Wireless Multiple Access Protocols Technological advances, coupled with the flexibility and mobility of wireless systems,are the driving force behind the anyone, anywhere, anytime features of networking. Atthe same time, we see a convergence of the telephone, cable and data networks into aunified network that supports multimedia and real-time applications like voice andvideo in addition to data. Medium access control protocols define rules for orderlyaccess to the shared medium and play a crucial role in the efficient and fair sharing ofscarce wireless bandwidth. The nature of the wireless channel brings new issues likelocation-dependent carrier sensing, time varying channel and burst errors. Low powerrequirements and half duplex operation of the wireless systems add to the challenge.Wireless MAC protocols have been heavily researched and a plethora protocols havebeen proposed. Protocols have been devised for different types of architectures,different applications and different media. The challenges in the design wireless MACprotocols, classifies them based on architecture and mode of operation, and describestheir relative performance and application domains in which they are best deployed. Wireless Mac Issues The unique properties of the wireless medium make the design of MAC protocols verydifferent from, and more challenging than, wireline networks. The unique properties ofwireless systems and their medium are, Half-Duplex Operation: In wireless systems it is very difficult to receive data when the transmitter is sendingdata. This is because when a node is transmitting data, a large fraction of the signalenergy leaks into the receive path. This is referred to as self-interference Thetransmitted and received power levels can differ by orders of magnitude. The leakagesignal typically has much higher power than the received signal, which makes itimpossible to detect a received signal while transmitting data. Hence, collisiondetection is not possible while sending data and so Ethernet-like protocols cannot beused. Due to the half-duplex mode of operation, the uplink and downlink need to bemultiplexed in time (TDD) or frequency (FDD). As collisions cannot be detected by thesender, all proposed protocols attempt to decrease the probability of a collision usingcollision avoidance principles. Time Varying Channel: Radio signals propagate according to three mechanisms: reflection, diffraction, andscattering. The signal received by a node is a superposition of time-shifted andattenuated versions of the transmitted signal. As a result, the received signal powervaries as a function of time. This phenomenon is called multipath propagation. Therate of variation of the channel is determined by the coherence time of the channel.Coherence time is defined as time within which the received signal strength changesby 3 dB. When the received signal strength drops below a certain threshold, the nodeis said to be in fade. Handshaking is a widely used strategy to mitigate time-varyinglink quality. When two nodes want to communicate with each other, they exchange
small messages that test the wireless channel between them. A successful handshakeindicates a good communication link between the two nodes. Burst Channel Errors: As a consequence of the time-varying channel and varying signal strength, errors aremore likely in wireless transmissions. In wireline networks, the bit error rates aretypically less than 10-6 and as a result the probability of a packet error is small. Incontrast, wireless channels may have bit-error rates as high as 10-3 or higher,resulting in a much higher probability of packet errors. In wireline networks theseerrors are usually due to random noise. In contrast, the errors on a wireless link occurin long bursts when the node is in fade. Packet loss due to burst errors can beminimized by using one or more of the following three techniques Smaller packets. Forward error correcting codes. Retransmission methods. A widely used strategy is to use link-layer retransmissions. Therefore, most protocolshave immediate acknowledgments (ACK) to detect packet errors. If an ACK is notreceived at the end of a transmission, the packet is retransmitted. Location-Dependent Carrier Sensing: It is well known that in free space, signal strength decays with the square of the pathlength. As a result, carrier sensing is a function of the position of the receiver relativeto the transmitter. In the wireless medium, because of multipath propagation, signalstrength decays according to a power law with distance. Only nodes within a specificradius of the transmitter can detect the carrier on the channel. This location-dependent carrier sensing results in three types of nodes in protocols that use carriersensing. Hidden Nodes: A hidden node is one that is within the range of the intended destination but out ofrange of the sender. Consider the case shown in Figure.
Figure: Location-dependent sensing, hidden nodes, exposed nodes and capture. Node A is transmitting to node B. Node C cannot hear the transmission from A. Duringthis transmission when C senses the channel, it falsely thinks that the channel is idle.If node C starts a transmission, it interferes with the data reception at B. In this casenode C is a hidden node to node A. Hence, hidden nodes can cause collisions on datatransmission. Exposed Nodes: Exposed nodes are complementary to hidden nodes. An exposed node is one that iswithin the range of the sender but out of range of the destination. Consider the casethat node B is attempting to transmit to A. Node C can hear the transmission from B.When it senses the channel, it thinks that the charnel is busy. However, anytransmission by node C does not reach node A, and hence does not interfere with datareception at node A. In theory, C can therefore have a parallel conversation withanother terminal out of range of B and in range of C. In this case, node C is anexposed node to node B. If the exposed nodes are not minimized, the bandwidth isunderutilized. Capture: Capture is said to occur when a receiver can cleanly receive a transmission from oneof two simultaneous transmissions, both within its range. In Figure when nodes A andD transmit simultaneously to B, the signal strength received from D is much higherthan that from A, and D's transmission can be decoded without errors in the presenceof transmission from A. Capture can improve protocol performance, but it results inunfair sharing of bandwidth with preference given to nodes closer to the BS. WirelessMAC protocols need to ensure fairness under such conditions.