JNTU B.Tech III DATA COMMUNICATIONS AND NETWORKS Previous Question Paper (March-2021 - R18)

In the Internet model, information is passed between layers through the process of encapsulation and decapsulation. At the sender side, each layer receives data from the layer above it, treats it as a payload, and adds its own control information in the form of a header (or sometimes a trailer) to create a Protocol Data Unit (PDU). This process continues down the stack until the physical layer transmits the bits over the medium.

At the receiver side, the process is reversed. Each layer receives the PDU from the layer below, strips off its specific header to extract the original data, and passes the remaining payload up to the next higher layer. This interaction is governed by the interface between adjacent layers, ensuring that each layer provides services to the layer above it while remaining independent of the internal implementation of other layers.

Code Division Multiple Access (CDMA) allows multiple users to share the same frequency channel simultaneously by assigning a unique orthogonal code (or chip sequence) to each user. Unlike TDMA or FDMA, which divide resources by time or frequency, CDMA uses spread spectrum technology.

• Each station is assigned a unique code sequence, typically represented as a series of bits (e.g., +1 and -1).
• To send a data bit 1, the sender transmits the code sequence; to send a bit 0, it transmits the inverse of the code sequence.
• The receiver, knowing the specific code of the sender, performs a mathematical operation (dot product) between the received signal and its own code sequence.
• Due to the orthogonality of the codes, the receiver can extract the intended signal from the composite signal containing transmissions from multiple users, as the inner product of different codes results in zero.

This allows all stations to transmit at the same time over the entire available bandwidth, with the receiver effectively filtering out the noise and signals from other users based on the unique code signature.