Introduction The transition from wired to wireless networks have created new research advances in the wireless arena which have seen massive growth both in terms of services provided and the type of technology that have become available. These have revolutionized the entire wireless networks and will play an important role in future generation wireless sensor network for multimedia such as video surveillance systems. |
Wireless Sensor Networks (WSNs) [1,2,3,4] consist of sensor nodes that use low power consumption which are powered by small replaceable batteries that collects real-world data, process it, transmit the data by radio frequencies to their destination. WSNs are usually static nodes that send data to a server or a sink node for processing. |
WSN based applications usually have relaxed bandwidth requirement and can be classified under a number of areas including security and military sensing, home automation, consumer electronics, agriculture and environmental purposes, industrial control and monitoring. |
Security and Military sensing applications are usually used for magnetic door opening, smoke detection, to locate and identify targets for potential attack. Home automation and consumer electronics include universal remote control; a personal digital assistant type of device, wireless keyboards, toys, light control and remote keyless entry. Industrial control and monitoring sensors may include heating, ventilating and air conditioning unit of buildings that can regulate the temperature, the monitoring and controlling of moving machinery and detection of the presence of poisonous or dangerous material. |
Other applications such as environmental monitoring over large areas may require frequent battery eplacement as such network nodes in this kind of WSN must employ other means of energy or obtain their energy from other sources such as energy scavenging [1] (photovoltaic cell, mechanical vibration). With the rapid development and fast growth of new technologies such as multimedia streaming over wireless medium arise the need for improved or new MAC and transport protocols in the WSN. |
In this paper, we have examined various MAC and transport protocols and proposed the need for cross-layer MAC-Transport scheme for WSN. |
Top Medium Access Control (MAC) Overview MAC protocol [2,3,4,5] is responsible for reliable, error free data transfer with minimum retransmissions; in order to meet performance requirements such as controlling bandwidth, power awareness, contention resolution, minimize interference and collision avoidance. |
Collection of data in WSN tends to suffer from heavy congestion especially nodes nearer to the sink node. MAC protocols, proposed in literature, to combat these problems can be categorized as contention free or contention based while in [4] has classified these protocols as scheduled and unscheduled or random protocols. |
MAC protocols MAC protocols can be categorized as Contention-free and Contention-based, as shown in Figure 2. Contention free The contention free [2,3,4] protocols are more efficient than those of the contention based, they do not make the assumption that network traffic is intrinsically random, instead traffic are ordered in a bounded channel assignment. These schemes are generally based on TDMA, FDMA or CDMA that utilizes the synchronization technique and the channel access mechanism of the physical layer, where the structure of the network is spatially divided into slots or cells [3]. These protocols works well for multimedia traffic and are more applicable for static networks with centralize control. However, these schemes are more complex, require centralize control, use multiple channels simultaneously, specialized sensor hardware and there is a dependency on the physical layer. We therefore focus mainly on the contention based and transport layer schemes, where WSN need to cope with congestion, fairness and packet loss. Contention based Most of the proposed contention based protocols use the Carrier Sense Multiple Access (CSMA) [1,6] scheme, where for a station (STA) to transmit, it must sense the medium to determine if another station is transmitting. If the medium is busy, the STA will defer until the end of the current transmission. After deferral or just before attempting to transmit again, the STA shall select a random back-off interval and shall decrement the back-off interval counter while the medium is idle. The transmitting and receiving STA exchange short control frames (RTS and CTS frames) after determining that the medium is idle and after any deferrals or back-offs, prior to data transmission. The CSMA/CA protocol is designed to reduce collision between multiple stations accessing the medium. However CSAM/CA tends to suffer from hidden and exposed node problems. Hidden Node In Figure 3, nodes A and C are in the range of node B, but they are not in the range of each other. If node A is transmitting to node B, and Node C wishes to transit to node B, node C may sense the channel and find it idle and transmit causing collision at the receiving node, B with node A's transmission. Exposed Node In Figure 3 if node B is transmitting to node A, and node C wishes to transmit to D, node C may sense the channel, find it busy by node B and refrain from transmitting even though a transmission by node C to node D would not cause an interference at Node A. To combat the problems encountered by CSMA a number of techniques have been developed to improve upon CSMA deficiencies such as:
Multiple Access Collision Avoidance (MACA) Floor Acquisition Multiple Access (FAMA) Power Aware Multi Access with Signaling (PAMAS) 802.11 Distributed coordination function (DCF)
The MACA [4,5] protocols is an improvement of CSMA/CA that eliminates some of the inefficiencies. It does not use carrier sensing instead it uses the Request-To-Send/Clear-To-Send (RTS/CTS) control to avoid collisions. The main idea of MACA is that any neighboring node which overhears a RTS packet has to refrain from sending for sometime. The RTS/CTS packets are much shorter than the data packets and as such collisions are much inexpensive and nodes hearing these messages can determine how long to delay before attempting to transmit. MACA has made an improvement over CSMA/CA in that the RTS/CTS packets are much shorter than the data packets. However, the hidden node problem is not completely solved and therefore collisions can occur when different nodes send RTS and CTS packets. In addition when a node receive a RTS that is destined for another node, but do not receive the CTS to begin data exchange, this can lead to exposed node inefficiencies. MACA also does not provide any acknowledgement of data transmission and if a transmission fails, retransmission has to be initiated by the transport layer. The FAMA [5,7] is a MACA based scheme that allows every transmitting station to have control of the medium before sending data packets. It requires that collision avoidance be performed at the sender and at the receiver. FAMA uses non-persistent packet (NPP) sensing or non-persistent carrier sensing (NCS) RTS with response with CTS that plays the role of a busy signal and contains the address of the sending node. The packets repeat long enough so that hidden nodes can overhear it and refrain from sending. The objective of FAMA-NCS is for a station that has data to send acquires control of the channel in the vicinity of the receiver before sending any data packet and to ensure that no data collides with any other packet at the receiver. The medium (the floor) is acquired using non-persistent carrier sensing with the RTS-CTS exchange. The length of a CTS in FAMA-NCS is larger than the aggregate length of an RTS plus one maximum roundtrip time across the channel, the transmit receive turnaround time, and any processing time. The length of the RTS is larger than the maximum channel propagation delay plus the transmit-to-receive turn-around time and any processing time. This is required to avoid one station hearing a complete RTS before another has started to receive it. The CTS is given dominance over the RTS based on its size. Once a station has begun transmission of a CTS, any other station within range of it that transmits an RTS simultaneously will hear at least a portion of the dominating CTS, which acts as a jamming signal and back off, thereby letting the data packet that will follow to arrive free from collision. PAMAS [5,8] was developed mainly for energy conservation, nodes would listen on the signaling channel to determine when to power off their transceivers. Similarly to MACA, PAMAS uses RTS/CTS packets and data packets which are sent over different channels utilizing two transceivers in order to prevent collision and save power in Figure 4. PAMAS devices power down under two conditions: the device has no data to transmit and a neighbour device begins transmitting to another device, or when the sender node has two neighbours involved in communication. The first case saves energy since the device cannot receive a data message without corruption, so the node may power down the transceivers. The second condition saves energy since the device cannot transmit or receive without a collision resulting at itself or its receiving neighbour. To determine the length of time to sleep, each data message includes the transmission duration so a device that overhears the start of the message can calculate the length of time to sleep. PAMAS also uses a busy tone signal on the RTS/CTS signaling channel such nodes that did not overhear the RTS and CTS would know that the data channel is busy. IEEE 802.11 DCF [4,5,6] is based on CSMA with collision avoidance (CSMA/CA), it is mostly used for wireless LANs. It is a combination of CSMA and MACA schemes. This protocol uses RTS-CTS- DATA -ACK sequence for data transmission. This scheme uses a virtual carrier sense mechanism known as network allocation vector (NAV) that predict the future traffic on the medium based on duration information that is announced in RTS/CTS frame. The RTS/CTS frames contain a duration field that defines the period of time that the medium is to be reserved to transmit the actual data from the returning ACK frame. Each device maintains the NAV, that indicates the channel activity whether it has a non-zero value. Each device update the NAV based on the length present in the control message they receive. Each device also periodically decrements its NAV so the current transmission ends when the NAV reaches zero. Using the NAV allows a device to quickly check for possible channel activity without having to activate the device transceiver. DCF also uses a back-off procedure that sets a back-off timer to a random time, all back-off slots occur following a DCF inter-frame space (DIFS) period during which the medium is determined to be idle for the duration of the DIFS period. All STA using DCF is allowed to transmit if its carrier sense (CS) mechanism determines that the medium is idle and its backoff time has expired. When a node successfully receives a data message it sends a short inter-frame space (SIFS). The SIF is the time from the end to the last symbol of the previous frame to the beginning of the first symbol of the preamble of the subsequent frame as seen at the wireless interface. This scheme will work well in WSN that have short transmission range. Collision can still occur based on the transmission range of the destination node that the packet is sent to. Top Transport Layer Overview Transport layer is used to mitigate congestion, reduce packet loss, provide fairness in bandwidth allocation and guarantee reliable end-to end delivery. TCP and UDP are two traditional transport methods use in providing transportation within the Internet and cannot directly implement for WSN. TCP, a connection-oriented protocol, assumes that all packet losses are due to network congestion, as well both congestion and reliability are coupled with receipt of an acknowledgement (ACK) where as wireless networks packet losses are mainly due to high bit error rate. |
UDP does not provide reliable delivery, no flow control and congestion control mechanism. |
WSNs transport protocols should be designed to support and cope with multiple applications, variable reliability, packet-loss recovery and congestion control owing to the fact that WSNs do not only facilitate existing small sensor network with limited processing and computing resources, but take a paradigm shift in supporting multimedia traffic and applications. A number of studies have proposed various techniques that can handle the congestion control and reliable transport. |
Transport Protocols A number of techniques have been proposed which are based on one or more of the following transport protocols [9] mechanism:
Congestion Control Reliable Transport Energy conservation
Congestion Control Mechanism Accurate and efficient congestion detection plays an important role in congestion control for sensor networks. A number of proposed congestion detection techniques have been designed such as:
Congestion Detection and Avoidance (CODA) [10] is a congestion protocol that based on queue length at intermediate nodes and channel status on the basis of channel sampling and monitoring the current buffer occupancy. The authors propose the CODA energy efficient congestion control scheme that comprises three mechanisms:
Congestion detection – this technique uses a combination of the present and past channel loading conditions and the current buffer occupancy to infer accurate detection at each receiver with low cost. CODA uses a sampling scheme that activates the local channel monitoring at the appropriate time to minimize cost while forming an accurate estimation. Nodes inform their upstream neighbors via a backpressure mechanism once congestion is detected. Open-loop, hop-by-hop backpressure – this technique broadcasts backpressure messages as long as it detects congestion. Back pressure signals are propagated upstream toward the source. When there is an impulse data event in dense networks the backpressure will propagate directly to the source. When an upstream node receives a backpressure message it decides whether or not to further send the message upstream, based on its own local network conditions. Closed-loop, multi-source regulation – this technique operates over a slower time scale and is capable of asserting congestion control over multiple sources from a single sink in the event of persistent congestion. When the source event rate is less than some fraction of the maximum theoretical throughput, the source regulates it. When the rate exceeds the maximum throughput a congestion control is triggered. At this point the source requires a constant, slow time-scale feedback from the sink to maintain its rate. If there is a failure from source in receiving acknowledgment in maintaining rates each nodes are forced to maintain their own rates.
Although CODA provides congestion control as well as conserves energy in this aspect, it does not provide reliability under such scenarios with sparse source and high data rate. FUSION [11] is similar to CODA and suffers from the similar deficiencies. This protocol uses a combination of three techniques to control congestion:
Hop-by-hop flow control – nodes signal local congestion to each other via backpressure, reducing packet loss rates and preventing the wasteful transmission of packets that are only destined to be dropped at the downstream. Source rate limiting – this alleviate the serious unfairness towards sources that have to traverse a larger number hops. The rate control used is similar to the token bucket mechanism. This mechanism assumes that the data rate of each sensor nodes is the same. Prioritized MAC layer – this gives a backlogged node priority over non-backlogged nodes for access to the shared medium, hence avoiding buffer drops.
Although this scheme uses a combination of three techniques to control congestion, a performance comparison need to be evaluated and a rate limitation algorithm need to be design to correctly handles node failures. PCCP [12] uses packet inter-arrival time and packet service to measure congestion. Congestion level is capture at the node or at the link through a parameter referred to as congestion degree which is the ratio of service over inter-arrival time. It employs weighted fairness to allow nodes to receive priority-dependent throughput. PCCP results in low buffer occupancy and as a result, it can avoid or reduce packet loss and therefore improve energy-efficiency as well as achieves high link utilization and low packet delay. PCCP also uses implicit congestion notification to avoid transmission of additional control messages and therefore help improve energy-efficiency. This scheme suffers from the same drawback as CODA and Fusion. Reliable Transport mechanism Reliable Multi-Segment Transport (RMST) in Figure 6 [13] and Reliable Bursty Convergecast (RBC) [14] are reliable transport protocols that provide reliability through a hop by hop loss recovery. RMST is designed to run in conjunction with directed diffusion. In diffusion, a sink subscribes to an interest that names a particular type and source of data. The naming of data is accomplished via attribute-value pairs. It uses a filter that could be attached to any diffusion node on an as needed basis without recompilation of the diffusion core or gradient filter. RMST provides segmentation and reassembly of data packets and also guarantees delivery of all packets from each source to sink. Receivers are responsible for detecting whether or not a fragment needs to be resent. In the non-caching mode, only sinks monitor the integrity of an RMST entity in terms of fragment received and in a caching mode, an RMST node collects fragments which is capable of initiating recovery for missing fragments to the next node along the path toward the source. Reliability for all packets is inherently wasteful in many to one data transmission environment and it does not exploit the redundancy of traffic. Therefore RMST mechanism is not suitable of WSNs. RBC design a window-less block acknowledgment scheme which guarantees continuous packet forwarding irrespective of the underlying link unreliability as well as the resulting packet- and ack-loss. It was shown to increase channel utilization, reduce the probability of loss in acknowledgment for a received packet. To improve retransmission incurred channel contention different contention control was introduced which rank nodes by their queuing conditions as well as the number of times that the queued packets have been transmitted. Congestion/Reliable/energy efficient mechanism Sensor Transmission Control Protocol (STCP) [15] and Event to Sink Reliability (ERST) [16] are transport protocols that attempt to resolve more than one of the transport protocol mechanisms, Table 1. STCP implements both congestion control and reliability in a single protocol, it offers different control policies to both guarantee application requirements and improve energy efficiency. Before STCP transmit packets, sensors node establishes an association with the base station by a session initiation packet. The session initiation packet informs the base station of the number of flows coming from the node, the type of data flow, transmission rate and required reliability. For continuous flow the base station calculates the running average for the reliability; reliability is measured as a fraction of successfully received packets. If there are multiple nodes transmitting, a single initiation packet is send with each packet detail. STCP uses ACK/NACK mechanism. Sensor nodes retransmit packets only on receiving a NACK. The transmitted packets are buffered but a timer is maintained to prevent buffer overflow, once the threshold is reached the buffer is cleared. For event driven flows, the base station cannot estimate arrival times of data thus ACK are used by source to know if a packet has reached the base station. The source node buffers each transmitted packets until an ACK is received, then the corresponding packet is deleted from the buffer. STCP only send NACK when reliability goes below the required level, even if base station does not receive a packet within the expected time interval. ESRT is a novel transport solution that seeks to achieve reliable event detection with minimum energy expenditure and congestion resolution. To achieve the desired event detection accuracy with minimum energy expenditure, ERST uses a control mechanism that serves dual purposes of reliable detection and energy conservation. To also achieve reliability, the reporting frequency rate is aggressively increased to attain the required reliability as soon as possible. Only the sink and not the sensor nodes can determine the reliability and act accordingly. The authors think that end-to-end transmissions and ACK/NACK overheads are a waste of limited sensor resources, hence the congestion detection mechanism is based on local buffer level monitoring in the sensor nodes. ERST also address multiple event detection and uses an event ID field to determine if there is a single event or multiple events. This is done by checking the event ID when data packets are received at the sink; if the event IDs are the same it is assume to be a single event otherwise it is a multiple event. Top Cross-Layer Design Traditional layered approach was designed for wired network, the Open System Interconnection (OSI) model [17], where all layers need not communicate with each other, as the architecture layout is built on top of the one below. Neither was there severe problems with sharing the medium as each layer offers services to the respective higher layer and provides an abstract interface for its service. |
In the wireless environments users communicate over scarce and changeable transmission medium which are prone to interference, weak signal strength and other channel conditions. With these challenges protocols can no more develop in isolations and as such the invention of cross-layer approach. The idea of cross-layer design is where layers (example MAC and Transport), as shown in Figure 7, can exchange information between them in an intelligent way during communication to improve the performances of the system. |
In [18] discussed useful cross-layer information and differentiate the channel state as it relates to signal strength, interference level, and channel response estimate in time and frequency domain. The layering approach to network design does not fit in the wireless network as mentioned by [19], in which an in-depth analysis of cross-layering approaches for wireless adhoc has been discussed. However a number of issues should be taken into consideration as it relates to cross-layer design in wireless network using the IEEE 802.11 medium which is based on shared media and node contentions. These include traffic flow(s) which will have impact on the available bandwidth of all its neighboring nodes, nodes transmit and receive data on a single channel; the delivery of a single traffic flow involves and the contention of channel resource within the node(s). As a result, different nodes (i.e., the source, the destination, intermediate nodes, and neighboring nodes along the end-to-end route) may consume different amount of bandwidth resource for the transmission of a specific traffic flow. |
In IEEE 802.11, the available bandwidth cannot be estimated directly from the overall throughput being achieved, because of the following reasons:
The maximum throughput is not a constant for a given data rate, is affected by the average packet length and the number of active contending nodes. The data rates of links are not the same either due to multi-rate supports.
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Therefore the cross-layer interactions is a technique to boost the performance by effectively adapt to the dynamic environment. |
There are many studied TCP flavors such as New Reno[20], and SACK [21], which differ in how they react to packet loss. There implementations differ by manipulating the window size of the TCP by calculating the throughput, setting threshold and checking packet drops. |
Having examined solutions for cross-layer design in WSN networks, our intention for the future is to design a novel approach for congestion control among MAC and Transport layer using multichannel assignment. As mentioned before, the most popular contention based MAC is the CSMA/CA where a number of improved techniques have derived such as 802.11 DCF. The transport layer, which provides the end to end communication service, mainly uses the user datagram protocol (UDP) and the transmission control protocol (TCP), that the improve techniques covered are based on to support reliable flow and congestion as well as error recovery. |
The challenges to be overcome as it relates to WSN are:
Sensor nodes are more constrained in computational, energy and storage resources because of its limited energy which are usually batteries and are difficult to replace when consumed. Interference among the transmission, since more nodes are deployed in a sensor network, up to hundred or thousand nodes, than in other wireless networks. Redundant information since in most case neighboring nodes often sense the same events from their environment thus forwarding the same data to the base station. Topology changes due to node failure even though most sensor nodes are usually stationary.
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The transport layer using TCP for wireless transmission will create additional challenges as TCP makes assumption that packet losses are due to congestion. In wireless networks a number of issues may cause packet losses such as:
Bit Error Rate (BER), which is usually high base on the changes within the environment. Bandwidth limitation Round Trip Time (RTT), the overall throughput and increase in delay will be affect because of longer latency within the wireless medium.
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Multiple non-overlapping channels present in the IEEE 802.11 ISM free frequency band have been exploited by mapping them to multiple-radios to increase the overall capacity and connectivity of the wireless mesh network's backbone. A centralized, graph based approach has been proposed in [22], [23] and [24] where links and nodes are considered as edges and vertices of a graph respectively and formulating radio/channel assignment by assigning edges to vertices. The limitation of these methods is that it is very difficult to capture network load information with a graph model. Network flow based centralized approaches can be found in [25], [26] and [27], where multi radio multi-channel (MRMC) is modeled based on network flows and therefore overcomes the limitations associated with graph based approaches. These approaches are not realistic as constant traffic sources are assumed all the time while network traffic can be bursty in nature. A distributed gateway centered multi-radio multi-channel approach has been developed by [28] and [29] where mesh gateways are considered as sink and source of data. |
Although the MRMC enormously increases network throughput, connectivity, robustness and resilience; it requires extra resources e.g. energy because addition of extra radios consumes more power. Keeping in view these constraints, applying MRMC techniques directly to WSN's needs further investigation for optimization. None of the research work done in this area has considered the power constraint as WSN's nodes have limited energy supplies. The use of multiple channels with a single radio can also be an interesting future study where the power limitation is kept in mind. Furthermore, the effect of channel assignment on the transport layer has been ignored by the researchers. Since the channel condition at the MAC layer has a considerable effect on the TCP congestion mechanism, it needs to be further investigated with a cross layer optimization. |
Top Conclusion and Future MAC-Transport This paper has presented an overview of MAC and transport layer protocols. A number of existing techniques were analyzed, each attempting to resolve one or more problems faced by the current layers; hidden and exposed nodes, congestion, fair utilization or reliable transportation within the medium while providing energy conservation. The MAC protocols mentioned in this paper mainly address the hidden or exposed node problem in the CSMA scheme but not both simultaneously, except for PAMAS whose focused was mainly to save energy. The 802.11 DCF scheme will work well for short transmission range, the back-off procedure used does not work well in noisy environment, therefore the need for longer range transmission need to be explored in WSNs, as well as the consideration of the effect for channel errors. |
The transport protocols for WSNs have implemented a number of techniques for energy efficiency, reliability and congestion. However these techniques mainly considered a single or multiple solutions but not a complete solution for the entire existing problem except ERST and STCP, they both attempt to resolve both congestion and reliability problem. ERST also resolved energy consumption to a lesser extent. Overall, both MAC and transport work in isolation in resolving the problems faced by both layers and as such cross-layer design was discussed as a means to optimized both layer to have them function as an entity to combat the problems and to obtain an energy efficient WSNs. |
For future work in this area, the implementation in real sensor network to realize the full potential and integrity of most of the studied techniques are recommended in a real sensor environment. A cross layer design to optimize and confer both MAC and transport is being recommended to maximize efficiency, allow both layers to communicate simultaneously, reduce packet overhead, to provide reliable transmission and to support multimedia traffic. Although most protocols mentioned do not consider, the finite energy resources available, owing to the fact that most WSNs use replaceable batteries as a mean of energy source which are deemed as scarce resources based on limited battery life time to power each node. This is a major concern in WSNs and as such other means of energy source may need to be investigated in the future. |
In our future work, these major limitations in WSNs will be address by improving on the deficiencies on one of the technique covered in MAC and transport protocol with the use of multichannel assignment to overcome congestion control. Multichannel assignment will create additional over head in terms of switching delays, synchronization among the nodes, extra control packets and hence more energy. In our future work, we are going to investigate the effect of multichannel use in WSN's on the transport layer congestion mechanism and develop a cross layer optimization technique where multiple non overlapping channels are exploited with minimum overhead for increased capacity and minimum power usage. |
Top Figures | Figure 1:: Wireless Sensor network architecture
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| | Figure. 2: MAC protocols
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| | Figure 3:: Hidden and Exposed node
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| | Figure 4:: The PAMAS protocol
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| | Figure 5:: Back-off procedure
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| | Figure 6:: Relation of RMST to a basic diffusion node
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| | Figure. 7: Cross-layer
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Table | Table: 1: Summary of transport protocols mechanism
| | Protocols | Mechanism | | Congestion control | Reliable | Energy conservation | | CODA | Yes | No | Yes | | Fusion | Yes | No | No | | PCCP | Yes | No | No | | RMST | No | Yes | No | | RBC | No | Yes | No | | ERST | Yes | Yes | Yes (minimum) | | STCP | Yes | Yes | No |
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