Multi-Channel Protocols for Wireless Mesh Networks

1
Multi-Channel Protocols for Wireless Mesh Networks

• Yu-Chee Tseng
• CS/NCTU

2
Outline

• Introduction to MANET
• Review of 3 Multi-channel Protocols
• Summary

3
Wireless Mesh Network
4
Observation Multi-channel MANET

• IEEE 802.11 provides several non-overlapping
  channels which could be used simultaneously
  within a neighborhood.

Ex (Assume Channel Capacity 100Mbps)
Interference
A
C
44 /50 Ch1
B
43/ 50 Ch1
Link Capacity 100/2
Goodput 87Mbps Single-Channel
Expected Load
5
Motivation

• The idea of exploiting multiple channels is
  appealing in wireless mesh networks because of
  their high capacity requirements to support
  backbone traffic.
• However, the channel assignment problem is
  NP-hard.

6
A Multi-channel Example
7
Problem Spectrum

• number of interfaces per node
• single interface
• fixed at a particular channel (traditional
  solution)
• may switch among different channels
• multiple interfaces
• each fixed at a particular channel
• may switch among different channels
• channel assignment algorithm
• Centralized
• assignment is done in longer period
• Distributed
• assignment can be done is shorter period
• more flexible and dynamic, depending on current
  loads

8
Review 1A Centralized Greedy Solution

• Ashish Raniwala, Kartik Gopalan, and Tzi-cker
  Chiueh, Centralized Channel Assignment and
  Routing Algorithms for Multi-Channel Wireless
  Mesh Networks, Mobile Computing and
  Communications Review, vol. 8, no. 2, pp. 5065,
  April 2004.

9
Problem Statement

• Input expected load on each link
• Output assignment of channels to network
  interfaces
• Goal to reduce interference between neighboring
  interfaces

10
An Example
Internet

• Number of channels 4 (1,2,3,4)
• Number of interface per node 2
• Per Channel Capacity 100 units
• Definition degree of interference
• The sum of expected load that a link may
  experience on a particular channel in its
  interference region.

B
30
G
A
50
C
40
15
D
20
E
25
expected load on this link
F
11
Example of Degree of Interference
Degree of Interference for link E-F on different
channels
Internet
Ch1 502575
Ch2 25201560
Ch3 25
Ch4 25
B
30 Ch3
G
A
50 Ch1
C
40 Ch1
15 Ch2
D
link E-F will only experience interferences from
links D-E, C-D, and D-G, but not from links
B-C, A-C
20 Ch2
E
25 Ch1
F
coverage of node E
coverage of node F
12
Outline of the Algorithm

• Links are sorted, and then visited in the
  decreasing order of their link loads.
• A greedy approach
• When a link is visited, it is assigned to a
  channel with the lowest degree of interference.
• Special cases
• If the interfaces of the incident nodes are all
  used out, we may need to change one interface to
  a used channel.
• If the interfaces of the incident nodes are all
  used out but they have a common channel, then
  assign the link to the common channel.

13
A Running Example
Internet

• Number of channels 4 (1,2,3,4)
• Number of interface per node 2
• Per Channel Capacity 100 units
• First
• sort links according to their link loads (in a
  decreasing order)

B
30
G
A
50
C
40
15
D
20
E
25
F
14
Connect (D,G)
Internet
Channel List
Degree of Interference
A
B
C
D 1
E
F
G 1
Ch1 0?50
Ch2 0
Ch3 0
Ch4 0
B
G
A
50 Ch1
C
D
E
F
15
Connect (A,C)
Internet
Channel List
Degree of Interference
A 2
B
C 2
D 1
E
F
G 1
Ch1 50
Ch2 0?40
Ch3 0
Ch4 0
B
G
A
50 Ch1
C
40 Ch2
D
E
F
16
Connect (B,C)
Internet
Channel List
Degree of Interference
A 2
B 3
C 2,3
D 1
E
F
G 1
Ch1 50
Ch2 40
Ch3 0?30
Ch4 0
B
30 Ch3
G
A
50 Ch1
C
40 Ch2
D
E
F
17
Connect (E,F)
Internet
Channel List
Degree of Interference
A 2
B 3
C 2,3
D 1
E 3
F 3
G 1
Ch1 50
Ch2 0
Ch3 0?25
Ch4 0
B
30 Ch3
G
A
50 Ch1
C
40 Ch2
D
E
25 Ch3
F
18
Connect (D,E)
Internet
Channel List
Degree of Interference
A 2
B 3
C 2,3
D 1,4
E 3,4
F 3
G 1
Ch1 50
Ch2 40
Ch3 3025
Ch4 0?20
B
30 Ch3
G
A
50 Ch1
C
40 Ch2
D
20 Ch4
E
25 Ch3
F
19
Connect (C,D)
Channel List
Degree of Interference
Internet
A 2
B 3
C 2,3
D 1,4
E 3,4
F 3
G 1
Ch1 50
Ch2 40
Ch3 55
Ch4 20
B
30 Ch3
G
A
50 Ch1
C
40 Ch2
15 ??
D
Switch from ch. to ch.
20 Ch4
E
interference of new ch.
25 Ch3
F
1?2 1?3 2?1 2?4 3?1 3?4 4?2 4?3
90 105 90 60 80 50 60 75
Explanation next page
20
Connect (C,D)
Internet
Channel List
Degree of Interference
A 2
B 4
C 2,4
D 1,4
E 3,4
F 3
G 1
Ch1 50
Ch2 40
Ch3 55?25
Ch4 20?50
B
30 Ch3?Ch4
G
A
50 Ch1
C
40 Ch2
15 ??
D
switch ch 3 to ch 4 (?????15)
20 Ch4
E
25 Ch3
F
1?2 1?3 2?1 2?4 3?1 3?4 4?2 4?3
90 105 90 60 80 50 60 75
21
Connect (C,D)
Internet
Channel List
Degree of Interference
A 2
B 4
C 2,4
D 1,4
E 3,4
F 3
G 1
Ch1 50
Ch2 40
Ch3 25
Ch4 50?65
B
30 Ch4
G
A
50 Ch1
C
40 Ch2
15 Ch4
D
20 Ch4
E
25 Ch3
1?2 1?3 2?1 2?4 3?1 3?4 4?2 4?3
90 105 90 60 80 50 60 75
F
select the one with min. int.
22
Final Result
Channel List
A 2
B 4
C 2,4
D 1,4
E 3,4
F 3
G 1
23
A Short Summary

• Adv. quite simple
• Disadv.
• need initial expected load on every link
• centralized algorithm (must know network
  topology)
• static network topology
• static traffic load

24
Review 2 (SSCH)A distributed, single-interface
solution

• Paramvir Bahl, Ranveer Chandra, and John Dunagan,
  SSCH Slotted Seeded Channel Hopping for
  Capacity Improvement in IEEE 802.11 Ad-Hoc
  Wireless Networks, in ACM Mobicom, 2004.

25
Protocol Outline SSCH

• Single interface hopping on multiple channels.
• time is slotted
• SSCH (Slotted Seeded Channel Hopping)
• Each node has its own channel hopping schedule.
• Each node transmits its schedule to neighboring
  nodes in the beginning of each slot.
• To transmit data, a node has to change its
  hopping schedule to adapt to receivers hopping
  patterns.
• The seeded hopping ensures through number theory
  that every pair of nodes have a common channel to
  exchange their schedulers.

26
??1 Channel Hopping Scheduling

• Time is slotted.
• Continuous slots are framed together.
• The i-th slots of all frame form the i-th virtual
  channel.
• Each virtual channel is represented by a
  (channel, seed) pair, denoted by (xi , ai ).
• xi current channel
• aihopping distance
• hopping rule xi ? (xi ai) mod 3
• Also, there is a special slot called parity
  slot.
• a common channel must be used.

27
Channel Schedule Example
1
0
2
(channel,seed) 1 (channel,seed) 2 (mod 3)
(channel,seed) 1 (channel,seed) 2 (mod 3)
28
Mathematical Properties

• For two nodes with an identical seed
• if they have identical channel
• these two nodes are always synchronized.
• if they have different channels
• they only overlap in the parity slots
• For two nodes with different seeds
• they overlap exactly once every 3 slots
• prime number theory
• ? These overlapping slots ensure that stations
  can exchange schedulers.

29
??2 Optimistic Synchronization

• If node A has packets to be sent to node B, A
  will select a virtual channel and match it with
  Bs corresponding virtual channel.
• receiver-based rule

(channel-A,seed-A) ? (channel-B,seed-B)
(channel-B, seed-B)
B
A
30
??3 Partial Synchronization

• For a multi-hop path, partial synchronization is
  used.
• Node B follows node Cs hopping schedule in some
  virtual channels, and leave some virtual channels
  to be synchronized by node A.
• goal better spatial reuse

31
A Naive Synchronization
32
Solution (??A?B,??B?C)
Receiving ch. If a slot always receives data, it
is marked as receiving slot.
Case 1 B preserves its receiving ch. and uses
its idle virtual ch. to sync. with C
4 (channel,seed) (x1,a1) (x2,a2) (x3,a3) (x4,a4)
Case 2 all slots are receiving, partial sync.
33
??4 De-synchronization

• To reduce interference, if too many nodes use the
  same (channel, seed) is the same virtual channel,
  de-synchronize some of them.
• simply choose a new (channel, seed) at a nodes
  own decision

may choose to de-sync.
Example 3 pairs use the same (channel, seed) in
the 2nd virtual channel
34
Short Summary

• Adv.
• an interesting partial synchronization technique
• an interesting de-synchronization technique
• Disadv.
• need global time synchronization
• only designed for one interface

35
Review 3 (MCR)A multi-interface channel
assignment protocol

• Pradeep Kyasanur and Nitin H. Vaidya,  "Routing
  and Interface Assignment in Multi-Channel
  Multi-Interface Wireless Networks", WCNC 2005.

36
Main Idea

• Each node has multiple interfaces.
• Fixed Interface assigned to some fixed channel
  for long intervals of time
• Switchable Interface dynamically assigned to
  channels over short time scales
• Transmission Rules
• receiver-based
• a sender adapts to a receiver by changing its
  switchable interface to the receivers fixed
  interface

37
Example

• 2 interfaces per node
• 1 fixed, 1 switchable
• 3 channels are available.
• Routing Path A?B?C

38
Fixed Interface assignment

• Goal
• to ensure that fixed interfaces of nodes in a
  neighborhood have better spatial reuse.
• A localized protocol, where each node maintains
  two tables
• NeighborTable containing the fixed channels
  being used by its neighbors
• ChannelUsageList (CUL) keeping the number of
  nodes using each channel by their fixed channels

39
Distributed Algorithm

• 1. Initially, each node chooses a random channel
  as its fixed interface.

40
Distributed Algorithm

• 2. Periodically, each node broadcasts on every
  channel its current fixed channel.

Hello
Hello
Hello
41
Distributed Algorithm

• 3. On receiving a hello packet, a node updates
  its NeighborTable and ChannelUsageList.

42
Distributed Algorithm

• 4. Each node periodically consults its CUL
  (relatively long period). If its fixed channel is
  detected to be too crowded, it has a probability
  p to change its fixed channel to a less crowded
  channel.

43
Distributed Algorithm

• 4. Each node periodically consults its CUL
  (relatively long period). If its fixed channel is
  detected to be too crowded, it has a probability
  p to change its fixed channel to a less crowded
  channel.

Change
44
Distributed Algorithm

• 4. Each node periodically consults its CUL
  (relatively long period). If its fixed channel is
  detected to be too crowded, it has a probability
  p to change its fixed channel to a less crowded
  channel.

Hello
Hello
Hello
45
Distributed Algorithm

• 4. Each node periodically consults its CUL
  (relatively long period). If its fixed channel is
  detected to be too crowded, it has a probability
  p to change its fixed channel to a less crowded
  channel.

46
Distributed Algorithm

• 4. Each node periodically consults its CUL
  (relatively long period). If its fixed channel is
  detected to be too crowded, it has a probability
  p to change its fixed channel to a less crowded
  channel.

Nothing
47
Distributed Algorithm

• 4. Each node periodically consults its CUL
  (relatively long period). If its fixed channel is
  detected to be too crowded, it has a probability
  p to change its fixed channel to a less crowded
  channel.

Change
48
Distributed Algorithm

• 4. Each node periodically consults its CUL
  (relatively long period). If its fixed channel is
  detected to be too crowded, it has a probability
  p to change its fixed channel to a less crowded
  channel.

Hello
Hello
49
Distributed Algorithm

• 4. Each node periodically consults its CUL
  (relatively long period). If its fixed channel is
  detected to be too crowded, it has a probability
  p to change its fixed channel to a less crowded
  channel.

50
Distributed Algorithm

• 4. Each node periodically consults its CUL
  (relatively long period). If its fixed channel is
  detected to be too crowded, it has a probability
  p to change its fixed channel to a less crowded
  channel.

Change
51
Distributed Algorithm

• 4. Each node periodically consults its CUL
  (relatively long period). If its fixed channel is
  detected to be too crowded, it has a probability
  p to change its fixed channel to a less crowded
  channel.

Hello
Hello
52
Distributed Algorithm

• 4. Each node periodically consults its CUL
  (relatively long period). If its fixed channel is
  detected to be too crowded, it has a probability
  p to change its fixed channel to a less crowded
  channel.

53
Distributed Algorithm
Threshold to determinate whenever a channel is
too crowded
54
Scheduling Rules for Interfaces

• Each node maintains a separate queue for each
  channel.
• When a packet arrives,
• if the sender has the same fixed channel as the
  receiver, enqueue to the fixed channel.
• otherwise, enqueue to the switchable channel.
• The use of each switchable channel is bounded by
  two parameters BurstLengh and MaxSwitchTime.
• For broadcast, add a packet to each queue.

Fixed1
Fixed3
FixedN
55
Summary

• Adv.
• a simple rule to use multiple interfaces and
  multiple channels
• Disadv.
• If a node always receives but doesnt send, the
  fixed interface may be overloaded while the
  switchable interface is always idle.

56
Review 4 A multi-interface routing protocol

• Richard Draves, Jitendra Padhye, and Brian Zill,
  Routing in Multi- Radio, Multi-Hop Wireless Mesh
  Networks, in ACM Mobicom, 2004.

57
Route Selection Metric
Which is the best routing path?
10ms
less delay, but using one channel?
A
B
10ms
10ms
D
S
40ms
30ms
C
longer delay, but shortest?
10ms
10ms
10ms
E
F
Ch2
using multiple channel?
58
How to Select a Good Path

• A good routing protocol should take the loss
  rate, the bandwidth of a link, and channel
  diversity into account.
• For a multi-channel MANET, the path metric should
  explicitly account for the interference among
  links that operate on the same channel.
• Multi-Radio Link-Quality Source Routing
• a combination of the LQSR protocol with a new
  metric that we call WCETT (Weighted Cumulative
  Expected Transmission Time).

59
Original WCETT

• Sum of expected time to successfully transmit a
  packet on the route

Example
20ms
15ms
5ms
10ms
5ms
Note?????
WCETT1015552055
60
WCETT by Channel Diversity

• Xj sum of transmission times of hops on channel
  j

Xj S ETTi 1?j?k
Hop i is on channel j
X1 105 15 X2 152010 45 X3 5 WCETT
max(X1,X2,X3) max(15,45,5) 45
Example
20ms Ch2
15ms Ch2
5ms Ch3
10ms Ch2
10ms Ch1
5ms Ch1
61
Proposed Combined WCETT
Example
Ch1 Ch2
ETT10
ETT5
ETT12
1 2 3 4
S
D
ETT10
ETT5
ETT12
ETT6
S
D
ETT9
ETT7
ETT11
ETT7
S
D
ETT2
ETT2
ETT2
ETT2
S
D
Path Sum Max WCETT (ß0.9) WCETT (ß0.1)
1 27 22 22.5 26.5
2 33 22 23.1 31.9
3 34 20 21.4 32.6
4 8 8 8 8
62
Conclusions

• Layer 2 Channel Assignment
• a centralized greedy Algorithm
• a distributed single-interface protocol
• hopping seed
• receiver-based channel switch
• a distributed multi-interface protocol
• fixed and switchable interfaces
• using switchable to adapt to fixed channel
• Layer 3 Multi-Channel Routing
• a new metric WCETT with channel diversity

63
References

1. Ashish Raniwala, Kartik Gopalan, and Tzi-cker
   Chiueh, Centralized Channel Assignment and
   Routing Algorithms for Multi-Channel Wireless
   Mesh Networks, Mobile Computing and
   Communications Review, vol. 8, no. 2, pp. 5065,
   April 2004.
2. Paramvir Bahl, Ranveer Chandra, and John Dunagan,
   SSCH Slotted Seeded Channel Hopping for
   Capacity Improvement in IEEE 802.11 Ad-Hoc
   Wireless Networks, in ACM Mobicom, 2004.
3. Pradeep Kyasanur and Nitin H. Vaidya,  "Routing
   and Interface Assignment in Multi-Channel
   Multi-Interface Wireless Networks",  Technical
   Report, October 2004 (A version will appear in
   WCNC 2005)
4. Richard Draves, Jitendra Padhye, and Brian Zill,
   Routing in Multi- Radio, Multi-Hop Wireless Mesh
   Networks, in ACM Mobicom, 2004.
5. Shih-Lin Wu, Chih-Yu Lin, Yu-Chee Tseng, and
   Jang-Ping Sheu, A New Multi-Channel MAC Protocol
   with On-Demand Channel Assignment for Multi-Hop
   Mobile Ad Hoc Networks, in International
   Symposium on Parallel Architectures, Algorithms
   and Networks (ISPAN), 2000.