Hidden and Exposed Nodes and Medium Access Control in Wireless Ad-Hoc Networks M. J. Saeed, M. Merabti, R. J. Askwith School of Computing and Mathematical Sciences Liverpool John Moores University Byrom Street, Liverpool L3 3AF, UK Email:
[email protected], { M.Merabti, R.J.Askwith}@ljmu.ac.uk
Abstract – A lot of research has taken place in the field of MANETs but it still has a lot of issues that need to be addressed for its proper and multi-purpose utilization. Design of MAC protocols is a very important element in solving some of the issues. A well designed MAC protocol should be able to maintain the fairness among nodes and deal with Hidden and Exposed Nodes. This paper explains the basics of the Ad-Hoc networks, hidden and exposed nodes problem, a general background that explains how IEEE 802.11 deals with these issues. This paper also proposes novel ideas to make the network aware of the communication and use the variable transmission range to avoid interference/collision.
Variable transmission range can be used as a way of reducing the communication area between the nodes thus freeing the medium of unnecessary occupation which should help in increasing the throughput of the whole network. In the following sections Ad-hoc Wireless Networks are discussed in detail to provide an understanding of the issues associated with it. Further a brief background and problem area is discussed. In Section 5 we have discussed a solution for the problem area followed by the conclusion and future work.
I. INTRODUCTION
II. FUNDAMENTALS OF AD-HOC WIRELESS NETWORKS
Mobile Wireless Ad-Hoc Networks (MANETs) can be set up when and where needed without the presence of a fixed infrastructure as the nodes may move around [1]. This features make MANETs very practical and easy to deploy in places where existing infrastructure is not capable enough to allow communication, e.g. in disaster zones, or infeasible to deploy. At the same time it creates huge problems as well. One problem lays in the design of the Medium Access Control (MAC) Protocols which defines how the wireless medium is shared by all nodes. It is possible to design a MAC protocol that can handle the sharing of the medium but at the same time has proved to be one of the most challenging tasks for the researchers. Due to the nature of the network distributed random access MAC is preferred over centralized MAC, however distributed random access protocols suffer from Hidden and Exposed nodes issues. Hidden and exposed nodes are explained in the following sections. A lot of research has taken place already to minimize this problem; this paper provides a solution to this problem with a different perspective.
ISBN: 1-9025-6013-9 © 2006 PGNet
A mobile ad hoc network (MANET) is a wireless network temporarily and spontaneously created by mobile stations without requiring any infrastructure or central control. Network managements and communications are typically performed in a distributed manner. Though ad hoc networks are treated with little difference in IEEE standards for wireless networks as a whole, some unique features make ad hoc networks distinct from other types of wireless networks such as wireless LANs. An ad-hoc wireless network is a network without any base stations, an 'infrastructureless' network. Such a network supports 'anytime' and 'anywhere' computing, allowing the spontaneous formation and deformation of mobile networks. In such a network each mobile host acts as a router, peer-to-peer communications are possible as well as peer-to-remote communications. Some important features of ad-hoc wireless networks: • a host is no longer just an end system, it also acts as an intermediate system. • the network topology may change over time
• •
every node may be mobile the mobile nodes may have a limited power capacity • there is limited wireless bandwidth for communication • the channel quality is varyable • There is no centralized entity, in other words the network is distributed. As a consequence of the above, the following interesting problems arise: how to support routing, how to support channel access [2], how to deal with mobility, how to conserve power and how to use bandwidth efficiently and the issue of Exposed and Hidden nodes. Wireless channel is a shared and very limited resource. The MAC protocols used over wireless networks are distributed protocols which try to avoid collisions and provide the nodes in a network with an access to the channel in a fair manner. A channel access protocol (MAC) is termed to be unfair if it fails to provide the channel access to individual nodes without giving preference to one node over others. Though the wired Ethernet protocol based on CSMA/CD is known to be fair, its wireless version 802.11 based on CSMA/CA is proven to be unfair. Fairness problems occur when some nodes tend to grab the shared channel thereby making other nodes suffer. Although receiver oriented approach of channel utilization is fairer than transmitter initialled approach, however, there is certain unfairness related to the receiver initiated approach as well. Hence this paper will focus on the following Objectives: 1. Discussion on MAC level issues and protocols. 2. Avoiding interference and hence focusing mainly on the hidden and exposed nodes problem [3]. As discussed [4] interference range can be much higher than the transmission range of the node thus, the research will have to keep these issues under consideration. 3. Develop a solution for enhancing the throughput of the whole network [5]. II. BACKGROUND In this section we review the latest information available on the MAC level protocols that have been proposed and have tried to solve the hidden and exposed node problems. Although these protocols have attempted to
solve the problem but have given rise to other problems on the way and in some cases have left question marks under certain conditions. A. Carrier Sense Multiple Access (CSMA) Carrier Sense Multiple Access (CSMA) is one of the earliest mechanisms adopted for ad hoc networks. In CSMA, a transmitter will first sense the wireless channel in the vicinity and refrain itself from transmission if the channel is already in use. Various methods such as ALOHA [6] and n-persistent algorithms [7] can be used to determine how long the deferred node should wait before the next attempt. CSMA introduces hidden node and exposed node problems, which are mentioned earlier. It is assumed that each node can communicate with another node only if there is a link (solid line) between them. In a typical exposed node problem a node within the range of the transmitter may be unnecessarily prohibited from accessing the medium and thus decreases the network throughput [8].
Figure 1: Hidden Node Problem
B. Medium Access Collision Avoidance (MACA/ MACAW) To solve the problems with physical sensing, the mechanism of "virtual sensing", has been proposed. The virtual sensing mechanisms typically rely on the transmitter and receiver to perform a handshake prior to the transmission of the data packet. More specifically, The Medium Access Collision Avoidance (MACA) method proposed by Karn [9] implements the handshake via a pair of Request-To-Send (RTS) and ClearTo-Send (CTS) messages. Any node that overhears the RTS signal but not CTS is ok to send out packets in a certain time period as either the RTS/CTS handshake is not complete or it is out of the range of the receiver. MACAW [10] is an enhancement of MACA that utilize a RTS-CTS-DS(Data Sending)-Data-ACK message exchange.
However, as indicated by Haas and Deng [11], new types of hidden/exposed node problems emerge when pure virtual sensing protocols are used. We further classify such case as an inefficiency problem where the network bandwidth is underused, in contrast to the original hidden node problem where packet collisions occur. Similarly, another type of exposed node problem also occurs in case of pure virtual sensing, the definitions of transmitter and receiver are associated with data packet, not control packets (i.e., RTS/CTS). Please note that collisions occur in such exposed node problem and the traditional problem with CSMA incurs underutilization of the network bandwidth.
Figure 2: Exposed Node Problem
C. 802.11 The IEEE 802.11 MAC protocol is another example of using both physical sensing and RTS/CTS handshake mechanisms. 802.11 is actually defined as the standard MAC and physical protocols for wireless LANs, not specially designed for multi-hop ad hoc networks. The MAC sublayer consists of two core functions: distributed coordination function (DCF) and point coordination function (PCF). DCF controls the medium accessing through the use of carrier sense multiple access with collision avoidance (CSMA/CA) and a random backoff time following a busy medium period. Carrier sense in CSMA/CA are performed both through physical and virtual mechanisms. In addition to RTS/CTS exchanges, all data packet receivers immediately sends back positive acknowledgment (ACK packet) so that retransmission can be scheduled by the sender if no ACK is received. PCF is not a good choice for the Ad-Hoc Wireless Networks as it requires a fixed infrastructure which is not available. Though the 802.11 MAC layer protocol intends to address the hidden node problem, it
pays almost no effort in resolving the exposed node problem. And both the hidden node inefficiency and exposed node collision problems still exist in 802.11. Based on simulation results, Xu [12] and also in [13] report that some serious instability and unfairness problems occur when TCP in used on top of 802.11 in ad hoc networks. IV. ANALYSIS Wireless communications in various forms has been the subject of much attention and research in recent years. With the emergence of Wireless Ad Hoc Networks also emerges the fact that ideally users of wireless networks will want the same services and capabilities that they have come to expect with wired networks. However, to meet these objectives certain challenges and constraints are faced that are not imposed by wired networks. In wireless networks, the nodes must use a common transmission medium instead of multiple point-to-point interfaces. The common transmission medium is a very scarce resource and so its efficient usage is highly needed. Medium Access Control (MAC) protocol in wireless Ad hoc networks controls the sharing of this common transmission medium and is responsible for the efficient use of the limited communication bandwidth of the wireless channel. Thus, the MAC scheme that governs the transmission of data packet between different nodes should attempt to maximize the number of packets exchanged per second which is calculated as the throughput and to minimize the delay in the network. However, designing a MAC protocol faces serious challenges in wireless networks because interference is inherent in all wireless systems and is one of the most important issues to be addressed in the design, operation and maintenance of wireless communication systems. To formulate, how should the participating nodes coordinate actions so that, the number of messages exchanged per second is maximized and time spent waiting for a chance to speak is minimized, is the core issue faced by wireless networks in particular and Adhoc wireless networks in particular. Uncontrolled transmission in a shared medium may lead to the time overlap of two or more packet receptions, called collision or interference resulting in corrupted packets at the destination. The topology dynamics of the ad-
hoc wireless networks with the use of a common transmission medium brings up the problem that, in some cases, a node may receive concurrent transmissions from multiple neighbours that cannot hear one another. We call these nodes as hidden from each other and this problem is defined as the hidden node problem. This can also be defined as the contention interference because two or more source nodes target the same receiver. Due the existence of the hidden node problem, unnecessary collisions of data packets occur which are followed by message retransmissions. Such message retransmissions cause wastage of the efficient bandwidth which could have been otherwise used for successful message transmission. There is also another serious issue with the design of efficient MAC schemes. Co-channel interference occurs when two or more pairs of nodes that are in communication range of each other try to transmit simultaneously in the same channel. The communication between one pair of nodes affects the packet transmission between other pairs. Such nodes are called as exposed nodes and the problem is defined as the exposed node problem [14]. Presence of exposed nodes also results in less efficient usage of the medium. The throughput degrades as multiple parallel transmissions are not possible even when they are feasible. A MAC protocol without the existence of hidden and exposed nodes is one of the fundamental problems faced by the ad-hoc wireless networks. The IEEE 802.11 was developed as a standard for MAC in wireless local area networks and subsequently applied to the ad-hoc wireless networks as well. IEEE 802.11 is based on the Carrier Sense Multiple Access Scheme with Collision Avoidance (CSMA/CA) and implemented a sender-initiated handshake mechanism of RTS-CTS-DATAACK for data transfer [12]. CSMA/CA always senses the channel before any transmission. If the channel is sensed busy, the sender waits for the channel to become available, also called the backoff period, before retrying. The Request-To-Send/Clear-To-Send (RTS/CTS) exchange before actual data transfer is there to reserve the channel before data transmission to minimize the number of data packets being dropped due to collisions. However, the standard was not able to completely eliminate the hidden node problem. The RTS and CTS sent using carrier sensing at
the sender can still collide at the receiver. Although the size of these control packets, RTS and CTS, is very small as compared to data packets, severe degradation in throughput occurs at high loads. IEEE 802.11 MAC is prone to inefficiencies at heavy loads, since with increasing traffic there is a higher wastage of bandwidth from collisions and backoff [11]. Also, the standard does not deal with the exposed node problem. Several MAC schemes have been proposed in the past that try to alleviate these problems. The aim of this paper is to introduce, study and compare a new channel access method which should reduces the number of collisions among data packets thereby providing better throughput performance and channel utilization than the standard 802.11 MAC. V. PROPOSED SOLUTION Keeping in mind the objectives mentioned in Section 2, a variable transmission range can be used as solution. Proposed plan is discussed here in brief. As discussed earlier the collision occurs when a receiving node comes within the transmission range of another node other than the node it is communicating with, that is transmitting as well. Thus it is proposed that variable transmission range can be viable in conjunction with the a concept that each node within its own transmission range can act as a ‘base station’, although this is not the base station as in case of a wireless LAN but just during the time of the transmission every node should be able to control how the other nodes within its transmission range should act. Thus it can feed other nodes when to completely go into a complete backoff stage or a partial backoff stage. Partial backoff stage means that the node should be able to communicate but make sure that its transmission can in no way interfere with the transmission that is already taking place, which can be done by feeding that node with the maximum transmission range at which it should communicate. For example if node A, B and C (Figure 3) are communicating and are at a distance of 20 meters from each other and there is a node D which is at 40 meters from node A but in the opposite direction from B then, according to the existing practice D will sense the medium and find out that the medium is in use and will drop into the backoff mode, but
with this proposed research it will be worked on that if that node can carry out successful transmission by limiting its transmission range in such a way that the communication between node A and B would not be effected then it should be allowed to carry on the transmission which can be called a partial backoff stage (Figure 3).
Figure 3
As A can also interfere with the transmission of D thus A, during its own communication, will limit its transmission range to the minimum required and feed that information to D as well and if at that stage D can carryout a transmission then it will be allowed to do so. Thus by dynamically adjusting the transmission range we should be able to increase the network throughput as nodes which would otherwise be stopped from transmitting would be allowed to carry out the transmission under certain limited conditions that it does not effect communication among other nodes, so whatever the transmission range of a node it will act as a controlling authority within that area, thus calling it a ‘Virtual Base Station’. The idea is that communication takes place with the already existing method of medium reservation by sending RTS/CTS before data transfer, so every node calculates the distance between itself and the node that it receives the RTS from with the help of Signal Strength to Distance(SSD) ratio. SSD ratio is calculated by every node and then sent with the every message that it sends across the network. Every node that receives the message calculates the distance from the transmitting node with the help of Signal Strength to Distance ratio and then determines the signal strength at which it needs to transmit. The second node that sends the CTS message also embeds the distance from the first node thus making sure that all the nodes within its transmission range are aware of the
communication range of the node and make sure that the signal strength at which they operate is low enough to make sure that there is no collision or interference in the ongoing communication. The first node as it receives the CTS message also gets the calculated distance between itself and the node it in communication with, that distance is then transmitted, as a Medium Reservation message (MR), to all the nodes within its range to make them aware of the communication range of the ongoing communication. With this new scheme the medium is not reserved when the RTS message is sent across but when the MR message is distributed. Where as the receiving node reserves the medium with the amended CTS message. As it is a Mobile network thus every node transmitting anything within a communication needs to be aware of the movement of the nodes it is communicating with. Sending SSD ratio with every message (i.e with CTS/RTS/Data/ACK) makes sure that the nodes are aware of any change in the position of the transmitting node. All the nodes that are not taking place in the transmission but are within the transmission range of the communicating nodes can use the SSD ratio to determine their own available transmission area and thus can go into a Partial Backoff instead or going into a complete backoff as suggested by other protocols. Kaixin Xu, Mario Gerla and Sang Bae in [13] have suggested through simulations that the interference range of a node can be much larger than the transmission range as very low signal can also cause interference at the receiving node thus acting as a hidden node. This interference range can be up to 1.78 times the transmission distance but they have considered that all the nodes in the network are transmitting at their maximum available signal strength. Thus this new proposed scheme should be able to deal with that extended interference rage as the transmission strength of the signal is reduced dynamically. And thus there should be a considerable reduction in the signal strength. Currently available protocols have mostly focused on the virtual or physical sensing of the medium to avoid interference/collision from the other nodes occupying the same medium and variable signal strength has only been used for the purpose of routing as in [15].
VI. CONCLUSION AND FURTHER WORK In this paper we have discussed the wireless mobile ad-hoc networks in detail, high lighting the fundamentals of wireless networks in general and mobile ad-hoc wireless networks in particular. Apart from that the core issues/problems associated with wireless ad-hoc have also been discussed in detail providing an overview of the complexities associated with the design and practical implementation of MAC layer protocols. Also the IEEE standard is discussed to analyse the Hidden and Exposed nodes problem faced by the wireless ad-hoc networks and how other available protocols have tried to deal with it. In the end a new MAC level protocol is proposed, which proposes new approach to the ‘Backoff’ criteria used by IEEE and the concept of ‘Virtual Base Station’. The results in this paper are preliminary in nature. We are currently working on formalising the mechanisms and developing a simulation and evaluation plan. REFERENCES [1]
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