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The A-ISDN proposal to bridge “personal computers” and “ISDN”
149
Short Communication
The A-ISDN Proposal to Bridge "Personal Computers" and "ISDN" 1. Introduction
Jaap VAN TILL James Martin Associates, Amsterdamseweg Amsteloeen, The Netherlands
204, 1182 HL
Abstract. The present recommendations for data communications on the ISDN seem to have been defined for the connection of dumb-terminals to computerports at 19.2 k b i t / s or slower. Personal computers can (and need to) communicate, with each other and with LANs, m u c h faster than dumb-terminals. 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 k b i t / s that digital (telephone) networks will offer for data communications. Keywords. ISDN, Personal Computers, Asynchronous Datacommunications, Dial-up.
Data Processing people regard their own world as "intelhgent systems interconnected by d u m b lines". Telecommunications Networks are regarded by their own supporters as complex yet reliably providing "interconnectabihty for d u m b 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 Networks, nearer to the user, will be the "Integrated Services" interface which is now available on the new public "Digital Networks" in several countries and on some digital PABXs.
2. How Can We Connect to the ISDN?
J.W.J. b a r o n v a n Till is Principal Consultant at James Martin Associates. He received the M.Sc. degree in Electronic Information Science from Delft University of Technology in 1972. He helped to design the (tele) communication architecture and strategy for several organisations in Europe. He also develop~l the high-speed TILLEGRAAF ® and PLECTOR ® baseband linedrivers for local data communication, of which about 20 000 are in use. The views expressed in this article are his own. North-Holland Computer Networks and ISDN Systems 17 (1989) 149-152
The end-user equipment " S " interface on the I S D N is a 4-wire interface for "basic rate access" communication: 2B + D. This means: two transparent " B " digital synchronous channels, both full duplex at 64 k b i t / s and one signalling " D " channel (16 k b i t / s , synchronous H D L C packet L A P D 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 I S D N " 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 communication.
0169-7552/89/$3.50 © 1989, Elsevier Science Publishers B.V. (North-Holland)
150
J. Van Till / The A - I S D N Proposal to Bridge PCs and I S D N
"R" ',q ',4
Iv
q
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I! t
q
I,I iiiiiiiiiiiiiii~
kbit/sli~i~:iii~#~iiiiiiii~i!!!iii!i! async L
19.2
48or
kbit/s sync
56 kbit/s sync
signalling
iiiiiii!iiiiiiii
~
~iiiii?iiiii
,B
N!iiiiiii
16 kbit]s packets
,
~
D
Fig. 1. X.30/V.110 rate adaptation.
- 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 connect 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. "Asynchronous 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 k b i 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 k b i t / s and one step for 48 and 56 k b i t / s synchronous terminals. For terminals with 64 k b i t / s synchronous transmission capabilities, a rate adaptation is not necessary. Some RA devices will offer handling of the " A T command set" to permit autodialing from PC software. These asynchronous commands request the " D " channel to establish the " B " connection.
(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) computer interface which allows multiple terminal data streams to flow through one PABX computer connection. 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 connected with an adapter or digital telephone.
4. What D o We Want to Connect?
The proposed ISDN connectivity looks promising 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 national or propriety networks. The effective datacommunication speed with such PC card will be 19.2 k b i t / s asynchronous for present PC software in combination with the mentioned RA standards. (2) Terminal adapters. Terminals operating at 48, 56 or 64 k b i t / s synchronous are rare. The proposed asynchronous LINE-speeds is what peo-
151
J. Van Till / The A-ISDN Proposal to Bridge PCs and ISDN
~N
3x Fig. 2. The A-ISDN proposal. ple already have and at much lower cost with modems than with an expensive I S D N Terminal Adapter, giving only asynchronous 19.2 k b i t / s or less. Also the error rate for terminal-based data communication through I S D N is still an open question if existing telecommunications cableplants are used for the new digital networks. Why not give on the I S D N the W O R K S T A T I O N speeds that people already have? Most PCs can communicate with each other in a serial asynchronous mode at 57.6 k b i t / s and some can even communicate at 880 k b i t / s ! These asynchronous workstation speeds are not generally known because of several c o m m o n assumptions which are not correct and which slow innovation: - " D a t a c o m m u n i c a t i o n 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 c e / 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 t a r t / s t o p character data communication) 19.2 k b i t / s is the top speed of asynchronous terminals and the COM1 or COM2 connectors of PCs". In fact this speed presents the m a x i m u m some modems can offer over analogue telephone links and also the maxi m u m 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: -PS/2 or PC-AT or compatible: m a x i m u m asynchronous speed of 115.2kbit/s. - P C / X T or PC or compatible: half of m a x i m u m = 57.6 kbit/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 communication with personal computers is concerned there is no reason to stop innovation at 19.2 k b i t / s ! Furthermore some form of " p a c k e t a s s e m b l y / 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 A p p l e / L o c a l T a l k LANs, - bridges between personal computer LANs, - serial T C P / I P (SLIP), - M N P protocols, SPEED > S document transfer, - X.PC protocol, - X m o d e m 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. The
A-ISDN
proposal
In June 1988 the author circulated an "openletter" (the Tillecom proposal) suggesting a PC asynchronous speed of 62.4 k b i t / s to be synchronised into a 64 k b 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 t o p 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 software modifications, the possibility is now proposed, 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 characters/s. This is a 300% increase over the present maximum of 19.2 k b i t / s and yet 90% of the 64 k b i t / s rate. A low-cost adapter is now under construction in the Netherlands, offering a synergy-bridge between the best of both worlds: 57.6 k b i t / s asynchronous PC user interface & software to the 64 k b i t / s synchronous ISDN rate. The proposed alternative Rate Adaptation proposal would also make ISDN more attractive for data communication users. ISDN will be implemented 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 " d a t a c o m interface for P E R S O N A L C O M PUTERS is offered for error-free dial-up data
communications at 57.6 kbit/s: Async-ISDN or "A-ISDN". I So Do kNow an advantage of getting 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 i c e / performance improvement (a concept not unknown 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.
Short Communication
The A-ISDN Proposal to Bridge "Personal Computers" and "ISDN" 1. Introduction
Jaap VAN TILL James Martin Associates, Amsterdamseweg Amsteloeen, The Netherlands
204, 1182 HL
Abstract. The present recommendations for data communications on the ISDN seem to have been defined for the connection of dumb-terminals to computerports at 19.2 k b i t / s or slower. Personal computers can (and need to) communicate, with each other and with LANs, m u c h faster than dumb-terminals. 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 k b i t / s that digital (telephone) networks will offer for data communications. Keywords. ISDN, Personal Computers, Asynchronous Datacommunications, Dial-up.
Data Processing people regard their own world as "intelhgent systems interconnected by d u m b lines". Telecommunications Networks are regarded by their own supporters as complex yet reliably providing "interconnectabihty for d u m b 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 Networks, nearer to the user, will be the "Integrated Services" interface which is now available on the new public "Digital Networks" in several countries and on some digital PABXs.
2. How Can We Connect to the ISDN?
J.W.J. b a r o n v a n Till is Principal Consultant at James Martin Associates. He received the M.Sc. degree in Electronic Information Science from Delft University of Technology in 1972. He helped to design the (tele) communication architecture and strategy for several organisations in Europe. He also develop~l the high-speed TILLEGRAAF ® and PLECTOR ® baseband linedrivers for local data communication, of which about 20 000 are in use. The views expressed in this article are his own. North-Holland Computer Networks and ISDN Systems 17 (1989) 149-152
The end-user equipment " S " interface on the I S D N is a 4-wire interface for "basic rate access" communication: 2B + D. This means: two transparent " B " digital synchronous channels, both full duplex at 64 k b i t / s and one signalling " D " channel (16 k b i t / s , synchronous H D L C packet L A P D 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 I S D N " 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 communication.
0169-7552/89/$3.50 © 1989, Elsevier Science Publishers B.V. (North-Holland)
150
J. Van Till / The A - I S D N Proposal to Bridge PCs and I S D N
"R" ',q ',4
Iv
q
S
I! t
q
I,I iiiiiiiiiiiiiii~
kbit/sli~i~:iii~#~iiiiiiii~i!!!iii!i! async L
19.2
48or
kbit/s sync
56 kbit/s sync
signalling
iiiiiii!iiiiiiii
~
~iiiii?iiiii
,B
N!iiiiiii
16 kbit]s packets
,
~
D
Fig. 1. X.30/V.110 rate adaptation.
- 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 connect 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. "Asynchronous 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 k b i 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 k b i t / s and one step for 48 and 56 k b i t / s synchronous terminals. For terminals with 64 k b i t / s synchronous transmission capabilities, a rate adaptation is not necessary. Some RA devices will offer handling of the " A T command set" to permit autodialing from PC software. These asynchronous commands request the " D " channel to establish the " B " connection.
(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) computer interface which allows multiple terminal data streams to flow through one PABX computer connection. 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 connected with an adapter or digital telephone.
4. What D o We Want to Connect?
The proposed ISDN connectivity looks promising 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 national or propriety networks. The effective datacommunication speed with such PC card will be 19.2 k b i t / s asynchronous for present PC software in combination with the mentioned RA standards. (2) Terminal adapters. Terminals operating at 48, 56 or 64 k b i t / s synchronous are rare. The proposed asynchronous LINE-speeds is what peo-
151
J. Van Till / The A-ISDN Proposal to Bridge PCs and ISDN
~N
3x Fig. 2. The A-ISDN proposal. ple already have and at much lower cost with modems than with an expensive I S D N Terminal Adapter, giving only asynchronous 19.2 k b i t / s or less. Also the error rate for terminal-based data communication through I S D N is still an open question if existing telecommunications cableplants are used for the new digital networks. Why not give on the I S D N the W O R K S T A T I O N speeds that people already have? Most PCs can communicate with each other in a serial asynchronous mode at 57.6 k b i t / s and some can even communicate at 880 k b i t / s ! These asynchronous workstation speeds are not generally known because of several c o m m o n assumptions which are not correct and which slow innovation: - " D a t a c o m m u n i c a t i o n 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 c e / 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 t a r t / s t o p character data communication) 19.2 k b i t / s is the top speed of asynchronous terminals and the COM1 or COM2 connectors of PCs". In fact this speed presents the m a x i m u m some modems can offer over analogue telephone links and also the maxi m u m 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: -PS/2 or PC-AT or compatible: m a x i m u m asynchronous speed of 115.2kbit/s. - P C / X T or PC or compatible: half of m a x i m u m = 57.6 kbit/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 communication with personal computers is concerned there is no reason to stop innovation at 19.2 k b i t / s ! Furthermore some form of " p a c k e t a s s e m b l y / 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 A p p l e / L o c a l T a l k LANs, - bridges between personal computer LANs, - serial T C P / I P (SLIP), - M N P protocols, SPEED > S document transfer, - X.PC protocol, - X m o d e m 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. The
A-ISDN
proposal
In June 1988 the author circulated an "openletter" (the Tillecom proposal) suggesting a PC asynchronous speed of 62.4 k b i t / s to be synchronised into a 64 k b 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 t o p 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 software modifications, the possibility is now proposed, 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 characters/s. This is a 300% increase over the present maximum of 19.2 k b i t / s and yet 90% of the 64 k b i t / s rate. A low-cost adapter is now under construction in the Netherlands, offering a synergy-bridge between the best of both worlds: 57.6 k b i t / s asynchronous PC user interface & software to the 64 k b i t / s synchronous ISDN rate. The proposed alternative Rate Adaptation proposal would also make ISDN more attractive for data communication users. ISDN will be implemented 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 " d a t a c o m interface for P E R S O N A L C O M PUTERS is offered for error-free dial-up data
communications at 57.6 kbit/s: Async-ISDN or "A-ISDN". I So Do kNow an advantage of getting 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 i c e / performance improvement (a concept not unknown 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.