There are two different ways to perform signaling. Our analog telephones provide in-band signaling, while ISDN phones use common channel signaling.
In-band signaling was the first type of signaling deployed in the telephone network. In-band signaling is defined as voice and signaling information sharing the same communications path. Think of this: When you are talking on your analog home phone and your fingers hit keys on the telephone's numeric keypad, what do you hear? You hear the DTMF beeps in the middle of your conversation. That is because analog telephones use in-band signaling. The signaling information (beeps) is carried in the main and only communications channel.
In common channel signaling, voice information and signaling information travel on separate paths. If you were dialing an ISDN phone that had common channel signaling, you could press the keypad keys and talk to someone at the same time without them hearing the keypad tones. The signaling information and voice information travel separate paths and do not interfere with each other. Sometimes this may be referred to as out-of-band signaling. Compare the channel capacity your voice requires to the channel capacity required to send telephone numbers (signaling information). Voice transmission requires much more capacity than call setup information (the telephone number dialed). Further, signaling information (the call setup information) is data transmission that comes at the beginning of the call and the end of the call with much wasted time between the signaling data packets. So why waste communications capacity to set up a phone call? A separate signaling network can easily accommodate signaling. This network is a data communications network that carries signaling information efficiently from many callers simultaneously because data chunks (packets or frames) can be interleaved on a single channel. This describes the technique used by Signaling System 7 (SS-7) that is discussed later in this chapter.
With ISDN phones, common channel (D channel) signaling is used. The signaling travels as chunks of digital data on the separate D channel. Inband signaling is used today only for signaling between our analog home phones and the telephone company. The telephone backbone (supporting) network employs common channel signaling, or out-of-band signaling, over separate D channels to manage all calls passing through it.
There are 2-wire and 4-wire channels in the telephone network. Service to our analog home phone is 2-wire service. Only two wires are required to communicate. As we saw earlier, one wire carries the analog signal from the telephone company to the phone while the other wire carries our analog voice signal to the telephone company. This is a full duplex connection—we can speak and listen at the same time. Common analog telephones use just two wires (although four are typically present in telephone cable) to transfer voice between the phone and the telephone company central office. This represents a single path carrying information in both directions.
Analog connections between telephone switching equipment such as trunk lines from telephone company end offices to telephone company toll offices use four wires to provide two separate message pathways. Each pathway has a full pass window. Thus, a 4-wire circuit has two separate pass windows and a greater bandwidth than a 2-wire circuit. Class 5 end offices support 2-wire switching, while toll offices switch 4-wire circuits.
4-wire circuits are much less important today with the backbone support network being largely high-speed digital transmission channels.
In telecommunications networks, Quality of Service (QoS) is guaranteeing a measurable minimum level of transmission delay, data loss, and transmission errors. The goal is to maintain the equivalent voice communications quality to that of the analog telephone network as telephony is moved from an analog circuit-switched network to a digital packet (or cell) delivery network.
A circuit-switched network defines a point-to-point guaranteed bandwidth that no one else can use. Imagine a room of people. To set up a connection from one side of the room to the other, each person in the room must hold another person's hand. These people are forming a point-to-point circuit, or channel. Once they hold someone else's hand, they cannot hold another hand. No one else in the room can add their hand to the point-to-point circuit. This guarantees a fixed, constant bandwidth and a defined path.
In contrast, a packet-switched network is a network that does not have a defined circuit path. In this case, imagine the group of people tossing small white, blue, and pink coffee sweetener packets to each other. They start at one side of the room and are tossed from person to person until all the white, blue, and pink coffee sweetener packets make it to the other side of the room.
The problem with tossing packets is now that they can travel the same pathways (reducing bandwidth), they can get out of sequence, there can be delays, and there is the risk of packet losses (transmission errors). QoS is a measure of how well a digital network (tossing sweetener packets) matches up with a circuit-switched network (holding hands).
The goal is to have the new digital packet networks perform as well as the older circuit-switched analog/digital networks.
Central to the QoS concept is the fact that transmission speeds, error rates, and other characteristics can be measured and guaranteed in advance. Continuous transmission of high-bandwidth video and multimedia information requires a high QoS. Transmitting this kind of content dependably is difficult in public networks using data communications protocols and products.
The Internet is a packet network that does not guarantee on-time or complete delivery of all packets passing through it. To carry voice communications across the Internet, there must be QoS guarantees that provide performance equivalent to that of the circuit-switched voice network. The Internet's Resource Reservation Protocol (RSVP) provides this QoS. With RSVP protocol data passing through the Internet, communications components can be expedited based on QoS policy and reservation criteria arranged in advance. This permits the Internet to effectively carry voice communications traffic. Similarly, Asynchronous Transfer Mode (ATM) also lets a company or user pre-select a level of quality for ATM service. In ATM, technical transmission details like the average delay at a gateway, the variation in delay in a group of cells (cells are 53-character (byte) transmission units), cell losses, and the transmission error rate can be measured and guaranteed to achieve a specific QoS level.
Types of Analog Signals
Pick up any phone and call an AOL access number. Wait until the modem answers, then hang up. What did you hear during this phone call?
1. When you dialed the AOL access port telephone number, did you hear the in-band DTMF tones while dialing?
2. Did you hear the ringing signal from the AOL number called? What created this ringing signal? Was it the telephone company switch, the Private Branch Exchange (PBX) switch in your office, or the digital telephone you were calling from? Try a phone at home to see if the ringing signal sounds different.
3. What did the modem sound like when it answered the phone? Did it at some time change frequencies? Did the answering modem change signal amplitude (get softer or louder)? What was the final sound it made? Was it that awful background static-type noise (sometimes referred to as white noise)? That was a phase encoded analog signal. Yup, humans cannot hear phase changes, but modems can.
The goal here is to illustrate the different types of signaling that are used in making a dial-up connection to the Internet.
The telephony network was originally designed to carry only human voice. Voice was carried over analog circuit-switched connections that amplified the signal so that it was audible at the other end of the circuit-switched connection.
Today our telephone networks are evolving rapidly into so much more. The original analog voice is digitally encoded into 64-Kbps data streams (64 Kbps is a very good number to remember). This has turned the backbone telephony network into a high-speed digital transport system that has the ability to transport data, digital video, and much more. The components in the voice network are still largely influenced by the original intent and design of the telephone network to carry voice.
The basic voice communications network connects from a telephone inside a customer premise to a telephone network Central Office (CO) facility through a local loop of copper wire (see Figure 3-12). This local loop is a single line for the voice telephone network subscriber. A telephone company CO connects to and serves up to 100,000 subscribers. In it resides a branch exchange switch. This is a high-end switch that connects the subscriber lines to one another or to the trunk lines that run to other voice network COs.
A trunk line is simply a line that connects two telephone switches together. Trunk lines are implemented using a variety of communications technologies, ranging from 4-wire analog channels to high-speed digital fiber optic SONET channels. Anyone having a phone system like Jason and I have trunk lines connecting them to the telephone company CO.
Some voice network COs connect directly to other subscribers, while others act as relay points to other voice communications networks. Trunk circuits run between COs. These are most often high-speed digital links composed of fiber. Sometimes these were 4-wire analog circuits. They were
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