g) and three low speed wireless LANs
802.11 Data Frame Structure The 802.11 standard define three different standards of frames on wire: data, control and management. Each of these has a header with variety of fields within MAC sub layer. The format of data frame is shown in figure. Following is the brief description of each:
Figure 3.4 802.11 Frame Structure
First is the Control field which has 11 subfields. The first of these is protocol version, which allows two versions of protocols to operate at the same time. Then comes the type field, which can be data, control or management. The subtype contains RTS or CTS. The To DS and From DS fields indicate whether the frame is going to or coming from intercell distribution. MF indicates more fragments follow. Retry means retransmission of frame sent earlier. Power management bit is used by base station to save power by putting the receiver to sleep or taking out of sleep state. More bit indicate that sender has more frames for receiver. W bit specifies that the frame body has been encrypted using WEP (Wired Equivalent Privacy) algorithm O bit indicates the sequence of bits needs to be processes in strict order. The Duration field indicates how long the channel will be occupied by the frame. This field is also contained in control frames. The frame header contains four addresses all in standard IEEE 802 format. First two addresses are for the source and destination other two are for source and destination of base station
Address 1: All stations filter on this address. Address 2: Transmitter Address (TA), Identifies transmitter to address the ACK frame to. Address 3: Dependent on To and From DS bits. Address4: Only needed to identify the original source of WDS (Wireless Distribution System) frames.
Sequence field allows fragments to be numbered. 12 bits identify the frame and 4 identify the fragment. The Data field can contain payload of up to 2312 bytes, followed by Checksum. Management frames have a format similar to data frames. The only difference is they don't have the base station address, because management frames are restricted to single address. Control frames are shorter having at the most two addresses with no data or sequence field.
IEEE 802.11 Architecture
editThe emergence of wireless networks as a communication channel allows seamless connectivity between different electronic devices. Based on the network structure, wireless networks can be divided into two classes: infrastructure-based and ad hoc. The infrastructure-based network is a pre-configured network that aims to provide wireless services to users in a fixed network area. On the other hand, the ad hoc network has no fixed infrastructure so that a network can be established anywhere to offer services to users.
Infrastructure Mode
The current existing wireless networks are mostly infrastructure-based, such as cellular networks and IEEE 802.11 wireless LANs. In a cellular network, whole service areas are divided into several small regions called cells. There is at least one base station to provide services to devices (i.e. cellular phone) in each cell. Each device connects to the network by establishing a wireless connection to the base station in order to transmit and receive packets. The base stations are connected through high bandwidth wired connections to exchange packets, making it possible for senders and receivers within different service areas to communicate. Note that all network traffic is constrained to either uplink (device to base station) or downlink (base station to device). Emphasis in this research area focuses on providing quality of service (QoS) guarantees, such as soft handoff to ensure a low probability of dropped call or no significant packet delay due to mobility of user from one cell to a neighboring cell. The drawback of this kind of network is its requirement for a fixed infrastructure, which is infeasible in certain situations. The ad hoc network is proposed to address this problem to allow network with infrastructureless architecture.
Figure 4.1 A Small-scaled Model of a Wireless Infrastructure Network
Ad-Hoc Mode
Unlike the conventional infrastructure-based wireless network, ad hoc network, as a distributed wireless network, is set of mobile wireless terminals communicating with each other without any pre-existing fixed infrastructure. The mobile ad hoc network has several unique features that challenge the network operation, such as the routing algorithm, Quality of Service (QoS), resource utilization, etc. The following figure depicts a small-scaled model of a wireless ad hoc network. All the terminals, also referred to as mobile nodes, exchange information among one another in a fully distributed manner through wireless connections within the ad hoc network. And due to the mobility of these nodes, the network topology is under constant changes without any centralized control in the system. These are several main concerns that needs to be considered when designing a specific application-layer protocol based on wireless ad hoc networks.
Figure 4.2 A Small-scaled Model of a Wireless Ad Hoc Network
802.11a
editWhat is 802.11a and history of 802.11a?
It is a Wireless LAN standard from the IEEE(Institute of Electronics and Electrical Engineers). It was released on October 11 in 1999.
Speed:
It can achieve a maximum speed of 54Mbit/s. Although the typical data rate transfer is at 22Mbit/s. If there is a need the data rate will be reduced to 48, 36, 24, 18, 12, 9 then 6Mbit/s respectively. This usually occurs as the distance between the access point or the wireless router and the computer gets further and further away.
Frequency:
It operates under the 5GHz frequency band. The advantage of this is that it has lesser interference compared to the 802.11b and 802.11g standards, which operate at 2.4GHz. It means that quite a number of electronic equipment use this frequency band such as microwaves, cordless phones, bluetooth devices etc. Therefore, the more electronic equipment that use the same frequency band, the more interferences it will cause among the equipment that are using that frequency band.
Interoperability:
802.11a will not operate readily with 802.11b or 802.11g due to the different frequency bands unless the equipment implements the both standards. E.g. Equipment that use both 802.11a and 802.11g standards.
Number of Channels
It has 12 non-overlapping channels. 8 are for indoor(within the area) and the other 4 are for point to point.
802.11b
editWhat is 802.11b and its history
It is also something like 802.11a. It is of course a wireless standard made by IEEE and guess what it was implemented on the same month and year as 802.11a which was in October 1999.
Speed
802.11b has the lowest speed after 802.11 legacy. It can reach a maximum speed of only 11 Mbit/s.
Frequency
802.11g
edit802.11n (Wi-Fi 4)
edit802.11ac (Wi-Fi 5)
edit802.11ax (Wi-Fi 6)
editWireless LANs Issues (CSMA/CA)
editAt the MAC sublayer, IEEE 802.11 uses the carrier sense multiple access with collision avoidance (CSMA/CA) media access control (MAC) protocol, which works in the following way:
• A wireless station with a frame to transmit first listens on the wireless channel to determine if another station is currently transmitting (carrier sense). If the medium is being used, the wireless station calculates a random backoff delay. Only after the random backoff delay can the wireless station again listen for a transmitting station. By instituting a random backoff delay, multiple stations that are waiting to transmit do not end up trying to transmit at the same time (collision avoidance).
The CSMA/CA scheme does not ensure that a collision never takes place and it is difficult for a transmitting node to detect that a collision is occurring. Additionally, depending on the placement of the wireless access point (AP) and the wireless clients, a radio frequency (RF) barrier can prevent a wireless client from sensing that another wireless node is transmitting. This is known as the hidden station problem, as illustrated in Figure 5.1(a).
Figure 5.1 (a)Hidden Station Problem (b)Exposed Station Problem
Hidden Station Problem: Wireless stations have transmission ranges and not all stations are within radio range of each other. Simple CSMA will not work! A transmits to B. If C “senses” the channel, it will not hear A’s transmission and falsely conclude that C can begin a transmission to B.
Exposed Station Problem: This is the inverse problem. C wants to send to D and listens to the channel. When C hears B’s transmission to A, C falsely assumes that it cannot send to D. This reduces network efficiency.
Multiple Access with Collision Avoidance
editTo provide better detection of collisions and a solution to the hidden station problem, IEEE 802.11 also defines the use of an acknowledgment (ACK) frame to indicate that a wireless frame was successfully received and the use of Request to Send (RTS) and Clear to Send (CTS) messages. When a station wants to transmit a frame, it sends an RTS message indicating the amount of time it needs to send the frame. The wireless AP sends a CTS message to all stations, granting permission to the requesting station and informing all other stations that they are not allowed to transmit for the time reserved by the RTS message. The exchange of RTS and CTS messages eliminates collisions due to hidden stations.
For example, the idea is to have a short frame transmitted from both sender and receiver before the actual transfer. As shown in Figure 5.2, A sending a short RTS (30 bytes) to B with length of L. B responding with a CTS to A. And whoever hears CTS shall remain silent for the duration of L. Then A can safely send data (length L) to B.
Figure 5.2 An illustration of Multiple Access with Collision Avoidance
Medium Access Control
editDistributed Coordination Function (DCF) is the fundamental MAC technique of the IEEE 802.11 wireless LAN standard. DCF employs a distributed CSMA/CA distributed algorithm and an optional virtual carrier sense using RTS and CTS control frames.
DCF mandates a station wishing to transmit to listen for the channel status for a DIFS interval. If the channel is found busy during the DIFS interval, the station defers its transmission or proceeds otherwise. In a network that a number of stations contend for the multi-access channel, if multiple stations sense the channel busy and defer their access, they will also find that the channel is released virtually simultansously and then try to seize the channel. As a result, collisions may occur. In order to avoid such collisions, DCF also specifies random backoff, which is to force a station to defer its access to the channel for an extra period. The length of the backoff period is determined by the following equation:
DCF also has an optional virtual carrier sense mechanism that exchanges short Request-to-send (RTS) and Clear-to-send (CTS) frames between the source and destination stations between the data frame is transmitted. This is illustrated in Figure 5.3 below. C (in range of A) receives the RTS and based on information in RTS creates a virtual channel busy NAV(Network Allocation Vector). And D (in range of B) receives the CTS and creates a shorter NAV.
Figure 5.3 The use of virtual carrier sensing using CSMA/CA
DCF also includes a positive acknowledge scheme, which means that if a frame is successfully received by the destination it is addressed to, the destination needs to send an ACK frame to notify the source of the successful reception. DCF is defined in subclause 9.2 of the IEEE 802.11 standard and is de facto default setting for WiFi hardware.
Fragmentation is a technique to improve network throughput. Due to unreliable ISM band causing high wireless error rates, long packets have less probability of being successfully transmitted. So the solution is to implement MAC layer fragmentation with stop-and-wait protocol on the fragments, as shown in figure below.
Figure 5.4 Fragmentation in 802.11 for better throughput
IEEE 802.11 standard also has an optional access method using a Point Coordination Function (PCF). PCF allows the Access Point (PC) acting as the network coordinator to manage channel access.
Point Coordination Function (PCF) is a Media Access Control (MAC) technique use in wireless networks which relies on a central station, often an Access Point (AP), to communicate with a node listening, to see if the airwaves are free (i.e., all other stations are not communicating). PCF simply uses the AP as a control system in wireless MAC. PCF seems to be implemented only in very few hardware devices as it is not part of the Wi-Fi Alliance's interoperability standard.
Since most APs have logical bus topologies using shared circuits, only one message can be processed at one time because it is a contention based system. Therefore, a media access control technique is required.
The problem with wireless is the hidden station problem, where some regular stations (which communicate only with the AP) cannot see other stations on the extreme edge of the geographical radius of the network (because the wireless signal attenuates before it can reach that far). Thus having an AP in the middle allows the distance to be halved, allowing all station to see the AP and consequentially have the maximum distance between two stations on the extreme edges of a circle-star topology (in a circled-star physical topology).
Co-Existence between distributed DCF and centralized PCF is possible using InterFrame Spacing as illustrated in Figure 5.5 below.
• SIFS (Short IFS) :: is the time waited between packets in an ongoing dialog (RTS,CTS,data, ACK, next frame)
• PIFS (PCF IFS) :: when no SIFS response, base station can issue beacon or poll.
• DIFS (DCF IFS) :: when no PIFS, any station can attempt to acquire the channel.
• EIFS (Extended IFS) :: lowest priority interval used to report bad or unknown frame.
Figure 5.5 Interframe Spacing in 802.11
IEEE 802.11 AP Services
editThe 802.11 AP service include two types of services:
1. Distribution services: The distribution services include many functionalities such as association - which is related to a particular station that reports identity, data rate,and power; disassociation, reassociation which is like a handover of controls, distribution using the routing protocols, and integration.
2. Intracell services: The intracell services include different functions such as authentication, deauthentication, privacy, and data deliver. Authentication is a process to authenticate the user once the association takes place. It is always conducted after association with an AP. The privacy is a wired equivalent privacy. More information on wireless security will be discussed later.
Lets take a look in detail how each of this process works.
Association Process: The association with an AP takes place in the following way -
When a Client comes on line, it will broadcast a Probe Request. An AP that hears this will respond with details. The client makes a decision who to associate with based on the information returned from the AP. Next the Client will send an authentication request to the desired AP. The AP authenticates the client, and sends an acknowledge back. Next the client sends up an association request to that AP. The AP then puts the client into the table, and sends back an association response. From that point forward, the network acts like the client is located at the AP. The AP acts like an Ethernet hub.
Re-association Process: When the client wants to associate back with the AP which was involved in the prior communication, re-association takes place. The process takes place in the following way - As the client is moving out of range of his associated AP, the signal strength will start to drop off. At the same time, the strength of anther AP will begin to increase. At some point in time, BEFORE communication is lost, the client will notify AP A that he is going to move to AP B. B and A will also communicate to assure any information buffered in A get to B over the backbone. This eliminates retransmitting packets over the air, and over the backbone. The same handoff can occur if the load on A become large, and the client can communicate with someone other than A.
Cellular and 802.11b
There are quite a few differentiating functionalities in both of these services. Lets see how these two communication protocols differ.
Bluetooth
Bluetooth is a radio standard; a technology by which phones, computers, and personal digital assistants(PDAs), can be easily interconnected using a short-range wireless connection. Following are some of the features of Bluetooth technology:
IEEE 802.11 Security
editThis is a new section that is introduced in this chapter. The contents are based on my understanding and prior work experience in embedded wireless technology field.
Wireless Security
After the emergence of 802.11 it was certain that the internet technology was no longer going to be the same. Many new protocols and communication devices were introduced. To communicate using these devices, and to be secure over the internet, it was going to be a new challenge. The wireless security was developed in such a way that both the tasks were accomplished - hence no interference and secured wireless connection. There are different types of wireless security involved which will be discussed in brief. Let us see the different wireless security features available currently.
1. WPA and WPA2:
Wi-Fi Protected Access (WPA and WPA2) is a class of systems to secure wireless Wifi, computer networks. It was created in response to several serious weaknesses researchers had found in the previous system, Wired Equivalent Privacy (WEP). WPA2 implements the full standard, but will not work with some older network cards. Both provide good security, with two significant issues:
• either WPA or WPA2 must be enabled and chosen in preference to WEP. WEP is usually presented as the first security choice in most installation instructions.
• in the "Personal" mode, the most likely choice for homes and small offices, a passphrase is required, for full security, it must be longer than the typical 6 to 8 character passwords users are taught to employ.
2. WEP:
Wired-Equivalent Privacy (WEP) protocol. A security protocol, specified in the IEEE 802.11 standard, that attempts to provide a wireless LAN (WLAN) with a minimal level of security and privacy comparable to a typical wired LAN. WEP encrypts data transmitted over the WLAN to protect the vulnerable wireless connection between users (clients) and access points (APs). WEP is weak and fundamentally flawed.
EAP in Wireless Technology In addition to these standards, wireless security also involves additional authentication protocol known as Extensible Authentication Protocol (EAP).
Extensible Authentication Protocol, or EAP, is a universal authentication framework frequently used in wireless networks and Point-to-Point connections. It is defined by RFC 3748. Although the EAP protocol is not limited to wireless LAN networks and can be used for wired LAN authentication, it is most often used in wireless LAN networks. Commonly used modern methods capable of operating in wireless networks include EAP-TLS, EAP-SIM, EAP-AKA, PEAP, LEAP and EAP-TTLS.
IEEE 802.16 / WiMax
editIEEE 802.22
editWith the continuous move to the digital, it becomes not only possible to compress signals but to take full advantage of a channel's capability. Tests of the IEEE 802.22 as a solution to make use of spare radio spectrum that become available with the move to Digital Terrestrial TV (DTV), including the so called White Space that exists between each DTV data channel, that is left free due to the possibility of interference have been going on for some time in the EU the move to digital TV is expected to be concluded by 2012. The possibility to utilize this unused spectrum would permit to deploy Internet coverage in even remote locations at very attractive prices.
Summary
edit802.11 dominates the field of Wireless LANs. The IEEE 802.11 committee came up with various standards which use different technology and achieve variable speeds. Its physical layer allows five different transmission modes which include infrared, spread spectrum and multi channel FDM system.
Wireless LANs have their own problem and solution. The biggest one is caused by hidden stations. To deal with this problem 802.11 supports two model of operation, the first one is called as DSF (Distributed Coordination Function) and the other PCF (Point Coordination Function). When DSF is employed 802.11 uses CSMA/CA. Distributed DCF and centralized PCF can also co–exist using InterFrame Spacing.
The 802.11 AP service include two types of services distribution services which include association, disassociation and reassociation and Intracell services which include different functions such as authentication, deauthentication, privacy, and data deliver.
Wireless Security plays an important role in current wireless technology. One should not overlook the features involved in wireless networks. The standards such as WPA, WEP, EAP, TKIP are the fundamentals of wireless security now.
Questions
editQ: What are IEEE 802.11a, 802.11b and 802.11g?
A: IEEE 802.11a, 802.11b and 802.11g are industry standard specifications issued by the Institute of Electrical and Electronic Engineers (IEEE). These specifications define the proper operation of Wireless Local Area Networks (WLANs). 802.11a—an extension to 802.11 that applies to wireless LANs and provides up to 54 Mbit/s in the 5 GHz band. 802.11a uses an orthogonal frequency division multiplexing encoding scheme rather than FHSS or DSSS. 802.11b—an extension to 802.11 that applies to wireless LANS and provides 11 Mbit/s transmission (with a fallback to 5.5, 2 and 1 Mbit/s) in the 2.4 GHz band. 802.11b uses only DSSS. 802.11b was a 1999 ratification to the original 802.11 standard, allowing wireless functionality comparable to Ethernet. 802.11g—applies to wireless LANs and provides 20+ Mbps in the 2.4 GHz band.
Q: When do we need an Access Point?
A: Access points are required for operating in infrastructure mode, but not for ad-hoc connections. A wireless network only requires an access point when connecting notebook or desktop computers to a wired network. If you are not connecting to a wired network, there are still some important advantages to using an access point to connect wireless clients. First, a single access point can nearly double the range of your wireless LAN compared to a simple ad hoc network. Second, the wireless access point acts as a traffic controller, directing all data on the network, allowing wireless clients to run at maximum speed.
Q: How many simultaneous users can a single access point support?
A: There are two limiting factors to how many simultaneous users a single access point can support. First, some access point manufacturers place a limit on the number of users that can simultaneously connect to their products. Second, the amount of data traffic encountered (heavy downloads and uploads vs. light) can be a practical limit on how many simultaneous users can successfully utilize a single access point. Installing multiple access points can overcome both of these limitations.
Q: Why do 802.11a WLANS operate in the 5 GHz frequency range?
A: This frequency is called the UNII (Unlicensed National Information Infrastructure) band. Like the 2.4 GHz ISM band used by 802.11b and 802.11g products, this range has been set aside by regulatory agencies for unlicensed use by a variety of products. A major difference between the 2.4 GHz and 5 GHz bands is that fewer consumer products operate in the 5 GHz band. This reduces the chances of problems due to RF interference.
