Ece 4450 427 527 computer networks spring 2014
This presentation is the property of its rightful owner.
Sponsored Links
1 / 19

ECE 4450:427/527 - Computer Networks Spring 2014 PowerPoint PPT Presentation


  • 70 Views
  • Uploaded on
  • Presentation posted in: General

ECE 4450:427/527 - Computer Networks Spring 2014. Dr. Nghi Tran Department of Electrical & Computer Engineering. Lecture 5.6: Wireless Networks - MAC. Wireless Mobile Networks. Brief Introduction Wireless Channel Characteristics WiFi : CSMA/CA - C ollision A voidance.

Download Presentation

ECE 4450:427/527 - Computer Networks Spring 2014

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Ece 4450 427 527 computer networks spring 2014

ECE 4450:427/527 - Computer NetworksSpring 2014

Dr. Nghi Tran

Department of Electrical & Computer Engineering

Lecture 5.6: Wireless Networks - MAC

ECE 4450:427/527


Wireless mobile networks

Wireless Mobile Networks

  • Brief Introduction

  • Wireless Channel Characteristics

  • WiFi: CSMA/CA - Collision Avoidance

ECE 4450:427/527


Elements of wireless networks

Elements of Wireless Networks

wireless hosts

  • laptop, PDA, IP phone

  • run applications

  • may be stationary (non-mobile) or mobile

    • wireless does not always mean mobility

network

infrastructure

ECE 4450:427/527


Elements of wireless mobile networks

Elements of Wireless Mobile Networks

base station

  • typically connected to wired network

  • relay - responsible for sending packets between wired network and wireless host(s) in its “area”

    • e.g., cell towers, 802.11 Access Points (AP)

  • Handoff: Mobile changes station

network

infrastructure

ECE 4450:427/527


Characteristics of selected wireless link standards

Characteristics of selected wireless link standards

200

802.11n

54

802.11a,g

802.11a,g point-to-point

data

5-11

802.11b

802.16 (WiMAX)

3G cellular

enhanced

4

UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO

Data rate (Mbps)

1

802.15

.384

3G

UMTS/WCDMA, CDMA2000

2G

.056

IS-95, CDMA, GSM

Indoor

10-30m

Outdoor

50-200m

Mid-range

outdoor

200m – 4 Km

Long-range

outdoor

5Km – 20 Km

ECE 4450:427/527


Wireless mobile networks1

Wireless Mobile Networks

ad hoc mode

  • no base stations

  • nodes can only transmit to other nodes within link coverage

  • nodes organize themselves into a network: route among themselves

ECE 4450:427/527


Wireless network taxonomy

Wireless Network Taxonomy

multiple hops

single hop

host may have to

relay through several

wireless nodes to

connect to larger

Internet: mesh net

host connects to

base station (WiFi,

WiMAX, cellular)

which connects to

larger Internet

infrastructure

(e.g., APs)

no base station, no

connection to larger

Internet. May have to

relay to reach other

a given wireless node

MANET, VANET

no

infrastructure

no base station, no

connection to larger

Internet (Bluetooth,

ad hoc nets)

ECE 4450:427/527


Wireless channel characteristics

Wireless Channel Characteristics

Differences from wired link ….

  • decreased 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); devices (motors) interfere as well

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

    …. make communication across (even a point to point) wireless link much more “difficult”

ECE 4450:427/527


Multipath propagation

Multipath propagation

  • Constructive and destructive interference: channel strengths change randomly with time→ Fading

  • When channel is weak, i.e., bad quality→Low reliability

ECE 4450:427/527


Hidden terminal problem

Hidden Terminal Problem

C

B

A

Multiple wireless senders and receivers create additional problems (beyond multiple access):

Hidden terminal problem: Signals blocked

  • B, A hear each other

  • B, C hear each other

  • A, C can not hear each other

    means A, C unaware of their interference at B

ECE 4450:427/527


Hidden terminal problem1

Hidden Terminal Problem

B

A

C

C’s signal

strength

A’s signal

strength

space

  • Hidden terminal problem due to signal attenuation:

  • B, A hear each other

  • B, C hear each other

  • A, C can not hear each other interfering at B

ECE 4450:427/527


Exposed node problem

Exposed Node Problem

  • Suppose B is sending to A. Node C is aware of this communication because it hears B’s transmission.

  • It would be a mistake for C to conclude that it cannot transmit to anyone just because it can hear B’s transmission.

  • Suppose C wants to transmit to node D. This is not a problem since C’s transmission to D will not interfere with A’s ability to receive from B.

ECE 4450:427/527


Ieee 802 11 wireless lan

IEEE 802.11 Wireless LAN

  • 802.11a

    • 5-6 GHz range

    • up to 54 Mbps

  • 802.11g

    • 2.4-5 GHz range

    • up to 54 Mbps

  • 802.11n:multiple antenna

    • 2.4-5 GHz range

    • up to 600 Mbps

  • 802.11b

    • 2.4-5 GHz unlicensed spectrum

    • up to 11 Mbps

    • direct sequence spread spectrum (DSSS) in physical layer

      • all hosts use same chipping code

  • all use CSMA/CA for multiple access

  • all have base-station and ad-hoc network versions

ECE 4450:427/527


80211 wireless lan architecture

80211. Wireless LAN Architecture

AP

AP

Internet

  • wireless host communicates with base station

    • base station = access point (AP)

  • Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains:

    • wireless hosts

    • access point (AP): base station

    • ad hoc mode: hosts only

hub, switch

or router

BSS 1

BSS 2

ECE 4450:427/527


Ieee 802 11 multiple access

IEEE 802.11 Multiple Access

  • avoid collisions: 2+ nodes transmitting at same time

  • 802.11: CSMA - sense before transmitting

    • don’t collide with ongoing transmission by other node

  • 802.11: no collision detection as in Ethernet!

    • difficult to receive (sense collisions) and transmitting at the same time due to weak received signals: swamps the receiving circuitry

    • can’t sense all collisions in any case: hidden terminal, fading

    • goal: avoid collisions: CSMA/C(ollision)A(voidance)

ECE 4450:427/527


Ieee 802 11 mac protocol csma ca

IEEE 802.11 MAC Protocol: CSMA/CA

DIFS

data

SIFS

ACK

To deal with transmitting and sensing problems

802.11 sender

1 if sense channel idle for DIFSthen transmit entire frame (no CD) – cannot sense when transmitting

2 if sense channel busy then

- start random backoff time

  • timer counts down while channel idle. If busy, timer frozen

  • transmit when timer expires. Wait for ACK

    - ACK: New frame, go to step 1; if no ACK, increase random backoff interval, repeat 2

    802.11 receiver

    - if frame received OK

    return ACK after SIFS

sender

receiver

ECE 4450:427/527


Ieee 802 11 mac protocol csma ca1

IEEE 802.11 MAC Protocol: CSMA/CA

To deal with hidden terminal problem: Using RTS and CTS

idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of data frames

  • sender first transmits smallrequest-to-send (RTS) packets to BS using CSMA

    • RTSs may still collide with each other (but they’re short)

  • BS broadcasts clear-to-send (CTS) in response to RTS

  • CTS heard by all nodes

    • sender transmits data frame

    • other stations defer transmissions

avoid data frame collisions completely

using small reservation packets!

ECE 4450:427/527


Ieee 802 11 mac protocol csma ca2

IEEE 802.11 MAC Protocol: CSMA/CA

RTS(B)

RTS(A)

reservation collision

RTS(A)

CTS(A)

CTS(A)

DATA (A)

ACK(A)

ACK(A)

B

A

AP

defer

time

ECE 4450:427/527


Ieee 802 11 mac protocol csma ca3

IEEE 802.11 MAC Protocol: CSMA/CA

  • Applets to check out:

  • http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/csma-ca/withhidden.html

  • http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/csma-ca/withouthidden.html

ECE 4450:427/527


  • Login