Aspects Of Satellite Systems Computer Science Essay

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Person to person text messaging: Person-to-person text messaging is the most commonly used SMS application. In these kinds of text messaging applications, a mobile user types an SMS text message using the keypad of his/her mobile phone, then he/she inputs the mobile phone number of the recipient and clicks a certain option on the screen, such as "Send" or "OK", to send the text message out. When the recipient mobile phone receives the SMS text message, it will notify the user by giving out a sound or vibrating. The user can read the SMS text message some time later or immediately and can send a text message back if he/she wants.

A chat application is another kind of person- to-person text messaging application that allows a group of people to exchange SMS text messages interactively. In a chat application, all SMS text messages sent and received are displayed on the mobile phone's screen in order of date and time. SMS text messages written by different mobile users may be displayed in different colors for better readability.

Provision of Information: Many content providers make use of SMS text messages to send information such as news, weather report and financial data to their subscribers. Many of these information services are not free. Reverse billing SMS is a common way used by content providers to bill their users.

Downloading: SMS messages can carry binary data and so SMS can be used as the transport medium of wireless downloads. Objects such as ringtones, wallpapers, pictures and operator logos can be encoded in one or more SMS messages depending on the object's size. Like information services, wireless download services are usually not free and reverse billing SMS is a common way used by content providers to bill their customers.

Q3. With example, discuss structure of Third Generation Telephone Mobile Systems. How does it overcome the limitations or shortcomings of the 2nd Generation?

Ans. Universal Mobile Telecommunications System (UMTS), standardized by the 3GPP, is the 3G mobile communication technology successor to GSM and GPRS. UMTS combines the W-CDMA, TD-CDMA, or TD-SCDMA air interfaces, GSM's Mobile Application Part (MAP) core, and the GSM family of speech codecs. W-CDMA is the most popular cellular mobile telephone variant of UMTS in use. UMTS, using W-CDMA, supports up to 14.0 Mbit/s data transfer rates in theory with High Speed Downlink Packet Access (HSDPA), although the performance in deployed networks could be much lower for both uplink and downlink connections. A major difference of UMTS compared to GSM is the air interface forming Generic Radio Access Network (GeRAN). It can be connected to various backbone networks like the Internet, ISDN, GSM or to a UMTS network. GeRAN includes the three lowest layers of OSI model. The network layer (OSI 3) protocols form the Radio Resource Management protocol (RRM). They manage the bearer channels between the mobile terminals and the fixed network including the handovers. The UMTS standard is an extension of existing networks based on the GSM and GPRS technologies. In UMTS release 1, a new radio access network UMTS terrestrial radio access network (UTRAN) is introduced. UTRAN, the UMTS radio access network (RAN), is connected via the Iu to the GSM Phase 2+ core network (CN). The Iu is the UTRAN interface between the radio network controller (RNC) and CN; the UTRAN interface between RNC and the packet-switched domain of the CN (Iu-PS) is used for PS data and the UTRAN interface between RNC and the circuit-switched domain of the CN (Iu-CS) is used for CS data. UTRAN is subdivided into individual radio network systems (RNSs), where each RNS is controlled by an RNC. The RNC is connected to a set of Node B elements, each of which can serve one or several cells. Two new network elements, namely RNC and Node B, are introduced in UTRAN.

The major benefits of 3G over 2G network are:

The major factor that led to the adoption of 3G network by the users is the data rates. 3G provides a data rate of 2mbps for stationary users and 384kbps for mobile users which is far more than 2G data rates of 9.6kbps.

With the evolution of 3G it became possible to provide services like mobile broadband i.e. providing users with the speed of broadband over a wireless network.

3G networks offer a greater degree of security than 2G predecessors. By allowing the UE (User Equipment) to authenticate the network it is attaching to, the user can be sure the network is the intended one and not an impersonator. 3G networks use the KASUMI block crypto instead of the older A5/1 stream cipher.

Q4. Draw a comparative analysis of the Operation and Channel Allocation techniques used for FDMA, CDMA & TDMA





With Frequency Division Multiple Access (FDMA), different signals are assigned frequency channels. A channel is a frequency. FDMA is a basic technology in the analog Advanced Mobile Phone System (AMPS). With FDMA, each channel can be assigned to only one user at a time. FDMA is also used in the Total Access Communication System (TACS).

If channel is not in use, it sits idle.

Channel bandwidth is relatively narrow (30kHz), i.e., usually narrowband systems can operate.

In Time Division Multiple Access (TDMA), it makes use of the same frequency spectrum as FDMA but allows more users on the same band of frequencies by dividing the time into "slots" and shares the channel between users by assigning them different time slots. TDMA is utilized by Digital-Advanced Mobile Phone System (D-AMPS) and Global System for Mobile communications (GSM). However, each of these systems implements TDMA in a somewhat different and incompatible way.

Only 1 user is allowed to either transmit or receive in a slot.

Slots are assigned cyclically.

In Code Division Multiple Access (CDMA), each user is assigned a different pseudorandom binary sequence that modulates the carrier, spreading the spectrum of the waveform and giving each user a unique code pattern. This technology is used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands.

All users use same frequency and may transmit simultaneously.

Each user has its own pseudo-codeword (orthogonal to others).

Q5. Cite examples to explain the concept of "Man in the middle attacks", "Denial of Service Attack" "Spoofing" & "Phishing". Tell, how they cause harm and not how can we protect us from them.


Man-in-the-Middle attacks are often referred to as "session hijacking attacks", suggesting that the intruder aims to gain access to a legitimate user's session to tamper it. The attack usually starts with sniffing and eavesdropping on a network stream, and ends with trying to alter, forge or reroute the intercepted data. An example is :

In a denial-of-service (DoS) attack, an attacker attempts to prevent legitimate users from accessing information or services. By targeting your computer and its network connection, or the computers and network of the sites you are trying to use, an attacker may be able to prevent you from accessing email, websites, online accounts (banking, etc.), or other services that rely on the affected computer. An example is :

In a spoofing attack, the attacker creates misleading context in order to trick the victim into making an inappropriate security-relevant decision. A spoofing attack is like a con game: the attacker sets up a false but convincing world around the victim. The victim does something that would be appropriate if the false world were real. Unfortunately, activities that seem reasonable in the false world may have disastrous effects in the real world. An example is:

Phishing is essentially an online con game and phishers are nothing more than tech-savvy con artists and identify thieves. They use SPAM, malicious Web sites, email messages and instant messages to trick people into divulging sensitive information, such as bank and credit card accounts. An example is: