Title: uCast Unified Connectionless Multicast for Energy Efficient Content Distribution in Sensor Networks
1uCast Unified Connectionless Multicast for
Energy Efficient Content Distribution in Sensor
Networks
- Qing Tao, Tian He, Tarek Abdelzaher
- Presented By
- Andrew Connors
2Introduction
- This paper introduces uCast
- Connection-less protocol
- Does not keep state in any intermediate node
- Keeps list of destination addresses in message
headers - Forwarding decisions made at each node
- Uses underlying unicast protocols
- Defines simple interface to use distance
embedded in unicast protocol - And demonstrates performance improvements
3Sensor Challenges
- Extremely Energy Constrained
- Short battery life
- Use conservation protocols
- Limited Memory
- 4K bytes on Mica2/MicaZ
- Dynamic
- For this paper this means topology changes due to
nodes entering sleep states - Not changes due to sensor movement
4Unicast
- Used to send packets to single destination
5Unicast
- Multiple addressing schemes
- Identifier
- Geographical location
- Network Encoding
6Unicast
- Identifier
- No topology information
- Requires routing tables
- Uses flooding to establish routes
- Examples for ad-hoc networks include
- Dynamic Source Routing (DSR)
- Ad-Hoc, On Demand Distance Vector Routing (AODV)
7Unicast
- Geographical location
- Each node is location aware using GPS or
localization - Location approximate relative topology
- Do not need flooding as only local information is
used for routing - Examples include
- Greedy Perimeter Stateless Routing (GPSR)
- GEographical DIstance Routing (GEDIR)
- GEDIR FACE-2 GEDIR (GFG)
- Location Aided Routing (LAR)
8Unicast
- Network Encoding
- Topology information encoded in identifier
- Identifiers directly used for routing
- No flooding needed
- Examples include
- Virtual location-based geographical routing
- Logical coordinate-based routing (LCR)
- Graph embedded-based routing (GEM)
9Multicast
- Used to send packets to multiple destinations
10Multicast
- Three types of multicast
- Sensor Networks
- Ad-Hoc Networks
- Internet
11Multicast
- Sensor Network Multicast Protocols
- Geocast
- Destinations located within geographical region
- Mobicast
- Spatiotemporal multicast where destinations are
in a moving zone and the goal is to deliver
packets just in time to zone for tracking
purposes - Data Caching Placement
- Uses multicast for asynchronous data updates
- Two-Tier Data Dissemination (TTDD)
- Optimized for mobile sinks and uses a grid
structure combined with localized flooding to
track sinks (users that collects these data
reports from the sensor network)
12Multicast
- Ad-Hoc Network Multicast Protocols
- Tree-Based
- For example Ad-hoc On-Demand Distance Vector
Routing (AODV), that builds multicast trees
on-demand to connect members - Mesh-Based
- For example, core-assisted mesh protocol (CAMP)
forms multicast meshes (higher connectivity
graphs than trees) for each multicast group - Group-Based
- For example, On-Demand Multicast Routing Protocol
(ODMRP) also mesh based but also uses forwarding
groups
13Multicast
- Internet Multicast Protocols
- Internet Group Management Protocol (IGMP)
- Used to maintain groups of multicast members by
IP and routes through existing routers to
optimize delivery through network - Distance Vector Multicast Routing Protocol
(DVMRP) - Used to share information between routers to
transport multicast packets, and each router
generates a router table for multicast group - Explicit Multi-Unicast (Xcast)
- Does not use multicast addresses but places IP
addresses of destinations into headers, but still
relies on routing tables and a single unicast
protocol
14Related Work
- Existing protocols for sensor, ad-hoc or IP
networks are not suitable for dynamic sensor
networks - Either do not use unicast or only one specific
unicast protocol and difficult to maintain
multiple protocols in small memory footprint - Construction of overlays expensive uses
flooding to maintain topology uses too much
energy - Designed more for laptops not sensors
- Rely on routing tables and/or connection state
again difficult to implement in small memory
15Connection-Less Threshold
- When to use a connection-less protocol versus
connection based
Cost per Member
Cost Threshold (Conceptual)
Application Domain of uCast
Connection-based Multicast
Fewer Members/Light Traffic per Session
More Members/Heavy Traffic per Session
16uCast Design
- Uses underlying unicast protocol through single
interface to facilitate a pair-wise comparison to
obtain closest to destination - Implements a scoreboard algorithm executed at
intermediate nodes using destination list and
current node neighbors and generates a multicast
task allocation of a list of next hop nodes that
should receive multicast packet
17Unicast Interface
- Defines only one method
- compare (Node N1, Node N2, Node Dest)
- Which returns the selected nearest Node to the
Destination node
18Scoreboard Algorithm
- INPUT Destination Set (DS), Neighbor Set NS, and
Current Node (S) - FOR EACH node in NS that are in DS, set selected
in NS and move from DS into Covered Set (CS) - FOR EACH node in DS, if only one neighbor in NS
closer than S, set that node in NS to selected,
and move from DS into CS - FOR EACH node is DS, if no neighbor in NS closer
than S move from DS to Local Maximum Set (LS) - FOR EACH node in SN, find all destinations for
which it closer compared to S, move those from DS
to CS
19Scoreboard Algorithm
- WHILE DS is NOT EMPTY
- FOR EACH node in DS, find all unselected nodes in
NS, set each node with a score of 0, assign one
more score to node closer to S to node in DS - FIND unselected node K in NS with highest score,
break ties randomly or using node ID, set K to
selected - FOR EACH node is DS, find nodes for which K is
closer than current node S and move them from DS
to CS - FOR EACH node in SN, find all destinations for
which it closer compared to S, move those from DS
to CS
20Scoreboard Algorithm
- FINALLY PERFORM OPTIMIZATION
- FOR EACH node in NS that are selected insert into
SN - FOR EACH destination in CS choose the best node,
snode, among nodes in SN (i.e. closest to that
CS node) add destination to SD set of snode - FOR EACH node is NS, remove nodes with empty SD,
for other nodes form individual delivery tasks
based on SD - IF LS is not empty, switch to underlying unicast
protocol and corresponding local maximum handling
approach to deliver packets to destination in LS
21Detailed Example
S N53 DS N51,N55 NS
N52,N54,N43 Score 1 1 0 CS
N51 DS N55 NS N52,N54,N43 Score
_ 1 0 CS N51,,N55 SN
N52,N54 N51 gt N52 N55 gt N54
S N53 DS N51,N55 NS
N52,N54,N43
S N22 DS N51,N15,N55 NS
N21,N31,N32,N33,N23,N13,N12 Score 1 1 2
3 2 1 1 CS N51,N15,N55
SN N33 N51,N15,N55 gt N33
S N22 DS N51,N15,N55 NS
N21,N31,N32,N33,N23,N13,N12
S N33 DS N51,N15,N55
NSN32,N42,N43,N44,N34,N24,N23,N22 Score 1 1
2 1 2 1 1 0 CS
N51,N55 DS N15 NSN32,N42,N43,N44,
N34,N24,N23,N22 Score 0 0 _ 0 0
1 0 0 CS N51,N55,N15 SN
N43,N24 N51,N55 gt N43 N15 gt N24
S N33 DS N51,N15,N55 NS
N32,N42,N43,N44,N34,N24,N23,N22
S N11 DS NS N21,N22,N12
Score 2 3 2 CS N51,N15,N55
SN N22 N51,N15,N55 gt N22
S N11 DS N51,N15,N55 NS
N21,N22,N12
22Handling Local Minima
42
44
34
32
23Other Examples
24Design Tradeoffs
- Uses greedy algorithm may not be globally
optimal - Limit to maximum number of destinations due to
packet header size limitations - However, is at least as good as the NP-Complete
Set Cover problem - To be globally optimal would need another
NP-Complete problem - Steiner tree generation
but cannot be generated in reasonable time due to
large number of nodes
25Optimality Analysis
- Use simulation
- With nodes having range of 50m
- In 500m x 500m region
- Source node placed an (250, 250)
- 6 destination nodes in 60 degree region
- At least six hops in each route
- Each scenario tested for 100 rounds
- Same topology used for minimum cover selection,
scoreboard, and plain unicast
26Optimality Analysis
27Destination Encodings
- Imposes limit to size of multicast destinations
- Three possible trade-offs to mitigate
- With longer packets such as used in video
streams size not an issue - Compress destination header trading space for
computational time - In network aggregation use train of packets
that share destination list but need
synchronization and retransmission mechanisms - In any case uCast is designed for small-group
multicast
28Performance Evaluation
- Compare with connection-based protocols
- Shortest Path Tree (SPT)
- Source node sends packets along shortest paths to
destinations and aggregates common paths to form
tree structure - Greedy Incremental Tree (GIT)
- Centralized construction and requires full
knowledge of topology and is computationally
intensive - Plain unicast
- Uses geographical forwarding with the GPSR
traversing technique to handle local minimum set
29Destination Placement
- Uses four parameters
- Polar angle of dispersion (AOD)
- Radius which is furthest destination node
- Density number of nodes within communication
range - Number of destination nodes
30Default Parameters
- Communication range 50m
- Area 500m x 500m
- Density 20 nodes per communication range
- AOD 900
- Number of destinations 10
- Radius 250m
- Total nodes 636
- Data rate 6 packets / minute
- Use PicaZ nodes with CC2420 radio
31Energy Efficiency
GIT is best but impractical uCast performs better
than SPT Unicast
32Energy Efficiency
GIT is best but impractical uCast performs better
than SPT Unicast
33Energy Efficiency
GIT is best but impractical uCast performs better
than SPT Unicast
34Energy Efficiency
GIT is best but impractical uCast has longer path
length then SPT due to GPSR but in reality
voids are not common
35Average Path Length
uCast GIT have longer path lengths due to path
aggregation leading to higher end-to-end
delay SPT Unicast find near optimal paths But
trading energy consumption for longer path lengths
36Average Path Length
uCast GIT have longer path lengths due to path
aggregation leading to higher end-to-end
delay SPT Unicast find near optimal paths But
trading energy consumption for longer path lengths
37Topological Changes
- Introduce topological changes by using energy
saving protocols - Using parameters
- Toggle cycle time interval between sleep state
transitions - Scale size of multicast area larger the area
then cost of reconstruction greater - Packet Delivery Rate use 6 and 12 packets per
minute
38Topological Changes
Shows stateless multicast is superior with node
state transitions As toggle periods shorten SPT
degrades considerably, but uCast achieves 96
delivery ratio
- Impact of Toggle Period (Rate 10ppm)
39Topological Changes
Connection based multicast is less scalable than
uCast as range increased there is higher
probability of state loss
40Topological Changes
Shows 1000s of control packets are needed to
rebuild tree
- Impact of Toggle Cycle Range 250m
- On SPT
41Topological Changes
Shows 1000s of control packets are needed to
rebuild tree
- Impact of Toggle Cycle Range 500m
- On SPT
42Topological Changes
Shows effect of increased data rate which
decreases delivery ratio
- Increasing Data Rate to 12 ppm
43Unicast Protocols
Geo forwarding and logical coordinates-based
routing similar in their performances. However,
uCast based on GEM shows quite different
performance characteristics due to convoluted
delivery paths and polar coordinates
44Running System
- Used actual sensor platform with
- 25 MICA2 motes
- Code Size of 992 bytes
- 3V supplies
- 19.2 kbps
- 12 byte payload
45System Evaluation
uCast significantly reduces energy consumption
46uCast unicast
uCast significantly reduces data load
Recorded data load at each node
47Conclusions
- uCast is generally as efficient as
connection-based protocols even with static
networks - uCast is more robust in a dynamic network due to
its connectionless nature - uCast can be implemented on different unicast
routing protocols - A real implementation supports these conclusions