study goals & talk summary

Wireless Internet Access: the BT approachTyrrhenina Workshop, Sept. 2000

11

University of Padova

Study Goals & Talk summaryStudy Goals & Talk summary

• Aims

Evaluation of FTP performance over Bluetooth (BT) radio link in different environmental conditions

Influence of BT radio packet format on system performance

• Outline

TCP over wireless link

Bluetooth overview

Methodology of analysis & Performance metrics

Main results

Conclusions & Future Work


22


TCP over Wireless LinksTCP over Wireless Links

• A hard coexistence

TCP is tuned to work well in wired networks

Wireless Link can produce packet losses not related to congestion

These events may trigger useless congestion reaction mechanisms,

resulting in sub-optimal performances

• The “Link Layer” solution

Idea:– providing radio link reliability by using local retransmissions

Drawback:– possibility of bad interaction between TCP and Link Layer retransmission

mechanisms


33


Bluetooth overview: piconet Bluetooth overview: piconet

Two up to eight Bluetooth units sharing the

same channel form a piconet.

In each piconet, a unit acts as master.

Channel access is organised on the bases of a

centralised polling scheme. active slavemaster

parked slavestandby

slave1

slave2

slave3

master


44


Bluetooth overview: Frequency Hopping Bluetooth overview: Frequency Hopping

625 s

t

t

master

slave

f(2k) f(2k+1) f(2k+2)

Each piconet is characterised by a pseudo-random frequency hopping sequence, imposed by master.

All the units in the same piconet hop synchronously.

Time is divided into slots of 625 s; each slot corresponds to a different hop frequency.

Consecutive packets are transmitted on different RF carriers.


55


Bluetooth overview: ARQ schemeBluetooth overview: ARQ scheme

MASTER

SLAVE 1

SLAVE 2

A B B CX

Z Z

G F H

NAK

ACK

An Automatic Retransmission Query (ARQ) mechanism grants the

reliability of asynchronous data traffic (ACL)

– 1-bit fast ACK/NAK

– 1-bit sequence number

– header piggy-backing


66


Bluetooth overview: multi-slot packets Bluetooth overview: multi-slot packets

f(k)

625 s

f(k+1) f(k+2) f(k+3) f(k+4)

f(k+3) f(k+4)f(k)

f(k)

f(k+5)

f(k+5)

f(k+5)

A baseband packet can extend over one, three or five consecutive slots.

The carrier frequency remains unchanged for the whole packet duration.

Multi-slot packets reduce bandwidth losses due to packet header and PLL settling time (220)


77


Bluetooth overview: DH & DM packetsBluetooth overview: DH & DM packets

Asynchronous data packet can be optionally protected by a 2/3 Forward Error Correction (FEC).

Protected packet formats realise medium

data rate and are noted with DM.

Unprotected packet formats realise

higher payload capacity but are more

subject to errors. They are noted with DH.0

50

100

150

200

250

300

350

Byt

es

1 slot 3 slots 5 slots

Paylod Capacity

Medium rate High rate


88


Bluetooth overview: packet formatBluetooth overview: packet format

• Access Code (AC) All packet exchanged in a piconet have the same AC.

Packets that don’t satisfy AC test are immediately discarded.

• Packet Header

Contains, among other information, slave active member receiver address, ARQ flags, payload format, header checksum field (HEC).

If the HEC test fails, the packet is immediately discarded.

• Payload If the CRC test fails, the packet is negative acknowledged.

AC HECaccess code packet header payload

72 54 0-2745

CRC


99


Methodology of AnalysisMethodology of Analysis

• Bluetooth radio link connects a nomadic

client to a FTP server.

• Snooping programs collect end-to-end and

point-to-point transmission statistics.

• A series of large bulk data transfers have

been performed, with notebook in different

positions.

• Data collected have been analysed to extract

system performances.

Measurement PlatformMeasurement Platform

Notebook

Client

Notebook

Router

FTP Server

Ethernet Bluetooth

TCPSnooper

BTStatistics


1010


Packet Dropping ProbabilityPacket Dropping Probability

• Packet Dropping Probability (Packet Dropping Probability (PDPPDP))

Probability of packet drop due to Access Code or Header Checksum failures.

• Packet Error Probability (Packet Error Probability (PEPPEP))

Probability of packet retransmission due to bad reception: Access Code or

Header Checksum or CRC failures

AC HEC CRC

PDP

PEP


1111


Performance indexesPerformance indexes

• Radio Link performance metrics

Goodput:– average number of bit transmitted successfully in master to slave

direction, in the unit of time.

• End-to-end performance metrics

Segment Service Time (SST):– time employed by the BT entities to transmit a TCP segment through

the radio link.

TCP sender transmission window size.


1212


Goodput VS Packet Drop ProbabilityGoodput VS Packet Drop Probability


1313


Segment Service TimeSegment Service Time


1414


End-to-end performance: transmission windowEnd-to-end performance: transmission windowTitolo:P:\WIRELE~1\docs\UniPD\THYRRE~1\IMA_FL~1.PSAutore:GSview from P:\WIRELE~1\docs\UniPD\THYRRE~1\IMA_FL~1.PSAnteprima:L'immagine EPS non è stata salvata con l'anteprima inclusa in essa.Commento:L'immagine EPS potrà essere stampata con una stampante PostScript e non con altri tipi di stampante.


1515


End-to-end performance: spurious retransmissionEnd-to-end performance: spurious retransmission

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1616


ConclusionsConclusions

• Longest unprotected packet format (DH5) realises the highest performance

in almost all the situations considered.

• FEC protected packets overrun unprotected ones only in particularly hostile

channel conditions.

• Mean and standard deviation of the Segment Service Time grow rapidly

when PDP moves near one.

• In general, TCP well follows RTT oscillations except when PDP changes

drastically during the transmission.


1717


Open Issues and Future WorkOpen Issues and Future Work

• Definition of a mathematical model for the Bluetooth radio connection.

• Study of methods to protect TCP sender against drastic variations of

environmental conditions.

• Performance analysis in systems with more than two units per piconet.

• Study of hand-off problem between piconets.

• Routing algorithms for scatternet.


1818


Segment Service Time: measurement problemsSegment Service Time: measurement problems

• At the moment, we cannot directly measure the SST, because

probing programs suffer of some drawbacks:

– master & slave statistics are collected independently,

– probing time is not always constant;

we are not able to distinguish radio packets belonging to the same TCP

segment.

• We can estimate the SST statistic by using traditional queue theory.


1919


PST, SST and RTTPST, SST and RTT

DH5DH5 DH5 DH5

L2CAP Packet

Datagram

Source Destination

BT link (ARQ S&W)

L2CAP Packet

Datagram

PEP

DH5 DH5 DH5 DH5

SST

RTT

PST


2020


Segment Service Time: statistic estimationSegment Service Time: statistic estimation

• Let PST be the number of transmissions attempts until positive acknowledgement.

• PST is a modified geometric random variable, with mean 1/PEP:

• Let N be the number of radio packets needed to carry a whole TCP segment.

• Since each packet requires PST transmission, SST is given by the sum of N i.i.d. random variables:

• Hence, SST results a random variable with modified Pascal distribution.

PEPPST 'G

N

PSTSST


2121


Packet Error ProbabilitiesPacket Error Probabilities


2222


Packet Error ProbabilityPacket Error Probability

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