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Integrated Services Digital Network, or ISDN, is a digital connection that consists of two full-duplex 64Kbps circuits called channels (or "B" channels), and one 16Kbps Delta channel (or the "D" channel), which is used for control and management. You could say that ISDN is a mutt of PSTN technologies (the dedicated B channels) and PSDN technologies (the packet-based D channel).

The B channels are used for carrying the digital information, whether computer data, digitized voice, or motion video. The D channel is dedicated between the phone company and you, and handles out-of-band circuit management. This eliminates inbound management traffic from the B channels, generally providing you with a full 64Kbps bandwidth.

The D channel uses packets to establish phone call connections, send billing information, enable caller ID, enact charge reversal requests, and perform other nifty telco features.

A requirement of ISDN is that your office must be within 18,000 feet (about 3.4 miles or 5.5km) of the telephone company central office for Basic Rate Interface (BRI) service; beyond that, expensive repeater devices are required, or ISDN service might not be available at all.

Originally an ISDN connection was difficult to install, configure, and troubleshoot. Although many advances in ISDN equipment have made inroads toward easing the installation and initial configuration of ISDN, it is still significantly more difficult to troubleshoot than other WAN technologies. That is mainly due to the way that ISDN was developed, nearly stillborn by committee. ISDN was first proposed in the late 1970s and spent more than 10 years in the definition and approval process.

In 1988, after ISDN had been fully defined and approved, the world had changed. In North America, AT&T had been broken up, and it was no longer cost-effective for the regional Bells to foot the conversion bill necessary to get end-to-end digital. Only where entire telephone infrastructures were being rebuilt, such as those in Germany and France, was ISDN an attractive choice for the incumbent phone company.

Like many other networking protocols, ISDN covers the bottom three of the standard ISO levels: the physical, data link, and network layers.

As expected, ISDN's physical layer is oriented toward telephone wiring. There are two basic versions of this physical-layer interface that the public can choose from. These are the Basic Rate Interface (BRI) and Primary Rate Interface (PRI).

Basic Rate Interface (BRI)

BRI is designed to run through the last copper mile, or subscriber loop, of twisted-pair wires running from your office to the local telephone exchange.

The subscriber loop is a twisted pair of 22- to 26-gauge copper wire that was designed to handle a rather narrow band of frequencies. The original specifications called for handling a range of 20–50 volts with frequencies less than 4000Hz. This isn't enough to carry a single channel of 64,000bps.

The interesting thing with ISDN is that there are three logical circuits. Two, each rated at 64,000bps, are known as B channels. These are called bearer channels because they carry the actual digitized data. The third channel in BRI is the D channel. ISDN BRI service is often referred to by its designation of 2B+D.

Although most ISPs describe ISDN as 128Kbps, this is misleading. There are two separate 64,000-bit channels and no official specification for concatenating them into a single 128,000bps channel. There are, however, two competing industry standards for making use of both B channels to achieve 128Kbps throughput:

  • Bandwidth On Demand Interoperability Group, called BONDING, is a set of protocols that lets you simultaneously use more than one of ISDN's 64Kbps B channels. BONDING is often referred to as multilink, channel aggregation, channel bonding, and load balancing.

  • Multilink PPP is a method for splitting, recombining, and sequencing datagrams across multiple logical data links. RFC-1717, the original specification for multilink PPP, was defined in 1994 and obsoleted in 1996 by RFC-1990. Although ISDN was the original motivation for multilink PPP, it's not specific to ISDN and can be applied to any number of multiple-link technologies.

The two B channels can be used independently of one another, to handle two different voice/data calls at once or to provide bandwidth on demand.

In the case of the BRI D channel, the bandwidth is 16Kbps. In practice, the channel is used exclusively by the telephone companies for telephony signaling. Although the D channel can be used to carry data as well as control information, the capability to carry data on the D channel is generally vendor-specific.


The NT-1 is a relatively simple device that converts the two-wire U interface into the four-wire S/T interface. Today, many devices have NT-1s built into their design. This has the advantage of making the devices less expensive and easier to install, but it often reduces flexibility by preventing additional devices from being connected.

Technically, ISDN devices must connect through a Network Termination 2 (NT-2) device, which converts the T interface into the S interface (the S and T interfaces are electrically equivalent). However, virtually all ISDN devices now include an NT-2 in their design. The NT-2 communicates with terminal equipment and handles the Layer 2 and 3 ISDN protocols. Almost all ISDN devices expect either a U interface connection (and thus have a built-in NT-1) or an S/T interface connection.

S/T Interface

The S/T interface supports multiple devices (up to seven devices can be placed on the S/T bus) because, although it is still a full-duplex interface, there is now a pair of wires for receiving data and another for transmitting data. The S/T interface is on the customer (user) side of the NT-1.

Devices that connect to the S/T (or S) interface include ISDN-capable telephones and fax machines, video teleconferencing equipment, bridge/routers, and terminal adapters. All devices that are designed for ISDN are designated terminal equipment 1 (TE1). All other communication devices that are not ISDN-capable but that have a POTS telephone interface (also called the R interface), including ordinary analog telephones, fax machines, and modems, are designated terminal equipment 2 (TE2). A terminal adapter (TA) connects a TE2 to an ISDN S/T bus.

Outside the United States, the telephone company supplies the NT-1 and the customer is provided an S/T interface.

U Interface

In the United States, the telephone company provides its BRI customers with a U interface. The U interface is a two-wire (single-pair) interface from the phone switch on the far (switch) side of the NT-1. It supports full-duplex data transfer over a single pair of wires, so only a single device can be connected to a U interface.

The U interface is directly connected to the NT-1. This makes physical installation nearly as convenient as connecting an analog telephone.

Primary Rate Interface (PRI)

The Primary Rate Interface is designed explicitly for the American T1 and European E1 long-distance phone circuits. In the case of T1, the PRI supports 23 B channels at 64,000bps and 1 D channel at 64,000bps, for at total of 24 channels. Not coincidentally, this is the exact number of channels in a North American T1. The European E1 version uses 30 B channels each at 64,000bps and 1 D channel at 64,000bps. The remaining E1 channel is still used for timing.

Unlike BRI, there are officially defined methods to aggregate several B channels to form larger capacity channels in PRI. These methods are the H channels:

  • H0 = 384Kbps (6 B channels)

  • H10 = 1472Kbps (23 B channels)

  • H11 = 1536Kbps (24 B channels)

  • H12 = 1920Kbps (30 B channels)—international [E1] only

These 6, 23, 24, and 30 standard B channels are designed to work either on the American T1 (H0 and H11) or the European E1 (H0 and H12) cables.

ISDN Layer 1: Physical

The ITU I-series and G-series documents specify the ISDN physical layer. The U interface provided by the telephone company for BRI is a two-wire, 160Kbps digital connection. The mojo used to provide 160Kbps over the last mile is powerful. Powerful invocations of echo cancellation are used to reduce noise. Data-encoding schemes (2B1Q in North America, 4B3T in Europe) permit this relatively high data rate over ordinary single-pair local loops.


2B1Q (2 Binary 1 Quaternary) is the most common signaling method on U interfaces (see Table 3). This protocol is defined in detail in the 1988 ANSI spec T1.601. In summary, 2B1Q provides the following:

  • 2 bits per baud

  • 80,000 baud per second

  • Transfer rate of 160Kbps

Table 3 2B1Q Signaling Interface Values


Quaternary Symbol

Voltage Level













This means that the input voltage level can be one of four distinct levels (0 volts is not a valid voltage under this scheme). These four voltage levels are called quaternaries. Each level represents 2 data bits, and because there are four possible ways to represent 2 bits, that works out to be a clever implementation.

Frame Format

Each U interface frame is 240 bits long. At the defined data rate of 160Kbps, each frame is therefore 1.5 milliseconds long. The frame header is shown in Figure 8.

Figure 8 The U interface frame format.

The Sync field consists of nine quaternaries (2 bits each) in the pattern 1010 0000 0010 0010 00.

(B1 + B2 + D) is 18 bits of data consisting of 8 bits from the first B channel, 8 bits from the second B channel, and 2 bits from the D channel.

The Maintenance field contains CRC information, block error-detection flags, and additional maintenance commands used for loopback testing without disrupting user data.

Data is transmitted in a superframe format consisting of eight 240-bit frames, for a total of 1920 bits (240 octets). The sync field of the first frame in the superframe is inverted (that is, 0000 1010 1000 1000 10).

ISDN Layer 2: Data Link

The ISDN data Link layer is specified in the ITU Q-series documents Q.920 through Q.923. Link Access Protocol D channel (LAP-D) is defined in Q.921. LAP-D is almost identical to the X.25 LAP-B protocol.

LAP-D works in Asynchronous Balanced Mode (ABM). This mode is totally balanced (that is, no master/slave relationship). Each station can initialize, supervise, recover from errors, and send frames at any time. The protocol treats the DTE and DCE as equals.

Figure 9 shows the structure of a LAP-D frame.

Figure 9 The structure of a LAP-D frame.

The following are the bits and pieces of the LAP-D frame:

  • Flag (one octet)—This bit pattern is always 0111 1110. To ensure that this pattern never occurs in the data portion of the frame, bit stuffing is used. You can see this in Figure 10.

Figure 10 Bit stuffing keeps the bit pattern 0111 1110 from accidentally occurring in nature.

  • SAPI (service access point identifier)—This is a 6-bit field that identifies the point where Layer 2 provides a service to Layer 3 (see Figure 11).

Figure 11 The service access point identifier.

  • C/R (Command/Response) bit—This indicates whether the frame is a command or a response. Frames from the user with this bit set to 0 are command frames, as are frames from the network with this bit set to 1. Other values indicate a response frame.

  • EA0 (address extension) bit—This is always set to 0.

  • EA1 (address extension) bit—This is always set to 1.

  • TEI (terminal endpoint identifier)—This is a 7-bit device identifier. TEIs are unique IDs given to each device (the terminal endpoint) on an ISDN S/T bus (see Figure 12). This identifier can be dynamically assigned by the ISDN switch, or the value can be assigned statically when the device is installed.

Figure 12 Terminal endpoint identifiers identify the endpoints of an ISDN device.

  • Control (two octets)—The frame level control field indicates the frame type (information, supervisory, or unnumbered) and sequence numbers, control features, and error tracking.

  • Information—This is Layer 3 protocol information and user data.

  • CRC (two octets)—Cyclic redundancy check is a low-level test for bit errors on the user data.

ISDN Layer 3: Network

The ISDN network layer is specified by the ITU Q-series documents Q.930 through Q.939. The networking layer of ISDN is used for the establishment, maintenance, and termination of logical network connections between two devices.


The service profile identifier, or SPID, is used to identify the ISDN device to the telephone network, much as an Ethernet address uniquely identifies a network interface card. A SPID (or more than one, if necessary) is assigned when you order the ISDN Basic Rate Interface (BRI) from the phone company. Although they're optional, without a SPID ISDN, devices won't work on most lines. If an ISDN line requires a SPID but it isn't correctly supplied, then Layer 2 initialization will take place, but Layer 3 initialization will not, and the device will not be able to place or accept calls.

Beginning in 1998, most phone companies began to use a generic SPID format. In this format, the SPID is a 14-digit number that includes your 10-digit telephone number, a 2-digit sharing terminal identifier, and a 2-digit terminal identifier (TID). With the introduction of the generic SPID format, the installation of an ISDN line was simplified because the SPID could easily be communicated to users.

Information Field Structure

The Information field is a variable-length field that contains the Q.931 protocol data (see Figure 13).

Figure 13 The structure of the Information field.

  • Protocol Discriminator (one octet)—Identifies the Layer 3 protocol.

  • Length (one octet)—Indicates the length of the next field, the call reference value. The call reference might be one or two octets long, depending on the size of the value being encoded.

  • Call Reference Value (CRV) (one or two octets)—Used to uniquely identify each call on the user-network interface between the device making the call and the ISDN switch. This is an arbitrary value assigned at the beginning of a call and allocated for the entire call duration.

  • Message Type (one octet)—Identifies the message type. This determines what additional information is required and allowed.

  • Mandatory and Optional Information Elements (variable length)—Consists of options that are set depending on the message type.

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