Voice communication has evolved considerably since the first successful recorded voice transmission over wires occurred on March 10th, 1876. Today we have different means at our disposal to communicate with one another, from conventional landline telephones, to terrestrial mobile phones, to voice over IP internet phones. Each has its advantages, and we all use one or all of them to communicate everyday.
Having options is good for the end-user; however, it brings interesting challenges for service providers. Service providers have been trying to maximize their revenue since the first commercial telephone exchange in the world opened in 1878 in New Haven Connecticut, with only 21 subscribers. As each service provider tries to maximize its revenue per user (RPU), they are faced with the challenge of combining the different networks that carry each form of communication into one central network. Over time, the line card has evolved to address this challenge and enable the sought-after convergence.
In order to understand the evolution of the line card, we must take a look at how the first person-to-person voice calls were made. Almon B. Strwoger patented the first switch, called the step-by-step office, in 1891. To make a call, one subscriber would have to lift their headset, ask the operator to connect them to another subscriber, and wait. This manual process sometimes took as long as 15 minutes. The first commercial step-by-step central office was opened in 1893. What we all recall from the first days of telecom is the phone operator sitting in front of rows and rows of plug boards, connecting the conductors from one calling party to another.
These phone calls were placed with manual service exchanges. Customers had to hand-crank the phone to generate sufficient ringing current and to attract the operator's attention. Once the operator saw that the subscriber was trying to place a call, the operator would respond by dropping a metal tab above the subscriber's line jack and sounding a buzzer. Dry cell batteries placed in the customers' phone would provide the DC voltage required to place the call. This type of system was in place as late as the early 1940s.
1. Early telephone systems relied on manual service exchanges.
Over the years, significant progress has been made in the way calls are placed, the way central offices look, and the amount of traffic that can be handled. Much of this is due to the line card, whose fundamental components include:
- Tip Ring interface circuit
- CODEC (analog to digital and digital to analog)
- Call control and voice supervision state machine
- Power supply and power supply control
- Ringing generator
- DTMF generation and detection circuit
- PCM (Pulse Code Modulation) ability: Digital representation of an analog signal where the magnitude of the signal is sampled regularly at uniform intervals then quantized a series of symbols in a digital (usually binary) code.
Throughout the years, technological improvements in the electronics industry and semiconductor processes have led to significant advances in the line card, turning this circuit board into an efficient machine capable of performing tasks that only a few decades ago could not even be imagined.
Until a few years ago, and in some cases even today, most line cards used discreet components for the tip ring interface circuit. The tip ring interface circuit is a group of circuits, also know by the acronym SLIC (Subscriber Line Integrated Circuit), that provide a number important functions we almost take for granted, including:
- Power Management/Shutdown
- Zero Crossing Ring Control: Zero Voltage On/Zero Current Off
- Tip/Ring Disconnect
- Pulse Metering Capability
- 4 Wire Loopback
- Programmable Current Feed
- Programmable Resistive Feed
- Programmable Loop Detect Threshold
- Programmable On-Hook and Off-Hook Overheads
- Programmable Overhead for Pulse Metering
- Programmable Polarity Reversal Time
- Selectable Transmit Gain
- 2 Wire Impedance Set by Single Network
- Loop and Ground Key Detectors
- On-Hook Transmission
While many of these features may sound simple or even mundane, they are critical to the telecommunications experience. For example, as consumers we know that when we lift the phone handset we should hear a dial tone. We know when we have an incoming call because the phone rings. Much of this functionality is performed by the SLIC. This high voltage analog chip is usually designed on a high voltage process technology capable of handling over 100 volts DC. There are some semiconductor companies, such as Zarlink, that own process technologies greater than 175V. These process technologies allow for further integration of line card functionality, such as building the ringing function directly inside the SLIC. .