What is ISDN ? To begin, "ISDN" should not be thought of as just one more product. is actually a family of services that are central to the fundamental transportation of the entire world-wide telecommunications network.... t United States and the world are going digital. A global economy requires a global telecommunications technology. Overseeing this world-wide evolution is the International Telegraph Telephone Consultative Committee (CCITT), an agency of the United Nation The CCITT's membership consists of major governments, manufacturers service providers from around the world , and it stablishes common international ISDN standards. In the past many different ISDN "flavors" have been developed by competing manufacturers and countries. It is the CCITT's job to reconcile these differing flavors into one harmonious who These evolving universal standards will not only ensure that everyo can effectively communicate with everyone else, but by the very fact of their being universal they tend to protect ISDN providers' investment in switches and infrastructure and their customers' investment in terminals other equipment. Now the shift to digital transmission is certainly nothing new. It began in the early 1960's with the conversion (to T - carrier technology of the inter-office trunk facilities - the backbone network that connect telephone companies' central offices. Since then, the other elements o of the network have in turn also been included in this conversion proces The conversion process continues at a furious pace. Toady in the Ameritech region some of the last pieces of the puzzle are falling into place. The last of the old 1ESS analog central offices are going digit It is interesting to not that a similar process has occured in the world. Today it is difficult to find an analog PBX being offered (let a purchased). In fact, virtually all major manufacturers also offer some type of ISDN compatability in their lines. This does not mean, of cour that analog telephone sets are dead - many will continue to be sold ( especially in the lower end markets), but teir days are numbered in the business world. Digital technology alone - as important as it is - is not the whole story, however. To state that the network is digital is like saying th all automobiles have engines. Ther are gasoline engines, but there are also diesel engines; there are big ones and small ones, foreign and domestic. And almost none of the parts are interchangeable. What ISDN is doing is building on that digital foundationa set of r specifications detailing common features and services so the everyone ca work from the same set of blueprints. And the first building block of I is the Basic Rate Interface (BRI). Here in Ameritech, we call this service ISDN Direct. How does ISDN Work? At it's most basic level, ISDN provides a standard "pipe" called th Basic Rate Interface. BRI is transmitted over the normal 2-wire copper cable facilities familar to telephone trnsmission all over the world. But unlike ordinary analog transmission which restricts this pipe to conversation at a time, BRI combines, or multiplexes, three communicatio channels into one pipe - all of which can be used simultaneously. The BRI consists of two 64 Kbs "B" (or Bearer) channels and one 16 "D" (Delta) channel. This is the infamouse 2B + D many people have hear about. We can see that one on the more obvious achievements of ISDN technology is that the capacity of the local copper loop is effectively trippled. SIGNALING Beyond the issue of capacity, ISDN signaling schemes not only provide another break with the past, but also present opportunities for unique telecommunications applications. Today's analog transmission uses a system called "in-band" signaling, which means that all call processing (such as sending "off" or "on" hooks," call set-up, and ringing) is done on the same path, or circuit, that the conversation takes place. This signaling "overhead" greatly impairs network efficiency. ISDN, on the other hand, utilizes the D channel for all signaling -"out of band". In simplified terms, what this means is that when a user lifts the reciever off the hook and dials a number, the telephone itself sends a short message, or packet, over the D-channel to the central office saying, in effect, "I want to send a certain kind of message to this address." The central office first looks at this packet and determines what kind of route the actual message will need to take (i.e. circuit switched or packet switched). It then hands off this signaling packet to a seperate network used only for signaling, while reserving a future path or route in the circuit-switched network for the upcoming message. This signaling network usually referred to as Signaling System 7 (SS7 or sometimes CCS) repeats the process - passing along the packet - until the far-end telephone is reached. Only when that telephone says "OK, go ahead" is the transmission path actually established and the call put through. When out-of-band signaling is examined we can see another key element in the ISDN picture. That signaling packet as it is passed from node to node, finally reaching the terminal at the far end must contain all the information for the call setup - and that includes the number of the calling party. Many of the most exciting applications being developed to utilize ISDN make use of this information called Automatic Number Identification (ANI) or sometimes Calling Line Identification (CLID). This signaling network usually referred to as Signaling System 7 (SS7 or sometimes CCS) repeats the process - passing along the packet - until the far-end telephone is reached. Only when that telephone says "OK, go ahead" is the transmission path actually established and the call put through. When out-of-band signaling is examined we can see another key element in the ISDN picture. That signaling packet as it is passed from node to node, finally reaching the terminal at the far end must contain all the information for the call setup - and that includes the number of the calling party. Many of the most exciting applications being developed to utilize ISDN make use of this information called Automatic Number Identification (ANI) or sometimes Calling Line Identification (CLID). For example. an incoming ISDN call could be automatically routed to the proper department or person (while simultaneously displaying all the customer's records on his or her computer screen) - all before the call is even answered. THE B-CHANNELS With all signaling tasls being performed by the D-channel, this leaves the two B-channels free for full 64 Kbs, "Clear Channel", transmission. Each may be used for circuit-switched voice or data calls. ("Circuit-switched" means that the entire transmission path, or circuit, remains "nailed up" end to end for the duration of the call.) It should be noted the full, clear-channel 64 Kbs transmissiom between central offices requires seperate SS7 deployment between all the srvving offices. Until the new SS7 network is fully extended throughout the nation some links may still be restricted to "only" 56 Kbs. Incidentally, this situation is one of the primary causes for the "ISDN islands" many people talk about. In addition to the circuit-switched network described above, the B-channel can also be utilized for high-speed packet data transmission. The packet network is seperate from both the circuit-switched network with which most people are familiar and the new SS7 signaling network. In packet transmission the message is broken down into small discrete blocks of information called packets - similar to the signaling packet described earlier. These packets are then addressed and sent out individually over the network much like mailing a letter. Packet data transmission is extremely efficient for "bursty" type of transmission today. One interesting option for users is to combine two B-channels for a total of 128 Kbs of available bandwidth. Video conferencing applications, for example, can effectively use this speed. D-channel packet: Another benefit of ISDN derives from the fact that not all the 16 Kbs capacity of the D-channel is needed for signaling. Up to 9.6 Kbs is available for medium speed packet transmissionwhich provides still another communications option to the user. Actual throughput will depend on the number of packet devices hooked up to the D-channel and how bursty their transmission patterns are. To sum up then, it would be possible for one rather energetic ISDN user to simultaneously talk to a colleague over one B-channel, exchange data files over the other B-channel and check his electronic mail over the D-channel. And, instead if needing to use multiple telephone lines and modems, everything is accomplished over one BRI ISDN line. So how does it all fit together? First of all, ISDN starts out on a special line card in the Bell central office switch. It is then carried over the normal(non-loaded) 2-wire telephone plant cable mentioned earlier using a special transmission scheme called 2B + 1Q. (By the way, the term "2B + 1Q" should not be confused with "2B + D." 2B + D is a generic term that describes the actual transmission scheme of the 2-wire loop from the C.O. to the customer.) Distances of up to 18,000 feet form the C.O. can be accomodated. (Distances over 18 Kft are possible, but would require "boosters" and thus cost more.) At the customer's premise the cable terminates in a devise called the NT1 (Network Termination 1). Among other things, this device establishes the basic boundary point between telephone company facilities and the customer's inside facilities. The NT1 also takes the outside, 2-wire transmission mode and converts it to the inside mode(and vice versa). This inside mode, or passive bus, can vary, but it usually consists of 6 wires - 2 for transmitting, 2 for recieving and 2 for local power. Customers need to be aware that local power is needed. This will be no surprise to many of our business customers as some of them already have commercial telephone systems. But Residence and smaller users need to be that in order to run ISDN the unti that makes this possible needs power. Up to eight devices can be connected to one BRI in the "passive bus" arrangement. "Passive bus" is just a term used to describe the way in which all the devices can share the same 6-wire transmission medium and all transmit and/or recieve at the same time. This arrangement allows each device can have its own individual directory number. There are a few restrictions. One important one is that only up to two circuit-switched devices (such as voice terminals) can be hooked up to a single BRI - all other devices must be packet-switched devices. (Upon reflection this arrangement makes sense since two circuit-switched terminals would be able to access one of the two B-channels at the same time while all the devices could share the "virtual" packet network.) Any terminal equipment designed to "talk ISDN" (called a "TE1" in ISDN terminology) can be connected to the passive bus. A complicating factor here is the exsistance of those different, propietary "flavors" of ISDN mentioned earlier. The bad news is that until this year it was impossible to mix and match equipment designed for a particular manufacturer's central office switch with those of another. The good news is that equipment built to the new National ISDN-1 (NI-1 or sometimes NISDN) standard that is now in place will solve that problem - all ISDN CPE will be interchangeable. Of course other equipment not specifically designed to work with ISDN (called a TE2) can still be hooked up to the circuit, but it must work through a terminal adaptor ("TA") which translates the communications protocols from non-ISDN to ISDN. A typical situation might involve a PC with normal RS-232 connections. BRI and RS-232 are not compatable, but the user need not throw out his existing PC in order to work with ISDN. The user has a choice: For example, the PC could work through a stand-alone TA such as the Nothern telecom M5000, or it might be connected through a circuit card plugged into one of the computer's expansion slots like the Apple ISDN NB card, or it might plug directly into a telephone with a TA built in like the AT&T 7507. BEYOND BASIC RATE INTERFACE: ISDN Centrex ISDN Centrex is offered as an enhancement of the standard Centrex service with which we are familiar. Working across the standard Basic Rate Interface, customers have access not only to all the features of ISDN Direct but also to all the Centrex voice features that have made that product so successful. ISDN Prime Primary Rate Access (PRI) is for our customers, particularly PBX users, who need larger access rates to the network. Instead of BRI's 2-wire, 2B + D interface, PRI is presented to the customer over a 4-wire interface with 23 B-channels and one D-channel. All channels (including the D-channel) work at 64 Kbs. If a customer needs more capacity than one PRI (roughly equivalent to a T-1 circuit), Pri's can be aggregated with one D-channel controlling multiple PRI's which increase circuit efficiency. Since ISDN is a switched service, a particular strength of ISDN Prime is the ability to consolidate PBX trunks and data services. This dynamic call allocation feature often virtually eliminates the need for expensive private lines and greatly reduces the number of trunks needed. Broadband ISDN Broad band ISDN (BISDN) is the ultimate goal of the digital revolution taking place. Transmitted over the fiber-optic based Synchronous Optical Network (SONET), this optical technology has the bandwidth capacity to handle any conceivable application well into the next century. Like digital network evolution before it, the transition to a SONET backbone will begin with interoffice trunks and then gradually work it;s way out towards the customer. Full deployment will take many years. However, our customers' rapidly expanding needs for bandwidth cannot wait for the complete SONET network to be in place. So, the first elements of BISDN such as Switched Multimegabit Digital Service (SMDS) are comming online right now with transmission speeds up to 34 Mbs. Higher and higher speeds will become available as the network and our customers' needs evolve. Ultimately, switched speeds in the billions of bits per second("gigbits") will be possible. THE BIG PICTURE: We are all in the early stages of a fundamental transformation of worl-wide telecommunications. It is not an exaggeration to say that this transformation is comparable in scope to changes that took place in society with the introduction of the automobile... or television. (SMDS) are comming online right now with transmission speeds up to 34 Mbs. Higher and higher speeds will become available as the network and our customers' needs evolve. Ultimately, switched speeds in the billions of bits per second("gigbits") will be possible. THE BIG PICTURE: We are all in the early stages of a fundamental transformation of worl-wide telecommunications. It is not an exaggeration to say that this transformation is comparable in scope to changes that took place in society with the introduction of the automobile... or television. And, like those befoer us who were also "in at the beginning" we can only dimly perceive where this new road will take us. You may have noticed already in your area that new phone line are being installed and these are the digital lines that the ISDN architecture will be using. ISDN is already being used here in the United States right now. More manufacturers of telephone equipment have started to produce telephone systems the is compatible with the ISDN structure (such as Macrotel 80D). For More information on ISDN Networks please call your local phone company.