Content Overview Integrated Services Digital Network (ISDN) is a network that provides end-to-end digital connectivity to support a wide range of services including voice and data services.ISDN allows multiple digital channels to operate simultaneously through the same regular phone wiring used for analog lines, but ISDN transmits a digital signal rather than analog. Latency is much lower on an ISDN line than on an analog line. Dial-on-demand routing (DDR) is a technique developed by Cisco that allows the use of existing telephone lines to form a wide-area network (WAN), instead of using separate, dedicated lines. Public switched telephone networks (PSTNs) are involved in this process. DDR is used when a constant connection is not needed, thus reducing costs. DDR defines the process of a router connecting using a dialup network when there is traffic to send, and then disconnecting when the transfer is complete. Students completing this module should be able to:
Content 4.1 ISDN Concepts 4.1.1 Introducing ISDN There are several WAN technologies used to provide network access from remote locations. One of these technologies is ISDN. ISDN can be used as a solution to the low bandwidth problems that small offices or dial-in users have with traditional telephone dial-in services. The traditional PSTN was based on an analog connection between the customer premises and the local exchange, also called the local loop. The analog circuits introduce limitations on the bandwidth that can be obtained on the local loop. Circuit restrictions do not permit analog bandwidths greater than approximately 3000 Hz. ISDN technology permits the use of digital data on the local loop, providing better access speeds for the remote users. Telephone companies developed ISDN with the intention of creating a totally digital network. ISDN allows digital signals to be transmitted over existing telephone wiring. This became possible when the telephone company switches were upgraded to handle digital signals. ISDN is generally used for telecommuting and networking small and remote offices into the corporate LAN. Telephone companies developed ISDN as part of an effort to standardize subscriber services. This included the User-Network Interface (UNI), better known as the local loop. The ISDN standards define the hardware and call setup schemes for end-to-end digital connectivity. These standards help achieve the goal of worldwide connectivity by ensuring that ISDN networks easily communicate with one another. In an ISDN network, the digitizing function is done at the user site rather than the telephone company. ISDN brings digital connectivity to local sites. The following list provides some of the benefits of ISDN: ISDN is a versatile service able to carry voice, video, and data traffic. It is possible to use multiple channels to carry different types of traffic over a single connection. ISDN uses out-of-band signaling, the delta (D channel), for call setup and signaling. To make a normal telephone call, the user dials the number one digit at a time. Once all the numbers are received, the call can be placed to the remote user. ISDN delivers the numbers to the switch at D-channel rates, thus reducing the time it takes to set up the call. ISDN also provides more bandwidth than a traditional 56 kbps dialup connection. ISDN uses bearer channels, also called B channels, as clear data paths. Each B channel provides 64 kbps of bandwidth. With multiple B channels, ISDN offers more bandwidth for WAN connections than some leased services. An ISDN connection with two B channels would provide a total usable bandwidth of 128 kbps. Each ISDN B channel can make a separate serial connection to any other site in the ISDN network. Since PPP operates over both synchronous and asynchronous serial links, ISDN lines can be used in conjunction with PPP encapsulation. Web Links Integrated Services Digital Network (ISDN) http://www.cisco.com/univercd/cc/ td/doc/cisintwk/ ito_doc/ isdn.htm
Content 4.1 ISDN Concepts 4.1.2 ISDN standards and access methods Work on standards for ISDN began in the late 1960s. A comprehensive set of ISDN recommendations was published in 1984 and is continuously updated by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T), formerly known as the Consultative Committee for International Telegraph and Telephone (CCITT). The ISDN standards are a set of protocols that encompass digital telephony and data communications. The ITU-T groups and organizes the ISDN protocols according to the following general topic areas: ISDN standards define two main channel types, each with a different transmission rate. The bearer channel, or B channel, is defined as a clear digital path of 64 kbps. It is said to be clear because it can be used to transmit any type of digitized data in full-duplex mode. For example, a digitized voice call can be transmitted on a single B channel. The second channel type is called a delta channel, or D channel. There can either be 16 kbps for the Basic Rate Interface (BRI) or 64 kbps for the Primary Rate Interface (PRI). The D channel is used to carry control information for the B channel. When a TCP connection is established, there is an exchange of information called the connection setup. This information is exchanged over the path on which the data will eventually be transmitted. Both the control information and the data share the same pathway. This is called in-band signaling. ISDN however, uses a separate channel for control information, the D channel. This is called out-of-band signaling. ISDN specifies two standard access methods, BRI and PRI. A single BRI or PRI interface provides a multiplexed bundle of B and D channels. BRI uses two 64 kbps B channels plus one 16kbps D channel. BRI operates with many Cisco routers. Because it uses two B channels and one D channel, BRI is sometimes referred to as 2B+D. The B channels can be used for digitized speech transmission. In this case, specialized methods are used for the voice encoding. Also, the B channels can be used for relatively high-speed data transport. In this mode, the information is carried in frame format, using either high-level data link control (HDLC) or PPP as the Layer 2 protocol. PPP is more robust than HDLC because it provides a mechanism for authentication and negotiation of compatible link and protocol configuration. ISDN is considered a circuit-switched connection. The B channel is the elemental circuit-switching unit. The D channel carries signaling messages, such as call setup and teardown, to control calls on B channels. Traffic over the D channel employs the Link Access