channel access protocols

1

Channel Access Protocolsfor

Ad Hoc Networks

Instructor: Carlos Pomalaza-Ráez

Fall 2003University of Oulu, Finland

2

Media Access Control (MAC)

• Protocols that enable multiple users to share a finite amount of frequency and time resources

• Needed for efficient operation and good performance for wireless systems

• An important goal is to minimize overhead while maximizing overall network capacity

• Ad Hoc networks have no pre-existing infrastructure or centralized administration, e.g. base stations, and as such they require distributed MAC protocols

3

Wireless MAC issues

Half-duplex operation

Time varying channel

Burst channel errors• bit-error rate can be as high as 10-3 or higher

• packet loss can be minimized by using smaller packets, forward error correcting codes and retransmission methods such as acknowledgement packets

• reflection

• diffraction

• scattering

multipathfading

received signalpower varies asfunction of time

4

Wireless MAC issues

Location dependent carrier sensing• Hidden nodes. A hidden node is one that is within the

range of the intended destination but out of range of the sender

• Exposed nodes. An exposed node is one that is within the range of the sender but out of range of the destination

• Capture. This event occurs when a receiver can correctly receive a transmission from one of two (or more) simultaneous transmissions, all within its range, because the signal strength of the correctly received signal is much higher than strength of the other signals

5

Hidden and Exposed Node Problems

• A is transmitting to B

• C is out of range of A and is unaware of the transmission

• If C transmits to B it will cause a collision at B

A B C

A B C D

• B is transmitting to A

• C wants to transmit to D

• C senses transmission & declines even if its transmission will not cause any collision at A

Hidden node

Exposed node

6

Capture Problem

A B

C

D

dAB

dCB

If A and C transmitsimultaneously to B then the signal power of C, received at B, is higher than the one from A (because dCB < dAB) and there is a good probability that C’s signal can be correctly decoded in the presence of A’s transmission

This capture of C’s signal can improve protocol performance, but it results in unfair sharing of the channel with preference given to nodes closer to the receiver. Wireless MAC protocols need to ensure fairness under such conditions

7

Classification of MAC Protocols

Wireless MAC protocols can be classified into two categories, distributed and centralized, according of the type of network architecture for which they have been designed. Protocols can be further classified, based on the mode of operation, into random access protocols, guaranteed access protocols, and hybrid access protocols

Wireless MAC protocols

DistributedMAC protocols

CentralizedMAC protocols

Randomaccess

Randomaccess

Guaranteedaccess

Hybridaccess

Since we are interested in Ad Hoc networks we will focus our discussions on distributed type protocols

8

Performance Metrics

Delay – Average time spent by a packet in the MAC queue. It is a function of protocol and traffic characteristics

Throughput – Fraction of the channel capacity used for data transmission

Fairness – A fair MAC protocol does not exhibit preference to any single node when multiple nodes are trying to access the channel

Stability – A stable system can handle instantaneous loads that are greater than the maximum sustained load

Robustness against Channel Fading

Power Consumption – Most wireless devices have limited battery power, it is then important to design MAC protocols that have power saving features

Support for multimedia – Protocols need to treat packets from various applications based on their delay constraints. Common methods are the use of access priorities and scheduling

9

Distributed MAC Protocols

These protocols are based on carrier sensing and collision avoidance

Carrier sensing refers to listening to the physical medium to detect anyongoing transmission

Collision avoidance techniques must be implemented to minimize the probability of collisions. Two mechanisms can be used:

Collision Avoidance with Out-of-Band Signaling such as the one used in the BTMA (busy tone multiple access) protocol → F.A. Tobagi and L. Klienrock, “Packet Switching in Radio Channels: Part II – The Hidden Terminal Problem in Carrier Sense Multiple Access and the Busy Tone Solution,” IEEE Trans. Commun., COM-23, 1975, pp. 1417-1433

Collision Avoidance with Control Handshaking such as one used in MACA (multiple access with collision avoidance) protocols → P. Karn, “MACA – A New Channel Access Method for Packet Radio.” ARRL/CRRL Amateur Radio 9th Computer Networking Conf., 1990

10

Distributed Foundation Wireless MAC

It is a derivative of the MACA protocol

It is the basic access protocol of the IEEE 802.11 standard

Consists of a four-way exchange, RTS-CTS-DATA-ACK

Sender node

Receiver node

Others

RTS

CTS

DIFS

DATASIFS

SIFS

ACKSIFS

NAV(RTS)NAV(CTS)

NAV- Network Allocation Vector

NAV(DATA)

11

Distributed Foundation Wireless MAC

DIFS (distributed inter frame space) → a node must sense channel idle for DIFS interval before making an RTS attempt

SIFS (short inter frame space) → a node has to sense channel idle for SIFS before sending an ACK.

SIFS < DIFS NAV (Network Allocation Vector): Packets contain

time field (NAV) to indicate duration of current transmission.

All nodes that hear RTS/CTS back off NAV amount (time) before sensing the channel.

To give preference to a stations trying to send and ACK different waiting intervals are specified:

12

Performance Analysis of 802.11 DCF

Assumptions Fixed number of stations, each always having a packet available for

transmission (saturation condition) Each packet needs to wait for random backoff time before transmitting Constant & independent collision probability for each transmitted packet Ideal channel condition (no hidden terminals and capture)

Notation b(t) = stochastic process representing the backoff time counter CWmin = minimum contention window = W

m = maximum backoff stage → CWmax = 2mW

Wi = 2iW, where i (0,m) is called “backoff stage” s(t) = stochastic process representing the backoff stage at time t p = probability of each transmitted packet being collided

13

802.11 DCF Example

ACK

RTS

Busy channel NAV(RTS)

DATA

CTS

RTSStation A

Station B

DIFS

DIFS

BO=3

BO=7

SIFS SIFS

ACK

DIFS

BO=4

DIFS

BO=4

RTS

collision

BO=7

BO=2 RTS

Busy channel

BO=4

BO=5

Others

Busy channel NAV(CTS)

t0 …...

{s(t0),b(t0)} = {0,3}

t1 t2 t3 t4 t8t9

{s(t9),b(t9)} = {1,7}

Station A has a packet to transmit to station B. At the end of the ACK station A waits for a DIFS and then chooses a backoff time equal to 3 before sending an RTS to station B. Another station also waits for a DIFS and chooses a backoff time equal to 7. At t3 station A initiates the RTS/CTS transaction with station B. The station that had a backoff time of 7 “freezes” its timer until the channel is idle again. After B’s ACK station A has another packet to transmit and this time it chooses a backoff time of 4 which brings it to a collision condition with the station that had resumed its counter. Because of the collision station A increments its contention window size and chooses a backoff time of 7.

14

802.11 DCF - Markov Chain Model

A bi-dimensional process model {s(t),b(t)} can be used (*) which leads to a discrete-time Markov chain model with the following transition probabilities:

P{i,k | i, k+1} = 1; k (0, Wi – 2), i (0,m)

1

This equations accounts for the fact that, at the beginning of each slot time, the backoff time is decremented as long as the counter has not reached zero and that the channel has been sensed idle for a mini-slot

(*) G. Bianchi, “Performance Analysis of the IEEE 802.11 Distributed Coordinated Function,” IEEE Journal on Selected Areas in Communications, vol. 18, no. 3, March 2003, pp. 535-547

i,k+1 i,k

15


P{0,k | i, 0} = (1-p)/ W0 ; k (0, W0 – 1), i (0,m)

This equations accounts for the fact that a new packet following a successful packet transmission starts with backoff stage 0, and that the backoff is initially uniformly chosen in the range (0, W0 – 1)

i,0

0,0 0,1 0,W0 – 2 0, W0 – 1. . .

(1-p)/ W0

TxSuccess

16


P{i,k | i-1, 0} = p/ W1 ; k (0, Wi – 1), i (1,m)

When an unsuccessful transmission occurs at backoff stage i-1, the backoff stage increases, and the new initial backoff value is uniformly chosen in the range (0, Wi)

i-1,0

i,0 i,1 i,Wi – 2 i, Wi – 1. . .

p/ Wi

TxCollision

17


P{m,k | m, 0} = p/ Wm ; k (0, Wm – 1)

This equation models the fact that once the backoff stage reaches the value m, it is not increased in subsequent packet transmissions

m,0

m,1 m,2 m,Wm – 2 m, Wm – 1. . .

p/ Wm

(1-p)/ W0

0,0 0,1 0,W0 – 2 0, W0 – 1. . .

18

Steady State Probabilities

The stationary transition probability distribution π(i,k) is then

)1,0(),,0()0,(),(

)0,0(1

)0,(

0)0,0()0,(

ii

i

m

i

WkmiiW

kWki

p

pm

mipi

π(0,0) is determined by using the normalization condition

m

i

W

k

i

ki0

1

0

1),(

from which

))2(1()1)(21(

)1)(21(2)0,0(

mppWWp

pp

19

Steady State Probabilities

The probability that a station transmits in a randomly chosen slot time can be derived since any transmission occurs when the backoff time counter is zero

))2(1()1)(21(

)21(2)0,(

0m

m

i ppWWp

pi

p, the probability that a transmitted packet encounters a collision, is the probability that in a time slot at least one of the n-1 remaining stations transmits. At steady state, each remaining station transmits a packet with probability τ, this results in

1)1(1 np

From this two equations the values for p and τ can be found using numerical techniques

20

Throughput

Let Ptr be the probability that there is at least one transmission in the considered slot time, i.e.,

ntrP )1(1

The probability Ps that a transmission on the channel is successful is given by the probability that exactly one station transmits on the channel, conditioned on the fact that at least one station transmits, i.e.,

n

n

tr

n

s

n

P

nP

)1(1

)1()1( 11

The normalized throughput S, defined as the fraction of the time the channel is used to successfully transmit payload bits is then

]slot time a oflength [

]slot time ain tedn transmitinformatio payload[

E

ES

21

Throughput

If E[P] is the average packet payload then S can be expressed as

cstrsstrtr

trs

TPPTPPP

PEPPS

)1()1(

][

where

collision a duringstation each by busy sensed is channel the timeaverage

ion transmisssuccessful a of becausebusy sensed is channel the timeaverage

slot timeempty an ofduration

c

s

T

T

For the basic access mechanism (no RTS/CTS transaction) the packet header is

hdrhdr MACPHY H

And let δ be the propagation delay

22

Throughput – Basic Access

PHYhdr MAChdr PAYLOAD ACKSIFS DIFS

Tsuccess (basic access)

PHYhdr MAChdr PAYLOAD DIFS

Tcollision (basic access)

DIFSACKSIFS][PEHT basics

DIFS][ *PEHT basicc

E[P*] is the average length of the longest packet payload involved in a collision

1

2 0*

)1()1(1

))(1()1(

][

max

nn

n

k

P kknk

n

dxxFk

n

PE

Where F(.) is the probability distribution function for the packet payload size

23

Throughput – RTS/CTS

PHYhdr MAChdr PAYLOAD ACKSIFS DIFS

Tcollision (RTS/CTS)

CTSSIFS SIFSRTS

DIFSRTS

Tsuccess (RTS/CTS)

DIFSACKSIFS][

SIFSCTSSIFS RTS/

PEH

T ctsrtss

DIFS RTS/ ctsrtscT

24

Model Validation

The model is validated by comparing its results with those obtained via simulation. The parameters used for the validation study are summarized in the following table. A PHY layer using a FHSS modulation system is assumed.

Packet payload 8184 bits

MAC header 272 bits

PHY header 128 bits

ACK 112 bits + PHY header

RTS 160 bits + PHY header

CTS 112 bits + PHY header

Channel Bit Rate 1Mbit/s

Propagation Delay 1μs

Slot Time 50 μs

SIFS 28 μs

DIFS 128 μs

ACK-Timeout 300 μs

CTS-Timeout 300 μs

25

Model Validation- Simulation vs. Analysis

Analytic results are represented by lines, simulation results by symbols

26

Elimination Yield – Non-Preemptive Multiple Access (EY-NPMA)

EY-NPMA is the channel access protocol used in the HYPERLAN system. HIPERLAN is a high-speed (24 Mb/s) wireless LAN standard. The protocol works as follow:

A node that has data to transmit senses the channel

If the channel is free for at least 1,700 bits then it is considered free and the node can start transmitting

Each data frame must be explicitly acknowledge with an ACK packet

If the channel is sensed busy the node synchronizes itself with the end of the current transmission interval

At that point a channel access cycle begins according to the EY-NPMA scheme

This scheme has 3 phases: prioritization, contention, and transmission

27

Channel Access (EY-NPMA)

Prioritizationphase

Contention phase Transmission phase

Elimination phase (B) Yield phase (D)

ACK

Data Frame

CycleSynchronization

Interval

PriorityAssertion

PriorityDetection

SurvivalVerification

Interval

In the prioritization phase only the nodes with the highest channel access priority frame are allowed to continue to the next phase

The contention phase has 2 sub-phases: elimination, and yield

In the elimination phase each node transmits for a random number of slots after which it listen to the channel. If the channel is busy the node loses contention and waits for the next cycle. If the channel is idle the node moves to the yield phase

In the yield phase the node listens to the channel for a random number of slots. If no transmission is detected during this time, the node starts and completes its data transmission

28

Channel Access (EY-NPMA)For the prioritization phase each frame is assigned a priority level h (0, H-1). Each node that has a frame with priority h senses the channel for the first h prioritization slots. If the channel is idle during this interval, then the node transmits a burst in the h + 1 slot and it is admitted to the contention phase, otherwise it stops contending and waits for the next channel access cycle.

The elimination phase consists of at most n slots followed by an elimination survival verification slot. Starting from the first elimination slot, each node transmits a burst for a number B, 0≤ B ≤ n, of subsequent elimination slots.

nbq

nbqqbB

n

b 0)1(}{Pr

The yield phase consists of at most m yield slots. Each node listens to the channel for a number D, 0≤ D ≤ m, of yield slots before beginning a transmission.

B has a truncated geometric probability distribution function, i.e.,

D has also a truncated geometric probability distribution function, i.e.,

mbp

mdppdD

m

d 0)1(}{Pr

29

Performance Analysis (EY-NPMA)

Analysis of the HIPERLAN MAC protocol via a mathematical model is very difficult because of the complexity and level of interdependence of the various processes that are involved. Therefore the analysis is carried out using simulation models. For the study carried by Anastasi et. al. (*) the simulation model compared the performance of the protocol with different arrival processes. The nodes have infinite buffers and they contribute with the same offered load.Poisson and ON/OFF processes are used. The ON/OFF period lengths are Weibull distributed with α = 1 (Markov Modulated Markov Process) and α < 1. Studies have shown that experimental data related to WWW applications can be satisfactorily modeled by ON/OFF processes where the ON and OFF time-lengths distributions are heavy tailed, e.g. Weibull, Pareto. The Weibull probability distribution function of a random variable X is:

0,1}{ )/( xexXP x

(*) G. Anastasi, L. Lenzini, and E. Mingozzi, “Stability and Performance Analysis of HIPERLAN,” Proceedings of IEEE INFOCOM’98, pp. 134-141.

30

Simulation Analysis (EY-NPMA)

31

Multi-Channel Assignment Approach

1

3

2

4

5

6 7

8

Proposed for CDMA Ad-Hoc Networks

Nodes use different channels (codes) to transmit data

The codes must be “locally” unique but they can be reused

The code or channel can be used as an implicit address for the node

Similar approach is used in cellular networks but there the topology is static

32

Channel Assignment in Ad Hoc Networks

Let G(V,E) be the graph representation of the network and Δ the maximum degree of the graph. The problem of channel assignment becomes then one of graph coloring

For any node: All its neighbors have different colors (ROCA), or All two-hop neighbors have different colors (TOCA)

ROCA = Receiver-Oriented-Code-Assignment TOCA = Transmitter-Oriented-Code-Assignment

},1)1(min{needed colors ofNumber VBrooks and Vizing theorem:

33

Channel Assignment in Ad Hoc Networks

In Ad-Hoc networks:

There is no base station

Nodes are free to connect or disconnect

Nodes move about

Increase or decrease their transmission range

Should minimize or avoid the primary and secondary conflicts

These features call for distributed, dynamic, code assignment algorithms

Two transmissions with the same code arrive at a node simultaneously

A transmission arrives at a node that is receiving a signal with a different code

34

Minimal Recoding Strategies A change in the code assignment (recoding) is needed to eliminate

potential collisions caused by nodes joining the network, moving, or changing the power of their transmissions

The recoding strategy must be distributed and local

The number of events required for recoding should be minimized

The maximum number of codes needed for a proper recoding should be also minimized

Used TOCA method since it has better performance1

Assign a code ci to each node such that,

CA1 - For every edge (vi,vj) E, ci ≠ cj (primary collision avoidance)

CA2 - For every pair of edges (vi,vk), (vj,vk) E & i ≠ j, ci ≠ cj (secondary collision avoidance)

Graph coloring problems of this nature are NP-complete(1) L. Hu, “Distributed Code Assignment for CDMA Packet Radio Networks,” IEEE Trans.

on Networking, vol. 1, no. 6, December 1993, pp. 668-677

35

Recoding on Node Join

Consider the event where a new node n joins an Ad Hoc network

n

Node n joins

Set 4n

Set 1n Set 2n

Set 3n

……

…… ……

……

Nodes in 3n need not change their color since n will be assigned a new color different from any color in 3n. To minimize the amount of recoding there is only to recode the nodes in the set 1n U 2n U {n}

36

Recoding on Node JoinIf the set of old colors in 1n U 2n is {C1, C2,…, Cm} and the number of nodes in 1n U 2n

with these corresponding colors is {K1, K2,…, Km}, then apart from recoding n, at least

nn

m

ii

m

ii mKK 21in nodes theof )1(

11

need to be recoded with different new colors to avoid conflicts into the new code assignment after node n joins the network. To find out which nodes to chose for recoding one can use the following approach(1).Consider the undirected graph G’= (V1 U V2, E’) where,V1 = 1n U 2n U {n} V2 = {i : i Z+ & i ≤ max = maximum color constraint in the vicinity of 1n U 2n U {n} }

E = {(u,v) : u V1, v V2, u is not constrained to be newly colored with v}“maximum color constraint in the vicinity of 1n U 2n U {n}” means the maximum integer among all constraints due to 3n U 4n on nodes 1n U 2n U {n}, e.g. the set of old colors in 3n U 4n forbidden to be assigned to nodes in 1n U 2n U {n} due to the

constraints CA1 and CA2, and all old colors in 1n U 2n . The edges in E are weighted;

edges (u,v), u 1n U 2n , v V2 where v is the old color assigned to node u are given a weight of 3; all other edges have weight 1.(1) I. Gupta, “Minimal CDMA Recoding Strategies in Power Controlled Ad-Hoc Wireless Networks,” Technical Report,

Dept. of Computer Science, Cornell University, 2001.

37

Recoding on Node Join (Example)

D

C

A B

G

E

F

H

1n = {G}, 2n = {A,B,F,C}, 3n = { }, 4n = {D,F}. The original color assignment, before node H joins the network, is indicated next to each node (in the square box).Next G’ is built as previously defined. Note that G’ is a bipartite graph.

11

2

2

3

3

3

1n U 2n

4n

38


A

B

C

G

F

H

1

3

21

3

1

1

1

1

1

3

3

3

1

1

3

Consider now the maximum matching MEM

on this bipartite graph. M is a set of edges with no common end-vertices and with the maximum total weight among all such sets. There are many efficient algorithms to find a maximum matching of a weighted bipartite graph(1). Once M is found assign a node u in V1 to the color in V2 that M matches into, and for all the remaining nodes in V1 not matched by M, assign them consecutive colors one by one starting from | V2 | + 1 onwards.

(1) Z. Galil, “Efficient Algorithms for Finding Maximum Matching in Graphs,” ACM Computing Surveys, vol. 18, March 1998, pp. 23-28.

39


A

B

C

G

F

H

1

3

23

3

3

The maximum matching M for this example is then {(A,2), (B,3), (C,1)}. The color (code) assignment for the unmatched nodes is (F,4), (G,5) and (H,6). This recoding strategy is local since the task of recoding the nodes in 1n U 2n U {n} is locally centralized at node n, and uses only local information.

Claims(1)

Minimal Recoding: For the event where n joins the network this algorithm recodes the minimum number of nodes

Optimal among Minimal: Among all minimal strategies that recode nodes only one hop away from the event, e.g. nodes in 1n U 2n U {n}, this strategy re-assigns the least maximum color to any node

(1) I. Gupta, “Minimal CDMA Recoding Strategies in Power Controlled Ad-Hoc Wireless Networks,” Technical Report, Dept. of Computer Science, Cornell University, 2001.

40

Recoding on Node Join (Pseudo Code)

1. Obtain the constraints (u, oldcolor (v)) of the from-neighbors u of n,

2. Obtain the constraints (n, oldcolor (v)) for n,

3. Let max = maximum color seen in these constraints on old colors in 1n U 2n

4. Let V1 = 1n U 2n U {n}, V2 = (1,2,…,max}

Build the bipartite graph G’ by joining edges from each vertex v in V1 to each color k in V2 that it can be assigned without conflicting with the constraints with any of the nodes not in 1n U 2n U {n}

Assign a weight of 3 to the edge if it connects to the old color assigned to v, otherwise assign it a weight of 1

5. Run the bipartite matching algorithm on G’

6. For each edge v in V1 that is matched to some edge k in V2 assign k as the new color. For all the remaining m vertexes in V1 not assigned a color assigned them the colors max + 1, …, max + m

7. Transmit this information to all concerned nodes and coordinate on when to change colors

}{21,21 nvu nnnn

nnv 21

41

Recoding on Move

D

C

A B

GE

F

1

1

2

2

3

3

3

1n U 2n

B

1n = { }, 2n = {E,F,G}, 3n = { }, 4n = {A,D,C}. The original color assignment, before node B moves, is indicated next to each node (in the square box).

The event when a node n moves can be handled in almost the same way as when a node joins the network, e.g. for the following example,

4n

42

B

E

G

F

1

3

2

1

3

1

1

1

3

3

1

3

The bipartite graph G’ is

Recoding on Move (Example)

B

E

G

F

1

3

23

3

3

The maximum matching M is

The final color (code) is (A,2), (B,4), (C,1), (D,3), (E,3), (F,1), and (G,2)

43

Recoding on Node Move (Pseudo Code)

1. Obtain the constraints (u, oldcolor (v)) of the from-neighbors u of n,

2. Obtain the constraints (n, oldcolor (v)) for n,

3. Let max = maximum color seen in these constraints on old colors in 1n U 2n

4. Let V1 = 1n U 2n U {n}, V2 = (1,2,…,max}

Build the bipartite graph G’ by joining edges from each vertex v in V1 to each color k in V2 that it can be assigned without conflicting with the constraints with any of the nodes not in 1n U 2n U {n}

Assign a weight of 3 to the edge if it connects to the old color assigned to v, otherwise assign it a weight of 1

5. Run the bipartite matching algorithm on G’

6. For each edge v in V1 that is matched to some edge k in V2 assign k as the new color. For all the remaining m vertexes in V1 not assigned a color assigned them the colors max + 1, …, max + m

7. Transmit this information to all concerned nodes and coordinate on when to change colors

}{21,21 nvu nnnn

nnv 21

0. Define 1n, 2n, 3n, and 4n, for the node n in its new position

channel access protocols

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