ISDN and Data Networking

A Technology Overview

By Robyn Aber and Najib Khouri-Haddad

Note: This document downloaded from the 3Com corporation, and is on their server at URL <http://www.3com.com/nsc/500606.html>

The Integrated Services Digital Network (ISDN) is becoming a mainstream telecommunications network that will serves a wide variety of users' needs. Experts call ISDN the telecommunications network of the 21st century, the foundation upon which to further build the information age. With its narrowband and broadband aspects, the ISDN data highway will evolve from today's switched telephone and dedicated leased-line networks to become a unified global network carrying voice, data, video, interactive pictures, and other services to homes and businesses.

This network technology is becoming more available on a global scale and is providing a highly reliable and flexible infrastructure that can support high-bandwidth applications and a variety of services. Asynchronous Transfer Mode (ATM) switching technology, which is the basis for broadband ISDN, will be ideal for carrying various types of multimedia traffic in the next few years. Today, switched services and narrowband ISDN represent the first steps in the move toward digitized information.

Narrowband ISDN as a means of switched WAN access in data networks is an attractive solution for branch or satellite locations, small businesses, or home offices because of the reliability that is inherent in its digital nature, its fast call setup (needed to support routing protocols), its flexibility in supporting different services, and its ability to support existing WAN protocols such as Frame Relay and X.25. ISDN provides a migration path for remote sites and individual remote users to meet the bandwidth demands of critical applications today and to evolve to higher speeds as needs change in the future.

This paper presents an overview of narrowband ISDN technology from a data networking perspective and reports on its current deployment status worldwide. It compares ISDN with other existing and developing WAN services, covering point-to-point and switched, private, and public services. Next it looks at how ISDN addresses the growing need for remote LAN-to-LAN, remote node-to-LAN, and Internet and on-line information access connectivity. Finally, the paper provides information on ISDN planning and cost of ownership. An ISDN glossary and a list of 3Com ISDN products for central sites, remote sites, and individual user environments are included at the end of the paper.

Table of Contents:

• What Is ISDN?
• ISDN Development History
• The Benefits of ISDN for Data-Intensive Applications
• Comparing WAN Service Transmission Rates
• How ISDN Works
• ISDN Deployment--A Status Update
• Comparing WAN Services
• Evaluating WAN Service Cost-of-Ownership Options
• ISDN Data Networking Applications
• Planning and Installing an ISDN Service
• Summary
• ISDN Glossary
• About the Authors

What Is ISDN?

The Integrated Services Digital Network (ISDN) is a currently available and growing public telecommunications network with a flexible infrastructure designed to integrate voice, data, video, images, and other applications and services. ISDN can be thought of as a replacement for the existing analog telephone network. Narrowband ISDN provides lower-speed services, from 56 Kbps to 2.0 Mbps; while broadband ISDN, based on the evolving cell-based Asynchronous Transfer Mode (ATM) technology, addresses high-speed service needs, from 2 to 600 Mbps.

With ISDN, smaller regional and international sites can connect to enterprise networks and to one another much more cost-effectively than with dedicated leased lines. Dial-up ISDN links can replace leased-line connections altogether or be used as supplemental bandwidth for overflow and redundancy. Remote sites and individual remote users can dial in to central networks worldwide and enjoy reliable, high-speed digital connectivity to critical resources, or access and download the ever-increasing wealth of multimedia information on the Internet.

ISDN Development History

• It is digital from one end of a connection to the other.
• It defines a small set of internationally standardized user/network interface protocols, so that all ISDN devices can use the same type of physical connection and the same set of signaling protocols to request services.

ISDN combines the coverage of a geographically extensive telephone network with the data-carrying capacity of digital data networks into a well-defined structure that can support simultaneous voice, data, video, imaging, and multimedia applications.

ISDN technology has been available for years, but only in the past year have tariffs and deployment reached the point where it has become an affordable and viable WAN option, particularly in North America. In most of Europe and Japan, deployment is widespread; however, ISDN availability is just over the 70 percent mark in North America. ISDN is available in most North American metropolitan areas, while more rural regions still do not have access.

Tariffing, as with any public service, has a large political/bureaucratic component. In North America, a Basic Rate Interface (BRI) is the most commonly used ISDN line and generally costs slightly more than a POTS (plain old telephone service) line. Service charges are slightly higher, but because less time is spent on-line, usage costs are lower. Given the advantages of a digital network, ISDN can be a cost-effective alternative to analog services. In some European countries, a Primary Rate Interface (PRI) access line is more affordable than BRI service.

The Benefits of ISDN for Data-Intensive Applications

ISDN offers many benefits for organizations where data applications use public switched telephone network facilities. These benefits make ISDN particularly attractive for small regional and international branch sites that need to connect to central enterprise networks and to one another.

• Because ISDN can carry multiple services--voice, video, and data--on a single network over existing twisted-pair copper wire, telecommunications service providers and subscribers can dramatically reduce their infrastructure and maintenance costs. And voice and data integration promotes more synergistic capabilities, such as high-function telephones and electronic directories or voice-annotated electronic mail.
• With speeds up to 128 Kbps, ISDN BRI provides much higher bandwidth than analog modem-based solutions that range from 4.8 Kbps to 28.8 Kbps. Using ISDN compression ratios from 2:1 to 4:1, it's possible to deliver effective transmission rates ranging from 256 Kbps to 632 Kbps.
• ISDN provides clearer, quieter voice telephone service, with the built-in security of digital transmission, managed by easy-to-use call control features.
• Remote users working from home or on the road can use high-speed ISDN to access critical central site resources at higher performance levels.
• ISDN caller identification features can screen incoming calls based on the caller's phone number, and accept or reject the call based on user-specified preferences. It can link to a directory and forward the call accordingly, or map to a database to pull the caller's record. It can even bypass the local site and link the call to a remote-site IP address for routing purposes.
• ISDN's dynamic bandwidth allocation feature can aggregate data channels in real time to accommodate even the most bandwidth-intensive applications.
• ISDN is increasingly used to provide WAN dial backup overflow and disaster recovery facilities. An ISDN connection can act as a low-cost backup for a leased line on a "pay only for use" dial-up connection basis with line speeds comparable to current T1/E1 leased lines. ISDN eliminates the expense of a second leased line that may go unused.
• ISDN can handle multiple devices on a single line. Up to eight telephones, computers, workstations, faxes, credit card readers, cash registers, or other devices can be directly attached to a single ISDN line, all in use simultaneously.
• ISDN's end-to-end digital transmission delivers more accurate and reliable connectivity than analog technology, with lower error rates and fewer dropped connections.
• ISDN's technology provides quicker connect times to better support LAN protocols such as IP and IPX(TM), which require lower latency across the WAN connection. This is particularly useful for Internet and other on-line services as well as in retail credit verification applications.
• ISDN interoperates with other WAN services such as existing analog services, X.25, Frame Relay, and Switched Multimegabit Data Service (SMDS), and higher-speed services like ATM.
• ISDN provides dial-up access for IBM users: for example, cluster controller to front-end processor (FEP) links and inter-FEP links at T1 channel extension rates.
• Attractive tariffs and expanded availability make ISDN a cost-effective alternative to private leased lines for low- and high-speed data networking.

Comparing WAN Service Transmission Rates

Data call connection with ISDN takes about one to four seconds to establish, while a Switched 56 or analog modem connection can take as long as 40 seconds. Fast call setup enables data applications to establish a connection on demand, transfer data, and disconnect, thereby minimizing usage charges. ISDN can also support concurrent voice, data, and video applications, whereas other switched digital services are designed to support only data.

How ISDN Works

ISDN Logical Channels

• B (bearer) channels transmit at 64 Kbps and carry circuit-mode or packet-mode user information such as voice, data, fax, and user-multiplexed information streams. All network services are available through B channels. (H channels are functionally equivalent to B channels but operate at speeds greater than 64 Kbps.)
• The D (data) channel transmits at 16 Kbps for BRI and 64 Kbps for PRI. It carries call signaling and setup information to establish a network connection, request network services, route data over B channels, and tear down the call when complete. This information is designed to travel through a totally separate, dedicated communications network from the bearer channels. It is this out-of-band signaling network that gives ISDN faster connection times--from one to four seconds as opposed to 10 to 40 seconds for analog dial-up lines. However, for full efficiency, bandwidth not required for signaling and control on the D channel can be used to transport user packet or frame data when needed.

ISDN User Interface Standards

The ITU-TSS has defined two ISDN user interface standards (shown in Figure 1):

• The Basic Rate Interface consists of two B channels and one D channel for signaling (2B+D).
• The Primary Rate Interface is a 23B+D interface in North America and Japan, and a 30B+D interface in Europe. It is the ISDN equivalent of the 1.544 Mbps or 2.048 Mbps interface over a T1/E1 line or trunk; the physical layer is identical for both. The D channel is channel 24 (or 31) of the interface, and it controls the signaling procedures for some or all of the B channels.

Figure 1. BRI and PRI User/Network Interfaces

Rate Adaption

Terminal adapters employ two common rate adaption protocols to handle the transition. In Europe, V.110 is the most popular rate-adaption protocol, while North American equipment manufacturers use the newer V.120 protocol. Both standards support both synchronous and asynchronous transmission.

Rate adaption also comes into play when access to 64 Kbps circuits is not available from one end of a connection to another. Since ISDN is currently not ubiquitous worldwide, telephone providers must sometimes create an end-to-end digital connection using Switched 56 Kbps digital services, even though the call originated on an ISDN link. In this case, the effective throughput of the link is limited to 56 Kbps. This type of rate adaption commonly occurs on international calls originating in North America or on calls that pass through multiple telephone carrier service areas. Most ISDN terminal equipment adjusts transparently to the lower rate.

Dynamic Bandwidth Allocation

Also known as bandwidth-on-demand or inverse multiplexing, channel aggregation is often abbreviated as Nx64 Kbps, where N stands for the number of channels logically combined. In practice, network managers can adjust the inbound or outbound calling flow to respond to time-of-day or day-of-week needs. For example, a network manager could combine a number of channels to support a full-color, full-motion video conference during a busy weekday morning, and arrange to combine channels after workday hours for high-speed file transfers to remote sites anywhere.

Multilink PPP Versus BONDing Channel Aggregation Support

The competing proposal for providing this functionality is called BONDing, which approaches synchronization between multiple streams at the bit level. BONDing was designed for video conferencing applications and will most likely require additional hardware for the end system. Because BONDing is hardware-oriented, it is efficient; but it is also expensive and inflexible, since once a pipe size is set, it cannot be changed until the session is finished. BONDing has been adopted more readily in the videoconferencing arena than in data networking.

Physical Connections to ISDN

The following list shows the different types of functional devices that connect a customer site to ISDN services at the telephone carrier central office, as well as the ITU-TSS-defined interfaces that link them to network services (see Figure 2). Equipment that complies with these ITU-TSS-defined interfaces is guaranteed to be compatible with both ISDN and the other functional devices connecting to ISDN from the customer premises.

There are two types of terminal equipment (TE): devices with a built-in ISDN interface, known as TE1, and devices without native ISDN support, known as TE2. Terminal equipment consists of devices that use ISDN to transfer information, such as a computer, a telephone, a facsimile machine, or a videoconferencing machine.

Terminal adapters (TAs) translate signaling from non-ISDN TE2 devices into a format compatible with ISDN. TAs are usually stand-alone physical devices.

The S interface is a four-wire interface that connects terminal equipment to a customer switching device, such as a PBX, for distances up to 590 meters. The S interface can act as a passive bus to support up to eight TE devices bridged on the same wiring. In this arrangement, each B channel is allocated to a specific TE for the duration of the call.

Devices that handle on-premises switching, multiplexing, or ISDN concentration, such as PBXs or switching hubs, qualify as NT2 devices. ISDN PRI can connect to the customer premises directly through an NT2 device, while ISDN BRI requires a different type of termination.

The T interface is a four-wire interface that connects customer site NT2 switching equipment and the local loop termination (NT1).

An NT1 is a device that physically connects the customer site to the telephone company local loop. For PRI access, the NT1 is a CSU/DSU device, while for BRI access the device is simply called by its reference name, NT1. It provides a four-wire connection to the customer site and a two-wire connection to the network. In Europe, the NT1 is owned by the telecommunications carrier and considered part of the network. In North America, the NT1 is located on the customer premises.

The U interface is a two-wire interface to the local or long-distance telephone central office. It can also connect terminal equipment to PBXs to support distances up to 3000 meters. The U interface is currently not supported outside North America.

Figure 2. ISDN Functional Devices and Physical Interfaces

Bandwidth Allocation Control Protocol

BACP manages bandwidth by allowing two peers (both of which support BACP) to negotiate the addition or deletion of a link in a multilink bundle. These links could be voice or data channels in an ISDN line. While not limited to ISDN, BACP is especially useful in ISDN environments because ISDN is a high-speed medium that uses multiple channels. With BACP, large hunt groups over multiple channels are easy to manage.

The benefits BACP offers are network flexibility, multivendor product interoperability, the ability to manipulate bandwidth to maximize network resources, and the ability for either end of a connection to manage the link, reducing operating costs.

ISDN Deployment--A Status Update

For remote users in Europe and the Pacific Rim, where the ISDN infrastructure is more mature, ISDN has supplanted analog lines for remote node-to-LAN connectivity. Current ISDN installation in Europe is estimated at two million lines, and in Japan, more than 850,000 lines. The Japanese telecommunications giant NTT has committed to having 97 percent deployment by 1997. The stumbling block for these remote users has been the availability at the central site of ISDN-capable remote access servers, which are required for high-performance access to central site LAN resources. However, remote access servers that support ISDN are becoming more available worldwide.

According to Dataquest, ISDN connections in the United States increased 81 percent from 1994 to 1995--from 247,000 to 448,000 BRI lines. Dataquest predicts that ISDN installations will increase another 69 percent this year, from 448,000 to 758,000 BRI connections. ISDN deployment in the U.S. varies widely among RBOCs and local exchange carriers (LECs). Although some carriers such as Pacific Bell and Bell Atlantic Corporation have upgraded more than 80 percent of their switches to ISDN technology, other carriers have as few as 46 percent of their switches upgraded. Figure 4 on shows the current and projected growth of average ISDN deployment by U.S.-based LECs.

ISDN availability in Canada is much like that in the U.S. The service is predominately available in metropolitan areas through a variety of service providers, including members of the Canadian consortium, Stentor.

Figure 3. Worldwide Installed ISDN Circuits

Figure 4. ISDN Switching Deployment Among U.S. Carriers

Comparing WAN Services

• Analog dial-up
• Switched 56
• X.25
• Point-to-point dedicated leased lines
• Frame Relay
• xDSL
• Cable
• SMDS
• ATM

Some of these technologies and services are normally only cost-effective with larger site LAN-to-LAN connectivity applications. In contrast, ISDN is a viable option for both large and small remote LAN-to-LAN and remote node-to-LAN applications, as well as for Internet and on-line service access. This section compares the most common WAN data communication service options with ISDN. Choosing the best option involves evaluating such factors as service availability in locations requiring connectivity, types of network applications to be supported, and the internetworking environment. Table 2 on compares the types, line speeds, applications, and relative strengths and weaknesses of common WAN services.

Analog Dial-Up Service

In most cases, analog dial-up service providers will not guarantee support for specific data rates even if users purchase analog modems capable of handling up to 28.8 Kbps transmission rates; the maximum data rate is typically 19.2 Kbps. Little or no diagnostics are available from the service provider other than normal testing performed on residential and business lines. Also, line quality varies widely, and the amount of noise on a line has a direct bearing on the maximum data transmission rate.

Table 2. (Part 1) WAN Services Comparison

	Analog Dial-Up	Switched 56	X.25	Point-to-Point Leased Lines	Frame Relay
Type	Circuit switched, public	Circuit switched, digital, private	Packet switched, public or private	Point-to-point, private	Packet switched, public or private
Bandwidth	64 Kbps voice 9.6-28.8 Kbps data (14.4 Kbps typical)	56 Kbps data	56 Kbps data	T1/E1: 1.544 Mbps data (N.A.); 2.048 Mbps data (Europe) T3/E3: 45 Mbps (N.A.); 34 Mbps (Europe)	64 Kbps-1.544 Mbps data
Applications	Voice and data on separate lines	Voice and data on separate lines	Protocol for terminal -to-host	High-speed voice and data transmission for transaction based environment applications and Internet access	Optimized for data data transmission Point-to-point environments
# of Sites for Cost- Effectiveness	Unlimited	Unlimited	Unlimited	Few	Private leased line replacement; cost-effective for fewer fixed sites
Strengths	Wide availability, any-to-any connectivity, low cost	Wide availability, any-to-any connectivity, moderate cost, uses standard telephone numbers, interoperates with ISDN	Wide availability, efficient for bursty traffic, any-to-any connectivity, automatic error detection and correction, security, standardized protocols	High speed; high degree of management, reliability, and security; standardized; direct connections to Internet	High speed, low latency, bandwidth- on-demand. easy scalability, standardized protocols, point-to-point connectivity
Weaknesses	Limited bandwidth, no efficiency gain supporting bursty versus continuous traffic, lacks multi-vendor management	Data only, limited bandwidth relative to ISDN, service providers de- emphasizing it, requires CSU/DSU equipment	Limited bandwidth since error detection limits speeds, increases cost; marginal for LAN inter- connection	Fully meshed topologies very expensive	Expensive (compared to ISDN), requires a dedicated access line, not widely deployed in capabilities Europe, expensive and complicated to make moves and changes
Pricing Elements	Installation (varies); average monthly charge plus usage charge	Installation (varies); average monthly charge plus usage charges	Installation (varies); average monthly charge, free usage charge up to ceiling	Installation (varies); flat rate based on bandwidth and distance	Installation (varies); typically flat rate options, some carriers offer usage options

Table 2. (Part 2) WAN Services Comparison

	xDSL	Cable	SMDS	ISDN	ATM (Broadband ISDN)
Type	Circuit mode	Point-to-point, private	Cell switched, public	Packet/circuit switched, public	Cell switched, public switching technology
Bandwidth	64 Kbps-52 Mbps (varies by technology)	500 Kbps-30 Mbps data and video	Nx56/64 Kbps data (or packet voice or video) (1.544-45 Mbps typical)	64-128 Kbps for BRI voice, video, and data 1.544-2.0 Mbps for PRI voice, video, and data	1.544 Mbps-622 Mbps voice, video, and data (25-155 Mbps typical)
Applications	Data dialtone (Internet access, remote LAN access), video dialtone (video conference, video on demand)	Consumer, on-line Internet and information access, LAN connections, video	Optimized for data Multipoint environment	Optimized for voice, data, and video integrated on a single digital line	Optimized for switching voice, data, video Switching , multiplexing technology
# of Sites for Cost- Effectiveness	Unlimited, but local access service only	Unlimited	Cost-effective for four or more sites	Unlimited	Power users, early adopters; initially LAN only backbone applications
Strengths	Broadband bandwidth, simultaneous digital services and lifeline POTS, dedicated (not shared), supports multimedia service	High speed, existing infrastructure, fast call setup	High speed, low latency, any-to-any connectivity, economical for virtual meshed networks, standardized protocols, easy to make changes	High speed; digital data, voice, images, video on integrated line; fast call setup; secure, reliable, stable digital connectivity; efficient for bursty traffic; standardized protocols	Very high-speed; data, voice, images, video on integrated line; fast call setup; secure, reliable, stable digital connectivity; efficient for bursty traffic
Weaknesses	Standards and infra-structure still under development, service area distance limitations, degree of data transport symmetry varies	Developmental stage only, voice on separate line, bandwidth split among users with no firewall capability, historically weak customer service and support, mostly one-way transmission	Not widely used in N.A., Europe, or Pacific Rim	Not yet ubiquitous, tariff rates inconsistent, can be complicated to install and configure	Not yet widely available; standard- ization details still under development, current products expensive, proprietary products have multi-vendor compatibility problems due to lack of standards
Pricing Elements	TBD	Installation (varies); average monthly charge plus usage charge	Installation (varies); usage and flat-rate options charge	Installation (varies); average monthly plus usage charge (varies)	Customer- by-customer basis

Switched 56 Service

X.25 Switched Service

X.25's packet-switching technology automatically allocates access to available bandwidth and efficiently handles the bursty traffic inherent in LAN environments. X.25 also provides data security and automatic error detection and correction facilities, although the in-band facilities consume valuable data transmission bandwidth. However, X.25's slow speed prevents it from being an effective medium for higher-speed LAN or WAN applications.

Point-to-Point Dedicated Leased Services

For locations transferring controlled amounts of steady data traffic, leased 56- and 64-Kbps lines are popular because of their low installation and monthly rental charges. Private dedicated lines do not provide any inherent efficiencies in transmitting bursty traffic and are often underutilized, sometimes running at only 5 to 20 percent of total capacity. In addition, point-to-point, fully redundant interconnectivity requires expensive mesh topologies and customer premises equipment.

Fractional T1 (E1 in Europe) is a high-capacity, private digital service designed to support multiple 64 Kbps channels. With this service, WAN subscribers can lease one of several 64 Kbps channels instead of the full T1 pipe. The service offers the same control, management, and security features as the full T1, and the same disadvantages, although costs are lower for fully meshed topologies.

When data traffic between remote sites is intermittent or infrequent, ISDN and other switched services can be a cost-effective and reliable alternative to leased lines.

Frame Relay Service

Used in a remote communications path for an enterprise network, Frame Relay is totally transparent to users. Frame Relay technology uses standard WAN interfaces and CSU/DSUs and can coexist easily with ISDN and ATM services. The disadvantages of Frame Relay are that it can be as much as 12 times more expensive than ISDN and it requires a dedicated access line. Like SMDS, it is not yet widely deployed in Europe or in Asia and the Pacific Rim.

Digital Subscriber Line Services

Cable Service

Cable data networks are being designed to eliminate the lengthy dial-up and sign-on process inherent in analog services. The cable access device includes modem technology to convert the analog signal into digital data used by the computer. The network pipe itself is increasingly a fiber optic or hybrid fiber-coaxial cable designed for two-way communication in an asymmetric configuration. Data is carried downstream to the user over wide electromagnetic bands, while smaller bands carry commands and responses upstream to the cable provider's head-end.

The disadvantages of existing cable networks are their reputation for unreliable service and the fact that bandwidth is shared; if one user ties up the WAN link with a high-bandwidth application, other users suffer. Cable companies are currently designing distributed network models that would connect smaller groups of users to multiple points of presence to solve this problem. Furthermore, unlike ISDN, cable networks are not currently optimized for two-way data communications (versus one-way broadcast transmission). ISDN is standardized worldwide, whereas cable addressing schemes and other internetworking standards have not yet been developed. The cable standardization process could take several years; ISDN is a standardized offering now.

SMDS

SMDS's connectionless nature eliminates the need for carrier switches to establish a call connection between two points before data transmission. SMDS access devices pass 53 byte datagrams that include addressing information to a carrier switch that forwards the cells over any available path to their destination. Data travels over the least congested routes in an SMDS network, providing faster transmission, security, and greater flexibility to add or drop network sites.

Between 200 and 500 customers in North America are using the service. In Europe and the Pacific Rim, service is just beginning to be deployed in about 16 countries. Internationally, SMDS is sometimes referred to as CBDS (Connectionless Broadband Data Service).

ATM

ATM has some of the advantages of narrowband BRI and PRI ISDN, with the significant added advantage of almost unlimited transmission speeds. However, many details of the ATM standard implementation are still evolving, and ATM WAN service is not yet widely available.

Evaluating WAN Service Cost-of-Ownership Options

ISDN Data Networking Applications

ISDN's high throughput capacity with dynamic bandwidth allocation features, signal quality, reliability, flexibility, fast call setup, and attractive tariffs make ISDN an excellent, cost-effective medium for the following major networking applications:

• Remote LAN-to-LAN and LAN-to-host internetworking
• Remote node-to-LAN and terminal-to-host internetworking
• Internet and on-line services access

Remote LAN-to-LAN and LAN-to-Host Internetworking

These smaller remote sites might also require dial-up access to central site network resources just for individual users. In either case, exchanging e-mail or retrieving information from central computers requires at least a temporary WAN connection.

ISDN and other switched services are well suited for on-demand remote LAN-to-LAN configurations since they provide a telephone circuit only when information needs to be transferred. And some internetworking devices have the intelligence to schedule these connections when telephone rates are more economical, for added cost savings.

Figure 5. Remote LAN-to-LAN Internetworking

Dial-on-Demand Internetworking Applications

With ISDN's fast call setup capability, remote LAN-to-LAN or remote node-to-LAN access can take advantage of dial-on-demand during an individual session. When there is no client data traffic destined for the LAN, the client can disconnect an ISDN WAN link transparently to the running application to save dial-up charges. The application continues to see a logical link to the LAN through a process known as "spoofing." When communication with the LAN is required, the remote client automatically reestablishes a dial-up session and passes the data traffic over the WAN.

Figure 6. Dial-on-Demand Internetworking Application

Redundancy and Overflow Internetworking Applications

A dial-up connection, such as an ISDN BRI line, is a more affordable back-up solution to a primary leased line. As shown in Figure 7, the dial-up line is automatically activated by the central internetworking device when a failure occurs on the primary line, with no apparent degradation of service in the network. If the primary line is a high-speed pipe running at T1 or E1 rates, several lower-speed dial-up circuits can be aggregated to achieve comparable high-bandwidth capacity.

ISDN can also be used to carry the overflow of data connections when the data load increases. When the primary line reaches maximum capacity, the bridge or router can detect the bandwidth bottleneck, dial one or more ISDN circuits in real time, and route overflow traffic through the B channels.

Figure 7. Redundant WAN Link Internetworking Application

Remote Node-to-LAN Internetworking

Remote node-to-LAN internetworking opens new avenues of communication for employees who travel extensively, work at home, or require after-hours access to the office. At the central site, the internetworking device receives incoming data calls from various locations through pools of dial-in lines, provides security authentication and validation of callers via login procedures, and routes the calls over the central site network. Alternatively, the internetworking device can hang up after identifying the caller and call back later for security or economy. With ISDN, remote and mobile computer users can get secure and reliable access to multiprotocol LANs and SNA mainframe resources.

ISDN's fast call setup is ideal for the point-of-sale transaction-based networks of small businesses and retailers that need immediate responses to credit and debit card authorization, and for medical center and other health-care providers that need immediate verification of health-care eligibility. Using ISDN, the verification process typically takes less than one second compared to the 15 second or more wait experienced with 14.4 Kbps analog modem-based systems. ISDN's high speed and rapid response times offer a competitive advantage in responsive customer service.

Figure 8. Remote Node-to-LAN Internetworking Application

Internet and On-Line Information Access

As shown in Figure 9, ISDN's high-speed service solves the bandwidth access problems for these remote users, as well as for Internet users on an enterprise LAN. Remote users can connect to the Internet directly or dial in to the central site access server and gain access to the Internet through the central site LAN, taking advantage of its faster link to the Internet service provider. More and more public and private Internet service providers are tapping this market and supporting ISDN gateways that provide 64- and Nx64-Kbps access to the Internet or other on-line information access services.

Figure 9. Internet Access Internetworking Application

ISDN Interoperability with WAN Services

As Figure 10 shows, client-to-LAN applications such as remote node-to-LAN internetworking are point-to-point applications. LAN protocols such as IP and IPX, transmitted from a client application or network operating system, are transported over the dial-up ISDN link using the Point-to-Point Protocol (PPP) or Multilink PPP (ML-PPP).

Once the data packet reaches the LAN, the remote access server can route it as is using PPP, or repackage the data and route it over the backbone using any available network service, including X.25, Frame Relay, or SMDS. The service is entirely transparent to the B channel. The remote access server can also establish a LAN-to-LAN connection on demand via a dial-up link, and route the data packet using PPP or ML-PPP.

Figure 10. ISDN Access to Other WAN Services

Planning and Installing an ISDN Service

ISDN Services

• Through a direct BRI (2B+D) connection. The ISDN lines can be connected directly to ISDN equipment in an organization or residence, or through a PBX, key system, computer, or front-end processor so that users can communicate without calling through an outside connection.
• Through an ISDN Centrex service. One or more BRIs can also be linked to an ISDN Centrex service, which then functions as the PBX for the organization.
• Through a PRI (23B+D) connection.
  ISDN lines can be connected through a PBX, automatic call distributor, or central computer or computer network, which then distributes the B channels as needed throughout the organization.

ISDN BRI Wiring Alternatives

• ISDN as the only line. This alternative relies on ISDN lines as the main connectivity medium for all voice, data, and video transmissions. It requires either an ISDN telephone or PC and/or fax with a terminal adapter that supports a standard, non-ISDN analog telephone or fax, depending on whether ISDN or non-ISDN terminal devices are to be used.
• ISDN as the second line. This alternative boosts the level of connectivity for higher-speed applications while existing analog lines serve analog phones and faxes. The drawback of this alternative is that you cannot "roll over" calls from analog lines to ISDN lines, or vice versa.

ISDN Customer Premises Equipment Requirements

• Bridge/routers to provide remote LAN-to-LAN or node-to-LAN ISDN connections to the central site LAN. TE1 (native ISDN) bridge/routers are usually equipped with either an S/T or U interface, while some support both interfaces.
• Terminal adapters (TAs) to be used in T2 (nonnative ISDN) bridge/routers and other devices to provide ISDN interface connections. Some terminal adapters include analog ports to support analog telephones and faxes concurrently.
• Aggregation devices, specialized TAs that aggregate or bond the B channels into a single higher-speed connection when needed. Some PC vendors also include an Ethernet bridge with this device.
• Access servers, the central site connection devices that provide dial-up access to users of remote node-to-LAN applications.

Residential and small-site users will need some of the following special equipment to access ISDN lines:

• A network termination device to serve as the network interface for each BRI connection and to offer line connection and testing capabilities; it often includes a back-up power supply. Some CPE vendors incorporate this NT1 device into their ISDN interface equipment (for example, equipment with an integrated U interface for the North American market).
• A power supply for power to the ISDN line connection, since unlike analog lines, ISDN connections are not powered. Some CPE vendors integrate the power supply with the NT1 device.
• A terminal adapter to convert protocols used in PCs, workstations, and other nonnative ISDN equipment to a format that can be sent over the ISDN lines. Some terminal adapters include analog ports to support analog telephone and faxes concurrently. CPE vendors often integrate terminal adapters in ISDN telephones as well.
• An external ISDN modem to convert serial interface protocols used in LAN equipment and computers to a format that can be sent over the ISDN lines. It is an alternative to the internally installed TA. Some digital modems include analog ports to support analog telephones and faxes concurrently.
• An ISDN telephone to make the ISDN signal available for voice communication. The ISDN telephone often comes equipped with an LCD screen for message and extended feature control.

Table 3. 3Com ISDN Product Offerings

ISDN Ordering Codes

The ISDN ordering codes are a standardized list of network services associated with specific U.S.-based ISDN customer premises equipment for a specific application. Seventeen standard capabilities were defined based on the most popular network configurations -- combinations of features, services, and network parameters--already in use by thousands of ISDN customers across the country. To date, more than ten international service providers and over 40 CPE vendors support ISDN ordering codes.

Specifying the CPE vendor's equipment ISDN ordering code when placing an order with an ISDN service provider circumvents line provisioning or other complexities. The standard code ensures that the line will work properly with the equipment and will be provisioned more quickly and accurately. A vendor should be able to provide the IOCs for its products to its U.S.-based ISDN users. Bellcore alerts carriers to the products and switch translations these IOCs represent.

The North American ISDN User's Forum (NIUF) Ad Hoc Group on the Simplification of ISDN Ordering, Provisioning, and Installation has been focusing on developing easier ways for customers to order ISDN service. The EZ-ISDN codes that they have proposed reduce even further the number of IOCs and are geared toward assisting mass market users of ISDN.

ISDN Configuration

Summary

A single BRI connection can link remote users or smaller sites to a central LAN site with simultaneous voice and data communication. Switched digital network access is particularly well adapted to users in remote locations who often cannot justify the cost of a dedicated connection to central site LANs. In addition, ISDN's high-speed capabilities make it a cost-effective option for organizational and residential users who want to take full advantage of the information-rich resources on the Internet and World Wide Web.

Evaluating WAN services for your enterprise network entails weighing the advantages, limitations, and cost of ownership of each option against your company's short- and long-term internetworking needs. ISDN provides some clear advantages for switched WAN access: the reliability and built-in security of digital technology, fast call setup for data applications, interoperability with other services such as Frame Relay and X.25, and cost-effective integrated support of voice, data, and video services on a call-by-call basis. ISDN also delivers the bandwidth demands of critical applications today, while providing a migration path to the higher speeds of broadband ISDN using ATM switching as application demands grow in the future.

Successful ISDN deployment involves identifying and understanding all service provisioning and equipment configuration and maintenance options, as well as other cost-of-ownership issues, in advance of implementation. Careful planning will ensure that your ISDN installation runs smoothly and that the network system you've implemented meets your organization's short-term and long-term WAN connectivity requirements.