ISDN at Home

ISDN for Home Users

Note: this is kind of old info, but is left here for historical interest. Most people will be much better off using xDSL.

Here's the short story on ISDN:

ISDN comes into your house in a single pair, just like a regular phone line does. You can have a 1D ISDN line that has one phone number and supports 64KB/sec data or a 2D ISDN line that has two phone numbers and supports 128KB/sec of data.
You plug the ISDN line into an ISDN router using an ordinary telephone cable (silver satin).
Two modular phone jacks are provided on the router; these are bound to the 2 phone numbers that are assigned to an ISDN line. You can connect one of these jacks to the wiring that goes to all your regular telephones. You can use the phone line and data at the same time if you have a 2D line. The telephone line "uses up" one of the two ISDN channels, so your data drops to 64KB/sec. The router handles this automatically.
Router usually has a built-in hub with four or more RJ-45 connectors for Ethernet. Plug your computers into these. Otherwise, you need a separate hub (~$50) or a cross-over cable if you're only connecting one computer.
You program the router with your account setttings from your ISP: phone number to dial, user name and password, and some other stuff. When any computer requests a tcp/ip connection, the router will automatically dial the ISP and establish a connection in about 3 or 4 seconds.
Router will probably support DHCP and NAT. This allows you to set each of your computers to "obtain IP address automatically". The router will pass out IP information to each computer and manage all the traffic properly. That way you don't have to get a block of IP addresses from your ISP.

Why ISDN?
What is ISDN?
How do you get an ISDN connection?
Hooking it up
How Good Is It?

Why ISDN?

I put ISDN in because I always have to be the first on my block with anything techie. I'm the only one I know with DirecTV dish too. I can't stop.

Other less important reasons:

Reasonable rates were available from Pac Bell
High performance - up to 128Kbits/sec, relative to 28.8Kbits/sec on a modem.
Fast, quiet connections

What ISDN Is

ISDN is a method establishing a digital connection between two points in the telephone company's switched network. An ordinary telephone makes an analog connection between two points on the network: you dial to select the receiving point, and, once the connection's established, you talk. Computers need to exchange digital data - 1s and 0s - instead of sounds. To accomplish this, modems convert digital data into analog sounds to go over the telco's voice system. Just like a telephone, the modem dials to establish a connection (with another modem). Then, instead of "talking", it "whistles". The whistles are a representation of digital data, and can be converted back to digital by the modem at the other end.

The phone companies long ago stopped actually making analog connections between phones. When your voice or modem-whistles get to the phone company, they promptly digitize the sounds and then transfer the digital information to the receiving phone exchange. At the receiving exchange, the digital info is converted back into analog sounds. All the connections between exchanges are digital.

[computer]-digital-[modem]-analog-[telco]-digital-[telco]-analog-[modem]-digital-[computer]

Since the phone company wants to send many phone conversations over a single wire or connection, they severely limit the "bandwidth" that a voice line can consume. The less bandwidth a connection occupies, the more conversations can be sent over a single connection. That's why an ordinary voice line can only support 28.8 modems. That's the fastest that digital data can be transferred using "whistles" down the wire and still stay inside the bandwidth allocated to an analog phone line.

ISDN is different. Instead of transmitting sounds, it skips the analog part and makes a direct digital connection to the phone company.

[computer]-digital-[ISDN]-digital-[telco]-digital-[telco]-digital-[ISDN]-digital-[computer]

Your bandwidth limitation is mainly controlled by what a pair of copper wires can carry between your house and the telco - typically a distance of few miles. (Longer distances means the telco has to upgrade wires, which makes them very very unhappy.) Of course, it's also controlled by the quality of the equipment at both ends (yours and the telco) and by how much you pay the phone company to carry your traffic.

The standard ISDN connection for home users is called "Basic Rate". It is carried by a single pair of copper wires, just like an analog phone line. Most people already have 2 pairs coming to their home from the phone pole. One is used for your phone connection; the other is for a second phone line or, more likely, not used. If you only have one phone line, the ISDN can come in on your other pair. If you have two, then the phone company will have to string another line, which will probably upset them or you, depending on who pays.

Basic Rate ISDN divides the signal on this single pair into 3 channels: two "B" channels that can carry 64Kbits/second each, and a "D" channel that is lower bandwidth and is used for signaling and control. So "1B" connections can carry 64Kbits and 2B connections can carry 128Kbits. With ISDN, it is possible to use one or both of the channels. Most ISPs charge based on total connect time per B channel. That is, 20 hours of 1B costs the same as 10 hours of 2B. Good ISDN hardware can be "demand sensitive"; it starts out using 1B, and then if the channel maxes out it kicks in the second B to give you twice the bandwidth.

More details kindly submitted by a johnscloset fan:

ISDN, you get 2 B channels at 64 kbps for data, and 1 D channel at 9600 bps for signaling.
Each B channel can carry voice traffic too, either of the B channels can be used for either purpose simultaneously, and if you have a cool ISP, you can use both B channels at the same time.
The other thing is PRI (Primary Rate ISDN) has 32 64k channels (2 mbps) and is often known as an E1. You can either have 30 B channels and 2 D channels, 31 B channels and 1 D channel, or 32 B channels and inband signaling.
B channels are always data of some sort, and D channels are always signalling. You can use any of the B channels for anything on a PRI, provided you ask the telco to support it. For example, some places have 4 B channels as a "leased line" internet connection, then say 8 B channels connected to a switch for their phone system and the rest spare. nb: I've seen T1's used like this too - john
cool huh?

Connections

Once the phone company provides you with an ISDN service, you need to connect something to it. A phone won't do. You must connect:

A NT1 network termination interface. This converts your 2-wire ISDN into a 4-wire digital service. The 4-wire service can be run around your house like a phone line, and multiple things can be connected to it, provided they have ISDN S/T interfaces (4-wire). The ISDN line from the telco MUST go directly into an NT1 interface.
AND
An ISDN "modem". This is a misnomer, since it doesn't whistle, like a modem does. It is a interface between your computer's serial port and the ISDN NT1 interface. Some ISDN modems have the NT1 interface built-in, so you can just plug in the ISDN line. You won't be able to connect anything else to the ISDN line if you do this. ISDN modems cost from $250 (Motorola Bitsufr) to $650.
OR
An ISDN router. This is an interface between the ISDN line (may or may not have the NT1 interface built-in) and an Ethernet. Most popular models are from Ascend, makers of the Pipeline series. They cost around $400, with the software you'll probably need. A very user-friendly router, especially in a Macintosh environment, is the Farallon Netopia.

Phone Numbers and Analog lines

If you get the full two-channel ISDN service, you will generally be assigned two phone numbers. (It depends on your telco's switch). Your ISDN interface may also have two regular RJ-11 modular phone plugs on it for analog phone lines. You can assign each analog plug to one of the two phone numbers for incoming calls, and you can use these lines just like any other phone line. You can run 128Kb data, or 64Kb data plus an analog call, or two analog calls simultaneously. If you are running data over the ISDN, and you have two channels available, you can pick up either of the analog phone lines and the ISDN hardware will send your analog traffic over the unused B channel. If you have an incoming call on one of the lines, the ISDN hardware will see if it's data, and if it isn't will "ring it through" to the assigned analog plug. If both B channels are in use, it will drop one of them, ring the analog call through, and then reconnect the data call when the analog call hangs up. Pretty slick, huh?

Connecting it up

OK, this is important: ISDN is easy to hook up.

The ISDN line that comes to your house is an ordinary phone pair. You need to route it to a phone connector close to your ISDN router/modem using your ordinary phone wiring. Most likely you will use the Yellow/Black pair, while your regular voice line uses the Red/Green pair.

Connect the pair carrying the ISDN signal to the Center pins of an ordinary RJ-11 telephone socket, NOT an RJ-45 data socket. The center pins are the ones labeled red and green. Use an ordinary telephone connector cable to connect the RJ-11 plug in the wall to your ISDN equipment.

Finally connect the RJ-45 Ethernet connector on your router to your Ethernet hub or, using a cross-over cable, to the Ethernet connector on a single PC. Many routers now include a hub, so you have 3 or 4 RJ-45 connectors on the back of the router.

So How's It Work?

Well, pretty good.

The connection is made so quickly (and quietly of course: no dialing) that it's almost like being on line all the time.

The performance is highly dependent on time of day and where you're going. During the day, the network itself seems to limit performance to about 2Kbits/sec, equivalent to a 28.8 modem. When traffic is less of a problem, I seem to get about 9 to 11 KBytes/sec from most sites. Many sites, however, are simply not capable of delivering information that quickly. There's always going to be some stuff that comes over at a few hunded bytes per second.

On the whole, I'm happy with the performance.

After getting a few bills, I can see why Pac Bell wants to do something about the rates. There are two of us here on the network, and we both are on line a fair amount. The March 1996 bill looks like this:

      Rate      Calls    Initial Mins     Addl Mins     Charge
      -------- -------   ------------     ---------     -------
Home ISDN basic rate
   includes base rate, taxes, etc.                        26.10
Measured Service on First B line
      Day        162           162           488          10.51
      Evening    171           171           781            .00
      Night      177           177           952            .00
Second B Line (kicks in when first line is at 80% of capacity)
      Day         32            32            21           1.29
      Evening     28            28            15            .00
      Night      121           121            21            .00
Call Allowances
    (Huh?)                                                (3.00)
                                                           =====
Total Bill                                                 34.90
                                                         = $0.0029/minute

Just for reference, this is a total of 2969 minutes, or just under 50 hours. Prior to having ISDN, we probably averaged 100 to 120 hours a month of 28.8 connect time.

Another issue that's raised its head is line capacity. When I started, every connection went through fine. Now, however, I sometimes fail to connect. This is the ISDN equivalent of a busy signal, and it results from too few lines between the telco and the ISP (a high capacity line called a "PRI"). My previous provider, Scruznet, had this problem so badly I switched providers; my current provider, Internex, has the problem too, but not as badly. PacTel is giving the ISPs very long lead times (up to 6 months) for adding capacity. Could there be some non-technical reason? Could PacTel be getting ready to provide ISDN service themselves? Inquiring minds are a bit suspicious.

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Mail John

6 Sept 2000