Medium Access Control (MAC) for Distortion Based Networking
Distortion based networking is a concept where the quality of received multimedia content (i.e. video and audio) is used in the design, implementation and selection of networking protocols at all layers of the networking stack. This is in contrast to the typical evaluation metrics (throughput, delay, ect.) in that multimedia quality is directly observable by the end user. The medium access control (MAC) layer of the networking stack is responsible for, as the name implies, controlling how each node accesses the physical medium. The main objective is typically to allow multiple access to the channel without collision between users while maximizing the use of the channel. Selecting the MAC protocol and the parameters of that protocol can have a huge impact on the quality of the received multimedia content. There are a number of MAC protocols commonly used in wireless networking. Presented below are a few common ones which have some particular interest in distortion based networking.
Carrier sense medium access with collision avoidance (CSMA/CA) is the most typically used MAC protocol as it is the basis of the well known 802.11 series of protocols. IN CSMA or CSMA/CA protocols, before transmitting, each node waits a random amount of time and then senses the channel to determine whether any other transmitter is currently active. If the channel is busy, the node will wait until the other nodes are finished before attempting to access the channel. In the case of a collision, both nodes will wait for an increasing “random backoff” before attempting to transmit again, attempting to avoid repeated collisions. The main advantage of CSMA/CA for this application is that there is no central controller needed between nodes allowing for a fully distributed protocol. For multimedia traffic, though, the randomness of the protocol makes it difficult to guarantee any level of quality at the application layer. If a number of nodes attempt to send data at once, the video (or audio) stream could be delayed or interrupted, resulting in distortion if the received content.
For multimedia traffic, a more promising MAC protocol is time division multiple access (TDMA), which is used in some 3G cellular networks. TDMA networks separate the channel in the time domain into slots, and each node is assigned one or more of these time slots. This is ideal for multimedia data because the slots, and therefore the data rate, can be guaranteed. In some cases, the time slots can even be allocated based on the requirements of the multimedia content being transmitted. Collisions are avoided in this type of MAC by assigning the time slots such that two nodes within interfering distance of each other are not transmitting simultaneously. However, this type of MAC needs a central controller to assign the time slots.
Another promising MAC scheme for multimedia content is orthogonal frequency division multiple access (OFDMA), where each user is allocated a number of orthogonal subchannels. Because these subchannels are orthogonal, different nodes can simultaneously access different subchannels without creating interference. Like TDMA, collisions are avoided by assigning the subchannels such that two nodes are not using the same subchannel in the same physical space which requires a central controller. Also like TDMA, these subchannels can be allocated based on the requirements of the multimedia traffic.