Low Bandwidth Infra-Red Networks and Protocols for Mobile Communicating Devices Andy Harter and Frazer Bennett Olivetti Research Limited 24a Trumpington Street Cambridge CB2 1QA Telephone: +223 343000 October, 1993
1 Introduction 1.1 Scope This document is intended as a source of technical reference for the infra-red network developed at Olivetti Research, and in use within Olivetti, Digital, Xerox, Bellcore and elsewhere. The document is restricted in scope to the details of the physical communication medium and formatting of simple data packets over the medium. The ISO Reference Model for Open System Interconnection (the OSI model), is used as a descriptive framework, and within this model the document describes the Physical and Data Link Layers of the IR network. Elements of network architecture which would be described in terms of the Network Layer and above are beyond the scope of this document. Where possible, speci c details of actual devices making use of the IR network are included in appendices.
1.2 Historical background A simple low bandwidth unidirectional IR network was devised in 1989 by Roy Want at Olivetti Research as a component part of the Active Badge system. The Active Badge is personal device which periodically transmits an IR packet. The transmissions are detected by IR base sensors which are in xed known locations and which relay the information to some central point over a wire network. In this way, the badge is dynamically located. The signaling rate of this system was around 1k baud. The system was extensively reworked by Roy Want, Andy Harter and Tom Blackie in 1991. A key feature of the redesign was the inclusion of a full bidirectional capability, 1
with IR transceivers in both badges and base sensors. The signaling rate was increased by approximately one order of magnitude. It was apparent that physically the network was now a more general entity, which other types of cordless or mobile device might use. The protocols were explicitly extended to embrace this concept. Other mobile devices have been built to use the network. At Xerox Parc, the Parc TAB is a sophisticated mobile personal device with LCD display and touch sensitive screen. At Olivetti Research, a small equipment badge is primarily intended as an asset tag, but has digital inputs to allow cordless and mobile telemetry. Also at Olivetti Research, IR interfaces are being added to portable notebook PC's such as the Olivetti Quaderno and the HP Palmtop.
1.3 Network requirements The properties of the IR network are targeted on a particular class of object, namely mobile personal devices. These devices have several common characteristics. Most importantly, they are lightweight, small and tetherless. By implication, they have their own power source which might be rechargeable or disposable batteries, or solar cell. Another characteristic of the target devices is that they are simple, cheap and practical to build. This implies the use of simple transceiver circuitry which avoids any complex signal processing. The power available for such devices to communicate over the IR network is a limiting factor in the achievable signaling rate. Devices need to communicate over a range of distances, and the inverse square law of propagation of electromagnetic radiation rapidly reduces the incident signal energy at the receiver from a distant source. Since receivers must be simple, the margin of the incident signal above ambient noise at the receiver must be easily detectable. By implication, energetic but low rate signaling is needed.
2 IR Network Model 2.1 Topology The IR network is implicitly formed between mobile devices and xed base sensors which communicate in the IR spectrum. A third essential component is some form of backbone network over which base sensors relay information to entities managing the IR network. The physical extent of the IR network is determined by the topology of the base sensors which is an important part of the network model. The properties of IR light greatly in uence the topology of base sensor positions. In particular, the property that light is re ected or absorbed by many materials rather than transmitted through them tends to de ne conveniently bounded and disjoint zones of IR signal. In order to equip a particular environment with an IR network, the space is modelled as a collection of light zones and each zone is spanned with a number of base sensors. The number and position of base sensors in a zone is determined by heuristics involving bidirectional range, zone shape and material composition. 2
Zoning is a useful property in a wireless network, since the same carrier can be reused in each zone. The spatial diversity which results yields a higher aggregate bandwidth in a collection of zones. In each zone, a modest bandwidth is likely to be sucient, which corresponds well with the requirements of low power and simple transceiver design. The properties of a spatially indiscriminate carrier such as RF are less able to satisfy these requirements.
2.2 Mobiles Mobiles must be able to transmit over the IR network, and are obliged to periodically broadcast their identity which should be a globally unique code. This enables a dynamic route to the device to be maintained. For some devices, for example a simple Active Badge, this may be the only information originating from the mobile. Other mobiles with input devices, for example notebooks or the Parc TAB, may originate data of many dierent forms. Most mobiles will have receive capabilities, though some will not have permanently enabled receivers for power saving reasons. Such devices will typically have windows of reception surrounding transmission by the mobile. Mobiles which transmit frequently should have a receive capability to enable a collision avoidance scheme to be implemented. This scheme is a period of reception prior to transmission, during which the detection of a modulated signal will prevent the transmission. Mobiles which transmit infrequently may not need to receive, but their transmission may collide with another transmission particularly in a zone operating near the bandwidth limit. The frequency of transmission can be based on a statistical treatment of assumptions about the density and type of trac in a zone. This approach is in keeping with the nature of the inherently imperfect medium, and the impossibility of guaranteeing to prevent or detect collisions. A simple technique to avoid synchronisation is to ensure variation in the periodicity of address broadcasts amongst mobiles of the same type. Furthermore, a particular mobile should have variation in its period. In this way, the number of broadcast collisions can be modelled statistically and will not be prone to locking or beating. In devices capable of attempting to avoid collisions, back-o strategy should be similarly constructed to avoid locking.
2.3 Bases Bases must be able to transmit and receive over the IR network, and also over the backbone network. Bases require buering of two types. First, bases require buers to implement a store and forward capability for outgoing IR messages to mobiles which only have an intermittent receive capability. A transmission from such a mobile represents a rendezvous point when the base can transmit in the knowledge that the mobile is receiving. Bases should also have buering or memory to provide exibility in backbone network access method, and to cope with mismatches in communications characteristics between the IR 3
and backbone networks. Many aspects of base sensor design are less critical than those for mobiles. Size is less important, and since bases are in xed locations they are more likely to have an external source of power. However, one disadvantage of the zoning property is that base sensors will be numerous. This implies that base sensors should be cheap, which implies simple and inexpensive backbone network interfaces. The backbone network might be a dedicated or standard network infrastructure, such as ethernet. The network could be wire based or even a wireless radio local area network. However, a simple, cheap interface probably implies the use of a dedicated wire network with simple protocols requiring the minimum of hardware and software support. Furthermore, it is particularly convenient for low-voltage power distribution to be run in parallel with the network, since the cost and logistics of individually supplying power to base sensors in potentially inaccessible positions must be considered.
3 IR Physical Layer 3.1 Carrier characteristics The physical layer transmission medium is incoherent radiation in the near IR range with a wavelength of between 880 and 920 nm. Components transmitting and receiving at this wavelength are widely available. A carrier of this wavelength suers negligible interference from stray sources of higher frequency electromagnetic radiation such as radio frequency emissions. The necessary power levels of the carrier are determined by a number of factors. The required range between mobile and base is clearly important. The radiometric properties of the emitter and detector are also in uential. The power level of the emitter should be such that the radiance of the IR source results in a detectable irradiance at the required range. This will depend on the radiant intensity of the transmitter and the photosensitivity and radiant sensitive area of the detector. A typical IR photodiode has a photosensitivity such that an irradiance of around 1W/cm2 is detectable. At a distance of 25 metres, this would require a source of radiant intensity 1W/sr, which can be produced by a small number of typical IR emitters in combination. Detectors can also be used in combination to improve receiver sensitivity. This is useful in the detection of re ected signals which are attenuated. The use of incoherent IR light for signaling is remarkably free from restriction. Equipment operating in the IR spectrum is not believed to be subject to licencing or approval by any regulatory agency. Current health and safety guidelines in a number of countries relate safe average levels of IR to those found in bright sunlight.
3.2 Carrier modulation A baseband modulation scheme is used. Broadband schemes are possible, and are the subject of some interest for higher bandwidth applications. The techniques involved preclude the use of a broadband scheme for simple practical applications requiring low power, low bandwidth mobile communication using currently available optoelectronic technology. A pulse position modulation scheme is used. In this scheme, a sequence of short pulses of carrier are transmitted. The relative timing between the pulses of carrier is used to code information. A binary pulse position modulation is used, where two dierent relative positions are used to code bit values `0' and `1'. Additional relative positions are used to code synchronisation information. The carrier is modulated by a transmitter to form discrete sequences of pulses which denote a message. Messages are broadcast without synchronisation or reservation of the carrier, since the level of complexity required to synchronise all potential transmitters within a zone is unwarranted. Furthermore, since the carrier is inherently unreliable, synchronisation is in general not possible. Collision detection at the source is also not possible, because of the hidden terminal problem. The collision avoidance scheme mentioned above is a simple, pragmatic approach to making good use of the carrier.
3.3 Physical Layer Protocol Table 1 describes the physical format of all messages transmitted over the IR network. The message format is further illustrated in Figure 1. The following notes expand the description. message start-sync preamble
:= := := value packet-sync := packet := byte := coding bit := byte-sync := zero := one :=
start-sync preamble packet-sync packet 125s gap byte `0x7f' 300s gap byte byte-sync byte ... byte-sync byte bit ... bit msb ... lsb zero one 200s gap 150s gap 100s gap j
Table 1: Physical Layer Protocol Pulse width. In the physical layer protocol of the IR network, pulses of width 3s are used in the modulation scheme. Pulses of this width are well within the response characteristics of typical emitters and detectors, and are comfortable for manipulation by hardware or software implementing the physical layer.
Figure 1: Physical Layer Protocol
4 IR Data Link Layer 4.1 A connectionless network In the OSI model, the function of the data link layer can be split into two groups. The lower level Media Access Control (MAC) group takes the transmission system provided by the physical layer and superimposes a scheme for transmitting frames of user data, addressing, and control information. The higher level Logical Link Control (LLC) group adds ow and error control by implementing an end-to-end logical connection. The data link layer of the IR network has an explicit mechanism for detecting errors and acknowledging the receipt of error-free packets. It does not however, include the necessary addressing information to establish logical end-to-end connections. Network architectures of this kind without the functions of an LLC layer are said to be connectionless.
Figure 2: Data Link Layer Protocol 7
A Olivetti Research Active Badge A.1 Description Active Badges are small mobile devices about 5cm square, weighing about 50g, with internal lithium batteries lasting a year or more. The device transmits and receives over the IR network, and is intended to be worn by personnel in an environment appropriately equipped with base sensors (see Appendix D). The primary function is to behave as a personnel locating tool and to provide an extremely simple ubiquitous means of interacting with the environment. Each badge has a globally unique device address. Under normal circumstances, a badge periodically broadcasts an IR message containing its address. The period is around 10 seconds, with in-built variation as described earlier. The badge can be stimulated into transmitting at any time by depressing one of two pushbuttons, or by the presence of an external radio frequency eld of around 290kHz and appropriate strength. Stimulating radio elds can be simply modulated and the badge is able to distinguish between a small number of patterns (see Appendix C). The cause of the stimulating event is coded in the broadcast message. All external stimuli reset the 10 second interval. The badge has a receive capability, which for power saving reasons consists of a window of receive opportunity after each address broadcast. Badges receive messages which have been stored and forwarded from a sensor or sensors in the appropriate zone. The window is approximately 1ms wide. Received messages contain commands addressed to the badge, which are decoded and executed. The badge replies to all commands received. The badge has a small amount of internal state, and there are commands to set and read the state. The badge also has three simple output devices which are a pair of visible LEDs and a piezo-ceramic speaker, and there are commands to light the LEDs and to sound the speaker. Each badge has a private 128 bit key and an encryption algorithm. An authentication command presents a sequence of random bytes which are encrypted by the badge and returned in the reply.
Figure 3: Active Badge Data Link Layer Protocol block 9
A.3 Data from Active Badge (sssd=`1000') The address broadcast message is the only unsolicited data from the Active Badge. The message consists of the badge address, and event speci c data payload. The badge speci c codes `cccc' in the type byte are interpreted as event codes. Table 4 describes the list of possible badge events. event
periodic broadcast side button middle button badge in eld side button in eld middle button in eld badge in unrecognised eld side button in unrecognised eld middle button in unrecognised eld
0000 0010 0001 0100 0110 0101 1000 1010 1001
sequence sequence sequence sequence eld sequence eld sequence eld sequence sequence sequence
:= coding := coding
byte unsigned integer [byte [byte]] see Appendix C
Table 4: Data from Active Badge Sequence. The sequence number is incremented by 1 for each address broadcast message. 0x wraps round to 0x01, with value 0x00 reserved. The sequence number provides a simple mechanism for a system to lter duplicate messages propagated by the reception of the same broadcast at more than one base. Field. A variable length code representing the modulation pattern of the stimulating eld. The encoding scheme is described in Appendix C.
A.4 Data to Active Badge (sssd=`0000') For messages from a base to an Active Badge the format of the data portion depends on the value of the badge speci c code `cccc' which is interpreted as a command for the badge to execute. Table 5 describes the list of Active Badge commands. command cccc
challenge sound set-state get-state set-home get-home
random tones state
identi er identi er identi er
0000 0010 0011 0100 0001 0111 where
identi er random tones tone ddd ppppp state status r g home protocol length
:= coding := := := coding := value := value := := coding := value := value := := value := coding
[byte] unsigned integer length byte ... byte[length-1] length tone ... tone[length-1] byte `dddpppp' bit ... bit duration bit ... bit pitch status tones byte `xxxxxxrg' bit red LED o (`0') or on (`1') bit green LED o (`0') or on (`1') length protocol byte ... byte[length-2] byte `0x00' (ip address) byte unsigned integer
Table 5: Data to Active Badge Identi er. If included, the command identi er byte allows the badge to execute commands idempotently. A system can arrange for successive commands to contain dierent identi ers. On receipt, the badge compares the identi er with that of the last command executed. If the identi er is dierent, the command is executed and the identi er is stored in the badge, otherwise the command is ignored. Challenge. The system generated random sequence of at most 5 bytes is encrypted by the badge using an algorithm and a private 128 bit key set at manufacture. The encrypted bytes form the data payload of the reply. In this symmetric authentication scheme, both the algorithm and the key are shared secrets with the system, which can therefore verify the identity of the badge.
Sound. A sequence of at most 5 tone bytes are interpreted and sounded on the speaker by the badge. The pitch is coded as a number of semitones from `00001' (low) to `11111' (high). Pitch `00000' is silent, and can be used as a rest. The duration is coded as a number of beats as show in Table 6. where each beat lasts approximately 1/16th of a second ddd beats 000 001 010 011 100 101 110 111
48 24 12 6 4 3 2 1
Table 6: Tone duration codes Set state. The status byte of the badge is set. The status of the visible LEDs will be displayed for the duration of the accompanying sequence of tones which is of maximum length 4. The status of the visible LEDs will be redisplayed for the duration of each subsequent button press. The sequence of tones is not stored and cannot be replayed. Get state. The value of the status byte is retrieved from the badge. Set home. Up to ve bytes of additional state information is stored in the badge. It is intended that this state is used to contain the address of an agent responsible for the badge at the home organisation of the badge. The rst byte is an address protocol character. Value `0x00' denotes an ip address. Get home. The home address state bytes are retrieved from the badge. This can be used to enable a system to interrogate an unknown badge and contact the agent of the badge to negotiate the exchange of information.
A.5 Replies from Active Badge (sssd=`0100') For reply messages from an Active Badge the format of the data portion depends on the value of the badge speci c code `cccc' which is interpreted as the code of the command which has been executed. Table 7 describes the list of Active Badge command replies. command bbbb
challenge sound set-state get-state set-home get-home
response tones set-status get-status home home
`0000' `0010' `0011' `0100' `0001' `0111'
identi er identi er identi er identi er
where identi er
:= coding response := tones := tone := coding ddd := value ppppp := value set-status := status := get-status := version := home := protocol := value length := coding
[byte] unsigned integer length byte ... byte[length-1] length tone ... tone[length-1] byte `dddpppp' bit ... bit duration bit ... bit pitch status tones byte version status byte length protocol byte ... byte[length-2] byte `0x00' (ip address) byte unsigned integer
Table 7: Reply Data from Active Badge Version. A version number of the Active Badge. The byte is coded as two 4 bit unsigned integers. The most signi cant 4 bits represent a major version number, and the least signi cant 4 bits represent a minor revision number.
B Olivetti Research Equipment Badge B.1 Description The equipment badge is derived from the Active Badge designed to be attached to pieces of oce equipment. The equipment badge is smaller than the Active Badge, about 3cm x 5cm and weighing 25g. The device uses the IR physical layer protocol and periodically transmits a globally unique address. The interval between transmissions is about six minutes, increasing the battery life of the equipment badge over the Active Badge. The equipment badge may be stimulated into immediate transmission by an external radio eld, or by pressing a single pushbutton. The equipment badge is able to decode and report modulated radio elds in the same way as the Active Badge. The equipment badge has no IR receive capability, which helps to improve the battery life. The equipment badge also monitors two digital inputs which can be connected externally via a small socket. The value of the inputs is reported in each equipment badge transmission.
B.2 Data Link Layer Protocol block The equipment badge data link layer protocol block is identical to Active Badge data link layer protocol described in Table 3.
B.3 Data from equipment badge (sssdcccc=`1001001') The address broadcast message is the only message from the equipment badge. The message consists of the badge address and the event speci c payload, and is described in Table 8. data eld
:= := coding status := coding e := f
eld status [byte [byte]] see Appendix C byte `000efpqb' bit `unrecognized eld' bit bit `device in eld' bit bit bit `input port status' bits bit `button press' bit
Table 8: Data from equipment badge Field. A variable length code representing the modulation pattern of the stimulating eld. The encoding scheme is described in Appendix C.
C Proximity Fields A low-power radio eld with a frequency of 290kHz is generated at a xed location within an IR zone. This eld is then pulse-width modulated with a xed mark/space ratio. The size of the eld generator's aerial and the eld strength are chosen to give the eld an envelope which typically surrounds an oce desk. A tuned circuit in the Active Badge and equipment badge is able to detect this eld, and the on-board processor will attempt to determine the mark/space ratio of the eld that it nds itself in. Once interpreted, the identity of the eld is encoded in the periodic address broadcast.
C.1 Olivetti Research Field Generator The eld generator is a stand-alone unit, requiring an external 12v supply. An external aerial is connected, which typically consists of 12m of wire coiled into a 4 turns of a 3m loop. The eld strength is adjustable with an internal potentiometer. The modulation pattern is determined by the con guration of an internal 28 pin header as de ned in Table 9. pin to be grounded 2-24-25-26-27 2 2-6-27 2-5-26 2-5-6-26-27 2-4-25 2-4-6-25-27 2-4-5-25-26 2-4-5-6-25-26-27 2-3-24 2-3-6-24-27 2-3-5-24-26 2-3-5-6-24-26-27 2-3-4-24-25 2-3-4-6-24-25-27 2-3-4-5-24-25-26 2-3-4-5-6-24-25-26-27
modulation pattern constant eld 1ms/1ms 2ms/2ms 3ms/3ms 4ms/4ms 5ms/5ms 6ms/6ms 7ms/7ms 8ms/8ms 9ms/9ms 10ms/10ms 11ms/11ms 12ms/12ms 13ms/13ms 14ms/14ms 15ms/15ms 16ms/16ms
Table 9: Header con guration for modulation patterns;
C.2 Radio Field Sampling The Active Badge and equipment badge sample the radio eld by monitoring a digital input which indicates whether the radio eld is detected or not. The badge samples the input continuously for a period, building an internal representation of the eld modulation pattern. The badge will sample for 32ms, or until two mark/space pairs or a eld error is detected, whichever is the sooner. A eld error is deemed to have occurred if a pulse 16
shorter than 0.5ms is detected. The internal representation of the modulation pattern is then coded into an IR address broadcast message as described below.
C.3 Radio Field Encoding The Active Badge and equipment badge will transmit zero, one or two bytes of eld code in the payload of the IR address broadcast message. If the eld is constant and unmodulated, or a eld error was detected, no extra bytes are sent and bit or bit of the badge speci c code in the message type byte are set (see Table 4). Otherwise, the mark/space pattern is coded in 2 or 4 nibbles as described in Table 10. eld byte nibble p vvv
:= := coding := value value := value
byte [byte] nibble nibble `pvvv' bit `0' (space) `1' (mark) bit ... bit `000' (1ms) `001' (2ms) .. . `110' (7ms) `111' (8ms)
Table 10: Mark/space coding in nibbles The most signi cant bit of each nibble distinguishes between mark and space codes. The bottom three bits of each nibble indicate the number of milliseconds, to the nearest millisecond, for which the mark / space was observed. For pulses longer than 8ms, the following nibble continues the count of the pulse length. If the second byte is identical the rst byte, then only one byte is encoded in the message.
D Olivetti Research Base Sensor D.1 Description The base sensor is a topologically xed device which receives and transmits over the IR network. The base sensor is also equipped with a backbone network interface, and performs a bridging function between the two networks. Sensors are identi ed and addressed by a single byte set at manufacture. A red LED on the sensor indicates a message being received over the IR network, and a green LED on the sensor indicates a message being received over the backbone network. Two FIFO memories buer messages being bridged between networks. The sensor has a single 16 byte buer for use in store and forward IR communications to intermittently receiving devices, for example the Active Badge. The sensor can also transmit directly over the IR network to devices which are permanently receiving. The backbone network is constructed using a simple four conductor cable such as twisted pair telephone cable. Two of the conductors are used to supply power to the base sensors, and the remaining two are signal conductors. The topology of the backbone network is an arbitrary rooted acyclic tree where the root or hub is the power source. In practice, the physical extent and topology of the network are constrained by the need to supply power to sensors. Amongst the factors to consider are the voltage and current supply of the source and the power requirements of the sensors. Sensors typically consume 50mA, and require a supply of between 9V and 12V. The resistivity and length of a wire run and the load at the end of the run are important considerations. The optimal topology of the backbone network for a given arrangement of sensors is a star con guration with the hub at a point which minimises the total length of wire. Simple applications of Ohms law are useful in determining whether a topology is practical. Logically, the signal conductors carry RS232 style signals, and the base sensors are con gured to operate at 9600 baud, 8 data bits, no parity, 2 stop bits. Physically, however, the representations of the logic levels are shifted to form a wired-or network. A simple level-shifter at the network hub is used to convert to true RS232 levels. A variant of the base sensor has a true RS232 level interface, and can be used where a single base sensor is required. A dedicated power supply is required for this device. The RS232 level signal is connected to the serial interface of a network control host. The host can be a standard workstation with the network control function as a background task, or a dedicated system. For applications where access to the IR network is part of a generally available communications infrastructure, the host will have additional networking and further bridging functionality.
D.2 Backbone network host to sensor protocol The host to sensor protocol is described in Table 11. message := preamble := value address := value packet := type := coding sss := value value value ccccc := value value length := coding block :=
preamble address [packet] byte `0x' byte `0x00' to `0xfe' type length block byte `sssccccc' bit ... bit `000' (data for immediate ir transmission) `110' (data for buered ir transmission) `111' (data to base sensor) bit ... bit base sensor speci c code (sss=`111') mobile device speci c code (otherwise) byte unsigned integer byte ... byte[length - 1]
Table 11: Backbone network host to sensor protocol Message. There are three types of message. First, a control message can be issued to a sensor. Second, a message for relay over the IR network can be transmitted. Third, a message can be issued to eect a poll of a sensor which results in received buered IR packets being relayed back to the host. Sensors do not transmit unsolicited data, and every byte received over the IR and buered in the sensor, or reply to a control message, must be extracted by explicit polling. Preamble. The preamble is an escape sequence to delimit address bytes. All instances of byte values `0x' in the packet are transmitted twice over the backbone network. Address. In the protocol between the host network controller and base sensors, all messages from the host contain the explicit address of a sensor. There are 255 possible sensor addresses in the range `0x00' to `0xfe' inclusive, which determines the maximum logical size of any one backbone network. Packet. The optional packet is one of three types. Type codes sss=`000' and sss=`110' indicate messages directed at mobiles intended for relay over the IR network. Type code sss=`111' indicates a control message for the sensor. Block. The format of the block is dependent on the type of the message. For messages intended for mobiles, the format is as described in the generic data link layer protocol, Table 2. In particular, for messages intended for Active Badges the format of the block is as described in Tables 3 and 5. Messages intended for the sensor itself are described below.
D.3 Control commands from host to sensor For backbone network host to sensor control messages, the format of the block is as described in Table 12. command
reset-buer reset- fos get-state output-on output-o persistence-on persistence-o
00000 00001 00010 00011 00100 00101 00110
null null null null null null null
Table 12: Control commands to sensor Reset buer. Cause any data in the 16 byte store and forward buer to be discarded. Reset fo's. Cause any data in the FIFO's bridging the IR and backbone networks to be discarded. Useful at sensor initialisation. Get state. Retrieve internal sensor status. Output on. The base sensor has a single digital output, which is turned on by this command. Typically, the digital output is connected to a circuit generating the radio frequency eld which can stimulate the Active Badge. Output o. Turn the digital output o. Persistence on. Do not automatically ush the 16 byte store and forward buer when the buered message has been transmitted. In this mode, the message is eligible for repeated transmission. Persistence o. Automatically ush the 16 byte store and forward buer when the buered message has been transmitted.
Figure 4: Backbone network protocol interleaving Packet. The packet is one of three types. Type codes sss=`100' and sss=`010' indicate messages received from mobiles being relayed over the backbone network. Type code sss=`011' indicates a reply to a sensor control message. Block. The format of the block is dependent on message type. For messages received from mobiles, the format is as described in the generic data link layer protocol, Table 2. In particular, for messages received from Active Badges the format of the block is as described in Tables 3, 4 and 7. Messages generated by the sensor are described below.
D.5 Control replies from sensor to host For backbone network sensor to host control replies, the format of the block is as described in Table 14. command ccccc
status length version status status
:= := coding value := := :=
length version status status byte unsigned integer `0x03' byte byte byte
Table 14: Control replies from sensor Version. A version number of the base sensor. The byte is coded as two 4 bit unsigned integers. The most signi cant 4 bits represent a major version number, and the least signi cant 4 bits represent a minor revision number.