Communication theory 5:

Chapter 10: Integrated Services Digital Network

(ISDN)

Overview:

This chapter will describe the concept of the Integrated Services Digital Network (ISDN) and will

focus specifically on the following topics:

Ø   Its original goals

Ø   How it can be used

Ø   Some of the alternatives

The world's telephone companies conceived ISDN in the early 1980s as the next generation

network. The existing voice networks didn't deal well with data for the following reasons:

Ø  One had to use modems to transmit data.

Ø  The data rates were around 9600 bps.

Ø  Connections (worldwide) were unreliable.

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 telephone 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 classic 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).

  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 kilobit/s. 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.

Interfaces

The customer interface I.45x specifies the Basic Rate Interface (BRI). It was intended to become the standard subscriber interface.

The BRI specifies two bearer channels and a data channel.

The two bearer channels would bear the customer's information. The initial concept had this as being everything from analog telephone

calls (digitized) to teleconferencing data and these would be switched channels. The only 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. (Note in the current network, your analog telephone circuit is

digitized to 64 KBps at the local Telco office before being switched across the network. It is then

turned back into analog at the far end before being delivered to the called party.)

Now with BRI, we have not one, but two such telephone circuits. Since it is digital, we have

switched digital 64 KBps to (theoretically) anywhere in the world.

The problem with the existing Telco network is that the signaling information shares the telephone

channel with the user information. With plain voice circuits, a customer doesn't notice or care. With

the advent of modems, this represents a loss of channel bandwidth, and with digital transmission, it

means a loss of usable bits per second. The customer is therefore stuck with 56 KBps, instead of

the actual channel rate of 64 KBps.

The BRI is created by the Network Terminal type 1 (NT1).

The NT1 creates a four−wire bus called the T interface onto which each ISDN device is connected.

The S and T interfaces are logically and physically identical. They have separate identities to allow us to describe the functionality of the NT2 element. The NT2 could create multiple S interfaces and perform the switching to adjudicate access to the B channels on the T interface. Since the two interfaces are the same, they are frequently referred to as the S/T interface. 

Architecture of the ISDN interface:

The packet−switching access is not universally implemented, but it permits access to the worldwide

X.25 packet−switching network. The telemetry system access is also generally unimplemented.

Several telemetry experiments have been done in Europe and in the United States, but few resulted

in a cost−effective solution. We may have to wait for new lower−cost technology. The concepts,

discussed in the following paragraphs, are sound ones.

The higher−layer services could be almost anything. A typical example might be video conferencing.

Here the higher−layer services would be the functions of the Codec.

ISDN B channels are used for Internet access.

The B and D channels are then just timeslots on the bus that can be grabbed by any of the connected devices.

v  It has 48 KBps inbound bus.

v  the highest address will win since the one bits would overlay any zero bits of a device with a lower address.

v  This can get a little sticky when bonding channels. Essentially, bonding (that is, one device using both 64 KBps channels to get 128 KBps) must be done at call initiation.

Figure of Architecture of the ISDN interface:

 

Interface Components:

The previous interface doesn't allow for switching on premises, which would be necessary if there

were multiple telephones.

NT1

It creates the T interface for premise devices (from the U interface). In the original CCITT concept, the NT1 was provided by the Telco as part of the ISDN service.

NT2

This device would do the switching, permitting more than the standard eight devices to share the T

bus by creating perhaps multiple S buses. Therefore, an ISDN terminal equipment (TE) device can't

really tell if it is connected to an NT1 or NT2.

TE1

The terminal equipment type 1 (TE1) is a standard (there is that word again) ISDN terminal that is

capable of dealing with the B and D channels.

TE2

The terminal equipment type 2 (TE2) is a standard device having an RS−232 or V.35 interface. (In

ITU parlance, this is called a V−series interface.) It may be intelligent, but it doesn't have an ISDN

Interface capable of handling the D and B channels.

TA

The terminal adapter (TA) is the semi−intelligent device that lets a TE2 connect to the S/T ISDN

interface. The primary function of the TA is to run the ISDN interface for our TE2. The functionality

varies widely due to the manufacturers. Some are simple and support only one TE2; others support

two TE2s and an analog telephone.

Physical Delivery

One of the more interesting parts of the ISDN service is the solution to the local loop problem.

Remember our local loop is ancient and designed for voice. We expect it to support 192 KBps bidirectionally! This would not be a problem if we only had to go a few hundred feet. Unfortunately, there aren't that many customers within a few hundred feet of the central office.

what is the average length of the cable to them and what are the quality and

characteristics of that cable?

In cities, 95 percent of the customers are within 15,000 cable feet of the central office. When we go to suburbs and the rural areas in particular, then all bets are off.

Figure 10−4 shows a typical local loop layout with emphasis on the fact that local loops, particularly

those in older parts of a city, are comprised of different gauges of cable and may have several

bridge taps. Although slightly exaggerated, one could imagine that all of the cable taps are on one

pair. That is, at one time or another during the 60−year life of this particular cable plant, that pair

was used to provide telephone service to each of those different locations. Note also the gauge

change from 24 to 19 gauge. This was obviously not a problem for the analog telephone network

because it worked fine for 60 years on that cable.

Typical local loop layout:

High−speed digital transmission presents a whole new set of problems. We will outline just the main

issues here. The fundamental problem centers on the fact that a wire is an antenna. This is OK if we are in the radio or TV transmission business, but it is an unfortunate side effect of the telephone transmission business. The problem is worse at higher frequencies that characterize digital signals.

Ø  The low frequency portion of the signal goes a relatively long way.

Ø  The longer the wire, the worse the problem.

Ø  the 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.

      

The U Interface

The U interface is unique to North America and the open telephone network interconnection. Figure

10−5 shows the U interface connecting to the NT1 and the NT1 in turn creating the internal S/T bus

to which up to eight ISDN devices can be connected. This U interface can be either a two−wire or a

four−wire connection. In the following discussion, we concentrate on the two−wire connection

because it is the more technologically challenging. The four−wire interface requires much less

technology and can be delivered over a greater distance.

AT&T (Bell Labs) came to the rescue with a technique known as 2B1Q (At this time, this was all part

of AT&T. Since then, Bell Labs was spun off from AT&T as part of Lucent Technologies).

 2B1Q technique for ISDN

Ø   It is easy to generate (that is, it is a simple wave form).

Ø   It minimizes crosstalk.

Ø   It will work on most local loops.

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

all possible bit combinations (there are only 16 combinations). The receiver at the transmitting end

then monitors the resultant complex signal.

The 2 binary 1 quaternary (2B1Q) works so well that this basic technique is now the primary

technology used by the local Telcos to deliver T−carrier service, instead of the old alternate mark

version (AMI) technique. It will go twice as far before repeaters are required. Of course, the name

has been changed to confuse us and is now called High bit−rate Digital Subscriber Line (HDSL).

The Physical Interface

Another clever design feature of both the S/T and U interfaces is that they all use the same RJ−45

type connector. Figure 10−7.

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.

Although the augment lasted several years, three powering mechanisms are provided

across the interface:

Ø   The customer provides power to the NT1.

Ø   The carrier provides power to the customer from the NT1.

Ø   The carrier provides a small amount of keep−alive power on the actual bus leads.

Applications of the ISDN Interface

The following section describes the areas in which ISDN functions, including multiple channels,

telephone services, digital fax, analog fax, computer/video conferencing, signaling, telemetry, and

packet switching.

Telephone

The obvious starting point is the telephone, which is now a digital telephone. Instead of the

telephone conversation being analog from the handset to the central office where it becomes

digitized, the conversation can be digitized directly at the source and passed digitally all the way

through the network to the other end.

Digital Fax

Fax machines now have to be digital. Therefore, the Group IV fax standard specifies 64 KBps fax

operation.

Analog Fax

Analog fax machines use a modem, so it has to plug into the telephone (or similar device) that

would take the analog modem tones and digitize them at 64 KBps. This would provide compatibility

with all existing Group III fax machines.

Computer/Video Conferencing

Our computer or video conferencing equipment can use one of the 64 KBps or bond both bearer

channels together for a 128 KBps digital channel across the network.

Signaling

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. At 16 KBps, the data channel has more bandwidth

than is needed for signaling alone. Therefore, when it is not being used for its primary and

high−priority signaling function, it could be used for other things.

Telemetry

This feature has never been well defined. The concept is that many household devices can be

connected to the data channel.

Packet Switching

The 16 KBps data channel has bandwidth to spare. Therefore, the local carrier can provide a data

service on this excess bandwidth.

Primary−Rate ISDN

The BRI interface offers 2B + D. The Primary Rate Interface (PRI) provides 23B + D and, in this

case, all are 64 KBps channels. This should sound familiar. What technology provides 24 64−KBps

channels? Of course, the answer is T1. The physical interface for PRI then is simply a T1.

Technically, it is a T1 with extended superframe framing (ESF).

H0 Channels

Equally interesting is that aggregated channels have been defined. These are known as H

channels. An H0 channel is 384 KBps or six B channels treated and switched as a single channel.

This means our PRI NT2 could call for an H0 to Atlanta, another to Phoenix, and one to Chicago

and still have five B channels to individually switch.

H11 Channels

The H11 channel is a 1.536 Mbps (T1) switched channel too. If our PRI NT2 had, let's say, 5 T1s, it

could configure any one of them as an H11 channel and another as 2 H0 channels with 12 B

channels. This essentially gives us dial−up T1, 384 KBps, and 64 KBps clear channel service.

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.

Signaling on the D Channel

The signaling packets on the D channel are the same for BRI and PRI. In the early days of defining

the interfaces, an argument arose about whether to grant direct access to the signaling system by

the customers. As we indicated, 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 particular call.

Because there could be many (479) calls in progress at once, you might need more than a

fixed−length value. (In order to prevent confusion about which call is doing what, you don't want to

reuse those numbers too quickly.) All signaling packets associated with a given call will have the

same call reference value.

BRI Application

One of the major uses of ISDN is in video conferencing. This is normally done by installing three

BRI lines. The video conferencing equipment has a built−in inverse multiplexer (mux). Figure 10−9

shows a typical configuration. The three BRI interfaces are connected to the inverse mux and the

control panel enables the operator to specify how much bandwidth is to be used for the

videoconference. Although one could use 64 KBps for a videoconference, it is quite impractical. A

minimal usable videoconference is 128 KBps, and as long as someone else is paying for the

bandwidth, 384 KBps is acceptable.

The inverse mux takes the commands from the control panel and provides the appropriate packets

on the D channels to set up the amount of bandwidth required. The inverse mux is needed because

it is essentially making up to six independent 64 KBps calls.

Broadband ISDN

There has been much confusion over exactly what is broadband ISDN. This confusion stems from

at least two sources. First, the definition of the word broadband, and second from the fact that even

after we are over that hurdle, no one knows what it means.

Source:

http://en.wikipedia.org/wiki/Integrated_Services_Digital_Network#Basic_Rate_Interface