wireless law

Luis Corrales, PhD EPN-DACI 1

SMART SENSOR

• ES UN SENSOR PROVISTO DE UN PROTOCOLO DE COMUNICACIÓN DIGITAL


SMART SENSOR

• SENSOR CIRCUIT: PROVEE ACONDICONAMIENTO DE LA SEÑAL QUE ENTREGA EL SENSOR. ESTO LE CONVIERTE A LA TARJETA EN ÚNICA.


SMART SENSOR

• SENSOR CIRCUIT: CONVIERTE LA SEÑAL ANALOGA A DIGITAL


SMART SENSOR

• NETWORK CHIP: ES EL QUE PORVEE EL PROTOCOLO DE COMUNICACIONES, P.E. ETHERNET.


SMART SENSOR

• NETWORK TRANSCEIVER: ES EL QUE COVIERTE LA SEÑAL DIGITAL LÓGICA EN VALORES REALES DE VOLTAJE, DEPENDIENDO DEL MEDIO.


SMART SENSOR

• SI EL MEDIO ES COBRE: CONVIERTE A VOLTIOS. • FIBRA ÓPTICA: CONVIERTE A LUZ • INLÁMBRICA: CONVIERTE A ONDAS

ELECTROMAGNÉTICAS.


SMART SENSOR

• UN PROBLEMA QUE SE TIENE CON LOS SENSORES Y ACTUADORES INTELIGENTES ES LA OBTENCIÓN DE LA ALIMENTACIÓN ELÉCTRICA.


SMART SENSOR

• UN SENSOR O ACTUADOR CARECE DE ALIMENTACIÓN ELÉCTRICA Y TOCA ALIMENTAR AL CONJUNTO POR EL MISMO CABLE DE DATOS.


SMART SENSOR

• UNA SOLUCIÓN MUY CONOCIDA EL PoE. • POWER OVER ETHERNET. • HAY SWITCHES ESPECIALES QUE PROPORCIONAN ESTA

ALTERNATIVA.


What is a wireless LAN?

• Wireless LAN (WLAN) - provides all the features and benefits of traditional LAN technologies such as Ethernet and Token Ring, but without the limitations of wires or cables.



• WLAN, like a LAN, requires a physical medium to transmit signals.

• Instead of using UTP, WLANs use:

– Infrared light (IR)

• 802.11 does include an IR specification

• limitations, easily blocked, no real 802.11 products (IrDA)

– Radio frequencies (RFs)

• Can penetrate ‘most’ office obstructions

http://earlyradiohistory.us/1920au.htm



• WLANs use the 2.4 GHz and 5-GHz frequency bands.

• ISM (Industry, Scientific, Medical) license-free (unlicensed) frequency bands.

• S-Band ISM

– 802.11b and 802.11g: 2.4- 2.5 GHz

• C-Band ISM

– 802.11a: 5.725 – 5.875 GHz

More later!


IEEE 802.11 and the Wi-Fi Alliance

• IEEE LAN/MAN Standards Committee (LMSC)

– First 802.11 standard released in 1997, several since then

• Wireless Ethernet Compatibility Alliance (WECA)

– Advertises its Wi-Fi (wireless fidelity) program

– Any 802.11 vendor can have its products tested for interoperability

– Cisco is a founding member


Wi-Fi™ • Wi-Fi™ Alliance

– WECA changed its name to Wi-Fi

– Wireless Fidelity Alliance

– 170+ members

– Over 350 products certified

• Wi-Fi’s™ Mission – Certify interoperability of WLAN products (802.11)

– Wi-Fi™ is the “stamp of approval”

– Promote Wi-Fi™ as the global standard


Other Wireless Technologies

Not discussed in this course:

• Cellular

• Bluetooth or PAN (Personal Area Network)

• 3G (3rd Generation)

• UWB (Ultra Wide Band)

• FSO (Free Space Optics)

• Radio waves off meteor trails!


Why Wireless?

Luis Corrales, PhD EPN-DACI

860 Kbps

900 MHz

1 and 2 Mbps

2.4 GHz

Proprietary

WLAN Evolution •Warehousing

•Retail

•Healthcare

•Education

•Businesses

•Home

802.11

Ratified

802.11a,b

Ratified

802.11g

Drafted

1986 1988 1990 1992 1994 1996 1998 2000 2002

1 and 2 Mbps

2.4 GHz

11 Mbps 54 Mbps

Standards-based

5 GHz Radio

Network

Speed

IEEE 802.11Begins

Drafting

17


Current Standards – a, b, g

• 802.11a – Up to 54 Mbps – 5 GHz – Not compatible with either 802.11b or 802.11g

• 802.11b – Up to 11 Mbps – 2.4 GHz

• 802.11g – Up to 54 Mbps – 2.4 GHz

860 Kbps

900 MHz

1 and 2 Mbps

2.4 GHz

Proprietary

802.11

Ratified

802.11a,b

Ratified

1986 1988 1990 1992 1994 1996 1998 2000 2003

1 and 2 Mbps

2.4 GHz

11 Mbps 54 Mbps

Standards-based

5 GHz Radio

Network

Speed

IEEE 802.11Begins

Drafting

802.11g is backwards compatible

with 802.11b, but with a drawback

(later)

802.11g

Ratified

More later!


802.11 PHY (Physical Layer) Technologies

• Infrared light

• Three types of radio transmission within the unlicensed 2.4-GHz frequency bands:

– Frequency hopping spread spectrum (FHSS) 802.11b (not used)

– Direct sequence spread spectrum (DSSS) 802.11b

– Orthogonal frequency-division multiplexing (OFDM) 802.11g

• One type of radio transmission within the unlicensed 5-GHz frequency bands:

– Orthogonal frequency-division multiplexing (OFDM) 802.11a

860 Kbps

900 MHz

1 and 2 Mbps

2.4 GHz

Proprietary

802.11

Ratified

802.11a,b

Ratified

802.11g

Ratified

1986 1988 1990 1992 1994 1996 1998 2000 2003

1 and 2 Mbps

2.4 GHz

11 Mbps 54 Mbps

Standards-based

5 GHz Radio

Network

Speed

IEEE 802.11Begins

Drafting

More later!


Atmosphere: the wireless medium

• Wireless signals are electromagnetic waves • No physical medium is necessary • The ability of radio waves to pass through walls and cover great distances

makes wireless a versatile way to build a network.


WLAN Devices

In-building Infrastructure

• 1200 Series (802.11a and 802.11b)

• 1100 Series (802.11b)

• 350 Series (802.11b) not shown

Bridging

• 350 Series (802.11b)

•BR350

•WGB350

• 1400 Series (802.11a)


Antennas Antenna

•2.4GHz Antennas

•5 GHz Antennas


Four main requirements for a WLAN solution

1. High availability — High availability is achieved through system redundancy and proper coverage-area design.

2. Scalability — Scalability is accomplished by supporting multiple APs per coverage area, which use multiple frequencies. APs can also perform load balancing, if desired.

3. Manageability — Diagnostic tools represent a large portion of management within WLANs. Customers should be able to manage WLAN devices through industry standard APIs, including SNMP and Web, or through major enterprise management applications like CiscoWorks 2000, Cisco Stack Manager, and Cisco Resource Monitor.

4. Open architecture — Openness is achieved through adherence to standards such as 802.11a and 802.11b, participation in interoperability associations such as the Wi-Fi Alliance, and certification such as U.S. FCC certification.


Other requirements

• Security — It is essential to encrypt data packets transmitted through the air. For larger installations, centralized user authentication and centralized management of encryption keys are also required.

• Cost — Customers expect continued reductions in price of 15 to 30 percent each year, and increases in performance and security. Customers are concerned not only with purchase price but also with total cost of ownership (TCO), including costs for installation.

Challenges and Issues



Radio Signal Interference

• Network managers must ensure that different channels are utilized.

• Interference cannot always be detected until the link is actually

implemented.

• Because the 802.11 standards use unlicensed spectrum, changing

channels is the best way to avoid interference.

• If someone installs a link that interferes with a wireless link, the

interference is probably mutual.


Radio Signal Interference

• To minimize the possible effects of electromagnetic interference (EMI), the best course of action is to isolate the radio equipment from potential sources of EMI.


Power Consumption

• Power consumption is always an issue with laptops, because the

power and the battery have limited lives.

• 802.11a uses a higher frequency (5 GHz) than 802.11a/g (2.4 GHz)

which requires higher power and more of a drain on batteries.


Interoperability

• Non-standard (for now) 802.11 devices include:

• Repeater APs

• Universal Clients (Workgroup Bridges)

• Wireless Bridges

• Cisco bridges, like many other vendor bridges, are proprietary

implementations of the 802.11 standard and therefore vendor

interoperability cannot be attained.


Wireless LAN Security: Lessons

“War Driving”

Hacking into WEP

Lessons:

• Security must be turned on (part of the installation process)

• Employees will install WLAN equipment on their own (compromises security of your entire network)

• WEP keys can be easily broken (businesses need better security)


Wireless LAN Security

• Security in the IEEE 802.11 specification—which applies to 802.11b, 802.11a, and 802.11g—has come under intense scrutiny.

• Researchers have exposed several vulnerabilities. • As wireless networks grow, the threat of intruders from the inside and

outside is great. • Attackers called “war drivers” are continually driving around searching for

insecure WLANs to exploit.


Installation and Site Design Issues—Bridging


Installation and Site Design Issues—WLAN


Health Issues


IEEE 802.11 Standards Activities • 802.11a: 5GHz, 54Mbps • 802.11b: 2.4GHz, 11Mbps • 802.11d: Multiple regulatory domains • 802.11e: Quality of Service (QoS) • 802.11f: Inter-Access Point Protocol (IAPP) • 802.11g: 2.4GHz, 54Mbps • 802.11h: Dynamic Frequency Selection (DFS)

and Transmit Power Control (TPC) • 802.11i: Security • 802.11j: Japan 5GHz Channels (4.9-5.1 GHz) • 802.11k: Measurement

802.11 Standards



Overview of Standardization

• Standardization of networking functions has done much to further the development of affordable, interoperable networking products.

• This is true for wireless products as well. • Prior to the development of standards, wireless systems were plagued with

low data rates, incompatibility, and high costs. • Standardization provides all of the following benefits:

– Interoperability among the products of multiple vendors – Faster product development – Stability – Ability to upgrade – Cost reductions


IEEE and 802.11

• IEEE, founded in 1884, is a nonprofit professional organization

• Plays a critical role in developing standards, publishing technical works, sponsoring conferences, and providing accreditation in the area of electrical and electronics technology.

• In the area of networking, the IEEE has produced many widely used standards such as the 802.x group of local area network (LAN) and metropolitan area network (MAN) standards,


IEEE 802 Architecture

Some you may recognize:

• 802.3 – CSMA/CD (Carrier Sense Multiple Access with Collision Detection), often mistakenly called Ethernet

• 802.1d – Spanning Tree

• 802.1Q – VLANs

• 802.5 – Token Ring


IEEE 802.11 Architecture

• 802.11 is a family of protocols, including the original specification, 802.11, 802.11b, 802.11a, 802.11g and others.

• Officially called the IEEE Standard for WLAN MAC and PHY specifications.

• 802.11 “is just another link layer for 802.2”

• 802.11 is sometimes called wireless Ethernet, because of its shared lineage with Ethernet, 802.3.

• The wired network side of the network could be Ethernet, Token Ring, etc.(we will always use Ethernet in our examples)

• Access Points and Bridges act as “translation bridges” between 802.11 and 802.3 (or other other protocol)

Overview of WLAN Topologies

IBSS BSS ESS

Access Points Quick Preview: Station/AP Connectivity



Overview of WLAN Topologies

• Three types of WLAN Topologies:

– Independent Basic Service Sets (IBSS)

– Basic Service Set (BSS)

– Extended Service Set (ESS)

• Service Set – A logical grouping of devices.

• WLANs provide network access by broadcasting a signal across a wireless radio frequency.

• Transmitter prefaces its transmissions with a Service Set Identifier (SSID)

• A station may receive transmissions from transmitters with the same or different SSIDs.


Independent Basic Service Sets (IBSS)

• IBSS consists of a group of 802.11 stations directly communicating with each other.

• No Access Point used

• Also known as an ad-hoc network.

• Usage: Few stations setup up for a specific purpose for a short period of time. (ex. file transfers.)

• We will have a an IBSS lab, but our main focus will be BSSs and ESSs.


Basic Service Set (BSS)

• BSS, also known as an Infrastructure BSS (never called IBSS)

• Requires an Access Point (AP)

– Converts 802.11 frames to Ethernet and visa versa

– Known as a translation bridge

• Stations do not communicate directly, but via the AP

• APs typically have an uplink port that connects the BSS to a wired network (usually Ethernet), known as the Distribution System (DS).


Extended Service Set (ESS)

• Multiple BSSs can be connected together with a layer 2 “backbone network” to form an Extended Service Set (ESS).

• 802.11 does not specify the backbone network

• The backbone network is also known as the Distribution System (DS) and could be wired or wireless.

• Stations are “associated” with only one AP at a time.

• The SSID is the same for all BSS areas in the ESS (unless creating multiple BSSs, i.e. one for Marketing and another for Sales).


• What if you want to be able to move between access points without the latency of re-association and re-authentication (these will be explained)?

• Roaming gives stations true mobility allowing them to move seamlessly between BSSs. (More later)

• APs need to be able to communicate between themselves since stations can only associate with one AP at a time.

• Currently, inter-access point communication can only be achieved with proprietary, non-standard technologies.

• IEEE 802.11 working group (Task Group F) is working on standardizing IAPP (Inter-Access Point Protocol)

Extended Service Set (ESS)


Access Points

• Access Point (AP)

– Translates (converts) 802.11 frames to Ethernet and visa versa

– Known as a translation bridge

– Typically provides wireless-to-wired bridging function

– All BSS communications must go through the AP, even between two wireless stations


Quick Preview: Station/AP Connectivity

SSID (Service Set Identity)

• At a minimum a client station and the access point must be configured to be using the same SSID.

• An SSID is:

– Between 2 and 32 alphanumeric characters

– Spaces okay

– Must match EXACTLY, including upper and lower case

– Sometimes called the ESSID

– Not the same as BSSID (MAC address of the AP, later)



• The Cisco APs have the default SSID tsunami.

SSID



• Your operating system (Windows) or wireless NIC client (Aironet) will tell you whether or not you have successfully connected (associated).

Windows Toolbar Icon

Aironet Toolbar Icon

Windows

Network

Properties



• This only associates your client with the AP.

• If you want to communicate with other devices on the network (wireless and wired), make sure your IP address and subnet mask are correct (or if using DHCP choose that setting).

• This is configured for your wireless NIC, not the wired NIC.



• In Windows this is done from the Start -> Control Panel -> Network

Connections (amongst other methods).

• Usually, need to have wired Ethernet disconnected or disabled.


802.11 Frames – This isn’t Ethernet!

• 802.11 has some similarities with Ethernet but it is a different protocol.

• Access Points are translation bridges.

• The “data/frame body” is re-encapsulated with the proper layer 2 frame.

• Certain addresses are copied between the two types of frames.

Distribution System (DS)

General 802.11 Frame

IP Packet

IP Packet L

L

C


802.11 Frames 802.11 Frames

• Data Frames (most are PCF)

– Data

– Null data

– Data+CF+Ack

– Data+CF+Poll

– Data+CF+Ac+CF+Poll

– CF-Ack

– CF-Poll

– CF-Cak+CF-Poll

• Control Frames

– RTS

– CTS

– ACK

– CF-End

– CF-End+CF-Ack

• Management Frames

– Beacon

– Probe Request

– Probe Response

– Authentication

– Deauthentication

– Association Request

– Association Response

– Reassociation Request

– Reassociation Response

– Disassociation

– Announcement Traffic

Indication


Medium Access – CSMA/CA

• Both CSMA/CD and CSMA/CA are half-duplex architectures

• Ethernet uses CSMA/CD – Collision Detection

– Ethernet devices detect a collision when the data is transmitted

• 802.11 uses CSMA/CA – Collision Avoidance

– 802.11 devices only detect a collision when the transmitter has not received an Acknowledgement.

– Stations also use CS/CCA.

– Stations also use a virtual carrier-sense function, NAV.

CSMA/CD CSMA/CA

ACK

All stations detect the collision


Medium Access – CSMA/CA

• The 802.11 standard makes it mandatory that all stations implement the DCF (Distributed Coordination Function), a form of carrier sense multiple access with collision avoidance (CSMA/CA).

• CSMA is a contention-based protocol making sure that all stations first sense the medium before transmitting (physically and virtually).

• The main goal of CSMA/CA is to avoid having stations transmit at the same time, which will then result in collisions and eventual retransmissions.

• However, collisions may still occur and when they do stations may or may not be able to detect them (hidden node problem).

CSMA/CD CSMA/CA

ACK

All stations detect the collision


DCF and PCF

• IEEE mandated access mechanism for 802.11 is DCF (Distributed Coordination Function)

– Basis for CSMA/CA

• There is also the PCF (Point Coordination Function)

– Point Coordinators (PC), ie.Access Points, provide point coordination for contention-free services.

– Restricted to Infrastructure BSSs

– Stations can only transmit when allowed to do so (AP).

– PCF is not widely implemented and will not be discussed


DCF Operation

• In DCF operation, a station wanting to transmit :

– Checks to see if radio link is clear, CS/CCA – Carrier Sense, Clear Channel Assessment .

– Checks its NAV timer to see if someone else is using the medium.

– If medium is available DCF uses a random backoff timer to avoid collisions and sends the frame.

• Transmitting station only knows the 802.11 frame got there if it receives an ACK.

• May also use RTS/CTS to reduce collisions.


Duration Field

• Duration/ID field – The number of microseconds that the medium is expected to remain busy for transmission currently in progress.

– Transmitting device sets the Duration time in microseconds.

– Includes time to:

• Transmit this frame to the AP (or to the client)

• The returning ACK

• The time in-between frames, IFS (Interframe Spacing)

• All stations monitor this field!

• All stations update their NAV (Network Allocation Vector) timer.

General 802.11 Frame (more on this later)


NAV Timer

• All stations have a NAV (Network Allocation Vector) timer.

• Virtual carrier-sensing function

• Protects the sequence of frames from interruption.

– Martha sends a frame to George.

– Since wireless medium is a shared medium, all stations including Vivian receive the frame.

– Vivian updates her NAV timer with the duration value.

– Vivian will not attempt to transmit until her NAV is decremented to 0.

• Stations will only update their NAV when the duration field value received is greater than their current NAV.


Broadcast-based shared medium • Host A is sending 802.11

frames to another host via the AP.

• All other 802.11 devices in BSS (on this channel) and within range of the signal will see the frame.

• 802.11 framing provides addressing, so only the AP knows it is the next-hop receiver.

• Other 802.11 devices within this BSS can sense that the medium is in use and will update their NAV values.

What if a station is in range of the AP but not

the Host A? (Hidden node problem )

802.11 Medium Access Mechanisms

DCF Operations Hidden Node Problem

RTS/CTS Frame Fragmentation



Hidden Node Problem

• What if a station is in range of the AP but not other hosts, like the transmitting host?

• Wireless networks have fuzzy boundaries, sometimes where may not be able to communicate/see every other node.

• Hidden nodes can be caused by:

– Hosts are in range of the AP but not each other.

– An obstacle is blocking the signal between the hosts.


Hidden Node Problem

• The problem is collisions.

– Collisions occur at the AP (or another station in an IBSS).

– Both stations assume the medium is clear and transmit near the same time, resulting in a collision.

– The AP cannot properly receive either signal and will not ACK either one.

– Both stations restransmit, resulting in more collisions.

• Throughput is significantly reduced, up to 40%.


Hidden Node Problem

• Solutions:

– Move the node

– Remove the obstacle

– Use RTS/CTS (Request to Send / Clear to Send)


RTS/CTS Solution


RTS/CTS Solution

• The hidden node stations cannot see the RTS.

• The AP replies to Vivian with a CTS, which all nodes, including the hidden node can see.

• Vivian transmits the frame.

• The AP returns an ACK to Vivian.

• The AP sends the message to George who returns an ACK to the AP.

• Vivian attempts to reserve the medium using

an RTS control frame to the AP.

• The RTS frame indicates to the AP and all

stations within range, that Vivian wants to

reserve the medium for a certain duration of

time, message, ACK, and SIFS.


RTS/CTS Solution

• RTS/CTS consumes a fair amount of capacity and overhead, resulting in additional latency.

• Normally used in high capacity environments.

• The RTS/CTS procedure can be enabled/controlled by setting the RTS threshold on the 802.11 client NIC.

• RTS/CTS is also used during frame fragmentation.


Setting the RTS Threshold on a Cisco Client

Specifies the data packet size beyond which the low-level RF protocol invokes RTS/CTS flow control.

A small value causes RTS packets to be sent more often, which consumes more of the available bandwidth.

Small values, however help the system recover from interference or collisions

RTS Threshold


RTS/CTS Example

• Stations C, D, E, and F can see traffic (signals) from all stations

including HN-A and HN-B (and visa versa).

• HN-A and HN-B can not see each other, but can communicate with the

AP.

• RTS/CTS is enabled on HN-A and HN-B, so that the AP will respond

with a CTS that the other HN station will see.

• If it wasn’t for the other HN station, neither HN would need RTS/CTS

HN-A

RTS/CTS

HN-B

RTS/CTS

C

D

E

F

AP


Frame Fragmentation

• Since we have already discussed RTS/CTS, let’s also discuss frame fragmentation.

• Later, we will see that RTS/CTS and fragmentation are typically combined.

• Frame fragmentation is a MAC layer function that is designed to increase the reliability of transmitting frames across a wireless medium.


Frame Fragmentation

• In a “hostile wireless medium” (interference, noise) larger frames may have more of a problem reaching the receiver without any errors.

• By decreasing the size of the frame, the probability of interference during transmission can be reduced.

• Breaking up a large frame into smaller frames, allows a larger percentage of frames to arrive undamaged (without errors).

• “Easier to poor sand down a hole than boulders.”


Frame Fragmentation

• Frame fragmentation can increase the reliability of frame transmissions but there is additional overhead:

– Each frame fragment includes the 802.11 MAC protocol header.

– Each frame fragment requires a corresponding acknowledgement.

• If a frame fragment encounters errors or a collision, only that fragment needs to be retransmitted, not the entire frame.

• The frame control field includes information that this is a fragmented frame.

802.11 Data Frames and Addressing

Helps to understand this because it is not dependent upon the

802.11 Physical layer.



Ethernet MAC Addressing


A C D

Access Point 1 Access Point 2

X

Y

xxx

yyy

yyy Pseudo MAC address of hosts

xxx

B

IP Packet

yyy xxx


802.11 MAC Addressing

• Four address fields

• The number and function of the address fields is dependent upon the source and destination for the 802.11 frame.

• Before we look at how these addresses are used, lets look at the different source and destination options.

• Address 4 is optional and not commonly used, except for WDS (wireless distribution system, bridge to bridge).


The LLC encapsulation will be

explained later in this presentation.


802.11 MAC Addressing - DS

• Distribution System (DS)

– “The distribution system is the logical component of 802.11 used to forward frames to their destination. 802.11 does not specify any particular technology for the distribution system.” Matthew Gast

– The DS is the exiting network from the AP. (For purposes of this discussion.)

– It can be a wired network (Ethernet) or a wireless network (wireless bridge) or something else.

– We will assume it is a wired network for these discussions.


A B

C D


X

Y


802.11 MAC Addressing – Frame Control Field

• To DS: indicates if frame is destined for the DS or AP (1 bit).

• From DS: indicates if frame is sourced from the DS or AP (1bit).



802.11 MAC Addressing – Frame Control Field

Function ToDS FromDS

IBSS (no AP) 0 0

To AP 1 0

From AP 0 1

Wireless bridge to bridge 1 1


Note: Some

documentation is

misleading stating that the

ToDS is set to 1 only when

the destination is on the

wired side of the AP.



• Options:

– Host A to Host B

– Host A to Host X

• Frames to and from a BSS (Basic Service Set) must go via the access point.

• The access point is a layer 2 bridge (translation bridge) between the 802.11 network and the 802.3 network.


A

B

C D


X

Y

aaa bbb 111 Pseudo MAC address of hosts and BSSID of

AP1

aaa

bbb

xxx

111



• Each BSS is assigned a BSSID.

– Not to be confused with SSID or ESSID.

• In a BSS, the BSSID is the MAC address of the wireless interface.


A B

C D


X

Y


aaa bbb

xxx The BSSID

111



• Besides the BSSID MAC address, the access point has a MAC address for other interfaces.

– Ethernet (LAN)

– Ethernet (WAN)

– 802.11a for dual mode APs


A B

C D


X

Y

General 802.11 Frame aaa bbb

xxx

The BSSID

111



• Address 1 – Receiver address

• Address 2 – Transmitter address

• Address 3 – Ethernet/wireless SA, Ethernet/wireless DA, or BSSID

• Transmitter: Sends a frame on to the wireless medium, but may not be the original source (didn’t necessarily create the frame), i.e. AP

• Receiver: Receives a frame on the wireless medium, but may not be the final destination, i.e. AP


A B

C D


X

Y


Host A to Host B

aaa bbb

xxx

111



• Address 1 – Receiver address

• Address 2 – Transmitter address

• Address 3 – Ethernet/wireless SA, Ethernet/wireless DA, or BSSID


A

B

C D


X

Y

Host A to Host B

aaa bbb

aaa 111 bbb

Host A to AP 1

AP1 to Host B 111 bbb aaa

xxx

Trans. Rec.

Rec. Trans.

DA

SA

111

1 0

0 1



• Access Points are translation bridges. • From 802.11 to Ethernet, and from Ethernet to 802.11 • The “data/frame body” is re-encapsulated with the proper layer 2 frame

(Ethernet or 802.11). • Certain addresses are copied between the two types of frames.



IP Packet

IP Packet L

L

C




A

B

C D


X

Y

Host A to Host X

aaa

bbb

aaa 111 xxx

Host A to AP 1

Host A to AP 1

aaa xxx

802.11 Frame

• The Ethernet DA and SA are the source and destination addresses just like on traditional Ethernet networks.

– Destination Address – Host X

– Source Address – Host A

xxx

Rec. Trans. DA

copied

111

1 0


LLC – Logical Link Control

• The IP Packet is in an LLC frame which is encapsulated in a MAC frame.

• 802.11 does not include a protocol type field.

• An 8 byte SNAP field is added to the LLC to indicate the layer 3 data being carried in the data field.

• The rest of the information within the LLC is not really relevant.

General 802.11 Frame IP Packet L

L

C


LLC – Logical Link Control

• The only word of caution is that there are two types of LLC encapsulation, RFC 1042 and 802.1h.

• On a rare occasion, you might find a problem with a client associating to an AP when their LLCs do not match.


Station Connectivity

• Station connectivity is a explanation of how 802.11 stations select and communicate with APs.

State 1

Unauthenticated

Unassociated

State 2

Authenticated

Unassociated

State 3

Authenticated

Associated

Successful

Authentication

Successful

Association

Deauthentication Disassociation



• Three processes:

– Probe Process (or scanning)

– The Authentication Process

– The Association Process

• Only after a station has both authenticated and associated with the access point can it use the Distribution System (DS) services and communicate with devices beyond the access point.

State 1

Unauthenticated

Unassociated

State 2

Authenticated

Unassociated

State 3

Authenticated

Associated

Successful

Authentication

Successful

Association


Probe

process

Authentication process Association process


Station Connectivity – Probe Process

• The Probe Process (Scanning) done

by the wireless station

– Passive - Beacons

– Active – Probe Requests

• Depends on device drive of wireless

adapter or the software utility you are

using.

• Cisco adapters do active scanning

when associating, but use passive

scanning for some tests.

• In either case, beacons are still

received and used by the wireless

stations for other things besides

scanning (coming).


Station Connectivity – Passive Scanning • Passive Scanning

– Saves battery power

– Station moves to each channel and waits for Beacon frames from the AP.

– Records any beacons received.

• Beacon frames allow a station to find out every thing it needs to begin communications with the AP including:

– SSID

– Supported Rates

• Kismet/KisMAC uses passive scanning


Station Connectivity – Passive Scanning



Note: Most of these

beacons are received

via normal operations

and not through

passive scanning.



• AP features (options)

– The SSID can be “hidden” or “cloaked” in the beacon frame (can be done on Cisco APs)

– Do not send AP broadcast beacons (not an option with Cisco APs)

• From some mailing lists: – “SSID cloaking and beacon hiding isn't necessarily a bad thing, but too many

places use it as the only protection because it leads to a false sense of security.”

– “Obscurity != security. Too many companies blindly trust that no beaconing or hiding their SSID means they're automatically safe.”


Station Connectivity – Active Scanning • Active Scanning: Probe Request

– This process is not mandatory on with 802.11.

– A Probe Request frame is sent out on every channel (1 – 11) by the client.

– APs that receive Probe Requests must reply with a Probe Response frame if:

• SSID matches or

• Probe Request had a broadcast SSID (0 byte SSID)

• NetStumber uses active scanning

From the client


Station Connectivity – Active Scanning • Active Scanning: Probe Response

– On BSSs the AP is responsible for replying to Probe Requests with Probe Responses.

– Probe Responses are unicast frames.

– Probe Responses must be ACKnowledged by the receiver (client).

• Like a beacon, Probe Response frames allow a station to find out every thing it needs to begin communications with the AP including:

– SSID

– Supported Rates

1

2

3

From the AP


• How a station chooses an AP is not specified in 802.11.

• It is left up to the vendor.

• It could be, Matching SSIDs, Signal Strength, Supported data rates.

Station Connectivity – Multiple APs

Most likely Vivian will

communicate with AP 2,

which matches her SSID

and has the stronger signal

strength.



• Access Points can be configured whether or not to allow clients with broadcast SSIDs to continue the connectivity process.

– If there is no authentication on the AP, then the client will most likely “associate” and be on their network!

• Cisco APs use a default SSID of tsunami known as the “guest mode” SSID. (coming)

• Unless this feature is disabled or authentication is enabled, anyone can easily associate with your AP and access your network (or the Internet).

Probe Request

Broadcast (no) SSID Probe Response

SSID = tsunami ACK

No SSID

Hey, I didn’t

do anything

and I am on

the Internet!



• Station connectivity processes:





State 1

Unauthenticated

Unassociated

State 2

Authenticated

Unassociated

State 3

Authenticated

Associated

Successful

Authentication

Successful

Association


Probe

process



Authentication Process

• On a wired network, authentication is implicitly provided by the physical cable from the PC to the switch.

• Authentication is the process to ensure that stations attempting to associate with the network (AP) are allowed to do so.

• 802.11 specifies two types of authentication:

– Open-system

– Shared-key (makes use of WEP)


Authentication Process – Open-System

• Open-system authentication really “no authentication”.

• Open-system authentication is the only method required by 802.11

– You could buy an AP that doesn’t support Shared-key

• The client and the station exchange authentication frames.


Authentication Process – Shared-Key

• Shared-key authentication uses WEP (Wired Equivalent Privacy) and can only be used on products that support WEP.

• WEP is a Layer 2 encryption algorithm based on the RC4 algorithm.

• 802.11 requires any stations that support WEP to also support shared-key authentication.

• WEP and WPA will be examined more closely when we discuss security.

• For now both the client and the AP must have a shared-key, password.


Authentication Process

• We’ll look at the configuration of the client and AP later!

• Example of open-system authentication.

• Note: On “some” systems you can configure authentication (WEP) and WEP encryption separately. On the ACU you can have open-system authentication and also have WEP encryption. However, if you have Shared-key (WEP) authentication, you must use WEP encryption.


Authentication Process • Authentication

– Open-System

– Shared-Key (WEP)

• Encryption

– None

– WEP

or only



• Station connectivity processes:





State 1

Unauthenticated

Unassociated

State 2

Authenticated

Unassociated

State 3

Authenticated

Associated

Successful

Authentication

Successful

Association


Probe

process



Association Process

• The association process is logically equivalent to plugging into a wired network.

• Once this process is completed, the wireless station can use the DS and connect to the network and beyond.

• A wireless station can only associate with one AP (802.11 restriction)

• During the 802.11 association process the AP maps a logical port known as the Association Identifier (AID) to the wireless station.

– The AID is equivalent to a port on a switch and is used later in Power Save Options.

• The association process allows the DS to keep track of frames destined for the wireless station, so they can be forwarded.

1. Association Request

2. Association Response


Association Process

• Association Request Frame (From client)

– Listen Interval – This value is used by the Power Save Operation (later). Informs AP how often it will wake-up to receive buffered frames.

– Supported Rates – What data rates the client station supports.

• Association Response Frame (From AP)

– Status Code – Indicates success or reason for failure.

– AID – A value assigned to this station for the Power Save Operation (later).

– Supported Rates - What data rates the AP supports.



• Traffic can now flow between the client and the AP.

• Disassociation and deauthentication can be due to:

– Inactivity

– The AP cannot handle all currently associated stations

– Station has left BSS

– etc.

State 1

Unauthenticated

Unassociated

State 2

Authenticated

Unassociated

State 3

Authenticated

Associated

Successful

Authentication

Successful

Association


Probe

process



Roaming

• A WLAN designer must determine whether clients will require seamless roaming from access point to access point.

• Not yet standardized by IEEE 802.11 (working on it), most vendors use IAPP (Inter-Access Point Protocol).

– Task Group F: A Standard IAPP

Not yet covered under

802.11.


Roaming

• Initial Association: – Probing (Probe Request, Probe Response)

• Note: 802.11 does not specify how the client determines which AP to associate with , so it depends on vendor implementation.

– Authentication (Authentication Request, Authentication Response) – Association (Association Request, Association Response)

• 802.11 does not allow associating with more than one AP.


Roaming

• Several factors need to be considered when designing a WLAN with seamless roaming capabilities:

– Coverage must be sufficient for the entire path.

– A consistent IP address should be available throughout the entire path.

– Until standardized by IEEE 802.11, access points will most likely need to be from the same vendor.


Roaming

• The client initiates the roaming (re-association) process.

• As the client is moving out of range of its associated AP, the signal strength will start to drop off.

• At the same time, the strength of another AP will begin to increase.

• The re-association process then occurs, including authentication.

IAPP: Please

send buffered

frames for…

IAPP: Ok!

* AP(B) must update MAC

address tables on

infrastructure switches to

prevent to loss of data.

AP(B) sends an Ethernet

frame to AP(A) with the

source MAC address of the

client so all the switches

can update their SAT/MAC

tables.

* Packet - Source

MAC of client…


Roaming • Scans for a better access point if

the signal strength falls below a threshold value.

• The following options define signal strength and wait thresholds that trigger a new scan.

• When Adapter Has Been Associated for at Least—The number of seconds the client adapter waits after connecting before searching for a better access point. This threshold keeps the client adapter from jumping from one access point to another too quickly after the initial connection.

• Signal Strength is Less Than—The signal strength threshold below which the client adapter should search for a better access point. This threshold keeps the client adapter from jumping from one access point to another when both have strong signals.

• Example: When using the default values of 20 seconds and 50%, the client adapter monitors the signal level 20 seconds after connecting and every second thereafter. If the client detects that the signal strength is below 50%, it scans for a better access point. After the access point connects to a better access point, this scanning process repeats.


Scalability

• Scalability is the ability to locate more than one access point in the same area.

• This will increase the available bandwidth of that area for all users local to that access point.

• The current Cisco Aironet products are frequency agile. • This means that they can look for and use the best channel. • Three non-overlapping and non-interfering channels, up to a theoretical 33

Mbps per cell. • Users still only operate at a maximum theoretical value of 11 Mbps

APs are on

different channels


Scalability

• In the case of 802.11a, there are eight non-overlapping channels, each up to a theoretical bandwidth of 54 Mbps.

• This means that a maximum of eight discrete systems can reside in the same area, with no interference.

• Therefore, the highest aggregate total data rate for an 802.11a system is a theoretical 432 Mbps, for a given cell area.

• Remember that any connected user will still only receive up to 54 Mbps.

APs are on

different channels


Scalability

• Specifies the channel number and frequency that the client adapter uses for communications. The channels conform to the IEEE 802.11 Standard for your regulatory domain.

• In infrastructure mode, this option is set automatically and cannot be changed. The client adapter listens to the entire spectrum, selects the best access point, and then uses the same channel as that access point.

• In ad hoc mode, the channel of the client adapter must match the channel used by the other clients in the wireless network. If the client adapter does not find any other ad hoc client adapters, this option specifies the channel on which the client adapter broadcasts beacons.


Access point coverage and comparison

• As a client roams away from the access point, the transmission signals between the two attenuate (weaken).

• Rather than decreasing reliability, the AP shifts to a slower data rate, which gives more accurate data transfer.

• This is called data rate or multi-rate shifting.

• As a client moves away from an 802.11b access point, the data rate will go from 11 Mbps, to 5.5Mbps, to 2 Mbps, and, finally, to 1 Mbps.

• This happens without losing the connection, and without any interaction from the user.


Access point coverage and comparison

• The Cisco Aironet 2.4 GHz radio delivers 100 mW of output and offers a high degree of receiver sensitivity.

• The 5 GHz client radio has a 20 mW transmit power and the 5 GHz access point has a 40 mW transmit power.

• It is possible to adjust the power level down, to create pico-cells, or smaller coverage cells.

• This would be done, for example, to prevent the coverage area of one AP from extending too far into the coverage area of another AP.


• Sets the transmit power level of the radio. Select a value for Transmit Power that is no greater than the maximum allowed by the regulatory body in your country (FCC in the United States, ETSI in Europe, and MKK in Japan). Reducing the transmit power conserves battery power, but it reduces the range of the radio. The default power level is the maximum power allowed by the regulatory agency in your country.

• Note: If World Mode is enabled, the transmit power is limited to the maximum level allowed by the regulatory agency of the country where the adapter is used.


Multirate implementation

• Provides for seamless roaming, but not at a constant speed.

• This example takes advantage of multi-rate technology, to step down in bandwidth and gain greater coverage distances, with a single access point.

• If 11 Mbps is required everywhere, the access points would need to be relocated, so that only the 11-Mbps circles are touching each other, with some overlap.

• This would require a greater number of APs, but consistent bandwidth would be achieved.


Channel usage and interference

• Remember that the 802.11 standard uses the unlicensed spectrum and, therefore, anyone can use these frequencies.

Bridge Topologies

More on Bridges Later



Distance limitations

• The 802.11 standard sets a time limit for the acknowledgement of packets.

• Remember that 802.11 also defines a Local Area Network, which means a typical wireless range of up to 305 m (1000 ft), not several kilometers or miles.

• The bridge products have a parameter that increases this timing, whereas the workgroup bridge and AP does not.

• The timing is increased, by violating the 802.11 standard.

• This allows the Cisco devices to operate at greater distances.

• Any wireless bridge that supports distances over one mile must violate 802.11.

• This means that radios of other 802.11 vendors may not work with the Cisco bridges when the distances are greater than 1.6 km (1 mile).


Root modes

• Cisco Aironet access points and bridges have two different root modes, in which to operate the following:

– Root = ON —

• The bridge or AP is a root.

• If it is a bridge, then it is called the master bridge.

– Root = OFF —

• The bridge or AP is not a root, non-root.


Root modes


Root modes

on on

off

off off

off


Point-to-point configuration

• When using point-to-point wireless bridges, two LANs can be located up to 40 km (25 miles) apart.

• The antennas must have line-of-site with each other. • Obstacles such as buildings, trees, and hills will cause communication

problems. • In this configuration, the Ethernet segments in both buildings act as if they

are a single segment. • The bridge does not add to the Ethernet repeater count because this

segment is viewed by the network as a cable.


Point-to-point configuration

• Many corporations would like to have more bandwidth between two locations, than the 11 Mbps provided by the 802.11b standard.

• Currently, with Cisco IOS, it is possible to use Fast Etherchannel or multi-link trunking, to bond or aggregate up to three bridges together.

• This gives the customer the potential for 33 Mbps.


Point-to-multipoint configuration

• For multipoint bridging, an omni directional antenna is typically used at the main site.

• Directional antennas are used at the remote sites. • In this configuration, again, all the LANs appear as a single segment. • Traffic from one remote site to another will be sent to the main site and then

forwarded to the other remote site. • Remote sites cannot communicate directly with one another. • Line of sight must be maintained between each remote site and the main

site.

root

Non-root Non-root


Basic Topologies

Peer-to-Peer (Ad Hoc)

Topology (IBSS) Basic Infrastructure

Topology (BSS)

Extended

Infrastructure Topology (ESS)

BLUETOOTH


Es una tecnología desarrollada por Ericsson en 1994, que hace factible la conectividad inalámbrica entre dispositivos a corta distancia, éstos pueden llegar a formar redes con diversos equipos de comunicación: computadoras móviles, radiolocalizadores, teléfonos celulares, PDAs, e, inclusive, electrodomésticos. Originally defined so as to replace wire/cable technology for cellular telephony. In fact, Bluetooth is far more than a communications protocol; it is a full communications application stack.

BLUETOOTH: protocol stack


BLUETOOTH


The lower communications layers of Bluetooth have been published as IEEE standard 802.15.1. For the original task of device connection, Bluetooth offers a rich suite of functionalities, including enabling walk-up linking without user interaction and establishing voice connection.

BLUETOOTH


La tecnología CMOS utilizada en el chip permite reducir tanto los costos como el consumo de energía; de esta forma se reduce a aproximadamente del 97% el uso de energía, comparado con un teléfono móvil.

BLUETOOTH


Bluetooth networking is intentionally limited to a maximum of eight Bluetooth nodes, which together form a piconet (Figura abajo).

BLUETOOTH


Se puede formar una Scatter net a través de la técnica de multiplexación de división de tiempo dúplex (Time-Division Duplex – TDD). Esta técnica de multiplexación emplea intervalos de tiempo de 625µs, para lograr una transmisión bidireccional (full-dúplex) entre los dispositivos conectados.

BLUETOOTH


When a node is included in more than one piconet, that node then assumes the task of forwarding messages to/from the other piconet, adding to the complexity of Bluetooth networking.

BLUETOOTH


Canales máximos de datos: 7 por piconet Rango esperado del sistema: hasta 721 kbit/s por piconet Número de dispositivos: 8 por piconet y hasta 10 piconets Alimentación: 2,7 voltios Consumo de potencia: desde 30 uA a 30 mA transmitiendo Tamaño del Módulo: 0.5 pulgadas cuadradas (9x9 mm) Interferencia: Bluetooth minimiza la interferencia potencial al emplear saltos rápidos en frecuencia÷1600 veces por segundo.

BLUETOOTH


The most attractive feature of Bluetooth for industrial automation purposes is its use of forward error correction (FEC) for delivering messages without error and without requiring retransmission. The drawback of FEC is loss of efficiency: a 1 Mbps communications channel can deliver only 721 Kbps.

BLUETOOTH


A multivendor consortium defined Bluetooth, not a standards organization. Just like 802.11b and 802.11g, it operates in the unlicensed 2.4 GHz frequency band, but uses frequency-hopping spread-spectrum technology that hops faster than the original FHSS of 802.11. As a result, the presence of Bluetooth in close proximity to Wi-Fi nodes causes the signal for the WLAN to degrade, spelling disaster for Wi-Fi transmissions.

BLUETOOTH


La distancia nominal de un enlace puede variar desde 10 centímetros a 10 metros, pero se puede aumentar a más de 100 m elevando la potencia de transmisión.

BLUETOOTH


While there is no protocol yet to help such nodes avoid signal degradation, many early suppliers of nodes with both Bluetooth and Wi-Fi have been able to synchronize transmissions to avoid degradation. Suppliers of 802.11a, which operates in the 5 GHz unlicensed band, are quick to point out that they avoid signal degradation from Bluetooth completely. Nevertheless, 802.11g suffers the same problems as 802.11b in the presence of Bluetooth.

Proprietary or Non-Standard Wireless Networks


Standards take a long time to be developed, much slower than the pace of technology. Commercial suppliers often cannot wait for the approval of a standard, or may have a product concept that adequately fulfills the network requirements more than any proposed standard. These companies will often introduce their network products hoping to establish a market in the absence of standardized networks.



Currently, two suppliers, Honeywell and Adaptive Instruments, both offer their own wireless networks for process control field instrumentation. Both networks use frequency hopping spread spectrum operating in the 915 MHz ISM(Industrial, Scientific, and Medical) band. These networks are capable of passing data at rates that vary from 4.8 to 76.8 Kbps over distances that vary from 780m to 175m, respectively.



Their devices are battery powered and have battery life estimated to be several years. Both of these networks are configured with a wired base-station located close to the field instruments, and form direct links to each instrument from the base station. Additionally, Dust Networks is another supplier using frequency hopping in the 915 MHz ISM band, but with integral mesh networking technology. Dust sells OEM modules to be used by other manufacturers to build wireless transmitters.

Wireless versus Wired Networks


Wired networks, such as Ethernet, are designed for communications between fixed locations. Wireless networks, such as Wi-Fi, are designed for communications between devices. The distinction is lost for fixed-location devices, but device mobility is a primary benefit of wireless. However, the primary applications for wireless in industrial automation is expected to be between fixed locations.



Wireless networks will often need a wired connection to a computer or to the wired network, a source of power, and radios. Estimating the cost of a wired network is easy. It is the sum of the cost of the network cable, junctions, and connecting wires; the cable and junction installation; the network interfaces; and the long-term maintenance of the installed wiring plant. Wireless networks are more difficult to estimate. They include the cost of wiring to access points, access point equipment, wireless interfaces, and long-term wireless troubleshooting and maintenance.



The other notable problem of wireless devices is that they still need a power source. Wired network nodes can draw power from the local AC receptacle, but mobile wireless devices depend on batteries or some alternative power source. Of course, you can always plug the wireless device into a local power source, but then you lose the mobility advantage and incur the cost of installing power connections at the device.



The recent PoE (Power over Ethernet) standard, IEEE 802.3af was created to help resolve this problem by transporting electrical power on the wired Ethernet network so it can be used by wireless access points. Nowadays, there is much acceptance for this standard, and it will become well accepted once products are sold for it. However, PoE still does not address the issue of powering the wireless device itself.

Wireless Network Topologies


Wired networks have a layout or topology that is determined by the location of the nodes and network components. Wireless networks are not so easily described. The topology of a wireless network is determined by the logical capabilities of the network components. Often the user must determine how the wireless network’s topology is to be configured after installation, or perhaps after some usage determinations.

Star


The most typical or default arrangement for a wireless network is a star cluster in which the wireless access point is at the center, as illustrated in the nest Figure. Each wireless device then communicates only with the common access point, which is usually connected via wires to a network switch. This arrangement then places all of the wireless devices into the same collision domain, presuming that this is an Ethernet-based network. Usually, this arrangement presents no problem since the access point itself will be unable to receive more than one message at a time and will ignore whichever began second.

Star


Tree


As in wired networks, wireless networks can be organized into a tree topology. Each field unit is configured to a network that is connected to a specific switch/access point. That access pointis then hierarchically connected to another access point closer to the wired network. The topology appears as illustrated in the following Figure.

Tree


Mesh


The newest and most revolutionary form of network is called a mesh. In a mesh network each station is both an end device and a network forwarding element. Mesh networks are naturally self-healing and redundant – exactly the property needed for industrial automation networks. In a mesh network, each station is responsible for forwarding a network transmission not intended for itself to other stations within its radio range.

Mesh


Those stations, in turn, send the transmission to at least one other station within its radio range, as illustrated in the next Figure. Therefore, the network becomes very redundant, fault-tolerant, and extended in range. The drawback is that each station must remove redundant messages.

Mesh


Mesh


Since mesh networks that are intended for industrial automation tend to have 256 or fewer nodes, routing tables can be small and the routing simple. Routing tables need to be updated when new nodes appear in the mesh or for any reason fail to respond to forwarded messages. Mesh networks are not new. The Internet itself is a very large wired mesh network with very complex routing algorithms. Since IP addresses do not imply anything about location, messages routed on the Internet “hop” from one node to another that is (hopefully) closer to the desired destination.

El protocolo de comunicación ZigBee


ZigBee (802.15.4) es un protocolo de comunicación inalámbrica desarrollado sin fines de lucro por una alianza de 100 fabricantes de semiconductores para tener una tecnología inalámbrica de bajo costo. Entre las 100 empresas se encuentran algunas muy exitosas como: Mitsubishi, Philips, Motorola, Honeywell, Samsung que trabajan en un sistema estándar de comunicaciones. Este protocolo es muy similar al Bluetooh aunque con marcadas diferencias que destacan:



Una red ZigBee puede constar de un máximo de 255 nodos, frente a los 8 máximos de una red Bluetooth. Menor consumo eléctrico que el, ya de por sí bajo, del Bluetooth. En términos exactos, ZigBee tiene un consumo de 30ma transmitiendo y de 3ma en reposo, frente a los 40ma transmitiendo y 20 ma en reposo que tiene el Bluetooth. Este menor consumo se debe a que el sistema ZigBee se queda la mayor parte del tiempo dormido, mientras que en una comunicación Bluetooth esto no se puede dar, y siempre se está transmitiendo y/o recibiendo.

http://es.wikipedia.org/wiki/Ma



Tiene un ancho de banda de 250 kbps, mientras que el bluetooth tiene 1 Mbps. Debido al ancho de banda diferente, uno es más apropiado que el otro para ciertas cosas. Por ejemplo, mientras que el Bluetooth se usa para aplicaciones como el Wireless USB, los teléfonos móviles y la informática casera, el ancho de banda del ZigBee se hace insuficiente para estas tareas, desviándolo a usos tales como controles remotos, productos dependientes de la batería, sensores médicos, y en artículos de juguetería, en los cuales la transferencia de datos es menor.

http://es.wikipedia.org/wiki/Wireless



ZigBee es un sistema ideal para redes domóticas, específicamente diseñado para reemplazar la proliferación de sensores/actuadores individuales. ZigBee fue creado para tener un estándar para redes Wireless de: • pequeños paquetes de información, • bajo consumo, • seguro y • fiable.



ZigBee: Stack de comunicaciones



ZigBee: Tramas



La seguridad de las transmisiones y de los datos son puntos clave en la tecnología ZigBee que utiliza el modelo de seguridad de la subcapa MAC IEEE 802.15.4, la cual especifica 4 servicios de seguridad: Control de accesos: el dispositivo mantiene una lista de los dispositivos ‘comprobados’ en la red. Datos Encriptados, se usa una encriptación con un código de 128 bits.



Integración de tramas para proteger que los datos no sean modificados por otros. Secuencias de refresco, para comprobar que las tramas no han sido reemplazadas por otras. El controlador de red comprueba estas tramas de refresco y su valor, para ver si son las esperadas. Depende del dispositivo final para tomar la decisión de dotarlo de más o menos seguridad.



Un dispositivo ZigBee típico incluye un circuito integrado de radio frecuencia (RF IC), una pequeña capa física (PHY) que se conecta a un microcontrolador de 8-bits de bajo consumo/pequeño voltaje, y periféricos que pueden estar conectados a un sensor o actuador. La pila (stack) de protocolos y aplicaciones está implementada en un chip de memoria tipo flash. Según empresas analistas, existen más de 300 millones de nodos o dispositivos equipados con la tecnología ZigBee, sólo en el sector de la domótica.



Tipos de dispositivos: Coordinador de red: es el encargado de mantener en todo momento el control del sistema. Es el más complejo de todos los dispositivos, y necesita memoria y capacidad de computación. Dispositivo de función completa (FFD): es capaz de recibir mensajes del estándar 802.15.4. Puede funcionar como un coordinador de red. Gracias a su memoria adicional y a su capacidad de computar, es ideal para funcionar como Router o para ser usado en dispositivos de red que actúen de interface con los usuarios.

http://es.wikipedia.org/wiki/Router



Tipos de dispositivos: Dispositivo de función reducida (RFD): tiene capacidad y funcionalidad limitadas (especificada en el estándar) con el objetivo de conseguir un bajo costo y una gran simplicidad. Básicamente son los sensores/actuadores de la red. Bandas en las que opera: 2.4 Ghz (mundial), 915 MHz (EEUU) y 868 MHz (Europa). Métodos de transmisión: DSSS, se focaliza en las capas inferiores de red (Física y MAC). Velocidad de transmisión: 20 kbit/s por canal. Rango: 10 y 75 metros.

http://es.wikipedia.org/wiki/DSSS



Tipos de dispositivos: Dispositivo de función reducida (RFD): tiene capacidad y funcionalidad limitadas (especificada en el estándar) con el objetivo de conseguir un bajo costo y una gran simplicidad. Básicamente son los sensores/actuadores de la red. La capa de red soporta múltiples configuraciones de red incluyendo estrella, árbol, y rejilla, como se muestra en la figura:



Modelo de red ZigBee



Las redes ZigBee se han diseñado para conservar la potencia en los nodos ‘esclavos’. De esta forma se consigue el bajo consumo de potencia. La estrategia consiste en que, durante mucho tiempo, un dispositivo "esclavo" está en modo "dormido", y solo se "despierta" por una fracción de segundo para confirmar que está "vivo" en la red de dispositivos de la que forma parte. Esta transición del modo "dormido" al modo "despierto" (modo en el que realmente transmite), dura unos 15ms, y la enumeración de "esclavos" dura alrededor de 30ms.



En la configuración en estrella, uno de los dispositivos tipo FFD asume el rol de coordinador de red y es responsable de inicializar y mantener los dispositivos en la red. Todos los demás dispositivos zigbee, conocidos con el nombre de dispositivos finales, ‘hablan’ directamente con el coordinador. En la configuración de rejilla (mesh), el coordinador ZigBee es responsable de inicializar la red y de elegir los parámetros de la red, pero la red puede ser ampliada a través del uso de routers ZigBee.



Un algoritmo de enrutamiento utiliza una protocolo de pregunta-respuesta (request-response) para eliminar las rutas que no sean óptimas. La red final puede tener hasta 254 nodos (probablemente nunca se necesite tantos). Utilizando el direccionamiento local, tú puedes configurar una red de más de 65000 nodos (216).



La trama general de operaciones (GOF) es una capa que existe entre la de aplicaciones y el resto de capas. La GOF suele cubrir varios elementos que son comunes a todos los dispositivos, como el subdireccionamiento y los modos de direccionamientos y la descripción de dispositivos, como el tipo de dispositivo, potencia, modos de ‘dormir’ y coordinadores de cada uno. Utilizando un modelo, la GOF especifica métodos, eventos, y formatos de datos que son utilizados para constituir comandos y las respuestas a los mismos.



Esquema típico de un dispositivo ZigBee

wireless law

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