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Enhancing Performance of Asynchronous Data Traffic Over The Enhancing Performance of Asynchronous Data Traffic over the Bluetooth Wireless Ad-hoc Network Abhishek Das, Abhishek Ghose, Ashu Razdan, Huzur Saran Ý & Rajeev Shorey IBM India Research Laboratory, Block 1, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India Email: [email protected] Phone: 91-11-6861100; Fax: 91-11-6861555 Abstract—Emerging technologies such as Bluetooth are expected to be- These key features of Bluetooth will significantly impact the come a ubiquitous solution for providing short range, low power, low cost, performance of data traffic over Bluetooth. Fragmentation of pico-cellular wireless connectivity. Bluetooth is a Master driven Time Divi- sion Duplex (TDD) system that supports an asynchronous channel for data large data packets by performing SAR, which allows them to be traffic as well as synchronous channels for voice traffic. Data applications transmitted in small baseband packets, may increase their end- running over Bluetooth such as http, ftp and real audio will need transport to-end delay. Master driven scheduling at the MAC level will layer protocols such as TCP and UDP to send packets over the wireless affect throughput and queueing delay. The success rate of data links. In this paper we study several schemes designed to improve the per- formance of asynchronous data traffic over a Bluetooth piconet that sup- transmissions will be affected by the presence of FEC and ARQ ports multiple active slaves. We propose and compare a number of SAR mechanisms at the link level. The bandwidth available for data policies and MAC scheduling algorithms with a view towards enhancing traffic will be reduced in the presence of voice connections. The the performance of transport layer sessions. We investigate the effect of different FEC and ARQ schemes at the baseband level, using a two-state effect of these issues needs to be better understood in order to Markov channel model for the Bluetooth RF link. We also study how the enhance the performance of asynchronous data traffic over Blue- presence of circuit-switched voice impacts the performance of data traffic. tooth. Keywords— Medium Access Control (MAC), Scheduling, Time Division Since TCP [3] is the most widely used transport protocol for Duplex (TDD), Segmentation and Reassembly (SAR), Forward Error Cor- rection (FEC), Automatic Repeat Request (ARQ), TCP, UDP. reliable data services over the Internet, our primary focus in this paper is on the performance of TCP over Bluetooth. A key ob- servation for wireless environments is that since TCP interprets I. INTRODUCTION packet loss as a sign of congestion and cuts back its window, its Bluetooth technology [1], [2] allows for the replacement of performance may deteriorate in the presence of random losses the numerous proprietary cables that connect one device to an- that cannot be attributed to congestion [4]. Considerable atten- other with a universal short-range radio link. Beyond untether- tion is being given to the design of a better TCP over the wireless ing devices by replacing cables, Bluetooth provides a universal link [5], [6]. Since Bluetooth is distinct from existing wireless bridge to existing data networks, a peripheral interface, and a LANs, these studies are not directly applicable to Bluetooth. We mechanism to form small private ad-hoc groupings of connected also study the performance of Constant Bit Rate (CBR) applica- devices away from fixed network infrastructures. tions using UDP, a minimal non-guaranteed datagram service without any flow control or congestion avoidance. Bluetooth has a number of distinctive features compared to existing wireless LANs such as: In this paper, we propose two SAR policies with the aim of increasing link utilization and decreasing end-to-end delay of ¯ Support for both data and voice traffic data packets. When multiple data transfers share the wireless ¯ Frequency hopping to avoid interference link, as in a Bluetooth piconet, MAC scheduling algorithms are ¯ A master driven Time Division Duplex (TDD) system at needed to achieve fair sharing of bandwidth, high link utilization the Medium Access Control (MAC) layer to support full and low queue occupancy. We demonstrate that Round-Robin duplex transmission scheduling is unable to meet these requirements and propose ¯ Segmentation and Reassembly (SAR) to handle large data three new scheduling algorithms which meet these criteria ad- packets equately. We also incorporate Channel State Dependency [7] ¯ Support for link level Automatic Repeat Request (ARQ) in these algorithms in order to improve the performance in the and Forward Error Correction (FEC) schemes presence of bursty wireless errors. In accordance with observa- tions reported in earlier studies of packet loss behavior in wire- Ý Abhishek Das and Ashu Razdan are in the Computer Science and Engineer- ing Department, Indian Institute of Technology at Kharagpur and Guwahati, re- less LANs [5], [6], [18], a two state Markov model has been spectively. Abhishek Ghose is in the Electrical Engineering Department, Indian used to model the errors in the wireless channel. Bluetooth pro- Institute of Technology, Mumbai, India. Dr. Huzur Saran is a Professor in the Department of Computer Science and Engineering, Indian Institute of Technol- vides support for error correction at the link level through FEC ogy, New Delhi, India. He is currently on a sabbatical at Stanford University, and ARQ schemes. We investigate the performance improve- CA, USA. Email: [email protected]; Dr. Rajeev Shorey is a Research Staff ment provided by FEC and ARQ schemes. We also compare the Member in the IBM India Research Laboratory, New Delhi, India. This work was done while the first three authors were summer interns in the IBM India performance of different versions of TCP, namely, Tahoe, Reno, Research Laboratory, New Delhi from May to July’2000. New Reno and Sack [3], [8] over Bluetooth. 0-7803-7018-8/01/$10.00 (C) 2001 IEEE IEEE INFOCOM 2001 Prior research closest to our work is that of Johansson et al TCP UDP TCP UDP IP IP [9]. They address the performance of TCP/IP over a Bluetooth PPP PPP wireless network but with very simplistic assumptions. They RFCOMM RFCOMM assume only two nodes (master and slave) in a Bluetooth pi- Master Slave 1 conet and study the behavior of TCP Vegas. Further, the authors model bit errors with a constant loss probability. They do not L2CAP assume any FEC for data traffic arguing that doing so will yield SAR SAR largest ideal throughput. They do not specify any ARQ schemes Host Controller Interface at the baseband level to prevent packet loss. In [10], the au- From Slave 1 From Slave 6 From Slave 7 LMP thors have analysed and compared the behaviour of three dif- ferent scheduling algorithms for Bluetooth: strict round robin Baseband polling, exhaustive polling and fair exhaustive polling. They study average delay versus bitrate for the three scheduling al- To Slave 1 RADIO CHANNEL gorithms. The authors have demonstrated an increase in per- To Slave 6 formance (high throughput and low delays) when allowing data To Slave 7 packets to be sent in multi-slots (i.e., 3 or 5 slot baseband pack- ets). The simulation study uses very simplistic assumptions: (i) Fig. 1. The Bluetooth Protocol Stack the packet loss probability is constant for all packets, (ii) the master does not send any traffic to the slave but just forwards tion of duplex slots at regular intervals. The ACL link is a point- traffic from one slave to another, (iii) the buffers are assumed to-multipoint link between the master and all the slaves in the to be unbounded. Kalia et al [11] have proposed some simple piconet. A baseband packet may occupy one, three or five slots. SAR policies and MAC scheduling algorithms for Bluetooth. The desired baseband packet length can be decided based on They propose two scheduling policies that utilize information criterion such as the quantity of data to be transmitted or the about the size of the Head-of-the-Line packet at the master and number of contiguous slots available in the presence of voice slave queues to schedule the TDD slots effectively. These poli- traffic. cies achieve high throughput and greater fairness compared to the Round-Robin based scheduling policies. The work in [11] A fast unnumbered ARQ scheme is used to inform the source is restricted only to the link layer and hence is not optimized for of the success or failure of transfer of payload. The base- transport layer sessions. band packets are retransmitted till a positive acknowledgement =¿ (ACK) is returned or until timeout is exceeded. An optional ¾ The remainder of this paper is organized as follows. Section rate Forward Error Correction (FEC) can be used on the data II gives a brief introduction to the Bluetooth technology. In Sec- payload to reduce the number of retransmissions. The packet tion III, we propose SAR and MAC scheduling policies and also =¿ header is always protected by a ½ rate FEC since it contains discuss other important design issues in Bluetooth which affect valuable link information and should be able to sustain more the performance of asynchronous data traffic. The simulation errors. model is presented in Section IV. In Section V, we present our simulation results and analyses. Finally, we conclude by pre- B. Link Manager Protocol (LMP) and Logical Link Control and senting a summary of our results and some suggestions for fu- Adaptation Protocol (L2CAP) ture work in Section VI. LMP and L2CAP are layered above the Baseband Protocol II. BLUETOOTH TECHNOLOGY and reside in the datalink layer. LMP assumes the responsibility Bluetooth is a specification for the wireless communication of managing connection states, enforcing fairness among slaves, of voice and data using a short-range radio. It is defined by a power management and other management tasks.
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