The Mobile Switching Center: Evolution From Pure Circuit to Full IP

If you have a couple of minutes, join me for a tour of the incredible evolution of the Mobile Switching Center for mobile voice calls from a circuit switched elephant to an IP based leopard. As much as possible, I've tried to use general terms and reduce the number of telecom acronyms to have a blog entry that is understandable to a wider audience. The purpose? To explain at the end why the circuit switched and the packet switched IP world should loose the fear from each other.

Only 10 years ago, the mobile world was a very different place. GPRS was only in the standardization phase and GSM networks were purely circuit switched voice networks. The core component was the Mobile Switching Center, the MSC. A single MSC was composed of several long aisles of cabinets which were full of circuit boards and cabling. Voice calls were circuit switched, 30 of them multiplexed over 2 MBit/s E1/T1 links and many of these links connected the MSC to other MSCs and with the GSM radio network. But if you had seen the earlier generation of voice switches that was definitely state of the art!

Fast forward to today: The MSC still exists and to the user it still behaves like it has 10 years ago. However, today's MSCs have not only completely changed the way they look but also how they work. The switching power of aisles of cabinets is now contained in a single cabinet and the physical circuit switching of 64 kbit/s channels has been virtualized into IP packets running through gigabit optical cables. In effect the MSC has become a powerful server that communicates with the outside world over IP and instead of connecting voice circuits physically in a switching matrix they are now virtually connected by routing IP packets from one IP address to another.

Since 3GPP Release 4, the MSC is logically and in many cases also physically separated into two different boxes with a standardized interface in between them:

The MSC-Server: This is the brain of the voice switching system if you want. When a mobile subscriber initiates a call, the mobile exchanges messages with the MSC-Server which then finds the destination party and signals the incoming call to it. If the destination is available the MSC-Server then establishes a voice connection between the two parties and the conversation can begin once the other side accepts the call. With the old MSC architecture, the Signaling System number 7 has been used for this purpose. Today, the MSC-Server is an IP based system and all messages between the MSC-Server and the radio access network pass an SS7 to IP gateway that exchanges the lower layer of the protocol stack so SS-7 messages can be transported over an IP connection. A mobile network usually contains many MSC-Servers at different physical locations and messages between them are directly transported over IP. Interested in how an MSC-Server looks like? Here's a cool 3D example.

The Media Gateway: The MSC-Server only sends messages through the system via IP links, it is not involved with actually setting up the speech path between two phones. This is the responsibility of the Media Gateway. The MSC-Server is connected to the Media Gateway via an IP connection and the protocol for establishing voice connections is called H.248 or Megaco (Media Gateway Control Protocol). Basically the MSC-Server uses these messages to tell the Media Gatway to connect a virtual or physical circuit coming in on one interface with a virtual or physical circuit on another. I call them virtual and physical because there are different kinds of interfaces. To the GSM radio network, a real physical circuit switched channel is used. Thousands of them are usually multiplexed into an optical link. IP is not used on this link. To the UMTS radio network, it's already a bit more virtual, as ATM (Asynchronuous Transfer Mode) links are used, usually again over an optical link. Also, no IP used here. To other Media Gateways in the network IP links are used and the protocol to transport the voice stream is called Real Time Transfer protocol (RTP). Interested how a Media Gateway looks like? Here's a cool 3D example.

Below I will use the term MSC system to describe the combination of MSC-Server and Media Gateway.

SIP

Speaking of the RTP protocol for sending a voice stream through an IP network. It's probably not a coincidence that the same protocol is also used by SIP, which is the protocol used for most of the voice over IP solutions that are in use today. SIP stands for Session Initiation Protocol and if the MSC System is enhanced to understand this protocol, it could also accept and forward voice calls to SIP lines. That's not done in practice today as far as I know but let's keep that in mind for the future.

IP based GSM and UMTS access

Today, the links in most wireless networks to the radio network for the call establishment signaling and the voice calls are not based on IP. However, there are now 3GPP standard enhancements to offer an IP alternative for these interfaces as well for both GSM and UMTS. As a consequence it is likely that over time, network operators will convert these links to IP, too. This could be part of a radio network backhaul convergence, where all data from base stations that support several radio technologies (e.g. GSM, UMTS and LTE) use the same IP based backhaul link instead of several different ones based on different lower layer standards as is mostly still the case today. In short, there is also a trend to replace the current circuit switched based radio network links with IP based connections.

Interconnection between different networks

To allow calls between different networks in the same country and between different countries, MSC-Servers and Media Gateways of different network operators are interconnected. Today, these interconnections are also still based on E-1 circuit switched interfaces and SS-7 signaling. However, as we go forward network operators will start replacing these legacy interfaces with IP links which saves the SS-7 to IP message converters mentioned above reduces cost as E-1 circuit links are much more expensive than using an IP interconnect.

VOLGA (Voice over LTE over Generic Access)

A special case is the LTE radio network. As it is fully IP based system it has no connection to the MSC system today. As reported recently, the VOLGA forum is about to change that as discussed here. Effectively, VOLGA introduces a gateway between the LTE network and the MSC system. To the MSC system, the gateway looks like a GSM BSC or a UMTS RNC. This way, no code has to be changed on the MSC system and the link can either be based on circuit switched or IP technology.

To the LTE network the VOLGA gateway just looks like an IP server and the communication between it and mobile devices flows transparently through the LTE core and radio networks. An already standardized interface is used to announce handovers to the gateway, so the LTE network can inform the VOLGA gateway and the MSC system that an ongoing session is to be handed over to a GSM cell. To the MSC Server, the handover is no different from a GSM/UMTS to GSM handover.

Circuit to Circuit, IP to IP

So when looking at this evolution from a slightly different angle one could say that the MSC looks like to each world what it would like to see. For the circuit switched world, the split MSC-Server and Media Gateway still looks like a circuit switched box. For the IP world, however, the MSC-Server and Media Gateway are just IP based devices which send and receive IP packets. For the voice stream, RTP is used, a protocol well known in the IP world. The call establishment signaling also uses IP packets but the protocol used is originally from the circuit switched world. It doesn't change the principle that it's now IP based but protocols like M3UA, SCCP, TCAP, MAP, etc. are not yet in the toolkit of IP engineers. But fear not, Wireshark and other well known IP tracing tools understand these protocols so they are as easy to trace and analyze as any other IP based protocol. The beauty of the approach is that old and new, circuit and IP can be mixed and matched for each voice call as required.

As we go into the future, more and more links connecting the MSC system will be converted from legacy circuit switched to IP and physical circuits are put into virtual circuits, embedded into RTP IP packets. At some point, some MSCs in the wireless network will only use IP based links between different servers, to the radio access network and even servers in another mobile network. This might still be a couple of years away but the standards are all in place and the changes are well underway.

So from my point of view there is still a lot of life left in the MSC architecture and it will serve us well into the future, possibly even in LTE networks unless a miracle happens and alternatives such as the IMS based system suddenly emerge from hibernation.