Category Archives: Mobile Networks

Tales from the Trenches: The issue with Emergency Calling URNs in IMS Networks

A lot of countries have a single point of contact for emergency services; in Europe you’d call 112 in an emergency, 000 in Australia or 911 in the US. Calling this number in the country will get you the emergency services.

This means a caller can order an ambulance for smoke inhalation, and the fire brigade, in one call.

But that’s not the case in every country; many countries don’t have one number for the emergency services, they’ve got multiple; a phone number for police, a different number for fire brigade and a different number for an ambulance.

For example, in Brazil if you need the police, you call 190, while a for example, uses 193 as the emergency number for the fire department, the police can be reached at 190 or 191 depending on if it’s road policing or general, and medical emergencies are covered by 192. Other countries have similar setups.

This is all well and good if you’re in Brazil, and you call 192 for an ambulance, the phone sends a SIP INVITE with a Request URI of sip:[email protected], because we can put a rule into our E-CSCF to say if the number is 192 to route it to the answer point for ambulances – But that’s not often the case on emergency calls.

In IMS, handsets generally detect the number dialed is on the Emergency Calling Code (ECC) list from the USIM Card.

The use of the ECC list means the phone knows this is an emergency call, and this is really important. For countries that use AML this can trigger sending of the AML SMS that process, and Emergency Calls should always be allowed to be made, even without credit, a valid SIM card, or even a SIM in the phone at all.

But this comes with a cost; when a user dials 911, the phones doesn’t (generally) send a call to sip:[email protected] like it would with any other dialled number, but rather the SIP INVITE is sent to urn:service:sos which will be routed to the PSAP by the E-CSCF. When a call comes through to these URNs they’re given top priority in the network

This is all well and good in a country where it doesn’t matter which emergency service you called, because all emergency calls route to a single PSAP, but in a country with multiple numbers, it’s really important when you call and ambulance, your call doesn’t get routed to animal control.

That means the phone has to look at what emergency number you’ve dialed, and map the URN it sends the call to to match what you’ve actually requested.

Recently we’ve been helping an operator in a country with a numbering plan like this, and we’ve been finding the limits of the standards here.
So let’s start by looking at what the standards state:

IMS Emergency Calling is governed by TS 103.479 which in turn delegates to IETF RFC 5031, but for the calling number to URN translation, it’s pretty quiet.

Let’s look at what RFC 5031 allows for URNs:

  • urn:service:sos.ambulance
  • urn:service:sos.animal-control
  • urn:service:sos.gas
  • urn:service:sos.marine
  • urn:service:sos.mountain
  • urn:service:sos.physician
  • urn:service:sos.poison
  • urn:service:sos.police

The USIM’s Emergency Calling Codes EF would be the perfect source of this data; for each emergency calling code defined, you’ve got a flag to indicate what it’s for, here’s what we’ve got available on the SIM Card:

  • Bit 1 Police
  • Bit 2 Ambulance
  • Bit 3 Fire Brigade
  • Bit 4 Marine Guard
  • Bit 5 Mountain Rescue
  • Bit 6 manually initiated eCall
  • Bit 7 automatically initiated eCall
  • Bit 8 is spare and set to “0”

So these could be mapped pretty easily you’d think, so if the call is made to an Emergency Calling Code flagged with Bit 4, the URN would go to urn:service:sos.mountain.

Alas from our research, we’ve found most OEMs send calls to the generic urn:service:sos, regardless of the dialled number and the ECC flags that are set on the SIM for that number.

One of the big chip vendors sends calls to an ECC flagged as Ambulance to, which is totally infuriating, and we’ve had to put a rule in our E-CSCF to handle this if the User Agent is set to one of their phones.

Is there room for improvement here? For sure! Emergency calling is super important, and time is of the essence, while animal control can probably transfer you to an ambulance, an emergency is by very nature time sensitive, and any time wasted can lead to worse outcomes.

While carrier bundles from the OEMs can handle this, the global ability to take any phone, from any country and call an emergency number is so important, that relying on a country-by-country approach here won’t suffice.

What could we do as an industry to address this?

Acknowledging that not all countries have a single point of contact for emergency service, introducing a simple mechanism in the UE SIP message to indicate what number (Emergency Calling Code) the user actually dialled would be invaluable here.

URNs are important, but knowing the dialed number when it comes to PSAP routing, is so important – This wouldn’t even need to be its own SIP header, it could just be thrown into the Contact header as another parameter.

Highly developed markets are often the first to embrace new tech (for us this means VoLTE and VoNR), but this means that these issues seen by less developed markets won’t appear until long after the standard has been set in stone, and often countries like this aren’t at the table of the standards bodies to discuss such requirements.

This easy, reasonable update to the standard, has the potential to save lives, and next time this comes up in a working group I’ll be advocating for a change.

Playing back AMR streams from Packet Captures

The other day I found myself banging my head on the table to diagnose an issue with Ringback tone on an SS7 link and the IMS.

On the IMS side, no RBT was heard, but I could see the Media Gateway was sending RTP packets to the TAS, and the TAS was sending it to the UE, but was there actual content in the RTP packets or was it just silence?

If this was PCM / G711 we’d be able to just playback in Wireshark, but alas we can’t do this for the AMR codec.

Filter the RTP stream out in Wireshark

Inside Wireshark I filtered each of the audio streams in one direction (one for the A-Party audio and one for the B-Party audio)

Then I needed to save each of the streams as a separate PCAP file (Not PCAPng).

Turn into AMR File

With the audio stream for one direction saved, we can turn it into an AMR file, using Juan Noguera (Spinlogic)’s AMR Extractor tool.

Clone the Repo from git, and then in the same directory run:

python3 -i AMR_B_Leg.pcap -o AMR_B_Leg.3ga

Playback with VLC / Audacity

I was able to play the file with VLC, and load it into Audacity to easily see that yes, the Ringback Tone was present in the AMR stream!

A look at Advanced Mobile Location SMS for Emergency Calls

Advanced Mobile Location (AML) is being rolled out by a large number of mobile network operators to provide accurate caller location to emergency services, so how does it work, what’s going on and what do you need to know?

Recently we’ve been doing a lot of work on emergency calling in IMS, and meeting requirements for NG-112 / e911, etc.

This led me to seeing my first Advanced Mobile Location (AML) SMS in the wild.

For those unfamiliar, AML is a fancy text message that contains the callers location, accuracy, etc, that is passed to emergency services when you make a call to emergency services in some countries.

It’s sent automatically by your handset (if enabled) when making a call to an emergency number, and it provides the dispatch operator with your location information, including extra metadata like the accuracy of the location information, height / floor if known, and level of confidence.

The standard is primarily driven by EENA, and, being backed by the European Union, it’s got almost universal handset support.

Google has their own version of AML called ELS, which they claim is supported on more than 99% of Android phones (I’m unclear on what this means for Harmony OS or other non-Google backed forks of Android), and Apple support for AML starts from iOS 11 onwards, meaning it’s supported on iPhones from the iPhone 5S onards,.

Call Flow

When a call is made to the PSAP based on the Emergency Calling Codes set on the SIM card or set in the OS, the handset starts collecting location information. The phone can pull this from a variety of sources, such as WiFi SSIDs visible, but the best is going to be GPS or one of it’s siblings (GLONASS / Galileo).

Once the handset has a good “lock” of a location (or if 20 seconds has passed since the call started) it bundles up all of this information the phone has, into an SMS and sends it to the PSAP as a regular old SMS.

The routing from the operator’s SMSc to the PSAP, and the routing from the PSAP to the dispatcher screen of the operator taking the call, is all up to implementation. For the most part the SMS destination is the emergency number (911 / 112) but again, this is dependent on the country.

Inside the SMS

To the user, the AML SMS is not seen, in fact, it’s actually forbidden by the standard to show in the “sent” items list in the SMS client.

On the wire, the SMS looks like any regular SMS, it can use GSM7 bit encoding as it doesn’t require any special characters.

Each attribute is a key / value pair, with semicolons (;) delineating the individual attributes, and = separating the key and the value.

Below is an example of an AML SMS body:

0.18305;rd=50;top=20130717141935;lc=90;pm=W;si=123456789012345;ei=1234567890123456;mcc=234;mnc=30; ml=128

If you’ve got a few years of staring at Wireshark traces in Hex under your belt, then this will probably be pretty easy to get the gist of what’s going on, we’ve got the header (A”ML=1″) which denotes this is AML and the version is 1.

After that we have the latitude (lt=), longitude (lg=), radius (rd=), time of positioning (top=), level of confidence (lc=), positioning method (pm=) with G for GNSS, W for Wifi signal, C for Cell
or N for a position was not available, and so on.

AML outside the ordinary

Roaming Scenarios

If an emergency occurs inside my house, there’s a good chance I know the address, and even if I don’t know my own address, it’s probably linked to the account holder information from my telco anyway.

AML and location reporting for emergency calls is primarily relied upon in scenarios where the caller doesn’t know where they’re calling from, and a good example of this would be a call made while roaming.

If I were in a different country, there’s a much higher likelihood that I wouldn’t know my exact address, however AML does not currently work across borders.

The standard suggests disabling SMS when roaming, which is not that surprising considering the current state of SMS transport.

Without a SIM?

Without a SIM in the phone, calls can still be made to emergency services, however SMS cannot be sent.

That’s because the emergency calling standards for unauthenticated emergency calls, only cater for

This is a limitation however this could be addressed by 3GPP in future releases if there is sufficient need.

HTTPS Delivery

The standard was revised to allow HTTPS as the delivery method for AML, for example, the below POST contains the same data encoded for use in a HTTP transaction:


Implementation of this approach is however more complex, and leads to little benefit.

The operator must zero-rate the DNS, to allow the FQDN for this to be resolved (it resolves to a different domain in each country), and allow traffic to this endpoint even if the customer has data disabled (see what happens when your handset has PS Data Off ), or has run out of data.

Due to the EU’s stance on Net Neutrality, “Zero Rating” is a controversial topic that means most operators have limited implementation of this, so most fall back to SMS.

Other methods for sharing location of emergency calls?

In some upcoming posts we’ll look at the GMLC used for E911 Phase 2, and how the network can request the location from the handset.

Further Reading

VoLTE / IMS – Analysis Challenge

It’s challenge time, this time we’re going to be looking at an IMS PCAP, and answering some questions to test your IMS analysis chops!

Here’s the packet capture:

Easy Questions

  • What QCI value is used for the IMS bearer?
  • What is the registration expiry?
  • What is the E-UTRAN Cell ID the Subscriber is served by?
  • What is the AMBR of the IMS APN?

Intermediate Questions

  • Is this the first or subsequent registration?
  • What is the Integrity-Key for the registration?
  • What is the FQDN of the S-CSCF?
  • What Nonce value is used and what does it do?
  • What P-CSCF Addresses are returned?
  • What time would the UE need to re-register by in order to stay active?
  • What is the AA-Request in #476 doing?
  • Who is the(opens in a new tab)(opens in a new tab)(opens in a new tab) OEM of the handset?
  • What is the MSISDN associated with this user?

Hard Questions

  • What port is used for the ESP data?
  • Which encryption algorithm and algorithm is used?
  • How many packets are sent over the ESP tunnel to the UE?
  • Where should SIP SUBSCRIBE requests get routed?
  • What’s the model of phone?

The answers for each question are on the next page, let me know in the comments how you went, and if there’s any tricky ones!

Mobile IPv6 Tax?

Recently a Tweet from Dean Bubly got me thinking about how data is charged in cellular:

In the cellular world, subscribers are charged for data from the IP, transport and applications layers; this means you pay for the IP header, you pay for the TCP/UDP header, and you pay for the contents (the cat videos it contains).

This also means if an operator moves mobile subscribers from IPv4 to IPv6, there’s an extra 20 bytes the customer is charged for for every packet sent / received, which the customer is charged for – This is because the IPv6 header is longer than the IPv4 header.

Source: ServerFault -

In most cases, mobile subs don’t get a choice as to if their connection is IPv4 or IPv6, but on a like for like basis, we can say that if a customer moves is on IPv6 every packet sent/received will have an extra 20 bytes of data consumed compared to IPv4.

This means subscribers use more data on IPv6, and this means they get charged for more data on IPv6.

For IoT applications, light users and PAYG users, this extra 20 bytes per packet could add up to something significant – But how much?

We can quantify this, but we’d need to know the number of packets sent on average, and the quantity of the data transferred, because the number of packets is the multiplier here.

So for starters I’ve left a phone on the desk, it’s registered to the network but just sitting in Idle mode – This is an engineering phone from an OEM, it’s just used for testing so doesn’t have anything loaded onto it in terms of apps, it’s not signed into any applications, or checking in the background, so I thought I’d try something more realistic.

So to get a clearer picture, I chucked a SIM in my regular everyday phone I use personally, registered it to the cellular lab I have here. For the next hour I sniffed the GTP traffic for the phone while it was sitting on my desk, not touching the phone, and here’s what I’ve got:

Overall the PCAP includes 6,417,732 bytes of data, but this includes the transport and GTP headers, meaning we can drop everything above it in our traffic calculations.

Everything except the data encapsulated in GTP can be dropped

For this I’ve got 14 bytes of ethernet, 20 bytes IP, 8 bytes UDP and 5 bytes for TZSP (this is to copy the traffic from the eNB to my local machine), then we’ve got the transport from the eNB to the SGW, 14 bytes of ethernet again, 20 bytes of IP , 8 bytes of UDP and 8 bytes of GTP then the payload itself. Phew.
All this means we can drop 97 bytes off every packet.

We have 16,889 packets, 6,417,732 bytes in total, minus 97 bytes from each gives us 1,638,233 of headers to drop (~1.6MB) giving us a total of 4.556 MB traffic to/from the phone itself.

This means my Android phone consumes 4.5 MB of cellular data in an hour while sitting on the desk, with 16,889 packets in/out.

Okay, now we’re getting somewhere!

So now we can answer the question, if each of these 16k packets was IPv6, rather than IPv4, we’d be adding another 20 bytes to each of them, 20 bytes x 16,889 packets gives 337,780 bytes (~0.3MB) to add to the total.

If this traffic was transferred via IPv6, rather than IPv4, we’d be looking at adding 20 bytes to each of the 16,889 packets, which would equate to 0.3MB extra, or about 7% overhead compared to IPv4.

But before you go on about what an outrage this IPv6 transport is, being charged for those extra bytes, that’s only one part of the picture.

There’s a reason operators are finally embracing IPv6, and it’s not to put an extra 7% of traffic on the network (I think if you asked most capacity planners, they’d say they want data savings, not growth).

IPv6 is, for lack of a better term, less rubbish than IPv4.

There’s a lot of drivers for IPv6, and some of these will reduce data consumption.
IPv6 is actually your stuff talking directly to the remote stuff, this means that we don’t need to rely on NAT, so no need to do NAT keepalives, and opening new sessions, which is going to save you data. If you’re running apps that need to keep a connection to somewhere alive, these data savings could negate your IPv6 overhead costs.

Will these potential data savings when using IPv6 outweigh the costs?

That’s going to depend on your use case.

If you’ve extremely bandwidth / data constrained, for example, you have an IoT device on an NTN / satellite connection, that was having to Push data every X hours via IPv4 because you couldn’t pull data from it as it had no public IP, then moving it to IPv6 so you can pull the data on the public IP, on demand, will save you data. That’s a win with IPv6.

If you’re a mobile user, watching YouTube, getting push notifications and using your phone like a normal human, probably not, but if you’re using data like a normal user, you’ve probably got a sizable data allowance that you don’t end up fully consuming, and the extra 20 bytes per packet will be nothing in comparison to the data used to watch a 2k video on your small phone screen.


Ask someone with headphones and a lanyard in the halls of a datacenter what transport does DNS use, there’s a good chance the answer you’d get back is UDP Port 53.

But not always!

In scenarios where the DNS response is large (beyond 512 bytes) a DNS query will shift over to TCP for delivery.

How does the client know when to shift the request to TCP – After all, the DNS server knows how big the response is, but the client doesn’t.

The answer is the Truncated flag, in the response.

The DNS server sends back a response, but with the Truncated bit set, as per RFC 1035:

TC TrunCation – specifies that this message was truncated due to length greater than that permitted on the transmission channel.

RFC 1035

Here’s an example of the truncated bit being set in the DNS response.

The DNS client, upon receiving a response with the truncated bit set, should run the query again, this time using TCP for the transport.

One prime example of this is DNS NAPTR records used for DNS in roaming scenarios, where the response can quite often be quite large.

If it didn’t move these responses to TCP, you’d run the risk of MTU mismatches dropping DNS. In that half of my life has been spent debugging DNS issues, and the other half of my life debugging MTU issues, if I had MTU and DNS issues together, I’d be looking for a career change…

Improving WiFi Calling quality for WiFi Operators

I had a question recently on LinkedIn regarding how to preference Voice over WiFi traffic so that a network engineer operating the WiFi network can ensure the best quality of experience for Voice over WiFi.

Voice over WiFi is underpinned by the ePDG – Evolved Packet Data Gateway (this is a fancy IPsec tunnel we authenticate to using the SIM to drop our traffic into the P-CSCF over an unsecured connection). To someone operating a WiFi network, the question is how do we prioritise the traffic to the ePDGs and profile it?

ePDGs can be easily discovered through a simple DNS lookup, once you know the Mobile Network Code and Mobile Country code of the operators you want to prioritise, you can find the IPs really easily.

ePDG addresses take the form so let’s look at finding the IPs for each of these for the operators in a country:

The first step is nailing down the mobile network code and mobile country codes of the operators you want to target, Wikipedia is a great source for this information.
Here in Australia we have the Mobile Country Code 505 and the big 3 operators all support Voice over WiFi, so let’s look at how we’d find the IPs for each.
Telstra has mobile network code (MNC) 01, in 3GPP DNS we always pad network codes to 3 digits, so that’ll be 001, and the mobile country code (MCC) for Australia is 505.
So to find the IPs for Telstra we’d run an nslookup for – The list of IPs that are returned, are the IPs you’ll see Voice over WiFi traffic going to, and the IPs you should provide higher priority to:

For the other big operators in Australia will get you Optus and will get you VHA.

The same rules apply in other countries, you’d just need to update the MNC/MCC to match the operators in your country, do an nslookup and prioritise those IPs.

Generally these IPs are pretty static, but there will need to be a certain level of maintenance required to keep this list up to date by rechecking.

Happy WiFi Calling!

SMS Transport Wars?

There’s old joke about standards that the great thing about standards there’s so many to choose from.

SMS wasn’t there from the start of GSM, but within a year of the inception of 2G we had SMS, and we’ve had SMS, almost totally unchanged, ever since.

In a recent Twitter exchange, I was asked, what’s the best way to transport SMS?
As always the answer is “it depends” so let’s take a look together at where we’ve come from, where we are now, and how we should move forward.

How we got Here

Between 2G and 3G SMS didn’t change at all, but the introduction of 4G (LTE) caused a bit of a rethink regarding SMS transport.

Early builders of LTE (4G) networks launched their 4G offerings without 4G Voice support (VoLTE), with the idea that networks would “fall back” to using 2G/3G for voice calls.

This meant users got fast data, but to make or receive a call they relied on falling back to the circuit switched (2G/3G) network – Hence the name Circuit Switched Fallback.

Falling back to the 2G/3G network for a call was one thing, but some smart minds realised that if a phone had to fall back to a 2G/3G network every time a subscriber sent a text (not just calls) – And keep in mind this was ~2010 when SMS traffic was crazy high; then that would put a huge amount of strain on the 2G/3G layers as subs constantly flip-flopped between them.

To address this the SGs-AP interface was introduced, linking the 4G core (MME) with the 2G/3G core (MSC) to support this stage where you had 4G/LTE but only for data, SMS and calls still relied on the 2G/3G core (MSC).

The SGs-AP interface has two purposes;
One, It can tell a phone on 4G to fallback to 2G/3G when it’s got an incoming call, and two; it can send and receive SMS.

SMS traffic over this interface is sometimes described as SMS-over-NAS, as it’s transported over a signaling channel to the UE.

This also worked when roaming, as the MSC from the 2G/3G network was still used, so SMS delivery worked the same when roaming as if you were in the home 2G/3G network.

Enter VoLTE & IMS

Of course when VoLTE entered the scene, it also came with it’s own option for delivering SMS to users, using IP, rather than the NAS signaling. This removed the reliance on a link to a 2G/3G core (MSC) to make calls and send texts.

This was great because it allowed operators to build networks without any 2G/3G network elements and build a fully standalone LTE only network, like Jio, Rakuten, etc.

VoLTE didn’t change anything about the GSM 2G/3G SMS PDU, it just bundled it up in an SIP message body, this is often referred to as SMS-over-IP.

SMS-over-IP doesn’t address any of the limitations from 2G/3G, including limiting multipart messages to send payloads above 160 characters, and carries all the same limitations in order to be backward compatible, but it is over IP, and it doesn’t need 2G or 3G.

In roaming scenarios, S8 Home Routing for VoLTE enabled SMS to be handled when roaming the same way as voice calls, which made SMS roaming a doddle.

4G SMS: SMS over IP vs SMS over NAS

So if you’re operating a 4G network, should you deliver your SMS traffic using SMS-over-IP or SMS-over-NAS?

Generally, if you’ve been evolving your network over the years, you’ve got an MSC and a 2G/3G network, you still may do CSFB so you’ve probably ended up using SMS over NAS using the SGs-AP interface.
This method still relies on “the old ways” to work, which is fine until a discussion starts around sunsetting the 2G/3G networks, when you’d need to move calling to VoLTE, and SMS over NAS is a bit of a mess when it comes to roaming.

Greenfield operators generally opt for SMS over IP from the start, but this has its own limitations; SMS over IP is has awful efficiency which makes it unsuitable for use with NB-IoT applications which are bandwidth constrained, support for SMS over IP is generally limited to more expensive chipsets, so the bargain basement chips used for IoT often don’t support SMS over IP either, and integration of VoLTE comes with its own set of challenges regarding VoLTE enablement.

5G enters the scene (Nsmsf_SMService)

5G rolled onto the scene with the opportunity to remove the SMS over NAS option, and rely purely on SMS over IP (IMS); forcing the industry to standardise on an option alas this did not happen.

Instead 5GC introduces another delivery mechanism for SMS, just for 5GC without VoNR, the SMSf which can still send messages over the 5G NAS messaging.

This added another option for SMS delivery dependent on the access network used, and the Nsmsf_SMService interface does not support roaming.

Of course if you are using Voice over NR (VoNR) then like VoLTE, SMS is carried in a SIP message to the IMS, so this negates the need for the Nsmsf_SMService.

2G/3G Shutdown – Diameter to replace SGs-AP (SGd)

With the 2G/3G shutdown in the US operators who had up until this point been relying on SMS-over-NAS using the SGs-AP interface back to their MSCs were forced to make a decision on how to route SMS traffic, after the MSCs were shut down.

This landed with SMS-over-Diameter, where the 4G core (MME) communicates over Diameter with the SMSc.

The advantage of this approach is the Diameter protocol stack is the backbone of 4G roaming, and it’s not a stretch to get existing Diameter Routing Agents to start flicking SMS over Diameter messages between operators.

This has adoption by all the US operators, but we’re not seeing it so widely deployed in the rest of the world.

State of Play

MAP2G/3G OnlyRelies on SS7 signaling and is very old
Supports roaming
SGs-AP (SMS-over-NAS)4G only relies on 2G/3GNeeds an MSC to be present in the network (generally because you have a 2G/3G network and have not deployed VoLTE)
Supports limited roaming
SMS over IP (IMS)4G / 5GNot supported on 2G/3G networks
Relies on a IMS enabled handset and network
Supports roaming in all S8 Home Routed scenarios
Device support limited, especially for IoT devices
Diameter SGd4G only / 5G NSAOnly works on 4G or 5G NSA
Better device support than 4G/5G
Supports roaming in some scenarios
Nsmsf_SMService5G standalone onlyOnly works on 5GC
Doesn’t support roaming
The convoluted world of SMS delivery options

A Way Forward:

While the SMS payload hasn’t changed in the past 31 years, how it is transported has opened up a lot of potential options for operators to use, with no clear winner, while SMS revenues and traffic volumes have continued to fall.

For better or worse, the industry needs to accept that SMS over NAS is an option to use when there is no IMS, and that in order to decommission 2G/3G networks, IMS needs to be embraced, and so SMS over IP (IMS) supported in all future networks, seems like the simple logical answer to move forward.

And with that clear path forward, we add in another wildcard…

Direct to device Satellite messes everything up…

Remember way back in this post when I said SMS over IP using IMS is a really really inefficient way of getting data? Well that hasn’t been a problem as we progressed up the generations of cellular tech as with each “G” we had more and more bandwidth than the last.

To throw a spanner in the works, let’s introduce NB-IoT and Non-Terrestrial Networks which rely on Non-IP-Data-Delivery.

These offer the ability to cover the globe with a low bandwidth / high latency service, that would ensure a subscriber is always just a message away, we’re seeing real world examples of these networks getting deployed for messaging applications already.

But, when you’ve only got a finite resource of bandwidth, and massive latencies to contend with, the all-IP architecture of IMS (VoLTE / VoNR) and it’s woeful inefficiency starts to really sting.

Of course there are potential workarounds here, Robust Header Correction (ROHC) can shrink this down, but it’s still going to rely on the 3 way handshake of TCP, TCP keepalive timers and IMS registrations, which in turn can starve the radio resources of the satellite link.

For NTN (Satelite) networks the case is being heavily made to rely on Non-IP-Data-Delivery, so the logical answer for these applications is to move the traffic back to SMS over NAS.

End Note

Even with SMS over 30 years old, we can still expect it to be a part of networks for years to come, even as WhatsApp / iMessage, etc, offer enhanced services. As to how it’s transported and the myriad of options here, I’m expecting that we’ll keep seeing a multi-transport mix long into the future.

For simple, cut-and-dried 4G/5G only network, IMS and SMS over IP makes the most sense, but for anything outside of that, you’ve got a toolbox of options for use to make a solution that best meets your needs.

Verify Android Signing Certificate for ARA-M Carrier Privileges in App

Part of the headache when adding the ARA-M Certificate to a SIM is getting the correct certificate loaded,

The below command calculates it the SHA-1 Digest we need to load as the App ID on the SIM card’s ARA-M or ARA-F applet:

apksigner verify --verbose --print-certs "yourapp.apk"

You can then flash this onto the SIM with PySIM:

pySIM-shell (MF/ADF.ARA-M)> aram_store_ref_ar_do --aid FFFFFFFFFFFF --device-app-id 40b01d74cf51bfb3c90b69b6ae7cd966d6a215d4 --android-permissions 0000000000000001 --apdu-always

What’s the maximum speed for LTE and 5G?

Even before 5G was released, the arms race to claim the “fastest” speeds on LTE, NSA and SA networks has continued, with pretty much every operator claiming a “first” or “fastest”.

I myself have the fastest 5G network available* but I thought I’d look at how big the values are we can put in for speed, these are the Maximum Bitrate Values (like AMBR) we can set on an APN/DNN, or on a Charging Rule.

*Measurement is of the fastest 5G network in an eastward facing office, operated by a person named Nick, in a town in Australia. Other networks operated by people other than those named Nick in eastward facing office outside of Australia were not compared.

The answer for Release 8 LTE is 4294967294 bytes per second, aka 4295 Mbps 4.295 Gbps.

Not bad, but why this number?

The Max-Requested-Bandwidth-DL AVP tells the PGW the max throughput allowed in bits per second. It’s a Unsigned32 so max value is 4294967294, hence the value.

But come release 15 some bright spark thought we may in the not to distant future break this barrier, so how do we go above this?

The answer was to bolt on another AVP – the “Extended-Max-Requested-BW-DL” AVP ( 554 ) was introduced, you might think that means the max speed now becomes 2x 4.295 Gbps but that’s not quite right – The units was shifted.

This AVP isn’t measuring bits per second it’s measuring kilobits per second.

So the standard Max-Requested-Bandwidth-DL AVP gives us 4.3 Gbps, while the Extended-Max-Requested-Bandwidth gives us a 4,295 Gbps.

We add the Extended-Max-Requested-Bandwidth AVP (4295 Gbps) onto the Max-Requested Bandwidth AVP (4.3 Gbps) giving us a total of 4,4299.3 Gbps.

So the short answer:

Pre release 15: 4.3 Gbps

Post release 15: 4,4299.3 Gbps

Using Wireshark to search a SIM

Today I was updating a SIM profile for work, the client is rebranding and we need to remove all references to their old brand from the SIM profile.

I’ve written about using Wireshark to view APDU traces on SIM cards before, but today I had a simple need, to find all files with the client’s brand name in them.

I started off just updating the SPN, OPN, etc, etc, but I had a suspicion there were still references.

I confirmed this pretty easily with Wireshark, first I started a trace in Wireshark of the APDUs: I enabled capturing on a USB Interface:

modprobe usbmon

Then we need to find where our card reader is connected, running ‘lsusb‘ lists all the USB devices, and you can see here’s mine on Bus 1, Device 49.

Then fired up Wireshark, selected USB Bus 01 to capture all the USB traffic on the bus.

Then I ran the “export” command in PySIM to read the contents of all the files on the SIM, and jumped back over to Wireshark. (PySIM decodes most files but not all – Whereas this method just looks for the bytes containing the string)

From the search menu in Wireshark I searched the packet bytes for the string containing the old brand name, and found two more EFs I’d missed.

For anyone playing along at home, using this method I found references to the old brand name in SMSP (which contains the network name) and ADN (Which had the customer support number as a contact with the old brand name).

Another great use for Wireshark!

Cisco ITP STP – Network Appearance

Short one,
The other day I needed to add a Network Appearance on an SS7/SS7 M3UA linkset.

Network Appearances on M3UA links are kinda like a port number, in that they allow you to distinguish traffic to the same point code, but handled by different logical entities.

When I added the NA parameter on the Linkset nothing happened.

If you’re facing the same you’ll need to set:

cs7 multi-instance

In the global config (this is the part I missed).

Then select the M3UA linkset you want to change and add the network-appearance parameter:

network-appearance 10

And bingo, you’ll start seeing it in your M3UA traffic:

Huawei BBU 3900 Architecture

Huawei BTS3900 eNB Configuration

Last year I purchased a cheap second hand Huawei macro base station – there’s lots of these on the market at the moment due to the fact they’re being replaced in many countries.

I’m using it in my lab environment, and as such the config I’ve got is very “bare bones” and basic. Keep in mind if you’re looking to deploy a Macro eNodeB in production, you may need more than just a blog post to get everything tuned and functioning properly…

In this post we’ll cover setting up a Huawei BTS3900 eNodeB from scratch, using the MML interface, without relying on the U2020 management tool.

Obviously the details I setup (IP Addressing, PLMN and RF parameters) are going to be different to what you’re configuring, so keep that in mind, where I’ve got my MME Addresses, site IDs, TACs, IP Addresses, RFUs, etc, you’ll need to substitute your own values.

A word on Cabinets

Typically these eNodeBs are shipped in cabinets, that contain the power supplies, alarm / environmental monitoring, power distribution, etc.

Early on in the setup process we’ll be setting the cabinet types we’ve got, and then later on we’ll tell the system what we have installed in which slots.

This is fine if you have a cabinet and know the type, but in my case at least I don’t have a cabinet manufactured by Huawei, just a rack with some kit mounted in it.

This is OK, but it leads to a few gotchas I need to add a cabinet (even though it doesn’t physically exist) and when I setup my RRUs I need to define what cabinet, slot and subrack it’s in, even though it isn’t in any. Keep this in mind as we go along and define the position of the equipment, that if you’re not using a real-world cabinet, the values mean nothing, but need to be kept consistent.

The Basics

Before we get started, familiarise yourself with the Huawei MML we’ll use for configuring the unit, and log into the Web UI and bring up an MML shell.

To begin we’ll need to setup the basics, by disabling DHCP and setting an local IP Address for the unit.


Obviously your IP address details will be different.
Next we’ll add an eNodeB function, the LMPT / UMPT can have multiple functions and multiple eNodeBs hosted on the same hardware, but in our case we’re just going to configure one:

 ADD ENODEBFUNCTION: eNodeBFunctionName="LTE", ApplicationRef=1, eNodeBId=9527;
 SET NE: NENAME="HUAWEI", LOCATION="NewSite", DID="NewSite12345", SITENAME="NewSite1", USERLABEL="NewInitSite";

Again, your eNodeB ID, location, site name, etc, are all going to be different, as will your location.

Next we’ll set the system to maintenance mode (MNTMODE), so we can make changes on the fly (this takes the eNB off the air, but we’re already off the air), you’ll need to adjust the start and end times to reflect the current time for the start time, and end time to be after you’re done setting all this up.

 SET MNTMODE: MNTMode=INSTALL, ST=2013&09&20&15&00&00, ET=2013&09&25&15&00&00, MMSetRemark="NewSite Install";

Next we’ll set the operator details, this is the PLMN of the eNodeB, and create a new tracking area.

 ADD CNOPERATOR: CnOperatorId=0, CnOperatorName="NickTest", CnOperatorType=CNOPERATOR_PRIMARY, Mcc="001", Mnc="01";
ADD CNOPERATORTA: TrackingAreaId=0, CnOperatorId=0, Tac=1;

Next we’ll be setting and populating the cabinets I mentioned earlier. I’ll be telling the unit it’s inside a APM30 (Cabinet 0), and in Cabinet Number 0, Subrack 0, is a BBU3900.

 //To modify the cabinet type, run the following command:
//Add a BBU3900 subrack, run the following command:
//To configure boards and RF datas, run the following commands:

And inside the BBU3900 there’s some cards of course, and each card has as slot, as per the drawing below.

In my environment I’ve got a LMPT in slot 7, and a LBBP in Slot 3. There’s a fan and a UPEU too, so:
We’ll add a board in Slot No. 7, of type LMPT,
We’ll add a board in Slot No. 3, of type LBBP working on FDD,
We’ll add a fan board in Slot No. 16, and a UPEU in Slot No. 18.


Huawei publish design guides for which cards should be in which slots, the general rule is that your LMPT / UMPT card goes in Slot 7, with your BBP cards (UBBP or LBBP) in slots 3, then 2, then 1, then 0. Fans and UPEUs can only go in the slots designed to fit them, so that makes it a bit easier.

Next we’ll need to setup our RRUs, for this we’ll need to setup an RRU chain, which is the Huawei term for the CPRI links and add an RRU into it:



With our RRU chains defined, we’ll need to setup our transport network to get the traffic back to the S-GW / MME:

ADD S1: S1Id=0, CnOperatorId=0, EpGroupCfgFlag=CP_UP_CFG, CpEpGroupId=0, UpEpGroupId=0;

We’ll need clocking and time as well, we’ll use NTP and GPS:



Next we’ll need to define a sector, sector equipment & cell, then link it to a sector equipment group:





Alright, now we can activate it:

//Modify the reference signal power.
MOD PDSCHCFG: LocalCellId=1, ReferenceSignalPwr=-81;

//Add an operator for the cell.
ADD CELLOP: LocalCellId=0, TrackingAreaId=0;

//Activate the cell.
ACT CELL: LocalCellId=1;

And lastly we can define some neighboring cells:

//Configure neighboring cells. 
ADD EUTRANINTERNFREQ: LocalCellId=1, DlEarfcn=3100, UlEarfcnCfgInd=NOT_CFG, CellReselPriorityCfgInd=NOT_CFG, SpeedDependSPCfgInd=NOT_CFG, MeasBandWidth=MBW100, PmaxCfgInd=NOT_CFG, QqualMinCfgInd=NOT_CFG;
ADD EUTRANEXTERNALCELL: Mcc="460", Mnc="02", eNodeBId=236, CellId=0, DlEarfcn=3100, UlEarfcnCfgInd=NOT_CFG, PhyCellId=236, Tac=33;
ADD EUTRANINTERFREQNCELL: LocalCellId=1, Mcc="460", Mnc="02", eNodeBId=236, CellId=0;

BSF Addresses

The Binding Support Function is used in 4G and 5G networks to allow applications to authenticate against the network, it’s what we use to authenticate for XCAP and for an Entitlement Server.

Rather irritatingly, there are two BSF addresses in use:

If the ISIM is used for bootstrapping the FQDN to use is:

But if the USIM is used for bootstrapping the FQDN is

You can override this by setting the 6FDA EF_GBANL (GBA NAF List) on the USIM or equivalent on the ISIM, however not all devices honour this from my testing.

Will 5GC be used in Wireline Access? No. Here’s why.

One of the hyped benefits of a 5G Core Networks is that 5GC can be used for wired networks (think DSL or GPON) – In marketing terms this is called “Wireless Wireline Convergence” (5G WWC) meaning DSL operators, cable operators and fibre network operators can all get in on this sweet 5GC action and use this sexy 5G Core Network tech.

This is something that’s in the standards, and that the big kit vendors are pushing heavily in their marketing materials. But will it take off? And should operators of wireline networks (fixed networks) be looking to embrace 5GC?

Comparing 5GC with current wireline network technologies isn’t comparing apples to apples, it’s apples to oranges, and they’re different fruits.

At its heart, the 3GPP Core Networks (including 5G Core) address one particular use cases of the cellular industry: Subscriber mobility – Allowing a customer to move around the network, being served by different kit (gNodeBs) while keeping the same IP Address.

The most important function of 5GC is subscriber mobility.

This is achieved through the use of encapsulating all the subscriber’s IP data into a GTP (A protocol that’s been around since 2G first added data).

Do I need a 5GC for my Fixed Network?

Wireline networks are fixed. Subscribers don’t constantly move around the network. A GPON customer doesn’t need to move their OLT every 30 minutes to a new location.

Encapsulating a fixed subscriber’s traffic in GTP adds significant processing overhead, for almost no gain – The needs of a wireline network operator, are vastly different to the needs of a cellular core.

Today, you can take a /24 IPv4 block, route it to a DSLAM, OLT or CMTS, and give an IP to 254 customers – No cellular core needed, just a router and your access device and you’re done, and this has been possible for decades.
Because there’s no mobility the GTP encapsulation that is the bedrock for cellular, is not needed.

Rather than routing directly to Access Network kit, most fixed operators deploy BRAS systems used for fixed access. Like the cellular packet core, BRAS has been around for a very long time, with a massive install base and a sea of engineering experience in house, it meets the needs of the wireline industry who define its functions and roles along with kit vendors of wireline kit; the fixed industry working groups defined the BRAS in the same way the 3GPP and cellular industry working groups defined 5G Core.

I don’t forsee that we’ll see large scale replacement of BRAS by 5GC, for the same reason a wireless operator won’t replace their mobile core with a BRAS and PPPoE – They’re designed to meet different needs.

All the other features that have been added to the 3GPP Core Network functionality, like limiting speed, guaranteed throughput bearers, 5QI / QCI values, etc, are addons – nice-to-haves. All of these capabilities could be implemented in wireline networks today – if the business case and customer demand was there.

But what about slicing?

With dropping ARPUs across the board, additional services relating to QoS (“Network Slicing”) are being held up as the saving grace of revenues for cellular operators and 5G as a whole, however this has yet to be realized and early indications suggest this is not going to be anywhere near as lucrative as previously hoped.

What about cost savings?

In terms of cost-per-bit of throughput, the existing install base wireline operators have of heavy-metal kit capable of terabit switching and routing has been around for some time in fixed world, and is what most 5G Cores will connect to as their upstream anyway, so there won’t be any significant savings on equipment, power consumption or footprint to be gained.

Fixed networks transport the majority of the world’s data today – Wireline access still accounts for the majority of traffic volumes, so wireline kit handles a higher magnitude of throughput than it’s Packet Core / 5GC cousins already.

Cutting down the number of parts in the network is good though right?

If you’re operating both a Packet Core for Cellular, and a fixed network today, then you might think if you moved from the traditional BRAS architecture fore the wired network to 5GC, you could drop all those pesky routers and switches clogging up your CO, Exchanges and Data Centers.

The problem is that you still need all of those after the 5GC to be able to get the traffic anywhere users want to go. So the 5GC will still need all of that kit, all your border routers and peering routers will remain unchanged, as well as domestic transmission, MPLS and transport.

The parts required for operating fixed networks is actually pretty darn small in comparison to that of 5GC.


While cellular vendors would love to sell their 5GC platform into fixed operators, the premise that they are willing to replace existing BRAS architectures with 5GC, is as unlikely in my view as 5GC being replaced by BRAS.

Authenticating Fixed Line Subscribers into IMS

We recently added support in PyHSS for fixed line SIP subscribers to attach to the IMS.

Traditional telecom operators are finding their fixed line network to be a bit of a money pit, something they’re required to keep operating to meet regulatory obligations, but the switches are sitting idle 99% of the time. As such we’re seeing more and more operators move fixed line subs onto their IMS.

This new feature means we can use PyHSS to serve as the brains for a fixed network, as well as for mobile, but there’s one catch – How we authenticate subscribers changes.

Most banks of line cards in a legacy telecom switches, or IP Phones, don’t have SIM slots to allow us to authenticate, so instead we’re forced to fallback to what they do support.

Unfortunately for the most part, what is supported by these IP phones or telecom switches is SIP MD5 Digest Authentication.

The Nonce is generated by the HSS and put into the Multimedia-Authentication-Answer, along with the subscriber’s password and sent in the clear to the S-CSCF.

Subscriber with Password made up of all 1's MAA response from HSS for Digest-MD5 Auth

The HSS then generates the the Multimedia-Auth Answer, it generates a nonce (in the 3GPP-SIP-Authenticate / 609 AVP) and sends the Subscriber’s password in the 3GPP-SIP-Authorization (610) AVP in response back to the S-CSCF.

I would have thought a better option would be for the HSS to generate the Nonce and Digest, and then the S-CSCF to just send the Nonce to the Sub and compare the returned Digest from the Sub against the expected Digest from the HSS, but it would limit flexibility (realm adaptation, etc) I guess.

The UE/UA (I guess it’s a UA in this context as it’s not a mobile) then generates its own Digest from the Nonce and sends it back to the S-CSCF via the P-CSCF.

The S-CSCF compares the received Digest response against the one it generated, and if the two match, the sub is authenticated and allowed to attach onto the network.

IMS iFC – SPT Session Cases

Mostly just reference material for me:

Possible values:


In the past I had my iFCs setup to look for the P-Access-Network-Info header to know if the call was coming from the IMS, but it wasn’t foolproof – Fixed line IMS subs didn’t have this header.


But now I’m using the Session Cases to know if the call is coming from a registered IMS user:

        <!-- SIP INVITE Traffic from Registered Sub-->

How much computing power is in a SIM (And is it enough to get humans to the Moon?)

The first thing people learn about SIMs or the Smart Cards that the SIM / USIM app runs on, is that “There’s a little computer in the card”. So how little is this computer, and what’s the computing power in my draw full of SIMs?

So for starters the SIM manufacturers love their NDAs, so I can’t post the chip specifications for the actual cards in my draw, but here’s some comparable specs from a seller selling Java based smart cards online:

Specs for Smart Card

4K of RAM is 4069 bytes.
For comparison the Apollo Guidance Computer had 2048 words of RAM, but each “word” was 16 bits (two bytes), so actually this would translate to 4069 bytes so equal with one of these smart cards in terms of RAM – So the smart card above is on par with the AGC that took humans to the moon in terms of RAM, althhough the SIMs would be a wee bit larger if they were also using magnetic core memory like the AGC!

The Nintendo Entertainment System was powered by a MOS Technology 6502, it had access to 2K of RAM, two the Smart Card has twice as much RAM as the NES, so it could get you to the moon and play Super Mario Bros.

What about comparing Non-Volatile Memory (Storage)? Well, the smart card has 145KB of ROM / NVM, while Apollo flew with 36,864 words of RAM, each word is two bits to 73,728 Bytes, so roughly half of what the Smart Card has – Winner – Smart Card, again, without relying on core rope memory like AGC.

SIM cards are clocked kinda funkily so comparing processor speeds is tricky. Smart Cards are clocked off the device they connect to, which feeds them a clock signal via the CLK pin. The minimum clock speed is 1Mhz while the max is 5Mhz.

Now I’m somewhat of a hoarder when it comes to SIM Cards; in the course of my work I have to deal with a lot of SIMs…

Generally when we’re getting SIMs manufactured, during the Batch Approval Process (BAP) the SIM vendor will send ~25 cards for validation and testing. It’s not uncommon to go through several revisions. I probably do 10 of these a year for customers, so that’s 250 cards right there.

Then when the BAP is done I’ll get another 100 or so production cards for the lab, device testing, etc, this probably happens 3 times a year.

So that’s 550 SIMs a year, I do clean out every so often, but let’s call it 1000 cards in the lab in total.

In terms of ROM that gives me a combined 141.25 MB, I could store two Nintendo 64 games, or one Mini CD of data, stored across a thousand SIM cards – And you thought installing software from a few floppies was a pain in the backside, imagine accessing data from 1000 Smart Cards!

What about tying the smart cards together to use as a giant RAM BUS? Well our 1000 cards give us a combined 3.91 MB of RAM, well that’d almost be enough to run Windows 95, and enough to comfortably run Windows 3.1.

Practical do do any of this? Not at all, now if you’ll excuse me I think it’s time I throw out some SIMs…

SSH into Cisco STPs

If it ain’t broke don’t fix is an addage that the telecom industry has well and truly applied to the SS7 space.

If you’ve got an SS7 network (especially one built on TDM links) the general philosophy is don’t touch it and hope to retire before it dies.

The Cisco STP (Internet Transfer Point) is a good example of this, and for that reason I still work on them.

But OpenSSH and standards have moved on, and SSHing into them these days requires some extra (insecure) parameters to access, so here they are:

ssh -oKexAlgorithms=+diffie-hellman-group1-sha1 -oHostKeyAlgorithms=+ssh-rsa -caes128-cbc [email protected]

Will get you into an Version 12.3(4r)T4 Cisco ITP. Be sure to run sho ver and marvel at that uptime!