cis 632 / eec 687 mobile computing•rts must include some information on how long. •for rts, the...

1

CIS 632 / EEC 687Mobile Computing

Wireless PHY & MAC

Chansu Yu

Evolution of 802.11

2

Radio Improvements

IEEE 802.11b

11Mbps 2.4GHz

802.11n

150Mbps 2.4GHz

600Mbps 5GHz

802.11ac

Gigabit

1000Mbps

5GHz

802.11a

54Mbps 5GHz

802.11i

Security

TKIP +AES Encryption

802.11g

54Mbps 2.4GHz

802.11ad

Multi-Gig

2000Mbps

60GHz

20121999 2000 2001 2003 2004 2005 2007 2008 2009 2010 2011

MAC Improvements802.11e

Wi-Fi

Quality of

Service

802.11k

Radio

Resouce

Mgmt

802.11v

Wireless

Network

Mgmt

802.11r

Fast

Roaming

802.11w

Protected

Mgmt

Frames

802.11af

TV

White

Space

802.11n –Draft

150Mbps 2.4GHz

600-Mbps 5GHz

802.11s

Mesh

2

3

IEEE 802.11 Standard

4

802.11 - Physical Layer

3

5

802.11/a/bPhysical layer

6

Application

TCP packet

IP packet

4

7

IP packet

Frame

controlDuration Addr1 Addr2 Payload

2 2 6 6 0-2312

Addr3

6

CRC

4Seq.

control

2

Addr4

6

synchronization SFD signal service HEC Payload (MPDU)

PLCP preamble PLCP header

128 16 8 8 16 variable

length

16

802.11 MAC packet

802.11 PHY packet

8

802.11 DSSS PHY Packet Format



128 16 8 8 16 variable

length

16

Synchronization: synch., gain setting, energy detection, frequency offset compensation

SFD (Start Frame Delimiter): 1111001110100000

Signal: data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK)

Service (future use, 00: 802.11 compliant)

Length: length of the payload

HEC (Header Error Check)

protection of signal, service and length, x16+x12+x5+1

5

802.11 DSSS PHY Packet Format



128 16 8 8 16

length

16

Always 1Mbps with BPSK modulation

Data rate for MPDU:

Up to 11Mbps (802.11b, DSSS) in steps of 100kbps

Up to 2Mbps (802.11, DSSS) in steps of 100kbps

Up to 2Mbps (802.11, FHSS) in steps of 0.5Mbps

Variable (<4KB)

10

802.11 MAC Format (MPDU)

PreambleStart

DelimiterDA SA PadPayload

7 1 6 6 0-1500

length

2

(cf. 802.3)Chksum

0-46 4

Frame


2 2 6 6 0-2312

Addr3

6

CRC

4Seq.

control

2

Addr4

6

Differences- No preamble, start delimiter and length fields.- Four addresses instead of two addresses.- Larger payload.- Frame control, duration, sequence control.

6

Frame


2 2 6 6 0-2312

Addr3

6

CRC

4Seq.

control

2

Addr4

6

A B C

RTS

CTSCTS

• “Listen-and-Talk” protocol

• CTS from B to A is also

heard by C, which then waits

• But, the question is “how long?”

• RTS must include some information on how long.

• For RTS, the value is the time, in microseconds, required to

transmit the pending data or management frame, plus one CTS frame,

plus one ACK frame,

plus three SIFS intervals.

802.11 MAC: DCF with RTS/CTS

Sending unicast packets Station can send RTS with reservation parameter after waiting for DIFS

reservation determines amount of time the data packet needs the medium

If ready to receive, CTS after SIFS by receiver Sender can now send data at once, acknowledgement via ACK Other stations store medium reservations distributed via RTS and CTS

NAV (network allocation vector) is used to solve the hidden terminal problem

t

SIFS

DIFS

data

ACK

other

stations

receiver

senderdata

DIFS

contention

RTS

CTSSIFS SIFS

defer access

NAV (RTS)NAV (CTS)

7

13

DCF: Contention & Backoff Algorithm to Reduce Collisions

CW = CWmin

Medium idle

Wait for IFS duration

Still idle

Transmit frame

Yes

Yes

Data Ready

Wait for ACK or ACK timeout

ACK received

Done

DIFS

(50us)

Transmit frame if

medium is free DIFS

Check

medium

Check

medium

Data

ready

14

CW = CWmin

Medium idle


Still idle

Transmit frame

Yes


ACK received

Yes

Done

Data Ready t

medium busy

(other station starts comm)

DIFSDIFS

next frame

contention window (32)

(randomized back-off

mechanism)

slot time (20usec)medium becomes

free again

Wait until current transmission ends

Still idle No No


Yes

Choose backoff time between (0, 31) and wait. (Use frozen backoff time if it exists)

Backoff time

expired

Medium busy(Freeze backoff time)

The mediumhas been idle

The mediumhas NOT been idle

8

15

CW = CWmin

Medium idle


Still idle

Transmit frame

Yes

Wait until current transmission ends

Still idle No

Yes

No

Choose backoff time between (0, CW-1) and wait. (Use frozen backoff time if it exists)


ACK received

No

Yes


Done

Data Ready

Backoff time

expired

Medium busy(Freeze backoff time)

Yes

Maximumretransmissions

reached

CW = CW 2 if CW is not CWmax

No

Drop packet

ACK timeout

Congestion control

DCF: Congestion ControlB

inar

y E

xpo

nent

ial

Bac

koff

16

802.11 Power Management

Mobile devices are battery poweredCurrent LAN protocols assume stations are always ready to receive (promiscuous mode)Idle receive state dominates LAN adapter power consumption over time Absolute value is slightly less than transmit/receive power But, time duration in idle state is larger

How can we power off during idle periods, yet maintain an active session?PS (Power Save) mode A station sleeps most of the time But wakes up periodically to receive regular beacon from AP

(Access Point) This is to check if there is any packet destined to it

9

17

Power Save Mode in IEEE 802.11

768 Kbps, 10-100 meters2 Mbps, 250 meters

802.11 (WaveLAN-II) Bluetooth (Nokia)Hardware State Mode of Operation Mode of Operation Hardware State

Transmit

(300mA)

Receive

(250mA)

Idle(Listen)

(230mA)

Active

Awake

Power Save

Sleep

(9mA)

Doze

Active

(40-60mA)

Sniff

Hold

Park

Connection

StandbyStandby

(0.55mA)

18

Power Save Mode in IEEE 802.11

AP can transmit data frames to an active node at any time

For nodes in PS mode, AP buffers data frames

AP announces buffered traffic at a predetermined time

AP transmits the data frames

AP

Node in active mode

Node in PS mode

Requires synchronization

(beacons)

Uses TIM (Traffic Indication Map)

AP maintains nodes’ mode of operation

10

19

Power Management Procedure

20

Power Management Procedure (Example)

*1) Stations do not wake up every BI.*2) Buffered packet can be dropped due to many reasons (e.g., buffer full).*3) How does the AP manage the packet buffer? Separate ones for CAM and PSM?*4) Packet delay is about BI (e.g., 100ms). Is it ok for VoIP, Skype, or FaceTime?

11

21

Power Management: Buffer

Buffer management at AP Two separate buffers; one for CAM (continuous

active mode) and the other for PSM devices When an AP receives a PS-poll, the corresponding

packets will be moved from PSM buffer to normal buffer

High-priority buffer (those packets gets a priority), Normal-priority buffer

Packet delay problem (e.g., iPhone) When a device receives a packet from an AP, it

changes its status to CAM for 20msec (timeout). If it does not receive any further packet from the

AP, it changes its status back to PSM (must inform to AP).

22* Eric Rozner, Vishnu Navda, Ramachandran Ramjee, and Shravan Rayanchu, NAPman: Network-Assisted Power Management for WiFi Devices, ACM MobiSys, 2010.

12

IEEE 802.11 Standard

Communication Standards (802.15)

IEEE 802.15 is a working group of IEEE 802 standards committee which specifies Wireless Personal Area Network (WPAN) standards.

It includes seven task groups. Task Group 1: WPAN / Bluetooth (802.15.1) Task Group 2: Coexistence Task Group 3: High Rate WPAN (802.15.3) Task Group 4: Low Rate WPAN (802.15.4)

WPAN Low Rate Alternative PHY (4a) Revision and Enhancement (4b) PHY Amendment for China (4c) PHY and MAC Amendment for Japan (4d) MAC Amendment for Industrial Applications (4e) PHY and MAC Amendment for Active RFID (4f) PHY Amendment for Smart Utility Network (4g)

Task Group 5: Mesh networking Task Group 6: Body Area Networks (802.15.6) Task Group 7: Visible light communication* Wireless Next Generation Standing Committee

1Mbps, 1mW, 8mJ/MB

(Bluetooth 4.0: 1Mbps, a few uW)

55Mbps, 200uW, 0.03mJ/MB

0.25Mbps

WLAN standards802.11b: 11Mbps, 50mW, 36mJ/MB802.11g: 54Mbps, 50mW, 7.4mJ/MB

13

900MHz2.4GHz

5GHz (802.11a, 802.11n, 802.11ac)

60GHz (802.11ad)

2626

802.15 Wireless Personal Area Network (WPAN) Working Group

802.15

802.15.1 802.15.2

802.15.4b802.15.3a 802.15.3b

802.15.4802.15.3

802.15.1 : WPAN/Bluetooth

802.15.2 : Coexistence Group

802.15.3 : High Rate (HR) WPAN Group

802.15.4 : Low Rate (LR) WPAN Group (Zigbee)

802.15.5: Mesh networking

802.15.6: Body area networking

802.15.7: Visible light communication

802.15.4a

14

IEEE 802.15.1 Bluetooth

FHSS: frequency hopping spread spectrum Channels: 2.402 GHz + k MHz, k=0, …, 78 Hopping rate: 1,600 hops per second Dwell time in each carrier frequency: 625-µs During each 625-µs time slot, it transmits

one packet 220us switching timeGFSK modulation (Version 1.1) 1 Mb/s symbol ratetransmit power 0 dbm (up to 20dbm with power control)

. . .

1Mhz

1 2 3 79

83.5 Mhz

Hedy Lamarr

IEEE 802.15.4 ZigBee

DSSS250 Kbps at 2.450 GHz (ISM) O-QPSK modulation 16-ary quasi-orthogonal modulation

(4 bits/symbol) => 62.5 Ksymbol/s 32-chip (per symbol) => 2 Mchips/s

20-40 Kbps at 868/915 MHz BPSK modulation 15-chip m-sequence as a spreading code 0.3-0.6 Mchips/s, 20-40 Ksymbols/s

868.3 MHz

Channel 0 Channels 1-10928 MHz902 MHz

2 MHz

2.4 GHz

Channels 11-26

2.4835 GHz

5 MHz

15

Bluetooth vs IEEE 802.15.4 (ZigBee)

Bluetooth based WPANFew devices (8)Power consumption is a low.Battery life is low.Star only.

IEEE 802.15.4 LR-WPANMany devices (64K)Power consumption is ultra low.Battery lasts years.Peer to peer, Star.

3030

Coexistence between Bluetooth and IEEE 802.11

16

31

Bluetooth: Baseband

m

s1

s2

625 sec

f1 f2 f3 f4

(1600 hops/sec)

f5 f6

FH/TDD

Master starts at odd slots.Slave starts at even slots.

32

Physical Link Types

m

s1

s2

SCO SCO SCOSCO SCO SCOACL ACL ACLACL ACL ACL

Reserved at fixed interval

17

33

Bluetooth Network: Piconet

M=Master

S=Slave

P=Parked

SB=Standby

M

S

P

SB

S

S

P

P

SB

Master can connect to 7 active slaves and 200 parked slaves Active Member Address (AMA, 3 bit) Parked Member Address (PMA, 8 bit)

Each piconet has max capacity (1 Mbps)“Hopping pattern” is determined by the masterPolling scheme: A slave may send in a slave-to-master slot when it has been addressed by its MAC address in the previous master-to-slave slot

3434

Forming a Piconet

Unit establishing a piconet automatically becomes the master It sends an inquiry to discover

which other devices are out there

Device in standby listens periodically (inquiry scan) and may respond to the inquiry with its device address (inquiry response) It will become a slave to that

master

18

3535

Page and Connect StatesAfter receiving a response from devices, the master can connect to each device individually Page (M): Used by a master to activate

and connect to a slave (use slave’s device access code or DAC in different hop channels)

Page scan (S): device is listening for a page with its own DAC

Slave response (S): a slave responds to a page message

Master response (M): a master receives a page response from slave.

In active state (connection), master and slaves listen, transmit and receive Slaves synchronize to the hopping

sequence established by the master

36

Complete State Diagram

slavemaster

19

3737

Low Power States

Sniff state Slaves listen to the piconet at a

reduced rate

Master designates certain slots to transmit to slaves in sniff state

Hold state Slave stops ACL transmission, but

can exchange SCO packets

Park state Slave releases its AMA

Still FH synchronized and wakes up periodically to listen to beacon

standby

inquiry page

Transmit

AMA

Connected

AMA

Park

PMA

Hold

AMA

Sniff

AMA

3838

Low Power Mode: Sniff

Master

Slave

Sniff period

Sniff offset

Sniff duration

A master can only transmit to devices in sniff mode during particular sniff-designated time slots

Therefore, these devices listen only during these special time slots and sleep the rest of the time

20

3939

Low Power Mode: Hold

Slave

Hold duration

Hold offset

Master

A slave in hold mode, alternatively, does not receive any asynchronous packets and listens only to determine if it should become active again (but still participate in SCO exchanges)

4040

Low Power Mode: Park

Master

Slave

Beacon interval

Beacon instant

A device in park mode not only stops listening, but gives up its active member address (AM_ADDR). Parking member address (PM_ADDR) is kept

Power saving + keep more than 7 slaves in a piconet

Communication via broadcast LMP messages

21

4141

Low Power Modes

768 Kbps, 10-100 meters2 Mbps, 250 meters

802.11 (WaveLAN-II) Bluetooth (Nokia)Hardware State Mode of Operation Mode of Operation Hardware State

Transmit

(300mA)

Receive

(250mA)

Idle(Listen)

(230mA)

Active

Awake

Power Save

Sleep

(9mA)

Doze

Active

(40-60mA)

Sniff

Hold

Park

Connection

StandbyStandby

(0.55mA)

42

Bluetooth and Zigbee

22

43

Hop Selection (Volume 2, Part B, Section 2.6 of Bluetooth 2.0 Spec.)

Which hops to use for data communications? A basic channel hopping sequence An adapted channel hopping sequence

When devices is in standby mode, which hops does the master use for discovering the devices(page, inquiry)? A page hopping sequence A page response hopping sequence An inquiry hopping sequence An inquiry response hopping sequence

44

Hop Selection (Volume 2, Part B, Section 2.6 of Bluetooth 2.0 Spec.)

A basic channel hopping sequence which has a very long period length, which does not show repetitive patterns over a short time interval, and which distributes the hop frequencies equally over the 79 MHz during a short time interval.

An adapted channel hopping sequence derived from the basic channel hopping sequence which uses the same channel mechanism and may use fewer than 79 frequencies. The adapted channel hopping sequence is only used in place of the basic channel hopping sequence. All other hopping sequences are not affected by hop sequence adaptation.

23

45

Hop Selection (Volume 2, Part B, Section 2.6 of Bluetooth 2.0 Spec.)A page hopping sequence with 32 wake-up frequencies distributed equally over the 79 MHz, with a period length of 32;A page response hopping sequence covering 32 response frequencies that are in a one-to-one correspondence to the current page hopping sequence. The master and slave use different rules to obtain the same sequence;An inquiry hopping sequence with 32 wake-up frequencies distributed equally over the 79 MHz, with a period length of 32;An inquiry response hopping sequence covering 32 response frequencies that are in a one-to-one correspondence to the current inquiry hopping sequence.

46

Hop Selection Scheme

24

47

Quiz: Connection Latency

A device becomes a master by initiating a connection (if he knows the address of the receiver) Send 16 identical page messages on 16 different hop

frequencies and then remaining 16 frequencies (send on 2 channels in one slot and listen in the next slot => takes 16 slots for trying all 16 channels)

A device in unconnected listens for paging messages Tune to 32 wakeup carriers (chosen based on device’s identity)

every 2048 slots (1.28 sec) for 18 slots (11.25 msec) Gets the DeviceAccessCode, goes into “connected” state and

he becomes a “slave” The slave then synchronize to the master's clock and to the

correct frequency hopping pattern

What is the maximum delay for connection ?

* Volume 2, Part B, Sections 2, 8.3 of Bluetooth 2.0 Spec

4848

* Some Hints

1600 hops/s for data communications

3200 hops/s for paging because the paging message is shorter

Master sends page messages on fk & fk+1

Master listens slave Response on fk & fk+1

Master sends page messages on fk+2 & fk+3

25

4949

* Some Hints

Master tried the first 16 channels (around the most probable channels, train A), which takes 16x625usec = 10 msecMaster repeats Npage times, which is, e.g., 128 or 1.28sMaster tries another 16 channels (train B), which is repeated Npage timesMaster will continue train A & B until the timeout pageTO expires

Slave scans channel #1 for 18 slots, or 11.25msThis is continued for all 32 channels, which takes 360msIt will be repeated every 1.28s

* Additional hint at slides 53-59.

50

* References

Improving Connection Times for Bluetooth Devices in Mobile Environments, Erik Welsh, Patrick Murphy, J. Patrick Frantz

Analysis of Bluetooth Device Discovery and Some Speedup Mechanisms, Jehn-Ruey Jiang, Bing-Rong Lin, and Yu-Chee Tseng

Analysis of the Bluetooth device discovery protocol, Goutam Chakraborty, Kshirasagar Naik, Debasish Chakraborty, Norio Shiratori, David Wei

50

26

51

Application

TCP packet

IP packet

52

IP packet

Frame


2 2 6 6 0-2312

Addr3

6

CRC

4Seq.

control

2

Addr4

6



128 16 8 8 16 variable

length

16

802.11 MAC packet

802.11 PHY packet

27

53

Packet Format

Packets may consist of Access code only (in which case the access

code is 68 bits, not 72)

Access code + header

Access code + header + payload

72 bits 54 bits 0 - 2744 bits

Access

codeHeader Payload

54

Access Code

Access code is used for synchronization and identificationAll packets sent in a piconet use the same channel access codeThere are specific access codes for signaling and inquiry (for instance, to discover what other BT devices are in range)Types Channel Access Code (CAC): identifies a piconet Device Access Code (DAC): used for paging/response Inquiry Access Code (IAC): used for inquiry


Access

codeHeader Payload

28

55

Header (1)

AM_ADDR:

3-bit slave address

temporary, assigned while the slave is active, and specific to the piconet

Messages from slave to master and from master to slave carry this address

All zeros: broadcast address

56

Header (2)

TYPE:

Distinguishes between synchronous and asynchronous links, indicates how many slots the packet will occupy

FLOW:

Asynchronous flow control in asynchronous links

ARQN:

ACK (ARQN = 1) or NAK (ARQN = 0)

Null, POLL, FHS,

DM1, DM3, DM5,

DH1, DH3, DH5,

HV1, HV2, HV3,

DV, AUX1

29

57

Header (3)

SEQN:

Sequence number

1 bit is sufficient for very simple ARQHEC:

Header error check

58

Header (Summary)

Addressing (3) : max 7 active slavesPacket type(4) : 16 packet typesFlow control(1)1-bit ARQ (1)Sequencing (1) : for filtering retransmitted packetsHEC (8) : verify header integrity

Access

codeHeader Payload

54 bits

Purpose

Encode with 1/3 FEC to get 54 bits

(simply triple modular redundancy)18 bitstotal

ss

m

s

30

59

Voice & Data Packet


Access

codeHeader Payload

DataVoice CRC

No CRC

No retries

625 µs

master

slave

ARQ

FEC (optional) FEC (optional)

60

Voice Packets (HV1, HV2, HV3)

Access

codeHeader

Payload

72 bits 54 bits 240 bits

30 bytes

= 366 bits

10 bytes

+ 2/3 FEC

+ 1/3 FEC

20 bytes

30 bytesHV3

HV2

HV1

3.75ms (HV3: sent every 6 slots=>64kbps)

2.5ms (HV2: sent every 4 slots=> 64kbps)

1.25ms (HV1: sent every two slots => 80bits/1.25msec = 64kbps)

TMR

Hamming code

High-quality voice packets

31

61

Data Packet Types

625 µs

1 2

1875 µs

1 2 3 4

3125 µs 625 µs

1 2 3 4 5 6

DM1/DH1(Data 30B,

Total 366bits)

DM3/DH3(Data 187B,

Total 1626bits)

DM5/DH5(Data 343B,

Total 2870bits)

2/3 FEC

No FEC

Data medium/ Data high

62

Data Packet Types

Access code (72), header (54) is common to all cases

DM1:Data header (8), payload (0-136), 2/3 FEC, CRC (16)DH1:Data header (8), payload (0-216), CRC (16)DM3:Data header (16), payload (0-968), 2/3 FEC, CRC (16)DH3:Data header (16), payload (0-1464), CRC (16)DM5:Data header (16), payload (0-1792), 2/3 FEC, CRC (16) Asymmetric links between Master and Slave

DM5 link (5-slot packet) by M, DM1 link (1-slot packet) by S 1792 bits/(625usec*6 slots) = 477.8kbps 136 bits/(625usec *6 slots) = 36.3kbps

Symmetric links between M and S, all uses DM5 links 1792 bits/(625usec*10 slots) = 286.7 kbps for each direction

DH5:Data header (16), payload (0-2712), CRC (16)

32

63

Quiz: Packet Types

1. Bluetooth supports asynchronous and asymmetric data traffic up to 723.2/57.6 Kbps. Which data packet types are used for that data rate?

2. Which combination of data packet types produces the maximum throughput?

3. Bluetooth supports three types of voice packets, HV1, HV2 and HV3. Do they differ in packet rate (number of packets per second)? Do they differ in resulting data rate? (Yes, No)

4. A group of piconets is called a scatternet. Can a slave of a piconet belong to another piconet as a slave or a master? Can a master of a piconet belong to another piconet as a slave or a master? (Yes, No)

6464

IEEE 802.15.4

IEEE 802.15.4 task group began to develop a standard for low-rate WPAN (LR-WPAN).

The goal of this group was to provide a standard with ultra-low complexity, cost, and power for low-data-rate wireless connectivity among inexpensive devices.

The Zigbee Alliance is an association of companies involved with building higher-layer standards based on IEEE 802.15.4. This includes network, security, and application protocols.

33

65

IEEE 802.15.4 MAC

Applications

IEEE 802.15.4

2400 MHz

PHY

IEEE 802.15.4

868/915 MHz

PHY

802.15.4 / ZigBee Architecture

ZigBee

66

Bluetooth vs IEEE 802.15.4 (ZigBee)

Bluetooth based WPANFew devices (8)Power consumption is a low.Battery life is low.Star only.

IEEE 802.15.4 LR-WPANMany devices (64K)Power consumption is ultra low.Battery lasts years.Peer to peer, Star.

34

67

TelosB

CC2420

IEEE 802.15.4 radio

PIFA Antenna

USB-serial10 pin expansion

6 pin expansionSHT11 humidity / temp

TI MSP430 F1611

reset button

user button

TSR photodiode

PAR photodiodeLEDs

(bottom)

ST M25P80 flash

reset support

jtag

serial ID

68

cis 632 / eec 687 mobile computing•rts must include some information on how long. •for rts, the...

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