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Telecommunications Fundamentals, Chapter 4: Fundamentals of Switching

FUNDAMENTALS OF SWITCHING

Objectives

Identify the major functions of switching. State the meaning of electronic switching systems (ESS) and stored program control (SPC) switching systems. Describe how this family of switches differs from earlier switches. Describe the major components of a digital switch and the main functions of those components. Trace a call through an analog switch and a digital switch. Explain the differences. Overview of remote switching and reasons for implementing. Identify and describe the basic traffic measurements.

Functions of a Switch

The purpose of a switch is to provide a path for the call. To process a call the switch performs three main functions:

1) Identifies the customer

2) Sets up the communication path

3) Supervises the call

Identify the Customers

Initially customers were identified by the jack position they occupied on the switchboard. With the introduction of electromechanical switches, customers were as signed telephone numbers. (Also called line or station numbers.) The customer's cable pair is terminated and cross-connected to the office equipment at the main distributing frame. Office equipment terminated on the MDF represents a physical location in the switch and a specific telephone number. With the introduction of electronic switches, a telephone number is no longer wired to a specific component of the switch. The telephone number is now associated with a customer record which exists in the translations (or memory) of the switch (Figure 4.1).

Set Up the Path

Early in the processing of a call, the switch needs to determine what type of a call is being made (Figure 4.2). By analyzing either the first digit (is it a 0 or a 1?) or the first three digits (prefix), the switch will determine whether the call is intraswitch or inter-switch. If the call being processed is an intra-switch call, the path that the switch will allocate is called a line (i.e., "on the line side of the network"). If the call is an inter-switch call, the path that the switch will allocate is a trunk.

Supervise the Call

The supervision functions of the switch tend to be overlooked because they are transparent to the customer. They are, however, extremely important because they directly impact the efficient functioning of the switch itself. Supervision functions include (Figure 4.3):

The Direct Connection Network (Fig. 4.4)

Relatively soon after the introduction of the telephone it became obvious that the direct connection network had some very distinct trade-offs. Advantages were:

Speed of call processing

Privacy Control over the call

Disadvantages, however, included:

Each destination required separate phones and cable pairs

Limited connectivity

Expense Lack of standards

Switchboards and the Centralized Network

When Theodore Vail hired the out-of-work railroad engineers to become the earliest AT&T employees, the concept of a centralized point of switch ing for telephone calls became real. As stated in Lesson 1, it was the railroad engineers who had created a hierarchical process for the routing of local and long distance railroad trains (Figure 4.5). The long distance trains were routed from switch house to switchhouse along "express rail" lines which were known as trunks. Once the train reached the furthermost switch house or hub, the passengers would be required to change trains and board a local train which would travel the distribution network to the end destination. The trains that traveled across the distribution network were slower speed trains with many more stops than the express trunks that connected the switch houses. The same concept was employed with the newly developing telephone network.

Manual Switchboards and Centralized Networks

To realize the vision of a Centralized Network it was necessary to create a central switching point, thus the development and deployment of the manual switchboard. At this location, later known as the central office, each customer had a single connection and the ability to be connected to any other customer.

Please click here for a manual switchboard depiction

To complete a call, a patch cord which terminated in two jack locations was used. One jack represented the originator. The other jack represented the called party, or outgoing trunk. The implementation of centralized switching dramatically increased the interconnectivity in the network, and along with it the value of the tele phone to the customer. The concept of long distance was a natural evolution of the switchboard. Long distance calls were processed initially by the operator physically daisy chaining switchboards together with patch cords on outgoing/incoming trunks. This became an automatic function of the electromechanical switch.

While the centralized network did have its advantages, its early trade-offs were: Lack of control over the call by the customer (all processing was done by the operator). Dependency on the operator (for call set up and billing). Lack of privacy (the operator supervised the call for disconnection and billing). As the switches became more automatic, these issues diminished greatly.

Automatic Switching

The invention of automatic switching opened up a whole new set of possibilities for telephony.

Analog Switches The "first generation" of automatic switches were electromechanical, analog type switches.

Step-by-Step Switch (SXS)

Here are a list of features of step by step switches

The first "automatic" switch, invented by Almon Strowger in 1890, was the "SXS" electromechanical switch. It was patented in 1891 and first installed in 1892 in New Haven, Connecticut. Also introduced the rotary dial. Strowger was an undertaker by trade, but had experienced firsthand some of the disadvantages of the centralized network. From Strowger's perspective, the biggest problem with the centralized network was that the originator no longer had any control over the call. His intent in inventing the first electromechanical switch was to give control of the call back to the customer.

It was said that Strowger worked out his original design with a card board collar box, some straight pins and a pencil. The name Step-by-Step comes from the way the path is set up -- step-by step -- as each digit is dialed (Figure 4.7). The fundamental process is that telephone calls progress through switches in steps, each step being made in response to the dialing of a digit.

Unique features of the Step-by-Step are:

Introduction of the concentration ratio concept (10:1)

Third digit analysis, resulting in the reduction of operator requirements.

10,000 line capacity with the capability of processing 1,000 calls simultaneously.

Flat rate billing for intra-switch calls.

Figure 4.7 illustrates the basic internal elements of a step-by-step switch that were needed for call flow. Let's process a call through the switch.

Trade-offs of the Step-by-Step switch are:

Power: Required a large amount of electricity to move switch components for call processing

Floor Space: Major switch components were made of brass, copper and magnets which were so heavy they required steel reinforcement in the floors where they were installed

High Maintenance: Due to the large number of moving parts in this switch, it required constant dusting and oiling to ensure connections could be made inside the switch

Noise: The contacts are chattering as a physical path is set up through the switch for call processing

Direct Control: A feature designed into the switch which gave the customer control over the common equipment used to process a call.

Crossbar Switch

In the late 1930s and throughout the 1940s, AT&T introduced various versions of the Crossbar switch (most common of which was the #5XB). Following are the unique features of the switch:

Common control allowed the customer and the switch to share control over common equipment used to process the call.

Wire logic computer allowed specified routine functions of call processing to be hardwired into the switch.

Flexible concentration ratios allowed the system designer to select the appropriate concentration ratio for a specific switch based on customer mix in a specific location.

Easier to maintain because the switch had significantly fewer moving parts than the SXS. 36,000 line capacity.

Introduction of an Automated Message Accounting (AMA) (perforated paper tape).

Similarities with the Step-by-Step switch:

Approximately the same footprint (floor space required)

Noisy Electromechanical Manual maintenance

Power Principal

Components of the crossbar switch are sets of contacts called crosspoints (Figure 4.8) mounted one above the other along the vertical bars. These crosspoints are essential to setting up the call path.

To close a set of crosspoints and complete a connection through the switch, a horizontal bar moves and then a vertical bar. Up to ten simultaneous electrical paths can be established when connections are made in a crossbar switch. It is over these electrical paths customers talk to each other.

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