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@@ -1476,9 +1476,9 @@ Every TDMA frame is assigned a unique integer number which is then repeated and
every 3h:28m:53s:760ms, also known as \textit{hyperframe} \citep[Chapter 7]{0890064717}. In the hierarchy pyramid,
a layer lower of the hyperframe is the \textit{superframe}. There are two types of superframes, consisting of two types
of \textit{multiframes}, differing in their length \citep[Chapter 7]{0890064717}. The relations can be seen in figure
-\ref{img:GSMHierarchy} with their duration periods. The multiframe with 26 TDMA frames carries only traffic channels (TCH) and associated
-control channels (CCH). The other multiframe type, with 51 TDMA frames carries solely signalling data. This hierarchy constrain
-was defined due to internal synchronization and cyphering between the MS and the Base Transceiver Station (BTS)
+\ref{img:GSMHierarchy} with their duration periods. The multiframe with 26 TDMA frames carries only traffic channels and associated
+control channels. The other multiframe type, with 51 TDMA frames carries solely signalling data. This hierarchy constrain
+was defined due to internal synchronization of the GSM network and cyphering between the MS and the Base Transceiver Station (BTS)
\citep[Chapter 7]{0890064717}.
@@ -1520,7 +1520,29 @@ from 64 kbps to 16 or 8 kbps depending if it is a full or half rate channel.
The next subsystem block is the Network Switching Subsystem (NSS), as it can be seen on figure \ref{img:GSMBig}, on right
side inside of the gray dashed line rectangle. The main task of NSS is to connect the GSM with other telephony networks
(GSM networks from other providers or the Public Switched Telephone Network) \citep[Chapter 4]{0470742984}. It consists of
-Mobile Switching Center
+Mobile Switching Center (MSC), Gateway Mobile Switching Center (GMSC) and databases.
+
+MSC's main function is to route incoming and outgoing calls between the moving mobile users,
+``the assignment of user channels toward the BSS'' \citep[Chapter 4]{0890064717} \citep{konrad}. GMSC is
+a type of MSC for external networks, GSM networks from other providers or telephone networks are
+routed through the GMSC\citep[Chapter 4]{0890064717}.
+
+There are four databases: Home Location Register (HLR), Visitor Location Register (VLR), Authentication Center (AUC)
+and Equipment Identity Register (EIR). HLR database stores data about the GSM subscribers of a network provider. The data that
+can be found in HLR: the unique International Mobile Subscriber Identity (IMSI) - that is stored on the SIM card of a mobile user;
+usage statistics; subscriber's number (MSISDN) and the current location of the mobile user acquired by knowing the location
+of the BSC controlling the BTS that provides at the current moment the GSM air interface to the mobile user \citep{konrad}.
+VLR serves as a temporary data storage of important parts of HLR data (not all data known for the particular user)
+of all the visiting mobile subscribers served by the current MSC. i.e. if a MS from its home MSC enters an area covered
+by the newly entered MSC, its VLR will request some of the HLR data from the HLR database of the MSC where
+the MS is registered \citep{dennis} \citep[Chapter 4]{0470742984}. AUC contains confidential keys for each mobile subscriber
+required for encrypting the data before they are transmitted to the MS from the BTS \citep[Chapter 3]{0470030704}.
+The keys located in AUC are also required for the MS to register
+in the network \citep{konrad}. EIR is an optional database but contains data about approved types of
+mobile equipment (not stolen cell phones), black listed cell phones (they are identified by their International
+Mobile Equipment Identity number which is unique for every manufactured cell phone) and cell phones which
+ought to be tracked if they register \citep[Chapter 4]{0890064717}.
+
\newpage
\section{Logical channels and the SDCCH channel}