-: Troubleshooting wireless networks. 2 2
In this section, we're going to discuss troubleshooting 3 3
of our wireless networks. 4 4
For the purposes of this section, 5 5
when I talk about wireless, 6 6
I'm specifically referring to wifi 7 7
or the 802.11 family standards, not Bluetooth, cellular, 8 8
satellite, near-field communication or RFID, 9 9
even though some of the topics 10 10
that we're going to cover like interference and antennas 11 11
and attenuation are also applicable 12 12
to those technologies as well. 13 13
Before we do, though, 14 14
it's important for us to do a quick review 15 15
of the different specifications and limitations 16 16
involved with wireless networks. 17 17
First, let's consider the different speeds and throughputs 18 18
of our various 802.11 wireless networks. 19 19
When we talk about the speed 20 20
we're normally referring to the bandwidth 21 21
of a given connection. 22 22
Bandwidth refers to the theoretical speed 23 23
of data going across a network. 24 24
Throughput, on the other hand, is the actual speed 25 25
of data on that network. 26 26
So, if you're using an 802.11a wireless network, 27 27
the maximum bandwidth or theoretical speed is 54 Mbps. 28 28
Now, while that's the theoretical maximum, 29 29
in reality, you're commonly going to see throughput 30 30
of only 20 to 30 Mbps. 31 31
If you're using 802.11b, this is going to have 32 32
a maximum bandwidth or theoretical speed of 11 Mbps. 33 33
Now again, while that's the theoretical maximum, 34 34
in reality, you're commonly going to see throughputs 35 35
of only 5 to 7 Mbps. 36 36
If you're using 802.11g wireless networks, 37 37
the maximum bandwidth is going to be 54 Mbps. 38 38
But in reality, it's going to be common 39 39
to see throughputs of 30 to 32 Mbps. 40 40
If you're using an 802.11n wireless network, 41 41
the maximum bandwidth is 600 Mbps, 42 42
but the common throughput you're going to see 43 43
is usually around 140 to 150 Mbps. 44 44
If you're using 802.11ac wireless networks, 45 45
the maximum bandwidth is 1300 Mbps or 1.3 Gbps. 46 46
Now, while that's the advertised speed, 47 47
and even sometimes you'll see it as high as 1900 Mbps, 48 48
when they start adding the 1300 Mbps 49 49
for the 5GHz radio and another 600 Mbps for the 50 50
backwards compatible wireless n, embedded 2.4GHz 51 51
radio on the access point, 52 52
that is just a theoretical number. 53 53
So, your theoretical number is somewhere between 54 54
1300 Mbps and 1900 Mbps, 55 55
depending on the model of wireless ac that you're using. 56 56
But the throughput is really going to be somewhere between 57 57
100 and 500 Mbps, in reality. 58 58
Now, if you're using an 802.11ax model, 59 59
this is going to have a maximum bandwidth 60 60
or theoretical speed of 10 Gbps. 61 61
Now, again, that's a theoretical maximum. 62 62
In the real world though, we're only seeing throughput 63 63
of around 600 to 900 Mbps. 64 64
All right, next, let's consider the distance 65 65
associated with each variant of wifi 66 66
that you may come across in the field. 67 67
Wireless a, b, and g networks can reach distances 68 68
of about 35 meters indoors and about 100 meters outdoors. 69 69
When you go to wireless n networks 70 70
you can reach a distance of up to 70 meters indoors 71 71
and about 250 meters outdoors. 72 72
With wireless ac and ax, 73 73
you can reach distances of around 50 meters indoors 74 74
and about 100 meters outdoors. 75 75
Now you may be wondering, do I need to memorize 76 76
all these different distances? 77 77
And the answer is, no, you don't. 78 78
But you should be aware that wireless n 79 79
doubled the distance over earlier wireless a, 80 80
b, and g networks. 81 81
You should also be aware that wireless ac and ax 82 82
cover a distance between wireless n 83 83
and the earlier technologies, 84 84
but at much higher speeds and throughputs. 85 85
Now, when it comes to distance, 86 86
really what we're worried about is how strong the signal is, 87 87
once it's received by a client. 88 88
This is measured in terms of the RSSI 89 89
or received signal strength indication. 90 90
Sometimes you'll hear people simply call this 91 91
the signal strength. 92 92
Now, RSSI is an estimate and measure of the power level 93 93
that a radio-frequency client device is receiving 94 94
from a wireless access point or wireless router. 95 95
As the distance increases, the signal will get 96 96
weaker and weaker at the client. 97 97
And the data rates in turn are going to go down 98 98
as you're going to get lower throughput 99 99
over this wireless network, as it gets slower and slower 100 100
when you get further and further from the signal strength. 101 101
Now, when you look at the received signal strength indicator 102 102
on the client, you should see it 103 103
in the negative 60 to negative 65 decibel range. 104 104
This would mean you have a strong signal. 105 105
It indicates a good strong signal that can support 106 106
heavy network based applications like voiceover IP, 107 107
video streaming, and other bandwidth intensive applications. 108 108
The RSSI value measures itself from zero to 100 109 109
in the negative range. 110 110
The closer the signal is to zero the stronger 111 111
the signal is going to be at the client. 112 112
For example, my computer is currently receiving 113 113
an RSSI value of negative 38 decibels, 114 114
which is extremely strong because my wireless access point 115 115
is actually located about five feet from my computer. 116 116
When you're looking at the RSSI, 117 117
anything that's over negative 90 decibels, 118 118
like 90 or 100 decibels, that is going to be considered 119 119
extremely weak. 120 120
If the signal is around negative 65 decibels, 121 121
this is considered fairly strong and a usable signal. 122 122
If you have an RSSI of negative 55 decibels, 123 123
this is a strong signal. 124 124
If you get a signal of negative 30 or above, 125 125
meaning somewhere between zero and negative 30, 126 126
this is extremely strong and you're probably sitting right 127 127
next to your wireless access point. 128 128
All right, finally, let's talk about 129 129
the Effective Isotropic Radiated Power or EIRP. 130 130
The effective equivalent isotropic radiated power 131 131
is the maximum amount of power that could be rated 132 132
from an ideal isotropic antenna, 133 133
given its antenna gain and the transmitter power 134 134
of the radio frequency system. 135 135
That is a lot of technical words, 136 136
but essentially this is going to be written down 137 137
as a dBi or decibels over isotropic when you measure it. 138 138
An omnidirectional antenna is considered 139 139
an isotropic antenna and it distributes power 140 140
equally out in all directions. 141 141
Now, why should you care about the EIRP levels? 142 142
Well, because these EIRP levels tell you 143 143
how strong the signal is being rated 144 144
out of your wireless access point towards your client. 145 145
This is the other half of the RSSI discussion 146 146
that we just had. 147 147
Remember, RSSI is used to measure the amount of signal 148 148
being received by the client from the access point. 149 149
We're measuring it on the client. 150 150
When we talk about EIRP, 151 151
we're measuring it at the access point. 152 152
So, if you're having signal issues 153 153
with your wireless network, 154 154
you can look at it from two different perspectives. 155 155
If you look at it at the RSSI level, 156 156
you're measuring it on the client and determining 157 157
if they're having an issue receiving the data. 158 158
On the other hand, you could look at the EIRP levels 159 159
from the access point and determine if the issue 160 160
is instead being caused by the access point, 161 161
its antennas or its power levels not being sufficient 162 162
to send out the signal. 163 163
All right, we just covered a lot of key terms 164 164
and information that you need to understand 165 165
as we move forward throughout this section 166 166
on wireless network troubleshooting. 167 167
In this section, we're going to focus on domain 5, 168 168
network troubleshooting and objective 5.4. 169 169
Objective 5.4 states, given a scenario, 170 170
you must troubleshoot common wireless connectivity issues. 171 171
So let's dive into this section 172 172
on troubleshooting wireless networks.