by Travis Griggs
Sprint 4G Rollout Updates
Wednesday, January 30, 2013 - 10:00 AM MST

A PRL file is a Preferred Roaming List. In simple terms, it tells the device how to scan for various wireless cell systems, which ones are native, and which priority to use them in. If there isn't a native Sprint signal available, the PRL defines which roaming partners to scan for, which ones should be used, and in what order of preference to scan for them in. Contrary to belief and what some Sprint reps may tell say, a PRL is not a list of cell sites. You do not need a new PRL update to receive service from a new cell site. Nor will a PRL update result in faster Sprint EVDO (3G) speeds either.

Of course there are a few exceptions to these rules with roaming agreements and/or Network Vision in the picture, but we will explain that later. PRL updates have nothing do with 4G WiMax coverage either. On some 4G LTE chipsets such as Qualcomm, the PRL determines if LTE is enabled for the geographic region you are in.

So how does a PRL really work?

Before I can start to explain the inner workings of a PRL, there are few terms for reference:

A PRL is broken down into a three tier system:

• GEO - Geographic areas (regions), they are commonly referred to as a GEO.
• SID - System IDs assigned to the various carriers.
• NID - Network IDs are assigned by carriers to break a SID up.

Common wireless bands found in US CDMA PRLs:

• PCS Band - 1900mhz PCS band in the US (A block, B block, etc) - Band Class 1 or 25
• Cellular band - 850mhz cellular band in the US (A and B side) - Band Class 0
• SMR band - 800mhz band used previously by Nextel. CDMA 1xA is in active deployment - Band Class 10

Other terms:

• Channel – assigned frequency within a band (200, 476, 350, etc)
• Negative (Neg) Network – SID/NID is prohibited (only 911 calls allowed)
• Preferred (Pref) Network – SID/NID is allowed for acquisition and usage
• Preferred Only PRL - only the SIDs specified in the PRL are allowed for acquisition

When a device is powered up for the very first time, the phone will start at the top of the PRL and start searching through the list of SIDs for a native Sprint signal. This usually happens very quickly. Once your phone acquires a SID in your GEO, the devices will stay within the GEO for any additional searching for SIDs before it goes out looking in other GEOs again. This gives your phone a quicker response time of finding another SID when it needs to. If you have ever noticed it takes a little longer to find a signal when the flight attendant states you may now use your wireless devices, this is your phone searching the last known GEO, the devices then gives up and starts searching the other GEOs until it finds one to acquire.

The SID/NID records within the GEO have their various priorities and channel/band scans assigned to them. A SID is the regional number assigned to wireless system. A NID is used by a cellular carrier to break up a large SID into smaller pieces for further localizing scans/rules. For instance a SID that has two large metro areas could have a NID of 51 for one area and 52 for the other area. The record would be listed as 4159/51 and 4159/52.

If Sprint needs to apply different rules and/or acquisition channels to either NID it will put a record for each one. If no local rules are needed, the NID is listed as 65535 to encompass all NIDs within the one SID. In the PRL analysis reports, any NID of 65535 is suppressed as it is not needed. It may sound confusing at times but it is a simple three tiered system; GEO area, SID, then NID.

In the PRL example above there are 5 SIDs assigned to Geo #4. The first two have a roaming indicator of 0, meaning a native Sprint signal. 22411 and 4159 have a priority of 1. These two SIDs do not necessarily have a preference in which either is used since they are the same priority but the device will scan for 22411 first. If 4159 is acquired, the device will not actively seek another network to use. During various sleep periods and/or timers the device could scan/acquire 22411 though. Once the device finds itself without a usable signal from 4159 or 22411, the scan will proceed into the next priority group.

The next priority group of 2 has SID 4279 and a roaming indicator presented to the user. The device will acquire 4279 and notify the network carrier of its presence. The device will actively and aggressively continue to search for a non-roaming signal. Due to this continued scanning this may cause the radio chipset to not enter into the power saving sleep modes causing increased battery usage. As long as SID 4279 is available, the device will not search for SID 4160 with the priority of 3. 85 is a NEG network meaning your phone is not allowed to use this network for any reason other than 911 calls.

What happens when Sprint installs a new cell site?

I will say it again and again. You do not need a PRL update to use a new cell site, you do not need a PRL update to use a new cell site. Many Sprint reps will swear up and down that a PRL update is required to use new cell sites. This is incorrect! Many Airaves are activated and deactivated everyday but yet we don't see new PRL updates for these everyday. Using the example above, the phone is attached to Sprint 4159/51 using the same cell sites that were active on the previous day. Today the Sprint crews activated a new cell site to extend coverage a few more miles down the highway. Sprint will configure this cell site with the same licensed channels for the area and also configure it as a 4159/51 site. The devices in this area will use this new site without ever needing any type of PRL update.

I've only scratched the surface of the various inner workings of the PRL file. Stay tuned for part 2 of this article. The next article will take a more in-depth look on EVDO records, MCC/MNC records for LTE, 800mhz SMR for Network Vision, and much more.