Question

1. A digital signal has four levels. How many bits are needed per level? Draw the digital signal assume the baud rate is 8 baud per second.
2. Increasing the levels of a signal increases the probability of an error occurring, in other words it reduces the reliability of the system. Why and how can this be resolve?
3. What determines how successful a receiver will be in interpreting an incoming signal?

4. Spectrum of a channel between 5 MHz and 4 MHz; SNR_dB = 24 dB. Find the capacity and the signal level?
5. Explain the five (5) types of interfaces in GSM Architecture?
6. Draw a well labeled GSM Network Architecture and explain each components.
7. What is Mobile Roaming? Explain.
8. Consider an extremely noisy channel in which the value of the signal-to-noise ratio is almost zero (0 dB). In other words, the noise is so strong that the signal is faint. For this channel the capacity C is calculated as?

9. The A/D conversion is performed by using a process called Pulse Code    Modulation (PCM). Explain the three main steps involve?
10. Consider a noiseless channel with a bandwidth of 3GHz transmitting with 1024 signal levels (for each level, we send 2 bits). The maximum bit rate can be calculated as?

Answer 1

GSM stands for Global System for Mobile Communication. It is a digital cellular technology used for transmitting mobile voice and data services.

The primary goal of GSM was provide a mobile phone system that allow roaming to the user through Europe and provide the voice service comptaible ISDN and other PSTN System.

Draw a well labeled GSM Network Architecture and explain each components.

  

A GSM System mainly consists of three subsystems:

i. Radio Subsystem (Rss)

II.Network and Switching Subsystem (Nss)

iii.Operation Subsystem(Oss)

1.Radio Subsystem (Rss):

As the name implises ,the radio subsystem (RSS) comprises all radio specific entries i.e the mobile Station (MS) and Base Station SubSystem

i.Mobile Station: The MS comprises all the user equipment and software needed for the communication with GSM network. An MS consists of user independant hard and software and of the subscriber Identity Module (SIM) which store the all specific data

While MS can identified by (IMIE) International Mobile Equipment Ifentity a user can personalize  my MS using his or her sim.

SIM can contain many identifier and table such as card-type,serial number,a list of subscribed services a personal

Identity number(PIN) ,and Internatinal Mobile Subscriber Identity The pin is used to unlock the MS whereas using wrong pin 3 times will lock the sim in such case puk is needed to unlock the sim.

Typical Mss for example mobile phones ,PIN,calenders.

ii.Base Station SubSystem:

A GSM nwtwork comprises many BSSs ,each controlled by a Base Station Controller(BSC).

The BSS performs all function necessary to maintain radio connections to an MS coding/decoding voice ,and rate adaption to/from the wirelesee network part BSS contains several BTSs.

iii.Base Transreciver station:

A BTS can form a radio cell or, using sectorized antennas, several and is connected to MS viathe Um interface, and to the BSC via the Abis interface.

The Um interface contains all the mechanisms necessary for wireless transmission (TDMA, FDMA etc.)The BTS is the radio equipment (transceivers and antennas) needed to service each cell in the network.

A group of BTS’s are controlled by an BSC.

iv.Base station controllers (BSC): The BSC provides all the control functions and physical links between the MSC and BTS.

It is a high capacity switch that provides functions such as handover, cell configuration data, and control of radio frequency (RF) power levels in BTS.

A number of BSC’s are served by and MSC.

2. Network Switching Subsystem: The NSS i PCS provider, he or she is registered in the HLR of that operator.s responsible for performing call processing and subscriber related functions.

The switching system includes the following functional units:

Home location register (HLR): It is a database used for storage and management of subscriptions.

HLR stores permanent data about subscribers, including a subscribers service profile, location information and activity status.

When an individual buys a subscription from the Visitor location register (VLR): It is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers.

VLR is always integrated with the MSC. When a MS roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR.

Later if the mobile station needs to make a call, VLR will be having all the information needed for call setup

.Authentication center (AUC): A unit called the AUC provides authentication and encryption parameters that verify the users identity and ensure the confidentiality of each call.

Equipment identity register (EIR): It is a database that contains information about the identity of mobile equipment that prevents calls from stolen, unauthorized or defective mobile stations

Mobile switching center (MSC): The MSC performs the telephony switching functions of the system. It controls calls to and from other telephone and data systems.

3. Operation and Support system:

The operations and maintenance center (OMC) is connected to all equipment in the  switching system and to the BSC. Implementation of OMC is called operation and support system (OSS).

The OSS is the functional entity from which the network operator monitors and controls the system.

The purpose of OSS is to offer the customer cost-effective support for centralized, regional and local operational and maintenance activities that are required for a GSM  network.

OSS provides a network overview and allows engineers to monitor, diagnose and  troubleshoot every aspect of the GSM network.

The mobile station (MS) consists of the mobile equipment (the terminal) and a smart card called the Subscriber Identity Module (SIM).

The SIM provides personal mobility, so that the  user can have access to subscribed services irrespective of a specific terminal. By inserting the SIM card into another GSM terminal, the user is able to receive calls at that terminal, make calls from that terminal, and receive other subscribed services.

The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI). The SIM card contains the International Mobile Subscriber Identity (IMSI) used to identify the subscriber to the system, a secret key for authentication, and other information.

The IMEI and the IMSI are independent, thereby allowing personal mobility. The SIM card may be protected against unauthorized use by a password or personal identity number.

Explain the five (5) types of interfaces in GSM Architecture?

1.Um interface   The "air" or radio interface standard that is used for exchanges between a mobile (ME) and a base station (BTS / BSC). For signalling, a modified version of the ISDN LAPD, known as LAPDm is used.

2..Abis interface   This is a BSS internal interface linking the BSC and a BTS, and it has not been totally standardised. The Abis interface allows control of the radio equipment and radio frequency allocation in the BTS.

3.A interface   The A interface is used to provide communication between the BSS and the MSC. The interface carries information to enable the channels, timeslots and the like to be allocated to the mobile equipments being serviced by the BSSs. The messaging required within the network to enable handover etc to be undertaken is carried over the interface

4.B interface   The B interface exists between the MSC and the VLR . It uses a protocol known as the MAP/B protocol. As most VLRs are collocated with an MSC, this makes the interface purely an "internal" interface. The interface is used whenever the MSC needs access to data regarding a MS located in its area.

5.C interface   The C interface is located between the HLR and a GMSC or a SMS-G. When a call originates from outside the network, i.e. from the PSTN or another mobile network it ahs to pass through the gateway so that routing information required to complete the call may be gained. The protocol used for communication is MAP/C, the letter "C" indicating that the protocol is used for the "C" interface. In addition to this, the MSC may optionally forward billing information to the HLR after the call is completed and cleared down.

What is Mobile Roaming? Explain.

.Roaming enables a mobile subscriber to automatically make and receive voice calls, send and receive data, or access other services when travelling outside the geographical coverage area of their home network, by means of using a visited network.

Roaming is technically supported by mobility management, authentication and billing procedures. Establishing roaming between network operators.

GSM Roaming, which involves roaming between GSM networks, offers the mobile subscriber the convenience of being able to use a single number, a single bill and a single phone in up to 219 countries.

The convenience of GSM Roaming has been a key driver behind the global success of the GSM Platform.

Que. The A/D conversion is performed by using a process called Pulse Code    Modulation (PCM). Explain the three main steps involve?

Modulation is the process of varying one or more parameters of a carrier signal in accordance with the instantaneous values of the message signal.

The message signal is the signal which is being transmitted for communication and the carrier signal is a high frequency signal which has no data, but is used for long distance transmission.

There are many modulation techniques, which are classified according to the type of modulation employed. Of them all, the digital modulation technique used is Pulse Code Modulation PCM

Three Steps Invole are:

Pulse Code Modulator circuit consists of Sampling, Quantizing and Encoding, which are performed in the analog-to-digital converter section

The low pass filter prior to sampling prevents aliasing of the message signal

Sampler

This is the technique which helps to collect the sample data at instantaneous values of message signal, so as to reconstruct the original signal.

The sampling rate must be greater than twice the highest frequency component W of the message signal, in accordance with the sampling theorem.

Quantizer

Quantizing is a process of reducing the excessive bits and confining the data. The sampled output when given to Quantizer, reduces the redundant bits and compresses the value.

Encoder

The digitization of analog signal is done by the encoder. It designates each quantized level by a binary code. The sampling done here is the sample-and-hold process. These three sections LPF,Sampler,andQuantizerLPF,Sampler,andQuantizer will act as an analog to digital converter. Encoding minimizes the bandwidth used.

Que:Consider a noiseless channel with a bandwidth of 3GHz transmitting with 1024 signal levels (for each level, we send 2 bits). The maximum bit rate can be calculated as?

BitRate = 2*3000*log24=12,000bps

que.Consider an extremely noisy channel in which the value of the signal-to-noise ratio is almost zero (0 dB). In other words, the noise is so strong that the signal is faint. For this channel the capacity C is calculated as?

C=Blog2 (1 + SNR) =B10gz (l + 0) =Blog2 1 =Bx 0 = 0This means that the capacity of this channel is zero regardless of the bandwidth. In other words,we cannot receive any data through this channel.

que: A digital signal has four levels. How many bits are needed per level? Draw the digital signal assume the baud rate is 8 baud per second.

It is apparent that 8 levels require three binary digits, or bits; 16 levels require four bits; and 256 levels require eight bits. In general 2ⁿ levels require n bits.

Increasing the levels of a signal increases the probability of an error occurring, in other words it reduces the reliability of the system. Why and how can this be resolve?

Signal-to-noise ratio (SNR) is a well-known measure of how the signal and noise power compare against each other.

In terms of this, the bit error rate, BER, can also be defined in terms of the probability of error or POE.

It is important to note that POE is proportional to Eb/No and is a form of signal to noise ratio.

Obviously, it directly affects the probability of error performance of a system.

It is possible to define the bit error rate in terms of a probability of error.

POE=12 (1−erf)EbNo

Where:
erf = error function
E_b = energy in one bit
N_o = power spectral density (noise in 1Hz bandwidth).

It is important to note that E_b / N_o is a form of signal to noise ratio.