Frequencies and Channels
Frequencies => what distinguishes one RF signal from another
Channel => generally a portion of the entire frequencies assigned to specific technology such as 802.11b or 802.11a
Most modern devices hide the frequencies information from us in two ways.
First, 802.11b devices can communicate with other 802.11b devices because they all use the same total frequencies range
so the conclusion is : you don't have to know the frequencies used necessarily ,as long as you know you're using all 802.11b devices
Second,most modern devices such as Cisco use a method by hiding the frequencies through the use of simple channel assignments.most access points,for example provide an easy to use drop down box that list the available channel
Frequencies
two major frequencies are used in Wi-Fi networks at this time
1. 2,4 Ghz ISM ( industrial,Scientific and medical ) band
2. 5 Ghz UNII ( Unlicensed national information infrastructure)
The 2,4 Ghz band is used for 802.11,802,11b,and 802.11g devices,while the full range from 2.4000 Ghz to 2.5000 Ghz is included in the designated band,wireless networks use only the range from 2.4000 Ghz to 2.4835 Ghz because the FCC has provided power output specifications for this range of frequencies onlu
The 5 Ghz band is used for 802.11a devices and it's divided into three bands known as the lower,middle and upper bands.
The 2,4 Ghz devices use FHSS or DSSS modulation and 5 Ghz 802.11a devices use OFDM modulation
Channel
OFDM Channel
Each of three UNII bands ( Lower,middle,and upper) provides four channels for communications in 802.11a network
In Most case lower and middle are used indoors and upper are used outdoor
802.11a OFDM Channel
Channel Center Frequencies Lower(L)/Middle(M)
36 5,18Ghz L
40 5,20 Ghz L
44 5,22 Ghz L
48 5,24 Ghz L
52 5,26 Ghz M
56 5,28 Ghz M
60 5,30 Ghz M
64 5,32 Ghz M
DSSS Channel
Channel Indetifier Frequencies (In Ghz ) Americas EMEA Israel China Japan
1 2,412 X X X X
2 2,417 X X X X
3 2,422 X X X X X
4 2,427 X X X X X
5 2,432 X X X X X
6 2,437 X X X X X
7 2,442 X X X X X
8 2,447 X X X X X
9 2,452 X X X X X
10 2,457 X X X X
11 2,462 X X X X
12 2,467 X X
13 2,472 X X
14 2,484 X
FHSS Hopping Sequences
A Frequency hopping spread spectrum system does not use channels in the same way as DSSS or OFDM,instead,FHSS systems hop from frequency to frequency using pseuadorandom hopping sequence.
This hopping sequence (pattern) is defined as the channel ( 79 Total channel,frequency hop pattern)
Facility Coverage
After completed this chapter,you may be wondering why we need to know all this information about frequency and channel.the answer is simple : by using channel effictively,you can provide the needed coverage in most facilities
You can determine for the strategic placement of wireless devices such as Access point,wireless Bridge,and routers based on channel within frequencies,different standards ( 802.11g VS 802.11a) or both.
In order to implement colocated systems effictively,you must also understand channel interference and the potential problem created by personal area network devices
Colocation
To provide proper coverage in any facilty,you need to strike balance between bandwith and range.
While an access point might be able to provide service to a client this is 100 feat away,it might not be provide sufficient speeds for the needs of user
Standards and Advertised Range
802.11 802.11a 802.11b 802.11g
Frequency 2,4Ghz 5 Ghz 2,4 Ghz 2,4 Ghz
Data Rate(s) 1,2 Mbps 5,9,12,18,24,36,48,54 Mbps 1,2,5,5,11 Mbps 6,9,12,15,24,36,48,54 Mbps
Modulation FHSS,DSSS OFDM DSSS OFDM
Advertised Range 300 Feet 225 Feet 300 Feet 300 Feet
BANDWITH
In the world of Wi-Fi bandwith can have a twofold meaning.
In one sense ,bandwith can refer to the width of the RF band;in another sense,it can refer to the available bandwith or data rates supported by technology
1.Data Rates
2.Throughput
3.Dynamic Rate Selection
For point 1 and 2 we have been studied in a previous section.now we are talking about Dynamic Rate Selection
Dynamic Rate selection is the term used to refer to a method of dynamically adjusting the speed of wireless LAN Client Devices
Generally Speaking,to achieved a higher a higher data rate,you must shorten the distance between you and the access point
when a client moves farther away from the access point,data is more likely to be corrupted at higher data rates.theoratically you can achieve greater throughput at a lower data rate because there are a fewer transimittions of data.this will,in the end,provide greater throughput to those client closer to access point as well
Rate Distance (In Feet ) Frequency Power
54 Mbps 60 5 Ghz 40 mW
48 Mbps 80 5 Ghz 40 mW
36 Mbps 100 5 Ghz 40 mW
24 Mbps 120 5 Ghz 40 mW
18 Mbps 130 5 Ghz 40 mW
12 Mbps 140 5 Ghz 40 mW
9 Mbps 150 5 Ghz 40 mW
6 Mbps 170 5 Ghz 40 mW
11 Mbps 140 2,4 Ghz 100 mW
5,5 Mbps 180 2,4 Ghz 100 mW
2 Mbps 250 2,4 Ghz 100 mW
1 Mbps 350 2,4 Ghz 100 mW
Operatinal Modes
Ad Hoc
Benefits of Ad Hoc mode include :
1. No Access Point Needed
2. Can be Formed Dynamically
3. Support by most wireless hardware
4. Simple to set up and operate
Problem with Ad Hoc :
1. No Access to internet or network infrastructure
2. No centralization of management
3. More difficult to secure ( WEP keys can be used,but are difficult to manage among client )
4. Limited in size because of bandwith consumption just as in Ethernet peer to peer network
Infrastructure
Benefits of infrastructure mode include :
1.Centralized management
2.Greater security control
3.Access to internet or network infrastructure
4.High Bandwith availability with proper implementation
Problem with infrastructure mode include :
1. Greater Cost
2. Longer and more complex implementation
3. Less dynamic than ad hoc mode
4. Requires one or more access points
Naming the network – SSID
You are probably beginning to wonder what this name does for your network.the answer is with determine SSID ( Service Set indentifier )
There are three types of service sets :
1.Independent basic service set (IBSS )
2.Basic Service Set (BSS)
3.Extended Service Set (ESS )
IBSS ( The Independent Basic Service Set )
is also known as Ad Hoc network,IBSS network are useful only for smaller network in confined space
Basic Service Set ( BSS )
is also known as an infrastructure basic service set,use access point as the connector to the client
ESS ( Extended Service Set )
is two or more BSS that share the same network name or SSID.The concept of the ESS allow user to roam around on the network and still connect to the same network with the same name
Roaming Around the network
in order to be called “true” roaming ,the user should be able to move seamlessly from one BSS to another without losing their network connection
and how they work ?
The client determines if the received signals are getting to weak in relation to access point the user is moving away from and request reassociation with another closer and stronger access point in the same ESS.
In the end,the client request reassociation with a different access point,and the two access point perform the hand off
Reassociation
Client devices use various parameters to determine when they should attempt to reassociate with a different access point than the one they are currently associated with.depending on the access point and client devices,these can include :
1.Data Rates
2.Number of connected stations
3.Average Throughput
Four reassociation process :
1.The wireless client sends a reassociation request to the new access point with which it desires to connect
2.The new access point notifies the old access point that reassociation request has been made
3.The Old access point acknowledges that this reassociation is about to occur
4.The new access point sends a reasociattion response frame to the wireless client
802.11F
IEEE's 802.11F recommendation will alow roaming from one access point to another regardless of the vendor.
802.11F is a definition of the inter,Access-Point Protocol (IAPP).IAPP describe three protocol sequences :
1.Adding a station's information after it associates
2.Moving status information after reasociattion
3.Caching actions that improve performance when a station reassociates
Fault Tolerance and Load Balancing
Installing multiple access point in the same coverage area can provide fault-tolerance and load balancing.this concept is similar to roaming,but in this case..the goal is not to provide mobility as a user roams,the goal with fault tolerance or load balancing is to provide availability as the user remains stationary
NB:In order to increase bandwith,if you place two access point in a network,you have to configure one access point to channel 1 and other to other channel ( Let said channel 11 )by doing this thing you can provide much greater bandwith to client.
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