So dedicated appliances are dead and all our network functions are VMs or Containers, but there’s a performance hit when going virtual as the L2 processing has to be handled by the Hypervisor before being passed onto the relevant VM / Container.
If we have a 10Gb NIC in our server, we want to achieve a 10Gbps “Line Speed” on the Network Element / VNF we’re running on.
When we talked about appliances if you purchased an P-GW with 10Gbps NIC, it was a given you could get 10Gbps through it (without DPI, etc), but when we talk about virtualized network functions / network elements there’s a very real chance you won’t achieve the “line speed” of your interfaces without some help.
When you’ve got a Network Element like a S-GW, P-GW or UPF, you want to forward packets as quickly as possible – bottlenecks here would impact the user’s achievable speeds on the network.
To speed things up there are two technologies, that if supported by your software stack and hardware, allows you to significantly increase throughput on network interfaces, DPDK & SR-IOV.
DPDK – Data Plane Development Kit
Usually *Nix OSs handle packet processing on the Kernel level. As I type this the packets being sent to this WordPress server by Firefox are being handled by the Linux 5.8.0-36-generic kernel running on my machine.
The problem is the kernel has other things to do (interrupts), meaning increased delay in processing (due to waiting for processing capability) and decreased capacity.
DPDK shunts this processing to the “user space” meaning your application (the actual magic of the VNF / Network Element) controls it.
To go back to me writing this – If Firefox and my laptop supported DPDK, then the packets wouldn’t traverse the Linux kernel at all, and Firefox would be talking directly to my NIC. (Obviously this isn’t the case…)
So DPDK increases network performance by shifting the processing of packets to the application, bypassing the kernel altogether. You are still limited by the CPU and Memory available, but with enough of each you should reach very near to line speed.
SR-IOV – Single Root Input Output Virtualization
Going back to the me writing this analogy I’m running Linux on my laptop, but let’s imagine I’m running a VM running Firefox under Linux to write this.
If that’s the case then we have an even more convolted packet processing chain!
I type the post into Firefox which sends the packets to the Linux kernel, which waits to be scheduled resources by the hypervisor, which then process the packets in the hypervisor kernel before finally making it onto the NIC.
We could add DPDK which skips some of these steps, but we’d still have the bottleneck of the hypervisor.
With PCIe passthrough we could pass the NIC directly to the VM running the Firefox browser window I’m typing this, but then we have a problem, no other VMs can access these resources.
SR-IOV provides an interface to passthrough PCIe to VMs by slicing the PCIe interface up and then passing it through.
My VM would be able to access the PCIe side of the NIC, but so would other VMs.
So that’s the short of it, SR-IOR and DPDK enable better packet forwarding speeds on VNFs.
While we’ve already covered the inputs required by the authentication elements of the core network (The HSS in LTE/4G, the AuC in UMTS/3G and the AUSF in 5G) to generate an output, it’s worth noting that the Confidentiality Algorithms used in the process determines the output.
This means the Authentication Vector (Also known as an F1 and F1*) generated for a subscriber using Milenage Confidentiality Algorithms will generate a different output to that of Confidentiality Algorithms XOR or Comp128.
To put it another way – given the same input of K key, OPc Key (or OP key), SQN & RAND (Random) a run with Milenage (F1 and F1* algorithm) would yield totally different result (AUTN & XRES) to the same inputs run with a simple XOR.
Technically, as operators control the network element that generates the challenges, and the USIM that responds to them, it is an option for an operator to implement their own Confidentiality Algorithms (Beyond just Milenage or XOR) so long as it produced the same number of outputs. But rolling your own cryptographic anything is almost always a terrible idea.
So what are the differences between the Confidentiality Algorithms and which one to use? Spoiler alert, the answer is Milenage.
Milenage
Milenage is based on AES (Originally called Rijndael) and is (compared to a lot of other crypto implimentations) fairly easy to understand,
AES is very well studied and understood and unlike Comp128 variants, is open for anyone to study/analyse/break, although AES is not without shortcomings, it’s problems are at this stage, fairly well understood and mitigated.
There are a few clean open source examples of Milenage implementations, such as this C example from FreeBSD.
XOR
It took me a while to find the specifications for the XOR algorithm – it turns out XOR is available as an alternate to Milenage available on some SIM cards for testing only, and the mechanism for XOR Confidentiality Algorithm is only employed in testing scenarios, not designed for production.
Instead of using AES under the hood like Milenage, it’s just plan old XOR of the keys.
Comp128 was originally a closed source algorithm, with the maths behind it not publicly available to scrutinise. It is used in GSM A3 and A5 functions, akin to the F1 and F1* in later releases.
Due to its secretive nature it wasn’t able to be studied or analysed prior to deployment, with the idea that if you never said how your crypto worked no one would be able to break it. Spoiler alert; public weaknesses became exposed as far back as 1998, which led to Toll Fraud, SIM cloning and eventually the development of two additional variants, with the original Comp128 renamed Comp128-1, and Comp128-2 (stronger algorithm than the original addressing a few of its flaws) and Comp128-3 (Same as Comp128-2 but with a 64 bit long key generated).
Our BTS is going to need an accurate clock source in order to run, so without access to crazy accurate Timing over Packet systems or TDM links to use as reference sources, I’ve opted to use the GPS/GLONASS receiver built into the LMPT card.
Add new GPS with ID 0 on LMPT in slot 7 of cabinet 1:
Check GPS has sync (May take some time) using the Display GPS command;
DSP GPS: GN=0;
Assuming you’ve got an antenna connected and can see the sky, after ~10 minutes running the DSP GPS:; command again should show you an output like this:
+++ 4-PAL0089624 2020-11-28 01:06:55
O&M #806355684
%%DSP GPS: GN=0;%%
RETCODE = 0 Operation succeeded.
Display GPS State
-----------------
GPS Clock No. = 0
GPS Card State = Normal
GPS Card Type = M12M
GPS Work Mode = GPS
Hold Status = UNHOLDED
GPS Satellites Traced = 4
GLONASS Satellites Traced = 0
BDS Satellites Traced = 0
Antenna Longitude(1e-6 degree) = 144599999
Antenna Latitude(1e-6 degree) = -37000000
Antenna Altitude(m) = 613
Antenna Angle(degree) = 5
Link Active State = Activated
Feeder Delay(ns) = 15
GPS Version = NULL
(Number of results = 1)
--- END
Showing the GPS has got sync and a location fix,
Next we set BTS to use GPS as time source,
SET TIMESRC: TIMESRC=GPS;
Finally we’ll verify the Time is in sync on the BTS using the list time command:
DSP TIME:;
+++ 4-PAL0089624 2020-11-28 01:09:22
O&M #806355690
%%DSP TIME:;%%
RETCODE = 0 Operation succeeded.
Time Information
----------------
Time = 2020-11-28 01:09:22 GMT+00:00
--- END
Optionally you may wish to add a timezone, using the SET TZ:; command, but I’ve opted to keep it in UTC for simplicity.
In our last post we covered the file system structure of a smart card and the basic concepts of communication with cards. In this post we’ll look at what happens on the application layer, and how to interact with a card.
For these examples I’ll be using SIM cards, because admit it, you’ve already got a pile sitting in a draw, and this is a telco blog after all. You won’t need the ADM keys for the cards, we’ll modify files we’ve got write access to by default.
Commands & Instructions
So to do anything useful with the card we need issue commands / instructions to the card, to tell it to do things. Instructions like select this file, read it’s contents, update the contents to something else, verify my PIN, authenticate to the network, etc.
The term Command and Instruction are used somewhat interchangeably in the spec, I realise that I’ve done the same here to make it just as confusing, but instruction means the name of the specific command to be called, and command typically means the APDU as a whole.
The “Generic Commands” section of 3GPP TS 31.101 specifies the common commands, so let’s take a look at one.
The creatively named SELECT command/instruction is used to select the file we want to work with. In the SELECT command we’ll include some parameters, like where to find the file, so some parameters are passed with the SELECT Instruction to limit the file selection to a specific area, etc, the length of the file identifier to come, and the identifier of the file.
The card responds with a Status Word, returned by the card, to indicate if it was successful. For example if we selected a file that existed and we had permission to select, we’d get back a status word indicating the card had successfully selected the file. Status Words are 2 byte responses that indicate if the instruction was successful, but also the card has data it wants to send to the terminal as a result of the instruction, how much data the terminal should expect.
So if we just run a SELECT command, telling the card to select a file, we’ll get back a successful response from the card with a data length. Next need to get that data from the card. As the card can’t initiate communication, the GET RESPONSE instruction is sent to the card to get the data from the card, along with the length of the data to be returned.
The GET RESPONSE instruction/command is answered by the card with an APDU containing the data the card has to send, and the last 2 bytes contain the Status Word indicating if it was successful or not.
APDUs
So having covered the physical and link layers, we now move onto the Application Layer – where the magic happens.
Smart card communications is strictly master-slave based when it comes to the application layer.
The terminal sends a command to the card, which in turn sends back a response. Command -> Response, Command -> Response, over and over.
These commands are contained inside APplication Data Units (APDUs).
So let’s break down a simple APDU as it appears on the wire, so to speak.
The first byte of our command APDU is taken up with a header called the class byte, abbreviated to CLA. This specifies class coding, secure messaging options and channel options.
In the next byte we specify the Instruction for the command, that’s the task / operation we want the card to perform, in the spec this is abbreviated to INS.
The next two bytes, called P1 & P2 (Parameter 1 & Parameter 2) specify the parameters of how the instruction is to be to be used.
Next comes Lc – Length of Command, which specifies the length of the command data to follow,
Datacomes next, this is instruction data of the length specified in Lc.
Finally an optional Le – Length of expected response can be added to specify how long the response from the card should be.
Crafting APDUs
So let’s encode our own APDU to send to a card, for this example we’ll create the APDU to tell the card to select the Master File (MF) – akin to moving to the root directory on a *nix OS.
For this we’ll want a copy of ETSI TS 102 221 – the catchily named “Smart cards; UICC-Terminal interface; Physical and logical characteristics” which will guide in the specifics of how to format the command, because all the commands are encoded in hexadecimal format.
So here’s the coding for a SELECT command from section 11.1.1.1 “SELECT“,
For the CLA byte in our example we’ll indicate in our header that we’re using ISO 7816-4 encoding, with nothing fancy, which is denoted by the byte A0.
For the next but we’ve got INS (Instruction) which needs to be set to the hex value for SELECT, which is represented by the hex value A4, so our second byte will have that as it’s value.
The next byte is P1, which specifies “Selection Control”, the table in the specification outlines all the possible options, but we’ll use 00 as our value, meaning we’ll “Select DF, EF or MF by file id”.
The next byte P2 specifies more selection options, we’ll use “First or only occurrence” which is represented by 00.
The Lc byte defines the length of the data (file id) we’re going to give in the subsequent bytes, we’ve got a two byte File ID so we’ll specify 2 (represented by 02).
Finally we have the Data field, where we specify the file ID we want to select, for the example we’ll select the Master File (MF) which has the file ID ‘3F00‘, so that’s the hex value we’ll use.
So let’s break this down;
Code
Meaning
Value
CLA
Class bytes – Coding options
A0 (ISO 7816-4 coding)
INS
Instruction (Command) to be called
A4 (SELECT)
P1
Parameter 1 – Selection Control (Limit search options)
00 (Select by File ID)
P2
Parameter 1 – More selection options
00 (First occurrence)
Lc
Length of Data
02 (2 bytes of data to come)
Data
File ID of the file to Select
3F00 (File ID of master file)
So that’s our APDU encoded, it’s final value will be A0 A4 00 00 02 3F00
So there we have it, a valid APDU to select the Master File.
In the next post we’ll put all this theory into practice and start interacting with a real life SIM cards using PySIM, and take a look at the APDUs with Wireshark.
The pins on the terminal / card reader are arranged so that when inserting a card, the ground contact is the first contact made with the reader, this clever design consideration to protect the card and the reader from ESD damage.
Operating Voltages
When Smart Cards were selected for use in GSM for authenticating subscribers, all smart cards operated at 5v. However as mobile phones got smaller, the operating voltage range became more limited, the amount of space inside the handset became a premium and power efficiency became imperative. The 5v supply for the SIM became a difficult voltage to provide (needing to be buck-boosted) so lower 3v operation of the cards became a requirement, these cards are referred to as “Class B” cards. This has since been pushed even further to 1.8v for “Class C” cards.
If you found a SIM from 1990 it’s not going to operate in a 1.8v phone, but it’s not going to damage the phone or the card.
The same luckily goes in reverse, a card designed for 1.8v put into a phone from 1990 will work just fine at 5v.
This is thanks to the class flag in the ATR response, which we’ll cover later on.
Clocks
As we’re sharing one I/O pin for TX and RX, clocking is important for synchronising the card and the reader. But when smart cards were initially designed the clock pin on the card also served as the clock for the micro controller it contained, as stable oscillators weren’t available in such a tiny form factor. Modern cards implement their own clock, but the clock pin is still required for synchronising the communication.
I/O Pin
The I/O pin is used for TX & RX between the terminal/phone/card reader and the Smart Card / SIM card. Having only one pin means the communications is half duplex – with the Terminal then the card taking it in turns to transmit.
Reset Pin
Resets the card’s communications with the terminal.
Filesystem
So a single smart card can run multiple applications, the “SIM” is just an application, as is USIM, ISIM and any other applications on the card.
These applications are arranged on a quasi-filesystem, with 3 types of files which can be created, read updated or deleted. (If authorised by the card.)
Because the file system is very basic, and somewhat handled like a block of contiguous storage, you often can’t expand a file – when it is created the required number of bytes are allocated to it, and no more can be added, and if you add file A, B and C, and delete file B, the space of file B won’t be available to be used until file C is deleted.
This is why if you cast your mind back to when contacts were stored on your phone’s SIM card, you could only have a finite number of contacts – because that space on the card had been allocated for contacts, and additional space can no longer be allocated for extra contacts.
So let’s take a look at our 3 file types:
MF (Master File)
The MF is like the root directory in Linux, under it contains all the files on the card.
DF (Dedciated File)
An dedicated file (DF) is essentially a folder – they’re sometimes (incorrectly) referred to as Directory Files (which would be a better name).
They contain one or more Elementary Files (see below), and can contain other DFs as well.
Dedicated Files make organising the file system cleaner and easier. DFs group all the relevant EFs together. 3GPP defines a dedicated file for Phonebook entries (DFphonebook), MBMS functions (DFtv) and 5G functions (DF5gs).
We also have ADFs – Application Dedicated Files, for specific applications, for example ADFusim contains all the EFs and DFs for USIM functionality, while ADFgsm contains all the GSM SIM functionality.
The actual difference with an ADF is that it’s not sitting below the MF, but for the level of depth we’re going into it doesn’t matter.
DFs have a name – an Application Identifier (AID) used to address them, meaning we can select them by name.
EF (Elementary File)
Elementary files are what would actually be considered a file in Linux systems.
Like in a Linux file systems EFs can have permissions, some EFs can be read by anyone, others have access control restrictions in place to limit who & what can access the contents of an EF.
There are multiple types of Elementary Files; Linear, Cyclic, Purse, Transparent and SIM files, each with their own treatment by the OS and terminal.
Most of the EFs we’ll deal with will be Transparent, meaning they ##
ATR – Answer to Reset
So before we can go about working with all our files we’ll need a mechanism so the card, and the terminal, can exchange capabilities.
There’s an old saying that the best thing about standards is that there’s so many to choose, from and yes, we’ve got multiple variants/implementations of the smart card standard, and so the card and the terminal need to agree on a standard to use before we can do anything.
This is handled in a process called Answer to Reset (ATR).
When the card is powered up, it sends it’s first suggestion for a standard to communicate over, if the terminal doesn’t want to support that, it just sends a pulse down the reset line, the card resets and comes back with a new offer.
If the card offers a standard to communicate over that the terminal does like, and does support, the terminal will send the first command to the card via the I/O line, this tells the card the protocol preferences of the terminal, and the card responds with it’s protocol preferences. After that communications can start.
Basic Principles of Smart Cards Communications
So with a single I/O line to the card, it kind of goes without saying the communications with the card is half-duplex – The card and the terminal can’t both communicate at the same time.
Instead a master-slave relationship is setup, where the smart card is sent a command and sends back a response. Command messages have a clear ending so the card knows when it can send it’s response and away we go.
Like most protocols, smart card communications is layered.
At layer 1, we have the physical layer, defining the operating voltages, encoding, etc. This is standardised in ISO/IEC 7816-3.
Above that comes our layer 2 – our Link Layer. This is also specified in ISO/IEC 7816-3, and typically operates in one of two modes – T0 or T1, with the difference between the two being one is byte-oriented the other block-oriented. For telco applications T0 is typically used.
Our top layer (layer 7) is the application layer. We’ll cover the details of this in the next post, but it carries application data units to and from the card in the form of commands from the terminal, and responses from the card.
Coming up Next…
In the next post we’ll look into application layer communications with cards, the commands and the responses.
I know a little bit about SIM cards / USIM cards / ISIM Cards. Enough to know I don’t know very much about them at all.
So throughout this series of posts of unknown length, I’ll try and learn more and share what I’m learning, citing references as much as possible.
So where to begin? I guess at the start,
A supposedly brief history of Smart Cards
There are two main industries that have driven the development and evolution of smart cards – telecom & banking / finance, both initially focused on the idea that carrying cash around is unseemly.
This planet has – or rather had – a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most of these were largely concerned with the movement of small green pieces of paper, which was odd because on the whole it wasn’t the small green pieces of paper that were unhappy.
Douglas Adams – The Hitchhiker’s Guide to the Galaxy
When the idea of Credit / Debit Cards were first introduced the tech was not electronic, embossed letters on the card were fed through that clicky-clacky-transfer machine (Google tells me this was actually called the “credit card imprinter”) and the card details imprinted onto carbon copy paper.
Customers wanted something faster, so banks delivered magnetic strip cards, where the card data could be read even more quickly, but as the security conscious of you will be aware, storing data on magnetic strips on a card to be read by any reader, allows them to be read by any reader, and therefore duplicated really easily, something the banks quickly realised.
To combat this, card readers typically would have a way to communicate back to a central bank computer. The central computer verified the PIN entered by the customer was correct, confirmed that the customer had enough money in their balance for the transaction and it wasn’t too suspicious. This was, as you would imagine in the late 1980’s early 1990’s, rather difficult to achieve. A reliable (and cheap) connection back to a central bank computer wasn’t always a given, nor instant, and so this was still very much open to misuse.
“Carders” emmerged, buying/selling/capturing credit card details, and after programming a blank card with someone else’s fraudulently obtained card details, could write them on a blank card before going on a spending spree for a brief period of time. Racking up a giant debt that wasn’t reconciled against the central computer until later, when the card was thrown away and replaced with another.
I know what you’re thinking – I come to this blog for ramblings about Telecommunications, not the history of the banking sector. So let’s get onto telco;
The telecom sector faced similar issues, at the time mobile phones were in their infancy, and so Payphones were how people made calls when out and about.
A phone call from a payphone in Australia has sat at about $0.40 for a long time, not a huge amount, but enough you’d always want to be carrying some change if you wanted to make calls. Again, an inconvenience for customers as coins are clunky, and an inconvenience for operators as collecting the coins from tens of thousands of payphones is expensive.
Telcos around the world trailed solutions, including cards with magnetic strips containing the balance of the card, but again people quickly realised that you could record the contents of the magnetic stripe data of the card when it had a full balance, use all the balance on the card, and then write back the data you stored earlier with a full balance.
So two industries each facing the same issue: it’s hard to securely process payments offline in a way that can’t be abused.
Enter the smart card – a tiny computer in a card that the terminal (Payphone or Credit Card Reader) interacts with, but the card is very much in charge.
When used in a payphone, the caller inserts the smart card and dials the number, and dialog goes something like this (We’ll assume Meter Pulses are 40c worth):
Payphone: “Hey SmartCard, how much credit do you have on you?”
Smart Card: “I have $1.60 balance”
*Payphone ensures card has enough credit for the first meter pulse, and begins listening for Meter Pulses*
*When a meter pulse received:*
Payphone: “Please deduct $0.40 from your Balance”
Smart Card: “Ok, you have $1.20 remaining”
This process repeats for each meter pulse (Payphone metering is a discussion for another day) until all the credit has been used / Balance is less than 1 meter pulse charge.
While anyone could ask the smart card “Hey SmartCard, how much credit do you have on you?” it would only return the balance, and if you told the smart card “I used $1 credit, please deduct it” like the payphone did, you’d just take a dollar off the credit stored on the card.
Saying “Hey SmartCard set the balance to $1,000,000” would result in a raised eyebrow from the SmartCard who rejects the request.
After all – It’s a smart card. It has the capability to do that.
So in the telecom sector single use smart cards were rolled out, programmed in the factory with a set dollar value of credit, sold at that dollar value and thrown away when depleted.
The banking industry saw even more potential, balance could be stored on the card, and the PIN could be verified by the card, the user needs to know the correct PIN, as does the smart card, but the terminal doesn’t need to know this, nor does it need to talk back to a central bank computer all the time, just every so often so the user gets the bill.
It worked much the same way, although before allowing a deduction to be made from the balance of the card, a user would have to enter their PIN which was verified by the card before allowing the transaction.
Eventually these worlds collided (sort of), both wanting much the same thing from smart cards. So the physical characteristics, interface specs (rough ones) and basic communications protocol was agreed on, and what eventually became ISO/IEC 7816 was settled upon.
Any card could be read by any terminal, and it was up to the systems implementer (banks and telecos initially) what data the card did and what the terminal did.
Active RFID entered the scene and there wasn’t even a need for a physical connection to the card, but the interaction was the same. We won’t really touch on the RFID side, but all of this goes for most active RFID cards too.
Enter Software
Now the card was a defined standard all that was important really was the software on the card. Banks installed bank card software on their cards, while telcos installed payphone card software on theirs.
But soon other uses emerged, ID cards could provide a verifiable and (reasonably) secure way to verify the card’s legitimacy, public transport systems could store commuter’s fares on the card, and vending machines, time card clocks & medical records could all jump on the bandwagon.
These were all just software built on the smart card platform.
Hello SIM Cards
A early version Smart card was used in the German C-Netz cellular network, which worked in “mobile” phones and also payphones, to authenticate subscribers.
After that the first SIM cards came into the public sphere in 1991 with GSM as a way to allow a subscriber’s subscription to be portable between devices, and this was standardised by ETSI to become the SIM cards still used in networks using GSM, and evolved into the USIM used in 3G/4G/5G networks.
Names of Smart Cards & Readers
To make life a bit easier I thought I’d collate all the names for smart cards and readers that are kind of different but used interchangeably depending on the context.
Smart Card
|
Terminal
UICC (Universal Integrated Circuit Card) – Standards name for Smart Card
Card Reader (Generic)
SIM (Mobile Telco application running on UICC)
Phone (Telco)
USIM (Mobile Telco application running on UICC)
SIM Slot (Telco)
Credit / Debit / EFTPOS Card (Banking)
UE (Telco)
Java Card (Type of Smart Card OS)
EFTPOS Terminal (Banking)
Phone Card (Telco / Payphone)
And then…
From here we’ll look at various topics:
Introduction to Smart Cards (This post)
Meet & Greet (The basics of Smart Cards & their File System)
APDUs and Hello Card (How terminals interact with a smart cards)
(Interacting with real life cards using Smart Card readers and SIM cards)
Mixing It Up (Changing values on Cards)
Other topics we may cover are Javacard and Global Platform, creating your own smart card applications, a deeper look at the different Telco apps like SIM/USIM/ISIM, OTA Updates for cards / Remote File Management (RFM), and developing for SimToolkit.
Recently I’ve been wrapping my head around Cell Broadcast in LTE, and thought I’d share my notes on 3GPP TS 38.413.
The interface between the MME and the Cell Broadcast Center (CBC) is the SBc interface, which as two types of “Elementary Procedures”:
Class 1 Procedures are of the request – response nature (Request followed by a Success or Failure response)
Class 2 Procedures do not get a response, and are informational one-way. (Acked by SCTP but not an additional SBc message).
SCTP is used as the transport layer, with the CBC establishing a point to point connection to the MME over SCTP (Unicast only) on port 29168 with SCTP Payload Protocol Identifier 24.
The SCTP associations between the MME and the CBC should normally remain up – meaning the SCTP association / transport connection is up all the time, and not just brought up when needed.
Elementary Procedures
Write-Replace Warning (Class 1 Procedure)
The purpose of Write-Replace Warning procedure is to start, overwrite the broadcasting of warning message, as defined in 3GPP TS 23.041 [14].
Write-Replace Warning procedure, initiated by WRITE-REPLACE WARNING REQUEST sent by the CBC to the MMEs contains the emergency message to be broadcast and the parameters such as TAC to broadcast to, severity level, etc.
A WRITE-REPLACE WARNING RESPONSE is sent back by the MME to the MME, if successful, along with information as to where it was sent out. CBC messages are unacknowledged by UEs, meaning it’s not possible to confirm if a UE has actually received the message.
The request includes the message identifier and serial number, list of TAIs, repetition period, number of broadcasts requested, warning type, and of course, the warning message contents.
Stop Warning Procedure (Class 1 Procedure)
Stop Warning Procedure, initiated by STOP WARNING REQUEST and answered with a STOP WARNING RESPONSE, requests the MME inform the eNodeBs to stop broadcasting the CBC in their SIBs.
Includes TAIs of cells this should apply to and the message identifier,
Error Indication (Class 2)
The ERROR INDICATION is used to indicate an error (duh). Contains a Cause and Criticality IEs and can be sent by the MME or CBC.
Write Replace Warning (Class 2)
The WRITE REPLACE WARNING INDICATION is used to indicate warning scenarios for some instead of a WRITE-REPLACE WARNING RESPONSE,
PWS Restart (Class 2)
The PWS RESTART INDICATION is used to list the eNodeBs / cells, that have become available or have restarted, since the CBC message and have no warning message data – for example eNodeBs that have just come back online during the period when all the other cells are sending Cell Broadcast messages.
Returns a the Restarted-Cell-List IE, containing the Global eNB ID IE and List of TAI, of the restarted / reconnected cells.
PWS Failure Indication (Class 2)
The PWS FAILURE INDICATION is essentially the reverse of PWS RESTART INDICATION, indicating which eNodeBs are no longer available. These cells may continue to send Cell Broadcast messages as the MME has essentially not been able to tell it to stop.
Contains a list of Failed cells (eNodeBs) with the Global-eNodeB-ID of each.
This post is one in a series documenting my adventures attempting to configure a used BTS 3900 to function as a eNB in my lab.
There are 5 network ports on the LMPT card:
2x SFP cages – SFP 0 and SFP 1
1x 10/100 Ethernet port – ETH – Used to access the Local Maintenance terminal
2x Fe/Ge ports – Fe/Ge0 and Fe/Ge1
Configuring the Ethernet Ports
What took me a little while to realise is that SFP0 and Fe/Ge0 are paired, they’re really only one interface. This means you can only use one at a time – you can’t use SFP0 and Fe/Ge0 simultaneously- Same with SFP1 and Fe/Ge1.
Before we get started we’ll list the current interfaces:
DSP ETHPORT:;
Assuming the interfaces aren’t there, we’ll need to add the interfaces, in my case the LMPT card is in Chassis 1, Slot number 7.
And then we’ve got to add an IP to one of the interfaces, in the below example I’ve added 10.0.1.210/24 to port 0 (which can be either SFP0 or Fe/Ge0).
At this point I plugged into the Fe/Ge0 port into my switch, and from my laptop on the same 10.0.1.0/24 subnet, I was able to ping the eNodeB.
And now we can check the status of the port:
DSP ETHPORT: SRN=1, SN=7, SBT=BASE_BOARD, PN=0;
+++ 4-PAL0089624 2020-11-28 00:19:13
O&M #806355532
%%DSP ETHPORT: SRN=1, SN=7, SBT=BASE_BOARD;%%
RETCODE = 0 Operation succeeded.
DSP ETHPORT Result
------------------
Cabinet No. = 0
Subrack No. = 1
Slot No. = 7
Subboard Type = Base Board
Port No. = 0
Port Attribute = Copper
Port Status = Up
Physical Layer Status = Up
Maximum Transmission Unit(byte) = 1500
ARP Proxy = Enable
Flow Control = Open
MAC Address = DCD2-07FC-A9E8
Loopback Status = No Loop
In Loopback Mode or Not = No
Ethernet OAM 3AH Flag = Disable
Number of RX Packets(packet) = 1682
Number of RX Bytes(byte) = 163929
Number of RX CRC Error Packets(packet) = 2
RX Traffic(byte/s) = 259
Number of TX Packets(packet) = 53
Number of TX Bytes(byte) = 13952
TX Traffic(byte/s) = 0
Local Configuration Negotiation Mode = Automatic Negotiation
Local Actual Negotiation Mode = Automatic Negotiation
Local Speed = 100M
Local Duplex = Full Duplex
Peer Actual Negotiation Mode = Automatic Negotiation
Peer Speed = 100M
Peer Duplex = Full Duplex
Number of IPs = 1
IP Address List = 10.0.1.210 255.255.255.0
(Number of results = 1)
--- END
And with that, you’ve got the network side of the config done on the eNodeB.
At this stage you’re able to unplug from the ETH port you’ve got the WebLMT connection to, and just connect to it like any other network device.
There’s a few more steps before we bring cells on the air, we’ve got to set timing sources, configure a connection to an MME and S-GW, configure the Carrier settings and add the radios and sectors, but this will get you to the stage where you no longer need to plug directly into the eNB to configure it.
How do humans talk to base stations? For Huawei at least the answer to this is through MML – Man-Machine-Language,
It’s command-response based, which is a throwback to my Nortel days (DMS100 anyone?),
So we’re not configuring everything through a series of parameters broken up into sections with config, it’s more statements to the BTS along the lines of “I want you to show me this”, or “Please add that” or “Remove this bit”,
The instruction starts of with an operation word, telling the BTS what we want to do, there’s a lot of them, but some common examples are; DSP (Display), LST (List), SET (Set), MOD (Modify) and ADD (Add).
After the operation word we’ve got the command word, to tell the BTS on what part we want to execute this command,
A nice simple example would be to list the software version that’s running on the BTS. For this we’d run
LST SOFTWARE:;
And press F9 to execute, which will return a list of software on the BTS and show it in the terminal.
Note at the end the :; – the : (colon) denotes the end of a command word, and after it comes the paratmeters for the command, and then the command ends with the ; (semi-colon). We’ll need to put this after every command.
Let’s look at one more example, and then we’ll roll up our sleves and get started.
Note: I’m trying out GIFs to share screen recordings instead of screenshots. Please let me know if you’re having issues with them.
So once you’ve logged into WebLMT, selecting MML is where we’ll do all our config, let’s log in and list the running applications.
So far we’ve only got some fairly basic data, listing and displaying values, so let’s try something a bit more complex, taking a backup of the config, in encrypted mode, with the backup label “blogexamplebackup”,
If you’ve made it this far there’s a good chance you’re thinking there’s no way you can remember all these commands and parameters – But I’ve got some good news, we don’t really need to remember anything, there’s a form for this!
And if we want to upload the backup file to an FTP server, we can do this as well, in the navigation tree we find Upload Backup Configuration, fill in the fields and click the Exec button to execute the command, or press F9.
These forms, combined with a healthy dose of the search tab, allow us to view and configure our BTS.
I’ve still got a lot to learn about getting end-to-end configuration in place, but this seems like a good place to start,
Note: This is one part of a series of posts where I cover my adventures attempting to bring on air a commercial Macro cell site for my lab, with scrounged components.
So the Huawei BTS3900 unit I’ve ended up with, is only one part of the overall picture for building a working LTE RAN. Power systems, feeders, connectors, CPRI, antennas, baseband processing and transmission are all hurdles I’ve still got to overcome. So today, let’s talk about antennas!
For the output/TX side (downlink) of the RF Unit, I’ve ordered some 25w 50 ohm dummy loads (I’ll still need to work out how to turn down the RF power to less than 25w on the RF units). Even with the dummy load, a tiny bit of RF power is leaked, which should be enough to provide the downlink signal for my UEs – Time will tell if this works…
This option is fine for the power being pushed out of the RF unit, into the dummy load, where we have a lot of power available (too much power), but what about our very weak uplink signals from UEs?
For this I’d need some decent antennas to pickup the signals from the UEs, so I ended up with some Kathrein (Now owned by Ericsson) indoor multi-band omni antennas I found on an online auction site for $10 each. (I bought 4 so I can play with MIMO.)
Unfortunately, the RFUs I have are Band 28 (roughly 700Mhz-750Mhz uplink and 758Mhz to 798Mhz downlink), and reading the datasheet it seems this doesn’t cover the bands I need;
But beggars can’t be choosers, so I ran a calibration on the NanoVNA and swept the antenna from 700Mhz-750Mhz (Band 28 uplink frequencies) to see how it will perform when I get the rest of the solution together;
At the upper end of Band 28 Uplink (748Mhz) I’m getting a fairly respectable VSWR of 1.6 (Return Loss of -12.4dB), so I should be able to get away with these for what I’m doing,
I’v seen these white domes inside shopping centers and office buildings, so I was keen to crack open the case and see what magic inside, what I found was kind of underwhelming, just an aluminum plate with an aluminum reflector cone…
My ideas of putting the parts into the lathe and trying to lower it’s operating frequency by taking material off, were dashed when I realised taking material off would raise the operating frequency, not lower it…
Meta: The Australian government made up it’s mind some time ago that Huawei would be blacklisted from providing equipment for 5G networks. Several other countries have adopted the same policy in regards, and as such, deployed Huawei tech is being replaced, and some of it filters down to online auction sites…
So I kind of purchased an item described as “Huawei BBU3900” with a handful of unknown cards and 2 LRFU units, for just over $100.
My current lab setup is a single commercial picocell and a draw of SDR hardware that works with mixed results, so the idea of having a commercial macro cell to play with seemed like a great idea, I put lowball offer in and the seller accepted.
Now would be a good time to point out I don’t know much about RAN and it’s been a long time since I’ve been working on power systems, so this is shaping up to be a fun project.
Photo from the listing
Photo from the listing
I did a Huawei RAN course years ago and remembered the rough ingredients required for LTE:
You needed either RRUs (Remote Radio Units) or RFUs (Radio Frequency Units) to handle the RF side of things. RRUs are designed for outdoor use (such as mounting on the tower) and RFUs are designed for indoor use, like mounting in a cabinet. I’ve ended up with two LRFUe units, which I can join together for 2x MIMO, operate on Band 28 and can put out a whopping 80W of transmit power, yes I’m going to need some big attenuators…
You need a Baseband Processor card to tell the Radio units what do do. The card connects the CPRIs (Typically optic fiber links) between the radio units and the baseband. The chassis I purchased came with a stack of WBBP (For WCDMA) cards and a single LBBP card for LTE. The LBBP card has 6 SFP ports for the CPRI interfaces, which is more than enough for my little lab. (You can also daisy-chain CPRIs so I’m not even limited to 6 Radio Units.)
You need a backplane and a place for the cards to live – this is the BBU3900 chassis. It’s got basic switching to allow communication between cards, a chassis to distribute power and cooling. (Unlike the Ericson units there is actually a backplane for communications in the Huawei chassis – the Ericsson RBS series has is just power and cooling in the chassis)
Optional – Dedicated transmission card, I’ve ended up with a Universal Transmission Processor (UTRP9) with 2x Gig Ethernet and 2x Fast Ethernet ports for transmission. This will only work for GSM and UMTS though, not LTE, so not much use for me.
You need something to handle main processing (LTE / Universal Main Processing and Transmission Unit (LMPT / UMPT)). Unfortunately the unit I’ve ended up with only came with a WMPT (For WCDMA), so back online to find either an LMPT (LTE) or UMPT (Universal (2G/3G/4G))…
You need a Universal Power and Environment Module (UPEU) to power up the chassis and handle external IO for things like temperature alarms, door sensors and fire detectors. This chassis has two for redundancy / extra IO & extra power capacity.
So in order to get this running I still need quite a few components:
Attenuators – I’ll be able to turn the power down, sure, but not to the levels required to be legal.
Antennas – These are FDD units, so I’ll need two antennas for each RFU, on Band 28
Feeder Cables – To connect the antennas
SMF cables and SFPs – I’ve got a pile in my toolbox, but I’ll need to work out what’s supported by these units
A big -48vDC rectifier (I got the BBU3900 unit powered up with an existing supply I had, but I’m going to need something bigger for the power hungry RFUs)
DC Distribution Unit – Something to split the DC between the RFUs and the BBU, and protect against overload / short
USB-Network adapter – For OAM access to the unit – Found these cheaply online and got one on the way
The LTE Main Processing & Transmission (LMPT) card – Ordered a second hand one from another seller
I powered up the BTA3900 and sniffed the traffic, and can see it trying to reach an RNC.
Unfortunately with no open source RNC options I won’t be posting much on the topic of UMTS or getting the UMTS/WCDMA side of things on the air anytime soon…
So that’s the start of the adventure.
I don’t know if I’ll get this all working, but I’m learning a lot in the process, and that’s all that really matters…
As the standardisation for 5G-SA has been completed and the first roll outs are happening, I thought I’d cover the basic architecture of the 5G Core Network, for people with a background in EPC/SAE networks for 4G/LTE, covering the key differences, what’s the same and what’s new.
The AMF – Authentication & Mobility Function, serves much the same role as the MME in LTE/EPC/SAE networks.
Like the MME, the AMF only handles Control Plane traffic, and serves as the gatekeeper to the services on the network, connecting the RAN to the core, authenticating subscribers and starting data / PDN connections for the UEs.
While the MME connects to eNodeBs for RAN connectivity, the AMF connects to gNodeBs for RAN.
The Authentication Functions
In EPC the HSS had two functions; it was a database of all subscribers’ profile information and also the authentication centre for generating authentication vectors.
5GC splits this back into two network elements (Akin to the AuC and HLR in 2G/3G).
The UDM (Unified Data Management) provides the AMF with the subscriber profile information (allowed / barred services / networks, etc),
The AUSF (Authentication Server Function) provides the AMF with the authentication vectors for authenticating subscribers.
Like in UMTS/LTE USIMs are used to authenticate subscribers when connecting to the network, again using AKA (Authentication and Key Agreement) for mutual subscriber & network authentication.
Other authentication methods may be implemented, R16 defines 3 suporrted methods, 5G-AKA, EAP-AKA’, and EAP-TLS.
This opens the door for the 5GC to be used for non-mobile usage. There has been early talk of using the 5G architecture for fixed line connectivity as well as mobile, hence supporting a variety of authentication methods beyond classic AKA & USIMs. (For more info about Non-3GPP Access interworking look into the N3IWF)
The Mobility Functions
When a user connects to the network the AMF selects a SMF (Session Management Function) akin to a P-GW-C in EPC CUPS architecture and requests the SMF setup a connection for the UE.
This is similar to the S11 interface in EPC, however there is no S-GW used in 5GC, so would be more like if S11 were instead sent to the P-GW-C.
The SMF selects a UPF (Akin to the P-GW-C selecting a P-GW-U in EPC), which will handle this user’s traffic, as the UPF bridges external data networks (DNs) to the gNodeB serving the UE.
Moving between cells / gNodeBs is handled in much the same way as done previously, with the path the UPF sends traffic to (N3 interface) updated to point to the IP of the new gNodeB.
When a UE attempts to connect to the network their signalling traffic (Using the N1 reference point between the UE and the AMF), is sent to the AMF.
an authentication challenge is issued as in previous generations.
Upon successful authentication the AMF signals the SMF to setup a session for the UE. The SMF selects a UPF to handle the user plane forwarding to the gNodeB serving the UE.
Key Differences
Functions handled by the MME in EPC now handled by AMF in 5GC
Functions of HSS now in two Network Functions – The UDM (Unified Data Management) and AUSF (Authentication Server Function)
Setting up data connections “flatter” (more info on the User Plane differences can be found here)
Non 3GPP access (Potentially used for fixed-line / non mobile networks)
As the standardisation for 5G-SA has been completed and the first roll outs are happening, I thought I’d cover the basic architecture of the 5G Core Network, for people with a background in EPC/SAE networks for 4G/LTE, covering the key differences, what’s the same and what’s new.
The idea behind this, is that by removing the S-GW removes extra hops / latency in the network, and allows users to be connected to the best UPF for their needs, typically one located close to the user.
However, there are often scenarios where an intermediate function is required – for example wanting to anchor a session to keep an IP Address allocated to one UPF associated with a user, while they move around the network. In this scenario a UPF can act as an “Session Anchor” (Akin to a P-GW), and pass through “Intermediate UPFs” (Like S-GWs).
Unlike the EPCs architecture, there is no limit to how many I-UPFs can be chained together between the Session Anchoring UPF and the gNB, and this chaining of UPFs allows for some funky routing options.
The UPF is dumb by design. The primary purpose is just to encapsulate traffic destined from external networks to subscribers into GTP-U packets and forward them onto the gNodeB serving that subscriber, and the same in reverse. Do one thing and do it well.
SMF – Session Management Function
So with dumb UPFs we need something smarter to tell them what to do.
Control of the UPFs is handled by the SMF – Session Management Function, which signals using PFCP down to the UPFs to tell them what to do in terms of setting up connections.
This means the interface between the SMF and UPF (the N4 interface) is more or less the same as the interface between a P-GW-C and a P-GW-U seen in CUPS.
When a subscriber connects to the network and has been authenticated, the AMF (For more info on the AMF see the sister post to this topic covering Control Plane traffic) requests the SMF to setup a connection for the subscriber.
Interworking with EPC
For deployments with an EPC and 5GC interworking between the two is of course required.
The P-GW-C and P-GW-U communications using PFCP are essentially the same as the N4 interface (between the SMF and the UPF) so the P-GW-U is able to act as a UPF.
This means handovers between the two RATs / Cores is seamless as when moving from an LTE RAT and EPC to a 5G RAT and 5G Core, the same UPF/P-GW-U is used, and only the Control Plane signalling around it changes.
When moving from LTE to 5G RAT, the P-GW-C is replaced by the SMF, When moving from 5G RAT to LTE, the SMF is replaced by the P-GW-C. In both scenarios user plane traffic takes the same exit point to external Data Networks (SGi interface in EPC / N6 interface in 5GC).
Interfaces / Reference Points
N3 Interface
N3 interface connects the gNodeB user plane to the UPF, to transport GTP-U packets.
This is a User Plane interface, and only transports user plane traffic.
This is akin to the S1-UP interface in EPC.
N4 Interface
N4 interface connects the Session Management Function (SMF) control plane to the UPF, to setup, modify and delete UPF sessions.
It is a control plane interface, and does not transport User Plane traffic.
This interface relies on PFCP – Packet Forwarding Control Protocol.
This is akin to the SxB interface in EPC with CUPS.
N6 Interface
N6 interface connects the UPF to External Data Networks (DNs), taking packets destined for Subscribers and encapsulating them into GTP-U packets.
This is a User Plane interface, and only transports user plane traffic.
This is akin to the SGi interface in EPC.
N9 Interface
When Session Anchoring is used, and Intermediate-UPFs are used, the connection between these UPFs uses the N9 interface.
This is only used in certain scenarios – the preference is generally to avoid unnecessary hops, so Intermediate-UPF usage is to be avoided where possible.
As this is a User Plane interface, it only transports user plane traffic.
When used this would be akin to the S5 interface in EPC.
N11 Interface
SMFs need to be told when to setup / tear down connections, this information comes from the AMF via the N11 interface.
As this is a Control Plane interface, it only transports control plane traffic.
This is similar to the S11 interface between the MME and the S-GW in EPC, however it would be more like the S11 if the S11 terminated on the P-GW.
The Packet Gateway connects users of an LTE network to external networks like the Internet, by encapsulating IP packets inside GTP and forwarding them on to reach our subscriber wherever in the network they are.
So we use GTP to encapsulate user’s traffic, making it easy to carry it transparently from outside networks (Like the Internet) to the eNodeB and onto our UE / mobile phones, and more importantly redirect where the user’s traffic it’s going while keeping the same IP address.
But we need a network element to take plain old IP from external networks / Internet, and encapsulate the traffic into the GTP packets we’ll send to the subscriber.
This network element will have to do the same in reverse and decapsulate traffic coming from the subscriber to put it back onto the external networks / Internet.
That’s the role of the Packet Gateway (P-GW). The P-GW sits on the border between the outside network (An interface / reference point known as the SGi Interface) and the rest of the packet core (Serving-Gateway then onto eNodeB & UE) via the S5 Interface.
Let’s look at how the P-GW handles an incoming packet:
An IP packet comes in from the Internet destined for IP 1.2.3.4 and routed to the P-GW.
The P-GW looks up in it’s internal database what Tunnel Endpoint Identifier (TEID) IP Address 1.2.3.4 is associated with.
The P-GW encapsulates the IP packet (Layer 3 & up) into a GTP packet, adding the Tunnel Endpoint Identifier (TEID) to the GTP header.
The P-GW looks up in it’s internal database which Serving Gateway is handling traffic for that TEID.
The P-GW then sends this GTP packet containing our IP packet to the Serving Gateway.
In order to start relaying traffic to/from the S5 & SGi interfaces, the P-GW needs a set of procedures for setting up these sessions, (IP Address allocation and TEID allocation) known as bearers. This is managed using GTPv2 (aka GTPv2-Control Plane / GTPv2-C).
GTPv2-C has a set of procedures for creating these sessions, the key ones used by the P-GW are:
The Create Session Request is sent by the S-GW to the P-GW and contains the APN of the network to be setup, the IP Address to be assigned (if static) and information regarding the maximum throughput the user will be permitted to achieve.
If the P-GW was able to setup the connection as requested, a Create Session Response is sent back to the P-GW, with the IP Address for the UE to use, and the TEID (Tunnel Endpoint Identifier).
At this stage the tunnel is up and ready to go, traffic to the P-GW to the IP of the UE will be encapsulated in GTP-U packets with the TEID for this bearer, and forwarded on to the S-GW serving the user.
As our subscribers are mobile, moving between base stations / cells, the destination of the incoming GTP-U packets needs to be updated every time the subscriber moves from one cell to another.
As we covered in the last post, the Packet Gateway (P-GW) handles decapsulating and encapsulating this traffic into GTP from external networks, and vise-versa. The Packet Gateway sends the traffic onto a Serving Gateway, that forwards the GTP-U traffic onto the eNodeB serving the user.
So why not just route the traffic from the Packet Gateway directly to the eNodeB?
As our users are inherently mobile, the signalling load to keep updating the destination of the incoming GTP-U traffic to the correct eNB, would put an immense load on the P-GW. So an intermediary gateway – the Serving Gateway (S-GW), is introduced.
The S-GW handles the mobility between cells, and takes the load of the P-GW. The P-GW just hands the traffic to a S-GW and let’s the S-GW handle the mobility.
It’s worth keeping in mind that most LTE connections are not “always on”. Subscribers (UEs) go into “Idle Mode”, in which the data connection and the radio connection is essentially paused, and able to be bought back at a moments notice (this allows us to get better battery life on the UE and better frequency utilisation).
When a user enters Idle Mode, an incoming packet needs to be buffered until the Subscriber/UE can get paged and come back online. Again this function is handled by the S-GW; buffering packets until the UE comes available then forwarding them on.
A question that seems to come up often, is how to provide a static IPs to UEs on Open5GS EPC.
By default all UEs are allocated an internal IP that the server the P-GW is running on NATs out, but many users want to avoid NAT, for obvious reasons.
Open5GS has the ability to set a Static IP for each APN a subscriber has, but let’s get one thing out of the way first;
LTE is not Ethernet. No broadcast, no multicast. Each IP Address is best thought of as a single /32 network. This means you can’t have the UEs in your LTE network in the same 192.168.1.x subnet as your home network along with your laptop and printer, it’s not how it works.
So with that out of the way, let’s talk about how to do static IP address allocation in Open5GS EPC.
Assigning a Subscriber a Static IP Address
From the HSS edit the Subscriber and in the UE IPv4 or UE IPv6 address, set the static address you want to use.
You can set any UE IP Address here and it’ll get allocated to that UE.
But – there’s an issue.
The problem is not so much on the Open5GS P-GW implementation, but just how TCP/IP routing works in general.
Let’s say I assign the UE IPv4 address 1.2.3.4 to my UE. From the UE it sends a packet with the IPv4 Source address of 1.2.3.4 to anywhere on the internet, the eNB puts the packet in GTP and eventually the it gets to the P-GW which sends it out onto the internet from the source address 1.2.3.4.
The problem is that the response will never get back to me, as 1.2.3.4 is not allocated to me and will never make it back to my P-GW, so never relayed back to the UE.
For TCP traffic this means I can send the SYN with the source address of 1.2.3.4, but the SYN/ACK will be routed back to the real 1.2.3.4, and not to me, so the TCP socket will never get opened.
So while we can set a static IPs to be allocated to UEs in Open5GS, unless the traffic can be routed back to these IPs it’s not much use.
Routing
So let’s say we have assigned IP 1.2.3.4 to the UE, we’d need to put a static route on our routers to route traffic to the IP of the PGW. In my case the PGW is 10.0.1.121, so I’ll need to add a static route to get traffic destined 1.2.3.4/32 to 10.0.1.121.
In a more common case we’d assign internal IP subnets for the UE pool, and then add routes for the entire subnet to the IP of the PGW.
3GPP release 14 introduced the concept of CUPS – Control & User Plane Separation, and the Sx interface, this allows the control plane (GTP-C) functionality and the user plane (GTP-U) functionality to be separated, and run in a distributed fashion, allowing the node a user’s GTP-U traffic flows through to be in a different location to where the Control / Signalling traffic (GTP-C) flows.
In practice that means for an LTE EPC this means we split our P-GW and S-GW into a minimum of two network elements each.
A P-GW is split and becomes a P-GW-C that handles the P-GW Control Plane traffic (GTPv2-C) and a P-GW-U speaking GTP-U for our User Plane traffic. But the split doesn’t need to stop there, one P-GW-C could control multiple P-GW-Us, routing the user plane traffic. Sames goes for S-GW being split into S-GW-C and S-GW-U,
This would mean we could have a P-GW-U located closer to a eNB / User to reduce latency, by allowing GTP-U traffic to break out on a node closer to the user.
It also means we can scale better too, if we need to handle more data traffic, but not necessarily more control plane traffic, we can just add more P-GW-U nodes to handle this.
To manage this a new protocol was defined – PFCP – the Packet Forwarding Control Protocol. For LTE this is refereed to as the Sx reference point, it’ also reused in 5G-SA as the N4 reference point.
When a GTP-C “Create Session Request” comes into a P-GW for example from an S-GW, a PFCP “Session Establishment Request” is sent by the P-GW-C to the P-GW-U with much the same information that was in the GTP-C request, to setup the session.
So why split the Control and User Plane traffic if you’re going to just relay the GTP-C traffic in a different format?
That was my first question – the answer is that keeping the GTP-C interface ensures backward compatibility with older MMEs, PCRFs, charging systems, and allows a staged roll out and bolting on extra capacity.
GTP-C disappears entirely in 5G Standalone architecture and is replaced with the N4 interface, which uses PFCP for the Control Plan and GTP-U again.
Here’s a capture from Open5Gs core showing GTPv2C and PFCP in play.
The S1 interface can be pretty noisy, which makes it hard to find the info you’re looking for.
So how do we find all the packets relating to a single subscriber / IMSI amidst a sea of S1 packets?
The S1 interface only contains the IMSI in certain NAS messages, so the first step in tracing a subscriber is to find the initial attach request from that subscriber containing the IMSI.
Luckily we can filter in Wireshark to find the IMSI we’re after;
Quick note – Not all IntialUEMessages will contain the IMSI – If the subscriber has already established comms with the MME it’ll instead be using a temporary identifier – M-TMSI, unless you’ve got a way to see the M-TMSI -> IMSI mapping on the MME you’ll be out of luck.
Next up let’s take a look at the contents of one of these packets,
Inside the protocolIEs is the MME_UE_S1AP_ID – This unique identifier will identify all S1 signalling for a single user.
The MME_UE_S1AP_ID is a unique identifier, assigned by the MME to identify which signaling messages are for which subscriber.
(It’s worth noting the MME_UE_S1AP_ID is only unique to the MME – If you’ve got multiple MMEs the same MME_UE_S1AP_ID could be assigned by each).
So now we have the MME_UE_S1AP_ID, we can filter all S1 messaging containing that MME_UE_S1AP_ID, we’ll use this Wireshark filter to get it:
s1ap.MME_UE_S1AP_ID == 2
Boom, there’s a all the signalling for that subscriber.
Alternatively you can just right click on the value and apply it as a filter instead of typing everything in,
Hopefully that’ll help you filter to find what you’re looking for!
