Introduction :
Wireless USB is a short range, high bandwidth wireless radio communication protocol created by the wireless USB promoter Group.
WUSB is based on the wimedia version of ultra wideband.
Wireless USB is a short range, high bandwidth wireless radio communication protocol created by the wireless USB promoter Group.
WUSB is based on the wimedia version of ultra wideband.
Overview : Unwiring USB
Imagine if all the devices in a home office – such as printer, scanner, internal hard drive and digital camera could be connected to your PC without any wires. Imagine if all the components for an entire home entertainment centre could be setup and connected without a single wire Imagine if digital pictures could be transferred to a photo print Kiosk for instant printing without the need for a cable. These are just some of the possible scenarios for high speed wireless USB (WUSB) connectivity, the latest technology developed to bring even greater convenience & mobility to devices.
Universal serial bus (USB) technology has been a popular connection type for PC’s and its migrating into consumer electronics (CE) and mobile devices. Now this high – speed and effective connection interface is unwiring to provide the functionality of wired USB without the burden of cables. This next iteration of USB technology is the focus of the new wireless USB promoter group, which will define the specifications that will eventually provide standards for the technology.
Imagine if all the devices in a home office – such as printer, scanner, internal hard drive and digital camera could be connected to your PC without any wires. Imagine if all the components for an entire home entertainment centre could be setup and connected without a single wire Imagine if digital pictures could be transferred to a photo print Kiosk for instant printing without the need for a cable. These are just some of the possible scenarios for high speed wireless USB (WUSB) connectivity, the latest technology developed to bring even greater convenience & mobility to devices.
Universal serial bus (USB) technology has been a popular connection type for PC’s and its migrating into consumer electronics (CE) and mobile devices. Now this high – speed and effective connection interface is unwiring to provide the functionality of wired USB without the burden of cables. This next iteration of USB technology is the focus of the new wireless USB promoter group, which will define the specifications that will eventually provide standards for the technology.
Features of wireless – USB :
Wireless USB will build on the success of wired USB. An important goal of the WUSB promoter group is to ensure that wireless USB offers users the experience they have come to expect from wired USB.
Wireless USB standard is being designed to support the following features.
Backward Compatibility :
Wireless USB will be fully backward compatible with the one billion wired USB connections already in operation. More over, wireless USB will be compatible with current USB drivers and firmware and provide bridging from wired USB devices and hosts.
High Performance :
At launch, wireless USB will provide speeds up to 480 mbps, a performance comparable to the wired USB 2.0 standard and high enough to provide wireless transfer of rich digital multimedia formats. As UWB technology and process technologies evolve, bandwidth may exceed 1 Gbps.
Simple, Low – Cost Implementation :
Implementation will follow the wired USB connectivity mode as closely as possible to reduce development time and preserve the low – cost, case of use model that has made wired USB the interconnect of choice.
An Easy Migration Path :
To enable an easy migration path from wired USB, wireless USB will maintain the same usage models and architecture as wired USB.
Security :
Wireless USB will provide the same level of security as wired USB. All certified wireless USB devices will incorporates standard, non-removable security features. Connection – level security will be designed to ensure that devise are associated and that both hosts and devices are authenticated before operation. Higher levels of security involving encryption will be implemented at the application level. At the same time, an important goal of the specification is to ensure that security requirements do not impact the performance or cost of wireless USB applications.
Host to device Architecture :
Wireless USB will use a point to point connection to pology similar to the host to device architecture used for wired USB. For ease of use, wireless USB ill employ an asymmetric host centric model that confines complexity to the host.
The following is a brief survey of wireless protocol that address the variety of performance requirements in todays wireless infrastructure. As noted previously, extreme USB is agnostic to the physical layer upon which it is implemented, and any of these radio protocol could be integrated with an extreme USB core to provide a wireless USB solution.
Wireless USB will build on the success of wired USB. An important goal of the WUSB promoter group is to ensure that wireless USB offers users the experience they have come to expect from wired USB.
Wireless USB standard is being designed to support the following features.
Backward Compatibility :
Wireless USB will be fully backward compatible with the one billion wired USB connections already in operation. More over, wireless USB will be compatible with current USB drivers and firmware and provide bridging from wired USB devices and hosts.
High Performance :
At launch, wireless USB will provide speeds up to 480 mbps, a performance comparable to the wired USB 2.0 standard and high enough to provide wireless transfer of rich digital multimedia formats. As UWB technology and process technologies evolve, bandwidth may exceed 1 Gbps.
Simple, Low – Cost Implementation :
Implementation will follow the wired USB connectivity mode as closely as possible to reduce development time and preserve the low – cost, case of use model that has made wired USB the interconnect of choice.
An Easy Migration Path :
To enable an easy migration path from wired USB, wireless USB will maintain the same usage models and architecture as wired USB.
Security :
Wireless USB will provide the same level of security as wired USB. All certified wireless USB devices will incorporates standard, non-removable security features. Connection – level security will be designed to ensure that devise are associated and that both hosts and devices are authenticated before operation. Higher levels of security involving encryption will be implemented at the application level. At the same time, an important goal of the specification is to ensure that security requirements do not impact the performance or cost of wireless USB applications.
Host to device Architecture :
Wireless USB will use a point to point connection to pology similar to the host to device architecture used for wired USB. For ease of use, wireless USB ill employ an asymmetric host centric model that confines complexity to the host.
The following is a brief survey of wireless protocol that address the variety of performance requirements in todays wireless infrastructure. As noted previously, extreme USB is agnostic to the physical layer upon which it is implemented, and any of these radio protocol could be integrated with an extreme USB core to provide a wireless USB solution.
Flow Control :
Flow control imposed by the USB protocol will have to accommodated by the wireless dongle.
HID class devices use two USB endpoints. Control and Interrupt in the control endpoint returns data given a get – report or get – descriptor command and receives data when given a set – report command. Any low level flow control on the control endpoint is strictly a function of how long the device takes to process the current command and setup for the next.
However, the interrupt in endpoint works differently. It will NAK on read attempts until some event occurs in the device that requires it to send data to the host. A wireless HID device must be able to make this distinction in the messages that it sends to the host. That is immediate transfers that take place over the control endpoint and the vent drives transfers that take place over the interrupt in endpoint.
Wireless USB LS theory of Operation
Wireless USB LS adds to the existing Wireless USB portfolio a low cost 2.4 GHz wireless solution that enables the wireless gaming console peripheral device market and displaces the 27 MHz solutions currently used for low end retail PC Human Interface Device (HID) applications, A Wireless USB LS system typically consists of a wireless USB LS Bridge and at least one Wireless USB LS HID. The host PC is not aware of the wireless connection, since the interface to the host acts like a normal wired USB HID connection. Therefore, there is no special software required on the host PC in order to support Wireless USB LS.
Direct Sequence Spread Spectrum :
Wireless USB LS utilizes a 2.4 GHz direct sequence spread spectrum (DSSS) radio interface. DSSS generates a redundant bit pattern for each bit to be transmitted. This bit pattern is called a “chip” or a pseudo noise code. Notice in figure that the pseudo noise code is a binary signal that is produced at a much higher frequency than the data that is to be transmitted. Because it has a higher frequency, it has a large bandwidth that spreads the signal in the frequency domain (that is it spreads its spectrum). The nature of this signal makes it appear that it is random noise.
Contrast the spread wave from of a DSSS signal with the narrowband waveform of a traditional radio signal, both represented in figure. The wide bandwidth provided by the pseudo noise code allows the signal power to drop below the noise threshold without losing any information. This allows DSSS signals to operate in noisy environments and reduces the interference cause by traditional narrowband signals. The longer the chip is, the greater the probability that the original data can be recovered, and, of course, the more bandwidth required. See the section below on Gold Codes for more information on the 32 – bit and 64 – bit pseudo noise codes used in Wireless USB LS.
The receiver uses a locally generated replica pseudo noise code and a receiver correlator to separate only the desired coded information from all possible signals. A correlator can be throught of as a very special matched filter : it responds only to signals that are encoded with a pseudo noise code that matches its own code. Thus a correlator can be “tuned” to different codes simply by changing its local code. This correlator does not respond to man-made, natural, or artificial noise or interference. It responds only to signals with identical matched signal characteristics and encoded with the identical pseudo noise code. Even if one or more bits in the chip are damaged during transmission, statistical techniques embedded in the radio can recover the original data without the need for retransmission.
Flow control imposed by the USB protocol will have to accommodated by the wireless dongle.
HID class devices use two USB endpoints. Control and Interrupt in the control endpoint returns data given a get – report or get – descriptor command and receives data when given a set – report command. Any low level flow control on the control endpoint is strictly a function of how long the device takes to process the current command and setup for the next.
However, the interrupt in endpoint works differently. It will NAK on read attempts until some event occurs in the device that requires it to send data to the host. A wireless HID device must be able to make this distinction in the messages that it sends to the host. That is immediate transfers that take place over the control endpoint and the vent drives transfers that take place over the interrupt in endpoint.
Wireless USB LS theory of Operation
Wireless USB LS adds to the existing Wireless USB portfolio a low cost 2.4 GHz wireless solution that enables the wireless gaming console peripheral device market and displaces the 27 MHz solutions currently used for low end retail PC Human Interface Device (HID) applications, A Wireless USB LS system typically consists of a wireless USB LS Bridge and at least one Wireless USB LS HID. The host PC is not aware of the wireless connection, since the interface to the host acts like a normal wired USB HID connection. Therefore, there is no special software required on the host PC in order to support Wireless USB LS.
Direct Sequence Spread Spectrum :
Wireless USB LS utilizes a 2.4 GHz direct sequence spread spectrum (DSSS) radio interface. DSSS generates a redundant bit pattern for each bit to be transmitted. This bit pattern is called a “chip” or a pseudo noise code. Notice in figure that the pseudo noise code is a binary signal that is produced at a much higher frequency than the data that is to be transmitted. Because it has a higher frequency, it has a large bandwidth that spreads the signal in the frequency domain (that is it spreads its spectrum). The nature of this signal makes it appear that it is random noise.
Contrast the spread wave from of a DSSS signal with the narrowband waveform of a traditional radio signal, both represented in figure. The wide bandwidth provided by the pseudo noise code allows the signal power to drop below the noise threshold without losing any information. This allows DSSS signals to operate in noisy environments and reduces the interference cause by traditional narrowband signals. The longer the chip is, the greater the probability that the original data can be recovered, and, of course, the more bandwidth required. See the section below on Gold Codes for more information on the 32 – bit and 64 – bit pseudo noise codes used in Wireless USB LS.
The receiver uses a locally generated replica pseudo noise code and a receiver correlator to separate only the desired coded information from all possible signals. A correlator can be throught of as a very special matched filter : it responds only to signals that are encoded with a pseudo noise code that matches its own code. Thus a correlator can be “tuned” to different codes simply by changing its local code. This correlator does not respond to man-made, natural, or artificial noise or interference. It responds only to signals with identical matched signal characteristics and encoded with the identical pseudo noise code. Even if one or more bits in the chip are damaged during transmission, statistical techniques embedded in the radio can recover the original data without the need for retransmission.
Auto Correlation of USB
Auto correlation is the correlation of a variable with itself over successive time intervals, in our case the pseudo noise code. Pseudo noise codes should have a high autocorrelation factor so that the receiver’s correlator correctly matches the received pseudo noise code with its own code. For example, if “1010” was used as the pseudo noise code, the following sequence shows that the correlator would match the code in two separate time positions (1010 – 0101 – 1010 – 0101). If “1001” was used instead, the correlator would only match the code in a single time position allowing the correlator to correctly process the incoming data stream (1001-0011-0110-1100). Wireless USB LS uses pseudo noise codes that have a high auto correlation factor.
Cross Correlation of USB
Cross correlation is the statistical correlation between two different signals as a function of relative time between the signals. In other words, cross correlation measures how unique different pseudo noise codes are. If pseudo noise codes are used that have a low cross correlation factor their signals will not interfere with each other. For example, if “1001” and “1100” are used as pseudo noise codes, if “1100” is time shifted it becomes “1001” and can be incorrectly matched to by the correlator looking for “1001” and not “1100” if “1001” and “1011” are used instead, “1011” will never be incorrectly matched by the correlator looking for “1001” because these two pseudo noise codes never match no matter how they are shifted. Wireless USB LS uses pseudo noise codes that have a low cross correlation factor.
Gold Codes of USB
Wireless USB LS uses sets of Gold Codes as pseudo noise codes in order to enable multiple devices to simultaneously transmit on the same frequency. Gold Codes exhibit high auto correlation and Minimal, well defined, cross correlation levels with all other members of the set. Gold Codes are excellent pseudo noise codes for code division multiple access (CDMA) systems.
Wireless USB LS can use 32-bit or 64-bit Gold Codes. There are pros and cons to each code length. 32 bit Gold Codes allow a data rate of 32-kbps, while 64-bit Gold Codes allow a data rate of 16 kbps. On the other hand, using 64-bit Gold Codes has a greater probability of recovering the data due to the longer chip length. Analysis has shown that in order to avoid more than one false correlation in one “day” of use, the maximum number or errors allowed in a 64-bit code is ten, and for 32-bit codes it is one. Tolerance to errors can be improved through the use of error correction techniques implemented in firmware. Also there are twice as many 64 – bit Gold Codes as there are 32 – bit Gold Codes.
Frequency Division Multiple Access
Wireless USB LS not only separates transmissions by code it also separates transmissions by frequency. Wireless USB LS divides the 2.4 GHz ISM band into 79 distinct frequency channels. This allows devices to transmit distinct signals by either using a unique pseudo noise code or a unique frequency. Two signals will not interfere unless they are using the same frequency channel and the same pseudo noise code. Observe that signals A and B in figure use the same Gold Code but transmit on different frequencies, while signals C and D transmit on the same frequency but use different Gold Codes. Theoretically, hundreds of wireless USB LS devices could be operating in the same physical space at the same time.
Wireless USB LS Systems
There are two varieties of Wireless USB LS devices : transmit – only devices and transceiver devices. Transmit – only HID devices are used in 1-way Wireless USB LS networks, while transceiver HID devices are used in 2-way Wireless USB LS networks. Bridge devices always use the Wireless USB LS transceiver. The current 1-way and 2-way Wireless USB LS HID protocols are targeted at 1-to-1 and 2-to-1 networks. The protocol is executed in external Micro-controllers that interface to the Wireless USB LS Chip. Many other applications including non-HID and N:1 HID applications can easily be implemented with Wireless USB LS, but different protocols may be more applicable.
Wireless USB LS 1-way HID Networks :
Wireless USB LS 1-way networks utilize a communication protocol that emphasizes transmitter simplicity and is an ideal low cost, low power wireless solution for HID applications. Each HID device contains a Wireless USB LS transmitter while bridges contain a Wireless USB LS transceiver as shown in Figure. For more information please read Wireless USB LS 1-way HID Networks.
Wireless USB LS 2-way HID Systems :
Wireless USB LS 2-way networks contain a back channel allowing a HID to receive messages from the bridge. This back channel allows Wireless USB ls 2-way HIDs to establish a connection to the bridge, receive Ack/Nak messages from the bridge and receive Data messages from the bridge. All devices in Wireless USB LS 2-way networks contain transceivers as shown in figure. For more information please read Wireless USB LS 2-way HID networks.
Other Wireless USB LS Non-HID Systems :
Non –HID applications may benefit from customized protocols specifically designed for each network.
Non-HID networks could use a polling scheme to reduce the amount of overlapping HID transmissions. Wireless USB LS is flexible and robust enough to be used in a variety of environments including barcode scanners, point of sale (POS) terminals, TV remotes, and wireless game – pads with rumble packs.
Auto correlation is the correlation of a variable with itself over successive time intervals, in our case the pseudo noise code. Pseudo noise codes should have a high autocorrelation factor so that the receiver’s correlator correctly matches the received pseudo noise code with its own code. For example, if “1010” was used as the pseudo noise code, the following sequence shows that the correlator would match the code in two separate time positions (1010 – 0101 – 1010 – 0101). If “1001” was used instead, the correlator would only match the code in a single time position allowing the correlator to correctly process the incoming data stream (1001-0011-0110-1100). Wireless USB LS uses pseudo noise codes that have a high auto correlation factor.
Cross Correlation of USB
Cross correlation is the statistical correlation between two different signals as a function of relative time between the signals. In other words, cross correlation measures how unique different pseudo noise codes are. If pseudo noise codes are used that have a low cross correlation factor their signals will not interfere with each other. For example, if “1001” and “1100” are used as pseudo noise codes, if “1100” is time shifted it becomes “1001” and can be incorrectly matched to by the correlator looking for “1001” and not “1100” if “1001” and “1011” are used instead, “1011” will never be incorrectly matched by the correlator looking for “1001” because these two pseudo noise codes never match no matter how they are shifted. Wireless USB LS uses pseudo noise codes that have a low cross correlation factor.
Gold Codes of USB
Wireless USB LS uses sets of Gold Codes as pseudo noise codes in order to enable multiple devices to simultaneously transmit on the same frequency. Gold Codes exhibit high auto correlation and Minimal, well defined, cross correlation levels with all other members of the set. Gold Codes are excellent pseudo noise codes for code division multiple access (CDMA) systems.
Wireless USB LS can use 32-bit or 64-bit Gold Codes. There are pros and cons to each code length. 32 bit Gold Codes allow a data rate of 32-kbps, while 64-bit Gold Codes allow a data rate of 16 kbps. On the other hand, using 64-bit Gold Codes has a greater probability of recovering the data due to the longer chip length. Analysis has shown that in order to avoid more than one false correlation in one “day” of use, the maximum number or errors allowed in a 64-bit code is ten, and for 32-bit codes it is one. Tolerance to errors can be improved through the use of error correction techniques implemented in firmware. Also there are twice as many 64 – bit Gold Codes as there are 32 – bit Gold Codes.
Frequency Division Multiple Access
Wireless USB LS not only separates transmissions by code it also separates transmissions by frequency. Wireless USB LS divides the 2.4 GHz ISM band into 79 distinct frequency channels. This allows devices to transmit distinct signals by either using a unique pseudo noise code or a unique frequency. Two signals will not interfere unless they are using the same frequency channel and the same pseudo noise code. Observe that signals A and B in figure use the same Gold Code but transmit on different frequencies, while signals C and D transmit on the same frequency but use different Gold Codes. Theoretically, hundreds of wireless USB LS devices could be operating in the same physical space at the same time.
Wireless USB LS Systems
There are two varieties of Wireless USB LS devices : transmit – only devices and transceiver devices. Transmit – only HID devices are used in 1-way Wireless USB LS networks, while transceiver HID devices are used in 2-way Wireless USB LS networks. Bridge devices always use the Wireless USB LS transceiver. The current 1-way and 2-way Wireless USB LS HID protocols are targeted at 1-to-1 and 2-to-1 networks. The protocol is executed in external Micro-controllers that interface to the Wireless USB LS Chip. Many other applications including non-HID and N:1 HID applications can easily be implemented with Wireless USB LS, but different protocols may be more applicable.
Wireless USB LS 1-way HID Networks :
Wireless USB LS 1-way networks utilize a communication protocol that emphasizes transmitter simplicity and is an ideal low cost, low power wireless solution for HID applications. Each HID device contains a Wireless USB LS transmitter while bridges contain a Wireless USB LS transceiver as shown in Figure. For more information please read Wireless USB LS 1-way HID Networks.
Wireless USB LS 2-way HID Systems :
Wireless USB LS 2-way networks contain a back channel allowing a HID to receive messages from the bridge. This back channel allows Wireless USB ls 2-way HIDs to establish a connection to the bridge, receive Ack/Nak messages from the bridge and receive Data messages from the bridge. All devices in Wireless USB LS 2-way networks contain transceivers as shown in figure. For more information please read Wireless USB LS 2-way HID networks.
Other Wireless USB LS Non-HID Systems :
Non –HID applications may benefit from customized protocols specifically designed for each network.
Non-HID networks could use a polling scheme to reduce the amount of overlapping HID transmissions. Wireless USB LS is flexible and robust enough to be used in a variety of environments including barcode scanners, point of sale (POS) terminals, TV remotes, and wireless game – pads with rumble packs.
Extreme USB ® Wireless :
An industrial strength solution :
Extreme USB was developed to enable USB devices in industrial and commercial environments where operational requirements often exceed those of the desktop for which USB was designed. To achieve long reach USB connectivity, Extreme USB overcomes the limitations imposed by the Turnaround Timer. Removing this limitation enables conventional RF techniques such as error correction to be employed.
Referring back to the requirements discussed in the section on cable replacement, Extreme USB supports all three USB speeds – LS, FS and HS. Extreme USB also contains unique features that enable each of the four USB transfer types to be handled. Any particular implementation can combine support for speed and transfer type variants as required. Just like standard USB, devices with different speed and transfer type attributes can be attached to and detached from the system at random. Extreme USB recognizes each device automatically and provides the appropriate protocol handling. For the host controller, no additional software installations are required to support the Extreme USB system. The Extreme USB core simply enumerates as a generic hub, allowing transparent connectivity of any device on the host controller.
Throughput on the link is independent of the Extreme USB protocol, and is determined by simple performance characteristics of the physical medium that include the maximum available bandwidth, and the round trip latency of the link.
Operating at the USB protocol layer, Extreme USB is independent of the physical media used for data transmission. Extreme USB has been implemented over both copper and fibre media providing wired extension up to 100s of meters over standard Category – 5 UTP cabling and kilometers over fiber optics. In the wireless medium, Extreme USB has been combined with a standard 802.11g radio to enable a four port wireless hub, dubbed WiRanger.
The structure of the local (LEX) and remote (REX) dongles is very similar. Each can be logically partitioned into three district functional layers. The top layer is the Extreme USB packets in a manner that is more suitable for transmission over RF. It is here that features such as encryption and error correction are added if required. The bottom layer represents the actual RF transceiver hardware and associated baseband modems. This architecture provides a flexible system in which the RF convergence layer can be tailored to suit the requirements of the chosen RF link technology.
A wide variety of other physical media, wired or wireless, can be connected to an Extreme USB core to provide USB connectivity : 802.11, UWB, Broadband over Powerline transceivers, Gigabit Ethernet transceivers, etc.
The following sequence diagrams provide a simplified view of how extreme USB works.
The below figure shows the progress of an IN transaction between a USB host and a USB device. The host initiates the transaction by issuing an IN request to the device. The device responds with a DATA0 or DATA1packet as appropriate and the host completes the transaction by sending an acknowledgement ACK to the device. For a successful transaction, the host must see the start of the data packet within a defined period after completing transmission of the IN request. Similarly, the device must see the start of the acknowledgement packet within a defined period after completing transmission of the data packet.
The same scenario with Extreme USB. In this case there are two additional subsystems involved. These subsystems are identified here as LEX and REX.
As before, the host initiates the transaction by issuing an IN request. The LEX subsystem recognizes that the data packet cannot be returned within the allotted time and responds to the host with a negative acknowledgement (NAK). Concurrently, the LEX forwards the IN request to the REX unit and subsequently to the device itself.
The device responds to the IN request with a data packet, which is forwarded to the LEX and stored. Concurrently, the REX subsystem generates a local acknowledgement to the device.
At some later time the host issues a second IN request. This time, the LEX subsystem recognizes that it has the desired data packet stored in memory and supplies it to the host.
In practice, each separate USB transfer type requires slightly different handling and additional algorithms are provided to deal with error situations such as when a complete packet is lost. However, the preceding does illustrate the general approach.
UWB : The underlying technology
The basic transport mechanism for Wireless USB is the ultra – wide band (UWB) radio platform, which has been the focus of recent efforts by the Multiband OFDM Alliance (MBOA) and the WiMedia Alliance. The platform consists of two core layers : the UWB radio layer and the convergence layer.
UWB Radio Layers
UWB technology is fundamentally different from conventional narrow – band radio frequency (RF) and spread – spectrum technologies (SS) such as Bluetooth and 802.11 a/g. Conventional radios transmit a single continuous carried wave over a specified frequency. In contrast, UWB transmits short, fast, low – power wavelets of energy over a very wide band of frequencies.
In 2002, the Federal Communications Commission (FCC) legalized commercial use of UWB communications in the 3.10 to 10.6 GHz slice of the radio spectrum. At the same time, the FCC imposed stringent limitations on UWB power emissions to enable the co-existence of UWB and other services that operate in this spectrum. This combination of a very broad band and restricted power provides the high speed and limited range of UWB – based applications. It also enables spectrum reuse : unlike narrow – band RF wireless technologies, a wireless USB cluster can communicate on the same channel as another cluster in proximity. An additional advantage of UWB technology is that the radio circuitry can be implemented in cost – effective CMOS.
The Convergence Layer of USB :
The convergence layer serves as an interface between the UWB radio layer and UWB-based applications. It allows multiple applications to share a single radio. Wireless USB is the first of several wireless applications that will run on the UWB platform.
An industrial strength solution :
Extreme USB was developed to enable USB devices in industrial and commercial environments where operational requirements often exceed those of the desktop for which USB was designed. To achieve long reach USB connectivity, Extreme USB overcomes the limitations imposed by the Turnaround Timer. Removing this limitation enables conventional RF techniques such as error correction to be employed.
Referring back to the requirements discussed in the section on cable replacement, Extreme USB supports all three USB speeds – LS, FS and HS. Extreme USB also contains unique features that enable each of the four USB transfer types to be handled. Any particular implementation can combine support for speed and transfer type variants as required. Just like standard USB, devices with different speed and transfer type attributes can be attached to and detached from the system at random. Extreme USB recognizes each device automatically and provides the appropriate protocol handling. For the host controller, no additional software installations are required to support the Extreme USB system. The Extreme USB core simply enumerates as a generic hub, allowing transparent connectivity of any device on the host controller.
Throughput on the link is independent of the Extreme USB protocol, and is determined by simple performance characteristics of the physical medium that include the maximum available bandwidth, and the round trip latency of the link.
Operating at the USB protocol layer, Extreme USB is independent of the physical media used for data transmission. Extreme USB has been implemented over both copper and fibre media providing wired extension up to 100s of meters over standard Category – 5 UTP cabling and kilometers over fiber optics. In the wireless medium, Extreme USB has been combined with a standard 802.11g radio to enable a four port wireless hub, dubbed WiRanger.
The structure of the local (LEX) and remote (REX) dongles is very similar. Each can be logically partitioned into three district functional layers. The top layer is the Extreme USB packets in a manner that is more suitable for transmission over RF. It is here that features such as encryption and error correction are added if required. The bottom layer represents the actual RF transceiver hardware and associated baseband modems. This architecture provides a flexible system in which the RF convergence layer can be tailored to suit the requirements of the chosen RF link technology.
A wide variety of other physical media, wired or wireless, can be connected to an Extreme USB core to provide USB connectivity : 802.11, UWB, Broadband over Powerline transceivers, Gigabit Ethernet transceivers, etc.
The following sequence diagrams provide a simplified view of how extreme USB works.
The below figure shows the progress of an IN transaction between a USB host and a USB device. The host initiates the transaction by issuing an IN request to the device. The device responds with a DATA0 or DATA1packet as appropriate and the host completes the transaction by sending an acknowledgement ACK to the device. For a successful transaction, the host must see the start of the data packet within a defined period after completing transmission of the IN request. Similarly, the device must see the start of the acknowledgement packet within a defined period after completing transmission of the data packet.
The same scenario with Extreme USB. In this case there are two additional subsystems involved. These subsystems are identified here as LEX and REX.
As before, the host initiates the transaction by issuing an IN request. The LEX subsystem recognizes that the data packet cannot be returned within the allotted time and responds to the host with a negative acknowledgement (NAK). Concurrently, the LEX forwards the IN request to the REX unit and subsequently to the device itself.
The device responds to the IN request with a data packet, which is forwarded to the LEX and stored. Concurrently, the REX subsystem generates a local acknowledgement to the device.
At some later time the host issues a second IN request. This time, the LEX subsystem recognizes that it has the desired data packet stored in memory and supplies it to the host.
In practice, each separate USB transfer type requires slightly different handling and additional algorithms are provided to deal with error situations such as when a complete packet is lost. However, the preceding does illustrate the general approach.
UWB : The underlying technology
The basic transport mechanism for Wireless USB is the ultra – wide band (UWB) radio platform, which has been the focus of recent efforts by the Multiband OFDM Alliance (MBOA) and the WiMedia Alliance. The platform consists of two core layers : the UWB radio layer and the convergence layer.
UWB Radio Layers
UWB technology is fundamentally different from conventional narrow – band radio frequency (RF) and spread – spectrum technologies (SS) such as Bluetooth and 802.11 a/g. Conventional radios transmit a single continuous carried wave over a specified frequency. In contrast, UWB transmits short, fast, low – power wavelets of energy over a very wide band of frequencies.
In 2002, the Federal Communications Commission (FCC) legalized commercial use of UWB communications in the 3.10 to 10.6 GHz slice of the radio spectrum. At the same time, the FCC imposed stringent limitations on UWB power emissions to enable the co-existence of UWB and other services that operate in this spectrum. This combination of a very broad band and restricted power provides the high speed and limited range of UWB – based applications. It also enables spectrum reuse : unlike narrow – band RF wireless technologies, a wireless USB cluster can communicate on the same channel as another cluster in proximity. An additional advantage of UWB technology is that the radio circuitry can be implemented in cost – effective CMOS.
The Convergence Layer of USB :
The convergence layer serves as an interface between the UWB radio layer and UWB-based applications. It allows multiple applications to share a single radio. Wireless USB is the first of several wireless applications that will run on the UWB platform.
USB Cluster Topology :
Wireless USB clusters use a simple hub and spoke topology with point – to – point connections b/n the host and the devices connected to it. The host – which can logically connect to as many as 127 devices initiates and schedules data transfers to the devices in the cluster, allotting time slots and bandwidth to each connected device clusters will be able to physically overlap with minimal interference.
Wireless USB clusters use a simple hub and spoke topology with point – to – point connections b/n the host and the devices connected to it. The host – which can logically connect to as many as 127 devices initiates and schedules data transfers to the devices in the cluster, allotting time slots and bandwidth to each connected device clusters will be able to physically overlap with minimal interference.
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