Multiple Access with Collision Avoidance (MACA) is a slotted media access control protocol used in wireless LAN data transmission to avoid collisions caused by the hidden station problem and to simplify exposed station problem.

The basic idea of MACA is a wireless network node makes an announcement before it sends the data frame to inform other nodes to keep silent. When a node wants to transmit, it sends a signal called Request-To-Send (RTS) with the length of the data frame to send. If the receiver allows the transmission, it replies the sender a signal called Clear-To-Send (CTS) with the length of the frame that is about to receive.Meanwhile, a node that hears RTS should remain silent to avoid conflict with CTS; a node that hears CTS should keep silent until the data transmission is complete.

WLAN data transmission collisions may still occur, and the MACA for Wireless (MACAW) is introduced to extend the function of MACA. It requires nodes sending acknowledgements after each successful frame transmission, as well as the additional function of Carrier sense.+

Dbtma

In packet radio networks, the hidden terminal problem and the exposed terminal problem can severely reduce the utilization of a medium access control (MAC) protocols. To avoid these problems, request-to-send/clear-to-send (RTS/CTS)-based schemes were proposed. However, as shown in this paper, utilization of these schemes is still degraded, especially in the cases in which the propagation and the transmission delays are long. We propose a new MAC protocol, termed dual busy tone multiple access (DBTMA), and we evaluate its performance. In DBTMA, two busy tones are used to separate the use of the forward and the reverse communication directions. the simulations show that the network utilization of DBTMA is about twice as that of RTS/CTS-based schemes. We also discuss the effect of nodal mobility on the network utilization in packet radio networks, concluding that it is negligible under normal operational conditions.

2The dual busy tone multiple access (DBTMA) scheme was designed for decentralized networks such as packet radio networks (PRNs). We use the RTS/CTS dialogue to reserve the channel in DBTMA. In addition, we use two busy tones to notify neighbor nodes of the on-going transmission and reception after a successful channel reservation. Our analysis and simulations

show that DBTMA provides higher network utilization, which is 100% more than the basic RTS/CTS schemes. We present the details of our analysis in this paper. In order to evaluate our protocol in a more general network, our analysis and simulations are based on a fully-meshed decentralized multi-hop network topology.

3

In ad hoc networks, the hidden- and the exposed-terminal problems can severely reduce the network capacity on the MAC layer. To address these problems, the ready-to-send and clear-to-send (RTS/CTS) dialogue has been proposed in the literature. However, MAC schemes using only the RTS/CTS dialogue cannot completely solve the hidden and the exposed terminal problems, as pure “packet sensing” MAC schemes are not safe even in fully connected networks.We propose a new MAC protocol, termed the dual busy tone multiple access (DBTMA) scheme. The operation of the DBTMA protocol is based on the RTS packet and two narrow-bandwidth, out-of-band busy tones. With the use of the RTS packet and the receive busy tone, which is set up by the receiver, our scheme completely solves the hidden- and the exposed-terminal problems. The busy tone, which is set up by the transmitter, provides protection for the RTS packets, increasing the probability of successful RTS reception and, consequently, increasing the throughput. This paper outlines the operation rules of the DBTMA scheme and analyzes its performance. Simulation results are also provided to support the analytical results. It is concluded that the DBTMA protocol is superior to other schemes that rely on the RTS/CTS dialogue on a single channel or to those that rely on a single busy tone. As a point of reference, the DBTMA scheme out-performs FAMA-NCS by 20–40% in our simulations using the network topologies borrowed from the FAMA-NCS paper. In an ad hoc network with a large coverage area, DBTMA achieves performance gain of 140% over FAMA-NCS and performance gain of 20% over RI-BTMA.

Maca bi A novel wireless MAC protocol named MACA-BI (MACA By Invitation) is introduced. MACA-BI is a simplified version of the well known MACA (Multiple Access Collision Avoidance) protocol, which is based on the request to send/clear to send (RTS/CTS) handshake and which has inspired the IEEE 802.11 wireless LAN standard. In MACA-BI, the RTS part of the RTS/CTS handshake is suppressed, leaving only the clear to send a control message which can be viewed as an “invitation” by the receiver to transmit. This reduction greatly improves the efficiency when radio turn-around time is significant with respect to packet transmission time. Yet, it preserves the “data” collision free property of MACA. Simulation results for various multihop topologies show that, when the traffic characteristics are stationary or predictable, MACA-BI outperforms several known multiple access protocols, especially when “hidden terminal” conditions are predominant.

2

This paper introduces a new wireless MAC protocol, MACA-BI (MACA by invitation). The protocol is a simplified version of the well known MACA (multiple access collision avoidance) based on the request to send/clear to send (RTS/CTS) handshake. The clear to send (CTS) control message is retained, while the request to send (RTS) part of the RTS/CTS handshake is suppressed. MACA-BI, preserving the data collision free property, is more robust than MACA to problems such as protocol failures (control packet collision and corruption) and finite turn-around time. Analytic results for a 1 Mbps single-hop far-field wireless network, and simulation results for a 10 Mbps multi-hop near-field ATM wireless indoor network, show that MACA-BI outperforms other multiple access protocols in high speed, steady traffic environments (e.g. ATM VBR and CBR) and where the propagation delay can be neglected (typically indoor)

3

Abstruct- A novel wireless MAC protocolnamed MACA-BI (MACA By Invitation) is introduced.MACA-BI is a simplifiedversion of thewell known MACA (Multiple Access CollisionAvoidance) protocol, which is based on the Requestto Send/Clear to Send (RTS/CTS) handshakeand which has inspired the IEEE 802.11wireless LAN standard. In MACA-BI, the RTSpart of the RTS/CTS handshake is suppressed,leaving only the Clear to Send control messagewhich can be viewed as an “invitation” by thereceiver to transmit. This reduction greatly improvesefficiency when radio turn-around time issignificant with respect to packet transmissiontime. Yet, it preserves the “data” collision freeproperty of MACA. Simulation results for variousmultihop topologies show that, when trafficcharacteristics are stationary or predictable,MACA-I31 outperforms several known multipleaccess protocols, especially when “hidden terminal”conditions are predominant.I. INTRODUCTIONAn important component of a wireless network design isthe MAC (Medium Access Control) layer. CSMA (CarrierSense Multiple Access) was the MAC layer used inthe first generation packet radio networks [SI. CSMAprevents collision by sensing the carrier before transmission.A terminal, however, can sense the carrier onlywithin its transmitting range. Transmissions from terminalsout of range cannot be detected. Thus, in spite ofcarrer sensing a transmission could still collide at the receiverwith another transmission from an “out of range”terminal, often referred to as the “hidden terminal”.The Multiple Access with Collision Avoidance protocol(MACA), proposed by Karn [7], solves the hidden terminal

problem and outperforms CSMA in a wireless multihopnetwork. Fullmer and Garcia-Luna-Aceves [4] extendMACA by adding carrier sensing. The resultingFAMA-NTR protocol performs almost as well as CSMAin a single-hop wireless network. The same authors proposefurther improvements (FAMA-PJ [3], CARMA [5])achieveing even better performance at high loads. In theFAMA-PJ evaluation, an accurate radio model is used to‘This research was supported in part by a grant from the Stateof California and Teledyne, under a MICRO program, and in partby an Intel grant.O-7803-3871-5/97/$10.00 0 1997 IEEE 43 5account for the TX-RX turn-around time (the transitiontime from transmit to receive state). Their study revealsthe impact of the turn-around time on performance.Several modifications of MACA have been proposedwhich suppress RTS, mostly to transmit multipacketmessages or to support real time streams. For example,to increase the channel utilization for multipacketmessage transmissions, Fullmer and Garcia-Luna- Acevespropose in [6] to replace all RTS packets but the firstwith a MORE flag in the header of the data packet. In[4], the same authors propose to use FAMA-NTR in bulkmode to maximize the throughput. For a multimediaapplication,Lin and Gerla propose to use RTS/CTS onlyfor the first packet of a real time stream [9]. Subsequentpackets are transmitted with a reservation scheme thatrelies on the periodic nature of the multimedia traffic.Yet, other extensions to MACA have added even moreoverhead to the RTS/CTS exchange, mostly for errorrecovery purposes. For example, in [lo] an “invitationminipacket” is introduced to invite the transmitter to retransmitits last packet, in case it has been lost (NegativeAcknowledgment). In another case, the three-way handshakeis expanded to a five-way handshake (MACAW)with protected ACKs to guarantee transmission integrityin a multihop “nanocell” environment [2]. Unfortunately,each additional pass in the handshake contributesone TX-RX turn-around time plus preamble bits (forsynchronization), control bits (e.g. source-destinationinformation) and checksum bits. This overhead clearlyreduces the useful throughput.Let us focus for a moment on the TX-RX turn-aroundtime in order to appraise its impact on performance.According to the standard proposed in [l], the TX-RXturn-around time should be less than 25ps (includingradio transients, operating system delays and energy detection).Moreover, every transmission should be delayedby the TX to RX turn-around time (that is, upto 25ps) to give a chance to the previous transmitter toswitch to receive mode. This transmit-to-receive transitionoccurs precisely in the RTS/CTS mechanism ofMACA. The higher the channel speed, the higher the

turn-around time overhead in terms of bits. Thus, turnaroundtime will play a key role in future high speedwireless LANs.To reduce, in part, the turn-around overhead, we proposeMACA-BI (Multiple Access with Collision AvoidanceBy Invitation), a simplified version of MACA withonly a “two-way” handshake. A node ready to transmit,instead of “acquiring” the floor (Floor Acquisition MultipleAccess, FAMA) using the RTS (Ready to Transmit)signal, waits for an “invitation” by the intended receiverin the form of an RTR (Ready to Receive) control packet.This paper extents earlier simulation results presented in[ll] for a simple four node “in line” network. Here, differentmultihop topologies are selected with varying degreesof “hidden terminal” transmissions. Three protocolsare evaluated, namely CSMA, MACA and MACABI.Section 2 introduces MACA-BI for single and multi-hopoperation. Section 3 shows that MACA-BI, like MACA,is collision free. An analytical model of MACA-BI forsingle-hop networks is presented in section 4. Simulationresults for multi-hop wireless networks are presented in

section 5. Section 6 concludes the paper.

MARCH

The multiple access with reduced handshake (MARCH) protocol utilizes the broadcast characteristics of an omnidirectional antenna to reduce the number of control messages required to transmit a data packet in multihop ad hoc networks. In MARCH, the RTS-CTS handshake is used only by the first hop of a route to forward data packets while for the rest it utilizes a new CTS-only handshake. Since fewer control packets are transmitted, the probability of packet collision is reduced and therefore the channel throughput is increased. Simulation results demonstrate that MARCH outperforms MAC protocols that employ only the RTS-CTS handshake.

2

The multiple access with reduced handshake (MARCH) protocol utilizes the broadcast characteristics of an omnidirectional antenna to reduce the number of control messages required to transmit a data packet in multihop ad hoc networks. In MARCH, the RTS-CTS handshake is used only by the first hop of a route to forward data packets while for the rest it utilizes a new CTS-only handshake. Since fewer control packets are transmitted, the probability of packet collision is reduced and therefore the channel throughput is increased. Simulation results demonstrate that MARCH outperforms MAC protocols that employ only the RTS-CTS handshake

Intro adhocAn ad hoc wireless network is a collection of two or more devices equipped with wireless communications and

networking capability. Such devices can communicate with another node that is immediately within their radio

range or one that is outside their radio range. For the latter scenario, an intermediate node is used to relay or

forward the packet from the source toward the destination.

An ad hoc wireless network is self-organizing and adaptive. This means that a formed network can be deformed on-the-fly without the need for any system administration. The term “ad hoc” tends to imply “can take

different forms” and “can be mobile, standalone, or networked.” Ad hoc nodes or devices should be able to

detect the presence of other such devices and to perform the necessary handshaking to allow communications

and the sharing of information and services.

Since ad hoc wireless devices can take different forms (for example, palmtop, laptop, Internet mobile phone,

etc.), the computation, storage, and communications capabilities of such devices will vary tremendously. Ad hoc

devices should not only detect the presence of connectivity with neighboring devices/nodes, but also identify

what type the devices are and their corresponding attributes. Since an ad hoc wireless network does not rely

on any fixed network entities, the network itself is essentially infrastructureless. There is no need for any fixed

radio base stations, no wires or fixed routers. However, due to the presence of mobility, routing information

will have to change to reflect changes in link connectivity.

The diversity of ad hoc mobile devices also implies that the battery capacity of such devices will also vary.

Since ad hoc networks rely on forwarding data packets sent by other nodes, power consumption becomes a

critical issue.

3. 2. H eterogenei ty i n Mobile Devices

As shown in Figure 3.2, mobile devices can exist in many forms. There are great differences among these

devices, and this heterogeneity can affect communication performance and the design of communication protocols.

Macabi

4.5.2. MACA-BI (By Invi tati on )

A shift from the classic three-way handshake MAC protocol is MACA-BI (By Invitation). Invented by Fabrizio

Talucci, MACA-BI uses only a two-way handshake, as shown in Figure 4.10. There is no RTS. Instead, the

CTS message is renamed as RTR (Ready To Receive). In MACA-BI, a node cannot transmit data unless it

has received an invitation from the receiver. Note that the receiver node does not necessarily know that the

source has data to transmit. Hence, the receiver needs to predict if indeed the node has data to transmit to it.

The timeliness of the invitation will, therefore, affect communication performance.

Figure 4.10. An illustration of MACA-BI control handshake.

The author suggested the estimation of packet queue length and arrival rate at the source to regulate the transmission of invitations. One possible way to accomplish this is to piggyback such information into each data

packet so that the receiver is aware of the transmitter’s backlog. Hence, for constant bit rate (CBR) traffic, the

efficiency of MACA-BI will be high since the prediction scheme will work fine. However, for bursty traffic,

MACA-BI performance will be no better than MACA.

To enhance the communication performance of MACA-BI under non-stationary traffic situations, a node may

still transmit an RTS if the transmitter’s queue length or packet delay exceeds a certain acceptable threshold

before an RTR is issued. This means that MACA-BI now reverts back to MACA. Figure 4.11 clearly shows

the differences between MACA and MACA-BI.

Figure 4.11. Comparing MACA and MACA-BI MAC protocols.

In summary, MACA-BI results in reduced transmit/receive turn around time. Every transmission should be delayed by the transmit-to-receive turn-around time (i.e., up to 25 microseconds) to allow the previous transmitter

to switch to receive mode. Because MACA-BI only uses a single control message, this turn around limitation is

reduced. Additionally, MACA functionality is preserved in MACABI. This includes the collision-free feature

of MACA. In fact, MACA-BI is less likely to suffer from control packet collision since it uses half as many

control packets as MACA.

P2p

Peer-to-peer (P2P) computing or networking is a distributed application architecture that partitions tasks or workloads between peers. Peers are equally privileged, equipotent participants in the application. They are said to form a peer-to-peer network of nodes.

Peers make a portion of their resources, such as processing power, disk storage or network bandwidth, directly available to other network participants, without the need for central coordination by servers or stable hosts.[1] Peers are both suppliers and consumers of resources, in contrast to the traditional client–server model where only servers supply, and clients consume.

The peer-to-peer application structure was popularized by file sharing systems like Napster. The concept has inspired new structures and philosophies in many areas of human interaction. Peer-to-peer networking is not restricted to technology, but covers also social processes with a peer-to-peer dynamic. In such context, social peer-to-peer processes are currently emerging throughout society.

Architecture of P2P systemsPeer-to-peer systems often implement an abstract overlay network, built at Application Layer, on top of the native or physical network topology. Such overlays are used for indexing and peer discovery and make the P2P system independent from the physical network topology. Content is typically exchanged directly over the underlying Internet Protocol (IP) network. Anonymous peer-to-peer systems are an exception, and implement extra routing layers to obscure the identity of the source or destination of queries.

In structured peer-to-peer networks, peers (and, sometimes, resources) are organized following specific criteria and algorithms, which lead to overlays with specific topologies and properties. They typically use distributed hash table-based (DHT) indexing, such as in the Chord system (MIT).[2]

Unstructured peer-to-peer networks do not provide any algorithm for organization or optimization of network connections.[citation needed]. In particular, three models of unstructured architecture are defined. In pure peer-to-peer systems the entire network consists solely of equipotent peers. There is only one routing layer, as there are no preferred nodes with any special infrastructure function. Hybrid peer-to-peer systems allow such infrastructure nodes to exist, often called supernodes.[3] In centralized peer-to-peer systems, a central server is used for indexing functions and to bootstrap the entire system.[citation needed]. Although this has similarities with a structured architecture, the connections between peers are not determined by any algorithm. The first prominent and popular peer-to-peer file sharing system, Napster, was an example of the centralized model. Gnutella and Freenet, on the other hand, are examples of the decentralized model. Kazaa is an example of the hybrid model.

P2P networks are typically used for connecting nodes via largely ad hoc connections.[citation needed] Data, including digital formats such as audio files, and real time data such as telephony traffic, is passed using P2P technology.

A pure P2P network does not have the notion of clients or servers but only equal peer nodes that simultaneously function as both "clients" and "servers" to the other nodes on the network. This model of network arrangement differs from the client–server model where communication is usually to and from a central server. A typical example of a file transfer that does not use the P2P model is the File Transfer Protocol (FTP) service in which the client and server programs are distinct: the clients initiate the transfer, and the servers satisfy these requests.

The P2P overlay network consists of all the participating peers as network nodes. There are links between any two nodes that know each other: i.e. if a participating peer knows the location of another peer in the P2P network, then there is a directed edge from the former node to the latter in the overlay network. Based on how the nodes in the overlay network are linked to each other, we can classify the P2P networks as unstructured or structured.

Advantages and weaknessesIn P2P networks, clients provide resources, which may include bandwidth, storage space, and computing power. As nodes arrive and demand on the system increases, the total capacity of the system also increases. In contrast, in a typical client–server architecture, clients share only their demands with the system, but not their resources. In this case, as more clients join the system, less resource are available to serve each client.

The distributed nature of P2P networks also increases robustness,[citation needed] and—in pure P2P systems—by enabling peers to find the data without relying on a centralized index server[citation

needed]. In the latter case, there is no single point of failure in the system.[citation needed]

As with most network systems, unsecure and unsigned codes may allow remote access to files on a victim's computer or even compromise the entire network.[citation needed] In the past this has happened for example to the FastTrack network when anti P2P companies managed to introduce faked chunks into downloads and downloaded files (mostly MP3 files) were unusable afterwards or even contained malicious code.[citation needed] Consequently, the P2P networks of today have seen an enormous increase of their security and file verification mechanisms. Modern hashing, chunk verification and different encryption methods have made most networks resistant to almost any type of attack, even when major parts of the respective network have been replaced by faked or nonfunctional hosts.

Internet service providers (ISPs) have been known to throttle P2P file-sharing traffic due to the high-bandwidth usage.[7] Compared to Web browsing, e-mail or many other uses of the internet, where data is only transferred in short intervals and relative small quantities, P2P file-sharing often consists of relatively heavy bandwidth usage due to ongoing file transfers and swarm/network coordination packets. As a reaction to this bandwidth throttling several P2P applications started implementing protocol obfuscation, such as the BitTorrent protocol encryption. Techniques for achieving "protocol obfuscation" involves removing otherwise easily identifiable properties of protocols, such as deterministic byte sequences and packet sizes, by making the data look as if it were random.[8]

A possible solution to this is called P2P caching, where a ISP stores the part of files most accessed by P2P clients in order to save access to the Internet.

BroadcastingBroadcasting is the distribution of audio and video content to a dispersed audience via radio, television, or other. Receiving parties may include the general public or a relatively large subset of thereof.

Broadcasting antenna in Stuttgart

The original term broadcast referred to the literal sowing of seeds on farms by scattering them over a wide field.[1] It was first adopted by early radio engineers from the Midwestern United States to refer to the analogous dissemination of radio signals. Broadcasting forms a very large segment of the mass media. Broadcasting to a very narrow range of audience is called narrowcasting.

In computing, broadcasting refers to a method of transferring a message to all recipients simultaneously. Broadcasting can be performed as a high level operation in a program, for example broadcasting Message Passing Interface, or it may be a low level networking operation, for example broadcasting on Ethernet.

overview

In computer networking, broadcasting refers to transmitting a packet that will be received by every device on the network[1]. In practice, the scope of the broadcast is limited to a broadcast domain. Broadcast a message is in contrast to unicast addressing in which a host sends datagrams to another single host identified by a unique IP address.

Not all network technologies support broadcast addressing; for example, neither X.25 nor frame relay have broadcast capability, nor is there any form of Internet-wide broadcast. Broadcasting is largely confined to local area network (LAN) technologies, most notably Ethernet and token ring, where the performance impact of broadcasting is not as large as it would be in a wide area network.

The successor to Internet Protocol Version 4 (IPv4), IPv6 also does not implement the broadcast method to prevent disturbing all nodes in a network when only a few may be interested in a particular service. Instead it relies on multicast addressing a conceptually similar one-to-many routing methodology. However, multicasting limits the pool of receivers to those that join a specific multicast receiver group.

Both Ethernet and IPv4 use an all-ones broadcast address to indicate a broadcast packet. Token Ring uses a special value in the IEEE 802.2 control field.

Broadcasting may be abused to perform a DoS-attack. The attacker sends fake ping request with the source IP-address of the victim computer. The victim computer is flooded by the replies from all computers in the domain.

Forwarding is the relaying of packets from one network segment to another by nodes in a computer network.

A unicast forwarding pattern, typical of many networking technologies including the overwhelming majority of Internet traffic

A multicast forwarding pattern, typical of PIM

A broadcast forwarding pattern, typical of bridged Ethernet

The simplest forwarding model — unicasting — involves a packet being relayed from link to link along a chain leading from the packet's source to its destination. However, other forwarding strategies are commonly used. Broadcasting requires a packet to be duplicated and copies sent on multiple links with the goal of delivering a copy to every device on the network. In practice, broadcast packets are not forwarded everywhere on a network, but only to devices within a broadcast domain, making broadcast a relative term. Less common than broadcasting, but perhaps of greater utility and theoretical significance, is multicasting, where a packet is selectively duplicated and copies delivered to each of a set of recipients.

Networking technologies tend to naturally support certain forwarding models. For example, fiber optics and copper cables run directly from one machine to another form natural unicast media — data transmitted at one end is received by only one machine at the other end. However, as illustrated in the diagrams, nodes can forward packets to create multicast or broadcast distributions from naturally unicast media. Likewise, traditional Ethernet (10BASE5 and 10BASE2, but not the more modern 10BASE-T) are natural broadcast media — all the nodes are attached to a single long cable and a packet transmitted by one device is seen by every other device attached to the cable. Ethernet nodes implement unicast by ignoring packets not directly addressed to them. A wireless network is naturally multicast — all devices within a reception radius of a transmitter can receive its packets. Wireless nodes ignore packets addressed to other devices, but require forwarding to reach nodes outside their reception radius.

At nodes where multiple outgoing links are available, the choice of which, all, or any to use for forwarding a given packet requires a decision making process that, while simple in concept, is sometimes bewilderingly complex. Since a forwarding decision must be made for every packet

handled by a node, the total time required for this can become a major limiting factor in overall network performance. Much of the design effort of high-speed routers and switches has been focused on making rapid forwarding decisions for large numbers of packets.

The forwarding decision is generally made using one of two processes: routing, which uses information encoded in a device's address to infer its location on the network, or bridging, which makes no assumptions about where addresses are located and depends heavily on broadcasting to locate unknown addresses. The heavy overhead of broadcasting has led to the dominance of routing in large networks, particularly the Internet; bridging is largely relegated to small networks where the overhead of broadcasting is tolerable. However, since large networks are usually composed of many smaller networks linked together, it would be inaccurate to state that bridging has no use on the Internet; rather, its use is localized.

Impact pf mobility

2. REVIEW OF MOBILITY MODELSA mobility model is used to capture the movement of a real-world object in simulation studies.In MANETs, a mobility model is used to define the movement of a mobile wireless node(shortly referred hereafter as MN). There are two types of MANET mobility models: singleentityand group. In single-entity models, each MN moves independently of all the other MNswithin the network area. For simplicity, most of the mobility models are defined for arectangular network area enclosed by (0, 0), (0, ymax), (xmax, ymax), and (xmax, 0). A characteristicfeature of every mobility model is to ensure that a MN will not travel outside the network area.In group mobility models, nodes are assumed to be organized in groups and the mobility of a

node is often reflective of the movement pattern of the entire group.

MAC

The Media Access Control (MAC) data communication protocol sub-layer, also known as the Medium Access Control, is a sublayer of the Data Link Layer specified in the seven-layer OSI model (layer 2). It provides addressing and channel access control mechanisms that make it possible for several terminals or network nodes to communicate within a multi-point network, typically a local area network (LAN) or metropolitan area network (MAN). The hardware that implements the MAC is referred to as a Medium Access Controller.

ADDRESING MECHANISM

The MAC sub-layer acts as an interface between the Logical Link Control (LLC) sublayer and the network's physical layer. The MAC layer emulates a full-duplex logical communication channel in a multi-point network. This channel may provide unicast, multicast or broadcast communication service.

The MAC layers addressing mechanism is called physical address or MAC address. A MAC address is a unique serial number. Once a MAC address has been assigned to a particular network interface (typically at time of manufacture), that device should be uniquely identifiable amongst all other network devices in the world. This guarantees that each device in a network will have a different MAC address (analogous to a street address). This makes it possible for data packets to be delivered to a destination within a subnetwork, i.e. a physical network consisting of several network segments interconnected by repeaters, hubs, bridges and switches, but not by IP routers. An IP router may interconnect several subnets.

An example of a physical network is an Ethernet network, perhaps extended by wireless local area network (WLAN) access points and WLAN network adapters, since these share the same 48-bit MAC address hierarchy as Ethernet.

A MAC layer is not required in full-duplex point-to-point communication, but address fields are included in some point-to-point protocols for compatibility reasons.

Contention based protocol

A contention-based protocol (CBP) is a communications protocol for operating wireless telecommunication equipment that allows many users to use the same radio channel without pre-coordination. The "listen before talk" operating procedure in IEEE 802.11 is the most well known contention-based protocol.

Section 90.7 of Part 90 of the United States Federal Communication Commission rules define CBP as:

A protocol that allows multiple users to share the same spectrum by defining the events that must occur when two or more transmitters attempt to simultaneously access the same channel and establishing rules by which a transmitter provides reasonable opportunities for other transmitters to operate. Such a protocol may consist of procedures for initiating new transmissions, procedures for determining the state of the channel (available or unavailable), and procedures for managing retransmissions in the event of a busy channel.

This definition was added as part of the Rules for Wireless Broadband Services in the 3650-3700 MHz Band.[1]

2

What is contention? - A Word Definition From the Webopedia ...The contention protocol defines what happens when this occurs. The most widely used

contention protocol is CSMA/CD, used by Ethernet. Also see polling and token passing.www.webopedia.com/TERM/C/contention.ht… - Cached

Contention | Define Contention at Dictionary.comContention protocol ... –noun. 1. a struggling together in opposition; strife. 2. a striving in rivalry ...dictionary.reference.com/browse/conten… - Cached

Contention-based protocol - Wikipedia, the free encyclopediaA contention-based protocol (CBP) is a communications protocol for operating wireless telecommunication equipment that allows many users to use the same radio channel without pre-coordination. The "listen before talk" operating procedure in IEEE 802.11 is the most well known contention-based protocol.en.wikipedia.org/wiki/Contention_based… - Cached.

Answers.com - What is Contention-based protocolsCan you answer this question? Answer it or... get updates discuss research share Facebook Twitter Search Related answers: Is TCPIP a host- based protocol ? Yes. Upon ...wiki.answers.com/Q/What_is_Contention-… - Cached

Performance Analysis of Contention Based Medium Access ...This paper studies the performance of contention based Medium Access Control (MAC) protocols. This paper provides a simple and accurate method for estimating the ...whitepapers.techrepublic.com.com/abstr… - Cached

Answers.com - What is contention and contention free protocolCan you answer this question? Answer it or... get updates discuss research share Facebook Twitter Search Related answers: What is contention ? mainly all cargo gear's ...wiki.answers.com/Q/What_is_contention_… - Cached

Contention (telecommunications) - Wikipedia, the free ...Collision...|Collision...|Common examples|Other examplesIn packet mode communication networks, contention is a media access method that is used to share a broadcast medium.en.wikipedia.org/wiki/Contention_(tele… - Cached.

contention - definition of contention by the Free Online ...contention - the act of competing as for profit or a prize; "the teams were in fierce ... Content-Vectoring Protocol Contentation contented contented contented contentedwww.thefreedictionary.com/contention - Cached

Media Access Control: Information from Answers.comThe most widespread multiple access protocol is the contention based CSMA/CD protocol used in Ethernet networks. This mechanism is only utilized within a network collision ...www.answers.com/topic/media-access-con… - Cached

What is CSMA/CD? - A Word Definition From the Webopedia ...

CSMA/CD is a type of contention protocol. Networks using the CSMA/CD procedure are simple to implement but do not have deterministic transmission characteristics.www.webopedia.com/index.php/TERM/C/CSM… - Cached

With reservation protocol

In this paper, we present a novel contention-based medium access control (MAC) protocol, namely, the Channel Reservation MAC (CR-MAC) protocol. The CR-MAC protocol takes advantage of the overhearing feature of the shared wireless channel to exchange channel reservation information with little extra overhead. Each node can reserve the channel for the next packet waiting in the transmission queue during the current transmission. We theoretically prove that the CR-MAC protocol achieves much higher throughput than the IEEE 802.11 RTS/CTS mode under saturated traffic. The protocol also reduces packet collision, thereby saving the energy for retransmission. We evaluate the protocol by simulations under both saturated traffic and unsaturated traffic. Our simulation results not only validate the theoretical analysis on saturated throughput, but also reveal other good features of the protocol. For example, under saturated traffic, both the saturated throughput and fairness measures of the CR-MAC are very close to the theoretical upper bounds. Moreover, under unsaturated traffic, the protocol also achieves higher throughput and better fairness than IEEE 802.11 RTS/CTS.

Sesor network

"WSN" redirects here. For the metasyntax, see Wirth syntax notation.

Typical Multihop Wireless Sensor Network Architecture

A Wireless Sensor Network (WSN) consists of spatially distributed autonomous sensors to monitor physical or environmental conditions, such as temperature, sound, vibration, pressure, motion or pollutants.[1][2], and to cooperatively pass their data through the network to a main location. The more modern networks are bi-directional, enabling also to control the activity of the sensors. The development of wireless sensor networks was motivated by military applications such as battlefield surveillance; today such networks are used in many industrial and consumer application, such as industrial process monitoring and control, machine health monitoring[3], environment and habitat monitoring, healthcare applications, home automation, and traffic control.[2][4]

The WSN is built of "nodes" - from a few to several hundreds or even thousands, where each node is connected to one (or sometimes several) sensors. Each such sensor network node has typically several parts: a radio transceiver with an internal antenna or connection to an external antenna, a microcontroller, an electronic circuit for interfacing with the sensors and an energy source, usually a battery. A sensor node might vary in size from that of a shoebox down to the size of a grain of dust, although functioning "motes" of genuine microscopic dimensions have yet to be created. The cost of sensor nodes is similarly variable, ranging from hundreds of dollars to a few pennies, depending on the complexity of the individual sensor nodes. Size and cost constraints on sensor nodes result in corresponding constraints on resources such as energy, memory, computational speed and communications bandwidth.[2]. The topology of the WSNs can vary from a simple star network to an advanced multi-hop wireless mesh network. The propagation technique between the hops of the network can be routing or flooding[5][6].

In computer science and telecommunications, wireless sensor networks are an active research area with numerous workshops and conferences arranged each year.

heteroginity in mobile devices

Heterogeneity in mobile computing devices and application scenarios complicates the development of collaborative software systems. Heterogeneity includes disparate computing and communication capabilities, differences in users' needs and interests, and semantic conflicts across different domains and representations. In this paper, we describe a software framework that supports mobile collaboration by managing several aspects of heterogeneity. Adopting graph as a common data structure for the application state representation enables us to develop a generic solution for handling the heterogeneities. The effect external forces, such as resource constraints and diverging user interests, can be quantified and controlled as relational and attribute heterogeneity of state graphs. When mapping the distributed replicas of the application state, the external forces inflict a loss of graph information, resulting in many-to-one correspondences of graph elements. A key requirement for meaningful collaboration is maintaining a consistent shared state across the collaborating sites. Our framework makes the best of maximizing the state consistency, while accommodating the external force constraints, primarily the efficient use of scarce system resources. Furthermore, we describe the mobility aspects of our framework, mainly its extension to peer-to-peer scenarios and situations of intermittent connectivity. We describe an implementation of our framework applied to the interoperation of shared graphics editors across multiple platforms, where users are able to share 2D and 3D virtual environments represented as XML documents. We also present performance results, namely resource efficiency and latency, which demonstrate its feasibility for mobile scenarios.