ISDN

ISDN
Integrated Services Digital Network (ISDN) is a digital system that allows voice and data to be transmitted simultaneously using end-to-end digital connectivity. ISDN allows multiple digital channels to be transmitted simultaneously over the same wiring infrastructure used for analog lines. Two kinds of channels are defined in ISDN. The B channel or bearer channel carries user traffic, whereas the D channel or data channel carries CCS signaling data. The bandwidths of B channels are 64 kbps. Some switches limit B channels to a capacity of 56 kbps. The D channel handles signaling at 16 kbps or 64 kbps, depending on the service type. Original recommendations of ISDN were in Consultative Committee for International Telegraph and Telephone (CCITT) Recommendation I.120 (1984), which described some initial guidelines for implementing ISDN. As regards ISDN in North America, members of the industry agreed to create the National ISDN 1 (NI-1) standard as an interoperable ISDN standard. A more comprehensive standardization initiative, National ISDN 2 (NI-2), was later adopted. Two basic types of ISDN services are offered: basic rate interface (BRI) and primary rate interface (PRI).

ISDN BRI
ISDN BRI (2B+D) consists of two 64-kbps B channels and one 16-kbps D channel for a total of 144 kbps. BRI service is designed to meet the needs of most individual users. BRI ISDN also uses a channel-aggregation protocol, such as BONDING or Multilink PPP, that supports an uncompressed data transfer speed of 128 kbps, plus bandwidth for overhead and signaling.

As illustrated in Figure 2-18, the U interface is a two-wire (single-pair) interface from the ISDN switch, the same physical interface provided for plain old telephone service (POTS) lines. It supports full-duplex data transfer over a single pair. Echo cancellation is used to reduce noise, and data-encoding schemes, such as 2 binary 1 quaternary (2B1Q) in North America and 4B3T in Europe, permit a relatively high data rate of 160 kbps over ordinary single-pair local loops.


Figure 2-18. ISDN Basic Rate Interface

[View full size image]





The U interface is terminated with a network termination 1 (NT-1) device at the CPE end. North American carriers provide customers with a choice of U or S/T interfaces. EMEA and Asia Pacific phone companies supply NT1s, thereby providing their customers with an S/T interface. The ISDN NT-1 converts the two-wire U interface into the four-wire S/T interface. The S/T interface supports up to seven devices on the full-duplex S/T bus. The BRI NT-1 provides timing, multiplexing of the B and D channels, and power conversion.

Devices that connect to the S/T interface include ISDN-capable telephones, videoconferencing equipment, routers, and terminal adapters. All devices that are designed for ISDN are designated terminal equipment 1 (TE-1). All other communication devices that are not ISDN capable, but have an asynchronous serial (EIA-232) or POTS telephone interface—including ordinary analog telephones, modems, and terminals—are designated terminal equipment 2 (TE-2). A terminal adapter (TA) connects a TE-2 to the ISDN S/T bus. ISDN services can be deployed as OPX services by carriers and service providers that operate carrier class 5 switches capable of ISDN PRI and BRI services. There are local loop distance limitations of 18,000 feet (5.5 km) of the CO point of presence (POP) for BRI service. Repeater devices are required for distances exceeding these guidelines.

ISDN PRI
ISDN PRI service is offered as T1/PRI or E1/PRI. T1/PRI (23B+D) has a channel structure that is 23 B channels plus one 64-kbps D channel for a total of 1536 kbps. In EMEA and the Asia Pacific, E1/PRI (30B+D) consists of 30 B channels plus one 64-kbps D channel for a total of 1984 kbps. It is also possible to support multiple PRI lines with one 64-kbps D channel using NFAS. H channels provide a way to aggregate B channels. They are implemented as follows:

H0 = 384 kbps (6 B channels)

H10 = 1472 kbps (23 B channels)

H11 = 1536 kbps (24 B channels)

H12 = 1920 kbps (30 B channels)

ISDN PRI services are offered over a two-pair T1/PRI or E1/PRI unbalanced facility. As shown in Figure 2-19, in the case of ISDN PRI, the NT-1 is a CSU/DSU-like device, whereas the NT-2 devices provide customer premises switching, multiplexing, or other forms of concentration. If a device performs NT-1 and NT-2 functions, it might be referred to as an NT-12. The NT-2 device converts the T interface into the S interface. The ISDN S and T interfaces are electrically equivalent. The NT-2 communicates with terminal equipment, and handles the Layer 2 and 3 ISDN protocols. The U interface local loop connects to ISDN line-termination equipment that provides the LT function. The connection between switches within the phone network is called exchange termination (ET). The LT and ET functions communicate via the V interface.


Figure 2-19. ISDN Primary Rate Interface

[View full size image]





ISDN Layer 1
The ITU I-Series and G-Series documents specify the ISDN physical layer. Echo cancellation is used to reduce noise, and data encoding schemes, such as 2B1Q and 4B3T, are used to encode data.

As illustrated in Figure 2-20, 2B1Q is the most common signaling method on U interfaces. In this method, each pair of binary digits represents four discrete amplitude and polarity values. This protocol is defined in detail in ANSI spec T1.601. In summary, 2B1Q provides 2 bits per baud, which results in 80-kilo baud (baud = one modulation per second) or a transfer rate of 160 kbps. This means that the input voltage level can be one of four distinct levels. These levels are called quaternaries. Each quaternary represents 2 data bits, because there are 4 possible ways to represent 2 bits, as shown in Figure 2-18. Each U interface frame is 240 bits long. At the prescribed data rate of 160 kbps, each frame is therefore 1.5 ms long. Each frame consists of a 16-kbps frame overhead, 16-kbps D channel, and two B channels at 64-kbps each.


Figure 2-20. ISDN Layer 1





The Sync field consists of 9 quaternaries (2 bits each) in the quaternary symbolic pattern +3 +3 –3 –3 –3 +3 –3 +3 –3. The (B1 + B2 + D) represent 18 bits of data consisting of 8 bits from the first B channel, 8 bits from the second B channel, and 2 bits of D-channel data. The Maintenance field contains CRC information, block error detection flags, and embedded operator commands used for loopback testing without disrupting user data. Data is transmitted in a superframe consisting of 8 * 240-bit frames for a total of 1920 bits (240 octets). The Sync field of the first frame in the superframe is inverted (–3 –3 +3 +3 +3 –3 +3 –3 +3).

ISDN Layer 2
The ISDN data link layer is specified by the ITU Q-Series documents Q.920 through Q.923. All the signaling on the D channel is defined in the Q.921 spec. ISDN uses the Link Access Protocol - D channel (LAP-D) as its Layer 2 protocol. LAP-D is almost identical to the X.25 LAP-B protocol. Figure 2-21 shows the LAP-D frame format.


Figure 2-21. ISDN Layer 2

[View full size image]





The Start Flag field is 1 octet long and its value is always 7E (hex) or 0111 1110 (binary). The Control field is 2 octets long and indicates the frame type (information, supervisory, or unnumbered) and sequence numbers (N(r) and N(s)). The Information field contains Layer 3 protocol information and user data. The CRC field is a 2-octet field that provides cyclic redundancy checks for bit errors on the user data. The End Flag field is also 1 octet long and its value is always set to 7E (hex) or 0111 1110 (binary).

The Address field contains the Service Access Point Identifier (SAPI) subfield, which is 6 bits wide; the C/R (command/response) bit, which indicates whether the frame is a command or a response; the EA0 (address extension) bit, which indicates whether this is the final octet of the address or not; the TEI (terminal endpoint identifier) 7-bit device identifier; and the EA1 (address extension) bit, which is similar to the EA0.

As detailed in Figure 2-21, the Service Access Point Identifier (SAPI) is a 6-bit field that identifies the point where Layer 2 provides a service to Layer 3. Terminal endpoint identifiers (TEIs) are unique IDs given to each device (TE) on an ISDN S/T bus. This identifier value could be dynamic, or the value can be assigned statically when the TE is installed.

ISDN Link-Layer Establishment
The following steps are used to establish Layer 2 communication between ISDN devices:

The TE and the network initially exchange receive ready (RR) frames, listening for someone to initiate a connection.


The TE sends an unnumbered information (UI) frame with a SAPI of 63 (management procedure, query network) and TEI of 127 (broadcast).


The network assigns an available TEI (in the range 64 to 126).


The TE sends a set asynchronous balanced mode (SABME) frame with a SAPI of 0 (call control, used to initiate a setup) and a TEI of the value assigned by the network.


The network responds with an unnumbered acknowledgement (UA), SAPI = 0, TEI = assigned.


The Layer 2 connection is now ready for a Layer 3 setup.


ISDN Layer 3
The ISDN network layer is also specified by the ITU Q-Series documents Q.930 through Q.939. Layer 3 is used for the establishment, maintenance, and termination of logical network connections between two devices. Service profile IDs (SPIDs) are used to identify what services and features the ISDN switch provides to the attached ISDN device.

NOTE

The reader must not confuse the ISDN Layer 3 with Layer 3 of the OSI model. Protocols, such as ISDN and X.25, have their own Layer 3. Network layer protocols, such as IP, perceive such protocol stacks as the data link layer.



SPIDs are accessed at device initialization prior to call setup. The SPID is usually the 10-digit phone number of the ISDN line along with a prefix or suffix. The suffix is also known as a tag identifier (TID). SPIDs are used to identify features on the line, but in reality they can be whatever the carrier decides the value(s) should be. If an ISDN line requires a SPID, but it is not correctly supplied, Layer 2 initialization will take place, but Layer 3 will not, and the device will not be able to place or accept calls. ITU spec Q.932 provides greater details on SPIDs.

The Information field is a variable-length field that contains the Q.931 protocol data. Figure 2-22 describes the various subfields contained in the Information field. The following fields are contained in the Q.931 header:

Protocol Discriminator (1 octet)— Identifies the Layer 3 protocol. If this is a Q.931 header, this value is always 0816.

Length (1 octet)— Indicates the length of the next field, the CRV.

Call Reference Value (CRV) (1 or 2 octets)— Used to uniquely identify each call on the user-network interface. This value is assigned at call setup, and this value becomes available for another call when the call is cleared.

Message Type (1 octet)— Identifies the message type (setup, connect, and so forth). This determines what additional information is required and allowed.

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


Figure 2-22. ISDN Layer 3





ISDN Call Setup
The following steps are used to establish ISDN calls from an ISDN Layer 3 perspective:

Caller sends a setup to the ISDN switch.


If the setup is okay, the switch sends a call proceeding to the caller, and a setup to the receiver.


The receiver gets the setup. If it is okay, it sends an alerting message to the switch.


The switch forwards the alerting message to the caller.


When the receiver answers the call, is sends a connect message to the switch.


The switch forwards the connect message to the caller.


The caller sends a connect acknowledge message to the switch.


The switch forwards the connect ack message to the receiver.


The call is now set up.


TDM Network Elements
A variety of TDM-based network elements are used to build TDM systems. Some of these elements are discussed in this section. Common handoff to optical systems takes place at the DS1/DS3 levels in the case of the T-carrier, and E1/E3 levels in the case of the E-carrier. Note that the various individual network elements presented in this section, such as repeaters, CSUs/DSUs, DACS, and channel banks, are commercially available as integrated units supporting a wide variety of low- and high-speed interfaces, encoding, signaling, and protocols. The TDM network elements integrate to form the digital loop carrier (DLC) supporting various TDM architectures and topologies.

Repeaters
Repeaters are four-wire T1/E1 unbalanced amplifiers and signal processors for use on T1 or E1 lines. Repeaters are used to extend in-house T1/E1 lines in campus and high-rise environments. Repeaters can also be used to extend the distance between any T1/E1 equipment, such as DSUs, channel banks, and routers with built-in CSU/DSUs. A pair of repeaters can be located up to 5000 feet apart. Solid copper 22 AWG two-twisted-pair is the preferred cable for connection between repeaters. Smaller wire sizes will reduce the functional distance between the repeater pairs. Connection is made through RJ-45 modular connectors or four-wire, screw-down barrier strips. Both types of connectors are commonly used standards.

CSU/DSU
Channel service units/digital service units are essentially CPE multiplexers that can assign channels or time slots to a circuit. For example, a 256-kbps circuit will have four time slots assigned to it (N1 to N4). Each of these time slots is 64 kbps. Most CSUs/DSUs also support 56-kbps time slots. Some CSUs/DSUs are equipped with multiple ports. This enables the user to allocate time slots to each physical port that might be attached to routers or other CPE equipment. For example, a CSU/DSU connected to a 256-kbps line could assign N1 to port 1 and N2+N3+N4 to port 2. This would allocate 64-kbps bandwidth to the CPE device attached to port 1, and 192 kbps to the CPE device attached to port 2. Another function supported by CSUs/DSUs is the drop and insert function. Drop and insert is used to terminate one or more DS0 channels of a T1 at the digital RS-530/V.35 interface of the FT. One or more of the remaining DS0 channels can be passed on to other equipment, typically a system using voice lines. For example, a single 112-kbps channel (56 kbps * 2) might be dropped off to support a router, and up to 22 of the remaining DS0 channels passed on to a private branch exchange (PBX) for voice lines. CSU/DSU devices are regarded as a demarcation point by some carriers. In such a case, the carrier would own and manage the CSU/DSU, permitting them to perform loopback tests in the event of local loop circuit outages.

Digital Access and Cross-Connect Systems
The modern DACS is truly an integrated access device (IAD) that integrates channel bank cross-connect and multiplexer functionality in one device. DACS cross-connect functionality enables carriers to physically wire user circuits and electrically groom these 64-kbps voice or data circuits to higher T1 or E1 levels. The higher-level T1 or E1s can be groomed into DS3s or E3s for back-haul to a carrier class 5 switch. The time-slot interchange (digital cross-connect) functionality of a DACS enables you to assign DS0s to higher-level T1/E1 circuits in any order you want. It also enables you to assign the order of T1/E1s within a DS3/E3. A DACS also enables you to perform T1-to-E1 format conversion. Most DACSs support console, Telnet, and Simple Network Management Protocol (SNMP) for configuration, maintenance, performance monitoring, and administration.

Channel Bank
Channel banks are devices implemented at a CO (public exchange) that convert analog signals from home and business users into digital signals to be carried over higher-speed lines between the CO and other exchanges. The analog signal is converted into a digital signal that transmits at a 64-kbps rate. The 64-kbps signal is multiplexed with other DS0 signals on the same line using TDM techniques to higher T1/E1 levels. Channel banks offer foreign exchange office (FXO), foreign exchange subscriber (FXS), special access office (SAO), dial pulse originating (DPO), dial pulse terminating (DPT), equalized transmission only (ETO), transmission only (TO), and pulse link repeater (PLR) facilities.