Basic Telephony
Back in early days, Telephones were originally connected directly together in pairs. Each user had separate telephones wired to the various places he might wish to reach. This became inconvenient when people wanted to talk to many other telephones, so the telephone exchange was invented.
Each telephone could then be connected to other local ones, thus inventing the local loop and the telephone call. Soon, nearby exchanges were connected by trunk lines, and eventually distant ones were as well.
When the user of a telephone wants to make a telephone call, equipment at the exchange examines the dialed telephone number and connects that telephone line to another in the same wire center, or to a trunk to a distant exchange.
Most of the exchanges in the world are connected to each other, forming the public switched telephone network (PSTN).
Basic telephony was developed only for narrow band services like voice calls. But later when the Fax machine was invented, the telephone network used to transfer faxes, which still requires a lower data rate.
Typical Human speech contains frequencies ranging from 100 Hz to 3400Hz. This range is identified as a requirement for effective communication between two parties through a telephone line.
Human voice is Analog-Mechanical and it is converted to Analog-Electrical using a transducer. Then amplified and transmitted to the exchange over a pair of copper wires. At the Exchange, the signal is filtered with a 4 KHz Low-pass filter then converted to a digital signal for to be transmitted over the digital network.
In the process of digitizing, the signal is
1. Sampled with 8000 samples per second
2. Digitized with 8 bits per each sample
Hence the output is (8000/sec) * 8bits = 64000bits/sec (64Kbps) (According to Nyquist theorem, it is required double the sample rate of the maximum signal frequency, to successfully reproduce the signal)
Therefore the digital network (Apart from the access network) should be able to carry 64Kbps data rate per subscriber.
Theoretically it is possible to allocate more bandwidth per subscriber but practically it is a waste of resources.
Thus to obtain maximum utilization of the network resources, the data rate per subscriber is limited to 64Kbps. This data rate is enough to communicate via Voice, Fax and use of a Dial-up Modem, which is used to connect to the internet or another computer over the PSTN.
Since the Dial-up modem uses the traditional PSTN circuits, it costs a lot of money for a considerable period of on line time.
Pre-ADSL technologies, for Data communication needs in today’s world
Path towards ADSL, Earlier technologies for Data communication
As the technology develops, there arises the requirement for high-speed data. A solution came by the name - ISDN (Integrated Services Digital Network)
After the middle 20th century Fax and data became important secondary uses of the network created to carry voices; and late in the century parts of the network were upgraded with ISDN to improve handling of such data traffic.
Digital telephony is the use of digital technology in the provision of telephone services and systems.
Almost all telephone calls are provided this way, but sometimes the term is restricted to cases in which the last mile (In between Subscriber and the Exchange) is digital, or where the conversion between digital and analog signals takes place inside the telephone. Telephony was digitized to cut the cost and improve the quality of voice services, but digital telephony was then found useful for new network services (such as ISDN) to transfer data speedily over telephone lines.
Integrated Services Digital Network (ISDN) is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network. It was first defined in 1988 in the CCITT red book. Prior to ISDN, the phone system was viewed as a way to transport voice, with some special services available for data.
The key feature of ISDN is that it integrates speech and data on the same lines, adding features that were not available in the basic telephone system.
There are several kinds of access interfaces to ISDN defined as Basic Rate Interface (BRI), Primary Rate Interface (PRI) and Broadband ISDN (B-ISDN).
ISDN is a circuit-switched telephone network system, which also provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in potentially better voice quality than an analog phone can provide.
It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64 Kbps. A major market application for ISDN in some countries is Internet access, where ISDN typically provides a maximum of 128 kbit/s in both upstream and downstream directions. Channel bonding can achieve a greater data rate; typically the ISDN B-channels of 3 or 4 BRIs (6 to 8 64 kbit/s channels) are bonded.
Basic Rate Interface (BRI)
A 128Kbps service delivered over a pair of standard telephone copper wires. The 144 Kbps rate is broken down into two 64 Kbps bearer channels ('B' channels) and one 16 Kbps signaling channel ('D' channel or delta channel).
BRI is sometimes referred to as 2B+D
BRI-ISDN is very popular in Europe but is much less common in North America. It is also common in Japan - where it is known as INS64.
The interface specifies the following network interfaces:
The U interface is a two-wire interface between the exchange and a network terminating unit, which is usually the demarcation point in non-North American networks.
The T interface is a serial interface between a computing device and a terminal adapter, which is the digital equivalent of a modem.
The S interface is a four-wire bus that ISDN consumer devices plug into; the S & T reference points are commonly implemented as a single interface labeled 'S/T' on an NT1
The R interface defines the point between a non-ISDN device and a terminal adapter (TA) which provides translation to and from such a device.
Primary Rate Interface (PRI)
The other ISDN service available is the Primary Rate Interface (PRI), which is carried over an E1 (2048 Kbps) in most parts of the world. An E1 is 30 'B' channels of 64 Kbps, one 'D' channel of 64 kbit/s and a timing and alarm channel of 64 Kbps. In North America PRI service is delivered on one or more T1s (sometimes referred to as 23B+D) of 1544 Kbps (24 channels). A T1 has 23 'B' channels and 1 'D' channel for signalling (Japan uses a circuit called a J1, which is similar to a T1).
In North America, Non-Facility Associated Signalling (NFAS{{ allows two or more PRIs to be controlled by a single D channel, and is sometimes called "23B+D + n*24B".
D-channel backup allows for a second D channel in case the primary fails. One popular use of NFAS is on a T3.
PRI-ISDN is popular throughout the world, especially for connection of PSTN circuits to PBXs.
Data channel
The bearer channel (B) is a standard 64 Kbps voice channel of 8 bits sampled at 8 kHz with G.711 encoding. B-Channels can also be used to carry data, since they are nothing more than digital channels.
Each one of these channels is known as a DS0.
Most B channels can carry a 64 Kbps signal, but some were limited to 56 Kbps because they traveled over RBS lines. This was commonplace in the 20th century, but has since become less so.
Signaling channel
The signaling channel (D) uses Q.931 for signaling with the other side of the link.
However, still the high speed data rates were not available for cheaper cost. So the developers looked in to another technology called Asymmetric Digital Subscriber Line (ADSL)
