Category Archives: EUTRAN

Authentication Vectors and Key Distribution in LTE

Querying Auth Credentials from USIM/SIM cards

LTE has great concepts like NAS that abstract the actual transport layers, so the NAS packet is generated by the UE and then read by the MME.

One thing that’s a real headache about private LTE is the authentication side of things. You’ll probably bash your head against a SIM programmer for some time.

As your probably know when connecting to a network, the UE shares it’s IMSI / TIMSI with the network, and the MME requests authentication information from the HSS using the Authentication Information Request over Diameter.

The HSS then returns a random value (RAND), expected result (XRES), authentication token (AUTN) and a KASME  for generating further keys,

The RAND and AUTN values are sent to the UE, the USIM in the UE calculates the RES (result) and sends it back to the MME. If the RES value received by the MME is equal to the expected RES (XRES) then the subscriber is mutually authenticated.

The osmocom guys have created a cool little utility called osmo-sim-auth, which allows you to simulate the UE’s baseband module’s calls to the USIM to authenticate.

Using this tool I was able to plug a USIM into my USIM reader, using the Diameter client built into PyHSS I was able to ask for Authentication vectors for a UE using the Authentication Information Request to the HSS and was sent back the Authentication Information Answer containing the RAND and AUTN values, as well as the XRES value.

Wireshark Diameter Authentication Information Response message body looking at the E-UTRAN vectors
Diameter – Authentication Information Response showing E-UTRAN Vectors

Then I used the osmo-sim-auth app to query the RES and RAND values against the USIM.

Osmocom's USIM Test tool - osmo-sim-auth

The RES I got back matched the XRES, meaning the HSS and the USIM are in sync (SQNs match) and they mutually authenticated.

Handy little tool!

Information stored on USIM / SIM Card for LTE / EUTRAN / EPC - K key, OP/OPc key and SQN Sequence Number

HSS & USIM Authentication in LTE/NR (4G & 5G)

I talked a bit in my last post about using osmo-sim-auth to authenticate against a USIM / SIM card when it’s not in a phone,

I thought I’d expand a little on how the Crypto side of things works in LTE & NR (also known as 4G & 5G).

Authentication primarily happens in two places, one at each end of the network, the Home Subscriber Server and in the USIM card. Let’s take a look at each of them.

On the USIM

On the USIM we’ve got two values that are entered in when the USIM is provisioned, the K key – Our secret key, and an OPc key (operator key).

These two keys are the basis of all the cryptography that goes on, so should never be divulged.

The only other place to have these two keys in the HSS, which associates each K key and OPc key combination with an IMSI.

The USIM also stores the SQN a sequence number, this is used to prevent replay attacks and is incremented after each authentication challenge, starting at 1 for the first authentication challenge and counting up from there.

On the HSS

On the HSS we have the K key (Secret key), OPc key (Operator key) and SQN (Sequence Number) for each IMSI on our network.

Each time a IMSI authenticates itself we increment the SQN, so the value of the SQN on the HSS and on the USIM should (almost) always match.

Authentication Options

Let’s imagine we’re designing the authentication between the USIM and the Network; let’s look at some options for how we can authenticate everyone and why we use the process we use.

Failed Option 1 – Passwords in the Clear

The HSS could ask the USIM to send it’s K and OPc values, compare them to what the HSS has in place and then either accept or reject the USIM depending on if they match.

The obvious problem with this that to send this information we broadcast our supposedly secret K and OPc keys over the air, so anyone listening would get our secret values, and they’re not so secret anymore.

This is why we don’t use this method.

Failed Option 2 – Basic Crypto

So we’ve seen that sending our keys publicly, is out of the question.

The HSS could ask the USIM to mix it’s K key and OPc key in such a way that only someone with both keys could unmix them.

This is done with some cryptographic black magic, all you need to know is it’s a one way function you enter in values and you get the same result every time with the same input, but you can’t work out the input from the result.

The HSS could then get the USIM to send back the result of mixing up both keys, mix the two keys it knows and compare them.

The HSS mixes the two keys itself, and get’s it’s own result called XRES (Expected Result). If the RES (result) of mixing up the keys by the USIM is matches the result when the HSS mixes the keys in the same way (XRES (Expected Result)), the user is authenticated.

The result of mixing the keys by the USIM is called RES (Result), while the result of the HSS mixing the keys is called XRES (Expected Result).

This is a better solution but has some limitations, because our special mixing of keys gets the same RES each time we put in our OPc and K keys each time a subscriber authenticates to the network the RES (result) of mixing the keys is going to be the same.

This is vulnerable to replay attacks. An attacker don’t need to know the two secret keys (K & OPc) that went into creating the RES (resulting output) , the attacker would just need to know the result of RES, which is sent over the air for anyone to hear.
If the attacker sends the same RES they could still authenticate.

This is why we don’t use this method.

Failed Option 3 – Mix keys & add Random

To prevent these replay attacks we add an element of randomness, so the HSS generates a random string of garbage called RAND, and sends it to the USIM.

The USIM then mixes RAND (the random string) the K key and OPc key and sends back the RES (Result).

Because we introduced a RAND value, every time the RAND is different the RES is different. This prevents against the replay attacks we were vulnerable to in our last example.

If the result the USIM calculated with the K key, OPc key and random data is the same as the USIM calculated with the same K key, OPc key and same random data, the user is authenticated.

While an attacker could reply with the same RES, the random data (RAND) will change each time the user authenticates, meaning that response will be invalid.

While an attacker could reply with the same RES, the random data (RAND) will change each time the user authenticates, meaning that response will be invalid.

The problem here is now the network has authenticated the USIM, the USIM hasn’t actually verified it’s talking to the real network.

This is why we don’t use this method.

GSM authentication worked like this, but in a GSM network you could setup your HLR (The GSM version of a HSS) to allow in every subscriber regardless of what the value of RES they sent back was, meaning it didn’t look at the keys at all, this meant attackers could setup fake base stations to capture users.

Option 4 – Mutual Authentication (Real World*)

So from the previous options we’ve learned:

  • Our network needs to authenticate our subscribers, in a way that can’t be spoofed / replayed so we know who to bill & where to route traffic.
  • Our subscribers need to authenticate the network so they know they can trust it to carry their traffic.

So our USIM needs to authenticate the network, in the same way the network authenticates the USIM.

To do this we introduce a new key for network authentication, called AUTN.

The AUTN key is generated by the HSS by mixing the secret keys and RAND values together, but in a different way to how we mix the keys to get RES. (Otherwise we’d get the same key).

This AUTN key is sent to the USIM along with the RAND value. The USIM runs the same mixing on it’s private keys and RAND the HSS did to generate the AUTN , except this is the USIM generated – An Expected AUTN key (XAUTN). The USIM compares XAUTN and AUTN to make sure they match. If they do, the USIM then knows the network knows their secret keys.

The USIM then does the same mixing it did in the previous option to generate the RES key and send it back.

The network has now authenticated the subscriber (HSS has authenticated the USIM via RES key) and the subscriber has authenticated the USIM (USIM authenticates HSS via AUTN key).

*This is a slightly simplified version of how EUTRAN / LTE authentication works between the HSS and the USIM – In reality there are a few extra values, such as SQN to take into consideration and the USIM talks to to the MME not the HSS directly.

I’ll do a follow up post covering the more nitty-gritty elements, AMF and SQN fields, OP vs OPc keys, SQN Resync, how this information is transfered in the Authentication Information Answer and how KASME keys are used / distributed.

Making use of Australian Elevation Data in Forsk Atoll

The Australian Government publishes elevation data online that’s freely available for anyone to use. There’s a catch – If you’re using Forsk Atoll, it won’t import without a fair bit of monkeying around with the data…

The data is published on a a system called ELVIS – Elevation – Foundation Spatial Data.

You draw around the area you want to download, enter your email address and you’re linked to a download of the dataset you’ve selected.

So now we download the data from the link, unzip it and we’re provided with a .tiff image with the elevation data in the pixel colour and geocoded with the positional information.

Problem is, this won’t import into Atoll – Unsupported depth.

Forsk Atoll - Unsupported Depth when importing

I found a tool called VTBuilder – A tool for messing with terrain data.

I fired it up, and imported the elevation tiff file we’d downloaded.

Selected “Elevation” waited a few seconds and presto!

We can export from here in the PNG 16 bit grayscale format Atoll takes, but there’s a catch, negative elevation values and blank data will show up as giant spikes which will totally mess with your propagation modeling.

So I found an option to remove elevation data from a set range, but it won’t deal with negative values…

So I found another option in the elevation menu to offset elevation vertically, I added 100 ft (It’s all in ft for some reason) to everything which meant my elevation data that was previously negative was now just under 100.

So if an area was -1ft before it was now 99ft.

Now I was able to use the remove range for anything from 0 100 ft (previously sea level)

Now my map only shows data above sea level

Now I offset the elevation vertically again and remove 100ft so we get back to real values

Now I was able to export the elevation data from the Elevation -> Export to menu

Atoll seems to like PNG 16 bit greyscale so that’s what we’ll feed it.

In Atoll we’ll select File -> Import and open the PNG we just generated.

Data type will be Altitude, Pixel size is 5m (as denoted in email / dataset metadata).

Next question is offset, which took me a while to work out…

The email has the Lat & Long but Atoll deals in WGS co-ordinates,

Luckily the GeoPlanner website allows you to enter the lat & long of the top corner and get the equivalent West and North values for the UTM dataum.

Enter these values as your coordinates and you’re sorted.

I can even able a Map layer and confirm it lines up:

Roll your own USIMs for Private LTE Networks

I wrote a while ago about USIM basics and talked about what each of the fields stored on a USIM manage, but I thought I’d talk a little about my adventures in getting custom USIMs.

I started working on a private LTE project a while ago; RAN hardware (eNodeBs) were on the way, down to a shortlist of a few EPC platforms, but I still needed USIMs before anyone was connecting to the network.

So why are custom USIMs a requirement? Can’t you just use any old USIM/SIMs?

In UMTS / LTE / NR networks there’s mutual network authentication, again I’ve written about this topic before, but unlike GSM where the network authenticates the UE, in later RAN standards, the UE also authenticates the network. (This mitigates any bad actor from setting up their own base stations and having UEs attach to it and have their traffic intercepted).

For roaming to work between carriers they’ve got to have their HSS / DRA connecting to the DRA or HSS of other carriers, to allow roaming subscribers to access the network, otherwise they too would fall foul of the mutual network authentication and the USIM wouldn’t connect to the network.

The first USIMs I purchased online through a popular online marketplace with a focus on connecting you to Chinese manufacturers.
They listed a package of USIMS, a USB reader/writer that supported all the standard USIM form factors and the software to program it, which I purchased.

The USIMs worked fairly well – They are programmable via a card reader and software that, although poorly translated/documented, worked fairly well.

USIM Programming Interface

K and OP/OPc values could be written to the card but not read, while the other values could be read and written from the software, the software also has the ability to sequentially program the USIMs to make bulk operations easier. The pricing worked out about $8 USD per USIM, which although expensive for the quantity and programmable element is pretty reasonable.

Every now and then the Crypto values for some reason or another wouldn’t get updated, which is exactly as irritating as it sounds.

Pretty quickly into the build I learned the USIMs didn’t include an ISIM service on the card, ISIM being the service that runs on the UCCID responsible for IMS / VoLTE authentication.

Again I went looking and reached out to a few manufacturers of USIMs.

The big vendors, Gemalto, Kona, etc, weren’t interested in providing USIMs in quantities less than 100,000 and their USIMs came from the factory pre-programmed, meaning the values could only be changed through remote SIM provisioning, a form of black magic.

In the end I reached out to an OEM manufacturer from China who provided programmable USIM / ISIMs for less than I was paying on the online marketplace and at any quantity I wanted with custom printing options, allocated ICCIDs, etc.

The non-programmable USIMs worked out less than $0.40 USD each in larger quantities, and programmable USIM/ISIMs for about $5 USD.

The software was almost identical except for the additional tab for ISIM operations.

USIM / ISIM programming
ISIM parameters

Smart Card Readers

In theory this software and these USIMs could be programmed by any smart card reader.

In practice, the fact that the ISO standard smart card is the same size as a credit card, means most smart card readers won’t fit the bill.

I tried a few smart card readers, from the one built into my Thinkpad, to a Bluedrive II from one of the USIM vendors, in the end the MCR3516 Smart Card Reader which reads 4FF USIMs (Standard ISO size smart card, full size SIM, Micro SIM and Nano SIM form factors, which saved on so much mucking about with form factor adapters etc.

4FF Smart Card Reader for programming SIM/USIM/ISIM

Future Projects

I’ve got some very calls “Multi Operator Neutral Host” (MoNEH) USIMs from the guys at Telet Research I’m looking forward to playing with,

eSIMs are on my to-do list too, and the supporting infrastructure, as well as Over the Air updating of USIMs.

LTE / EUTRAN – Idle Detach

In order to keep radio resources free, if a UE doesn’t send or receive data for a predefined threshold, it’ll detach from the network and call back to Idle mode.

If the UE has data to send to the network, the UE will re-attach to the network, whereas if the network has data to send to the UE, it’ll Page the UE in the tracking area it’s currently in, the UE is always listening for it’s identifier (s-TMSI) on the paging channel, and if it hears it’s identifier called, the UE will re-attach.

I’ve also attached a PCAP file of the packet flow between the eNB and the MME.

UEContextReleaseRequest [RadioNetwork-cause=user-inactivity]

The first packet is sent by the eNB to the serving MME to indicate the user wishes to detach from the network.

PCAP of UEContextReleaseRequest from eNB to MME

UEContextReleaseCommand [NAS-cause=normal-release]

The next packet is sent from the MME back to the eNB confirming UE is releasing from the network.



Finally after the UE has released it’s radio resources the eNB sends a UEContextReleaseComplete so the MME knows the UE is now in Idle state and will need to be paged.

UEContextReleaseComplete response


Recently we saw Open5Gs’s Update Location Answer response putting the Subscribed-Periodic-RAU-TAU-Timer AVP in the top level and not in the AVP Code 1400 (APN Configuration) Diameter payload from the HSS to the MME.

But what exactly does the Subscribed-Periodic-RAU-TAU-Timer AVP in the Update Location Answer response do?

Folks familiar with EUTRAN might recognise TAU as Tracking Area Update, while RAU is Routing Area Update in GERAN/UTRAN (UMTS).

Periodic tracking area updating is used to periodically notify the availability of the UE to the network. The procedure is controlled in the UE by the periodic tracking area update timer (timer T3412). The value of timer T3412 is sent by the network to the UE in the ATTACH ACCEPT message and can be sent in the TRACKING AREA UPDATE ACCEPT message. The UE shall apply this value in all tracking areas of the list of tracking areas assigned to the UE, until a new value is received.

Section 5.3.5 of 24301-9b0 (3GPP TS 24.301 V9.11.0)

So the Periodic Tracking Area Update timer simply defines how often the UE should send a Tracking Area Update when stationary (not moving between cells / tracking area lists).

The case for Header Compression in VoIP/VoLTE

On a PCM (G.711) RTP packet the payload is typically 160 bytes per packet.

But the total size of the frame on the wire is typically ~214 bytes, to carry a 160 byte payload that means 25% of the data being carried is headers.

This is fine for VoIP services operating over fixed lines, but when we’re talking about VoLTE / IMS and the traffic is being transferred over Radio Access Networks with limited bandwidth / resources, it’s important to minimize this as much as possible.

IMS uses the AMR codec, where the RTP payload for each packet is around 90 bytes, meaning up to two thirds of the packet on the wire (Or in this case the air / Uu interface) is headers.

Enter Robust Header Compression which compresses the headers.

Using ROHC the size of the headers are cut down to only 4-5 bytes, this is because the IPv4 headers, UDP headers and RTP headers are typically the same in each packet – with only the RTP Sequence number, RTP timestamp IPv4 & UDP checksum and changing between frames.

Building Android APN / Carrier Config

As anyone who’s setup a private LTE network can generally attest, APNs can be a real headache.

SIM/USIM cards, don’t store any APN details. In this past you may remember having to plug all these settings into your new phone when you upgraded so you could get online again.

Today when you insert a USIM belonging to a commercial operator, you generally don’t need to put APN settings in, this is because Android OS has its own index of APNs. When the USIM is inserted into the baseband module, the handset’s OS looks at the MCC & MNC in the IMSI and gets the APN settings automatically from Android’s database of APN details.

There is an option for the network to send the connectivity details to the UE in a special type of SMS, but we won’t go into that.

All this info is stored on the Android OS in apns-full-conf.xml which for non-rooted (stock) devices is not editable.

Instead the devices get updates through the OS updates which pull the latest copy of this file from Google’s Android Open Source Git repo, you can view the current master file here.

This file can override the user’s APN configuration, which can lead to some really confusing times as your EPC rejects the connection due to an unrecognized APN which is not what you have configured on the UE’s operating system, but it instead uses APN details from it’s database.

The only way around this is to change the apns-full-conf.xml file, either by modifying it per handset or submitting a push request to Android Open Source with your updated settings.

(I’ve only tried the former with rooted devices)

The XML file itself is fairly self explanatory, taking the MCC and MNC and the APN details for your network:

<apn carrier="CarrierXYZ"

Once you’ve added yours to the file, inserting the USIM, rebooting the handset or restarting the carrier app is all that’s required for it to be re-read and auto provision APN settings from the XML file.

Further reading

APN and CarrierConfig | Android Open Source Project

Carrier Configuration | Android Open Source Project

UICC Carrier Privileges | Android Open Source Project

/etc/apns-full-conf.xml – Master Branch

PyHSS – Python 3GPP LTE Home Subscriber Server

I recently started working on an issue that I’d seen was to do with the HSS response to the MME on an Update Location Answer.

I took some Wireshark traces of a connection from the MME to the HSS, and compared that to a trace from a different HSS. (Amarisoft EPC/HSS)

The Update Location Answer sent by the Amarisoft HSS to the MME over the S6a (Diameter) interface includes an AVP for “Multiple APN Configuration” which has the the dedicated bearer for IMS, while the HSS in the software I was working on didn’t.

After a bit of bashing trying to modify the S6a responses, I decided I’d just implement my own Home Subscriber Server.

The Diameter interface is pretty straight forward to understand, using a similar structure to RADIUS, and with the exception of the Crypto for the EUTRAN Authentication Vectors, it was all pretty straight forward.

If you’d like to know more you can download PyHSS from my GitLab page, and view my Diameter Primer post and my post on Diameter packet structure.

Diameter Packet Structure

We talked a little about what the Diameter protocol is, and how it’s used, now let’s look at the packets themselves.

Each Diameter packet has at a the following headers:


This 1 byte field is always (as of 2019) 0x01 (1)


3 bytes containing the total length of the Diameter packet and all it’s contained AVPs.

This allows the receiver to know when the packet has ended, by reading the length and it’s received bytes so far it can know when that packet ends.


Flags allow particular parameters to be set, defining some possible options for how the packet is to be handled by setting one of the 8 bits in the flags byte, for example Request Set, Proxyable, Error, Potentially Re-transmitted Message,

Command Code

Each Diameter packet has a 3 byte command code, that defines the method of the request,

The IETF have defined the basic command codes in the Diameter Base Protocol RFC, but many vendors have defined their own command codes, and users are free to create and define their own, and even register them for public use.

3GPP have defined a series of their own command codes.

Application ID

To allow vendors to define their own command codes, each command code is also accompanied by the Application ID, for example the command code 257 in the base Diameter protocol translates to Capabilities Exchange Request, used to specify the capabilities of each Diameter peer, but 257 is only a Capabilities Exchange Request if the Application ID is set to 0 (Diameter Base Protocol).

If we start developing our own applications, we would start with getting an Application ID, and then could define our own command codes. So 257 with Application ID 0 is Capabilities Exchange Request, but command code 257 with Application ID 1234 could be a totally different request.

Hop-By-Hop Identifier

The Hop By Hop identifier is a unique identifier that helps stateful Diameter proxies route messages to and fro. A Diameter proxy would record the source address and Hop-by-Hop Identifier of a received packet, replace the Hop by Hop Identifier with a new one it assigns and record that with the original Hop by Hop Identifier, original source and new Hop by Hop Identifier.

End-to-End Identifier

Unlike the Hop-by-Hop identifier the End to End Identifier does not change, and must not be modified, it’s used to detect duplicates of messages along with the Origin-Host AVP.


The real power of Diameter comes from AVPs, the base protocol defines how to structure a Diameter packet, but can’t convey any specific data or requests, we put these inside our Attribute Value Pairs.

Let’s take a look at a simple Diameter request, it’s got all the boilerplate headers we talked about, and contains an AVP with the username.

Here we can see we’ve got an AVP with AVP Code 1, containing a username

Let’s break this down a bit more.

AVP Codes are very similar to the Diameter Command Codes/ApplicationIDs we just talked about.

Combined with an AVP Vendor ID they define the information type of the AVP, some examples would be Username, Session-ID, Destination Realm, Authentication-Info, Result Code, etc.

AVP Flags are again like the Diameter Flags, and are made up a series of bits, denoting if a parameter is set or not, at this stage only the first two bits are used, the first is Vendor Specific which defines if the AVP Code is specific to an AVP Vendor ID, and the second is Mandatory which specifies the receiver must be able to interpret this AVP or reject the entire Diameter request.

AVP Length defines the length of the AVP, like the Diameter length field this is used to delineate the end of one AVP.

AVP Vendor ID

If the AVP Vendor Specific flag is set this optional field specifies the vendor ID of the AVP Code used.

AVP Data

The payload containing the actual AVP data, this could be a username, in this example, a session ID, a domain, or any other value the vendor defines.

AVP Padding

AVPs have to fit on a multiple of a 32 bit boundary, so padding bits are added to the end of a packet if required to total the next 32 bit boundary.

Diameter Basics

3GPP selected Diameter protocol to take care of Authentication, Authorization, and Accounting (AAA).

It’s typically used to authenticate users on a network, authorize them to use services they’re allowed to use and account for how much of the services they used.

In a EPC scenario the Authentication function takes the form verifying the subscriber is valid and knows the K & OP/OPc keys for their specific IMSI.

The Authorization function checks to find out which features, APNs, QCI values and services the subscriber is allowed to use.

The Accounting function records session usage of a subscriber, for example how many sessional units of talk time, Mb of data transferred, etc.

Diameter Packets are pretty simple in structure, there’s the packet itself, containing the basic information in the headers you’d expect, and then a series of one or more Attribute Value Pairs or “AVPs”.

These AVPs are exactly as they sound, there’s an attribute name, for example username, and a value, for example, “Nick”.

This could just as easily be for ordering food; we could send a Diameter packet with an imaginary command code for Food Order Request, containing a series of AVPs containing what we want. The AVPs could belike Food: Hawian Pizza, Food: Garlic Bread, Drink: Milkshake, Address: MyHouse.
The Diameter server could then verify we’re allowed to order this food (Authorization) and charge us for the food (Accounting), and send back a Food Order Response containing a series of AVPs such as Delivery Time: 30 minutes, Price: $30.00, etc.

Diameter packets generally take the form of a request – response, for example a Capabilities Exchange Request contains a series of AVPs denoting the features supported by the requester, which is sent to a Diameter peer. The Diameter peer then sends back a Capabilities Exchange Response, containing a series of AVPs denoting the features that it supports.

Diameter is designed to be extensible, allowing vendors to define their own type of AVP and Diameter requests/responses and 3GPP have defined their own types of messages (Diameter Command Codes) and types of data to be transferred (AVP Codes).

LTE/EPC relies on Diameter and the 3GPP/ETSI defined AVP / Diameter Packet requests/responses to form the S6a Interface between an MME and a HSS, the Gx Interface between the PCEF and the PCRF, Cx Interface between the HSS and the CSCF, and many more interfaces used for Authentication in 3GPP networks.

Qos in LTE (4G) – ARP

ARP in LTE is not the Ethernet standard for address resolution, but rather the Allocation and Retention Policy.

A scenario may arise where on a congested cell another bearer is requested to be setup.

The P-GW, S-GW or eNB have to make a decision to either drop an existing bearer, or to refuse the request to setup a new bearer.

The ARP value is used to determine the priority of the bearer to be established compared to others,

For example a call to an emergency services number on a congested cell should drop any other bearers so the call can be made, thus the request for bearer for the VoLTE call would have a higher ARP value than the other bearers and the P-GW, S-GW or eNB would drop an existing bearer with a lower ARP value to accommodate the new bearer with a higher ARP value.

ARP is only used when setting up a new bearer, not to determine how much priority is given to that bearer once it’s established (that’s defined by the QCI).


MBR stands for Maximum Bit Rate, and it defines the maximum rate traffic can flow between a UE and the network.

It can be defined on several levels:

MBR per Bearer

This is the maximum bit rate per bearer, this rate can be exceeded but if it is exceeded it’s QoS (QCI) values for the traffic peaking higher than the MBR is back to best-effort.


Aggregate Maximum Bit Rate – Maximum bit rate of all Service Data Flows / Bearers to and from the network from a single UE.


The APN-MBR allows the operator to set a maximum bit rate per APN, for example an operator may choose to limit the MBR for subscriber on an APN for a MVNO to give it’s direct customers a higher speed.

(This is only applied to Non-GBR bearers)

QoS in LTE (4G) – QCI

The QCI (Quality Class Indicator) is a value of 0-9 to denote the service type and the maximum delays, packet loss and throughput the service requires.

Different data flows have different service requirements, let’s look at some examples:

A VoLTE call requires low latency and low packet loss, without low latency it’ll be impossible to hold a conversation with long delays, and with high packet loss you won’t be able to hear each other.

On the other hand a HTTP (Web) browsing session will be impervious to high latency or packet loss – the only perceived change would be slightly longer page load times as lost packets are resent and added delay on load of a few hundred ms.

So now we understand the different requirements of data flows, let’s look at the columns in the table above so we can understand what they actually signify:


Guaranteed Bit Rate bearers means our eNB will reserve resource blocks to carry this data no matter what, it’ll have those resource blocks ready to transport this data.

Even if the data’s not flowing a GBR means the resources are reserved even if nothing is going through them.

This means those resource blocks can’t be shared by other users on the network. The Uu interface in the E-UTRAN is shared between UEs in time and frequency, but with GBR bearers parts of this can be reserved exclusively for use by that traffic.


With a Non-GBR bearer this means there is no guaranteed bit rate, and it’s just best effort.

Non-GBR traffic is scheduled onto resource blocks when they’re not in use by other non-GBR traffic or by GBR traffic.


The priory value is used for preemption by the PCRF.

The lower the value the more quickly it’ll be processed and scheduled onto the Uu interface.

Packet Delay Budget

Maximum allowable packet delay as measured from P-GW to UE.

Most of the budget relates to the over-the-air scheduling delays.

The eNB uses the QCI information to make its scheduling decisions and packet prioritisation to ensure that the QoS requirements are met on a per-EPS-bearer basis.

(20ms is typically subtracted from this value to account for the radio propagation delay on the Uu interface)

Packet Error Loss Rate (PELR)

This is packets lost on the Uu interface, that have been sent but not confirmed received.

The PELR is an upper boundary for how high this can go, based on the SDUs (IP Packets) that have been processed by the sender on RLC but not delivered up to the next layers (PDCP) by the receiver.

(Any traffic bursting above the GBR is not counted toward the PELR)

(The list is now larger than 0-9 with 3GPP adding extra QCI values for MCPTT, V2X, etc, the full list is available here in table 6.1.7A)

QoS in LTE (4G) – GBR & Non-GBR Bearers

GBR is a confusing concept at the start when looking at LTE but it’s actually kind of simple when we break it down.

GBR stands for Guaranteed Bit Rate, meaning the UE is guaranteed a set bit rate for the bearer.

The default bearer is always a non-GBR bearer, with best effort data rates.

Let’s look at non-GBR bearers to understand the need for GBR bearers:

As the Uu (Air) interface is shared between many UEs, each is able to transfer data. Let’s take an example of a cell with two UEs in it and not much bandwidth available.

If UE1 and UE2 are both sending the same amount of data it’ll be evenly split between the two.

But if UE1 starts sending a huge amount of data (high bit rate) this will impact on the other UEs in the cells ability to send data over the air as it’s a shared resource.

So if UE2 needs to send a stream of small but important data over the air interface, while UE2 is sending huge amounts of data, we’d have a problem.

To address this we introduce the concept of a Guaranteed Bit Rate. We tell the eNB that the bearer carrying UE2’s small but important data needs a Guaranteed Bit Rate and it reserves blocks on the air interface for UE2’s data.

So now we’ve seen the need for GBR there’s the counter point – the cost.

While UE1 can still continue sending but the eNB will schedule fewer resource blocks to it as it’s reserved some for UE2’s data flow.

If we introduced more and more UEs each requiring GBR bearers, eventually our non-GBR traffic would simply not get through, so GBR bearers have to be used sparingly.

Note: IP data isn’t like frame relay or circuit switched data that’s consistent, bit rate can spike and drop away all the time. GBR guarantees a minimum bit rate, which is generally tuned to the requirements of the data flow. For example a GBR for a Voice over IP call would reserve enough for the media (RTP stream) but no more, so as not to use up resources it doesn’t need.

RF Planning with Forsk Atoll - Importing environmental data

Forsk Atoll – WMS Map Tiles

A hack I found useful to add Google Maps / Google Satelite View / Bing Maps / Bing Arial / Open Street Maps in Forsk Atoll.

Close Atoll,

Go to C -> Program Files -> Atoll

Edit the file named atoll.ini

Paste the following into it:

Name1 = OpenStreetMap Standard Map
URL1 =
Name2 = MapQuest Open Aerial
URL2 =
Name3 = 2Gis
URL3 =,256
Name4 = 2Gis without logo
URL4 = 
Name5 = Bing Aerial
URL5 =
Name6 = Bing Hybrid
URL6 =
Name7 = Bing Road
URL7 =
Name8 = Yandex Road
URL8 =
Name9 = Yandex Aerial
URL9 =
Name10 = Yandex Hybrid
URL10 =,skl&key=
Name11 = ArcGIS
URL11 =
Name12 = opencyclemap
URL12 =
Name13 = Google Terrain
URL13 =
Name14 = Google Map
URL14 =
Name15 = Google Hybrid (Map + Terrain)
URL15 =
Name16 = Google Hybrid (Map + Satellite)
URL16 =
Name17 = Google Satellite
URL17 =
Name18 = Google Scheme
URL18 =
Name19 = Google Scheme2 
URL19 =

Save and open Atoll,

Open the Geo Tab,

Right click on Online Maps, click “New”

Select the map source (In this example I’m using OSM) & hit Ok.

Enable the Online Map layer by ticking the layer.

Bam, done.

RF Planning with Forsk Atoll - Laying out environmental data


We’ve already touched on how subscribers are authenticated to the network, how the network is authenticated to subscribers and how the key hierarchy works for encryption of user data and control plane data.

If the IMSI was broadcast in the clear over the air, anyone listening would have the unique identifier of the subscriber nearby and be able to track their movements.

We want to limit the use of the IMSI over the air to a minimum.

During the first exchange the terminal is forced to send it’s IMSI, it’s the only way we can go about authenticating to the network, but once the terminal is authenticated and encryption of the radio link has been established, the network allocates a temporary identifier to the terminal, called the Temporary Mobile Subscriber Identity (TMSI) by the serving MME.

The TMSI is given to the terminal once encryption is setup, so only the network and the terminal know the mapping between IMSI and TMSI.

The TMSI is used for all future communication between the Network and the Terminal, hiding the IMSI.

The TMSI can be updated / changed at regular intervals to ensure the IMSI-TMSI mapping cannot be ascertained by a process of elimination.

The TMSI is short – only 4 bytes long – and this only has significance for the serving MME.

For the network to ascertain what MME is serving what TMSI the terminal is also assigned a Globally Unique Temporary (UE) Identity (GUTI), to identify the MME that knows the TMSI to IMSI mapping.

The GUTI is made up of the MNC/MCC combination, then an MME group ID to identify the MME group the serving MME is in, a MME code to identify the MME that allocated the TMSI and finally the TMSI itself.

The decision to use the TMSI or GUTI in a dialog is dependant on the needs of the dialog and what information both sides have. For example in an MME change the GUTI is needed so the original IMSI can be determined by the new MME, while in a normal handover the TMSI is enough.

IMTx: NET02x (4G Network Essentials) – Mobility Management – 3. Processing Location Updates

These are my lecture notes from IMT’s NET02x (4G Network Essentials) course, I thought I’d post them here as they may be useful to someone. You can find my complete notes here.

Let’s look at how the Tracking Area Updates work from the point of view of the network.

Let’s take an example of a UE which has been sent the Tracking Area List TA0 and TA1, which is currently in ECM_IDLE state served by eNBs in Tracking Area 1.

The UE is moving towards another eNB in Tracking Area 2. As the UE listens on the Broadcast Channel the power of the new eNB overtakes that of the previous eNB, but the UE notes the Tracking Area of the new eNB, which is not on the UE’s Tracking Area List.

So the UE must make a Tracking Area Update to inform the network.

The first thing to do is to establish a radio connection.

Once the radio connection is setup a S1-AP connection is setup, upon which an NAS message – EMM Tracking Area Update Request is sent which contains the GUIT and old Tracking Area ID, which is sent to the MME.

The MME then sends back a new Tracking Area List for the UE and new TMSI to update the GUTI of the subscriber.

The UE updates it’s GUTI, updates it’s Tracking Area List, sends an EMM TRACKING AREA UPDATE COMPLETE and the UE returns to ECM_IDLE state.

Further Reading

IMTx: NET02x (4G Network Essentials) – Mobility Management – 4. Changing MME and SGW

These are my lecture notes from IMT’s NET02x (4G Network Essentials) course, I thought I’d post them here as they may be useful to someone. You can find my complete notes here.

As we’ve seen earlier, the eNB needs a connection to an MME and a S-GW.

However different eNBs may connect to different S-GWs or different MMEs, and our UE may connect to any eNB, so we need a way to handover between S-GWs and MMEs.

Handover to new S-GW

Let’s take a look at a scenario where a UE is moving from one eNB to another, and each of the two eNBs is in a different S-GW.

At the start we have a connection from the MME to the S-GW, a GTP-C tunnel for control information and a GTP-U tunnel (called the S5/58 bearer) that carriers the user data over GTP-U between the P-GW and the S-GW.

As the UE moves to the eNB in TA2 we need the MME to modify the tunnel from the P-GW to the S-GW to change it from connecting the P-GW to the old S-GW and instead connecting the P-GW to the new S-GW.

The MME establishes a new tunnel for control to the new S-GW, and sends a message to the new S-GW to modify the tunnel from the P-GW to the old S-GW to point to the new S-GW.

Handover to new MME

IMTx: NET02x (4G Network Essentials) – Mobility Management – 2. Balancing Location Update Load

These are my lecture notes from IMT’s NET02x (4G Network Essentials) course, I thought I’d post them here as they may be useful to someone. You can find my complete notes here.

As we saw before larger Tracking Areas minimize the number of UEs between terminals to update their location.

The problem is the cells/eNBs on the edge of the Tracking Area have to handle almost all of the Tracking Area Update requests, to inform the network the UE has moved to a new TA.

The cells on the edges of the tracking area are shaded & handle the vast majority of the Tracking Area / Location Update messages

There’s an obvious imbalance between edge cells that handle almost all of Tracking Area Updates and the central cells inside a Tracking Area that handle very few many Tracking Area Update messages.

As we know we only have one radio interface, and sending Tracking Area Updates eats into our valuable radio resources that can’t be used to carry user data. Because of this users can experience a lower bit rate on edge cells.

To get around this we group Tracking Areas together into Tracking Area Lists.

A Tracking Area List is provided by the network to the UE, and contains a list of Tracking Areas, so long as the UE stays within the list of Tracking Areas, there is no need for it to send a Tracking Area Update.

You might think this just makes our problem worse, as now at the edges of the cells in the Tracking Area List we have even more signaling traffic, the clever part comes from the fact the network gives out different Tracking Area Lists to different UEs.

In the example below we can see UE2 has a different Tracking Area List to UE1.

This means the cell edges are different for UE1 and UE2, which spreads the signaling load across Tracking Areas, so while UE2 will send a Tracking Area Update when it reaches the border from TA1 to TA4, UE1 will send a Tracking Area Update when it passes from TA6 to TA9.

The other limitation of this is now to reach a UE paging must be sent on all cells in the Tracking Area List.