Shared Access Networks

Acknowledgement: this lecture is partially based on the slides of Dr. Larry Peterson

Direct link networks:� point-to-point links� shared access networks

Problem: physically connecting hosts

Hongwei Zhang

http://www.cs.wayne.edu/~hzhang

Key issue?

Efficient, fair access control

Outline

� Bus (Ethernet)

� Token ring (FDDI)

� Wireless (802.11)

� Discussion

Outline

� Bus (Ethernet)

� Token ring (FDDI)

� Wireless (802.11)

� Discussion

Ethernet Overview

� History� developed by Xerox PARC in mid-1970s� roots in Aloha packet-radio network� standardized by Xerox, DEC, and Intel in 1978� similar to IEEE 802.3 standard

� Common topologiesTransceiver

Ethernet cable

Adaptor

Host

Repeater

Host

■■■■■■■■■■■■ Hub Hub

Ethernet (contd.)

� Frame Format

� Addresses

� unique, 48-bit unicast address assigned to each adapter

� example: 8:0:e4:b1:2

� broadcast: all 1s

� multicast: first bit is 1

Destaddr

64 48 32

CRCPreamble Srcaddr

Type Body

1648

Ethernet (contd.)

� Bandwidth: 10Mbps, 100Mbps, 1Gbps

� Length: 2500m (500m segments with 4 repeaters)

� CSMA/CD� carrier sense� multiple access� collision detection

� Challenge: how to design distributed algorithm for efficient, fair

channel access?

Transmit Algorithm

� If line is idle…

� send immediately

� upper bound message size of 1500 bytes

� must wait 9.6us between back-to-back frames � To allow for receiver to be ready for the next reception

� 96 bits (minimum # of bits transmitted in the presence of collision) time for 10Mbps Ethernet

� If line is busy…

� wait until idle and transmit immediately

� called 1-persistent (special case of p-persistent)

Algorithm (contd.)

� If collision…

� jam for 32 bits, then stop transmitting frame

� Transmitter will minimally transmits 96 bits in the presence of collision: 32 bit

jamming sequence + 64 bits preamble

� delay and try again: exponential backoff

� 1st time: 0 or 51.2us

� 2nd time: 0, 51.2, 102.4, or 153.6 us

� nth time: k x 51.2us, for randomly selected k=0..2n - 1

� give up after several tries (usually 16)

� minimum frame is 64 bytes/512 bits (why?)

� 14 bytes of header + 46 bytes of data + 4 bytes of CRC

Collision: worst case scenario

(a)

(b)

(c)

A B

A B

A B

A B

(d)

� A will not be able to detect the

collision until it hears the

jamming sequence from B.

� To enable effective collision

detection, network physical

length and minimum frame size

are carefully designed to ensure

that the sender can reliably

detect all possible collision

i.e., the time taken to finish transmitting the min-length packet is

no less than 2*d, where d is the max. latency between two nodes

B starts to transmit right at

the moment that the packet

from A arrives at B

Example: minimum frame size (512bits) for Ethernet

Max. 2500 meters long and up to 4 repeaters

between two hosts =>

51.2 microseconds of max. round-trip delay

+

10M bps Ethernet

=> 512 bits

Outline

� Bus (Ethernet)

� Token ring (FDDI)

� Wireless (802.11)

� Discussion

Token Ring Overview

� Examples

� 4Mbps/16Mbps IEEE 802.5 (based on earlier IBM ring)

� 100Mbps Fiber Distributed Data Interface (FDDI)

Token Ring (contd.)

� Idea

� Frames flow in one direction: upstream to downstream

� special bit pattern (token) rotates around ring

� must capture token before transmitting

� release token after done transmitting

� remove your frame when it comes back around

� stations get round-robin service

Token release

Token

Fram

eToken Frame

(a) (b)

Delayed release: originally used

Early release: allows better BW utilization

Token Ring: construct

� 802.5 (IBM token ring)

Host

From previoushost

To nexthost

Relay

(a)

Host

Host Host

From previoushost

To nexthost

Relay

(b)

Relay open – host active Relay closed – host bypassed

Token Ring: construct (contd.)

� FDDI

(a) (b)

Normal operation: second ring is only used back-up Failure of the primary ring

Token release

Token

Fram

eToken Frame

(a) (b)

Delayed release: originally used

Early release: allows better BW utilization

Our discussion here

focuses on FDDI, since 802.5 is pretty much a legacy

Frame format (FDDI)

� “control field”: access control (frame/reservation priority),

higher-layer protocol ID …

� 48-bit MAC address (same as Ethernet)

Control

8 8 8 24

CRCStart offrame

End offrame

Destaddr

Body

4848

Srcaddr

Status

32

Timed Token Algorithm (FDDI)

� Token Holding Time (THT)� upper limit on how long a station can hold the token

� Token Rotation Time (TRT)� how long it takes the token to traverse the ring� TRT <= ActiveNodes x THT + RingLatency

� Target Token Rotation Time (TTRT): application requirments� agreed-upon upper bound on TRT

� TTRT <= ActiveNodes x THT + RingLatency

Algorithm (contd.)

� Each node measures TRT between successive tokens� if measured-TRT >= TTRT: token is late so don’t send� if measured-TRT < TTRT: token is early so OK to send

� Two classes of traffic

� synchronous: can always send; delay sensitive� But total amount of synchronous data that can be sent during one token rotation is bounded from above by TTRT

� asynchronous: can send only if token is early

� Worst case: (2xTTRT+RingLatency) between seeing token� Nodes with asynchronous data consumes one TTRT, and � Nodes with synchronous data consumes another TTRT

� Back-to-back 2xTTRT rotations not possible?

Token Maintenance (FDDI)

� Lost Token� no token when initializing ring� bit error corrupts token pattern� node holding token crashes

� Monitoring for a Valid Token in the ring

� should periodically see valid transmissions, whether frame or

token

� maximum gap = ring latency + max frame < = 2.5ms

� set timer at 2.5ms and send claim frame if it fires

Maintenance (contd.)

� Generating a Token (and agreeing on TTRT)?

� execute when join ring or suspect a failure

� send a claim frame that includes the node’s TTRT bid

� when receive claim frame, update the bid (i.e., choose the lower

TTRT bid) and forward

� if your claim frame makes it all the way around the ring (when

your bid was the lowest), then

� everyone knows TTRT

� you insert new token

Outline

� Bus (Ethernet)

� Token ring (FDDI)

� Wireless (802.11)

� Discussion

Wireless LAN (IEEE 802.11?)

� 802.11 (1997)

� 2.4-2.485 GHz unlicensed radio spectrum

� 1Mbps or 2Mbps

� Physical layer coding: Frequency Hopping Spread Spectrum (FHSS)

or Direct Sequence Spread Spectrum (DSSS)

� 802.11b (1999)

� 2.4-2.485 GHz unlicensed radio spectrum

� up to 11 Mbps: 1, 2, 5.5, 11M depending on coding scheme

� DSSS only (at physical layer)

� all hosts use same chipping code

� widely deployed, using base stations; ad hoc mode/mesh network

at research/prototype stage

Wireless LAN (contd.)

� 802.11a (1999)

� 5-6 GHz range

� up to 54 Mbps

� Orthogonal Frequency Division Multiplexing (OFDM)

� 802.11g (2003)

� 2.4-2.485 GHz range

� up to 54 Mbps

� OFDM

� 802.11n (2009)

� Multi-input multi-output (MIMO) at physical layer

� 2.4 GHz or 5 GHz

� Up to 549 Mbps; ~50meters

Spread Spectrum

� Idea

� spread signal over wider frequency band than required

� originally designed to thwart jamming

� Frequency Hopping

� transmit over random sequence of frequencies

� sender and receiver share…

� pseudorandom number generator

� seed

� 802.11 uses 79 x 1MHz-wide frequency bands

Spread Spectrum (cont)

� Direct Sequence

� For each bit, send XOR of that bit and n random bits (also called chipping sequence)

� Random sequence known to both sender and receiver

� Transmitted values are called n-bit chipping codes

� 802.11 defines an 11-bit chipping sequence

Example 4-bit chipping sequence

Random sequence: 0100101101011001

Data stream: 1010 (NRZ)

XOR of the two: 1011101110101001

0

0

0

1

1

1

Collisions Avoidance

� Similar to Ethernet

� Problem: hidden and exposed nodes

A B C D

� Hidden node problem: A and C

cannot sense and thus are hidden

from each other

� Exposed node problem: C cannot

transmits to D when B is transmitting

to A, even though the transmission

from C to D may not interfere the

transmission from B to A

MACAW� Sender transmits RequestToSend (RTS) frame

� Receiver replies with ClearToSend (CTS) frame

� Neighbors…

� see CTS: keep quiet� see RTS but not CTS: “ok” to transmit

� In 802.11: does not transmit if see RTS

� Receiver sends ACK when receives frame

� neighbors silent until see ACK, or after a timer fires if CTS is lost (and no DATA is sent)

� Collisions: RTS or DATA

� Unlike in Ethernet, collision detection is hard in wireless networks� Becomes known when the sender don’t receive CTS or ACK� Exponential backoff after collision (as in Ethernet)

RTS-CTS based interference control

� RTS: request to send CTS: clear to send

� Used in MACAW, S-MAC (WSN), IEEE 802.11 MAC, etc

A B C D

After receiving CTS from B, C waits until A’s transmission to finish

Q: does MACAW eliminate hidden terminal problem?

RTS-CTS may not work well in multi-hop networks

� RTS-CTS based S-MAC

� Retransmission does not help

much, and may even

decrease reliability and

throughput;

Similar observations when

adjusting contention window.

� One major reason: RTS-CTS

based approach assumes

equal comm. & interference

ranges

Metrics RT= 0 RT= 1 RT= 2

Reliability (%) 51.05 54.74 54.63

Latency (sec) 0.21 0.25 0.26

throughput (pkt/sec) 4.01 4.05 3.63

0 5 10 150

20

40

60

80

100

Time (seconds)

# o

f p

acke

ts r

ece

ive

d RT = 0

RT = 1

RT = 2

A Line in the Sand:

Wireless interference model: a basis for interference-oriented scheduling

� Predicts whether a set of concurrent transmissions may

interfere with one another

� Two commonly-used interference models

� Protocol model

� Physical model

Protocol interference model

� Interference range = K × communication range� RTS-CTS based approach implicitly assumed ratio-1 model

� Strengths� Local, pair-wise interference relation

� Good for distributed protocol design

� Weakness� Approximate model

May well lead to low performance

SR C

),( range ceInterferen , RSDKI RS ×=Exclusionregion

Physical interference model

� Strength� High fidelity: based on communication theory

� Weaknesses� Interference relation is non-local and combinatorial: explicitly depends on all concurrent transmitters

� A transmission is successful if the signal-to-interference-plus-noise ratio (SINR) is above a certain threshold γ

� �� , ��

�� + ∑ � � , ���

≥ �

��

�� ?

Open question

Interference model for predictable interference control:

locality of protocol model + high-fidelity of physical

model?

H. Zhang, X. Che, X. Liu, X. Ju, “Adaptive Instantiation of the Protocol Interference Model in

Wireless Networked Sensing and Control”, ACM Transactions on Sensor Networks, 10(2), 2014

Behavior of ratio-K-based scheduling: optimal instantiation of K

Analytical models of network

throughput and link reliability

� Based on optimal spatial reuse in grid

and Poisson random networks

� Spatial network throughput: T(K, P)

� Other factors P: network traffic load,

link length, wireless signal attenuation

� Link reliability: PDR(K, P)Example: optimal scheduling based on the ratio-2 model in grid networks

Numerical analysis � A wide range of system and environmental settings

� Wireless path loss exponent: {2.1, 2.6, 3, 3.3, 3.6, 3.8, 4, 4.5, 5}

� Traffic load: instant transmission probability of {0.05, 0.1, 0.15, . . . , 1}

� Link length: 60 different lengths, corresponding to different interference-

free link reliability (1%-100%)

� Node distribution density: 5, 10, 15, 20, 30, and 40 neighbors on average

� In total, 75,600 system configurations

� Parameter K of the ratio-K model

� Grid networks: {√2, 2, √5, √8, 3, √10, √13, 4, √18, √20, 5, √26, √29,

√34, 6}

� Random networks: {1, 1.5, 2, 2.5, . . . , 10}

Sensitivity of ratio-K-based scheduling: an example

0 1 2 3 4 5 60

20

40

60

80

100

kLin

k R

elia

bili

ty (

%)

d = 0.01d = 0.04d = 0.07d = 0.1d = 0.2d = 0.3d = 0.4d = 0.7d = 1

0 2 4 60

2000

4000

6000

8000

10000

k

Thro

ughput: (

packet ×

mete

r)/s

ec.

d = 0.01d = 0.04

d = 0.07d = 0.1d = 0.2d = 0.3d = 0.4d = 0.7d = 1

Sensitivity: network/spatial throughput

Ratio-K-based scheduling is highly sensitive to the choice of K and traffic pattern

Sensitivity: link reliability

PDR req. = 80%

Throughput-reliability tradeoff in ratio-K-based scheduling

-5 0 5-100

-50

0

50

100

∆k

Possib

le p

erf

orm

ance g

ain

(%

)

Median PDR gain

Median throughput gain

-5 0 5-100

-50

0

50

100

150

200

∆k

Possib

le p

erf

orm

ance g

ain

(%

)

Median PDR gain

Median throughput gain

PDR req. = 40% PDR req. = 100%

� Highest throughput is usually achieved at a K less than the minimum K for ensuring a certain minimum link reliability;

This is especially the case when link reliability requirement is high, e.g., for mission-critical sensing and control.

� Explained inconsistent observations in literature: only focused on throughput, link reliability is not controlled in their studies.

� Idea: use link reliability requirement as the basis of instantiating the ratio-K model

� Model: given a transmission from node S to node R, a concurrent transmitter C does not interfere with the reception at R iff.

� Suitable for distributed protocol design

� Both signal strength and link reliability are locally measurable

� K can be searched via local, control-theoretic approach

� Signal strength based definition can deal with wireless channel irregularity

Physical-Ratio-K (PRK) interference model

),,(

),(),(

pdrTRSK

RSPRCP ≤

P(S,R)K(Tpdr)

SR

C

Optimality of PRK-based scheduling

10 20 30 40 50 60 70 80 90 95 99

0

5

10

15

20

25

Thro

ughput lo

ss(%

)

PDR requirement(%)

Throughput loss is small,

and it tends to decrease

as the PDR requirement

increases.

Measurement verification

NetEye: a WSN testbed

Irregularity of NetEye

2 3 4 6 7 8 9 1011121314151617181920212223

0

20

40

60

80

100

PD

R (

%)

Link length (feet)-102 -100 -98 -96 -94 -920

5

10

15

20

Background noise power (dBm)

Measurement results

Obj-8 Obj-5 Obj-T0

20

40

60

80

100

PD

R (

%)

PRK

SINR

Obj-8 Obj-5 Obj-T0

0.5

1

1.5

2

2.5

3

Thro

ughput

PRK

SINR

Physical Ratio-K (PRK) interference model

� Enables local protocols (e.g., localized, online search of K)

� Locality implies responsive adaptation (to dynamics in traffic

pattern etc)

� Enables measurement-based (instead of model-based) online

adaptation

� No need for precise PDR-SINR models

Challenges of PRK-based scheduling

� On-the-fly instantiation of the PRK model parameter

� Dynamics and uncertainties in application requirements as

well as network and environmental conditions

� Protocol signaling for identifying mutual interference

relation

� Large interference range

� Anisotropic, asymmetric, and probabilistic wireless

communication

RSTRSK,,,

H. Zhang, X. Liu, C. Li, Y. Chen, X. Che, F. Lin, L.Y. Wang, G. Yin, “PRK-Based Scheduling for Predictable Link

Reliability in Wireless Networked Sensing and Control”, Technical report WSU-CS-DNC-TR-14-01, 2014

SR

C

( )

RSTRSK

RSP

,,,

,

Predictable link reliability in distributed, PRK-based scheduling (PRKS)

� Predictable link PDR through localized PRKS model adaptation

� Concurrency and spatial reuse statistically equal or close to that in state-of-the-art centralized scheduling

70 80 90 95−60

−40

−20

0

20

40

PDR requirement (%)

PR

K p

ara

me

ter

(dB

)

70 80 90 9570

80

90

100

PD

R (

%)

PDR requirement (%)

70 80 90 950

5

10

PDR requirement (%)

Concurr

ency

PRKS iOrder

Temporally-varying PDR requirements:70%→80%→90%→95%→90%→80%→70%

0 200 400 6000

50

100

Time (# of control steps)

PD

R (

%)

PDR

PDR requirement

PDR of a typical link

Comparison with existing protocols: PDR

0 30 60 90 120 1500

0.2

0.4

0.6

0.8

1

PDR (%)

CD

F

PRKSCSMARTS−CTSRIDBCMACSCREAM

PDR requirement = 90%

Current practice (1): Improve reliability by retransmission

� Significantly longer delay in existing

protocols due to retransmission

Current practice (2): Improve reliability by reducing traffic load

� Significantly lower throughput in existing

protocols due to low utilization of channel

capacity

Summary: PRK-based scheduling (PRKS)

� PRKS as a field-deployable approach to the 40+ years old

problem of predictable co-channel interference control

� Control-theoretic approach to PRK model instantiation

� Signal maps as basis of protocol signaling

� Two-timescale approach to TDMA

� Predictable reliability as a basis for controllable reliability-

delay-throughput tradeoff and thus co-design of control and

networking

Supporting Mobility

� Case 1: ad hoc networking

� Case 2: access points (AP)� tethered� each mobile node associates with an AP

B

H

A

F

G

D

AP-2

AP-3AP-1

C E

Distribution system

How APs communicate with one another is not specified in 802.11.

Mobility (contd.)

� Active scanning (selecting an AP): when join or move

� node sends Probe frame

� all AP’s w/in reach reply with ProbeResponse frame

� node selects one AP; sends it AssociateRequest frame

� AP replies with AssociationResponse frame

� new AP informs old AP via tethered network

� Passive scanning: AP periodically sends Beacon frame

Q: mobility example

B

H

A

F

G

D

AP-2

AP-3AP-1

EC

C

Distribution system

What actions will be taken when C moves as shown in the figure?

Other wireless standards/ technologies

� 802.11ac� At 5GHz band, with channel bandwidth of 40/80/160MHz

� Up to 8*8 MIMO, MU-MIMO

� Transfer rate up to 78Mbps to 3.2Gbps

� 802.11ad� At 60GHz band, with channel bandwidth of 2160MHz

� 1*1 SISO

� Transfer rate up to 6.76Gbps

� 802.11e (2005)

� QoS support in both ad-hoc and AP modes

� 802.11s

� Mesh networking

Other wireless standards (contd.)

� 802.11p

� Draft amendment to 802.11 to support ITS (Intelligent Transportation Systems) applications: DSRC (Dedicated Short Range Communication) for vehicle-to-vehicle and vehicle-to-roadside communication

� Run in the licensed ITS band of 5.9 GHz (5.85-5.925 GHz)

� 802.16 (started in 2001)

� WirelessMAN, WiMAX

� Up to 75Mbps

� 802.15.4

� Sensor networks

� 250Kbps

� Etc.

Outline

� Bus (Ethernet)

� Token ring (FDDI)

� Wireless (802.11)

� Discussion

Study on low power wireless link properties

� Interference free

� Jerry Zhao, and Ramesh Govindan, Understanding Packet Delivery

Performance In Dense Wireless Sensor Networks, ACM SenSys’03

� Marco Zuniga, Bhaskar Krishnamachari, Analyzing the Transitional

Region in Low Power Wireless Links, IEEE SECON’ 04

� Interference

� Dongjin Son, Bhaskar Krishnamachari, John Heidemann, Experimental

Analysis of Concurrent Packet Transmissions in Low-Power Wireless

Networks, ACM Sensys’ 06

� Hongwei Zhang, Anish Arora, and Prasun Sinha, Learn on the Fly: Data-

driven Link Estimation and Routing in Sensor Network Backbones, IEEE

INFOCOM’06

Channel access control in wireless networks

� Interference range > communication range

� H. Zhang, X. Che, X. Liu, X. Ju, “Adaptive Instantiation of the Protocol

Interference Model in Wireless Networked Sensing and Control”, ACM

Transactions on Sensor Networks, 10(2), 2014

� Gang Zhou, Tian He, John Stankovic, and Tarek Abdelzaher, RID: Radio

Interference Detection in Wireless Sensor Networks, IEEE INFOCOM’05

� Energy efficiency

� Wei Ye, John Heidemann, and Deborah Estrin, An Energy-Efficient MAC

Protocol for Wireless Sensor Networks, IEEE INFOCOM’02

� Joseph Polastre, Jason Hill, and David Culler, Versatile Low Power Media

Access for Wireless Sensor Networks, ACM SenSys’04

� Hui Cao, Ken Parker, and Anish Arora, O-MAC: A Receiver Centric Power

Management Protocol, IEEE ICNP’06

Further reading

� Ethernet

� R. Metcalf and D. Boggs, Ethernet: Distributed Packet Switching for

Local Computer Networks, Communications of ACM, 19(7):395-403,

July 1976

� D. Boggs, J. Mogul, and C. Kent, Measured Capacity of an Ethernet,

ACM SIGCOMM’88

� Integrating high-speed network adaptors with system software

� P. Druschel, M. Abbot, M. Pagels, and L.L. Peterson, Network

Subsystem Design, IEEE Networks, 7(4):8-17, July 1993

Summary

� Point-to-point links

� Encoding

� Framing

� Error detection

� Reliable transmission

� Shared access networks

� Channel access control

TinyOS: wireless communication

� Example app: BlinkToRadio

� tinyos-2.x\apps\tutorials\BlinkToRadio

Assignment – Chapter 2

� TinyLab#1 (optional)

� Write a TinyOS program that let a sender continuously transmit packets (with increasing packet sequence number) to a receiver; run the program using TOSSIM

� Implement the sliding window algorithm in TinyOS, and test it using TOSSIM

� Exercise#1

� Exercises 1, 5, 26, and 33

� Exercise 42� Hint: 1) the relation between propagation delay, transmission rate, and minimum packet size in CSMA/CD networks; 2) too large a minimum packet size may require padding and thus bandwidth wastage

� TinyExam#1