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From GSM to LTE Advanced Pro and 5G > GSM > Flashcards

GSM Flashcards

1
Q

What is the size of a Location Area ?

A

It is set by the network operator.

Could be several dozen cells.

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2
Q

Describe the Authentication Process of a SIM to the AuC

A

The process is initiated when a subscriber establishes a signaling connection with the network (before the actual call establishment request or another request).

Then the MSC request an authentication triplet from the HLR/AuC. The AuC retrieves the Ki of the subscriber and the authentication algorithm (A3 Algorithm) based on the IMSI of the subscriber that is part of the message from the MSC.

The Ki is then used with the A3 Algorithm and a random number (RAND) to generate the authentication:

  • RAND (the random number generated): on 128 bits
  • SRES generated from A3 with the parameters Ki and RAND - on 32 bits
  • Kc generated from Ki and Rand through another algorithm

This tripplet is then sent to the MSC to authenticate the subscriber. The key Ki never leaves the HLR/AuC.

The MSC/VLR will therefore send the RAND number to Mobile Station. The Mobile Station will send this number to the SIM Card, which will then use the Ki and A3 algorithm to generate a Signed Response SRES* that will be sent back to the MSC.

If SRES and SRES* are equal, the MSC will consider the SIMCard as authenticated.

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3
Q

What is the AuC ?

A

It is the Authentication Center.

It contains for each subscriber an authentication secret key named Ki.

The Ki key is stored within the AuC (in the HLR) and also within the SIM in a way that prevent its from being read directly.

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4
Q

What is a burst ?

A

It is the timeframe of a GSM timeslot

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5
Q

In France which frequency bands are allocated to UMTS ?

A

around 2 GHz

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6
Q

Why are multiframes needed on the A Interface ?

A

Each frame has 8 timeslots (bursts).
However in order to work properly, gsm needs to have multiple channel type. Some are signaling channels and others are voice channels. The total number of channel types is far above 8. And each burst can encode only 114bits. Then, multiple frames are needed to encode and represent all type of channels.

The 2g Air Interface defined 2 types of multiframe:

  • Signaling multiframes formed by 51 frames and using the timeslots 1 and 2 of these 51 frames.
  • User multiframes formed by 26 frames and using the timeslots 3 to 8.

The spec TS 45.002 define the various type of Logical channels and how they are mapped on the physical channels of the multiframes.

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7
Q

What does BICN means ?

A

Bearer Independent Core Network

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8
Q

What does PCM means ?

A

Pulse Code Modulation

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9
Q

What does AGCH mean ?

A

Access Grant CHannel

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10
Q

Describe the ISUP message RLC

A

Release Complete

This message is sent by a switching center in response to a REL message, to confirm termination of the connection.

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11
Q

What was the only service that was provided by the 1g networks ?

A

Voice calls.

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12
Q

How many frequencies can be used by à base station for frequency hopping ?

A

Up to 64

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13
Q

What does TCAP means ?

A

Transaction Capability Application Part

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14
Q

What are the sizes possible for Micro QRCode ?

A

11 × 11 modules to 17 × 17 modules (Versions M1 to M4, increasing in steps of two modules per side)

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15
Q

Which error correction code is used with QR Code ?

A

Reed Solomon

L 7% — M 15% — Q 25% — H 30%

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16
Q

What does FCH mean ?

A

Frequency Correction Channel

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17
Q

What is the typical Value of the RSSI indicating that the Mobile Station is very close to the BTS ?

A
  • 60 dBm
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18
Q

What does SMSC means ?

A

Short Messaging Service Center

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19
Q

What does SIGTRAN means and what is it ?

A

SIGnaling TRANsmission

It is the new IP-based SS7 protocol stack or the IP-based transmission of SS7 messages.

The abbreviation originated from the name of the IETF working group that was created for the definition of these protocols.

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20
Q

What is the aim of the “tails” at the beginning and at the end of the burst ?

A

It is a well‐known bit pattern sent to enable the receiver to detect the beginning and end of a burst correctly

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21
Q

What are the standardized supplementary services ?

A
  • Call Forward Unconditional (CFU)
  • Call Foward Busy (CFB)
  • Call Forward No Reply (CFNRY)
  • Call Forward Not Reachable (CFNR)
  • Barring of Outgoing Calls (BAOC)
  • Barring of Incoming calls (BAIC)
  • Call Waiting (CW) - Signaling an incoming call while another one is already engaged.
  • Call Hold (HOLD) - Accept an incoming call during an already active call or start a second call.
  • Calling Line Identification Presentation (CLIP) - Forward the number of the caller to the callee.
  • Calling Line Identification Restriction (CLIR)
  • Connected Line Presentation (COLP) - show the MSISDN to which the call is forwarded
  • Multiparty (MPTY) : Allows to establish conferences with many subscribers.
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22
Q

What is the Broadcast Common Control CHannel (BCCH)?

A

It is 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, among many other things, the following information:

the MCC and MNC of the cell;

the identification of the cell, which consists of the location area code (LAC) and the cell ID;

and to simplify the search for neighboring cells for a mobile device, the BCCH also contains information about the frequencies used by neighboring cells. Thus, the mobile device does not have to search the complete frequency band for neighboring cells.

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23
Q

What was the data rate of the UMTS (initially)

A

1 Mbps

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24
Q

Describe the ISUP message REL

A

RELease Message

This message is sent by the switching center of a user who have terminated a call to the other switching center implied in the communication, after having resets the connection in its switching matrix.

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25
Q

At a high level, describe the protocol stack in a MSC.

A

Application (voice calls)

Connection Management

Mobility Management

BSSAP

MAP*
TCAP*

SCCP

STP 1,2, 3

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26
Q

What does MSISDN mean ?

A

Mobile Subscriber Integrated Services Digital Network.

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27
Q

Which spec does specify SMS ?

A

TS 23.040

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28
Q

What is the MSIN ?

A

It is the part of the IMSI that identifies uniquely a subscriber within the home network.

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29
Q

What are the types of Logical channels ?

A

From a functionality view :

  • Signaling channels
  • Traffic CHannel

From a destination view:

  • Dedicated Channel (single user)
  • Common channels (broadcast)
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30
Q

What does IMSI means ?

A

e International Mobile Subscriber Identity

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31
Q

What are the expected answers to a “Setup” message sent by MSC to a Mobile Station ?

A

= A “Called Confirmed” message

  • Alerting message
  • Connect message
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32
Q

What is the Standalone Dedicated Control Channel (SDCCH) ?

A

It is a pure signaling channel that is used during call establishment when a subscriber has not yet been assigned a TCH. Furthermore, the channel is used for signaling that is not related to call establishment, such as for the location update procedure or for sending or receiving a text message (SMS)

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33
Q

Where are the TRAU features implemented ?

A

Between the MSC and the BSC.

Within the Mobile Station.

While the TRAU is a logical component of the BSS, it is most often installed next to an MSC in practice. 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. As the BSCs of a network are usually located in the field and not close to an MSC, this helps to reduce transmission costs for the network operator.

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34
Q

Which software changes were needed by the circuit-switching virtualization over IP ?

A

The main differences are found in the lower protocol layers for call signaling and voice call transmission.

For mobile devices, that transition was completely transparent.

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35
Q

What are the dedicated channels of the GSM ?

A
  • The traffic channel (TCH)
  • The Fast Associated Control Channel (FACCH)
  • The Slow Associated Control Channel (SACCH)
  • The Standalone Dedicated Control Channel (SDCCH)
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36
Q

Describe the channel codec process ?

A

The channel coder takes in input the 260bits produced by the FR codec, and generate 456bits in outputs.

Additional bits are added for cyclic redundancy check and error correction.

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37
Q

What is an MSRN ?

A

It is a temporary identity allocated by a roaming network (specifically the V-MSC) to the called subscriber (in roaming) for an incoming call.

That MSRN is similar to a normal subscriber number. Then why do we need another number? The call can work without MSRN only if the subscriber is not roaming. Telecom network does the routing based on country code and network code.

The other reason why MSRN exists is that it is possible, and even likely, that there are transit switching centers between the G-MSC and the V-MSC. The MSRN identifies uniquely the V-MSC of the subscriber. If the MSISDN were used in all calls, those devices in transit would have to always query the HLR to know where to route the messages

As the MSRN is saved in the billing record for the connection, it is also possible to invoice the terminating subscriber for forwarding the call to a foreign network and to transfer a certain amount of the revenue to the foreign network operator.

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38
Q

What does HSDPA means ?

A

High Speed (Downlink) Packet Access

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39
Q

What was the data rate of the LTE (initially) ?

A

100Mb

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40
Q

With the Mobile Number Portability which HLR perform the conversion MSISDN - IMSI for a ported number ?

A

The HLR of the first operator to which belonged the number. The HLR of the operator owning the Network Destination Code (in the MSISDN).

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41
Q

What does MCC mean ?

A

Mobile Country Code

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42
Q

What are the most important parameters within the SIM Card ?

A

IMSI
Ki Secret Key

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43
Q

What is an SCP ?

A

Service Control Point.

It is either a database or an application software that can influence the establishment of a connection.

In a GSM network, an SCP can be used, for example, for storing the current location of a subscriber.

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44
Q

What are the protocols underlying the BSSMAP protocol ?

A

As BSSMAP is not a database query protocol/language, it is directly based on SCCP (rather than having TCAP in between).

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45
Q

How is the IMSI used when the Mobile Device is switched on ?

A

The IMSI is retrieved from the SIM Card and sent to the MSC. There the MCC and the MNC of the IMSI are analysed and the MSC/VLR is enabled to request the subscriber’s record from the HLR of the subscriber’s home network.

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46
Q

Where is the A8 algorithm executed ?

A

in the SIM Card

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47
Q

What does BTS mean ?

A

Base Transceiver Station

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48
Q

What does SCTP means ?

A

Stream Control Transmission Protocol

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49
Q

What is the DS0 ?

A

Digital Signal 0 (DS0) is a basic digital signaling rate of 64 kilobits per second (kbit/s), corresponding to the capacity of one analog voice-frequency-equivalent communication channel.

The DS0 rate was introduced to carry a single digitized voice call. For a typical phone call, the audio sound is digitized at an 8 kHz sample rate, or 8000 samples per second, using 8-bit pulse-code modulation for each of the samples. This results in a data rate of 64 kbit/s.

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50
Q

What does BCCH mean ?

A

Broadcast Common Control Channel

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51
Q

Which spec does specify the file and folder structure of a SIM Card ?

A

TS 31.102

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52
Q

On which interface the BSSMAP and DTAP protocol transit ?

A

the A Interface.

For the communication between the MSC and the BSS or the MS

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53
Q

Who is the main responsible of the creation of the SS7 standard ?

A

ITU.
International Telecommunication Union

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54
Q

What does VoLTE means ?

A

Voice over LTE

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55
Q

What is the Channel used by the “immediate assignment” message ?

A

AGCH (Access Grant CHannel)

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56
Q

How many timeslots of 64kbits/s are in an T1 connection ?

A

24

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57
Q

What is the Access Grant Channel (AGCH) ?

A

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. The message contains information about which SDCCH or TCH the subscriber is allowed to use.

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58
Q

What is the aim of the training sequence in the middle of a burst ?

A

The training sequence in the middle of the burst always contains the same bit pattern. It is used to compensate for interference caused, for example, by reflection, absorption, and multipath propagation. On the receiver side, these effects are countered by comparing the received signal with the training sequence and thus adapting the analog filter parameters for the signal. The filter parameters calculated this way can then be used to modify the rest of the signal and thus to better recreate the original signal.

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59
Q

When receiving a SMS send request, how does the MSC knows the SMSC to which it needs to be sent.

A

The SMSC number is in the message request.
It is configured within the SIMCARD and then sent as part of the message request by the Mobile Station

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60
Q

Which spec provides details on the call establishment procedures ?

A

TS 23.018

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61
Q

What are the main tasks of the Mobility Management (MM) function of an MSC ?

A
  • Authentication of subscribers at connection establishment
  • If no connection exists between the network and the Mobile Station, the MS has to report a change of location to the network to be reachable for incoming calls and messages. This procedure is called Location Update
  • 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 is the handover process.
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62
Q

How to compute the theoritical maximum distance between the BTS and a Mobile Station ?

A

64 * 550m = 35km

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63
Q

What are the Common channels of the GSM Air Interface ?

A
  • The Synchronization Channel (SCH)
  • The Frequency Correction Channel (FCCH)
  • The Broadcast Common Control Channel (BCCH)
  • The Paging Channel (PCH)
  • The Random Access Channel (RACH)
  • The Access Grant Channel (AGCH)
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64
Q

What does SIM mean ?

A

Subscriber Identity Module

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65
Q

What is the gross datarate of a T1 Connection ?

A

1.544 Mbits/s

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66
Q

What is the gross datarate of an E1 Connection ?

A

2.048 Mbits / s

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67
Q

What change on the radio transmission path when it comes to send circuit-switched data rather than frame ?

A
  • it is not possible to tolerate a single faulty bit. If even a single bit is faulty, a retransmission of at least one single frame has to be performed, as most applications cannot tolerate a faulty data stream.
  • To increase the likelihood of correctly reconstructing the initial data stream, the interleaver spreads the bits of a frame over a much larger number of bursts than the eight bursts used for voice frames.
  • Furthermore, the channel coder, which separates the bits of a frame into different classes based on their importance, had to be adapted for data calls as well, as all bits are equally important. Thus, the convolutional decoder has to be used for all bits of a frame.
  • Finally, it is also not possible to use a lossy data compression scheme for data calls. Therefore, the TRAU operates in a transparent mode for data calls.
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68
Q

Which channel is used by the Mobile Station to send a “Channel Request” message ?

A

RACH (Random Access CHannel)

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69
Q

What is an SSP ?

A

Service Switching Point.

It is a Switching Center that is more generally referred to as network element and is able to establish, transport, or forward voice and data connections.

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70
Q

What are the necessary interfaces in an IP-based NSS architecture ?

A
  • The A Interface between the BSS and the MSC
  • The C Interface: between the MSC and the HLR
  • The D Interface: between the VLR and the HLR
  • The Nb Interface between 2 MGW
  • The Nc Interface between 2 MSC-S
  • The Mc Interface between an MSC-S and a MGW
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71
Q

What does MAP means ?

A

Mobile Application Part

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72
Q

What happens if parts of a frame are destroyed beyond repair during transmission on the air interface ?

A

The complete 20‐millisecond voice frame is discarded by the receiver and the previous data block is used instead to generate an output signal.

Most errors that are repaired this way remain undetected by the listener. This trick, however, cannot be used indefinitely. If after 320 milliseconds a valid data block has still not been received, the channel is muted and the decoder keeps trying to decode the subsequent frames.

If, during the following few seconds no valid data frame is received, the connection is terminated and the call drops.

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73
Q

At which frequency the voice is sampled in the digitization process ?

A

8000 Hz (8 Khz)

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74
Q

What are the tech specificities of the LTE ?

A

OFDMA
IP Access is the only service offered. With a low latency. Other services must rely on IP.

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75
Q

What is a G-MSC ?

A

Gateway Mobile Switch Center, it is a normal MSC that is also connected to other networks (fixed or mobile-other operators), and act as a gateway to the mobile subscribers for those networks.

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76
Q

What does ITU means ?

A

International Telecommunication Union

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77
Q

Which message is sent by the Mobile Device when the customer ends the call ?

A

The “disconnect” message

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78
Q

What is the reference standard of the MAP protocol ?

A

3GPP TS 29.002

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79
Q

What is an STP ?

A

Signaling Transfer Point.

It is a network element responsible for the forwarding of signaling messages between SSP and SCPs as not all network nodes have a dedicated link to all other nodes of the network.

It can be compared to an IP router in the Internet. However STPs only forward signaling messages that are necessary to establish, maintain or clearing a call. The calls themselves are directly carried on dedicated links between the SSPs.

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80
Q

Describe the encryption process of the GSM:

A

Inputs: the encryption key Kc, the current frame number, the 114bits of data of the current frame

Output: 114bits ciphered

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81
Q

What is the SCCP SSN of an VLR ?

A

7

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82
Q

What are the services provided by 2g network ?

A

Voice Calls, SMS, USSD, Data transmission (10kbps)

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83
Q

What was the data rate of the UMTS (initially) ?

A

1 Mbps

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84
Q

What was the tech specificities of the GPRS-EDGE ?

A

Packet Switching + New modulation

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85
Q

What is MNP ?

A

It is a functionality that allows a subscriber to retain its MSISDN even if he wants to change its mobile network operator.

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86
Q

What are the codecs used on Nb Interface to transmit voice ?

A

PCM/G.711, Narrowband‐AMR, or Wideband‐AMR

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87
Q

In France which autorities assign the frequency bands to TelCo ?

A

ARCEP

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88
Q

What does RSSI mean ?

A

Received Signal Strength Indication

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89
Q

How is Kc generated ?

A

It is computed by the AuC based on the A8 algorithm

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90
Q

What does SSN means ?

A

SubSystems Number

It is the equivalent of TCP/UDP port in SCCP

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91
Q

What are the protocols underlying the MAP protocol ?

A

In order : TCAP, SCCP and the MTPs.

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92
Q

What does PDP mean ? (GPRS)

A

Packet Data Protocol (IPv4, IPv6, PPP)

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93
Q

How is the encryption key (kc) generated ?

A

Kc is calculated in the AuC and on the SIM card by using a random number (RAND) and the secret key (Ki) as input parameters for the A8 algorithm.

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94
Q

Which nodes does initiate the “authentication” procedure ?

A

HLR/AuC

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95
Q

What does SABM means ?

A

Set Asynchronous Balance Mode

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96
Q

Describe the ISUP message ACM

A

Address Complete Message

A switching center send back this message in response of the IAM message, once it has analyzed the phone number, established the connection to the B line via the switching matrix, and applied a periodic current to the B line so that the B phone can generate an alerting tone.

By receiving this message, the other switching center knows that the number is complete and that the terminating party is being alerted about the incoming call.

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97
Q

What is the function of the interleaver unit ?

A

The convolutional decoder has one weak point: if several consecutive bits are changed during transmission over the air interface, the convolutional decoder on the receiver side is not able to correctly reconstruct the original frame.

To decrease this effect, the interleaver changes the bit order of a 456‐bit data frame in a specified pattern over eight bursts and consecutive frames are thus interlocked with each other.

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98
Q

What does HLR means ?

A

Home Location Register

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99
Q

How the mobile station knows the frequencies of the neighbouring cells he has to measure the signal strengths ?

A

Their transmission frequency is announced on the broadcast channel (BCCH) of the currently serving cell

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100
Q

How many physical contacts are defined for the communication between the SIM Card and the Mobile Device.

A

8 contacts.
5 required.

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101
Q

What is the A Law Standard ?

A

An A-law algorithm is a standard companding algorithm, used in European 8-bit PCM digital communications systems to optimize, i.e. modify, the dynamic range of an analog signal for digitizing. It is one of two versions of the G.711 standard from ITU-T, the other version being the similar μ-law, used in North America and Japan.

The reason for this encoding is that the wide dynamic range of speech does not lend itself well to efficient linear digital encoding.

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102
Q

What are the one-act of MNP ?

A
  • it is no longer possible to use the NDC to decide which tariff to apply to a call. Which complexity routing and billing.

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.

  • it is no longer possible to discern the Mobile Network Operator from the NDC.
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103
Q

After which frequency the waves transmission in the Air become very difficult ?

A

3GHz

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104
Q

What was the data transmission debit in first 2g networks ?

A

Around 10kbps

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105
Q

What does MTP means ?

A

Message Transfer Part.

There are 3 layers - MTP-1, MTP-2, MTP-3

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106
Q

What does ISDN means ?

A

Integrated Services Digital Network

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107
Q

What are the technology specificities of GSM (first 2G) networks ?

A

TDMA
digital
Circuit Switched

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108
Q

Where is the VAD (Voice Activity Detection) implemented ?

A

In the TRAU (downlink) and in the Mobile Station (uplink)

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109
Q

What does UICC mean ?

A

Universal Integrated Circuit Card

It is the SIM Card

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110
Q

At a high level, describe the protocol stack in a BTS.

A 
Data Link (Radio) | Data Link (BSC) 
Physical (Radio) | Physical (BSC)
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111
Q

What is the MCC ?

A

The MCC is the part of the IMSI that identifies the subscriber’s home country. On 3 digits.

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112
Q

How many steps are in the timing advance Control system of the GSM ?

A

64 steps

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113
Q

What does ETSI means ?

A

European Telecommunication Standards Institute

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114
Q

What was the data rate of the LTE (initially) ?

A

100Mb

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115
Q

Which specs specify the BICC protocol ?

A

ITU Q.1901 [7] and 3GPP TS 29.205

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116
Q

Describe the bsc handover procedure

A

Once the BSC decides to perform a handover, a TCH is activated in the new cell as shown in Figure 1.30. Afterward, the BSC informs the mobile device via the old cell with a Handover Command message that is sent over the FACCH. Important information elements of the message are the new frequency and timeslot number of the new TCH. The mobile device then changes its transmit and receive frequency, synchronizes to the new cell if necessary, and sends a Handover Access message in four consecutive bursts. In the fifth burst, a Set Asynchronous Balanced Mode (SABM) message is sent, which is acknowledged by the BTS to signal to the mobile device that the signal can be received. At the same time, the BTS informs the BSC of the successful reception of the mobile device’s signal with an Establish Indication message. The BSC then immediately redirects the speech path to the new cell.

From the mobile’s point of view, the handover is now finished. The BSC, however, has to release the TCH in the old cell, and has to inform the MSC of the performed handover before the handover is finished from the network’s point of view. The message to the MSC is only informative and has no impact on the continuation of the call.

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117
Q

What does “Connection establishment of a Mobile Station” concretely mean ?

A
  • Sending a channel request via the RACH ?
  • Receive an Immediate Assignment response containing an SDCCH channel number reserved
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118
Q

What are the parameters of the A8 algo ?

A

Rand and Ki

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119
Q

What does USSD mean ?

A

Unstructured Supplementary Service Data

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120
Q

What is a carrier system ?

A

A carrier system is a telecommunications system that transmits information, such as the voice signals of a telephone call and the video signals of television, by modulation of one or multiple carrier signals above the principal voice frequency or data rate.[1]

Carrier systems typically transmit multiple channels of communication simultaneously over the shared medium using various forms of multiplexing. Prominent multiplexing methods of the carrier signal are time-division multiplexing (TDM) and frequency-division multiplexing (FDM). A cable television system is an example of frequency-division multiplexing. Many television programs are carried simultaneously on the same coaxial cable by sending each at a different frequency. Multiple layers of multiplexing may ultimately be performed upon a given input signal. For example, in the public switched telephone network, many telephone calls are sent over shared trunk lines by time-division multiplexing. For long-distance calls several of these channels may be sent over a communications satellite link by frequency-division multiplexing. At a given receiving node, specific channels may be demultiplexed individually.

The purpose of carrier systems is to save money by carrying more traffic on less infrastructure.

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121
Q

How many timeslots (each per Subscriber) are in à GSM frame ?

A

8

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122
Q

What is frequency hopping ?

A

It is the use alternating frequencies for a single channel.

it changes the carrier frequency for every burst during a transmission. This increases the probability that only few bits are lost if one carrier frequency experiences a lot of interference from other sources like neighboring cells.

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123
Q

What does ISUP means ?

A

Integrated Service digital network User Part

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124
Q

What does MNC means ?

A

Mobile Network Code

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125
Q

What does FLO means ?

A

Flexible Layer One

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126
Q

What does BCSM mean ?

A

Basic Call State Model (BCSM)

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127
Q

Why is it important to have standardized interfaces between network nodes ?

A

This allows network operators to acquire different components for the network from different network equipment vendors.

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128
Q

How does the A5 algorithm is selected ?

A

The selection of the ciphering algorithm depends on the capabilities of the mobile device. During the establishment of a connection, the mobile device informs the network about the ciphering algorithms that it supports. The network can then choose an algorithm that is supported by the network and the mobile device.

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129
Q

Why is TCAP protocol used for ?

A

TCAP is the protocol designed on top of SCCP to access databases. It defines a number of different modules and messages that can be used to query all kinds of different databases in a uniform way.

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130
Q

What was the HSDPA data rate ?

A

10Mbps

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131
Q

What does PCH mean ?

A

Paging Channel

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132
Q

What is the burst duration ?

A

577ms

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133
Q

What does TRAU mean ?

A

Transcoding and Rate Adaptation Unit

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134
Q

in GSM, how the secret key shared only by the SIM and the AuC is called ?

A

Ki

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135
Q

Which spec does specify the Logical channels and their mapping on the physical one ?

A

TS 45.002

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136
Q

Describe the ISUP message ANM

A

ANswer Message

This message is sent by the switching center who have received an IAM message and have already sent back an ACM message, to notify the other originating switching center that the terminating user have picked the call and the conversation can start.

137
Q

Which 3GPP spec specify the splitting of MSC into an MSC-Server and and Media GateWay ?

A

3GPP TS 23.205

138
Q

What are the codecs used on Nb Interface to transmit voice ?

A

PCM/G.711, Narrowband‐AMR, or Wideband‐AMR

139
Q

What is the duration of the frame in GSM ?

A

4.615 milliseconds

140
Q

Where is the A5 algorithm executed ?

A

In the Mobile Station

141
Q

What are the advantages of using 2 identifiers (IMSI within the Network and MSISDN outside) for a subscriber?

A
  • A single SIM Card can be assigned multiple MSISDN. Usually there is a 1:1 relationship between IMSI and MSISDN in the HLR.
  • The MSISDN of a subscriber can be changed without replacing the user’s SIM Card or changing any information on it. Only the HLR record of the subscriber needs to be changed.
142
Q

In the specification, how is the root directory of a SIM Card called ?

A

MF (Main File)

143
Q

How was FDMA applied on GSM frequency 900Mz Band ?

A

The bandwith of 25 MHz was splitted into 125 channels of 200KHz each identified by a number called ARFCN (Absolute Radio Frequency Channel Number)

144
Q

Why is the BSSMAP protocol used for ?

A

BSSMAP is used for the communication between the MSC and the Radio Network, for example, to establish a dedicated radio channel for a new communication to a Mobile Subscriber.

145
Q

Which protocols are used during the Authentication Procedure of a Mobile Station ?

A
  • DTAP between the MS and the MSC
  • MAP between the MSC and the HLR/AuC
146
Q

In terms of transmission switching type, how was initially designed the GSM ?

A

The GSM were initially designed as a circuit-switched network in a similar way to fixed-line phone networks of the time. At the beginning of a call, the network established a direct connection between 2 parties, which was then used exclusively for that conversation.

147
Q

What are the part of an IMSI number ?

A
  • The Mobile Country Code (MCC). The MCC identifies the subscriber’s home country. (3 digits)
  • 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 EE and 20 for Three. (2-3 digits)
  • The Mobile Subscriber Identification Number (MSIN). The remaining digits of the IMSI form the MSIN, which uniquely identifies a subscriber within the home network. (9-10 digits)
148
Q

Which protocol is used on the Mc Interface ?

A

H.248/MEGACO

This protocol is used, for example, to estable voice channels to 2 parties and to logically connect the 2 channels within the MGW.

The protocol is also used to instruct the MGWs to play annoucements to inform users of events, for example, where the called party is currently not available or is busy, and to establish conference calls between more than 2 subscribers.

149
Q

What is a Slow Associated Control CHannel (SACCH)?

A

It is a channel assigned to a dedicated connection. It is used in the uplink 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. In the downlink direction, the SACCH is used to send power control commands to the mobile device. Furthermore, the SACCH is used for timing advance control.

150
Q

What is the Random Access Channel (RACH) ?

A

It 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. This is done by sending a Channel Request message. Requesting a channel has to be done via a ‘random’ channel because subscribers in a cell are not synchronized with each other.

151
Q

Which spec does standardize the USSD codes ?

A

To 22.030

152
Q

What does PDN mean ? (GPRS)

A

Packet Data Network

153
Q

What does SACCH mean ?

A

Slow Associated Control CHannel

154
Q

How does the Channel Coder unit proceed to add detection and correction error ?

A

It splits the 260 input bits into classes:

  • class 1a: 50 bits. 3 bits added on it for error detection via CRC.
- class 1b: 132 bits. 4 filles bits added on top of t
It.
  • class 2: 78bits. No check added.

Class 1a and 1b output bits(189bits) are Passed through a convolutional coder that will produce 378bits. Those additional bits permit error correction. For each input bit, the convolutional decoder calculates two output bits. For the computation of the output bits, the coder uses not only the current bit but also the information about the values of the previous bits. For each input bit, two output bits are calculated.

155
Q

What is the function of the channel coder unit ?

A

The channel coder adds error detection and correction information to the data stream.

This step is very important, as the transmission over the air interface is prone to frequent transmission errors due to the constantly changing radio environment.

Furthermore, the compressed voice information is very sensitive and even a few bits that might be changed while the frame is transmitted over the air interface create an audible distortion.

156
Q

How many subscribers can simultaneously communicate on a gsm carrier of 200khz ?

A

8

157
Q

Which device is the end target of a Channel Request message ?

A

The BSC

158
Q

What is the minimum feature of the layer 2 protocol in OSI Model (or MTP-2) ?

A

To frame the physical signal into packets/frames by inserting into the datastream a start and stop identifications at the beginning and end of each packet.

159
Q

What is the Paging CHannel (PCH) ?

A

It is a channel 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. The most important information element of the message is the IMSI of the subscriber or a temporary identification called the Temporary Mobile Subscriber Identity (TMSI).

160
Q

What is the SCCP SSN of an MSC ?

A

8

161
Q

What does PLMN means ?

A

Public Land Mobile Network

162
Q

What are the security functions needed on the Base Station Subsystem ?

A
  • Authentication
  • Encryption
  • Anonymity
163
Q

Example of SAT features ?

A
  • SAT allows the programs on the SIM Card to access functionalities of the mobile devices such as waiting for user input after showing a text message, or sending or receiving SMS without user intervention
  • SAT is used to send notification to the network, for example when the SIM Cards detect that it has been inserted in a new device, to trigger the transfer of welcome and configuration messages.
  • SAT allows to receive “SILENT” SMS from the network to update information on the SIM Card such as the list of preferred roaming network.
164
Q

In the specification, how are the normal files called ?

A

EF (Elementary Files)

165
Q

Why is the MAP protocol used for ?

A

MAP (Mobile Application Part) is used for the communication between the MSC and the HLR, which maintains subscriber information.

It is also used for the communication between 2 MSCs if the subscriber move into the coverage area of a different MSC WHILE A CALL IS ONGOING.

166
Q

What does SS7 means ?

A

Signaling System Number 7

167
Q

What is ISDN ?

A

It is the evolution of the fixed-line network to a network where the technologies were digital both with the switching center and at the last mile.

With this system the transmission became fully digital and the conversion back to an analog audio signal was done directly in the phone.

ISDN is also a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network

168
Q

Who is the main responsible of the creation of the GSM standard ?

A

ETSI.
European Telecommunication Standards Institute

GSM was meaning Groupe Spécial Mobile

169
Q

What is the role of the TMSI ?

A

By the use of a constantly changing temporary ID, the identity of a subscriber is not revealed to listeners during the first phase of the call, which is not ciphered

170
Q

What is a V-MSC ?

A

Visited Mobile Switching Center, it is the MSC currently responsible for a subscriber.

171
Q

How does the MS know the Frequency of the neighboring cells it needs to measure their signal ?

A

The network sends the frequencies of neighboring cells via the SACCH during an ongoing call.

172
Q

What is the GSM burst format ?

A

Tail | Data (57bits) | S | Training Sequence | S | Data (57bits) | Tail | Guard Time

173
Q

What is the size of the MSISDN

A

15 digits

174
Q

What does ARFCN mean ?

A

Absolute Radio Frequency Channel Number

175
Q

What is the goal of the ISUP protocol ?

A

The ISUP protocol is used to establish and clear call in the Fixed-Line and GSM Networks.

176
Q

Transmission path in the downlink direction between the network and the mobile device ?

A

Voice -> PCM -> TRAU (codecs - FR, EFR, HR, AMR) -> Channel Coder -> Interleaver -> Cipherer -> Modulation -> Demodulation -> Decipherer -> De Interleaver -> Channel Decoder -> Speech decoder -> PCM -> Voice

177
Q

Which organization is today responsible of the Mobile Network standardization ?

A

3GPP.
Third Generation Partnership Project.

ETSI is a regional standardization body of 3GPP.

178
Q

What are the speech codecs used by the TRAU ?

A
  • The Full Rate (FR) codec: 64kbps -> 13kbps
  • The Enhanced Full Rate (EFR) codec: 64kbps -> 13kbps with a much better voice quality. However, the algorithms are more complex and require more processing powers.
  • The Half Rate (HR) Codec: 64kbps -> 7kbps. Noticeably inferior voice quality
  • The Adaptative MultiRate (AMR) algorithm: Instead of using a single codec, which is selected at the beginning of the call, AMR allows a change of the codec during a call.
  • The AMR‐Wideband (AMR‐WB): The algorithm allows digitization of a wider frequency spectrum than is possible with the PCM algorithm that was described earlier. 7000 Hz. As a consequence, the caller’s voice sounds much clearer and more natural on the other end of a connection. A high compression rate is used in practice to reduce the datarate of a voice stream down to 12.65 kbit/s.
179
Q

In theory, what is the maximum cell radius of a BTS ?

A

35km

180
Q

What are the services provided by 2g network ?

A

Voice Calls, SMS, USSD, Data transmission (10kbps), FAX

181
Q

What is STM ?

A

Synchronous Transport Module.

It is the SDH ITU-T fiber optic network transmission standard. There are many versions going from STM-1 to STM-256.
This standard allows for higher transmission speeds on long distance.

182
Q

From a logical point of view, how are data stored in a GSM SIM Card ?

A

In directories and files.

183
Q

What does SCCP means ?

A

Signaling Connection and Control Part

184
Q

Which spec does specify AMR-WB ?

A

TS 26.190

185
Q

Why do optical connections like STM-1 were used for some interfaces (A, E, etc.), instead of 1 or multiple E1 Connections ?

A
  • An E1 connection can support only 31 channels so multiple E1 were needed, and it was more efficient and less expensive in operations and maintenance to use a single STM-1.
  • The electrical signals can only be carried over long distances with great effort.
186
Q

What is the downside of having Location Area ?

A

The network operator is only aware of the current location area of the subscriber but not of the exact cell. Therefore, the network has to search for the mobile device in all cells of a location area for an incoming call or SMS; this procedure is called ‘Paging.’

187
Q

What are the necessary nodes in a classic NSS architecture:

A
  • MSC: Mobile Switching Center
  • G-MSC: Gateway Mobile Switching Center (optional)
  • VLR: Visitor Location Register
  • HLR: Home Location Register
188
Q

What is the MNC ?

A

It is the part of the IMSI that identifies within the country the subscriber’s network.

189
Q

What is the function of the VAD (Voice Activity Detection) ?

A

The VAD aims at reducing the interference on the air interface and increasing the operating time of the mobile device, by sending data burst only if a speech signal is detected.

This method is called discontinuous transmission (DTX) and can be activated independently in the uplink and downlink directions.

Since only one person speaks at a time during a conversation, one of the two speech channels can usually be deactivated.

190
Q

What is the delay introduced by the transmission path of voice over the radio ?

A

120ms.

In addition to the delay of 20 milliseconds generated by the FR coder, the interleaver adds another 40 milliseconds, as a speech frame is spread over eight bursts instead of being transmitted consecutively in four bursts. Compared to a voice call in a fixed‐line network, a mobile network thus introduces a delay of at least 60 milliseconds. If the call is established between two mobile devices, the delay is at least 120 milliseconds as the transmission chain is traversed twice.

191
Q

How is the encryption key called in GSM ?

A

Kc

192
Q

What does MSIN mean ?

A

Mobile Subscriber Identification Number

193
Q

What are the 3 files formats in a SIM Card ?

A
  • Transparent (sequence of bytes)
  • Linear Fixed (records of a fixed length)
  • Cyclic (similar to Liner fixed but with a pointer to the last modified record)
194
Q

Which software changes were necessary to adapt a fixed-line switching center to a mobile switching center ?

A

The replacement of the “Fixed line Subscriber and Call control layer” by a Mobility Management layer (Mobile Subscriber Management, Mobility Management, Call Control).

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 to route calls to them at anytime.

The lower layers were mostly unchanged:

  • Switching and Signaling software
  • OS of the switching center
195
Q

In France which frequency bands are allocated to GSM?

A

900MHz and 1800MHz

196
Q

What is BICN ?

A

Bearer Independent Core Network.

It is the approach that consist in eliminating the physical circuit-switched infrastructure and replace it with an IP network and media gateways, that will virtualize circuit-switching and transfer voice over IP Packets.

197
Q

How are the MSC responsibilities distributed among the MSC-S and the MGW ?

A

MSC-S is responsible of the Call Control (CC) and the Mobility Management (MM) (signaling)

MGW is responsible of the transmission of the virtual voice circuit (the user data)

198
Q

What was the motivation operatos had to virtualize Circuit Switching over IP ?

A

The introduction of the GPRS (General Packet Radio Services) was adding an IP network in addition to the circuit-switched network. The maintenance and operation of both networks were difficult, inefficient and costly. Therefore network operators have replaced the “Switching Matrix” of the MSC by a Media Gateway, helping to virtualize circuit-switching and to transfer voice calls over IP packets.

This approach has been standardized under the name “Bearer Independent Core Network”

199
Q

What is the “transport” protocol used for communication between switching centers (SSP) and BSS and Mobile Stations ?

A

SCCP (Signaling Connection and Control Part)

200
Q

What is the SRI message ?

A

Send Routing Information

It is a request message sent to the HLR to locate a subscriber in the network. This request message contains the MSISDN of the subscriber to locate.

Once received by the HLR, the HLR identify the IMSI of the subscriber based on its MSISDN, and then identify the V-MSC, engage that V-MSC with a PRN (Provide Roaming Message) request message to get the Mobile Subscriber Roaming Number (MSRN), and will forward that MSRN to the SRI initiator.

201
Q

In the specification, how are the directories under the root directory of a SIM Card called ?

A

DF (Dedicated Files)

202
Q

What is the aim of the “Stealing bits” placed at the left and right of the training sequence ?

A

These bits indicate whether the data fields contain user data, or are used (‘stolen’) for urgent signaling information.

However, user data from bursts that carry urgent signaling information are lost. As we will see, the speech decoder is able to cope with short interruptions of the data stream quite well, and thus the interruptions are normally not audible to the user.

203
Q

What are the basic GSM network services ?

A
  • Telephony
  • SMS
  • Data Service
  • FAX
204
Q

What is a Fast Associated Control CHannel (FACCH) ?

A

It is a channel that is transmitted on the same timeslot as a TCH. It is used to send urgent signaling messages like a handover command. As 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. To inform the mobile device of this, the stealing bits to the left and right of the training sequence.

205
Q

which device is responsible for assigning SDCCH and TCH of a base station ?

A

The BSC (Base Station Controller)

206
Q

What has changed in the transition from the classical SS7 stack to the SS7 stack over IP (SIGTRAN) ?

A

MTP-1 and MTP-2 were replaced by the IP and the classical layer2 protocols of the TCP/IP stack (eg. Ethernet)

SCTP has been introduced as a transport protocol. However, it is part of layer 3.

M3UA (MTP-3 User Adaptation Layer) has been introduced. It aims at simulating all functionalities of MTP3 so that the changes at the lower layer be transparent to all higher-layer protocols.

ISUP has been replaced by BICC (Bearer Independant Call Control) because voice is now transported over IP (and no more on a 64kbits/s timeslot).

207
Q

What was the data transmission debit in first GPRS networks ?

A

20-30kbps

208
Q

What does DTX mean ?

A

Discontinuous transmission

209
Q

When do the analog technologies within the Switching Center were superseded by the digital technologies ?

A

In the middle of 1980s.

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. There, the digital signal was again converted back to an analog signal, which was then sent over the copper cable to the terminating party.

210
Q

Where is the A3 algorithm executed ?

A

in the SIM Card

211
Q

What does SONET means ?

A

Synchronous Optical Network

212
Q

Explains the terms A-MSC and R-MSC and their relation

A

A-MSC : Anchor Mobile Switching Center
R-MSC : Relay Mobile Switching Center

This is the terminology to refer to the MSCs involved in an Inter-MSC handover procedure.
The MSC who firstly delivered the call is the A-MSC. The MSC managing the new cell is the R-MSC.

213
Q

What is the Frequency Correction CHannel (FCCH) ?

A

It is a channel used by the mobile devices to calibrate their transceiver units, and to detect the beginning of a multiframe.

214
Q

Describe the ISUP message IAM

A

Initial Address Message

This message is sent to a switching center to establish a call and reserve a voice channel.

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

215
Q

What does FACCH mean ?

A

Fast Associated Control CHannel

216
Q

What does O-BCSM mean ?

A

Originator Basic Call State Model (BCSM)

217
Q

Which spec does specify the SIM Application Toolkit (SAT) ?

A

TS 31.111

218
Q

What is the expected response to a Channel Request message ?

A

The BSC activité the Channel in the BTS and assign that Channel to the mobile station requesting.

  • Channel Activation (Sent to the BTS)
  • Immediate Assignment (Sent to the Mobile Station)
219
Q

What does MNP means ?

A

Mobile Number Portability

220
Q

What is the role of the CPU/microcontroller in the SIM Card ?

A
  • It is the only interface between the Mobile Device and the different memories of the SIM Card, allowing to restrict direct access to sensitive information.
  • It is used to generate the SRES during the network authentication procedure, avoiding by doing so the disclosure of the Ki secret key to the mobile device.
  • It can execute programs that the network operator may have installed on the SIM Card. This is done via the SIM Application Toolkit (SAT)
221
Q

In terms of transmission, what are the main reference units of a telecommunication network ?

A

E-1 Connection in Europe
T-1 Connection in the United States

222
Q

What are the 3 subsystems of a GSM Network ?

A
  • The BSS (Base station SubSystem) also called the radio network connects wirelessly over the radio/air interface the mobile subscribers to the network.
  • The NSS (Network SubSystem): switching of calls, subscriber and mobility management.
  • The IN (Intelligent Network SubSystem): comprises SCP databases that add optional functionality to the network (Example: the prepaid service).
223
Q

What does UMTS means ?

A

Universal Mobile Telecommunications System

224
Q

What is the effective data rate of a TCH (considering bits used for error control)

A

13kbps

225
Q

What does SDCCH mean ?

A

Standalone Dedicated Control Channel

226
Q

What is the function of the TRAU ?

A

Transcoding and Rate Adaptation Unit

The TCH effective data rate is 13kbps. However the Voice transit within the network at 64kbps.

It is not easily possible to enfance the radio interface so that it can support 64kbps. Because the Frequency are expensive and because of some engineering limits. Then, the only solution is speech compression and decompression between the MSC and the BSC. This compression and decompression are ensure by the TRAU.

227
Q

What is SCTP ?

A

SCTP (Stream Control Transmission Protocol) is the equivalent of TCP/UDP in the SIGTRAN stack protocol.

Its advantages over TCP/UDP are found when many signaling connections between 2 network nodes are active at the same time.

228
Q

What is the protocol used on the Nc interface ?

A

Bearer Independent Call Control (BICC) protocol

229
Q

Which spec do specify the authentication procedures of a Mobile Station ?

A

TS 24.008

230
Q

What does BSSMAP means ?

A

Base station SubSystem Mobile Application Part

231
Q

What is the packet sent by a Mobile Station to update its location ?

A

MM LOCATION UPDATING REQUEST

232
Q

What is the raw data rate of a TCH ?

A

114 bits every 4.165ms (on 26 frames) + 0 bits during 4.165ms × 2 (SACCH and empty)

22.8kbps

233
Q

What is the distance before moving to the next step of the timing advance control system of the GSM ?

A

550m

234
Q

What does 5G CN means ?

A

5G Core Network

235
Q

Which message is sent to Mobile Station to Control its transmission power ?

A

a Power Control message is sent to the BTS. The BTS in turn forwards the message to the mobile device and repeats the message on the SACCH in every frame.

236
Q

What is the Mc Interface ?

A

It is the interface between the MSC-S and the MGW used to establish voice connections.

237
Q

What does MSRN mean ?

A

Mobile Subscriber Roaming Number

238
Q

What is the nominal traffic flow of an SMS ?

A

Subscriber A (sender) -> MSC (DTAP) -> HLR (Check the localization of the receiver) -> MSC receiver -> Subscriber B (receiver)

239
Q

What is the aime of the Guard Time at the end of every burst ?

A

To prevent any overlap between bursts that may happen because the signal of à faraday Subscriber takes longer time to reach the vase station compared to that of a subscriber who is closer to the base station.

240
Q

What is an IMSI ?

A

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 (Figure 1.13).

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.

241
Q

What was the smallest transmission speed unit in a classic circuit-switched telecommunication network ?

A

It is the DS0 (Digital Signal level 0) channel.
It has a fixed transmission speed of 64kbits/sec

242
Q

What does DTAP means ?

A

Direct Transfer Application Part

243
Q

What is the role of the VLR ?

A

Each MSC has an associated Visitor Location Register (VLR), which holds the record of each subscriber that is currently served by the MSC (Figure 1.12).

These records are only copies of the original records, which are stored in the HLR (see Section 1.6.3).

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 is copied to the VLR of the MSC and are thus locally available for every connection establishment.

The VLR is a cache memory.

244
Q

What does VAD means ?

A

Voice Activity Detection

245
Q

What is the effective data rate of a TCH (considering bits used for error control)

A

13kbps

246
Q

Why is the Nc Interface used for ?

A

Virtual speech channels that have been negotiated over the Nc interface are transmitted between MGWs over the Nb interface

247
Q

When was the “Cells Mobile Telecommunication” invented and by who ?

A

in the middle 1970s
by Bell Labs

248
Q

What is the Synchronization CHannel ?

A

It is a channel used by mobile devices during network and cell searches.

249
Q

What are the main tasks of the Call Control (CC) function of an MSC ?

A
  • Registration of Mobile Subscribers: When the Mobile Device is switched on, it registers to the network and is then reachable by all other subscribers.
  • Call establishment and call routing between 2 subscribers
  • Forwarding of SMS Messages
250
Q

What is the basic SS7 protocol stack ?

A

ISUP APPLICATION
MAP
TCAP
SCCP
MTP-3
MTP-2
MTP-1

251
Q

How is the encryption key (kc) generated ?

A

Kc is calculated in the AuC and on the SIM card by using a random number (RAND) and the secret key (Ki) as input parameters for the A8 algorithm.

252
Q

What are the sizes possible with QR Codes ?

A

21 × 21 modules to 177 × 177 modules (Versions 1 to 40, increasing in steps of four modules per side).

253
Q

How does the encryption process work in GSM ?

A

The encryption key (kc) and the current frame number are processed by the encryption algorithm (A5/x). The output is 114bits that will be xor-ed with the 114 original bits of the frame.

The result of the XOR operation are the ciphered bits.

254
Q

Examples of carrier systems ?

A

E-Carrier (E0, E1, E3, etc.)
T-Carrier (T0, T1, T3, etc.)

255
Q

What was the data transmission debit in first EDGE networks ?

A

100kbps

256
Q

What was the data transmission debit in first 2g networks ?

A

Around 10kbps

257
Q

What is the Mobile Station expected response to a “Disconnect” message ?

A
  • A Release message sent by the MSC to which its will reply with a “Release Complete”
258
Q

Which protocol is used for the handover signaling between 2 MSCs ?

A

MAP

259
Q

What are the parameters of a LOCATION UPDATING REQUEST ?

A

IMSI or TMSI + Location Area Code

260
Q

Why is the DTAP protocol used for ?

A

DTAP is used for the communication between the Mobile Station (MS) and the MSC. To establish a voice call, the MS sends a ‘Setup’ message, containing the phone number of the called subscriber, to the MSC

261
Q

What does 5G NR means ?

A

5G New Radio

262
Q

What is the PDN ? (GPRS)

A

The PDN is the generic name to designate the packet-swictched network to which the GPRS system need to provide access to.

263
Q

What does BSSAP means ?

A

Base station SubSystem Application Part.

It is the set formed by the BSSMAP and DTAP protocols

264
Q

When does the Mobile Station triggers an update request ?

A

Once the signal of a neighboring cell becomes stronger than the signal of the current cell by a value that can be set by the network operator, the mobile reselects the new cell and reads the BCCH.

If the LAC that is broadcast is different from that of the previous cell, a location update procedure is started. After a signaling connection has been established, the mobile device sends a Location Update Request message to the MSC, which is transparently forwarded by the radio network.

Before the message can be sent, however, the mobile device needs to authenticate itself and ciphering is usually activated as well.

265
Q

What are the necessary interfaces in a classic NSS architecture ?

A
  • The A Interface: Between the BSS and the MSC
  • The C Interface: between the MSC and the HLR
  • The D Interface: between the VLR and the HLR
  • The E Interface: between 2 MSC
266
Q

At a high level, describe the protocol stack in a mobile phone.

A

Application (Voice calls)

Connection Management

Mobility Management

Radio Resource Management

Data Link

Physical (radio)

267
Q

What is a Slow Associated Control CHannel (SACCH)?

A

It is a channel assigned to a dedicated connection. It is used in the uplink 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. In the downlink direction, the SACCH is used to send power control commands to the mobile device. Furthermore, the SACCH is used for timing advance control.

268
Q

What are the 3 basic types of network nodes defined by the SS7 standard ?

A
  • SSP: Service Switching Point
  • SCP: Service Control Point
  • STP: Signaling Transfer Point
269
Q

What are the 2 special burst of a traffic multiframe ?

A
  • the 12th timeslot is a Slow Associated Control CHannel (SACCH)
  • the 25th timeslot is empty. This gap is used to enable the mobile device to perform signal strength measurements of neighboring cells on other frequencies. This is necessary so that the network can redirect the connection to a different cell (handover) to maintain the call while the user is moving.
270
Q

Where to find GSM standards documents ?

A

http: //www.etsi.org/
http: //www.3gpp.org/

271
Q

What does EDGE mean ?

A

Enhanced Datarates for GSM Evolution

272
Q

What is an NDC ?

A

National Destination Code.
It is a part of the phone number.

This part of the phone number can be used by a switching center to identify the terminating switching center during the call establishment.

273
Q

What is a point code ?

A

It is the equivalent of an IP Address in a SS7 Network.
It is used to identify source and destination of messages and to route those messages within the network.

274
Q

What is the “transport” protocol used for communication between switching centers (SSP) and databases (SCPs) ?

A

SCCP (Signaling Connection and Control Part)

275
Q

What is the function of the BSC ?

A

The BSC (Base Station Controller) is responsible for the establishment, release, and maintenance of all connections for cells that are connected to it.

The BSC is also responsible for establishing signaling channels for incoming calls or SMS messages.

The BSC is also responsible of maintaining the connection while the user is roaming by Performing handovers.

To reduce interference, the BSC is also in charge of controlling the transmission power for every air interface connection.

276
Q

On which carriers frequency hopping must not be used ?

A

Carriers that transport the SCH, FCCH, and BCCH channels.

This restriction is necessary because it would be very difficult for mobile devices to find neighboring cells

277
Q

What is the reason Location Area where introduced ?

A
  • To reduce the signaling load in the radio network when roaming into neighboring cells, Location Area are regrouping multiple neighbouring cells behind a single code: the LAC (Location Area Code).
  • Location Area do also reduce the power consumption of the Mobile Station
278
Q

Which shift keying Method is used for gsm frequence Modulation ?

A

GMSK
Gaussian Minimum Shift Keying

279
Q

What has changed since 2010 in the way frequency are assigned to operators ?

A

Before 2010, frequency were assigned and the technology to use on that frequency were also specified (2G, 3G, etc.)

After 2010, only the frequencies are assigned. The operator can use any normalized technology.

280
Q

What does BSC mean ?

A

Base Station Controller

281
Q

How does the new architecture splitting MSC into MSC-S and MGW improve redundancy ?

A

For load balancing reasons, multiple MSC-S and MGW can be interconnected in a mesh.

If an MSC-S fails, a MGW can thus still continue to operate, and is then controlled by another server.

282
Q

What are the part of an MSISDN ?

A
  • The Country Code : which is the international code of the subscriber’s home country. 1-3 digits.
  • The NDC usually represents the code with which the network operator can be reached. It is normally three digits in length.
  • The Subscriber Network which is unique in the network.
283
Q

What does LTE means ?

A

Long Term Evolution. It is the other name of the 4G Network.

284
Q

How do the circuit switching virtualization over IP has improved the A interface ?

A

On the radio network side, the A interface continues to be used to connect the radio network to the MSC‐Ss and MGWs over an IP‐based link. In addition, the 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.

285
Q

What does VLR means ?

A

Visitor Location Register

286
Q

Which channel is used to send signal measurements ?

A

The SACCH

287
Q

In France which frequency bands are allocated to UMTS ?

A

around 2 GHz

288
Q

What was the technology specificities of UMTS ?

A

CDMA

289
Q

Describe succinctly the FR codecs process

A

The FR codec waits on its inputs 160 values every 20ms. With 8bits per Value, it corresponds to 1280 bits.

The FR codec will output 260 bits after having Passed the 1280bits through some filters that emulate the human speech.

290
Q

What is the Air Interface or the Um Interface ?

A

It is the transmission path between the BTS and the Mobile Device

291
Q

How is the mobile device informed about the use of frequency hopping ?

A

The Immediate Assignment message used during the establishment of a signaling link contains all the information about the frequencies that are used and the hopping pattern that is applied to the connection.

292
Q

How many frequencies can be used by a base station for frequency hopping ?

A

Up to 64

293
Q

When is the protocol SCCP used ?

A

SCCP is the transport protocol of the SS7 stack.

It can be compared to TCP or UDP in the TCP/IP stack.

It is used for communications between switching centers (SSPs) and databases (SCPs) or between switching centers and the Base Station Subsystem, or between Switching Centers and the Mobile Station.

294
Q

How is it possible that the Mobile Device or even the SIM Card is not aware of its own phone number ?

A

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.

295
Q

What are some security tech used inside the Network Subsystem ?

A

VPN (IPSEC), MPLS

296
Q

What is an inter‐MSC location update ?

A

It is a location update procedure that happen when the new cell is managed by a different MSC than the one managing the previous cell.

It doesn’t change anything from a Mobile Station perspective. However additional messages are exchanged within the network to update the HLR about the new location (MSC) of the customer and to delete the record of that Customer in the old MSC.

297
Q

What does TCH mean ?

A

Traffic CHannel

298
Q

What is timing advance Control ?

A

It is the procédure that carefully monitor and adapt the timing of transmission by the mobile stations.

Indeed, owing 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 were 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

299
Q

At a high level, describe the protocol stack in a BSC.

A

Radio Resource Management | BSSAP, SCCP
Data Link | STP-1, 2, 3
Physical | STP-1, 2, 3

300
Q

What was the technologies particularities of the 1st generation networks ?

A

Based on FDMA
Analogic
Multiple technologies (not compatible among themselves - R2000, NMT)

301
Q

What is the specificity of the GPRS compare to GSM ?

A

The resource in the radio and in the core are allocated only on a needed basis.

On the radio, a TCH is not reserved for a single full call even if no voice is transmitted.

In the Core, circuits are not reserved for a single call, but the resources (transmission, switches, etc) are used when packet comes and release once these packets are transmitted.

302
Q

What are the parameter of the A3 algorithm ?

A

RAND and Ki

303
Q

When did the first standardization activities of the GSM started ?

A

middle of the 1980s

304
Q

Which spec does specify the Full Rate (FR) codec ?

A

TS 46.010

305
Q

What does SDH means ?

A

Synchronous Digital Hierarchy

306
Q

In GSM, how is the algorithm used to compute the challenge while the terminal is authenticating called ?

A

A3

307
Q

Why is the Nc Interface used for?

A

The Nc interface is used to transport voice calls within the core network and to gateways to other mobile or to fixed networks.

308
Q

What does T-BCSM mean ?

A

Terminating Basic Call State Model (BCSM)

309
Q

What does TMSI mean ?

A

Temporary Mobile Subscriber Identification

310
Q

What is the typical Value of the RSSI indicating that the Mobile Station is very far away from the BTS ?

A
  • 100dBm
311
Q

How does the MSC Manage the billing ?

A

For each call the MSC create a billing record which is transferred to a billing server.

The billing record contains information like the caller number and the callee number, cell ID of the cell from which the call originated, time of call origination, duration of the call, and so on.

For prepaid, the billing is managed differently, and is done mostly in the IN SubSystem.

312
Q

Which spec does specify how the mobile device and SIM Card communicate (APDU) ?

A

TS 102.221

313
Q

What does APDU mean ?

A

Application Protocol Data Unit

314
Q

What is a TCH (Traffic CHannel) ?

A

It 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.

315
Q

Which node do initiate the establishment of a TCH for the voice path ?

A

The MSC request the establishment of the TCH to the BSC

316
Q

What does SCH mean ?

A

Synchronization Channel (SCH)

317
Q

Which protocol is used on the A Bis interface ?

A

LAP-D (Link Access Protocol)

318
Q

What are the protocols underlying the DTAP protocol ?

A

DTAP rely on BSSMAP.

However all nodes between the Mobile Station and the MSC are not required to understand DTAP.

319
Q

What is BCSM ?

A

Basic Call State Model (BCSM)

It is the CAMEL state model for Call Control that divide a call procedure into different states. For example, the states defined for the originator of a call are:

call establishment
analysis of the called party’s number
routing of the connection
notification of the called party (alerting)
ongoing call (active)
disconnection of the call
no answer from the called party
and called party busy

320
Q

What was the data transmission debit in first GPRS networks ?

A

20-30kbps

321
Q

How are files identified in a SIM Card and why ?

A

Files are identified by hexadecimal numbers with a length of four digits.

This is because the amount of memory is very limited. 2 bytes only needed for the file names.

322
Q

Why is it necessary to usually change the IMSI into a different TMSI ?

A

Because some exchanges (as the authentication procedure) are done in clear text on the radio.

And in these exchanges the MSISDN need to share its identity (either the IMSI or the last TMSI).

If the IMSI were always used, it would have been easier to track GSM subscriber based on their IMSI.

323
Q

What are the necessary nodes in an IP-based NSS architecture ?

A
  • MSC are divided in 2 parts:
    * MSC-S: MSC Server
    * MGW: Media Gateway
  • HLR
  • VLR
  • SGW
324
Q

In France which frequency bands are allocated to LTE ?

A

2.6 GHz

325
Q

What is a mobile terminated call ?

A

It is an incoming call for a mobile Subscriber.

326
Q

What is the PDP ? (GPRS)

A

The PDP is the generic name to designate the packet-swictched protocol used in the PDN to which the GPRS system need to provide access to.

327
Q

Which spec specify the Mc Interface ?

A

3GPP TS 29.232

328
Q

How many timeslots of 64kbits/s are in an E1 connection ?

A

32

329
Q

What are the type of memories in a SIM Card

A

ROM (1-3Kb)
EEPROM (16-64kb)
RAM (1-3Kb)

330
Q

What does RACH mean ?

A

Random Access CHannel

331
Q

What does GPRS means ?

A

General Packet Radio Service

332
Q

What does SAT mean ?

A

SIM Application Toolkit

333
Q

What is the traditional/historical frequency band of the GSM ?

A

The 900Mhz Band:

Uplink: 890-915 MHz
Downlink: 935-960 MHz

334
Q

What is APDU ?

A

Application Protocol Data Unit

It is the layer of command and response message defined to be used on the interface between the Mobile Device and the SIM Card.

335
Q

What are the most import functions of the NSS (Network SubSystem) ?

A
  • Call Establishment
  • Call Control
  • Routing of Calls betweend different fixed and mobile switching centers and other networks
  • Subscriber Management
336
Q

What is the HLR ?

A

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

337
Q

What does GSM means ?

A

Global System for Mobile Communications

338
Q

What are the step of the voice digitization in the classic Telco Network ?

A
  • Band pass filter 300hz - 3.4Khz
  • Sampler : 8KHz
  • Quantitizer and encoding: PCM - 8bits
339
Q

Which device do the conversion between BICC and ISUP ?

A

The SGW

From GSM to LTE Advanced Pro and 5G (2 decks)

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