“receiver” state in a random fashion. The results show that the

0-7803-6534-8/00/$10.00 0 2000 IEEE

+.(2) Start io INQUIRY INQUIRY at+ !;-;;-;I-

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which imposes each node to alternate between the “sender” and (1) stprt in the

connection establishment delay can be reduced if appropriate decisions are made in the choice of parameters.

I. INTRODUCTION

Bluetooth [5] is a promising new technology that enables portable devices to form short-range wireless ad hoc networks and relies on spread spectrum frequency hopping (FH) techniques [4] for link establishment and communication. The Bluetooth Baseband Specification [ 11 defines the Bluetooth point to point connection establishment as a two step procedure. The Inquiry Procedure is first used (6) Enter in order to discover the neighborhood synchronization information and then the Paging Procedure establishes the actual connection between the peers almost instantaneously if

P A G E ~ W ~

($2;;;;; it is performed shortly afier the Inquiry Procedure.

(Inquiring) (Inquired)

SCAN itate

‘ ___-

(7) Conoection Established

starts in INQUIRY SCAN and they both remain at this state until they connect. Thus there is a need for a symmetric protocol such that link can be formed between two units without a pre-assignment of states. This can be achieved by forcing the two nodes altemate between the INQUIRY and INQUIRY SCAN states until they form a connection during an overlap “on” interval where the devices are in different states. Typically, the connection establishment delay of the symmetric protocol will be larger than that of the asymmetric protocol. The reason is that at each “on” interval, the two units will connect after a random delay T = FS, + RB + Fs, , only if they both remain fixed at their (complementary) states for an amount of time greater than T. Otherwise, they have to wait for the next “on” interval. A first question on the proposed approach is: “should the units change states in a periodic or a random fashion?’ In [3] it has been analytically proven that the mean connection time can be arbitrarily large when each unit changes states in a deterministic fashion. Altematively, a random schedule can be imposed on the state residence times. Each unit will altemate between INQUIRY and INQUIRY SCAN with the residence time at each state following a random distribution. The figure below compares uniform and exponential distribution that have the same mean state residence times. 2 2000 - a 1800 = 1600

1400 2 1200 g 1000 r x 800 = 600 2 400 ‘I 200

W O

U

50 200 400 550 650 800 1000 2000 3000 4000

mean state residence time (ms)

I -uniform - exDonentia1 I We see that both distributions yield “U shaped” curves. For very small mean state residence times the “on” intervals of the merged schedule are very short and thus many of them are needed in order for the units to finally connect. For larger values of mean state residence times, devices may start altemating at the same state and they may have to wait a long time before one of them changes state. Hence for both distributions, there is a state residence time that yields the minimum link formation delay. The graphs show that this time is around 60Oms for both distributions. The exponential distribution performs slightly better for larger mean state residence times. However they both have about the same minimum delay region. Thus, by choosing the best mean state residence for each distribution, we can have about the same expected link formation delay. The similarity of the curves indicates that the choice of uniform mean state residence times may be a satisfactory one. This is an encouraging result since the uniform distribution is easily implemented in

hardware and is already supported through the Periodic-Inquiry-Mode command [2]. Of course the question of which distribution would provide the minimum expected link formation delay and whether this minimum will be radically less than the uniform or exponential, still exists and is under investigation.

IV. Discussion and future work

This paper can be regarded as an introduction to the issues that should be taken into consideration when forming ad hoc networks based on the Bluetooth technology (scattemets). The symmetric protocol introduced is a mechanism that guarantees an ad hoc point to point connection between two Bluetooth devices. When more than two devices exist and wish to form a scattemet “on the fly”, a protocol should be devised on top of this mechanism, that ensures that the resulting network will fulfill the requirements and structure of a Bluetooth scattemet. The next issue is that such a protocol should be able to connect in the minimum possible time. This issue stems from the envisioned usage model of Bluetooth scattemets where scattemet formation will be triggered by a service request (connection set up delay), perform the service requested (communication delay) and then tear down the network. This birth-live-die cycle will be repeated whenever there is a need for a service to be performed. During the network connection set up, random delays will occur during the formation of individual links. If there is no mechanism that tries to minimize these delays, then over many birth-live- die cycles, the network connection establishment delay can be a large portion of the subsequent communication delay. Since the network connection establishment protocol will be based on the symmetric point to point protocol, the parameters affecting the network delay will be affected by the ones that pertain to the single link formation delay. In the case of many nodes, collisions in the senders and receivers will introduce an additional term in the network connection set up delay. A topic of current research under way, is to devise a mechanism of appropriately choosing the altemating distribution, the mean state residence time per state and the back-off interval of each unit in order to find the optimum operating point that yields network connection establishment in the minimum possible time.

REFERENCES [ 11 J. Haartsen, “Bluetooth Baseband Specification, version 1 .O”, www.Bluetooth.com. [2] K. Fleming, “Bluetooth Host Controller Interface Functional Specification, version 1 .O”, www.Bluetooth.com. [3] T. Salonidis, P. Bhagwat, and L. Tassiulas, “Modeling and calculating the synchronization delay on pairs of Bluetooth devices”, work in progress. [4] J. Proakis, “Digital Communications”. [5] J. Haartsen, “Bluetooth-The universal radio interface for ad hoc wireless connectivity”, Ericsson Review, no3, 1998.

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