Telecommunications |  | by John Woody |
This article is about bandwidth and digital communications. Just as we are getting settled into 28.8 kbps modems, 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 flows",1 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".2
Integrated Digital Network Services (ISDN) is an 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.3 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.
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 eliminates anyone else from connecting until the call is terminated. This is called Plain Old Telephone Service (POTS). POTS 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 greatly. 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, i.e., 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.
Basic Rate Interface (BRI) consisting of two B (bearer) channels and one D (delta-control) channel makes up a pipeline bandwidth of two 56 or 64 kbps circuit switches 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, i.e., two voice conversations, or two data transmissions, 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 upon 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).
The ISDN standards are spelled out by the International Telecommunications Union (ITU)4. 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 dialup 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.5 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 a bonded two B channel ISDN TA will be 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.
Nor are compression of ISDN operations on PPP completely standardized yet. An Internet draft standard which proposes a compression control protocol (CCP) is being circulated. 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 cannot be agreed upon. 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. 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 Windows network protocol, is being 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.
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 its noise and length. 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 is 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, i.e., 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 a 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 that interface with the telephone or computer equipment. Most TA equipment is internal. The TA is where the two B-channels are controlled and used. The TA separates the data and 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 customer's 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 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 Zydacron are among the U.S. vendors currently providing equipment. Netmanage Chameleon is currently one of the few TCP/IP communications programs with ISDN capability. With two other firms, Netmanage is developing WinISDN, which will feature full dial-up PPP connectivity.
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 to $2,490, including NT-1. Standalone NT-1 devices range in price from $130 to $300. PCMCIA ISDN cards are available and cost in the range of $450 to $550.
Additionally, the CO connection is costly. 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 4.6 ISDN ready communications program is approximately $380.
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 is 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.
The potential for ISDN is great. Full use of the digital packet transmission capability of 64kbps 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 is 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.
Reprint from P.C. Ala-mode, The News Magazine of the Alamo P. C. Organization, Inc., December 1995.
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