Signaling is the language of telecommunications that machines and computers use to communicate with each other. In particular, the signals that a user enters need to be converted to a format that is appropriate for machines and then transmitted to a remote entity. The signals (e.g., the identity of a called party) are not part of the communication as such, that is, they are not a payload or a revenue-earning entity.
How is Signaling Performed?
When calls were set up manually, signaling consisted mostly of direct current impulses, which allowed a central office to determine the dialed digits. The smallest unit of a signal is called a bit and can, for example, be represented by an electric voltage, which a receiver can measure during a specified period of time. If the receiver measures the voltage as “low” over the specified time period, it interprets the value as a 0. If the voltage is “high,” the receiver interprets the value as a 1. It does not matter which level represents which value, so long as both the sender and the receiver agree on which is which. A sequence of bits allows the coding and sending of complex messages, which, in turn, allows a process to be controlled or information to be conveyed. The result is a bitstream.
Pulse code modulation (PCM) is the worldwide process for transmission of digital signals. PCM is used to transmit both signaling data and payload. PCM is categorized into hierarchies, depending on the transmission rate. The PCM link of 2 megabits per second (Mbps) (one that is referred to frequently in this book) is only one variant of many. By utilizing a time-division multiplexing technique, such a 2-Mbps PCM link can, among others, be partitioned into 32 independent channels, each capable of carrying 64 kilobits per second (Kbps). Another aspect of the change that the digital technology has enabled reveals its advantage only after a second look. Almost all signaling standards, like Signaling System Number 7 (SS7) and Link Access Protocol for the D-channel (LAPD) separate the traffic channel from the signaling or control channel. This is referred to as outbound signaling, in contrast to inband signaling. In the case of in-band signaling, all the control information is carried within the traffic channel. Although outbound signaling requires the reservation of a traffic channel, it makes a more efficient use of resources overall. The reason for that lies in the reduced occupation time of the traffic channel, which is not needed during call setup. Both call setup and call release can be carried out for many connections via one control channel since signaling data use the resources more economically. One 64-Kbps time slot out of a 2-Mbps PCM link typically is used for signaling data; a call setup consumes about 1 to 2 Kbps.
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