1. ECE 4450:427/527 - Computer Networks Spring 2015
Dr. Nghi Tran
Department of Electrical & Computer Engineering
Lecture 5.6: Wireless Networks - MAC
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

2. Wireless Mobile Networks
Brief Introduction
Wireless Channel Characteristics
WiFi: CSMA/CA - Collision Avoidance
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

3. 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
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

4. 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, Access Points (AP)
Handoff: Mobile changes station
network
infrastructure
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

5. Characteristics of selected wireless link standards
200
802.11n 54
802.11a,g
802.11a,g point-to-point
data
5-11
802.11b
(WiMAX) 4
3G cellular
enhanced
UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
Data rate (Mbps) 1
802.15
.384
UMTS/WCDMA, CDMA2000 3G
.056
IS-95, CDMA, GSM 2G
Indoor
10-30m
Outdoor
50-200m
Mid-range
outdoor
200m – 4 Km
Long-range
outdoor
5Km – 20 Km
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

6. 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
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

7. Wireless Network Taxonomy
single hop
multiple hops
host connects to
base station (WiFi,
WiMAX, cellular)
which connects to
larger Internet
host may have to
relay through several
wireless nodes to
connect to larger
Internet: mesh net
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)
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

8. 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-University of Akron
ECE 4450:427/527

9. Multipath propagation
Constructive and destructive interference: channel strengths change randomly with time → Fading
When channel is weak, i.e., bad quality→Low reliability
ECE-University of Akron
ECE 4450:427/527

10. Hidden Terminal Problem
Multiple wireless senders and receivers create additional problems (beyond multiple access): A B C
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-University of Akron
ECE 4450:427/527

11. Hidden Terminal ProblemC
A’s signal
strength
space
C’s signal
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-University of Akron
ECE 4450:427/527

12. 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-University of Akron
ECE 4450:427/527

13. IEEE Wireless LAN
802.11a
5-6 GHz range
up to 54 Mbps
802.11g
2.4-5 GHz range
802.11n: multiple antenna
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-University of Akron
ECE 4450:427/527

14. 80211. Wireless LAN Architecture
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 AP
hub, switch
or router AP
BSS 1
BSS 2
ECE-University of Akron
ECE 4450:427/527

15. IEEE 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-University of Akron
ECE 4450:427/527

16. IEEE 802.11 MAC Protocol: CSMA/CA
To deal with transmitting and sensing problems
sender
1 if sense channel idle for DIFS then 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
receiver
- if frame received OK
return ACK after SIFS
sender
receiver
DIFS
data
SIFS
ACK
ECE-University of Akron
ECE 4450:427/527

17. 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 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!
ECE-University of Akron
ECE 4450:427/527

18. IEEE 802.11 MAC Protocol: CSMA/CAB AP
RTS(A)
RTS(B)
reservation collision
RTS(A)
CTS(A)
DATA (A)
ACK(A)
defer
time
ECE-University of Akron
ECE 4450:427/527

19. IEEE 802.11 MAC Protocol: CSMA/CA
Applets to check out:
ECE-University of Akron
ECE 4450:427/527