1. Ubiquitous Networks WLAN
Lynn Choi
Korea University

2. 802.11 - Infrastructure Mode Terminology Infrastructure mode
Station (STA)
Wireless terminals
Basic Service Area (BSA)
Coverage area of one access point
Basic Service Set (BSS)
Group of stations controlled by the same AP
Distribution System (DS)
Fixed infrastructure used to connect several BSS to create an Extended Service Set (ESS)
Portal
Bridge to other (wired) networks
Infrastructure mode
Every transmission is with AP
No peer to peer communication
Multiple BSSs form an ESS
LAN
802.x LAN
STA1
BSS1
Portal
Access
Point
Distribution System
Access
Point
ESS
BSS2
STA2
STA3
LAN 9

3. 802.11 – Ad Hoc mode Multiple stations and no AP
LAN
Multiple stations and no AP
Peer to peer communication only
Independent BSS -> IBSS
STA1
BSS1
STA3
STA2
BSS2
STA5
STA4
LAN

4. Two Coordination Functions
DCF (Distributed Coordination Function)
Mandatory function
For both infrastructure mode and ad hoc mode
99% of WLAN implements DCF only
For distributed contention-based channel access
PCF (Point Coordination Function)
Optional
For infrastructure mode only
For centralized contention-free channel access
Polling-based
Seldom-used

5. Distributed Coordination Function (DCF)
DIFS
PIFS
SIFS
Contention
window
Next frame
Defer access
Wait for
reattempt time
Time
Busy medium
slot time
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
Contention-based: all stations contend for access to medium
Stations wait as long as channel is busy
When channel becomes idle,
All stations must wait Interframe Space (IFS)
SIFS (Short IFS), PIFS (PCF IFS), DIFS (DCF IFS)
Use random backoff to avoid collision
Slot Time
SIFS
PIFS
DIFS
802.11a
9 usec
16 usec
25 usec
34 usec
802.11b
20 usec
10 usec
30 usec
50 usec

6. CSMA (Carrier Sense Multiple Access)
ALOHA
Transmit whenever they have data.
On a collision, wait a random amount of time and retransmit
Pure ALOHA, slotted ALOHA
CSMA
Listen before transmit
If channel sensed idle, transmit entire frame
If channel sensed busy, defer transmission
Don’t interrupt others!

7. CSMA (Carrier Sense Multiple Access)
CSMA/CA (Collision Avoidance)
Transmit after a random backoff even if an idle channel is detected
A variation of this called CSMA-CA is used in WLAN
CSMA-CD
Abort transmission as soon as they detect a collision
Used in Ethernet (802.3)
Exponential backoff
Backoff counter is randomly selected from [0, CW], where CW is contention window
For each unsuccessful transmission, CW doubles to reduce the collision probability

8. Hidden Terminal ProblemRA RC
transmission
Node A
Node B
Node C
Node A is not aware that node B is currently busy receiving from node C, and therefore may start its own transmission, causing a collision.

9. Exposed Node Problem RD RA Node D Node A Node B Node C
transmission
Node D
Node A
Node B
Node C
Node B wants to transmit to node C but mistakenly thinks that this will interfere with A’s transmission to D, so B refrains from transmitting (loss in efficiency).

10. RTS-CTS Exchange
A way to handle hidden terminal problem
RTS (Request to Send) / CTS (Clear to Send) to reserve medium
Work with virtual carrier sensing AP
STA
Stations do not hear each other.
But they hear the AP
RTS-Range
CTS-Range

11. Net Allocation Vector This is called Virtual Carrier Sensing..
DIFS
RTS
data
sender
SIFS
SIFS
CTS
SIFS
ACK
receiver
NAV (RTS)
DIFS
Next frame
other
stations
NAV (CTS) t
defer access
backoff
The RTS packet has a duration field, which consists of information about the length of data packet.
Other stations hear the RTS packet set their NAV accordingly.
The CTS packet also has the duration field.
Other stations hear the CTS packet set their NAV accordingly.

12. PHY Evolution History 11b 11a 11g 11n
Published in 1999, market introduction in 1999
WLAN became popular due to 11b
Up to 11Mbps (unregulated 2.4GHz band)
11a
Published in 1999, market introduction in 2002
Never have been popular
Up to 54Mbps (OFDM-based) by using (5GHz band)
11g
Published in 2003 (OFDM-based) to combine the best of 11a &11b
Market introduction in 2003
Emerging as a dominating form
Backward compatible with 11b
Up to 54Mbps by using 2.4GHz band
11n
Published in October 2009 (OFDM-based) to combine the best of 11a, 11b &11g
Add MIMO and 40MHz channel (compared to 20MHz channels in b) to the physical layer and frame aggregation to the MAC layer
High data rate (54~600Mbps by using 2.4GHz band and lesser used 5GHz band)
Backward compatible with 11g

13. Ubiquitous Networks 802.15: WPAN
Lynn Choi
Korea University

14. 802.15
IEEE
IEEE is the 15th working group of the IEEE 802 and specializes in Wireless PAN (Personal Area Network) standards (PHY and MAC layer).
7 Task groups
IEEE (Bluetooth)
IEEE (Coexistence): WPAN coexistence with other wireless devices operating in unlicensed frequency bands such as WLAN
IEEE (High Rate WPAN)
IEEE (Low Rate WPAN)
IEEE (Mesh Networking): mesh networking with WPANs
Network initialization, addressing, multi-hop routing, multicast, etc. for 15.3/4
IEEE (BAN): body area network, short-range wireless standards
IEEE (VLC): visible light communication

15. Body Area Network ( )
Wireless communication for wearable computing devices
Communication between several small body sensor units (BSU) and a single body central unit (BCU) worn at the human body by using WPAN technologies
Applications
Healthcare
Monitoring and logging vital parameters such as diabetes, asthma, and heart attacks
Sports, Military, and Security
Sensors
Vital sign monitoring sensors, motion detectors
ECG, SpO2, blood pressure, EEG
Physical
40MHz channel allocated for 2.4GHz band for medical applications

16. 802.15
IEEE
IEEE is the WPAN standard based on the Bluetooth specifications. It includes a media access control and physical layer specification.
Bluetooth is an open wireless technology standard for exchanging data over short distances (using short wavelength radio transmissions) from fixed and mobile devices, creating personal area networks (PANs) with high levels of security.
Created by telecoms vendor Ericsson in 1994, it was originally conceived as a wireless alternative to RS-232 data cables. It can connect several devices, overcoming problems of synchronization.
IEEE
MAC and PHY standard for high-rate (11 to 55 Mbit/s) WPANs.
IEEE
MAC and PHY standard for low data rate but very long battery life (months or even years) and very low complexity. The first edition of the standard was released in May 2003.

17.
IEEE
IEEE is a standard which specifies the physical layer and media access control for low-rate wireless personal area networks (LR-WPANs).
It is the basis for the ZigBee specification.
Can be used with 6LoWPAN to build a Wireless Embedded Internet.
ZigBee
A specification for a suite of high level communication protocols using small, low-power digital radios based on the IEEE standard.
For wireless home area networks (WHANs), such as wireless light switches, electrical meters with in-home-displays, consumer electronics equipment.
Simpler and less expensive than other WPANs, such as Bluetooth.
6LoWPAN
6lowpan (IETF working group) is an acronym of IPv6 over Low power WPAN
6lowpan group has defined encapsulation and header compression mechanisms that allow IPv6 packets to be sent /received from over IEEE networks.
Applications: wireless embedded internet, smart grid

18. Zigbee Protocol Stack

19. 802.15.4 PHY/MAC PHY layer MAC layer Use unlicensed frequency bands
868MHz (Europe) / 915 MHz (North America): 20~40 kbps up to 100~250kbps
2.4GHz (World): 250kbps
Modulation
DSSS (Direct Sequence Spread Spectrum)
Binary or offset quadrature phase shift keying, amplitude shift keying
Direct sequence UWB (<1GHz, 1~3GHz, 6~10GHz), CSS (Chirp Spread Spectrum, 2.4GHz)
MAC layer
MAC allows the transmission of MAC frames through the use of the physical channel. It manages access to the physical channel through network beaconing, controls frame validation, guarantees time slots and handles node associations.

20. Node Type
Node type
Full-function device (FFD) serve as the coordinator of a personal area network just as it may function as a common node. It implements a general model of communication which allows it to talk to any other device: it may also relay messages, in which case it is dubbed a coordinator (PAN coordinator )
Reduced-function devices (RFD) extremely simple devices with very modest resource and communication requirements; due to this, they can only communicate with FFD's and can never act as coordinators.

21. 802.15.4 Topology Topology Either peer-to-peer or star networks
P2P: ad-hoc multi-hop network
Cluster Tree
Mesh network
Star network
A FFD declare itself its coordinator. After that, other devices can join.
Every network needs at least one FFD to work as the coordinator of the network.
Network coordinator =
Root of the cluster tree

22. MAC
Frames
Frames are the basic unit of data transport, of which there are four fundamental types (data, acknowledgment, beacon and MAC command frames)
Superframe
Each superframe consists of 16 equally sized slots and is bounded by network beacons, which are periodically broadcast by a designated coordinator device.
During the contention access period, the slotted mode of the CSMA-CA algorithm is used, while transmissions in the contention free period take place according to pre-assigned guaranteed timeslots. Up to 7 slots can be allocated by a coordinator upon device’s request