A New Approach to Channel Access Scheduling in Ad Hoc Networks Lichun Bao School of ICS University...
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Transcript of A New Approach to Channel Access Scheduling in Ad Hoc Networks Lichun Bao School of ICS University...
A New Approach to Channel Access Scheduling in Ad Hoc Networks
Lichun BaoSchool of ICS
University of California, Irvine
J.J. Garcia-Luna-AcevesSchool of Engineering
University of California, Santa Cruz
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Existing Solutions for Channel Access:
• Random Access Scheme:– ALOHA, CSMA/CA (FAMA, MACA, MACAW,
IEEE 802.11) : with/without RTS/CTS handshakes.
– Difficulties providing fairness, QoS.
• Scheduled Access Scheme:– Node/Link Activation.– FDMA/TDMA/CDMA in multihop networks:
graph coloring problem — UxDMA.
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Our Solution: Scheduled Access
• Problem description:– Given a set of contenders Mi of an entity i in
contention context t, how does i determine whether itself is the winner during t ?
• Topology dependence: – Exactly two-hop neighbor information required
to resolve contentions.– Two-hop neighbors are acquired by each node
broadcasting its one-hop neighbor set.
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Example Settings:
• Omni-directional Antenna;
• Time slotted channel access;
• Equal transmission range;
• 4 nodes;
• Each node knows its one- and two-hop neighbors — Mi.
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Goals to Achieve:
• Collision-free — avoid hidden terminal problem, no waste on transmissions;
• Fair — the probability of accessing the channel is proportional to contention;
• Live — capable of yielding at least one transmission each time slot.
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Neighbor-aware Contention Resolution (NCR):
• In each contention context (time slot t):– Compute priorities
– i is the winner for channel access if:
�
}{,)( iMkktkRandp itk
tj
tii ppMj ,
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Attributes of NCR:
• Collision freedom;
• Fairness;
• Liveliness;
• 2-coloring: – An entity colors itself if it red has the highest
priority among its contenders.– Otherwise, it has transparent color.
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NCR-MI (Multiple Identities):
• Dynamic Resource Allocation.
• A node i may have Ii pseudo identities.– k-th identity is denoted as – Ii is dynamically chosen by i according to
traffic requirement.– Each identity of i gives i a chance to win a
contention. The more identities, the better chance of channel access.
.ki
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NCR-MI Specification:
• Compute the priority on each pseudo identity of every contender:– For l-th identity of contender k, we have:
• i is the winner for channel access one of its priority is the greatest among its contenders.
�
ki
tlk
IliMk
lktlkRandp
1},{
)(
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Channel Access Probability:
• Dependent on the number of pseudo identities and the density of the neighborhood.
• Channel access probability:– Bandwidth allocation
}{iMk k
ii
i
I
Iq
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• Channel access probability for individual nodes:
• Spatial channel reuse ratio:
Bandwidth Allocation Example:
ji
lk
3
1 4
1
3
1
4
1
67.13
1
4
1
4
1
3
1
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Delay & Throughput Analysis:
• Data packet service at entity i modeled as M/G/1 queuing system with server vacation.
• Delay (Pollaczek-Kinchin formula):
• Throughput:
2
3
)(2
)1(2
ii
iiii q
qqT
k
kk qS ),min(
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Channel Access Scheduling Protocols:
• Node Activation Multiple Access (NAMA):– Entity type: node– Time division:
• Block
• Section
• Part
• Time-slot
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NAMA Time Division Illustrated:
0 1Section
0 1 50....... 51Membership Section:
Neighbor Maintenance
Block
0 1 2Part
Time Slot
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NAMA Illustrated:
Fully connected network with 10 nodes. ID: 1~10.
1,5,6,8,10 2,3,4,7,9Part 0 Part 1
Contenders resolvecontention using NCR
8 1,5,6 10 9 3 2,4,7 Section 1
1,10 5,8 6 4,9 2,3,7 Section 02,3,4,7,9No occupied by anyoneEveryone tries to use
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Neighbor Protocol:
• One-hop neighbor information broadcasting.– New node starting up.– Link addition and deletion.– Old neighbor going down can be treated as
multiple link deletions.
• Membership section: send signals.
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Channel Access Scheduling Protocols (continued):
• Link Activation Multiple Access (LAMA):– Direct Sequence Spread Spectrum, available pseudo-
noise code set: Cpn
– Received-Oriented Code Assignment (ROCA)
– Contenders of node i :
– Once Mi is decided, LAMA follows NCR.
pnk
i CiRandkcc mod)(,
}{)(1,
11 iNNMcink
kii
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LAMA Illustrated:
i
j
k
a
b
c
g
f
e
dNode i tries to activate its adjacentlinks on code c
Both j and k are assigned code c
c
c
At time t, the priorityof each node is computed.
20
21
19
1
14
23
8
5
11
6i can activate eitherlink (i,j) or (i,k).
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Channel Access Scheduling Protocols (continued):
• Pair-wise Link Activation Multiple Access (PAMA):– Contending entities are directed edges;– Priorities are computed for each link;– Dynamic code assignment:
– Contenders of a link are its adjacent links.
pnk
u CutuRandkcc mod)(,
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PAMA Illustrated:
i
a
b
c
k
g
f
21
14
7
5
11
13
51
23
1. Directional links
2. Only one direction shown for simplicity
3. Hidden terminal avoidance: link (i,k) and (f,g) assigned the same code — compare node priorities of i and f.
c
c
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Summary — Unified Algorithm:
• Determine the entity type (node/link);
• Find out the contender set;
• Run NCR to determine if the entity is active in the current time slot;
• Resolve hidden terminal problem.
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Problems with NAMA
• Inefficient activation in certain scenarios.– For example, only one node, a, can be activated
according NAMA, although several other opportunities exist.
—— We want to activate g and d as well.
a
f g
c d
e
h
b10
1
6
4
7 3
8
5
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Node + Link (Hybrid) Activation
• Additional assumption– Radio tranceiver is capable of code division
channelization (DSSS —— direct sequence spread spectrum)
– Code set is C .
• Code assignment for each node is per time slot:
i .code = i .prio mod |C |
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Hybrid Activation Multiple Access (HAMA)
• Node state classification per time slot according to their priorities.– Receiver (Rx): intermediate prio among one-hop
neighbors.– Drain (DRx): lowest prio amongst one-hop.– BTx: highest prio among two-hop.– UTx: highest prio among one-hop.– DTx: highest prio among the one-hop of a drain.
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HAMA (cont.)
• Transmission schedules:– BTx —> all one-hop neighbors.– UTx —> selected one-hops, which are in Rx
state, and the UTx has the highest prio among the one-hop neighbors of the receiver.
– DTx —> Drains (DRx), and the DTx has the highest prio among the one-hops of the DRx.
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HAMA Operations• Suppose no conflict in code assignment.
• Nodal states are denoted beside each node:– Node D converted from Rx to DTx.– Benefit: one-activation in NAMA to four possible
activations in HAMA.
a
f g
c d
e
h
b10-BTx
1-DRx
6-Rx
4-DRx
7-UTx 3-DRx
8-Rx
5-DTx
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Neighbor Protocol (Need)
• Purpose: propagate neighbor updates.• Cannot be based on NCR — requires a priori
neighbor information.• Only way:
– Random access.
– Broadcast.
– No acknowledgement: why? Efficiency, broadcast.
– Use retransmission to improve reliability.
• Why not TSMA: Topology-dependent.
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Neighbor Protocol (Method)
• Insert random access section after ROMA.
• Send short signals carrying neighbor updates (256 bytes).
• Problem formulation:– How to regulate interval t and number n of
retransmissions to have low latency to deliver messages with given (high) probability p .
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Neighbor Protocol (Results)
– Reliability: deliver-probability p =99%.– Retransmission interval: t =1.44N — only
depends on N (the number of two hop neighbors).– Number of retransmission: n =6.7≈7 — only
depends on p .– Suppose 2Mbps bandwidth, 2 second delay, 20
two-hop neighbors — random access sections cost 9.6% of the channel resource.
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Performance Analysis
• Modeling– Infinite plane with node density ρ (100 nodes
per 1000mX1000m area). – Transmission range r (0m~500m).
• Derive average per-node throughput according to node-distribution and node geometric relations.
• Analyze both NAMA and HAMA.
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Comparison between NAMA and HAMA
• HAMA has higher throughput than NAMA:– Similar at low transmission range r .– 3-4 times higher throughput at higher r .
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Comparison withCSMA and CSMA/CA (1)
• Throughput of CSMA (CA) taken from the work of Yu et al. [ICNP’02].
• Load conversion:– CSMA (CA) always fully loaded. Differ at channel
access probability p’ and size ldata.– HAMA load depends on packet arrival rate λ
λ=p’ · ldata /(1+p’ · ldata )
• Compare the throughput S in the one-hop neighborhood N= ρπr² (ρ: node density; r Tx range).
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Comparison with CSMA and CSMA/CA (2)
• Two scenarios: long data packet (100 time slots) and short data packet (10 time slot)
• Different contention levels in each scenario.
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Comparison withCSMA and CSMA/CA (3)
• HAMA gives the constant S at high load, whereas CSMA and CSMA/CA degrade.
• HAMA differs by the shift reaching the highest S.• When the data packet is shorter, the collision
vulnerable period becomes longer relatively in CSMA and CSMA/CA, thus lower throughput.
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Comparison with NAMA and UxDMA through Simulations
• UxDMA schedules broadcast only, like NAMA does.
• Network generated by placing 100 nodes in 1000mX1000m area. No movement.
• Transmission range: 100m, 200m, 300m, 400m.
• Code set size |C |=30.
• Simulation duration: 100,000 time slots.
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Throughput (2)
• HAMA collected throughput of broadcast and unicast traffics separately.
• Overall throughput of HAMA and NAMA is compared with the theoretical analyses — matches well.
• NAMA is worse than UxDMA sometimes, HAMA is always better than UxDMA.
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Delay Explained
• UxDMA always has lower delay.
• HAMA has separate delay attributes for unicast and broadcast, because they are transmitted using separate transmission opportunities.
• NAMA and HAMA have the same broadcast delay.
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Conclusions:
• Collision-free scheduling algorithm;• Minimum topology information needed;• Better throughput than static scheduling algorithms.• More activation opportunities can be explored in NAMA
—— HAMA.• HAMA needs code division channelization.• Theoretical analyses reveal higher throughput in HAMA
than in NAMA.• Scheduled approach gives higher throughput than random
access approach (CSMA, CSMA/CA).