CE 536 – Networking and Microcomputer Systems

Day 11: Wireless Networking

• Wireless Networking
• IEEE 802.11
• IEEE 802.11b (Wi-Fi)
• IEEE 802.11g
• IEEE 802.11a

 Wireless Networking

• Any method of connecting two network devices without running a cable between them
• Best suited for
• Areas where wiring isn’t possible or would be prohibitively expensive
• When client computers (e.g., laptops) join and leave the network frequently
• When client computers don’t have direct access to a desk with a network connection
• When client computers move around a lot
• When trying to hide illicit activities…!
• IR – Infrared signals – high frequency and high speed, but easily disrupted
• Line of Sight – short distances, light doesn’t go around corners without help
• Building to building applications – transceivers seldom moved
• Hundreds of megabits per second (1,000GHz+)
• Light frequencies are unregulated and unlicensed
• A focused, tight beam offers greater immunity to light interference from the sun and certain light fixtures
• RF – Radio Frequencies – wider range, immune to most barriers, but typically lower transmission speed
• Thick walls or metal barriers may interfere
• Only certain frequencies can be used
• Wireless frequencies may be crowded as they are typically used by all different types of wireless devices
• Preparation – preparing for a wireless network isn’t significantly different than preparing for a wired network
• Wireless is an extension of a wired network
• Access Points – act as a bridge/router between wireless and wired networks
• Authentication – to determine whether a given wireless device can join the network
• Association – handshaking relationship to ensure that each client connects to only one access point at a time

 IEEE 802.11

• Wireless LAN Protocol that defines both the Physical Layer (OSI layer 1) and Media Access Control (MAC – lower portion of OSI layer 2)
• CSMA/CA – Carrier Sense Multiple Access with Collision Avoidance
• Collision Detection (CD) would require wireless devices to be able to send and receive at the same time (i.e., would be more expensive) and would require that all wireless stations be in position to hear all other wireless stations and not just the access point.
• Positive Acknowledge – station checks if medium is free; if so, the station can send. Receiving station sends an acknowledgment upon receipt. If no acknowledgment is received, the sending station will resend.
• Virtual Carrier Sense – minimizes collisions due to stations not being able to hear each other. Instead, stations will hear RTS, CTS, or both through the access point
• RTS – Request to Send – a packet containing the source address, destination address, and duration of the transmission
• CTS – Clear to Send – receiving station replies if medium is free
• FHSS – Frequency Hopping Spread Spectrum – transmits a short burst of data at one frequency, hops to another frequency, and does it again
• Hop Sequence – sequence of frequencies used – must be synchronized between both sending and receiving stations
• FCC requires 75 or more frequencies be used with no more than 400 milliseconds on any frequency at one time
• Secure – constantly hops to new frequencies
• Used by the majority of vendors for LANs operating at 1Mbit per second
• Popular with 2.4GHz phones
• DSSS – Direct Sequence Spread Spectrum – spreads a signal over a wide range (i.e., a channel) of a frequency band
• Broadband Carrier – uses a redundant bit pattern for every bit of data transmitted
• Works well even with weak signals
• Rejects background interference well
• Popular with 900MHz (902 – 928MHz) phones

IEEE 802.11b (Wi-Fi)

• DSSS only (no FHSS)
• Operates at 11Mbits per second – downward compatible with 1Mbit per second and 2Mbit per second wireless network devices utilizing DSSS
• Worldwide standard for 2.4GHz (2.4GHz to 2.48GHz)
• Divided into 14 (overlapping) channels; 11 in the United States
• Each channel 22MHz wide
• Channels in use must be 25MHz apart to avoid interference
• Basic Service Set (BSS) – in the United States, any given location can utilize up to three channels (1, 6, and 11) to avoid interference
• Extended Service Set (ESS) – a logical collection of more than one BSS
• Roaming – allows client stations to switch from one access point to another, transparently to the user
• ESS ID Number (ESSID) – all components within the Extended Service Set area must be set to the same ESSID for roaming to be possible

• Wired Equivalent Privacy (WEP) – shared key encryption mechanism
• Access point sends out a challenge
• Client stations use the shard key to encrypt the challenge issued by the access point which verifies the client is authorized to use the access point
• Can be hacked in a matter of minutes…

 IEEE 802.11g

• Operates at 54Mbits per second – downward compatible with 1, 2 and 11Mbit per second wireless network devices utilizing DSSS
• Worldwide standard for 2.4GHz (2.4GHz to 2.48GHz) – overlapping channels…

 IEEE 802.11a

• Operates at 54Mbits per second – typically not downward compatible with other wireless standards
• Worldwide standard for 5GHZ (5.160GHz to 5.815 with gaps in most countries)
• No overlapping channels
• Each channel 20MHz wide
• More prone to interference from objects such as walls