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What is a PRL?

Posted by S4GRU, in Author: Travis Griggs 30 January 2013 · 48,497 views

PRL Preferred Roaming List Devices Series New Sites GEO SID NID
What is a PRL? 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.

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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.




Awesome article digiblur!

What I'm curious about is, is there any way to then identify, using GEO/SID/NID, exactly what network is what? In the example of GEO 4, SID 85 is "Negative", and I've found myself curious as to what network and what band that could possibly be that is CDMA yet lacks a proper roaming agreement beyond 911.
koiulpoi, if you want to research the provenance of a particular SID, a good starting point is this list:

http://ifast.org/files/NationalSID.htm

AJ
Great read, thank you.
Great idea and solid article, digiblur. This was sorely needed. Hopefully, the article will become something of an Internet meme whenever people commonly misconstrue a PRL as a "list of towers."

One clarification I have, though, is "Band Class 25." There is no such thing. Band 25 is on the 3GPP (W-CDMA/LTE) side for the extended PCS A-G block range, while band class 14 is the 3GPP2 (CDMA2000) equivalent. Band class 14 is basically irrelevant now, since we know that Sprint is not going to deploy any CDMA2000 in the PCS G block.

AJ
A few years ago while in a subway tunnel in Chicago, I noticed that I was roaming with full bars but my phone said "911 only". I think this was the case that US Cellular owned the network that was installed underground in the tunnels, but only made it available to their customers. In a unique application like that, I'm sure it would be easy for the network to be overwhelmed if they allowed everyone to roam because no other carriers got service underground.

A few years ago while in a subway tunnel in Chicago, I noticed that I was roaming with full bars but my phone said "911 only". I think this was the case that US Cellular owned the network...


Was this with a Sprint handset? I have been in Chicago 3-4 times over the last seven years. Even back in 2006, I seem to recall that Sprint roamed on USCC in "L" tunnels.

AJ
I see "Emergency Calls Only" frequently on my Nexus 4 when off the T-Mobile network.

Robert

Great idea and solid article, digiblur. This was sorely needed. Hopefully, the article will become something of an Internet meme whenever people commonly misconstrue a PRL as a "list of towers."One clarification I have, though, is "Band Class 25." There is no such thing. Band 25 is on the 3GPP (W-CDMA/LTE) side for the extended PCS A-G block range, while band class 14 is the 3GPP2 (CDMA2000) equivalent. Band class 14 is basically irrelevant now, since we know that Sprint is not going to deploy any CDMA2000 in the PCS G block.AJ


Thanks for the kind words and fact checks, AJ!

Yes it was definitely needed on just about any forum. I was glad to be able to share a bit of knowledge I have learned and reverse engineered over the years.

Hopefully after the PRL articles are written all of you will be experts on PRLs.

To All, please take note that I will not be discussing how to create your own PRLs and/or load them with this article series.

I see "Emergency Calls Only" frequently on my Nexus 4 when off the T-Mobile network.


In other countries, GSM based carriers commonly seem to have wide ranging reciprocal roaming agreements, but that is not the case in the US. For whatever reason, GSM based carriers here cannot seem to get along, thus they limit roaming on a granular LAC basis simply because they can.

Our CDMA2000 carriers, on the other hand, are practically paragons of roaming openness. And Sprint is the tip top. In many markets, Sprint has roaming agreements with and includes in the PRL all other CDMA2000 carriers. That means at least one, often two (or more) roaming partners.

AJ
4159 is not a roaming SID. It's a Sprint native signal. What I was trying to show with numbers might not make much sense when you look at it without names. But that is of course how it looks in the PRL.

So let's try it with names to cut some of the confusion.

Sprint 800SMR & Sprint PCS SIDs are in priority group 1. Your phone will look for 800SMR first then go to PCS afterwards. They are on the same level of priority so your device is content with either of the SIDs since they are on the highest level of priority for your Geo.

Lose signal and your device could land on 4279, let's call this one Cricket. It is on a priority of 2 combined with a roaming indicator. I have found with the roaming indicator it is more of like an aggressive "flag" for the phone to do whatever it takes to find a non-roaming signal. Since 4160, Verizon, is in priority 3, your phone will not acquire it unless all attempts to connect to a SID on priority 1 & 2 have been exhausted.

This is the norm since Sprint most likely receives a lower roaming rate with Cricket over Verizon Wireless. So it only makes sense to have the phones try to use Cricket over Verizon Wireless but still allow your customers have the roaming coverage of Verizon where Cricket is not available.
This is interesting stuff; thanks for the write-up! Will part 2's LTE information include the mystery with the "LTE available" file on some devices?
No, that "mystery feature" has nothing to do with PRLs.

Many people including myself theorize the LTE available file is for your phone to "remember" the 1X base station IDs of where it found LTE. When you are in the area again it uses this to determine if it should scan for LTE more often or not.
Very nice digiblur.
I think you should make digi a contributing author now for his hard work.

I think you should make digi a contributing author now for his hard work.


Yep, that is typically part of the deal. I suspect the change in status will happen soon.

And let me throw this out as an advertisement. If you have an article in mind worthy of The Wall, make us a pitch. We accept, you write it, we publish it, and you, too, can become a Contributing Author (with all of the benefits endowed). The Wall has gone a bit spare of late. We could use some new blood, some fresh ideas.

AJ
Great article, thanx Digiblur
On some 4G LTE chipsets such as Qualcomm, the PRL determines if LTE is enabled for the geographic region you are in.


i like that
I'd love to see a similar write-up on how GSM works... I've always understood the PRL concept--I know the GSM side of things is more "network/server-based", but not really figured out the specifics.

Also with LTE being based on the GSM core, is Sprint and Verizon having to deal with 2 systems here, or are they able to merge them into one?

I remember reading about C-SIMS for Verizon, but wasn't sure if Sprint was using the same thing--I assumed they weren't and that was why they initially weren't allowing swappable SIMs. The way I understood it, without the C-SIM app installed by the network (and on the SIM), the ANSI (CDMA) and MAP (GSM) cores couldn't both be updated when SIMs got swapped in devices, causing all sorts of screw up... IIRC the CSIM can even bring the ESN/MEID from the ANSI side over to it.

I know the MAP system doesn't use PRLs, but isn't there some sort of acquisition table?

Nat

I'd love to see a similar write-up on how GSM works... I've always understood the PRL concept--I know the GSM side of things is more "network/server-based", but not really figured out the specifics.Also with LTE being based on the GSM core, is Sprint and Verizon having to deal with 2 systems here, or are they able to merge them into one?I remember reading about C-SIMS for Verizon, but wasn't sure if Sprint was using the same thing--I assumed they weren't and that was why they initially weren't allowing swappable SIMs. The way I understood it, without the C-SIM app installed by the network (and on the SIM), the ANSI (CDMA) and MAP (GSM) cores couldn't both be updated when SIMs got swapped in devices, causing all sorts of screw up... IIRC the CSIM can even bring the ESN/MEID from the ANSI side over to it.I know the MAP system doesn't use PRLs, but isn't there some sort of acquisition table?Nat



I wouldn't mind that write up myself. But it is not something I'm qualified to author.

Robert

I'd love to see a similar write-up on how GSM works... I've always understood the PRL concept--I know the GSM side of things is more "network/server-based", but not really figured out the specifics.Also with


A good read if you are interested:

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CDAQFjAA&url=http%3A%2F%2Fwww.qualcomm.com%2Fmedia%2Fdocuments%2Ffiles%2Fmultimode-system-selection-basic-provisioning.pdf&ei=qYQMUcvTBq6A2QWe2IHQDw&usg=AFQjCNEauk7xsZ5NzXcLcLXOPWOUh-fJSg&bvm=bv.41867550,d.b2I&cad=rja
Should be required reading for all Sprint call center staff!

I'd love to see a similar write-up on how GSM works... I've always understood the PRL concept--I know the GSM side of things is more "network/server-based", but not really figured out the specifics.


The simple answer is that GSM/W-CDMA and even LTE use MCCs, MNCs, and LACs. Mobile Country Code, Mobile Network Code, and Location Area Code. The latter two are more or less equivalent to CDMA2000 (or ANSI-41) SIDs and NIDs.

Whenever a GSM/W-CDMA or LTE device detects a new LAC, it tries to register, and the network confirms whether the device is allowed to use certain/all services in that LAC.

By the way, based upon some info that we have gleaned from a new Android signal app, Sprint may be reusing its CDMA2000 NIDs as LACs. But that info is still in the confirmation stages.

I know the MAP system doesn't use PRLs, but isn't there some sort of acquisition table?


That is a great question. Honestly, I do not know what, if any acquisition assistance tables that GSM/W-CDMA or LTE devices store locally.

AJ
Thanks digiblur, this is a great article. I'm really looking forward to the next part!
The Chicago Subway question is a good one. What's going to happen with the Subway once Sprint takes over for USCC in the coming months?
Here are some good corporate PRLs for Sprint. (Via XDA)
No need for root.
24006 is about the best one to use.

I can confirm these working.