ISDN

Contents:

Overview of ISDN
What is ISDN
Access Interfaces
     Channels
     Signaling
Network Interface Demark Points
    R: Legacy Equipment
     U: Local Loop
     V: Central Office Connections
Basic Rate Interface
     U Interface of the BRI
     2B1Q Line Coding
S/T Interface of the BRI
     Wiring Topology
Primary Rate Interface
     High-speed DSL
     NT and CPE at the PRI
     Physical/Electrical Interface
     Signaling Procedures
Ordering ISDN Lines
Configuring for Terminal Parameters
       Number of Channels
       Bearer Services
       Directory Number (ISDN Phone Number)
       Your Calling Party Number
       Subaddress Information
       Calling Number Delivery
       Early Cut Through
       IXC Presubscription
       Protocol Compatibility Information
       Number of Terminals
       Service Profile Identification (SPID)
       Terminal Endpoint Identifier
ISDN Ordering Codes (IOCs) for NI-1
       Generic Ordering Codes for NI-1
       Proprietary IOCs (NI-1)
ISDN Ordering Codes for NI-2
       Basic NI-2 Package
       Feature Modules for NI2-1

ISDN is a digital transmission and switching capability of a network. Standards documents assume it is a public network, but switches with the same functions could be installed in private networks.

The essence of ISDN is the ability to link your network access line to anyone else's access line, and to make that connection on demand. "Anyone else" could be:

A key component of ISDN BRI is the local loop transmission technology, 2B1Q. This stands for "2 Bits per 1 Quaternary," a way of coding two digital bits into each voltage change on the line (baud). The goal achieved with 2B1Q is to eliminate amplifiers and repeaters in the local loop for digital services. With only copper pairs in the outside plant, ISDN service is easier to install and maintain compared to other services (like traditional T-1) that needed loop repeaters. And 2B1Q makes ISDN available to almost all telco customers on the wires now in place. The result is seen in the low tariffs for BRI access.

While most links between access lines are circuit-switched, a secondary ISDN characteristic (used more outside the U.S.) is to switch a user's packet data on the D channel to some other location on the packet network (which may or may not be connected via ISDN). For packet data service on the D channel, the carrier must have sufficient capacity in the packet handlers that terminate those D channels in central offices. Because that capacity usually isn't in place, D channel data in the US most often handles only short transactions like credit card verification. TOP

Unfortunately the two names (basic and primary) have almost the same common meaning. Think of Basic as being closer to a B channel, so it is the smaller (2B+D). Primary is the other one, the larger capacity access Pipe. TOP

ISDN uses time division multiplexing (TDM) to create multiple channels on each access link between a user's site and the ISDN switch, usually in a telco central office:

This third type of channel is not available to end users directly, but may be affected by user actions. It is often ignored when discussing bandwidth and access line speeds. You too may choose to ignore it with no operational risk at all. TOP

To accomplish its circuit switching, ISDN relies on an "out of band" signaling system, based on data messages sent in the Data or D channel part of the access link. These messages are directed by customer premises equipment to the switch, which may take action on parts of a message and forward other parts to other switches in the network. Except for local calls, it takes multiple switches to complete a network connection.

ISDN out-of-band signaling between switches is Signaling System 7
(SS7/ previously known as Common Channel Signaling System, CCSS). It has been used for more than a decade. ISDN switches communicate with each other over SS7/ which is made up of 56 and 64 kbit/s links among packet switches (signal transfer points, STPs) devoted to signaling. Packet handlers in ISDN switches may also direct some user data from the D channel on an access loop to a separate packet data network.

ISDN signaling between the customer and the switch is similar but different: Digital Subscriber Signaling system 1 (DSS1).

US phone lines before ISDN used "in-band" or "channel associated" signaling. That is, the signaling is sent over the same channel as the voice. On analog lines, the channel is the wire pair and it carries either dial pulses, for rotary dialing, or "TouchTone" (technically, dual tone multifrequency or DTMF) for "push button" dialing. When a phone goes "off-hook" (to place a call) the switch returns dial tone. Any pulses or DTMF tones detected by the switch during and immediately after dial tone are considered dialing instructions. TOP

The CCITT (predecessor to the ITU-TS) created a reference model for ISDN access loops. The model defines certain points between the customers' equipment and the carrier's ISDN switch. The same model fits both the basic rate and primary rate interfaces.

What happens across each of these Demarcation points is the subject of extensive technical specifications. Even an incomplete library of ISDN-related specifications occupies more than 5 ft of bookshelf. In fact, it is these specifications for functions or the "functional groups" between adjacent demarcation points that define the ISDN.

Functions at the customer premises are assigned to network termination (NT) equipment and/or terminal equipment (TE). The internal structures of the NT and TE are not specified—only the functions they perform and the interfaces to other equipment or the network. Hardware vendors are free to implement NT any way they (or their customers) want. Functional groups may be combined into a single device which might, for example, hide the S/T demark points from the customer. TOP

The huge amount of pre-ISDN equipment now in operation cannot be changed out immediately. There will be a need to accom modate legacy interfaces for a long time, perhaps indefinitely.

The ISDN model recognizes the need to work with older terminal equipment (TE-2 in ISDN terminology) by offering the R demark. This can be any data or voice interface: RS-232/ analog voice FXS/ V.35/ fax machine, etc. In a sense, these are not part of the ISDN—older CCITT/ ISO/ and ANSI standards cover them.

The R interface is provided by a terminal adapter (TA) that connects on its network side to an S interface of an NT-2. Some TAs may include the NT-2 functions, which allows them to connect to a T interface on an NT-1. Some TAs will include the NT-1 functions as well, presenting a U interface directly to the network.

How TE-2 output is converted to ISDN formats is up to each vendor. But the resulting format sent to the ISDN network is very strictly defined at the other demark points
(S/ T/ U). TOP

When terminal equipment is called "ISDN," or TE1/ there is usually an S or T interface on it (some have a U interface). The ISDN phone, PBX/ or other customer premises equipment (CPE) may still need an external NT-1 or NT-2 device, or both, between it and the local loop from the carrier.

Outside of the U.S. and Canada, the carrier provides the NT-1, or more. It is the S or T interface that marks the end of the carrier facilities and the start of a customer's own equipment. It is here that the S/T interface on CPE makes perfect sense—it lets the CPE plug directly into what the carrier provides.

For example, Germany provides an S interface as the standard service. In other countries, it is the T interface that is the demark point. Because of the S interface point, in Europe the Basic Rate Interface (BRI) is known as SO; the Primary Rate Interface (PRI/ 30B+D), as S2.

The ISDN specifications as late as 1988 assumed that all customers would have service provided at the S or T interface, not the U interface. In the U.S., by contrast, the Federal Communications Commission prohibits the local exchange carrier (LEC) by from providing customer premises equipment—not even NT-1—as part of the service. When the customer has to provide NT-1, the interface to the network (demark) becomes the U point. TOP

The U interface in the US is the demarcation between the public network and CPE. It is here that the physical layer transmission format is standardized on 2B1Q for the BRI.

2B1Q encoding reduces by half the number of "voltage changes per second" (baud) needed to transmit a given number of bits per second (bit rate). That is, the bit rate is twice the baud rate. Baud rate determines signal attenuation, so lowering the baud rate allows the signal to travel farther on local loop copper wires. The beauty of 2B1Q is that it allows the great majority of existing local loops (up to 18/000 feet long, or 1400 ohm loop resistance) to carry more than twice as much traffic in digital form (BRI) as they carry in analog form. This covers 99% of US local loops.

The PRI in North America is based on the T-1 extended superframe (ESF) format with B8ZS (bipolar with 8-zero substitution). The bit rate and baud rate on the PRI are the same (1.544 M/s); each pulse represents one bit. In this same form at the S/ T/ and U points, two pairs of wire carry 24 channels in the traditional TDM format.

It is ironic that the T-1 signal is more and more often delivered via 2B1Q technology in the form of High-speed Digital Subscriber Loop (HDSL) transceivers, rather than CSUs. As a local loop technology, the U interface with 2B1Q signal format of HDSL offers many advantages for "high speed" data (up to about 750 kbit/s full duplex on a single twisted pair)—even without a real ISDN switched network. HDSL-2 offers the same speeds on a single pair.

Some PTTs use BRI equipment for leased lines of 64 kbit/s. A few US carriers are using the technology to provision higher speed services (T-1 and fractional T-1) without incurring the costs of repeaters in the outside plant. Over short loops, the U interface can carry enough information for compressed video. Some experiments in "dial-a-movie" used ISDN technology. TOP

There must be some device at the central office to terminate the local loop, generically called the Loop Termination (LT). An LT could be built into the switch. In many digital services (including 56K DDS and T-1) the LT is a specialized device that converts the signal from the loop to a voltage level, pulse shape, and impedance appropriate for distribution within the CO. A T-1 line signal is converted to the DSX-1 format, for example, by an office channel repeater or similar device. In effect, the LT is a CSU.

If the LT for a BRI or PRI is not built into the ISDN switch, then the interface between LT and switch is called the V interface. However, since this point is by definition within a CO and never available to a customer, its detailed specification is outside the scope of this book. Note that this demark point V is not one of the "V series" interfaces defined by CCITT/ITU—those are serial data ports and modems. TOP

Residential and small business customers find the capacity of the basic rate interface suited to their needs. Two bearer channels provide voice circuits, simultaneous voice and data, or may be combined into a single data channel of 112 or 128 kbit/s. It is common practice to install multiple BRI lines into a location.

Any ISDN service allows for an unrestricted digital connection (any bit pattern, including all Is and all Os) at 64 kbit/s, at least locally. Now that interconnections between carriers are almost all "clear channel," only rarely is a call between networks in the US restricted for various reasons, like a transmission line with a Is density requirement, or the potential for a switch to insert robbed bit signaling into the data stream. These cases limit a user to 56 kbit/s per B channel, rate adapted to the full 64 kbit/s.

In addition to bandwidth for the bearer channels and the D channel, there is also loop overhead for synchronization and testing (maintenance or M channel). Each of the various functions is assigned a channel based on time division multiplexing. Like T-1 multiplexing in the traditional digital hierarchy, the bit flow is organized into frames and superframes. For the BRI/ frame format varies by direction of transmission and demark point. TOP

In most of the world the ISDN customer never sees the U interface. The carrier provides the NT-1 device so the service is based on the S or T interface.

In the U.S. and Canada, the carrier presents the U interface on an 8-position modular jack on the end of the local loop. Customers furnish an NT-1 that meets the U-interface specifications (also at a modular jack). An Swire (4-pair) cord with two modular plugs connects them. NT-1 may be powered locally on the U connector.

Basic Rate Access (BRA) was designed to operate over existing analog voice-quality local loops. There is supposed to be no need for special pair selection, conditioning (removal of bridged taps), etc., for loops up to about 18/000 ft. long. Loading coils used to shape frequency response on long loops must be removed. The design limit is a signal loss of about 42 dB from the original signal of 13 to 14 dBm.

The ability to send more than two digital channels in place of one analog conversation depends on the way the digital signal is encoded. TOP

The BRI line signal from the network is coded in "2B1Q". Each pair of consecutive bits is coded into one of four (Quaternary) values. "Quat" is shorthand for a voltage level and the two bits it represents. There are four different quats to denote the four possible pairs of bits/ numbered for reference as ±3 and ±1. Their nominal voltages are defined in a 3:1 ratio, specifically at ±2.5 and ±5/6 V at the output of a transmitter.

This format is also called pulse amplitude modulation (PAM) because the size of the pulse conveys as much information as its polarity. The two bits in each quat, from this viewpoint, are the sign (polarity) and magnitude of the transmitted pulse.

An oscilloscope trace won't show the translation of bits to quats exactly as outlined for two reasons:

Both the central office and the customer equipment transmit at the same time, full duplex, over a single wire pair. A "hybrid" circuit, similar to the one in an analog telephone, couples both transmitter and receiver to the same wire pair. Loop current passes through this circuit, and may be fed to the rest of the NT-1 for its power. TOP

Keep in mind that NT-1 and NT-2 are "functional groups" that need not be separate physical devices. At each interface a functional group will provide bit timing (clocking recovery, based on the signal received from the network); framing (from unique bit patterns); delineation of B and D channels and octet timing
(based on framing); D channel access procedures (signaling); and power feeding.

Much of the difference between S and T is in the number of devices that may be attached to each.

Polarity of the two other wire pairs must be maintained, in case they are needed to deliver d.c. power. Reverse power polarity indicates the source is the backup or reserve supply and may inhibit some TE functions.

The transmitter of the NT-2 toward the terminals may have several receivers attached. All of the terminals' transmitters (one per terminal device) connect to the single receiver on the NT-2. There are procedures (see below) at the S point to control access to the B and D channels by multiple terminals. TOP

Compared to the basic rate interface (BRI)/ the Primary Rate Interface is relatively simple. PRI has the same pulse shape, framing, rate, and other electrical characteristics at the U/ T/ and S reference points. Even the R interface seen by older terminal equipment could be a "plain" T-1 sharing these layer 1 characteristics (for the physical/electrical interface).

A DS-1 Primary Rate Interface (PRI) is divided into TDM channels using standard T-1 frames. The pulse shape is the same as that defined for the T-1 or the DSX-1/ the digital cross-connect found most often in central offices. This shape is essentially a square wave/ nonreturn to zero (NRZ/ or 100% duty cycle) pulse/ with a peak value of 2.4 to 3.45 V at the transmitter. Some 20% overshoot on the leading and trailing edges is tolerated within defined limits.

PRI requires the Extended Superframe (ESF)/ which has an embedded operations channel (EOC) and a CRC-6 for error checking (two more TDM channels) in the framing bits. The older D4 superframe doesn't have these features. The EOC carries alarm notifications, statistics, and error indications.

On a T-1/ time slot 24 is the D channel, if there is one present on the interface. A signaling messge on the D channel of one T-1 can control a call that passes through up to 19 other T-1 interfaces. That is how an H^ channel of 1.536 Mbit/s is managed.

On an E-1, signaling messages use TS-15 (actually the sixteenth time slot, as they are numbered 0 to 31). This is the same TS occupied by ABCD signaling bits when that form of signaling is used on an E-1. The first time slot (TS-0) carries framing codes and a small amount of other overhead.

Traditional T-1 did not allow more than 15 zeroes in a row. While Is are sent as pulses (of alternating polarity) Os are represented by no pulses sent. The receiver of a string of Os has to count bit time intervals with no clocking information from the sender. This can soon lead to errors. Voice meets the ones density requirement by never generating an all-Os byte; there are no long intervals without a clock reference. Data, however, may contain long strings of Os that are meaningful and must be transmitted faithfully.

What's different between the reference points is defined above layer 1/ in the functional groups between those points. At the physical layer, time slot 24 (the last DS-0 channel in the frame) is the same as the other 23, but it is dedicated to the D (data) channel used for signaling. Clock rate for the line signal at S and T must be extracted from the received signal at the U interface. This requirement arises from the time division nature of the transmission—the network switch must operate at the same speed in both directions. And there is no way to adapt slower rate data to fit the B channel except, in some cases, from the R to the S interface.

Unlike the BRI, where NT-1 extracts receive clock and generates transmit clock, "turning the clock around" at a PRI necessarily becomes the responsibility of the NT-2. The terminal equipment, if it has a T-1 interface, is also loop-timed, so it too sends at the same bit rate it receives.

NT-1 on a PRI is transparent to the data, clock, and framing. It acts as a repeater, not a controller, for timing.

On equipment that has more than one PRI to the same network, the recommendation is to derive clock from the physical interface that carries the D channel from the CPE to that network.

The connector at the demark is an RJ-48/ 8-pole modular jack. The terminal side equipment presents an RJ-48 plug on a cable/ which itself may plug into the terminal. Pinout is given in .

Powering the PRI also differs from the BRI or, more precisely may differ when defined. At this writing, no power is to be applied to the signal leads at the S or T interfaces. The NT-1 at the U interface must not apply power to the loop, but the CO may arrange with the customer to deliver power or loop sealing current. TOP

The assumption in the standards is that the PRI is provisioned over a traditional T-1 or E-1 transmission system. The T-1 transmitter and receiver in most CPE is designed to the DSX-1 specification. Since DSX was created for use within a central office, the distance limit is about 1250 ft.

T-1 loops normally terminate in a channel service unit (CSU) which acts as a regenerator of the "digital" pulses and performs loopbacks for maintenance. CSUs reliably send and receive data pulses over a distance of about 1 mile (1.6 km) on one twisted pair for each direction (dual simplex transmission). Local loops often are much longer, in which case they need powered repeaters in the outside plant. T-1 is relatively intolerant of wire gauge changes, bridged taps, and sloppy splicing. Pairs must be selected for quality or specially engineered. This proves expensive and slow.

In practice these days, a technology called High-speed Digital Subscriber Loop (HDSL) is increasingly likely to be deployed. Eventually it should replace the traditional T-1 and E-1 lines. HDSL may use the same line coding as the BRI 'U' interface, 2B1Q, but at higher rates. More sensitive receivers (than in T-1 equipment) further improve performance. There are forms of HDSL besides 2B1Q that are available, for example Carrierless Amplitude and Phase modulation
(CAP, a modem-like signal). HDSL-2 needs only a single pair.

2B1Q encoding can carry 1.544 Mbit/s for a mile or more on a single pair
(full duplex), rather than the 2 pairs for T-1. Reducing the baud rate from 772/000
(1.5 Mbit/s) to 386/000 (half a T-1) more than doubles the maximum loop length. At 260 kbaud (1 /3 a T-1) the distance can double again, reaching more than 5 miles over plain cable, without repeaters. As explained under BRI/ 2B1Q transmissions also tolerate gage changes and bridged taps. Consequently, HDSL is much easier and less expensive to install than T-1.

Currently shipping HDSL equipment may have one to four twisted pairs in the local loop. Each pair carries a 0.5 or 0.75 Mbit/s stream of user data, plus some overhead. At both ends of multiple wire pairs, DS-Os are mapped onto time slots of the PRI U interface, in a T-1 or E-1 frame, after removing the additional overhead from each link. LT and NT-1 see only the DSX-1 framing. The HDSL is transparent.

The same sort of HDSL technology is also sold as a short-haul "T-1 modem" or line driver, over a single pair. This version may be useful at the T and U reference points. TOP

Acronym overload aside it is time to look at the user side of the local loop. The functions between the U interface and the user may be in one device or several (NT-1, NT-2, TA/ terminal). The network sees only what happens at U, so that is where the specifications are concentrated. The devices at the user's site are called, collectively, the CPE (customer premises equipment. ).

The terminal, say an analog phone or non-ISDN digital phone, goes off-hook and dials a number. The CPE interprets the dialed digits and converts them into the correct message format to send to the network over the D channel. Most of the analog to digital conversion will take place in a terminal adapter (TA) function for an analog phone. The remainder of the interworking could be relatively simple, done in a combined NT-1 /NT-2, or perhaps aPBX.

At a PRI/ recall, the S and T formats are the same as the U format and NT-1 is transparent to clocking.

For most users, the important events happen at R/ where existing equipment attaches. The network cares only about U. What happens between R and U is simply 'magic' performed by the CPE vendors. TOP

NT-1 must receive bits at U with an accuracy of one error in 10/000/000 bits, or better. The required layer 1 functions for an NT-1 are available in chips from many vendors of merchant semiconductors.

Because of attenuation along the local loop of up to 16.5 dB (and another 1.5 dB in an extension cord) the receiver in the NT-1 must detect pulses that are much smaller than the nominal 3 V sent. To ensure reliability, they are tested to 16.5 dB (with an objective of 18 dB) below 2.25V.

S and T requirements are less stringent. Within a given premises, the attenuation over the cable should be relatively small. For those cases, the DSX-1 specification may apply, with its limitation of about 1250 ft. between devices (half that distance from each device to any cross-connect panel). However, the objective is to reach 3000 ft, which is close to the capability of a CSU. At this writing, Bellcore makes B8ZS only an objective, so S and T could use AMI encoding (without B8ZS).

Standards impose many additional requirements that influence the design of hardware. That is, you must assume that any CPE (NT) certified to meet standards has a certain electrical longitudinal balance, tolerance to jitter and wander, and can handle phase transients. Since the user can have no control over these factors they are not covered here. TOP

The NT-1 and -2 portions of CPE have some housework to do when connected and powered up for the first time:

Note that there may be more than one facility (ISDN PRI line) under the control of one D channel. The second PRI has 1.536 Mbit/s available for an H channel. If there is no redundancy for the D channel, the limit is two PRIs per D channel. With redundancy from a backup D channel on another facility, non-Facility Associated Signaling on one D channel may extend over 478 B channels (20 PRIs).

The more interesting part of the PRI CPE is in the NT-2 ^function group," in the software that handles call control procedures. There are vendors of ISDN software who license the source code to various hardware vendors. Thus the NT products from different manufacturers may be running the same software for signaling and other higher layer functions (management, for example). Common software should improve compatibility.

NT-2 software must distinguish among the various modes/ speeds, and bearer services when they are requested in a call setup message. Voice and audio (modem) services are particularly affected, in different ways.

The public switched telephone network (PSTN) delivers many types of information in the form of call progress tones, recorded announcements, and intercept messages. Even when ISDN signaling has an equivalent D channel message, the user may need to have the audible form. For example, a modem can detect and act on an audible busy signal. Attached to an analog port on the NT/TA/ however, a modem will never see the digital message for a busy signal (and wouldn't know what to make of it anyway). The NT itself could generate the busy tone for the modem, but the network delivers the information more easily by telling the CPE to cut through the B channel to the modem while the switch sends digitally encoded tone. The CPE easily converts any bit stream to an audible form, without interpretation.

Likewise the ISDN switch in the central office must be configured to anticipate the need to deliver audible information as well as the digital messages. This explains in part the complexity of ordering ISDN lines. On a pure data line, the ISDN switch might simply send the disconnect message with cause #17/ user busy. But for a voice or audio line it would first send a progress message to tell the NT that "further call progress information may be available inband" or "inband information or appropriate pattern now available," and put the busy tone on the B channel. TOP

For a basic phone line (Plain Old Telephone Service or analog POTS) these days you usually don't even get to choose rotary dial or touch tone any more—you get both automatically. ISDN lines are not so simple. There are differences in the proprietary "flavors" of ISDN on the central office switch. Then there are the many options on how the switch provides service features and which features the customer wants (and doesn't want).

Fortunately, most of the difficulty for end users disappeared quietly, invisible compared to the Y2K problem being solved at the same time. Where the worst case used to require the user to pick dozens of parameters to specify ISDN BRI service, the latest ISDN customer premises equipment has a single ISDN order code (IOC) that tells the LEC everthing needed to configure the line.

Even if you don't have the manual (and so don't have the IOC) all you need do is tell the phone company which equipment you have (model number, etc.), and perhaps the application, and they will match it to the specific IOC or profile recommended by the hardware vendor. TOP

ISDN access tariffs are all local. ISDN by its nature is local access to the central office: from your serving office your traffic goes on as part of a regular transmission system, almost always over optical fiber in the U.S. What the telephone companies charge for almost identical equipment, features, and functions has an astonishing range. Like POTS, ISDN may have a business rate and a (usually lower) residential rate. Then again, everyone may be charged the business rate, though a residential tariff is offered in many areas.

What happens in the future can't be known, but as predicted in the first edition of this book the trend is toward lower ISDN pricing relative to analog POTS and especially relative to older digital services. In fact, DDS leased lines and Switched 56 circuit-switched channels are generally going up in price. These last two services provide a subset of what is available from a BRI service, yet involve much more manual effort within the telco to install and maintain. For example, most DDS and Switched 56 provides 56/000 bit/s on a 4-wire local loop. BRI provides more than twice that capacity on a 2-wire loop. With easily deployed capability, ISDN BRI continues to be an important offering for LECs facing competition from independent DSL-based CLECs.

For the sake of their own long-term costs, all telcos will have to migrate customers from leased lines and special services (often provided on custom engineered lines) to a generic digital service, ISDN (or frame relay, or ATM/ or IP, or something newer). The way to convince users to change services is to show them how to save money for comparable performance or how to increase performance for comparable cost. Thus a BRI, to be attractive, should be less than twice the cost of a POTS line for voice. If so, it would be much cheaper than Switched 56, though this is not always the case as some LECs have charged more to carry an ISDN bit labeled "data" than a bit labeled "voice."

The Federal Communications Commission threw a scare into ISDN fans in early 1995 with a ruling that the subscriber line charge (SLC) be applied to each bearer and data channel, not just the 2-wire or 4-wire "line" that provides the interface to ISDN service. Several RBOCs requested a rescinding of the FCCs order, which was granted, but with reservations that it may be reconsidered in the future.

Originally, the SLC applied to the "line" which, in basic phone service, is a twisted pair of copper wires. That same copper converted to ISDN BRI supports three channels (2B+D). Two pairs are needed for a primary rate interface (PRI/ 23B+D). The SLC/ $3.50 per residential line or $6 per business line and rising, makes a considerable difference in the economics of ISDN service for most users. For T-1 access the difference is $1728 per year.

The SLC was created by the FCC and levied on customers to replace revenue lost by the local exchange carriers when access charges to long distance companies were reduced. The result was a shift of costs away from long distance callers at the expense of those who didn't make many LD calls. TOP

The plug-and-play level of most ISDN products in early 1995 was about the same as add-in cards on IBM PC-AT clones. There were many hardware options to select for the application. Then those same selections had to be reflected in the configuration of the ISDN line from the phone company

Help arrived in the form of parameter sets that can be specified with a single designation—an ISDN Ordering Code, For National ISDN-1 (NI-1) service the many lOCs represent an advance but still reflect the legacy of "you gotta configure it" from earlier ISDN versions. The real advance comes with NI-2: lOCs are defined, for up to two terminals, in the form of a base package (there is only one) plus up to six "feature modules" for functions like forwarding and voicemail. Details follow the discussion of NI-1 lOCs.

But assuming the worst case, and you must face the full configuration task, here are the factors to deal with. They apply on either the BRI or PRI/ though ranges of permitted values could vary; (or example, the number of B channels allowed in a connection. TOP

Under some tariffs, one version of BRI service is offered for D channel access only: the number of B channels is 0. At the other extreme is a PRI with a D channel that controls a second PRI with 24 B channels. That leads to a maximum of 47 B channels possible.

Each ISDN interface must have at least one DN/ but there is no limit on the number that may be assigned. A DM is associated with only one interface. One DN will be the default for the interface.

A basic rate line can have a different phone number for each B channel, or both may be considered a hunt group with a single "directory number." Carriers who anticipate exhausting numbers within an area code will appreciate saving some numbers where they are not needed for additional ISDN B channels.

Does your equipment have to supply a calling party number with each call request? Default is no, but the line may be configured to require such an information element in the call setup request message. This choice makes sense when many users (many phones) call out on the same interface and you want to identify the individual user (or station) to the network, for billing, determining subscription parameters (like presubscribed IXC), or other purposes.

Screening of the outgoing calling party number, for validity is performed by the network when CPN is sent. If not valid, the network will deliver the number to the called party anyway, but also adds the default DN for your interface.

You have a separate choice on whether you want your CPN presented to the called device. Default is yes, to permit ANI to function, but the selection can be changed with an information element in the call request.

Each user may have another identification, the subaddress information, in addition to the DN. It may be used at the terminating end of a call to route the connection over a private network or to a specific station.

The network wants to know if you plan to send or will accept subaddresses, either yours or the called party's. If you don't want or can't use it/ tell the network not to deliver any. TOP

Normally the network waits for your CPE to confirm that it is ready to receive information (the called extension is off-hook) before opening the channel from the calling party. But when your CPE offers in-band ringing tone while waiting for an extension to answer, you can configure the line to cut through early. This action allows the caller to hear the call progress information.

There is a separate selection of early cutthrough for (1) ISDN and (2) non-ISDN terminal equipment or private network devices behind the NT.

Each interface can designate an interLATA carrier or InterExchange Carrier (IXC). The network will attempt to route circuit mode calls originating here to this carrier. Packet mode calls are handled differently.

End terminals may have special functions or needs. To negotiate compatibility with the equipment called over the ISDN, a TE may send what is called high level and low level compatibility information in the SETUP message. The network does not interpret this information/ and can't act on it. However the SPCS that first takes the signaling message will verify the size and format of each information element, including the compatibility information.

The S interface point will support a passive bus. That is, the terminal-side transmitter on the NT-2 has parallel connections to as many as 8 TE-ls or terminal adapters. The count limit is imposed in part by the signal reduction as each attached TE or TA soaks up some of the transmission power. Too many devices will reduce the signal strength to the point where none of them gets a usable signal.

Under older ISDN practices, carriers would provision up to four SPIDs per BRI/ which are still available under some business line tariffs. National ISDN-1 limited the number of TEs per S interface to two. NI-2 also allows only two terminal configurations on a BRI. For simplicity, and for a lack of "pure" ISDN devices, the number in the US seems unlikely to reach 8, the international standard.

This is the unique layer-3 identification for each circuit-switched terminal device: phone, fax, computer port, etc. There may be more than one such device associated with an NT-1, NT-2, or B channel. For example, an ISDN TA with two voice ports, a serial data port, and an S/T interface would need unique SPIDs for each port so that someone could dial into them. That is, one SPID/ through user configuration of the TA, would identify a fax port, another SPID the data port, and so on.

The SPID is assigned permanently by the carrier at subscription time, to be unique on a switch. The number may be the 10-digit directory number (DN), if there is only one device or port per B channel, or the DN plus a prefix and/or a suffix. The exact form of the SPID is determined by the carrier and depends on the switch make, the software level (National ISDN stage or earlier software), and local practice.

The National ISDN Council fostered the adoption of a standard SPID format that simplifies assignment. The SPID is the DN (NPA-Nxx-XXXX) plus a four-number suffix to identify a B chan nel (2 characters) and a specific device on that channel (2 characters). Most equipment then can use a SPID of DN+0101 for most applications.

The SPID traditionally has been programmed manually into the TE and the switch in the CO. The switch uses it when the TE is first installed and initializes the layer-3 link (the TA sends its SPIDs to the switch as a form of identification). Enhancements in NI-2 let an unconfigured TA initialize itself and receive the SPIDs assigned to the line by the LEC—no manual configuration for the end user.

The switch from then on uses the SPID to provide the service (mode) appropriate to that device, even when it shares the DN with another device that needs a different mode. For example/ a phone and a router may share a line and a DN, but have different SPIDs to identify which is calling. TOP

Each TE must support a layer-2 terminal end point identifier (TEI) for each logical connection on the interface. TEI is part of the frame address (2nd octet) at the S/T interface. Individual NT-2 devices may select which frames to capture based on a match of a stored TEI with the frame address. The 7-bit field allows 128 values, but 127 is the broadcast address, always active. Of the remainder, 0 to 63 may be allocated manually, for PVC connections to a frame handler (packet switch).

TEI values from 64 to 126 form a pool from which the CPE and network negotiate a value to associate with each new switched connection they set up.

The complexity that up until 1994 was the despair of ISDN customers (as well as carriers and hardware vendors) is almost gone. Telcordia (formerly Bellcore), the National ISDN Council, and the National Institute of Standards and Technology (NIST) run a program with NIUF to predefine configuration sets that specify the values of all the possible parameters to be input into the ISDN

switch (the "switch translations"). Once defined, these pre-set configurations may be used by anyone.

Generic configuration sets fit broad categories of equipment in typical applications. Vendors can then build default values into their equipment that will interoperate with an ISDN line configured for the same set. It's close to plugand-play, and really there if the TA has an autoSPID feature.

The standard or "Generic" lOCs are widely implemented, replacing those carriers had made up ad hoc. Bell Atlantic, for example, had lumped all the decisions necessary to support the ProShare video conference system under the label "Intel Blue." Rather, Bell Atlantic had three such sets to match the type of switch (AT&T or Northern Telecom) and whether the ISDN "flavor" is proprietary (AT&T Custom) or National ISDN 1. A European version could be yet another set. Now one of the generic lOCs covers almost any need.

Documentation for terminal equipment should state the code (or codes, depending on application) to use. The installer of the equipment need tell the ISDN carrier simply which IOC is wanted. This should be as brief as a few letters, not the individual values for all of the parameters to be set in the switch.

The idea behind lOCs is to select line configuration sets that fit common applications. A given piece of terminal equipment may require different line capabilities (ordered with different codes) for different situations. A hardware vendor might suggest IOC Capability XX for a general purpose terminal adapter, but Capability YY when the same TA is used exclusively for an intense graphics file transfer or LAN interconnection application.

One reason that ISDN equipment had been relatively expensive was that buyers needed extensive technical support to configure and install it. That effort had to be paid for in the price of the hardware, even when the cost to manufacture was modest. As the need for support droped to the level required for a modem, prices for ISDN CPE likewise dropped. A very functional TA/ router with voice ports is only a few hundred dollars (in 2000).

There are three tracks at for defining configurations in terms of an Ordering Code for NI-1. They are distinguished by whether an IOC is associated with a specific vendor's equipment or is intended for a broad application (generic). If generic, there are lOCs that require strict conformaty (one or two letters) and those that require only compatibility (EZ ISDN and a number/letter).

EZ ISDN allows the TA to ignore some features configured on the line, but the TA must not interfer with operation of the switch. This idea is appealing and deserves to take over entirely the specification of ISDN services and the compatibility of CPE.

Groups of vendors within the NIUF defined core configurations that they saw as fundamental sets of ISDN line features. Each generic core capability registered was proposed to and considered by multiple organizations. There was consensus that the configurations serve a broad purpose—more than to meet the needs of one vendor's equipment.

However, experience in the field with early lOCs determined that some of the original parameter sets are almost never used. These have been relegated to a status called "archived." This means no new lines will be installed based on them, no new CPE will be certified against them, and nobody will support them.

Each generic "capability" is designated by one or two letters. 'A' through 'Q' were published in the first edition of this book, but more were added. The list may still grow as carriers will support NI-1 for some time.

The Capability represents a fixed configuration for all parameters of line, switch, and service. No variations are supported.

In the mid-1990s, for only $100, you could have bought a Bellcore paperback book (SR-3480) that spells out all the arcane commands to program a CO switch for each IOC/ in cookbook fashion, with no background information or even explanations for the acronyms. It takes a book because each brand of switch "translates" these parameters differently. This Bellcore publication also described the process to obtain review, confirmation, and registration of lOCs. Bellcore changed its name to Telcordia when SAIC purchased them from the RBOCs.

Specialized terminal equipment may not fit into any of the generic "Capabilities" defined. New applications may call for unanticipated parameter combinations. Hardware vendors have the option of modifying a core or generic capability by making some (not many) changes. The IOC is then designated by the letter of the underlying generic IOC ('X') plus a number ('n') resulting in the form "Xn."

Changes might be the number of directory numbers (DNs)/ assignment of a feature key to a location on a smart phone, etc. Changes would not be accepted in this format if they affected the basic interface; e.g., the number of B channels. TOP

Ordering codes for National ISDN-2 are very different. There is only one basic function package, augmented by six optional feature modules. Each base package applies to one terminal device; there may be one or two basic packages assigned to a BRI line/ which give each terminal access to both B channels. The packages may be different for each terminal.