Network Security

Wireless LAN

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About WLAN IEEE 802.11 standard

Use wireless transmission medium such as radio, microwave, infrared to

provide network connectivity

An Access Point (AP) is installed to act as Bridge between wireless and

wired Network

The AP is connected to wired network and is equipped with antennae to

provide wireless connectivity

Range (distance between AP and WLAN client) depends on structural

hindrances and RF gain of the antenna at the AP

A client is always associated with one AP and when the client moves closer

to another AP, it associates with the new AP

Three flavors: 802.11b, 802.11a, 802.11g, etc.

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802.11 Architecture Access Point (AP):

Any entity that has station functionality & provides access to distributed system

They transmit and receive radio frequencies Basic Service Set (BSS):

Set of stations controlled by a single coordination function Every BSS has an identification (ID) called the BSSID which

is the MAC address of the AP servicing the BSS

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Architecture continued

Extended service set (ESS): A set of two or more connected BSSs with APs connected through a Distribution System. Each ESS has an ID called the ESSID which is a 32-byte character string.

Distribution system (DS): Connects APs in an ESS. The concept of a DS can be used to increase network coverage through roaming between cells.

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Services

Services provided by distribution system: Distribution Integration Association Re-association Disassociation

Services provided by stations: Authentication & de-authentication Privacy Data delivery

Station types

No-transition A station is either stationary or moving only inside a

BSS

BSS-transition A station can move from one BSS to another, but

inside one ESS

ESS-transition mobility A station can move from one ESS to another

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WLAN Protocol Architecture

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MAC Sublayers - DCF

Distributed Coordination Function It uses CSMA/CD, contention based (without pre-coordination)

The “listen before talk" operating procedure (first sense then transmit)

Before sending a frame, the source station senses the medium. After a station is found to be idle, the station waits for a period of time DIFS – Distributed InterFrame Space; then the station sends a control frame called RTS – Request to send

After receiving RTS and waiting for period of time SIFS – Short interframe space, the destination station sends control frame CTS – clear to send back to source. It indicates that destination is ready to receive data

The source station sends data after waiting for amount of time equal to SIFS

The destination sends ACK to ensure delivery of frame

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MAC Sublayers - PCF

Point Coordination Function

Optional access method, works only in infrastructure network

High priority than DCF

A centralized contention-free polling access method

AP performs polling of stations one after another

To give priority to PCF over DCF: PIFS (PCF IFS) and SIFS are defined

SIFS is same as in DCF

PIFS is shorter than DIFS: If at the same time, a station wants to use

only DCF and AP wants to use PCF, AP gets priority

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WLAN Benefits

Convenience Mobility: Nomadic access Productivity Deployment Expandability Low Cost: no wiring required Robust

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WLAN disadvantages

Security Range Reliability Speed Radio emissions Hidden and exposed station problem

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WLAN : 802.11b

The most popular 802.11 standard currently in deployment.

Supports 1, 2, 5.5 and 11 Mbps data rates in the 2.4 GHz ISM (Industrial-Scientific-Medical) band

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WLAN : 802.11a

Operates in the 5 GHz UNII (Unlicensed National Information Infrastructure) band

Incompatible with devices operating in 2.4GHz

Supports Data rates up to 54 Mbps.

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WLAN : 802.11g

Supports data rates as high as 54 Mbps on the 2.4 GHz band

Provides backward compatibility with 802.11b equipment

Standards in process

802.11ac : <6GHz throughput (Dec 2013) 802.11ad : 60 GHz throughput (Dec 2012) 802.11af : TV whitespace (Jun 2014) 802.11ah : 1 GHz throughput Sensor Network

(May 2015) 802.11ai : Fast initial link setup (Sep 2014)

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Recommended Reading

WLAN MAC layer frame format & frame types

Physical layer specifications: IEEE 802.11 FHSS, DSSS,

Infrared, OFDM etc

Network Allocation Vector

WEP security service

Wireless transmission media

Bluetooth architecture & frame format

CSMA, ALOHA