spring 2005cmpe2571 cmpe 257: wireless and mobile networking set 3b: medium access control protocols
TRANSCRIPT
Spring 2005 CMPE257 1
CMPE 257: Wireless and Mobile Networking
SET 3b:
Medium Access Control Protocols
Spring 2005 UCSC CMPE257 2
Channel Access Schemes Contention based schemes
ALOHA, CSMA/CA (FAMA, MACA, MACAW, IEEE 802.11) : with/without RTS/CTS handshakes.
Difficulties: not scalable, fairness, QoS. Scheduled schemes
FDMA/TDMA/CDMA in multi-hop networks: graph coloring problem — UxDMA.
Node/link activation based on NCR (Neighbor-aware Contention Resolution)
Spring 2005 UCSC CMPE257 3
UxDMA [R01] Channel assignments (code in CDMA,
time-slot in TDMA and frequency in FDMA) are abstracted as graph coloring problems.
Several atomic constraints are identified.
A
B
Node-based constraint
Edge-based constraint
A
B
C
0trV E.g.: Two adjacent cells
cannot use the same freq. set.
E.g.: A node (A) cannot transmit and receive at the same time.
0trE
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UxDMA (Cont’d) Channel assignments can be classified
based on certain sets of constraints. (T/F)DMA broadcast schedule/assignment
RTS/CTS protocols
Then a unified algorithm for efficient (T/F/C)DMA channel assignments is proposed using global topology.
10 , tttr VV
11000 ,,,, trtttrttrr EEEEE
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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. In ad hoc networks, two-hop
neighbors are acquired by each node broadcasting its one-hop neighbor set.
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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 ,
a bc
d
e
Contention Floor
69
5
4
2
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Channel Access Probability: Dependent on the number of
contenders in the neighborhood. Channel access probability:
Bandwidth allocation general formula to i
}{iMk k
ii
iI
Iq
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NAMA: Node Activation Multiple Access (Broadcast)
Channel is time-slotted. Transmissions are broadcasts via
omni-directional antenna: all one-hop neighbors can receive the packet from a node.
The contenders of a node for channel access are neighbors within two hops because of direct and hidden terminal contentions.
Spring 2005 UCSC CMPE257 12
NAMA Improvements 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
Spring 2005 UCSC CMPE257 13
Node + Link (Hybrid) Activation
Additional assumption Radio transceiver 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 |
Spring 2005 UCSC CMPE257 14
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.
Spring 2005 UCSC CMPE257 16
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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Other Channel Access Protocols
Other protocols using omni-directional antennas: LAMA: Link Activation Multiple Access PAMA: Pair-wise Activation Multiple Access
Protocols that work when uni-directional links exist. Node A can receive node B’ s transmission
but B cannot receive A’ s. Protocols using direct antenna systems.
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Channel Access Probability Analysis of NAMA
The channel access probability for a single node i is given by
We are interested in average probability of channel access in multi-hop ad hoc networks.
}{iMk k
ii
i
I
Iq
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Ad Hoc Network Settings Equal transmission range; Each node knows its one- and two-
hop neighbors — Mi . Nodes are uniformly distributed on
an infinite plane with density . A node may have different numbers
of neighbors in one-hop and two-hop.
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Counting One-Hop Neighbors The prob of having k nodes in an
area of size S is a Poisson distribution:
Average one-hop neighbors is:
Note: the mean of r.v. with Poisson dist is2rS
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Counting Two-hop Neighbors Two nodes become two-hop nbrs if
they share at least one one-hop neighbor. Average number in B(t):
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Counting Two-hop Neighbors Probability of becoming two-hop: Prob of a node staying at tr is 2t. Summation of nodes in ring (r,2r)
times the corresponding prob of becoming two-hop --- number of two-hop neighbors:
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Total One- and Two-hop Neighbors
Sum:
This is average number of one-hop and two-hop neighbors.
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Average Probability of Channel Access
Apply Poisson distribution with the mean (number of one- and two-hop neighbors)
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Delay per Node Delay is related with the probability
of channel access and the load at each node.
Channel access probability can be different at each node.
Delay is considered per node.
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Packet Arrival and Serving: M/G/1 with server vacation: Poisson arrival
(exponential arrival interval), service time distribution (any), single server.
FIFO service strategy: head-of-line packet waits for geometric distributed period Yi with parameter 1-qi N qi is the channel access probability of node i.
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Service Time:
Service time: Xi = Yi + 1. The mean and second moment of
service time:
Server vacation: V=1,
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Delay in The System Pollaczek-Kinchin formula:
Take in Xi and Vi :
Delay in the system: (q>)
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System Throughput Multi-hop networks have concurrent
transmissions >1. The system can carry as many
packets at a time as all nodes can be activate.
Simple!
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Comparisons with CSMA CSMA/CA by Analysis
Different slotting: NAMA long slots CSMA CSMA/CA short slots
CSMA(CA) assumptions: Heavy load (always have packets waiting) Channel access regulated by back-off
probability p’ in each slot. Convert the load to comparable one in
NAMA.
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Convert Load in CSMA(CA) to the Load in NAMA
Each attempt to access channel is a packet arrival p’.
Packet duration is geometric with average 1/q.
Two state Markov chain to compute the load.
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NAMA Load Relation:
i=b is the load for each node. qm is the channel access probability
of each node.
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Simulations
Two scenarios: Fully connected: 2, 5, 10, 20 nodes. Multi-hop network:
100 nodes randomly placed in 1000x1000 area.
Transmission range: 100, 200, 300, 400.
Compare with UxDMA:
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Conclusions NCR ensures collision-free transmissions. Only two-hop topology information is
needed. HAMA performs better than static
scheduling algorithms (UxDMA). HAMA performs better than contention-
based protocols. The use of directional antennas can
improve performance further. (Next topic)
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Comments
Scheduled-access protocols are evaluated in static environments and what about their performance in mobile networks?
Neighbor protocol will also have impact on the performance of these protocols
Need comprehensive comparison of contention-based and scheduled access protocols.
Spring 2005 UCSC CMPE257 44
References [R01] S. Ramanathan, A unified framework and
algorithm for channel assignment in wireless networks, ACM Wireless Networks, Vol. 5, No. 2, March 1999.
[BG01] Lichun Bao and JJ, A New Approach to Channel Access Scheduling for Ad Hoc Networks, Proc. of The Seventh ACM Annual International Conference on Mobile Computing and networking (MOBICOM), July 16-21, 2001, Rome, Italy.
[BG02] Lichun Bao and JJ, Hybrid Channel Access Scheduling in Ad Hoc Networks, IEEE Tenth International Conference on Network Protocols (ICNP), Paris, France, November 12-15, 2002.