Space Switch vs. Time Switch: Key Differences Explained

This article explores the differences between space switches and time switches, fundamental components in digital switching architectures. Digital switches often rely on time and space elements to facilitate Time Division Switching (TDM) and Space Division Switching (SDM). In addition to space switches and time switches, hybrid approaches like time-space-time switches and space-time-space switches exist, combining the functionalities of both.

Time Switch

The core concept of a time switch, also known as a Time Slot Interchanger (TSI), is illustrated below:

Figure 1: Time Switch

In a Pulse Code Modulation (PCM) system, a time slot typically consists of 8 bits, representing a single voice channel. These time slots are repeated 8000 times per second. For example, a DS1 frame contains 24 time slots, while an E1 frame contains 32. As depicted in the figure, a time switch establishes connectivity by re-arranging the order of the time slots. In the illustration, user C on slot C is connected to user G on slot G, but potentially in a different order.

The following operations are crucial for time switch functionality:

• Memory for Data Storage: A memory component is required to store speech data from the time slots. This data is written and read to rearrange the positions of the time slots.
• Control Memory: A separate memory section stores the control actions needed to manipulate the stored data. This memory dictates how the time slots are re-ordered.
• Time Slot Counter/Processor: This component keeps track of the time slots and controls the switching operation.

Space Switch

Figure 2 shows the basic structure of a time-division space switch.

Figure 2: Space Switch

A space switch is made up of a cross-point matrix, utilizing logic gates to switch time slots in the spatial domain. Imagine input horizontals and output verticals intersecting at crosspoints, with a logic gate positioned at each intersection. These logic gates control the routing of data from a specific input to a specific output.

Consider a tandem switching center or the route switch of a local exchange. These need the capability to connect any channel on an incoming PCM highway to any channel on an outgoing PCM highway. Because the incoming and outgoing highways are spatially separated, this requires a space switch.

In a typical scenario, a connection occupies different time slots on the incoming and outgoing highways. Therefore, the switching network needs to receive PCM samples from one time slot and re-transmit them in a different time slot. This is where time slot interchange, or simple time switching, comes into play.

Figure 2 highlights that data transmitted on time slots 1 and 2 are re-transmitted on time slots 24 of stream A and stream B at the output. This illustrates that the switching network of a tandem exchange (or the route switch of a local exchange) requires both time switching and space switching capabilities.

Essentially, a space switch is like a matrix of N x M physical lines, while a time switch functions as a memory unit holding data at different, consecutive address locations.

Summary of Key Differences

In essence:

• Space Switch: Routes data based on physical pathways, creating a spatial connection between input and output lines.
• Time Switch: Re-arranges the order of time slots within a digital stream, allowing data from one time slot to be transmitted in another.

Ultimately, complex switching systems often combine both space and time switching to achieve the necessary flexibility and capacity for modern telecommunications networks.