Chapter 10:
Cross Layer Protocols
Wireless Sensor Networks
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Traditional Layered Approach
Network Layer
MAC Layer
Physical Layer
Energy Management Plane
Transport Layer
Cross-Layer Management Plane
Application Layer
Each protocol designed
independently
 Limited information is passed
between layers
 Good for abstraction and
development
 Bad for energy efficiency,
overhead, performance
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Inter-Layer Effects
 Wireless channel (PHY)
 Channel characteristics drastically influence performance
[1,2]
 MAC and Routing
 Significant effect on each other due to interference
[1] M. Zuniga, et.al., ``Analyzing the transitional region in low power wireless links,’’ Proc. IEEE SECON ‘04, Santa
Clara, CA, Oct. 2004.
[2] C. Bettstetter, et.al., “Connectivity of Wireless Multihop Networks in a Shadow Fading Environment,’’
ACM/Springer Wireless Networks, vol. 11, no. 5, pp. 571-579, September 2005.
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Inter-Layer Effects
 Transport and PHY layer
 Congestion and contention are highly coupled due to
broadcast nature of the wireless channel [3]
 Transmission power control and congestion affect each
other (CDMA scheme) [4]
[3] M. C. Vuran, V. C. Gungor, and O. B. Akan, ``On the interdependency of congestion and contention in
wireless sensor networks,'' Proc. SENMETRICS'05, July 2005.
[4] M. Chiang, ``Balancing transport and physical Layers in wireless multihop networks: jointly optimal
congestion control and power control,’’ IEEE JSAC, vol. 23, no. 1, pp. 104 – 116, Jan. 2005.
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Our Vision
Application Layer
Energy Management Plane
MAC Layer
Cross-Layer
Melting
Cross-Layer Management Plane
Network Layer
Energy Management Plane
Transport Layer
Cross-Layer Management Plane
Application Layer
PHY Layer
Our View
Traditional Approach
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Cross-layer Communication
 Receiver-based routing [5] and [6]
 The next hop is determined based on receiver contention
(MAC + Routing)
[5] P. Skraba, et. al., ``Cross-layer optimization for high density sensor networks: Distributed passive routing
Decisions,’’ in Proc. Ad-Hoc Now’04, Vancouver, July 2004.
[6] M. Zorzi, et. al., ``Geographic random forwarding (GeRaF) for ad hoc and sensor networks: multihop
performance,’’ IEEE Trans. Mobile Computing, vol. 2, no. 4, pp. 337- 348, Oct.-Dec. 2003.
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Cross-layer Communication
 Performance analysis of receiver based routing [6, 7]
 Energy efficiency analysis
 Latency and multihop performance
 Only MAC & routing interaction is considered (no PHY
layer/lossless/simple channel model)
 MAC is based on a two radio node
[6] M. Zorzi, et. al., ``Geographic random forwarding (GeRaF) for ad hoc and
sensor networks: multihop performance,’’ IEEE Trans. Mobile Computing,
vol. 2, no. 4, pp. 337- 348, Oct.-Dec. 2003.
[7] M. Zorzi, et. al., ``Geographic random forwarding (GeRaF) for ad hoc and
sensor networks: energy and latency performance,’’ IEEE Trans. Mobile
Computing, vol. 2, no. 4, pp. 349 – 365, Oct.-Dec. 2003.
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Cross-layer Communication
 In [8], the scheme in [6,7] is extended for single radio nodes
 Relies purely on geographical information
 Considers perfect channel conditions
[8] M. Zorzi, ``A new contention-based MAC protocol for
geographic forwarding in ad hoc and sensor networks,” in Proc.
IEEE ICC ‘04, vol. 6, pp. 3481 - 3485, June 2004.
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Cross-layer Communication
 A cross-layer solution with a realistic channel model [9] (includ. fading
channel)
 Receivers contend based on a cost function as well as
geographical location
 Correlation between nodes’ cost functions are considered
 Based on MAC + routing interaction
 Does not consider transport layer and PHY layer issues
 Energy efficiency is not considered
[9] M. Rossi, et. al., ``Cost Efficient Localized Geographical Forwarding Strategies for
Wireless Sensor Networks,’’ in Proc. TIWDC 2005, Sorrento, Italy, 2005.
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Cross-layer Communication
 Joint routing, MAC, and link layer optimization [10]
 TDMA for MAC and MQAM for modulation is considered
 Analytical results favor single-hop communication instead of multihop
 No communication protocol is proposed
 Transport layer issues such as congestion control are not
considered
[10] S. Cui, et. al., ``Joint routing, MAC, and link layer optimization in sensor
networks with energy constraints,’’ in Proc. IEEE ICC ’05, vol 2, pp. 725 – 729, May
2005.
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Cross-layer Communications
 Existing work focus on pair wise cross-layering, e.g.,
PHY+MAC, MAC+routing.
 A complete cross-layer suite that replaces each layer is
required
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Motivation
 Unique characteristics of WSN
 High density
 Limited resources
Energy
Processing
Memory
 Wireless channel
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Motivation: High Density
 Very low reporting rates
 Duty cycle operation is required
 Traditional routing algorithms require neighborhood information
 Leads to contention and energy consumption
 Duty cycle introduces
 Delays
 Frequent re-routing
 Energy inefficiency
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Motivation: Wireless Channel
 Unit disk model leads to simple connectivity graphs
R
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Motivation: Wireless Channel
 Shadow fading model complicates things!
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Motivation: Wireless Channel
 Shadow fading model complicates things!
 Short links not guaranteed
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Motivation: Wireless Channel
 Shadow fading model complicates things!
 Short links not guaranteed
 Considerable amount of long links
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Motivation: Wireless Channel
 Shadow fading model complicates things!
 Short links not guaranteed
 Considerable amount of long links
 Link quality varies with time
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Motivation: Wireless Channel
 Traditional routing protocols
 Neighborhood information actually varies with time
 Requires frequent updates
 Duty cycle operation
 Nodes sleep for a certain fraction of time
  – duty cycle parameter
  = 0.1  Awake for 10%, sleep for 90% of the time
 Necessitates either frequent re-routing or scheduling
 Channel asymmetry
 Routing decisions made by the source node are not accurate
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XLP: Cross-Layer Protocol
I. F. Akyildiz, M. C. Vuran, and O. B. Akan, “A Cross-layer Protocol for Wireless Sensor Networks,” in Proc. Conference on
Information Science and Systems (CISS ’06), Princeton, NJ, March 22-24, 2006.
M. C. Vuran, I. F. Akyildiz, ``XLP: A Cross-Layer Protocol for Efficient Communication in Wireless Sensor Networks’’ to appear
in IEEE Trans. Mobile Computing, 2010.
Application Layer
Transport
Network
MAC



Initiative Concept
 Communication incentive is passed to the
receiver
Receiver Contention

Potential receivers contend for packets and become
next-hop
Local XL congestion control

Highly congested nodes do not participate in
communication

Angle-based routing

Channel adaptive operation
PHY



Adaptive to local minima in case of ‘voids’ in the
network
Receivers adapt communication parameters based
on channel conditions
Duty cycle operation

Energy consumption centric operation via duty cycle
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Initiative Concept
 Core of XLP
 A node participates in communication based on its
initiatives
 When a node has a packet to send, it broadcasts an RTS
packet
 A neighbor node contends for routing of the packet based
on its initiatives
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Initiative Concept
 Node Initiative

 RTS  Th 


Th 
relay  relay 
1, if 


max
I  
    

min 

E rem  E rem 

0, otherwise


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Initiative Concept
 Node Initiative depends on






 RTS

Th




Th
 relay  relay 

1, if 


m
ax
I 
 

 

min 

 Erem  Erem 




0
,
otherwise


Received RTS packet’s signal
to noise ratio (SNR) – channel
quality
Input packet rate for Cong. C.
Buffer level for Cong. C.
Remaining energy
 If all the inequalities are satisfied, node participates in communication
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Initiative Concept
 Node Initiative implications

  RTS   Th 




Th
 relay  relay 

1, if 


m
ax
I 
 



min 

 Erem  Erem 



0, otherwise

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
Guarantees high channel
quality

Eliminates congested nodes

Eliminates congested nodes

Leverages energy
consumption
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XLP Mechanisms
Source node
Router/Active node
Passive node
Source
Router
Router
Sink
Receiver-based contention & routing
Local congestion control
Angle-based routing
Duty cycle (d) based operation
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XLP Mechanisms
 Transmission initiation
 Receiver contention
 Angle-based Routing
 Local XL congestion control
 Duty cycle determination
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Transmission Initiation
 When a node has packet to send
 Listens to channel
 If channel is occupied, performs backoff with CWRTS
 If channel is idle, broadcasts an RTS packet
 Nodes receiving RTS packet
 Check their location relative to source and destination
 Measure RTS packet SNR, RTS
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Transmission Initiation
Sink
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Transmission Initiation
Infeasible nodes
Feasible nodes
Sink
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Receiver Contention
 Feasible nodes calculate initiative, I
 Nodes with I=1 contend for the packet
 Each node location corresponds to a priority region Ai with backoff


window size CWi
Nodes with longer progress (closer to the sink) have higher priority
over other nodes
Based on the location, each node determines its region Ai and backs
off for
i 1
 CW
j 1
j
 cwi , where cwi  [0, CWi ]
 Contention winner sends a CTS packet
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Receiver Contention

A3
Closer nodes to the sink are highly
likely to win the contention
A2
A1
Priority regions
Sink
RTS
A1
A2
A3
CTS
CTS
CTS
CW1
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CW2
CW3
CW4
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Receiver Contention
A3
A2
A1
Sink
RTS
CTS
CW1
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CW2
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Receiver Contention
A3
A2
A1
Sink
DATA
RTS
CTS
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Receiver Contention
A3
A2
A1
Sink
DATA
RTS
CTS
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ACK
34
Receiver Contention

A3

A2
A1

I = 1 may not hold for any node (high
congestion)
After CW4 if no CTS is heard, neighbors
send KEEP ALIVE packet
Sender determines congestion and
decreases transmission rate
Sink
A1
A2
A3
KEEP ALIVE
CW1
CW2
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CW3
CW4
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Angle-based Routing (ABR)
 In sparse networks,
‘voids’ exist
 Receiver contention may
lead to local minimum
 Packets need to be
routed around the ‘void’
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How to determine local minimum
 Node broadcasts an RTS
packet
 If no CTS or KEEP ALIVE

packets received, re-broadcasts
RTS (maybe previous RTS
packet lost)
If no response  local
minimum reached
 Switches to angle-based
routing (ABR)
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Angle-based Routing (ABR)
 Send RTS with
RTS
1 CW
 Angle-based routing
ABR bit Traverse
direction
(ABR) bit set
 Traverse direction –
clock wise
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Angle-based Routing (ABR)
 Neighbor nodes send CTS
RTS
1 CW
ABR bit Traverse
direction
based on their locations
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Angle-based Routing (ABR)


Enlarged view
Neighbor that has the smallest angle
with the source-sink vector reply
earlier
q1,2 < q1,3 < q1,4 < q1,5
2
q1,4
3
q1,3
1
q1,2
2
1
4
3
Source-sink
vector
5
RTS
4
5
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CTS
CTS
CTS
CTS
40
Angle-based Routing (ABR)
ABR  basic XLP
 Angle-based routing is
terminated if packet
traverses closer to the
sink than the starting point
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Angle-based Routing (ABR)
CW  CCW
 If network edge is reached,
perform counter-clock
wise angle-based routing
ABR  basic XLP
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Angle-based Routing
sink

g
0
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Sample route
 0-a: Basic XLP
 a-b: ABR with clock-wise
direction
 b-c: Basic XLP
 c-d: ABR with clock-wise
direction
 d-e: Basic XLP
 e-f: ABR with clock-wise
direction
 f-g: ABR with counter clockwise
 g-sink: Basic XLP
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Local XL Congestion Control
 Relay nodes
 Participate in communication if
relay  Th
relay
 Source nodes
 Explicitly control the rate of generated packets
 (ii )
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Local XL Congestion Control
lji
lii
mi
 Input packet rate
i  ii  relay  ii 
 Output packet rate

ji
jΝ iin
i  (1  ei )(ii  relay )
ei packet error rate
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Local XL Congestion Control:
Relay Nodes
 Node’s transmit & receive times during active period:
(assuming a node is active for average of d fraction of time)
Trx  relayTPKT
Ttx  (1  ei )( ii  relay )TPKT


Tlisten    (1  ei )ii  ( 2  ei )relay  TPKT
where TPKT is the average duration required to successfully transmit
a packet to another node.
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Local XL Congestion Control:
Relay Nodes
 Tlisten ≥0 (in order to prevent buffer overflow and maintain its
duty cycle), input relay packet rate is bounded by:
relay  
th
relay

th
relay

,

(2  ei )TPKT
(1  ei )

ii
(2  ei )
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Local XL Congestion Control: Source
Nodes
 Control the rate of generated packets, ( )
 Source node waits for CTS packets
 If no CTS packets are received, performs retransmission
 (assumes there may be RTS packet loss)
 If KEEP ALIVE packet is received, decrease rate
ii
ii  ii 
1

 If data transfer is successfully, increase rate
ii  ii  
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Local XL Congestion Control: Source
Nodes
  is the transmission rate throttle factor; assumed to be 2.
  is the increase factor and is assumed to be iio/10 where
the iio is the initial value of generated packet rate.
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Effect of Duty Cycle
 Total energy consumed as a flow-based energy consumption analysis
E flow ( D)  E per  hop E[nhops ( D)]
 Expected number of hops from a source to sink with distance D
E[nhops ( D)] 
D  Rinf
1
E[ d next hop ]
E[dnext-hop] is the expected hop distance. Rinf is the approximated transmission
range.
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Effect of Duty Cycle
 Consumed energy per hop in one hop for transmitting a
packet
E per hop  ETX  ERX  Eneigh
Eng. consumption of the
transmitter node
Eng. consumption of the
neighbor nodes
Eng. consumption of the
receiver node
 Detailed formula on the paper
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Energy Consumption (Eflow)
Effect of D on energy consumption
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Effect of Duty Cycle Parameter ()
 Energy consumption of a flow is minimal for ~0.002
 For small sized networks with <1K nodes, this operating
point may not provide connectivity in the network.
 Energy consumption has a local minima around =0.2,
which is a suitable operating region.
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Recap: XLP Cross-Layer Interactions
Application Layer
Transport
Network
MAC Layer
PHY
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Recap: XLP Cross-Layer Interactions
 A node monitors its channel
Application Layer
Transport
Network
MAC
quality using the received
packet.
 It participates in
communication based on the
recent channel state.
 Receiver-based initiative
concept provides accurate
channel-aware operation.
PHY
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Recap: XLP Cross-Layer Interactions
Application Layer
 Receiver-based contention & routing
 Potential receivers contend for the
Transport
Network
MAC
PHY

transmitted packets and become nexthop
Routing Algorithm
 Feasible receivers are determined
based on geographical information
of source and sink
 Voids are avoided by angle-based
routing
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Recap: XLP Cross-Layer Interactions
Application Layer
Transport
 Local congestion control
 Nodes monitor their buffer state
 Highly congested nodes do not
Network

MAC

participate in contention & routing
Relay rate is controlled to prevent
congestion
Source rate is controlled based on local
information
PHY
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Performance Evaluation
 C++ cross-layer simulator (XLS) developed in BWN lab
 300 nodes, 100x100m2 sensor field
 Sink at (80,80), event at (20,20) with event radius 20m
 Throughput, goodput, energy consumption, number of hops
and latency analysis
 Comparative analysis with 5 layered protocol stacks and 1
cross-layer protocol (ALBA-R)
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Protocol Stacks
 Flooding: At the MAC layer, a simple CSMA type broadcast


mechanism, at the transport layer, constant packet rate with no rate
control.
[GEO]: geographical routing [14], and CC-MAC [15] at transport,
routing, and MAC layers, respectively.
[PRR]: ESRT [13], PRR-based geographical routing [14], and CC-MAC
[15] at transport, routing, and MAC layers, respectively.
[13] Ö. B. Akan and I. F. Akyildiz, ``Event-to-Sink Reliable Transport in Wireless Sensor Networks,'‘ IEEE/ACM Trans.
on Networking, vol. 13, no. 5, pp. 1003-1016, October 2005.
[14] K. Seada, M. Zuniga, A. Helmy, B. Krishnamachari, ``Energy-efficient forwarding strategies for geographic
routing in lossy wireless sensor networks,'' in Proc. ACM Sensys '04, November 2004.
[15] M. C. Vuran, and I. F. Akyildiz, ``Spatial Correlation-based Collaborative Medium Access Control in Wireless
Sensor Networks,'‘ IEEE/ACM Trans. on Networking, August 2006.
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Protocol Stacks
 [PRR-SMAC]: ESRT [13], PRR-based geographical routing [14], and



SMAC [16]
[DD-RMST]: RMST [17], directed diffusion [18], and CSMA at transport,
routing, and MAC layers, respectively (only for  = 1).
[ALBA-R(x)]: ALBA-R protocol [19] where x represents the traffic rate l
= {3,4,6.25} pkts/sec
XLP: Our proposed cross layer protocol (XLP)
[16] W. Ye, J. Heidemann, and D. Estrin, ``Medium Access Control with Coordinated Adaptive Sleeping for Wireless
Sensor Networks,'‘ IEEE/ACM Transactions on Networking, vol. 12, no. 3, pp. 493-506, June 2004.
[17] F. Stann and J. Heidemann, ``RMST: Reliable data transport in sensor networks,'' in Proc. IEEE SNPA '03, pp.
102-112, Anchorage, Alaska, April, 2003.
[18] C. Intanagonwiwat, R. Govindan, D. Estrin, J. Heidemann, F. Silva, ``Directed diffusion for wireless sensor
networking,'‘ IEEE/ACM Transactions on Networking, vol. 11, no. 1, pp. 2 - 16, February 2003.
[19] P. Casari, M. Nati, C. Petrioli, and M.Zorzi, “Efficient Non Planar Routing around Dead Ends in Sparse
Topologies using Random Forwarding,” in Proc. ICC’07, Scotland, UK, June 2007.
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Performance Evaluation
 Effect of angle-based


routing
Up to 70% decrease in
route failure rate
For >0.2, ABR limits route
failure rate to <10%
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Performance Evaluation
Avg. Throughput
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Goodput
62
Performance Evaluation
Avg. Number of Hops
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Performance Evaluation
End-to-end Latency
Consumed Energy/Packet
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Conclusions
 Smaller number of hops does not mean faster
routes/minimum energy consumption
 XLP achieves significantly low energy consumption
with low latency
 Cross-layer protocol design outperforms layered
and existing cross-layer protocols
 Lower complexity in design and implementation
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