Mesh Capacity and Multichannel Meshes

1
Mesh Capacity and Multi-channel Meshes

• Y. Richard Yang
• 2/10/2009

2
Admin.

• I am out of town on Thursday and Firday
• Thursday class by Richard Alimi on superposition
  coding
• I will make up missed office hours on Thursday
  and Friday later in the semester

3
Recap Mesh Networks
infrastructure mode
AP
wired network
AP Access Point
AP
AP
ad-hoc (mesh) mode
4
Recap Two Constraints
Interference constraint
Radio interface constraint

• transmission successful if there are no other
  transmitters within a distance (1D)r of the
  receiver

• a single half-duplex transceiver at each node

5
Capacity Bound
Note
Let L be the average (direct-line) distance for
all ?T end-to-end bits.
6
Recap Random Networks

• Uniform distribution of n nodes
• n origin-destination (OD) pairs
• Each node chooses same power level P, and thus
  equal radius r(n)
• Equal throughput ?(n) bits/sec for all OD pairs

7
Recap Random Networks Supply/Demand

• Required bit transmissions per second
• What is the maximum number of transmissions (of
  bits) in one second?
• space used per transmission (interference
  limited)
• at least ¼ p Dr(n)/22 pD2r2(n)/16
• number of simultaneous transmissions at most
  (interference limited)
• total bits per second

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Random Networks Capacity

• Required offered
• ?
• ?

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Connectivity Constraint

• Need routes between origin-destination pairs -
  places a lower bound on transmit range r(n)

A
A
D
D
Connected
Not connected
To maintain connectivity with a high probability,
requires r(n) on the order
10
Random Networks Capacity

• Required offered
• ?
• ?
• ?

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Measurement

• Measured scaling law throughput declines worse
  with n than theoretically predicted 1/n1.68
• Remaining story line
• mesh networks with wide-area traffic may have low
  scalability, and need techniques to increase
  capacity

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Mesh Network Capacity Arbitrary
Interference constraint
Radio interface constraint

• transmission successful if there are no other
  transmitters within a distance (1D)r of the
  receiver

• a single half-duplex transceiver at each node

ratedistance capacity
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Improving Capacity Approximate Ideal Model

• Transmission power control
• MAC scheduling
• Routing

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Approximate Ideal ControlPower/Carrier-Sense
Control
Transmit Spatial Power Rate
reuse High High Low Low
Low High
A
B
C
D
A
B
C
D
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Improving Capacity Change Traffic Pattern

• To make communications local
• node placement change the demand patterns (thus
  L)
• e.g. base stations/access points with high-speed
  backhaul
• use mobility

infrastructure
BS1
BS2
B
C
D
T
E
A
F
S
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Improving Capacity Reduce Radio Interface
Constraint

• Multiple radio interfaces/codes

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Improving Capacity Reduce Interference Constraint

• Antenna design steered/switched directional
  antennas
• Non-interfering channels

B
C
A
D
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Outline

• Admin
• Capacity
• Improving capacity using all available spectrum

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Using All Available Spectrum

• Goal Using right channels at right nodes at
  right time may optimize network throughput
• Problem domains
• Wireless LANs
• APs determine the channel
• Clients share the same channel as their
  associated Aps
• Mesh networks
• Nodes use multiple channels to increase spatial
  reuse

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Distributed Sharing of Unlicensed Spectrum

• Federal Communications Committee (FCC) is
  increasing unlicensed spectrum allocation
• Industry, Science, and Medicine (ISM)
• Unlicensed Personal Communication Service (UPCS)
  1910-1930 MHz and 2390-2400 MHz (30 Mhz)
• National Information Infrastructure (NII) Band
  350 Mhz
• 59-64 Ghz Millermeter Wave band
• Coordination in unlicensed spectrum is difficult

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One Proposal for UPCS Spectrum Etiquette

• Upper bound on energy level official
• A station must Listen Before Talk (LBT)
  official
• be quite for a monitoring time M after the
  previous energy level stops
• Penalty for using a channel non-official
• if a station holds a channel for a duration H,
  the station cannot transmit for P(H) amount of
  time as a penalty

Question what property do you want P(H) to hold?
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How to Design P(H)?

• Assume an arrived bit of a user is transmitted
  immediately if the user is having the channel
  otherwise the bit has to wait until the next
  interval
• What is the average delay

M
H
P(H)
H
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Discussion

• Spectrum sharing is largely still an open field
• There are several proposals and evaluations
• for example, seehttp//dx.doi.org/10.1023/A10191
  29906297
• A potential term project topic

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Outline

• Admin
• Capacity
• Improving capacity using all available spectrum
• Improving capacity using multiple channels in
  802.11 mesh

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Multi-Channel 802.11 Mesh

• Wireless LANs
• APs determine the channel
• Clients share the same channel as their
  associated APs
• 802.11 mesh networks
• Each node can choose operating 802.11 channels to
  increase spatial reuse

1
2
1
2
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Operating Channels for 802.11b
Europe (ETSI)
channel 1
channel 7
channel 13
2400
2412
2483.5
2442
2472
MHz
22 MHz
US (FCC)/Canada (IC)
channel 1
channel 6
channel 11
2400
2412
2483.5
2437
2462
MHz
22 MHz
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Operating channels for 802.11a / US U-NII
channel
36
44
40
48
52
56
60
64
5150
5180
5200
5220
5240
5260
5280
5300
5320
5350
MHz
16.6 MHz
center frequency 5000 5channel number MHz
149
153
157
161
channel
5725
5745
5765
5785
5805
5825
MHz
16.6 MHz
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Multi-Channel 802.11 Mesh Goal

• Goal Using right channels at right nodes at
  right time to improve network throughput

1
2
1
2
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Multi-Channel Network Capacity
Network Capacity
c number of channels
Channels
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Setup

• A radio can use multiple orthogonal channels
  (e.g., 12 channels in 802.11a)

3 channels
8 channels
4 channels
26 MHz
100 MHz
200 MHz
100 MHz
2.45 GHz
915 MHz
5.25 GHz
5.8 GHz
Key question how to assign channelsto
interfaces?
1
c
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Interface Assignment Strategies

• Static assignment
• an interface is assigned a fixedchannel
• Dynamic assignment
• interface assignment changes with time
• Hybrid interface assignment
• some interfaces use static assignment, others use
  dynamic assignment
• To focus on the key issue assume a single
  interface

1
1
c
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Key Challenge

• Connectivity vs using multiple channels

Multiple channels not used
Network is disconnected
Additional constraints intermediate relay nodes
need to share the same channel as the upstream
and downstream node
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SSCH Slotted Seeded Channel Hopping Overview

• A dynamic assignment algorithm
• divides the time into equal sized slots (e.g. 10
  ms) and switches each radio across multiple
  orthogonal channels on the boundary of slots in a
  distributed manner
• Main aspect of SSCH
• channel scheduling
• self-computation of tentative schedule
• communication of schedules
• synchronization with other nodes

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SSCH Desired Properties

• No Logical Partition Ensure all nodes come into
  contact occasionally so that they can communicate
  their tentative schedule
• Synchronization Allow nodes that need to
  communicate to synchronize
• De-synchronization Infrequently overlap between
  nodes with no communication

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Channel Scheduling -Self-Computation

• Each node uses (channel, seed) pairs to represent
  its tentative schedule for the next slot
• Seed 1 , number of channels -1 Initialized
  randomly
• Focus on the simple case of using one pair
• Update rule
• new channel (old channel seed)
  mod (number of channels)

1
0
2
1
0
2
1
0
A Seed 2
0
1
2
0
1
2
0
1
B Seed 1
Example 3 channels, 2 seeds
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Channel Scheduling Logical Partition

• Are nodes guaranteed to overlap?
• same init channel, same seed (always overlap)
• same init channel, different seeds (overlap
  occasionally)
• different init channels, different seeds (overlap
  occasionally)
• special case Nodes may never overlap if they
  have the same seed but different channels

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Channel Scheduling Solution to Logical Partition

• Parity slot
• every (number of channels) slots, add a parity
  slot
• in parity slot, the channel number is the seed

Parity Slot
Parity Slot
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Channel Scheduling -Communication of Schedules

• Each node broadcasts its tentative schedule
  (represented by the pair) once per slot

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Channel Scheduling - Synchronization

• If node B needs to send data to node A, it
  adjusts its (channel, seed) pair to be the same
  as A.

Seed
A
Sync starts upon the parity slot
Flow starts
B
Seed
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Channel Scheduling Channel Congestion

• It is likely various nodes will converge to the
  same (channel, seed) pair and communicate
  infrequently after that.

(1,2)
(1,2)
(1,2)
(1,2)
(1,2)
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Channel Scheduling Solution to Channel
Congestion

• De-synchronization
• To identify channel congestion compare the
  number of the synchronized nodes and the number
  of the nodes sending data. De-synchronize when
  the ratio gt 2
• To de-synchronize, simply choose a new (channel,
  seed) pair for each synchronized and non-sending
  nodes

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Channel Scheduling Synchronizing with Multiple
Nodes

• Examples
• a sender with multiple receivers
• a forwarding node in a multi-hop network
• Solution Use multiple seeds per node
• use one seed to synchronize with one node
• add a parity slot every cycle ( number of
  channels number of seeds) the channel number
  of the parity slot is the first seed.

Green slots are generated by seed 1 Yellow
slots are generated by seed 2
2
2
1
0
1
1
0
2
2
1
0
0
1
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Channel Scheduling Partial Synchronization
Seed
A
Flow starts
B
Seed
Partial Sync Sync the second seed only
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Evaluations of SSCH

• Simulate in QualNet
• 802.11a, 54Mbps, (used) 13 orthogonal channels
• Slot switch time 80 µs
• 4 seeds per node, slot duration 10 ms
• UDP flows CBR flows of 512 bytes sent every 50
  µs (enough to saturate the channel)

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Evaluation Throughput (UDP)
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Evaluation Multi-hop Mobile Networks
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Backup Slides
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Preview Network Layer

• Transport packet from source to dest
• network layer protocol in every host, router
• Basic functions
• Control plane location management, path
  determination
• locates hosts determines route taken by packets
  from source to dest., and quality of service
  packets receive
• Data plane forwarding
• move packets from routers input to appropriate
  router output use the setup from the control
  plane

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Packet Scheduling Main Idea

• Send packets to receivers in the same channel and
  delay sending packets to receivers in other
  channels

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Packet Scheduling Basic Scheme

• Within a slot, a node transmits packets in a
  round robin fashion among all flows
• For a single flow, the packet is transmitted in
  FIFO order
• Failed transmission causes the relevant flow to
  be inactive for half a slot. An inactive flow
  does not participate in transmission unless there
  are no active flows.

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Packet Scheduling Absent Destination

• Problem The destinations are in other channel
• Solution Retransmission
• broadcast 6 transmission
• unicast Until successful or the cycle ends
• Question Can SSCH distinguish
• destinations in other channels?
• failure because of bad channel condition or node
  crash
• collision

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Types of Conflicts in WLAN
D lt R
APs within communication range of each other
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Types of Conflicts in WLAN
R lt D lt 2R
APs within interference range of each other
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Types of Conflicts in WLAN
2R lt D lt RBSS
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Types of Conflicts in WLAN
RBSS lt D lt RBSSR
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Channel Assignment Algorithm

• Objective maximize conflict free clients
• Client c is conflict-free if at least one AP in
  the range set is assigned a color j and no other
  AP in the range set is assigned color j
• Random compaction
• Initialize AP assignment
• X random-permute(X)
• While true do
• ncf num_conflict_free(T,T)
• for each AP i in X
• T(xi) compaction_step(T, T, xi, k)
• end
• if num_conflict_free(T, T) ncf then
• stop
• end
• end

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Channel Assignment Algorithm

• Objective maximize conflict free clients
• Client c is conflict-free if at least one AP in
  the range set is assigned a color j and no other
  AP in the range set is assigned color j
• Random compaction
• Initialize AP assignment
• X random-permute(X)
• While true do
• ncf num_conflict_free(T,T)
• for each AP i in X
• T(xi) compaction_step(T, T, xi, k)
• end
• if num_conflict_free(T, T) ncf then
• stop
• end
• end
• Any drawback?

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Protocol Separation Implementation