Every question of this course

Q1.1 Which of the three stages does TS 23.501 belong to?

• Stage 1, the requirements
• Stage 2, the architecture
• Stage 3, the messages on the wire

The first sentence of the scope says the document defines the Stage 2 system architecture for the 5G System. §1

Q1.2 Which two documents does the scope name as the companions of this one?

• TS 24.501 and TS 29.500
• TS 38.300 and TS 38.401
• TS 23.502 and TS 23.503

TS 23.502 carries the procedures and flows, TS 23.503 the policy control and charging. All three are stage 2 and were written together. §1

Q1.3 Somebody quotes TS 23.501 to settle an argument about how a field is encoded in a message. Are they right to?

• Yes, the document defines every message it mentions
• No, encodings are stage 3 and live in other documents
• Only for messages between the device and the core

Stage 2 says what the pieces are and what they are responsible for. It does not define one byte on the wire. §1

Q1.4 A version of the document is numbered 20.2.0. What does the leading 20 say?

• It belongs to Release 20
• Twenty working groups have approved it
• It has been revised twenty times

Once a specification is under change control, the first digit is the release. The change history shows 20.0.0 as the opening version of Release 20. §Annex X

Q2.1 The document says "some or all of the UPF functionalities may be supported in a single instance of a UPF". What does that tell you about a network function?

• It is a named list of jobs, and a real network may run many instances of it
• It is a product a vendor sells under that name
• It is one server, running one program

A function is a bundle of responsibilities with a name. How many machines carry it, and which jobs each one takes, is a deployment choice the specification stays out of. §6.2.3

Q2.2 Which function actually forwards a user's packets?

• The AMF
• The SMF
• The UPF

The UPF is the data path — routing, forwarding, inspection, rate enforcement. The AMF and the SMF only ever handle signalling. §6.2.3

Q2.3 How may two control-plane functions of the 5G core talk to each other?

• Only over service-based interfaces
• Either way, whichever the vendor prefers
• Only over a point-to-point link between them

One sentence of clause 4.2.1 settles it — control-plane network functions shall only use service-based interfaces for their interactions. §4.2.1

Q2.4 Which of these is a real link of its own rather than a name for a set of service calls?

• N7, between the SMF and the PCF
• N4, between the SMF and the UPF
• N11, between the AMF and the SMF

Six reference points are listed before the sentence that says the rest are realized by service-based interfaces. N1, N2, N3, N4, N6 and N9 are those six. §4.2.7

Q3.1 A registered phone lies on a bedside table all night, sending nothing. Which pair of states is it in?

• Registered and connected
• Deregistered and idle
• Registered and idle

The core still holds its context and can page it, but there is no signalling connection to the radio network, no N2 connection and no N3 connection anywhere. §5.3.3.2.2

Q3.2 Where is it recorded which AMF is currently serving a subscriber?

• In the UDM
• In the NRF
• In the radio network

Registration writes the serving AMF's identifier into the UDM, so the rest of the network can find out who is looking after this subscriber. §5.3.2.1

Q3.3 What does a DNN name?

• The subscriber paying for the traffic
• The data network the session reaches
• The radio cell the device is camped on

A PDU session carries packets between a device and one data network, and the DNN — the Data Network Name — is what identifies that network. §5.6.1

Q3.4 A device with a session to the operator's internet service now wants one to a factory network. What does it do?

• Deregister and register again
• Modify the session to name the other data network
• Establish a second PDU session

The data network name is fixed for the life of a session. A different network, a different packet type or a different continuity promise means another session. §5.6.1

Q4.1 What is the finest thing a 5G network can give a separate quality treatment to?

• A whole session
• A QoS Flow
• A single packet

All traffic marked with the same flow identifier inside one session gets the same forwarding treatment, so the flow is the unit — not the session and not the packet. §5.7.1.1

Q4.2 What does a 5QI value carry?

• A speed in megabits per second
• A reference to a set of characteristics such as priority, delay and error rate
• The radio channel the traffic will use

A 5QI is a plain number used as a pointer into a table both ends already hold. Bit rates are separate parameters alongside it. §5.7.2.1

Q4.3 What identifies a network slice?

• A DNN
• A QFI
• An S-NSSAI

The S-NSSAI is the name of a slice. It is made of a Slice/Service type and, optionally, a Slice Differentiator that tells apart several slices of the same type. §5.15.2.1

Q4.4 A slice carries the standardised Slice/Service type 2, URLLC. What speed and delay does that guarantee?

• None — the value says what the slice is meant for, not what it promises
• Whatever the roaming agreement says
• The values in the standardised table

The table exists so two operators recognise each other's kinds of slice. No network has to support every value, and the same service can be carried on a different one. §5.15.2.2