by Allen Miu, Hari Balakrishnan and C.E. Koksal Improving Loss Resilience with Multi-Radio Diversity in Wireless Networks by Allen Miu, Hari Balakrishnan and C.E. Koksal Appeared in ACM MOBICOM 2005, was considered as a candidate for the best paper award.

What is the paper about? The paper looks at multi-radio diversity and uses the fact that the fading experienced at the different radios are different to improve performance in WLANs. The major contribution of this paper is that it proposes a new technique called “Frame Combining” using which, it tries to combine two frames received by the two radios, in error, to reconstruct the transmitted frame. The effort is “thorough”; the authors do an implementation of their proposed approach, provide some analytical insights and also do simulations to study the problem deeper.

What did I learn from this paper? Obtained insights into how much, and why, multi-radio diversity can help improve performance. How does any kind of diversity affect “rate control” ? (to some degree) Some interesting interactions between the physical, MAC and network layers which I will highlight in the presentation.

The MRD System The idea is to use multiple radios or multiple APs in a wireless LAN to simultaneously receive transmitted frames. Why does this help ? -- The radios are not spatially collocated. Thus, the wireless channel to the two radios differ (path diversity) Thus the errors experienced at one radio, would differ from that at the other. So, you would get two copies of a transmitted frame, but possibly at errors at different locations. Of course, if one of the frames is error free, that can be delivered to the IP layer (selection diversity). If not, can we combine the corrupted frames ?

A High level view Note: Unless I state otherwise, all figures that I have are from the paper itself. Of the two APs, one is called an active AP -- it is in communication with the client. The others passively listen and try to gather frames.

What are the challenges ? How does this frame combining (done at a layer that sits above the MAC) interact with the 802.11 MAC and the PHY layer ? Specifically, the frame combining make take some time, and so how can you acknowledge and provide retransmissions of “reconstructed” or “salvaged” frames ? How does this frame recombining work with the auto rate control ? How are the bit errors distributed ? Can frames even be salvaged and if so how ? The paper tries to address these questions while proposing a new technique for frame combining.

Frame Combining The idea is simple. Divide each frame into NB blocks, each of fixed size (the last block may possibly have fewer bits). Let us say we want to see if we can reconstruct the frame from two copies that are received. Clearly if any of the copies is ok (CRC is ok), then, the packet is successful. We look at those blocks that differ -- as an example, the ith block of the first copy might differ from that of the second copy. Assemble a combined frame with different possibilities --choosing different blocks from either of the copies. If CRC passes, then a success.

More about frame combining What should the block size be ? Note that the previous combining method is simple, but its running time is exponential in terms of the number of differing blocks D. If you have two copies, then you need about 2D CRC check operations. So, clearly you want to keep D small; which means that you may want to reduce the total number of blocks i.e., increase B, the number of bits per block. However, if you do this, then, the possibility of successfully recombining reduces (Why ?).

Analyzing Frame Combining Let the frame combining failure probability be pf. Let there be a bit error model characterized by “bursts” of “b” bit errors. pf is the fraction of frames that cannot be corrected with combining out of those that could not be corrected by the soft selection. The assumption made is that the loss rates observed at the two receivers are independent of each other. -- The paper corroborates this claim by experimental results. The errors are clustered and occur with a periodicity.

Bit Errors The authors claim that the error pattern is in line with what is used. QAM-64 modulation on OFDM with a rate 2/3 code. With QAM-64, you transmit 6 bits/symbol. This means, that for each transmission on 50 OFDM carriers, you have ~ 50 x 6 = 300 bits. Since you use a rate 2/3 code, you decode 3 symbols at a given time -- each carrier carries 3 symbols. Thus, you have approximately 900 bit transmission patterns on the different carriers that repeat. Since each carrier is likely to experience similar fades periodically (static), the error distribution repeats about 1000 bits.

Assumptions made for computing pf In order to compute the frame combining failure probability pf, the authors make the following assumptions: The burst of errors is of fixed to “b” bits. The number of bits per block “B” is much larger than b and thus: the probability that two blocks have errors that overlap is negligible. they ignore the possibility that the errors can spread over more than one block -- i.e., the errors are completely contained within a block. the number of burst errors that a block can hold are not fixed. Note that given that b ~ 300 bits and a block is more than say 200 bytes, these are reasonable assumptions.

Notations and some details Db,i -- number of b-bit sequences with errors in a given frame received at receiver Ri. N1 and N2 represent the sets of blocks that contain errors in frames received at receivers 1 and 2. Note that two receivers are considered. Then, the intersection of the two sets N1 and N2 represent those blocks that have errors in both frames. Now, if this intersection (N1 N2 ), contains no errors, then, it means that the frame can be decoded.

Computing pf First, assume that bit error sequences (b bits in error) occur uniformly over the frame. NB is the number of blocks Let frame 1 have d1 errors and frame 2 have d2 errors. Then, Why is this true ?

This is similar to the problem wherein we have NB buckets, and d1 balls; we put the balls (probably more than NB) into those NB buckets. We want to compute the number of ways in which we can put balls into these buckets. Note that some buckets may have multiple balls and so we can have empty buckets.

So, we have the first NB buckets, and in addition have (d1-1) dummy buckets. Note that there is at least one bucket which contains balls (all balls). If a ball falls into a non-empty bucket, we put the ball into one of the dummy buckets. Thus, the total ways we can do what we want is to choose d1 buckets out of the NB + d1 -1 buckets. Numerator: First place d red balls (first frame) and d blue balls (second frame) in d blocks. Distribute the rest of the balls in all blocks. Thus, we have at least d errors. It is an upper bound since some combinations are counted more than once.

Given this.... We remove the conditioning to get: where: Note : This is an upper bound on the frame combining failure probability.

Looking at pf Note that if the burst error size is small, errors are more uniform, and even for large NB, probability of combining successfully is small. With bursty errors (as observed), pf gets lower with NB. But beyond a certain point, increasing NB does not help much.

What does this mean? It is necessary to keep NB small, so as to reduce complexity. So, NB can be set to a small value (6-10) and still performance is ok.

Retransmissions Clearly link layer ACKs can result in erroneous conclusions. So the authors disable link layer retransmissions. Retransmissions are always invoked by the MRD layer. If packet reception is successful, synchronous ACK is received (at link layer). Else, this indicates either a frame with errors or an ACK failure. A frame with errors is either recovered using soft decision or frame combining; if this fails, the frame may be stored with the hope of trying to combine with later retransmitted versions. The sender expects an ACK sometime in the future. If prior to a time-out no ACK is forthcoming, it can request for an ACK -- to explicitly denote success or a failure. This is called the RFA (request for ACK).

RFA The RFA needs to explicitly state which frame is in question. Use of a flag in the frame header to indicate an RFA. When an link layer ACK fails, the MRDS (sender) simply stores the packet and proceeds with subsequent transmissions. It can perform upto a certain number (N) of future transmissions (from the first unACKed frame). Frame removed from buffer after K retransmissions. If the MRD-ACK indicates a frame recovery failure, the frame is retransmitted. If no MRD-ACK (higher layer) is received, retransmission after a time-out.

Link layer ACKs ? Why did they not disable Link Layer ACKs ? Needed for carrier sensing (virtual). Second, the synchronous ACKs have already a reserved channel. Loss is less probable. MRDS ACKs, on the other hand, need to contend for the channel. So they may be either lost or delayed.

Rate Adaptation With autorate or rate adaptation, the data rate is lowered if loss is encountered and increased with successful packet delivery. Not good with MRD -- not all radio receptions taken into account. The authors implement their protocol on the Atheros 5212 chipset driven by the Multiband Atheros driver. The authors modify the driver to make it fit with the MRD implementation.

How ? The original driver MADWIFI -- invokes TXCALLBACK to update numtx and numtxok after each frame transmission. Rate is adjusted every T seconds. If frame delivery rate is above 90 % for S consecutive observation periods, then increase bit rate. If frame delivery rate drops below threshold D, then reduce rate. The authors introduce a new function MRD_CALLBACK. Very simple fix -- count the number of transmissions ACKED by MRD_ACKs. Note that MRD_ACKs are cumulative -- so they have a clear picture of what was received even if some were lost.

Implementation Pentium PCs, Linux Kernel 2.4.20, 802.11 a/b/g wireless interfaces based on the Atheros 5212 chipset. They have modified the MADWiFi driver. Simple experiments -- they set retry limit to zero at the MAC layer (so no retransmissions there). This however, disables CSMA backoff and they say that they will look at it in the future. Some discussion on the implications. Since MRD-ACKs are not ACKed, they are transmitted in broadcast mode.

Implementation (contd). CTX -- Combiner Transmit header. NTX -- number of attempted transmissions, 1 RFA bit to indicate that the sender has pending frames. seq -seq number. useq -- oldest transmitted data frame in MRDS that has not been ACKed.

Identifying combinable frames Look at the MAC layer source address and seq number in CTX to identify copies of the same network layer frame. Since MRDS has to correctly identify the frames, a 4 byte CRC protection is used to protect the CRC and CTX headers. If either of these headers are corrupted, the frame is dropped.

MRDS-ACK Implementation Magic value distinguishes the MRD-ACK packet from other downlink data payload. N bit transmit state -- indicates the success failure of up to N consecutive frames. seq number -- seq value of the first data frame in the bit vector being acknowledged. Link layer checksum used to detect errors in this packet.

Experiments The authors perform experiments with low variability (LOVAR) -- where client is static and high variability (HIVAR) where the client is mobile. Two APs -- one is the Master and the other is the Monitor. The client is run in the 802.11 Managed mode (i.e., it is in the LAN access config.). They pick NB (number of blocks) to be six. (look at discussion on complexity tradeoffs).

Setup for HIVAR experiments

Some Results Note that only a small fraction of frames recovered using the combining process. Throughput with just R1 or R2 were 8.25 and 6.42 Mbps. With MRD-R1 and MRD-R2, the average throughputs were 18.7 and 18.36 Mbps. Still lower than 31 Mbps UDP throughput (computed theoretically).

Failure of frame combining They do thorough simulations to see why this happens. They argue that NB was too small. They also look at complexity versus efficiency trade-offs that I will not discuss here.

Rest.. I won’t discuss the rest of the paper but hopefully, this has shown what it contains. The final set is on LOVAR experiments, what happens there and finally, some discussion on back-offs -- if CSMA backoffs were invoked, then they wrongly cause nodes to back off The packets may still be recovered.