Ubiquitous Networks WLAN

Lynn ChoiKorea University

Distribution System

Portal

802.x LAN

Access Point

802.11 LAN

BSS2

802.11 LAN

BSS1

Access Point

802.11 - Infrastructure Mode Terminology

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)

PortalBridge to other (wired) networks

Infrastructure modeEvery transmission is with AP

No peer to peer communicationMultiple BSSs form an ESS

STA1

STA2 STA3

ESS

802.11 – Ad Hoc mode

Multiple stations and no APPeer to peer communication onlyIndependent BSS -> IBSS

802.11 LAN

BSS2

802.11 LAN

BSS1

STA1

STA4

STA5

STA2

STA3

Two Coordination Functions

DCF (Distributed Coordination Function)Mandatory functionFor both infrastructure mode and ad hoc mode

99% of WLAN implements DCF only

For distributed contention-based channel access

PCF (Point Coordination Function)OptionalFor infrastructure mode onlyFor centralized contention-free channel access

Polling-basedSeldom-used

Distributed Coordination Function (DCF)

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)Contention-based: all stations contend for access to mediumStations 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

DIFS

DIFS

PIFS

SIFS

Contentionwindow

Next frame

Defer access Wait for reattempt time

Time

Busy medium

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

slot time

CSMA (Carrier Sense Multiple Access)

ALOHATransmit whenever they have data. On a collision, wait a random amount of time and retransmitPure ALOHA, slotted ALOHA

CSMAListen before transmit

If channel sensed idle, transmit entire frameIf channel sensed busy, defer transmission

Don’t interrupt others!

CSMA (Carrier Sense Multiple Access)

CSMA/CA (Collision Avoidance)Transmit after a random backoff even if an idle channel is detectedA variation of this called CSMA-CA is used in WLAN

CSMA-CDAbort transmission as soon as they detect a collisionUsed in Ethernet (802.3)

Exponential backoffBackoff counter is randomly selected from [0, CW], where CW is con-tention windowFor each unsuccessful transmission, CW doubles to reduce the collision probability

Hidden Terminal Problem

RA RC

Node A Node B Node C

transmission

• 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.

Exposed Node Problem

RARD

Node A Node B Node C

transmission

• 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).

Node D

RTS-CTS Exchange

AP

STASTA

Stations do not hear each other. But they hear the AP

RTS-Range

CTS-Range

A way to handle hidden terminal problem– RTS (Request to Send) / CTS (Clear to Send) to reserve

medium– Work with virtual carrier sensing

Net Allocation Vector

t

SIFS

DIFS

Next frame

ACK

defer access

otherstations

receiver

senderdata

DIFS

backoff

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

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.

This is called Virtual Carrier Sensing..

PHY Evolution History11b

Published in 1999, market introduction in 1999WLAN became popular due to 11bUp to 11Mbps (unregulated 2.4GHz band)

11aPublished in 1999, market introduction in 2002Never have been popularUp to 54Mbps (OFDM-based) by using (5GHz band)

11gPublished in 2003 (OFDM-based) to combine the best of 11a &11bMarket introduction in 2003Emerging as a dominating formBackward compatible with 11bUp to 54Mbps by using 2.4GHz band

11n Published in October 2009 (OFDM-based) to combine the best of 11a, 11b &11gAdd MIMO and 40MHz channel (compared to 20MHz chan-nels in 802.11b) to the physical layer and frame aggregation to the MAC layerHigh data rate (54~600Mbps by using 2.4GHz band and lesser used 5GHz band)Backward compatible with 11g

Ubiquitous Networks 802.15: WPAN

Lynn ChoiKorea University

802.15

IEEE 802.15IEEE 802.15 is the 15th working group of the IEEE 802 and specializes in Wireless PAN (Personal Area Network) standards (PHY and MAC layer).

7 Task groupsIEEE 802.15.1 (Bluetooth)IEEE 802.15.2 (Coexistence): WPAN coexistence with other wireless devices oper-ating in unlicensed frequency bands such as WLANIEEE 802.15.3 (High Rate WPAN)IEEE 802.15.4 (Low Rate WPAN)IEEE 802.15.5 (Mesh Networking): mesh networking with WPANs

Network initialization, addressing, multi-hop routing, multicast, etc. for 15.3/4IEEE 802.15.6 (BAN): body area network, short-range wireless standardsIEEE 802.15.7 (VLC): visible light communication

Body Area Network (802.15.6)

Wireless communication for wearable computing devicesCommunication between several small body sensor units (BSU) and a single body central unit (BCU) worn at the human body by using WPAN technologies

ApplicationsHealthcare

Monitoring and logging vital parameters such as diabetes, asthma, and heart attacks

Sports, Military, and Security

SensorsVital sign monitoring sensors, motion detectors

ECG, SpO2, blood pressure, EEG

Physical40MHz channel allocated for 2.4GHz band for medical applications

802.15

IEEE 802.15.1IEEE 802.15.1 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 de-vices, 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 802.15.3MAC and PHY standard for high-rate (11 to 55 Mbit/s) WPANs.

IEEE 802.15.4MAC 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 802.15.4 standard was released in May 2003.

802.15.4IEEE 802.15.4

IEEE 802.15.4-2006 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.

ZigBeeA specification for a suite of high level communication protocols using small, low-power digital radios based on the IEEE 802.15.4-2003 standard.For wireless home area networks (WHANs), such as wireless light switches, elec-trical meters with in-home-displays, consumer electronics equipment.Simpler and less expensive than other WPANs, such as Bluetooth.

6LoWPAN6lowpan (IETF working group) is an acronym of IPv6 over Low power WPAN6lowpan group has defined encapsulation and header compression mechanisms that allow IPv6 packets to be sent /received from over IEEE 802.15.4 networks.Applications: wireless embedded internet, smart grid

Zigbee Protocol Stack

802.15.4 PHY/MAC

PHY layerUse unlicensed frequency bands

868MHz (Europe) / 915 MHz (North America): 20~40 kbps up to 100~250kbps2.4GHz (World): 250kbps

ModulationDSSS (Direct Sequence Spread Spectrum)Binary or offset quadrature phase shift keying, amplitude shift keyingDirect sequence UWB (<1GHz, 1~3GHz, 6~10GHz), CSS (Chirp Spread Spec-trum, 2.4GHz)

MAC layerMAC allows the transmission of MAC frames through the use of the physical chan-nel. It manages access to the physical channel through network beaconing, controls frame validation, guarantees time slots and handles node associations.

802.15.4 Node TypeNode 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 mes-sages, in which case it is dubbed a coordinator (PAN coordinator )Reduced-function devices (RFD) extremely simple devices with very modest re-source and communication requirements; due to this, they can only communicate with FFD's and can never act as coordinators.

802.15.4 TopologyTopology

Either peer-to-peer or star networksP2P: ad-hoc multi-hop network

Cluster TreeMesh network

Star networkA 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

802.15.4 MACFrames

Frames are the basic unit of data transport, of which there are four fundamental types (data, acknowledgment, beacon and MAC command frames)

SuperframeEach 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 algo-rithm is used, while transmissions in the contention free period take place accord-ing to pre-assigned guaranteed timeslots. Up to 7 slots can be allocated by a coor-dinator upon device’s request