(SEM-VII) THEORY EXAMINATION 2018-19 DATA COMMUNICATION NETWORKS

DATA COMMUNICATION & NETWORKS (NEC-702 / BEEC-702)

B.Tech – Semester VII

SECTION A

(Attempt all questions)

(a) Space Division Multiple Access (SDMA)

Space Division Multiple Access is a multiple access technique in which users are separated spatially so that the same frequency band and time slot can be reused simultaneously by different users located in different geographical regions. SDMA is achieved by using directional antennas, sectorization, or beamforming techniques. By focusing transmission energy in specific directions, SDMA increases system capacity and reduces interference. It is widely used in cellular communication systems where cells and sectors allow frequency reuse.

(b) Multiple access techniques in data communication

Multiple access techniques are used to allow multiple users to share a common communication medium efficiently. These techniques manage access to bandwidth, time, or code resources so that users can transmit data without unacceptable interference. Proper multiple access schemes improve spectrum utilization, system capacity, and quality of service in communication networks.

(c) Throughput in random access protocols

Throughput represents the average rate of successful data transmission over a communication channel. In random access protocols such as ALOHA and CSMA, throughput depends on collision probability, offered load, and retransmission strategy. Higher throughput indicates better channel utilization and improved network performance.

(d) Orthogonality in communication systems

Orthogonality refers to the property by which multiple signals can coexist on the same channel without interfering with each other, provided they are mathematically orthogonal. This concept is widely used in modern systems such as OFDM and CDMA to allow multiple users or subcarriers to transmit simultaneously with minimal interference.

SECTION B / SECTION C (Long Answer)

(Attempt any one)

(a) Comparison of FDMA, TDMA, and CDMA

Frequency Division Multiple Access divides the available bandwidth into several frequency channels, with each user assigned a separate frequency band. This technique is simple and provides continuous transmission, but it suffers from inefficient bandwidth usage due to guard bands.
 

Time Division Multiple Access divides the channel into time slots and allocates each user a specific time slot in a repeating frame. TDMA improves bandwidth efficiency compared to FDMA but requires precise synchronization.
 

Code Division Multiple Access allows all users to transmit simultaneously over the same frequency band by using unique spreading codes. CDMA provides high capacity, resistance to interference, and better security, but it requires complex receivers and power control mechanisms. Each technique has its own advantages and applications depending on system requirements.

(b)(i) Reason for doubled throughput in Slotted ALOHA compared to Pure ALOHA

In Pure ALOHA, data frames can be transmitted at any time, leading to a high probability of collisions. Slotted ALOHA improves performance by dividing time into discrete slots and allowing transmissions only at the beginning of a slot. This reduces the vulnerable period for collisions by half. As a result, the maximum throughput of Slotted ALOHA is twice that of Pure ALOHA, making it more efficient and reliable.

(b)(ii) Modulation and demodulation in an OFDM system

Orthogonal Frequency Division Multiplexing is a multicarrier modulation technique in which the available bandwidth is divided into many narrowband subcarriers that are orthogonal to each other. During modulation, the input data stream is divided into parallel streams, each modulating a separate subcarrier using schemes such as QPSK or QAM. An inverse fast Fourier transform is used to generate the composite time-domain signal.
 

At the receiver, demodulation is performed by applying a fast Fourier transform to separate the subcarriers, followed by symbol detection and data reconstruction. OFDM provides high spectral efficiency, robustness against multipath fading, and is widely used in modern communication systems such as Wi-Fi, LTE, and 5G.

(c) Role of multiple access in modern networks

Multiple access techniques play a crucial role in enabling efficient sharing of limited communication resources among multiple users. They improve network scalability, support high data rates, and ensure fair access. With the rapid growth of wireless communication, advanced multiple access schemes are essential for meeting the increasing demand for bandwidth and reliability.