JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year TELECOMMUNICATION SWITCHING SYSTEMS AND NETWORKS Previous Question Paper (February/March - 2022 - R15)

Memory-controlled time division switching uses a time-slot interchange (TSI) mechanism to reorder data samples in time. The operation relies on two primary memory structures: the Speech Memory and the Control Memory.

The Speech Memory stores incoming pulse code modulation (PCM) samples from various time slots. It is typically implemented as a random-access memory (RAM) where each memory location corresponds to a specific incoming time slot. The Control Memory stores the sequence of time slots to be read out, effectively determining the switching path.

The switching process occurs in two modes: write-in and read-out. In the write-in phase, incoming samples are written into the Speech Memory sequentially. In the read-out phase, the Control Memory provides the addresses in a non-sequential order, allowing samples to be read out in the desired time slot for the outgoing highway. This process achieves time-slot interchange, enabling any incoming time slot to be connected to any outgoing time slot.

A block diagram would show the incoming PCM highway connected to the Speech Memory, which is addressed by a counter during the write cycle. The Control Memory, addressed by a time-slot counter, provides the address to the Speech Memory during the read cycle, directing the output to the outgoing PCM highway.

The Erlang Delay formula, also known as Erlang-C, calculates the probability that a call arriving at a system with a finite number of servers will be delayed because all servers are busy. It assumes an infinite source of callers, Poisson arrival process, and exponential service times with a first-in-first-out (FIFO) queue discipline.

The formula is given by:

Delay tables are essential in telecommunication network planning to quickly determine the Grade of Service (GoS) without performing complex manual calculations. They provide pre-computed values of the probability of delay for various combinations of traffic intensity (A) and server capacity (N). Engineers use these tables to dimension trunk groups and determine the number of circuits required to maintain a target delay probability, ensuring efficient resource allocation in switching systems.