How To Set Up and Use ISDN

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Integrated Services Digital Networks (ISDN) is a digital service offered by some telephone companies to provide data access up to 128 Kbps for residence and small office applications. The following material explains: what is ISDN, what equipment is needed, and how to set up ISDN hardware and software.  ISDN is being replaced in most areas by Digital Subscriber Line (DSL) services. DSL (see NetInfo) differs in that it is provided as a "piggyback" service using an existing voice phone line where ISDN is provided using a separate dedicated phone line. DSL is generally faster but may not be available in your location.

ISDN Overview

As background, telephone companies are now essentially all digital except for the wires that connect to your residence or small office (the infamous "last mile"). When you make a call, a digital "circuit switched" 64,000 bps connection is established between your local telephone central office and the called party’s office. Your voice travels in the analog form (Plain Old Telephone System (POTS)) developed by Alexander Graham Bell 100 years ago only as far as the local office where it is sampled 8000 times per second. Each sample is converted to an 8 bit number (sometimes only 7 bits are meaningful) and sent by fiber optics and other high tech means to the called party’s office where it is converted back to analog. This explains why it is physically impossible to send more than 64,000 bps through a telephone circuit. The local line has a dc voltage and ac ringing signal which power your phone even if your local power is off. The equipment that actually performs handling and connecting of calls is known as a "switch".

An ISDN Basic Rate Interface (BRI) line uses a bidirectional digital signal which carries about 150,000 bps of information in each direction, specifically two 64,000 bps "B" circuits and one 16,000 bps signaling circuit. The ISDN line comes into your residence or office on a single standard POTS twisted pair of wires. Because of the high frequencies involved ISDN is generally only available if you are close enough to your telephone company central office to result in a wire run of less than 18,000 feet. Although the standard RJ-11 telephone cables and jacks can be used the telephone company recommends that IDSN lines be wired with the larger RJ-45 jacks and cables to prevent someone from accidentally plugging IDSN equipment into a standard telephone line (might damage your ISDN equipment).

Some ISDN equipment is designed to plug directly into the two-wire ISDN line using a "U" interface. This is suitable if the equipment will be the only item connecting to the ISDN line which is probably the case. Other equipment is designed to interface to an "S/T" interface. The S/T interface is a 4 or 8 wire (RJ-45 cables) interface designed for use where multiple equipment such as digital telephones will be connecting to the ISDN line. An additional piece of equipment, the NT-1 adapter, is needed to connect between the ISDN line and S/T equipment. Most SOHO applications will use a single piece of equipment connected directly to the ISDN line; if so make sure your equipment has a "U" interface.

Your IDSN "modem" or router with "U" interface will have a RJ-45 jack but you can plug a RJ-11 cable into a RJ-45 jack.

An ISDN line can be used to establish two 64,000 bps connections which can be either voice calls or 64,000 bps data calls to/from other ISDN lines. The 16,000 bps channel is used by your ISDN equipment to exchange signals with the central office regarding number to dial, receive calls, etc. You can have two standard telephone numbers associated with your line. The primary number can accept one or two data calls from ISDN lines or one call from an analog phone. The secondary number can make or receive an analog call.

ISDN equipment at the user end can take the form of an ISDN "modem" which is functionally and physically similar to an internal or external modem, or ISDN "router" designed to connect an existing Local Area Network (LAN) to an ISDN line.

Your ISDN equipment (I would recommend external equipment vs computer card) may have two RJ-11 jacks which can be connected to two in-house analog lines for phones, faxes, answering machines, etc. To do this the equipment has digital to analog and analog to digital converters to simulate the telephone company office functions which would have been provided on analog lines. Incidentally you can make a modem call through such an analog port if you need to contact an ISP or other digital service that doesn’t support ISDN. You should even be able to operate a 56K modem through such an analog port.

Your equipment may convert touch-tone signals into the digital signaling required for outgoing analog calls and may generate ringing signals to activate phones, faxes, and answering machines when receiving an incoming analog call. Two digital calls to the same number may be "bonded" by your equipment to produce a 128,000 bps connection if your ISP also supports this. Keep in mind that functionally this is two telephone calls and will cost twice as much if it is a long distance connection.

Your equipment may have the ability to handle extra features on analog calls such as call waiting, call forwarding, caller ID, etc. if you subcribe to these services.

Your equipment may have "Bandwidth On Demand" (BOD) capability. With BOD your equipment will establish a second 64,000 bps connection to your ISP if data is flowing to/from the Internet at a rate of greater than a preset amount (say 50 kbps) averaged over a given period (say 10 seconds) . In other words if it looks like you could benefit from a second channel it will set up a second channel. The second channel will be disconnected after a set number of seconds if demand falls below a set amount Your equipment may also be set up so that if an analog call comes in to either of your ISDN analog ports or someone picks up an analog line to make an outgoing call the second digital connection is dropped to make room for the analog call.

Your equipment may have Access on Demand capability in which a connection is automatically established to your ISP when any software requests to send a packet to the Internet and the connection is dropped after a set number of seconds elapses with no traffic. Because ISDN connections typically only take a few seconds to establish (much faster than modem) the connection can be easily made and broken as demand warrants.

Because ISDN typically has a per minute charge from your telephone company and your ISP may also have a per minute charge you will want to make sure that your internet software such as email programs, ICQ, Instant Messengers, and web browsers is not set up to automatically access the net unless you are sure you want it to. Your equipment and software may provide access logs and/or lights showing when a B channel is accessed. You don’t want to find out your software has been playing with itself in the middle of the night and running up per minute charges. Windows 95/98 in normal operation can trigger unwanted calls if the network is not properly set up.

Connectivity software is available designed to take advantage of ISDN characteristics while performing LAN connectivity and IP spoofing functions. IP spoofing, also known as "single user account" allows multiple employees in a small office to simultaneously use a (cheaper) single user ISP account. Alternately an inexpensive stand-alone ISDN router such as the Zyxel Prestige 100 or Ascend Pipeline can be purchased which connects an Ethernet LAN to an ISDN line.

Telephone companies have several different kinds of computerized "switches" that handle calls. Your ISDN equipment must be able to be configured to interface with the particular switch used by your local telephone company. The Lucent 5ESS switch commonly used in North America can handle 64k (64,000bps), 56k, or DOVBS digital calls. With Digital over Voice Bearer Service (DOVBS) your equipment pretends it is making an analog call but then sends digital data to a cooperating ISP. This could be handy if your local phone company has different tariffs for "voice" calls than digital calls but doesn't mind if you use DOVBS.

Note that ISDN equipment and software varies widely. Make sure the equipment or software you get does perform the functions you need.

Since many phone companies send voice as a 56k stream it is apparently not unusual for there to be problems with 64k transmission. If your ISDN doesn't work or stops working, try connecting at 56k. If it works at 56k but not 64k it is probably a telephone company problem.

Single channel 56k ISDN is typically noticeably faster for web surfing than a 56k modem probably partly due to reduced latency. Times for "pings" (round trip message time to a remote site) seem significantly shorter which is especially important if you are using realtime games, voice on net, or other applications which need short latency. Of course your mileage might vary depending on your ISP.

Keep in mind that if you lose power or your ISDN equipment fails you will lose the analog phone capability through your ISDN equipment. The phone company recommends that you have at least one analog line. You might want to buy an uninterruptible power supply for your ISDN equipment.

Copyright 1997, 1998 Azinet LLC

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