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:
- 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.
- 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.
- 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