Unit 1 (GSM)

What does 'Downlink' mean?

'Downlink' refers to transmission from the network to the mobile device

What does 'Uplink' mean?

'Uplink' refers to the transmission from the mobile device to the network

Define the Synchronous Transfer Mode (STM) standard

- (existing or occurring at the same time.) - For higher transmission speeds and for long distances, optical systems that use the synchronous transfer mode (STM) standard are used. - Multiplexing several circuits over transmission links and switches where by time is divided into slots or buckets, and circuits are given time units whether or not they have any data to transmit.

Once the BSC decides to perform a handover what happens?

- A TCH is activated in the new cell - Afterward, the BSC informs the mobile device via the old cell with a Handover Command message that is sent over the FACCH

For the transmission of voice data, a TCH is used in GSM:

- A TCH uses all but two bursts of a 26‐burst multiframe, with one being reserved for the SACCH and the other remaining empty to allow the mobile device to perform neighboring cell measurement - As has been shown in the preceding section, a burst that is sent to or from the mobile every 4.615 milliseconds can carry exactly 114 bits of user data

How are IP packets transferred on a network?

- A link with a high bandwidth is used to transfer the packets of many users. - By using the destination address contained in each packet, each network node that the packet traverses decides over which outgoing link to forward the packet.

Describe the Layer (4-7) OSI/SS7

- A number of different protocols are used on layers 4-7 depending on the application

What is a E‐3 connection and what are the benefits?

- A single E‐1 connection with 31 DS0s is not enough to connect two switching centers with each other - An alternative is an E‐3 connection over twisted pair or coaxial cables. - Benefits: An E‐3 connection is defined at a speed of 34.368 Mbit/s, which corresponds to 512 DS0s.

Advantage of Aaptive Multirate (AMR) codec

- Advantage of this approach is the ability to switch to a speech codec with a higher compression rate during bad radio signal conditions to increase the number of error detection and correction bits. - If signal conditions permit, a lower rate codec can be used, which only uses every second burst of a frame for the call. - This in effect doubles the capacity of the cell as a single timeslot can be shared by two calls in a similar manner to the HR codec

What happens if the user clicks "sends" to make a phone call?

- After pressing the 'send' button, which is usually the button that is also used to start a phone call after typing in a phone number, the mobile device sends the string to the HLR via the MSC, where the string is analyzed and the requested operation is performed. - For example, call forwarding to another phone (e.g. 0782 192 8355) while a user is already engaged in another call - call forward busy (CFB) is activated with the following string: **67* 07821928355# + call button.

How can a call be established between two parties by using ISUP messages. (Part 3)

- As B is a fixed‐line subscriber, the next step for the MSC is to establish a voice channel to the fixed‐line switching center. - This is done by sending an ISUP Initial Address Message (IAM).

What is the purpose of Signaling Gateways (SGWs)

- As IP links cannot be introduced on all interfaces in live networks at once, Signaling Gateways (SGWs) have been defined to bridge E‐1‐based and IP‐based SS‐7 communication - The SGWs adapt the lower layers of the protocol stack and thus make the differences transparent for both sides - is a network component responsible for transferring signaling messages (i.e. information related to call establishment, billing, location, short messages, address conversion, and other services) between Common Channel Signaling (CCS) nodes that communicate using different protocols and transports.

How can a switch predict where the call should be forwarded to for a certain subscriber?

- As a mobile network consists of many switching centers, with each covering a certain geographical area, it is not even possible to predict in advance which switching center a call should be forwarded to for a certain subscriber. - This means that the software for subscriber management and routing of calls of fixed‐line networks cannot be used for GSM - Instead of a static call‐routing mechanism, a flexible mobility management architecture in the core network became necessary, which needed to be aware of the current location of the subscriber and thus able to route calls to them at any time.

Explain the Mobility Management (MM)

- As subscribers can roam freely in the network, the MSC is also responsible for the Mobility Management (MM) of subscribers such as: - Authentication of subscribers at connection establishment is necessary because a subscriber cannot be identified as in the fixed network by the pair of copper cables over which the signal arrives. - If no active connection exists between the network and the mobile device, the MS has to report a change of location to the network to be reachable for incoming calls and SMS messages - If the subscriber changes their location while a connection is established with the network, the MSC is part of the process that ensures that the connection is not interrupted and is rerouted to the next cell. This procedure is called handover

Explain billing with the MSC

- As the MSC controls all connections, it is also responsible for billing. This is done by creating a billing record for each call, which is later transferred to a billing server - The billing record contains information like the number of the caller and the calling party, cell ID of the cell from which the call originated, time of call origination, duration of the call, and so on

What happens if the user changes locations when their phone is turned on?

- As the message waiting flag is also set in the HLR, the SMS also reaches a subscriber who has switched off the mobile device in London, for example, and switches it on again after a flight to Los Angeles - When the mobile device is switched on in Los Angeles, the visited MSC reports the location to the subscriber's home HLR (location update). - The HLR then sends a copy of the user's subscription information to the MSC/VLR in Los Angeles including the message waiting flag and thus the SMSC can also be notified that the user is reachable again.

Why was the switching matrix in the MSC replaced with the media gateway device?

- As the simultaneous operation of two different networks is very inefficient and costly, most network operators have

What needs to be done before voice signal is forwarded?

- Before the voice signal can be forwarded, it needs to be digitized. - For an analog fixed‐line connection, this is done in the switching center, while an ISDN fixed‐line phone or a GSM mobile phone digitizes the voice signal itself.

How to calculate the total capacity of a base station?

- By combining the two multiple access schemes it is possible to approximately calculate the total capacity of a base station. - For the following example, it is assumed that the base station is split into three sectors and each sector is covered by an independent cell. Each cell is typically equipped with three transmitters and receivers (transceivers). In each sector, 3×8=24 timeslots are thus available.

The solution for the air interface would have been?

- Define air interface channels that can also carry 64 kbit/s PCM‐coded voice channels. - This has not been done because the scarce resources on the air interface have to be used as efficiently as possible

Analog vs Digital Signals

- Digital signals can convey information with less noise, distortion, and interference. - Digital circuits can be reproduced easily in mass quantities at comparatively low costs. - Digital signal processing is more flexible because DSP operations can be altered using digitally programmable systems.

Explain Timing Advance Control

- Due to the limited speed of radio waves, a time shift of the arrival of the signal can be observed when a subscriber moves away from a base station during an ongoing call - If no countermeasures are taken, this would mean that at some point the signal of a subscriber would overlap with the next timeslot despite the guard time of each burst - Thus, the signal of each subscriber has to be carefully monitored and the timing of the transmission of the subscriber has to be adapted with this procedure: Timing Advance Control.

How did network operators deal with the rise of the Internet and IP‐based applications?

- Due to the rise of the Internet and IP‐based applications, network operators thus had to maintain two separate networks: 1. A circuit‐switched network for voice calls 2. A packet‐switched network for Internet‐based services.

How much power is used during a Phone Call?

- During call establishment, the mobile device always uses the highest allowed power output level, which is then reduced or increased again by the network step by step - As the mobile device only sends on one of the eight timeslots of a frame, the average power output of the mobile device is only one‐eighth of this value. The average power output of a mobile device that sends with a power output of 2 W is thus only 250 mW

How did Early fixed‐line telecommunication networks function

- Early fixed‐line telecommunication networks were designed only for voice communication, for which an analog connection between the par- ties was established. - In the mid‐1980s, analog technology was superseded by digital technology in the switching center. - Which means that calls were no longer sent over an analog line from the originator to the terminator. - Instead, the switching center digitized the analog signal that it received from the subscribers, which were directly attached to it, and forwarded the digitized signal to the terminating switching center

Several components in the network are involved in the call establishment,

- Ex: if originating and terminating parties are not connected to the same switching center, it is also necessary that the different nodes in the network exchange information with each other

What is the reference unit of a telecommunication network?

- E‐1 connection in Europe - T‐1 connection in the United States - They use either a twisted pair or coaxial copper cable

Why is the Signaling Connection and Control Part (SCCP) is used on layer 4?

- For communication between the switching centers (SSPs) and the databases (SCPs), the Signaling Connection and Control Part (SCCP) is used on layer 4 - Protocols on layer 4 of the protocol stack enable the distinguishing of different applications on a single system - In the SS‐7 world, the task of forwarding incoming messages to the right application is done by SCCP - Stimulus-based protocol and is designed as a communications protocol for hardware endpoints and other embedded systems, with significant CPU and memory constraints. Some Cisco analog media gateways, such as the VG248 gateway, register and communicate with Cisco Unified Communications Manager using SCCP

What is the TCAP

- For database access, the Transaction Capability Application Part (TCAP) protocol has been designed as part of the SS‐7 family of protocols - TCAP defines a number of different modules and messages that can be used to query all kinds of different data- bases in a uniform way. - an Signaling System 7 telecommunications (SS7) protocol which provides functions to a large variety of applications distributed over switches and specialized centers in telecommunication network

What is the Stream Control Transmission Protocol (SCTP)

- For the transmission of SS‐7 messages, however, a new protocol has been specified, which is referred to as Stream Control Transmission Protocol (SCTP) - Transport-layer protocol that ensures reliable, in-sequence transport of data

Ethernet links

- For virtual circuit switching over IP, optical Ethernet links are often used between network nodes at the same location. - Transmission speeds of 1 Gbit/s or more are used on these links. - Ethernet is the de facto standard for IP‐based communication over fiber and copper cables and is widely used.

What is the Global System for Mobile Communications (GSM)

- GSM was designed as a circuit‐switched system that establishes a direct and exclusive connection between two users on every interface between all network nodes of the system - Similar way to fixed‐line phone networks - GSM is used worldwide by billions of people today. - A large percentage of mobile voice calls are still handled by GSM

What are gateways between mobile networks based on?

- Gateways between mobile networks are usually still based on ISUP and circuit‐ switched links, even though most networks are based on IP technology today - In the future, this is expected to change as advanced speech codecs such as Wideband‐AMR can only be used over BICN and IP‐based transport links.

What happens If B answers the call?

- If B answers the call, the fixed‐line switching center sends an Answer Message (ANM) to the MSC and conversation can start

A Dedicated Channel - The Access Grant Channel (AGCH)

- If a subscriber sends a Channel Request message on the RACH, the network allocates an SDCCH or, in exceptional cases, a TCH, and notifies the subscriber on the AGCH via an Immediate Assignment message

Explain the Europe GSM Uplink and Downlink speeds

- In Europe, GSM was initially specified only for operation in the 900 MHz band between 890 and 915 MHz in the uplink direction and between 935 and 960 MHz in the downlink direction

Why do different set of extensions needs to be implemented?

- In order for a vendor to be able to sell its equipment. - Furthermore, the interconnection of networks of different countries is complicated by this

In the fixed‐line network, how are connections made?

- In the fixed‐line network, analog phones signal their connection request when the receiver is lifted off the hook and a phone number dialed that is sent to the network either via pulses (pulse dialing) or via tone dialing, which is called dual tone multifrequency (DTMF) dialing - With fixed‐line ISDN phones and GSM mobile phones, the signaling is done via a separate dedicated signaling channel, and information such as the destination phone number is sent as messages.

What happens if the subscriber roams out of the coverage area of the cell in which the call was initially established?

- In this case, the BSC has to redirect the call to the appropriate cell. - This procedure is called handover. - To be able to perform a handover to another cell, the BSC requires signal quality measurements for the air interface

Why is the IMSI internationally unique?

- It enables a subscriber to use their phone abroad if a GSM network is available that has a roaming agreement with their home operator. - When the mobile device is switched on, the IMSI is retrieved from the SIM card and sent to the MSC. There, the MCC and MNC of the IMSI are analyzed and the MSC is able to request the subscriber's record from the HLR of the subscriber's home network.

How has the MNP increased the complexity of call routing and billing?

- It is no longer possible to use the NDC to decide which tariff to apply to a call. - Instead of a simple call‐routing scheme based on the NDC, the networks now have to query an MNP database for every call to a mobile subscriber to find out if the call can be routed inside the network or if it has to be forwarded to a different national mobile network.

What happens when the base station receives the burst

- It measures the delay and forwards the request, including a timing advance value required for this mobile device, to the BSC - The BSC reacts to the connection request by returning an Immediate Assignment message to the mobile device on the AGCH

What happens if the subscriber leaves a coverage area?

- It was also necessary to be able to flexibly change the routing of an ongoing call as a subscriber can roam freely and thus might leave the coverage area of the radio transmitter of the network over which the call was established.

Without standards, which are defined by the International Telecommunication Union (ITU), what happens?

- It would not be possible to make phone calls internationally and network operators would be bound to the supplier they initially select for the delivery of their network components.

A Dedicated Channel - The Slow Associated Control Channel (SACCH)

- It's also assigned to a dedicated connection. It is used in the direction to report signal quality measurements of the serving cell and neighboring cells to the network. - The network then uses these values for handover decisions and power control.

A Dedicated Channel - The Broadcast Common Control Channel (BCCH)

- It's the main information channel of a cell and broadcasts SYS_INFO messages that contain a variety of information about the network. - The channel is monitored by all mobile devices which are switched on but currently not engaged in a call or signaling connection (idle mode), and broadcasts

What is the Short Messaging Service Center (SMSC)

- It's used to store and forward short messages. - The SMS was only introduced about four years after the first GSM networks went into operation, as an add‐on, and has been specified in 3GPP TS 23.040

How do the C and D interfaces interact?

- Like in classic core networks, the C and D interfaces are used in a BICN network to communicate with the HLR

How are services displayed to the user on a mobile device?

- Most services can be configured by the subscriber via a menu on the mobile device. - The menu, however, is just a graphical front end for the user and the mobile device translates the user's commands into numerical strings which start with an '*' character. - These strings are then sent to the network by use of an Unstructured Supplementary Service Data (USSD) message - The codes are standardized in 3GPP, and are thus identical in all networks

How does the Interface A connect the radio network?

- On the radio network side, the A interface continues to be used to connect the radio network to the MSC‐Ss and MGWs - The connection can be made without any changes in the radio network over the classic E‐1‐based A interface or over an IP‐based A interface - A interface has been made more flexible and can now be connected to several media gateways. This adds redundancy toward the radio network as well, as a geographical region can still be served even if a media gateway fails

What happens if the call gets established?

- Once the connection has been successfully established, the BTS continually monitors the delay experienced for this channel and reports any changes to the BSC. - The BSC in turn instructs the mobile device to change its timing advance by sending a message on the SACCH

What happens once the connection is established via the switching matrix?

- Once the connection is established via the switching matrix, the switch applies a periodic current to the line of the fixed‐line subscriber so that the fixed‐line phone can generate an alerting tone

A Dedicated Channel - The Paging Channel (PCH)

- PCH is used to inform idle subscribers of incoming calls or SMS messages. - As the network alone is aware of the location area the subscriber is roaming in, the Paging message is broadcast in all cells belonging to the location area.

A Dedicated Channel - The Random Access Channel (RACH)

- RACH is the only common channel in the uplink direction. - If the mobile device receives a message via the PCH that the network is requesting a connection establishment or if the user wants to establish a call or send an SMS, the RACH is used for the initial communication with the network

Define The European Telecommunication Standards Institute (ETSI)

- Responsible for a number of other standards, was the main body responsible for the creation of the GSM standard. - The ETSI GSM standards are composed of a substantial number of standards documents, each of which is called a technical specification (TS) and describes a particular part of the system

How can SMS be used?

- SMS can be used for person‐to‐person messaging as well as for providing notification of other events such as a missed call that was forwarded to the voice mail system. - The transfer method for both cases is identical. - The sender of an SMS prepares the text for the message and then sends the SMS via a signaling channel to the MSC - The SMS message also contains the MSISDN of the destination party and the address of the SMSC, which the mobile device has retrieved from the SIM card

What does the control of the timing advance start with?

- Starts with the first network access on the RACH with a Channel Request message. This message is encoded into a very short burst that can only transport a few bits in exchange for large guard periods at the beginning and end of the burst. - This is necessary because the mobile device is unaware of the distance between itself and the base station when it attempts to contact the network - Thus, the mobile device is unable to select an appropriate timing advance value

Define Frame?

- Subscribers are time multiplexed by dividing the carrier into frames with durations of 4.615 milliseconds. - Each frame contains eight physically independent timeslots, each for communication with a different subscriber

What is the digital signal level 0 (DS0) channel used for?

- Such a channel can be used to transfer voice or data, and thus it is usually not called a speech channel but simply referred to as a user data channel.

How does the BSC establish signaling channels for incoming calls or SMS messages?

- The BSC receives a Paging message from the MSC, which contains the IMSI and TMSI of the subscriber as well as the location area ID in which the subscriber is currently located. - The BSC in turn has a location area database that it uses to identify all cells in which the subscriber needs to be paged. When the mobile device receives the Paging message, it responds to the network in the same way as in the example above by sending a Channel Request message.

What is the function of BTS and BSC?

- The BTS (Base Transceiver Station) is controlling the radio transmission and the interface to the mobile phones - Several BTS connect to BSC stations (Base Station Controller) - BSC are connected to the MSC (Mobile Switching Center)

Describe the C Interface

- The C interface is used to connect the MSCs of a network with the HLR of the mobile network. - While the A and E interfaces that were described previously always consist of signaling and speech path links, the C interface is a pure signaling link. - Speech channels are not necessary for the C interface as the HLR is purely a database, which cannot accept or forward call - Despite being only a signaling interface, E‐1 connections are used for this interface.

A Dedicated Channel - The Fast Associated Control Channel (FACCH)

- The Fast Associated Control Channel (FACCH) is transmitted on the same timeslot as a TCH - It is used to send urgent signaling messages like a handover command these messages do not have to be sent very often, no dedicated physical bursts are allocated to the FACCH. Instead, user data is removed from a TCH burst

HLR (Home Location Register) vs VLR

- The HLR contains information regarding users who are registered as subscribers in the area, whereas the VLR contains information regarding users who are registered as subscribers somewhere else but happen to be roaming in the area - HLR contains information about all subscribers within a network, while VLR contains more dynamic information relevant to subscribers roaming within the VLR area - HLR acts as a fixed reference point for a particular mobile station (subscriber), while its VLR can vary depending on mobility and network design

What is the ISUP protocol is used for the establishment of?

- The ISUP protocol is used for the establishment of voice calls between switching centers and the assignment of a 64 kbit/s timeslot. - In an IP‐based network, voice calls are transmitted in IP packets. - The resulting protocol is referred to as the Bearer‐Independent Call Control (BICC) protocol - The protocol used to support the signaling necessary to provide voice and non-voice services in telephone communications. - It is an extension of SS7, used as the interface protocol for voice and data within, and for ingression or egression to/from the Public Switched Telephone Network (PSTN.)

Explain The International Mobile Subscriber Identity (IMSI)

- The International Mobile Subscriber Identity (IMSI) is an internationally unique number that identifies a subscriber and is used for most subscriber‐related signaling in the network - The IMSI is stored in the subscriber's subscriber identity module (SIM) card and in the HLR and is thus the key to all information about the subscriber.

During an ongoing call..

- The MSC sends the 64 kbit/s PCM‐encoded voice signal toward the radio network and the TRAU converts the voice stream in real‐time into a 13 kbit/s compressed data stream, which is transmitted over the air interface - The BSC sends a continuous stream of compressed voice data toward the core network and the TRAU converts the stream into a 64 kbit/s coded PCM signal.

Describe the Message Transfer Part 1 (MTP‐1) protocol (OSI Layer 1)

- The Message Transfer Part 1 (MTP‐1) protocol describes the physical properties of the transmission medium on layer 1 of the OSI model - Definition of the different kinds of cables that can be used to carry the signal, signal levels and transmission speeds.

What does the NDC (Networked Data Center) represent?

- The NDC usually represents the code with which the network operator can be reached. It is normally three digits in length - It should be noted that mobile networks in the United States use the same NDCs as fixed‐line networks. Thus, it is not possi- ble for a user to distinguish whether they are calling a fixed‐line or a mobile phone. - This impacts both billing and routing, as the originating network cannot deduct which tariff to apply from the NDC - Networking resources — switching, routing, load balancing, analytics, etc. — to facilitate the storage and processing of applications and data.

A Dedicated Channel - The Standalone Dedicated Control Channel (SDCCH)

- The Standalone Dedicated Control Channel (SDCCH) is a pure signaling channel that is used during call establishment when a subscriber has not yet been assigned a TCH.

Main functions of the VLR

- The VLR is mainly used to reduce signaling between the MSC and the HLR - If a subscriber roams into the area of an MSC, the data are copied to the VLR of the MSC and are thus locally available for every connection establishment. - Verification of the subscriber's record at every connection establishment is necessary as the record contains information about the services that are active and the services from which the subscriber is barred - For example, to bar outgoing calls while allowing incoming calls, to prevent abuse of the system

Explain the AMR‐Wideband (AMR‐WB) codec algorithm:

- The algorithm allows, as its name implies, digitization of a wider frequency spectrum than is possible with the PCM algorithm that was described earlier. - Instead of an upper limit of 3400Hz, AMR‐WB digitizes a voice signal up to a frequency of 7000 Hz. - As a consequence, the caller's voice sounds much clearer and more natural on the other end of a connection.

What are GSM networks benefit for roaming agreements?

- The common standard substantially reduces research and development costs as hardware and software can now be sold worldwide with only minor adaptations for the local market

Describe the Message Transfer Part 2 (MTP‐2) protocol (OSI Layer 2)

- The data link layer, messages are framed into packets and a start and stop identification at the beginning and end of each packet are inserted into the data stream so that the receiver is able to detect where a message ends and where a new message begins.

The disadvantage of the EFR codec is

- The disadvantage of the EFR codec is the higher complexity of the compression algorithm, which requires more processing power. - However, the processing power available in mobile devices has increased significantly since the 1990s, and thus modern GSM phones easily cope with the additional complexity.

Explain the establishment of voice calls

- The establishment of a TCH for voice calls is always requested by the MSC for both mobile‐originated and mobile‐terminated calls. - Once the mobile device and the MSC have exchanged all necessary information for the establishment of a voice call via an SDCCH, the MSC sends an assignment request for a voice channel to the BSC - The BSC then verifies if a TCH is available in the requested cell and, if so, activates the channel in the BTS. Afterward, the mobile device is informed via the SDCCH that a TCH is now available for the call. - The mobile device then changes to the TCH and FACCH. To inform the BTS that it has switched to the new channel, the mobile device sends a message to the BTS on the FACCH, which is acknowledged by the BTS. - In this way, the mobile also has a confirmation that its signal can be decoded correctly by the BTS. - Finally, the mobile device sends an Assignment Complete message to the BSC, which in turn informs the MSC of the successful establishment of the TCH.

How does the fixed‐line switching center receives the message?

- The fixed‐line switching center receives the message, analyzes the phone number and establishes a connection via its switching matrix to subscriber B

What is the Mobile Switching Center (MSC)

- The hardware as well as the lower layers of the software which are responsible, for example, for the handling of the switching matrix were mostly identical.

What is the Call Control (CC) protocol,

- The management activities to establish and maintain a connection: - Registration of mobile subscribers: When the mobile device, also referred to as MS, is switched on, it registers to the network and is then reachable by all other subscribers of the network. - Call establishment and call routing between two subscribers. - Forwarding of SMS messages.

How can the timing advance can be controlled?

- The maximum distance between a base station and a mobile subscriber is in theory 64 × 550 m = 35.2 km. - In practice, such a distance is not reached very often as base stations usually cover a much smaller area for capacity reasons. - The transmission power of the mobile device is also not sufficient to bridge such a distance under non‐line‐of‐sight conditions to the base station. Therefore, one of the few scenarios where such a distance has to be overcome is in coastal areas, from ships at sea.

What happens at the beginning of a call?

- The network establishes a direct connection between two parties, which is then used exclusively for this conversation - The switching center uses a switching matrix to connect any originating party to any destination party. - Once the connection has been established, the conversation is then transparently transmitted via the switching matrix between the two parties. - The switching center only becomes active again to clear the connection in the switching matrix if one of the parties wants to end the call. - This approach is identical in both mobile and fixed‐line networks

Describe the Message Transfer Part 3 (MTP‐3) protocol (OSI Layer 3)

- The network layer, is responsible for packet routing. - To enable network nodes to forward incoming packets to other nodes, each packet gets a source and destination address on this layer. - MTP‐3 protocol uses so‐called point codes to identify the source and the destination of a message.

Describe the MEGACO (Media Gateway Control) protocol

- The protocol is used, to establish voice channels to two parties and then to logically connect the two channels in the MGW. - The protocol is also used to instruct the MGWs to play announcements to inform users of events, for example, where the called party is currently not available or is busy. - To establish conference calls between more than two subscribers

What is the sender only notified of?

- The sender is only notified that the SMS has been correctly received by the SMSC. - However, if supported by a device, it is also possible to request an end‐to‐end delivery notification from the SMSC.

How do subscribers interact in a GSM?

- The subscribers are mobile and can change their location at any time. - Thus, it is not possible to use the same input and output in the switching matrix for a user for each call as is the case in fixed‐line networks

What is the time frame of a timeslot?

- The time frame of a timeslot is called a burst and the burst duration is 577 microseconds - For example, if a mobile device is allocated timeslot number 2 for a voice call, then the mobile device will send and receive only during this burst. Afterward, it has to wait until the next frame before it is allowed to send again.

A Dedicated Channel - The Traffic Channel (TCH)

- The traffic channel (TCH) is a user data channel. - It can be used to transmit a digitized voice signal or circuit‐switched data services of up to 14.4 kbit/s.

The TRAU offers a number of different algorithms for speech compression:

- These algorithms are called speech codecs or simply codecs. - The first codec that was standardized for GSM is the full‐rate (FR) codec, which reduces the 64 kbit/s voice stream to about 13 kbit/s.

Benefits of this replacement to the Media Gateway (MG)?

- This allows them to virtualize circuit switching and to transfer voice calls over IP packet - The physical presence of a circuit‐ switched infrastructure is thus no longer necessary and the network operator can concentrate on maintaining and expanding a single IP‐based network - This approach has been standardized under the name 'Bearer‐Independent Core Network' (BICN), used for supporting narrowband Integrated Services Digital Network (ISDN) service over a broadband backbone network

Supplementary service - Call waiting (CW)

- This feature allows signaling of an incoming call to a subscriber while they are already engaged in another call - The first call can then be put on hold to allow the subscriber to accept the incoming call. - The feature can be activated or barred by the operator and switched on or off by the subscriber

Supplementary service - Barring of all outgoing calls (BAOC)

- This functionality can be activated by the network operator if, for example, the subscriber has not paid their monthly invoice in time. - It is also possible for the network operator to allow the subscriber to change the state of this feature together with a PIN (personal identification number) so that the subscriber can lend the phone to another person for incoming calls only

While the TRAU is a logical component of the BSS, it is most often installed next to an MSC in practice. What's the advantage?

- This has the advantage that four compressed voice channels can be transmitted in a single E‐1 timeslot. - After compression, each voice channel uses a 16 kbit/s sub‐timeslot. Thus, only one‐quarter of the transmission capacity between an MSC and BSC is needed in comparison to an uncompressed transmission

How has the GSM Changed?

- This physical circuit switching has been virtualized and many network nodes are connected over IP‐based broadband connections today

What is The Mobile Application Part (MAP)

- This protocol has been standardized in 3GPP TS 29.002 [3] and is used for the communication between an MSC and the HLR, which maintains subscriber information - MAP is also used between two MSCs if the subscriber moves into the coverage area of a different MSC while a call is ongoing - MAP protocol uses the TCAP, SCCP and MTP protocols on lower layers.

The Direct Transfer Application Part (DTAP).

- This protocol is used between the user's mobile device, which is also called mobile station (MS), and the MSC, to communicate transparently. - To establish a voice call, the MS sends a Setup message to the MSC. - As it is only the MSC's task to forward calls, all network nodes between the MS and the MSC forward the message transparently and thus need not understand the DTAP protocol - Facilitating the transfer of session management, mobility management and call control signalling associated with procedures such as Network Attach, Location Area Update and Call Establishment.

What is the Base Station Subsystem Mobile Application Part (BSSMAP)

- This protocol is used for communication between the MSC and the radio network. Necessary, for example, to establish a dedicated radio channel for a new connection to a mobile subscriber - As BSSMAP is not a database query language like the MAP protocol, it is based directly on SCCP instead of TCAP being used in between.

Explain the The Voice Group Call Service (VGCS) service:

- This service offers a circuit‐switched walkie‐ talkie functionality to allow subscribers who have registered to a VGCS group to communicate with all other subscribers in the area who have also subscribed to the group. - To talk, the user has to press a 'push to talk' button. If no other subscriber holds the uplink, the network grants the request and blocks the uplink for all other subscribers while the push to talk button is pressed. - All mobile devices that participate in the group call listen to the same timeslot in the downlink direction

What is the SS‐7 protocol?

- This signaling is transparent for the user, and a protocol called the SS‐7 is used for this purpose. - It's also used in GSM networks and the standard has been enhanced by ETSI to fulfill the special requirements of mobile networks - An example: subscriber mobility management.

How to add redundancy for load balancing concerns.

- To add redundancy and for load‐balancing reasons, several MSC‐Ss and MGWs can be interconnected in a mesh. - If an MSC‐S fails, an MGW can thus still continue to operate, and is then controlled by another serve - Thus, a single MSC‐S is no longer solely responsible for a single geographical area as was the case in the traditional model.

How to bridge voice calls between E‐1‐based and IP‐based networks?

- To bridge voice calls between E‐1‐based and IP‐based networks, Media Gateways (MGWs) are used. - Connected to an MSC‐Server, an MGW handles both IP‐based and E‐1‐based voice calls transparently as it implements both the classic and IP‐based signaling protocol stacks.

How can a call be established between two parties by using ISUP messages. (Part 2)

- To call B, the phone number of B is sent by A to the MSC - The MSC then analyzes the national destination code (NDC) of the phone number, which usually comprises the first two to four digits of the number, and detects that the number belongs to a subscriber in the fixed‐line network - MSC and the fixed‐line switching center are directly connected with each other. Therefore, the call can be directly forwarded to the terminating switching center.

How to change the MSISDN?

- To change the MSISDN, only the HLR record of the subscriber needs to be changed. - In effect, this means that the mobile device is not aware of its own phone number. - This is not necessary because the MSC automatically adds the user's MSISDN to the message flow for a mobile‐originated call establishment so that it can be presented to the called party.

How to enable the MSC to communicate with other nodes?

- To enable the MSC to communicate with other nodes of the network, it is connected to them via standardized interfaces - This allows network operators to acquire different components for the network from different network equipment vendors. - The interfaces discussed below are either transmitted over timeslots in circuit‐ switched E‐1 lines or over an IP‐based network.

Explain how the mobile device reports the quality of signals it receives from other cells?

- To enable the mobile device to perform these measurements, the network sends the frequencies of neighboring cells via the SACCH during an ongoing call - The mobile device then uses this information to perform the neighboring cell measurements while the network communicates with other subscribers and reports the result via measurement report messages in the uplink SACCH. - The network receives these measurement values and is thus able to periodically evaluate if a handover of an ongoing call to a different cell is necessary

How to establish a voice connection

- To establish a voice connection, MSC‐Ss and MGWs communicate over the Mc interface - This interface does not exist in the classical model, as the MSC contained both components

How to increase the capacity of a base station?

- To increase the capacity of a base station, the coverage area is usually split into two or three sectors which are then covered on different frequencies by a dedicated transmitter. - This allows a better reuse of frequencies in two‐dimensional space than is the case where only a single frequency is used for the whole base station. Each sector of the base station, therefore, forms its own independent cell.

Define the Address Complete Message (ACM)

- To indicate to the originating subscriber that the phone number is complete and the destination party has been found, the fixed‐line switch sends back an Address Complete Message (ACM) - The MSC then knows that the number is complete and that the terminating party is being alerted about the incoming call.

How to reduce interference?

- To reduce interference, the BSC is also in charge of controlling the transmission power for every air interface connection. - For the mobile device, an active power control has the advantage that the transmission power can be reduced under favorable reception conditions.

A high compression rate is used for?

- To reduce the datarate of a voice stream down to 12.65 kbit/s. - This way, an AMR‐WB data stream can be transmitted in a single GSM timeslot, and also requires no additional capacity in a UMTS network

Explain Transmission power

- Transmission power is controlled using the signal quality measurements of the BTS for the connection. - If the mobile device's transmission power has to be increased or decreased, the BSC sends a Power Control message to the BTS. - The BTS in turn forwards the message to the mobile device and repeats the message on the SACCH in every frame.

Describe IP‐based communication over a network:

- Typically, connectivity is then established over a single IP‐based link - The air interface between the mobile devices and the network is not affected by the transition from circuit to packet switching - For mobile devices, whether the network uses classic or virtual circuit switching is therefore completely transparent

Define Aaptive Multirate (AMR) codec

- Used by most devices and networks today. - Instead of using a single codec, which is selected at the beginning of the call, AMR allows a change of the codec dur ing a call.

Explain the Nb interface and how speech channels are transmitted.

- Virtual speech channels that have been negotiated over the Nc interface are transmitted between MGWs over the Nb interface - The combination of the Nb interface and Nc interface thus replaces the E interface of the classic network architecture

What happens B ends the call?

- When B ends the call, the fixed‐line switching center resets the connection in the switching matrix and sends a Release (REL) message to the MSC. - The MSC confirms the termination of the connection by sending back a Release Complete (RLC) message - If A had terminated the call, the messages would have been identical, with only the direction of the REL and RLC reversed.

What happens if a subscriber leaves a coverage area of an MSC?

- When a subscriber leaves the coverage area of an MSC, their record is copied from the HLR to the VLR of the new MSC, and is then removed from the VLR of the previous MSC. - The communication with the HLR is standardized in the D interface specification

What happens when the MSC receives an SMS from a subscriber?

- When the MSC receives an SMS from a subscriber, it transparently forwards the SMS to the SMSC. - As the message from the mobile device contains the address of the subscriber's SMSC, international roaming is possible and the foreign MSC can forward the SMS to the home SMSC without the need for an international SMSC database

Main use of Aaptive Multirate (AMR) codec:

- While AMR is optional for GSM, it has been chosen for the UMTS system as a mandatory feature. - In the United States, AMR is used by some network operators to increase the capacity of their network, especially in very dense traffic areas like New York, where it has become very difficult to increase the capacity of the network any further, with over half a dozen carrier frequencies per sector already used

Difference between circuit switching and the Internet?

- While circuit switching establishes an exclusive channel between two parties - The Internet is based on transferring individual data packets

Describe The Base Station Controller (BSC)

- While the base station is the interface element that connects the mobile devices with the network, the BSC is responsible for the establishment, release and maintenance of all connections for cells that are connected to it.

Define the ISDN (Integrated Services Digital Network) Line

- With this system, the transmission became fully digital and the conversion back to an analog audio signal was done directly in the phone - provides high-speed, high-bandwidth channels to every subscribers on the public switched telephone network, achieving end-to-end digital functions with standard equipment interface devices

Define Internet Protocol (IP)

- a set of rules governing the format of data sent over the internet or other network. - Services, such as voice calls, e‐mail, instant messaging (IM), social networks (e.g. Facebook), blogs, wikis, share the Internet Protocol (IP) as a transport protocol and globally connect people via the Internet.

The IAM message is not sent directly to the fixed‐line switching center. How is the message forwarded?

- an STP is used to forward the message.

The advantage for the network operator of the HR codec is?

- half‐rate (HR) codec has been defined for GSM that only requires a bandwidth of 7 kbit/s - The advantage for the network operator of the HR codec is that the number of simultaneous voice connections per carrier can be doubled. - With the HR codec, a single timeslot, which is used for a single EFR voice channel, can carry two (HR) TCHs.

Describe the A interface

- the BSSMAP and DTAP protocols are used over the A interface for communication between the MSC, the BSS and the mobile devices. As an E‐1 connection can only carry 31 channels, many E‐1 connections are necessary to connect an MSC to the BSS

The trend toward IP‐based communication can also be observed in the GSM radio network.

- when a radio base station site supports GSM, UMTS and LTE simultaneously

The SS‐7 standard defines three basic types of network nodes:

1. Service Switching Points (SSPs) are switching centers that are more generally referred to as network elements and that are able to establish, transport or forward voice and data connections. 2. Service Control Points (SCPs) are databases and application software that can influence the establishment of a connection. Used for storing the current location of a subscriber. During call establishment to a mobile subscriber, the switching centers query the database for the current location of the subscriber to be able to forward the call. 3. Signaling Transfer Points (STPs) are responsible for the forwarding of signaling mes- sages between SSPs and SCPs as not all network nodes have a dedicated link to all other nodes of the network.

A GSM network is split into three subsystems

1. The Base Station Subsystem (BSS), which is also called 'radio network,' contains all nodes and functionalities that are necessary to wirelessly connect mobile subscribers over the radio interface to the network. The radio interface is usually also referred to as the 'air interface.' 2. The Network Subsystem (NSS), which is also called 'core network', contains all nodes and functionalities that are necessary for switching of calls, for subscriber management and mobility management 3. The Intelligent Network Subsystem (IN) comprises SCP (Service control point) databases that add optional functionality to the network. One of the most important optional IN functionalities of a mobile network is the prepaid service, which allows subscribers to first fund an account with a certain amount of money which can then be used for network services like phone calls, Short Messaging Service (SMS) messages and, of course, data services

The IMSI consists of the following parts:

1. The Mobile Country Code (MCC). The MCC identifies the subscriber's home country. 2. The Mobile Network Code (MNC). This part of the IMSI is the national part of a subscriber's home network identification. A national identification is necessary because there are usually several independent mobile networks in a single country. In the United Kingdom, for example, the following MNCs are used: 10 for O2, 15 for Vodafone, 30 for T‐Mobile, 33 for Orange, 20 for Hutchison 3G, etc. 3. The Mobile Subscriber Identification Number (MSIN). The remaining digits of the IMSI form the MSIN, which uniquely identifies a subscriber within the home network.

To allow the base station to communicate with several subscribers simultaneously, two methods are used

1. The first method is "Frequency Division Multiple Access" (FDMA), which means that users communicate with the base station on different frequencies 2. The second method used is "Time Division Multiple Access" (TDMA). GSM uses carrier frequencies with a bandwidth of 200 kHz over which up to eight subscribers can communicate with the base station simultaneously

What is the gross datarate for a E-1 connection in Europe?

1.544 Mbit/s

What is the gross datarate for a T-1 connection in the U.S?

2.048 Mbit/s

Supplementary service - Multiparty (MPTY)

Allows subscribers to establish conference bridges with up to six subscribers

Ethernet VS Wifi

An Ethernet connection is much more secure than a WiFi connection. Data on an Ethernet network can only be accessed by physically attaching a device to the network, while data on a WiFi network travels through the air and can be more easily intercepted.

Explain handover in more detail.

As a subscriber can roam into the area that is controlled by a different MSC while a connection is active, it is necessary to change the route of an active connection to the new MSC (handover).

What is the "Ki"

As the Ki is secret, it is stored in the AuC and especially on the SIM card in a way that prevents it from being read directly. - A unique key associated with a GSM subscriber - For many operations in the network, for instance, during the establishment of a call, the subscriber is identified by use of this key. - Thus, it can be ensured that the subscriber's identity is not misused by a third party

Explain The Base Transceiver Station (BTS)

Base stations, which are also called base transceiver stations (BTSs), are the most visible network elements of a GSM system Base stations replace the wired connection to the subscriber with a wireless connection, which is also referred to as the air interface A fixed radio transceiver in any mobile network. The BTS connects mobile devices to the network

Explain the Serving Gateway (SGW)

By using an SGW, the protocol can be converted into ISUP allowing the forwarding of calls to other core networks that are still based on the classic model.

Describe The Visitor Location Register (VLR)

Each MSC has an associated Visitor Location Register (VLR), which holds the record of each subscriber that is currently served by the MSC

Explain what type of gateway architecture is used today for most networks?

Even though many networks having moved to an IP‐based architecture, the gateways between them are still based on the classic architecture

How can a call be established between two parties by using ISUP messages.

Ex: - party A is a mobile subscriber while party B is a fixed‐line subscriber. - A is connected to the network via an MSC, while B is connected via a fixed‐line switching center

How are E-1 Links signaling messages transported?

E‐1 link, signaling messages are transported in IP packets.

What happens for coastal communication?

For special applications, like coastal communication, the GSM standard offers an additional timeslot configuration to increase the maximum distance to the base station to up to 120 km

What is the GSM‐R

GSM-Railway, The frequencies of GSM-R are extended below the frequencies of the GSM-900 standard. GSM‐R networks are private networks To use GSM‐R, mobile phones need to be slightly modified to be able to send and receive in this frequency range. This requires only minor software and hard- ware modifications.

Explain speech paths?

ISUP is used for the establishment of the speech path between different MSCs, and the MAP protocol is used for the handover signaling between the MSCs

What happens If A had terminated the call?

If A had terminated the call, the messages would have been identical, with only the direction of the REL and RLC reversed.

Supplementary service - Connected line presentation restriction (COLR)

If COLR is activated at the called party, the calling party will not be notified of the MSISDN to which the call is forwarded

Supplementary service - Calling line identification presentation (CLIP)

If activated by the operator for a subscriber, the functionality allows the switching center to forward the number of the caller

Supplementary service - Calling line identification restriction (CLIR)

If allowed by the network, the caller can instruct the network not to show their phone number to the called party

What happens to the SMS if the user can't be reached?

If the subscriber is not reachable because the battery of the mobile device is empty, network coverage has been lost temporarily or the device is simply switched off, it is not possible to deliver the SMS: - The message waiting flag is set in the VLR and the SMS is stored in the SMSC. Once the subscriber communicates with the MSC, the MSC notifies the SMSC to reattempt delivery.

Supplementary service - Call forward unconditional (CFU)

If this service is activated, a number can be configured to which all incoming calls are forwarded immediately. This means that the mobile device will not be notified of the incoming call even if it is switched on

Supplementary service - Call forward no reply (CFNRY)

If this service is activated, it is possible to forward the call to a user‐defined number if the subscriber does not answer the call within a certain time. The subscriber can change the number to which to forward the call as well as the timeout value (e.g. 25 seconds)

What most important role of Network Subsystem (NSS)

Important roles include, call establishment, call control and routing of calls between different fixed and mobile switching centers and other networks

In a classic circuit‐ switched network, how are connections made?

In a classic circuit‐ switched network, all connections between subscribers are managed by the MSC and are always routed over the switching matrix even if two subscribers who have established a connection communicate over the same radio cell

What are Network Elementss

In computer networks, a network element is a manageable logical entity uniting one or more physical devices

Evolution of mobile voice networks:

In most of today's mobile voice networks, circuit‐switched components have been replaced with IP‐based devices

What is the SIGTRAN (signaling transmission)?

In the industry, the IP‐based SS‐7 protocol stack or the IP‐based transmission of SS‐7 messages is often referred to as SIGTRAN (signaling transmission).

IS communication done on E‐1 links or IP links

Instead of E‐1 links, however, communication is based on IP links today.

What are Subsystem Numbers (SSNs)?

Instead of port numbers (IP World), SCCP uses Subsystem Numbers (SSNs). Used between local applications and remote applications.

Define The Voice Broadcast Service (VBS)

It is similar to VGCS, with the restriction that only the originator of the call is allowed to speak

A Dedicated Channel - The Frequency Correction Channel (FCCH)

It's used by the mobile devices to calibrate their transceiver units, and is also used to detect the beginning of a multi-frame.

Explain Mobile Number Portability (MNP)?

Many countries have introduced functionality called mobile number portability (MNP), which allows a subscriber to retain their MSISDN even if they want to change their mobile network operator.

How are each connection examined via their timeslots?

One of the timeslots is used for synchronization: - E‐1 is divided into 32 timeslots of 64 kbit/s each (31 timeslots for an E‐1) - T‐1 is divided into 24 timeslots of 64 kbit/s each (23 timeslots for a T‐1)

A Dedicated Channel - The Synchronization Channel (SCH)

SCH is used by mobile devices during network and cell searches.

What is the M3UA (MTP‐3 User Adaptation Layer) protocol

SCTP is followed by the M3UA (MTP‐3 User Adaptation Layer) protocol. As the name implies, the protocol is used to transfer information that is contained in the classic MTP‐3 protocol.

Supplementary service - Barring of all incoming calls (BAIC)

Same functionality as provided by BAOC for incoming calls

What is the SMSC and how does it work?

Short Messaging Service Center (SMSC): 1. To deliver a message, the SMSC analyzes the MSISDN of the recipient and retrieves its current location (the MSC concerned) from the HLR. 2. The SMS is then forwarded to the MSC concerned. If the subscriber is currently attached, the MSC tries to contact the mobile device, and if an answer is received, the SMS is forwarded. 3. Once the mobile device has confirmed the proper reception of the SMS, the MSC notifies the SMSC as well and the SMS is deleted from the SMSC's data storage.

Supplementary service - Connected line presentation (COLP)

Shows the calling party the MSISDN to which a call is forwarded, if call forwarding is active at the called party side

One of the most important ITU standards?

Signaling System Number 7 (SS‐7) - used for call routing.

Please explain The Authentication Center (AuC) in the HLR

The AuC contains an individual key per subscriber (Ki), which is a copy of the Ki on the SIM card of the subscriber.

What connects all subscribers to the core network?

The BSS, which connects all subscribers to the core network, is connected to the MSCs via a number of 2 Mbit/s E‐1 connections.

At the end of the 1990s, the enhanced full‐rate (EFR) codec was introduced:

The EFR codec not only compresses the speech signal to about 13 kbit/s but also offers superior voice quality compared to the FR codec

Explain The Home Location Register (HLR)

The HLR is the subscriber database of a GSM network. It contains a record for each subscriber, with information about the individually available services.

What is the The Mobile Switching Center (MSC)

The MSC is the central element of a mobile telecommunication network, which is also called a Public Land Mobile Network (PLMN) in the standards

The MSC has been split into an MSC‐Server (MSC‐S) and an MGW.

The MSC‐Ss are responsible for CC and MM (signaling), and the MGWs handle the transmission of virtual voice circuits (user data).

Explain how the Nc interface transport voice calls?

The Nc interface is used to transport voice calls within the core network, for example, to gateways, to other mobiles or to fixed networks.

What is the bandwidth of 25 MHz?

The bandwidth of 25 MHz is split into 125 channels with a bandwidth of 200 kHz each.

The Mobile Subscriber Integrated Services Digital Network Number (MSISDN) consists of

The country code is the international code of the subscriber's home country. The country code has one to three digits such as +44 for the United Kingdom, +1 for the United States, and +353 for Ireland

What is the smallest transmission speed unit in a classic circuit‐switched telecommunication network?

The digital signal level 0 (DS0) channel: - It has a fixed transmission speed of 64 kbit/s.

Define Initial Address Message (IAM)

The message contains, among other data, the phone number of B and informs the fixed‐line switching center of the channel that the MSC would like to use for the voice path

What If a subscriber wants to establish a voice call, send an SMS?

The mobile device sends a Channel Request message to the BSC 1. The BSC then checks if an SDCCH is available and activates the channel in the BTS. 2. Afterward, the BSC sends an Immediate Assignment message to the mobile device on the AGCH that includes the number of the assigned SDCCH. 3. The mobile device then uses the SDCCH to send DTAP messages that the BSC forwards to the MSC.

Describe the E Interface

The necessary signaling connection

What is another advantage of using the IMSI as the key to all subscriber information instead of the MSISDN?

The phone number of the subscriber can be changed without replacing the user's SIM card or changing any information on it

What is the phone number of the user called?

The phone number of the user, which is called the Mobile Subscriber Integrated Services Digital Network Number (MSISDN) in the GSM standards, has a length of up to 15 digits

Where is the prepaid billing service is usually implemented?

The prepaid billing service is usually implemented on an IN system and not on the MSC

Explain The results of the downlink signal quality measurements

The results of the downlink signal quality measurements are reported to the BSC by the mobile device, which continuously performs signal quality measurements that it reports via the SACCH to the network

Explain Air Interface

The transmission path between the BTS and the mobile device is referred to, in the GSM specifications, as the air interface or the Um interface

Explain The results of the uplink signal quality measurements

The uplink signal quality is constantly measured by the BTS and also reported to the BSC.

What are the records that the VLR holds?

These records are only copies of the original records, which are stored in the HLR

Supplementary service - Call hold (HOLD)

This functionality is used to accept an incoming call during an already active call or to start a second call

Define Enhanced Multi‐Level Precedence and Preemption (EMLPP).

This functionality, which is specified in 3GPP TS 23.067 is used to attach a priority to a point‐to‐point, VBS or VGCS call. This enables the network and the mobile devices to automatically preempt ongoing calls for higher priority calls to ensure that emergency calls (e.g. a person has fallen on the track) are not blocked by lower priority calls and a lack of resources

As the BSCs of a network are usually located in the field and not close to an MSC, what's the advantage?

This helps to reduce transmission costs for the network operator substantially

What is the advantage of MNP?

This is a great advantage for subscribers and for competition between mobile operators, but it also implies that it is no longer possible to discern the mobile network to which the call will be routed from the NDC

How is the BSC is able to control the power output of the base station?

This is done by evaluating the signal measurements of the mobile devices in the current cell

Supplementary service - Call forward busy (CFB)

This service allows a subscriber to define a number to which calls are forwarded if they are already engaged in a call when a second call comes in

Supplementary service - Call forward not reachable (CFNR)

This service forwards the call if the mobile device is attached to the network but is not reachable momentarily (e.g. temporary loss of network coverage)

The narrow bandwidth of a TCH stands in contrast to how a voice signal is transported in the core network. Where is the PCM algorithm is used for:

To digitize the voice signal, which makes full use of the available 64 kbit/s bandwidth of an E‐1 timeslot to encode the voice signal

In the mobile network, the compression and decompression of the voice data stream is performed in the:

Transcoding and Rate Adaptation Unit (TRAU), which is located between the MSC and a BSC and controlled by the BSC

Combined Aaptive Multirate (AMR) codec and HR codec:

Unlike the HR codec, the AMR codecs, which only use every second burst and which are thus called HR AMR codecs, still have a voice quality which is comparable to that of the EFR codec

Explain Wideband-AMR

Wideband (AMR-WB) is a patented wideband speech audio coding standard developed based on Adaptive Multi-Rate encoding

What is a Switch Center (MSC) Mobile switch center

is the centerpiece of a network switching subsystem (NSS). The MSC is mostly associated with communications switching functions, such as call set-up, release, and routing

If a message needs to be sent for the establishment or clearing of a call, what protocol is needed?

the Integrated Services Digital Network User Part (ISUP) protocol is used. - provides the signaling functions required to control circuit-switched voice/data calls and supplementary services - ISUP is also used extensively in the GSM core network for controlling calls between MSCs and between the GMSCs and the external PSTN.

What is the Public Switched Telephone Network (PSTN)

the national fixed‐line network

What is a Switch matrix systems

typically used in medium- to high-density switching of multiple channels where any row can be connected to any column, making them ideal for routing multiple signals between instruments and the device under test.