Home > Articles > Cisco Network Technology > General Networking > Integrated Services Digital Network Primer

Integrated Services Digital Network Primer

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Oct 18, 2002.

Chapter Description

This chapter covers Integrated Services Digital Network (ISDN) basics, ISDN physical attributes, ISDN specifications, BRI, and PRI.

PRI

PRI is a larger offering of N-ISDN that accommodates more bandwidth and that offers more flexibility. For PBX uses or where SS7 is not readily available, PRI allows for the transmission of voice and data over a single link. In North America and Japan, PRI is deployed over a T1 facility and comprises 23 B channels and a full 64-kbps D channel. In most other international countries, PRI is deployed over E1 facilities using 30 B channels and a 64-kbps D channel. International PRI is also referred to as ISDN-30.

The easiest way to think about PRI is to remember that it is merely ISDN deployed over T- or E-carrier facilities. As a subscriber, this means that you have to configure the T1 or E1 controller first and then add the ISDN functionality. PRI does not use elements such as SPIDs for connectivity to the switch, and the TEI is always set to 0 (because it is a point-to-point connection).

ISDN PRI requires that ISDN facilities are available at the CO. In recent years it has become difficult to keep up with the demand. PRI is still a local loop technology, as is BRI, but it can be expensive depending on the tariff of the associated carrier circuit.

PRI is popular in enterprise voice applications because it does not require any knowledge of SS7 to provide voice service. In many areas, PRI is preferred to SS7 even though SS7 is less expensive. Typically, enterprise customers deploy PRI circuits from their corporate headquarters to remote locations or over dry copper within the same location to facilitate PBX connectivity between all sites. Dry copper is a portion of the circuit that does not go through the service provider's network but is completely contained within the customer's private network.

A more recent application of PRI connectivity is to have ISDN on the local loop and then use a private Voice over Internet Protocol (VoIP) network as the transport system for the long distance haul between sites. At the destination, the call is dumped out of the IP network, back onto PRI facilities, thus saving the customer long distance charges while maintaining the features of ISDN PRI.

Basic PRI deployment is the same as with E1 or T1 circuit deployment, with the addition of the ISDN cards at the CO and the ISDN support at the user end. For this reason, you need to have a channel service unit (CSU)/DSU to terminate the E1 or T1 circuit and then be able to add ISDN support on top of that. The line coding and framing associated with PRI is not discussed in detail because they are the same formats that are used on standard T1 and E1 circuits. For more information on these coding standards, refer to Chapter 5, "T1 Technology," and Chapter 6, "E1, R2, and Japanese Carrier Technology."

The big difference in PRI is how it is deployed in Japan. Japanese PRI, also called INS-Net 1500 by NTT, is normally deployed by using fiber optics to the customer's premises. Figure 8-26 shows the different deployment methods that are supported in Japan with PRI circuits. NTT in Japan offers two main services, one with 23 B+D service and one with 24 B+D service. The 24 B+D service is offered by sending the D channel through a separate trunk to the customer's premises. Therefore, the customer has access to all 24 B channels on the other circuit.

Figure 8-26Figure 8-26 Japanese Deployment of a PRI Circuit

NFAS

NFAS is a function of ISDN that is typically only supported in the U.S. and that allows your equipment to be more efficient with channel use if more than one PRI circuit is in use. The basic premise of NFAS is to take a single D channel and configure it so that it can control multiple PRI circuits (up to 479 B channels). NFAS is limited to 479 B channels because the D channel is only 64 kbps. If you have five PRI circuits coming into the same router, you can use NFAS to reclaim a 64-kbps B channel on four of the circuits. Figure 8-27 shows the use of one ISDN D channel to control multiple PRI circuits to reclaim DS0s for bearer traffic.

Figure 8-27Figure 8-27 NFAS Configuration with Multiple PRI Circuits

The configuration in Example 8-5 shows three T1 controllers, using PRI, that are controlled by the same D channel. The first interface is set up as the primary D channel. For the sake of redundancy, another controller can be set up as the backup D channel, in the case of a loss of signal (LOS) on the primary circuit.

As with any ISDN configuration, you must configure the ISDN switch type in global configuration mode first.

Example 8-5 Configuration of NFAS on a T1 PRI Circuit Group

isdn switch-type primary-ni
-=snip=-
controller T1 0
 framing esf
 clock source line primary
 linecode b8zs
 pri-group timeslots 1-24 nfas_d primary nfas_int 0 nfas_group 0
!
controller T1 1
 framing esf
 clock source line secondary 1
 linecode b8zs
 pri-group timeslots 1-24 nfas_d backup nfas_int 1 nfas_group 0
!
controller T1 2
 framing esf
 clock source line secondary 2
 linecode b8zs
 pri-group timeslots 1-24 nfas_d none nfas_int 2 nfas_group 0

Configuring a T1 and E1 PRI Connection

T1 and E1 PRI configuration is simple. The first thing that you must do is configure the ISDN switch type in global configuration mode and then configure the T1 or E1 controller. Example 8-6 shows a T1 configuration.

Example 8-6 T1 Configuration

	Raleigh#conf t
Raleigh(config)#isdn switch-type primary-ni
Raleigh(config)#controller t1 7/0
Raleigh(config-controller)#framing esf
Raleigh(config-controller)#linecode b8zs
Raleigh(config-controller)#clock source line primary
Raleigh(config-controller)#cablelength short 133
Raleigh(config-controller)#pri-group timeslots 1-24	

NOTE

In the T1 configuration there is a command for cable length. Remember, this is used in DSX-1 or short-haul applications for T1.

Example 8-7 shows an E1 configuration.

Example 8-7 E1 Configuration

Raleigh#conf t
Raleigh(config)#isdn switch-type primary-net5
Raleigh(config)#controller e1 7/0
Raleigh(config-controller)#framing no-crc4
Raleigh(config-controller)#linecode hdb3
Raleigh(config-controller)#clock source line primary
Raleigh(config-controller)#pri-group timeslots 1-31

NOTE

HDB3 is the default line coding and does not have to be configured if that is what you are using. Although AMI is an option, it is almost never used for E1 circuits. To set up a Japanese PRI, you can use the primary-ntt switch type in IOS.

After the controllers have been configured with the pri-group command, you can configure the D channel. Any of the group-based commands in IOS create a virtual serial interface. In this case, it is the D channel. Typically, it is referred to as the controller/D channel. T1 PRI can be something along the lines of 7/0:23 and E1 can be 7/0:15. The virtual serial interfaces that are created are zero-based count.

A typical D-channel configuration might look something similar to Example 8-8.

Example 8-8 Configuration of an ISDN PRI D Channel

interface serial 7/0:23
 no ip address
 isdn switch-type primary-ni
 isdn incoming-voice modem
 isdn protocol-emulate network
 isdn bchan-number-order ascending

This configuration is specifically for a back-to-back T1 PRI on an AS5400. You can tell it is back-to-back by the isdn protocol-emulate network command. That command sets this router to act as the network side of the ISDN connection.

The command isdn incoming-voice modem specifies that incoming calls are treated as inbound analog modem calls and are connected to the proper resources within the device. This same command can map inbound voice calls to the proper voice resources for VoIP when using an AS5300.

The command isdn bchan-number-order ascending specifies which direction the B channel selection occurs. The choices are ascending or descending (default).

Resolving Issues with ISDN PRI

Many of the same troubleshooting techniques involved with PRI circuits are standard with T1 and E1 circuits. First and foremost, check to make sure that your controller is up and that no alarms are being detected. As a refresher, a red alarm is if you are receiving a loss of frame (LOF) or LOS. This means that you are unable to receive the proper information from the service provider's network. Next, look for a yellow or blue alarm, also called and alarm indication signal (AIS). If you are receiving a red alarm, your equipment attempts to send a yellow alarm to the remote end to notify them that you are experiencing a problem. The blue alarm is sent from the service provider to both ends of the circuit to notify them that there is a problem within the service provider's network cloud.

After you have verified that there are no alarms detected, check your controller for path code violations and clock slips. Severe clocking slips can cause your circuit to malfunction even if the circuit hardware is operating properly.

The next thing that you should check is the ISDN network status (show isdn status). You are looking for Q.921 to be MULTIPLE_FRAME_ESTABLISHED. If that is the case, you should be able to make ISDN calls. If you are in a TEI_UNASSINED or AWAITING_ESTABLISHMENT state, you need to verify your configuration. Remember that if you are in a back-to-back configuration you have to set one of the sides to emulate the network. If you do not, you do not come up on Layer 2.

After you can place calls, the debug isdn q931 command becomes your greatest asset in troubleshooting ISDN calls. The first thing to monitor is the direction of the disconnect (are you sending or receiving it). You also want to make sure that the bearer capability is correct. In other words, if you are making a voice call, but it's coming in as data, you have a problem. Receiving a disconnect cause code of "Bearer Capability not Implemented" typically means that a command such as isdn incoming-voice modem is missing from the D channel configuration.

Next, take a look at the channel ID, and find out what the status of the attempted channel is with the show isdn status command. If the channel the call is being attempted on is B, for some reason it has been busied out. You can manually attempt to reset that B channel or the entire range (although the range isn't recommended in production environments).

Last but not least is certainly the cause code for the release. The q931 debug tells you what the cause code is, although it doesn't necessarily spell out the problem for you. They have a tendency to be rather cryptic.

Q.Sig

Q.Sig was developed for private integrated services network exchange (PINX) applications. Based on Q.931, Q.Sig is an evolving technology that allows communication with legacy PBX and key systems. When used with Cisco routers, Q.Sig can connect PBX systems with private IP networks or other service provider network service offerings. Using Q.Sig with private IP transport allows businesses long-haul transport of voice traffic without the need for inter-exchange carrier (IXC) interconnection, which saves on long distance charges.

Cisco Press Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Cisco Press and its family of brands. I can unsubscribe at any time.

Overview

Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Cisco Press products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@ciscopress.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security

Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children

This site is not directed to children under the age of 13.

Marketing

Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out

Users can always make an informed choice as to whether they should proceed with certain services offered by Cisco Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.ciscopress.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links

This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020