Ad hoc Sensor Networks

Development of Complex Curricula for Molecular Bionics and Infobionics Programs within a consortial* framework**

Consortium leader

PETER PAZMANY CATHOLIC UNIVERSITY

Consortium members

SEMMELWEIS UNIVERSITY, DIALOG CAMPUS PUBLISHER

The Project has been realised with the support of the European Union and has been co-financed by the European Social Fund ***

**Molekuláris bionika és Infobionika Szakok tananyagának komplex fejlesztése konzorciumi keretben

***A projekt az Európai Unió támogatásával, az Európai Szociális Alap társfinanszírozásával valósul meg.

Ad hoc Sensor Networks

Multiple channel access

Érzékelő mobilhálózatok

Többszörös hozzáférés

Dr. Oláh András

Lecture 5 review

• Signal space representation

• Optimal detection of signal in AWGN (Bayesian decision)

• Probability of error (BER and SER)

• Demodulation and detection for modulation schemes

• BER in fading channel

• Channel equalization

Outline

• The goal of medium access control

• Types of wireless networks

• Duplexing techniques

• Multiple Access

• Random Access

• MAC for Wireless Sensor Networks

Structure of a wireless communications link

Layered Communication Approaches

2. Data Link layer: it handles access to the unerlying channel and defines the data format. It is split into two sublayers:

– Logical Link Control: it manages frames to upper and lower layer (encapsulation, decapsulation) and it enforces errror control (checksum and parity bits).

– Medium Access Control (MAC): it coordinates transmission between users sharing the spectrum. In wireless systems it must address the hidden terminal problem and must exercise power control. Goals: prevent collisions while maximizing throughput and minimizing delay

Physical

OSI reference model Data link

Network Transport Middleware

Application

Recall from Chapter 1

Medium Access Control

• Thus the wireless spectrum (frequency band) is a very precious and limited resource we need to use this resource very efficiently.

• We also want our wireless system to have high user capacity with QoS constraints.

• The algorithms and protocols that enables this sharing by multiple users and controls/coordinates the access to the wireless channel (medium) from different users are called MEDIUM ACCESS, or MEDIA ACCESS or MULTIPLE ACCESS

Design:

• Goal Function: Maximum Utilization (eg.: throughput, user capacity, …)

• Constraints: QoS

(eg.: delay<10ms, BER<10^-5,…)

Medium Access Control (cont’)

• Multiple Access (Channel Partitioning or Coordinated Schemes)

– Techniques: TDMA, FDMA, CDMA, SDMA – Examples: GSM, 3G,…

• Random Access (or Random Schemes)

– Techniques : MACA, MACAW, Aloha, 802.11 MAC,…

– Examples: Wifi, WSN,…

• Polling based schemes

– Access is coordinated by a central node – Examples: Bluetooth, BlueSky,…

• Infrastructured networks

– base stations are the bridges

– a mobile host will communicate with the nearest base station (or access point)

– handoff is taken when a host roams from one base to another

– Medium access control assisted by BS

• Ad hoc networks

– infrastructureless: no fixed base stations

– without the assistance of base stations for communication

– Due to transmission range constraint, two wireless nodes need multi-hop routing for communication

– quickly and unpredictably changing topology

Two types of wireless networks

Two types of wireless networks (cont’)

• Mesh networks serve as access networks that employ multi-hop wireless forwarding by non-mobile nodes to relay traffic to and from the wired Internet. In such an environment, hybrid wireless network technologies and/or hierarchical network organization can be used for ad hoc and infrastructure wireless links

• Wireless Sensor Networks are a special category of ad hoc networks that are used to provide a wireless communication infrastructure among the sensors deployed in a specific application domain. [ see Chapter 9 - 11]

Infrastructured wireless networks Ad hoc wireless networks

Fixed infrastructure-based Infrastructureless

Guaranteed bandwidth Shared radio channel

Seamless connectivity (low call drops during handoffs)

Frequent path breaks due to mobility High cost and time of deployment Quick and cost-effective deployment

Reuse of frequency spectrum (cellular principle) Dynamic frequency reuse based on carrier sense mechanism

Application domains include mainly civilian and commercial sectors

Application domains include battlefields, emergency search and rescue operations

High cost of network maintenance Self-organization and maintenance properties are built into the network

Widely deployed and currently in the third generation of evolution

Several issues are to be addressed for successful commercial deployment even though widespread use exists in defense

Two types of wireless networks (cont’)

Duplexing

• If the communication between two parties is one way, then it is called simplex communication. Simplex communication is achieved by default by using a single wireless channel (frequency band) to transmit from sender to receiver.

• If the communication between two parties is two- way, then it is called duplex communication. Duplex communication achieved by:

– Time Division (TDD) (famous in cellular systems) – Frequency Division (FDD) (famous in cellular systems) – Some other method like a random access method

FDD

• A duplex channel consists of two simplex channels with different carrier frequencies

– Downlink band: carries traffic from base to mobile

– Uplink band: carries traffic from mobile to base

Duplexing(cont’)

TDD

• channel (carrier frequency) is shared in time in a deterministic manner.

– The time is slotted with fixed slot length (sec)

– Some slots are used for downlink channel

– Some slots are used for uplink channel

FDD

• FDD is used in radio systems that can allocate individual radio frequencies for each user.

• For example analog systems: AMPS

• In FDD channels are allocated by a base station.

• A channel for a mobile is allocated dynamically.

• All channels that a base station will use are allocated usually statically.

• More suitable for wide-area cellular networks: GSM, AMPS all use FDD

Duplexing(cont’)

TDD

• Can only be used in digital wireless systems (digital modulation).

• Requires rigid timing and synchronization

• Mostly used in short-range and fixed wireless systems so that propagation delay between base and mobile do not change much with respect to location of the mobile.

• Such as cordless phones…

Multiple Access: narrow- vs. wideband systems

• Narrowband System

– The bandwidth is small compared to the coherence bandwidth of the channel (B<B_coh)

• Wideband System

– The system bandwidth is much larger that the coherence bandwidth of the multipath channel (B>B_coh). A large number of users can access the same channel (frequency band) at the same time.

• Four major multiple access (MA) schemes

– Time Division Multiple Access (TDMA): it could be used in narrowband or wideband system.

– Frequency Division Multiple Access (FDMA): it is usually used in narrowband system.

– Code Division Multiple Access (CDMA): it is used in wideband system.

– Space Division Multiple Access (SDMA): for wireless systems with multiple antennas, it can be combined with all of the other MA methods.

Multiple Access: cellular standards

Cellular System Multiple Access Technique

AMPS (2G) FDMA/FDD

CT2 Cordless Phone FDMA/TDD DECT Cordless Phone FDMA/TDD

GSM (2G) TDMA/FDD

USDC (IS-54 and IS-136) (2G) TDMA/FDD Personal Digital Cellular (2G) TDMA/FDD

US IS-95 (2G) CDMA/FDD

W-CDMA (3G) CDMA/FDD, CDMA/TDD

cdma2000 (3G) CDMA/FDD, CDMA/TDD

Narrowband System

Wideband System

• Individual radio channels are assigned to individual users. Each user is allocated a W frequency band by BS.

• If the channel allocated to a user is idle, then it is not used by someone else: waste of resource.

• Mobile and base can transmit and receive simultaneously (FDD)

• The W bandwidth of FDMA channels are relatively low. Symbol time is usually larger (low data rate) than the delay spread of the multipath channel (implies that inter-symbol interference is low)

• Lower complexity systems than TDMA systems.

Multiple Access: FDMA

Multiple Access: FDMA (cont’)

• Capacity of FDMA systems:

N

= (B-2B

)/W

where B is the total spectrum allocation, B

is the guard band allocated at the edge of the spectrum band, W is the bandwidth of an individual channel.

• Erlang B and Erlang C system assumes:

–infinite population of sources which jointly offer traffic to N_c channel

–call attempts arrive following a Poisson process, so call arrivals are independent.

The average arrival rate is λ.

–message length (holding times) are exponentially distributed (Markovian system).

The average call length is h.

–the total amount of traffic offered in erlangs: L= λh.

–If a user is rejected, his next call attempt is made statistically independent of the previous attempt. (Erlang B)

–If all the channels are busy when a request arrives from a user, the request is queued. An unlimited number of requests may be held in the queue (Erlang C)

• Statictical measures of offered load N

:

– In Erlang B system the probability of call blocking:

– In Erlang C system the probabilty of waiting:

– And the average wait time:

c

c

c BLOCK

0

Pr !

!

N

N k

k

L N

L k

=

=

∑

c

c c

wait 1

c

c 0

Pr

! 1 !

N

N k

N

k

L

L L

L N

N k

−

=

=  

+  − 

 

∑

wait wait c

Pr h

t = N L

−

Multiple Access: FDMA (cont’)

Recall from Queuing theory

Multiple Access: example of Erlang B system

• In an Erlang B system, 40 channels are available. A blocking probabilty of less than 1% is required. What is the traffic that can be serve if there is one operator or three operators?

• (Solution: L

>L

)

• (Note: The number of users increases faster than linearly with the number of

available channels. The difference between actual increase and linear

increase is called the trunking gain. It is preferable to have a large pool of

available channels that serves all users (a single operator). The reasons for

not choosing this approach are political, not technical.)

Multiple Access: TDMA

• The allocated radio spectrum for the system is divided into time slots

– In each slot a user can transmit or receive – A user occupies a cyclically repeating slots.

– A channel is logically defined as a particular time slot that repeats with some period.

• TDMA systems buffer the data, until its turn (time slot) comes to transmit.

• In TDMA/TDD: half of the slots in the frame is used for uplink channels, the other is used for downlink channels.

• In TDMA/FDD: a different carrier frequency is used for uplink or downlink

Multiple Access: TDMA (cont’)

• Preamble contains address and synchronization info to identify base station and mobiles to each other.

• Guard times are used to allow synchronization of the receivers between different slots and frames

– Different mobiles may have different propagation delays to a base station because of different distances.

Multiple Access: TDMA (cont’)

• Each frame contains overhead bits and data bits. Efficiency of frame is defined as the percentage of data (information) bits to the total frame size in bits:

η =(1 – b

/b

) 100%

where b

= T

R is the total number of bits in a frame, T

is the frame duration in sec, b

is the number of overhead bits. (

• TDMA is usually combined with FDMA

– Neighboring cells are allocated and using different carrier frequencies (FDMA).

Inside a cell TDMA can be used. Cells may be re-using the same frequency if they are far from each-other.

– There may be more than one channel allocated and used inside each cell. Each carrier frequency (radio channel) may be using TDMA to further multiplex more user (i.e. having TDMA logical channels inside radio channels). For example, in GSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logical channels). Hence GSM is using FDMA combined with TDMA.

Multiple Access: TDMA (cont’)

GSM (Europa) IS-54 (USA) PDC (Japan) DECT

Bit Rate 270.8 Kbps 48.6 Kbps 42 Kbps 1.152 Mbps

Bandwidth 200 KHz 30 KHz 25 KHz 1.728 MHz

Time Slot 0.577 ms 6.7 ms 6.7 ms 0.417 ms

Upstream slots per frame 8 3 3 12

Duplexing FDD FDD FDD TDD

Efficiency 73 % 80 % 80 % 67 %

Modulation GMSK π/4 DQPSK π/4 DQPSK GMSK

Adaptive equalized Mandatory Mandatory Optional None

Multiple Access: TDMA (cont’)

• Enables the sharing of a single radio channel among N users.

• Requires high data-rate per radio channel to support N users simultaneously, which requires adaptive equalizers to be used in multipath environments.

• Transmission occurs in bursts (not continually). It enables power saving by going to sleep modes in unrelated slots. It discontinues transmission and also enables mobile assisted handoff.

• It equires synchronization of the receivers. Need guard bits and sync bits, which occurs large overhead per slot.

• Allocation of slots to mobile users should not be uniform. It

may depend on the traffic requirement of mobiles. This brings

extra flexibility and efficiency compared to FDMA systems.

Multiple Access: spread spectrum multiple access

• SSMA uses signals that have transmission bandwidth that is several orders of magnitude larger than minimum required RF bandwidth.

• It provides

– Immunity to multipath interference – Robust multiple access.

• Two techniques

– Frequency Hopped Multiple Access (FHMA)

– Direct Sequence Multiple Access (DSMA) or Code Division

Multiple Access (CDMA)

Multiple Access: FHMA

• The carrier frequency of users are varied in a pseudo-random fashion.

– Each user is using a narrowband channel (spectrum) at a specific instance of time.

– The random change in frequency make the change of using the same narrowband channel very low.

• The sender receiver change frequency (calling hopping) using the same pseudo-random sequence, hence they are synchronized.

• Rate of hopping versus Symbol rate

– If hopping rate is greather: Fast Frequency Hopping – If symbol rate is greater: Slow Frequency Hopping

• GSM and Bluetooth are example systems

Multiple Access: FHMA (cont’)

• The narrowband message signal is multiplied by a very large bandwidth signal called spreading signal (code) before modulation and transmission over the air. This is called spreading.

• Spreading signal use a pseudo-noise (PN) sequence (a pseudo-random sequence) called codeword which are orthogonal. (low autocorrelation)

• The receiver correlator distinguishes the senders signal by examining the wideband signal with the same time-synchronized spreading code (despreading).

• Advantages:

– Low power spectral density;

– Interference limited operation;

– Privacy;

– Reduction of multipath affects by using a larger spectrum;

– Users can start their transmission at any time (random access possible);

– Cell capacity is soft and higher than TDMA and FDMA;

• At a receiver, the signals may come from various sources (multiuser). The strongest signal usually captures the modulator. The other signals are considered as noise. Each source may have different distances to the base station.

• In CDMA, we want a BS to receive CDMA coded signals from various mobile users at the same time.

– Therefore the receiver power at the BS for all mobile users should be close to each other. This requires power control at the mobiles.

• Power Control: BS monitors the RSSI (Received Signal Strength Indicator)

values from different mobiles and then sends power change commands to the

mobiles over a forward channel. The mobiles then adjust their transmit

power.

• FDMA/CDMA

– Available wideband spectrum is divided into a number narrowband radio channels. CDMA is employed inside each channel.

• DS/FHMA

– The signals are spread using spreading codes (direct sequence signals are obtained), but these signal are not transmitted over a constant carrier frequency; they are transmitted over a frequency hopping carrier frequency.

• Time Division CDMA (TCDMA)

– Each cell is using a different spreading code (CDMA employed between cells) that is conveyed to the mobiles in its range.

– Inside each cell (inside a CDMA channel), TDMA is employed to multiplex multiple users.

• Time Division Frequency Hopping

– At each time slot, the user is hopped to a new frequency according to a pseudo-random hopping sequence.

– Employed in severe co-interference and multi-path environments.

– Bluetooth and GSM are using this technique.

Multiple Access: comparison TDMA/FDMA/CDMA

Techniques TDMA FDMA CDMA SDMA

Idea

segment sending time into disjoint time-slots

Segment the frequency band into disjoint sub- bands

Spread the spectrum using orthogonal codes

Segment space into sectors

Signal separation

Synchronization in the time

domain

Filtering in the frequency domain

Code Directed antennas

Advantage

Established, flexible

Simple, established, robust

Flexible, less frequency

planning needed, soft handover

Simple, increases capacity

Disadvantage

Quard space, synchronization difficult

Inflexible, scarce source

Complex

receiver, power control

inflexible

The user capacity C is defined as the total number of active users per cell that the system can support while meeting a common BER constrain. For orthogonal multiple access (as FDMA and TDMA):

C = N

,

where N

is the number of channels assigned to any given cell.

The total number of orthogonal channels of bandwidth B

that can be created from a total system bandwidth of B is B / B

. The reuse factor satisfies N = (B / B

)/ N

, this implies:

C = (B / B

)/ N

Recall from Chapter 1

1. Increasing the amount of spectrum used: very expensive 2. More efficient modulation format and coding

3. Better source coding

4. Adaptive modulation and coding 5. Discontinuous transmission

6. Multiuser detection: CDMA systems

7. Reduction of cell radius: effective but very expensive, smaller cells require more handovers

8. Use of sector cells: tripled cells have tripled BS antennas 9. Use of an overlay structure

10. Multiple antennas: diversity, MIMO systems, SDMA

• Distributed operation is required.

• Synchronization is required in TDMA-based systems.

• Hidden terminals are nodes hidden from a sender.

• Exposed terminals are exposed nodes preventing a sender from sending.

• Throughput needs to be maximized.

• Access delay should be minimized.

• Fairness refers to provide an equal share to all competing nodes.

• Real-time traffic support is required for voice, video, and real-time data.

• Resource reservation is required for QoS.

• Ability to measure resource availability handles the resources.

• Capability for power control reduces the energy

Random access in adhoc networks

• All random access techniques are based on packetized data or packet radio, where user date is collected into packets of M bits.

• Collision: if packets from different users overlap in time.

• The transmission time of a packet is τ = M / R, where R [bps] the data rate.

• Analysis of random access techniques assumes that the users generate packets according to memoryless Poisson process at a rate λ [packets per unit time], the probability that the number of packet arrivals in a time period [0,t], denoted by X(t), is equal to integer k is given by

Pr(X(t)=k) = (λt)^k/ k! e^-λt

• The traffic load is defined as L = λ τ. If L > 1 means on avarage more packets arrive in the system over a given time period than can be transmitted in that period, so systems with L>1 are unstable.

• The throughput is defined as the ratio of the average number of packets successfully transmitted in any given time interval divided by the number of attempted transmissions in that interval:

T = L Pr(succesful packet transmission)

• The effective data rate of the system is R·T.

• We will examine the following attributes:

– Channel utilization (Throughput) – Latency

– Collision avoidance (Hidden and exposed terminals problem) – Reliability (ACK)

– Energy efficiency (power control) – Fairness

– Throughput needs to be maximized.

• We will discuss the following techniques:

– Aloha and slotted Aloha

– CSMA Protocols (1-persistent, non-persistent, p-persistent CSMA, CSMA/CD) – MACA

– MACAW

– Energy efficient MAC in wireless sensor networks (eg. SMAC)

Random Access: ALOHA (1970)

• ALOHA was developed for a wireless system at the University of Hawaii (Abramson et al.).

– Multiple remote stations, plus one base station.

– Frame transmissions are made at one frequency from a remote station to the base station; the base station re-broadcasts frames on another frequency. Contention is for access to the base station.

• Frames can be sent at any time to the base station.

– If no acknowledgment (ACK) is received, assume that the frame is lost and retransmit after waiting a random amount of time.

– If more than one station broadcasts on the base station access frequency at the same time – a collision – interference will destroy the frames.

Random Access: ALOHA (cont’)

• The probability that no packets generated during the tima [-τ, τ] is given by (no collision):

Pr(X(t)=0) = (λ2τ)⁰/ 0! e^-2λτ = e^-2L, with corresponding throughput:

T = L e^-2L

• The maximum throughput:

max

1

max max

0 0.5 0.5e 0.18

L L

dT L T

dL

−

=

= → = → = ≈

Random Access: slotted ALOHA (1971)

• Improvement: Time is slotted and a packet can only be transmitted at the beginning of one slot. Thus, it can reduce the collision duration.

• The probability that no packets generated during the time [0, τ] is given by (no collision):

Pr(X(t)=0) = (λτ)⁰/ 0! e^-λτ = e^-L, with corresponding throughput: T = L e^-L

• The maximum throughput:

1

max max

0 1 e 0.37

dT L T

dL

−

=

= → = → = ≈

• Slotted Aloha has double the maximum throughput as pure Aloha, and achieves this maximum at a higher offered load.

The effective data rate is still less than 40% of the raw transmission rate.

• Aloha does not listen to the carrier before transmission, more sophisticated techniques are needed to increase efficiency.

• Further problem: the delay.

–Delay:

–The mean packet delay:

Random Access: throughput

(

) ( )

Pr τ = k = −1 e^{−L k} e^−L

{ }

( )

1

1 e ^{L k} e ^L e^L

k

MPD E τ ^∞ k ^{− −}

=

= =

∑

Random Access: Carrier Sense Multiple Access

• CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission and transmits if channel is idle.

• Detection delay and propagation delay are two important parameters:

– Detection delay: time required to sense the carrier and decide if it is idle or busy – Propagation delay: distance/speed_of_ligth. The time required for bit to travel

from transmitter to the receiver.

Random Access: CSMA variations

1-persistent CSMA: Non-persistent CSMA:

There will always be a collision if two nodes want to retransmit (usually you stop transmission

Random backoff reduces probability of collisions. Waste: idle time if the backoff time is too long

Random Access: CSMA variations (cont’)

p-persistent CSMA: CSMA/ CD (Collision Detection):

A good tradeoff between non- persistent and 1-persistent CSMA

Same with CSMA, however a station also listen to the carrier while transmitting to see if the transmission collides with someone else transmission. It can be used in listen- while-talk capable channels (full duplex). In single radio channels, the transmission need to be interrupted in order to sense the channel.

Random Access: throughput

Random Access: CSMA (cont’)

• How to select probability p?

– Assume that M nodes have a packet to send and the medium is busy, then Mp is the expected number of nodes that will attempt to transmit once the medium becomes idle. If Mp > 1, then a collision is expected to occur. Therefore, the network must make sure that Mp < 1 to avoid collision, where M is the maximum number of nodes that can be active at a time.

• Problems:

1. The mean packet delay increases exponently with increasing offered load.

2. CSMA protocols sense the carrier, but sensing the carrier does not always releases true information about the status of the wireless channel. There are two problems that are unique to wireless channels (different than wireline channels):

– Hidden terminal problem – Exposed terminal problem

Hidden terminal problem

• A sends to B, C cannot receive A

• C wants to send to B, C senses a

“free” medium (CS fails)

• collision at B, A cannot receive the collision (CD fails)

• A is “hidden” for C

Random Access: hidden and exposed terminal problem

Exposed terminal problem

• B sends to A, C wants to send to D

• C has to wait, CS signals a medium in use

• since A is outside the radio range of C waiting is not necessary

• C is “exposed” to B

Random Access: MACA

When a station wants to transmit data

• It sends an RTS (Ready-to-Send) packet to the intended receiver

– The RTS packet contains the length of the data that needs to be transmitted

– Any station other than the intended recipient hearing RTS defers transmission for a time duration equal to the end of the corresponding CTS reception

• The receiver sends back CTS (Clear-to-Send) packet back to sender if it is available to receive.

– The CTS packet contains the length of the data that original sender wants to transmit

– Any station other than the original RTS sender, hearing CTS defers transmission until the data is sent.

• The original sender upon reception of the CTS, starts transmitting.

Hidden terminal problem

Random Access: MACA (cont’)

Exposed terminal problem

Waiting time of node X is much smaller than waiting time of node C.

• C defers transmission upon hearing B’s RTS until B could get CTS from A.

• After that C can start transmission to D.

For that it first sends an RTS.

• C is not longer exposed to the data transmission of B.

Problem: It does not address the collision of RTS packet

Random Access: reliability

• Wireless links are prone to have errors. High packet loss rate detrimental to transport-layer performance. Mechanisms needed to reduce packet loss rate experienced by upper layers.

• When node B receives a data packet from node A, node B sends an Acknowledgement (Ack). This approach adopted in many protocols.

• If node A fails to receive an Ack, it will retransmit the packet.

Random Access: MACAW (CSMA/CA)

• Physical carrier sense, and virtual carrier sense using Network Allocation Vector (NAV)

– NAV is updated based on overheard RTS/CTS/DATA/ACK packets, each of which specified duration of a pending transmission

– Nodes stay silent when carrier sensed (physical/virtual)

• Backoff intervals used to reduce collision probability

– When transmitting a packet, choose a backoff interval in the range [0,w], where w is contention window.

– Count down the backoff interval when channel is idle.

– Count down is suspended if channel becomes busy.

– When backoff interval reaches 0, transmit RTS

Random Access: CSMA/CA (cont’)

• The time spent counting down backoff intervals is a part of MAC overhead

– Choosing a large w leads to large backoff intervals and can result in larger overhead – Choosing a small w leads to a larger number of collisions (when two nodes count down to 0 simultaneously)

– Since the number of nodes attempting to transmit simultaneously may change with time, some mechanism to manage contention is needed

• IEEE 802.11 DCF: contention window w is chosen dynamically depending on collision occurrence (binary exponential backoff):

– When a node fails to receive CTS in response to its RTS, it increases the contention window

– w is doubled (up to an upper bound)

– When a node successfully completes a data transfer, it restores w to w_min

Random Access: fairness

• Simplest definition of fairness: all nodes should receive equal bandwidth

• An example of unfairness:

– Assume that initially, A and B both choose a backoff interval in range [0,31] but their RTSs collide

– Nodes A and B then choose from range [0,63]

– Node A chooses 4 slots and B choose 60 slots

– After A transmits a packet, it next chooses from range [0,31]

– It is possible that A may transmit several packets before B transmits its first packet

Unfairness occurs when one node has backed off much more than some other node

Random Access: fairness (cont’)

MACAW solution for fairness:

• When a node transmits a packet, it appends value w to the packet, all nodes hearing value w use it for their future transmission attempts.

• Since w is an indication of the level of congestion in the vicinity of a specific receiver node, MACAW proposes maintaining w independently for each receiver.

• Using per-receiver w

is particularly useful in multi-hop environments, since

congestion level at different receivers can be very different

Random Access: energy efficiency

• Since many mobile hosts are operated by batteries, MAC protocols which conserve energy are of interest.

• Two approaches to reduce energy consumption

– Power save: turn off wireless interface when desirable – Power control: reduce transmit power

• Power control has some more potential benefits

– Reduced interference (in ad hoc networks): eg. it improves ALOHA efficiency, where user with high power can capture a packet even if there is a collision.

– Increased spatial reuse (in infrastructured wireless networks): eg. it is used

in CDMA to maintain target SIR of voice and data users, or generally it can

be used to maintain target SIR for different user classes (admission control).

Random Access: power control

• When C transmits to D at a high power level, B cannot receive A’s transmission due to interference from C

• If C reduces transmit power, it can still communicate with D

– Reduces energy consumption at node C

– Allows B to receive A’s transmission (spatial reuse)

• A special wireless ad hoc network

– Large number of nodes – Battery powered

– Topology and density change – Nodes for a common task – In-network data processing

• Sensor-net applications

– Sensor-triggered bursty traffic – Can often tolerate some delay

Characteristics of Sensor Network

Scalabilty and self configuration Energy efficiency

Adaptivity

Fairness not important Latency

Adaptivity

Energy-delay tradoff

Wireless sensor node and sensor network

MAC for WSN: attributes

• Attributes:

– Collision avoidance (Basic task)

– Energy efficiency (most primary in WSN) – Scalability and adaptivity

– Channel utilization – Latency

– Throughput – Fairness

• What causes energy waste?

– Collisions

– Control packet overhead

– Overhearing unnecessary traffic

– Long idle time (it consumes 50-100% of the power)

Primary

Secondary

Dominant factor

MAC for WSN

• Contention-based protocols need to work hard in all directions for energy savings:

– Reduce idle listening – support low duty cycle – Better collision avoidance

– Reduce control overhead

– Avoid unnecessary overhearing

Scheduled Protocols Contention Protocols

Collisions No Yes

Energy efficiency Good Bad

Scalability and adaptivity Bad Good

Multi-hop communication Difficult Easy

Time synchronization Strict Loose or not required

MAC for WSN: classification

MAC for WSN: SMAC (2002)

• It has been developed by Ye, Heidemann and Estrin.

• Tradeoffs between (latency, fairness) and energy

• Major components in S-MAC

– Periodic listen and sleep: turn off radio when sleeping, the reduced duty cycle is ~ 10% (eg. 120ms on /1.2s off)

– Collision avoidance: is based on contention similarly to IEEE 802.11 ad hoc mode

– Overhearing avoidance: the node is sleeping when neighbors talk

– Massage passing: long message is fragmented & sent in burst, operation

with extended Tx time, and re-transmit occurs immediately

Summary

• The algorithms and protocols that enable sharing by multiple users and controls the access to the wireless channel from different users are called MAC.

• There are two types of wireless networks: Infrastructured wireless networks and ad hoc wireless networks.

• We also want our wireless system to have high user capacity and we can identify 10 methods for increasing it.

• There are several MACs with no absolute advantage, it should be tailored to the system requirements .

• There are primary concerns for designing a WSN MAC (collision avoidance, energy efficiency, scalability and adaptivity), while the secondary concerns are as follows (channel utilization, latency, throughput, fairness).

• Next lecture: Routing protocols