Integrated Services Digital Network (ISDN) is a digital service designed to run over existing telephone networks. ISDN can support both data and voice—a telecommuter’s dream. But ISDN applications require bandwidth. Typical ISDN applications and implementations include high-speed image applications (such as Group IV facsimile), high-speed file transfer, videoconferencing, and multiple links into homes of telecommuters. ISDN is actually a set of communication protocols proposed by telephone companies that allows them to carry a group of digital services that simultaneously convey data, text, voice, music, graphics, and video to end users, and it was designed to achieve this over the telephone systems already in place. ISDN is referenced by a suite of ITU-T standards that encompass the OSI model’s Physical, Data Link, and Network layers. The ISDN standards define the hardware and call-setup schemes for end-to-end digital connectivity.
PPP is typically used with ISDN to provide data encapsulation, link integrity, and authentication. These are the benefits of ISDN:
* Can carry voice, video, and data simultaneously
* Has faster call setup than a modem
* Has faster data rates than a modem connection
ISDN Components:
The components used with ISDN include functions and reference points. Figure 6 shows how the different types of terminal and reference points can be used in an ISDN network.
In North America, ISDN uses a two-wire connection into a home or office. That is called a “U” reference point. The NT1 device is used to convert the two-wire connection to a four-wire connection that is used by ISDN phones and terminal adapters (TAs). Most routers can now be purchased with a built-in NT1 (U) interface.
Figure 7 shows the different reference points and terminal equipment that can be used with Cisco ISDN BRI interfaces.
ISDN Terminals: Devices connecting to the ISDN network are known as terminal equipment (TE) and network termination (NT) equipment. There are two types of each: TE1 Terminal equipment type 1 refers to those terminals that understand ISDN standards and can plug right into an ISDN network.
TE2 Terminal equipment type 2 refers to those that predate ISDN standards. To use a TE2, you have to use a terminal adapter (TA) to be able to plug into an ISDN network.
NT1 Network termination 1 implements the ISDN Physical layer specifications and connects the user devices to the ISDN network.
NT2 Network termination 2 is typically a provider’s equipment, such as a switch or PBX. It also provides Data Link and Network layer implementation.
It’s very rare at a customer premises.
TA Terminal adapter converts TE2 wiring to TE1 wiring that then connects into an NT1 device for conversion into a two-wire ISDN network.
ISDN Reference Points
Reference points are a series of specifications that define the connection between the various equipment used in an ISDN network. ISDN has four reference points that define logical interfaces:
R reference point Defines the reference point between non-ISDN equipment (TE2) and a TA.
S reference point Defines the reference point between the customer router and an NT2. Enables calls between the different customer equipment.
T reference point Defines the reference point between NT1 and NT2 devices. S and T reference points are electrically the same and can perform the same function. Therefore, they are sometimes referred to as an S/T reference point.
U reference point Defines the reference point between NT1 devices and line-termination equipment in a carrier network. (This is only in North America where the NT1 function isn’t provided by the carrier network.)
ISDN Protocols:
ISDN protocols are defined by the ITU, and there are several series of protocols dealing with diverse issues:
* Protocols beginning with the letter E deal with using ISDN on the existing telephone network.
* Protocols beginning with the letter I deal with concepts, aspects, and services.
* Protocols beginning with the letter Q cover switching and signaling.
ISDN Switch Types
We can credit AT&T and Nortel for the majority of the ISDN switches in place today, but additional companies also make them. In Table 10.1 under “Keyword,” you’ll find the right keyword to use along with the isdn switch-type command to configure a router for the variety of switches it’s going to connect to. If you don’t know which switch your provider is using at their central office, simply call them to find out.
Switch Type Keyword
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AT&T basic rate switch - Basic-5ess
Nortel DMS-100 basic rate switch - Basic-dms100
National ISDN-1 switch - Basic-ni1
AT&T 4ESS (ISDN PRI only) - Primary-4ess
AT&T 5ESS (ISDN PRI only) - Primary-5ess
Nortel DMS-100 (ISDN PRI only) - Primary-dms100
Basic Rate Interface (BRI): ISDN Basic Rate Interface (BRI, also known as 2B+1D) service provides two B channels and one D channel. The BRI B-channel service operates at 64Kbps and carries data, while the BRI D-channel service operates at 16Kbps and usually carries control and signaling information.
The D-channel signaling protocol spans the OSI reference model’s Physical, Data Link, and Network layers. The D channel carries signaling information to set up and control calls. The D channel can also be used for other functions like an alarm system for a building, or anything that doesn’t need much bandwidth, since it is only a whopping 16k. D channels work with LAPD at the Data Link layer.
When configuring ISDN BRI, you will need to obtain SPIDs (Service Profile Identifiers), and you should have one SPID for each B channel. SPIDs can be thought of as the telephone number of each B channel. The ISDN device gives the SPID to the ISDN switch, which then allows the device to access the network for BRI or PRI service. Without a SPID, many ISDN switches don’t allow an ISDN device to place a call on the network.
To set up a BRI call, four events must take place:
1. The D channel between the router and the local ISDN switch comes up.
2. The ISDN switch uses the SS7 signaling technique to set up a path to a remote switch.
3. The remote switch sets up the D-channel link to the remote router.
4. The B channels are then connected end-to-end.
Primary Rate Interface (PRI):
In North America and Japan, the ISDN Primary Rate Interface (PRI, also known as 23B+D1) service delivers 23 64Kbps B channels and one 64Kbps D channel for a total bit rate of up to 1.544Mbps. In Europe, Australia, and other parts of the world, ISDN provides 30 64Kbps B channels and one 64Kbps D channel for a total bit rate of up to 2.048Mbps.
ISDN with Cisco Routers:
Accessing ISDN with a Cisco router means that you will need to purchase either a router with a built-in NT1 (U reference point) or an ISDN modem (called a TA). If your router has a BRI interface, you’re ready to rock. Otherwise, you can use one of your router’s serial interfaces if you can get ahold of a TA. A router with a BRI interface is called a TE1 (terminal endpoint 1), and one that requires a TA is called a TE2 (terminal endpoint 2). ISDN supports virtually every upper-layer network protocol (IP, IPX, AppleTalk, you name it), and you can choose PPP, HDLC, or LAPD as your encapsulation protocol.
For each ISDN BRI interface, you need to specify the SPIDs that are using the isdn spid1 and isdn spid2 interface subcommands. These are provided by the ISDN provider and identify you on the switch, sort of like a telephone number. However, some providers no longer require SPIDs to be configured on the router. Check with your provider to be sure.
The second part of the SPID configuration is the local dial number for that SPID. It is optional, but some switches need to have those set on the router in order to use both B channels simultaneously.
An example is shown below:
RouterA#config t
Enter configuration commands, one per line. End with CNTL/Z.
RouterA(config)#isdn switch-type basic-ne1
RouterA(config)#int bri0
RouterA(config-if)#encap ppp (optional)
RouterA(config-if)#isdn spid1 086506610100 8650661
RouterA(config-if)#isdn spid2 086506620100 8650662