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At the beginning of Section 2.1, the types, i.e. CS as well as PS, of services being provided by a typical mobile communications network was mentioned. In the case of a CS call, an end‐to‐end dedicated physical circuit establishment is required prior to the starting of voice call service on it. However, no such dedicated physical resources or path is required to be established for a PS call. A refresher illustration showing this basic difference between a CS and PS call is shown in Figure 2.5.

In Figure 2.5 illustration, User A wants to start a CS voice conversation (bold solid line) with User B. Prior to that, a dedicated and exclusive physical path/circuit (dotted line) for the entire duration of the call is required to be established between them through the interconnected switching or network elements. Similarly, User C wants to visit a web site, say www.abc.com, which is a typical PS data transfer scenario. In this case, the user’s mobile device sends a burst of data to the concerned network element NE1. Following this, depending on the concerned timer value and response from the webserver, the mobile device may release the ongoing signaling path that was established with the network element NE1. Network element NE1, in turn, shall forward the received packets to its peer network element NE2, en route to the web site server, say www.abc.com.

Table 2.1 Network elements comparisons.

	Mobile communication systems
Network elements	GSM/GPRS	UMTS	LTE	5G
GERAN	UTRAN	E‐UTRAN	NG‐RAN
SGSN	SGSN	S‐GW	UPF
GGSN	GGSN	PDN gateway	SMF (partially)
HLR	HLR	HSS	UDM and AUSF

Figure 2.5 Illustration of circuit switched and packet switched data transfer.