1. TCP-Cognizant Adaptive Forward Error Correction in Wireless Networks
Benyuan Liu , Dennis L. Goeckel , Don Towsley
Department of Computer Science
Department of Electrical and Computer Engineering
University of Massachusetts
IEEE INFOCOM 2002

2. System Goals
Introducing a new link layer protocol using adaptive forward error correction (AFEC) to improve TCP performance over wireless links
FEC code is selected according to a TCP goodput formula to maximize TCP goodput
FEC code is adaptive to the link characteristics
Performance comparison of AFEC with some other protocols

3. Goodput Definition
The “goodput” of a flow is the “effective bandwidth” delivered to the receiver, excluding overhead and duplicate packets caused by retransmission.

4. Related Work Split-Connection approach Snoop protocol
Automatic repeat request (ARQ)
Explicit loss notification (ELN)
TCP-SACK
Forward Error Correction (FEC)

5. TCP Goodput Formula
Gf : TCP goodput
p: packet loss rate
Wmax: maximum congestion window size of TCP sender
b: effect of delayed acknowledgement
T0: TCP transmission timeout value
The above formula predicts accurate TCP goodput over a wide range of packet loss rates
Reducing p helps to increase TCP goodput

6. Effect of FEC on TCP Goodput
FEC can reduce the packet loss rate and increase TCP goodput
FEC also causes overhead and reduces effective channel bandwidth for the real payload
Real TCP goodput can be approximated by the minimum of the achievable TCP goodput and the effective channel bandwidth, it is maximized when the achievable TCP goodput equals to the effective channel bandwidth

7. FEC Code Selection
Assumption: wireless link errors are mostly due to bit errors in packets
Focus only on bit errors and use error correction codes
(N, K) Reed-Solomon code
K: size of data in a codeword (link layer packet), fixed value
N: size of codeword, varied to adjust the redundancy level of the code
N-K: size of parity symbols in a codeword

8. Optimal FEC Code Selection
Effective link bandwidth
Bc: raw link bandwidth
PER: packet error rate
Real TCP goodput:
Optimal Reed-Solomon code (N0, K)
For an (N, K) Reed-Solomon code, N can only be integer numbers within a certain range, so the code is chosen from this available set

9. Wireless Link Characteristic
Signal to interference plus noise ratio (SINR) is used to calculate the packet loss rate of the wireless link
Average SINR measurement is available in wireless communication systems for use in functions such as power control, handoff and adaptive rate control
Assumption: SINR is constant during a given TCP session (suitable for systems without mobility, but systems with highly mobile hosts may have significant differences)

10. Mapping from SINR to Packet Loss Rate
Case 1: Independent fading from symbol to symbol within a packet, independent fading between packets
Applies to systems with relatively quickly varying multipath fading and with interleaving of symbols within a packet
For 2m-ary symbols, symbol error rate:
Frame error rate:

11. Mapping from SINR to Packet Loss Rate
Case 2: Identical fading for symbols within a packet, independent fading between packets
Applies to systems with moderate fading rate (approximately once per packet)
α: fading value
Nf: the number of frames of the packet
pα(x): the probability density function of the channel fading gain α

12. Mapping from SINR to Packet Loss Rate
Case 3: Identical fading for symbols within a packet, identical fading for packets within a TCP window
Applies to systems with slow fading, where the fading varies over the RTT of the system but constant over a TCP window duration, most likely occur in systems with low mobility environments such as WLAN
PER of case 2 can also be used in case 3

13. TCP-AFEC Protocol
A link layer agent is added to the base station and the mobile host
The agent at the upstream node of the data flow estimates the channel BER and TCP session RTT, computes the optimal FEC code for each packet and transmits the packets over the wireless link
The agent at the downstream node recovers the packet if the errors are correctable, otherwise the packet is discarded
There is no retransmission in this protocol

14. Simulation Model
The data source is a fixed host in the wired network, the path from the source to the base station is a link with 10Mbps bandwidth and 40ms propagation delay, there is no error and congestion on the wired link

15. Simulation Model Two types of wireless links are considered:
WLAN with 2Mbps bandwidth and 4ms delay, packet size of 1460 bytes
WWAN with 19.2Kbps bandwidth and 200ms delay, packet size of 512bytes
Only the loss model of case 1 is considered, the range of SER is from 8x10-6 ~ 8x10-3
Following results are from WLAN scenarios, WWAN results are similar

16. Comparison with TCP-SACK and Snoop with no FEC
The symbol error rates, ranging from 8x10-6 to 8x10-3, produce packet error rates ranging from 1.2 % to 99 % in TCP-SACK and Snoop
Under low error rates, TCP-AFEC and Snoop achieve similar goodput, much better than TCP-SACK
Under high error rates, the transmission of TCP-SACK and Snoop stalls due to congestion control, while the performance of TCP-AFEC only drops slightly

17. Comparison with TCP-SACK and Snoop with FEC of 5% overhead
Under low error rates, the performance of TCP-SACK and Snoop are close to TCP-AFEC
Under high error rates, the goodput of TCP-SACK decreases dramatically

18. Comparison with TCP-SACK and Snoop with FEC of 30% overhead
The redundancy level is large enough to account for the high error rates
The large overhead also results in lower goodput than TCP-AFEC

19. Comparison with a Physical Layer Optimization Scheme
A physical layer scheme can only try to optimize a quantity that is understood by the physical layer
While TCP-AFEC maximizes min(Gc, Gf), a physical layer scheme can only maximize the first term, which is the effective bandwidth, it does not consider the TCP goodput limitation

20. Comparison with a Physical Layer Optimization Scheme
Under low error rates, the physical scheme achieves the same goodput as TCP-AFEC
Under high error rates, TCP-AFEC outperforms the physical layer scheme
TCP performance knowledge is beneficial to link layer protocols

21. Conclusion
TCP-AFEC can effectively improve the TCP goodput over wireless links for a wide range of channel conditions

22. Comments
As TCP goodput formula is used to calculate the optimal FEC code, this is a TCP-aware link layer protocol
As there is no retransmission, data reliability cannot be guaranteed
The FEC code is used to correct error bits in packets, whole packet loss is not considered. But in WLAN systems packet loss might be even more serious than bit errors