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Where Does SDN Start and End in 5G?

So, here’s the deal: we live in a world filled with acronyms, right? I think most of us are accustomed to this, and pretty much ok with this way of life. We can laugh about it at the water cooler, which doesn’t make it less painfully true, but at least we share the pain with our fellow human telco beings.

The problem, however, is that some of the acronyms mean real things and some don’t. And some are somewhere in between. I have lived through 2G, 3G and 4G and can quite comfortably talk about things like a BTS, an RNC, and a PGW. I can share a room with words like SIP, GTP and ISUP without getting uneasy. I know what they mean, not just what the letters stand for.

There have always been somewhat ambiguous words and then we’ve had to deal with that. The more they are, though, and the more important they are, the ambiguity rises to become actual real-life problems.

There are currently a few big revolutions happening in the world of telecommunications. If I mention Cloud, Virtualization, SDN and 5G, I’m quite sure you’ve at least heard these words before. But what do they actually mean? None of these words are in any way reserved or owned and their meaning is not controlled by any organization in the world.

  • 5G – Is it the 3GPP specifications or is it more of a concept/era/way of life/conglomerate of a multitude of technologies? Is it just a marketing label on evolved “old” technology?
  • Virtualization – Is it just hypervisors or are containers part of it? Is Dropbox virtual storage or not?
  • Cloud – Is it just… well, what the heck is it? I mean, really?
  • SDN – Aha, this is where I would like to focus your attention in this article.

One easy way to answer all of this would be to say: just take our Cloud, NFV and SDN” training, and life will be so much easier in all imaginable ways for you. And while that may be true, it would be such a short article, so with your permission I’ll elaborate a little more right here.

SDN means Software-Defined Networking

So we know that there’s software involved and some form of networking. Before I go on, let me just make it brain-numbingly clear that SDN is not a well-defined word. The meaning of the letters in the acronym are universally agreed-upon, it’s “Software-Defined Networking”, but what that exactly means is very much up for debate. Software could be any form of software, in a separate controller or built-into a device, and networking could be any form of network.

The software in SDN is usually taken to mean a controller computer program installed somewhere affecting the behavior in other nodes, someplace else. Networking is, again usually, taken to mean the routing of data packets in a packet data network such as a TCP/IP network.

A trivial picture of SDN could be this.

SDN - 1

A group of generic packet-forwarding boxes, centrally managed by a controller (that’s where the software in SDN is). The controller is making the boxes behave in such a way that they replicate the function of an old, old-fashioned, network (left side of the picture) where all packet forwarding was done with semi-static logic on specially designed hardware (switches and routers and firewalls etc.).

A different interpretation of SDN could be software in clever endpoints (the VMs in the picture) that set up tunnels to other clever endpoints. The tunnels are quite smart and flexible, and movable, but the underlying network doesn’t have to be.

SDN - 2


It is again the matter of software affecting the path that data packets take to traverse a network, but quite different from the first picture. While VMWare is very much into the second meaning of SDN (sometimes referred to as “Overlay Abstractions”), the Telco world has a history of the first.

You could argue that SDN was introduced with “split architecture” in 3GPP, where we replaced mobile telephony switches (MSCs) with brain-body pairs (MSC Server and Media Gateways). This evolution has continued and almost all data plane nodes now have a separate controller, be it MGC/MGW or MRFC/MRFP or IBCF/TrGW (I won’t go through these functions here, just pick your favorite example!).

In 5G the data plane comprises one or more User Plane Functions (UPFs). You could say they are the successor of the SGW/PGW combo in 4G (or SGSN/GGSN in 3G), only now you can have any number of them (higher than zero).  The UPF is controlled by something called a Session Management Function (or SMF).

So is SMF an SDN controller? Good question, thanks for asking!

There is a lot of talk about SDN in telco networks these days, but it is almost exclusively about evolving the underlying transport networks into something more flexible, and in the end, cheaper to run. This is the good old traditional cables/fibers/switches/routers that have connected the telco network functions. Because let me tell you: it has all been IP for quite a long time! (Some may think that voice-over-IP is a new cool Internet:y thing, and it is to consumers, but standard circuit-switched PSTN telephony has been carried inside the operators by IP transport networks for quite some time.)

Where does that leave us?

Well, a 5G operator with a number of these UPFs (remember, those are the big traffic handlers carrying user data from your phone to e.g. the Internet and back) will have transport networks that connect them. These transport networks will benefit a lot from being controlled in a flexible manner by an SDN controller. This would also mean the transport network has to be built with SDN-controllable devices, as opposed to stone age (relatively speaking, ok?)  switches and routers.

The answer to the question above, the one about whether the SMF is an SDN controller, is: yes! Or at least “The SMF can absolutely be called an SDN controller, and I can back this statement up with good reasoning and sound arguments”.

This means we have several levels of SDN in normal telco networks. Or perhaps you want to see it as SDN in several domains. We have the transport network, which is just now seeing an overhaul as a flood of SDN-ification spreads through the world. But then there’s also the higher layer SDN, which is present in 5G, but which was actually also present in 2G although nobody at the time referred to it as SDN.

The summary is that we have several SDN operations going on in most modern telco networks. The SMF/UPF is a standardized pair of controller/SDN device, and they will be connected over a transport network under non-standardized SDN control.

Maybe we will see an evolution where we centralize this and the SMF (or perhaps the 6G equivalent when that time comes?) becomes the controller for both higher and lower layer SDN-devices. But for now, that’s just not the case. Or maybe something else will happen?

As always: we live in interesting times.


Until next time,

The Apis IP-Solutions Team

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