Chapter - 1 INTRODUCTION

Transcription

1 Chapter - 1 INTRODUCTION Worldwide Interoperability for Microwave Access (WiMAX) is based on IEEE standard. This standard specifies the air interface of fixed Broadband Wireless Access (BWA) system supporting multiple services. The Medium Access Control (MAC) layer supports a primarily point to multipoint architecture, with an optional mesh topology. The Medium Access Control (MAC) layer is structured to support multiple Physical (PHY) layers specification, each suited to a particular operational environment. This standard enables rapid worldwide deployment, cost effective, alternatives to wired broadband access and high speed. Transmission Control Protocols (TCP) has built in reliability features include sequence numbering with resending, which is used to detect and resend missing or out of sequence segments. TCP also includes a complete flow control mechanism (called windowing) to prevent any sender from overwhelming a receiver. Neither of these built in TCP features which are good for real time audio and video on the Internet. These applications cannot pause and wait for missing segments, nor should they slow down or speed up as traffic loads vary on the internet. To deal with the problem of high speed wireless internet service, this work find out the optimal medium access control layer, physical layer parameters and TCP variants. 1.1 Problem Statement High speed internet service is not available in many rural areas due to unavailability of wired network or areas where establishment of wired network are not possible. To provide high speed internet services in those areas WiMAX technology can be useful. a) Many researchers have worked on Medium Access Control (MAC) layer parameters, Physical (PHY) layer parameter and operating parameters of WiMAX network, but not focused to find out the optimal Medium Access Control (MAC) layer parameters, Physical (PHY) layer parameter and operating parameters of WiMAX network with WiMAX network asymmetry. Bhagwant University, Ajmer, Rajasthan, India 1

2 b) The researchers evaluated the performance of TCP variants (like TCP New Reno, TCP Vegas, TCP Linux and TCP Sack1) in Wi-Fi, but not evaluated the performance of TCP variants in WiMAX network with bandwidth asymmetry. Bandwidth asymmetry means that channel characteristics in one direction do not matched with the other direction and how can bandwidth asymmetry affect TCP performance, since TCP relies on the timely arrival of acknowledgments (ACKs) to increase its congestion window and data sending rate. Under normal network conditions, an ACK is duly received for packets sent, and this helps the sender to increase the data sending rate. In the case of congestion, typically indicated by packet loss, TCP abruptly decreases its congestion window, and retransmits the lost packets. The retransmission may aggravate the congestion. Normally, there are two ways to indicate the packet loss or congestion: (1) Expiry of retransmission timer (2) Receipt of tree or more duplicate ACKs. In the presence of an imperfect ACK channel, the ACK clocking is disrupted, i.e., packets sent are not duly acknowledged. Consequently, at the sender side, the timer expires which TCP interprets as congestion, the congestion window plummets and the packets are retransmitted, even though these packets may have correctly reached to the receiver. This implies that the TCP throughput and goodput not only depends on the characteristics of the data sending channel, but also on the reverse channel used by ACKs. 1.2 Objective of Research Work The emergence of broadband access enables users to tap the potential of high performance backbone networks and high speed internet services. Broadband Wireless Access (BWA) is touted as the next logical step in broadband proliferation after a decade of Digital Subscriber Line (DSL) and cable modem. BWA networks can more easily be deployed in difficult terrains where other wired infrastructures are not economically feasible. Worldwide Interoperability for Microwave Access (WiMAX) is the industry name for the standard being developed for broadband wireless access by the IEEE Working Group. WiMAX is able to fulfill the promise of high speed Internet service and it should efficiently support the core transport protocol of the Internet Transmission Control Bhagwant University, Ajmer, Rajasthan, India 2

3 Protocol (TCP). In the proposed thesis the performance of different TCP variants are evaluated in WiMAX bandwidth asymmetry. The objective of proposed work is to investigate the dependence of Transmission Control Protocol variants performance on several Medium Access Control (MAC) layer parameters, Physical (PHY) layer parameter and operating parameters of WiMAX network such as: a) DL:UL Ratio. b) Offered Load. c) Channel Bandwidth. d) Cyclic Prefix. e) Modulation and Coding Schemes. f) Frame Duration. g) Simultaneous Two Way Data Transfer. h) Propagation Model. The second objective of proposed work is to find suitable DL:UL ratio in WiMAX network for better performance of transmission control protocol variants in one way data transfer and simultaneous two way data transfer with different offered load and also analyze the performance of transmission control protocol variants in WiMAX network by considering different channel bandwidth, cyclic prefix, modulation coding schemes, frame duration and propagation model. 1.3 Overview of Proposed Work The proposed work focuses on WiMAX networks using Time Division Duplexing (TDD), and Point to Multi Point (PMP) network topology. In TDD, Downlink (DL) and Uplink (UL) transmissions use the same frequency, but occur at different times during the frame duration shown in figure 1.1. The first portion of the frame is called downlink sub-frame and it is used for transmission from the Base Station (BS) to Subscriber Stations (SSs). The second portion of the frame is called uplink frame and it is used for transmission from SSs to base station. The ratio of the sub- Bhagwant University, Ajmer, Rajasthan, India 3

4 frames, henceforth, called DL:UL ratio which can be adaptive. In PMP, both downlink and uplink transmissions are controlled by the BS via Downlink MAP (DL-MAP) and Uplink MAP (UL-MAP) messages respectively (Figure 1.1). The MAP messages define the allocation start times and profiles that are to be used by each burst and are sent at the beginning of each frame. There are several forms of asymmetry such as bandwidth asymmetry, access asymmetry, delay asymmetry and packet loss asymmetry in WiMAX network [1]. In WiMAX, the DL:UL ratio plays an important role in the bandwidth asymmetry. In the situations of download only, TCP packets are sent in the downlink subframe, and ACKs returned in the uplink sub-frame which is shown in figure 1.2. While high DL:UL ratio values are good for large DL traffic; they may aggravate the bandwidth asymmetry and prohibit the smooth flow of ACKs. The protocol overheads also contribute to the asymmetry by consuming a significant portion of the bandwidth shown in figure 1.1 and figure 1.2. Moreover, bandwidth asymmetry, access asymmetry, delay asymmetry and packet loss asymmetry are also inherent in WiMAX. For best effort class of service, particularly in WiMAX where TCP typically operates, the uplink access mechanism is mainly based on a request grant process. Before SS can send traffic, it must first contend with other SSs within the radio range for bandwidth requests. A collision may happen when two SSs request received on BS at the same time; in this case an algorithm is invoked based on truncated binary exponential back-off scheme. Even if the bandwidth request succeeds, the SS still needs to receive the grant prior to sending any data. Therefore, there are bandwidth waste (to bandwidth request), delays (due to the request grant process) and unpredictability (due to back-off) in uplink resource access. On the contrary, the bandwidth on the downlink is fully controlled by the BS which schedules transmissions without much overhead, loss and delay. WiMAX is able to fulfill the promise of high speed Internet service and it should efficiently support the core transport protocol of the internet and it is called Transmission Control Protocol (TCP). Bhagwant University, Ajmer, Rajasthan, India 4

5 This study is useful to enhance the data rate of WiMAX network by using different TCP variants in bandwidth asymmetry, also useful for voice calls with good quality and more useful where large downloading traffic occurred. Figure IEEE TDD Frame and Protocol Overheads Figure Downloading Frame The proposed simulation study is presented for the performance of different TCP variants under the influence of WiMAX network asymmetry [1]. The Network Simulator (NS-2.31), WiMAX module, mobility module are required to analyze the performance of Transmission Control Protocol (TCP) variants in WiMAX network with certain network asymmetry because the network simulator is powerful tool for simulation study in the area of networking. It could analyze the performance of Transmission Control Protocol (TCP) variants in WiMAX network with the help of some performance metrics like Throughput, Goodput and Packet Loss. The Linux environment (Fedora 8) is suitable for Network Simulator (NS-2.31) and AWK script is useful for extracting the information from trace file. 1.4 Organization of Thesis The remaining part of the thesis report is organized as follows: Chapter 2 contains the survey of research which has already been done on Medium Access Control (MAC), Physical Layer (PHY), Quality of Service (QoS), Bhagwant University, Ajmer, Rajasthan, India 5

6 scheduling techniques, bandwidth allocation, Transmission Control Protocol (TCP) and WiMAX network asymmetry which helps to identify the problem statement. Chapter 3 describes WiMAX technology, Physical (PHY) layer, Medium Access Control (MAC) layer of WiMAX network and IEEE (WiMAX) network asymmetry. This helps to find out different design or operating parameters. Chapter 4 gives a description of Transmission Control Protocol (TCP), concepts and its variants. Simulation tool and methodology used to solve the problem describes in Chapter 5 and Chapter 6 presents the network topology, results and discussion. Chapter 7 concludes with some open questions and future directions to continue this work. Last part of the thesis is Chapter 8 which gives the list of references and at last is the list of publications considered during this research work. Bhagwant University, Ajmer, Rajasthan, India 6