Sniffing client to server socket communication - sockets

Question

I was developing an IoT application with a BBB and a Quectel GSM modem to send data over internet.
I configured the modem on the debian OS with PPP. Then with something similar to this example I stablished communication between my server and my client. Data was trivial, I send just a simple "Close!", and receive a simple "OK!". Sniffing the serial port, I was expecting something simpler, but here is what I saw:
BBB >> GSM modem
ATZ
ATQ0 V1 E1 S0=0 &C1 &D2
AT+CGDCONT=1,"IP","internet.comcel.com.co"
ATD*99***1#
~}#!}!}!} }4}"}&} } } } }%}&"}'}"}(}"0~~}#!}!}"} }4}"}&} }*} } }%}&"}'}"}(}"}>~~}#!}$}!} }(}#}$#,~~}#!}"}"} }.}"}&} }*} } }'}"}(}"P~! "?~xx/2~!-/~!
!~!
j~!
_3~~!E<##_
_-7`99
}3BT~~!E<##^
_-7`99
}3D~!E4##e
_-7`:l5s
}3FGcE~!E:##^
_-7`:l5d
}3FGcEClose!=~~!E4##c
_-7`#l5
}3HcEh$~!E4##b
_-7`#l5
}3HcE~!E4##a
_-7`#l8
}3IycG~!E4##`
_-7`#l8}1
}3Iyc~!E4##_
_-7`Al9
}3I|c%~~!E4##^
_-7`#l9}1
}3Sc
!~~! "=~~!E4##\
_-7`#l9}1
}3cws~
Modem >> BBB
+CFUN: 1
+CPIN: READY
ATZ
OK
ATQ0 V1 E1 S0=0 &C1 &D2
OK
AT+CGDCONT=1,"IP","internet.comcel.com.co"
OK
ATD*99***1#
CONNECT
~}#!}!}!} }2}"}&} }*} } }#}$#}'}"}(}"U~~}#!}#}!} }*}"}&} }*} } lP~~}#!}"}"} }4}"}&} }*} } }%}&gHT}'}"}(}"Q~~}#!}!}!} }2}"}&} }*} } }#}$#}'}"}(}"U~~}#!}!}"} }.}"}&} }*} } }'}"}(}"~~!
H~~!
k~~!xx/~~!
-0~~!
xP`~~!
xP~~!E<#1-7
xPyCdOq x
d/2mQ~~!E<#1-7
xPyCdOq _x
d0N2~~!E4"#1Bw-7
xPzCdUrN
d0ev ~~!E7##1Bs-7
xPzCdUrd
d0eOK!3~~!E4$#1Bu-7
xP}]CdU}1rN
d0e~
I'm familiar with those AT commands, the "Close!" is there, and on the other side the "OK! is there too, but I didn't expect all those strange characters. I must say that I'm not very familiar with low level development for the TCP/IP network model, but I've always thought that sockets communication was the lowest layer (not counting physical layers).
What am I missing and where can I find more info related to this topic?
Thank you.