1. Announcement Office hour this week: Office hour next week
Today 4:30-5:30pm
Friday 10-11am
Office hour next week
Friday 3-4pm

2. Leveraging Diversity Leverage diversity Antenna diversity
MIMO, MUP, MRD
Topology diversity
Channel assignment, SSCH, partial overlapping channels, power control
Path diversity
Routing metrics, opportunistic routing
Application diversity
Delay tolerant vs. delay in-tolerant

3. Atul Adya, Paramvir Bahl, Jitendra Padhye, Alec Wolman, Lidong Zhou
A Multiple-Radio Unification Protocol for IEEE Wireless Networks
Atul Adya, Paramvir Bahl, Jitendra Padhye, Alec Wolman, Lidong Zhou
Microsoft Research

4. Question?
Given multiple radios on each node, how to effectively utilize them?

5. Proposed solution: MUP
Overview
“Unifies” the operation of multiple radios tuned to different channels
Objective
Use as much of the available bandwidth for improved performance with existing hardware/software
Assumption
Nodes equipped with multiple NICs with similar properties

6. 4 Goals of MUP design Must not require changes to existing hardware
does require IEEE e-based hardware
Must not require changes to existing application, transport, or routing protocols
Must inter-operate with legacy hardware
Must not require knowledge of network topology

7. MUP high level architecture
Single virtual MAC
Periodically monitors “channel quality” across all NICs
Prior to transmission, selects the channel with highest quality

8. Packet transmission using MUP: initialization
Application
Transport
Network
Logical Link Control
ARP
MUP
Neighbor table
Updated periodically
NIC1
NIC2
NICn
Tuned to C0
Tuned to C1
Tuned to Ck
Set at power-up

9. Packet transmission using MUP
Application
Pkt
Transport
Network
Logical Link Control
ARP
MUP
Neighbor table
get neighbor info
NIC1
NIC2
NICn
SRTT0
SRTT1
SRTTk
select interface to transmit packet

10. Which interface to use? MUP-Random Based on “Channel Quality” metric
Potentially reduces contention
or NOT, could select a channel currently being used
Based on “Channel Quality” metric
Send periodic probe messages across all interfaces
Measure time delay to when an acknowledgment is received
Compute smoothed round-trip time (SRTT)
Completely autonomous operation

11. 2 Major components of MUP
Neighbor discovery and classification
Construct MUP Neighbor Table
Communication protocol

12. Neighbor discovery Intercepts ARP requests/responses
Logical Link Control
ARP
MUP
Who has Dest_IP, Tell Src_IP
Broadcast across all interfaces
NICa
NICb
NICn
MAC_addra
MAC_addrb
MAC_addrn

13. Neighbor discovery (continued)
Logical Link Control
ARP
MUP
Neighbor table entry
NICa
NICb
NICn
Dest_IP is at MAC_addrq
Dest_IP is at MAC_addrr
Dest_IP is at MAC_addrs

14. Neighbor classification
MUP-enabled node broadcasts CS (channel select) message across all resolved interfaces
ARP initiated repeatedly up to N-times to resolve unresolved interfaces
MUP-enabled node responds with CS-ACK
Legacy nodes will ignore the CS message
Update neighbor table entry
Delete entry that timeouts with no activity

15. MUP communication protocol
Two MUP-enabled nodes periodically exchange probe packets
Probe packets are CS messages
Sent approximately every 500 msec
Node immediately replies with CS-ACK
Node measures packet latency
Node computes “Channel Quality” as smoothed round-trip time (SRTT)

16. SRTT Motivation for SRTT
Heavily used channels will typically experience large delay to gain access to the medium
Interference from external devices can cause excessive packet delays or even loss
Problem: probe packets can experience very large queuing delay if node is sending a large amount of data
Solution: require NICs support priority queues

17. IEEE e overview
Relatively new standard to add Quality of Service support to
Hardware to be available “soon”
Provides 8 separate priority queues per station
Ranges from best-effort to voice
Each priority level has specific backoff settings
Abitration interframe spacing

18. Switching channels
Channel is chosen if it provides at least 10% improvement in SRTT (to avoid flapping)
Otherwise current channel is used
Randomize time interval between changing channels
Avoids synchronized switching across node
SRTT adjusted for lost CS and CS-ACK packets
Once decided to switch, when to actually switch?

19. Current policy: switch immediately
Possible problem: new packets will likely be transmitted before the old queue empties
If 3 out-of-order packets are received, TCP will halve its congestion window
Possible solution: allow old queue to drain before queuing in new interface. However
Could introduce significant delay

20. Interference experiments
How many orthogonal channels are there really?
3 configurations
Netgear WAB501 cards in a and b modes
Cisco Aironet 340 in b mode
TCP
6” separation for Netgear NICs
3” vertical separation for Cisco NICs A B C D
TCP
Tuned each hop for each experiment

21. Interference experiment results

22. Protocol evaluation Through implementation and NS-2 simulations
NDIS driver under Windows XP
Channel switching and impact of queue experiments
CBR (50 Kbps)
TCP

23. Channel switching results
A switches to ch1
A switches to ch11
start C to D transfer over ch11
end transfer
start new D transfer over ch1

24. Impact of queuing A sends TCP traffic to B over channel 1
C sends TCP traffic to D over channel 11
IEEE e is needed to accurately measure channel quality.

25. Simulation setup Modified NS-2 to send CS and CS-ACK at high priority
12 wireless nodes, all in communication range
Traffic pattern
2 of 12 send probe packets and measure SRTT
Other 10 (5 pairs) send CBR traffic at 200 Kbps
Repeated using Web-like traffic

26. Computed SRTT results

27. Benefits of intelligent channel selection
16-node grid using AODV routing protocol
Traffic patterns
S-node sends FTP traffic to D-node over multiple hops
4 UDP flows with source/ destination pairs chosen randomly
On/Off time set randomly
Traffic intensity is ratio of mean On time to mean Off time

28. Intelligent channel selection results
Tunable parameters

29. Traffic striping
Packets are transmitted concurrently across multiple interfaces (instead of single interface)
Potentially increases throughput
Simple striping
If a source node can communicate with destination node over multiple interfaces, packets are transmitted in round-robin fashion
Load-aware striping
Same as above but only stripes if measured SRTT is within 10%

30. Striping channel selection results
Percentage Improvement over single channel

31. Web traffic in suburban topology
Simulated topology
Seattle suburban area
35 houses selected at random for Internet access
4 houses surfing the web
Web server located at access point
3 Scenarios
All legacy, half MUP, all MUP

32. Web traffic results

33. Discussion Other metrics for channel quality? Unicast or broadcast?
MUP or striping?

34. Improving Loss Resilience with Multi-Radio Diversity in Wireless Networks
Allen Miu, Hari Balakrishnan
MIT Computer Science and Artificial Intelligence Laboratory
C. Emre Koksal
Computer and Communication Sciences, EPFL

35. Motivation Wireless channels are loss-prone
Current solutions to cope with loss
Automatic repeat request (ARQ)
FEC
Rate adaptation
MIMO
Pros & Cons?

36. Today’s wireless LAN (e.g., 802.11)
May use only one path
Uses only one communication path
AP1
Internet

37. Today’s wireless LAN (e.g., 802.11)
Multi-Radio Diversity (MRD) – Uplink
May use only one path
Allow multiple APs to simultaneously receive transmissions from a single transmitter
10%
MRDC
AP1
20%
Internet
AP2
Loss independence 
simultaneous loss = 2%

38. Multi-Radio Diversity (MRD) – Downlink
Allow multiple client radios to simultaneously receive transmissions from a single transmitter
MRDC
AP1
Internet
AP2

39. Are losses independent among receivers?
Broadcast experiment at fixed bit-rate: 6 simultaneous receivers and 1 transmitter
Compute loss rates for the 15 receiver-pair (R1, R2) combinations
Frame loss rate FLR(R1), FLR(R2) vs. simultaneous frame loss rate FLR(R1 ∩ R2)

40. Individual FLR > Simultaneous FLR
y = x
FLR R1 R2
R1*R2
FLR(R1 ∩ R2)

41. Challenges in developing MRD
How to correct simultaneous frame errors?
Frame combining
How to handle retransmissions in MRD?
Request-for-acknowledgment protocol
How to adapt bit rates in MRD?
MRD-aware rate adaptation

42. Bit-by-bit frame combining
Combine failure
TX:
2. Select bit combination at unmatched bit locations, check
CRC
Patterns
CRC Ok 1 R1 -- R2 X
1. Locate bits with
unmatched value X O
Corrected frame
Problem: Exponential # of CRC checks in # of unmatched bits.

43. Block-based frame combining
Observation: bit errors occur in bursts
Divide frame into NB blocks (e.g., NB = 6)
Attempt recombination with all possible block patterns until CRC passes
# of checks upper bounded by 2NB
Explore bursty bit-error
Failure rate increases with NB when uniform error

44. Failure decreases with NB and burst size
1.0
Frame size = 1500B
Probability of failure
0.8
0.6
NB = 2
0.4
NB = 4
0.2
NB = 6 …
NB = 16 10 20 30 40 50
Burst error length parameter

45. How to Perform Error-Control?

46. Option 1: Directly use 802.11 retransmission scheme
Conventional link-layer ACKs do not work
Final status known only to MRDC

47. Option 2: Disable 802.11 retransmission scheme
Problems
Sending our own ACK is more expensive than sending ACK (Why?)
Hard to do rate control

48. Retransmission in MRD Two levels of ACKs
Use ACK for per-packet acknowledgement
ACK can be directly used for CSMA
No need to content for the medium
Send MRD-ACK via ACK compression
Sender retransmits when MRD-ACK not received upon timeout

49. Request-for-acknowledgment (RFA) for efficient feedback
DATA IP IP
RFA
MRD
MRD
MRD-ACK
DATA
DATA
link
link
link
ACK
MRDC

51. MRD-aware rate adaptation
Standard rate adaptation does not work
Reacts only to link-layer losses from 1 receiver
Uses sub-optimal bit-rates
MRD-aware rate adaptation
Reacts to losses at the MRD-layer
Implication: First use multiple paths,
then adapt bit rates.

52. Experimental setup ~20 m R2 R1 L
802.11a/b/g implementation in Linux (MADWiFi)
L transmits 100,000 1,472B UDP packets w/ 7 retries
L is in motion at walking speed, > 1 minute per trial
Variants: R1, R2, MRD (5 trials each)

53. MRD improves throughput
18.7 Mbps
2.3x Improvement
Throughput (Mbps)
8.25 Mbps R1 R2
MRD
Each color shows a different trial

54. MRD maintains high bit-rate
Fraction of transmitted frames
Frame recovery data
(% of total losses at R1)
via R %
frame combining 7.3%
Total % 6 9 12 18 24 36 48 56
Selected bit rate (Mbps)

55. Delay Analysis 10-4 10-3 10-2 10-1 1 Fraction of delivered packets
User space implementation caused high delay
10-4
10-3
10-2
10-1 1
One way delay (10x s)