Tag Archives: EPC

Making use of Australian Elevation Data in Forsk Atoll

Australian Telco, EUTRAN, Mobile Networks, Software, , ,

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:

Why GTP for Mobile Networks?

EPC, LTE, Mobile Networks, RFCs & Standards, , , , ,

Let’s take a look at GTP, the workhorse of mobile user plane packet data.

This post covers all generations of mobile data (2.5 -> 5G), so I’m using generic terms.

GSM, UMTS, LTE & NR all have one protocol in common – GTP – The GPRS Tunneling Protocol.

So why do every generation of mobile data networks from GSM/GPRS in 2000, to 5G NR Standalone in 2020, rely on this one protocol for transporting user data?

So Why GTP?

GTP – the GPRS Tunnelling Protocol, is what encapsulates and tunnels IP packets from the internet / packet data network, to and from the User.

So why encapsulate the packets? What if the Base Station had access to the internet and routed the traffic to the users?

Let’s say we did that, we’d have to have large pools of IP addresses available at each Base Station and when a user connected they’d be assigned an IP Address and traffic for these users would be routed to the Base Station which would forward it onto the user.

This would work well until a user moves from one Base Station to another, when they’d have to get a new IP Address allocated.

TCP/IP was never designed to be mobile, an IP address only exists in a single location.

Breaking out traffic directly from a base station would have other issues, such as no easy way to enforce QoS or traffic policies, meter usage, etc.

How to fix IP’s lack of mobility? GTP.

GTP addressed the mobility issue by having a single fixed point the IP Address is assigned to (In GSM/GRPS/UMTS this is the Gateway GPRS Support Node, in LTE this is the P-GW and in 5G-SA this is the UPF), which encapsulates IP traffic to/from a mobile user into GTP Packet.

You can think of GTP like GRE or any of the other common encapsulation protocols, wrapping up the IP packets into a GTP packet which we can rerouted to different Base Stations as the users move from being served by one Base Station to another.

This easy redirecting / rerouting of user traffic is why GTP is used for NR (5G), LTE (4G), UMTS (3G) & GPRS (2.5G) architectures.

GTP Packets

When looking at a GTP packet of user data you’d be forgiven for thinking nothing much goes on,

Example GTP packet containing a DNS query

Like in most tunneling / encapsulation protocols we’ve got the original network / protocol stack of IPv4 and UDP, and a payload of a GTP packet.

The packet itself is pretty bare bones, there’s flags, denoting a few basics like version number, the message type (T-PDU), the length of the GTP packet and it’s payload (used for delineating the end of the payload), a sequence number an a Tunnel Endpoint Identifier (TEID).

In the payload, we can see the network / protocol stack and application layer of the contents of the GTP packet.

From a mobility standpoint, the beauty of GTP is that it takes IP packets and puts them into a media stream of sorts, with out of band signalling, this means we can change the parameters of our GTP stream easily without touching the encapsulated IP Packet.

When a UE moves from one base station to another, all that has to happen is the destination the GTP packets are sent to is changed from the old base station to the new base station. This is signalled using GTP-C in GPRS/UMTS, GTPv2-C in LTE and HTTP in 5G-SA.

Traffic to and from the UE would look the same as the screenshot above, the only difference would be the first IPv4 address would be different, but the IPv4 address in the GTP tunnel would be the same.

Roll your own USIMs for Private LTE Networks

EPC, EUTRAN, IMS / VoLTE, LTE, Mobile Networks, SDM, Security, SIM Cards, , , , , , , ,

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

EPC, EUTRAN, LTE, Mobile Networks, , , ,

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.

LTE_EUTRAN_UE_Idle_DetachPCAP of LTE / EUTRAN UE Idle Detach process

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.

UEContextReleaseCommand

UEContextReleaseComplete

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

The case for Header Compression in VoIP/VoLTE

EPC, EUTRAN, IMS / VoLTE, LTE, Mobile Networks, RFCs & Standards, VoIP, , , , , ,

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.

Open5Gs- Python HSS Interface

EPC, LTE, Mobile Networks, Python, RF, SDM, , , ,

Note: NextEPC the Open Source project rebranded as Open5Gs in 2019 due to a naming issue. The remaining software called NextEPC is a branch of an old version of Open5Gs. This post was written before the rebranding.

I’ve been working for some time on Private LTE networks, the packet core I’m using is NextEPC, it’s well written, flexible and well supported.

I joined the Open5Gs group and I’ve contributed a few bits and pieces to the project, including a Python wrapper for adding / managing subscribers in the built in Home Subscriber Server (HSS).

You can get it from the support/ directory in Open5Gs.

NextEPC Python Library

Basic Python library to interface with MongoDB subscriber DB in NextEPC HSS / PCRF. Requires Python 3+, mongo, pymongo and bson. (All available through PIP)

If you are planning to run this on a different machine other than localhost (the machine hosting the MongoDB service) you will need to enable remote access to MongoDB by binding it’s IP to 0.0.0.0:

This is done by editing /etc/mongodb.conf and changing the bind IP to: bind_ip = 0.0.0.0

Restart MongoDB for changes to take effect.

$ /etc/init.d/mongodb restart

Basic Example:

import NextEPC
NextEPC_1 = NextEPC("10.0.1.118", 27017)

pdn = [{'apn': 'internet', 'pcc_rule': [], 'ambr': {'downlink': 1234, 'uplink': 1234}, 'qos': {'qci': 9, 'arp': {'priority_level': 8, 'pre_emption_vulnerability': 1, 'pre_emption_capability': 1}}, 'type': 2}]
sub_data = {'imsi': '891012222222300', \
             'pdn': pdn, \
             'ambr': {'downlink': 1024000, 'uplink': 1024001}, \
             'subscribed_rau_tau_timer': 12, \
             'network_access_mode': 2, \
             'subscriber_status': 0, \
             'access_restriction_data': 32, \
             'security': {'k': '465B5CE8 B199B49F AA5F0A2E E238A6BC', 'amf': '8000', 'op': None, 'opc': 'E8ED289D EBA952E4 283B54E8 8E6183CA'}, '__v': 0}

print(NextEPC_1.AddSubscriber(sub_data))                        #Add Subscriber using dict of sub_data

print(NextEPC_1.GetSubscriber('891012222222300'))               #Get added Subscriber's details

print(NextEPC_1.DeleteSubscriber('891012222222300'))            #Delete Subscriber

Subscriber_List = NextEPC_1.GetSubscribers()
for subscribers in Subscriber_List:
  print(subscribers['imsi'])
Open5Gs Logo

Open5GS – Splitting Network Elements

EPC, LTE, Mobile Networks, Software, ,

Note: NextEPC the Open Source project rebranded as Open5Gs in 2019 due to a naming issue. The remaining software called NextEPC is a branch of an old version of Open5Gs. This post was written before the rebranding.

I’ve been working for some time on Private LTE networks, and wrote my own HSS (See PyHSS – Python Home Subscriber Server).

The packet core I’m using is NextEPC, it’s well written, flexible and well supported.

I joined the Open5Gs group and I’ve contributed a few bits and pieces to the project.

One of which was how to split all the network elements in NextEPC:

NextEPC Splitting Network Elements

In a production network network elements would typically not all be on the same machine, as is the default example that ships with NextEPC.

NextEPC is designed to be standards compliant, so in theory you can connect any core network element (MME, PGW, SGW, PCRF, HSS) from NextEPC or any other vendor to form a functioning network, so long as they are 3GPP compliant.

To demonstrate this we will cover isolating each network element onto it’s on machine and connect each network element to the other. For some interfaces specifying multiple interfaces is supported to allow connection to multiple

In these examples we’ll be connecting NextEPC elements together, but it could just as easily be EPC elements from a different vendor in the place of any NextEPC network element.

ServiceIPIdentity
P-GW10.0.1.121pgw.localdomain
S-GW10.0.1.122 
PCRF10.0.1.123pcrf.localdomain
MME10.0.1.124mme.localdomain
HSS10.0.1.118hss.localdomain

External P-GW

In it’s simplest from the P-GW has 3 interfaces:

  • S5 – Connection to home network S-GW (GTP-C)
  • Gx – Connection to PCRF (Diameter)
  • Sgi – Connection to external network (Generally the Internet via standard TCP/IP)

S5 Interface Configuration

Edit /etc/nextepc/pgw.confand change the address to IP of the server running the P-GW for the listener on GTP-C and GTP-U interfaces.

pgw:
    freeDiameter: pgw.conf
    gtpc: 
      addr:
        - 10.0.1.121
     gtpu: 
      addr:
        - 10.0.1.121

Gx Interface Configuration

Edit /etc/nextepc/freeDiameter/pgwd.conf

Update ListenOn address to IP of the server running the P-GW:

ListenOn = "10.0.1.121";

Update ConnectPeer to connect to the PCRF on it’s IP.

ConnectPeer = "pcrf.localdomain" { ConnectTo = "10.0.1.123"; No_TLS; };

Restart Services

Restart NextEPC PGW Daemon:

$ sudo systemctl restart nextepc-pgwd

External S-GW

In it’s simplest form the S-GW has 2 interfaces:

  • S11 – Connection to MME (GTP-C)
  • S5 – Connection to the home network P-GW (GTP-C)

S5 Interface Configuration

Edit /etc/nextepc/sgw.confand change the address to IP of the server running the S-GW for the listener on GTP-C interface.

sgw:
    freeDiameter: pgw.conf
    gtpc: 
      addr:
        - 10.0.1.122

Restart NextEPC SGW Daemon:

$ sudo systemctl restart nextepc-sgwd

External PCRF

In it’s simplest from the PCRF has 1 network interface:

  • Gx – Connection to P-GW (Diameter)

Gx Interface Configuration

Edit /etc/nextepc/freeDiameter/hss.conf

Update ListenOn address to IP of the server running the HSS on it’s IP:

ListenOn = "10.0.1.123";

Update ConnectPeer to connect to the MME.

ConnectPeer = "pgw.localdomain" { ConnectTo = "10.0.1.121"; No_TLS; };

MongoDB Interface Configuration (NextEPC HSS only)

Edit /etc/nextepc/freeDiameter/hss.conf and change the db_uri: to point at the HSS: db_uri: mongodb://10.0.1.118/nextepc

Restart NextEPC PCRF Daemon:

$ sudo systemctl restart nextepc-pcrfd

External HSS

In it’s simplest form the HSS has 1 network interface:

  • S6a – Connection to MME (Diameter)

S6a Interface Configuration

Edit /etc/nextepc/freeDiameter/hss.conf

Update ListenOn address to IP of the server running the HSS on it’s IP:

ListenOn = "10.0.1.118";

Update ConnectPeer to connect to the MME.

ConnectPeer = "mme.localdomain" { ConnectTo = "10.0.1.124"; No_TLS; };

Restart NextEPC HSS Daemon:

$ sudo systemctl restart nextepc-hssd

MongoDB Interface Configuration (NextEPC specific)

If you are using NextEPC’s HSS you may need to enable MongoDB access from the PCRF. This is done by editing ‘‘/etc/mongodb.conf’’ and changing the bind IP to: bind_ip = 0.0.0.0

Restart MongoDB for changes to take effect.

$ /etc/init.d/mongodb restart

External MME

In it’s simplest form the MME has 3 interfaces:

  • S1AP – Connections from eNodeBs
  • S6a – Connection to HSS (Diameter)
  • S11 – Connection to S-GW (GTP-C)

S11 Interface Configuration

Edit /etc/nextepc/mme.conf, filling the IP address of the S-GW and P-GW servers.

sgw:
    gtpc:
      addr: 10.0.1.122

pgw:
    gtpc:
      addr:
        - 10.0.1.121

S6a Interface Configuration

Edit /etc/nextepc/freeDiameter/mme.conf

Update ListenOn address to IP of the server running the MME:

ListenOn = "10.0.1.124";

Update ConnectPeer to connect to the PCRF on it’s IP.

ConnectPeer = "hss.localdomain" { ConnectTo = "10.0.1.118"; No_TLS; };

Restart Services

Restart NextEPC MME Daemon:

$ sudo systemctl restart nextepc-mmed

Building Android APN / Carrier Config

EPC, EUTRAN, GSM, IMS / VoLTE, LTE, Mobile Networks, SIM Cards, Software, ,

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"
      mcc="123"
      mnc="123"
      apn="carrierxyz"
      type="default,supl,mms,ims,cbs"
      mmsc="http://mms.carrierxyz.com"
      mmsproxy="0.0.0.0"
      mmsport="80"
      bearer_bitmask="4|5|6|7|8|12"
/>

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

Qos in LTE (4G) – ARP

EPC, EUTRAN, LTE, Mobile Networks, RF, , , , , , , ,

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).

QoS in LTE (4G) – MBR/AMBR/APN-MBR

EPC, EUTRAN, LTE, Mobile Networks, RF, , , , , , , ,

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.

AMBR

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

APN-MBR

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

EPC, EUTRAN, LTE, Mobile Networks, RF, , , , , , ,

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:

GBR

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.

Non-GBR

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.

Priority

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

EPC, EUTRAN, LTE, Mobile Networks, RF, , , , , , , , ,

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.

LTE (4G) – Authenticating the Network

EPC, LTE, Mobile Networks, RF, RFCs & Standards, Security, SIM Cards, , , ,

In my last post we discussed how the network authenticated a subscriber, now we’ll look at how a subscriber authenticates to a network. There’s a glaring issue there in that the MME could look at the RES and the XRES and just say “Yup, OK” even if the results differed.

To combat this LTE networks have mutual authentication, meaning the network authenticates the subscribers as we’ve discussed, and the subscribers authenticate the network.

To do this our HSS will take the same random key (RAND) we used to authenticate the subscriber, and using a different cryptographic function (called g) take the RAND, the K value and a sequence number called SQN, and using these 3 inputs, generate a new result we’ll call AUTN.

The HSS sends the RAND (same as RAND used to authenticate the subscriber) and the output of AUTN to the MME which forwards it to the eNB to the UE which passes the RAND and AUTH values to the USIM.

The USIM takes the RAND and the K value from the HSS, and it’s expected sequence number. With these 3 values it applies the cryptographic function g generates it’s own AUTN result.

If it matches the AUTN result generated by the HSS, the USIM has authenticated the network.

LTE (4G) – Authenticating Subscribers

EPC, LTE, Mobile Networks, RF, RFCs & Standards, SDM, SIM Cards, , , ,

The USIM and the HSS contain the subscriber’s K key. The K key is a 128 bit long key that is stored on the subscriber’s USIM and in the HSS along with the IMSI.

The terminal cannot read the K key, neither can the network, it is never transmitted / exposed.

When the Terminal starts the attach procedure, it includes it’s IMSI, which is sent to the MME.

The MME then sends the the HSS a copy of the IMSI.

The HSS looks up the K key for that IMSI, and generates a random key called RAND.

The HSS and runs a cryptographic function (called f) using the input of RAND and K key for that IMSI, the result is called XRES (Expected result).

The HSS sends the output of this cryptographic function (XRES), and the random value (RAND) back to the MME.

The MME forwards the RAND value to the USIM (via eNB / Terminal), and stores a copy of the expected output of the cryptographic function.

The USIM take the RAND and the K key and performs the same cryptographic function the HSS did on it with the input of the K key and RAND value to generate it’s own result (RES).

The result of this same function (RES) is then sent from the USIM to the terminal which forwards it to the MME.

The MME and comparing the result the HSS generated (XRES) with the result the USIM generated. (RES)

If the two match it means both the USIM knows the K key, and is therefore the subscriber they’re claiming to be.

If the two do not match the UE is refused access to the network.

Next up, how the UE authenticates the network.

LTE (4G) – USIM Basics

LTE, Mobile Networks, RF, RFCs & Standards, Security, SIM Cards, , , , , , , ,

I’ve been working on private LTE recently, and one of the first barriers you’ll hit will be authentication.

LTE doesn’t allow you to just use any SIM to authenticate to the network, but instead relies on mutual authentication of the UE and the network, so the Network knows it’s talking to the right UE and the UE knows it’s talking to the right network.

So because of this, you have to have full control over the SIM and the network. So let’s take a bit of a dive into USIMs.

So it’s a SIM card right?

As a bit of background; the ever shrinking card we all know as a SIM is a “Universal integrated circuit card” – a microcontroller with it’s own OS that generally has the ability to run Java applets.

One of the Java applets on the card / microcontroller will be the software stack for a SIM, used in GSM networks to authenticate the subscriber.

For UMTS and LTE networks the card would have a USIM software stack allowing it to act as a USIM, the evolved version of the SIM.

Because it’s just software a single card can run both a USIM and SIM software stack, and most do.

As I’m building an LTE network we’ll just talk about the USIM side of things.

USIM’s role in Authentication

When you fire up your mobile handset the baseband module in it communicates with the USIM application on the card.

When it comes time to authenticate to the network, and authenticate the network itself, the baseband module sends the provided challenge information from the network to the USIM which does the crypto magic to generate responses to the authentication challenges issued by the network, and the USIM issues it’s own challenges to the network.

The Baseband module provides the ingredients, but the USIM uses it’s secret recipe / ingredients combo, known only to the USIM and HSS, to perform the authentication.

Because the card challenges the network it means we’ve got mutual authentication of the network.

This prevents anyone from setting up their own radio network from going all Lionel Ritche and saying “Hello, is it me you’re looking for” and having all the UEs attach to the malicious network. (Something that could be done on GSM).

It’s worth noting too that because the USIM handles all this the baseband module, and therefore the mobile handset itself, doesn’t know any of the secret sauce used to negotiate with the network. It just gets the challenge and forwards the ingredients down to the USIM which spits back the correct response to send, without sharing the magic recipe.

This also means operators can implement their own Crypto functions for f and g, so long as the HSS and the USIM know how to generate the RES and AUTN results, it’ll work.

What’s Inside?

Let’s take a look at the information that’s stored on your USIM:

All the GSM stuff for legacy SIM application

Generally USIMs also have the ability to operate as SIMs in a GSM network, after all it’s just a different software stack. We won’t touch on GSM SIMs here.

ICCID

Because a USIM is just an application running on a Universal Integrated Circuit Card, it’s got a ICCID or Universal Integrated Circuit Card ID. Generally this is the long barcode / string of numbers printed on the card itself.

The network generally doesn’t care about this value, but operators may use it for logistics like shipping out cards.

PIN & PUK

PINs and PUKs are codes to unlock the card. If you get the PIN wrong too many times you need the longer PUK to unlock it.

These fields can be written to (when authenticated to the card) but not read directly, only challenged. (You can try a PIN, but you can’t see what it’s set too).

As we mentioned before the terminal will ask the card if that’s correct, but the terminal doesn’t know the PIN either.

IMSI

Each subscriber has an IMSI, an International Mobile Subscriber Identity.

IMSIs are hierarchical, starting with 3 digit Mobile Country Code MCC, then the Mobile Network Code (MNC) (2/3 digits) and finally a Mobile Subscription Identification Number (MSIN), a unique number allocated by the operator to the subscribers in their network.

This means although two subscribers could theoretically have the same MSIN they wouldn’t share the same MNC and MCC so the ISMI would still be unique.

The IMSI never changes, unless the subscriber changes operators when they’ll be issued a new USIM card by the new operator, with a different IMSI (differing MNC).

The MSIN isn’t the same as the phone number / MSISDN Number, but an IMSI generally has a MSISDN associated with it by the network. This allows you to port / change MSISDN numbers without changing the USIM/SIM.

K – Subscriber Key

Subscriber’s secret key known only to the Subscriber and the Authentication Center (AuC/ HSS).

All the authentication rests on the principle that this one single secret key (K) known only to the USIM and the AuC/HHS.

OP – Operator Code

Operator Code – same for all SIMs from a single operator.

Used in combination with K as an input for some authentication / authorisation crypto generation.

Because the Operator Code is common to all subscribers in the network, if this key were to be recovered it could lead to security issues, so instead OPc is generally used.

OPc – Operator Code (Derived)

Instead of giving each USIM the Operator Code a derived operator code can be precomputed when the USIM is written with the K key.

This means the OP is not stored on the USIM.

OPc=Encypt-Algo(OP,Key)

PLMN (Public Land Mobile Network)

The PLMN is the combination of MCC & MNC that identifies the operator’s radio access network (RAN) from other operators.

While there isn’t a specific PLMN field in most USIMs it’s worth understanding as several fields require a PLMN.

HPLMNwAcT (HPLMN selector with Access Technology)

Contains in order of priority, the Home-PLMN codes with the access technology specified.

This allows the USIM to work out which PLMN to attach to and which access technology (RAN), for example if the operator’s PLMN was 50599 we could have:

  • 50599 E-UTRAN
  • 50599 UTRAN

To try 4G and if that fails use 3G.

In situations where operators might partner to share networks in different areas, this could be set to the PLMN of the operator first, then it’s partnered operator second.

OPLMNwACT (Operator controlled PLMN selector with Access
Technology)

This is a list of PLMNs the operator has a roaming agreement with in order of priority and with the access technology.

An operator may roam to Carrier X but only permit UTRAN access, not E-TRAN.

FEHPLMN (Equivalent HPLMN)

Used to define equivalent HPMNs, for example if two carriers merge and still have two PLMNs.

FPLMN (Forbidden PLMN list)

A list of PLMNs the subscriber is not permitted to roam to.

HPPLMN (Higher Priority PLMN search period)

How long in seconds to spend between each PLMN/Access Technology in HPLMNwAcT list.

ACC (Access Control Class)

The ACC allows values from 0-15, and determines the access control class of the subscriber.

In the UK the ACC values is used to restrict civilian access to cell phone networks during emergencies.

Ordinary subscribers have ACC numbers in the range 0 – 9. Higher priority users are allocated numbers 12-14.

During an emergency, some or all access classes in the range 0 – 9 are disabled.

This means service would be could be cut off to the public who have ACC value of 0-9, but those like first responders and emergency services would have a higher ACC value and the network would allow them to attach.

AD (Administrative Data)

Like the ACC field the AD field allows operators to drive test networks without valid paying subscribers attaching to the network.

The defined levels are:

  • ’00’ normal operation.
  • ’80’ type approval operations.
  • ’01’ normal operation + specific facilities.
  • ’81’ type approval operations + specific facilities.
  • ’02’ maintenance (off line).
  • ’04’ cell test operation.

GID 1 / 2 – Group Identifier

Two group identifier fields that allow the operator to identify a group of USIMs for a particular application.

SPN (Service Provider Name)

The SPN is an optional field containing the human-readable name of the network.

The SPN allows MVNOs to provide their own USIMs with their name as the operator on the handset.

ECC (Emergency Call Codes)

Codes up to 6 digits long the subscriber is allowed to dial from home screen / in emergency / while not authenticated etc.

MSISDN

Mobile Station International Subscriber Directory Number. The E.164 formatted phone number of the subscriber.

This is optional, as porting may overwrite this, so it doesn’t always match up.

References:

https://www.etsi.org/deliver/etsi_ts/131100_131199/131102/12.05.00_60/ts_131102v120500p.pdf

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

EPC, EUTRAN, LTE, Mobile Networks, Notes, RF, , , , ,

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 – 1. Managing Location

EPC, EUTRAN, LTE, Mobile Networks, Notes, RF, , , , , ,

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 with the Network Triggered Service Request, the network needs to know which eNB / cell the UE is currently being served by.

The UE knows which cell it should use as it’s always listening on the broadcast channel to know the received power levels of the nearby eNBs.

Paging

If our UE is in ECM IDLE state and the network needs to contact the UE, the eNB sends sends a Paging Request on the Beacon (Broadcast) Channel with the UE’s RNTI.

The UE is always listening on the Beacon Channel for it’s own RNTI, and when it hears it’s own RNTI it follows the process to come back from ECM_IDLE state to ECM_CONNECTED state.

For this to work the network needs to know which eNB to send the Paging request to.

For this to work our UE would need to inform the network each time it changes eNB, but, as we’ve touched upon several times, minimizing power consumption is a constant architecture constraint in LTE.

So if the UE has to transmit each time a UE moves to a different eNB / Cell, the UE power consumption would be high and the battery life of the UE would be low.

If we imagine driving along a freeway at speed, with each eNB serving an area of 1km, at 60kph, our UE would change cells every minute, and if the UE needs to transmit to let the network know it’s changing location, we’d be transmitting data every 60 seconds even if the UE is sitting in our pocket, all these transmissions would lead to lower battery life on the UE.

Tracking Areas

To work around the power wastage of each UE transmitting data to the network to let it know each time it changes eNB, 3GPP designers decided to group eNBs in the same geographic area into Tracking Areas or TAs.

This means instead of the network knowing exactly which eNB a UE is located in, it has it’s location down to a tracking area made up of several eNBs. (Tens to hundreds of cells per TA)

To go back to our freeway example, we might group all the eNBs along a freeway into one Tracking Area, all of which broadcast the ID of each eNB and the Tracking Area of each eNB.

As the UE moves from one eNB to another eNB in the same Tracking Area, there’s no need for the UE to send a Tracking Area Update message as it’s reamining in the same Tracking Area.

Tracking Area Update messages only need to be sent when the UE moves to an eNB in a different Tracking Area.

UEs can move from cell to cell inside TA1 without needing to update the network.
Only when a UE moves from a eNB / cell in TA1 to TA2, does it need to send a Tracking Area Update message to the network.

Paging a Tracking Area

As the network knows the location of our UE down to a tracking area, when it comes time to Page a UE a Paging Request is simply sent from the MME to all eNBs in the Tracking Area that the UE is in.

This means the RNTI of the UE is broadcast out of all eNBs in that tracking Area, and the UE establishes connectivity once again with it’s nearest eNB.

IMTx: NET02x (4G Network Essentials) – Management of Sporadic Data Flows – 4. UE Triggered Service Request

EPC, EUTRAN, LTE, Mobile Networks, Notes, RF, , , , , , , , ,

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 just saw when a terminal moves to ECC-Idle while in EMM-Registered state, it releases it’s radio resources, so what happens when the UE needs to send / receive data again?

UE is disconnected from Radio Resources (ECM-Idle & EMM Registered)

While one option could have been to go through the full attach procedure again when the UE is triggered, the 3GPP team wanted the re-connection process to be as fast as possible.

As we saw in the last post we don’t drop the S-GW <-> P-GW tunnel, which saves time on re-establishing a connection. The S1 tunnel is also not completely released; the TEID value from the S-GW end of the tunnel is saved by the MME so it can be reused by the new tunnel when the UE reconnects, without needing to inform the S-GW.

IMTx: NET02x (4G Network Essentials) – Management of Sporadic Data Flows – 2. UE Connection to the Network

EPC, EUTRAN, LTE, Mobile Networks, Notes, RF, , , , , , , , ,

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.

One of the common themes we cover over and over in the 4G discussion is the desire to preserve energy on the UE RF side of things, to extend battery life as much as possible.

The 3GPPs requirements for LTE also included the smallest round trip times, defining less than 5 ms in unload condition, so traffic to the UE must be routed as quickly as possible.

Mobiles are by their very nature, mobile.

This requires UEs to constantly monitor the RF conditions and the signal measurements from different base stations so the UE can determine if it’s time to handoff to another cell due to going further from one eNB and closer to another, or another eNB offering better RF conditions (Strong signal etc).

This requires regular exchanges of messages and checks, but this would take a lot of energy and eat up battery usage.

Instead we avoid maintaining the radio connection all the time with the aid of an inactivity timer on the eNB.

For as long as user data is flowing over the air interface the connection is maintained, for example web browsing, the inactivity timer is constantly reset as traffic flows.

However when the eNB detects no packets sent or received by the UE the timer starts counting down from it’s set value.

When the inactivity timer reaches 0 the RRC Connection is released and the UE no longer has an RNTI.

The UE is still listening to an eNB, it’s just not sending data to it it and visa-versa.

As the radio bearer has been removed the UE the S1-AP and S1-UP bearers between the eNB and the MME and the eNB and the S-GW respectively, can be torn down.

This means the MME is no long sure of exactly which eNB the UE is listening on.

This is referred to as ECM_IDLE state as there is no radio connection, and the network is unaware of the precise location of the UE.

An ECM_ACTIVE state is the state when the UE is connected to an eNB with an RNTI and it’s inactivity timer has not reached 0.

The dotted line bearers shown in the image above frequently change between active and inactive based on the ECM_ACTIVE / ECM_INACTIVE state of the bearers.

IMTx: NET02x (4G Network Essentials) – Management of Sporadic Data Flows – 1. Attach and Detach Procedures

EPC, EUTRAN, LTE, Mobile Networks, Notes, RF, , , , , , ,

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.

A LTE UE has permanent IP connectivity for as long as it is connected.

As soon as the UE powers up it requests the establishment of one or more bearers for it’s IP connectivity through GTP tunnels.

An EPS Connectivity Request message is sent by the UE.

The network needs to know if a UE can be reached or not, so the network must store state for each terminal,

EPS Session Management (ESM) manages EPS bearer contexts.

EPS Mobility Management (EMM) has two states – EMM-Registered (UE reachable) and EMM-Deregistered (UE not reachable).

A UE is in the deregistered state when it is not rechable, for example not currently powered up or in flight mode.

The MME memorizes the state of each UE and it’s context elements such as it’s most recent GUTI, IMSI, security parameters etc.

Attach Procedure

To attach to the network a UE sends an EMM Attach Request with it’s most recent GUTI to the MME.

In the same request the UE also includes an ESM PDN Connectivity Request to gain access to the external networks.

The Authentication & Key Agreement procedure is followed between the UE and the MME/HSS to authenticate the network and the subscriber.

One this is done the MME looks at the connectivity requested and the APN of the subscriber, the MME then selects a Serving-Gateway and Packet-Gateway based on the APN.

The MME then sends a GTP-C Create Session Request along with the connectivity requested (IPv4/6), APN and IMSI of the subscriber and it’s allocated TEID for this tunnel to the S-GW.

The S-GW also sends a GTP-C Create Session Request along with
the connectivity requested (IPv4/6), APN and IMSI of the subscriber to the P-GW, along with the S-GW’s allocated TEID for this tunnel too.

The P-GW then sends a GTP-C Create Session back to the S-GW containing it’s TEID and it also includes the IP Address to be allocated to the UE.

A GTP session is now setup between the P-GW and the S-GW for this bearer, with the TEID values added to the TEID management tables on both devices. This GTP tunnel is referred to an S5 (home) or an S8 (roaming) Bearer in 3GPP parlance.

Another GTP-C Create Session message with it’s own TEID is also sent from the S-GW to the MME.

The MME, S-GW and P-GW now each know TEID for each of the 2 tunnels setup (MME<->S-GW, S-GW<->P-GW) so have what they need to fill their TEID management tables.

When the MME recieves the GTP-C Create Session with the IP Address for the UE it sends an EMM Attach Accept and a EPS Bearer Context Setup Request containing the IP Address the P-GW allocated to the UE to the UE itself.

The UE stores the allocated IP and sends an acknowledgement to the MME in the form of an EMM Attach Complete message back to the MME.

The MME sends a GTP-C Modify Bearer Request which transfers the bearer setup between MME and SGW and modifies it to be between the SGW and the eNB.

The S-GW sends back a GTP-C Modify Bearer Complete message and modifies the GTP tunnel to be between the SGW and the eNB. A S1 bearer is now established for carrying user data from the eNB to the SGW.

Once this procedure is complete the UE is now in the EMM Registered State meaning it is known to the MME, it has a security association and has an IP Address.

The S-GW and the P-GW also stores the TEIDs for the UE.

Detach Procedure

When a UE detaches from the network (for example it powers down), the network must release all the tunnels for that UE, the MME state must be updated to EMM Deregistered and the MME must also keep a record for the last GUTI and security keys,

To detach from the network the UE sends a RLC UL Information Transfer message containing an EMM Detach Request which includes it’s current GUTI.

As soon as the UE recivers confirmation from the eNB the UE can power down, but the eNB must inform the network of the disconnection so the resources can be released.

The eNB sends a S1Ap Uplink NAS Transport message containing a EMM Detach Request with the UE’s GUTI to the MME.

The MME can then release the security context,

The MME then sends a GTP-C Delete Session Request to the S-GW.

Upon recipt of this request the S-GW requests the P-GW tears down it’s tunnel between the P-GW and S-GW (aka the S5/S8 Bearer) by sending it’s own GTP-C Delete Session Request to the P-GW.

Once the S-GW has confirmation the tunnel has been taken down (In the form of a GTP-C Delete Session Response) the S-GW sends a GTP-C Delete Session Response to the MME.

The MME must signal to the eNB it can release the RNTI and the radio resources. To do this it sends a S1-AP UE Context Release Command which releases the radio bearers and tears down the S1-UP bearer between the eNB and the S-GW.

The eNB then sends a S1-AP UE Context Release Completeto the MME.

Finally the MME sends a Diameter Notification Request (PGW and APN Removed) to the HSS to update the HSS of the user’s status, the HSS signals back with a Diameter Notification Answer and the HSS knows the user is no longer reachable.