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Executive Summary:

How to fix iPhone Wi-Fi problems.  Why can't that shiny new iPhone stay connected to the wireless network?  FYI, it's probably not your IT department's fault.  There are steps, however, that can be made to improve the overall connection.  Target audience is for IT professionals but this information is helpful for all end-users.












Problem:

Complaints of poor iPhone (Wi-Fi) performance. iPhones are inconsistently kicked off the Wi-Fi.  iPhones may only connect at low data rates.  End-uses are frustrated when iPhones and iPads disconnect.

Topology:

Corporate Wi-Fi uses Juniper wireless controllers and access point (APs).  Most smart phones connect to dedicated guest network. Wireless network seems to work well for most Android and Windows devices. Majority of problems are with iPhones and iPads.  N.B., topology is similar to Cisco controllers.  Topology is also similar, to a lesser extent, to personal wireless systems.

Symptoms:

iPhones and iPads experience disconnects, delays, packet drops, or total loss from Wi-Fi network. In other instances, Apple devices consistently connect to 802.11n networks at 24Mbps.

Cause:

Aggressive roaming causes most Wi-Fi problems for iPhones and iPads.  Roaming occurs when multiple APs broadcast identical SSIDs.  These Apple devices are quick to jump ship compared to other clients (e.g., Android). Excessive roaming causes wireless drops, slow data rates, and poor battery life.  What's more, all roaming decisions are made by wireless client (i.e., iPhone) –not the wireless AP or controller.

Additional factors can also influence wireless grief:  iPhone standby settings; spectrum preference; legacy rates; encryption; and signal strength -too much or too little.

Influencing Factors: 


  1.  Power save handler. iPhones enter standby/ sleep mode based on application activity. iPhones generally disconnect Wi-Fi in standby mode. End-users should understand this behavior is by design (à la Apple). It is not a limitation of their corporate Guest network.
  2. Roaming. Clients may disconnect if the roaming process takes too long.
  3. Spectrum preference. iPhones prefer the 2.4GHz spectrum over the 5Ghz spectrum. Wireless clients work best on the 5GHz spectrum. Consider disabling 2.4GHz radios from the APs.
  4. Data Rates. Legacy 2.4GHz data rates (e.g., 802.11b and 802.11g) harm 802.11n wireless networks. Disable 802.11b and consider disabling 802.11g (i.e., adjust roaming cell size). Wireless networks realize 30% to 50% increases in throughput after disabling 1,2,5.5, and 11 Mbps data rates.
  5. Encryption and Ciphers. WPA and TKIP caps/limits Wi-Fi networks to 54Mbps. Juniper recommends only implementing WPA2 with CCMP cipher for 802.11n transmissions. Pre-shared key is acceptable. 802.1X (i.e., WPA2-Enterprise) is better.
  6. Range. Signal strength affects client roaming decisions. 802.11b (2.4GHz) has nearly twice the range of 802.11a (5GHz). Adjust radio power levels (i.e., transmit power) so that 5GHz RSSI is great than the 2.4GHz RSSI. For example, set 2.4GHz to 4 dBm; and set 5GHz to 12dBm. 

Power Save:

Apple IOS devices (e.g., iPhones and iPads) use standby/ sleep mode to improve battery life. The phone enters standby five minutes after it locks (i.e., requires PIN to unlock). Active sessions (e.g., web download) transition to standby after thirty minutes.

In most situations, the iPhone disconnects Wi-Fi after it enters standby. It makes sense –who enjoys one hour power? Non-persistent Wi-Fi becomes less of an issue because most devices automatically reconnect when unlocked. N.B., auto-connect introduces security risks via Man-in-the-Middle (MMTM) –we’ll save that discussion for another day.

In some circumstances, there are exceptions to Apple’s non-persistent Wi-Fi policy. For example, some applications use a background task handler (e.g., Exchange ActiveSync) that keeps Wi-Fi connected at all times. Other users experience persistent Wi-Fi while streaming music via Spotify or Pandora.

Provide a power source (i.e., plug it in) or entirely disable the screen lock (also not recommended) are alternate methods for Wi-Fi persistence.

Roaming Rates

Normally, iPhones roam between APs within a few milliseconds. However, in some circumstances clients may take up-to five seconds to complete –ay caramba! Most APs disconnect the session after three minutes. End-users without auto-connect require manual reconnection. Either way, it’s best to keep roaming to a minimum.

Spectrum Preference:

iPhones often prefer the 2.4GHz over 5Ghz spectrum. This 2.4GHz preference is not necessarily by design. The preference results from a combination of roaming thresholds, dual-band APs, and imprecise interpretation of the RSSI signals.

Why does spectrum matter? 2.4 GHz only provides three non-overlapping channels: 1, 6, and 11. In addition, the majority of wireless devices operate on the 2.4GHz spectrum. In contrast, the 5 GHz spectrum provides 23 non-overlapping channels. Relatively few devices use the 5GHz spectrum.

For example, a quick wireless scan shows a total of 81 neighbor APs within range of my office.


This scan shows 86% of my neighbors transmit on the 2.4GHz spectrum; compared to only 13% on the 5GHz spectrum.



This data also shows only six 5GHz channels are in use. That leaves us with 16 pure unused virgin 5GHz channels for unadulterated throughput! Contrast that with the surrounding APs (all 70 of them) saturating the only three 2.4GHz channels: 1, 6, and 11. Additionally, microwaves and cordless phones operate on the same 2.4GHz spectrum.

What is the actual impact of an entire office building sharing the same three channels? Expect a wide range of clients and usage in large office buildings. Neighbor APs share the limited bandwidth across wireless channels. 802.11g radios provide roughly 300Mbps of total throughput. Wi-Fi bandwidth increases (per SSID) with 802.11ng and MIMI antennas. These innovations may further reduce neighbors’ throughput by up to 90%.


Table 1. 
Comparison of Wi-Fi Rates and Channels
Standard
802.11a
802.11b
802.11g
802.11ng
802.11na
Spectrum
(5GHz)
(2.4GHz)
(2.4GHz)
(2.4GHz)
(5GHz)
Max Speeds
54 Mbps
11Mbps
54Mbps
300Mbps
 300Mbps
Range
50 feet
100 feet
100 feet
50 feet
 50 feet
Non-overlapping Channels
24
3
3
3
24

Spectrum Throughput

Data can transmit roughly twice as far over the 2.4GHz spectrum compared to the 5GHz spectrum. The range is impressive but it’s not necessarily better. For example, it’s prudent to keep wireless beacons confined to the office perimeter (i.e., security).

Additionally, 2.4GHz has a negative impact on client data rates and roaming. Sticky clients (e.g., Android) may connect from the parking lot at 1Mb connection rate. These clients may be stuck with low connection rates throughout the day because of their low roaming aggressiveness. Recall the negative impact legacy rates have on the entire network. Likewise, non-sticky clients may experience unnecessary roams. Also, consider the negative impact of signal noise. Wi-Fi works better when it doesn’t detect every neighbor on the block.

RSSI, SNR, and Throughput:

Data transmission rates (i.e., throughput) can be estimated in relation to RSSI and Signal Noise Ratio (SNR). RSSI is the RF signal strength. Large RSSIs represent strong signal strength. For example, -60 dBm is greater than -70 dBm.
SNR is an expression of signal strength minus signal noise. SNR generates a positive number expressed in decibels (dB). For example, SNR is 20 if the RSSI value is -75db and Noise value is -95db. In general, SNR above 20 dB is an acceptable level for transmission.

Table 2.  Legacy Data Rates Estimates.
Rate (Mb/s)
1
2
5.5
11
6
9
12
18
24
36
48
54
SNR (dB)
4
6
8
10
4
5
7
9
12
16
20
21
Signal level (dBm)
-81
-79
-77
-75
-81
-80
-78
-76
-73
-69
-65
-64

Table 3.  802.11ng (2.4GHz) Data Rate Estimates.
2.4GHz Rate (Mb/s)
(20MHz)
14.4
28.9
43.3
57.8
86.7
115.6
130
144.4
SNR (dB)
11
14
16
19
23
27
28
29
Signal level (dBm)
-82
-79
-77
-74
-70
-66
-65
-64

Table 4.  802.11na (5GHz) Data Rate Estimates.
5 GHz Rate (Mb/s)
(40MHz)
30
60
90
120
180
240
270
300
SNR (dB)
11
14
16
19
23
27
28
29
Signal level (dBm)
-79
-76
-74
-71
-67
-63
-62
-61

iPhone Roaming Process:

iPhone roaming algorithm is based on AP signal strength and client activity:

  1. iPhone compares current BSSID (based on radio Mac) and other available AP BSSID signal levels.
  2. The roaming decision based on client activity: (a) idle session (phone in pocket) or (b) active session (e.g., web surfing). iPhone searches for a better signal level in both 2.4GHz and 5GHz bands.
     (a.) Idle clients search for new BSSIDs when current signal strength is -70dB or less (e.g. -71dB).

  • iPhone roams when it locates another AP with a signal at least +12dB better.
  • iPhone scans in 90 second intervals until its current signal strength improves (e.g., -69 dB) or it locates a different BSSID with 12dB stronger signal strength. 
     (b.)  Active client attempts to locate a stronger signal if current signal strength is -70dB or less.

  • iPhone chooses new BSSID if it has +8dB or greater RSSI.
  • iPhone continues to scan in 90 second intervals until current signal improves or locates a different BSSID at least 8 dB or better. 
Extenuating factors also influence iPhone roaming decisions: (a) Different spectrum signals and (b) client detection imprecision.

The 2.4GHz signal is roughly 7dB higher than the 5GHz signal. Physical attributes of BSSID antenna accounts for the difference in signal strength -it is not a limitation of the AP. By itself, the +7dB difference between RSSI strength is not enough to initiate roaming. Recall, the iPhone only initiates roaming after it identifies a BSSID  with RSSI at least +8dB or greater.

Wireless roaming reaches critical mass the client does not measure RSSI accurately (i.e. client imprecision).  This imprecision accounts for an additional plus or minus 4dBs. In other words, iPhones generally prefer 2.4GHz radios over 5GHz radios

Encryption and Ciphers

WPA and TKIP slows Wi-Fi traffic connection rates to a maximum 54Mbps. These standards are superseded by WPA2 encryptions and CCMP cipher. Do not enable WPA2 and WPA simultaneously. Likewise, do not enable WPA2 and CCMP and TKIP.

*Read the following post for specific fixes, recommendations, and best practices.

References: 

http://www.cisco.com/c/en/us/td/docs/wireless/controller/technotes/8-0/iPhone_roam/b_iPhone-roaming.html
http://kb.juniper.net/InfoCenter/index?page=content&id=KB20295&actp=search&viewlocale=en_US&searchid=1301446020120
http://www.greatwhitewifi.com/2015/07/12/fixing-hallway-fi-vol-1/
http://community.arubanetworks.com/t5/Controller-Based-WLANs/How-does-the-quot-beacon-rate-quot-profile-work/ta-p/179242
http://kb.juniper.net/InfoCenter/index?page=content&id=KB28153&actp=search&viewlocale=en_US&searchid=1373982600826
https://en.wikipedia.org/wiki/IPhone
http://www.juniper.net/documentation/en_US/network-director1.5/topics/concept/wireless-encryption-and-ciphers.html
http://blogs.cisco.com/wireless/wi-fi-taxes-digging-into-the-802-11b-penalty
http://www.cisco.com/c/en/us/td/docs/wireless/technology/apdeploy/8-0/Cisco_Aironet_3700AP.html
https://www.wireless.att.com/support_static_files/KB/KB3895.html
http://kb.juniper.net/InfoCenter/index?page=content&id=KB20248&actp=search&viewlocale=en_US&searchid=1456780940943
https://www.youtube.com/watch?v=tihSXW6Yg1M

About Steven M. Jordan

Steven Jordan is an infrastructure and process management specialist. Steven holds a Master of Science degree in ICT from the University of Wisconsin Stout. Steven is also a Cisco Certified Network Professional (CCNP) and Master Gardener.
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1 comments:

  1. Any recommendations on what to do when an iphone starts a large download but never seems to complete? This seems to happen with IOS updates. Thanks!

    ReplyDelete