Lecture 16: QoS and 802.11 CSE 123: Computer Networks Chris Kanich - - PDF document

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CSE 123: Computer Networks Chris Kanich

Lecture 16: QoS and 802.11

Prj2 due for full credit 11:59pm

Lecture 16 Overview

 QoS followup: Packet Scheduling

 Fair Queueing  Fluid Flow

 802.11 Wireless

 CSMA/CA  Hidden Terminals  RTS/CTS

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 So far we’ve done flow-based traffic policing

 Limit the rate of one flow regardless the load in the network

 In general, need scheduling

 Dynamically allocate resources when multiple flows compete  Give each “flow” (or traffic class) own queue (at least

theoretically)

 Weighted fair queuing

 Proportional share scheduling  Schedule round-robins among queues in proportion to some

weight parameter

Scheduling

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Flow 1 Flow 2 Flow n

I/P O/P

(Weighted) Fair Queuing

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 Maintain a queue for each flow

 What is a flow?

 Implements max-min fairness: each flow receives

min(ri, f) , where

 ri – flow arrival rate  f – link fair rate (see next slide)

 Weighted Fair Queuing (WFQ) – associate a weight

with each flow

Fair Queuing

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 If link congested, compute f such that

8 6 2 4 4 2 f = 4: min(8, 4) = 4 min(6, 4) = 4 min(2, 4) = 2 10

Fair Rate Computation

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TCP vs. UDP

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TCP vs. UDP w/Fair Queuing

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 Associate a weight wi with each flow i  If link congested, compute f such that

8 6 2 4 4 2 f = 2: min(8, 2*3) = 6 min(6, 2*1) = 2 min(2, 2*1) = 2 10 (w1 = 3) (w2 = 1) (w3 = 1) If Σk wk <= C, flow i is guaranteed to be allocated a rate >= wi Flow i is guaranteed to be allocated a rate >= wi*C/(Σk wk)

Another Example

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 Flows can be served one bit at a time  WFQ can be implemented using bit-by-bit weighted

round robin

 During each round from each flow that has data to send, send a

number of bits equal to the flow’s weight

Fluid Flow

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15 2 10 4 6 8 5 1 1 1 1 1



Orange flow has packets backlogged between time 0 and 10



Other flows have packets continuously backlogged



All packets have the same size

flows link weights

Fluid Flow Example

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 Packet (Real) system: packet transmission cannot be

• preempted. Why?

 Solution: serve packets in the order in which they

would have finished being transmitted in the fluid flow system

Packet-Based Implementation

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2 10 4 6 8 2 10 4 6 8

 Select the first packet that finishes in the fluid flow

system

Service in fluid flow system Packet system

Packet-Based Example

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Infrastructure vs. Ad hoc

infrastructure network ad-hoc network

AP AP AP wired network AP: Access Point CSE 123 – Lecture 16: QoS and 802.11 15

IEEE 802.11 Infrastructure

mobile terminal access point fixed terminal application TCP 802.11 PHY 802.11 MAC IP 802.3 MAC 802.3 PHY application TCP 802.3 PHY 802.3 MAC IP 802.11 MAC 802.11 PHY LLC infrastructure network LLC LLC

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CSE 123 – Lecture 16: QoS and 802.11 16

802.11 Frame Format

synchronization SFD signal service HEC payload PLCP preamble PLCP header 128 16 8 8 16 variable bits 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 Length



future use, 00: 802.11 compliant  payload length



HEC (Header Error Check)



protection of signal, service and length, x16+x12+x5+1 CSE 123 – Lecture 16: QoS and 802.11 17

WLAN: IEEE 802.11b

 Data rate

 1, 2, 5.5, 11 Mbit/s  User data rate max. approx. 6 Mbit/s

 Transmission range

 300m outdoor, 30m indoor  Max. data rate ~10m indoor

 Frequency

 Free 2.4 GHz ISM-band

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Physical Channels

US (FCC)/Canada (IC) 2400 [MHz] 2412 2483.5 2437 2462 channel 1 channel 6 channel 11 22 MHz

 12 channels available for use in the US

 Each channel is 22 MHz wide  Only 3 orthogonal channels  Using any others causes interference

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CSE 123 – Lecture 16: QoS and 802.11 19

CSMA: listen before transmit:

 If channel sensed idle: transmit entire pkt  If channel sensed busy, defer transmission  Persistent CSMA: retry immediately with

probability p when channel becomes idle (may cause instability)

 Non-persistent CSMA: retry after random

interval

Carrier Sense Multiple Access

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A B C

Hidden Terminal Problem



B can communicate with both A and C



A and C cannot hear each other



Problem

 When A transmits to B, C cannot detect the transmission using the

carrier sense mechanism

 If C transmits, collision will occur at node B 

Solution

 Hidden sender C needs to defer

CSE 123 – Lecture 16: QoS and 802.11 21

CSMA/CA

 Cannot detect collision w/half-duplex radios  Wireless MAC protocols often use collision

avoidance techniques, in conjunction with a (physical or virtual) carrier sense mechanism

 Collision avoidance

 Nodes negotiate to reserve the channel.  Once channel becomes idle, the node waits for a

randomly chosen duration before attempting to transmit.

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A B C

 When A wants to send a packet to B, A first sends a

Request-to-Send (RTS) to B

 On receiving RTS, B responds by sending Clear-to-

Send (CTS), provided that A is able to receive the packet

 When C overhears a CTS, it keeps quiet for the

duration of the transfer

 Transfer duration is included in both RTS and CTS

RTS/CTS (MACA)

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Backoff Interval

 Problem: With many contending nodes, RTS

packets will frequently collide

 Solution: When transmitting a packet, choose a

backoff interval in the range [0, CW]

 CW is contention window

 Wait the length of the interval when medium is idle

 Count-down is suspended if medium becomes busy  Transmit when backoff interval reaches 0

 Need to adjust CW as contention varies

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Non-symmetric ranges

C F A B E D DATA Transmit “range” Interference “range” Carrier sense range F A

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802.11 MAC Modes

 Distributed Coordination Function (DCF) CSMA/CA

 collision avoidance via randomized “back-off“ mechanism  minimum distance between consecutive packets  ACK packet for acknowledgements (not for broadcasts)

 DCF w/ RTS/CTS

 Distributed Foundation Wireless MAC  avoids hidden terminal problem

 Point Control Fuction (PCF) - optional

 Access point polls terminals according to a list  We’re not going to discuss…

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IEEE 802.11 DCF

 DCF is CSMA/CA protocol

 Uses a Network Allocation Vector (NAV) to implement

collision avoidance

 DCF suitable for multi-hop ad hoc networking  Optionally uses RTS/CTS exchange to avoid hidden

terminal problem

 Any node overhearing a CTS cannot transmit for the duration

• f the transfer

 Uses ARQ to provide reliability

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C F A B E D RTS RTS = Request-to-Send

Pretending a circular range

IEEE 802.11

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C F A B E D RTS RTS = Request-to-Send

NAV = 10

NAV = remaining duration to keep quiet

IEEE 802.11

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C F A B E D CTS CTS = Clear-to-Send

IEEE 802.11

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C F A B E D CTS CTS = Clear-to-Send

NAV = 8

IEEE 802.11

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C F A B E D DATA

• DATA packet follows CTS. Successful data reception

acknowledged using ACK.

IEEE 802.11

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C F A B E D ACK

Reserved area

IEEE 802.11

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 When a node fails to receive CTS in response to its

RTS, it increases the contention window

 CW is doubled (up to an upper bound)  More collisions  longer waiting time to reduce collision

 When a node successfully completes a data transfer, it

restores CW to CWmin

Binary Exponential Backoff

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802.11 Backoffs

 SIFS (Short Inter Frame Spacing)

 highest priority, for ACK, CTS, polling response

 PIFS (PCF IFS)

 medium priority, for time-bounded service using PCF

 DIFS (DCF, Distributed Coordination Function IFS)

 lowest priority, for asynchronous data service

t medium busy SIFS PIFS DIFS DIFS next frame contention direct access if medium is free  DIFS CSE 123 – Lecture 16: QoS and 802.11 35

DCF Example

data wait B1 = 5 B2 = 15 B1 = 25 B2 = 20 data wait

B1 and B2 are backoff intervals at nodes 1 and 2 cw = 31

B2 = 10

CSE 123 – Lecture 16: QoS and 802.11 36

Fragmentation

t SIFS DIFS data ACK1

• ther

stations receiver sender frag1 DIFS contention RTS CTS SIFS SIFS NAV (RTS) NAV (CTS) NAV (frag1) NAV (ACK1) SIFS ACK2 frag2 SIFS