Chapter 10 Integrated Services Digital Network (ISDN)

Chapter 10

Integrated Services Digital Network

(ISDN)

ISDN was developed by the Consultative Committee on International Telegraph and Telephone (CCITT).  CCITT is a consultative committee to the International Telecommunication Union (ITU) and have recently changed their name to ITU−T. The ITU−T (CCITT) is a UN treaty organization and each country is entitled to send representatives to any committee meeting. The CCITT is comprised of study groups (SG), each SG has its own area of expertise. CCITT and ITU−T publish recommendations of standards in which the philosophy of the ISDN committee is to specify the customer interface first and then figure out how to support it in the network.

The customer interface I.45x specifies the Basic Rate Interface (BRI). It was intended to become the standard subscriber interface.BRI also specifies a multifunctional data channel at 16 KBps that could handle signaling  and network data (X.25) when not needed for signaling. It specifies two bearer channels and a data channel. The difference between a conventional telephone circuit and a bearer channel is that the bearer channel would be 64 KBps all the way to the customer. BRI indicates the bandwidth allocation on the ISDN interface and created by the Network Terminal type 1 (NT1).

One of the interesting parts of the ISDN service is the solution to the local loop problem because local loop is ancient and designed for voice. Another set of problem of ISDN service is high−speed digital transmission problem because center considers the fact that a wire is an antenna. At higher frequencies that characterize digital signals. So, if we use our local loop to send digital signals containing high frequencies, the low frequency portion of the signal goes a relatively long way. Long wire results signal dribbling out the end of the wire is weak and distorted because all the components didn't arrive in time or with the right strength. Adjacent wires in the cable pick up the components of our digital signal that are radiated. We call this crosstalk in the telephone business and the unwanted signal picked up is noise. We can solve this problem by sending a stronger signal makes more signals available at the destination, but also increases the amount of energy radiated which we called Brute force method.

U Interface is unique to North America and the open telephone network interconnection. It is either a two−wire or a four−wire connection. Four−wire interface requires much less technology and can be delivered over a greater distance while two-wire is  more technologically challenging.AT&T (Bell Labs) made a technique known as 2B1Q.  This technique able to solve problems, in which it is easy to generate, minimizes crosstalk and it work on most local loops.

Another problem is that old local loop has different gauge wire and has bridge taps.  when sending pulses down the line, we are going to get reflections from these gauge changes and bridge taps. These reflections will always be ff the same magnitude and at the same relative time from each of the cable plant anomalies.

When the ISDN NT1 goes off−hook, it transmits a known pattern. That pattern contains

all possible bit combinations The receiver at the transmitting end then monitors the resultant complex signal. Since it knows what it sent and can subtract that signal, it memorizes the resulting reflected energy. Each transmitter can subtract its own transmitted signal and the reflections, which it knows to be there. After subtracting our transmitted, whatever is left over must be the data from the far end. Unfortunately the old local loop doesn't handle all frequencies equally, so some of the signal components arrive out of the precise time (or phase) with the other parts of the signal, as mentioned above. This effectively distorts the signal. The U interface hardware therefore uses the old modem technique of equalizing the line. While each end trains itself on reflections, the opposite end receives the known signal, recognizes the distortion, and tunes its equalizer to make the signal appear correct. If we were to independently measure the line characteristics (amplitude and phase) and the equalizer, we would (as you might expect) find them equal and opposite. The result is that the equalization process takes the distortion out of the line.

            Another clever design feature of both the S/T and U interfaces is that they all use the same RJ-45 type connector. The S/T and U interfaces carefully select the pin assignments so that accidentally plugging an S/T connector into a U interface and vice versa doesn't hurt anything. In the current analog system, the carrier provides power to the telephones so that they work, although commercial power is off. It has mechanism  that is three powering mechanisms are provided across the interface which is, the customer provides power to the NT1,  carrier provides power to the customer from the NT1, and carrier provides a small amount of keep−alive power on the actual bus leads.

            The primary function of the data channel is to provide for signaling, that is, the setting up and tearing down of the switched bearer channels. The BRI interface offers 2B + D. The Primary Rate Interface (PRI) provides 23B + D ,in which all are 64 KBps channels. T1 the physical interface for PRI. H0 Channels known as H channels. An H0 channel is 384 KBps or six B channels treated and switched as a single channel.H11 Channels is a 1.536 Mbps (T1) switched channel. H12 Channels the Europeans use an E1 system that has 32 channels, each 64 KBps. One channel is used for timing and alarms and one is used for common channel signaling. The H12 channels are only in the E1 system and are 1,920 KBps.

The signaling packets on the D channel are the same for BRI and PRI. Signaling System 7 (SS7) is the mechanism for managing the network and it's logical to simply let the D channel use SS7 packets. Signaling System 7 (SS7) is the mechanism for managing the network and it's logical to simply let the D channel use SS7 packets. Unfortunately, the more paranoid faction won the argument, so the D channel signaling packets require a small amount of conversion to change them from D channel signaling packets to SS7 packets. The call reference value is essentially a random number chosen to identify a pa The message type indicates the format of the packet. The setup message, for example, would have the called and calling telephone numbers in the information elements field. The call−clearing message would have a single−byte cause code in the information element field particular call.