Ch 2. Multiradio Multichannel Mesh
Networks
Myungchul Kim
[email protected]
Introduction
- Hot spot using IEEE 802.11
– Cellular networks: the key to one-hop capacity scaling is based
on frequency reuse spatially.
– Three orthogonal channels in IEEE 802.11b and 12 in 802.11a
– For any given system bandwidth, optimizing network
performance necessarily requires improving the entire protocol
stack.
• Multihop network
• Effective topology modification mechanisms including
power control, node clustering, and channel assignments
(CA)
• Multiple radio nodes
• Multiradio multichannel (MRMC)
• Intelligent CA and routing in MRMC
Introduction
• 802.11 mesh architecture
Introduction
• 802.11 mesh architecture
Introduction
• Capacity scaling
– Single-radio multihop wireless networks, e.g. packet radio networks
in 1970s
– Throughput with the number of hops?
• Half-duplex and interference
• The number of simultaneous transmissions per channel
• Single-radio, single-channel mesh networks: A baseline
– The per-node share of the aggregate throughput of a singlechannel multihop IEEE 802.11 = 1/ nα where n is the number of
nodes and the α is influenced by topology and traffic
characteristics.
– α = 0.5 for MANET by analytical way
– α = 1.68 for MANET by experimentation
– α = 1 for n node linear chain
Introduction
• Single-radio, multichannel mesh networks
– Channel scanning, selection and switching a radio
– Utilize all the available orthogonal channels in a manner
that maximizes spatial reuse -> simultaneous
transmission
– The switching delay for 802.11 ranges from a few
milliseconds to a few hundred microseconds.
• Multiradio mesh
– Effectively full duplex
Introduction
– One radio at node 2 and the two flows 1->2->3 and 4->2->5: if
node 1 and 4 are R bps and are scheduled at different time, node 3
and 5 are R/2, respectively
– One radio at node 2: if the two flows are simultaneous, R/4
– Two radios at node 2 with orthogonal channels: R/2
Introduction
• Example
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CA algorithm is a simple heuristic
Two radio-nodes
Even number of channels for horizontal and vertical links
360 flows
Shortest path routing
Packet loss rate 10%: full MAC layer buffer and
retransmission limit of 7
– As the number of channels C increases, T max (max traffic)
initially grows linearly with C, but the rate of growth slows as
C increases beyond 6.
– Increasing the number of channels to eight will not yield
additional throughput increase.
Introduction
• Example
Radio usage policies
– Static binding: each interface is assigned to a channel when the
system is intialized. e.g., 802.11
– MRMC
• Striping: use the multiple interfaces on a packet-by-packet basis > packet reorder
• Multiradio unification protocol (MUP)
• Hybird: one interface for a fixed channel and the other interface
is dynamically switched.
• Disadv: a node communicating with several neighbors
simultaneously and no support of a per-packet basis in 802.11
Radio usage policies
• Hybird: a common control channel
• Disadv: the control channel becomes a scarce source
Channel assignment and routing
– CA and routing: link and network layers, wireless
network
– Network load?
Channel assignment and routing
• Interfering edges (on same channel)
Channel assignment and routing
• Formulations and algorithms
– Integer linear programming
• Maximize the number of simultaneous transmissions
• Find the largest possible independent set in the
conflict graph
– Graph-theoretic approaches
• Coloring problem
• NP-hard
Channel assignment and routing
Channel assignment and routing
– Limitations
• A transmission fails if there exists a simultaneous
cochannel transmitter within a certain range of the
receiver.
• Does not consider the effect of aggregate interference
from several simultaneous transmitters.
– Routing metrics
• Hop count
• ETX
• ETT
• WCETT: channel reuse along a path
Channel assignment and routing
• WCETT: channel reuse along a path
• Reducing the first term reduces delay while reducing
the second term increases throughput.
• No interflow interference
• Metric of interface and channel switching (MIC)
– Joint approaches
• Iterative approach
• Joint CA and routing
Open issues
– Radio usage policies: switching cost
– CA algorithms
• Centralized off-line algorithms
• 802.11: not much information to the higher layers
with which to make CA decisions, e.g., RSSI
– Routing metrics
• Measuring the link conditions
• Estimating transmission time using probing is
unreliable.