11 chapter 4. wireless local area networks departamento de tecnología electrónica some of these...

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Chapter 4.Wireless Local Area Networks

Departamento deTecnología Electrónica

Some of these slides are copyrighted by:

Computer Networking: A Top Down Approach 5th edition. Jim Kurose, Keith RossAddison-Wesley, April 2009.

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Chapter 4: Wireless Local Area Networks 4.1 Introduction 4.2 Standard IEEE 802.11

4.2.1 Standards 4.2.2 IEEE 802.11 network components 4.2.3 Standard IEEE 802.11 4.2.4 IEEE 802.11 network topologies

Wireless Local Area Networks

33Wireless Local Area Networks



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IntroductionIntroduction No cables They use the electromagnetic spectrum Generally they are integrated within wired LANs

Why? Allow mobility Allow installation in places where cables can’t be

installed (or are expensive)


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Introduction


Introduction

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Advantages: Allow the same features as wired LANs, but without

cable limitations. Mobility

Reduce installation time/cost Flexibility May work inside buildings or between buildings

Disadvantages: Need a trasnsmission medium based on radio

frequency (RF) -> Electromagnetic spectrum is limited Transmission rates are slower than in wired LANs Security problems


Introduction

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Use of electromagnetic spectrum 0-200 MHz: Radio, television, remote controls,

wireless phones, etc. 200 MHz- 1GHz: alarms, medical implants, walkie-

talkies, television, mobile phones. 1- 2 GHz: GPS, medical telemetry, phone mobiles 2.4 GHz: free band… satellite radio, satellite phones,

microwave ovens, weather radars, WI-FI, BLUETOOTH.

2.5- 5 GHz: satellite communications (e.g, TV) 5-50 GHz: Wi-fi, police radars 50-300 GHz: short-distance signals.


Electromagnetic spectrum

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ISM frequency bands: 900 – 928 MHz 2,400 – 2,4835 GHz 5,725 – 5,850 GHz


Electromagnetic spectrum

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Standards

Standars Standardization of WLANs is in charge of

IEEE & WIFI Alliance. IEEE in standard 802.11 is in charge of:

Define specifications of high performance WLANs Ensures Interoperability

SecurityQuality of Service.

WIFI Alliance is in charge of:Certificates that a manufacturer’s product may

interoperate with another manufacturer’s one.Promote the use of WLANs


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Standards

LAN/MAN Standards


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802.11 Network componentsIEEE 802.11Network components Networks that follow standard 802.11 are formed by

four basic elements

Distribution system

Access Point

Station or client

Wireless medium


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Station or client: Device using a NIC that follows standard IEEE 802.11

PC, laptop, PDA,…

Wireless Network Adapter


802.11 Network components

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Station or client Wireless Network Adapters

Radio units Several types of Wi-Fi adapters

PCI CardsUSB Adapters



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Access Point (AP): Device in charge of “Medium Control Access” for the

clients of a WLAN and allows connection to a wired network (bridge)

An AP is a wireless hub. Different from a wireless router (very common

nowadays). A wireless router is a combination of an AP and a router and may execute more complex functions than a simple AP.



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Access Point (AP): Bridge: allows interconnecting different

networks, regardless of the protocol they are using. Works in 1 & 2 OSI layers.

A router also allows interconnecting several networks, but the network layer protocol must be common (E.g, IP)

If we want to interconnect two networks that are using the same protocol, it is desirable to use a router.



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802.11 Network components Wireless Medium: Use of

RF for the transport of MAC_PDUs.


Frequency Standard Wavelength

2.4 Ghz 802.11b/g 12.5 cm5.x GHz 802.11a 5-6 cm

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Distribution System: LAN or WLAN technology used to make WLAN range wider. Wireless case:

Several AP.WDS AP: Master AP (WDS = Wireless Distribution

System).The other APs are slaves and act as relay stations: WDS

Stations.All in the same channel SSID may be the same one or different Not standard. Not supported by all hosts and

there may be incompatibilities.Incompatible with some security mechanisms



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Standard IEEE 802.11

Standard IEEE 802.11

PHY

MAC

LLC (802.2)


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Physical Layer (PHY) Cell topology.

Half-duplexFrequency bands: 2,4 GHz & 5 GHz

A license is not neccessary. Every band has a set of channels


Standard IEEE 802.11. Physical Layer

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Physical Layer (PHY) Cell topology.

Physical layer options

Standard Band Tx Rate802.11a 5 GHz 54 Mbps802.11b 2,4 GHz 11 Mbps802.11g 2,4 GHz 54 Mbps802.11n Both 200 Mbps


Standard IEEE 802.11. Physical Layer

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MAC Sublevel Every MAC_PDU containing data is ACKed by the rcvr Encryptation and authenticaction algorithms are

implemented. A MAC_PDU may contain up to 4 MAC addresses

(source, destination, transmitter & receiver).


Standard IEEE 802.11. Data Link Layer



802.11 Data frame

Ethernet frame

Frame Control

Duration Dest Address

Source Address

Address3

Seq. Address4

Data CRC

2 Bytes 2 Bytes 6 Bytes 6 Bytes 6 Bytes 2 Bytes 6 Bytes 0-2312 Bytes 4 Bytes

Dest Address

Source Address

EType Data CRC

6 Bytes 6 Bytes 2 Bytes 46-1500 Bytes 4 Bytes

IP

IP

LLC/SNAP header (802.2)

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MAC Sublevel Three types of MAC_PDUs:

DataControlManagement



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MAC level Data MAC_PDUs:

Superior layer info (MAC_SDU) is carriedMAC_PCI is 34 bytes long.MTU is 2312.

In case of the existance of many interferences MAC_SDU fragmentation is enabled



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MAC sublevel Control MAC_PDUs:

Used for the “reservation” of the medium and for ACKs ACK: sent by MAC sublevel to ACK a Data MAC_PDU.

• Does not indicate that MAC_PDU destination has received it.

RTS (Request to Send) / CTS (Clear to Send) for the reservation of the medium.

• RTS is sent by MAC sublevel to request the use of the medium and indicate the duration of the reservation (time needed).

• CTS is sent by MAC sublevel as a response to RTS. Indicates that the sender is able to transmit and the duration of the reservation (time left)



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MAC sublevel Management MAC_PDUs:

To manage wireless links. Beacon. Sent periodically by MAC sublevel to inform about the

existance of a wireless network• Interval: configurable parameter.

Probe request. Enable MAC sublevel to search for wireless networks within an area.

• Informs about transmission rates. Probe response. Response to Probe Request. Association request. MAC sublevel requests the connection to a

wireless network. Association response. Connection confirmation. Others.



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MAC Sublevel Medium Access Control (MAC), changes for adapting

to wireless medium.• In Ethernet, CSMA/CD (Carrier Sense Multiple

Access / Collision Detection). • In Wireless, this mechanism is hard to

implement, as transmission errors are more frequent.

• New mechanisms for MAC are added: CSMA/CA y MACA.



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MAC Sublevel Medium Access Technique: CSMA/CA

1) Before transmitting info, a station must determine the medium state (free or busy)

2) If the channel is not busy, there is an additional wait: InterFrame Space (IFS)

3) If the channel is busy or it gets busy while IFS, the tx must wait until current transaction ends.



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4) After current transaction ends: Backoff algorithm is executed Additional and random wait, chosen in an interval

called contention window (CW) Measured in units of slot time (slots)

5) If during this wait, medium is not free for a time equal or higher than IFS, wait is suspended until the condition is fulfilled.



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StationA

B

C

D

E

IFS IFS IFS IFSCW

CW

CW

CW

Data

Data

Data

Data

DataBackoff

Backoff

Backoff

Data to tx arrivalWait



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Problems in WLAN:Hidden nodes. Channel is busy by a station that is not

heard by the correspondent node Exposed nodes. A station thinks that the channel is busy

though it is, in fact free, as other nodes does not intereferes in the communication.



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Standard IEEE 802.11. Data Link Layer MAC Sublevel

Medium Access Technique: CSMA/CAProblems in WLAN:

Hidden nodes. Exposed nodes


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MAC Sublevel Medium Access Technique: MACA

Contention that allows reservations to avoid collisions (CSMA/CA, CA = Collision Avoidance)

More resstrictive than standard CSMA/CA:RTS (Request to Send) / CTS (Clear to Send)DIFS (Distributed IFS)/ SIFS (Short IFS)NAV (similar to backoff algorithm time)

Not employed (overload) if: There are a few stations Very dense network: all the stations are in range of every station. Small frames.



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Standard IEEE 802.11. Data Link Layer MAC Sublevel

Medium Access Technique: MACA


38Wireless Local Area Networks 38



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802.11 Network topologies

The basic communication set in a 802.11 network is the BSS (Basic Service Set) or cell. Every BSS has a coverage area, so that all the stations

belonging to the BSS can communicate to the others A name known as SSID (Service Set Identifier) is assigned to

them. According to the number of BSSs and the kind of devices

within a network, there are three types of 802.11 networks: Ad hoc networks or Independent BSS (IBSS).

There are only clients. Infrastructure BSS.

There are clients and an AP. EBSS (Extended BSS)

Multiple BSS to allow bigger coverage area.


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Ad hoc 802.11 Network



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Infrastructure BSS



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EBSS 802.11 Network

BSS BSS

EBSS



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Functioning in Infrastructure Mode / EBSS

Every AP has a BSSID – Basic Service Set Identifier- (MAC of its wireless interface) and a SSID (configured by the network administrator). In EBSS, every cell has the same SSID, but is

distinguished by its AP’s BSSID. 802.11 does not limit the number of clients per AP. To connect to a wireless network, a client must

know BSSID & SSID in a cell. APs send Beacon periodically, with BSSID and, optionally

with SSID The client sends Probe request with SSID, waiting a Probe

response, from the AP, containing its BSSID.



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A client who know BSSID & SSID of a cell request the association (connection) with an AP by means of the Association Request If the AP accepts the client, it sends an Association

Response with an Association Id.AP registers client’s MAC in its Addressing Table.

An AP controls the communication in all its associated clients Clients never communicate with each other directly

They only proccess MAC_PDUs coming from their associated AP.



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APs keep Addressing tables, like bridges do. They learn from the traffic that goes through it. They forward info based on the dest MAC addr

An AP that is connected to a Distribution System acts as a bridge, but Injects traffic to the wireless interface if the dest is

one of its clients or if it is broadcast/multicast Injects traffic in the Distribution System (like a bridge)

AP adapts logical addressing if it’s neccessary. E.g: Distribution System based on 802.3



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802.11 Network topologiesAssociation


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802.11 Network topologiesSending of data MAC-

PDU


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