New questions . . .
What is sealing current (mentioned in my ISDN NT-1 documentation) and why do I need it?
Sealing current is defined in the Bellcore and CCITT Layer 1 specifications. This is a low current (1-20mA) DC signal applied to the copper pair at the U interface. It serves two purposes:
Brad Quicomm, a former researcher that did the sealing current work while at Bellcore, recently wrote in the newsgroup comp.dcom.isdn about his experience with sealing current; "...sealing current does effectively keep a copper loop intact (through a process called electromigration ... it does work on copper loops which have splices. ... for direct copper loops (CO to customer sites) it is ALWAYS suppose to be applied (and is built into the line cards). ....sealing current helps to maintain good electrical connections. In both theory and practice."
I am working on bringing up a router using ISDN between Colorado (US WEST) and the Washington DC area (Bell Atlantic). I get the following message when trying to telnet from one router to another across the ISDN link :
". . . Call not end-to-end ISDN . . . "
The debug messages indicated a bad bearercap on B1. What does bearercap mean?
The term "bearercap" refers to Bearer Capability, also called Bearer Services. Bearer Services characterize the communication capabilities of an ISDN line. An example would be the support for circuit switched data bearer service, or support for circuit switched voice bearer service. In this case you should check two areas. First, check to be sure that your line supports data services, as not all lines are provisioned for data on both B channels (see examples in our ISDN Ordering codes). Then, check your Long Distance PIC. This is often the problem if a "interLATA" data call fails. If your LD carrier does not have Feature Group D access trunks in your LATA, your data call will fail, but your voice calls will work. Most second tier LD carriers can only support the analog (i.e. voice) traffic at this time. AT&T is a good choice if you simply are not sure about your carrier. You can force a call out over any LD carrier with the equal access dial code preceding your area code and telephone number. The code for AT&T is 10288. You can also try MCI (10222), and Sprint (10333). Most others fail, but I have found some success on a regional basis. Here is an example for the dial string:
Format (AT&T version) for equal access: 10288 + 1 + Area Code + Telephone number
Archived questions . . .
What does ISDN mean?
ISDN is an acronym for Integrated Services Digital Network. This type of digital service currently provides two speeds of digital products. The first is Basic Rate ISDN running at 144 Kbps and Primary Rate ISDN which operates over a T-1 at 1.536 Mbps.
Where can I get ISDN?
ISDN is a telephone company product. You get ISDN by ordering ISDN digital telephone service from your local telephone company. We have gathered a list of contact numbers and some web sites to help you get started. Jump to Where to get ISDN page for more information.
Does ISDN work everywhere?
ISDN is delivered to your home or place of business over a single pair of copper wires, just like your regular telephone service. Theoretically, it should work anywhere there is copper, but not so. You must also have a newer digital central office. A central office is the large switching center your local telephone company uses to supply you with telephone service. If your central office is not digital and it is not scheduled for replacement, try to get your telephone company to bring service in from another central office, or to install Remote Node off another central office.
Is there such a thing as dedicated ISDN access, i.e. can I stay connected 24 hours a day for flat monthly rate?
This would be a "dedicated circuit" by definition. Currently, you can use 56/64 DDS (dedicated) circuits for around $200 month, or T-1 at $350 to over $900 per month (distance sensitive), or Frame Relay (often more than T-1) for this. ISDN is billed on a per call basis, determined by time and distance (see my Ameritech ISDN usage rates). One option is ISDN Centrex. Centrex is a Central Office based telephone service where the CO is actually your telephone system. Centrex is used by some ISP's to deliver ISDN to customers connected to the same CO as the ISP. For the group using Centrex, there are no time and distance issues, or per call charges, because the caller can only call the ISP, and the call is considered an "intercom" call, and therefore is not billed. The result is "flat rate" ISDN. The down side is that the ISP needs to order, and pay for the Centrex service (more expensive than ISDN direct) and take financial responsibility for the lines. This requires re-billing the end user (a process not clearly covered in the tariffs, and overlooked by most ROBC's). Also, the line generally would be restricted for a closed user group (you could only call certain telephone numbers, like your Internet POP). This means that you could not use your ISDN line for regular telephone service. Generally, this protects the ISP (who’s is financially responsible for the line) form huge phone bills racked up by clients using the ISDN Centrex.
Can I use a 28.8 Kbps modem in the analog port of my ISDN NT-1 or Terminal Adapter? Would it pass 28.8 OK?
Yes. To support your existing modem you will need an analog port (RJ-11 jack) on your equipment. This is available as an option to some Terminal Adapter's (TA), and NT-1 devices. An alternative would be to use one of the TA's that incorporates an integrated V.32 or V.34 chipset into the design and will communicate with either analog or digital devices. This feature is found on many of the "Digital Modems" being sold today.
I have an ISDN telephone with a Terminal Adapter (TA) and V.24 interface incorporated into the device. Do I still need a modem or is the V.24 interface a modem ?
My working definition for V.24 is:
"a CCITT recommendation that defines the pin assignments between Data Communications Equipment (DCE) and Data Terminal Equipment (DTE), similar to and operationally compatible with RS-232."
The existence of what looks like a COM port does not necessarily mean that there is a modem incorporated into the device. You must look to the manufacturers' specifications to see just what the device is capable of.
Probably, you do need a modem, if you want to talk to another modem. The TA in the telephone will allow you to connect your computer to the network and talk to other TA's via one of the available protocols (V.120, V.110, PPP, SLIP, MLPPP). Be sure to check the speed supported by the Telephone's data port. The early ones only supported 19.2 Kbps, then 38.4 Kbps. Recently, I have seen some specifications on telephones now supporting 64 Kbps through the integrated TA.
Can a analog subscriber fax a ISDN subscriber and vice versa?
This could be trick question. There are two types of fax machines on the market today (Group 3 and Group 4). Most of us use a Group 3 machine, which connects to a regular analog telephone line, or to an RJ-11 analog jack on your ISDN equipment (see above). In either of these cases, the fax connected to the ISDN line could connect to the fax connected to the analog line. A Group 4 fax machine is digital and designed to connect to only with other Group 4 machines. Some of the newest Group 4 machines incorporate a modem chip that will allow it to emulate Group 3 fax machine. This would allow it to connect to a fax on an analog line. Any Group 3 machine connected to the ISDN through the analog port of a TA would be able to communicate with any Group 3 fax connected to an analog line. Both the analog and digital networks are fully interconnected for voice services, and fax operates over voice lines.
What types of hardware will I need to access the Internet from home?
For dial up access via ISDN you will need an ISDN Terminal Adapter, sold as a "digital modem" by some companies. The faster ones are the internal cards, but these may be harder to install. If you can install a controller card or internal modem, then you can install an internal ISDN card. Otherwise, stick with the external type. Most external adapters will connect to your COM port, like an external modem. One exceptions is the new US Robotics parallel port ISDN TA or digital modem (very fast throughput). Just plug it in to the parallel port of your PC (then the Printer piggybacks on the TA). For the more sophisticated user, get the internal board type TA that supports TAPI. Look for ML-PPP support, and be sure the board emulates a Network Card rather than a Modem. This will add 30% to your throughput.
Also, I recommend using an external NT-1, probably one with analog ports like the Super NT-1 from ATI (2 "S/T" ports, and 2 POT's ports). This will allow you to add other cool ISDN stuff in the future. (I use a digital telephone, on-demand router, internal TA, video conference at 128 Kbps, and my modem/fax server/voicemail all off my ISDN BRI line.
If you use Win95 you can use "dial up networking" and the Microsoft TCP stack to get on-line. Mac has equivalent capabilities, but I am not the one to ask. Of course you will need an account with an Internet Service Provider (ISP), and they will most likely have a "suite" of shareware applications (TCP stack, dialer, WEB browser, email client, etc.) for you to start out with. They may have special numbers to dial for ISDN access. Most everything you will need can be purchased at your local computer store.
Do you know where I can find the significance of the SAPI's number?
To understand the significance of SAPI you must understand what it does. SAPI (Service Access Point Identifier) is Layer 2 information used to specify the Layer 2 data link entity that will process a layer 2 frame, and the Layer 3 entity which is to receive that information. The SAPI allows 64 service access points to be specified, where bit 3 of the address field octet containing the SAPI is the least significant binary digit and bit 8 is the most significant. Currently, very few assignments have been made. They include the following potential selections, while all other selections are reserved for future standardization:
SAPI = 0 Frame contains L3 Q.931 message
SAPI = 1 packet mode communications using Q.931 call control procedures
SAPI = 16 packet mode communications using X.25 layer 3 procedures
SAPI = 63 Layer 2 management procedures
Recently, I came across a listing for SAPI 12. ETSI is the European Telecommunications Standards Institute. To my knowledge, this standard has not yet been approved by ANSI, but ANSI and ETSI work together for international interoperability.
SAPI = 12 ETSI defines SAPI 12 for Connectionless Network Applications
The SAPI is one part of the Data Link Connection Identifier (DLCI). It will conform to the definitions contained in Recommendation Q.920 as published by the ITU.
Other Information not covered here can be found at another ISDN FAQ web site linked here. This site covers the basics of ISDN and is worth a read. There is also a good deal of information about ISDN on the WEB sites of the telephone companies. Pacific Bell even has a complete ISDN primer. Check out "Other WEB sites" for links of interest to ISDN users.
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