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Figure 12.21 Pseudoternary line code, example of application.

tion accounts for only 144 kbps. The remaining 48 kbps are overhead bits, whose functions are briefly reviewed as follows.

The functions covered at the interface include bit timing at 192 kbps to enable the TE and NT to recover information from the aggregate bit stream. This octet timing provides 8-kHz octet timing for the NT and TE to recover the time-division multiplexed channels (i.e., 2B + D multiplexed). Other functions include D-channel access control, power feeding, deactivation, and activation.

Interchange circuits are required, of which there is one in either direction of transmission (i.e., to and from the NT); they are used to transfer digital signals across the interface. All of the functions described previously, except for power feeding, are carried out by means of a digitally multiplexed signal. In both directions of transmission the bits are grouped into frames of 48 bits each. However, the frame structures are different in each direction of transmission. A dc balancing bit is periodically inserted to move the signal energy away from 0 Hz.

12.4.6.2.2 Line Code. For both directions of transmission, a pseudoternary coding is used with 100% pulse width, as illustrated in Figure 12.21. Coding is performed such that a binary 1 is represented by no line signal, whereas a binary 0 is represented by a positive or negative pulse. The first binary signal following the framing balance bit is the same polarity as the balance bit. Subsequent binary 0s alternate in polarity. A balance bit is a 0 if the number of 0s following the previous balance bit is odd. A balance bit is a binary 1 if the number of 0s following the previous balance bit is even. As mentioned, balance bits tend to limit the build-up of a dc component on the line.

12.4.6.2.3 Timing Considerations. The NT derives its timing from the network clock. A TE synchronizes its bit, octet, and frame timing from the NT, which has derived its timing from the ISDN bit stream being received from the network. The NT uses this derived timing to synchronize its transmitter clock.

12.4.6.2.4 BRI Differences in the United States. The Bellcore/ ANSI ISDN standards differ considerably from their CCITT counterparts in the I and several Q recommendations. The various PSTN administrations (telephone companies) in the United States are at variance with most other countries of the world. Bellcore stated its intention at the outset to produce equipment that was cost effective and marketable and that would easily interface with existing North American telephone plant.

One point of variance, of course, is where the telephone company responsibilities end and customer responsibilities begin. This is called the U-interface, which is peculiar to U.S. ISDN operation (see Figure 12.15). For example, U.S. ISDN calls for a two-wire customer interface; CCITT uses a four-wire connectivity. Rather than a pseudoternary line waveform, the United States uses 2B1Q, a four-level waveform. The line bit rate is 160 kbps rather than the CCITT recommended 192 kbps. The 2B + D frame overhead differs significantly from its CCITT counterpart. Bellcore uses the generic term DSL for digital subscriber line, and the ISDN is one of a large class of digital subscriber lines.

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The effective 160 kbps signal is divided up into 12 kbps for synchronization words, 144 kbps for 2B + D customer data, and 4 kbps of DSL overhead.

The synchronization technique used is based on transmission of nine (quarternary) symbols every 1.5 ms, followed by 216 bits of 2B + D data and 6 bits of overhead. The synchronization word provides a robust method of conveying line timing and establishes a 1.5-ms DSL “basic frame” for multiplexing subrate signals. Every eighth synchronization word is inverted (i.e., the 1s become 0s and the 0s become 1s) to provide a boundary for a 12-ms superframe composed of eight basic frames. This 12-ms interval defines an appropriate block of customer data for performance monitoring and permits a more efficient suballocation of the overhead bits among various operational functions.

2B + D Customer data bit pattern. There are 216 2B + D bits placed in each 1.5-ms basic frame, for a customer data rate of 144 kbps. The bit pattern (before conversion to quaternary form and after reconversion to binary form) for the 2B + D data is

B1B1B1B1B1B1B1B1B2B2B2B2B2B2B2B2DD,

where B1 and B2 are bits from the B1- and B2-channels and D is a bit from the D-channel. This 18-bit pattern is repeated 12 times per DSL basic frame.

DSL Line code—2B1Q. The average power of a 2B1Q transmitted signal is between +13 dBm and +14 dBm over a frequency band from 0 Hz and 80 kHz, with the nominal peak of the largest pulse being 2.5 V. The maximum signal power loss at 40 kHz is about 42 dB. As mentioned earlier, the bit rate is 160 kbps and the modulation rate is 80 kbaud.

2B1Q Waveform. It is convenient to express the 2B1Q waveform as +3, +1, −1, −3 because this indicates symmetry about zero, equal spacing between states, and convenient integer magnitudes. The block synchronization word (SW) contains nine quaternary elements repeated every 1.5 ms:

+3, +3, −3, −3, −3, +3, −3, +3, +3.

As we are aware, North American BRI connects to an ISDN user on a two-wire basis, which operates full-duplex. It can do this without interference between transmit and receive sides by the use of local echo suppressors at the U-interface.

12.4.7 Primary Rate Interfaces

The primary rate interfaces cover two standard bit rates: 1.544 Mbps in North America and 2.048 Mbps when in areas of the world that utilize E1.

12.4.7.1 Interface at 1.544 Mbps

12.4.7.1.1 Bit Rate and Synchronization—Network Connection Characteristics. The network delivers (except as noted in the following) a signal synchronized from a clock having a minimum accuracy of 1 × 10−11 (stratum 1; see Section 6.12 for strata definitions). When synchronization by a stratum 1 clock has been interrupted, the signal delivered by the network to the interface should have a minimum accuracy of 4.6 × 10−6 (stratum 3).

12.4.7.1.2 DS1 Interface. The ISDN interface for the 1.544 Mbps rate is the standard line interface described in Chapter 6. Note that time slot 24 is assigned to the D-channel when the D-channel is present (thus 23B + D).

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A channel occupies an integer number of time slots and in the same time-slot positions in every frame. A B-channel may be assigned any time slot in the frame, an H0- channel may be assigned any six slots in a frame in numerical order (not necessarily consecutive), and an H11-channel may be assigned slots 1 to 24. The assignments may vary on a call-by-call basis.

12.4.7.2 Interface at 2.048 Mbps

12.4.7.2.1 Frame Structure. This is the standard E1 frame structure described in Chapter 6. Channel 16, in compliance with the E1 standard (CCITT Rec. G.703) carries the signaling information. Channel 0, the synchronization channel, is the responsibility of the user. Thus primary service is 30B + D (i.e., not 30B + D + “S,” where S means synchronization).

12.4.7.2.2 Timing Considerations. The NT derives its timing from the network clock. The TE synchronizes its timing (bit, octet, and frame) from the signal received from the NT and synchronizes its transmitted signal accordingly. In an unsynchronized condition—that is, when the access that normally provides network timing is unavailable—the frequency deviation of the free-running clock shall not exceed ±50 ppm.

12.4.8 Overview of Layer 2, ISDN D-Channel, LAPD Protocol

The link access procedure (LAP) for the D-channel (LAPD) is used to convey information between layer 3 entities across the ISDN user-network interface (UNI) using the D-channel.8

A service access point (SAP) is a point at which the data-link layer provides services to its next higher OSI layer or layer 3.9 Associated with each data-link layer is one or more data-link connection endpoints (see Figure 12.22). A data-link connection endpoint is identified by a data-link connection endpoint identifier, as seen from layer 3, and a data-link connection identifier (DLCI), as seen from the data-link layer.

Figure 12.22 Entities, service access points (SAPs), and end-points. (From ITU-T Rec. Q.920, Figure 2/ Q.920, [Ref. 14].)

8The discussion here only covers aspects of BRI service, namely, the 16-kbps signaling channel. It does not cover the PRI service.

9Remember that the data-link layer is synonymous with OSI layer 2.

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Figure 12.23 Data-link layer reference model. (From ITU-T Rec. Q.920, Figure 5/ Q.920, p. 5, [Ref. 14].)

Cooperation between data-link layer entities is governed by a specific protocol to the applicable layer. In order for information to be exchanged between two or more layer 3 entities, an association must be established between layer 3 entities in the data-link layer using a data-link layer protocol.

Figure 12.23 shows the data-link layer reference model. All data-link layer messages are transmitted in frames delimited by flags, where a flag is a unique binary sequence pattern (i.e., 01111110). The governing data-link layer protocol is LAPD (link access protocol D-channel). The frame structure is described in ITU-T Q.921 (Ref. 15).10

The LAPD includes functions for:

1. The provision of one or more data-link connections on a D-channel; discrimination between the data-link connections is by means of a data-link connection identifier (DLCI) contained in each frame;

2. Frame delimiting, alignment, and transparency, allowing recognition of a sequence of bits transmitted over a D-channel as a frame;

3. Sequence control, which maintains the sequential order of frames across a datalink connection;

4. Detection of transmission, format, and operational errors on a data link;

5. Recovery from detected transmission, format, and operational errors, and notification to the management entity of unrecovered errors; and

6. Flow control.

10LAPD is a direct descendent of HDLC (see Section 10.10.3). LAPD modified is at the very heart of frame relay described in Section 12.5.

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There is unacknowledged and acknowledged operation. With unacknowledged operation, information is transmitted in unnumbered information (UI) frames. At the data-link layer the UI frames are unacknowledged. Transmission and format errors may be detected, but no recovery mechanism is defined. Flow control mechanisms are also not defined. With acknowledged operation, layer 3 information is transmitted in frames that are acknowledged at the data-link layer. Error-recovery procedures based on retransmission of unacknowledged frames are specified. For errors that cannot be corrected by the data-link layer, a report to the management entity is made. Flow-control procedures are also defined.

Unacknowledged operation is applicable for point-to-point and broadcast information transfer. However, acknowledged operation is applicable only for point-to-point information transfer.

There are two forms of acknowledged information that are defined:

1. Single-frame operation; and

2. Multiframe operation.

12.4.8.1 Layer 2 Frame Structure for Peer-to-Peer Communications. There are two frame formats used for layer 2 frames:

1. Format A, for frames where there is no information field; and 2. Format B, for frames containing an information field.

These two frame formats are illustrated in Figure 12.24. The following discussion briefly describes the frame content (sequences and fields) for the LAPD layer 2 frames.

Figure 12.24 Frame formats for LAPD frames. (From ITU-T Rec. Q.921, Figure 1/ Q.921, p. 20, [Ref. 15].)

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Figure 12.25 LAPD address field format. (From CCITT Rec. Q.921, Figure 5/ Q.921, p. 23, [Ref. 15].)

Flag sequence. Identical to HDLC described in Section 10.10.3, it is the binary sequence 01111110. The flag opens and closes individual frames. For LAPD frames sent in sequence, the closing flag of one frame is the opening flag of the next frame.

Address field. As shown in Figure 12.25, the address field consists of two octets and identifies the intended receiver of a command frame and the transmitter of a response frame.

Control field. The control field consists of one or two octets. It identifies the type of frame, either command or response. It contains sequence numbers where applicable. Three types of control field formats are specified:

1. Numbered information transfer (I format);

2. Supervisory functions (S format); and

3. Unnumbered information transfers and control functions (U format).

Information field. The information field of a frame, when present, follows the control field and precedes the frame check sequence (FCS). The information field contains an integer number of octets:

•For a SAP supporting signaling, the default value is 128 octets.

•For SAPs supporting packet information, the default value is 260 octets.

Frame check sequence (FCS) field. Identical to HDLC, Section 10.10.3, it is 16 bits long and is based on the generating polynomial:

X16 + X12 + X5 + 1.

Transparency, mentioned previously, ensures that a flag or abort sequence is not initiated within a frame. On the transmit site the data-link layer examines the frame content between the opening and closing flag sequences and inserts a 0 bit after all sequences with five contiguous 1 bits (including the last five bits of the FCS). On the receive side the data-link layer examines the frame contents between the opening and closing flag sequences and discards any 0 bit that directly follows five contiguous 1 bits.

Address field format. The address field is illustrated in Figure 12.25. It contains address field extension bits (EA), command/ response indication bit (C/ R), a data-link

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layer service access point identifier (SAPI) subfield, and a terminal end-point identifier (TEI) subfield.

Address field extension bit (EA). The address field range is extended by reserving the first transmitted bit of the address field to indicate the final octet of the address field. The presence of a 1 in the first bit position of an address field octet signals that it is the final octet of the address field. The double octet address field for LAPD operation has bit 1 of the first octet set to a 0 and bit 1 of the second octet set to a 1.

Command/ response field bit (C/ R). The C/ R bit identifies a frame as either a command or a response. The user side sends commands with the C/ R bit set to 0, and it sends responses with the C/ R bit set to 1. The network side does the opposite; that is, commands are sent with the C/ R bit set to 1, and responses are sent with the C/ R bit set to 0.

In keeping with HDLC rules, commands use the address of the peer data-link entity while responses use the address of their own data-link layer entity.11 In accordance with these rules, both peer entities on a point-to-point data-link connection use the same datalink connection identifier (DLCI) composed of an SAPI and TE1.

12.4.9 Overview of Layer 3

The layer 3 protocol, of course, deals with the D-channel and its signaling capabilities. It provides the means to establish, maintain, and terminate network connections across an ISDN between communicating application entities. A more detailed description of the layer 3 protocol may be found in ITU-T Rec. Q.931 (Ref. 16). Layer 3 utilizes functions and services provided by its data-link layer, as described in Section 12.4.8 under LAPD functions.

Layer 3 performs two basic categories of functions and services in the establishment of network connections. The first category directly controls the connection establishment. The second category includes those functions relating to the transport of messages in addition to the functions provided by the data-link layer. Among these additional functions are the provision of rerouting of signaling messages on an alternative D-channel (where provided) in the event of D-channel failure. Other possible functions include multiplexing and message segmenting and blocking. The D-channel layer 3 protocol is designed to carry out establishment and control of circuit-switched and packet-switched connections. Also, services involving the use of connections of different types, according to user specifications, may be provided through “multimedia” call control procedures. Functions performed by layer 3 include:

1. The processing of primitives for communicating with the data-link layer;

2. Generation and interpretation of layer 3 messages for peer-level communications;

3. Administration of timers and logical entities (e.g., call references) used in callcontrol procedures;

4. Administration of access resources, including B-channels and packet-layer logical channels (e.g., ITU-T X.25); and

5. Checking to ensure that services provided are consistent with user requirements, such as compatibility, address, and service indicators.

The following functions may also be performed by layer 3:

11LAPD, as we know, is a derivative of HDLC.

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1. Routing and Relaying. Network connections exist either between users and ISDN exchanges or between users. Network connections may involve intermediate systems that provide relays to other interconnecting subnetworks and that facilitate interworking with other networks. Routing functions determine an appropriate route between layer 3 addressees.

2. Network Connection. This function includes mechanisms for providing network connections making use of data-link connections provided by the data-link layer.

3. Conveying User Information. This function may be carried out with or without the establishment of a circuit-switched connection.

4. Network Connection Multiplexing. Layer 3 provides multiplexing of call control information for multiple calls onto a single data-link connection.

5. Segmentation and Reassembly (SAR). Layer 3 may segment and reassemble layer 3 messages to facilitate their transfer across user–network interface.

6. Error Detection. Error-detection functions are used to detect procedural errors in the layer 3 protocol. Error detection in layer 3 uses, among other information, error notification from the data-link layer.

7. Error Recovery. This includes mechanisms for recovering from detected errors. 8. Sequencing. This includes mechanisms for providing sequenced delivery of layer 3 information over a given network connection when requested. Under normal conditions, layer 3 ensures the delivery of information in the sequence it is sub-

mitted by the user.

9. Congestion Control and User Data Flow Control. Layer 3 may indicate rejection or unsuccessful indication for connection establish requests to control congestion within a network. Typical is the congestion control message to indicate the establishment or termination of flow control on the transmission of user information messages.

10. Restart. This function is used to return channels and interfaces to an idle condition to recover from certain abnormal conditions.

12.4.10 ISDN Packet Mode Review

12.4.10.1 Introduction. Two main services for packet-switched data transmission are defined for packet-mode terminals connected to the ISDN:

Case A: Access to a PSPDN (PSPDN services) (PSPDN c packet-switched public data network); and

Case B: Use of an ISDN virtual circuit service (Refs. 11, 18).

12.4.10.2 Case A: Configuration When Accessing PSPDN Services. This con-

figuration is shown in Figure 12.26 and refers to Case A, which implies a transparent handling of packet calls through an ISDN. Only access via the B-channels is possible. In this context, the only support that an ISDN gives to packet calls is a physical 64-kbps circuit-mode semipermanent or demand transparent network connection type between appropriate PSPDN port and the X.25 DTE + TA or + TE1 at the customer premises.

In the case of semipermanent access, the X.25 DTE + TA or TE1 is connected to the corresponding ISDN port at the PSPDN (AU [access unit]). The TA, when present, performs only the necessary physical channel rate adaption between the user at the R reference point and the 64-kbps B-channel rate. D-channel layer 3 messages are not used in this case.

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Figure 12.26 Case A, configuration when accessing PSPDN services. (From ITU-T Rec. X.31, Figure 2-1/ X.31, p. 3, [Ref. 18].)

In the case of demand access to the PSPDN, which is shown in the upper portion in Figure 12.26, the X.25 DTE + TA or TE1 is connected to an ISDN port at the PSPDN (AU). The AU is also able to set up 64-kbps physical channels through the ISDN.

In this type of connection, an originating call will be set up over the B-channel toward the PSPDN port using the ISDN signaling procedure prior to starting X.25 layer 2 and layer 3 functions. This is done by using either hot-line (e.g., direct call) or complete selection methods. Moreover, the TA, when present, performs user rate adaption to 64 kbps. Depending on the data rate adaption technique employed, a complementary function may be needed at the AU of the PSPDN.

In the complete selection case, two separate numbers are used for outgoing access to the PSPDN:

1. The ISDN number of the access port of the PSPDN, given in the D-channel layer 3 setup message (Q.931); and

2. The address of the called DTE indicated in the X.25 call request packet.

The corresponding service requested in the D-channel layer 3 setup message is ISDN circuit-mode bearer services.

For calls originated by the PSPDN, the same considerations apply. In fact, with reference to Figure 12.26, the ISDN port of the PSPDN includes both rate adaption (if required) and path setting-up functions. When needed, DTE identification may be provided to the PSPDN by using call establishment signaling protocols in D-Channel layer 3 (ITU-T Rec. Q.931). Furthermore, DCE identification may be provided to the DTE, when needed, by using the same protocols.

For the demand access case, X.25 layer 2 and layer 3 operation in the B-channel, as well as service definitions, are found in ITU-T Rec. X.32. Some PSPDNs may operate

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Figure 12.27 Case B, configuration for the ISDN virtual circuit services (access via a B-channel). (From ITU-T Rec. X.31, Figure 2-2/ X.31, p. 5, [Ref. 18].)

the additional DTE identification procedures defined in Rec. X.32 (Ref. 17) to supplement the ISDN-provided information in Case A.

12.4.10.3 Case B: Configuration for the ISDN Virtual Service. This configuration refers to the case where a packet handling (PH) function is provided within the ISDN. The configuration shown in Figure 12.27 relates to the case of X.25 link and packet procedures conveyed through the B-channel. In this case, the packet is routed, within the ISDN, to a PH function, where the complete processing of the X.25 call can be carried out.

There is still another configuration where X.25 packet procedures are conveyed through the D-channel. In this case a number of DTEs can operate simultaneously through a D-channel by using connection identifier discrimination at ISDN layer 2. The accessed port of the PH is still able to support X.25 layer 3 procedures.

It should be pointed out that the procedures for accessing the PSDTS (packet-switched data transmission services) through the ISDN user–network interface over a B- or D-channel are independent of where the service provider chooses to locate PH functions such as:

•In a remote exchange or packet-switching module in an ISDN; or

•In the local exchange.