![[previous]](../../../graphics/l_arrow.gif){kind=icon}
![[next]](../../../graphics/r_arrow.gif){kind=icon}
This chapter describes how to configure the Integrated Services Digital Network (ISDN) interface on model 42x and 52x SuperStack II NETBuilder bridge/routers.
Table 88 lists the steps you must perform to configure the ISDN interface and where to find the information related to each step. 3Com recommends performing these steps in the order in which they are listed.
| 1
The WAN Cabling and Connectivity Guide can be found on the 3Com Corporation World Wide Web site by entering: http://www.3com.com/
|
ISDN operates at the physical layer of the Open System Interconnection (OSI) Model. Since bridging and all routing protocols and Point-to-Point Protocol (PPP) operate at higher layers of the OSI Model, you can configure these protocols exactly as you would over a LAN or any other type of WAN interface. To configure these protocols to run over an ISDN interface, you do not need to perform additional ISDN-related steps.
For conceptual information, see "How the ISDN Interface Works" later in this chapter.
In this chapter, the term ISDN interface refers to the two B channels and the D channel. The term B channel refers to a specific B channel. The term ISDN line refers to the physical line that connects one ISDN device to another. When the ISDN line is used, it is not assumed that both B channels are being used.
3Com offers the following ISDN systems:
3Com also recommends some BRI terminal adapters (TAs) that allow a non-ISDN bridge/router, such as the NETBuilder II bridge/router, to connect to an ISDN network. For information on accessing TA recommendations, see the WAN Cabling and Connectivity Guide. You can find this guide on the 3Com World Wide Web site by entering:
http://www.3com.com/
The ISDN systems described above are commonly used in a few different topologies. The first is a Boundary Routing topology; the second is a traditional routed environment where all devices are meshed (connected to one another).
Figure 308 shows a Boundary Routing topology with a NETBuilder II system as the central node and three model 421 SuperStack II NETBuilder bridge/routers as peripheral nodes. Three 3Com-recommended TAs with BRIs connect to the NETBuilder II system with an HSS V.35 3-port module installed.
Figure 308 
Boundary Routing Topology Using Multiple TAs with BRIs
Figure 309 shows a traditional routed topology where four SuperStack II NETBuilder bridge/routers that represent small offices are connected to one another. In this topology, each small office communicates directly with one another.
Figure 309 
Traditional Routed Topology (Meshed)
Although the ISDN topologies discussed in this section are the most common, many others can be used. For complete information on the 3Com-recommended TAs, see the documentation that accompanies these devices.
Before cabling and configuring the ISDN interface, you need to decide how you want to configure the B and D channels.
The following are some basic questions that should help you decide:
Dial-up is the method through which a call is placed through the D channel from one ISDN device to another ISDN device, and a 64 kbps circuit-switched B channel connection is established between the two devices. After a connection is established, data is transmitted over the B channel.
Table 309-1 lists three common scenarios in which the ISDN interface is used. The figures that follow this table provide more information.
Bandwidth management is a process that applies static bandwidth, dynamic bandwidth, or a combination of these, to provide the ISDN and serial ports using PPP with the bandwidth they need to meet current requirements. Bandwidth management thinks in terms of unrestricted, available resources, or resources configured for a specific function such as disaster recovery only, instead of in terms of primary and secondary lines. Bandwidth management dynamically allocates or de-allocates available resources as necessary to manage link traffic. After reading the conceptual information, see the Configuring Port Bandwidth Management chapter for configuration steps.
Figure 310 shows a Boundary Routing topology where one NETBuilder II bridge/router is connected to a SuperStack II NETBuilder bridge/router through a DTE interface over which Frame Relay or X.25 is running, while the other
NETBuilder II bridge/router is connected to the same SuperStack II NETBuilder bridge/router through an ISDN line over which PPP is running. The line running Frame Relay or X.25 is considered the primary line, while the ISDN line is considered the secondary line. In this topology, the secondary line is configured to come up only if the primary line fails, which provides a redundant route for network resiliency.
Figure 310 
ISDN Used as Backup to Cloud Technology
Figure 311 shows additional Boundary Routing topologies. These topologies illustrate using an ISDN line as a backup to the same site or device, or as a backup to a different site or device.
In the first topology shown in this figure, two lines connect the NETBuilder II bridge/router to the SuperStack II NETBuilder bridge/router. The first line connects the two devices through a DTE interface over which PPP is running, while the second line connects the two devices through an ISDN path over which PPP is running. Both interfaces or paths are mapped to port 3. The DTE line is considered the primary line, and the ISDN line is considered the secondary line. In this topology, the secondary line is configured to come up only if the primary line fails (disaster recovery), or is overwhelmed by traffic and needs additional bandwidth (bandwidth-on-demand).
In the second topology shown in this figure, two lines connect two NETBuilder II bridge/routers to a SuperStack II NETBuilder bridge/router. The first line connects the two devices using a DTE interface running PPP, while the second line connects the two devices using an ISDN path running PPP. The DTE line is considered the primary line, while the ISDN line is considered the secondary line. In this topology, the secondary line is configured to come up only if the primary line fails (redundant route for network resiliency).
Figure 311 
ISDN as Backup to Serial Line Running PPP (Same or Different Site)
Figure 312 shows a traditional routed topology where a model 527 SuperStack II NETBuilder bridge/router is connected to another model 527 bridge/router through an ISDN path over which PPP is running. Because the ISDN line provides the only connection between these two devices, it is considered the primary line. In this topology, this line is configured to come up only when there is a demand for it (dial-on-demand).
Figure 312 
ISDN as Primary Using Dial-on-Demand
Although the examples described in the preceding paragraphs are the most common, many other examples exist. For more information on Boundary Routing using network resiliency, disaster recovery, and bandwidth-on-demand, see the Configuring Boundary Routing System Architecture chapter. For information on dial-up, including more information on disaster recovery and bandwidth-on-demand, see the Configuring Port Bandwidth Management chapter.
A Network Termination 1 (NT1) and a power supply are required for every ISDN line in North America. Your service provider or telephone company may provide you with an NT1 and power supply for a small monthly fee, or, you may want to purchase it from an ISDN equipment vendor. The NT1 and power supply may come in a single standalone box or the two may be in separate units. In this discussion, the two units together will be referred to as an NT1.
Two kinds of NT1s are currently in use in North America, differentiated by the data encoding scheme used in the transmission of data between the NT1 and the telephone company's equipment. The two data encoding schemes are called 2B1Q (two bits mapped into one quaternary symbol) and AMI (Alternate Mark Inversion). The 2B1Q scheme is the dominant method in use today. The AMI scheme is older and rarely used.
Two power sources are available from an NT1 for CPE equipment. An ISDN telephone uses one power source. The SuperStack II NETBuilder bridge/router does not use either one for power. Instead, it detects the presence or absence of phantom power and can determine whether or not a telephone cord is plugged in.
However, not all NT1s provide phantom power. The AMI NT1 from AT&T does not. If you are connecting the SuperStack II NETBuilder bridge/router to an NT1 that does not provide phantom power, you must turn off phantom power detection before you can dial successfully. To turn off phantom power detection, set the value of the -PATH PhantomPower parameter to Disable. For more information on this parameter, see the PATH Service Parameters chapter in Reference for Enterprise OS Software.
Phantom power is not supported on the HSS 8-port BRI modules.
After you have configured the ISDN device at the other end of the ISDN network (use the documentation that accompanies that device), no additional configuration is necessary for that device to interoperate with your SuperStack II NETBuilder bridge/router with an ISDN interface.
This section provides conceptual information on aspects of the ISDN interface that require further explanation.
SuperStack II NETBuilder bridge/routers with an ISDN interface provide ISDN connectivity through a BRI. This interface consists of two full-duplex B channels operating at 64 kbps and one full-duplex D channel operating at 16 kbps (2B + D). The two B channels transmit data, while the D channel is used for call processing with the ISDN switch.
Because the BRI consists of multiple B channels over which data can be transmitted, a path numbering convention has been devised. For complete information on this convention, see the Configuring Basic Ports and Paths chapter or the installation guide that you received with your SuperStack II NETBuilder bridge/router.
Paths 2.1 and 2.2 correspond to the B channels. However, a particular B channel is not statically bound to a particular path. At one time path 2.1 could use B channel 1, while at another time the same path could use B channel 2.
Some parameters that you must configure to set up the ISDN environment are connector-specific, that is, they are generically applicable to the ISDN operating environment, not to any one specific B channel. Connector-specific parameters require that you specify the connector number only. Other parameters are channel-specific, that is, they apply specified configurations to an individual channel and the physical connector which it is associated with.
Channel-specific parameters require that you specify the connector number and the channel number. If you are unsure as to whether you need to specify the connector and channel numbers or just the connector number, see the description of that particular parameter in Reference for Enterprise OS Software.
Your SuperStack II NETBuilder bridge/router with an ISDN interface can be a device in a point-to-point or point-to-multipoint configuration. A point-to-point configuration is a topology where a single device is connected to an ISDN line. A point-to-multipoint configuration is a topology where up to eight devices, including bridge/routers (SuperStack II NETBuilder bridge/router with an ISDN interface and other non-ISDN 3Com bridge/routers connected to a TA) and telephones (ISDN and non-ISDN connected to a TA), are connected to an ISDN line. The point-to-multipoint configuration is implemented using a passive S-bus.
Figure 313 shows two network topologies. The first topology (far left) is a point-to-point configuration; the second topology shows a point-to-multipoint configuration. Each configuration is connected to an ISDN switch through an ISDN line.
Figure 313 
Point-to-Point and Point-to-Multipoint Configurations
This section explains how SuperStack II NETBuilder bridge/routers with an ISDN interface decide to accept an incoming call from an ISDN switch.
SuperStack II NETBuilder bridge/routers with an ISDN interface use the following types of call compatibility criteria to determine whether or not to accept an incoming call from an ISDN switch:
Some bearer capability criteria are fixed and cannot be changed, while others are determined by user configuration. The following sections describe each type of call compatibility criteria.
The ISDN specifications provide other compatibility criteria called low-layer compatibility and high-layer compatibility information elements that can be used to determine incoming call acceptability. SuperStack II NETBuilder bridge/routers with an ISDN interface do not use low-layer compatibility and high-layer compatibility information elements as criteria to determine whether or not to accept an incoming call from an ISDN interface.
SuperStack II NETBuilder bridge/routers with an ISDN interface have the following fixed bearer capability criteria for an incoming call:
Calls that do not fulfill these criteria are rejected or ignored.
You can specify the rate at which data is to be transferred on a B channel that is to be connected by a call by using the -PATH RateAdaption parameter.
After an incoming call fulfills the bearer capability criteria, the following items must be determined:
In a point-to-point configuration, where a single device is connected to an ISDN line, it is assumed that the bridge/router on the ISDN line will answer the call. Therefore, it is not necessary to configure the bridge/router as the device that will answer incoming calls.
In a point-to-multipoint configuration, where up to eight devices, including bridge/routers and ISDN and non-ISDN telephones, can be connected to an ISDN line, you must configure at least one bridge/router to answer the incoming calls.
After the incoming call is answered by at least one bridge/router in either the point-to-point and point-to-multipoint configurations, the bridge/router must also determine whether the call is to be connected to path 2.1 or 2.2. The ISDN switch selects a B channel over which to transmit a call and the bridge/router must determine the path that is to be connected with that B channel.
The selection of which bridge/router will answer a call and subsequently which path will accept a call is determined by how you address your ISDN paths. ISDN addresses consist of the following components:
For more information, see "ISDN Addressing" later in this chapter. You can assign a phone number and a subaddress to an ISDN path using the -PATH LocalDialNo and -PATH LocalSubAddr parameters, respectively.
Assign a phone number to an ISDN path under the following circumstances:
Figure 314 
Assigning a Unique Phone Number to Multiple ISDN Paths
Suppose an incoming call specifying phone number 408-555-1111 arrives. As long as it is not engaged in another call, path 2.1 accepts the call based on the phone number specified in the incoming call. If path 2.1 was already engaged in another call or the phone number specified in the incoming call is different from that assigned to path 2.1, the call is rejected. If path 2.2 also cannot be used, the call is rejected by the bridge/router.
In addition to assigning a phone number, you should assign a subaddress to an ISDN path if the phone number is the same one that you plan to assign to another ISDN path in your point-to-point or point-to-multipoint configuration. Assigning the same phone number to all or some of the ISDN paths in a topology presents a problem: more than one ISDN path may attempt to accept a call. To resolve this problem, you can assign a subaddress to each of the bridge/router's ISDN paths with the same phone number using the -PATH LocalSubAddr parameter. For example, in the point-to-multipoint topology shown in Figure 315, four ISDN paths have been assigned the phone number 408-555-1234. Unique subaddresses have also been assigned to the paths of each of these devices.
Figure 315 
Assigning the Same Phone Number to Multiple ISDN Paths
Suppose an incoming call specifying phone number 408-555-1234, subaddress 99, arrives. The SuperStack II NETBuilder bridge/router answers the call based on the phone number specified, and as long as it is not engaged in another call, path 2.1 accepts the call based on the subaddress specified. If path 2.1 was already engaged in another call or the phone number and subaddress specified in the incoming call is different from that assigned to path 2.1, the call is rejected. If path 2.2 also cannot be used, the call is rejected by the bridge/router.
Not all telecommunications carriers allow you to assign the same phone number to multiple paths. When you contact your carrier to acquire support services, verify that they support this feature. You must also specify that you will be using subaddresses.
Do not assign a phone number or a subaddress to an ISDN path if you have a point-to-point configuration and plan to use dynamic dial path pooling. Figure 316 shows a point-to-point configuration where a phone number and subaddress have not been assigned for both ISDN paths.
Figure 316 
Point-to-Point Configuration Without Specified Phone Number and Subaddress
Suppose an incoming call arrives that specifies a particular phone number and subaddress. The bridge/router ignores the ISDN addressing information provided by the incoming call and not use it as criteria to determine which path should accept the call. Either path can accept the call provided that they are not engaged in another call. Criteria at higher layers of the OSI Model will determine the port to which the path will be bound to transmit this particular call.
After you have assigned phone numbers to ISDN paths and an incoming call arrives, an algorithm attempts to match the incoming phone number with the phone number specified using the -PATH LocalDialNo parameter. This algorithm compares the numbers in sequence from the end of the numbers toward the beginning. The length of the incoming phone number can be shorter than the length of the phone number configured using the -PATH LocalDialNo parameter. For example, although you can specify an international phone number using elements such as a dial prefix, country code, area code, and phone number through the -PATH LocalDial No parameter, a phone number composed of only an area code and phone number will be considered a match as long as the phone number you specified and the phone number that is received through the incoming call are the same.
Specifying an international phone number using the -PATH LocalDialNo parameter allows you to accept all calls, including international and local. If you want to restrict incoming calls to local calls only, then specify at most an area code and local phone number using the -PATH LocalDialNo parameter.
An algorithm also attempts to match the incoming subaddress with the subaddress specified using the -PATH LocalSubAddr parameter. The characters for the subaddress in the incoming message must exactly match those specified in the -PATH LocalSubAddr parameter.
For more information on static and dynamic paths and dynamic dial path pools, see the Configuring Port Bandwidth Management chapter. For more information on the -PATH LocalDialNo and -PATH LocalSubAddr parameters, see the PATH Service Parameters chapter in Reference for Enterprise OS Software.
An ISDN address is a phone number provided by your telecommunications carrier. The address can consist of the following elements:
The telecommunications carrier does not provide a subaddress; you must create your own subaddress. For information on when to use a subaddress, see "ISDN Addressing Compatibility" earlier in this chapter.
Not all telecommunications carriers allow you to assign the same phone number to multiple paths. When you contact your carrier to acquire support services, verify that they support this feature. You must also specify that you will be using subaddresses.
When setting up certain parameters such as -PORT DialNoList, you may need to specify a dial or phone number string consisting of a phone number and, if applicable, a subaddress. If you specify a subaddress, you must separate the phone number and the subaddress with a semicolon (;). The phone number can consist of a maximum of 30 characters, while the subaddress can consist of a maximum of 20 characters.
When specifying a phone number, valid characters include the digits 0 through 9, an asterisk (*), and the pound sign (#). Because the software ignores all other characters to the left of the semicolon that separates the phone number and subaddress, you can also specify special characters such as parentheses and dashes to distinguish the different elements that compose a phone number, and text characters to embed descriptive text in the string.
When you specify a phone number, each character entered (whether the software considers it valid or invalid) counts toward the maximum allowable number of characters.
When you specify a subaddress, valid characters include all ASCII or IA5 characters.
The following string is an example of a dial or phone number string that includes a subaddress:
In this dial or phone number string, the phone number consists of long distance dial prefix 1 (assuming that the bridge/router being configured is located in Santa Clara), phone number 213-555-1000, and the subaddress 200. The descriptive text to the left of the semicolon indicates that the phone and subaddress numbers are for the Los Angeles office.
An Austel Semi Permanent Circuit (ASPC) is an ISDN 64 KB B-channel circuit that uses the ISDN D-channel Signaling Protocol to initiate a call. The ASPC is unique in that the circuit is controlled like a dial line, but is tariffed and operated as a leased line.
The ASPC line type designates that the interface supports the features of a leased line with a dial number. The ASPCNumber parameter allows you to associate a dial number with this class of ISDN leased line.
The basic rate ASPC relies on existing ISDN signalling to bring up a call. A special telephone number, the ASPC number, signals that an ASPC should be used for the call. Once connected, the line can be disconnected by the service provider at any time. The interface may deactivate or a DISCONNECT/RELEASE ACK may be received. If this situation occurs and the connection is again required, the originator redials the call until another connection is established.
Generally the port and path based dial parameters have no affect on an ASPC call. An ASPC call does not use the port and path parameters to support the call. The path is seen as a leased line.
When you change a BRI line (2.1 for example) to ASPC, the other BRI line on the same interface changes to ASPC unless it is a dialup line. With a dial line and an ASPC on the same interface, ASPC can be configured on B1 while B2 serves as a disaster recovery backup line.
For information about configuring ASPC on ISDN leased lines, see "Configuring Leased LInes" in the Configuring Port Bandwidth Management chapter.