The 5G Core – Part 7 – More About Session Management

In part 6, I’ve been musing a bit about the inter-play between Connection-, Registration- and Session Management. In this part, let’s have a closer look at Session Management and in particular at the standard features that are required for mobile Internet access.

The standard functionality of Session Management, i.e. assigning an IP address to the user pretty much still works the same as in 2G, 3G and 4G but most concepts have received new names:

Getting an IPv4 address or IPv6 prefix in 2G/3G was referred to as establishing a ‘PDP Context’ (PDP = Packet Data Protocol). In LTE, the same operation was referred to as establishing a ‘Default Bearer’. In the 5G core, this operation is now referred to as establishing a ‘PDU Session‘ (PDU = Protocol Data Unit). Note: If you come across a 3GPP abbreviation you are not sure about what it stands for exactly, have a look at the ‘Vocabulary for 3GPP Specifications’ that can be found in TS 21.905.

As a mobile device can (and usually does have) have several IP addresses (or IPv6 prefixes) at once, e.g. one for Internet access and one for operator services such as VoLTE, each is bound to a different virtual network interface on the device. In this case, several PDU sessions are established simultaneously. Each is identified with a (human readable) name which in 2G, 3G and 4G networks was referred to as the APN (Access Point Name). In the 5G core, it is referred to as the DNN (Data Network Name).

And finally we have Secondary PDP Contexts in 2G/3G (which were never really used) and Dedicated Bearers in 4G which are in effect packet filters to prefer some packets over others in the network. Dedicated Bearers are mostly used for VoLTE voice calls over LTE. When a call (an IMS session to be precise) is established, the network can instruct the mobile device to activate a ‘Dedicated Bearer’ by sending it a corresponding message that contains a set of packet filtering rules to prefer speech packets exchanged between certain IP addresses and ports over other packets. Furthermore, these rules can be used to enforce maximum data transmission speeds for certain traffic such as voice packets. In the 5G core, Dedicated Bearers are referred to as ‘Packet Filter Sets‘. I am surprised 3GPP has decided for once to name something after what it actually does. Well done! An interesting side note: 23.501 even mentions in Chapter in Note 2 that the packet filter sets ‘[…] may be needed by the UE e.g. for the purpose of IMS precondition. […]’.

So for mobile Internet connectivity, pretty much the same ideas as already present in previous generations of mobile systems are going to be used again. But the 5G core wants to be a lot more than just a core for Internet connectivity so there are a number of new features for other uses. More about that in the next part.