5G/NR - Converged Connectivity

 

 

 

Converged Connectivity - ATSSS/5WWC

Converged Network is a collective name for various technologies to utilize 5G core network for the data path (and some control plane in some cases) of non 3gpp communication as illustrated below. Final goal is to converge the core part of existing non-3gpp technology (e.g, WiFi or IPTV etc) to UPF / DN and in some cases use 5G AMF as well for the control plane.

Before going into each of the technologies, take a look at the illustration shown below. What you see in the upper part is nothing special. It is just the ordinary 5G core network drawn in the reference point representation : AMF, SMF, UPF and the other network functions (NSSF, NEF, NRF, PCF, UDM, AUSF, AF), and the numbers in the small ovals are the reference points between them (2 means N2, 3 means N3, 6 means N6 and so on). This part does not change at all when we talk about Converged Connectivity.

What is worth looking at is the bottom left part of the picture. This is where the user device gets attached to the network, and three different types of access are drawn there at the same time.

  • 5G UE (top row) : the ordinary case. The device has a 5G modem and it reaches the core network through the 5G RAN. Nothing new here.
  • 5G ATSSS UE (middle row) : the device has a 5G modem and a WiFi radio, and both of them can be connected at the same time - one path through the 5G RAN and the other path through a WLAN AP. This is the case that ATSSS is about.
  • 5WWC UE (bottom row) : a device that has no 3GPP radio at all. In this example it is an IPTV set-top box, and it is connected over a cable network through a DOCSIS RG (Residential Gateway). This is the case that 5WWC is about.

Now follow the three lines coming out of these access nodes and see where they end. This is the whole point of the picture.

  • For the user data, all three lines arrive at the same UPF over N3 (the three ovals labelled 3), and from that UPF the data goes out to the data network over N6 (the oval labelled 6). This is the thick red path marked as "Converged Data Path" in the picture.
  • For the control plane, the lines also climb up to the same AMF over N2, which means the subscriber can be authenticated and managed by the 5G core (AUSF, UDM, PCF) no matter which access it came from.

So, if I have to summarize the whole picture in one sentence : the access technology on the left can be anything (5G radio, WiFi, cable), but the core network on the right is always the 5G core. Everything else in this page - ATSSS, 5WWC, N3IWF, DOCSIS RG - is just the detail of how each access type is converted so that it can plug into that one common core.

Whatever the existing technology to be the candidate for the Converged Connectivity, all of them has the same goal, that is, getting connected to 5G UPF. However they need to develop different gateway that translate their existing access protocols to fit into 5G standard interfaces (N3, N9, N2). Not only for the standard interfacing, each of the technology (candidates) would employ additional technologies to faciliate further the combined connection of 5G and non-3GPP network.

ATSSS

ATSSS stands for Access Traffic Steering, Switching and Splitting. To me, ATSSS looks to be an envolved version of WiFi Offloading/ePDG that we used in LTE.  

What has been evolved in ATSSS comparing to WiFi Offload ? As the name implies, ATSSS can support much diverse technology to managing / coordinating data flow to increase throughput, robustness and reliability of the data traffic. In WiFi Offloading, only one possible scenario of data transfer was possible between 4G (3GPP) and WiFi (non 3GPP). It was one or the other, i.e, flowing data through LTE or WiFi. However in ATSSS, more diverse scenario is possible.

  • The UE maintain the connection to both 5G and WiFi and steer the data flow either through 5G or WiFi depending on situation ==> Steering
  • The UE can maintain the connection only to one of the access network (i.e, 5G or WiFi) at a specific moment and switch to another access network depending on situation ==> Switching
  • The UE maintain the connecto to both 5G and WiFi and flow the data through both channels ==> Splitting

To best use of this kind of capability, the device (and/or server) should support specical IP protocol called MPTCP (Multi Path TCP), implying that the success of ATSSS would depend on how soon MPTCP will be employed by various servers and UE IP stack.

Formal description of ATSSS is well summarized in 23.501 - 5.32.1 as follows. But the details of ATSSS is scattered around the whole 23.501.

The ATSSS feature enables a multi-access PDU Connectivity Service, which can exchange PDUs between the UE and a data network by simultaneously using one 3GPP access network and one non-3GPP access network and two independent N3/N9 tunnels between the PSA and RAN/AN. The multi-access PDU Connectivity Service is realized by establishing a Multi-Access PDU (MA PDU) Session, i.e. a PDU Session that may have user-plane resources on two access networks. This assumes both 3GPP access and non-3GPP access are allowed for the S-NSSAI of the PDU Session.

The overall structure of protocol stack of ATSSS are depicted in 23.501 as shown below.  As you see, the UE side protocol stack is very similar to ePDG stack for LTE / WiFi offloading and I think N3IWF of ATSSS is equivalent to ePDG of WiFi Offload.  Via N3IWF, the protocol is converted to 5G standard interface protocl N3 and eventually reaches 5G data plane UPF.

< 23.501-Figure 5.32.5.4-1: UE/UPF measurements related protocol stack for 3GPP access and for an MA PDU Session with type IP >

< 23.501-Figure 5.32.5.4-2: UE/UPF measurements related protocol stack for Untrusted non-3GPP access and for an MA PDU Session with type IP>

< 23.501-Figure 5.32.5.4-3: UE/UPF measurements related protocol stack for Trusted non-3GPP access and for an MA PDU Session with type IP >

5WWC

5WWC stands for 5G Wireless and Wireline Convergence. In one sentence, it is the work of letting a fixed (wireline) broadband access - the DSL, fiber or cable line coming into your house - attach to the 5G core network, so that the operator can run the fixed service and the mobile service on one single core.

To understand why anybody would want this, think about a typical operator. The same company very often owns two completely separate networks : a mobile network with its own core, and a fixed broadband network with its own (BNG based) core. Two networks means two subscriber databases, two policy systems, two charging systems, two operation teams and two sets of hardware, even though a large number of customers are exactly the same people. 5WWC is the answer to the question "why not use the 5G core for both ?".

The important point, and the one that is often misunderstood, is what is being converged. The access is not converged at all. The house is still connected by a DSL, PON or DOCSIS line, exactly as before, and no 5G radio is involved. What is converged is everything behind the access : the wireline access network is made to look like just another access network of the 5G system, so that the same AMF, SMF, UPF, UDM and PCF can serve it.

To make this possible, 3GPP defined a few new names in TS 23.316. They look complicated at first, but there are really only three things to remember.

  • RG (Residential Gateway) : the box in your home, i.e, the modem/router that terminates the fixed line. In the 5G world this box plays the role that a UE plays in the mobile world.
  • W-5GAN (Wireline 5G Access Network) : the whole wireline access network between the RG and the 5G core. Depending on who defines it, it comes in two flavours : the BBF (Broadband Forum) one for DSL/PON, and the cable one based on DOCSIS which is defined by CableLabs.
  • W-AGF (Wireline Access Gateway Function) : the most important new box. It sits at the border between the wireline access network and the 5G core, and it terminates N2 toward the AMF and N3 toward the UPF. In other words, from the point of view of the 5G core, the W-AGF looks exactly like a gNB. This is the trick that makes the whole thing work.

Now, one more thing that makes 5WWC a little bit tricky. There are two kinds of home gateway in the real world, and 5WWC has to support both of them.

  • 5G-RG : a new gateway that understands 5G NAS. It can register to the AMF and establish PDU Sessions by itself, just like a normal UE does. It simply uses a wire instead of a radio. (A 5G-RG may also be able to connect through NG-RAN, which is how Fixed Wireless Access fits into the same picture.)
  • FN-RG (Fixed Network RG) : an old gateway that is already installed in millions of homes and knows nothing about 5G. It cannot be upgraded, and an operator cannot ask every customer to replace it. So in this case the W-AGF performs the NAS signalling on behalf of the FN-RG. The old box keeps doing what it always did, and the network pretends to the 5G core that a proper 5G capable device is registering.

This is also the reason why the figures below come in pairs. Some of them show the stack for a 5G-RG (where NAS goes all the way to the RG) and some show it for an FN-RG (where NAS stops at the W-AGF).

Finally, it is worth comparing 5WWC with ATSSS, because the two are easy to mix up.

  • ATSSS is about one device using two accesses at the same time. The device is already a 5G UE, and the question is how to steer, switch or split its traffic between 5G and WiFi. It is mostly a user plane story.
  • 5WWC is about bringing a completely new access type into the 5G system. There is no traffic splitting here. The question is instead how the fixed line can be registered, authenticated, policed and charged by the 5G core, so the control plane matters as much as the user plane.

The specification work is also split between several organizations, which is why the references at the bottom of this page point to different places : 3GPP TS 23.316 defines the 5GS side, the Broadband Forum defines the DSL/PON access side, and CableLabs (WR-TR-5WWC-ARCH) defines the DOCSIS/cable side.

5GWWW is for converging Cable Modem based IP service (e.g, IPTV) to 5G core network and 5G data path. Since the type of DUT (assuming IPTV type of DUT) would not require much of interplay with 5G network, it wouldn't propose diverse type of flow control between non 3GPP and 3GPP data path, but 5WWC seems to pay more attenation to C Plane as well as U plane. With converging to 5G C plane core, it would be able to do subscriber management with 5G core.

< 23.316-Figure 6.2.2-1: Control Plane stack for W-5GAN for FN-RG>

< 23.316-Figure 6.3.1-1: User Plane stack for W-5GAN for 5G-RG>

< 23.316-Figure 6.3.2-1: User Plane stack for W-5GAN for FN-RG >

< WR-TR-5WWC-ARCH-V03-200618 : Figure 4 - Integration Model Control Plane Protocol Stacks for 5G Converged Network Architecture>

< WR-TR-5WWC-ARCH-V03-200618 : Figure 5 - User Plane Protocol Stacks for 5G Converged Network Architecture>

< WR-TR-5WWC-ARCH-V03-200618 : Figure 9 - High-Level Cable Modem Registration Message Flow >

Reference