ADVANCED NETWORKING TRENDS - Module 2

ISDN Introduction ISDN stands for Integrated Service Digital Network, and as the name suggests it allows digital communication. This is favorable as digital technology is a lot fast faster, er, and and more more accu accurat rate e than than the the old old anal analog ogue ue line lines s as they they no long longer er require the process of modulation and demodulation. ISDN relies on already exist existin ing g copp copper er cabl cable e syste systems ms,, caus causin ing g its its inte integr grat atio ion n into into our our exist existin ing g communications system to be smoother and less disruptive. According to ITU-T (formerly CCITT) "an ISDN is a network, in general evolving from a telephony IDN, that provides end-to-end digital connectivity to support a wide wide range range of servic services, es, includ including ing voice voice and non-voic non-voice e service services, s, to which which users have addressed by a limited set of standard multi-purpose user-network inte interf rfac aces. es."" The The ke key y poin pointt of this this defi defini niti tion on is the the abil abilit ity y to supp suppor ortt voic voice e services adequately; this has not been achieved using any other concept. ISDN has has four four majo majorr aspec aspects; ts; tele teleph phon one e netw networ ork; k; inte integr grat ated ed servi service ces; s; digi digita tal; l; network. •

Integrated Services Voice, Video, Image, Data, Mixed media at a number of standard data rates

•

Digital Digital terminal equipment, local loops, trunks, switching, signaling

•

Network Worldwide, Worldwide, interoperat interoperating ing communicati communications ons fabric fabric under distributed distributed control using common standards

ISDN ISDN stan standa dard rds s have have been been defi define ned d by the the ITUITU-T, T, a bran branch ch of the the Unit United ed Nations' International Telecommunications Union (ITU), in the series I and Q recommendations. Integrated Services

  The The curr curren entt telep elepho hone ne net network work uses uses a mix mixture ture of anal analo og and digi digittal transmission methods and diverse access techniques and standards to provide different services: • • • • •

Switched voice telephony Centrex Dedi edicated point-to -to-point data ata car arrrier Packet-switched data carrier Dedi edicated po point-to -to-point di digital car carrrier

Future telephone networks will also provide full-motion video, voice/video/graphics conferencing, high-speed facsimile, and electronic mail. ISDN ISDN inte integr grat ates es all all thes these e serv servic ices es by prov provid idin ing g a smal smalll set set of stan standa dard rd interfaces and access protocols that apply to all services. Because ISDN is an international standard, the same interfaces and access protocols should be available anywhere in the world, across international boundaries, and among equipment from any set of vendors.

Digital

ISDN provides all of its services over an entirely digital transmission system. In pre-ISDN telephony, only interoffice trunks and certain high-capacity dedicated customer circuits use digital transmission. ISDN ISDN employ employs s digita digitall transm transmissi ission on from from the custom customer-p er-prem remises ises equipm equipment ent (CPE; i.e., telephones, data terminals, fax machines, etc.), through the local access loop , and across the carrier's trunk network. All central- and end-office swi switching is performed by digital swi switches, es, and all sig signalling (call establishment, "dial tone," ringing, on-hook/off-hook, service requests) occurs through digital protocols. Network 

Fina Finall lly, y, ISDN ISDN defi define nes s a NETWO NETWORK RK,, not not a loos loose e coll collec ecti tion on of stan standa dard rds s for for privat private-li e-line ne service services. s. Ultima Ultimatel tely, y, ISDN ISDN define defines s a single single worldw worldwide ide fabric fabric of  tran transm smis issi sion on and and swit switch chin ing g serv servic ices es oper operat atin ing g unde underr a comm common on set set of  standards, with control distributed among all the various operating companies and national telecommunications authorities. PRINCIPLES OF ISDN

Support of voice and non-voice applications in the same network – Interfaces and data transmission facilities standardized by ITU-T • Switched and non-switched connections – Packet & circuit switching, leased lines • 64-kbps channel – chosen because at the time was the standard rate for digitized voice. Layered protocol structure – mapped into OSI model (advantages in utilizing existing standards as well as in developing new ones) • Variety of configurations – According to specific national situations & state of  technology ISDN Services

ISDN provides three types of services:

• • •

Bearer services  Teleservices Supplementary services

Tele and supplementary services represent the type of features and functions which are visible to end-users, while bearer services represent the parts of the network which remain hidden from end-users. Bearer services facilitate the real-time communication of digital information between end-users. These services mainly relate to network functions and account for OSI layers 1-3. An example of a bearer services is the 64 kbps, 8 kHz structured, speech service. This service uses a data rate of 64 kbps together with 8 kHz timing information (which structures the data into octet intervals) for transmitting a Pulse Code Modulated (PCM) speech signal. The fact that the signal represents speech is known to the network, allowing it to employ transformations which may not preserve bit integrity but will result in good quality audio reproduction. Teleservices provide a set of higher-level functions on top of bearer services. These services account for OSI layers 4-7. Examples of teleservices are:   Telephony services which provide speech communication over a B channel with control signaling over the D channel. Facsimile services which facilitate the communication of bitmap images over a B channel with control signaling over the D channel.  Teletex services which facilitate the interchange and communication of  textual as well as formatted documents over a B channel with control signaling over the D channel. •

•

•

Supplementary services enhance bearer and teleservices in an independent fashion. Examples of supplementary services are:  The Centrex service emulates a private network and provides specialized features to a set of subscribers.   The Call Transfer service allows a user to transfer an active call to a third-party.  The Call Waiting service allows a user already engaged in a call to be informed of another incoming call.   The Calling Line ID service provides the calling party's address information to the called party. •

•

•

•

Although these services all appear geared toward circuit-switched telephone calls, they are equally applicable to packet-switched data calls. DEFINITION

ISDN stands for Integrated Services Digital Network. It was first introduced by NEC in Japan. There basic purpose was integration of traditionally different computer and communication (C&C) services into a single one. The integration basically means incorporation of three types of services: •

Voice (telephone)

• •

Data (internet) Entertainment (TV)

  The integration should be most comfortably and efficiently done in digital domain, so the switching, multiplexing, signaling and transmission, everything should be digital. It was first named integrated digital network (IDN), which received lukewarm response as only the enterprises, not the general public, realized the potential behind that acronym. Later on it was named ISDN which more clearly states the idea (of integrating different services) behind it.

ISDN Channels A CHANNEL is the basic unit of ISDN service. The ISDN Standards define three basic types of channels: • • •

Bearer channels (B channels) Delta (or "Demand") channels (D channels) High-capacity channels (H channels)

B Channel A B channel is a 64-Kbps unit of clear digital bandwidth. Based on the data rate required to carry one digital voice conversation, a B channel can carry any type of digital information (voice, data, or video) with no restrictions on format or protocol imposed by the ISDN carrier. B-channel is the basic user channel – can carry digital data, PCM-encoded digital voice,or a mixture of lowerrate traffic – with mixed traffic, all traffic must be destined for the same end-point (carried over the same circuit) – supports circuit-switched, packet-switched (exchange of data via X.25) and semipermanent connections – in the case of circuit-switched connections, common channel signaling is used

D Channel A D channel is a signaling channel. It carries the information needed to connect or disconnect calls and to negotiate special calling parameters (i.e., automatic number ID, call waiting, data protocol). The D channel can also carry packetswitched data using the X.25 protocol.  The D channel is not a clear channel. It operates according to a well-defined pair of layered protocols: • •

Q.921 (LAPD) at the Data Link Layer (Layer 2) Q.931 at the upper layers (Layers 3 and above)

 The data rate of a D channel varies according to the type of access it serves: a Basic Rate Access D channel operates at 16 Kbps and a Primary Rate Access D channel operates at 64 Kbps. D-channel is dual-purpose – carries signaling information to control circuit switched calls on Bchannel – may be used to carry low-speed data applications (e.g., videotex, telemetry) Signalling on the D Channel

  The ISDN D channel carries all signalling between the customer's terminal device and the carrier's end switching office. Signalling information with end-to-end significance (i.e., which must be received by the terminal device at a call's destination, such as Automatic Calling Number Identification information) travels between the carrier's switching offices on the carrier's common-channel signalling network and on to the destination terminal through the receiving user's D channel.

H Channel An H channel is a special, high-speed clear channel. H channels, designed primarily for full-motion color video, are not yet in common use. There are currently three kinds of H channel: H0 ("H-zero") H11 ("H-one-one") H12 ("H-one-two") • • •

An H0 channel operates at 384 Kbps (roughly one fourth of a North American Primary Rate Access or one fifth of a European Primary Rate Access). An H1 channel operates at 1.536 Mbps and occupies one whole North American Primary Rate Access. An H12 channel occupies an entire European Primary Rate Access. H-channel is a high-speed channel – can be used as a single trunk or subdivided by the user – fast fax, video, high-speed data, high-quality audio and multiplexed information streams at lower data rates Only D channels can be used for carrying signaling information. B and H channels can only be used for carrying user data. In practice, channels are offered to users in a packaged form. Two such packages have been defined: basic access and primary access. The Basic Rate Access (BRA) package (also called 2B+D) is primarily intended for residential subscribers and consists of  the following: * Two B channels * One 16 kbps D channel * Overhead of 48 kbps for framing, synchronization, etc.

 This produces a total bit rate of 192 kbps. The channels may be used for a variety of purposes. For example, the two B channels can be used for two independent voice services, or one of them can be used for voice and the other for a data service such as fax, teletex, or remote LAN access. Modest data communication requirements (e.g., remote banking transactions) may be met by the D channel alone. Other permitted combinations for basic access are: B+D or just D.  The Primary Rate Access (PRA) package is aimed at business users with greater bandwidth requirements. Primary access comes in two configurations: At a bit rate of 1.544 mbps (North America and Japan) and consisting of: -23 B channels -One 64 kbps D channel -Overhead of 8 kbps At a bit rate of 2.048 mbps (Europe) and consisting of: -30 B channels -One 64 kbps D channel -Overhead of 64 kbps As with the basic access, lower configurations are also possible, depending on requirements. Primary access can also support H channels. Standard bit rates:

B-channel : 64 kbps D-channel : 16 or 64 kbps H-channel : 384 (H0), 1536 (H11), 1920 (H12) kbps

ISDN Access Types ISDN offers two general types of access: • •

BASIC RATE ACCESS (BRA) PRIMARY RATE ACCESS (PRA)

 These differ from one another by the amount of information they can carry. Basic Rate Access

It should be viewed as a replacement for POTS (Plain Old Telephone Service). Basic Rate Access provides two 64-Kbps B channels and one 16-Kbps D channel (referred to as 2B+D). In other words, it provides transmission facilities for one voice conversation (one B channel), one medium-speed data session (the other B channel), and the signaling exchanges needed to make them work (the D channel). The separate channel for signaling results in a significantly faster setup time.  Two B channels at 64 Kbps plus one D channel at 16 Kbps equals 144K bps.  The ISDN Basic Rate transmission protocol uses an additional 48 Kbps of 

bandwidth for maintenance and synchronization, so an ISDN Basic Rate Access actually uses 192 Kbps.

Fig. 2-42. (a) Basic rate digital pipe. (b) Primary rate digital pipe Primary Rate Access

Primary Rate Access, which is based on pre-ISDN digital carrier technology, is designed to provide high-capacity service to large customers for applications such as PBX-to-PBX trunking. There are two kinds of Primary Rate Access: 23B+D and 30B+D. Each depends on the kind of digital carrier available in a given country. In North America and Japan, 23B+D Primary Rate Access operates at 1.544 Mbps and offers 23 B channels plus 1 64-Kbps D channel (usually located in time-slot 23), or 4 H0 channels, or 1 H11 channel. In most of the rest of the world, 30B+D Primary Rate Access operates at 2.048 Mbps and offers 30 B channels plus 1 64-Kbps D channel (located in time-slot 16), or 5 H0 channels, or 1 H12 channel. 23B + 1D (US and Japan) or 30B + 1D (Europe). It is intended for use at the T reference point for businesses with a PBX. Hybrid: 1A + 1C

Because ISDN is so focused on 64 kbps channels, it is referred to as N-ISDN (Narrowband ISDN), in contrast to broadband ISDN (ATM). Functional Groupings and Reference Points

User access to ISDN is provided at a number of different levels of abstraction.  These levels are defined by functional groupings, which encompass functions equivalent to those denoted by one or more OSI layers. The interfaces between the functional groupings are called reference points.

ISDN Devices

In the context of ISDN standards, STANDARD DEVICES refers not to actual hardware, but to standard collections of functions that can usually be performed by individual hardware units. The ISDN Standard Devices are: • • • • •

 Terminal Equipment (TE)  Terminal Adapter (TA) Network Termination 1 (NT1) Network Termination 2 (NT2) Exchange Termination (ET)

Terminal Equipment (TE)

A TE is any piece of communicating equipment that complies with the ISDN standards. Examples include: digital telephones, ISDN data terminals, Group IV Fax machines, and ISDN-equipped computers. In most cases, a TE should be able to provide a full Basic Rate Access (2B+D), although some TEs may use only 1B+D or even only a D channel. Terminal Equipment 1 (TE1) denotes ISDN terminals which use a 4-wire physical link to the S or S/T interface. TE1 devices conform to ISDN standards and protocols and are especially designed for use with ISDN. A digital ISDN telephone and a PC with an ISDN card are examples. Terminal Equipment 2 (TE2) denotes non-ISDN terminal equipment. Ordinary terminals and personal computers are examples. These devices can be connected to ISDN at the R (Rate) reference point. RS-232 and V.21 are examples of the type of standards that may be employed for the R reference point. The mapping between the R interface and the S or S/T interface is performed by a Terminal Adapter (TA), which performs the necessary protocol conversions and data rate adaptations between the two interfaces.

Terminal Adapter (TA)

A TA is a special interface-conversion device that allows communicating devices that don't conform to ISDN standards to communicate over the ISDN.

  The most common TAs provide Basic Rate Access and have one RJ-type modular jack for voice and one RS-232 or V.35 connector for data (with each port able to connect to either of the available B channels). Some TAs have a separate data connector for the D channel. Network Termination (NT1 and NT2)

 The NT devices, NT1 and NT2, form the physical and logical boundary between the customer's premises and the carrier's network. NT1 performs the logical interface functions of switching and local-device control (local signalling). NT2 performs the physical interface conversion between the dissimilar customer and network sides of the interface. In most cases, a single device, such as a PBX or digital multiplexer, performs both physical and logical interface functions. In ISDN terms, such a device is called NT12 ("NT-one-two") or simply NT. The Network Termination 1 (NT1) functional grouping provides OSI layer 1 capabilities and deals with signal transmission and physical connectors for interfacing Customer Premises Equipment (CPE) to ISDN. The NT1 transforms the U interface into a 4-wire subscriber S/T interface which supports 2B+D channels (in case of basic access) or T interface which supports 23B+D or 30B+D (in case of primary access). NT1 multiplexes these channels using TDM into a continuous bit stream for transmission over the U interface. NT1 also supports up to eight CPEs connected in a multidrop line arrangement to basic access. The NT1 device may be owned and operated by the service provider, baring the customer from direct access to the U interface, or it may be a CPE. The Network Termination 2 (NT2) functional grouping provides additional OSI layer 2 and 3 capabilities on top of NT1. NT2 is a CPE which transforms the  T (Terminal) interface into an S (System) interface. The S interface supports 2B+D channels. NT2 may perform switching and concentration functions. A typical NT2 device would be a digital PBX, serving a set of digital phones, or a LAN, serving a set of personal computers. Exchange Termination (ET)

 The ET forms the physical and logical boundary between the digital local loop and the carrier's switching office. It performs the same functions at the end office that the NT performs at the customer's premises. In addition, the ET: 1. Separates the B channels, placing them on the proper interoffice trunks to their ultimate destinations 2. Terminates the signalling path of the customer's D channel, converting any necessary end-to-end signalling from the ISDN D-channel signalling protocol to the carrier's switch-to- switch trunk signalling protocol

ISDN Interfaces (Standard Reference Points)

  The ISDN standards specify four distinct interfaces in the customer's connection to the network: R, S, T, and U. From the standards viewpoint, these are not "real" physical interfaces, but simply STANDARD REFERENCE POINTS where physical interfaces may be necessary. However, in common practice, the names of reference points are used to refer to physical interfaces. The R Interface

 The interface at reference point R is the physical and logical interface between a non-ISDN terminal device and a terminal adapter (TA). The R interface is not really part of the ISDN; it can conform to any of the common telephone or data interface standards. The S Interface

 The interface at reference point S is the physical and logical interface between a TE (or TA) and an NT. The S interface uses four wires and employs a bipolar transmission technique known as Alternate Mark Inversion (AMI). A special feature of the S interface is the "Short Passive Bus" configuration, which allows up to eight ISDN devices (TE or TA) to contend for packet access to the D channel in a prioritized, round-robin fashion. Only one device at a time can use a given B channel. The T Interface

 The interface at reference point T is the physical and logical interface between NT1 and NT2, whenever the two NTs are implemented as separate pieces of  hardware. The specification for the T interface is identical to the specification for the S interface. In most implementations, NT1 and NT2 exist in the same physical device, so there is no real T interface. The U Interface

 The interface at reference point U is the physical and logical interface between NT (or NT2) and the ISDN carrier's local transmission loop. It is also the legal demarcation between the carrier's loop and the customer's premises.  The U interface is implemented with two wires and uses a special quaternary signal format (i.e., four possible electrical states, with one pulse encoding a predefined combination of 2 bits) called 2B1Q. Quaternary encoding allows the U interface to carry data with a logical bit rate of 192 Kbps over a signal with a physical pulse rate of only 96 Kbps. The slower pulse rate is better suited to the less-predictable environment of the outside-plant loop carrier system.

The User Interface Figure A.l is a conceptual view of the lSDN from a user, or customer, point of  view. The user has access to the ISDN by means of a local interface to a "digital pipe" of a certain bit rate. Pipes of various sizes are available to satisfy differing needs. For example, a residential customer may require only sufficient capacity to handle a telephone and a videotex terminal. An office will

undoubtedly wish to connect to the ISDN via an on-premise digital PBX, and will require a much higher capacity pipe. At any given point in time, the pipe to the user's premises has a fixed capacity, but the traffic on the pipe may be a variable mix up to the capacity limit. Thus, a user may access circuit-switched and packet-switched services, as well as other services, in a dynamic mix of signal types and bit rates. To provide these services, the ISDN requires rather complex control signals to instruct it how to sort out the time multiplexed data and provide the required services. These control signals are also multiplexed onto the same digital pipe.

An important aspect of the interface is that the user may, at any time, employ less than the maximum capacity of the pipe, and will be charged according to the capacity used rather than "connect time." This characteristic significantly diminishes the value of current user design efforts that are geared to optimize circuit utilization by use of concentrators, multiplexers, packet switches, and other line sharing arrangements.

Architecture Figure A.2 is a block diagram of ISDN. ISDN supports a new physical connecter for users, a digital subscriber loop (link from end user to central or end office), and modifications to all central office equipment.

 The area to which most attention has been paid by standards organizations is that of user access. A common physical interface has been defined to provide, in essence, a DTE-DCE connection. The same interface should be usable for telephone, computer terminal, and videotex terminal. Protocols are needed for the exchange of control information between user device and the network. Provision must be made for high-speed interfaces to, for example, a digital PBX or a LAN.  The subscriber loop portion of today's telephone network consists of twisted pair links between the subscriber and the central office, carrying 4-kHz analog signals. Under the ISDN, one or two twisted pairs are used to provide a basic fullduplex digital communications link.  The digital central office connects the numerous ISDN subscriber loop signals to the IDN. In addition to providing access to the circuit-switched network, the central office provides subscriber access to dedicated lines, packet-switched networks, and time-shared, transaction-oriented computer services. Multiplexed access via digital PBX and LAN must also be accommodated. 3.10.1 Physical Interfaces

An understanding of the format of interfaces and channel type is critical to any analysis of ISDN because they provide the framework through which the protocols and applications flow. ISDN defines a full network architecture as shown Fig. 1. This architecture separates access functions from actual network functions.

Network termination (NT) equipment handles the communications from the network while the terminal equipment(TE) is responsible for the communications from the user. NT1 includes functions equivalent to the OSI's Physical layer. These functions include line termination, Layer 1 line maintenance and performance monitoring, timing, and Layer multiplexing. In contrast, NT2 devices are usually more intelligent than NT1 devices. NT2 devices perform the link layer functions and usually network functions as well. an NT2 devices may be a switch, multiplexer, LAN, PBX, or a terminal controller. NT12 is a single device that has the functionality of an NT1 and NT2. NT12 will handle the physical, link, and network layer protocols.  TE includes functions equivalent to Layer 1 and higher. Examples of TE are digital telephones, data terminal equipment, and integrated workstations. TE functions include protocol handling, maintenance, and interfacing. TE1's interface compiles with the ISDN user-network interface (UNI); TE2's does not.  TA includes functions equivalent to Layer 1 and higher. It enables a TE2 to be served by an ISDN user network interface.

  The S/T interface is intended for use on the two twisted-wire pairs(four-wire twisted pairs)-one to transmit and one to receive- ,allowing full-duplex communication, between the subscriber terminal equipment and the remote PBX terminal. U interface is a full-duplex running over a single pair of twisted wires between the subscriber terminal equipment and the central office switch, or between a network termination 1(ISDN destination NT1) and the central office switch. The conversion from four to two wires (NT1 to LT), needed to maintain consistency with current telephone networks, is done by an echocanceling algorithm.   The range of S and T interface is 3300 feet (1000 m) in a point-to point configuration or up to 500 feet (150 m) in a multi drop passive-bus configuration. The U interface will have a maximum length of 2500 to 6500 m.  The T bus is a multipoint bus. It is sometimes called passive bus because there are no repeaters on the line between the NT1 and the devices (TE1). It can be implemented using the same cable and connectors as is 10 base T Ethernet.  There may be up to 8 devices on the S/T bus. The bus may be formed with splitters and T connectors- it is bus, not a star. The D channel is used to control the attachment of the one to eight devices to the two B channels. No two devices attach to the same B channel at the same time.

ISDN System Architecture  The key idea behind ISDN is that of the digital bit pipe between the customer and the carrier through which bits flow in both directions. Whether the bits originate from a digital telephone, a digital terminal, a digital facsimile machine, or some other device is irrelevant.  The digital bit pipe can support multiple independent channels by time division multiplexing of the bit stream. Two principal standards for the bit pipe have been developed: •

a low bandwidth standard for home use, and

•

a higher bandwidth standard for business use that supports multiple channels identical to the home use channels.

Normal configuration for a home consists of a network terminating device NT1 (Fig. 2-41(a)) placed on the customer's premises and connected to the ISDN exchange in the carrier's office using the twisted pair previously used to connect the telephone. The NT1 box has a connector into which a bus cable can be inserted. Up to 8 ISDN telephones, terminals, alarms, and other devices can be connected to the cable. From the customer's point of view, the network boundary is the connector on NT1.

Fig. 2-41. (a) Example ISDN system for home use. (b) Example ISDN system with a PBX for use in large businesses

For large businesses, the model of Fig. 2-41(b) is used. There is a device NT2 called PBX (Private Branch eXchange - conceptually the same as an ISDN switch) there connected to NT1 and providing the interface for ISDN devices. CCITT defined four reference points (Fig. 2-41): U reference point = connection between the ISDN exchange and NT1,  T reference point = connector on NT1 to the customer, S reference point = interface between the ISDN PBX and the ISDN terminal, • • •

R reference point = the connection between the terminal adapter and nonISDN terminal.

ISDN Protocol Architecture Figure A.5 illustrates, in the context of the OSI model, the protocols defined or referenced in the ISDN documents. As a network, ISDN is essentially

unconcerned with user layers 4-7. These are end-to-end layers employed by the user for the exchange of information. Network access is concerned only with layers 1-3.

Layer 1, specifies the physical interface for both basic and primary access. Because B and D channels are multiplexed over the same physical interface, these standards apply to both types of channels. Above this layer, the protocol structure differs for the two channels. For the D channel, a new data link layer standard, LAPD (Link Access Protocol, D channel) has been defined. This standard is based on HDLC, modified to meet ISDN requirements. All transmission on the D channel is in the form of  LAPD frames that are exchanged between the subscriber equipment and an ISDN switching element. Three applications are supported: control signaling, packet switching, and telemetry. For control signaling, a call control protocol has been defined (1.451|Q.931).  This protocol is used to establish, maintain, and terminate connections on B channels; thus, it is a protocol between the user and the network. Above layer 3, there is the possibility for higher-layer functions associated with user-to-user control signaling. These functions are a subject for further study. The D channel can also be used to provide  packet-switching services to the subscriber. In this case, the X.25 level-3 protocol is used, and X.25 packets are transmitted in LAPD frames. The X.25 level-3 protocol is used to establish virtual circuits on the D channel to other users and to exchange packetized data. The final application area, telemetry, is a subject for further study.

 The B channel can be used for circuit switching, semipermanent circuits, and packet-switching. For circuit switching, a circuit is set up on a B channel, on demand. The D-channel call control protocol is used for this purpose. Once the circuit is set up, it may be used for data transfer between the users. A semipermanent circuit is a B-channel circuit that is set up by prior agreement between the connected users and the network. As with a circuit-switched connection, it provides a transparent data path between end systems. With either a circuit-switched connection or a semipermanent circuit, it appears to the connected stations that they have a direct, full-duplex link with each other. They are free to use their own formats, protocols, and frame synchronization. Hence, from the point of view of ISDN, layers 2 through 7 are not visible or specified. In the case of  packet-switching, a circuit-switched connection is set up on a B channel between the user and a packet-switched node using the D-channel control protocol. Once the circuit is set up on the B channel, the user may employ X.25 layers 2 and 3 to establish a virtual circuit to another user over that channel and to exchange packetized data. As an alternative, the frame relay service may be used. Frame relay can also be used over H channels and over the D channel. Link Access Protocol - D Channel (LAPD) Layer 2 protocol Almost identical to LAP-B used w/ X.25 (based on HDLC) Provides unacknowledged information-transfer service (unnumbered frames, error detection to discard frame but no error control or flow control) and acknowledged information transfer. • • •

Layer-3 information is transferred in unnumbered frames. Error detection is used to discard damaged frames, but there is no error control or flow control. Layer-3 information is transferred in frames that include sequence numbers and are acknowledged. Error-control and flow-control procedures are included in the protocol. This type is also referred to in the standard as multiple-frame operation. Physical Layer Frame Format

Each frame 1.5 msec long • SYNC field (9 quaternaries) : +3 +3 -3 -3 -3 +3 -3 +3 -3 • 12 * (8 bits from each B channel + 2 bits from D) • Maintenance contains CRC, other operation info.

ISDN Connections ISDN provides four types of service for end-to-end communication: Circuit-switched calls over a B channel. Semipermanent connections over a B channel. Packet-switched calls over a B channel. Packet-switched calls over the D channel. • • • •

Circuit-Switched Calls  The network configuration and protocols for circuit switching involve both the B and D channels. The B channel is used for the transparent exchange of user data. The communicating users may employ any protocols they wish for endto-end communication. The D channel is used to exchange control information between the user and the network for call establishment and termination, as well as to gain access to network facilities.

 The B channel is serviced by an NT1 or NT2 using only layer-1 functions. On the D channel, a three-layer network access protocol is used and is explained below. Finally, the process of establishing a circuit through ISDN involves the cooperation of switches internal to ISDN to set up the connection. These switches interact by using an internal protocol: Signaling-System Number7. Semipermanent Connections

A semipermanent connection between agreed points may be provided for an indefinite period of time after subscription, for a fixed period, or for agreedupon periods during a day, a week, or some other interval. As with circuitswitched connections, only Layer-1 functionality is provided by the network interface. The call-control protocol is not needed because the connection already exists. Packet-Switched Calls over a B Channel

 The ISDN must also permit user access to packet-switched services for data traffic (e.g., interactive) that is best serviced by packet switching. There are two possibilities for implementing this service: Either the packet-switching capability is furnished by a separate network, referred to as a packet-switched public data network (PSPDN), or the packet-switching capability is integrated into ISDN. In the former case, the service is provided over a B channel. In the latter case, the service may be provided over a B or D channel. We first examine the use of a B channel for packetswitching. When the packet-switching service is provided by a separate PSPDN, the access to that service is via a B channel. Both the user and the PSPDN must therefore be connected as subscribers to the ISDN. In the case of the PSPDN, one or more of the packet-switching network nodes, referred to as packet handlers, are connected to ISDN. We can think of each such node as a traditional X.25 DCE supplemented by the logic needed to access ISDN. That is, the ISDN subscriber assumes the role of an X.25 DTE, the node in the PSPDN to which it is connected functions as an X.25 DCE, and the ISDN simply

provides the connection from DTE to DCE. Any ISDN subscriber can then communicate, via X.25, with any user connected to the PSPDN, including • •

Users with a direct, permanent connection to the PSPDN. Users of the ISDN that currently enjoy a connection, through the ISDN, to the PSPDN.

 The connection between the user (via a B channel) and the packet handler with which it communicates may be either semipermanent or circuit-switched. In the former case, the connection is always there and the user may freely invoke X.25 to set up a virtual circuit to another user. In the latter case, the D channel is involved, and the following sequence of steps occurs (Figure A.6):

1. The user requests, via the D-channel call-control protocol (1.451/Q.931), a circuit-switched connection on a B channel to a packet handler. 2. The connection is set up by ISDN, and the user is notified via the D channel call-control protocol. 3. The user sets up a virtual circuit to another user via the X.25 call establishment procedure on the B channel (described in Section 3.2).  This step requires first that a data link connection, using LAPB, must be set up between the user and the packet handler. 4. The user terminates the virtual circuit, using X.25 on the B channel. 5. After one or more virtual calls on the B channel, the user is done and signals via the D channel to terminate the circuit-switched connection to the packetswitching node. 6. The connection is terminated by ISDN. Figure A.7 shows the configuration involved in providing this service. In the figure, the user is shown to employ a DTE device that expects an interface to an X.25 DCE. Hence, a terminal adapter is required. Alternatively, the X.25 capability can be an integrated function of an ISDN TE1 device, dispensing with the need for a separate TA.

When the packet-switching service is provided by ISDN, the packet-handling function is provided within the ISDN, either by separate equipment or as part of the exchange equipment. The user may connect to a packet handler either by a B channel or the D channel. On a B channel, the connection to the packet handler may be either switched or semipermanent, and the same procedures described above apply for switched connections. In this case, rather than establish a B-channel connection to another ISDN subscriber that is a PSPDN packet handler, the connection is to an internal element of ISDN that is a packet handler.

Packet-Switched Calls over a D Channel

When the packet-switching service is provided internal to the ISDN, it can also be accessed on the D channel. For this access, ISDN provides a semipermanent connection to a packet-switching node within the ISDN. The user employs the X.25 level-3 protocol, as is done in the case of a B-channel virtual call. Here, the level-3 protocol is carried by LAPD frames. Because the D channel is also used for control signaling, some means is needed to distinguish between X.25 packet traffic and ISDN control traffic; this is accomplished by means of the link-layer addressing scheme, as explained below. Figure A.8 shows the configuration for providing packet-switching within ISDN.  The packet-switching service provided internal to the ISDN over the B and D channels is logically provided by a single packet-switching network. Thus, virtual calls can be set up between two D-channel users, two B-channel users, and between a B and D channel user. In addition, it will be typical to also provide access to X.25 users on other ISDNs and PSPDNs by appropriate interworking procedures.

ISDN Specifications  This section describes the various ISDN specifications for Layer 1, Layer 2, and Layer 3.

Layer 1 ISDN physical layer (Layer 1) frame formats differ depending on whether the frame is outbound (from terminal to network) or inbound (from network to terminal). Both physical layer interfaces are shown in Figure 12-2.

 The frames are 48 bits long, of which 36 bits represent data. The bits of an ISDN physical layer frame are used as follows:

Figure 12-2 ISDN Physical Layer Frame Formats Differ Depending on Their  Direction

•F—Provides synchronization •L—Adjusts the average bit value • E—Ensures contention resolution when several terminals on a passive bus contend for a channel •A—Activates devices •S—Is unassigned •B1, B2, and D—Handle user data Multiple ISDN user devices can be physically attached to one circuit. In this configuration, collisions can result if two terminals transmit simultaneously.  Therefore, ISDN provides features to determine link contention. When an NT receives a D bit from the TE, it echoes back the bit in the next E-bit position.  The TE expects the next E bit to be the same as its last transmitted D bit.  Terminals cannot transmit into the D channel unless they first detect a specific number of ones (indicating "no signal") corresponding to a pre-established priority. If the TE detects a bit in the echo (E) channel that is different from its D bits, it must stop transmitting immediately. This simple technique ensures

that only one terminal can transmit its D message at one time. After successful D-message transmission, the terminal has its priority reduced by requiring it to detect more continuous ones before transmitting. Terminals cannot raise their priority until all other devices on the same line have had an opportunity to send a D message. Telephone connections have higher priority than all other services, and signaling information has a higher priority than nonsignaling information. Layer 2

Layer 2 of the ISDN signaling protocol is Link Access Procedure, D channel (LAPD). LAPD is similar to High-Level Data Link Control (HDLC) and Link Access Procedure, Balanced (LAPB) (see Chapter 16, "Synchronous Data Link Control and Derivatives," and Chapter 17, "X.25," for more information on these protocols). As the expansion of the LAPD acronym indicates, this layer is used across the D channel to ensure that control and signaling information flows and is received properly. The LAPD frame format (see Figure 12-3) is very similar to that of HDLC; like HDLC, LAPD uses supervisory, information, and unnumbered frames. The LAPD protocol is formally specified in ITU-T Q.920 and ITU-T Q.921. Figure 12-3 LAPD Frame Format Is Similar to That of HDLC and LAPB

  The LAPD Flag and Control fields are identical to those of HDLC. The LAPD Address field can be either 1 or 2 bytes long. If the extended address bit of the first byte is set, the address is 1 byte; if it is not set, the address is 2 bytes.  The first Address-field byte contains the service access point identifier (SAPI), which identifies the portal at which LAPD services are provided to Layer 3. The C/R bit indicates whether the frame contains a command or a response. The  Terminal Endpoint Identifier (TEI) field identifies either a single terminal or multiple terminals. A TEI of all ones indicates a broadcast. Layer 3  Two Layer 3 specifications are used for ISDN signaling: ITU-T (formerly CCITT) I.450 (also known as ITU-T Q.930) and ITU-T I.451 (also known as ITU-T Q.931).  Together, these protocols support user-to-user, circuit-switched, and packetswitched connections. A variety of call-establishment, call-termination, information, and miscellaneous messages are specified, including SETUP, CONNECT, RELEASE, USER INFORMATION, CANCEL, STATUS, and DISCONNECT.

  These messages are functionally similar to those provided by the X.25 protocol. Figure 12-4, from ITU-T I.451, shows the typical stages of an ISDN circuit-switched call.

ISDN Connection Establishment and Release

. Benefits of ISDN

ISDN affords many benefits to service providers and customers. The increasing popularity of ISDN allows pricing that continues to fall and compete with standard analog service. Some of the many benefits are: •

•

•

•

•

•

•

Simultaneous audio, video, and data services over a single pair of copper wires reduces infrastructure and maintenance costs for service and subscribers. ISDN BRI service can use data compression which boosts the 128 Kbps transmission rate to between 256 Kbps and 632 Kbps, depending upon the compression ratio used. Digital transmissions produce clearer and quieter voice telephone service and more reliable and accurate connectivity than analog technology. Remote computer users benefit from high performance ISDN connections at home or on the road. ISDN’s dynamic bandwidth allocation feature accommodates the bandwidth-intensive applications. Up to eight different devices can be operated simultaneously over a single ISDN line. LAN protocols such as IP and IPX are better supported by ISDN connections across WANs due to faster connect times (between 1 and 4 seconds) than analog service (between 10 and 40 seconds).

•

ISDN is compatible with other WAN services like X.25, Frame Relay, Switched Multi-megabit Data Services (SMDS) and higher speed services like Asynchronous Transfer Mode (ATM).

Applications Of ISDN •

ISDN in Business

For business users and even residential subscribers, videoconferencing is the biggest communication advancement that ISDN has to offer. With the simultaneous high speed transfer of voice and video, ISDN can provide real time video communication on a PC that once was only capable on sophisticated systems costing upwards of $100,000. A shared electronic chalk board is another tool available through ISDN. Ideas and illustrations can be distributed in real time to remote locations so people in other cities or other countries can participate in meetings.  Telecommuting is becoming a rule more than an exception; more and more people are working from home. ISDN provides the facilities for users to tap into central network resources from the privacy of their own homes and do so with the functionality of a network node. Node connections are possible with Serial Line Interface Protocol (SLIP) and Point-to-Point Protocol (PPP). •

ISDN in Education

Students will also reap the benefits of videoconferencing by relating with other students worldwide. Using the video capabilities of ISDN allows students to see the surroundings of other countries or speak with pen-pals. The value of  videoconferencing in educational settings is unlimited. Computers have become important learning tools for students. Children are introduced to computers and networking at an early age, and ISDN allows the high speed connections to vast amounts of information and resources

BROADBAND ISDN In the 1980s the telecommunications industry expected that digital services would follow much the same pattern as voice services did on the public switched telephone network, and conceived a grandiose vision of end-to-end circuit switched services, known as the Broadband Integrated Services Digital Network (B-ISDN). This was designed in the 1990s as a logical extension of the end-to-end circuit switched data service, ISDN. CCITT modestly defines B-ISDN as "a service requiring transmission channels cable of supporting rates greater than the primary rate." Behind this innocuous statement lie plans for a network and a set of services that will have far more impact on business and residential customers than ISDN. With B-ISDN, services, especially video services, requiring data rates in excess of those that can be delivered by ISDN will become available. To contrast this new network

and these new services to the original concept of ISDN, that original concept is now being referred to as narrowband ISDN.  The technology for B-ISDN was going to be Asynchronous Transfer Mode (ATM), which was intended to carry both synchronous voice and asynchronous data services on the same transport. The B-ISDN vision has been overtaken by the disruptive technology of the Internet. The ATM technology survives as a lowlevel layer in most DSL technologies, and as a payload type in some wireless technologies such as WiMAX. Broandband Integrated Services Digital Network (BISDN or Broadband ISDN) is designed to handle high-bandwidth applications. BISDN currently uses ATM technology over SONET-based transmission circuits to provide data rates from 155 to 622Mbps and beyond, contrast with the traditional narrowband ISDN (or N-ISDN), which is only 64 Kbps basically and up to 2 Mbps.  The designed Broadband ISDN (BISDN) services can be categorized as follows: Conversational services such as telephone-like services, which was also supported by N-ISDN. Also the additional bandwidth offered will allow such services as video telephony, video conferencing and high volume, high speed data transfer. Messaging services, which is mainly a store-and-forward type of service. Applications could include voice and video mail, as well as multi-media mail and traditional electronic mail. Retrieval services which provides access to (public) information stores, and information is sent to the user on demand only. No user control of presentation. This would be for instance, a TV broadcast, where the user can choose simply either to view or not. User controlled presentation. This would apply to broadcast information that the user can partially control.  The B-ISDN is designed to offer both connection oriented and connectionless services. The broadband information transfer is provided by the use of  asynchronous transfer mode (ATM), in both cases, using end-to-end logical connections or virtual circuits. Broadband ISDN uses out-of-band signaling (as does N-ISDN). Instead of using a D Channel as in N-ISDN, a special virtual circuit channel can be used for signaling. However, B-ISDN was not widely deployed so far. •

•

•

•

•

Broadband ISDN Architecture B-ISDN differs from a narrowband ISDN in a number of ways. To meet the requirement for high-resolution video, an upper channel rate of approximately 150 Mbps is needed. To simultaneously support one or more interactive and distributive services, a total subscriber line rate of about 600 Mbps is needed. In terms of today's installed telephone plant, this is a stupendous data rate to sustain. The only appropriate technology for widespread support of such data rates is optical fiber. Hence, the introduction of B-ISDN depends on the pace of  introduction of fiber subscriber loops. Internal to the network, there is the issue of the switching technique to be used. The switching facility has to be capable of handling a wide range of  different bit rates and traffic parameters (e.g., burstiness). Despite the increasing power of digital circuit-switching hardware and the increasing use of  optical fiber trunking, it is difficult to handle the large and diverse

requirements of B-ISDN with circuitswitching technology. For this reason, there is increasing interest in some type of fast packet-switching as the basic switching technique for B-ISDN. This form of switching readily supports ATM at the user-network interface. Functional Architecture Figure A.12 depicts the functional architecture of B-ISDN. As with narrowband ISDN, control of B-ISDN is based on common-channel signaling. Within the network, an SS7, enhanced to support the expanded capabilities of a higherspeed network, is used. Similarly, the user-network control-signaling protocol is an enhanced version of I.451lQ.931. B-ISDN must, of course, support all of the 64-kbps transmission services, both circuit-switching and packet-switching, that are supported by narrowband ISDN; this protects the user's investment and facilitates migration from narrowband to broadband ISDN. In addition, broadband capabilities are provided for higher datarate transmission services. At the user-network interface, these capabilities will be provided with the connection-oriented asynchronous transfer mode (ATM) facility.

User-Network Interface

 The reference configuration defined for narrowband ISDN is considered general enough to be used for B-ISDN. Figure A.13, which is almost identical to Figure A.4, shows the reference configuration for B-ISDN. In order to clearly illustrate the broadband aspects, the notations for reference points and functional groupings are appended with the letter B (e.g., B-NT1, TB). The broadband functional groups are equivalent to the functional groups defined for narrowband ISDN, and are discussed below. Interfaces at the R reference point may or may not have broadband capabilities.

Transmission Structure In terms of data rates available to B-ISDN subscribers, three new transmission services are defined. The first of these consists of a full-duplex 155.52-Mbps service. The second service defined is asymmetrical; providing transmission from the subscriber to the network at 155.52 Mbps, and in the other direction at 622.08 Mbps; and the highest-capacity service yet defined is a full-duplex, 622.08-Mbps service. A data rate of 155.52 Mbps can certainly support all of the narrowband ISDN services. That is, such a rate readily supports one or more basic- or primaryrate interfaces; in addition, it can support most of the B-ISDN services. At that rate, one or several video channels can be supported, depending on the video resolution and the coding technique used. Thus, the full-duplex 155.52-Mbps service will probably be the most common B-ISDN service.   The higher data rate of 622.08 Mbps is needed to handle multiple video distribution, such as might be required when a business conducts multiple simultaneous videoconferences. This data rate makes sense in the network-tosubscriber direction. The typical subscriber will not initiate distribution services and thus would still be able to use the lower, 155.52-Mbps service. The fullduplex, 622.08 Mbps service would be appropriate for a video-distribution provider.