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Chapter 2
Introducing ISDN Hardware
Your GT 70 Series router provides Integrated Services Digital Network (ISDN) capability. ISDN is a switched digital technology that upgrades an ordinary analog telephone network to an all-digital, end-to-end network. ISDN is capable of transmitting all types of information, such as voice, data, and video. With ISDN, you no longer need to incur the expense of leasing an all-digital line to receive the high-speed benefits of digital technology.
Routers and other internetworking equipment that use ISDN can take advantage of the high speed Wide Area Network (WAN) services that ISDN provides, as well as the ability to send multiple types of data.
This chapter introduces you to ISDN hardware and assists you with ordering ISDN from your telephone service. It covers the following topics:
How ISDN Works
Ordering ISDN
How ISDN Works
To best understand how ISDN works, it is helpful to understand the following:
To configure your GT 70 Series router, you need to use the numbers, such as, telephone number, TEI, and SPID, that you get from your telephone company when you order ISDN.
Access Interfaces and Data Transmission Channels
An access interface is the connection between you and the telephone company's central switching office. This interface is what allows you to access and obtain ISDN services.
A channel is a conduit through which information flows in both directions. Until the advent of ISDN, people used Plain Old Telephone Service (POTS) lines for WANs and connections to the Internet. POTS transmits analog information through one channel.
ISDN provides two types of access interfaces: Basic Rate Interface (BRI) and Primary Rate Interface (PRI), each of which provides more than one channel. The GT 70 Series routers support BRI.
Basic Rate Interface (BRI)
BRI transmits and controls information through the use of two 64 Kbps B (bearer) channels and one 16 Kbps D (data) channel. This is called 2B+D. The telephone company provides these three separate channels over the same type of wire that carries only one channel of analog information.
BRI is an affordable access interface that most small businesses and individuals use for ISDN. The user-data rate on the BRI for the 2 B channels is 2 x 64 Kbps or 128 Kbps, and the signalling rate for the D channel is 16 Kbps.
B Channels
The B channels are the data transmission connections for ISDN. They are circuit-switched digital data flow channels used to carry data, voice, video, or fax across the network.
Circuit-switching is a form of communication in which there is a transmission path between two end-point devices that is routed through one or more Central Office switches. The path is assigned for the duration of the call. At this time, the GT 70 Series routers only transport data.
D Channel
The D channel is primarily used to control the flow of data through the network: out-of-band call setup and teardown and maintenance of overhead functions, such as signaling between telephone company switches and network monitoring.
Out-of-band signaling uses signaling channels that are separate from the user's data transmission channel. ISDN uses out-of-band signaling via the D channel. This allows for fast call setup, minimizing the amount of time wasted on retries, and much more efficient routing.
Figure 2-1 illustrates the two B channels and one D channel.
Figure 2-1 BRI With 2 B Channels and 1 D Channel
Functional Devices and Reference Points
Figure 2-2 illustrates the ISDN connection. It shows the ISDN functional devices and reference points.
Figure 2-2 The ISDN Connection
Functional devices are pieces of hardware that perform tasks on an ISDN. See Table 2-1. The PC and FAX machine are Terminal Equipment 2 (TE2) functional devices that are not capable of connecting to an ISDN without using a Terminal Adapter (TA). The combination of a TE2 and a TA has the same functionality as a Terminal Equipment 1 (TE1) device.
The ISDN line between the TA and the NT1 can support up to eight devices. You need to bus the devices together.
The GT 70 Series routers are TE1 devices that are ISDN-ready. The GT 70 Series U models are devices that can carry out more than one function. They combine a TE1 and an NT1. The GT 70 Series S/T models are TE1s that require an external NT1.
The following table describes the tasks that each functional device performs in an ISDN connection.
Table 2-1 Functional Devices
Device-to-device interfaces are called reference points. Table 2-2 explains the location of the reference points in an ISDN connection.
Table 2-2 Reference Points
Network Termination (NT1) Devices
An NT1 is the boundary to the ISDN network. It includes the physical and electrical termination functions of ISDN. It provides an interface between the twisted-pair wires used by the telephone company in the BRI and the eight-wire cables used by ISDN equipment. It is located between the S/T and U reference points.
Figure 2-3 shows the GT 70 Series S/T and U models in relation to the switches in North America and outside North America.
Figure 2-3 Function of NT1
The U model has an internal NT1. The S/T model does not come with an NT1; you must provide it. In North America, where the NT1 is not built into the wall, using an integrated piece of equipment, such as the GT 70 Series U model, is convenient.
Switch Variants
The telephone company's switch variant, or type of switch, determines the capabilities of your ISDN service. It is your connection into the ISDN network. The switch variant determines how many ISDN devices you can handle, as well as what configuration options you can have. Some switch variants support eight devices, others limit you to two devices. The switch also determines which B channels support voice, video, data, or both. Most telephone companies provide a switch variant from one of the following ISDN switch variants. The GT 70 Series routers support all of the following:
Note: As of October 18, 1996, VN3 is obsolete in Europe.
When you call your telephone company, ask for the switch variant they use. When you configure your GT 70 Series router, you need to provide the switch variant.
ISDN Telephone Numbers
When you order ISDN, the telephone company assigns you one or more telephone numbers (directory numbers) for the ISDN line. Depending on the switch variant your telephone company uses, you may need only one number for both B channels or one for each B channel. These numbers do not have a direct association with any one of the B channels.
When you configure your GT 70 Series router for ISDN, you need to define the ISDN telephone number(s) assigned to the B channels.
Point-to-Point and Multipoint Configurations
In Figure 2-4, the GT 70 Series S/T model is in a point-to-point configuration, where there is one router on the ISDN line. A point-to-point configuration for the U model looks the same except that the NT1 is inside the GT 70 Series router itself.
Figure 2-4 ISDN Point-to-point Configuration
In Figure 2-5, the GT 70 Series S/T model is sharing the ISDN line with other devices in a multipoint configuration.
Note: When the GT 70 Series router is the only device at the end of an ISDN line, you are not sharing the line with other devices. However, if your telephone company uses a switch variant that requires multipoint configuration, you need to configure the line for multipoint, regardless of how many devices you have on your ISDN line.
Figure 2-5 ISDN Multipoint Configuration
Terminal Endpoint Identifier (TEI)
The Terminal Endpoint Identifier (TEI) is a number that identifies equipment connected to an ISDN line. In Figure 2-2, the GT 70 Series, S/T and U models, as well as the Terminal Adapter, have TEIs. The Terminal Adapter has the TEI for non-ISDN terminal equipment.
The telephone company either assigns TEIs automatically or gives you a pre-assigned value to enter in the configuration of your GT 70 Series router. When you configure your GT 70 Series router, you either enter the assigned TEI(s) or select automatic, as directed by your telephone company.
Service Profile Identifier (SPID)
Switch variants in North America use SPIDs. A SPID is a number that lets the telephone company know what kinds of ISDN services a device can access. If your telephone company uses SPIDs, you need a unique SPID for every device you have on the ISDN line or one for each B channel. A SPID is usually a 10-, 12-, or 14-digit number containing the telephone number of the line and a prefix or suffix, or both. For example, 31466699990101. Whether or not your telephone company uses SPIDs depends on the kind of switch in use.
When you configure your GT 70 Series router for ISDN, if your ISDN provider requires them, you need to specify the SPIDs used. You can configure up to two SPIDs on your GT 70 Series router, one for each B channel.
Ordering ISDN
When you configure your GT 70 Series router, you must know the switch variant your telephone company has, your ISDN telephone number, TEIs, and possibly SPIDs. You can get this information from your telephone company. Be sure to save this information.
To order an ISDN line,
- 1. Call your telephone company.
- 2. If you are using Data Over Speech Bearer Service (DOSBS), order ISDN Odering Code (IOC) EZ-ISDN-1. This provides data and voice capabilies on one or both B channels.
- 3. If you are using circuit switched data only on both B channels, it may be cost effective to order ISDN Ordering Code (IOC) package R. This provides data only capabilities on both B channels.
- 4. Ask for the telephone company's ISDN switch variant.
The telephone company must make certain provisions to ensure that ISDN can work for your needs and configuration. The telephone company's switch (see Switch Variants) must be provisioned to supply the services you need.You may want to use a Centrex switch, if your local telephone company provides it. Centrex provides switching applications like an on-site Private Branch Exchange (PBX), but all your connections go back to the telephone company office. For Centrex, the telephone company charges an installation and monthly fee and operates much like a leased-line connection so you can use it for your own internal network. The lines remain up all the time, and there is no per-call usage charge.
- 5. Get an ISDN telephone number (directory number).
Depending on the switch variant, you may need only one number for both B channels or one for each B channel. The telephone company tells you.
- 6. Find out how many digits you need to use to dial to your destination.
Sometimes you need the area code and telephone number, such as for long-distance calls, other times just a seven-digit number, or a 9 + seven digits, or just four digits as in an extension number. The number of digits may vary depending on the call destination.
- 7. Find out whether you need to configure your ISDN line as point-to-point or multipoint.
Some switches require you to configure your line as multipoint (having multiple devices using the same BRI line) regardless of how many devices you actually have at the end of your ISDN line.
- 8. Get a TEI for each device connected to your ISDN line.
Generally, the telephone company tells you to use Automatic TEIs, where the switch assigns the TEI value automatically when the router initializes the ISDN line. However, in some circumstances, the telephone company assigns you a Fixed TEI, a number between 0 and 63 that is permanently assigned to the ISDN line.
- 9. In North America only, find out whether or not your telephone company uses SPID numbers. If so, you need to know the SPIDs associated with this line and should have at least one SPID for each B channel.
ISDN Call Setup and Teardown
Figure 2-6 illustrates what happens physically when two GT 70 Series S/T models, GT X and GT Y, make a connection.
Figure 2-6 Call Setup and Teardown
When you place a call using ISDN equipment, the following steps occur from setup to teardown.
- 1. A physical/electrical connection is made between each GT 70 Series router and its local switch.
- 2. Using the D channel, the switch and each GT 70 Series router exchange TEI messages so that each GT 70 Series router ends up with an identifier.
- 3. Using the D channel, each GT 70 Series router sends SPIDs to the switch for initialization. This typically happens for North American switches that are configured for multipoint operation.
If TEIs are assigned and SPIDs are successfully initialized, you are ready to receive or make calls over one or both of the B channels, simultaneously.
- 4. The control information, which the D channel carries, sets up one or two calls on the B channels, one call per B channel.
- 5. The two devices, for example, GT X and GT Y pass data back and forth on the B-channel calls.
- 6. Suppose GT X requests to shut down a call.
- 7. Using the D channel, the switches disconnect the B channel carrying that call between the two routers. (If two calls are up, the other B channel remains up.)
- 8. The call is terminated and the B channel is then available for another call.
One or both calls can set up or tear down in this manner.
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