GSM MSC Architecture

GSM with Osmocom Part 8: The Mobile Switching Center

We’re nearing the end of our “setup” story – So far we’ve covered the access network (BTS & BSC) and our subscriber database (The HLR) so now let’s talk about one of the key “Core” elements of the network – the Mobile Switching Center (MSC).

The MSC’s name kind of says it all, it’s a switching center for mobiles.

The MSC handles switching of voice calls and SMS/text messages between local & remote subscribers and networks.

Switching Function

Because GSM was designed to be voice centric (Keep in mind the first GSM network went live in 1991) the MSC’s primary function is switching phone calls between subscribers.* For this the MSC has to keep track of which subscribers it’s currently serving, their capabilities and how to reach them -which BSC they’re being served by and therefore which BTS they’re being served by.

The OsmoMSC also features a minimalistic SMSC (Short Message Service Server) for routing SMS traffic between subscribers on the network.
This basic SMSC acts in a store-and-forward fashion. Production networks would typically use an external SMSC for handling SMS, OsmoMSC has the SMSC functionality built in by default, but the interfaces are there if you wanted to use an external SMSC.

Any calls/texts to subscribers/destinations outside the MSC (for example a call to a mobile subscribers on a different carrier or on the PSTN) are typically routed to another MSC known as the Gateway MSC.
The GMSC handles the interconnection with other networks. We’ll touch upon this later with the SIP connector, but for now we’ll focus just on on-net calls between subscribers.

It’s worth noting that the MSC does not sit in the media stream, it just sets up and tears down the calls, we’ll cover more on the nitty-gritty of calling in GSM soon.

Visitor Location Register Function

The MSC also acts as the interface to the HLR for AAA, as we covered in our last post, the HLR provides the authentication role and also provides the subscriber data to the MSC. Subscriber data is copied from the HLR to the internal HLR cache on the MSC known as the Visitor Location Register (VLR) after a subscriber attaches.

Authentication, Ciphering and EIR Queries

In the last post we talked about the role of the HLR in terms of Authentication on the network, the authentication vectors but the policies that enforce this are set on the MSC.

The MSC queries the admission control info from the HLR, but it’s the MSC that’s responsible for enforcing these rules.

Core Network Identity

The MCC (Mobile Country Code) and MNC (Mobile Network Code) of the network (Together the MCC + MNC are referred to as the PLMN ID), along with the network name, are configured on the MSC.

While this may seem like a rather small detail, the PLMN ID is analogous to the SSID of a WiFi network – it’s what identifies your network out of all the others on the air, and the network name shows up on your phone when you’re connected showing your network name.

Setup & Connections

The BSC we setup earlier communicates with the MSC via SS7 Point Codes. We’ll go into how point codes route requests in a later post, but so long as you’re running Osmo-BSC, Osmo-MGW, Omso-MSC and Osmo-HLR on the same machine you won’t need to link them to each other like we had to do with adding our BTS to the BSC.

Instead we’ll just need to start everything required:

systemctl restart osmo-stp
systemctl restart osmo-hlr
systemctl restart osmo-mgw
systemctl restart osmo-msc

The GSUP connection between the MSC and the HLR will be established at startup, but BSCs will only establish a connection to the MSC when they need something from the MSC.

Once we’ve got everything started we can Telnet into the MSC to confirm it’s running and check it’s status:

root@gsm # telnet localhost 4254

Assuming you can connect that’s another network element online. – We’ll leave the default the Point Codes in place so the BSC will be able to connect to the MSC, but keep in mind that the BSC will only establish a connection when it needs something from the MSC.

Follow Up

There’s a few topics we skipped over in this topic, stuff like SS7/SIGTRAN, how real world GSM calls route using MNCC-SAP, the Media Gateway and anchoring media streams and what an SMSC does.

I’ll do posts covering each of these topics in more depth.

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