Digital Wireless Corporation WIT2410 Guide de configuration rapide

WIT2410

2.4GHz Spread Spectrum

Wireless Industrial Transceiver

Integration Guide

June 15, 1999

One Meca Wa

Norcross, Georgia 30093

www.digital-wireless.com

(770) 564-5540

Note: This device has not been authorized

as required by the rules of the Federal

Communications Commission. This device

is not, and may not be, offered for sale or

lease, or sold or leased, until authorization

is obtained.

About This Manual

This manual is designed to allow integration of the Digital Wireless Corporation WIT2410 OEM

module into complete products. Care has been taken to try and make sure all of the information

in this manual is accurate. However, specifications can change over time and Digital Wireless

cannot guarantee the accuracy of this information. If you have any questions on any information

in this manual, please contact Digital Wireless Technical Support at (770) 564-5540.

TABLE OF CONTENTS

1. INTRODUCTION..................................................................................................................................1

1.1 Why Spread Spectrum?.....................................................................................................................1

1.2 Frequency Hopping vs. Direct Sequence.........................................................................................2

2. RADIO OPERATION............................................................................................................................4

2.1. Synchronization and Registration...................................................................................................4

2.2. Data Transmission...........................................................................................................................5

2.2.1. Point-to-Point ........................................................................................................................5

2.2.2. Point-to-Multipoint................................................................................................................6

2.2.3. TDMA Mode.........................................................................................................................6

2.2.4. CSMA Mode .........................................................................................................................8

2.2.5. Full Duplex Communication.................................................................................................8

2.2.6. Error-free Packet Transmission Using ARQ.........................................................................8

2.3. Modes of Operation.........................................................................................................................9

2.3.1. Control and Data Modes........................................................................................................9

2.3.2. Sleep Mode..........................................................................................................................10

2.3.3. Low Power Mode and Duty Cycling...................................................................................10

3. PROTOCOL MODES..........................................................................................................................11

3.1. Packet Formats..............................................................................................................................12

3.1.1. Data Packet..........................................................................................................................13

3.1.3. Connect Packet....................................................................................................................13

3.1.4. Disconnect Packet (base only, receive only)......................................................................13

4. MODEM INTERFACE........................................................................................................................14

4.1. Interfacing to 5 Volt Systems........................................................................................................15

5. MODEM COMMANDS......................................................................................................................16

5.1. Serial Commands..........................................................................................................................16

5.2. Network Commands......................................................................................................................17

5.3. Protocol Commands......................................................................................................................19

5.4. Status Commands..........................................................................................................................21

5.5. Memory Commands......................................................................................................................22

5.6. Modem Command Summary........................................................................................................23

6. WIT2410 DEVELOPER’S KIT...........................................................................................................24

6.1. COM24..........................................................................................................................................24

6.2. Demonstration Procedure..............................................................................................................25

6.3. Troubleshooting ............................................................................................................................26

7. APPENDICES......................................................................................................................................28

7.1. Technical Specifications ...............................................................................................................28

7.1.1. Power Specifications...........................................................................................................28

7.1.2. RF Specifications.................................................................................................................28

7.2.2. Mechanical Specifications...................................................................................................28

7.3. Serial Connector Pinouts...............................................................................................................29

7.4. Approved Antennas.......................................................................................................................29

7.5. Technical Support .........................................................................................................................29

7.6. Mechanical Drawing.....................................................................................................................30

7.7. Warranty........................................................................................................................................31

1. INTRODUCTION

The WIT2410 radio transceiver provides reliable wireless connectivity for either

point-to-point or multipoint applications. Frequency hopping spread spectrum technology

ensures maximum resistance to noise and multipath fading and robustness in the presence of

interfering signals, while operation in the 2.4 GHz ISM band allows license-free use and

worldwide compliance. A simple serial interface supports asynchronous data up to 230400

bps. An on-board 3 KB buffer and an error-correcting over-the-air protocol provide smooth

data flow and simplify the task of integration with existing applications.

-Multipath fading impervious

frequency hopping technology

with 75 frequency channels

(2401-2475 MHz).

- Supports point-to-point or

multipoint applications.

- Meets FCC rules 15.247 and ETS

300.328 for worldwide license-

free operation.

- Superior range to 802.11 wireless

LAN devices.

- Transparent ARQ protocol

w/3KB buffer ensures data

integrity.

- Digital addressing supports up to

32 networks, with 62 remotes per

network.

- Low power 3.3v CMOS signals

- Simple serial interface handles both

data and control at up to 230400

bps.

- Fast acquisition typically locks to

hopping pattern in 5 seconds or less.

- Selectable 10 mW or 100 mW

transmit power.

- Support for diversity antenna.

- Built-in data scrambling reduces

possibility of eavesdropping.

- Nonvolatile memory stores

configuration when powered off.

- Smart power management features

for low current consumption.

- Dynamic TDMA slot assignment

that maximizes throughput.

1.1 Why Spread Spectrum?

The radio transmission channel is very hostile, corrupted by noise, path loss and

interfering transmissions from other radios. Even in a pure interference-free

environment, radio performance faces serious degradation through a phenomenon

known as multipath fading. Multipath fading results when two or more reflected rays of

the transmitted signal arrive at the receiving antenna with opposing phase, thereby

partially or completely canceling the desired signal. This is a problem particularly

prevalent in indoor installations. In the frequency domain, a multipath fade can be

described as a frequency-selective notch that shifts in location and intensity over time as

reflections change due to motion of the radio or objects within its range. At any given

time, multipath fades will typically occupy 1% - 2% of the 2.4 GHz band. This means

that from a probabilistic viewpoint, a conventional radio system faces a 1% - 2% chance

of signal impairment at any given time due to multipath.

Spread spectrum reduces the vulnerability of a radio system to both interference from

jammers and multipath fading by distributing the transmitted signal over a larger region

of the frequency band than would otherwise be necessary to send the information. This

allows the signal to be reconstructed even though part of it may be lost or corrupted in

transit.

Figure 1

Narrowband vs. spread spectrum in the presence of interference

1.2 Frequency Hopping vs. Direct Sequence

The two primary approaches to spread spectrum are direct sequence (DS) and frequency

hopping (FH), either of which can generally be adapted to a given application. Direct

sequence spread spectrum is produced by multiplying the transmitted data stream by a

much faster, noise-like repeating pattern. The ratio by which this modulating pattern

exceeds the bit rate of the baseband data is called the processing gain, and is equal to the

amount of rejection the system affords against narrowband interference from multipath

and jammers. Transmitting the data signal as usual, but varying the carrier frequency

rapidly according to a pseudo-random pattern over a broad range of channels produces a

frequency hopping spectrum system.

Figure 2

Forms of spread spectrum

One disadvantage of direct sequence systems is that due to spectrum constraints and the

design difficulties of broadband receivers, they generally employ only a minimal amount

of spreading (typically no more than the minimum required by the regulating agencies).

For this reason, the ability of DS systems to overcome fading and in-band jammers is

relatively weak. By contrast, FH systems are capable of probing the entire band if

necessary to find a channel free of interference. Essentially, this means that a FH

system will degrade gracefully as the channel gets noisier while a DS system may

exhibit uneven coverage or work well until a certain point and then give out completely.

Because it offers greater immunity to interfering signals, FH is often the preferred

choice for co-located systems. Since direct sequence signals are very wide, they tend to

offer few non-overlapping channels, whereas multiple hoppers may interleave with less

interference. Frequency hopping does carry some disadvantage in that as the transmitter

cycles through the hopping pattern it is nearly certain to visit a few blocked channels

where no data can be sent. If these channels are the same from trip to trip, they can be

memorized and avoided; unfortunately, this is generally not the case, as it may take

several seconds to completely cover the hop sequence during which time the multipath

delay profile may have changed substantially. To ensure seamless operation throughout

these outages, a hopping radio must be capable of buffering its data until a clear channel

can be found. A second consideration of frequency hopping systems is that they require

an initial acquisition period during which the receiver must lock on to the moving carrier

of the transmitter before any data can be sent, which typically takes several seconds. In

summary, frequency hopping systems generally feature greater coverage and channel

utilization than comparable direct sequence systems. Of course, other implementation

factors such as size, cost, power consumption and ease of implementation must also be

considered before a final radio design choice can be made.

As an additional benefit, RF spectrum has been set aside at 2.4 GHz in most countries

(including the U.S.) for the purpose of allowing compliant spread spectrum systems to

operate freely without the requirement of a site license. This regulatory convenience

alone has been a large motivation for the industry-wide move toward spread spectrum.

2. RADIO OPERATION

2.1. Synchronization and Registration

As discussed above, frequency hopping radios periodically change the frequency at which

they transmit. In order for the other radios in the network to receive the transmission, they

must be listening to the frequency over which the current transmission is being sent. To do

this, all the radios in the net must be synchronized and must be set to the same hopping

pattern. All radios in a net must be set to the same hopping pattern before attempting to

communicate.

In point-to-point or point-to-multipoint arrangements, one radio module is designated as the

base station. All other radios are designated remotes. One of the responsibilities of the base

station is to transmit a synchronization signal to the remotes to allow them to synchronize

with the base station. Since the remotes know the hopping pattern, once they are

synchronized with the base station, they know which frequency to hop to and when. Every

time the base station hops to a different frequency, it immediately transmits a synchronizing

signal.

When a remote is powered on, it rapidly scans the frequency band for the synchronizing

signal. Since the base station is transmitting over 75 frequencies and the remote is scanning

75 frequencies, it can take several seconds for a remote to synch up with the base station.

Once a remote has synchronized with the base station, it must request registration from the

base station. The registration process identifies to the base station the remotes from which

transmissions will be received and not discarded. Registration also allows tracking of

remotes entering and leaving the network. The base station builds a table of serial numbers

of registered remotes. To improve efficiency, the 24-bit remote serial number is assigned a

6-bit “handle” number. Two of these are reserved for system use, thus each base station can

necessary, the automatic handle assignment can be overridden to explicitly tie certain handles

to certain remotes. See the section on Network Commands for details on the Set Default

Handle command.

To detect if a remote has gone offline or out of range, the registration must be “renewed”

once every 256 hops. Registration is completely automatic and requires no user application

intervention. When the remote is registered, it will receive several network parameters from

the base. This allows the base to automatically update these network parameters in the

remotes over the air. Once a parameter has been changed in the base, it is automatically

changed in the remotes. The parameters automatically changed are hop duration, the duty

cycle and the multiple access mode, i.e., TDMA or CSMA.

At the beginning of each hop, the base station transmits a synchronizing signal. After the

synchronizing signal has been sent, the base will transmit any data in its buffer unless packet

transmit delay has been set. The packet transmit delay parameter allows for the transmission

of groups of continuous data in transparent mode (protocol mode 0). In TDMA mode the

amount of data that the base station can transmit per hop is determined by the hop duration

and the number of remotes registered with the base. In CSMA mode, the maximum amount

of data sent is determined by maximum data length. In any event, the maximum amount of

data sent by a base station or remote per hop is 127 bytes. If there is no data to be sent, the

base station will not transmit until the next frequency.

The operation of the remotes depends on whether the remote is set up for TDMA mode or

CSMA mode. In TDMA mode, the operation of the remotes is the same as the base station

without the synchronization signal.

In CSMA mode, remotes compete on an ad hoc basis for transmission time. The likelihood

that a remote with will attempt to transmit immediately is affected by the persistence

parameter. If a collision is detected with another radio, the remote will wait a random period

of time before trying to retransmit. The backoff parameter controls the maximum time a

remote will wait before attempting to retransmit. Unregistered remotes can request

registration any time after the base station transmission. Refer to the section Protocol

Commands for details on the persistence and backoff parameters.

Except for the registration process which occurs only when a remote logs onto the network,

the whole procedure is repeated on every frequency hop. Refer to the section on Modem

Commands for complete details on parameters affecting the transmission of data.

2.2. Data Transmission

The WIT2410 supports two network configurations: point-to-point and point-to-multipoint.

In a point-to-point network, one radio is set up as the base station and the other radio is set up

as a remote. In a point-to-multipoint network, a star topology is used with the radio set up as

a base station acting as the central communications point and all other radios in the network

set up as remotes. In this configuration, all communications take place between the base

station and any one of the remotes. Remotes cannot communicate directly with each other.

It should be noted that point-to-point mode is a subset of point-to-multipoint mode and

therefore there is no need to specify one mode or the other.

2.2.1. Point-to-Point

In point-to-point mode, unless packet transmit delay has been set, the base station will

transmit whatever data is in its buffer limited to 127 bytes or by the length of the base slot

size. If the base station has more data than can be sent on one hop, the remaining data will be

sent on subsequent hops. In addition to the data, the base station adds some information to

the transmission over the RF link. It adds the address of the remote to which it is

transmitting, even though in a point-to-point mode there is only one remote. It also adds a

sequence number to identify the transmission to the remote. This is needed in the case of

acknowledging successful packets and retransmitting errored packets. Also added is a 24-bit

CRC to allow the base to check the received transmission for errors. When the remote

receives the transmission, it will acknowledge the transmission if it was received without

errors. If no acknowledgment is received, the base station will retransmit the same packet on

the next frequency hop.

In point-to-point mode, a remote will transmit whatever data is in its buffer up to the limit of

its maximum packet length. If desired, a minimum packet length can also be set, which forces

the remote to wait until a certain amount of data is available or the specified packet transmit

delay is exceeded before transmitting. If the remote has more data than can be sent on one

hop, it will send as much data as possible as a packet, adding its own address, a packet

sequence number and 24-bit CRC. These additional bytes are transparent to the user

application if the protocol mode is 00 (which is the default). In the event a remote has more

data to send, the data will be sent on subsequent hops. If the packet is received by the base

station without errors, the base station will acknowledge the packet. If the remote does not

receive an acknowledgment, it will retransmit the packet on the next frequency hop. To the

user application, acknowledgments and retransmissions all take place behind the scenes

without the need for user intervention.

2.2.2. Point-to-Multipoint

In point-to-multipoint mode, data sent from the user application to the base station must be

packetized by the user application. This is necessary to identify the remote to which the base

station should send data. When the user packet is received by the remote, if the remote is in

transparent mode (protocol mode 0), the packetization bytes are stripped by the remote. In

this instance the remote host receives just data. If the remote is not in transparent mode, the

remote host will receive the appropriate packet header as specified by the remote’s protocol

mode. Refer to the section Protocol Modes for details on the various packet formats.

When a remote sends data to a base station in point-to-multipoint mode, the remote host does

not need to perform any packetization of the data. The remote will add address, sequence

and CRC bytes as in the point-to-point mode. When the base station receives the data, the

base station will add packetization header bytes according to its protocol mode setting.

2.2.3. TDMA Mode

For applications needing guaranteed bandwidth availability, the TDMA mode of the

WIT2410 can meet this requirement. This is the default mode of the WIT2410. In TDMA

mode, each remote has an assigned time slot during which it can transmit. The base station

time slot is set independently of the remote time slots through the Set Base Slot Size

command. The base station assigns each remote a time slot and informs the remotes of the

size of the time slot. All remote time slots are the same size which is determined by the

number of remotes registered with the base station. The slot size is a dynamic variable that

changes as the number of registered remotes changes. The remotes are continually updated

with the time slot size. This approach continually maximizes the data throughput. The base

station divides the amount of time available per hop by the number of registered remotes up

to a maximum of 16 times slots per hop. If the number of registered remotes is greater than

16, the time slots will be spread across the required number of hops. For networks with more

than 16 possible remotes, the Set Duty Cycle command must be used to specify a duty cycle -

- the number of hops over which the time slots must be spread. For 1 to 16 remotes, no duty

cycle is required; for 17 to 32 remotes a duty cycle of at least ½ is required; and for 33 to 62

remotes a duty cycle of ¼ or more is necessary. An added benefit of using the power save

mode to set a duty cycle is improved average current consumption efficiency. Refer to the

Status Commands section for details of this command.

When setting up a TDMA network, keep in mind that time slot length, maximum packet size

and hop duration are all interrelated. The hop duration parameter will determine the time

slot size and the maximum amount of data that can be transmitted per hop by the remotes.

There is a hard limit of the absolute maximum amount of data that can be sent on any given

hop of 212 bytes regardless of any parameters. The base station requires 1.7 ms overhead for

tuning, the synchronization signal and parameter updating, as well as a guard time of 500 us

between each remote slot. Thus the amount of time allocated per remote slot is roughly:

hop duration – base slot – 1.7ms - ( # of registered remotes-1)·500us

( # of registered remotes)

Take for example a network comprised of a base station and 10 remotes. A hop duration of

15 ms is chosen. We decide that the base station needs to be able to send up to 32 bytes each

hop (equivalent to a capacity for the base of ~ 21 kbps). Counting the 1.7 ms overhead for

the base packet and making use of the fact that our RF rate is 460.8 kbps, we determine that

the base slot requires approximately:

Each remote time slot will be:

15 ms – 2.3 ms – (9)·0.5 ms

From our RF data rate of 460.8kbps we see that it takes 17.36 µs to send a byte of data, so

each remote will be able to send up to

= 47 bytes of data per hop.

Note that the 47 bytes is the actual number of data bytes that can be sent. If the WIT2410 is

using a protocol mode, the packet overhead does not need to be considered. So in this

example, the total capacity per remote would be:

If we figure a minimum margin of safety for lost packets and retransmissions of about 20%,

we see that this would be more than sufficient to support 19.2 kbps of continuous data per

remote. It is also useful to remember that the asynchronous data input to the WIT2410 is

= 0.82 ms

0.82 ms

17.36

32·8

460.8kbps+ 1.7 ms = 2.3 ms

47 bytes

15 ms = 25 kbps

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