Readings for Lecture Sep 30, 2008

MACAW: A Media Access Protocol for Wireless LAN’s

The emerging of a wide variety of mobile computing devices like palmtops, personal digital assistants, and portable computers with the demand for network connection has motivated a new generation of wireless network technology and WLAN is a crucial enabling technology. The MAC layer of WLAN can be very different from traditional wired LAN due to its nature of shared, and scarce, resource.

The WLAN infrastructure consists of a “base station”, and pads, which are custom, built portable computing devices. All the wireless communication is between a pad and a base station. The range of transmission is 3 to 4 meters, and the near field signal strength decays very rapidly. Thus around each base station, a very small cell with very sharply defined boundaries, is called “nanocell”.

A collision occurs when a receiver is in the reception range of two transmitting stations and is unable to cleanly receive signal from either station. Capture occurs when a receiver is in the reception range of two transmitting stations, but is able to cleanly receive signal from the closer station. Interference occurs when a receiver is in range of one transmitting station and slightly out-of-range of another transmitting station but unable cleanly receive the signal of the closer station due to the interfering presence of other signal.

The common wireless multiple access algorithm is CSMA, in which every station senses the carrier before transmitting. This algorithm has problem at the receiver since the senders cannot sense the carrier at the receiver so the senders are hidden from each other. The MACA algorithm introduces a protocol with RTS-CTS exchange to initiate a connection. MACA uses the binary exponential backoff (BEB) to determine the retransmission time.

The ACK is implemented at the link layer to enable fast error recovery at the link layer due to frequent packet collision and corruption by noise. To further improve RTS-CTS model, a Data-Sending packet is sent at the sender to let other stations know the RTS-CTS has successfully implemented. The RRTS is added to help the receiver inform the sender that it is available to send. The efficiently implement the backoff algorithm, each station should maintain a separate backoff counter for each stream.

A Comparison of Mechanisms for Improving TCP Performance over Wireless Links
TCP reponds to all losses by invoking congestion control and avoidance algorithm, resulting in degraded end-to-end performance in wireless and lossy systems. The paper classify the schemes into three broad categories: end-to-end protocols, where loss recovery is performed by the sender, link-layer protocols, the provide local reliability, and split-connection protocols that break the end-to-end connection into two parts at the base station.

The end-to-end throughput and goodput are used as performance metrics to seek the answer of the specific questions: What combination of mechanisms results in best performance for each of the protocol classes? How important is it for link layer schemes to be aware of TCP algorithm to achieve high end-to-end throughput? How useful are selective acknowledgements in dealing with lossy links, especially in the presence of burst losses? Is it important for the end-to-end connection to be split in order to effectively shield the sender from the wireless losses and obtain the best performance? The goodput for any path (or link) is defined as the ratio of the actual transfer size to the total number of bytes transmitted over that path.

The link-layer protocols have two main classes of techniques: error correction, using techniques such as forward error correction (FEC), and retransmission of lost packets in response to automatic repeat request (ARQ) message.

The split connection protocols split each TCP connection between a sender and receiver into two separate connections at the base station. Over the wireless hop, a specialized protocol tuned to the wireless environment may be used. One proposed two protocols: one in which wireless hop uses TCP, and another in which the wireless hop uses a selective repeat protocol (SRP) on top of UDP.

The snoop protocol introduces a module, called snoop agent, at the base station. The agent monitors every packet that passed through the TCP connection in both directions and maintains a cache of TCP segments sent across the link that have not yet been acknowledged by the receiver. A packet loss is detected by duplicate acknowledgement or a local timeout. The snoop agent retransmits the lost packet if it has it cached and suppresses the duplicate acknowledgement.

Selective acknowledgements contain information about up to three non-contiguous blocks of data that have been received successfully by the receiver. An alternate proposal, SMART, uses acknowledgements that contain the cumulative acknowledgement and the sequence number of the packet that caused the receiver to generate the acknowledgement.
The link layer protocols using knowledge of TCP improve the throughput by 10-30%. The SMART-based selective acknowledgement yields good throughput.