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Wireless Extends Range Using 2.4GHz PRISM® Chipset
Authors: Petrick Zyren
This application note addresses ways extend range Universal Serial (USB) data communications interface using PRISM wireless (WLAN) bridge. PRISM chipset coupled with interface allows data connectivity between personal computer hosts remote peripherals. class peripherals include printers, optical scanners audio speakers. This application note focuses composite device configurations illustrating examples protocols network topologies. Described detail timing constraints aware wireless bridge implementations.
cabling PRISM provides that capability. PRISM capability extend range between host peripheral 400ft indoors.
Packet Formats
protocol supports four types packet formats. They include OUT, SETUP. maximum packet size limited 1024 bytes. Each packet protected with 16-bit CRC. handshaking process includes ACK, STALL. indicates that device (peripheral) unable accept data from host STALL indicates that serious problems have occurred host assistance necessary. There four data transactions each endpoint device defined support one. Transactions isochronous, interrupt, control bulk. Isochronous transactions those which carry data containing either video voice/audio information. Bulk transactions those associated with printer scanner peripherals. When host communicates device, i.e., printer, transaction issued, signifying that write operation executed. device completes transaction responding with host. data rate Bulk Isochronous transactions Mbps bounded frame rate. Lower speed devices such keyboards, mice, joysticks less time critical. They make speed class communicate Mbps with frame rate.
PRISM Overview
PRISM 2.4GHz WLAN chipset designed meet direct sequence spread spectrum physical layer (radio) specifications IEEE802.11 WLAN standard. PRISM uses DPSK modulation scheme. PRISM radio architecture provides half duplex wireless communications packet data data rate Mbps. PRISM provides 100mW power antenna. This allows continuous data connectivity 400ft indoors 1000ft outdoors. more details chipset, refer data sheet application note list reference section.
Overview
peripheral interface standard developed telecom industry leaders. Plug-n-Play computer peripherals will brought outside enclosure eliminate need install cards into dedicated computer slots. expected become mainstream coming year, will supersede traditional serial communication ports. protocol limited single host, allowing computer peripherals connected. They include: printers, optical scanners, joysticks, keyboards, speakers, microphones, PSTN modems, mice others. allows data rates Mbps Mbps. data transfers formatted into frames which occur boundaries. Most configurations architecture. intelligent device that acts repeater router with power management control. When end-device (peripheral) connected disconnected from hub, network emulates true Plug-n-Play system. physical interface between host peripheral 4-wire cable limited length five meters. extend range cable interface, repeaters used. However, wireless bridges another alternative
Timing
When considering wireless bridge between host peripheral device, timing network most critical. signal timing designed wired-cabled transmission mediums. turnaround device response time critical factor. Bulk transactions, host executes packet device. device must respond with ACK/NAK/STALL frame within times including transmission wired-cable delays. This translates into turnaround time approximately 1µs. This calculation assumes Mbps rate meter cable length. turnaround time violated, peripheral risk missing response window eventually being disconnected host. turnaround time quite aggressive wireless communications. Throughput delays over medium transmit receive switching settling times virtually make this impossible meet. Therefore minimize timing violation, recommended that device (before bridge), buffer incoming data memory. This emulates "virtual" device peripheral interface. transaction completed with ACK/NAK response data stored memory. data transferred from memory PRISM radio,
http://www.intersil.com 407-727-9207 Copyright Intersil Corporation 1999 PRISM PRISM logo trademarks Intersil Corporation.
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Application Note 9781
transmitted receiving radio where data buffered. When Bulk transaction completed, data from memory formatted passed onto device peripheral. transaction peripheral completed ACK/NAK responds. Because wireless bridge operates Mbps, there possibilities latencies Isochronous transfers. However careful memory management able handle data latencies. This will require further analysis. (PLCP) header Protocol Data Unit (MPDU). PLCP header processed HFA3824 baseband processor MPDU layer controller. Figure illustrates transaction data embedded data field IEEE802.11 data packet. transaction data Figure consists four fields. Transaction Type field indicates type transaction, i.e., Bulk, Isochronous. Frame Type field indicates frame either frame. data frame contains Bulk Isochronous information bytes, followed check field. Host field added access point track route transaction data. This used distinguish between multiple hosts accessing shared peripherals. Device either serial number port address embedded part IEEE802.11 MPDU header.
Adding Interface IEEE802.11 Access Point
Figure illustrates block diagram IEEE802.11 access point. engine added existing access point architecture. Flexibility memory partitioning access points allows interfaces added minimal cost. previously described, interface configured "virtual" end-device. Modifications memory management structure access point allow buffering incoming Bulk Isochronous transactions available transmission over wireless bridge using IEEE802.11 WLAN networks. IEEE802.11 WLAN networks operate Mbps data rate. contrast Bulk transactions peripherals such printers scanners, throughput performance should issue. However, Isochronous Mbps data transmitting over Mbps IEEE802.11 WLAN network will exhibit latency. Therefore, recommended that FIFO memory management architectures implemented overcome this problem.
IEEE802.11 Distributed Network with interface
Figure illustrates typical distributed network using IEEE802.11 wireless protocol PRISM radios. this configuration, peripherals such printers, scanners speakers located remote locations. Both Bulk Isochronous data transfers supported. PRISM radios used wireless communications bridge between host peripherals. host transaction data transfers over interface access point. access point interface acts "virtual" device buffering peripheral data memory with respective enddevice serial number and/or port addresses. access point builds IEEE802.11 message frame illustrated Figure transfers transaction data respective peripheral over 2.4GHz wireless WLAN network. ACK/NAK handshaking peripheral end-device recognized access point. responsibility access point distribute data from host respective peripheral end-devices.
2.4GHz IEEE802.11 ENGINE OPTIONAL ETHERNET BASE-T CONTROLLER MEMORY RAM/ROM CONNECTOR ENGINE PRISM RADIO CHIPSET
BASE-T CONNECTOR
IEEE802.11 DSSS ACCESS POINT
FIGURE IEEE802.11 ACCESS POINT WITH INTERFACE
Using IEEE802.11 CSMA/CA Protocol
IEEE802.11 wireless protocol direct sequence physical layer uses Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) mechanism over medium. Figure illustrates message frame specified IEEE802.11 WLAN standard. message frame consists Physical Layer Convergence Protocol
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Application Note 9781
IEEE802.11 MESSAGE FRAME
PLCP HEADER SYNC SIGNAL SERVICE LENGTH CRC-16
DATA PACKET MPDU
TRANSACTION TRANSACTION TYPE FRAME TYPE DATA
FIGURE IEEE802.11 MESSAGE FRAME WITH TRANSACTION DATA FIELD
IEEE802.11 HOST DESKTOP PSTN MODEM 12Mbps MOUSE JOYSTICK KEYBOARD IEEE802.11 PRISM ACCESS POINT 2.4GHz 2Mbps PRISM RADIO
12Mbps
PRINTER
IEEE802.11 PRISM RADIO
12Mbps
SCANNER
IEEE802.11 PRISM RADIO
12Mbps
SPEAKERS
FIGURE IEEE802.11 DISTRIBUTED NETWORK WITH
WIRELESS PROTOCOL IEEE802.11 PLCP HEADER HOST END-DEVICE TRANACTION TYPE TRANSACTION DATA FRAME TYPE TRANSACTION DATA
SYNC
SIGNAL
SERVICE
LENGTH
CRC-16
FIGURE SIMPLIFIED WIRELESS PROTOCOL
Simplified Wireless Protocol
networks requiring performance IEEE802.11, implementation wireless bridge achievable with simplified version IEEE802.11 CSMA/CA protocol. transaction data extracted reformatted into wireless protocol shown Figure wireless protocol uses IEEE802.11 PLCP header radio acquisition synchronization packet length tracking maximum size packets. transaction packet configured into fields. They include; Host Device Transaction Type, Frame Type, Transaction Data CRC. transaction type used determine transaction data either Bulk Isochronous. frame type field used
complete ACK/NAK handshaking. field last field message. used detect errors after transmission over medium. wireless bridge data rate supports rates. IEEE802.11 PLCP header sent Mbps. transaction data sent either Mbps Mbps. data rate selected configuring internal registers HFA3824 baseband processor. details data rate selection, refer HFA3824 data sheet.
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Application Note 9781 Simple Wireless Bridge
simple Wireless bridge illustrated Figure This network referred Ad-Hoc network. This network useful when printer remotely located office home when cabling host physically impossible. PRISM based radios used implement Mbps wireless bridge. same principles discussed throughout this application note buffering transactions should followed this configuration. wireless communications protocol illustrated Figure used this network. 500mA including downstream thereafter. maximum current draw specified device configuration descriptor. maximum power consumption PRISM radio chipset 350mA. This occurs when PRISM radios transmitting data over medium. When PRISM radios receiving messages, power consumption considerably less. Depending upon application, power consumption function transmit power required range. 350mA range 400ft indoors 1000ft outdoors. applications home where range 100ft sufficient, transmit power scaled back, reflecting power consumption approximately 100mA. This well within power consumption constraints specified standard. maximum power consumption violated hub, recommended that PRISM radios powered external AC/DC power supply.
HOST DESKTOP PSTN MODEM 12Mbps JOYSTICK JOYSTICK 2.4GHz PRISM RADIO 12Mbps PRINTER KEYBOARD OPTIONAL POWER PRISM RADIO 2Mbps
References
Intersil documents available internet, site http://www.intersil.com/ Intersil AnswerFAX (407) 724-7800. HFA3824 Data Sheet, Intersil Corporation, AnswerFAX Doc. 4308. HFA3860 Data Sheet, Intersil Corporation, AnswerFAX Doc. 4347. AN9614 Application Note, Intersil Corporation, "Using PRISMChip Data Rate Applications", AnswerFAX Doc. 99614. AN9673 Application Note, Intersil Corporation, "PRISMDSSS Radio Operation Continuous Links", AnswerFAX Doc. 99673. AN9616 Application Note, Intersil Corporation, "Programming HSP3824", AnswerFAX Doc. 99616. TB337, Tech Brief, Intersil Corporation, Brief Tutorial Spread Spectrum Packet Radio", AnswerFAX Doc. 82337.
FIGURE SIMPLE WIRELESS BRIDGE
Power Management
host interface sources power 5VDC over 4-wire cable. power management constrains each device draw 100mA before configured host
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information regarding Intersil Corporation products, site http://www.intersil.com
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