Cross-Layer Strategies for Efficient Video Delivery over Multihop Wireless Networks

Group Members:

Faculty:         Tzi-cker Chiueh

Students:        Pradipta De
                       Ashish Raniwala
                       Srikant Sharma

Project Description:

The fragile nature of the wireless links compared to the wired links is well-known. Typically signal quality is affected by a multitude of factors, starting from the prresence of interfering devices in the environment in the same radio range (e.g. presence of microwave, cordless phones in a 802.11b network ), distance,  and  even movement of people around the wireless cards. Poor signal quality leads to bit corruption and subsequent packet loss at the receiver. The bit corruption problem is accentuated further in a multihop scenario.

The IEEE 802.11 standard incorporates mechanisms, like link-layer retransmission, frame size adaptation at the link layer to reduce packet error rate, automatic adaptation of the transmission rate. While these mechanisms appear quite reasonable for data applications, where absolute bit-level reliability is required, they may not be the optimal design choice for one-way streaming, as in video playback, and/or two-way media communication, such as VoIP applications, which can tolerate certain degree of bit error.

In this project we are investigating three possible ways to utilize the available bandwidth for video delivery.  These are:
  1. Bit Error Tolerant Streaming: Since video streaming can tolerate bit error to a ceratin extent, therefore, it may be worthwhile to send a partly corrupted packet to the upper layer instead of discarding it after checksumming. This will also save the bandwidth for the link-layer retransnmissions.
  2. Semi Reliable Transport Protocol: Bit errors in radio channels  tend to be bursty in nature and may last for a sustained period of time. We investigate a way to reduce the adverse effect of these burst losses, given the limited channel bandwidth. Most media streaming protocols take a no-retransmission approach with the assumption that the retransmitted packets may arrive too late to be useful. However, the observation that most video playback applications support a large playback buffer suggests that limited retransmission may be feasible. Our goal is to make a judicious choice of which packets to be retransmitted given that we may not have enough bandwidth available to ask for complete retransmission. The goal is to reduce the effect of burst losses as much as possible so that "user experience" can be made better.
  3. Link-Layer Aware Transport Protocol: In case of reliable transport layer protocol, the receiver sends back an ACK at the transport layer to indicate successful transmission of a packet. On a single hop Wi-Fi network, which supports link-layer ACK this is redundant. Hence, we have designed a reliable transport layer protocol that eliminates the performance overhead of the tranpsort layer ACKs by exploiting link-layer ACKs in Wi-Fi networks.
Project Status:

The results of the project are available in the form of a technical report which can be made available on request.

Related Links:
Bit-Error Tolerant Streaming
Error Characteristics of Wireless Channels