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Radio and Medium Access Control

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Learning Objectives

• Understand important concepts about radio signals

• Understand radio properties of WSNs

• Understand schedule-based medium access protocols in WSNs

• Understand contention-based medium access protocols in WSNs

• Understand S-MAC, B-MAC, and X-MAC

3

Prerequisites

• Basic concepts of wireless communications

• Basic concepts of medium access control protocols

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Radio Properties

5

Some Basic Concepts

• RSSI

• dbm

• Noise floor: see wikipedia

• CCA thresholding algorithms

• Duty cycle

• LPL

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Signal Transmission

Ref: Fig. 2.9 of “Wireless Communications and Networks” by William Stallings

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Packet Reception and Transmission

• Ref: [Hardware_1] Figure 5

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Signal

• An electromagnetic signal– A function of time– Also a function of frequency

• The signal consists of components of different frequencies

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802.15.4 Physical Layer

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dB• dB (Decibel)

– Express relative differences in signal strength– dB = 10 log10 (p1/p2)– dB = 0: no attenuation. p1 = p2– 1 dB attenuation: 0.79 of the input power survives:

10 * log10(1/0.79)– 3 dB attenuation: 0.5 of the input power survives:

10 * log10(1/0.5)– 10 dB attenuation: 0.1 of the input power survives:

10 * log10(1/0.1)• http://en.wikipedia.org/wiki/Decibel

• http://www.sss-mag.com/db.html

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dBm

• The referenced quantity is one milliwatt(mW)

• dBm = 10 log10 (p1/1mW)

• 0 dBm: p1 is 1 mW

• -80 dBm: p1 is 10-11W = 10pW

• http://en.wikipedia.org/wiki/DBm

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Received Signal Strength Indicator (RSSI)

• The strength of a received RF signal

• Many current platforms provide hardware indicator– CC2420, the radio chip of MicaZ and TelosB,

provides RSSI indicator and LQI (Link Quality Indicator)

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LQI (Link Quality Indicator)

• A measure of chip error rate

• Error rate– The rate at which errors occur– Error

• 0 is transmitted while 1 is received

• 1 is transmitted while 0 is received

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Noise Floor

• The measure of the signal created from the sum of all the noise sources and unwanted signals

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Signal Noise Ratio (SNR)

• The ratio of the power in a signal to the power contained in the noise that is present

• Typically measured at the receiver• Take CC2420 as the example:

– Noise Floor: the RSSI register from the CC2420 chip when not receiving a packet

• For example -98dBm

– The strength field from the received packet: RSSI of the received packet

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Radio Spectrum Frequency Allocation

• http://www.ntia.doc.gov/osmhome/allochrt.pdf

ref: [radio_1]

Radio Irregularity

• Spherical radio range is not valid

• When an electromagnetic signal propagate, the signal may be– Diffracted– Reflected– Scattered

• Radio irregularity and variations in packet loss in different directions

[Radio_1] Figure 1 18

Radio Signal Property

• Anisotropic Signal Strength: Different path losses in different directions

-65-64-63-62-61-60-59-58-57-56-55

0 25 50 75

Beacon SeqNo

South NorthWest East

Figure 1: Signal Strength over Time in Four Directions

[Radio_1]: Figure 3 19

Radio Signal Property

• Anisotropic Packet Loss Ratio: Packet Reception Ratio (PRR) varies in different directions

[Radio_1]: Figure 4 20

Radio Signal Property• Anisotropic Radio Range: The communication

range of a mote is not uniform

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Medium Access Control (MAC)

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Introduction

• A radio channel cannot be accessed simultaneously by two or more nodes that are in a radio interference range– Nodes may transmit at the same time on the same

channel

• Medium Access Control– On top of Physical layer– Control access to the radio channel

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MAC Protocol Requirements • Energy Efficiency

– Sources of energy waste• Collision, Idle Listening, Overhearing, and Control Packet

Overhead

• Effective collision avoidance– When and how the node can access the medium and send

its data

• Efficient channel utilization at low and high data rates– Reflects how well the entire bandwidth of the channel is

utilized in communications

• Tolerant to changing RF/Networking conditions• Scalable to large number of nodes

Ref: [MAC_2] Section I, II

[MAC_3]: Section 4 24

Two Basic Classes of MAC Protocol – Slotted and Sampling

• Slotted Protocols (Synchronous Protocols)– Nodes divides time into slots

– Radio can be in receive mode, transmit mode, or powered off mode

– Communication is synchronized

– Data transfers occur in “slots”

– TDMA, IEEE 802.15.4, S-MAC, T-MAC, etc.

• Also Ref: J. Polastre Dissertation – Section 2.4: http://www.polastre.com/papers/polastre-thesis-final.pdf

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Two Basic Classes of MAC Protocol – Slotted and Sampling

• Sampling Protocols– Nodes periodically wake up, and only start

receiving data if they detect channel activity– Communication is unsynchronized– Data transfer wakes up receiver– Must send long, expensive messages to wake up

neighbors– B-MAC, Preamble sampling, LPL, etc.

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Slotted Protocol Example: 802.15.4

• Each node beacons on its own schedule

• Other nodes synchronize with the received Beacons

Bea

con

Bea

con

Superframe Duration

Beacon Frame Duration

sleep

Dat

a

Dat

a

Ack

CSMA Contention Period

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IEEE 802.15.4 Superframe

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Using 802.15.4

RF Channel

15.4

SPwake forbeacon period

start radiosend beacon

Be

aco

n

beaconTX

are messagespending?

If yes,wake up

send

Da

ta

beaconRX

senddone

TX firstpacket

packetRX

Da

ta

Ack

TXdone

Ackreceived

send donereliability set

packetreceived

stop radiosuperframe complete

Stopradio

SP

15.4

Coordinator

Neighbors

Updateschedule

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Main MAC ProtocolsWireless medium access

Centralized

Distributed

Contention-based

Schedule-based

Fixedassignment

Demandassignment

Contention-based

Schedule-based

Fixedassignment

Demandassignment

[MAC_2]: Figure 1 30

Scheduled Protocols

• TDMA divides the channel into N time slots

[MAC_2]: Section IV 31

Contention-based Protocols

• A common channel is shared by all nodes and it is allocated on-demand

• A contention mechanism is employed• Advantages over scheduled protocols

– Scale more easily– More flexible as topologies change– No requirement to form communication clusters– Do not require fine-grained time synchronization

• Disadvantage– Inefficient usage of energy

• Node listen at all times• Collisions and contention for the media

[MAC_2]: Section IV 32

CSMA

• Listening before transmitting

• Listening (Carrier Sense)– To detect if the medium is busy

• Hidden Terminal Problem

[MAC_2]: Section IV 33

Hidden Terminal Problem

• Node A and C cannot hear each other

• Transmission by node A and C can collide at node B

• On collision, both transmissions are lost

• Node A and C are hidden from each other

BA C

[MAC_2]: Section IV 34

CSMA-CA

• CA– Collision Avoidance: to address the hidden

terminal problem

• Basic mechanism– Establish a brief handshake between a sender and a

receiver before transmission– The transmission between a sender and a receiver

follows RTS-CTS-DATA-ACK

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Centralized Medium Access• Idea: Have a central station control when a node may access the

medium– Example: Polling, centralized computation of TDMA schedules– Advantage: Simple, quite efficient (e.g., no collisions), burdens

the central station• Not directly feasible for non-trivial wireless network sizes• But: Can be quite useful when network is somehow divided into

smaller groups– Clusters, in each cluster medium access can be controlled

centrally – compare Bluetooth piconets, for example

! Usually, distributed medium access is considered

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Schedule- vs. Contention-based MACs

• Schedule-based MAC – A schedule exists, regulating which participant may use

which resource at which time (TDMA component) – Typical resource: frequency band in a given physical

space (with a given code, CDMA)– Schedule can be fixed or computed on demand

• Usually: mixed – difference fixed/on demand is one of time scales

– Usually, collisions, overhearing, idle listening no issues – Needed: time synchronization!

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Schedule- vs. Contention-based MACs

• Contention-based protocols– Risk of colliding packets is deliberately taken – Hope: coordination overhead can be saved,

resulting in overall improved efficiency– Mechanisms to handle/reduce probability/impact

of collisions required

– Usually, randomization used somehow

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Possible Solutions

• CSMA (Carrier Sense Multiple Access)– Advantage:

• No clock synchronization required

• No global topology information required

– Disadvantage• Hidden terminal problem: serious throughput

degradation

• RTS/CTS can alleviate hidden terminal problem, but incur high overhead

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Possible Solutions

• TDMA (Time-division multiple access)– Advantage

• Solve the hidden terminal problem without extra message overhead

– Disadvantage• It is challenging to find an efficient time schedule• Need clock synchronization

– High energy overhead

• Handling dynamic topology change is expensive• Given low contention, TDMA gives much lower

channel utilization and higher delay

40# of Contenders

Channel Utilization(The fraction of time that the channel is transmitting data)

TDMACSMA

IDEAL

Effective Throughput CSMA vs. TDMA

Sensitive to Time synch. errors,

Topology changes,

Slot assignment errors.

Do not use any topology or time synch. Info.

Thus, more robust to time synch. errors and changes.

[MAC_2]: Section II.B 41

Four important sources of wasted energy in WSN:

– Idle Listening (required for all CSMA protocols)– Overhearing (since RF is a broadcast medium)– Collisions (Hidden Terminal Problem)– Control Overhead (e.g. RTS/CTS or

DATA/ACK)

MAC Energy Usage

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S-MAC

• During sleep, the node turns off its radio, and sets a timer to awake itself

[S-MAC]: Figure 2

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S-MAC

• Requires periodic synchronization among neighboring node– Negotiate a schedule– Prefer that neighboring nodes listen at the same

time and go to sleep at the same time– Use SYNC message

[S-MAC]

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S-MAC

• All senders perform CS (Carrier Sense) before initiating a transmission

– Broadcast packets are sent without using RTS/CTS

– Unicast packet follow RTS/CTS/DATA/ACK

• To avoid collision

[S-MAC]

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S-MAC – Overhearing Avoidance

• To avoid overhearing: let interfering nodes go to sleep after they hear an RTS or CTS packet

[S-MAC]

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S-MAC – Adaptive Listening

• To improve latency caused by the periodic sleep of each node in the multi-hop network

• Let each node who overhears its neighbor’s transmissions (RTS and CTS) wake up a short period of time at the end of transmission

[S-MAC]

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S-MAC with Adaptive Listening

Ref: Figure 1 of [DW-MAC]

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B-MAC• A set of primitives that other

protocols may use as building block• Provide basic CSMA access• Optional link level ACK, no link

level RTS/CTS• CSMA backoffs configurable by

higher layers• Carrier Sensing using Clear Channel

Assess (CCA)• Sleep/Wake scheduling using Low

Power Listening (LPL)• Ref: Section 1, 3 of ref. [MAC_1]

• LPL: See Section 2.1 of ref. [Energy_1]

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B-MAC

• Does not solve hidden terminal problem• Duty cycles the radio through periodic channel

sampling – Low Power Listening (LPL)

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B-MAC Clear Channel Assessment

- Before transmission – take a sample of the channel

- If the sample is below the current noise floor, channel is clear, send immediately.- If five samples are taken, and no outlier found => channel busy, take a random backoff- Noise floor updated when channel is known to be clear e.g. just after packet transmission

A packet arrives between 22 and 54ms. The middle graph shows the output of a thresholding CCA algorithm. ( 1: channel clear, 0: channel busy)

• Ref: Section 1, 3 of ref. [MAC_1]

[MAC_1]: Figure 3 51

A Trace of Power Consumption

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B-MAC Low Power Listening

Receive data

Carrier sense

Receiver

Long Preamble Data TxSender

CheckInterval

Similar to ALOHA preamble sampling Wake up every Check-Interval Sample Channel using CCA If no activity, go back to sleep for Check-

Interval Else start receiving packet Preamble > Check-Interval

Goal: minimize idle listening

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Low Power Listening• Purpose

– Energy cost = RX + TX + Listen– Save energy

• How– Duty cycle the radio while ensuring reliable message

delivery– Periodically wake up, sample channel, and sleep

• The duty cycling receiver node performs short and periodic receive checks

• If the channel is checked every 100ms– The preamble must be at least 100 ms long for a node to

wake up, detect activity on the channel, receive the preamble, and then receive the message.

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X-MAC

• Asynchronous duty-cycled MAC protocol

• Provide the following advantages over B-MAC– Avoid overhearing problem: embedding the Target

ID in the Preamble– Reduce excessive preamble: strobed preamble

• Ref: Section 1, 3 of ref. [MAC_4]

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X-MAC and B-MAC

• Ref: Section 1, 3 of ref. [MAC_4]

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802.15.4 Frame Format

• Page 36 of CC2420 Data Sheet

http://www.mail-archive.com/tinyos-help@millennium.berkeley.edu/msg07600.html

TinyOS Implementation of CSMA o CC2420 - CCA

• Hardware– CC2420 has CCA as a pin that can be sampled to

determine if another node is transmitting– See CC2420 Data Sheet – Figure 1 CC2420 Pinout

• Software– CC2420Transmit has the option to send the

message with or without CCA• See CC2420TransmitP.send();

https://www.millennium.berkeley.edu/pipermail/tinyos-help/2008-February/031061.html

TinyOS Implementation of CSMA of CC2420 - Ack

• Hardware Ack– If MDMCTRL0.AUTOACK of CC2420 is enabled

• Software Ack– SACK strobe in CC2420ReceiveP can be used to

set software ack