The idea
A WiFi access point covers a room. A cellular network covers a country, and the user is allowed to be on a train. Two things follow. A device must prove who it is to a network it does not own, and a device that moves must be passed from one base station to the next without the call dropping. This topic covers the generations, the 5G architecture, how a device joins and how a handover works.
How it works
Generations
The lecture’s list of cellular generations is a list of names.
| Generation | Technologies |
|---|---|
| 1G | AMPS |
| 2G | D-AMPS, GSM, IS-95 (CDMA-ONE) |
| 2.5G | GPRS |
| 2.75G | EDGE |
| 3G | UMTS (WCDMA), CDMA2000 |
| 3.5G | HSPA |
| 3.75G | HSPA+ |
| 4G | LTE, LTE-A, WiMAX (some versions) |
| 5G | NR |
How it works
5G radio access network
The 5G RAN is the edge network connecting devices to a base station. It provides link-layer service as the first hop between devices and the larger network. It has limited geographic scope, sits under the control of a single service provider and is somewhat analogous to a WiFi LAN.
Its components are:
- many devices, called user equipment (UE)
- a radio channel, New Radio (NR)
- one base station, the Next Generation Node B (gNodeB, gNB)
The RAN protocol stack at the device and the base station has a user plane and a control plane. The slide’s layers are SDAP, RRC, PDCP, RLC, MAC and Physical. The lecture gives no further detail for the layers on these slides.
How it works
5G core network
The core sits between the RAN and the other endpoints, the larger Internet. There is a single core and multiple RANs. It consists of links, routers and servers providing services to devices and base stations. It is an “all IP” core, but its services are very different from traditional Internet applications. The control plane and user plane are logically separate, called CUPS.
| Function | What it does |
|---|---|
| UPF (User Plane Function) | Each device is assigned one. Relays traffic between the UE and the Internet. Policy enforcement, lawful intercept, traffic usage measurement, QoS policing. Tunnels traffic to and from the base station over the N3 interface |
| AMF (Access and Mobility Management Function) | Connection and reachability management, mobility management, access authorisation, location services |
| SMF (Session Management Function) | Manages each UE session: IP address allocation, selecting the UP function, QoS control, aspects of UP routing |
| AUSF (Authentication Server Function) | Authenticates devices |
| UDM (Unified Data Management) | Manages user identity, including generating authentication credentials |
| UDR (Unified Data Repository) | Manages static subscriber-related information |
| UDSF (Unstructured Data Storage Network Function) | Stores unstructured data, similar to a key/value store |
User plane traffic is tunnelled. A datagram from an Internet sender at address w to a device at address z is wrapped by the UPF in an outer packet addressed to the base station, then carried over the backhaul network inside GTP-U over UDP over IP. The lecture’s figure marks the UPF address as x and the base station address as y.
How it works
Identity and joining a 5G network
In 4G and 5G, a user is a customer of a wireless service provider with a home cellular network. The SIM card (or vSIM) identifies the user and the home network to all cellular networks globally, and holds cryptographic key information also known to the home network. The lecture contrasts this with the Internet approach, where a user is a name and password that differs by application, or a name and a public key certificate.
Joining has two phases.
- Registration. The device identifies and authenticates itself to the network, and the network authenticates itself to the device.
- PDU session establishment. Core network functions allocate an IP address to the device and create the data-plane path between the device and its UPF. PDU means protocol data unit, otherwise known as a packet.
The AMF plays the key role in bringing the arriving device into the network. The signalling figure is partly garbled, but its first steps are an RRC connection setup request and response, an identity request and response, authentication and security, and a registration accept. The session phase begins with a PDU session establishment request and ends with the first uplink datagram in the data plane.
How it works
Handover
Handover is when a mobile device changes its point of attachment to the network, so the data flow to the device switches from the source base station to the target base station. The lecture gives two reasons: a stronger signal from the target base station, or a target base station with fewer devices and less traffic.
- The current (source) base station selects the target base station and sends it a Handover Request message.
- The target base station pre-allocates radio time slots and responds with a handover request ACK carrying information for the mobile.
- The source base station informs the mobile of the new base station. The mobile can now send via the new base station, and the handover looks complete to the mobile.
- The source base station stops sending datagrams to the mobile and forwards them to the new base station, which forwards them to the mobile over the radio channel.
- The target base station informs the AMF and SMF that it is the new base station for the mobile.
- The SMF instructs the UPF to change the tunnel endpoint to the target base station. The target base station ACKs back to the source base station that handover is complete, so the source can release its resources.
- The mobile’s datagrams now flow through the new tunnel from the target base station to the UPF.
The lecture’s signalling timeline labels one part of the exchange as making the handover decision, one as handover in the RAN and one as handover in the core. The timeline slide that shows the signalling between device, base stations, AMF, SMF and UPF extracted only partly, and its message names are not reproduced here.
Where marks get lost
Two steps carry the number 6
On the lecture’s second handover slide, two actions are both numbered 6, the SMF instructing the UPF and the target base station ACKing the source. They are written as one step above. If a question asks for the order, the safe statement is that the core is told first and the source releases its resources once the target ACKs.
Aside
The lecture also has a retrospective slide called A day in the life, tracing an HTTP/3 request in a UDP segment, in an IP datagram, into a 5G frame, with the device sending a scheduling request and the base station granting an upstream transmission. Its figure labels did not extract cleanly, so it is not expanded here.
In the exam
- Generations. Know which technology sits under which generation, such as GSM in 2G and UMTS (WCDMA) in 3G.
- RAN parts. UE, NR and gNodeB.
- Core functions. UPF, AMF and SMF are the three the lecture spends most time on. Be able to say what each does in one line.
- Joining. Registration, then PDU session establishment.
- Handover. Reasons, then the seven steps in order, with the source forwarding data in step 4 and the SMF changing the UPF tunnel endpoint in step 6.
Check yourself
- The generations run from AMPS through GSM, UMTS and LTE to 5G NR.
- The 5G RAN is UE, NR and a gNodeB. The core separates control and user planes.
- A device registers first and then gets a PDU session with an IP address and a path to its UPF.
- In a handover the target is prepared first, the source forwards data meanwhile, and then the core moves the tunnel to the target.