Wireless Multiple Access Protocols

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. At the same time, we see a convergence of the telephone, cable and data networks into a unified network that supports multimedia and real-time applications like voice and video in addition to data. Medium access control protocols define rules for orderly access to the shared medium and play a crucial role in the efficient and fair sharing of scarce wireless bandwidth. The nature of the wireless channel brings new issues like location-dependent carrier sensing, time varying channel and burst errors. Low power requirements and half duplex operation of the wireless systems add to the challenge. Wireless MAC protocols have been heavily researched and a plethora protocols have been proposed. Protocols have been devised for different types of architectures, different applications and different media. The challenges in the design wireless MAC protocols, classifies them based on architecture and mode of operation, and describes their 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 very different from, and more challenging than, wireline networks. The unique properties of wireless systems and their medium are, Half-Duplex Operation: In wireless systems it is very difficult to receive data when the transmitter is sending data. This is because when a node is transmitting data, a large fraction of the signal energy leaks into the receive path. This is referred to as self-interference The transmitted and received power levels can differ by orders of magnitude. The leakage signal typically has much higher power than the received signal, which makes it impossible to detect a received signal while transmitting data. Hence, collision detection is not possible while sending data and so Ethernet-like protocols cannot be used. Due to the half-duplex mode of operation, the uplink and downlink need to be multiplexed in time (TDD) or frequency (FDD). As collisions cannot be detected by the sender, all proposed protocols attempt to decrease the probability of a collision using collision avoidance principles. Time Varying Channel: Radio signals propagate according to three mechanisms: reflection, diffraction, and scattering. The signal received by a node is a superposition of time-shifted and attenuated versions of the transmitted signal. As a result, the received signal power varies as a function of time. This phenomenon is called multipath propagation. The rate 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 changes by 3 dB. When the received signal strength drops below a certain threshold, the node is said to be in fade. Handshaking is a widely used strategy to mitigate time-varying link quality. When two nodes want to communicate with each other, they exchange small messages that test the wireless channel between them. A successful handshake indicates a good communication link between the two nodes. Burst Channel Errors: As a consequence of the time-varying channel and varying signal strength, errors are more likely in wireless transmissions. In wireline networks, the bit error rates are typically less than 10-6 and as a result the probability of a packet error is small. In contrast, 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 these errors are usually due to random noise. In contrast, the errors on a wireless link occur in long bursts when the node is in fade. Packet loss due to burst errors can be minimized 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 protocols have immediate acknowledgments (ACK) to detect packet errors. If an ACK is not received 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 path length. As a result, carrier sensing is a function of the position of the receiver relative to the transmitter. In the wireless medium, because of multipath propagation, signal strength decays according to a power law with distance. Only nodes within a specific radius of the transmitter can detect the carrier on the channel. This locationdependent carrier sensing results in three types of nodes in protocols that use carrier sensing. Hidden Nodes: A hidden node is one that is within the range of the intended destination but out of range 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. During this 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 case node C is a hidden node to node A. Hence, hidden nodes can cause collisions on data transmission. Exposed Nodes: Exposed nodes are complementary to hidden nodes. An exposed node is one that is within the range of the sender but out of range of the destination. Consider the case that 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, any transmission by node C does not reach node A, and hence does not interfere with data reception at node A. In theory, C can therefore have a parallel conversation with another terminal out of range of B and in range of C. In this case, node C is an exposed node to node B. If the exposed nodes are not minimized, the bandwidth is underutilized. Capture: Capture is said to occur when a receiver can cleanly receive a transmission from one of two simultaneous transmissions, both within its range. In Figure when nodes A and D transmit simultaneously to B, the signal strength received from D is much higher than that from A, and D's transmission can be decoded without errors in the presence of transmission from A. Capture can improve protocol performance, but it results in unfair sharing of bandwidth with preference given to nodes closer to the BS. Wireless MAC protocols need to ensure fairness under such conditions.