Notebooks may have an IrDA port, a Bluetooth adapter, and a Wi-Fi interface.
IrDA port common, but not very convenient to use. When using it, you need to place the "eyes" of the infrared ports located on both connected devices in the line of sight and at a small distance from each other (no more than 10 cm, no matter what the manufacturers claim), and also ensure their almost complete immobility during the entire communication session. Even a small shift of ports, as a rule, leads to a connection break. Therefore, it is practically impossible to use an IrDA connection, for example, in transport. In addition, even when both connected devices are stationary relative to each other, the naughty infrared connection can break for no apparent reason.
More recently, IrDA was the most common wireless interface. Such a port was present in most laptops, in all pocket computers that allow stand-alone use, printers, and in most mobile phones. The last one is the most important because cellular telephone is the most common means of accessing the Internet from a laptop. The data transfer rate through the infrared port reaches up to 115.2 Kbps.
The laptop may have two infrared ports: one for establishing communication with other digital devices, and the other for the remote remote control(Fig. 4.5). The "eye" of the port intended for the remote control is usually located on the front end of the laptop, but it can also be external (in this case, it is "fastened" to the USB port). The remote control (in common parlance - "sloth") is relevant when using a laptop as a player for audio and video files. For other purposes, the IrDA port for the console cannot be used: it will not provide communication with digital devices.
Rice. 4.5. You can connect a remote control to your laptop for easy presentations
Bluetooth- a device that transmits data at speeds up to 722 Kbps, no doubt, is a serious competitor to IrDA.
The use of a radio channel to provide a wireless connection does not require the placement of the connected devices in the line of sight. For example, you can connect to the phone without taking the device out of the case, print on a printer located in the far corner of the room, etc. Moreover, the radio connection is more stable than that established through the infrared port. In addition, Bluetooth has been successfully used to create personal access points. More and more popular are models in which the modem - cable or ADSL - uses a Bluetooth connection to communicate with a laptop. At first glance, this solution looks overly sophisticated, but upon closer examination it turns out to be very convenient. Agree, it is silly to have a laptop, the mobility of which, even in your apartment, is limited by wired connections.
Wireless interface WiFi, also known as IEEE 802.11, RadioEthernet, or in Apple terminology, AirPort Extreme, used for wireless LAN access. There are many IEEE 802.11 standards. The data transfer rate through the most common of them - IEEE 802.11a - is 54 Mbps. Appropriate solutions appeared quite a long time ago, but were mainly used in corporate networks and only relatively recently became available to the mass user.
Today, the word hotspot is probably known to everyone. This is the name of a public area with Wi-Fi coverage, that is, a place where you can come with your laptop and connect to local network resources (usually the Internet, but other options are possible). Access can be free, paid or provided under certain conditions (for example, restaurant visitors ordering food and drinks). Today in the West, such points exist in all major hotels, train stations, airports and other places where mobile users are concentrated: in many cafes, restaurants, Internet cafes, libraries, business centers (see sites www.jiwire.com, www. wifinder.com, www.totalhotspots.com, etc.). Zones with Wi-Fi coverage (both paid and free) are becoming more widespread in Russia as well. Sites with data on the location of such points in different cities (for example, www.freewifi.ru, http://wifi.yandex.ru or http://wifi.ru/) are becoming one of the most popular categories of Internet resources. One glance at their list is enough to understand that Wi-Fi connection is not a European or metropolitan “thing”, since an access point can be found in a more or less large city in any country. This means that the presence of an appropriate adapter in a laptop with which you plan to move not only within your own apartment or office is an urgent need.
Note
Wi‑Fi's rise in popularity is in no small part due to Intel's policies. The corporation is actively promoting this method of wireless access to Internet resources and is promoting Centrino technology, an integral part of which is a Wi-Fi adapter. As a result, Wi-Fi adapters are much more common in laptops than Bluetooth modules.
Most laptops manufactured today have built-in Wi‑Fi‑adapters. However, if you don’t have one in your laptop, don’t worry: you can purchase an external Wi-Fi adapter for almost any laptop that connects to a USB port or is made in the form of a PC card.
Connectors and ports
All modern laptops are equipped with USB ports, to which you can connect almost all modern peripheral devices. USB interface 2.0 provides data transfer rates up to 60 Mbps and is backwards compatible with USB 1.1. This tricky term means that USB ports 2.0, you can connect devices that support USB 1.1, and these devices will work correctly, although the data exchange rate will not exceed 12 Mbps (that is, it will be the one provided by the "younger" version of the standard).
It is good form to equip a laptop FireWire ports(The official name of the interface is IEEE 1394, it is also known as i.Link). This interface is not necessary, but may be convenient when connecting peripheral devices with which intensive data exchange is carried out: digital video cameras, memory card readers (Card-Reader), external drives(both CD and DVD drives, and based on hard drive), digital cameras with large sensors, etc. Data transfer rate via FireWire is up to 400 Mbps.
From ports of old formats - LPT, COM And PS/2(they are called legacy - inherited) - laptop manufacturers are gradually refusing. This is correct, since fewer people are working, for example, with printers connected via LPT and with mice using a COM interface. Thus, these ports are practically not used anymore, and the user has to carry additional cargo with him. Let it be only a few tens of grams, and yet ...
The exception is the PS/2 port. Its presence in the laptop is still relevant. First, USB keyboards are slightly more expensive than PS/2 keyboards. Secondly, it is still used a large number of mice connected through this interface, and any user will prefer to work with a familiar manipulator.
All laptops are equipped VGA connector, allowing you to connect an external monitor or projector to computers (Fig. 4.6).
Rice. 4.6. Some laptops (usually models designed for professional use) allow you to connect two external monitors at once
Some manufacturers equip their laptops proprietary interfaces. For example, some ThinkPad systems (formerly from IBM and now from Lenovo) have a proprietary UltraPort connector that allows you to connect an IR module, Bluetooth module, PC camera, and some other devices to the system. Other companies also have their own interface standards. For example, Asus laptops were equipped with a proprietary Ai-Box interface that allows you to connect disk drives. However, the range of peripheral devices connected to proprietary interfaces is not numerous, they are not widely used and are quite expensive, and therefore are used extremely rarely.
When choosing a laptop, pay attention to the relative position of the ports (Fig. 4.7). If their connectors are located close to each other, then it will be inconvenient to work: connecting one external device can practically block access to neighboring ports. As practice shows, there is no benefit from interfaces whose connectors are located one above the other: when a device is connected to one of them, the second one is inaccessible.
Rice. 4.7. The mutual arrangement of ports significantly affects the usability
Advice
The laptop must have more USB ports; legacy ports are redundant, FireWire is rarely used (however, if you have a DV camcorder, you need such an interface), and proprietary ports are generally irrelevant.
fax modem
Modems for telephone lines are built into all modern laptops. We have not been able to find a model on the market that does not have an integrated 56 Kbps modem.
Notebook modems are no different from each other, the data transfer speed achieved when using Russian telephone networks about the same.
Of course, modems installed in laptops can send and receive faxes, but today this form of communication is considered obsolete and is rapidly being replaced. email. However, faxes continue to be used to send documents, images, greetings, etc., so the fax component of the laptop communication subsystem seems to be relevant for quite some time.
It should also be noted that the usual connection via a modem is beginning to be actively replaced by ADSL and satellite technologies. Many users may not need the modem built into the laptop.
Network adapter
An adapter for connecting to a local network is present in any laptop. In most cases, this is 10/100 Ethernet, but today there are laptop computers equipped with Ethernet cards that support 1 Gb / s connections. Significant differences for the user between different network adapters there are no laptops.
Keyboard
A comfortable keyboard is essential for comfortable work on a laptop! However, users often forget about this when choosing laptop computer attention to anything but the keyboard.
It is impossible to assess the convenience of the I / O system by eye, so before purchasing a laptop, you should type at least a short text on its keyboard to understand whether you are comfortable working. Don't expect any special comfort when you first get acquainted with an unusual keyboard, but there shouldn't be any pronounced irritation from touching the keys with your fingers. If the keyboard of the laptop you are planning to buy annoys you, it is better to try to find another model - the keyboard in mobile computer is built-in and cannot be replaced!
Attention!
The laptop keyboard should not bend under the fingers when hitting the keys! A portable computer with this feature should be discarded.
Additional tricks that improve the ergonomics of the keyboard are almost never used in laptops. Unless Acer sometimes has slightly curved rows of keys on its models, but this does not change the situation much.
The laptop keyboard usually has additional keys (Fig. 4.8). Often they perform hard-coded functions that are limited to launching certain applications - a browser, an email system, a program to establish communication with a provider, etc. In some laptops, additional keys can be programmed for actions other than those set by default.
Rice. 4.8. The laptop keyboard usually has additional keys that can be configured to launch the most commonly used applications.
Lecture 13. Wireless interfaces of peripheral devices
1. Infrared IrDA interface
2.
radio interfaceBluetooth
1. Infrared IrDA interface
Wireless (wireless) interfaces allow you to free devices from interface cables connecting them, which is especially attractive for small-sized peripherals, commensurate in size and weight with cables. Wireless interfaces use infrared (IrDA) and radio frequency (Blue Tooth) electromagnetic waves. In addition to these interfaces of peripheral devices, there are also wireless methods for connecting to local networks.
The use of emitters and receivers in the infrared (IR) range makes it possible to carry out wireless communication between a pair of devices separated by several meters. Infrared - IR (InfraRed) Connection - safe for health, does not cause interference in the radio frequency range and ensures the confidentiality of the transmission. IR rays do not pass through walls, so the reception area is limited to a small, easily controlled area. Infrared technology is attractive for communicating laptops with stationary computers or PU. Some models of printers have an infrared interface, they are equipped with many modern small-sized devices: PDAs, mobile phones, digital cameras, etc.
There are infrared systems:
Low (up to 115.2 Kbps)
Medium (1.152 Mbps)
High (4 Mbps) speed.
Low-speed systems are used for the exchange of short messages.
High-speed - for exchanging files between computers, connecting to computer network, output to a printer, projector, etc. Higher bit rates are expected, which will allow the transfer of "live video".
In 1993, an association of developers of infrared data transmission systems was established. IrDA (Infrared Data Association), designed to ensure compatibility of equipment from different manufacturers. The current standard IrDA 1.1 along with which there are also Hewlett Packard's own systems - HP-SIR(Hewlett Packard Slow Infra Red) And Sharp - ASKIR (Amplitude Shifted Keyed IR). These interfaces provide the following transfer rates:
IrDA SIR (Serial Infra Red), HP-SIR - 9.6-115.2 Kbps;
· in IrDA HDLC, also known as IrDA MIR (Middle Infra Red) - 0.576 and 1.152 Mbps;
IrDA FIR (Fast Infra .Red) - 4 Mbps;
ASKIR - 9.6-57.6 Kbps.
The emitter for IR communication is an LED having a peak power spectral characteristic of 880 nm. The LED gives a cone of effective radiation with an angle of about 30°. As a receiver, PIN diodes are used, which effectively receive IR rays in a cone of 15 °. The IrDA specification defines the requirements for transmitter power and receiver sensitivity, with both minimum and maximum IR beam power specified for the receiver. The receiver will not “see” pulses of too low power, and too much power “blinds” the receiver - the received pulses will merge into an indistinguishable signal.
In addition to the useful signal, the receiver is affected by interference: exposure to sunlight or incandescent lamps, which gives a constant component of optical power, and interference from fluorescent lamps, giving a variable (but low-frequency) component. These noises have to be filtered. The IrDA specification provides a bit error rate (BER - Bit Error Ratio) of no more than 10 9 at a distance of up to 1 m and daylight (illuminance - up to 10 klux).
Specification IrDA defines a multi-level system of protocols, which we will consider from the bottom up.
The following are possible options IrDA on physical level.
· IrDA SIR - for speeds of 2.4-115.2 Kbps, a standard asynchronous transmission mode is used (as in COM ports): start bit (zero), 8 data bits and stop bit (single). The zero value of a bit is encoded by a pulse with a duration of 3/16 bit interval (1.63 μs at a rate of 115.2 Kbps), a unit value is encoded by the absence of pulses (mode IrDA SIR-A). Thus, in the pause between sendings, the transmitter does not shine, and each sending begins with a start-bit pulse. Specification 1.1 provides for another mode - IrDA SIR-B, with a fixed pulse duration of 1.63 µs for all these speeds.
· ASK IR - for speeds of 9.6-57.6 Kbps, the asynchronous mode is also used, but the coding is different: the zero bit is encoded by sending pulses with a frequency of 500 kHz, and the single bit is encoded by the absence of pulses.
· IrDA HDLC- for speeds of 0.576 and 1.152 Mbps, a synchronous transmission mode and coding similar to SIR, but with a pulse duration of 1/4 bit-interval, is used. The frame format corresponds to the HDLC protocol, the beginning and end of the frame are marked with flags 01111110, inside the frame this bit sequence is excluded using bit stuffing. For validity control, the frame contains a 16-bit CRC-code.
· IrDA FIR (IrDA4PPM)- For 4 Mbps, the synchronous mode is also applied, but the encoding is somewhat more complicated. Here, each pair of adjacent bits is encoded with a position-pulse code: 00 - 1000, 01 - 0100, 10 - 0010, 11 - 0001 (in quadruples of characters, one means sending an impulse in the corresponding quarter of a two-bit interval). This coding method allows you to halve the frequency of switching on the LED compared to the previous one. The constancy of the average frequency of the received pulses facilitates adaptation to the level of external illumination. To improve reliability, a 32-bit CRC code is used.
Above the physical level access protocol IrLAP (IrDA Infrared Link Access Protocol)- modification of the HDLC protocol, reflecting the needs of IR communication. It converts data into frames and prevents device conflicts; if there are more than two devices "seeing" each other, one of them is assigned as primary, and the rest - as secondary. Communication is always half duplex. IrLAP describes the procedure for establishing, numbering, and closing connections. The connection is established at a speed of 9600 bps, after which the exchange rate is negotiated to the maximum of those available to both (9.6, 19.2, 38.4, 57.6 or 115.2 Kbps) and logical channels are established (each channel controlled by one master).
Above IrLAP located connection control protocol IrLMP (IrDA Infrared Link Management Protocol). With its help, the device informs others about its presence in the coverage area (device configuration IrDA can change dynamically: to change it, it is enough to bring a new device or take it away). Protocol IrLMP allows you to discover the services provided by a device, inspect data streams, and act as a multiplexer for configurations with many available devices. Applications can use IrLMP to find out if the device they want is in range. However, this protocol does not provide guaranteed data delivery.
transport layer provided by the protocol Tiny TP (IrDA Transport Protocols)- virtual channels between devices are served here, errors are processed (lost packets, data errors, etc.), data is packed into packets and initial data is assembled from packets (the protocol resembles TCP). The protocol can also work at the transport layer IrTP.
Protocol IrCOMM allows you to emulate a conventional wired connection via IR communication:
· 3-wire over RS-232C (TXD, RXD and GND);
· 9-wire via RS-232C (the whole set of COM port signals);
· Centronics (parallel interface emulation).
Protocol IrLAN provides access to local networks; it allows the transmission of network frames ethernetAndtoken ring. IR LAN connection requires a device provider with an interface IrDA, connected in the usual (wired) way to the local network, and the corresponding software support in the client device (which must enter the network).
Object Exchange Protocol IrOBEX (Object Exchange Protocol)- a simple protocol that defines the PUT and GET commands for the exchange of "useful" binary data between devices. This protocol is located above the protocol TinyTR. At the protocol IrOBEX there is an extension for mobile communications that defines the transmission of information related to GSM networks (notebook, calendar, call control, digital transmission voices, etc.), between your phone and computers of different sizes (from desktop to PDA).
These protocols do not exhaust the entire list of protocols related to IR communications. Note that for remote control of household appliances (TVs, VCRs, etc.) the same 880-nm range is used, but different frequencies and physical encoding methods.
Transceiver IrDA can be connected to the computer in various ways; towards system unit it can be either internal (placed on the front panel) or external, placed anywhere. The transceiver should be placed taking into account the angle of "view" (30° at the transmitter and 15° at the receiver) and the distance to the required device (up to 1 m).
Internal transceivers at speeds up to 115.2 Kbps (IrDA SIR, HP-SIR, ASK-IR) are connected via conventional UART chips compatible with 16450/16550 through relatively simple modulator-demodulator circuits. In a number of modern motherboards the COM2 port can be configured to use infrared (up to 115.2 Kbps). To do this, in addition to the UART, the chipset contains modulator and demodulator circuits that provide one or more infrared communication protocols. To use the COM2 port for infrared communication, CMOS Setup you need to select the appropriate mode (infrared prohibited means normal use COM2). There are internal adapters in the form of expansion cards (for tires ISA, PCI, PC Card), for the system they look like additional COM- ports.
At medium and high exchange rates, specialized controller chips are used IrDA, focused on intensive program exchange (PIO) or DMA, with direct bus control. Here, the conventional UART chip is unsuitable because it does not support synchronous mode and high speed. Controller IrDA FIR performed in the form of an expansion card or integrated into the system board; as a rule, such a controller also supports modes SIR.
The transceiver is connected to the connector IR Connector system board, directly (if it is installed on the front panel of the computer) or through an intermediate connector (mini-DIN) located on the blank bracket on the back of the case. Unfortunately, there is no single circuit layout on the internal connector, and for greater flexibility, the transceiver (or intermediate connector) is supplied with a cable with separate connector pins.
For application use IrDA in addition to the physical connection of the adapter and the transceiver, installation and configuration of the appropriate drivers is required.
In Windows 9x/ME/2000 controller IrDA enters the "Network Neighborhood". The configured software allows
Establish a connection to the local network (to access the Internet, use network resources);
Transfer files between a pair of computers;
Print data;
Synchronize data of PDA, mobile phone and desktop computer;
Upload captured images from the camera to a computer and perform a number of other useful actions without worrying about any cable management.
2. Radio interfaceBluetooth
Bluetooth (blue tooth) is the de facto standard for miniaturized, inexpensive means of transmitting information to short distances via radio communication between mobile (and desktop) computers, mobile phones and any other portable devices.
The specification is developed by a group of leading companies in the fields of telecommunications, computers and networks - 3Com, Agere Systems, Ericsson, IBM, Intel, Microsoft, Motorola, Nokia, Toshiba. This group, which formed the Bluetooth Special Interest Group, brought the technology to market. The Bluetooth specification is freely available on the Web (www.blueto6th.com), although it is quite voluminous (about 15 MB PDF files). The openness of the specification should contribute to its rapid dissemination, which is already observed in practice. Here we allow ourselves to shorten the name of the technology to "BT" (this is an unofficial abbreviation). The name itself originates from the nickname of the Danish king who united Denmark and Norway - this is a hint at the universal unifying role of technology.
Each BT device has a radio transmitter and receiver operating in the 2.4 GHz frequency band. This range in most countries is reserved for industrial, scientific and medical equipment and does not require licensing, which ensures the ubiquitous applicability of devices. For VT, radio channels with discrete (binary) frequency modulation are used, the carrier frequency of the channels is F = 2402 + k (MHz), where k is 0,...,78. For several countries (for example, France, where the military operates in this range), an abbreviated version with F = 2454 + k (k = 0,...,22) is possible. Encoding is simple - a positive frequency deviation corresponds to a logical unit, a negative frequency deviation corresponds to zero. Transmitters can be of three power classes, with a maximum power of 1, 2.5 and 100. MW, and it must be possible to reduce the power in order to save energy.
The transmission is carried out with carrier frequency hopping from one radio channel to another, which helps in the fight against interference and signal fading. The physical communication channel is represented by a certain pseudo-random sequence of used radio channels (79 or 23 possible frequencies).
A group of devices sharing the same channel (that is, knowing the same hopping sequence) forms the so-called piconet (piconet), which can include from 2 to 8 devices.
Each piconet has one master and up to 7 active slaves. In addition, “parked” slave devices can be within the coverage area of the master device in its own piconet: they also “know” the sequence of hops and are synchronized (by hops) with the master, but cannot exchange data until the master allows them to activity. Each active piconet slave has its own temporary number (1-7); when the slave device is deactivated (parked), it gives its number to others. Upon subsequent activation, it may already receive a different number (that's why it is temporary).
Piconets can overlap with coverage areas, forming a "scattered" network (scatternet). At the same time, there is only one master in each piconet, but slave devices can be included in several piconets through time sharing (part of the time the device works in one piconet, part in another). Moreover, the master of one piconet can be the slave of another piconet. These piconets are not synchronized in any way, each of them uses its own channel (sequence of hops).
The channel is divided into time slots with a duration of 625 µs, the slots are sequentially numbered with a cycle of 2 27 . Each timeslot corresponds to one carrier frequency in a hop sequence (1600 hops per second). The frequency sequence is determined by the address of the piconet master device. Transmissions are carried out in packets, each packet can take from 1 to 5 time slots. If the packet is long, then it is all transmitted at the same carrier frequency, but the count of slots of 625 µs continues, and after a long packet, the next frequency will correspond to the next slot number (that is, several hops are skipped). The master and slaves transmit alternately: in even slots, the master transmits, and in odd slots, the slave device addressed to it (if it has something to “say”).
Two types of physical connections can be established between the master and slave devices: synchronous and asynchronous.
Synchronous communications (they are isochronous) with connection establishment, SCO link (Synchronous Connection-Oriented), are used to transmit an isochronous graph (for example, digitized audio). These point-to-point links are pre-established by the master with selected slaves, and each link is given a period (in slots) after which slots are reserved for it. The connections are symmetrical two-way. There are no retransmissions of packets in case of receive errors. The master can establish up to three SCO links with the same or different slaves. A slave can have up to three links to one master, or one SCO link to two different masters. According to the network classification, SCO communications are classified as circuit switching.
Asynchronous communications connectionless , ACL link (Asynchronous Connection-Less), implement packet switching point-to-multipoint between the master and all slaves in the piconet. The master can communicate with any of the piconet slaves in slots not occupied by SCO by sending it a packet and requesting a response.
5.12 Wireless interfaces
5.12.1 IrDA infrared interface
The use of emitters and receivers in the infrared (IR) range allows for wireless communication between a pair of devices remote at a distance of up to several meters. Infrared -IR ( infra Red) connection - safe for health, does not cause interference in the radio frequency range and ensures the confidentiality of the transmission. IR rays do not pass through walls, so the reception area is limited to a small, easily controlled area. Some models of printers have an infrared interface; they are equipped with many modern small-sized devices: pocket computers ( PDA ), mobile phones, digital cameras, etc.
There are infrared systems of low (up to 115.2 Kbps), medium (1.152 Mbps) and high (4 Mbps) speeds. In 1993, an association of developers of infrared data transmission systems was created.IrDA(Infrared Data Association ). The current standard IrDA 1.1.
The emitter for IR communication is an LED having a peak power spectral characteristic of 880 nm. The LED gives a cone of effective radiation with an angle of about 30°. As a receiver, PIN diodes are used, which effectively receive IR rays in a cone of 15 °.
The options available at the IrDA physical layer are listed below:
1. IrDA SIR - for speeds of 2.4-115.2 Kbps, a standard asynchronous transmission mode is used (as in COM ports).
2. ASK IR- Forspeeds of 9.6-57.6 Kbps, asynchronous mode is also used.
3. IrDA HDLC - for speeds of 0.576 and 1.152 Mbps, synchronous transmission mode and protocol-like coding are used SIR.
4. IrDA FIR- For 4 Mbps, synchronous mode is also applied.
The IrDA transceiver can be connected to a computer in a variety of ways; in relation to the system unit, it can be either internal (placed on the front panel) or external, placed in an arbitrary place. The transceiver should be placed taking into account the angle of "view" (30° at the transmitter and 15° at the receiver) and the distance to the required device (up to 1 m).
Internal transceivers at speeds up to 115.2 Kbps ( IrDA SIR , ASK IR ) are connected through conventional microcircuits of asynchronous transceivers UART compatible with 16450/16550 through relatively simple IR modulator/demodulator circuits. In a number of modern motherboards, the COM2 port can be configured to use infrared communication (up to 115.2 Kbps).
Exist external IR adapters with interface RS-232C (for connection to COM port) and with interface USB . Bandwidth USB even enough for FIR , The COM port is only suitable for SIR.
5.12.2 Serial infrared SIR
SIR (Serial InfraRed port) interface - serial infrared port) was developed by the association IrDA (Infrared Data Association) ). The first versions of this standard were published in 1994.
According to its properties, the interface SIR close to standard interface RS-232C . The infrared transmission rate is up to 115 Kbps (up to 4 Mbps in the latest specifications). Data exchange is asynchronous (without synchronization), that is, sequential. Algorithms for checking the checksum of data packets are used to detect and eliminate transmission errors.
A significant disadvantage infrared port is a limited range (transfer rate of 4 Mbps is achieved at a distance of about 1 m). In addition, there should be no foreign objects between the receiver and transmitter.
5.12.3 Radio interface Bluetooth
Bluetooth (blue tooth) is the de facto standard for miniature low-cost means of transmitting information via radio between mobile (and desktop) computers, mobile phones and any other portable devices over short distances.
Every device Bluetooth has a radio transmitter and receiver operating in the 2.4 GHz frequency band. For Bluetooth radio channels with discrete (binary) frequency modulation are used. Encoding is simple - a positive frequency deviation corresponds to a logical unit, a negative frequency deviation corresponds to zero. The transmitters are available in three power classes, with a maximum power of 1, 2.5 and 100 MW.
Within specification Bluetooth several protocols are defined.
Service discovery protocol SDP(Service Discovery Protocol) ) that allows the device to use the functionality of the surrounding hardware.
ProtocolRFCOMMprovides emulation of a serial port (9-wire RS -232). With its help, traditional cable connections of devices can be easily replaced with radio communication, without any modifications to upper-layer software.
Host controller interface HCI(Host Controller Interface ) is a uniform method of accessing lower-level firmware Bluetooth . It provides a set of commands for radio control, status information, and actual data transmission. Physical hardware Bluetooth can be connected to various interfaces: expansion bus (for example, PC Card ), USB bus , COM port. For each of these connections, there is a corresponding transport layer protocol. HCI - a layer that ensures the independence of HCI from the connection method.
Bluetooth (blue tooth) is the de facto standard for miniature, low-cost means of transmitting information via radio between mobile (and desktop) computers, mobile phones, and any other portable device over short distances. The specification is developed by a group of leading companies in the fields of telecommunications, computers and networks - 3Com, Agere Systems, Ericsson, IBM, Intel, Microsoft, Motorola, Nokia, Toshiba. This group, which formed the Bluetooth Special Interest Group, brought this technology To the market. The Bluetooth specification is freely available on the Web (www.bluetooth.com), although it is quite large (about 15 MB PDF files). The openness of the specification should contribute to its rapid dissemination, which is already observed in practice. Here we allow ourselves to shorten the name of the technology to "BT" (this is not an official abbreviation). The name itself is a nickname for the Danish king who united Denmark and Norway - a hint at the universal unifying role of technology.
Each BT device has a radio transmitter and receiver operating in the 2.4 GHz frequency band. This range in most countries is reserved for industrial, scientific and medical equipment and does not require licensing, which ensures the ubiquitous applicability of devices. For VT, radio channels with discrete (binary) frequency modulation are used, the carrier frequency of the channels is F = 2402 + k (MHz), where k = 0, ..., 78. For several countries (for example, France, where the military operates in this range), it is possible abbreviated version with F=2454+k (k=0,..., 22). Coding is simple - a positive frequency deviation corresponds to a logical unit, a negative one corresponds to zero. Transmitters can be of three power classes, with a maximum power of 1, 2.5 and 100 MW, and it must be possible to reduce the power in order to save energy.
The transmission is carried out with carrier frequency hopping from one radio channel to another, which helps in the fight against interference and signal fading. Physical channel communication is represented by a certain pseudo-random sequence of used radio channels (79 or 23 possible frequencies). A group of devices sharing one channel (that is, "knowing" the same sequence of hops) forms a so-called piconet, which can include from 2 to 8 devices. Each piconet has one master and up to 7 active slaves. In addition, “parked” slave devices can be within the coverage area of the master device in its own piconet: they also “know” the sequence of hops and are synchronized (by hops) with the master device, but cannot exchange data until the master device allow their activity. Each active piconet slave has its own temporary number (1-7); when the slave device is deactivated (parked), it gives its number to be used by others. With subsequent activation, it may already receive a different number (that's why it is temporary). Piconets can overlap with coverage areas, forming a "scattered" network (scatternet). At the same time, there is only one master device in each piconet, but slave devices can be included in several piconets using time sharing (part of the time it works in one, part in another piconet. Moreover, the master device of one piconet can be a slave device of another piconet. These piconets are not synchronized in any way, each of them uses its own channel (sequence of hops).
The channel is divided into time slots with a duration of 625 μs, the slots are sequentially numbered with a cycle of 2". Each time slot corresponds to one frequency, the carrier in the sequence of hops (1600 hops per second). The frequency sequence is determined by the address of the piconet master. Transmissions are carried out in packets, each a packet can take from 1 to 5 time slots.If the packet is long, then it is all transmitted on the same carrier frequency, but the count of slots of 625 µs continues, and after a long packet, the next frequency will correspond to the next slot number (that is, several hops will be skipped The master and slave devices transmit alternately: in even slots, the master device transmits, and in odd slots, the slave device addressed to it (if it has something to “say”).
Two types of physical connections can be established between master and slave devices: synchronous and asynchronous.
Synchronous communications(they are also isochronous) with a connection, SCO link (Synchronous Connection-Oriented), are used to transmit isochronous traffic (for example, digitized audio). These point-to-point links are pre-established by the master with selected slaves, and each link is defined for a period (in slots) after which slots are reserved for it. The connections are symmetrical two-way. Packet retransmissions in case of receive errors are not used. The master can establish up to three SCO links with the same or different slaves. A slave can have up to three links to one master, or one SCO link to two different masters. According to the network classification, SCO communications are classified as circuit switching.
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Asynchronous communications connectionless, ACLlink (Asynchronous Connection-Less), implement packet switching point-to-multipoint between the master and all slaves in the piconet. The master can communicate with any of the piconet slaves in slots not occupied by SCO by sending it a packet and requesting a response. The slave device has the right to transmit only after receiving a request from the master device addressed to it (having correctly decoded its address). For most types of packets, retransmission is provided if a reception error is detected. The master device can also send unaddressed broadcast packets to all slave devices in its piconet. The master can only establish one ACL connection with each of its slaves.
Information is transmitted in packets, in which the data field can be 0-2745 bits long. For ACL links there are several types of packets with CRC code protection (if an error is detected, retransmission is provided) and 1 unprotected (no retransmissions). For SCO connections the data is not protected by a CRC code and therefore retransmissions on a receive error are not provided.
Protection of data from distortion and reliability control is carried out in several ways. The data of some types of packets is protected by a CRC code, and the receiver of the information must acknowledge receipt of the correct packet or report a reception error. Redundant coding FEC (Forward Error Correction code) is used to reduce the number of repetitions. In the EEC 1/3 scheme, each useful bit is transmitted three times, which allows you to choose the most plausible option by majorization. The FEC 2/3 scheme is somewhat more complex, using a Hamming code, which allows you to correct all single errors and detect all double errors in each 10-bit block.
Every voice channel provides a speed of 64 Kbps in both directions. The channel can be encoded in PCM (pulse code modulation) or CVSD (Continuous Variable Slope Delta Modulation) format. PCM encoding allows G.711 compression; it provides only a purely "telephone" signal quality (meaning digital telephony, 8-bit samples with a frequency of 8 Kbit/s). The CVSD encoder provides more high quality- it packs the input PCM signal with a sampling rate of 64 kbps, however, even in this case, the spectral density of the signal in the frequency band 4-32 kHz should be negligible. Voice (speech) channels of the VT are unsuitable for transmitting a high-quality audio signal, however, a compressed signal (for example, an MP3 stream) can be completely transmitted over an asynchronous data transmission channel.
Asynchronous channel can provide a maximum speed of 723.2 Kbps in an asymmetric configuration (leaving return channel bandwidth 57.6 Kbps) or 433.9 Kbps each way in a symmetrical configuration.
To ensure security in BT, it is used data authentication and encryption at the link layer, which, of course, can be supplemented by means of upper protocol layers.
An important part of BT is SDP service discovery protocol(Service Discovery Protocol), which allows the device to find an "interesting interlocutor". In the future, by establishing a connection with it, the device will be able to use the required services (for example, print documents, connect to the Web, etc.).
RFCOM protocol provides serial port emulation (9-wire RS-232) via L2CAP. With its help, traditional cable connections of devices (including null-modem ones) can be easily replaced by radio communication, without any modifications to upper-level software. The protocol allows for multiple connections (one device to several), and radio communication will replace bulky and expensive multiplexers and cables. Through the RFCOMM protocol, the OBEX protocol used in infrared wireless connections (in the IrDA protocol hierarchy) can work. The PPP protocol, over which the protocols of the TCP / IP stack stand above, can also work through RFCOMM - this opens the way to all applications for the Internet. AT commands that control telephone connections and fax services also work through RFCOMM (the same commands are used in modems for dial-up lines).
Special bit-oriented telephone protocol TCS BIN (Telephony Control protocol - Binary), which defines call signaling for communication between BT devices (voice communication and data exchange), also works via L2CAP. The protocol also provides means for managing groups of TCS devices.
HCI Coimmeter Host Interface(Host Controller Interface) is a uniform method of accessing the hardware and software of the lower levels of the BT. It provides a set of commands for radio control, status information, and actual data transmission. Through this interface, the L2CAP protocol interacts with the BT equipment. Physically, VT equipment can be connected to various interfaces: expansion bus (for example, PC Card), USB bus, CQM port. For each of these connections, there is a corresponding HCI transport layer protocol, a layer that makes HCI independent of the connection method.
