Telephone companies developed ISDN with the intention of creating a totally digital network. ISDN was developed to use the existing telephone wiring system, and it works very much like a telephone. When you make a data call with ISDN, the WAN link is brought up for the duration of the call and is taken down when the call is completed; it's very similar to how you call a friend on the phone and then hang up when you are done talking.
ISDN allows digital signals to be transmitted over existing telephone wiring. This became possible when the telephone company switches were upgraded to handle digital signals. ISDN is generally viewed as an alternative to leased lines, which can be used for telecommuting and networking small and remote offices into LANs.
Telephone companies developed ISDN as part of an effort to standardize subscriber services. This included the User-Network Interface (UNI), which is how the screen looks when the user dials into the network, and network capabilities. Standardizing subscriber services makes it more possible to ensure international compatibility. The ISDN standards define the hardware and call setup schemes for end-to-end digital connectivity, which help achieve the goal of worldwide connectivity by ensuring that ISDN networks easily communicate with one another. Basically, the digitizing function is done at the user site rather than the telephone company.
ISDN components include terminals, terminal adapters (TAs), network-termination (NT) devices, line-termination equipment, and exchange-termination equipment. ISDN terminals come in two types, Type 1 or Type 2. Specialized ISDN terminals are referred to as Terminal Equipment type 1 (TE1). Non-ISDN terminals such as Data Terminal Equipment (DTE) that predate the ISDN standards are referred to as Terminal Equipment type 2 (TE2). TE1s connect to the ISDN network through a four-wire, twisted-pair digital link. TE2s connect to the ISDN network through a TA. The ISDN TA can be either a standalone device or a board inside the TE2. If the TE2 is implemented as a standalone device, it connects to the TA via a standard physical-layer interface.
Beyond the TE1 and TE2 devices, the next connection point in the ISDN network is the Network Termination type 1 (NT1) or Network Termination type 2 (NT2) device. These are network-termination devices that connect the four-wire subscriber wiring to the conventional two-wire local loop. In North America, the NT1 is a Customer Premises Equipment (CPE) device. In most parts of the world besides North America, the NT1 is part of the network provided by the carrier. The NT2 is a more complicated device, typically found in digital Private Branch eXchanges (PBXs), that performs Layer 2 and Layer 3 protocol services. An NT1/2 device also exists; it is a single device that combines the functions of an NT1 and an NT2.
Because Customer Premise Equipment (CPE) covers a wide variety of capabilities and requires a variety of services and interfaces, the standards refer to interconnects by reference points rather than specific hardware requirements. Reference points are a series of specifications that define the connection between specific devices, depending on their function in the end-to-end connection. It is important to know about these interface types because a CPE device, such as a router, may support different reference types; the reference points supported will determine what specific equipment is required for purchase.
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KEY CONCEPTS |
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ANALOG |
Signal transmission over wires or through the air in which information is conveyed through variation of some combination of signal amplitude, frequency, and phase. |
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DIGITAL |
Language of computers comprising only two states, on and off which are indicated by a series of voltage pulses. |
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INTEGRATED SERVICES DIGITAL NETWORK (ISDN) |
Communication protocol, offered by telephone companies, that permits telephone networks to carry data, voice, and other source traffic. |
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USER NETWORK INTERFACE (UNI) |
ATM Forum specification that defines an interoperability standard for the interface between ATM-based products (a router or an ATM switch) located in a private network and the ATM switches located within the public carrier networks. |