Network Plus
Unit 4
Section 2: Wireless Networking
1/28/2010
1/28/2010
Wireless Concepts
• In this section we will
discuss wireless
concepts.
– Antennas
– Frequency ranges
– Signal types
Antennas
• Antenna
– Omnidirectional
Common types of low-gain omnidirectional antennas are the whip antenna, "Rubber Ducky", ground plane
antenna, vertically oriented dipole antenna, discone antenna, mast radiator, horizontal loop antenna
(sometimes known colloquially as a 'circular aerial' because of the shape), and the halo antenna.
– Directional
•
A directional antenna or beam antenna is an antenna which radiates greater power in one or more
directions allowing for increased performance on transmit and receive and reduced interference from
unwanted sources. Directional antennas like Yagi-Uda antennas provide increased performance over
dipole antennas when a greater concentration of radiation in a certain direction is desired.
Network+ Guide to Networks, 5th Edition
3
Figure 8-3 Multipath signal propagation
Network+ Guide to Networks, 5th Edition
8
Frequency Ranges
• 2.4-GHz band
– Frequency range: 2.4–2.4835 GHz
– 11 unlicensed communications channels
• Most commonly used 1 – 6 - 11
– Susceptible to interference
• Microwave, cordless phone, WAP compete
• 5-GHz band
– Shorter distance
– Less interference
Network+ Guide to Networks, 5th Edition
10
Narrowband, Broadband, and Spread
Spectrum Signals
• Defines wireless spectrum use:
– Narrowband
• Transmitter concentrates signal energy at single
frequency, very small frequency range
– Broadband
• Relatively wide wireless spectrum band
• Higher throughputs than narrowband
– Spread-spectrum
• Multiple frequencies used to transmit signal
• Offers security
Network+ Guide to Networks, 5th Edition
11
Spread Spectrum Signals
• FHSS (frequency hopping spread spectrum)
– Signal jumps between several different frequencies
within band
– Synchronization pattern known only to channel’s
receiver, transmitter
– Used by Blue Tooth
• DSSS (direct-sequence spread spectrum)
– Signal’s bits distributed over entire frequency band at
once
– Used by 802.11x
• Receiver reassembles original signal upon receiving
bits
12
WLAN (Wireless LAN) Architecture
• Ad hoc WLAN
– Wireless nodes transmit directly to each other
– Use wireless NICs
• No intervening connectivity device
– Poor performance
• Many spread out users, obstacles block signals
• Access point (AP)
– Accepts wireless signals from multiple nodes
• Retransmits signals to network
– Base stations, wireless routers, wireless gateways
Network+ Guide to Networks, 5th Edition
13
AdHoc
– An Ad Hoc network uses a connection between two or more
devices without using a wireless access point: the devices
communicate directly when in range.
– An Ad Hoc network is used in situations such as a quick data
exchange or a multiplayer LAN game because setup is easy and
does not require an access point.
– Due to its peer-to-peer layout, Ad Hoc connections are similar to
Bluetooth ones and are generally not recommended for a
permanent installation
Ad Hoc Configuration
Figure 8-4 An ad hoc WLAN
14
Windows Ad Hoc Configuration
Access Point Configuration
Infrastructure
• Infrastructure WLAN
– Stations communicate with access point
• Not directly with each other
– Access point requires sufficient power, strategic
placement
• WLAN may include several access points
– Dependent upon number of stations
– Maximum number varies: 10-100
Network+ Guide to Networks, 5th Edition
15
WLAN Configuration
Figure 8-5 An infrastructure WLAN
Network+ Guide to Networks, 5th Edition
16
WLAN Bridged
Figure 8-6 Wireless LAN interconnection
Network+ Guide to Networks, 5th Edition
18
Understanding 802.11x WAN
Technology
• In this section we will discuss the technologies
behind the 802.11x standard.
802.11 WLANs
• Wireless technology standard
– Physical and Data Link layers
– Uses CSMA/CA (Collision Avoidance)
– 802.11b, 802.11a, 802.11g, 802.11n
• Share characteristics
– Halfduplexing, access method
– Frame contains 4 addresses
• 2 for WAP and 2 for device
19
Access Methods
• CSMA/CA (Carrier Sense Multiple Access with
Collision Avoidance)
– Minimizes collision potential
– Uses ACK packets to verify every transmission
• Requires more overhead than 802.3
• Real throughput less than theoretical maximum
• RTS/CTS (Request to Send/Clear to Send) protocol
–
–
–
–
Optional
Ensure packets not inhibited by other transmissions
Efficient for large transmission packets
Further decreases overall 802.11 efficiency
Network+ Guide to Networks, 5th Edition
21
RTS/CTS Mode
• CTS Mode - CTS (Clear To Send)
– Function used to minimize collisions among wireless devices on a wireless local
area network (WLAN). CTS will make sure the wireless network is clear before a
wireless client attempts to send wireless data. Enabling CTS will add overhead
and may lower wireless throughput.
• Settings
– Auto - CTS will monitor the wireless network and automatically decide whether to
implement CTS based on the amount of traffic and collisions that occur on the
wireless network.
– Always - CTS will always be used to make sure the wireless LAN is clear before
sending data.
– None - Typically used in a pure 802.11g environment. If CTS is set to None in a
mixed mode environment populated by 802.11b clients, wireless collisions may
occur frequently.
– If you have a mixed environment, both 802.11b and 802.11g clients, the
recommended setting is Auto. Auto is the default setting. Note: If you are in a
pure 802.11g environment (802.11g devices only), you can optimize the
performance by setting the CTS to None.
Association Process
• Process that connects to WAP
• Scanning
– Surveying surroundings for access point
– Active scanning transmits special frame
• Probe
– Passive scanning listens for special signal called a
Beacon fame
Network+ Guide to Networks, 5th Edition
22
Association (cont’d.)
• SSID (service set identifier)
– Unique character string identifying access point
• In beacon fame information
– Configured in access point
• Can turn broadcast off for increased security
• BSS (basic service set)
– Station groups sharing access point
– BSSID (basic service set identifier)
• Station group identifier
Network+ Guide to Networks, 5th Edition
23
Basic Service Set
• The basic service set (BSS) is the basic building block
of an 802.11 wireless LAN. In infrastructure mode, a
single access point (AP) together with all associated
stations (STAs) is called a BSS.[1] This is not to be
confused with the coverage of an access point, which is
called basic service area (BSA). [2] An access point acts
as a master to control the stations within that BSS.
• In ad hoc mode a set of synchronized stations, one of
which acts as master, forms a BSS. Each BSS is
identified by a BSSID. The most basic BSS consists of
one access point and one station
Extended Service Set
• ESS (extended service set)
– Access point group connecting same LAN
• Share ESSID (extended service set identifier)
– Allows roaming
• Station moving from one BSS to another without losing
connectivity
• Re-association
• Several access points detected
– Select strongest signal, lowest error rate
– Poses security risk
• Powerful, rogue access point
Network+ Guide to Networks, 5th Edition
24
Extended Service Set
•
•
•
•
An extended service set (ESS) is a set of one or more interconnected
BSSs and integrated local area networks that appear as a single BSS to the
logical link control layer at any station associated with one of those BSSs.
The set of interconnected BSSs must have a common service set identifier
(SSID). They can work on the same channel, or work on different channels
to boost aggregate throughput
The Extended Service Set Identification (ESSID) is one of two types of
Service Set Identification (SSID). In an ad hoc wireless network with no
access points, the Basic Service Set Identification (BSSID) is used. In an
infrastructure wireless network that includes an access point, the ESSID is
used, but may still be referred to as SSID.
An SSID is a 32-character (maximum) alphanumeric key identifying the
name of the wireless local area network. Some vendors refer to the SSID as
the network name. For the wireless devices in a network to communicate
with each other, all devices must be configured with the same SSID.
Figure 8-7 A network with a single BSS
Network+ Guide to Networks, 5th Edition
25
Figure 8-8 A network with multiple BSSs forming an ESS
Network+ Guide to Networks, 5th Edition
26
Switching between WAPs
• Question:
– I'd like to add wireless to my network, and I need multiple access points
to cover the whole area. I'd like to set them up so that there's only one
"wireless network" that the clients see, and it switches them as
seamlessly as possible between access points as they wander around
(if that's not possible, then at least have it so that they don't need to set
up the security by hand on each one the first time, if possible).
• Answer:
– The last time I did this, I setup the same SSID and WPA keys on two access
points and set them to different channels. I've been told that due to overlapping
frequencies, the channels 1, 6, and 11 are the best ones to use. I set mine up
on 1 and 6. So far everything seems to be working well. My Windows-based
clients connect to either one seamlessly, and the software will normally choose
the one with the strongest signal automatically.
Windows WAP Settings
802.11b
• DSSS (direct-sequence spread spectrum) signaling
• 2.4-GHz band
– Separated into 22-MHz channels
• Throughput
– 11-Mbps theoretical
– 5-Mbps actual
• 100 meters node limit
• Oldest, least expensive
• Being replaced by 802.11g
Network+ Guide to Networks, 5th Edition
31
802.11a
• Released after 802.11b
• 5-GHz band
– Not congested like 2.4-GHz band
• Lower interference, requires more transmit power
• Throughput
– 54 Mbps theoretical
– 11 and 18 Mbps effective
• Attributable to higher frequencies, unique modulating
data method, more available bandwidth
• 20 meter node limit
• More expensive, least popular
Network+ Guide to Networks, 5th Edition
32
802.11g
• Affordable as 802.11b
• Throughput
– 54 Mbps theoretical
– 20 to 25 Mbps effective
• 100 meter node range
• 2.4-GHz frequency band
– Compatible with 802.11b networks
Network+ Guide to Networks, 5th Edition
33
802.11n
• Draft: expected ratification in late 2009
• Manufacturers
– Selling 802.11n-compatible transceivers
• Primary goal
– Wireless standard providing much higher effective
throughput
• Maximum throughput: 600 Mbps
• Backward compatible with 802.11a, b, g standards
• 2.4-GHz or 5-GHz frequency range
Network+ Guide to Networks, 5th Edition
34
802.11n (cont’d.)
Figure 8-10 802.11n access point with three antennas
• MIMO (multiple input-multiple output) – page 380
– Multiple access point antennas may issue signal to
one or more receivers
– Increases network’s throughput, access point’s range
Network+ Guide to Networks, 5th Edition
36
Bluetooth Networks
• Version 1.1
–
–
–
–
Maximum theoretical throughput: 1 Mbps
Effective throughput: 723 Kbps
10 meter node difference
Designed for PANs (personal area networks)
• Version 2.0 (2004)
– Different encoding schemes
• 2.1-Mbps throughput
– 30 meters node difference
– Usage: cellular telephones, phone headsets,
computer peripherals, PDAs
Network+ Guide to Networks, 5th Edition
41
Summary of WLAN Standards
Table 8-1 Wireless standards
Network+ Guide to Networks, 5th Edition
42
WAP Configuration
• In this section we will discuss basic Wireless Access
configuration using a Linksys router running the ddwrt firmware.
Configuring Wireless Connectivity
Devices
• Set SSID
– Determine if beacon on or off
• Security
– WPA2 is best
– Secure password
• Verify DHCP settings
• Optional Port forwarding
Network+ Guide to Networks, 5th Edition
49
dd-wrt Configuration
dd-wrt Configuration - SSID
dd-wrt Configuration - DHCP
dd-wrt Configuration - DynDNS
dd-wrt Configuration
Port Forwarding
802.16 (WiMAX) Internet Access
• WiMAX (Worldwide Interoperability for Microwave
Access)
– Current version: 802.16e (2005)
• Improved mobility, QoS characteristics
• Digital voice signals, mobile phone users
• Functions in 2 and 66 GHz range
– Licensed, nonlicensed frequencies
• line-of-sight paths between antennas
– Throughput potential maximized
• Non-line-of-sight paths
– Exchange signals with multiple stations at once
Network+ Guide to Networks, 5th Edition
62
802.16 (WiMAX) Internet Access
(cont’d.)
• Two distinct advantages over Wi-Fi
– Much greater throughput (70 Mbps)
– Much farther range (30 miles)
• Appropriate for MANs and WANs
• Highest throughput achieved over shortest
distances between transceivers
• Possible uses
–
–
–
–
Alternative to DSL, broadband cable
Well suited to rural users
Internet access to mobile computerized devices
Residential homes
Network+ Guide to Networks, 5th Edition
63
802.16 (WiMAX) Internet Access
(cont’d.)
Figure 8-22 WiMAX residential service installation
Network+ Guide to Networks, 5th Edition
64
Satellite Internet Access
• Used to deliver:
– Digital television and radio signals
– Voice and video signals
– Cellular and paging signals
• Provides homes and businesses with Internet
access
Network+ Guide to Networks, 5th Edition
67
Satellite Orbits
• Geosynchronous orbit
– Satellites orbit the Earth at the same rate as the Earth
turns
– Downlink
• Satellite transponder transmits signal to Earth-based
receiver
– Typical satellite
• 24 to 32 transponders
• Unique downlink frequencies
• LEO (low Earth orbiting) satellites
– Orbit Earth with altitude 100 miles to 1240 miles
– Not positioned over equator
Network+ Guide to Networks, 5th Edition
68
Satellite Orbits (cont’d.)
Figure 8-25 Satellite communication
Network+ Guide to Networks, 5th Edition
69
The End
Network+ Guide to Networks, 5th Edition