SENSE: Scalable and
Efficient Networking of
Sensor Elements
J.J. Garcia-Luna-Aceves
CCRG
Computer Engineering Department
University of California, Santa Cruz
Discussion Topics

Implications of fundamental limitations to the
scaling of ad hoc networks

Cross-layer optimization
Impact of the physical layer on communication
protocol stack.
 Importance of modular protocol stacks and
good understanding of their distributed
algorithms.

2
2
Scaling
Known Results on Network Capacity

Definition: A source-destination throughput of λ(n)
bits/sec is feasible if every source node can send
information at a rate of λ(n) bits/sec to its destination.

Gupta and Kumar (for static networks)

 n 

1
n  n 



 
 ( n)  

 0 and D(n)  
 

 n log( n) 
 log (n) 



Grossglauser and Tse (Multiuser diversity: One-copy
two phase packet relay to nearest neighbor strategy for
mobile networks)
 (n)  1 and D(n)  (n)
4
4
Preliminary Results

Multiuser diversity with multi-copy twophase packet relay to close neighbors strategy
for mobile networks where
 (n)  1 , D(n)  (n) and Var (n)  n
2

delay reduction  69%

 bounded delay
 flooding time speeds up


For fixed n

 ,   2
Interference analysis: SIR n
 cte
5
5
Preliminary Results:
Node Trajectories Are IID
Single-copy forward
Multi-copy forward
r0
r0
n total
users
r0
t'  t
Only one relay looking
for destination
n total
users
r0
First relay reaching destination
delivers the packet
(More than one relay looking for destination)
6
6
Outlook:
Need More than Min-Hop Routing
D
S
Conventional
close straight
line path
Path of least
interference subject
to constraints
How can we reduce interference subject to multiple constraints
(power consumption, e-t-e delays, bandwidth requirements)?
Exploit diversity (user, space, time, code, freq) and cross-layer
optimization!
7
7
Need for Cross-Layer Optimization
scheduling establishes links and decides which nodes
are awake; needs multicast group affiliations and routes
to destinations of flows
topology control
determines nodes &
links that can be
used for certain
functions; needs
links for collision-free
transmission of
control packets, and
dissemination of
neighborhood data
S
Scalable &
Efficient
Network Control
T
R
Signaling to support
functions should not
be redundant
routing needs links
for collision-free
transmission of
control packets;
packet forwarding
needs links for
collision-free
transmission of data
packets
Multicasting needs a
convenient topology
8
8
Importance of
analytical models
Why Do We Need Analytical Models?
Simulations:
Specific to each
scenario and setup
 Results for each
parameter value of
interest
 Statistical fitting not a
trivial task
 Many physical layer
features not readily
available
 Physical layer has to
be implemented
 How far can we go?

Analytical Models:
Aim to cover different
scenarios: general
behavior!
 Quick answers for
the impact of
different parameter
values on system
performance
 Upper/lower bounds
 Insights: help in the
design
 Physical layer issues
at least as accurate
as in simulations

10
10
Limits of Simulation Effort
Consider executing a simulation in a Sun blade
100 running Solaris 5.8
 50 seeds of a 100-node, 5-min data traffic
scenario required 16.41 hours for a given set of
PHY-level parameters.
 Analyzing the impact of different combinations of
PHY-level parameters will take a very long time,
and testbeds are hard to control.

11
11
Multihop Networks
CTS
RTS
Interference is network-wide!
12
12
Previous Work






Single-hop (mostly) or “weak-interactions” approach (to
avoid interference from distant nodes)
Scheduling rates are independent Poisson point
processes
Packet lengths exponentially distributed and
independently generated at each transmission attempt =
backoff retransmissions ignored!
Instantaneous acknowledgments
Error-free Links
Assumptions on spatial distributions (e.g., Poisson)
13
13
Modeling the Effect of the PHY:
Highlights [Mobicom 04]






Framework for any MAC protocol in ad hoc networks
Focus on PHY / MAC layer interactions
No assumptions on spatial probability distributions or
specific arrangement of nodes
Individual (per-node) performance metrics for any given
network topology (node location) and radio channel model
Linear model that provides remarkable correlation with
simulation results.
Key Benefit: Analytical results are obtained much faster
than in simulations (same example as before takes 0.44
sec in Matlab).
M. Carvalho and J.J. Garcia-Luna-Aceves, " A Scalable Model for Channel Access
Protocols in Multihop Ad Hoc Networks," Proc. ACM Mobicom 2004, Philadelphia,
Pennsylvania, Sept. 26--Oct. 1, 2004.
14
14
Modeling Rationale
Focus on the essentials of MAC and PHY layers:



MAC/PHY interactions depend on connectivity among the
nodes:


Network topology is key!
Model each layer’s functionality in probabilistic terms:




PHY: Ensure that frames are correctly received
MAC: Scheduling discipline to share the channel
PHY: successful frame reception probability
MAC: transmission probability
Model topology with an interference matrix
15
15
Application:
Modeling IEEE 802.11 [Mobicom 04]

Based on the works by


M. Carvalho and J. J. Garcia-Luna-Aceves,
“Delay Analysis of IEEE 802.11 in Single-Hop
Networks,” Proc. ICNP, Atlanta, 2003.
G. Bianchi, “Performance Analysis of the IEEE
802.11 Distributed Coordination Function,”
IEEE JSAC, 2000.
16
16
Application: Modeling IEEE 802.11
[Mobicom 04]

Per-node performance metric: throughput
Simulator used: Qualnet 3.5
17
17
Percentage of Prediction Error
[Mobicom 04]
Sample topologies
Histogram over 10 random topologies
(100 nodes)
18
18
Modular protocols and
distributed algorithms
Modular Protocol Stack
collaborative sensor
processing applications…
APPLICATION
TRANSPORT
NETWORK
end-to-end transport protocols…
routing-structure
maintenance
opportunistic
packet forwarding
node interconnection
LINK
synchronization
PHYSICAL
transmission
scheduling
prototype
radios
neighborhood
discovery
simulated
PHY
20
20
Routing Issues

Routing protocols are monolithic
One flavor of signaling for all destinations
 One flavor of routes (single path) for all traffic to
destinations.

Routing layer in MANETs assumes that routing
takes place over a given topology, just like
Internet routing protocols like OSPF and RIP do.
 The existence of radio connectivity does not imply
the availability of a logical link in a MANET.

21
21
Not All Nodes and Traffic Are Created
Equal!
Most communication is
multipoint and for particular
purposes
Image from
sensor
command center
22
22
Need for Cross-Layer Optimization
scheduling establishes links and decides which nodes
are awake; needs multicast group affiliations and routes
to destinations of flows
topology control
determines nodes &
links that can be
used for certain
functions; needs
links for collision-free
transmission of
control packets, and
dissemination of
neighborhood data
S
Scalable &
Efficient
Network Control
T
R
Signaling to support
functions should not
be redundant
routing needs links
for collision-free
transmission of
control packets;
packet forwarding
needs links for
collision-free
transmission of data
packets
Multicasting needs a
convenient topology
23
23
Routing Issues

Timers and sequence numbers can be a problem
when the networks become very large and
partitions can happen (disruption tolerance):
 How long should a node remember its “state” for a
destination?
 What are the implications of forgetting?

Similarly, path information becomes obsolete very
quickly in large dynamic/disrupted networks.


How should path information be used to ensure
correct routing?
Same mechanisms repeated in different protocols.
24
24
Outlook:
Develop Flow Adaptive Routing
Mechanisms (FARM)





Develop routing techniques that are “role”-centric (no
clusters) and adapt dynamically to the flows in the network.
How a routing table entry for a destination is obtained and
maintained is a function of the type of flow towards the
destination.
Proactive and on-demand mechanisms used according to
flow types.
Different flows are given resources (paths) according to their
types and priorities.
Routing works in coordination with scheduling and topology
management.
25
25
Outlook:
Integrated Routing and Multicasting
i
g
multicast group
C1
R
f
h
e
c
b
d
R
C2
a
special services,
sink of data
Each common node keeps paths to the cores of groups and well-known
nodes.
Paths to common nodes are found on demand.
Much of the traffic in sensor nets is to groups and common nodes!
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26
Thanks!