ECE 4450:427/527 - Computer Networks Spring 2014
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Transcript of ECE 4450:427/527 - Computer Networks Spring 2014
ECE 4450:427/527 - Computer NetworksSpring 2015
Dr. Nghi TranDepartment of Electrical & Computer Engineering
Lecture 5.6: Wireless Networks - MAC
Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 1
Wireless Mobile Networks
Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 2
• Brief Introduction
• Wireless Channel Characteristics
• WiFi: CSMA/CA - Collision Avoidance
Elements of Wireless Networks
Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 3
network infrastructure
wireless hosts laptop, PDA, IP phone run applications may be stationary (non-
mobile) or mobile wireless does not always
mean mobility
Elements of Wireless Mobile Networks
Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 4
network infrastructure
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
Characteristics of selected wireless link standards
Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 5
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m – 4 Km
Long-rangeoutdoor
5Km – 20 Km
.056
.384
1
4
5-11
54
IS-95, CDMA, GSM 2G
UMTS/WCDMA, CDMA2000 3G
802.15
802.11b
802.11a,g
UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO 3G cellularenhanced
802.16 (WiMAX)
802.11a,g point-to-point
200 802.11n
Dat
a ra
te (
Mbp
s)
data
Wireless Mobile Networks
Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 6
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
Wireless Network Taxonomy
Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 7
single hop multiple hops
infrastructure(e.g., APs)
noinfrastructure
host connects to base station (WiFi,WiMAX, cellular)
which connects to larger Internet
no base station, noconnection to larger Internet (Bluetooth,
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger Internet: mesh net
no base station, noconnection to larger Internet. May have torelay to reach other a given wireless node
MANET, VANET
Wireless Channel Characteristics
ECE-University of Akron ECE 4450:427/527 Computer Networks 8
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”
Multipath propagation
ECE-University of Akron ECE 4450:427/527 Computer Networks 9
• Constructive and destructive interference: channel strengths change randomly with time → Fading
• When channel is weak, i.e., bad quality→Low reliability
Hidden Terminal Problem
ECE-University of Akron ECE 4450:427/527 Computer Networks 10
Multiple wireless senders and receivers create additional problems (beyond multiple access):
AB
C
Hidden terminal problem: Signals blocked B, A hear each other B, C hear each other A, C can not hear each othermeans A, C unaware of their interference at B
Hidden Terminal Problem
ECE-University of Akron ECE 4450:427/527 Computer Networks 11
A B C
A’s signalstrength
space
C’s signalstrength
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
Exposed Node Problem
ECE-University of Akron ECE 4450:427/527 Computer Networks 12
• 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.
IEEE 802.11 Wireless LAN
ECE-University of Akron ECE 4450:427/527 Computer Networks 13
• 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
• 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
all use CSMA/CA for multiple access all have base-station and ad-hoc network versions
80211. Wireless LAN Architecture
ECE-University of Akron ECE 4450:427/527 Computer Networks 14
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
BSS 1
BSS 2
Internet
hub, switchor routerAP
AP
IEEE 802.11 Multiple Access
ECE-University of Akron ECE 4450:427/527 Computer Networks 15
• 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)
IEEE 802.11 MAC Protocol: CSMA/CA
ECE-University of Akron ECE 4450:427/527 Computer Networks 16
sender receiver
DIFS
data
SIFS
ACK
802.11 sender1 if sense channel idle for DIFS then transmit entire
frame (no CD) – cannot sense when transmitting2 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
To deal with transmitting and sensing problems
IEEE 802.11 MAC Protocol: CSMA/CA
ECE-University of Akron ECE 4450:427/527 Computer Networks 17
idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of data frames
• sender first transmits small request-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!
To deal with hidden terminal problem: Using RTS and CTS
IEEE 802.11 MAC Protocol: CSMA/CA
ECE-University of Akron ECE 4450:427/527 Computer Networks 18
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
IEEE 802.11 MAC Protocol: CSMA/CA
ECE-University of Akron ECE 4450:427/527 Computer Networks 19
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