Ajay Chandra V. Gummalla

Wireless medium access control protocols

Technological advances, coupled with the flexibility and mobility of wireless systems, are the driving force behind the Anyone, Anywhere, Anytime paradigm 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 of protocols have been proposed. Protocols have been devised for different types of architectures, different applications and different media. This survey discusses the challenges in the design of 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.

Design of an access mechanism for a high speed distributed wireless LAN

The medium used by indoor wireless LANs is a shared, scarce resource. Hence, efficient use of this medium is important, which in turn requires an efficient MAC protocol. Moreover, next generation wireless LANs will have to support data rates from 10 to 100 Mbits/s. At these data rates, turnaround times in half-duplex radios are a large overhead. This makes the current wireless standards inefficient. However, busy-tone solutions can get around this problem. In spite of their better performance, the busy-tone protocols are not widely used because of the hardware cost, i.e., the requirement of two transceivers-one each for the data and feedback channel. A novel wireless transceiver architecture which mitigates this hardware cost by overlaying the data channel and the feedback channel in the same frequency band is designed. It is shown that this transceiver can be built with current technology. Based on this wireless transceiver, a new MAC protocol called wireless collision detect (WCD) is proposed. This protocol uses a carrier detect signal to decrease the collision probability and receiver initiated feedback to handle hidden nodes. We derive a mathematical expression for the throughput of the protocol. The simulation results match the analysis. At 100 Mbits/s, WCD achieves 77% efficiency with 192 byte packets. For this scenario, the throughput of the IEEE 802.11 MAC protocol and RI-BTMA is 3% and 52%, respectively.

Wireless collision detect (WCD): multiple access with receiver initiated feedback and carrier detect signal

An ad-hoc wireless network is a collection of wireless nodes that can communicate with each other without the aid of any pre-existing communication infrastructure. The medium access control protocol plays a crucial role in such networks by enabling efficient communication among multiple devices on the shared medium. At high data rates, busy-tone protocols will be much more efficient than collision avoidance protocols because of transceiver turn-around times. Wireless collision detect (WCD) is proposed to enhance the performance of existing busy-tone solutions. This improves the performance and eliminates hidden and exposed nodes. This protocol is based on collision detection by the destination node (DN) and performs similarly to the Ethernet protocol. We analyze the protocol and get very close agreement between analytical and simulation results. At 100 Mbps, WCD achieves 77% efficiency even with packets as short as 192 bytes which is a 10 fold improvement over the IEEE 802.11 MAC protocol. When the time during which the node is vulnerable to a collision is three times the time to detect a carrier, WCD performs 20% better than RI-BTMA.

An access protocol for a wireless home network

Wireless technology is an attractive option to network devices in the home as it does not require rewiring the home. The media access protocol (MAC) plays a crucial role in allowing multiple devices to transmit and receive on the shared wireless medium and thereby determining the efficiency of a wireless home network (WiHN). It is found that many of the protocols proposed to date like the Distributed Foundation Wireless MAC (DFWMAC) in the IEEE 802.11 standard have poor performance at high data rates (100 Mbps). We modify the slotted-idle sense multiple access protocol and propose a simple and efficient MAC protocol which fits well with the WiHN architecture. We present a queuing analysis and simulation results of the performance of the wireless home network MAC (WiHNMAC) protocol.

Effect of Turn-Around Times on the Performance of High Speed Ad-hoc MAC Protocols

Many random access protocols have been proposed for ad-hoc networks and they are based on collision avoidance principles and are designed for low data rates (≤ 2Mbps). A key element in the design of high speed MAC protocols is the ability to send feedback from the destination to the source about the state of current transmission. We present an analysis to show that when hardware constraints like transceiver turn-around times are involved, multiplexing the feedback using a different frequency channel is a better design choice. We validate this analysis by comparing the performance of three current ad-hoc MAC protocols as the data rates are increased using a power law model for the variation of the turn-around time. We show that at high data rates, the busy tone protocols will be much more efficient than the collision avoidance protocols. Further, the results show that unless the turnaround times scale in proportion to data rate the performance of CSMA/CA protocols will be worse than slotted-ALOHA

Receiver architecture for high-speed wireless LAN protocols

Random access protocols are best suited for an ad-hoc wireless network because they require no central control. Many random access protocols have been proposed for such networks and all of them are based on collision avoidance principles and are designed for low data rates (/spl les/2 Mbps). These protocols are very inefficient at high data rates and small packet sizes where transceiver turnaround times become a large overhead. A feedback channel from the source to the destination enables the design of efficient random access protocols which are rate scalable. Two transceivers (one for the data channel and the second for the feedback channel) are required to enable a feedback channel. This paper proposes a novel receiver architecture which allows the overlay of the feedback channel and data channel in the same spectrum. The design challenges and solutions for building such a receiver are discussed. A MATLAB simulation is used to test the feasibility of the proposed receiver.