Question $1(15$p$)($System Overview$)$ Consider a communication session that is initiated by a smart

phone via FaceTime over $4G$ LTE$/5$G system and answered by another laptop using NCSU campus WLAN.

Design$/$Enumerate step$-$by$-$step operations $($as detailed as possible$)$ by highlighting the transmitter$/$sender$,$

up$/$down $($radio$)$ links, and receiver. Refer to the example on page pp$15$ in the lecture notes. Considering

$($or answer$)$ following issues:

$[$ Step $1$: Cell phone $$ Base Station$],$ say eNodeB or gNodeB

$[$ Step $2$: Base station $$ NCSU WLAN access point$]$

$[$ Step $3$: NCSU Access point $$ cell phone$]$

cdots

For example, here are the first several messages from the sender:

$[$Step $1$: cellular phone $$ base station $]$

$($radio links over AT&T mobility $--$ this is to highlight the LINKS and entities$)$

$1\mathrm{.}1.$ dial the receiver's number on FaceTime app

$($cellular phone $-$ a connected device$)$

$1\mathrm{.}2$ send a call request to base station

$($uplinks $-$ mobility management$)$

$1\mathrm{.}3$ base station of the caller forwards the request to MSO$/$MME

$($high$-$speed backhaul $($up$)$links between MSO$/$RAN and base stations$)$