The mobility management and connection management process in 5GC focuses on Connection Management (CM) and Registration Management (RM).
Registration Management (RM)
The Registration Management state (RM) of a UE can either be RM-Registered or RM-Deregistered. This is akin to the EMM state used in LTE.
RM-Deregistered Mode
From the Core Network’s perspective (Our AMF) a UE that is in RM-Deregistered state has no valid location information in the AMF for that UE. The AMF can’t page it, it doesn’t know where the UE is or if it’s even turned on.
From the UE’s perspective, being in RM-Deregistered state could mean one of a few things:
UE is in an area without coverage
UE is turned off
SIM Card in the UE is not permitted to access the network
In short, RM-Deregistered means the UE cannot be reached, and cannot get any services.
RM-Registered Mode
From the Core Network’s perspective (the AMF) a UE in RM-Registered state has sucesfully registered onto the network.
The UE can perform tracking area updates, period registration updates and registration updates.
There is a location stored in the AMF for the UE (The AMF knows at least down to a Tracking Area Code/List level where the UE is).
The UE can request services.
Connection Management (CM)
Connection Management (CM) focuses on the NAS signaling connection between the UE and the AMF.
To have a Connection Management state, the Registration Management procedure must have successfully completed (the UE being in RM-Registered) state.
A UE in CM-Connected state has an active signaling connection on the N1 interface between the UE and the AMF.
CM-Idle Mode
In CM-Idle mode the UE has no active NAS connection to the AMF.
UEs typically enter this state when they have no data to send / recieve for a period of time, this conserves battery on the UE and saves network resources.
If the UE wants to send some data, it performs a Service Request procedure to bring itself back into CM-Connected mode.
If the Network wants to send some data to the UE, the AMF sends a paging request for the UE, and upon hearing it’s identifier (5G-S-TMSI) on the paging channel, the UE performs the Service Request procedure to bring itself back into CM-Connected mode.
CM-Connected Mode
In CM-Connected mode the UE has an active NAS connection with the AMF over the N1 interface from the UE to the AMF.
When the access network (The gNodeB) determines this state should change (typically based on the UE being idle for longer than a set period of time) the gNodeB releases the connection and the UE transitions to CM-Idle Mode.
To make more Money (This post, congratulations, you’re reading it!)
Because they have to (Regulatory compliance, insurance, taxes, etc) – That’s the next post
So let’s look at SA in this context.
5G-SA can drive new revenue streams
We (as an industry) suck at this.
Last year on the Telecoms.com podcast, Scott Bicheno made the point that if operators took all the money they’d gambled (and lost) on trying to play in the sports rights, involvement in media companies, building their own streaming apps, attempts at bundling other utilities, digital identity, etc, and just left the cash in the bank and just operated the network, they’d be better off.
Uber, Spotify, “OTTs”, etc, utilize MNOs to enable their services, but operators don’t see this extra revenue. While some operators may talk of “fair share” the truth is, these companies add value to our product (connectivity) which as an industry, we’ve failed to add ourselves.
If the Metaverse does turn out to be a cash cow, it is unlikely the telecommunications industry will be the ones milking it.
Claim: Customers are willing to pay more for 5G-SA
This myth seems to be fairly persistent, but with minimal data to support this claim.
While BSS vendors talk about “5G Monetization”, the truth is, people use their MNO to provide them connectivity. If the coverage is adequate, and the speed enough to do what they need to do, few would be willing to pay any additional cash each month to see higher numbers on a speedtest result (enabled by 5G-NSA) and even fewer would pay extra cash for, well, whatever those features only enabled by 5G-Standalone are?
With most consumers now also holding onto their mobile devices for longer periods of time, and with interest rates reining in consumer spending across the board, we are seeing the rise of a more cost conscious consumer than ever before. If we want to see higher ARPUs, we need to give the consumer a compelling reason to care and spend their cash, beyond a speed test result.
We talk a little about APIs lower down in the post.
Claim: Users want Ultra-Low Latency / High Reliability Comms that only 5G-SA delivers
Wanting to offer a product to the market, is not the same as the market wanting a product to consume.
Telecom operators want customers to want these services, but customer take up rates tell a different story. For a product like this to be viable, it must have a wide enough addressable market to justify the investment.
Reliability
The URLCC standards focus on preventing packet loss, but the world has moved on from needing zero packet loss.
The telecom industry has a habit of deciding what customers want without actually listening. When a customer talks about wanting “reliable” comms, they aren’t saying they want zero packet loss, but rather fewer dropouts or service flaps. For us to give the customer what they are actually asking for involves us expanding RAN footprint and adding transmission diversity, not 5G-SA.
The “protocols of the internet” (TCP/IP) have been around for more than 50 years now.
These protocols have always flowed over transport links with varied reliability and levels of packet loss.
Thanks to these error correction and retransmission techniques built into these protocols, a lost packet will not interrupt the stream. If your nuclear command and control network were carried over TCP/IP over the public internet (please don’t do this), a missing packet won’t lead to worldwide annihilation, but rather the sender will see the receiver never acknowledged the receipt of the packet at the other end, and resend it, end of.
If you walk into a hospital today, you’ll find patient monitoring devices, tracking the vital signs for patients and alerting hospital staff if a patient’s vital signs change. It is hard to think of more important services for reliability than this.
And yet they use WiFi, and have done for a long time, if a packet is lost on WiFi (as happens regularly) it’s just retransmitted and the end user never knows.
Autonomous cars are unlikely to ever rely on a 5G connection to operate, for the simple reason that coverage will never be 100%. If your car stops because you’re in a not-spot, you won’t be a happy customer. While plenty of cars have cellular modems in them, that are used to upload telemetry data back to the manufacturer, but not to drive the car.
One example of wireless controlled vehicles in the wild is autonomous haul trucks in mines. Historically, these have used WiFi for their comms. Mine sites are often a good fit for Private LTE, but there’s nothing inherent in the 5G Standalone standard that means it’s the only tool for the job here.
Slicing
Slicing is available in LTE (4G), with an architecture designed to allow access to others. It failed to gain traction, but is in networks today.
The RAN a piece of the latency puzzle here, but it is just one piece of the puzzle.
If we look at the flow a packet takes from the user’s device to the server they want to talk to we’ve got:
Time it takes the UE to craft the packet
Time it takes for the packet to be transmitted over the air to the base station
Time it takes for the packet to get through the RAN transmission network to the core
Time it takes the packet to traverse the packet core
Time it takes for the packet to get out to transit/peering
Time it takes to get the packet from the edge of the operators network to the edge of the network hosting the server
Time it takes the packet through the network the server is on
Time it takes the server to process the request
The “low latency” bit of the 5G puzzle only involves the two elements in bold.
If you’ve got to get from point A to point B along a series of roads, and the speed limit on two of the roads you traverse (short sections already) is increased. The overall travel time is not drastically reduced.
I’m lucky, I have access to a well kitted out lab which allows me to put all of these latency figures to the test and provide side by side metrics. If this is of interest to anyone, let me know. Otherwise in the meantime you’ll just have to accept some conjecture and opinion.
You could rebut this talking about Edge Compute, and having the datacenter at the base of the tower, but for a number of fairly well documented reasons, I think this is unlikely to attract widespread deployment in established carrier networks, and Intel’s recent yearly earning specifically called this out.
Claim: Customers want APIs and these needs 5G SA
Companies like Twilio have made it easy to interact with the carrier network via their APIs, but yet again, it’s these companies producing the additional value on a service operated by the MNOs.
My coffee machine does not have an API, and I’m OK with this because I don’t have a want or need to interact with it programatically.
By far, the most common APIs used by businesses involving telco markets are APIs to enable sending an SMS to a user.
These have been around for a long time, and the A2P market is pretty well established, and the good news is, operators already get a chunk of this pie, by charging for the SMS.
Imagine a company that makes medical booking software. They’re a tech company, so they want their stack to work anywhere in the world, and they want to be able to send reminder SMS to end users.
They could get an account manager with each of the telcos in each of the markets they work in, onboard and integrate the arcane complexities of each operators wholesale SMS system, or they could use Twilio or a similar service, which gives them global reach.
Often the cost of services like Twilio are cheaper than working directly with the carriers in each market, and even if it is marginally more expensive, the cost savings by not having to deal with dozens of carriers or integrate into dozens of systems, far outweighs this.
While it’s a great idea, in the context of 5G Standalone and APIs, it’s worth noting that none of the use cases in OpenGateway require 5G Standalone (Except possibly Edge discovery, but it is debatable).
Critically, from a developer experience perspective:
I can sign up to services like Twilio without a credit card, and start using the service right away, with examples in my programming language of choice, the developer user experience is fantastic.
Jump on the OpenGateway website today and see if you can even find a way to sign up to use the service?
Claim: Fixed Wireless works best with 5G-SA
Of all the touted use cases and applications for 5G, Fixed Wireless (FWA) has been the most successful.
The great thing about FWA on Cellular networks is you can use the same infrastructure you use for your mobile customers, and then sell excess capacity in the network to deliver Fixed Wireless Access services, better utilizing an asset (great!).
But again, this does not require Standalone 5G. If you deploy your FWA network using 5G SA, then you won’t be able to sweat that same asset for both mobile subscribers and FWA subscribers.
Today at least, very few handsets short of this generation of flagship phones, supports 5G SA. Even the phones sold as supporting 5G over the past few years, are almost all only supporting 5G-NSA, so if you rolled out your FWA network as Standalone, you can’t better utilize the asset by sharing with your existing LTE/5G-NSA customers.
Claim: The Killer App is coming for 5G and it needs 5G SA
This space is reserved for the killer app that requires 5G Standalone.
Whenever that comes?
Anyone?
I’m not paying to build a marina berth for my mega yacht, mostly because I don’t have one. Ditto this.
Could you explain to everyone on an investor call that you’re investing in something where the vessel of the payoff isn’t even known to exist? Telecom is “blue chip”, hardly speculative.
The Future for Revenue Growth?
Maybe there isn’t one.
I know it’s an unthinkable thought for a lot of operators, but let’s look at it rationally; in the developed world, everyone who wants a mobile service already has one.
This leaves operators with two options; gaining market share from their competitors and selling more/higher priced services to existing customers.
You don’t steal away customers from other operators by offering a higher priced product, and with reduced consumer spending people aren’t queuing up to spend more each month.
But there is a silver lining, if you can’t grow revenues, you can still shrink expenditure, which in the end still gets the same result at the end of the quarter – More cash.
Simplify your operations, focus on what you do really well (mobile services), the whole 80/20 rule, get better at self service, all that guff.
There’s no shortage of pain points for consumers telecom operators could address, to make the customer experience better, but few that include the word Slicing.
No one spends marketing dollars talking about the problems with a tech and vendors aren’t out there promoting sweating existing assets. But understanding your options as an operator is more important now than ever before.
Sidebar; This post got really long, so I’m splitting it into 3…
We’re often asked to help define a a 5G strategy for operators; while every case is different, there’s a lot of vendors pushing MNOs to move towards 5G standalone or 5G-SA.
I’m always a fan of playing “devil’s advocate“, and with so many articles and press releases singing the praises of standalone 5G/5G-SA, so as a counter in this post, I’ll be making the case against the narratives presented to operators by vendors that the “right” way to do 5G is to introduce 5G Standalone, that they should all be “upgrading” to Standalone 5G.
With Mobile World Congress around the corner, now seems like a good time to put forward the argument against introducing 5G Standalone, rebutting some common claims about 5G Standalone operators will be told. We’ll counterpoint these arguments and I’ll put forward the case for not jumping onto the 5G-SA bandwagon – just yet.
On a personal note, I do like 5G SA, it has some real advantages and some cool features, which are well documented, including on this blog. I’m not looking to beat up on any vendors, marketing hype or events, but just to provide the “other side” of the equation that operators should consider when making decisions and may not be aware of otherwise. It’s also all opinion of course (cited where possible), but if you’re going to build your network based on a blog post (even one as good as this) you should probably reconsider your life choices.
Some Arcane Detail: 5G Non-Standalone (NSA) vs Standalone (SA)
5G NSA (Non Standalone) uses LTE (4G) with an additional layer “bolted on” that uses 5G on the radio interface to provide “5G” speeds to users, while reusing the existing LTE (Evolved Packet Core) core and VoLTE for voice / SMS.
From an operator perspective there is almost no change required in the network to support NSA 5G, other than in the RAN, and almost all the 5G networks in commercial use today use 5G NSA.
5G NSA is great, it gives the user 5G speeds for users with phones that support it, with no change to the rest of the network needed.
Standalone 5G on the other hand requires an a completely new core network with all the trimmings.
While it is possible to handover / interwork with LTE/4G (Inter-RAT Handovers), this is like 3G/4G interworking, where each has a different core network. Introducing 5G standalone touches every element of the network, you need new nodes supporting the new standards for charging, policy, user plane, IMS, etc.
Scope
There’s an old adage that businesses spend money for one of three reasons:
To Save Money (Which we’ll cover in this post)
To make more Money (Covered next – Will link when published)
Because they have to (Regulatory compliance, insurance, taxes, etc)
Let’s look at 5G Standalone in each of these contexts:
5G Cost Savings – Counterpoint: The cost-benefit doesn’t stack up
As an operator with an existing deployed 4G LTE network, deploying a new 5G standalone network will not save you money.
From an capital perspective this is pretty obvious, you’re going to need to invest in a new RAN and a new core to support this, but what about from an opex perspective?
Claim: 5G RAN is more efficient than 4G (LTE) RAN
Spectrum is both finite and expensive, so MNOs must find the most efficient way to use that spectrum, to squeeze the most possible value out of it.
In rough numbers, we can say we get 5x the spectral efficiency by moving from 3G to 4G. This means we can carry 5.2x more with the same spectrum on 4G than we can on 3G – A very compelling reason to upgrade.
The like-for-like spectral efficiency of 5G is not significantly greater than that of LTE.
In numbers the same 5Mhz of spectrum we refarmed from UMTS (3G) to 4G (LTE) provided a 5x gain in efficiency to deliver 75Mbps on LTE. The same configuration refarmed to 5G-NR would provide 80Mbps.
Refarming spectrum from 4G (LTE) to 5G (NR) only provides a 6% increase in spectral efficiency.
While 6% is not nothing, if refarmed to a 5G standalone network, the spectrum can no longer be used by LTE only devices (Unless Dynamic Spectrum Sharing is used which in itself leads to efficiency losses), which in itself reduces the efficiency and would add additional load to other layers.
The crazy speeds demonstrated by 5G are not due to meaningful increases in efficiency, but rather the ability to use more spectrum, spectrum that operators need to purchase at auction, purchase equipment to utilize and pay to run.
Claim: 5G Standalone Core is Cheaper to operate as it is “Cloud Native”
It has been widely claimed that the shift for the 5G Core Architecture to being “Cloud Native” can provide cost savings.
Operators should regard this in a skeptical manner; after all, we’ve been here before.
Did moving from big-iron to VNFs provide the promised cost savings to operators?
For many operators the shift from hardware to software added additional complexity to the network and increased the headcount to support this.
What were once big-iron appliances dedicated to one job, that sat in the corner and chugged away, are now virtual machines (VNFs). Many operators have naturally found themselves needing a larger team to manage the virtual environment, compared to the size of the team they needed to just to plug power and data into a big box in an exchange before everything was virtualized.
Introducing a “Cloud Native” Kubernetes layer on top of the VNF / virtualization layer, on top of the compute layer, leaves us with a whole lot of layers. All of which require resources to be maintain, troubleshoot and kept running; each layer having associated costs for staffing, licensing and support.
Many mid size enterprises rushed into “the cloud” for the promised cost savings only to sheepishly admit it cost more than the expected.
Almost none of the operators are talking about running these workloads in the public cloud, but rather “Private Clouds” built on-premises, using “Cloud Native” best practices.
One of the central arguments about cloud revolves around “elastic scaling” where the network can automatically scale to match demand; think extra instances spun up a times of peak demand and shut down when the demand drops.
I explain elastic scaling to clients as having to move people from one place to another. Most of the time, I’m just moving myself, a push bike is fine, or I’ve got a 4 seater car, but occasionally I’ll need to move 25 people and for that I’d need a bus.
If I provide the transportation myself, I need to own a bike, a car and a bus.
But if use the cloud I can start with the push bike, and as I need to move more people, the “cloud” will provide me the vehicle I need to move the people I need to move at that moment, and I’ll just pay for the time I need the bus, and when I’m done needing the bus, I drop back to the (cheaper) push bike when I’m not moving lots of people.
While telecom operators are going to provide the servers to run this in “On-prem-cloud”, they need to dimension for the maximum possible load. This means they need to own a bike/car/bus, even if they’re not using it most of the time, and there’s really no cost savings to having a bus but not using it when you’re not paying by the hour to hire it.
Infrastructure aside, introducing a Standalone 5G Core adds another core network to maintain. Alongside the Circuit Switched Core (MSC/GGSN/SGSN) serving 2G/3G subscribers, Evolved Packet Core serving 4G (LTE) and 5G-NSA subscribers, adding a 5G Standalone Core to for the 5G-SA subscribers served by the 5G SA cells, is going to be more work (and therefore cost).
While the majority of operators have yet to turn off their 2G/3G core networks, introducing another core network to run in parallel is unlikely to lead to any cost savings.
Claim: Upgrading now can save money in the Future / Future Proofing
Life cycles of telecommunications are two fold, one is the equipment/platform life cycle (like the RAN components or Core network software being used to deliver the service) the other is the technology life cycle (the generation of technology being used).
The technology lifecycles in telecommunications are vastly longer than that for regular tech.
GSM (2G) was introduced into the UK in 1991, and will be phased out starting in 2033, a 42 year long technology life cycle.
No vendor today could reasonably expect the 5G hardware you deploy in 2024 to still be in production in 2066 – The platform/equipment life cycle is a lot shorter than the technology life cycle.
Operators will to continue relying on LTE (4G) well into the late 2030s.
I’d wager that there is not a single piece of equipment in the Vodafone UK GSM network today, that was there in 1991. I’d go even further to say that any piece of equipment in the network today, didn’t even replace the 1991 equipment, but was probably 3 or 4 generations removed from the network built in 1991.
For most operators, RAN replacements happen between 4 to 7 years, often with targeted augmentation / expansion as needed in the form of adding extra layers / sectors between these times.
The question operators should be asking is therefore not what will I need to get me through to 2066, but rather what will I need to get to 2030?
The majority of operators outside the US today still operate a 2G or 3G network, generally with minimal bandwidth to support legacy handsets and devices, while the 4G (LTE) network does most of the heavy lifting for carrying user traffic. This is often with the aid of an additional 5G-NSA (Non-Standalone) layer to provide additional capacity.
Is there a cost saving angle to adding support for 5G-Standalone in addition to 2G/3G/4G (LTE) and 5G (Non-Standalone) into your RAN?
A logical stance would be that removing layers / technologies (such as 2G/3G sunsetting) would lead to cost savings, and adding a 5G Standalone layer would increase cost.
All of the RAN solutions on the market today from the major vendors include support for both Standalone 5G and Non Standalone, but the feature licensing for a non-standalone 5G is generally cheaper than that for Standalone 5G.
The question operators should be asking is on what timescale do I need Standalone 5G?
If you’ve rolled out 5G-NSA today, then when are you looking to sunset your LTE network? If the answer is “I hope to have long since retired by that time”, then you’ve just answered that question and you don’t need to licence / deploy 5G-SA in this hardware refresh cycle.
Other Cost Factors
Roaming: The majority of roaming traffic today relies on 2G/3G for voice. VoLTE roaming is (finally) starting to establish a foothold, but we are a long way from ubiquitous global roaming for LTE and VoLTE, and even further away for 5G-SA roaming. Focusing on 5G roaming will enable your network for roaming use by a miniscule number of operators, compared to LTE/VoLTE roaming which covers the majority of the operators in the developed world who can utilize your service.
I decided to split this into 3 posts, next I’ll post the “5G can make us more money” post and finally a “5G because we have to” post. I’ll post that on LinkedIn / Twitter / Mailing list, so stick around, and feel free to trash me in the comments.
One of the hyped benefits of a 5G Core Networks is that 5GC can be used for wired networks (think DSL or GPON) – In marketing terms this is called “Wireless Wireline Convergence” (5G WWC) meaning DSL operators, cable operators and fibre network operators can all get in on this sweet 5GC action and use this sexy 5G Core Network tech.
This is something that’s in the standards, and that the big kit vendors are pushing heavily in their marketing materials. But will it take off? And should operators of wireline networks (fixed networks) be looking to embrace 5GC?
Comparing 5GC with current wireline network technologies isn’t comparing apples to apples, it’s apples to oranges, and they’re different fruits.
At its heart, the 3GPP Core Networks (including 5G Core) address one particular use cases of the cellular industry: Subscriber mobility – Allowing a customer to move around the network, being served by different kit (gNodeBs) while keeping the same IP Address.
The most important function of 5GC is subscriber mobility.
This is achieved through the use of encapsulating all the subscriber’s IP data into a GTP (A protocol that’s been around since 2G first added data).
Do I need a 5GC for my Fixed Network?
Wireline networks are fixed. Subscribers don’t constantly move around the network. A GPON customer doesn’t need to move their OLT every 30 minutes to a new location.
Encapsulating a fixed subscriber’s traffic in GTP adds significant processing overhead, for almost no gain – The needs of a wireline network operator, are vastly different to the needs of a cellular core.
Today, you can take a /24 IPv4 block, route it to a DSLAM, OLT or CMTS, and give an IP to 254 customers – No cellular core needed, just a router and your access device and you’re done, and this has been possible for decades. Because there’s no mobility the GTP encapsulation that is the bedrock for cellular, is not needed.
Rather than routing directly to Access Network kit, most fixed operators deploy BRAS systems used for fixed access. Like the cellular packet core, BRAS has been around for a very long time, with a massive install base and a sea of engineering experience in house, it meets the needs of the wireline industry who define its functions and roles along with kit vendors of wireline kit; the fixed industry working groups defined the BRAS in the same way the 3GPP and cellular industry working groups defined 5G Core.
I don’t forsee that we’ll see large scale replacement of BRAS by 5GC, for the same reason a wireless operator won’t replace their mobile core with a BRAS and PPPoE – They’re designed to meet different needs.
All the other features that have been added to the 3GPP Core Network functionality, like limiting speed, guaranteed throughput bearers, 5QI / QCI values, etc, are addons – nice-to-haves. All of these capabilities could be implemented in wireline networks today – if the business case and customer demand was there.
But what about slicing?
With dropping ARPUs across the board, additional services relating to QoS (“Network Slicing”) are being held up as the saving grace of revenues for cellular operators and 5G as a whole, however this has yet to be realized and early indications suggest this is not going to be anywhere near as lucrative as previously hoped.
What about cost savings?
In terms of cost-per-bit of throughput, the existing install base wireline operators have of heavy-metal kit capable of terabit switching and routing has been around for some time in fixed world, and is what most 5G Cores will connect to as their upstream anyway, so there won’t be any significant savings on equipment, power consumption or footprint to be gained.
Fixed networks transport the majority of the world’s data today – Wireline access still accounts for the majority of traffic volumes, so wireline kit handles a higher magnitude of throughput than it’s Packet Core / 5GC cousins already.
Cutting down the number of parts in the network is good though right?
If you’re operating both a Packet Core for Cellular, and a fixed network today, then you might think if you moved from the traditional BRAS architecture fore the wired network to 5GC, you could drop all those pesky routers and switches clogging up your CO, Exchanges and Data Centers.
The problem is that you still need all of those after the 5GC to be able to get the traffic anywhere users want to go. So the 5GC will still need all of that kit, all your border routers and peering routers will remain unchanged, as well as domestic transmission, MPLS and transport.
The parts required for operating fixed networks is actually pretty darn small in comparison to that of 5GC.
TL;DR?
While cellular vendors would love to sell their 5GC platform into fixed operators, the premise that they are willing to replace existing BRAS architectures with 5GC, is as unlikely in my view as 5GC being replaced by BRAS.
The Network Repository Function plays matchmaker to all the elements in our 5G Core.
For our 5G Service-Based-Architecture (SBA) we use Service Based Interfaces (SBIs) to communicate between Network Functions. Sometimes a Network Function acts as a server for these interfaces (aka “Service Producer”) and sometimes it acts as a client on these interfaces (aka “Service Consumer”).
For service consumers to be able to find service producers (Clients to be able to find servers), we need a directory mechanism for clients to be able to find the servers to serve their needs, this is the role of the NRF.
With every Service Producer registering to the NRF, the NRF has knowledge of all the available Service Producers in the network, so when a Service Consumer NF comes along (Like an AMF looking for UDM), it just queries the NRF to get the details of who can serve it.
Basic Process – NRF Registration
In order to be found, a service producer NF has to register with the NRF, so the NRF has enough info on the service-producer to be able to recommend it to service-consumers.
This is all the basic info, the Service Based Interfaces (SBIs) that this NF serves, the PLMN, and the type of NF.
The NRF then stores this information in a database, ready to be found by SBI Service Consumers.
This is achieved by the Service Producing NF sending a HTTP2 PUT to the NRF, with the message body containing all the particulars about the services it offers.
Simplified example of an SMSc registering with the NRF in a 5G Core
Basic Process – NRF Discovery
With an NRF that has a few SBI Service Producers registered in it, we can now start querying it from SBI Service Consumers, to find SBI Service Producers.
The SBI Service Consumer looking for a SBI Service Producer, queries the NRF with a little information about itself, and the SBI Service Producer it’s looking for.
For example a SMF looking for a UDM, sends a request like:
Previous generations of core mobile network, would only allocate a single IP address per UE (Well, two if dual-stack IPv4/IPv6 if you want to be technical). But one of the cool features in 5GC is the support for Framed Routing natively.
You could do this on several EPC platforms on LTE, but it’s support was always a bit shoe-horned in, and the UE was not informed of the framed addresses.
If you’ve worked in a wireline ISP you’re probably familiar with the concept of framed routing already, in short it’s one or more static routes, typically returned from a AAA server (Normally RADIUS) that are then routed to the subscriber.
Each subscriber gets allocated an IP by the network, but other IPs can also be routed to the subscriber, based on the network and CIDR mask.
So let’s say we allocate a public IP of 1.2.3.4/32 to our subscriber, but our subscriber is a fixed-wireless user running a business and they want a extra public IP Addresses.
How do we do this? With Framed Routing.
Now in our UDM we can add a “Framed IP”, and when the SMF sets up a session for our subscriber, the extra networks specified in the framed routes will get routed to that UE.
If we add 203.176.196.0/30 in our UDM for a subscriber, when the subscriber attaches the UPF will be setup to forward traffic to 1.2.3.4/32 and also traffic to 203.176.196.0/30 to the UE.
Update: I previously claimed: Best of all this is signaled to the UE during the attach, so the UE is say a router, it becomes aware of the Framed IPs allocated to it. This is incorrect! Thanks to Anonymous Telco Engineer from an Anonymous Nordic Country for pointing this out, it is not signaled to the UE.
More info in 3GPP TS 23.501 section 5.6.14 Support of Framed Routing.
Reflective QoS is a clever new concept introduced in 5G SA networks.
The concept is rather simple, apply QoS in the downlink, and let the UE reply using the QoS in the uplink.
So what is Reflective QoS? If I send an ICMP ping request to a UE with a particular QoS Flow setup on the downlink, if Reflective QoS is enabled, the ICMP reply will have the same QoS applied on the uplink. Simple as that.
The UE looks at the QoS applied on the downlink traffic, and applies the same to the uplink traffic.
Let’s take another example, if a user starts playing an online game, and the traffic to the user (Downlink) has certain QoS parameters set, if Reflective QoS is enabled, the UE builds rules based on the incoming traffic based on the source IP / port / protocol of the traffic received, and the QoS used on the downlink, and applies the same on the uplink.
But actually getting Reflective QoS enabled requires a few more steps…
Reflective QoS is enabled on a per-packet basis, and is indicated by the UPF setting the Reflective QoS Indication (RQI) bit in the encapsulation header next to the QFI (This is set in the GTP header, as an extension header, used on the N3 and N9 reference points).
But before this is honored, a few other parameters have to be setup.
A Reflective QoS Timer (RQ Timer) has to be set, this can be done during the PDU Session Establishment, PDU Session Modification procedure, or set to a default value.
SMF has to set Reflective QoS Attribute (RQA) on the QoS profile for this traffic on the N2 reference point towards gNodeB
SMF must instruct UPF to use uplink reflective QoS by generating a new UL PDR for this SDF via the N4 reference point
When these requirements have been met, the traffic from the UPF to the gNodeB (N3 reference point) has the Reflective QoS Indication (RQI) bit in the encapsulation header, which is encapsulated and signaled down to the UE, which builds a rule based on the received IP source / port / protocol, and sends responses using the same QoS attributes.
Like in EPS / LTE, there are two ways to send SMS in Standalone 5G Core networks.
SMS over IMS or SMS over NAS – Both can be used on the same network, or just one, depending on operator preferences.
SMS over IMS in 5G
SMS over IMS uses the IMS network to send SMS. SIP MESSAGE methods are used to deliver SMS between users. While most operators have deployed IMS for 4G/LTE subscribers to use VoLTE some time ago, there are some changes required to the IMS architecture to support VoNR (Voice over New Radio) on the carrier side, and support for VoNR in commercial devices is currently in its early stages. Because of this many 5G devices and networks do not yet support SMS over IMS.
I’ve read in some places that RCS – The GSMA’s Rich Communications Service will replace SMS in 5GC. If this is the case, it reflected in any of the 3GPP standards.
SMS over NAS
To make a voice call on a device or network that does not support VoNR, EPS (VoLTE) fallback is used. This means when making or receiving a call, the UE drops from the 5G RAN to using a 4G (LTE) basd RAN, and then uses VoLTE to make the call the same as it would when connected to 4G (LTE) networks, because it is connected to a 4G network. This works technically, but is not the prefered option as it adds extra signaling and complexity to the network, and delays in the call setup, and it’s expected operators will eventually move to VoNR,but works as a stop-gap measure.
But mobile networks see a lot of SMS traffic. If every time an SMS was sent the UE had to rely on EPS fallback to access IMS, this would see users ping-ponging between 4G and 5G every time they sent or received an SMS.
5GC reintroduces the SMS-over-NAS feature, allowing the SMS messages to be carried over NAS messaging on the N1 interface. Voice calls may still require fallback to EPS (4G) to make calls over VoLTE, but SMS can be carried over NAS messaging, minimizing the amount of Inter-RAT handovers required.
The Nsmsf_SMService
For this a new Service Based Interface is introduced between the AMF and the SMSF (SMS Function, typically built into an SMSc), via the N20 / Nsmsf SBI to offer the Nsmsf_SMService service.
There are 3 operations supported for the Nsmsf_SMService:
Active – Initiated by the AMF – Used to active the SMS service for a given subscriber,
Deactivate – Initiated by the AMF – Used to deactivate the SMS over NAS service for a given subscriber.
UplinkSMS – Initiated by the AMF to transfer the SMS payload towards the SMSF.
The UplinkSMS is a HTTP post from the AMF with the SUPI in the Request URI and the request body containing a JSON encoded SmsRecordData.
Astute readers may notice that’s all well and good, but that only covers Mobile Originated (MO) SMS, what about Mobile Terminated (MT) SMS?
Well that’s actually handled by a totally different SBI, the Namf_Communication action “N1N2MessageTransfer” is resused for sending MT SMS, as that interface already exists for use by SMF, LMF and PCF, and 5GC attempts to reuse interfaces as much as possible.
There’s no such thing as a free lunch, and 5G is the same – services running through a 5G Standalone core need to be billed.
In 5G Core Networks, the SMF (Session Management Function) reaches out to the CHF (Charging Function) to perform online charging, via the Nchf_ConvergedCharging Service Based Interface (aka reference point).
Like in other generations of core mobile networks, Credit Control in 5G networks is based on 3 functions: Requesting a quota for a subscriber from an online charging service, which if granted permits the subscriber to use a certain number of units (in this case data transferred in/out). Just before those units are exhausted sending an update to request more units from the online charging service to allow the service to continue. When the session has ended or or subscriber has disconnected, a termination to inform the online charging service to stop billing and refund any unused credit / units (data).
Initial Service Creation (ConvergedCharging_Create)
When the SMF needs to setup a session, (For example when the AMF sends the SMF a Nsmf_PDU_SessionCreate request), the CTF (Charging Trigger Function) built into the SMF sends a Nchf_ ConvergedCharging_Create (Initial, Quota Requested) to the Charging Function (CHF).
Because the Nchf_ConvergedCharging interface is a Service Based Interface this is carried over HTTP, in practice, this means the SMF sends a HTTP post to http://yourchargingfunction/Nchf_ConvergedCharging/v1/chargingdata/
Obviously there’s some additional information to be shared rather than just a HTTP post, so the HTTP post includes the ChargingDataRequest as the Request Body. If you’ve dealt with Diameter Credit Control you may be expecting the ChargingDataRequest information to be a huge jumble of nested AVPs, but it’s actually a fairly short list:
The subscriberIdentifier (SUPI) is included to identify the subscriber so the CHF knows which subscriber to charge
The nfConsumerIdentification identifies the SMF generating the request (The SBI Consumer)
The invocationTimeStamp and invocationSequenceNumber are both pretty self explanatory; the time the request is sent and the sequence number from the SBI consumer
The notifyUri identifies which URI should receive subsequent notifications from the CHF (For example if the CHF wants to terminate the session, the SMF to send that to)
The multipleUnitUsage defines the service-specific parameters for the quota being requested.
The triggers identifies the events that trigger the request
Of those each of the fields should be pretty self explanatory as to their purpose. The multipleUnitUsage data is used like the Service Information AVP in Diameter based Credit Control, in that it defines the specifics of the service we’re requesting a quota for. Inside it contains a mandatory ratingGroup specifying which rating group the CHF should use, and optionally requestedUnit which can define either the amount of service units being requested (For us this is data in/out), or to tell the CHF units are needed. Typically this is used to define the amount of units to be requested.
On the amount of units requested we have a bit of a chicken-and-egg scenario; we don’t know how many units (In our case the units is transferred data in/out) to request, if we request too much we’ll take up all the customer’s credit, potentially prohibiting them from accessing other services, and not enough requested and we’ll constantly slam the CHF with requests for more credit. In practice this value is somewhere between the two, and will vary quite a bit.
Based on the service details the SMF has put in the Nchf_ ConvergedCharging_Create request, the Charging Function (CHF) takes into account the subscriber’s current balance, credit control policies, etc, and uses this to determine if the Subscriber has the required balances to be granted a service, and if so, sends back a 201 CREATED response back to the Nchf_ConvergedCharging_Create request sent by the CTF inside the SMF.
This 201 CREATED response is again fairly clean and simple, the key information is in the multipleQuotaInformation which is nested within the ChargingDataResponse, which contains the finalUnitIndication defining the maximum units to be granted for the session, and the triggers to define when to check in with CHF again, for time, volume and quota thresholds.
And with that, the service is granted, the SMF can instruct the UPF to start allowing traffic through.
Update (ConvergedCharging_Update)
Once the granted units / quota has been exhausted, the Update (ConvergedCharging_Update) request is used for requesting subsequent usage / quota units. For example our Subscriber has used up all the data initially allocated but is still consuming data, so the SMF sends a Nchf_ConvergedCharging_Update request to request more units, via another HTTP post, to the CHF, with the requested service unit in the request body in the form of ChargingDataRequest as we saw in the initial ConvergedCharging_Create.
If the subscriber still has credit and the CHF is OK to allow their service to continue, the CHF returns a 200 OK with the ChargingDataResponse, again, detailing the units to be granted.
This procedure repeats over and over as the subscriber uses their allocated units.
Release (ConvergedCharging_Release)
Eventually when our subscriber disconnects, the SMF will generate a Nchf_ConvergedCharging_Release request, detailing the data the subscriber used in the ChargingDataRequest in the body, to the CHF, so it can refund any unused credits.
The CHF sends back a 204 No Content response, and the procedure is completed.
More Info
If you’ve had experience in Diameter credit control, this simple procedure will be a breath of fresh air, it’s clean and easy to comprehend, If you’d like to learn more the 3GPP specification docs on the topic are clear and comprehensible, I’d suggest:
TS 132 290 – Short overview of charging mechanisms
TS 132 291 – Specifics of the Nchf_ConvergedCharging interface
The common 3GPP charging architecture is specified in TS 32.240
TS 132 291 – Overview of components and SBIs inc Operations
Today, we’re going to look at one of the simplest Service Based Interfaces in the 5G Core, the Equipment Identity Register (EIR).
The purpose of the EIR is very simple – When a subscriber connects to the network it’s Permanent Equipment Identifier (PEI) can be queried against an EIR to determine if that device should be allowed onto the network or not.
The PEI is the IMEI of a phone / device, with the idea being that stolen phones IMEIs are added to a forbidden list on the EIR, and prohibited from connecting to the network, making them useless, in turn making stolen phones harder to resell, deterring mobile phone theft.
In reality these forbidden-lists are typically either country specific or carrier specific, meaning if the phone is used in a different country, or in some cases a different carrier, the phone’s IMEI is not in the forbidden-list of the overseas operator and can be freely used.
The dialog goes something like this:
AMF: Hey EIR, can PEI 49-015420-323751-8 connect to the network?
EIR: (checks if 49-015420-323751-8 in forbidden list - It's not) Yes.
or
AMF: Hey EIR, can PEI 58-241992-991142-3 connect to the network?
EIR: (checks if 58-241992-991142-3 is in forbidden list - It is) No.
(Optionally the SUPI can be included in the query as well, to lock an IMSI to an IMEI, which is a requirement in some jurisdictions)
As we saw in the above script, the AMF queries the EIR using the N5g-eir_EquipmentIdentityCheck service.
The N5g-eir_EquipmentIdentityCheck service only offers one operation – CheckEquipmentIdentity.
It’s called by sending an HTTP GET to:
http://{apiRoot}/n5g-eir-eic/v1/equipment-status
Obviously we’ll need to include the PEI (IMEI) in the HTTP GET, which means if you remember back to basic HTTP GET, you may remember means you have to add ?attribute=value&attribute=value… for each attribute / value you want to share.
For the CheckEquipmentIdentity operation, the PEI is a mandatory parameter, and optionally the SUPI can be included, this means to query our PEI (The IMSI of the phone) against our EIR we’d simply send an HTTP GET to:
AMF: HTTP GET http://{apiRoot}/n5g-eir-eic/v1/equipment-status?pei=490154203237518
EIR: 200 (Body EirResponseData: status "WHITELISTED")
And how it would look for a blacklisted IMEI:
AMF: HTTP GET http://{apiRoot}/n5g-eir-eic/v1/equipment-status?pei=490154203237518
EIR: 404 (Body EirResponseData: status "BLACKLISTED")
Because it’s so simple, the N5g-eir_EquipmentIdentityCheck service is a great starting point for learning about 5G’s Service Based Interfaces.
The SUPI (Subscription Permanent Identifier) replaces the IMSI as the unique identifier for each Subscriber in 5G.
One of the issues with using IMSI in LTE/EUTRAN is there were a few occasions where the IMSI was sent over the clear – meaning the IMSIs of subscribers nearby could be revealed to anyone listening.
So what is a SUPI and what does it look like? Well, most likely it’ll look like an IMSI – 15 or 16 digits long, with the MCC/MNC as the prefix.
If you’re using a non-3GPP RAT it could be a RFC 4282 Network Access Identifier, but if it’s on a SIM card or in a Mobile Device, it’s probably exactly the same as the IMSI.
SUCI Subscription Concealed Identifier
Our SUPI is never sent over the air in the clear / plaintext, instead we rely on the SUCI (Subscription Concealed Identifier) for this, which replaces the GUTI/TMSI/IMSI for all plaintext transactions over the air.
Either the UE or the SIM generate the SUCI (if it’s done by the SIM it’s much slower), based on a set of parameters defined on the SIM.
The SUCI has to be generated by the UE or SIM in a way the Network can identify the SUPI behind the SUCI, but no one else can.
In LTE/EUTRAN this was done by the network randomly assigning a value (T-MSI / GUTI) and the network keeping track of which randomly assigned value mapped to which user, but initial attach and certain handovers revealed the real IMSI in the clear, so for 5G this isn’t an option.
So let’s take a look at how SUCI is calculated in a way that only the network can reveal the SUPI belonging to a SUCI.
The Crypto behind SUCI Calculation
As we’ll see further down, SUCI is actually made up of several values concatenated together. The most complicated of these values is the Protection Scheme Output, the cryptographically generated part of the SUCI that can be used to determine the SUPI by the network.
Currently 3GPP defines 3 “Protection Scheme Profiles” for calculating the SUCI.
Protection Scheme Identifier 1 – null-scheme
Does nothing. Doesn’t conceal the SUPI at all. If this scheme is used then the Protection Scheme Output is going to just be the SUPI, for anyone to sniff off the air.
Protection Scheme Identifier 2 & 3 – ECIES scheme profile A & B
The other two Protection Scheme Identifiers both rely on Elliptic Curve Integrated Encryption Scheme (ECIES) for generation.
So if both Profile A & Profile B rely on Elliptic Curve Integrated Encryption Scheme, then what’s the difference between the two?
Well dear reader, the answer is semantics! There’s lots of parameters and variables that go into generating a resulting value from a cryptographic function, and Profile A & Profile B are just different parameters being used to generate the results.
For crypto nerds you can find the specifics in C.3.4.1 Profile A and C.3.4.1 Profile B outlined in 3GPP TS 33.501.
For non crypto nerds we just need to know this;
When the SIM is generating the SUCI the UE just asks for an identity by executing the GET IDENTITY command ADF against the SIM and uses the response as the SUCI.
When the UE is generating the SUCI, the UE gets the SUCI_Calc_Info EF contents from the SIM and extracts the Home Network Public Key from it’s reply. It uses this Home Network Public Key and a freshly created ephemeral public/private key pair to generate a SUCI value to use.
Creating the SUCI
After generating a Protection Scheme Output, we’ll need to add some extra info into it to make it useful.
The first digit of the SUCI is the SUPI type, a value of 0 denotes the value contained in the Protection Scheme Output is an IMSI, while 1 is used for Network Access Indicator for Non 3GPP access.
Next up we have the Home Network Identifier, which in a mobile environment is our PLMN (MCC/MCC).
Then a Routing Indicator, 1-4 digits long, is used with the Home Network Identifier to route the Authentication traffic to the UDM that contains that subscriber’s information, ie you may have MVNOs with their own UDM. If the routing indicator of 10 is assigned to the MVNOs SIMs then the AMF can be set to route traffic with a routing indicator of 10 to the UDM of the VMNO.
The Protection Scheme we covered earlier, with the 3 types of protection scheme (Null & two relying on Elliptic Curve Integrated Encryption Scheme).
Home Network Public Key Identifier identifies which Public Key was used to generate the Protection Scheme Output.
Finally we have the Protection Scheme Output which we covered generating in the previous session.
Usage in Signaling
The SUPI is actually rarely used beyond the initial attach to the network.
After authenticating to the network using AKA and the SUCI, in 5GC, like in LTE/EUTRAN, a shorter GUTI is used which further protects the subscriber’s identity and changes frequently.
Next up we’ll need to configure the NRF on the domain “nrf.5gc.mnc001.mcc001.3gppnetwork.org”, for this we’ll edit /etc/open5gs/nrf.conf and set the binding IP.
nrf:
sbi:
- addr:
- 10.0.1.252
port: 7777
Now for each Network Element we’re bringing online we’ll need to point it at our NRF’s address (or IP).
Mobile networks are designed to be redundant and resilient, with N+1 for everything.
Every network element connects to multiple other network elements.
The idea being the network is architected so a failure of any one network element will not impact service.
To take an LTE/EPC example, your eNodeBs connect to multiple MMEs, which in turn connect to multiple HSSs, multiple S-GWs, multiple EIRs, etc. The problem is when each eNodeB connects to 3 MMEs, and you want to add a 4th MME, you have to go and reconfigure all the eNodeBs to point to the new MME, and all the HSSs to accept that MME as a new Diameter Peer, for example.
The more redundant you make the network, the harder it becomes to change.
This led to development of network elements like Diameter Routing Agents (DRAs) and DNS SRV for service discovery, but ultimately adding and removing network elements in previous generations of mobile core, involved changing a lot of config on a lot of different boxes.
The Solution
The NRF – Network Repository Function serves as a central repository for Network Functions (NFs) on the network.
In practice this means when you bring a new Network Function / Network Element online, you only need to point it at the NRF, which will tell it about other Network Functions on the network, register the new Network Function and let every other interested Network Function know about the new guy.
Take for example adding a new AMF to the network, after bringing it online the only bit of information the AMF really needs to start placing itself in the network, is the details of the NRF, so it can find everything it needs to know.
Our new AMF will register itself to the NRF, advertising what Network Functions it can offer (ie AMF service), and it’ll in turn be able to learn about what Network Functions it can consume – for example our AMF would need to know about the UDMs it can query data from.
It is one of the really cool design patterns usually seen in modern software, that 3GPP have adopted as part of the 5GC.
In Practice
Let’s go into a bit more detail and look at how it looks.
The NRF uses HTTP and JSON to communicate (anything not using ASN.1 is a winner), and looks familiar to anyone used to dealing with RESTful APIs.
Let’s take a look at how an AMF looks when registering to a NRF,
NF Register – Providing the NRF a profile for each NF
In order for the NRF to function it has to know about the presence of all the Network Functions on the network, and what they support. So when a new Network Function comes online, it’s got to introduce itself to the NRF.
It does this by providing a “Profile” containing information about the Network Functions it supports, IP Addresses, versions, etc.
Going back to our AMF example, the AMF sends a HTTP PUT request to our NRF, with a JSON payload describing the functions and capabilities of the AMF, so other Network Functions will be able to find it.
Let’s take a look at what’s in the JSON payload used for the NF Profile.
Each Network Function is identified by a UUID – nfInstanceId, in this example it’s value is “f2b2a934-1b06-41eb-8b8b-cb1a09f099af”
The nfType (Network Function type) is an AMF, and it’s IP Address is 10.0.1.7
The heartBeatTimer sets how often the network function (in this case AMF) sends messages to the NRF to indicate it’s still alive. This prevents a device registering to an NRF and then going offline, and the NRF not knowing.
The nfServices key contains an array of services and details of those services, in the below example the key feature is the serviceName which is namf-comm which means the Namf_Communication Service offered by the AMF.
The NRF files this info away for anyone who requests it (more on that later) and in response to this our NRF will indicate (hopefully) that it’s successfully created the entry in its internal database of Network Functions for our AMF, resulting in a HTTP 201 “Created” response back from the NRF to the AMF.
NRF StatusSubscribe – Subscribe & Notify
Simply telling the NRF about the presence of NFs is one thing, but it’s not much use if nothing is done with that data.
A Network Function can subscribe to the NRF to get updates when certain types of NFs enter/leave the network.
Subscribing is done by sending a HTTP POST with a JSON payload indicating which NFs we’re interested in.
Contents of a Subscription message to be notified of all AMFs joining the network
Whenever a Network Function registers on the NRF that related to the type that has been subscribed to, a HTTP POST is sent to each subscriber to let them know.
For example when a UDM registers to the network, our AMF gets a Notification with information about the UDM that’s just joined.
NRF Update – Updating NRF Profiles & Heartbeat
If our AMF wants to update its profile in the NRF – for example a new IP is added to our AMF, a HTTP PATCH request is sent with a JSON payload with the updated details, to the NRF.
The same mechanism is used as the Heartbeat / keepalive mechanism, to indicate the NRF is still there and working.
Summary
The NRF acts as a central repository used for discovery of neighboring network functions.
As the standardisation for 5G-SA has been completed and the first roll outs are happening, I thought I’d cover the basic architecture of the 5G Core Network, for people with a background in EPC/SAE networks for 4G/LTE, covering the key differences, what’s the same and what’s new.
The AMF – Authentication & Mobility Function, serves much the same role as the MME in LTE/EPC/SAE networks.
Like the MME, the AMF only handles Control Plane traffic, and serves as the gatekeeper to the services on the network, connecting the RAN to the core, authenticating subscribers and starting data / PDN connections for the UEs.
While the MME connects to eNodeBs for RAN connectivity, the AMF connects to gNodeBs for RAN.
The Authentication Functions
In EPC the HSS had two functions; it was a database of all subscribers’ profile information and also the authentication centre for generating authentication vectors.
5GC splits this back into two network elements (Akin to the AuC and HLR in 2G/3G).
The UDM (Unified Data Management) provides the AMF with the subscriber profile information (allowed / barred services / networks, etc),
The AUSF (Authentication Server Function) provides the AMF with the authentication vectors for authenticating subscribers.
Like in UMTS/LTE USIMs are used to authenticate subscribers when connecting to the network, again using AKA (Authentication and Key Agreement) for mutual subscriber & network authentication.
Other authentication methods may be implemented, R16 defines 3 suporrted methods, 5G-AKA, EAP-AKA’, and EAP-TLS.
This opens the door for the 5GC to be used for non-mobile usage. There has been early talk of using the 5G architecture for fixed line connectivity as well as mobile, hence supporting a variety of authentication methods beyond classic AKA & USIMs. (For more info about Non-3GPP Access interworking look into the N3IWF)
The Mobility Functions
When a user connects to the network the AMF selects a SMF (Session Management Function) akin to a P-GW-C in EPC CUPS architecture and requests the SMF setup a connection for the UE.
This is similar to the S11 interface in EPC, however there is no S-GW used in 5GC, so would be more like if S11 were instead sent to the P-GW-C.
The SMF selects a UPF (Akin to the P-GW-C selecting a P-GW-U in EPC), which will handle this user’s traffic, as the UPF bridges external data networks (DNs) to the gNodeB serving the UE.
Moving between cells / gNodeBs is handled in much the same way as done previously, with the path the UPF sends traffic to (N3 interface) updated to point to the IP of the new gNodeB.
When a UE attempts to connect to the network their signalling traffic (Using the N1 reference point between the UE and the AMF), is sent to the AMF.
an authentication challenge is issued as in previous generations.
Upon successful authentication the AMF signals the SMF to setup a session for the UE. The SMF selects a UPF to handle the user plane forwarding to the gNodeB serving the UE.
Key Differences
Functions handled by the MME in EPC now handled by AMF in 5GC
Functions of HSS now in two Network Functions – The UDM (Unified Data Management) and AUSF (Authentication Server Function)
Setting up data connections “flatter” (more info on the User Plane differences can be found here)
Non 3GPP access (Potentially used for fixed-line / non mobile networks)
