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.

AB

Node-based constraint

Edge-based constraint

AB

C0trV 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

Spring 2005 UCSC CMPE257 4

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

Spring 2005 UCSC CMPE257 5

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

54

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

Spring 2005 UCSC CMPE257 9

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.

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Algorithm

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Illustration of NAMA

A B

C D E

F

HG

8

6

5

3 4

1

29

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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 gc d

e

h

b10

1

64

7 3

85

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.

Spring 2005 UCSC CMPE257 15

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 gc d

e

h

b10-BTx

1-DRx

6-Rx

4-DRx

7-UTx 3-DRx

8-Rx5-DTx

Spring 2005 UCSC CMPE257 17

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.

Spring 2005 UCSC CMPE257 18

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

iI

Iq

Spring 2005 UCSC CMPE257 19

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.

Spring 2005 UCSC CMPE257 20

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):

Spring 2005 UCSC CMPE257 22

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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Plotting Channel Access Probability

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Comparison of Channel Access Probability

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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 ̶ qi is the channel access probability of node i.

Spring 2005 UCSC CMPE257 29

Service Time: Service time: Xi = Yi + 1. The mean and second moment of

service time:

Server vacation: V=1,

Spring 2005 UCSC CMPE257 30

Delay in The System Pollaczek-Kinchin formula:

Take in Xi and Vi :

Delay in the system: (q>)

Spring 2005 UCSC CMPE257 31

Plotting System Delay

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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!

Spring 2005 UCSC CMPE257 33

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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Protocol Throughput Comparison

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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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Fully Connected Network (Throughput)

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Fully Connected Network (Delay)

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Multi-hop Network (Throughput)

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Multi-hop Network (Delay)

Spring 2005 UCSC CMPE257 42

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)

Spring 2005 UCSC CMPE257 43

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.