Table of contents
- Wireless networks are half duplex on a shared medium
- Channel and power levels
- Analogy for Wi-Fi
- Why we don't allow rogues
- Simple troubleshooting
- Reporting problems
- User recommendations
- Connecting on campus
- Wi-Fi - 802.11 wireless standard
- Access Point (AP) - device that allows computers, laptops, handhelds, etc, to connect to a network using Wi-Fi. Sometimes called Wireless Router.
- RF - Radio Frequency
- Half duplex communication allow transmitting and receiving, but not at the same time.
- If more than one device transmits at the same time on the same channel the traffic will be garbled, requiring retransmission.
- Shared medium serves multiple users at the same time.
- As the number of devices competing to transmit increases, the number of failures also increases, requiring retransmission.
- If the access point has a data rate of 144 Mbps and there are 10 people actively using it, each person would have a maximum theoretical throughput of (144/2)/10 = 7.2Mbps (around 921.6 kilobytes/second), assuming no retransmission's are required.
- Wi-Fi uses radio waves.
- Radio waves from your device or from the AP are in most cases transmitted in all directions (directional antenna allow for increased signal strength in certain directions).
- If the transmit power is higher the radio waves will travel farther.
- Radio waves can be absorbed, reflected, and scattered by physical objects.
- Installing additional access points beyond a certain limit at a location on a channel that is already in use does not increase capacity, because the same radio frequency is being shared.
Suppose you were sitting at table with 10 other people all trying to ask questions to one instructor. What would you do? Would you listen first to make sure no one is talking before asking a question? If you and another person spoke simultaneously would you then stop, listen for silence and try again?
As the instructor what would you do? The same pause for silence before speaking applies, but what if two people asked questions before you gave an answer, would you direct your answers by using someone's name and would this mean no one else hears what you say and can talk?
What if two instructors wanted to hold sessions, ideally they would find their own tables and be completely separate. But what if they had to share one table, so now there are 20 people and 2 instructors at one table. Could two people speak at the same time now because they are asking questions to different instructors? Obviously no, they would still be interrupting each other.
What about how loud or soft people may be speaking and the ability of others to hear, would these have any effect on the discussions?
The same things are happening between your Wi-Fi device and the wireless access point. Consider what is going on when you see 10, 20, or 100 other people nearby all using Wi-Fi. Everyone is taking turns.
Wireless lans exist on 2.4Ghz & 5Ghz
- 2.4 GHz, or the 'legacy' frequency is limited to 3 non overlapping channels and is suitable for low density of users.
- Adjacent channel interference: with 2.4 GHz there are 13 channels (1-13), However the only non overlapping channels that should be used are 1,6,11. For example, if there is an AP on channel 1 and an AP on channel 2, their RF spectrum overlaps such that if both transmitted at the same time, the traffic would be garbled.. But it gets worse, because they are on different channels, they cannot understand the management information that improves speed and performance in the shared medium, and as such continue to try transmitting with all traffic being garbled.
- Non Wi-Fi interference: these create noise and corrupt or prevent Wi-Fi data communication. Examples: Microwave ovens (2.4GHz), cordless phones (2.4GHz or 5GHz), blue-tooth devices (2.4GHz), zigbee (2.4GHz)
- 5 GHz has 23 non overlapping channels and is recommended for areas with a high density of users (example: a university).
- For a definition of a rogue AP see the AP classification process page and review the Rogue AP checklist
- Additional access points on a channel reduce throughput. Devices need to take turns using the frequency and worse still, each SSID sends out multiple beacon frames a second at the lowest configured data rate using up additional air time. Even worse is that most low cost access points have only a single power setting (Max power) and can cause problems over greater distances.
- Security risks to users: Malicious individuals may try to trick users into associating to their rogue access point in order to steal credentials or other sensitive information.
- Security risk to the campus network: connecting a private AP exposes the campus network to anyone who can circumvent security settings on the AP.
- Rogue APs can create hidden nodes where devices on a channel cannot detect each other, resulting in transmissions which are garbled.
- Rogue APs create near/far issues where a powerful rogue and client talking to it can drown out a distant client talking to a legitimate AP.
- Wireless traffic is shaped on campus.
- Shaping occurs after wireless traffic has entered the wired network.
- The current shaper is designed to allow for quick web browsing and video streaming but to slow large downloads and high throughput users down so other users have more opportunity to access the wireless network.
- Make sure you are on the best AP possible by disconnecting and then reconnecting to eduroam when you arrive to your usage destination. Client devices control when they change access points and tend to hold on to a connection that is poor but usable even when there is a better one available.
- Check your drivers; keep your wlan driver up-to-date from the manufacturer website.
- Disable the "power save" features of your adapter so it stays awake/connected (performance is improved at the cost of battery life).
- Visit a nearby helpdesk to troubleshoot your wireless issue
- If you cannot get connected to the wireless network, please visit a campus helpdesk for assistance (i.e. you can see the eduroam network but cannot connect).
- If you are connected, but feel there is a problem, please complete the Wi-Fi Incident Report form.
- Complete the form from your wireless device at the problem location.
- The data you provide will help us determine if the current service level meets the configured profile.
- We will also use the location provided for a Wi-Fi spot test to determine if a problem exists.
- If you cannot detect a wireless network at the location, please submit a new Request Tracker (RT) ticket with the following information:
- Building and room
- Your wireless device type: (laptop, phone, tablet)
- We will test the wireless signal in the provided location with our standard devices. If coverage at the location does not meet the Wi-Fi requirement to provide coverage indoors in public areas, it will be added to a list of locations that require further upgrades to fill coverage gaps.
- Buy a laptop with 5GHz Wi-Fi
Note: 802.11n does not necessarily mean it supports 5GHz
- 802.11agn supports 5GHz
- 802.11bgn is 2.4GHz only