Linux.com Article DB: The Commands Remain the Same: BlueTooth Technology and Linux

Understanding Frequency Hopping

Imagine data hopping around you in bits and pieces. That's precisely the way frequency hopping works. It takes the data signal and modulates it with a carrier signal that hops from frequency to frequency as a function of time over a wide band of frequencies.

It is the repeated switching of frequencies during radio transmission to minimize the effectiveness of "electronic warfare" - that is, the unauthorized interception or jamming of telecommunication technologies that helps make Bluetooth effective. Frequency hopping also is known as frequency- hopping code division multiple access (FH-CDMA).

How it Takes Place

Whenever a connection is first established between 2 (or more) units (in a piconet), the Master-unit establishes a frequency-hopping scheme, which is communicated to the other units. This frequency selection scheme consists of two parts:

selecting a sequence;
mapping this sequence onto the hop frequencies.

The mapping from the input to a particular hop frequency is performed in the selection box.

Basically, the input is the native clock and the current address. In the CONNECTION state, the native clock is modified by an offset equal to the master clock. Only the 27 MSBs of the clock are used. In the page and inquiry substates, all 28 bits of the clock are used. However, in the PAGE substate the native clock will be modified to the master?s estimate of the paged unit.

In a general FH-CDMA system, a transmitter "hops" between available frequencies according to a specified algorithm, which can be either random or pre-planned. The transmitter operates in synchronization with a receiver, which remains tuned to the same center frequency as the transmitter. A short burst of data is transmitted on a narrowband. Then, the transmitter tunes to another frequency and transmits again. The receiver thus is capable of hopping its frequency over a given bandwidth several times a second, transmitting on one frequency for a certain period of time, then hopping to another frequency and transmitting again. Frequency hopping requires a much wider bandwidth than is needed to transmit the same information using only one carrier frequency.

The Reduction of Interference

The frequency hopping technique reduces interference because an interfering signal from a narrowband system will only affect the spread spectrum signal if both are transmitting at the same frequency at the same time. Thus, the total sum of frequencies will have a very low interference presence, resulting in little or no bit errors. A frequency hopping radio, for example, will hop the carrier frequency over the 2.4 GHz frequency band between 2.4 GHzs and 2.483 GHz.This is important because Bluetooth has been designed to operate in noisy radio frequency environments.

Common BlueTooth Usage Models

When Bluetooth developers decide to put the technology into every day life, they followed a simple procedure: find an idea and develop models and protocols to illustrate them. This way the researchers hope that when manufacturers start developing products they won't have to start from scratch but will have an existing model to work on and add functionality too.

File Transfer: This model allows file transfer between two Bluetooth enabled devices. Based on the GOEP (Generic Object Exchange Profile) protocols and procedures used by applications in handling object exchanges.

Internet Bridge: This model describes the use of AT commands and how to connect a mobile phone or a cordless modem to the internet via PC without the need of a DUN (Dial Up Networking) connection.

Lan Access: Based on the above model, the LAN Access usage model simply removes the AT commands and uses a LAN access point for a wireless connection to a LAN.

Synchronization: It allows various Bluetooth devices such as PDA's, Laptop's, Desktop PC's, Mobile Phone's to synchronize with one another using a common protocol and format.

Three in One Phone: The model explains how a simple phone can work as a cordless telephone being charged at a fixed pulse rate, how it can function as a walkie-talkie communicating with other similar phones and also act as a cell phone connected to the cellular network when moving around.

Ultimate Headset: This final fantasy device, though currently in the making, certainly adopts some of the existing features of Captain John Lu Picard's USS Enterprise Star Ship. This Bluetooth equipped device, probably a mobile phone, can act as a remote unit's audio input and output interface. It shall use AT commands to control the mobile phone and another stack to transfer speech for audio I/O.

Originally Published: Tuesday, 16 October 2001	Author: Devang Shah and Anurag Phadke
Published to: enhance_articles_hardware/Hardware News	Page: 3/6 - [Printable]
The Commands Remain the Same: BlueTooth Technology and Linux Linux.com is pleased to publish this well researched look at the emerging field of wireless networking, focusing on Bluetooth technology and Linux. Learn all about Bluetooth and even modifying the kernel (when needed) to support it.

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Understanding Frequency Hopping Imagine data hopping around you in bits and pieces. That's precisely the way frequency hopping works. It takes the data signal and modulates it with a carrier signal that hops from frequency to frequency as a function of time over a wide band of frequencies. It is the repeated switching of frequencies during radio transmission to minimize the effectiveness of "electronic warfare" - that is, the unauthorized interception or jamming of telecommunication technologies that helps make Bluetooth effective. Frequency hopping also is known as frequency- hopping code division multiple access (FH-CDMA). How it Takes Place Whenever a connection is first established between 2 (or more) units (in a piconet), the Master-unit establishes a frequency-hopping scheme, which is communicated to the other units. This frequency selection scheme consists of two parts: selecting a sequence; mapping this sequence onto the hop frequencies. The mapping from the input to a particular hop frequency is performed in the selection box. Basically, the input is the native clock and the current address. In the CONNECTION state, the native clock is modified by an offset equal to the master clock. Only the 27 MSBs of the clock are used. In the page and inquiry substates, all 28 bits of the clock are used. However, in the PAGE substate the native clock will be modified to the master?s estimate of the paged unit. In a general FH-CDMA system, a transmitter "hops" between available frequencies according to a specified algorithm, which can be either random or pre-planned. The transmitter operates in synchronization with a receiver, which remains tuned to the same center frequency as the transmitter. A short burst of data is transmitted on a narrowband. Then, the transmitter tunes to another frequency and transmits again. The receiver thus is capable of hopping its frequency over a given bandwidth several times a second, transmitting on one frequency for a certain period of time, then hopping to another frequency and transmitting again. Frequency hopping requires a much wider bandwidth than is needed to transmit the same information using only one carrier frequency. The Reduction of Interference The frequency hopping technique reduces interference because an interfering signal from a narrowband system will only affect the spread spectrum signal if both are transmitting at the same frequency at the same time. Thus, the total sum of frequencies will have a very low interference presence, resulting in little or no bit errors. A frequency hopping radio, for example, will hop the carrier frequency over the 2.4 GHz frequency band between 2.4 GHzs and 2.483 GHz.This is important because Bluetooth has been designed to operate in noisy radio frequency environments. Common BlueTooth Usage Models When Bluetooth developers decide to put the technology into every day life, they followed a simple procedure: find an idea and develop models and protocols to illustrate them. This way the researchers hope that when manufacturers start developing products they won't have to start from scratch but will have an existing model to work on and add functionality too. File Transfer: This model allows file transfer between two Bluetooth enabled devices. Based on the GOEP (Generic Object Exchange Profile) protocols and procedures used by applications in handling object exchanges. Internet Bridge: This model describes the use of AT commands and how to connect a mobile phone or a cordless modem to the internet via PC without the need of a DUN (Dial Up Networking) connection. Lan Access: Based on the above model, the LAN Access usage model simply removes the AT commands and uses a LAN access point for a wireless connection to a LAN. Synchronization: It allows various Bluetooth devices such as PDA's, Laptop's, Desktop PC's, Mobile Phone's to synchronize with one another using a common protocol and format. Three in One Phone: The model explains how a simple phone can work as a cordless telephone being charged at a fixed pulse rate, how it can function as a walkie-talkie communicating with other similar phones and also act as a cell phone connected to the cellular network when moving around. Ultimate Headset: This final fantasy device, though currently in the making, certainly adopts some of the existing features of Captain John Lu Picard's USS Enterprise Star Ship. This Bluetooth equipped device, probably a mobile phone, can act as a remote unit's audio input and output interface. It shall use AT commands to control the mobile phone and another stack to transfer speech for audio I/O.
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