Always OnDynamic ISDN (AODI)

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Always On/Dynamic ISDN (AO/DI)

When you need a wide area link that's faster than an analog telephone line, what comes to mind? Many think of ISDN in its Basic Rate Interface (BRI) incarnation as the next step up from the Plain Old Telephone System (POTS). Unlike leased lines, the endpoints of which are fixed, ISDN (like POTS) is a switched-circuit technology, so you can place a call to anyone who has an ISDN telephone number. (As a matter of fact, you can call POTS numbers from an ISDN number, and vice versa, but this is a least common denominator situation, so you have to settle for analog line performance.)

The classic ISDN BRI is divided through Time Division Multiplexing (TDM) into three independent channels: one 16Kbit/sec Data (D) channel, and two 64Kbit/sec Bearer (B) channels. The D channel is used by the telco for signaling, and the B channels carry user data. When you dial a number on an ISDN line, the D channel handles the linking of one, or both, of the B channels to the desired destination telephone number; this is known as call setup. Similarly, when you hang up and terminate the call, the D channel is used for call tear-down.

What's New

A new technology twist that ISDN equipment vendors and service providers are now unveiling allows for some user data to be carried over the D channel. Unlike the B channels, which are operational only during the course of a call, the D channel is always active, and always connected to the switch at the telephone company's Central Office (CO). Always On/Dynamic ISDN (AO/DI) takes advantage of this feature. Under AO/DI, a portion of the D channel is operated as a link in an X.25 packet-switched network. The Point-to-Point Protocol (PPP) is used over this link. Of the D channel's 16Kbit/sec total capacity, 9.6Kbits/sec is devoted to AO/DI user data traffic; the balance of the link continues to be used for telco signaling (call setup and tear-down, for example).

At the telco's CO, user data packets sent across the D channel link are stripped off by packet handlers placed in front of the ISDN switch; the data packets are then sent across an X.25 network to their final destination.

One frequently cited application for AO/DI is Internet access. Packets ride the D channel to the telco's CO, and are then put onto the X.25 packet network to be forwarded to the ISP. The ISP, in turn , routes the packets to the Internet.

Expanding Bandwidth-On-Demand

Under AO/DI, the D channel plays a role in some of the bandwidth-on-demand features that ISDN devices are noted for. If there is a minimal amount of traffic (that is, if there are a small number of packets to be sent), the B channels will be inactive, but the D channel will still be carrying packets. If the amount of data waiting to be sent across the 9.6Kbit/sec D channel exceeds 7500 bytes, a circuit-switched B channel link will be established, providing a throughput of 64Kbits/sec. At this point, the D channel will be idled for user data purposes; it will continue to be used for telco signaling. If utilization of the B channel exceeds a preconfigured threshold (typically 70 percent utilization), a second B channel will be established, providing a 128Kbit/sec data transfer rate.

If utilization of the 128Kbit/sec link should fall below some preset percentage, one of the B channels will be dropped, so the link throughput will fall back to 64Kbits/sec. If traffic falls further, the remaining B channel will be dropped, and the ISDN device will revert to using the 9.6Kbit/sec D channel.

Keep In Touch

Those of us who have a T1, or even a mere 56Kbit/sec leased-line, connection to the Internet know how useful it is to be constantly connected, and how frustrating it can be to have to wait for a modem to dial and establish a connection. The connection time for an analog modem operating on a POTS line ranges from about 10 seconds to about 30 seconds. ISDN cuts the connection time to a second or two, but ISDN with AO/DI eliminates it entirely.

At a time when users consider a 14.4Kbit/sec modem to be obsolete, a 9.6Kbit/sec communications channel may seem so slow as to be not worth the bother, but this is not necessarily the case. A low data-rate connection that's always available can enable some applications that just aren't feasible (or as useful) with occasionally-connected links. As one example, burglar and fire alarm companies could use the 9.6Kbit/sec X.25 link to poll alarm sensors at the customer's sitewithout the expense of placing a B channel call. It may require only one packet to poll a sensor, and one packet for the sensor's response, so this can be a very low data-rate application. Stock quotes and news headlines are also examples of information that can be pushed to user's computers at a data rate that's a mere trickle.

It's also possible to move data in the other direction: Utility companies are able to eliminate the expense of sending meter readers to each home and business if the meters are outfitted with a digital interface to the ISDN D channel, and retailers can use the D channel for credit card authorization.

Small business users and telecommuters that need electronic mail often depend on their ISP to run the mail server. If these customers are connected with analog lines, or even with non-AO/DI ISDN lines, they will have to periodically call up to see if they have new messages. With AO/DI, users could be notified automatically when they get a new mail message. The notification would come in the form of a packet (or several) that is sent across the X.25 D channel link. If the message in question is a short text file, it may be possible to download it without crossing the threshold set for the 9.6Kbit/sec D channel.

Development of AO/DI technology has been shepherded by the Vendors ISDN Association (VIA), a San Ramon, CA-based trade association. VIA members consist primarily of ISDN equipment providers. The organization has been putting together the proposed specification for AO/DI.

Clearly, for AO/DI to work, ISDN equipment vendors and service providers must support it. Bell Atlantic, BellSouth, and SBC Communications (the parent company of Southwestern Bell and Pacific Bell) have announced plans to support AO/DI in the first quarter of 1998. Equipment vendors who have announced their intention to support it include 3Com, Adtran, Arescom, Ascend Communications, BinTec Communications, Cisco Systems, Digi International, ECI Telecom, Eicon Technology, ITK Telecommunications, Jetstream Communications, Shiva, and Virtual Access. Release dates for products range from the first quarter of 1998 through the third quarter of 1998, depending on the vendor and product.

Eicon Technology is already shipping a product. Eicon's Diva T/A ISDN terminal adapter is an external terminal adapter that supports AO/DI. It also has Auto-SPID and SPID-Wizard features, which automate the task of getting an ISDN device connected and talking with the telco's CO switch.

If you're planning to take advantage of AO/DI for your Internet connections, your telecommunications carrier and your ISP must both support AO/DI. The telco will need to strip packets off the D channel and put them onto the X.25 network. Whichever ISP you use will also have to have a connection to your telco's X.25 network to receive and send your packets.

ISPs use high-density ISDN routers that let them terminate a large number of B channels on a single chassis. Ascend Communications has a big chunk of the service provider market with its Max product. Mike Baccala, senior technical manager for product engineering at Ascend, says the company will offer support for AO/DI in the "first part of the second quarter of 1998." It will be a free firmware upgrade for existing equipment, he says.

Telco pricing for X.25 packet data over the D channel is still in flux, but figures around $5 per month over the price of regular ISDN service are being discussed; this could make it quite attractive for users.

AO/DI's attraction for telcos is that it could dramatically lower B channel usage. Telcos base their projections of how many switch ports will be needed for each local exchange upon the number of users in that exchange, as well as upon the assumption that each line will be used only occasionally. This is a reasonable assumption for voice calls, but Web surfers can be connected for long periods of time.

Many telcos price ISDN using a pricing model that's similar to analog lines: local calls are not charged over and above the monthly rate for service. This has led most ISPs to place a POP in each local exchange in which they have customers. Subscribers to the ISP's service can place local calls and remain connected to their ISP's POP for long periods of time, without incurring significant per-minute charges, so there's little incentive for users to keep their connect-times short. AO/DI can give users constant connectivity, while simultaneously freeing up B channels for data transfers that truly require B channels' higher throughput.

Because the D channel X.25 packets are handled at the CO by the X.25 packet handler, it's possible to route these packets without first crossing the circuit-switched fabric of the CO's switch. This reduces the impact on the telephony network because, from the switch's viewpoint, there's no activity until a B channel is raised.

Ready When?

When will AO/DI be available to you? It's somewhat difficult to predict whether the ramp-up for AO/DI will match ISDN's initial slow, painful birth and development, or whether it will all happen quickly and smoothly. From the equipment providers' standpoint, support for AO/DI appears to be coming quickly. It's also possible that much existing ISDN equipment can be upgraded by loading new softwareanother spur to rapid deployment. We're not talking about AO/DI competing with ISDN; it's simply an extension to ISDN's existing feature set. As such, AO/DI can leverage ISDN's hard-won gains in market development and telco deployment.

Interoperability between different vendors' equipment will also be a key issue. VIA has already sponsored an early interoperability test, but deployments of production units into the field will be the litmus test.

The critical path in AO/DI deployment and usage is likely to be telco and ISP support for it. Although you may have customer premises equipment that's AO/DI-enabled, your local telephone company will need to offer support for AO/DI before you can use it. According to Ascend's Baccala, the telco's CO switch must be AO/DI-enabled. However, he says, "all the switches are already X.25-capable, so this is not a big deal."

In addition, your ISP will need ISDN routers or concentrators that support AO/DI, as well as have the necessary connection to your telco's X.25 network.

Resources

If you'd like to explore Always On/Dynamic ISDN (AO/DI) in more depth, check the Vendors ISDN Association (VIA, San Ramon, CA) Web site at www.via-isdn.org. VIA maintains an FTP site that contains, among other things, the proposed RFC for AO/DI. Look for the document named aodirfc.doc.

For more ISDN information, you can also surf to these Web sites:

European ISDN Users Forum www2.echo.lu/eiuf/en/eiuf.html

North American ISDN Users Forum www.niuf.nist.gov

Dan Kegel's ISDN Web Page www.alumni.caltech.edu/~dank/isdn

This tutorial, number 117, by Alan Frank, was originally published in the April 1998 issue of Network Magazine.

 
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Network Tutorial
Lan Tutorial With Glossary of Terms: A Complete Introduction to Local Area Networks (Lan Networking Library)
ISBN: 0879303794
EAN: 2147483647
Year: 2003
Pages: 193

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