Hospital networks are undoubtedly one of the most challenging network to work with specially when it comes to wireless. We can not deny that people’s life depend on effective communication inside the hospitals and a lot of those communication system rely on 802.11 standard based wireless systems. Below are some tips which I concluded to be useful working with one of the major hospital. There can be multiple other tips, but these tips I am sure will help you get started on the right foot.
Tip 1 – In most of the design documentation, its very common to use RSSI (Recieved Signal Strength Indicator) value of -67 dBm when building VoWiFi (voice over wireless) network, but this may not be necessarily true. These values are based on receive sensitivity of Cisco wireless phones. However there are multiple other vendors who have different requirements, for e.g. Spectralink model 8440, if you look at spec sheet and best practices for Spectralink 8440, from the link below, you will realize their receive sensitivity is very different from Cisco 7925G for instance. Spectralink 8440 requires Signal to be heard at -60dBm at 5Ghz in order to decode the signal at 12 mbps datarate. So if you build the network for -67dBm and experience voice quality issues with spectralink phones don’t be surprised. Same can hold true for other vendors like Zebra T51 or T56. In some cases the datasheets does not mention the receive sensitivity, for those devices its best advisable to contact the vendor and find out this information. So to conclude, the first tip is to understand what end points you are building the wireless VoWiFi network for ? Use all the information available about the radio installed on the endpoint. Use the most sensitive radio parameters to build the RF infrastructure.
Refer to page 5 of this document from spectralink – https://www.spectralink.com/sites/default/files/spectralink_best_practices_84-series_handsets_091313.pdf
Tip 2 – For VoWiFi communication, don’t even think 2.4Ghz band. I have constantly found that for any given AP 2.4Ghz radio has high channel utilized, compared to 5GHz range. 5GHz is just wider, cleaner and convenient to build VoWiFi network, most devices as of this writing support 5GHz range at least 802.11a standard.
Tip 3 – Range is inversely proportional to frequency and directly proportional to power. So if same power if used for both 2.4Ghz and 5Ghz radio than the coverage for 2.4 Ghz and 5Ghz will be very different. In practice, I have mostly found that 2.4Ghz range is almost double of 5GHz range. For optimum design, one must keep the coverage pattern (cell size) of 2.4Ghz and 5Ghz radio same. This should be crafted carefully during survey by using lower power for 2.4Ghz and higher power for 5Ghz radio. Additionally some of 2.4GHz radios can be disabled for to match the coverage pattern as much as possible. read tip 6 for more details.
Tip 4 – Use 20Mhz channel width. Use channel bonding only if we have assigned a free channel to every AP and you have more channels left (which is most cases will not be true). While channel bonding might help you use some more subcarriers but it also increase SNR requirements, and VoWiFi is not about bandwidth, its more about reliability.
Tip 5 – If you plan to use 802.1X/EAP authentication for VoWiFi clients, you have to use some form of FT (Fast Transition) to keep the RTT under 150ms (VoIP requirement). Check support for 802.11r, 802.11k, most Cisco phones are CCX compliant so you can also consider CCKM (which has similar end result as 802.11r for 802.1x in terms of roaming time).
Tip 6 – Build for location services (based on triangulation), not for presence (based on proximity). Even if the hospital do not need any location services or location based analytic at present, still try to convince and build the network to be location ready. The cost of redesigning for location from voice/data RF design would be much higher and complicated than building it for location to start with. you can use the following parameters for location. Remember that client need to be seen by 3 APs for triangulation and local accuracy of 10meters (30 feet) unless using hyper location, but all 3 AP not necessarily need to perform data processing. Many APs can be in monitor mode only, contributing to location analytical but not processing data and not adding to co-channel interference. Whatever survey tool you plan to use, you can use following parameters for survey. Along with survey results strategically place APs at the edges in monitor mode for location triangulation.
|Signal Strength||Neg 67 to Neg 58dBm||Depends on Vendor of choice|
|SNR||20 or above||depends on Signal Strength|
|Datarate||12mbps or above||12mbps can be set mandatory, however keep in mind that every time there is shift in the datarate, it leads to re-transmission which is not a great thing to have on a wireless network, so its often recommended to disable some of the other datarate between 12mbps and 54mbps, for e.g. keep 12mbps mandatory, 18, 24 as disable, 36,48,54 as supported|
|Number of AP||Client should be seen by minimum 3 AP at all times, building redundancy for AP failure.||At minimum -62dBm, remember about non client serving APs in monitoring mode for location purposes only, usually placed at edges.|
|Round trip time||max 150 ms|
|Packet Loss||max 2%|
|Channel||Perform survey with no less than 3 AP at 3 separate channel and repeat pattern by changing channel|
|Power||max 10mW for 2.4 and max 24mW for 5Ghz EIRP||UNII 1 do not let you use more than 50mW of power (17dBm), do not use more than 1/2th of available power, leaving room for AP to increase power using TPC in case of failure of 1 AP. Remember the value recommended is EIRP, not actual power, so actual power set on AP will be lower so that effective EIRP matches to recommended value.|
I hope you enjoyed reading it as much as I enjoyed writing it.