 |
What a difference a phone line makes.
Because of the enormous number of hardware types and uses and purposes of digital ISDN communication, the process of ordering a line may be a bit frustrating. The B channel is basically a phone line, almost no one gets just one B channel. In fact the prices are strongly in favor of what is called an ISDN BRI (Basic Rate ISDN). BRI's consist of 2 B channels plus 1 D channel. Your other B could be voice or circuit or both, depending on what hardware you may have or what you want to do. Voice B's are generally used for voice communications (ie. regular phone calls). CircuitB's are generally used for high speed data (ie. Internet access). The D channel is used to dial your ISDN line, (dial tone and dial up.)
Another ISDN line is called a Primary Rate Interface (PRI). Inside North America and Japan, this consists of 24 channels, usually divided into23 B channels and 1 D channel, and runs over the same physical interface as T1. Outside of these areas the PRI has 31 user channels,usually divided into 30 B channels and 1 D channel and is based on the E1 interface. It is typically used for connections such as one betweena PBX (private branch exchange, a telephone exchange operated by the customer of a telephone company) and a CO (central office, of the telephone company) or IXC (inter exchange carrier, a long distance telephone company).
How it is possible to get 128Kbps. (NT-1)
It is possible under certain circumstances to take two circuit provisioned B's and connect simultaneously to two other circuit B's to obtain 128Kbps data speeds. Unlike your regular phone line, ISDN requires a special piece of equipment called an NT-1, NT standing for Network Terminator.
In the United States, the NT-1 is purchased by the consumer and generally comes in two varieties, internal and external. ExternalNT-1's are little devices that plug into the wall jack with a thicker kind of phone cord. They cost from $200-$300 and usuallyhave multiple jacks. The advantage of externals is you can easily add multiple devices to your ISDN lines. This is the same as having twophones around the house sharing a single phone line. For simplicity and some cost savings, most ISDN devices come with the NT-1 built in. Generally manufactures recommend ISDN routersand modems with internal NT-1's for these reasons. However, if you plan to expand, you need to decide this up front. Fortunately,most brands of ISDN devices now have internal or external NT-1's as an option at usually a $100 difference.
If you want to purchase an external NT-1, be aware there are a significant number of kinds of them. Ask your salesperson aboutyour options and costs. Some hardware manufacturers interchange products with or without NT-1's and some don't. In general, you purchase an internal NT-1 if all you wantis high-speed Internet access and will connect just one computer or network.
How a Network Terminator is used.
At the customer premises the U-loop is terminated by an NT1 (network termination 1) device. The NT1 drives an S/T-bus which is usually 4wires, but in some cases it may be 6 or 8 wires. In these optional cases, the extra wires are used to provide power to operate telephoneswhen normal power fails. Alternately, 'phantom' power may be derived from the standard four wires. Outside of North America emergency modeoperation provides power for basic voice service only in the case of loss of local power. In emergency mode operation the NT1 receives upto 1.2W from the central office. In North America there is no provision for emergency mode operation.
More about BRI's. The length of wiring makes a big difference.
A Basic Rate Interface (BRI) is two 64K bearer ("B") channels and a single delta ("D") channel. The B channels are used for voice or data, and the D channel is used for signaling and/or X.25 packet networking. This is the variety most likely to be found in residential service. An ISDN BRI U-Loop is 2 conductors from the CO (telephone company central office) to the customer premises. Its maximum length may be5.5 km (18000 ft). The equipment on both sides of the U loop has to be carefully designed to deal with the long length of the U loop and thenoisy environment it operates in.
This picture shows what a residential ISDN connection looks like.
 |
The name of the S/T bus comes from the letters used in the ISDN specifications used to refer to two reference points, S and T. Point Trefers to the connection between the NT1 device and customer supplied equipment. Terminals can connect directly to NT1 at point T, or theremay be a PBX (private branch exchange, i.e. a customer-owned telephone exchange). When a PBX is present, point S refers to the connectionbetween the PBX and the terminal. Note that in ISDN terminology, "terminal" can mean any sort of end-user ISDN device, such as dataterminals, telephones, FAX machines, etc. Telephones and fax machines can be attached to the BRI, when they have the proper interfacehardware and software.
T Bus (Passive Bus) Information.
The T bus is a multipoint bus in this configuration. It is sometimes called the passive bus because there are no repeaters on the linebetween the NT1 and the devices. It can be implemented using the same cable and connectors as is 10 base T Ethernet (Networks). There may be up to 8devices on the S/T bus. The bus may be formed with splitters and T connectors - it is a bus, not a star. The D channel is used to controlthe attachment of the one to eight devices to the two B channels. No two devices attach to the same B channel at the same time.
Why a NT-1.
In this configuration, the major function of the NT is to allow more than one device to have access to the 2 B channels provided by the ISDNBRI. For instance, you may have an ISDN telephone, an ISDN fax and an ISDN computer interface attached to the BRI. Each device can listenfor calls and only connect to a B channel when it identifies a message requesting a service it can provide.
 |
The NT1 also performs other functions.
The NT1 only implements part of the channel sharing scheme; the other devices participate as well, and the communication protocol used by theNT1 and the other devices is an integral part of the scheme. The NT1 also performs other functions; it translates the bit encoding schemeused on the lines between it and the telephone company (the U loop) to the encoding used between it and the devices. These schemes aredifferent because the device to NT encoding was designed to enable channel sharing whereas the NT to telephone company encoding was designed to allowtransmission across long distances with no sharing.
Termianl Adapters
Equipment known as a Terminal Adapter (TA) can be used to adapt these channels to existing terminal equipment standards such as RS-232 andV.35. This equipment is typically packaged in a similar fashion to modems, either as standalone units or as interface cards that plug intoa computer or various kinds of commmunications equipment (such as routers or PBXs). TAs do not interoperate with the modem; theyreplace the modem.
There may be cases where there is no need to interface to existing terminal equipment or to emulate exisiting terminal equipment, or theremay be equipment all ready in place with a synchronous interface present. In these cases, standalone units or computer interfaces can provide high speedsynchronous connections to the B channels without converting to an asynchronous standard.
Bridges.
Another common type of equipment can be used to implement a bridge between local area networks using the ISDN channel to transport thedata. These devices typically provide features such as demand dialing and/or data compression.