149

Short Communication

The A-ISDN Proposal to Bridge "Personal Computers" and "ISDN"

J a a p V A N T I L L James Martin Associates, Amsterdamseweg 204, 1182 HL Amsteloeen, The Netherlands

Abstract. The present recommendations for data communica- tions on the ISDN seem to have been defined for the connec- tion of dumb-terminals to computerports at 19.2 kb i t / s or slower. Personal computers can (and need to) communicate, with each other and with LANs, much faster than dumb-termi- nals. The A-ISDN proposal is made to make better use of the new ISDN connections, which allow most unchanged installed personal computers to communicate at 90 percent of the 64 kb i t / s that digital (telephone) networks will offer for data communications.

Keywords. ISDN, Personal Computers, Asynchronous Data- communications, Dial-up.

1. Introduction

Data Processing people regard their own world as "intelhgent systems interconnected by dumb lines". Telecommunications Networks are re- garded by their own supporters as complex yet reliably providing "interconnectabihty for dumb terminals". It is therefore only recently and with difficulty that these two separate worlds start to overlap. Besides leased hnes and modems we see an increase in the use of T 1 / T 3 or CEPT-G.703 digital pubhc network trunks for both data and voice transmission.

Another meeting place of Systems and Net- works, nearer to the user, will be the "Integra ted Services" interface which is now available on the new public "Digital Networks" in several coun- tries and on some digital PABXs.

2. How Can We Connect to the ISDN?

article are his own.

J .W.J . baron van Till is Principal Con- sultant at James Martin Associates. He received the M.Sc. degree in Elec- tronic Information Science from Delft University of Technology in 1972. He helped to design the (tele) communi- cation architecture and strategy for several organisations in Europe. He also develop~l the high-speed TIL- L E G R A A F ® and P L E C T O R ® baseband linedrivers for local data communication, of which about 20 000 are in use. The views expressed in this

North-Holland Computer Networks and ISDN Systems 17 (1989) 149-152

The end-user equipment "S" interface on the ISDN is a 4-wire interface for "bas ic rate access" communication: 2B + D. This means: two trans- parent "B" digital synchronous channels, both full duplex at 64 kb i t / s and one signalling " D " chan- nel (16 kbi t / s , synchronous H D L C packet LAPD format) to set up the connection for B1 and B2 and for other outband signalhng requirements.

3. What Can We Connect to the "S" Interface?

The ISDN "S" interface bus is implemented in such a way that up to eight devices can use the two digital "B" channels, B1 and B2. Possible types of devices that can be connected are - " I S D N Terminal" offering data and voice com-

munication.

0169-7552/89/$3.50 © 1989, Elsevier Science Publishers B.V. (North-Holland)

150 J. Van Till / The A- ISDN Proposal to Bridge PCs and ISDN

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Fig. 1. X.30/V.110 rate adaptation.

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- Digital telephone: B1 for voice, " R " interface connector for datacommunications on B2.

- Personal computer with internal ISDN adapter board ( + connector for an analogue telephone). Such a PC card replaces a modem card.

- Terminal Adapter box: " R " interface to con- nect non-ISDN data terminals to B1 or B2 at the "S" interface. Terminals are connected at V24/V28-RS232C on the " R " interface. Such Terminal Adapter boxes have been announced for synchronous terminals like the X.21 or S D L C / S N A compatible control units. "Asyn- chronous terminals" effect data communication with serial start stop characters coded in, for example, ASCII. On all four types of devices Rate Adaptation

(RA) techniques are used to connect non-64 kbi t / s bitstreams to the B channel(s). Three CCITT RA techniques (and B formats) are recommended in 1.460:

(a) X.30/V.110 bit repetition & positioning, including ECMA 102 RA: for multiple (TDM) circuit-mode use of the B channel, see Fig. 1. The RA is performed in three steps for asynchronous terminals at or below 19.2 kbit /s , two steps for synchronous terminals at or below 19.2 kbi t / s and one step for 48 and 56 kbi t / s synchronous termi- nals. For terminals with 64 kbi t / s synchronous transmission capabilities, a rate adaptation is not necessary. Some RA devices will offer handling of the " AT command set" to permit autodialing from PC software. These asynchronous commands re- quest the " D " channel to establish the "B" con- nection.

(b) X.31 with X.25 Packet Link Protocol over LAPB: for packet-mode use of a "B" channel.

(c) V.120 statistical multiplexing of several data connections in circuit-switched mode. An example is the Digital Multiplexed Interface (DMI) com- puter interface which allows multiple terminal data streams to flow through one PABX computer con- nection. A similar interface is Computer-to-PBX Interface (CPI).

To summarize: New ISDN terminals and all existing "terminals" can communicate through ISDN either by

(1) using PCs with internal cards connected to the "S" interface, or

(2) ~multiple) terminals/computerports con- nected with an adapter or digital telephone.

4. What Do We Want to Connect?

The proposed ISDN connectivity looks prom- ising but is, in my opinion, not very realistic since

(1) PC cards. These are expensive compared to modem cards with the same speed and this thus poses a threshold to present PC users. Also some different "S" interfaces are offered on some na- tional or propriety networks. The effective data- communication speed with such PC card will be 19.2 kbi t / s asynchronous for present PC software in combination with the mentioned RA standards.

(2) Terminal adapters. Terminals operating at 48, 56 or 64 kb i t / s synchronous are rare. The proposed asynchronous LINE-speeds is what peo-

J. Van Till / The A-ISDN Proposal to Bridge PCs and ISDN 151

Fig. 2. The A-ISDN proposal.

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ple already have and at much lower cost with modems than with an expensive ISDN Terminal Adapter, giving only asynchronous 19.2 k b i t / s or less. Also the error rate for terminal-based data communication through ISDN is still an open question if existing telecommunications cable- plants are used for the new digital networks.

Why not give on the ISDN the WORKSTA- T I O N speeds that people already have? Most PCs can communicate with each other in a serial asynchronous mode at 57.6 kb i t / s and some can even communicate at 880 kbi t / s !

These asynchronous workstation speeds are not generally known because of several common as- sumptions which are not correct and which slow innovation: - "Datacommunica t ion with PC's is 1200 or 2400

b i t / s ' . These are in fact the line-speeds of popular dial-up modems with acceptable p r i ce / performance.

- " T h e V24/V28 interface (alias RS232C) will collapse above 19 200 b i t / s , so we need a V.35 connector".

- "Asynchronous (serial s ta r t / s top character data communication) 19.2 kb i t / s is the top speed of asynchronous terminals and the COM1 or COM2 connectors of PCs". In fact this speed presents the maximum some modems can offer over analogue telephone links and also the max- imum bit rate most multi-user computerports can handle. The actual transmission rates between personal

computers that can be achieved when a direct serial (null-modem) Cable is used in combination with file transfer software are as follows: - P S / 2 or PC-AT or compatible: maximum

asynchronous speed of 115.2kbit/s. - P C / X T or PC or compatible: half of maximum

= 57.6 kbi t / s .

- The Apple Macintosh can communicate much faster: more than 230 k b i t / s on both serial asynchronous connectors. The point is that as far as data communicat ion

with personal computers is concerned there is no reason to stop innovation at 19.2 kb i t / s ! Further- more some form of "packe t a s sembly /d i s - assembly" of ASCII characters is performed by PC-software to do error-correction in file transfer mode and to be able to open multiple virtual sessions.

Examples are - remote dial-up into App le /Loca lTa lk LANs, - bridges between personal computer LANs, - serial T C P / I P (SLIP), - MNP protocols, SPEED > S document trans-

fer, - X.PC protocol, - Xmodem and K E R M I T file transfer, - V.42 asynchronous dial-up error correction and

V.42bis compression. All these ASCII- and other asynchronous based

facilities need the highest possible speed links and take advantage of the intelligence of the personal computer.

5 . T h e A - I S D N p r o p o s a l

In June 1988 the author circulated an "open- letter" (the Tillecom proposal) suggesting a PC asynchronous speed of 62.4 k b i t / s to be synchro- nised into a 64 kb i t / s B2 channel on a digital telephone. An adapter would perform the one step rate adaptation to effect synchronised s t a r t / s top characters. This speed of 62.4 k b i t / s was found to be less practical in the PC hardware because at this speed range clock divisions for speeds other than half of 115.2 k b i t / s cannot be programmed.

152 J. Van Till / The A-ISDN Proposal to Bridge PCs and ISDN

In order to avoid any PC hardware and soft- ware modifications, the possibility is now pro- posed, see Fig. 2, to connect most of the installed millions of personal computers to the new (ISDN) digital channels at a bit-transfer rate of 57600 b i t / s asynchronous, which means 5760 char- acters/s. This is a 300% increase over the present maximum of 19.2 kbi t / s and yet 90% of the 64 kbi t / s rate.

A low-cost adapter is now under construction in the Netherlands, offering a synergy-bridge be- tween the best of both worlds:

57.6 kbi t / s asynchronous PC user interface & software to the 64 kbi t / s synchronous ISDN rate.

The proposed alternative Rate Adaptation pro- posal would also make ISDN more attractive for data communication users. ISDN will be imple- mented to meet voice needs anyway by the PTTs, and low-cost digital telephones for that function can be offered much more attractively if a " R " da tacom interface for P E R S O N A L COM- PUTERS is offered for error-free dial-up data

communications at 57.6 kbi t /s : Async-ISDN or "A-ISDN". I So Do kNow an advantage of get- ting ISDN connections!

Applications can be, for example, to connect from your home to the LAN in the office. Error protection and file transfer would be performed by the personal computer software. This looks especially attractive because long file-transfer times'at for example 2400 b i t / s now restrict that type of connection.

6. Conclusions

The new asynchronous data transmission method would present a considerable p r ice / performance improvement (a concept not un- known in the Data Processing world) for the use of PCs, as compared to dial-up modems + telephone links. Also a number of new voice & data functions can be invented for the proposed personal c o m p u t e r - I S D N combination.