"H. Peter Anvin" <hpa@casbah.acns.nwu.edu> (05/19/91)
A few questions about ISDN: 1. Is is feasible/not feasible to use ISDN as a link in an IP or Ethernet network? 2. Is ISDN a worldwide standard, or another one of them "we decide what we want" USA standards? 3. Does the 64 kbit/s B-channel rate over ISDN include error correction? 4. Does the D-channel protocol include service identification (say IP, video, voice, modem)...? 5. Is is possible to call a POTS line with a modem from an ISDN connection? 6. What are typical rates for ISDN? Is it billed per minute or per block? Peter A. is Curious (blue&yellow... yes I am Swedish)
oberman@ptavv.llnl.gov (05/20/91)
In article <telecom11.375.7@eecs.nwu.edu>, hpa@casbah.acns.nwu.edu (H. Peter Anvin) writes: > A few questions about ISDN: > 1. Is is feasible/not feasible to use ISDN as a link in an IP or Ethernet > network? An ISDN B channel is simply a bit pipe. IP or anything else is perfectly feasible. It should be noted that Ethernet is a 10 Mhz system and ISDN is only 64Kb (B channel), so some sort of remote bridge would be required and protocols with tight timing requirements (IP is NOT one of these) might have problems. > 2. Is ISDN a worldwide standard, or another one of them "we decide > what we want" USA standards? Yes, ISDN is a worldwide standard. Yes, it is another one of them "we decide what we want US standards." It's "Two ... two ... two standards in one." Actually the problem is that the basic telephone link speed is different in the US and, as a result, the ISDN must be different to work. The US uses 1.5 Mhz T1 lines while much of the world uses 2 Mhz links. So there are two ISDNs, one carrying more B channels (30) than the other (23). But, since the B channels are the same, the phone company can handle conversion of the 2 Mhz version to the US 1.5. It does mean that the terminal equipment is totally incompatible. > 3. Does the 64 kbit/s B-channel rate over ISDN include error correction? It is just an HDLC bit pipe and has very little difference form LAPB, the link layer standard used for OSI networking. The error detection/correction is very limited because LAPD (the HDLC protocol used for ISDN) is very light weight. > 4. Does the D-channel protocol include service identification (say IP, > video, voice, modem)...? D-channel is a very different bird. It does include a "service identification, but the only defined protocols I am aware of are X.25 and native ISDN. I wouldn't be surprised to see others added. There are plenty of bits available. But you need to understand that D-channel is NOT a bit pipe and is a packet protocol. It run IP over a D channel (IMHO a silly idea) would have to be like running it over X.25. It would not be a protocol at this level. In fact, it probably would be over X.25 as would modem (by connection to an ISDN PAD). Voice and video on a shared 16 Kb channel? You're kidding, right? > 5. Is is possible to call a POTS line with a modem from an ISDN connection? Not directly. As mentioned above, you would not use a modem but an ISDN PAD. Our site has a pool of modems for outgoing calls from ISDN lines. You call a magic number to be assigned a modem and then enter the command sequence to dial the analog modem. You can't hook up any POTS equipment to an ISDN line. This includes modems, answering machines and any other analog component. > 6. What are typical rates for ISDN? Is it billed per minute or per block? ISDN is currently tariffed in California for Centrex service only. ISDN runs about $30 / mo. with all other services charged the same as POTS. I believe there is a new tariff on the docket, but I wouldn't swear to it. At this time there is no residential ISDN tariff in this state. But other states may vary. In the "For what it's worth department", Pac*Bell is hoping to have ISDN in my central office by late this year and SS7 for connection to other ISDN islands like Berkeley and Sunnyvale. Who knows. Some year I might even be able to get a connection to another state. R. Kevin Oberman Lawrence Livermore National Laboratory Internet: oberman@icdc.llnl.gov (415) 422-6955 Disclaimer: Don't take this too seriously. I just like to improve my typing and probably don't really know anything useful about anything.
"Fred R. Goldstein" <goldstein@delni.enet.dec.com> (05/21/91)
In article <telecom11.375.7@eecs.nwu.edu>, hpa@casbah.acns.nwu.edu (H. Peter Anvin) writes... > A few questions about ISDN: > 1. Is is feasible/not feasible to use ISDN as a link in an IP or Ethernet > network? Sure. There's even an Internet mailing list on the topic of IP over ISDN. Ethernet, on the other hand, is a LAN subnet, and ISDN is a WAN subnet, so you're comparing apples to pie there. You could however run Ethernet remote bridges over ISDN. Or anything else, since it's a bit pipe. > 2. Is ISDN a worldwide standard, or another one of them "we decide what we > want" USA standards? Worldwide. CCITT I-series, to be specific, albeit with national standards for some of the finer details. > 3. Does the 64 kbit/s B-channel rate over ISDN include error correction? In circuit mode, no, it's raw bits. You run the error correction. In packet mode, it's like X.25, with a checksum on each packet. > 4. Does the D-channel protocol include service identification (say IP, > video, voice, modem)...? Generally, yes. It has specific codepoints for CCITT and ISO standard protocols at various layers. IP, on the other hand, is not CCITT-standard, so it'll probably be a) unidentified or b) identified using the "user specified" codespace. > 5. Is is possible to call a POTS line with a modem from an ISDN connection? Yes, if you run a modem over ISDN. ISDN is essentially an access arrangement, and it interworks fine with the analog phone world. > 6. What are typical rates for ISDN? Is it billed per minute or per block? Up to your local telco. Figure that circuit mode ISDN is billed like a phone call, maybe a surcharge for clear-channel data calls (but regular price for voice calls). Packet mode will probably have the usual per-packet (X.25-style) rates. Plus monthly line charges, usually between one and two times the usual phone rate, for the Basic Rate (2B+D) ISDN line. Fred R. Goldstein Digital Equipment Corp., Littleton MA goldstein@delni.enet.dec.com voice: +1 508 952 3274 Do you think anyone else on the planet would share my opinions, let alone a multi-billion dollar corporation?
tnixon@uunet.uu.net> (05/21/91)
In article <telecom11.375.7@eecs.nwu.edu>, hpa@casbah.acns.nwu.edu (H. Peter Anvin) writes: > A few questions about ISDN: I will answer as best I can, not being an employee of an operating company but an employee of a company that manufacturers ISDN terminal adapters. > 1. Is is feasible/not feasible to use ISDN as a link in an IP or Ethernet > network? Yes, if you can find somebody else with ISDN at the other end, and to the extent that it makes sense to have these connection be switched instead of leased. Bridging Ethernets over ISDN will be a major application (just as it already is for Switched 56k service). > 2. Is ISDN a worldwide standard, or another one of them "we decide what we > want" USA standards? The ISDN standards are being developed in the CCITT, so they are "worldwide". Eventually, when Signalling System 7 is implemented on international circuits, you'll actually be able to use ISDN on these circuits. Each country does specify which subset of ISDN options it will actually support; the T1S1 standards committee and the ISDN-1 industry group have been doing this in the USA. > 3. Does the 64 kbit/s B-channel rate over ISDN include error correction? No. The 64kbps is "clear channel". You certainly CAN run an error correction protocol on top of it, such as V.120 terminal adaption or X.25, but the basic service is not error-corrected. That makes sense, since error correction is neither necessary nor desirable for circuit-switched voice traffic (because the human ear filters out occassional noise, and the delay of error correction would be worse than the noise itself). > 4. Does the D-channel protocol include service identification (say IP, > video, voice, modem)...? Yes. > 5. Is is possible to call a POTS line with a modem from an ISDN connection? Yes, but exactly how that is accomplished varies. Hayes terminal adapters, for example, provide an analog phone jack into which a modem can be plugged, to allow you to communicate with remote PSTN modems through an ISDN voice call. Another way is through a modem pool at the ISDN-PSTN gateway (switch), to which you connect using V.120 or some other adaption protocol. T1S1 and CCITT Study Groups XVII and XVIII are currently looking at expanding the Q.931 protocol to accommodate automatic fallback from ISDN terminal adaption protocols to PSTN modulations, if the station called is turns out to be on the PSTN; this would be useful for interworking with both PSTN data modems and fax machines. We envision devices that would have a DSP implementation of a modem that speaks directly to a B channel (called a "digital modem"), built right onto the terminal adapter. This would help the transition from PSTN to ISDN. > 6. What are typical rates for ISDN? Is it billed per minute or per block? That is still very much open. Current ISDN services are B-channel circuit switched (which are charged by the minute) or D-channel packet-switch (which are often FREE on the same switch, depending on the RBOC, because they haven't really figured out how to charge for it yet). Charges for frame relay service are still being discussed. Charges for packet switched services, once these are provided other than point-to-point (i.e., by gateways to the PSPDNs), will probably be by segments or characters, as they are on the PSPDNs now. I haven't done much research on ISDN charges, but my understanding is that the time-and-distance charges for circuit-switched ISDN connections are identical to PSTN charges. Since most PSTN connections are circuit-switched 64kbps digital connections anyway, this makes perfect sense. Toby Nixon, Principal Engineer | Voice +1-404-840-9200 Telex 151243420 Hayes Microcomputer Products Inc. | Fax +1-404-447-0178 CIS 70271,404 P.O. Box 105203 | UUCP uunet!hayes!tnixon AT&T !tnixon Atlanta, Georgia 30348 USA | Internet hayes!tnixon@uunet.uu.net [Moderator's Note: A new mailing list is getting started, devoted to the topic of ISDN. I received a note on it today, and will run it here soon, so watch for it if you would like to join an interesting new group devoted to this topic. PAT]