Tag Archives: GPRS Tunneling Protocol

GSM with Osmocom: OsmoSGSN for Packet Data

In our last Osmocom post we talked about the basics of packet data, and configuring our BTSs to support it.

In this post we’ll take a look at using Osmocom’s Serving Gateway Support Node (SGSN) named OsmoSGSN.

At the BSC traffic is divided into two categories, Circuit Switched (CS) traffic (Like voice calls & SMS) which is handed by the MSC, and Packet Switched (PS) traffic (Mobile data) is handled by the SGSN.

The SGSN acts as an anchor point for our packet data, it connects our BSC (that handles our RAN) to the GGSN (that handles the connection to external data networks).

Although it’s not technically possible to run a data only 2G/3G network (you require the MSC) it almost could be.
The SGSN handles authentication of subscribers, and runs the PS network completely standalone from the CS network. The SGSN does it’s own handover management, authentication, etc, without any connection to the MSC.

Basic SGSN Config

Like the previous Osmocom network elements we’ve covered, we’ll access the SGSN via Telnet on localhost (the server running the Osmocom stack) on port 4254.

Once we’ve accessed the terminal we’ll escalate our privileges using the enable command, and run configure terminal to start configuring,

We’ll begin by setting the local IP our SGSN will listen on, the gtp local-ip, we’ll need this to be externally accessible for our BTSs, so set it to the IP of the server.

  gtp local-ip

Next we’ll need to configure the IP of our GGSN. It gets a bit messy if we’re running everything on one box, as we’re going to have the SGSN and the GGSN trying to communicate on the same ports for GTP, so best to assign an IP in the loopback range, like in my case, for the GGSN:

   gtp local-ip
   ggsn 0 remote-ip
   ggsn 0 gtp-version 1
   ggsn 0 no echo-interval
   apn * ggsn 0

We can also steer GGSN selection based on the APN, for example an APN for a corporate network, you may want to have a dedicated GGSN for, for example, we could create a second GGSN – GGSN 1 and route any traffic on our “special.access.net” APN to that GGSN, and everything else to GGSN0:

   gtp local-ip
   ggsn 0 remote-ip
   ggsn 0 gtp-version 1
   ggsn 0 no echo-interval
   ggsn 1 remote-ip
   ggsn 1 gtp-version 1
   ggsn 1 no echo-interval
   apn special.access.net ggsn 1
   apn * ggsn 0

You may notice that APNs look like domain names – that’s because they can be,

If we owned the domain special.access.net we could set it to resolve to the GGSN IP we’re using for the special.access.net GGSN at, and instead of hardcoding the IP in our config use a DNS server (like to resolve these.

   gtp local-ip
   ggsn dynamic

But for now, in order to keep our config simple we’ll just configure the one GGSN (GGSN 0) and route all APNs to it:

   gtp local-ip
   ggsn 0 remote-ip
   ggsn 0 gtp-version 1
   ggsn 0 no echo-interval
   apn * ggsn 0


So the SGSN has it’s own connection to the HSS in order to authenticate subscribers.

Because GSM doesn’t employ Mutual Network Authentication on the SIM we can set the authentication policy on the SGSN to just allow anyone in with any SIM card and they’ll be able to attach and access packet data.

We can easily set this through the VTY:

    auth-policy accept-all

To enable authentication we’d need to setup the Subscriber in the HLR, like we did for CS only connections, and change the access mode to cs+ps in the HLR.

Then we can change our config to use a remote HLR for authentication,

auth-policy remote

A Word on Compression & Encryption

As the demand for traffic on GPRS & EDGE grew, there were still limitations on the bandwidth of the system.

To try and make the best of what’s available, header compression is available, similar to what we’ve seen with ROHC in VoLTE.

To learn more about setting up compression and encryption of the data, take a look in the Osmo-SGSN Manual.


Charging in mobile networks is a topic we could spend weeks on, but we’re not going to!

OsmoSGSN implements a simple CDR based charging mechanism that writes to a text file a simple CSV file with most importantly the IMSI and bytes in / out for each subscriber, that can be used to implement offline charging (Post paid) if required, and with some hacky scripts can even cut off sessions after reaching a certain amount of throughput (online charging aka pre-paid).

By adding the below to our config OsmoSGSN will write CDRs into /home/nick/sgsn.cdr every 60 seconds.

  cdr filename /home/nick/sgsn.cdr
  cdr interval 30

The complete Setup

Here’s a complete copy of my running config, you’ll obviously need to change the IP that I’m using to the IP you’re using for your server.


Getting TEID up with GTP Tunnels

If you’re using an GSM / GPRS, UMTS, LTE or NR network, there’s a good chance all your data to and from the terminal is encapsulated in GTP.

GTP encapsulates user’s data into a GTP PDU / packet that can be redirected easily. This means as users of the network roam around from one part of the network to another, the destination IP of the GTP tunnel just needs to be updated, but the user’s IP address doesn’t change for the duration of their session as the user’s data is in the GTP payload.

One thing that’s a bit confusing is the TEID – Tunnel Endpoint Identifier.

Each tunnel has a sender TEID and transmitter TEID pair, as setup in the Create Session Request / Create Session Response, but in our GTP packet we only see one TEID.

There’s not much to a GTP-U header; at 8 bytes in all it’s pretty lightweight. Flags, message type and length are all pretty self explanatory. There’s an optional sequence number, the TEID value and the payload itself.

So the TEID identifies the tunnel, but it’s worth keeping in mind that the TEID only identifies a data flow from one Network Element to another, for example eNB to S-GW would have one TEID, while S-GW to P-GW would have another TEID.

Each tunnel has two TEIDs, a sending TEID and a receiving TEID. For some reason (Minimize overhead on backhaul maybe?) only the sender TEID is included in the GTP header;

This means a packet that’s coming from a mobile / UE will have one TEID, while a packet that’s going to the same mobile / UE will have a different TEID.

Mapping out TIEDs is typically done by looking at the Create Session Request / Responses, the Create Session Request will have one TIED, while the Create Session Response will have a different TIED, thus giving you your TIED pair.

GSM with Osmocom: GPRS & Packet Data

So far we’ve focused on building a plain “2G” (voice and SMS only) network, which was all consumers expected twenty years ago.

As the number of users accessing the internet through DSL, Dial Up & ISDN grew, the idea of getting this data “on the go” became more appealing. TCP/IP was becoming the dominant standard for networking, the first 802.11 WiFi spec had recently been published and demand for mobile data was growing.

There’s a catch however – TCP/IP was never designed to be mobile.

An IP address exists in a single location.

(Disclaimer: While you can “move” a subnet by advertising itself out in a different location via BGP peering relationships with other operators, it’s cumbersome, can only be done per /24 or larger, and most importantly it’s painfully slow. IPv6 has MIPv6 which attempts to fix some of these points, but that’s outside of this scope.)

GPRS addressed the mobility issue by having a single fixed point the IP Address is assigned to (the Gateway GPRS Support Node), which encapsulates IP traffic to/from a mobile user into GTP Packet (GPRS Tunnelling Protocol), like GRE or any of the other common routing encapsulation protocols, allowing the traffic to be rerouted to different destinations as the users move from being served by one BTS to another BTS.

I’ve written about GTP here if you’d like to learn more.

So now we’ve got a method of encapsulating our data we’ve got to work out how to get that data out over the air.

BTS Time Slots

Way back when we were first setting up our BSC and adding our BTS(s) you will have configured timeslots for each BTS configured on your BSC.

Chances are if you’ve been following along with this tutorial, that you configured the first time slot (timeslot 0) as a CCCH+SDCCH4, meaning Common Control Channel and 4 standalone dedicated control channels, and all the subsequent timeslots (timeslot 1 – 7) as Traffic Channels (full rate) – TCH/F.

This works well if we’re only carrying voice, but to carry data we need timeslots to put the data traffic on.

For this we’ll re assign a timeslot we were using on our BSC as a voice traffic channel (TCH/F) as a PDCH – a Packet Data Channel.

This means that on the BSC your timeslot config will look something like this:

   timeslot 6
    phys_chan_config PDCH
    hopping enabled 0
   timeslot 7
    phys_chan_config PDCH
    hopping enabled 0

In the above example I’ve assigned two timeslots for Packet Data Channels,

The more timeslots you allocate for data, the more bandwidth available, but the fewer voice resources available.

(Most GSM networks today have few data timeslots as more recent RATs like 3G/4G are taking the data traffic, and GSM is used primarily for voice and low bandwidth M2M communications)


GPRS comes in two flavors, GPRS and EDGE.

GPRS (General Packet Radio Services) was the first of the two, standardised in R97, and allowed users to reach a downlink speeds of up to 171Kbps using GMSK on the air interface to encode the data.

Users quickly expected more speed, so EDGE (Enhanced Data rates for Global Evolution) was standardised, from a core perspective it was the same, but from a BTS / Air interface perspective it relied on 8PSK instead of GMSK allowed users to reach a blistering 384Kbps on the downlink.

These speeds are the theoretical maximums.

As the difference between GPRS and EDGE is encoding on the air interface, from a core perspective it’s treated the same way, however as our BTS gets all it’s brains from the BSC, we’ll need to specify if the BTS should use EDGE or GPRS it in the BSC’s BTS config.

BSC Config

On the BSC for each BTS we want to enable for packet data, we’ll need to define the parameters.

There’s two other values we’ll introduce when setting this up,

The first is NSEI – the Network Service Entity Identifier, which is the identifier of the BTS’s Packet Control Unit, like the cell identity.

The second value we’ll touch on is the BVCI – the BSSGP Virtual Connections Identifier, which is used for addressing between the BTS PCU and the SGSN.

bts 0
  gprs mode egprs
  gprs 11bit_rach_support_for_egprs 0
  gprs routing area 0
  gprs network-control-order nc0
  gprs cell bvci 2
  gprs cell timer blocking-timer 3
  gprs cell timer blocking-retries 3
  gprs cell timer unblocking-retries 3
  gprs cell timer reset-timer 3
  gprs cell timer reset-retries 3
  gprs cell timer suspend-timer 10
  gprs cell timer suspend-retries 3
  gprs cell timer resume-timer 10
  gprs cell timer resume-retries 3
  gprs cell timer capability-update-timer 10
  gprs cell timer capability-update-retries 3
  gprs nsei 101
  gprs ns timer tns-block 3
  gprs ns timer tns-block-retries 3
  gprs ns timer tns-reset 3
  gprs ns timer tns-reset-retries 3
  gprs ns timer tns-test 30
  gprs ns timer tns-alive 3
  gprs ns timer tns-alive-retries 10
  gprs nsvc 0 nsvci 101
  gprs nsvc 0 local udp port 23001
  gprs nsvc 0 remote udp port 23000
  gprs nsvc 0 remote ip

The OsmoBSC docs cover each of these values, they’re essentially defaults.

There are quite a few changes required on the BSC for each BTS we’re setting this up for. Instead of giving you info on what fields to change, here’s the diffs.

In the next post we’ll cover the GGSN and the SGSN and then getting a device on “the net”.

Why GTP for Mobile Networks?

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.