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+\section{The GSM core network}
+
+\subsection{GSM core network components}
+
+\begin{frame}{GSM core network components}
+    \begin{description}[MSC]
+      \item[MSC] (Mobile Switching Center): The central switch
+      \item[HLR] (Home Location Register): Database of subscribers
+      \item[AUC] (Authentication Center): Database of authentication keys
+      \item[VLR] (Visitor Location Register): For roaming users
+      \item[EIR] (Equipment Identity Register): To block stolen phones
+    \end{description}
+\end{frame}
+
+\begin{frame}{GSM network structure}
+\begin{description}[BTS]
+\item[MSC] Actual call switching and top-level mobility functions.  May serve dozens of location areas
+\item[VLR] Temporary cache of subscriber data from HLR + TMSI
+\item[HLR] Subscriber databases + subscriber location information
+\item[AUC] Generation of authentication tuples
+\item[SMSC] SMS Service Centre, store+forward for SMS
+\end{description}
+\end{frame}
+
+\begin{frame}{GSM core network integration}
+\begin{itemize}
+	\item VLR often integrated into MSC
+	\item AUC often integrated with AUC
+	\item integration so common, many graphs/diagrams are actually
+not 100\% correct
+\end{itemize}
+\end{frame}
+
+\begin{frame}{GSM Network Structure}
+\includegraphics[width=100mm]{gsm_network.png}
+\end{frame}
+
+\begin{frame}{GSM network interfaces}
+  \begin{description}[D]
+    \item[C] Interface between GMSC and HLR
+    \item[D] Interface between MSC and HLR
+    \item[E] Interface between MSC and MSC
+  \end{description}
+All of them based on MAP, so C/D/E not commonly distinguished
+\end{frame}
+
+\subsection{GSM core network protocols}
+
+\begin{frame}{core network protocol stack}
+Traditional telephony based on SS7 / CS7, GSM too
+\begin{itemize}
+	\item Lower layers (MTP2/MTP3) re-used
+	\item ISUP used for actual call control signalling
+	\item SCCP for routing / GTT
+	\item TCAP for transaction supprt
+	\item MAP for actual GSM related signalling
+\end{itemize}
+\end{frame}
+
+\begin{frame}{SS7 networks}
+\begin{itemize}
+	\item STP - Signalling Transfer Point
+	\begin{itemize}
+		\item {\em Router} for SCCP
+		\item performs GTT (see below)
+	\end{itemize}
+	\item SCP - Signalling Control Point
+	\begin{itemize}
+		\item {\em End-node} like MSC/HLR
+		\item SCP has GT, PC, ..
+	\end{itemize}
+\end{itemize}
+\end{frame}
+
+\begin{frame}{SS7 addresses}
+\begin{itemize}
+	\item Point Code (PC)
+	\begin{itemize}
+		\item typically unique within PLMN / country
+	\end{itemize}
+	\item Global Title (GT)
+	\begin{itemize}
+		\item world-wide unique address
+		\item translated into PC by GTT at STP
+	\end{itemize}
+	\item Subsystem Number (SSN)
+	\begin{itemize}
+		\item logical function address inside network (MSC, VLR, HLR, ...)
+		\item not used on international links
+	\end{itemize}
+\end{itemize}
+\end{frame}
+
+\begin{frame}{SS7 GTT (Global Title Translation)}
+Global Title Translation
+\begin{itemize}
+	\item can happen at any STP
+	\item translates a Destination GT into new destination address
+	\item new dest address can be any address, such as
+	\begin{itemize}
+		\item new global title (GT)
+		\item point code (PC)
+		\item sub-system number (SSN)
+	\end{itemize}
+	\item GTT rules explicitly configured by operator, e.g.
+	\begin{itemize}
+		\item prefix or range based match
+		\item (inter)nationalize numbering plan
+		\item add digits at beginning or end
+	\end{itemize}
+\end{itemize}
+\end{frame}
+
+\begin{frame}{SS7 physical layer}
+\begin{itemize}
+	\item{classic SS7 signalling over TDM circuits}
+	\begin{itemize}
+		\item E1 timeslot (64kbps)
+		\item multiple E1 timeslots (N*64kbps)
+		\item MTP Level 2 / MTP Level 3
+	\end{itemize}
+	\item modern networks use SIGTRAN
+	\begin{itemize}
+		\item IP as network layer replaces E1 lines
+		\item SCTP on top(no TCP/UDP!)
+		\item many different SIGTRAN stacking options
+	\end{itemize}
+	\item some vendor-proprietary protocols like SCCPlite
+\end{itemize}
+\end{frame}
+
+\begin{frame}{SIGTRAN stacking options}
+SIGTRAN != SIGTRAN
+\begin{itemize}
+	\item IP/SCTP/M2PA/MTP2/MTP3/SCCP/TCAP/MAP
+	\item IP/SCTP/M2UA/MTP3/SCCP/TCAP/MAP
+	\item IP/SCTP/M3UA/SCCP/TCAP/MAP
+	\item IP/SCTP/SUA/TCAP/MAP
+\end{itemize}
+\end{frame}
+
+\begin{frame}{SCCP}
+SCCP takes care of
+\begin{itemize}
+	\item Global Title based addressing
+	\item Global Title Translation
+	\item connection-oriented or connectionless semantics
+	\item GSM core network interfaces with MAP/CAP only use
+connection-less UDT service
+\end{itemize}
+\end{frame}
+
+\begin{frame}{TCAP}
+\begin{itemize}
+	\item Idea: decouple transaction logic from actual application
+	\item transaction semantics can be used by multiple higher-layer protocols
+	\item state machines on both sides maintained outside of application
+	\item protocol specified in ASN.1, BER encoding
+\end{itemize}
+\end{frame}
+
+\begin{frame}{MAP - Mobile Application Part}
+\begin{itemize}
+	\item used between all classic GSM core network components
+	\item application protocol on top of TCAP
+	\item protocol specified in ASN.1, BER encoding
+\end{itemize}
+\end{frame}
+
+\begin{frame}{CAP - Camel Application Part}
+\begin{itemize}
+	\item used for CAMEL entities (gsmSCF, gsmSSF, gprsSSF, gsmSRF)
+	\item application protocol on top of TCAP
+	\item protocol specified in ASN.1, BER encoding
+\end{itemize}
+\end{frame}
+
+%\section{Roaming interfaces}
+%
+%\subsection{Roaming introduction}
+%
+%\begin{frame}{Introduction to Roaming}
+%Roaming enables subscribers to use other operators' networks
+%\begin{itemize}
+%	\item Home Network is called HPLMN
+%	\item Visited Network is called VPLMN
+%	\item Roaming requres between HPLMN and VPLMN
+%	\begin{itemize}
+%		\item Roaming agreement (contract)
+%		\item SS7 connectivity (ISUP/MAP/CAP)
+%		\item IP connectivity (for packet data)
+%	\end{itemize}
+%\end{itemize}
+%\end{frame}
+%
+%\begin{frame}{Roaming principle}
+%\begin{itemize}
+%	\item MS, MSC, VLR and SGSN are in VPLMN
+%	\item HLR, AUC, GMSC and GGSN are in HPLMN
+%	\item they talk to each other via MAP, just like in non-roaming case
+%	\item selection of HPLMN based on IMSI of subscriber
+%	\item non-roaming caes: HPLMN == VPLMN
+%\end{itemize}
+%\end{frame}
+%
+%\begin{frame}{MVNO - Mobile Virtual Network Operators}
+%A MVNO setup is a special case of roaming
+%\begin{itemize}
+%	\item MNO operates PLMN with RAN and CN
+%	\item MVNO operates HPLMN without RAN (BSC/BTS)
+%	\item MVNO subscribers always roam into MNO network
+%\end{itemize}
+%\end{frame}
+%
+%\subsection{Roaming transactions}
+%FIXME
+
+%\subsection{Traditional Billing}
+%
+%\begin{frame}{Traditional Billing}
+%Initially, GSM was designed for business users
+%\begin{itemize}
+%	\item Billing was always post-paid
+%	\item Each PLMN simply logs all call/sms
+%	\item Logs called CDR (Call Data Record)
+%	\item At the end of the month, invoices are generated
+%	\item CDR records are exchanged between roaming partners
+%\end{itemize}
+%\end{frame}
+%
+%\begin{frame}{Billing for Roaming}
+%\begin{itemize}
+%	\item CDR files often vendor-specific / custom
+%	\item GSMA established a standard called TAP
+%	\item TAP is the standard for exchange of billing records
+%between roaming partners
+%	\item Summary: Intra-PLMN: CDR, Inter-PLMN: TAP
+%	\item TAP has many versions/generations
+%	\item Specified in ASN.1
+%\end{itemize}
+%\end{frame}
+%
+%\begin{frame}{The advent of pre-paid}
+%\begin{itemize}
+%	\item At some point, users wanted pre-paid services
+%	\item Difficult to implement in traditional billing architecture
+%	\item In HPLMN, every operator could come up with custom
+%solution
+%	\item Thus, pre-paid initially not supported in roaming
+%	\item In the early pre-paid days, there were lots of ways to exceed pre-paid balance 
+%\end{itemize}
+%\end{frame}
+%
+%\begin{frame}{Pre-paid required fundamental changes}
+%\begin{itemize}
+%	\item The pre-paid balance / account is maintained in HPLMN
+%	\item HPLMN needs much more control over user while roaming
+%	\item A new protocol (CAMEL) was introduced, as well as new
+%entities in the network
+%	\item Lots of changes all over netowrk elements (MSC, SGSN, HLR)
+%\end{itemize}
+%\end{frame}
+%
+%\subsection{CAMEL}
+%
+%\begin{frame}{CAMEL - Customized Applications Mobile Enhanced Logic}
+%\begin{itemize}
+%	\item gsmSCF - Service Control Function
+%	\begin{itemize}
+%		\item receives per-subscriber specific config from HLR
+%(CSI: CAMEL Subscription Information)
+%		\item remotely controls call, SMS, etc. processing
+%	\end{itemize}
+%	\item gsmSSF - Service Switching Function
+%	\begin{itemize}
+%		\item built into MSC
+%		\item hooks / triggers at key state changes
+%		\item allows gsmSCF to alter/override/abort transactions
+%	\end{itemize}
+%	\item gprsSSF provides similar feature inside SGSN
+%\end{itemize}
+%\end{frame}