Wireless Networked Systemsnadeem/classes/cs795-WNS-S...Page 24 Spring 2013 CS 795/895 - Wireless Networked Systems 802.11 Data Rates • The data rates supported by 802.11b standard

Wireless Networked Systems� �

CS 795/895 - Spring 2013 �

Tamer Nadeem �Dept. of Computer Science�

Lec #3: Medium Access Control�Multirate Adaptation, Data Fragmentation, Association/

Authentication, and Roaming

Page 2 Spring 2013 CS 795/895 - Wireless Networked Systems

Performance Analysis of the IEEE 802.11 Distributed Coordination Function

(Giuseppe Bianchi)


802.11 DCF Throughput Analysis (Bianchi)

• Objective: •  Analytical Evaluation of Saturation Throughput

• Assumptions: •  Fixed number of stations having packet for transmission

•  Each packet collide with constant and independent probability

• Model bi-dimensional process {s(t) , b(t)} with discrete-time Markov chain

• Analysis divided into two parts: •  Study the behavior of single station with a Markov model

•  Study the events that occur within a generic slot time & expressed throughput for both Basic & RTS/CTS access method


Markov Chain Model


•  Closed form solution for Markov chain

Markov Chain Model

•  Stationary Probability


•  In general τ depends on conditional collision probability p

Markov Chain Model

• Probability τ that a station transmits in randomly chosen slot time

• When m =0 no exponential backoff is considered probability τ results independent of p


Throughput Analysis

• Normalized system throughput S

• Probability of transmission Ptr

• Probability of successful transmission Ps


Normalized system throughput

Throughput Analysis

Specify Ts and Tc to compute throughput for DCF access mechanism


• Considering System via Basic Access mechanism •  Packet header H = PHYhrd +MAChrd

•  Propagation delay δ

Throughput Analysis


• Packet transmission via RTS/CTS Access mechanism

Throughput Analysis


Model Validation

• Compared analytical results with that obtained by means of simulation

• Analytical model extremely accurate

• Analytical results (lines) coincide with simulation results (symbols) in both Basic Access & RTS/CTS cases

Saturation throughput analysis vs. simulation


Performance Evaluation

Saturation throughput vs. initial window size for Basic Access mechanism

• Greater the network size lower is the throughput for basic access


• Throughput of Basic Access mechanism depends on W

• W depends on number of terminals

• High value of W gives excellent throughput performance


Saturation throughput vs. initial window size for Basic Access mechanism


• Throughput obtained with RTS/CTS mechanism

•  Independent of value of W


Saturation throughput vs. initial window size for RTS/CTS mechanism


• Number of transmissions per packet increases as W reduces & network size n increases.


Average number of transmissions per packet


IEEE 802.11 Frames


802.11 Data Unit

• Data-Link layer • MAC Service Data Unit (MSDU) • MAC Protocol Data Unit (MPDU )

• Physical layer • PLCP Service Data Unit (PSDU) • PLCP Protocol Data Unit (PPDU)


Data Link Layer

• MAC Service Data Unit (MSDU) •  Layer 3-7 information that is encapsulated

•  Max size of 2, 304 bytes

• MAC Protocol Data Unit •  802.11 frame

•  With 802.11 header and FCS


Physical Layer

• Physical Layer Convergence Procedure sublayer (PLCP)

•  Prepares the data link frame for transmission

• Physical Medium Dependant Sublayer (PMD) •  Modulates and sends data

• PLCP Service Data Unit (PSDU) •  Same as MPDU-but on physical side

• PLCP Protocol Data Unit (PPDU) •  Includes the preamble for synchronizations and the PHY header


Data Link and Physical Layer


Long PLCP Frame Format

Preamble and Header always at 1Mb/s DBPSK Barker

1Mbps DBPSK Barker 2Mbps DQPSK Barker 5.5, 11Mbps DQPSK CCK

1Mbps DBPSK

192us

PPDU

SYNC 128 bits

SFD 16 bits

SIGNAL 8 bits

SERVICE 8 bits

LENGTH 16 bits

CRC 16 bits

Long PLCP Preamble 144 bits in 1 Mbps

Long PLCP Header 48 bits in 1 Mbps

PSDU/MPDU 1, 2, 5.5, 11 Mbps

•  Mandatory in 802.11b


IEEE 802.11 Multi-Rate


What is Data Rate ?

Number of bits that you transmit per unit time under a fixed energy budget

Too many bits/s:

Each bit has little energy -> Hi BER

Too few bits/s: Less BER but lower throughput


802.11 Data Rates

•  The data rates supported by 802.11b standard

1, 2, 5.5 and 11Mbps

•  The data rates supported by 802.11g standard

1, 2 ,5.5, 11, 6, 9, 12, 18, 24, 36, 48 and 54

•  The data rates supported by 802.11a standard

6, 12 and 24Mbps are mandatory and

9, 18, 36, 48 and 54Mbps are optional

•  AP and IBSS creators announce set of Basic rates and supported rates in the Beacons and Probe Response packets. Station announces supported rate information in Probe Request and (Re)Association packets


Some Basics

Floor Noise Data Rate

Received Power

Channel Bandwidth

•  Bit error (p) for BPSK and QPSK :

SNR

•  Friss’ Equation:

Varying with time and space

How do we choose the rate of modulation


Static Rates

SINR

time

# Estimate a value of SINR # Then choose a corresponding rate that would transmit packets correctly most of the times # Failure in some cases of fading à live with it


Adaptive Rate-Control

SINR

time

# Observe the current value of SINR # Believe that current value is indicator of near-future value # Choose corresponding rate of modulation # Observe next value # Control rate if channel conditions have changed


Impact Large-scale variation with distance (Path loss)

SNR

(dB

)

Distance (m) Distance (m)

Mea

n Th

roug

hput

(Kbp

s) Path Loss

1 Mbps

8 Mbps


Answer à Rate Adaptation

•  Dynamically choose the best modulation scheme for the channel conditions

Mea

n Th

roug

hput

(Kbp

s)

Distance (m)

Desired Result


Design Issues •  How frequently must rate adaptation occur?

•  Signal can vary rapidly depending on: •  carrier frequency

•  node speed

•  interference

•  etc.

•  For conventional hardware at pedestrian speeds, rate adaptation is feasible on a per-packet basis

Coherence time of channel higher than transmission time


•  Cellular networks •  Adaptation at the physical layer

•  Impractical for 802.11 in WLANs

•  For WLANs, rate adaptation best handled at MAC

Adaptation à At Which Layer ?

D

C

B A CTS: 8

RTS: 10

10

8 Sender Receiver

RTS/CTS requires that the rate be known in advance�

Why?


Who should select the data rate? •  Collision is at the receiver

•  Channel conditions are only known at the receiver

•  SS, interference, noise, BER, etc.

•  The receiver is best positioned to select data rate

A

B


Bigger Picture

•  Rate control has variety of implications •  Any single MAC protocol solves part of the puzzle

•  Important to understand e2e implications •  Does routing protocols get affected?

•  Does TCP get affected?

•  …

•  Good to make a start at the MAC layer •  RBAR

•  OAR

•  Opportunistic Rate Control

•  …


Lucent WaveLAN “Autorate Fallback” (ARF)

•  Sender decreases rate after •  N consecutive ACKS are lost

•  Sender increases rate after •  Y consecutive ACKS are received or

•  T secs have elapsed since last attempt

B A DATA 2 Mbps

2 Mbps Effective Range

1 Mbps Effective Range


Performance of ARF

Time (s) Rat

e (M

bps)

S

NR

(dB

)

Time (s)

–  Slow to adapt to channel conditions

–  Choice of N, Y, T may not be best for all situations

Attempted to Increase Rate During Fade

Dropped Packets

Failed to Increase Rate After Fade


RBAR Approach

• Move the rate adaptation mechanism to the receiver

•  Better channel quality information = better rate selection

• Utilize the RTS/CTS exchange to: •  Provide the receiver with a signal to sample (RTS)

•  Carry feedback (data rate) to the sender (CTS)


•  RTS carries sender’s estimate of best rate

•  CTS carries receiver’s selection of the best rate

•  Nodes that hear RTS/CTS calculate reservation

•  If rates differ, special subheader in DATA packet updates nodes that overheard RTS

Receiver-Based Autorate (RBAR) Protocol

C

B A CTS (1)

RTS (2)

2 Mbps

1 Mbps

D

1 Mbps DATA (1)

2 Mbps

1 Mbps


Performance of RBAR

Time (s)

SN

R (d

B)

Time (s)

Rat

e (M

bps)

R

ate

(Mbp

s)

Time (s)

RBAR

ARF


Question to the class

•  There are two types of fading •  Short term fading

•  Long term fading

•  Under which fading is RBAR better than ARF ?

•  Under which fading is RBAR comparable to ARF ?

•  Think of some case when RBAR may be worse than ARF


Implementation into 802.11

Performance Anomaly of 802.11 Martin Heusse, Frank Rousseau, Gilles-Berger Sabbatel,

Andrzej Duda

LSR-IMAG Laboratory Grenoble, France


Performance of DCF


Performance of DCF

Overall Transmission time (T) :

Constant Overhead (tov) :

Proportion of useful throughput (p):


Performance of DCF

Taking into account collisions and exponential backoff,

Overall Transmission Time (T(N)) becomes :

Time spent in contention (tcont(N)) :


Performance of DCF

Assuming that multiple successive collisions are negligible,

Proportion of collisions (Pc(N)) experienced for each packet acknowledged successfully :

Proportion (p) of useful throughput obtained by a host :


Performance Anomaly of 802.11b

Fast Host :

Slow Host :

R : transmission rate of ‘fast’ host (11Mbps)

r : transmission rate of ‘slow’ host (5.5, 2 or 1 Mbps)

tRov : overhead time of ‘fast’ host

trov : overhead time of ‘slow’ host


Model


Result :

The fast hosts transmitting at a higher rate ‘R’ obtain the same throughput as the slow host transmitting at a lower rate ‘r’.

i.e.

Performance Anomaly of 802.11b


Simulation Studies


Performance Measurements • Hosts with different rates, real mobility


IEEE 802.11 Fragmentation/Defragmentation


Fragmentation

•  Break a frame into smaller pieces •  Fragments

•  Actual amount of data is same, but causes additional overhead

•  Fragmenting can help with networks that have lots of data corruption

•  Less to retransmit if lots of errors

•  Not all cards allow you to do this


802.11 Fragmentation

t

SIFS

DIFS

data

ACK1

other stations

receiver

sender frag1

DIFS

contention

RTS

CTS SIFS SIFS

NAV (RTS) NAV (CTS)

NAV (frag1) NAV (ACK1)

SIFS ACK2

frag2

SIFS

•  Burst of Fragments which are individually acknowledged. •  For Unicast frames only.

•  Random backoff and retransmission of failing fragment when no ACK is returned.

•  Duration information in data fragments and Ack frames causes NAV to be set, for medium reservation mechanism.


802.11 Fragmentation

•  The length of a fragment MPDU shall be an equal number of octets for all fragments except the last, which may be smaller.

•  The length of a fragment MPDU shall always be an even number of octets, except for the last fragment.

•  The length of a fragment shall never be larger than aFragmentationThreshold unless WEP is invoked for the MPDU. Because the MPDU shall be expanded by IV and ICV.

•  The sequence number shall remain the same for all fragments of a MSDU or MMPDU.

•  The fragments shall be sent in order of lowest fragment number to highest fragment number (start at zero, and increased by one).

•  More Fragments bit is used to indicate the last (or only) fragment of the MSDU or MMPDU.


IEEE 802.11 Association/Authentication


Infrastructure Beacon Generation •  APs send Beacons in

infrastructure networks. •  Beacons scheduled at

Beacon Interval . •  Transmission may be

delayed by CSMA deferral. –  subsequent transmissions

at expected Beacon Interval to last Beacon transmission

Time Axis

Beacon Interval

X X X X

"Actual time" stamp in Beacon

Beacon Busy Medium

–  next Beacon sent at Target Beacon Transmission Time •  Timestamp contains timer value at transmit time to ensure

synchronization between client and AP •  Transmitted about 10 times per second

‒ Sometimes configurable •  AP sends the beacon with information about the BSS

‒ Client stations only transmit beacons when part of an IBSS


Beacon Management Frame

•  Time stamp: Synchronization information •  Spread spectrum parameter sets: FHSS-, DSSS-, or ERP-specific

information •  Channel information: Channel used by the AP or IBSS •  Data rates: Basic and supported rates •  Service set capabilities: Extra BBS or IBSS parameters •  SSID: Logical WLAN name •  Traffic indication map (TIM): A field used during the Power Save

process •  QoS capabilities: Quality of service and EDCA information •  Security capabilities: TKIP or CCMP cipher information •  Vendor proprietary information: Vendor-unique or vendor-specific

information

Pg 283


Authentication

• Not the traditional username and password •  Authenticating to the network

• Open Systems Authentication •  Null authentication

•  Everyone gets in

•  Now used with 802.1X/EAP to provide better security

• Shared Key Authentication •  Uses WEP key to respond to a challenge response

•  WEP key is then used for encryption as well

•  SECURITY RISK!!!!

Pg 286


Association

• After Authentication, Client sends a request to associate to the BSS

•  Association means the client can send data through the AP

• AP sends association response •  Grant or deny permission

•  Includes and Association Identifier (AID)

•  Unique number for each client

•  Used with power management

Pg 288


Open-System Authentication/Association


IEEE 802.11 Roaming


Scanning

•  Scanning required for many functions. •  finding and joining a network

•  finding a new AP while roaming

•  initializing an Independent BSS (ad hoc) network

•  Passive Scanning •  Find networks simply by listening for Beacons

•  Active Scanning •  On each channel

•  Send a Probe, Wait for a Probe Response

•  Beacon or Probe Response contains information necessary to join new network.


Active Scanning Example

Steps to Association:

Station sends Probe.

APs send Probe Response.

Station selects best AP.

Station sends Association Request to selected AP.

AP sends Association Response.

Initial connection to an Access Point - Reassociation follows a similar process

Access Point C Access Point A


Roaming

•  Mobile stations may move –  beyond the coverage area of their Access Point –  but within range of another Access Point

•  Reassociation allows station to continue operation

Access Point A

Access Point B

Station 4

Access Point C

Station 1

Station 2

Station 3

Station 5 Station 6

Station 7

Reassocication


Roaming Approach

•  Station decides that link to its current AP is poor

•  Station uses scanning function to find another AP •  or uses information from previous scans

•  Station sends Reassociation Request to new AP

•  If Reassociation Response is successful •  then station has roamed to the new AP

•  else station scans for another AP

•  If AP accepts Reassociation Request •  AP indicates Reassociation to the Distribution System

•  Distribution System information is updated

•  normally old AP is notified through Distribution System


Handoff Procedure


Result and Analysis Handoff Latencies Graph for different client using Lucent AP

1. Probe delay is dominating component (90%) : Its good to use technique/heuristics that either cache or deduce AP information without having to actually perform a complete active scan stands to benefit.


Result and Analysis Handoff Latencies Graph for different client using Cisco AP

2. Wireless hardware used(AP,STA) affects the handoff latency :

Keeping fixed AP it seen that client card affects the latency.


Result and Analysis

3. Keeping the client card fixed, the AP also affects the latency but to a much lower extent.

4. There are large variation in the handoff latency (Fig. 13)

5. Different wireless cards follow different sequence of messages


Probe Procedure

Further analysis of Probe(its contribution to delay is maximum) •  Probe is essentially a active scan, the wireless NIC do by default n  Transmit a probe request frame which contains the broadcast

address as the destination. n  Start a probe timer. n  Listen for probe response. n  If no response received by minChannelTime, scan next channel. n  If one or more responses are received by minChannelTime, stop

accepting probe responses at maxChannelTime and process all received responses.

n  Move to next channel and repeat above steps.


Probe Procedure


Probe Procedure

The scatter-plot shows two clusters being formed, which more-or-less correspond to the MinChannelTime and MaxChannelTime values from the above active scan procedure.

If there are two or less probe responses, wait time is between 0-20ms,otherwise its between 35 to 40ms.Number of probe response can creates difference of average of 25ms per channel.


Enhancement Ideas

1. Query an external agent that provides hints on the neighboring APs and channels i.e a map of the APs based on the location.

2. Interleave scan messages with data during normal connectivity and use that information to perform a partial active scan (or no scan at all) during the handoff. Also passive scanning (listening for beacon messages) might be performed during normal connectivity to build up the list of APs.

3. Since the probe-wait time depends on the number of probe responses received, another strategy might be to create an ordering among the APs such that a single AP or a small set of APs is responsible for probe requests (i.e. the number of probe responses is a constant).


Questions

Documents

Wireless Networked Systemsnadeem/classes/cs795-WNS-S...Page 24 Spring 2013 CS 795/895 - Wireless Networked Systems 802.11 Data Rates • The data rates supported by 802.11b standard