7.0 INTEGRATED SERVICES DIGITAL NETWORKS (ISDN) 7.1 Background and Goals of ISDN 7.2 User Access and Interface 7.3 ISDN Protocol Structure 7.3.1 ISDN and OSI 7.3.2 Layer 1 Interface, Basic Rate 7.3.3 Layer 1 Interface, Primary Rate 7.4 Layer Interface: Link Access Procedure for D Channel 7.5 Layer 3 Specification

What Is ISDN?

ISDN Defined Known as the Integrated Services Digital Network Data, audio, image and video transmission It is a switched digital telecommunication line that can be delivered over regular copper wires Possible to provide end-to-end digital communications

ISDN Application Examples On/off ramp to the information super-highway to communicate at speeds of 128 Kbps for a single ISDN line Multiple ISDN lines can be combined together to achieve higher communication speeds Home use On/Off ramp to the Internet Business world Provide remote access to LANs .

ISDN Standard There is a national ISDN standard known as the NI standard It is being implemented in phases Current implementation is termed as NI-1 to indicate that it is in phase 1 of the implementation process Can be obtained from a local telephone company in the same way an analog connection is obtained Phone companies offer different types of ISDN connections

Features of ISDN Uses Digital Signal Uses Existing telephone wiring Charges are generally based on the duration of call (How long the WAN link was used) Alternate to using leased lines Can transport many types of Network traffic (Voice, Data, Video, Text, Graphics etc) Faster Data transfer rate than modems Faster Call setup than Modems

ISDN Architecture Subscriber premises Digital subscriber Subscriber Network Subscriber premises >64kbps non-switched capabilities >64kbps switched capabilities Digital subscriber loop to C. O. Subscriber interface to ISDN 64kbps ckt non-switched capabilities Network Termination (NT) Subscriber site or service provider Terminal equipment (TE) ISDN switch >64kbps ckt switched capabilities ISDN switch Packet switched capabilities User-network signaling Frame mode capabilities Common- channel signaling capabilities User-network signaling

Fundamentals Service types Basic Rate Interface (2 B channels + 1 D channel (16 kb/s)) Primary Rate Interface (30 B channels + 1 D channel (64 kb/s)) Primary Rate Interface (23 B channels + 1 D channel (64 kb/s)) for North America

ISDN Service BRI (Basic Rate Interface) Connection from the ISDN office to the user location provides for access to three channels. The channels are two 64Kb B-channels and one 16Kb D-channel The B-channels and the D-channel provide the user with access to the circuit switched network

ISDN Service PRI (Primary Rate Interface) ISDN Primary Rate Interface service provides digital access via a T1 line. A T1 line provides a 1.544 bandwidth. This bandwidth is divided into 24 64Kb channels. The ISDN PRI service uses 23 B channel access and uses the 24th (D) channel for signaling purposes

Basic Rate Interface (BRI) Primary Rate Interface (PRI) ISDN Interfaces Basic Rate Interface (BRI) 144 Kbps 2B D } 64 Kbps 16 Kbps Primary Rate Interface (PRI) 1.544 Mbps in U.S. 2.048 Mbps in Europe 23B or 30B D 64 Kbps }

User Access and Interface

GENERAL ISDN transmission rates were to be accommodated on copper wire pairs. The objectives of any digital interface design and specifically of ISDN access and interface, are as follows: i. Electrical and mechanical specification ii. Channel structure and access capabilities iii. User network protocols iv. Maintenance and operation v. Performance vi. Services

ISDN SINGLE ISDN TERMINAL MULTIPLE ISDN TERMINAL INSTALLATION PBX/CBX/LAN PRIVATE NETWORK MAINFRAME COMPUTERS; BULK STORAGE OTHERS

ISDN Devices Terminal Adapter (TA) - Converter device that converts standard electrical signals into the form used by ISDN - allows non-ISDN devices to operate on an ISDN network. Terminal Equipment Type 1 (TE1) - Compatible with the ISDN network. Example:Telephones, personal computers, fax machine or videoconferencing machine.

ISDN Devices Terminal Equipment Type 2 (TE2) - Not compatible with the ISDN network. Example: Analog phone or modem, requires a TA (TE2 connects to TA). Network termination type 1 & 2 (NT1 and NT2) - A small connection box that physically connects the customer site to the Telco local loop, provides a four-wire connection to the customer site and a two-wire connection to the network (PRI – CSU/DSU).

ISDN Components and Reference Points

ISDN Reference Points U - Two wire cable that connects the customer’s equipment to the telecommunications provider R - Point between non-ISDN equipment (TE2) and the TA S - Four-wire cable from TE1 or TA to the NT1 or NT2 T - Point between NT1 and NT2

Analogies NT-1 (Network Terminator-1) An NT-1 is an interface box that converts ISDN data into something a PC can understand (and vice versa). It works a little like a cable TV descrambler for ISDN signals, and is often built into ISDN adapters..

Analogies TA (Terminal Adapter) This chunk of hardware converts the data it receives over ISDN to a form your computer can understand. Sometimes mistakenly called an ISDN modem or a digital modem, a terminal adapter handles data digitally and does not need to modulate or demodulate an analog signal. Terminal adapters can be an internal board or an external board that connects to the computer through the serial port.

ISDN Components and Reference Points #2

ISDN Reference Points

ISDN and the OSI Reference Model The ISDN Physical Layer The ISDN Data Link Layer The ISDN Network Layer

ITU-T Standards of the First Three Layers of ISDN

ISDN Protocols E-series protocols—Telephone network standards for ISDN. I-series protocols—Specify ISDN concepts and interfaces. Q-series protocols—Standards for ISDN switching and signaling. Operate at the physical, data link, and network layers of the OSI reference model Developed by the ITU-T are organized and grouped into three different series: (Know these – what is on the slide is adequate) A. E-series protocols – Telephone network standards for ISDN. For example, E.164 specifies international ISDN addressing B. I-series protocols – Specify ISDN concepts and interfaces. Grouped into four subcategories: 1. I.100 series – Specify ISDN concepts and structures 2. I.200 series – Deals with ISDN service aspects 3. I.300 series – Deals with network layer aspects 4. I.400 series – Deals with the user network interface (UNI) C. Q-series protocols – Specify standards for ISDN switching and signaling 1. Q.921 describes Link Access Procedure on the D Channel (LAPD). 2. Q.931 defines Layer 3 features Teaching Tip A simple way to tell these Q protocols apart is to use the 2nd digit, 2 for Layer 2 and 3 for Layer 3. Cute but effective. Recognizing the E, I and Q protocols and their general purpose (Q: switching and signaling) is critical for the CCNA exam. The E and I subcategories are not likely but recognizing what you see in C and the Teaching Tip above should be assumed D. The ISDN protocols operate at the physical, data-link, and network layers of the OSI reference model. Examples: 1. Physical Layer ISDN Protocols – BRI is defined by the ITU-T I.430 protocol (PRI is defined by ITU-T I.431), which describes the physical connections between the ISDN CPE and the ISDN local exchange. I.430 also defines two ISDN physical layer frame formats. a) Inbound frame formats (frames that travel from the local exchange to the ISDN customer) b) Outbound frame formats (frames that travel from the ISDN customer to the local exchange) 2. Data Link Layer ISDN protocols – LAPD signaling protocol, defined by ITU-T Q.920 (BRI) and Q.921 (PRI) for transmitting control and signaling information over the D channel between the ISDN CPE and local exchange a) LAPD frame format is similar to that of the ISO HDLC frame format. Like HDLC and PPP it include a flag, an address, a control, an information (data), and a frame check sequence (FCS) field. Don’t confuse LAPD with Link Access Procedure, Balanced (LAPB), the data-link layer protocol used in X.25 networks (Chapter 17) 3. Network Layer ISDN protocols - Responsible for specifying the switching and signaling in the end-to-end ISDN communication over the D channel. These protocols include ITU-T I.930) and ITU-T Q.931 There are two useful ISDN debug commands “debug isdn q921” to troubleshoot L2 connectivity to the switch. Also “debug isdn q931” to troubleshoot call processing

ISDN Protocol Operating OSI Layers 1 Through 3 Physical layer ISDN protocols BRI (ITU-T I.430) / PRI (ITU-T I.431) Defines two ISDN physical layer frame formats Inbound (local exchange to ISDN customer) Outbound (ISDN customer to local exchange) The ISDN protocols operate at the physical, data-link, and network layers of the OSI reference model. Examples: 1. Physical Layer ISDN Protocols – BRI is defined by the ITU-T I.430 protocol (PRI is defined by ITU-T I.431), which describes the physical connections between the ISDN CPE and the ISDN local exchange. I.430 also defines two ISDN physical layer frame formats. a) Inbound frame formats (frames that travel from the local exchange to the ISDN customer) b) Outbound frame formats (frames that travel from the ISDN customer to the local exchange) 2. Data Link Layer ISDN protocols – LAPD signaling protocol, defined by ITU-T Q.920 (BRI) and Q.921 (PRI) for transmitting control and signaling information over the D channel between the ISDN CPE and local exchange a) LAPD frame format is similar to that of the ISO HDLC frame format. Like HDLC and PPP it include a flag, an address, a control, an information (data), and a frame check sequence (FCS) field. Don’t confuse LAPD with Link Access Procedure, Balanced (LAPB), the data-link layer protocol used in X.25 networks (Chapter 17) 3. Network Layer ISDN protocols - Responsible for specifying the switching and signaling in the end-to-end ISDN communication over the D channel. These protocols include ITU-T I.930 and ITU-T Q.931 Teaching Tip There are two useful ISDN debug commands “debug isdn q921” to troubleshoot L2 connectivity to the switch. Also “debug isdn q931” to troubleshoot call processing Note: With Q.921/Q.931 the second digit indicates the OSI layer.

ISDN Protocol Operating OSI Layers 1 Through 3 Data link layer ISDN protocols LAPD signaling protocol (ITU-T Q.920 for BRI and Q.921 for PRI) for transmitting control and signaling information over the D channel LAPD frame format similar to ISO HDLC frame format Network layer ISDN protocols ITU-T I.930 and ITU-T Q.931 defines switching and signaling methods using the D channel. The ISDN protocols operate at the physical, data-link, and network layers of the OSI reference model. Examples: 1. Physical Layer ISDN Protocols – BRI is defined by the ITU-T I.430 protocol (PRI is defined by ITU-T I.431), which describes the physical connections between the ISDN CPE and the ISDN local exchange. I.430 also defines two ISDN physical layer frame formats. a) Inbound frame formats (frames that travel from the local exchange to the ISDN customer) b) Outbound frame formats (frames that travel from the ISDN customer to the local exchange) 2. Data Link Layer ISDN protocols – LAPD signaling protocol, defined by ITU-T Q.920 (BRI) and Q.921 (PRI) for transmitting control and signaling information over the D channel between the ISDN CPE and local exchange a) LAPD frame format is similar to that of the ISO HDLC frame format. Like HDLC and PPP it include a flag, an address, a control, an information (data), and a frame check sequence (FCS) field. Don’t confuse LAPD with Link Access Procedure, Balanced (LAPB), the data-link layer protocol used in X.25 networks (Chapter 17) 3. Network Layer ISDN protocols - Responsible for specifying the switching and signaling in the end-to-end ISDN communication over the D channel. These protocols include ITU-T I.930 and ITU-T Q.931 Teaching Tip There are two useful ISDN debug commands “debug isdn q921” to troubleshoot L2 connectivity to the switch. Also “debug isdn q931” to troubleshoot call processing Note: With Q.921/Q.931 the second digit indicates the OSI layer.

ISDN Physical Layer ISDN physical-layer frame formats are 48 bits long, of which 36 bits represent data

ISDN Data Link Layer Frame format is very similar to that of HDLC

ISDN Network Layer Two Layer 3 specifications are used for ISDN signaling: ITU-T I.450 (also known as ITU-T Q.930) ITU-T I.451 (also known as ITU-T Q.931) Together, these protocols support: User-to-user circuit-switched connections User-to-user packet-switched connections A variety of standards for: Call establishment Call termination

7.4 Layer Interface: Link Access Procedure for D Channel

Link access procedure (LAP) for D channel (LAPD) is used to convey information between layer 3 entities across the ISDN used network interface using D channel. 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. Associated with each data link layer (OSI layer 2) is one or more data link connection endpoints. Data link connection endpoint is identified by a data link connection endpoint identifier, from layer 3 and by data link connection identifier (DLDI), as from the data link layer

. Layer 3 entity Data Link Layer entity Data link layer service access point . Data link connection endpoint Data Link Layer entity Data link layer Entities, service access points (SAPs) and endpoints.

Between the data link layer and its adjacent layers there are four types of service primitives; Request – used where a higher layer is requesting service from the next lower layer Indication – used by a layer providing service to notify the next higher layer of activities related to the REQUEST primitive

Response – used by a layer to acknowledge receipt from a lower layer of the INDICATON primitive Confirm – the confirm primitive is used by the layer providing the requested service to confirm that the requested activity has been complete

FRAME STRUCTURE FOR PEER TO PEER COMMUNICATION LAYER 2 FRAME STRUCTURE FOR PEER TO PEER COMMUNICATION There are two frame formats used on layer 2 frames: Format A, for frames where there is no information field Format B, for frames containing an information field

Flag Field The opening and closing flags are used for frame synchronization. These flag are called the opening and closing flag. If sequential frames are transmitted, the closing flag of one frame is opening flag of the next frame. Address Field The address consists of two octets and identifies the intended receiver of a command frame and the transmitter of a response frame.

Control Field Consists of one or two octets. It identifies the type of frame, either command or response. It contains sequence numbers where applicable. There are three (3) types of control field formats. Information Field The information field of a frame, follows the control field and precedes the frame check sequence (FCS), contains an integer number of octets