ECE/CS 372 – introduction to computer networksLecture 14

1Chapter 6, slide

Announcements: Assign 4 is due this Thursday Lab 4 is due next Tuesday Assignment 5 posted soon…

Credit for lecture slides to Professor Bechir Hamdaoui

Adapted from Jim Kurose & Keith Ross (original copyright)

Wireless and Mobile Networks

Background: success of wireless: # wireless (mobile) phone

subscribers now exceeds # wired phone subscribers!

Internet: anytime & anywhere: laptops, PDAs, tablets, iPhones, MagicJack, IP-enabled devices

two important (but different) challenges wireless: communication over wireless link mobility: handling the mobile user who changes point of

attachment to network

2Chapter 6, slide

Elements of a wireless network

Chapter 6, slide 3

network infrastructure

wireless hosts

e.g.: laptop, iPhone run: applications stationary or mobile

wireless does not always mean mobility

Elements of a wireless network

Chapter 6, slide 4

network infrastructure

base station bridge: typically

connected to wired network

relay: responsible for sending pkts between backbone network and wireless host(s) e.g., cell towers,

802.11 access points

Elements of a wireless network

Chapter 6, slide 5

network infrastructure

wireless link connects: mobiles

to base station multiple access

protocol: coordinates link access

various data rates, transmission distance

Elements of a wireless network

Chapter 6, slide 6

network infrastructure

infrastructure mode bridge: base station

connects mobiles into wired network

handoff: mobile changes base station providing connection into wired network

Elements of a wireless network

Chapter 6, slide 7

ad hoc mode no base stations limited range:

nodes can only transmit to other nodes within link coverage

multi-hop: nodes organize themselves into a network: route among themselves

Wireless network taxonomy

Chapter 6, slide 8

Infrastructure Infrastructure-less

SingleHop

Multiple hops

1. hosts connect to base station 2. base station connects to larger InternetE.g.: WiFi/Cellular

1. no base station 2. no connection to larger InternetE.g.: Bluetooth

1. hosts may have to relay via multiple nodes (multi-hop) 2. connects to larger Internet E.g.: mesh network

1. no base station 2. no connection to larger Internet3. may have to relay via others to reach a given node E.g.: MANET

large Internet

WiFi Network

Mesh Network

large Internet

Bluetooth Network

Mobile Ad-Hoc Network (MANET)

Outline

Wireless wireless characteristics multiple access schemes

TDMA/FDMA CDMA

Wi-Fi wireless LANs CSMA/CA IEEE 802.11

Mobility principles:

addressing routing to mobile

users mobile IP

9Chapter 6, slide

Wireless Link CharacteristicsDifferences from wired link ….

decayed signal strength: radio signal attenuates as it propagates through matter (path loss)

interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone)

multipath propagation: radio signal reflects off objects ground, arriving at destination at slightly different times

…. make communication across (even a point to point) wireless link much more “difficult” Chapter 6, slide 10

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access):

Chapter 6, slide 11

AB

C

Hidden terminal problem B, A hear each other B, C hear each other A, C can not hear each other,

meaning that A, C are unaware of their interference at B

A B C

A’s signalstrength

space

C’s signalstrength

Signal attenuation: B, A hear each other B, C hear each other A, C can not hear each

other can interfere at B

Outline

Wireless wireless characteristics multiple access schemes

TDMA/FDMA CDMA

Wi-Fi wireless LANs CSMA/CA IEEE 802.11

Mobility principles:

addressing routing to mobile

users mobile IP

12Chapter 6, slide

Multiple Access SchemesQ: How can multiple users share the medium?

FDMA: Frequency Division Multiple Access

(seen before)

TDMA: Time Division Multiple Access(seen before)

CDMA: Code Division Multiple Access(will be discussed next)

CSMA/CA: Carrier-Sense Multiple Access (i.e., Wi-Fi)(will be discussed later)

Chapter 6, slide 13

FDMA and TDMA (review)

Chapter 6, slide 14

FDMA

frequency

time

TDMA

frequency

time

4 users

Example:

CDMA all users use all frequency (like TDMA) all users send at all time (like FDMA) allows multiple users to “coexist” and

transmit simultaneously

a unique “code” assigned to each user encoding at sender: (original data) x (code) decoding at receiver: (encoded signal) x

(code)

Chapter 6, slide 15

CDMA Encode/Decode: one sender only

Chapter 6, slide 16

2nd bit 1st bit

d1 = -1

1 1 1 1

1- 1- 1- 1-

d0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

2nd bitchanneloutput

1st bitchanneloutput

senderCode: cm

Sent bits: di

Channel output

Zi,m= di.cm

2nd bit 1st bit

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1- d1 = -1

decoded2nd bit

Decoding at receiver:

di = (Zi,m.cm)/8 = (di

.cm.cm)/8 = di

Each user is assigned a unique code: cm = [-1 -1 -1 1 -1 1 1 1]

(length of cm is M = 8 in this example)

We use “-1” to mean “0”

receiverReceived bits

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-Code: cm

Z0,m= d0.cmZ1,m= d1

.cm

d0 = 1

decoded1st bit

Z0,m= d0.cm

Z1,m= d1.cm

Note that cm .cm = 8

CDMA: two-senders and interference

Chapter 6, slide 17

CDMA: two-senders and interference

Chapter 6, slide 18M = length of cm

CDMA: Example of multiple users See board notes for a CDMA example.

Chapter 6, slide 19

ECE/CS 372 – introduction to computer networksLecture 15

20Chapter 6, slide

Announcements: • Assignment 4 due now• Lab 4 due next Tuesday• Assignment 5 posted, due next

Thursday

Credit for lecture slides to Professor Bechir Hamdaoui

Adapted from Jim Kurose & Keith Ross (original copyright)

Outline

Wireless wireless characteristics multiple access schemes

TDMA/FDMA CDMA

Wi-Fi wireless LANs CSMA/CA IEEE 802.11

Mobility principles:

addressing routing to mobile

users mobile IP

21Chapter 6, slide

IEEE 802.11: multiple accessThere are two access operating modes

Chapter 6, slide 22

Infrastructure based mode

Ad hocbased mode

IEEE 802.11: multiple access

Point Coordination Function (PCF)

TDMA-like access Point Coordinator (PC)

polls users in a round-robin fashion

No contention Synchronous Infrastructure mode

Distributed Coordination function (DCF)

CSMA-like access Random access: listen-

before-talk Contention-like medium Asynchronous Both infrastructure and

adhoc modes

23Chapter 6, slide

There are two multiple access functions:

IEEE 802.11 multiple access

Contention-Free Period (CFP)

Synchronous traffic Point Coord. Fct (PCF) is

the access method

Contention Period (CP)

Asynchronous traffic Distr. Coord. Fct

(DCF) is the access method

24Chapter 6, slide

Contention and contention free periods:

CFP CP CFP CP

Access point alternates between CFP and CP modes

IEEE 802.11 DCF MAC one at a time: 2+ nodes send at same time => collision CSMA - sense before transmitting

don’t collide with ongoing transmission by other node

no collision detection difficult to sense collision when transmitting due to weak sigl goal: avoid collisions: CSMA/C(ollision)A(voidance) use Acknowledgment mechanism to recover from collision

Chapter 6, slide 25

AB

CA B C

A’s signalstrength

space

C’s signalstrength

Two scenarios where collision cannot be detected

IEEE 802.11 DCF MACSimple MACif channel idle for SIFS=10 µsec then

sender transmits frame receiver waits for SIFS=10 µsec and sends ACK (SIFS stands for Short InterFrame Space,

and allows HW to switch from rx to tx)

Chapter 6, slide 26

sender receiver

SIFS

data

SIFS

ACK

Challenges/issues(1) more than one communication

higher collision rate (100%)(2) when collision occurs, it at least should not

occur when ACK is being sent allow ACK to be sent successfully

IEEE 802.11 DCF MACSimple MAC w/ minor

improvementif channel idle for DIFS=50 µsec then

sender transmits frame receiver waits for SIFS=10 µsec only and sends ACK SIFS < DIFS ACK gains access before (DIFS stands for DCF InterFrame Space)

Chapter 6, slide 27

sender receiver

DIFS

data

SIFS

ACKChallenges/issues(1) sure, it now prevents collision between

DATA & ACK

(2) but collision can still occur between DATAs

How to reduce collision even further?

IEEE 802.11 DCF MACMAC w/ backoffAfter medium sensed ‘busy’

don’t send right away wait for a random time, then transmit

idea: different senders will hopefully pick different times to transmit so as to avoid colliding with each other again

Chapter 6, slide 28

IEEE 802.11 DCF MACMAC w/ backoffAfter medium sensed ‘busy’

Chapter 6, slide 29

contention window W: define new parameter W = 8, 16, 32, etc

backoff counter b: pick a random number b from [0,W] E.g., if W = 64, perhaps b = 23 Decrement counter b by 1 every idle slot If someone else goes first, reset ‘b’ but don’t change ‘W’ If counter reaches 0, transmit

DIFS DIFS

Medium busy

Defer access

W = contention window

Backoff counter b

Random counter

Data readyTransmit!

IEEE 802.11 DCF MACMAC w/ backoffAfter collision

Chapter 6, slide 30

contention window W: define new parameter W = 8, 16, 32, etc

backoff counter b: pick a random number b from [0,W] E.g., if W = 64, perhaps b = 23 Decrement counter b by 1 every idle slot If counter reaches 0, transmit If another collision, reset ‘b’ & increase ‘W’

DIFS DIFS

Medium busy

Defer access

W = contention window

Backoff counter b

Random counter

Collision?Transmit!

IEEE 802.11 DCF MACMAC w/ backoffAfter collision

Chapter 6, slide 31

ChallengeHow to choose W?

lower W higher collision waste of bandwidthhigher W lower collision but wasted, unused slots

Solution: W should adapt/adjust to loads/# of users W increases as # of users increases and vice-versa Exponential Backoff

DIFS DIFS

Medium busy

Defer access

W = contention window

Backoff counter b

Random counter

Collision?Transmit!

IEEE 802.11 DCF MACMAC w/ exponentional backoff

exponential backoff:(1) increase W when collision(2) decrease W when success

This is how it works: set W = W0 (this is an initial value) pick random backoff counter b from [0,W] transmit when counter reaches 0 if collision occurs, set W 2W (double

window) and repeat Chapter 6, slide 32

IEEE 802.11 DCF MACMAC w/ exponentional backoff

Example Initial window W0 = 8

1st collision: set W1=W0=8; pick b from [0,1,…,7= W1-1]

2nd collision: set W2=2W0=16; pick b from [0,1,…,15= W2-1]

mth collision: set Wm=2(m-1)W0; pick b from [0,1,…, Wm-1]

Maximum window size = Wmax = 2(m-1)W0

After mth collision, Window is set to Wmax

For example: (m+1)th collision, Wm+1= Wmax

at ith collision: Wi = min {2(i-1)W0, Wmax}

Chapter 6, slide 33

IEEE 802.11 DCF MACMAC w/ exponentional backoff

How do you detect a collision without collision detection? You don’t!

You intuit them! Any failed transmission (i.e. you didn’t get an ACK) is

interpreted as a collision.

Chapter 6, slide 34

More challenges: Hidden terminal problem

Chapter 6, slide 35

AB

C

Hidden terminal problem B, A hear each other B, C hear each other A, C can not hear each

other, and hence, may interfere at B

A B C

A’s signalstrength

space

C’s signalstrength

Signal attenuation: B, A hear each other B, C hear each other A, C can not hear each

Problem:If A is transmitting to B, C won’t be able to sense that medium is busy

RTS/CTS handshaking mechanismidea: allow sender to “reserve” channel rather than

random access of data frames Solves the hidden terminal problem Avoids collisions of long data frames

Sender: transmits small request-to-send (RTS) packets Receiving neighbors all hear this RTS defer receipt RTSs may still collide with each other (but they’re short)

Receiver: transmits small clear-to-send (CTS) packets in response to RTS Sending neighbors all hear CTS defer transmission

Chapter 6, slide 36

Collision Avoidance: RTS-CTS exchange

Chapter 6, slide 37

BA C

time

RTS(A)RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

Final words

Carrier sense mechanisms: there are two types

Physical carrier sensingactual sensing of medium to determine whether it is busy or not

Virtual carrier sensingprovided by MAC via RTS/CTS frames. Predicts future traffic based on information/duration indicated in RTS/CTS frames

Chapter 6, slide 38

Outline

Wireless wireless characteristics multiple access schemes

TDMA/FDMA CDMA

Wi-Fi wireless LANs CSMA/CA IEEE 802.11

Mobility principles:

addressing routing to mobile

users mobile IP

39Chapter 6, slide

What is mobility?

spectrum of mobility, from the network perspective:

Chapter 6, slide 40

no mobility high mobility

mobile wireless user, using same access point

mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone)

mobile user, connecting/ disconnecting from network using DHCP.

Mobility: Vocabulary

Chapter 6, slide 41

home network: permanent “home” of mobile(e.g., 128.119.40/24)

Permanent address: address in home network, can always be used to reach mobilee.g., 128.119.40.186

home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote

wide area network

correspondent

Mobility: more vocabulary

Chapter 6, slide 42

Care-of-address: address in visited network.(e.g., 79,129.13.2)

wide area network

visited network: network in which mobile currently resides (e.g., 79.129.13/24)

Permanent address: remains constant (e.g., 128.119.40.186)

foreign agent: entity in visited network that performs mobility functions on behalf of mobile.

correspondent: wants to communicate with mobile

How do you contact a mobile friend:

expect her to update the new phone book: So, search all phone

books for new address?

expect her to let you know where she is? So, use her new

address

expect her to let her parents know So, call her parents?

Chapter 6, slide 43

I wonder where Alice moved to?

Consider a friend who frequently changes addresses, how do you find her?

Mobility: approaches

Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange. routing tables indicate where each mobile

located no changes to end-systems

Let end-systems handle it: Let mobile inform his home agent E.g. let Alice inform her parents Alice’s friend must contact her parent before

Chapter 6, slide 44

Mobility: approaches

Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange. routing tables indicate where each mobile

located no changes to end-systems

Let end-systems handle it: Let mobile inform his home agent E.g. let Alice inform her parents Alice’s friend must contact her parent before

Chapter 6, slide 45

not scalable

to millions of mobiles

Mobility: registration

End result: Foreign agent knows about mobile Home agent knows location of mobile

Chapter 6, slide 46

wide area network

home network

visited network

1

mobile contacts foreign agent on entering visited network

2

foreign agent contacts home agent home: “this mobile is resident in my network”

Mobility via Indirect Routing

Chapter 6, slide 47

wide area network

homenetwork

visitednetwork

3

2

41

correspondent addresses packets using home address of mobile

home agent intercepts packets, forwards to foreign agent

foreign agent receives packets, forwards to mobile

mobile replies directly to correspondent

indirect routing: communication from correspondent to mobile goes via home agent, then forwarded to remote

Mobility via Direct Routing

Chapter 6, slide 48

wide area network

homenetwork

visitednetwork

2

1correspondent requests, receives foreign address of mobile

4

foreign agent receives packets, forwards to mobile

5

mobile replies directly to correspondent

correspondent forwards to foreign agent

3

direct routing: correspondent gets foreign address of mobile, then sends directly to mobile

Outline

Wireless wireless characteristics multiple access schemes

TDMA/FDMA CDMA

Wi-Fi wireless LANs CSMA/CA IEEE 802.11

Mobility principles:

addressing routing to mobile

users mobile IP

49Chapter 6, slide

Mobile IP

RFC 3344 has many features we’ve seen:

home agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet)

Uses indirect routing of datagrams

Chapter 6, slide 50

Mobile IP: indirect routing

Chapter 6, slide 51

Permanent address: 128.119.40.186

Care-of address: 79.129.13.2

dest: 128.119.40.186

packet sent by correspondent

dest: 79.129.13.2 dest: 128.119.40.186

packet sent by home agent to foreign agent: a packet within a packet

dest: 128.119.40.186

foreign-agent-to-mobile packet

Wireless, mobility: last words…

Challenges/issues…

packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoff

congestion: TCP interprets loss as congestion, will decrease congestion window unnecessarily

bandwidth: limited bandwidth of wireless linksChapter 6, slide 52