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Wireless network interfaces. Local networks are closed networks, access to which is allowed to a limited number of users

Lecture 6. Connection topologies. Network adapters and hubs.

Basic descriptive characteristics of the computer (processor, amount of RAM and external memory, multimedia and network capabilities, peripheral and other components)

Early approaches to database organization. Systems based on inverted lists, hierarchical and network DBMSs. Examples. Strengths and weaknesses of early systems

The advantage of a wireless network is obvious: the user is not tied to the place where the network cable is connected, but can move freely within the range of the wireless signal.

However, do not forget that wireless networks have a lower data transfer speed compared to cable networks. The throughput of wireless networks is tens and hundreds of megabits/sec, while cable network based on twisted pair, the speed is two orders of magnitude higher: tens of gigabits/sec. When working with large amounts of data, this can be important.

Wi-Fi is a set of IEEE 802.11 standards for wireless networks. Any equipment that meets IEEE requirements may be eligible to display the Wi-Fi logo after being tested by the Wi-Fi Alliance. Wi-Fi Alliance is an organization that tests and certifies wireless network devices.

When created, the term Wi-Fi stood for Wireless Fidelity, which translates to “wireless precision” or “ wireless quality" But then the Wi-Fi Alliance decided to remove this interpretation, and now the term Wi-Fi is not deciphered in any way. Wi-Fi is Wi-Fi and nothing more.

Wi-Fi connection was first created in 1991, and the original 802.11 standard was published in 1997. To date, several standards have been developed on its basis, the characteristics of which are given in the table.

Table. Wi-Fi standards.

Nowadays, most devices support the 802.11n standard, although there are still many devices running on older standards. Here, as elsewhere, the principle of top-down compatibility is observed: a standard created later supports the operation of network adapters for earlier versions. However, modern operating systems (for example, Windows XP, 7, 8) may not release drivers that support outdated network adapters. In the future, obviously, adapters and drivers for 802.11ac will be released and there will be a gradual transition to this standard.

Wi-Fi can operate at two frequencies, one of which (2.4 GHz) is the same as Bluetooth. If a device simultaneously operates in both technologies at the same frequency, then it constantly switches from one technology to another, which leads to a slight loss of time.

Obviously, Wi-Fi is not capable of independently transmitting a signal over any significant distances. A Wi-Fi base station (or access point) is limited to one building, part of a building, and its immediate surroundings. And the signal is supplied to the access point in other ways, usually using cable technology.

operating room Windows system 7 has an intuitive, well-thought-out interface for connecting to all common types of wireless networks. Having it at hand, the Windows 7 user in the vast majority of cases does not have any compelling reason to use the command line for control. Nevertheless, such a possibility exists, and knowledge of it is at least useful for general development. Let's look at how you can manage wireless networks using a standard utility netsh.

Wireless network interfaces

To connect to a wireless network, we need an appropriate interface associated with the wireless network card. The list of available interfaces can be found with the following command:
netsh wlan show interface

The team reports that I have only one wireless interface on my computer, and its name is “Wireless Network Connection 3”.

List of WiFi networks

Find out which WiFi networks available, you can command
netsh wlan show networks
For example, in the following screenshot you can see that my neighbor distributes “feeds” of the Internet to everyone who connects to him:

Connecting to a WiFi network

To connect to a WiFi network use the command
netsh wlan connect name=NetworkProfileName

About profiles - just below.

You can specify a specific interface through which to connect. The syntax is:
netsh wlan connect name=NetworkProfileName interface=InterfaceName
For me it would look like this:
netsh wlan connect name=TRENDnet interface="Wireless Network Connection 3"

Disconnecting from a WiFi network

To disconnect from the WiFi network, you need to run the following command
netsh wlan disconnect
Or specify a specific interface
netsh wlan disconnect interface=InterfaceName

WiFi network profiles

WiFi network profiles are one of the key players in the “game” with wireless networks. The profile stores all the information necessary for a successful installation wireless connection, including authentication method and passwords. A profile is created when you successfully connect from a wireless network. With help netsh you can view all available profiles:
netsh wlan show profile

And, in fact, connect to the network with the selected profile:
netsh wlan connect ssid=NetworkName name=NetworkProfileName

The Netsh utility allows you to perform export to XML file and import of wireless network profiles, export command syntax:
netsh wlan export profile name=ProfileName folder=Path:\To\Folder\ForStorage\XML-files
You can also specify the wireless interface to which the profile corresponds.
netsh wlan export profile name=ProfileName folder=Path:\To\Folder\ForStorage\XML-files interface=InterfaceName
The export command has an option that allows you to place the network connection key in clear, unencrypted form. If this is required, you must supplement the command with the key=clear option:
netsh wlan export profile name=ProfileName folder=Path:\To\Folder\ForStorage\XML-files key=clear

For import profile from XML file using a command like:
netsh wlan add profile filename="D:\profiles\Wireless Network Connection 3-TRENDnet.xml"

Automatic creation of a script for connecting to a WiFi network

The Netsh utility allows you to display the script used to connect to a WiFi network. The command for this is
netsh wlan dump
By redirecting the output to text file, you can use it later to connect to the network, for example, on another computer:
netsh wlan dump > d:\script.txt
The resulting script can be specified to the Netsh utility:
netsh exec d:\script.txt
The Netsh utility is a powerful network configuration tool, and its capabilities are far from limited to the techniques described above. You can find out full list Netsh options by running it with a command like:
netsh?
You can get all the commands related specifically to WiFi management using a command like
netsh wlan?

Practice: creating a home network wirelessly

Long gone are the days when having a personal computer at home was an extraordinary event, and grandmothers near the entrance whispered about the owner of this “overseas miracle.” Today, it can be considered quite normal to have several computers at home (ideally for each family member), especially since the role of “personal computers” is often played by laptops, PDAs and other mobile gadgets. A characteristic feature of this computer park is the need to periodically (or constantly) connect them to each other. Information exchange, data synchronization, access to the Internet, joint games - this is an incomplete list of reasons that push the user, in the end, to create home network.

Several years ago, the solution to this problem was quite clear - a wired local network. However, it is at least unreasonable to consider that such an option is acceptable today. Wireless technologies have gradually turned from a transcendental dream into an objective reality and provide an excellent chance to create a modern and convenient home network that is a paradise for home occupants and guests. A wireless home offers many benefits over a traditional wired network, so today we'll look at the basic principles of creating a wireless home network. Despite the fact that the main method of creating a network is Wi-Fi, we will still pay a little attention to Bluetooth, since in some cases creating a network based on this protocol is a completely suitable option.

The option is slow and not always convenient - BlueTooth

Generally speaking, Bluetooth is not a protocol for implementing a wireless network; it is intended for connecting various devices with each other, since it has a low data transfer rate (just over 700 Kbps) and a short range. As for the latter, it is regulated by the corresponding device class: Class 3 - 10 meters, Class 2 - 20 meters and Class 1 - 100 meters. Note that the distance is calculated without barriers to the propagation path, so the ideal option for an apartment or office where there are several interior partitions is to use first-class devices. The main disadvantage of this protocol in the network version is the low bandwidth, but if you do not download gigabytes of information every day, Bluetooth can serve very well.

BlueTooth modem and special module

To implement the network, each device must have a Bluetooth module. Built-in options are quite rare, so most often you have to turn to external modules. For a desktop computer or laptop this will be an adapter connected via USB (the so-called USB Dongle), for a pocket computer - a card of the appropriate form factor (Compact Flash, Secure Digital, etc.). Network participants can also include other devices that support this protocol, for example, Cell phones, digital cameras, etc. By the way, there are also Bluetooth adapters equipped with a parallel interface; they are most often used for wirelessly connecting printers, but they can also find other uses.

Connecting an external Bluetooth module usually does not cause problems, although some troubles may await the user during the software installation process. Most of the products we came across were equipped with software from Widcomm, and the installation process requires a fair amount of patience and endurance. Interestingly, the user does not influence the installation process in any way (except that you can choose the location where to install the files), you only need to agree with the messages displayed. The result of the program will be the appearance on the desktop of the Bluetooth environment icon (My Bluetooth Places) and the appearance huge amount virtual ports, which, nevertheless, work as if they were quite real.

List of services available when working with a BlueTooth device

Perhaps many will find it interesting to connect Bluetooth devices to an already existing local network. This process is very simple, and is one of the ways to expand an existing LAN. Essentially, a Bluetooth hotspot is installed and connected to the wired network. The ability to serve up to seven Bluetooth devices with one access point at a time (using PPP over Bluetooth) is a good opportunity to extend the life of an already established network.

Option fast and promising - WiFi

However, using Bluetooth to create a wireless network is only suitable in rare cases (for example, if the speed of the network is not important to you) and does not fully implement the task. Wi-Fi (or IEEE 802.11) is a technology that is designed not only to replace the existing wired network, but also to significantly improve its characteristics. Bandwidth up to 54 Mbit/s, sharing of files and network resources, shared Internet connection (all computers in the house can use one common broadband access channel), a minimum of settings, etc. have ensured the enormous success of this technology.

WiFi routers differ in design, but not much

The first and mandatory attribute of home organization Wi-Fi networks is an access point that is responsible for creating a “zone” of a wireless network (by analogy with a wired network, it can be considered a network hub or hub). Usually it is a small plastic box with one or two short antennas. On the front panel there are several LEDs indicating the status of the access point, and on the back there are several connectors. This is a socket for connecting a power supply, a standard RJ-45 connector for connecting network cable 10/100BaseT, sometimes there may be a serial port connector that allows you to fine-tune the access point (in our case it will not be needed). There is usually a “Reset” button nearby, which can be useful in case of any problems with the device or the need to change the operating configuration. Some access points have an additional connector for connecting an external antenna, which makes sense when organizing a relatively long network (for example, between several houses).

Let's make a small digression regarding version selection Wi-Fi standard. Of the three currently existing options (a,b,g), preference should be given to 802.11g, which provides the highest speed, operates in the standard 2.4 Hz range and is backward compatible with 802.11b (in this case, this participant has a speed will be lower than that of faster modules on the network). As for 802.11a, in some situations this version may be the only solution. The 2.4 GHz range is “pseudo-free”, and moving to 5 GHz implies the presence of a special permit in our country (although here the speed is higher and the distance that information travels is greater).

Best Selling Wireless Access Points

	Model	Price
1	D-Link DWL-2000AP+	91 $
2	Pheecom WAP-154G	77 $
3	Cisco 1231G	140 $
4	Linksys WAP54G	259 $
5	Pheecom W-118C+	103 $

Source: ZOOM.CNews based on store data

The ideal place to install an access point, oddly enough, is the ceiling, which provides the greatest range of the network (mounting holes are usually provided on the case). Setting up a network usually does not cause problems and is similar to setting up any network equipment. A traditional interface, the main parameter of which is the IP address of the access point, which must be specified in the properties of any browser. Now any device with a Wi-Fi module (built-in version, PCMCIA card for a laptop, CF/SD card for a pocket computer, USB module for a computer, etc.) can access the wireless network.

However, this is the simplest option and the least valuable. In most cases, you want to provide wireless Internet access to multiple devices, as long as your computer is connected to world wide web via cable modem or ADSL. Of course, this problem can be solved by installing on a computer with Internet access a program that distributes network resources (for example, the well-known WinGate). This option is quite workable, but such a load on the computer actually turns it into a network server, which is not always desirable. Therefore, our next object is a router.

An ADSL router is distinguished by the presence of a corresponding connector

A router (sometimes called a gateway) is precisely the dedicated device that is capable of distributing high-speed cable or ADSL Internet access between all devices on the wireless network. The advantages of using it are ease of setup and ample capabilities; the only disadvantages include the relatively high cost. The set of connectors is similar to that of the access point, the main difference is three (or more) RJ-45 connectors. In the vast majority of cases, a WiFi router includes the following devices:

Hub (in the photo – for 4 ports);
The router itself (smart router, which, for example, allows you to “distribute” rights and mac addresses);
ADSL modem or port for connecting to a leased line;
WiFi access point;
FireWall (this doesn’t always happen, but lately finding a device without it has become a problem).

Now about its capabilities. Firstly, you can use the router as a regular network switch, but this will be of little use. More interesting is the installation of address translation mode, which allows all members of the home network to access the Internet without special problems. You can forget about the painful setup of proxy servers and simply specify the router address on any computer. If desired (or necessary), Internet access for home network users can be configured in the most intricate way (denying specific devices from certain sites or services, port forwarding, sharing Internet access, organizing a DHCP server, IP sharing, firewall, VPN pass-through etc.).

The built-in hub will make life much easier for those who have more than one PC

As for the router settings, all the latest models of these devices use an HTML interface for these purposes. After making all the connections and typing the IP address of the router in the browser, the user gains access to all settings. To ensure access to the Internet, simply enter the data provided to you by your provider. All setup information is usually described in detail in the user manual (paper or electronic version); by the way, the login and password for the initial login are also located there.

Web interface for managing the router

Basically, that's all that's needed to create home Wi-Fi network, the only thing that remains behind the scenes is its security. This issue is quite serious (you don’t want “left-wing” users to access the Internet “through you”) and deserves a separate discussion. In general terms, the use of WEP encryption, which is a built-in Wi-Fi feature, is not only desirable, but also mandatory.

Best Selling Wireless Routers

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Future-proof wireless LAN interfaces

Introduction

1.1 General concepts
1.4 Access points

2.1 802.11 standard
2.4 Wi-Fi
2.5 HiperLAN/2

Conclusion
Bibliography

Introduction

For several decades now, people have been using computer networks to provide communication between staff, computers and servers in offices, large companies, and educational institutions. Recently, there has been a trend towards increasing use of wireless networks.

Wireless networks have been around us for many years. Thus, primitive forms of wireless communication include the smoke signals of the American Indians, when they threw buffalo skins into the fire to transmit some kind of message over a long distance. Or the use of intermittent light signals to transmit information via Morse code between ships, this method was and remains an important form of communication in navigation. And, of course, so popular now Cell Phones, allowing people to communicate across vast distances can also be classified as wireless communications.

Today, the use of wireless networks allows people to "extend" their workplace and get a number of benefits as a result. During business trips you can, for example, send emails waiting to board a plane at the airport. Homeowners can easily share an Internet connection across many PCs and laptops without running cables.

Thus, the topic of this work is undoubtedly relevant.

The subject of the research is technologies for constructing local networks, the object of research is wireless interfaces of local networks.

The purpose of the work is to study promising wireless interfaces of local networks. To achieve this goal, it is necessary to solve the following tasks:

Learn the basic aspects of building wireless local networks

Research technologies used to build wireless local networks.

The works of domestic and foreign authors, reference literature, periodical press materials, and information from specialized Internet resources are used as methodological support.

1. Basic aspects of building wireless local networks

1.1 General concepts

A local area network is usually called a network that has a closed infrastructure before reaching the service provider. This can be a small office network, consisting of several computers located in several offices, and a network of a large plant, which occupies an area of several hectares. There are local networks (orbital centers, space stations), the nodes of which are separated from each other at distances of more than 10,000 km.

Local networks are closed networks, access to which is allowed to a limited number of users.

In a local network, computers are connected to each other through various access media, such as copper or optical conductors, radio channels.

Wired communication in the local network is provided by Ethernet technology, wireless - BlueTooth, Wi-Fi, GPRS, etc. To ensure communication between computers on a local network, various models of equipment that support the corresponding technologies are used. In this case, the connection point between the user’s computer and the local network is called a network interface or local network interface.

In general, an interface is a certain set of rules, methods and tools that provide the conditions for interaction between the elements of a system.

Currently, there is a trend towards increasing use of wireless networks. Indeed, wireless interfaces are now available that allow network services, email, and Web browsing no matter where the user is located.

There are many types of wireless communications, but the most important feature of wireless networks is that communication occurs between computer devices. These include personal digital assistants(personal digital assistance, PDA), laptops, personal computers (PCs), servers and printers. Computer devices are those that have processors, memory, and means of interacting with some kind of network. Cell phones are not typically classified as computer devices, but latest phones and even headsets (headphones) already have certain computing capabilities and network adapters. The trend is that soon most electronic devices will be able to connect to wireless networks.

Wireless networks use radio waves or infrared (IR) as a transmission medium to enable interaction between users, servers, and databases. This transmission medium is invisible to humans. In addition, the actual transmission medium (air) is transparent to the user. Many manufacturers now integrate network interface cards (NICs), called network adapters, and antennas into computer devices so that they are not visible to the user. This makes wireless devices mobile and easy to use.

Wireless local area networks provide high performance data transmission inside and outside offices, industrial premises and buildings. Users of such networks typically use laptops, PCs and PDAs with large screens and processors capable of running resource-intensive applications. These networks fully satisfy the requirements for connection parameters for computer devices of this type.

Wireless LANs easily provide the features needed to run high-level applications smoothly. Thus, users of these networks can receive large attachments in email messages or streaming video from the server.

These networks are similar in characteristics, components, cost, and operations to traditional wired Ethernet-type LANs.

Because wireless LAN adapters are already built into most laptops, many public wireless network providers have begun offering wireless LANs to provide mobile broadband Internet access.

Users of some public wireless networks in hot zones, such as airports or hotels, can send and receive email or access the Internet for a fee (unless the establishment provides free access). The rapid growth of public wireless networks is making the Internet accessible to users in crowded areas.

The predominant standard for wireless local area networks is IEEE 802.11, various versions of which regulate data transmission in the 2.4 and 5 GHz bands. The main problem with this standard is that it does not adequately ensure interoperability between devices that comply with its different versions. For example, 802.11a WLAN computer device adapters do not provide connections to 802.11b computer devices. There are other unresolved issues with the 802.11 standard, such as insufficient security.

In order to somehow resolve the problems associated with the use of 802.11 devices, the Wi-Fi Alliance organization has combined all its compatible functions into a single standard called Wireless Fidelity (Wi-Fi). If a wireless LAN device is Wi-Fi compliant, it is virtually guaranteed to work with other Wi-Fi compliant devices. The openness of the Wi-Fi standard allows different users using different platforms to work on the same wireless LAN, which is extremely important for public wireless LANs.

1.2 Features of the wireless network structure

The structure (or architecture) of a network defines the protocols and components needed to meet the requirements of the applications running on it. One popular standard that can be used to consider network structure is the Open System Interconnection (OSI) reference model developed by the International Standards Organization (ISO). The OSI model covers all network functions, grouping them into so-called layers, the tasks of which are performed by various network components (Figure 1.1). The OSI reference model is also useful when considering different standards and interoperability of wireless networks.

The OSI layers provide the following network functions.

Level 7 is the application level. Ensures user communication and operation of basic communication services (file transfer, Email). Examples of software running at this layer are Simple Mail Transfer Protocol (SMTP), Hypertext Transfer Protocol (HTTP), and File Transfer Protocol (FTP).

Level 6 is the data presentation level. Regulates the data transfer syntax for the application layer and, if necessary, converts data formats. For example, this layer can transform code representing data to enable communications between remote systems from different manufacturers.

Figure 1.1 Layers of the OSI Reference Model

Level 5 is the session level. Establishes, manages, and terminates sessions between applications. Intermediate software and access controllers provide this form of communication via a wireless network. If the wireless network is disrupted by interference, the session layer's job is to suspend communication until the interference level is reduced to an acceptable level.

Level 4 is the transport layer. Provides mechanisms for creating, maintaining, and properly terminating virtual circuits without requiring higher layers to worry about network implementation details. In general, these circuits are connections established between applications running at different ends of the communication circuit (for example, between a laptop's Web browser and a server's Web page). At this level, for example, the Transmission Control Protocol (TCP) operates.

Layer 3 -- network layer. Provides routing of packets as they travel from sender to recipient. A routing mechanism that ensures that packets are sent in a direction leading to a specified destination. Works at this level Internet protocol(Internet Protocol, IP).

Level 2 is the link level. Provides access to the environment, as well as synchronization between network objects and error control. In wireless networks, this layer also coordinates access to the shared medium and retransmits in case of errors in the transmission of data from the sender to the recipient. Most types of wireless networks use a common method of performing functions at the data link layer, regardless of the actual transmission media used.

Level 1 is the physical level. Ensures the actual transmission of information through the medium. The physical level includes radio waves and infrared radiation.

By combining layers, network structures provide the necessary functions, but wireless networks directly use only the lower layers of the above model. For example, the network interface card performs the functions of the data link and physical layers. Other components, such as wireless network middleware, provide session layer-specific functionality. In some cases, adding a wireless network may only affect the lower layers, but to ensure efficient work applications, if the wireless network characteristics deteriorate, do not forget about higher levels.

Each layer of the OSI model provides the needs of the higher layer.

Thus, TCP, running at the transport layer, establishes connections with applications running on a remote host, without considering how lower layers provide synchronization and signaling.

As Figure 1.1 shows, the protocols at each layer interact through the network with the layer of the corresponding rank. However, the actual data transfer occurs at the physical layer. As a result, this structure enables a layering process in which a particular layer inserts its protocol information into frames residing in the frames of lower layers. The frame sent at the physical layer actually contains frames from all upper layers.

At the destination, each layer forwards the corresponding frames to all higher layers, ensuring that protocols operate on layers of the same rank.

1.3 Wireless LAN Interfaces

Wireless networks use the same components as wired networks, but wireless networks must be able to convert information into a form suitable for transmission over the air (medium). Although the wireless network directly includes only a part of the entire network infrastructure, the degradation of the entire network is undoubtedly caused by the degradation caused by the use of the wireless transmission medium.

Wireless networks include computer devices, base stations, and wireless infrastructure.

A network interface card, or network interface card, provides the interface between a computer device and a wireless network infrastructure. It is installed inside the computer device, but external network adapters are also used, which, after being turned on, remain outside the computer device.

Wireless standards define how the network interface card should function. For example, a card that complies with the IEEE 802.11b standard will only be able to communicate with a wireless network whose infrastructure complies with the same standard. Therefore, users must be careful to ensure that the card they choose matches the type of wireless network infrastructure they wish to access.

The main component of a wireless local area network is the radio network interface card, often implemented based on the 802.11 standard. These radio cards usually operate on the same physical layer - 802.11a or 802.11b/g. As a consequence, the radio card must implement a wireless LAN compatible version of the standard. Wireless LAN radio cards that implement multiple versions of this standard and therefore provide greater interoperability are becoming increasingly common.

A wireless network interface card is also characterized by a form factor that defines the physical and electrical parameters of the bus interface that allows the card to interact with a computer device.

Radio cards are available in various form factors: ISA, PCI, PC card, miniPCI and CF. PCs usually use ISA and PCI cards, while PDAs and laptops use PCcard, mini-PCI and CF adapters.

Industry-Standard Architecture (ISA)

Industry-Standard Architecture (ISA) - an architecture that complies with an industry standard. The ISA bus has been widely used since the early 80s. Although its characteristics were very low, almost all PC manufacturers until recently installed at least one connector for the ISA bus. But its performance could not improve as quickly as that of other computer components, and high-speed alternatives to this bus are now available. The ISA bus did not have a major impact on the performance of 802.lib wireless LANs. You should not buy new ISA cards as they are already outdated.

Peripheral Component Interconnect (PCI).

Today the local connection bus peripheral devices-- the most popular interface for PCs because it has high performance. PCI was originally developed and released by Intel in 1993, and this bus still meets the needs of the latest multimedia computers. PCI cards were the first to implement plug-and-play technology, making it much easier to install a network interface card into a computer. PCI circuit solutions can recognize compatible PCI cards and start working with operating system computer to configure each board. This saves time and avoids mistakes when installing boards by inexperienced users.

PC Card

PC Card design boards were developed in the early 90s by the International Memory Card Manufacturers Association for personal computers IBM PC (Personal Computer Memory Card International Association, PCMCIA). The PC Card is a device the size of credit card containing external memory, modems, devices for connecting to external devices, and also providing wireless network compatibility for small computing devices such as laptops and PDAs. The most widespread and even more popular than ISA or PCI bus cards, as they are used in laptops and PDAs, the number of which is growing rapidly. You can also use a PC Card in a desktop PC by using an adapter that converts a PC Card into a PCI card, i.e. one network interface card for two computers. You can take a PC Card on a business trip or to work and use it on your desktop PC in the office.

Mini-PCI.

A mini-PCI card is a smaller version of a standard desktop PCI card and is suitable for installation in small mobile computing devices. It provides almost the same capabilities as a regular PCI card, but is approximately four times smaller in size. A mini-PCI board can be installed in laptops (optional, at the request of the buyer). A serious advantage of this type of board (using a radio channel) is that it leaves a free slot for installing a PC Card, into which you can insert a memory expansion card or graphics accelerator. In addition, the cost of a wireless network interface card based on mini-PCI technology is generally lower. However, these boards also have disadvantages. To replace them, as a rule, you have to disassemble the laptop, which can void the manufacturer's warranty. Using a mini-PCI card can also result in reduced performance since they offload some (if not all) of the processing to the computer.

CompactFlash.

CompactFlash (CF) technology was first introduced by SanDisk in 1994, but wireless network interface cards in the CF form factor were not produced until recently. The CF card is small, weighing 15g (half an ounce) and half as thin as a PC Card. Its volume is four times less than that of a PC Card type radio card. It features low power consumption, so the batteries last much longer than when using devices with a PC Card.

The most common adapters for wireless LANs have the PC Card Type II form factor. For connection to a PC, they are equipped with either a 16-bit PCMCIA host interface, which can be compared to the old ISA computer bus, or a 32-bit CardBus host interface, which is similar to PCI buses. For normal operation of an 11-Mbps 802.11b adapter, the throughput of a 16-bit interface is sufficient, but faster 802.11a and 802.11b cards must have a CardBus interface - many laptops are equipped with it. Don't assume that just because a mobile computing device is new, it necessarily has a CardBus slot. For example, the PC Card expansion unit for the popular HP iPaq PDAs only supports 16-bit PCMCIA cards.

Most of the recently released laptops come with a built-in 32-bit mini-PCI host interface. Typically, the mini-PCI slot is located under a cover on the bottom of the laptop. Very often, mini-PCI wireless network adapters are pre-installed by manufacturers on their machines. If your laptop does not have such an adapter, you can buy and install it yourself.

A desktop PC connects to a wireless LAN using either a wireless PCI network adapter or a wireless USB interface. Installing a PCI adapter requires some skill, and it's worth noting that if system unit If the PC is located under the table, then the antenna of this adapter is also there - you must agree, this is not the best place for it from the point of view of ensuring reliable radio communications. Wireless interface USB is much more convenient to install, and it can also be placed so that nothing interferes with the reception and transmission of radio signals. However, if this interface is used, there may be a slight reduction in data transfer speed compared to that of a PCI adapter.

1.4 Access points

Communication between individual wireless network user devices and the network interface card is provided using an access point.

The access point system software allows the wireless LAN parts and the access point distribution system to interact. This software differentiates access points based on manageability, installation, and security features.

In most cases, the access point provides an HTTP interface that allows you to change its configuration using a user device equipped with a network interface and a Web browser. Some access points also have an RS-232 serial interface, so they can be configured via a serial cable or a user device that emulates a terminal and runs a Telnet program (hyperterminal).

2. Wireless LAN technologies

Most often, wireless local networks are created in accordance with the 802.11 and HyperLAN/2 standards. We will consider them.

2.1 802.11 standard

The IEEE 802.11 standard describes a common Media Access Control (MAC) protocol and several physical layers of wireless local area networks. The first edition of the 802.11 standard was adopted in 1997, but then wireless local area networks were not widely used. The situation changed radically in 2001, when component prices dropped sharply. The IEEE 802.11 Working Group is actively working to improve the standard in an effort to improve the performance and security of wireless LANs. The 802.11 standard specifies the implementation of the physical layer using infrared radiation, but there are currently no products on the market that comply with this version of the standard.

2.2 802.11 Link Layer MAC Layer

The 802.11 standard describes a single MAC layer that provides many functions to support 802.11 wireless LANs. The MAC layer manages and supports communications between 802.11 stations (radio network interface cards and access points), coordinating access to the shared medium (in this case, the airwaves). Considered the "brains" of the network, the 802.11 MAC layer controls the 802.11 physical layer, such as 802.11a, 802.11b, or 802.11g, to determine whether the medium is busy or unoccupied, and transmit and receive 802.11 frames. Before transmitting a frame, the station must gain access to the medium, i.e. radio channel shared between stations. The 802.11 standard specifies two forms of media access: distributed coordination function (DCF) and point coordination function (PSF). Support for DCF mode is mandatory and is based on a protocol that provides Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). When operating in DCF mode, stations compete for access to the medium and attempt to transmit frames if no other station is transmitting at the time (Figure 2.1). If one station transmits a frame, the others wait for the channel to become free.

Figure 2.1 Distributed form of access to the environment

As a condition of media access (Figure 2.1), the MAC layer checks the value of its network allocation vector (NAV), which is a counter located at each station whose value corresponds to the time required to transmit the previous frame. The NAV value must be zero for the station to attempt to send the frame. Before sending a frame, the station calculates the time required to transmit it based on the size of the frame and the data transfer rate of the network. The station places the value corresponding to the named time in the duration field of the frame header. When a station receives a frame, it checks the value in its duration field and uses it as the basis for setting its NAV. Through this process, the medium is reserved for use by the transmitting station.

An important aspect of DCF mode is the back-off timer, which the station uses if the transmission medium becomes busy. If the channel is being used by another station, the station wishing to transmit the frame must wait for a random period of time before attempting to access the medium again. This eliminates the possibility that multiple stations intending to transmit frames will begin sending them at the same time. Due to random delay, different stations wait for the right to transmit for different periods of time, so they do not check the medium for occupancy at the same time and, upon finding that the channel is free, do not start transmitting, thereby creating a collision. The rollback timer significantly reduces the number of collisions and therefore retransmissions, especially when the number of active users is large.

When using radio-based LANs, the transmitting station cannot monitor the medium for collisions while sending data because it is unable to use its receiver while transmitting data. Therefore, the receiving station must send an acknowledgment (ACK) that it did not detect errors in the received frame.

If the transmitting station does not receive the ACK within a certain period of time, it assumes that a collision has occurred or the frame has been damaged due to radio interference, and retransmits it.

In order to support the online transmission of frames (for example, video signals), the 802.11 standard optionally offers a PCF mechanism, in which the access point guarantees a specific station access to the medium by polling the station during a contention-free period. Stations cannot transmit frames until the access point polls them for frames to transmit. Time periods for data traffic based on the PCF mechanism (if possible) occur alternately with contention periods.

The access point polls stations according to the questionnaire, then enters contention mode, in which stations use the DCF mechanism.

Thanks to this, both operating modes are supported - synchronous and asynchronous. However, there are no wireless network interface cards or access points on the market that can operate in PCF mode.

One of the problems with PCF is that few vendors support it in their products. Therefore, the capabilities provided by this mechanism are usually not available to users. However, future products will support PCF because this mechanism provides the required Quality of Service (QoS).

Let's look at the main functions performed at the MAC level of the 802.11 standard.

Scanning

The 802.11 standard regulates both scanning options - active and passive. During this process, the radio network interface card locates an access point. Passive scanning is mandatory and requires each network interface card to scan separate channels in order to detect the best signal from the access point. Access points periodically send a beacon signal in broadcast mode. The network interface radio cards receive these beacon signals and take note of the corresponding signal strength. These beacons contain information about the access point, including its service set ifentifier (SSID) and supported data rate. The radio network interface card can use this information, along with signal strength data, to compare access points and decide which one to connect to.

The optional active scan is performed in a similar manner, except that the process is initiated by the radio network interface card. It sends a broadcast probe frame, and all access points within range send it a probe response. With active scanning, the radio network interface card can immediately receive responses from access points without waiting for a beacon signal to be transmitted. However, active network scanning incurs overhead due to the transmission of probe request frames and their responses.

Stations operating in unscheduled network mode are called independent basic service set (IBSS) in the 802.11 standard. When operating in this mode, one of the stations always sends beacon signals, thereby notifying new stations about the presence of the network. The responsibility for transmitting this beacon signal rests with each station, which waits for a random amount of time for the beacon interval to complete. A station transmits a beacon signal if, after the beacon interval and some random period of time, the station does not receive a beacon signal from any other station. Thus, the responsibility for transmitting beacon signals is distributed among all stations.

Authentication

Authentication is the process by which identity is verified. The 802.11 standard specifies two forms of authentication: open system authentication and shared key authentication. An open authentication system is mandatory and is carried out in two stages. The network interface radio card initiates the authentication process by sending an authentication request frame to the access point. The access point responds with an authentication response frame containing a grant or denial of authentication, as indicated in the status code field of the frame body.

Shared key authentication is optional and occurs in four steps. The process is based on determining whether the device being authenticated has the correct WEP key." The radio network interface card begins it by sending an authentication challenge frame to the access point. The access point, placing the challenge text in the body of the response frame, sends it to the radio network interface card The radio network interface card uses its WEP key to encrypt the call text and sends it back to the access point in a different authentication frame. The access point decrypts the call text and compares it with the original. If both texts are equivalent, the access point assumes that the radio network interface card has the correct key. The access point completes the sequence of exchanges by sending an authentication frame to the radio network interface card with an allow or deny. Many hackers know how to overcome the barrier created by shared key authentications, so rely on such a security system if you need to ensure high level safety, not worth it.

Binding

Once the authentication process is complete, the radio network interface card must bind to the access point before it can send data frames.

Association is required for exchange important information between the radio network interface card and the access point, such as the supported data rates. The radio network interface card initiates the binding process by sending a binding request frame containing information such as the SSID and supported baud rate. The access point responds by sending a binding response frame containing the association identifier and other information about the access point. After the radio network interface card and the access point complete the binding process, they can transmit data frames to each other.

WEP

If optional WEP mode is available, the wireless interface card encrypts the body (but not the header) using a pre-shared key before transmitting any frame. The receiving station, having received the frame, decrypts it using a shared key. The 802.11 standard does not specify a key distribution method, which makes 802.11 wireless LANs vulnerable to eavesdropping. However, version 802. Hi of this standard increases the level of security by introducing 802.11x mechanisms and more reliable encryption into the standard.

RTS/CTS

Optional mechanisms for determining readiness to send (request to send) and readiness to receive (clear to send) allow the access point to control the process of using the transmission medium by stations that have the RTS/CTS function activated. With most radio network interface cards, users can set a maximum frame size before the radio network interface card activates RTS/CTS mode. For example, if you set the frame size to 1000 bits, RTS/CTS mode will be used for all frames over 1000 bits. By using the RTS/CTS mode, hidden node problems (when two or more radio network interface cards cannot hear each other, although they are tied to the same access point) are mitigated.

If the radio network interface card has enabled RTS/CTS mode, it sends an RTS frame to the access point before sending a data frame. The access point responds with a CTS frame, indicating that the radio network interface card can send a data frame. At the same time as sending the CTS frame, the access point offers a value for the frame header duration field, which deters other stations from transmitting so that the station that sent the RTS frame can also send its data frame. This avoids collisions caused by the hidden node problem. The RTS/CTS frame exchange accompanies the transmission of each data frame whose volume exceeds the threshold set on the corresponding radio network interface card.

2.3 Physical layers of the 802.11 standard

The multiple physical layers of 802.11 address the different network requirements of different applications.

Original 802.11

The original 802.11 standard, ratified in 1997, includes physical layers that perform Frequency Hopping Spread Spectrum (FHSS) and high-rate direct sequence spread spectrum (HR). DSSS). The data transfer rate reaches 2 Mbit/s, communication is carried out in the 2.4 GHz band." When using FHSS technology, broadband signals occupy the entire 2.4 GHz band allocated for such purposes.

Access points operating in FHSS mode can be configured with 15 different frequency hopping patterns to ensure they do not interfere with each other. Thanks to this, up to 15 access points can effectively operate in FHSS mode over water in the same area.

Because the current version of the 802.11 standard with FHSS mode provides a maximum data rate of only 2 Mbps, few companies offer FHSS-based solutions for wireless LANs intended for indoor deployments. Faster networks are now available based on the 802.11a, 802.11b and 802.11g standards. In addition, the FHSS mechanism is not capable of interoperating with other physical layers of the 802.11 standard. However, FHSS-based networks are a good solution for point-to-multipoint systems intended for outdoor deployments. This is because FHSS technology is more resistant to radio interference, which can be quite high outdoors.

802.11 DSSS systems also provide transfer rates of only 2 Mbps, but are compatible with the latest physical layer, 802.11b. Therefore, a user whose laptop has an 802.11 DSSS radio network interface card installed can interact with 802.11b access points. However, this situation is unlikely because 802.11 DSSS radio network interface cards are no longer sold.

802.11a

In late 1999, IEEE released the 802.11a standard, which regulates data transmission in the 5 GHz band using orthogonal frequency division multiplexing (OFDM) technology, providing data rates of up to 54 Mbit/s. However, products implementing this technology were not available until 2000, mainly due to difficulties encountered during development electronic circuits operating in this range.

802.11a devices operate in the 5 GHz band, providing data transfer rates of up to 54 Mbps with a range of up to 90 m, which depends on the actual data transfer rate. Access points and radio network interface cards of the 802.11a standard appeared on the market at the end of 2001, so the share of installed equipment that complies with this standard is still insignificant compared to the number of 802.11b networks. It is recommended that you carefully review the compatibility issues that may arise when deploying an 802.11a network.

An important advantage of the 802.11a standard is that it offers increased throughput through the use of 12 separate, non-overlapping channels. It is a good choice when you need to support many, concentrated users in a small area and high-performance applications such as streaming video. In addition to better performance than 802.11b systems, 802.11a networks also have higher throughput than 802.11g networks.

Another advantage of the 802.11a standard is that the 5 GHz band is not yet widely used, allowing users to achieve high performance. Most interfering devices, such as microwave ovens and cordless phones, operate in the 2.4 GHz band. Because the potential for radio interference in the 5 GHz band is lower, wireless LAN deployment is less risky.

A potential problem with 802.11a networks is their limited range, which is primarily due to their operation over a range of high frequencies(5 GHz). When operating at speeds up to 54 Mbit/s, the range in most cases is limited to 90 m. In order to ensure network operation within a given area, it is necessary to install more access points than when using 802.11b devices.

However, if you compare the performance of 802.l1b and 802.11a networks, it turns out that a user of an 802.11a network is able to transmit data at higher speeds over the same distances as a user of an 802.11b network before losing connectivity. But at the same time, a user of an 802.11b network can continue to work at a low data transfer rate - 1 or 2 Mbps - over greater distances than typical for 802.11a networks.

The undoubted difficulty is that the 802.11a and 802.11b/g standards are incompatible. Thus, a user whose computer device is equipped with an 802.11b radio card cannot bind to an access point that complies with the 802.11a standard, and vice versa. Manufacturers are solving this problem by offering multi-mode radio cards that support both 802.11a and 802.11b standards.

An 802.11a modulator converts a binary signal into analog form using different modulation methods depending on what data rate has been selected. For example, when operating at 6 Mbps, the physical layer medium dependent (PMD) uses differential binary phase shift keying (DBPSK), which shifts the phase of the center frequency of the transmission to reflect different combinations of bits. . At higher transmission rates (54 Mbps), quadrature amplitude modulation (QAM) is used. In this case, data bits are represented by changing the center frequency of the transmission, as well as changing the amplitude of the signals in addition to phase shifts.

802.11b

Along with the 802.11a standards, the IEEE has ratified the 802.11b standard, which is an extension of the original 802.11 direct sequence spread spectrum standard in the 2.4 GHz band. The transmission speed reaches 11 Mbit/s. 802.11b access points and radio network interface cards have been on the market since 1999, and a significant number of networks installed today are 802.11b compliant.

An important advantage of the 802.11b standard is that compliant devices provide a relatively long range. In most indoor applications, you can expect the range to exceed 270 m. The increased range allows you to install significantly fewer access points when deploying a wireless LAN in the same building where an 802.11a network would otherwise be installed.

The disadvantage of 802.11b is that you can only select three non-overlapping channels in the 2.4 GHz band. The 802.11 standard defines 14 channels (only channels 1 through 11 are allowed in the US) that access points can be configured to operate on, but each transmission channel takes up about a third of the entire 2.4 GHz band. Many companies use only non-overlapping channels 1, 6 and 11 to prevent access points from causing interference. This limits the overall throughput of 802.11b networks, making them only good for mid-range performance applications such as email and Web browsing.

Another disadvantage of 802.11b networks is their potential for interference from other radio devices. For example, wireless phone, operating in the 2.4 GHz band, may cause serious interference to the 802.11b wireless LAN, causing users to experience degraded performance. Microwave ovens and other devices operating in the 2.4 GHz band may also cause interference.

802.11b devices use DSSS technology to scatter the data frame signal across 2.4 GHz subchannels, each 22 MHz wide. This leads to increased noise immunity of communications compared to when signal transmission is carried out in a narrow frequency band. Therefore, the FCC allows you to not have to purchase a license to use spread spectrum devices.

The 802.11b modulator converts the spread binary signal into analog form using different modulation techniques depending on the data rate at which the data is being transmitted. For example, when operating at 1 Mbps, the PMD layer uses differential binary phase shift keying (DBPSK). The modulator simply shifts the phase of the center frequency of the transmission so that a binary 1 can be distinguished from a binary 0 in the data stream.

For 2 Mbps transmission, PMD uses differential quadrature phase shift keying (DQPSK), which is similar to DBPSK except that it uses four possible phase shifts to represent every two bits of data. Thanks to this ingenious process, it is possible to transmit a data stream at 2 Mbps while using the same bandwidth required to transmit at 1 Mbps using other modulation methods. Similar methods are used when transmitting data at higher speeds - 5.5 and 11 Mbit/s.

802.11g

The IIEE ratified the 802.11g standard in 2003. It is compatible with the 802.11b standard and specifies higher transmission rates (54 Mbps in the 2.4 GHz band).

This uses orthogonal frequency division multiplexing (OFDM).

The strength of 802.11g is that it is backward compatible with 802.11b. Companies that have already deployed 802.11b networks can generally upgrade access points to be compatible with 802.11g devices simply by upgrading the firmware. This effective method taking the company's network to a new level. But existing 802.11b client devices operating on an 802.11g network require security mechanisms that limit the performance of the WLAN as a whole. This is because 802.11b devices, due to differences in modulation methods used, cannot detect when 802.11g devices are transmitting. Therefore, both types of devices must announce their intention to use the transmission medium using a modulation type that is mutually understandable.

The disadvantages of 802.11b, such as being susceptible to potential radio interference and having only three non-overlapping channels, are also present in 802.11g networks because they operate in the same 2.4 GHz band. Therefore, 802.11g networks have limited bandwidth compared to 802.11a networks.

2.4 Wi-Fi

The Wi-Fi Alliance, which began as the Wireless Ethernet Compatibility Association or simply WECA, is an international non-profit organization dedicated to marketing and interoperability issues. 802.11 wireless LAN components. The Wi-Fi Alliance is a group that promotes the "Wi-Fi" brand, which covers all types of wireless networks that comply with the 802.11 standard (802.11a, 802.11b and 802.11g), as well as all standards of this type that will appear in the future . The Wi-Fi Alliance is also promoting Wi-Fi Protected Access (WPA), a bridge between the much-criticized WEP mechanism and the 802.11 security standard.

The Wi-Fi Alliance has the following goals:

Provide worldwide certification to encourage manufacturers to adhere to 802.11 standards when developing wireless LAN components;

Promote the sale of Wi-Fi certified products for use in homes, small offices and enterprises;

Test and certify Wi-Fi products to ensure network interoperability.

Wi-Fi certification is a process that enables wireless LAN components, such as access points and radio cards, in different form factors to interoperate. To obtain a certificate for its products, a company must become a member of the Wi-Fi Alliance.

The Alliance uses established testing programs to certify products for interoperability with other certified Wi-Fi components. Once a product has been successfully tested, the manufacturer is authorized to use the "Wi-Fi Certified" logo on each individual product, as well as on its packaging and instructions for use.

Wi-Fi certification gives customers peace of mind. that they have purchased wireless LAN components that meet the requirements for interoperability with products from many other manufacturers. The "Wi-Fi" logo on a product means that it has passed interoperability testing and is likely to work with Wi-Fi certified products from other vendors.

WEP does not provide sufficient security for most applications running on enterprise wireless LANs.

Because it uses a static key, WEP is easy to crack using existing keys. software. This encourages information technology managers to use more dynamic forms of WEP.

However, these enhanced security mechanisms are proprietary, making them difficult for client devices from other vendors to support. Therefore, the Wi-Fi Alliance has made significant efforts to effectively standardize the security of wireless LANs by defining WPA as a mechanism that enables network interoperability. When using WPA, the network environment formed by the radio network interface cards different types 802.11 standard can take advantage of advanced forms of encryption.

wireless network interface protocol

2.5 HiperLAN/2

The HiperLAN/2 standard, which stands for the high performance radio LAN standard, is a wireless LAN standard developed by the broadband radio access networks (BRAN) division of the European Telecommunications Standards Institute. (European Telecommunications Standards Institute, ETSI). This standard defines the use of efficient, high-speed wireless LAN technology that meets all European spectrum regulatory requirements.

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