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Title: 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.
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