U1-Chip: Wireless Communication Module
for Fast Service Discovery
Yuichi Taguchi, Shunsuke Saruwatari,
Hiroyuki Morikawa, Tomonori Aoyama
The University of Tokyo
7–3–1 Hongo, Bunkyo-ku,Tokyo, 113–8656, Japan+81 3 5841 6710
{yuichi, saru, mori, aoyama}@mlab.t.u-tokyo.ac.jp
ABSTRACTThis paper describes a framework for location-based service
discovery using U1 -Chip, which is a wireless communication module specialized in fast device discovery and burst
data communication. These features are enabled by using
different frequency channels and media access control (MAC)
protocols between discovery stage and communication stage.
We show some application examples of the U1 -Chip, andthen present prototype implementation and evaluation.
Keywords

wireless communication module, service discovery, locationbased service
INTRODUCTION

In ubiquitous computing environment, many small smart devices provide various services anywhere, and we can enjoy services at anytime with a mobile terminal device likea PDA. To get connectivity easily and rapidly with them,
low-power short-range wireless communications have become a key technology. Therefore the Bluetooth and ZigBee have been developed as wireless personal area networks(WPANs) until now.

Mikio Hasegawa, Masugi Inoue
National Institute of Information andCommunications Technology
3–4 Hikarino-oka, Yokosuka,Kanagawa, 239–0847, Japan+81 468 47 5063
{mikio, inoue}@nict.go.jp

Figure 2 shows a message sequence chart of the U1 -Chip. A
master is a device requesting service discoveries (usually auser terminal device) and a slave is a device providing a service. At discovery stage, the adaptive tree walk protocol[1]is used in control channel to associate with slaves rapidly.
At communication stage, a master and a slave occupy one
data channel, and transmit data in bursts with carrier sense
multiple access (CSMA) protocol.
For these communications, we designed a protocol stackshown in Figure 3 and implemented it on the U1 -Chip prototype. The details of each layer are described in the following
sections.
Hardware Architecture
RF Transceiver(Chipcon CC1020)

Power Supply(USB or Battery)

Microcontroller(Renesas H8S/2215U)

Prototype

However, they are targeted at a continuous communication
forming a personal area network and are not always suited
to practical services. There are many services completed
within a short and burst communication of several Kbytes
data, for example, acquiring available information from surroundings, or getting control of devices around the user. We
call these services “instant services”. It is important for instant services to discover services rapidly and transmit datain bursts.
To realize these communications, we have developed a lowpower and low-cost wireless communication module called
U1 -Chip. The U1 -Chip uses very low-rate channel for reducing power consumption. However, using different frequency
channels and MAC protocols between discovery stage andcommunication stage, it can accomplish both fast device discovery and burst data communication.
IMPLEMENTATION

We have developed the U1 -Chip prototype shown in Figure
1. It consists of an H8S microcontroller from Renesas and a
CC1020 RF transceiver from Chipcon. The size of prototype
is 3 cm × 3 cm.

USB Interface
UART Interface

Figure 1: Overview of the U1 -ChipMaster

Slave
Query

Control
ChannelData
ChannelSlave Address

DiscoveryStage

Channel Assignment
Burst Communication
Channel Release

Communication
StageFigure 2: Message sequence chart

Application

Link Management LayerAt the beginning of the communication stage, this layer looks
for an available data channel with carrier sense and assigns
it to a slave. This data channel is released when a communication is finished.

APICommunication
Transport
Link Management
MAC( Adaptive Tree Walk )( CSMA )PHYControl
ChannelIn data channels, datagrams are divided into 1 Kbytes segments for reducing the risk of data loss. Fragmentation andreassembly are implemented in this layer.
Communication Layer

DataChannel

Figure 3: Protocol stack
Table 1: Physical layer specifications
Frequency band
Number of channels
ModulationRaw data rate
Range

Transport Layer

429.25 – 429.7375 MHz40 channels
FSK4800 bps
10 m

Physical Layer

The physical layer specifications of the U1 -Chip prototype
are shown in Table 1. A master only discovers physicallynear slaves because of the limitation of radio range. Thus
location-based services are provided for the user.
MAC Layer

The prototype has 40 frequency channels. One of them isa control channel, and the rest are data channels. We use
different MAC protocols in each channel: the adaptive tree
walk protocol in control channel, and CSMA protocol indata channels.
The adaptive tree walk protocol uses a tree algorithm forchannel arbitration. It is effective at discovery stage because
it acts as a contention protocol to provide low delay when
the number of slaves is small, but it acts as a collision-freeprotocol to provide good channel efficiency when the number of slaves is large. We use a quadtree composed of tree
IDs of slaves. A tree ID consists of a service class of 2 bytes
and an address of 8 bytes. The service class represents a category of service provided by the slave, and the address is aunique identifier for each U1 -Chip.
The protocol works as follows: First, a master sends a query
packet for all slaves. The slaves receiving this packet senda reply packet at different timings depending on their first 2
bits of tree ID, i.e. reply packets are sent to the master in
4 different time slots after the query packet. If there is 0 or
1 reply packet in a slot, the master can receive the packet
successfully. But if there are more packets, a collision occurs. In this case, there are many slaves having the sametree ID prefix, therefore the master continues to send a query
packet for this group of slaves recursively until discovering
all slaves.
On the other hand, CSMA protocol is used in data channel
for burst data communication. ACK packet is used to raise
reliability of communication.

The U1 -Chip uses request / response communication protocol like HTTP at the communication stage. This layer provides application programming interface (API) to use thisprotocol for application layer.
EVALUATIONAs a preliminary evaluation, we measured the discovery timein both experiment and simulation. Figure 4 shows the timethat a master spent for discovering all slaves at 4800 bps. In
experiment, the tree IDs of slaves were fixed. In simulation,
we assigned random tree ID to slaves at each simulation,and computed the average and the standard deviation of the
discovery time for 300 times.
The experimental result is mostly contained within the rangeof standard deviation of the simulation although the discovery time depends on the distribution of the tree IDs of slaves.
The master can discover about 5 slaves within a second at4800 bps.
CONCLUSIONS AND FUTURE WORKIn this paper, we have designed and developed the U1 -Chipthat is a low-power and low-cost wireless communication
module for fast device discovery and burst data communication. We have measured the device discovery time andpresented that we can accomplish fast device discovery inspite of low data rate. Currently we are developing practicalapplications of the U1 -Chip.
REFERENCES[1] Capetanakis, J. I. Tree Algorithms for Packet
Broadcast Channels. IEEE Transactions onInformation Theory, vol. IT-25, no. 5, Sept. 1979, pp.
505–515.
3
Time to discover all slaves [sec]MAC

U 1-Chip

2.5
2
1.5
1
Experimental result
Simulation

0.5
005

10
Number of slaves

15

Figure 4: Evaluation of the discovery time