Telecommunications Connections: ISDN

Reprinted from the December, 1995 issue of PC Alamode Magazine
© Copyright 1995, John Woody. All rights reserved.


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High Speed Connections

This month's article is about bandwidth and digital communications. Just as we are getting settled into 28.8kbps, the bandwidth technology springs ahead of us. What is digital communications? Just what is bandwidth?

 Gilster defines bandwidth as "The size of the data pipeline. The higher the bandwidth, the faster the information flow," and Fraase defines it as "The range of transmission frequencies that a network can use. The greater the bandwidth, the greater amount of information that can travel on the network at one time."

 Integrated Digital Network Services (ISDN) is a International Telecommunications Union (ITU) standard for all-digital transmissions end to end, using existing switches and wiring upgraded so that the basic "call" is a 64 kbps end-to-end channel. The basic ISDN connection contains two 64 kbps bearer ("B") channels and a single 16 kbps delta ("D") channel. Southwest Bell has defined digital communication in its offering of Integrated Digital Network Services (ISDN) as a high speed digital connection at 128,000 bps. This is indeed faster than 28,800 bps. SW Bell seemed to be pushing "integrated service". They see ISDN as a business tool, either for small to medium or home office. There seem to be many advantages, but there are many disadvantages as well. The biggest disadvantage is SW Bell's cost structure. SW Bell is not the only provider who has problems with its pricing. Articles abound which describe other providers who seem to be in trouble with State regulators. I will touch on this more later in this article.
 
 

Digital Communications

From the time Thomas Edison provided the first telephone, it has been an analog service from the telephone company's Central Office (CO) to the telephone connection. One call at a time can be made on an analog telephone. Once the call is made, that physical connection between the caller and the CO to the receiving party eliminate anyone else from connecting until the call is terminated. This is called Plain Old Telephone Service (POTS). POTS service completely controls how computer connections are made in that the communications hardware has to be analog, thus the analog modem. The modem exists to translate digital information into analog information. Thus, the ones and zeros are translated into sound tones within a frequency range of approximately 3000 Hz and a transmission rate of approximately 2400 baud in the telephone service. Protocols for speed, compression, and error correction have increased the computer data transmission much higher. The analog speed and compression protocols being used today allow for transmission speeds of approximately 28,800 bps. With POTS service, a separate line is needed for each user service, ie., telephone, fax, office.

 ISDN is digital allowing both voice and data to be sent at the same time as digital packets. Digital packets are transmitted in the same manner as letters in the Post Office. The packets are grouped with others and sent along the circuit together. Each packet contains calling information (addresses) so that it is routed to the proper exit or receiving party. Voice and data may be transmitted at the same time.
 
 

ISDN Description

Basic Rate Interface (BRI) consisting of two B (bearer) channels and one D (delta) (control) channel make up a pipeline bandwidth of two 56 or 64 kbps circuit switched or packet data or voice channels and one 16 kbps control or signal channel. The signaling channel is used to set up the call and ring the other party. It can be used to transmit low-volume data after the initial call is connected. It is important to note that even though SW Bell is advertising each B-channel as 64 kbps, this may be a 56 kbps data rate in some areas as some CO local and/or long distance switches are not capable of 64 kbps. With bonding, the total bandwidth can be either 114,000 or 128,000 bps depending on the CO switch capability. Add the D-channel capability of 16,000 bps and you could have 144,000 bps capability from your computer. BRI service provides two independent transmission channels, ie., two voice conversations, or two data transmission, or one voice and one data transmission, all occurring simultaneously and to different destinations.

 The second level of ISDN service is called Primary Rate Interface (PRI) and consists of either 24 or 31 channels, usually divided into 23 or 30 B-channels and one D-channel. The bandwidth of each B-channel is 56 or 64 kbps and the D-channel is 16 kbps. The 24 or 31 channel designation depends where in the world you are located. The U. S. and Japan standard is 24 PRI channels and the remainder of the world uses 31 PRI channels. PRI in the U. S. uses a T-1 interface. In the remainder of the world, the interface is E-1. This connection is used between a private branch exchange (PBX) and the CO, or between the CO and the inter exchange carrier (long distance company).
 
 

ISDN Standards

The ISDN standards are spelled out by the International Telecommunications Union (ITU). They are numerous and cover such areas as the ISDN user-network data link layer specification, the ISDN user-network interface layer 3 specification, pulse code modulation of voice frequencies, audio coding, coding of speech at 16 Kbits/s, narrow-band visual telephone systems and terminal equipment, frame structure for 64 to 1920 Kbits/s audiovisual teleservices, frame for establishing communications between audiovisual terminals using 2 mbits/s digital channels, video codec for audiovisual services at 64 Kbits/s, basic MCU Procedures for establishing communications between three or more audiovisual terminals at 2 mbits/s, ISDN frame mode bearer services, support of DTE's with V series type interfaces by an ISDN terminal, support by an ISDN of data terminal equipment with V series type interfaces with provision for statistical multiplexing, and calling mechanism under synchronous. It can be seen that from the above range of standards, ISDN is meant to be an all encompassing technology. Much of the standards development addresses audiovisual functions.

 The two standards which address the V series are of immediate interest to individual users as these functions cover the dial up devices which are called ISDN Terminal Adapters (TA). Some of the vendors are supporting the V.120 "Support by an ISDN of Data Terminal Equipment with V series Type Interfaces with Provision for statistical multiplexing" with the bonding protocol to mix the two B-channels into a single 112 or 128 kbps bandwidth pipe. This is not universal as some TA vendors who are manufacturing equipment for PPP (point-to-point) protocol do not support bonding. This is being worked out by vendors.

 Dial up PPP with an bonded two B channel ISDN TA will known as a multiplexed system. MP (multiplexed Point-to-point) are being designed under a proposed standard known as PPP RFC 1717. Vendors are working hard to deliver equipment and ISPs are beginning to offer access at rates similar to async dial up.

 Compression of ISDN operations on PPP are not completely standardized yet also. An Internet draft standard is be circulated which proposes a compression control protocol (CCP). Devices may negotiate this CCP each time to enter into document or file compression during the transmission. One of the problems being encountered is that several compression algorithms are being proposed and can not be agreed to. Stacker LZS, V.42, and Microsoft RAS are being considered to name some of the draft entries. The selection process will be prolonged as vendors push their interests. Compression may not be important as compared to the raw bps speed of two B-channels is used.

 Several interface strategies are being proposed to connect the ISDN to network connections. One technique is being written by NetManage, ISDN*Tek, and PSI which is an open API which is a Windows network protocol is be proposed. It will require that both network protocol stack vendors and ISDN vendors support a new API. This technique, WinISDN, provides for direct control on the connection similar to a modem providing for PPP control of the connection.

None of these approaches work if you are using a Mac or UNIX computer.
 
 

ISDN Description

A Basic Rate Interface (BRI), two B-channels and one D-channel, consists of a two conductor line from the CO (telephone company) to a dial up customer. This is called a U-loop and can be a maximum of 5.5 km (18,000 ft) in length. Equipment on both sides of the U-loop must be carefully designed due to the noise and length of it. The U-loop is terminated into a Network Termination 1 (NT-1) device. The NT-1 unit may be external or internal to the ISDN Terminal Adapter (TA) device. The NT-1 unit provides power to the terminal devices such as the telephone and the TA device. ISDN does not ring. The bus or cable leading from the NT-1 device to the TA device contains four to eight wire paths and called the S/T-loop. Some of these wire paths provide the power required for ring, etc. The power circuit is required in the case that electrical power is lost. The S/T-loop connects to multiple TA devices, ie., telephone, fax, computer.

 The NT-1 device provides the interface between the two wire CO wire to the customer and the four wire terminal equipment. NT-1 may be part of the TA equipment or it may be a separate unit. In the U. S., this is user supplied device. Some NT-1 devices have analog ports which allow the connection of regular analog modems and telephones.

 Terminal Adapters (TA) are the devices which interface with the telephone or computer equipment. Most TA equipment are internal devices. The TA is where the two B-channels are controlled and used. The TA separates the data and the phone calls or bonds the two B-channels together for full bandwidth operations.

 The ISDN line pairs are the same as those used for regular telephone service. ISDN service provides the 56 or 64 kbps data rates right up to the customers desktop. Each telephone call uses the entire 56 or 64 kbps because there is no need to convert signal to analog in the modem and back to digital form in the CO. Many other technological opportunities become available, such as the transmission of lots of data, high-fidelity compressed audio, encrypted audio, and compressed video.
 
 

ISDN Equipment

ISDN manufacturers include both U. S. and European companies. Remember that Europe and Japan are further along the ISDN path than the U. S. Advanced Micro Devices, American Telephone and Telegraph, Asend Communications Inc., Combinet, Digiboard, Digital Equipment Corp., Hayes ISDN Technologies, MITEL Corp., Motorola, IBM, US Robotics, ISDN*tek, and Zydacron are among the U. S. vendors currently providing equipment.

 Netmanage Chameleon is currently one of the only TCP/IP communications programs with ISDN capability. Netmanage is developing WinISDN with two other firms which will feature full dial up PPP connectivity.
 
 

ISDN Costs

The cost of ISDN comes from four areas. The first is the equipment/hardware cost. The second cost is from the CO provider (telephone company). The third cost is from the Internet Service Provider (ISP). The fourth cost comes from the communications programs for either network or dial up use.

 Price ranges for this equipment are from $380.00 to $2,490.00 including NT-1. Standalone NT-1 devices range in price from $130.00 to $300.00. PCMCIA ISDN cards are available and cost in the range of $450.00 to $550.00.

Additionally, the CO connection is costly also. There are installation and monthly charges from the CO provider. These range from $57.57 installation and $28.35 monthly charge at NYNEX to $71.50+ installation and $58.00+ at the SW Bell San Antonio startup. This rate is signed for a two year period. State regulators in the East have begun to overturn some of the rate schemes being used by the telephone companies.

 The Netmanage Chameleon 4.5 ISDN ready communications program is approximately $380.00.

ISDN Installation

The CO U-loop line is plugged into the NT-1 device. The NT-1 unit is mounted close to the phone wire inlet. This unit also requires its own electrical power. The TA card in placed in the computer and is attached from the NT-1 unit to it by an S/T loop. ISDN phones and fax machines contain the TA and are attached in the same manner.
 
 

Conclusions

The potential for ISDN is great. Full use of the digital packet transmission capability of 64 kbps single channel or 128 kbps two channel to transfer data or images opens many new avenues for remote offices. Full motion video or data and voice at the same time has great potential. The down side in the cost structures of the providers. The hardware is costly at the present time, but it will drop in price as usage increases. The main overriding problem will be how the access charges are administered.