Introduction

This Information Factory course "Introduction to ISDN, ATM and Broadband Transmission" is a continuation of our earlier course "Introduction to Digital Transmission." In the first course we described how the world's telephone networks were moving from their analog transmission methods onto a completely digital transmission network. In this training course we are going to describe how the telephone networks are extending the digital interfaces into the customer's homes and offices under a new service called "Integrated Services Digital Network" or ISDN.

We will begin here in Section 1 by describing how the existing telephone network has been changing from an analog network into an all digital network. It is necessary to look first at the existing network and its changes in order to understand how they form the basis for the future ISDN and eventually the Broadband or B-ISDN network.

Telephone Systems and Networks

The modern ISDN networks are being built on the existing telephone networks worldwide and as a result many of the views of the network and many of its principles are based on the same ideas. As an example, when you picked up a telephone and dialed a call, the call was connected through a series of relays or "stepper switches" as shown in Figure 1.1 (below).

Figure 1.1 Mechanical Telephone Switching

Notice that the process of making a telephone call involves more than just talking. The caller takes the phone off-hook, dials the call, listens to the ringing and finally talks when the far end phone is answered.

In the original mechanical telephone systems as shown in Figure 1.1 (above), all of the work of setting up the call was handled by the same relays that transmitted the voice. This meant when you picked up the phone and dialed a call, the stepper switches were being tied up as each digit was entered.

In the 1930's, telephone system manufacturers started to build switching systems that broke out the functions of setting up a call from the parts that switched the call or transmitted the voice. Figure 1.2 is an example of the organization of a Crossbar Telephone Switch. In this system the call was not put through the network until the full number was entered by the caller.

Figure 1.2 Crossbar Switching System

In the crossbar switch the first improvement was the addition of registers which held the number being dialed. When the phone went off-hook, an available register was connected to that telephone and the caller heard dial tone. As the call was being dialed the digits are held in the Incoming Register. After the last digit is dialed, the Incoming Register passes the number to the Common Control which analyses the number and computes a path through the network to the remote telephone.

The connections for the call were then made in matrices called Crossbar Frames. Because these were matrices, multiple simultaneous calls could be in progress through a single crossbar frame.

The telephone system Common Control was one of the earliest versions of a computer system. The common control could look at a telephone number and the available trunks and compute primary or alternate routes for a telephone call.

The ability to apply the common control and registers to the processing of a telephone call came about when the network was broken down into functions which could be analyzed and improved separately.

The Functions in a Telephone Call

The telephone network can be represented as three functions:

Figure 1.3 Signaling

Signaling - Provides the mechanism for moving numbers across the network to set up connections and provides progress signals like Dial Tone, Ringing or Busy.

Figure 1.4 Switching

Switching - Which provides the ability to connect any two telephones for the duration of the call.

Figure 1.5 Transmission

Transmission - Which moves the voice signal from one telephone to another and keeps it understandable.

This concept of looking at the function of the network in layers is important because it carries over into the ISDN networks where each function is being separated into its own systems.

Also by layering the network, parts of it could be improved without swapping out all of the systems or equipment. In fact the transition of the world's telephone network from carrying voice calls to an all digital network has been going on for many years.

Digital Transmission

The use of digital transmission systems began in the United States in about 1963 when the telephone companies installed a system called T1. The original use for T1 had been to replace the old analog transmission systems that connected the central offices.

As shown in Figure 1.6 the links between central offices had been based on an analog carrier system called "L-Carrier." This system multiplexed several voice channels using Frequency Division Multiplexing techniques.

Figure 1.6 Inter-Office L-Carrier Systems

L-Carrier provided a method for the telephone companies to put multiple signals on one cable but, it had several disadvantages:

• It used analog transmission techniques which were susceptible to noise each time the signal was amplified.
  
  

• It used a special coaxial cable which was different for each version of L-Carrier.
  
  

• It had to be taken off-line for maintenance or testing.
  
  

• It used analog transmission techniques which were susceptible to noise each time the signal was amplified.

The first task of the digital transmission systems in the United States and Europe was to replace the analog transmission systems used between the telephone company switching offices. In this case the L-Carrier Multiplexing Systems were replaced with T1 Channel Banks (or E1 Channel Banks in Europe) as shown in Figure 1.7.

Figure 1.7 T1/E1 Channel Bank in Central Office

The trunk lines from the switching system were brought into the T1 Channel bank and converted into digital signals for transmission. In the T1 system, 24 voice channels were carried on a 1.544 megabit per second digital data stream. In the E1 system, 30 voice channels were carried on one 2.048 megabit per second data stream. Digital transmission eliminated many of the problems that L-Carrier had including:

• It used plain copper wires rather than special coaxial cables.
  
  

• The digital transmission system was less susceptible to transmission noise.
  
  

• The system could perform self-diagnostics and automatically switch to back up units

Digital transmission provided a great improvement in the quality of long distance telephone calls. The effect on the network was very noticeable as digital transmission replaced the old analog interoffice trunks. The use of these digital systems was a cost saving for the telephone company due to the automatic maintenance and high reliability of digital transmission. In fact the maintenance and cost savings provided a great incentive for the telephone companies to switch over to digital systems.

The main problem at this point was that the telephone switches were still based on analog switching techniques. This meant that the telephone signal had to be converted between analog and digital as it passed through each office switching system as shown in Figure 1.8.

Figure 1.8 Digital Conversion Across the Network

This conversion back and forth introduced an error in the digital conversion called "quantizing" noise. This quantizing noise limited the number of times a signal could be digitized and converted back to analog.

Digital Switching

The next step for the telephone companies was to replace their analog switching systems with all digital switching systems. This meant the voice could be carried through the network in digital form eliminating the need for conversion at each switching and transmission point.

In the mechanical switching systems, a relay or stepper switch was used to make the physical connections through the network. In effect there was a wire path from one telephone to another (refer back to Figure 1.1).

Digital Switching systems are based on the use of electronic matrix switches. The switches are usually based on the same digital signals as the T1 and E1 transmission systems which means they switch the signals in eight bit groups of data equivalent to one voice sample.

Figure 1.9 Matrix Switch

As shown in Figure 1.9, the digital switching matrix has several input and output lines and a counter. The counter provides the timing for the digital multiplexing time slots. To provide a connection, the input and output lines are assigned a common number or time slot as shown in Figure 1.10. In our example we will use the number five (5).

Figure 1.10 Assigning a Time Slot

The attached counter rotates through its count until it reaches five (5), and at that time the input and output lines are connected as shown in Figure 1.11. While the two lines are connected, eight binary bits representing one voice signal sample are passed across the connection.

Figure 1.11 Data Passing Through Connection

As the counter advances, it will come to the number five (5) and at that time a connection between the input and output line will be made. While the connection is established, eight binary bits or one voice sample will pass through the matrix. When the counter advances to six (6), the connection will be dropped and another matrix connection established.

Most of the telephone switching built today use the same technique of switching the signal in eight bit binary samples (one octet). This allows them to switch voice signals in the standard digital form found on the T1 and E1 carrier systems. In these carrier systems, the voice is usually sampled 8,000 times per second and with one eight bit sample each time, this is a total of 64,000 bits per second. As a result the matrix must provide a time slot for eight bits every 1/8000 th of a second.

To build large switching systems, multiple matrices are placed in series and parallel as shown in Figure 1.12. Notice that the matrix switches are ganged together so that the first group switches the first digit, the second group the second digit and on. This grouping is similar to the way the stepping switches were arranged in Figure 1.1.

Figure 1.12 Grouping Matrix Switches into a Switching System

When the first digital switching systems were installed, they were mainly used on the long distance lines. The effect on the network was very noticeable to the telephone users because calls had a much higher signal quality and the delays due to call set up and switching disappeared.

Signaling

While the telephone companies were converting to digital switching and transmission, they also began to convert their method of providing network signaling. Signaling is the function that provides control and information over the progress and connection of calls. The most noticeable things about signaling are:

• Tones provided for call progress such as:

  • Dial Tone

  • Busy (called number engaged)

  • Ringing

  • All Circuits Busy (all circuits busy retry later)

   

• The transfer of the dialed digits across the network

In the old mechanical switching network, the signaling was carried on the same lines as the voice signal. This meant when you dialed a telephone number, a trunk was seized and used to attempt the call. The trunk line was in use to carry the dialed number and to bring the ringing or busy signal back to the caller (as shown in Figure 1.13).

Figure 1.13 In-Band Signaling

The telephone company doesn't make any revenue from the call unless it is answered (the remote phone goes off-hook). If the call was never answered or encountered a busy signal, then no money was made from the attempt. As can be seen the old "in-band" signaling was terribly inefficient.

In the 1970's telephone companies began deploying a new signaling system called "Common Channel Inter-Office Signaling" or "Common Channel Signaling" or CCIS for short. In this new signaling system the telephone signaling is carried on separate channels and is referred to as "out-of-band" signaling.

As shown in Figure 1.14, the new central offices are connected by transmission systems which carry the inter-office trunks. These trunks are only used to carry the telephone calls once a connection has been established. A separate set of communications links is provided to carry the signaling between the central offices.

Figure 1.14 Common Channel or Out-Of-Band Signaling

Figure 1.15 through Figure 1.17, shows the steps involved in a telephone call using CCIS:

1. When a phone goes off-hook, it receives dial tone from the local switching system.
   
   

2. After the caller dials in the number, the common control at the local Central Office generates a message for the remote or called number. The message is passed across the network via the signaling channels.

   

   Figure 1.15 Call Set Up Starting

   
   
   

3. At the remote end the switch verifies that the number is available and starts ringing the phone
   
   

4. The remote switch also returns a message back to the originating switch.
   
   

5. The message tells the originating switch to provide the ringing signal for the caller. If the called number had been busy, the remote switch would have returned a message that let the local switch generate a busy (or engaged) signal.

   

   Figure 1.16 Phone Starts Ringing

   
   
   

6. When the remote telephone is answered, a message is sent to notify the network.
   
   

7. The message tells the all of the switches on the network to set up a trunk to handle the telephone call.
   
   

8. The conversation takes place across the trunk line.

   

   Figure 1.17 Call Completed and Trunk Set Up

The Common Channel Signaling is based on the use of a communications network that uses a message switching protocol (similar to the protocols used on computer networks). Because this is a message network, the telephone switches can not only send call progress signals, they can also use the signaling network for sending maintenance messages, routing table updates and performing remote diagnostics on the health of the network or the telephone switches.

The use of Common Channel Signaling on telephone networks has given telephone companies a large improvement in:

• Revenues - Since trunks are not actually used unless the call is answered, the number of trunk circuits to support the network is reduced and the average revenue per trunk increases.
  
  

• Set Up Time - The average time from the end of dialing to the called phone starts ringing is significantly reduced. With the old signaling system, calls could take up to ten or fifteen seconds before ringing would start. Now calls begin ringing in one to two seconds.

Since the signaling on the network has been moved off of the telephone trunks and switches, it is easier to make changes to signaling.

Summing Up the Improvements

At this point we need to summarize the improvements that have been made to the telephone network.

(refer to Figure 1.18)

First, the telephone companies converted their inter-office transmission systems from analog to digital carrier systems.

Second, the telephone switching systems used in long distance and end offices were converted from mechanical and analog switching to electronic digital switching.

Third, a separate signaling network was established to improve the efficiency of connecting calls.

Figure 1.18 Improvements in Telephone Network

The telephone network now separates the functions of switching, transmission and signaling. This has made the network modular so that any of these areas can be upgraded without having to change equipment or systems in the other areas.

The only missing piece in this puzzle, is the requirement to provide an all digital network into the customer's home or office. This means that when a telephone call is made, the telephone company still has to deal with analog transmission and in-band signaling conversion on the wires or local loop to the customer.

ISDN The Next Generation

The next step would be to convert the links into your home or office into digital transmission. The telephone companies have been at work on this conversion to digital under a set of standards called "Integrated Services Digital Network" or "ISDN."

As the names implies, this is not just a telephone network but, a network of "Integrated Services" that will provide access to a full range of digital communication for voice, video, multimedia and data communications. In fact the telephone company can be thought of as a "digital company."

What we will show in the balance of this course is how ISDN will provide a connection that allows the customer to access the network digitally. Also the ISDN network will extend a part of the telephone company signaling system into the customer's premises to allow them a new level of control over incoming and outgoing calls.

SUMMARY SECTION 1

Let's summarize what we learned in Section 1 about where the telephone network has come from and how it is moving toward an all digital network.

First we covered the way the telephone network used to work with the mechanical switching of calls.

Second, we talked about how the telephone network can be looked at as layers that control the functions of switching, transmission and signaling.

Third, we showed how these functions of transmission, switching and signaling have changed the internal network to make it more a "digital network" with only the analog link into the customer's office missing from a totally end-to-end digital network.

Finally we introduced the concept of ISDN which will provide an all digital network for the customer.

Terms to Review Section 1

CCIS - See Common Channel Inter-Office Signaling.

Common Channel Inter-Office Signaling - The method that allows telephone companies to separate their signaling from voice transmission by placing it into its own transmission channel.

Common Channel Signaling - See Common Channel Inter-Office Signaling.

Common Control - The Common Control is the computer or "heart" of a telephone switching system. The main function of the common control is to route and connect the telephone calls. Modern telephone systems use computer based common controls with stored programs that give a large number of features and options for routing telephone calls.

E1 - The standard for digital transmission systems in Europe is based on the CEPT standards sometimes called the E Standards. E1 is the lowest rate of CEPT transmission at 2.048 megabits per second. The E1 system carries 30 voice channels, one signaling channel and one framing channel.

Integrated Services Digital Network - The ISDN network is the next generation of telephone network where all signals are carried in digital format right into the subscribers home or office. The intent is that the new network can be used for more services such as high speed data or multimedia.

ISDN - See Integrated Services Digital Network.

Local Loop - The pair of wires that connect the telephone system to a home or office. Generally one pair of wires (or one loop) is one telephone line.

Signaling - Signaling is the process that moves the information for call routing and network progress. Call routing would be the dialed number and other information. Network progress are the signals like dial tone, ringing and busy.

Switching - Switching is the process of connecting the telephone calls across the network.

Transmission - Transmission refers to the process of moving the voice signals across the telephone network. The measure of a good transmission system is that it will make the voice understandable enough so that the listener can identify the speaker.

T1 - The standard for digital transmission systems in the United States and Canada is based on the ANSI T1 standards which were derived from the Western Electric Company standards. T1 is the lowest rate of transmission at 1.544 megabits per second. The T1 system carries 24 voice channels.

Questions For Review Section 1

1. The Common Control improved the telephone network by?

2. What does the function of Signaling do in a telephone call?

3. How many voice channels are carried on T1 digital links?

4. Why was digital transmission an improvement over analog transmission?

5. Why was Common Channel Signaling an improvement for the network?

Answers For Review Section 1

1. The Common Control improved the telephone network by?

The Common Control removed the routing and contorl of the telephone system into a stored program computer. This computer allowed numbers to be dialed into registers rather than tying up the switching equipment and circuits.

2. What does the function of Signaling do in a telephone call?

Signaling provides the communciations between the parts of the network. It sends the numbers, provides progress tones and notifies of the call status.

3. How many voice channels are carried on T1 digital links?

The North American T1 can carry 24 standard voice channels.

4. Why was digital transmission an improvement over analog transmission?

With analog transmission the amplification of signals also amplified noise, digital system use a regeneration technique that eliminates most noise. Digital systems could also be tied into automatic maintenance equipment.

5. Why was Common Channel Signaling an improvement for the network?

The Common Channel Signaling or CCIS moved the signaling out of the voice channel. This allowed swicthes to look ahead before connecting calls that may go to busy or unavailable numbers. This look ahead made a vast improvement in the utilization of network trunks.

For Further Study

The following are some additional things that can be done to improve your understanding and expand your knowledge of how the telephone network works.

1. Look at the telephone equipment in your office today and contrast this equipment with the changes over the past twenty years. Most if all not all rotary dial telephones have disappeared from desktops and been replaced by push button dialing. If your office has a PBX, ask for a tour of the equipment room and have someone show you:

a. How are the trunks brought to your office? ON a fiber optic cable? On wires as individual trunks? Or, through a digital transmission system using T1 or E1 multiplexing?

b. What is the brand and type of PBX? Is the PBX digital all of the way to the telephone? Can you PBX support ISDN?

2. Contact your telephone company request a tour of the central office that serves your office. Ask if your telephone company has any mechanical switching systems still in operation and if they do, ask to see and contrast it with a modern central office.

3. Find out if any of the telephone companies provide ISDN to offices and homes in your area. If they don't have ISDN, ask about their plans and what features they expect to offer.

End Of Section 1

This completes Section 1 of our course Introduction To ISDN, ATM And Broadband Transmission. If you are having trouble with the material, we suggest that you review the videotape for Section 1. If you feel confident with the subject matter, go on to Section 2.