$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ $ P/HUN Issue #4 $ $ Volume 2: Phile 9 of 11 $ $ $ $ $ $ USDN VERSUS ISDN $ $ ---------------- $ $ $ $ by $ $ $ $ $ $ LORD MICRO $ $ ********** $ $ $ $ TOLL CENTER BBS - (718)-358-9209 $ $ A 2600 MAGAZINE BBS $ $ $ $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ PREFACE: The integrated services digital network (ISDN) is a long-range plan for systematically upgrading the televommunications networks of various countries to provide both voice and data services on a single physical network. European countries have been the major force behind degining ISDN. The U.S. however, will require a variant of ISDN, because its communications industry operates in a competitive user-oriented environment. This article describes the differences in implementation and services that can be expected with USDN (the U.S. version of ISDN) and identifies unresolved issues that should concern the data communications manager. INTRODUCTION ------------ The ISDN proposal has recieved worldwide attention for at least a decade. Constrained by an apathetic marketplace, technical limitations, economic considerations, and the slow pace involved in establishing acceptable world-wide standards, implementation of ISDN has occured principally in laboratories only. The International Telegraph and Telephone Consultative Committee (CCITT) has attempted to define and obtain general consesus regarding ISDN objectives, interfaces, services, and standards. The CCITT-backed ISDN principally represents European interests. Although the U.S. is repre- sented in the CCITT and offers support for its programs, the major telecommunications organizations in the U.S. are more interested in estab- lishing their own standards and programs. So, while ISDN seems to be gaining more U.S. support, it continues to reflect a European perspective. Recently, the term USDN has been used to distinguish the modifications to ISDN that are expected to evolve in the U.S. The USDN concept is one of integrated access to multiple networks, rather than the integrated services on one network approach of the ISDN proposal. The U.S. telecommunications industry has long recognized that ISDN would have a somewhat modified personality in the U.S. Several industry-wide ISDN conferences addressed the U.S. equivalent to ISDN, but none of the conference comittees proposed that the U.S. adopt ISDN totally, be- cause of the unique characteristics of the U.S. communications environment. THE U.S. COMMUNICATIONS ENVIRONMENT ----------------------------------- The unique U.S. communications industry characteristics that influence the USDN effort are described in the following section. These characteristics are summerized in Table 1, which compares the U.S. communications environment with the environment in other countries. The Competitive Marketplace: In the U.S., the privately managed telephone industry responds, rapidly to user demands for new services. In most other countries, however, services are established in a slow, deliberate program by one government-administrated source, usually the country's postal, telephone, and telegraph (PTT) agency. The users then decide if and how they will use the services offered. Technology Advances: Second only to the competitive marketplace, technology advances include component developments such as as memory devices, VLSI chip design, and optical elements. Developments in the system architecture, networking, and functional interfaces in transmission and switching technologies are equally important in defining the USDN concept. Less Restrictive National Standards: Because standards imply conformity, they can restrain innovation. Because they are not required to undergo a lengthy standards-approval cycle, U.S. manufacturers are free to produce systems that are incompatible with existing equipment. De facto standards are often established by the market's acceptance of a particular system. Comparing ISDN and USDN FEATURES CLASSICAL ISDN USDN ENVIRONMENT ENVIRONMENT -------- -------------- ---------------- Competition Essentially None Varied, encouraged by government Standard Inter- CCITT Essentially none exchange Carriers One, nationalized Many, equal access Existing Investment Due for replacement Huge, undepreciated investment Service Offerings By PTT schedule Entrepreneurial, competitive Implementation Cost Government-provided funds Private Capital Multiple Carriers and Competative Networks: The premise of ISDN is that a common national network will evolve, able to handle multiple voice and data services in an integrated fashion. ISDN thus precludes a carrier by-passing a local office or vying for customers' traffic through innovative techniques. In most countries other than the U.S., telephone companies are not legally required to provide or counter new service offerings. Integrated digital networks (IDNs) are emerging in the U.S. that provide digital access and transmission, in both circuit-switchhed and packet modes. The number of IDNs will probably increase regardless of whether an ISDN is proposed for the U.S. Embedded Investment: The U.S. has invested heavily in modern stored-program controlled (SPC) switching. However, other countries are only now facing conversion to SPC, as much of their existing systems investment is greatly depreciated. These countries can therefore converty to ISDN switching in a more orderly and economical fashion that the U.S. can. Thus the U.S. will have overlay networks, digital adjuncts to existing SPC switches, and multiple networks in the foreseeable future. EVOLUTION OF U.S. COMMUNICATIONS OFFERINGS ------------------------------------------ The American solutions to data transmission problems have tended to be faster, more practical, and less elegant than those evolving in Europe. Not surprisingly, the American solutions have generally ignored CCITT recommendations. For example, public packet-switched networks such as TYMNET, GTE Telenet, and Satellite Business Systems are offered to some customers with long-haul traffic. These systems were severly limited because they often required access through analog local office. Many major industries and private organizations thus established their own data networks, often point to point over leased circuts. Digital Terminating Service was introduced to provide 56K bit-per-second (up to 448K bit-per-second) links to the end user over special transmission links. Digital Electronic Message Service was recently approved to provide 1.5M bit-per-second service to the end user. Some suppliers are now offering PBXs with 64K bit-per-second local loops and direct pulse code-modulated (PCM) trunks to the public network. Two new standard 1.544M bit-per-second central office-to-PBX interfaces have been established, the Northern Telecom Computer-to-PBX-interface and the AT&T Information Systems Digital Multiplexed Interface. The Development of Local Area Networks: The increasing use of data terminals and the growth of distributed processing has led to the necessity of transporting data at rapid rates within a building or local area. This rapid local data transmission imposed impossible requirements on the conventional PBX. While PBX designers struggled to upgrade their data capabilities, computer manufacturers saw the oppertunity to offer local area networks (LANs) designed specifically to provide wideband data transport between users in a limited area. Again, expediency and the competitive climate produced a practical solution - several LANs with different architectures and protocols. In general, these LANs so not conform to the ISDN protocol levels identified in the International Standards Organization (OSI) models. However these LANs cannot be eadily replaced, so the USDN will have to accommodate them. A case in point is the apparent conflict between the ISO model of Open System Interconnection (OSI) and the IBM System Network Architecture (SNA). The OSI model of a seven-layered architecture for data networks has been defined for the first four layers only. International agreement on the remaining protocols will take several more years to obtain, if agreement on the remaining protocols will take several more years to obtain, if agreement is possible, Meanwhile, in the U.S., IBM defined a similar protocol, SNA, and has implemented numerous networks. Long before any ISO standard can be established, the U.S. will be well populated with SNA systems. The USDN must be at least compatible with SNA, and SNA could become the national standard. Because future PBXs will probably be able to switch synchronous data at 64K bits per second (and multiples therof, up to at least 1.544M bits per second), there may not be sufficient switched wideband traffic requirements to support a seperate LAN standard. Long-distance dedicated data services such as AT&T's ACCUNET and SKYNET are competing for data traffic. In addition, various data-over-voice (DOV) schemes have been employed over switched analog circuts. In short, many approaches, services, and facilities have already been implemented to satisfy the immediate market needs, without regard to an orderly transition to ISDN. Thus, the USDN will have to accomodate thesee established services and inteface with most of them. Introduction of Local Area Data Transports: Recently, electronic (carrier) serving areas have been replacing long local physical loops. These subscriber carrier systems provide such data capabilities as DOV and local area data transport (LADT). LADT offers a packet-switched data capability that may apply to both business and residential services; its low speed (up to 4.8K bits per second) and relatively low cost may make it universally attractive. LADTs may find widespread use for Videotex, meter reading, power load shedding, security reporting, and marketing transactions. Although LADT is restricted primarily to a local exchange area, the evolving USDN will provide transport between LADTs. LADT subscribers will access the USDN transport carrier through pooled local data concentrators. A typical LADT Data Subscriber Interface (DSI) unit will concentrate data from 124 subscribers to a 56K (or 9.6K) bit-per-second trunk to a packet network. A subscriber can thus dial up a DSI over a conventional voice loop and transport data through a modem (which may be a part of the terminal) by means of the switch. A direct access mode is also available with the subscriber loop terminating on the DSI, permitting independant simultaneous data and voice transmission. The X.25 link access protocol- balanced (LAPB) is used, but protocol conversion is restricted in many instances by federal rules. Although LADTs so not comply with any defined ISDN service, they are an integral requirement of the USDN. CSDC Technology: Circut-switched digital capability (CSDC) is another USDN service that has no ISDN counterpart. CSDC is an alternative voice- or data-switched circut with end-to-end 56K (or 64K) bit-per-second transparent connection ensured by dedicated trunk groups in each switching location. CSDC facilitates large, continuous, bulk data trans- fers, and its implementation requires added investment in each switch location as special terminal equipment. CSDC represents yet another expediant toward providing ISDN-like services while using existing investment. CSDC technology can also accomodate a full ISDN, if one ever evolves in the U.S. Digital Subscriber Loops: ISDN-compatible digital subscriber loops (two voice, plus one data channel at 144K bits per second) are recieving attention in the U.S., but the commercial implementation of this technology is not prograssing rapidly. Near-term subscriber loops will probably acquire data capability by data ober analog voice multiplexing. Although this step would not precluse the eventual inclusion of ISDN loops, it would tend to slow their introduction and widespread acceptance. Interfaces and Protocols: Although the ISDN revolves around the 30 channel PCM transmission format used in Europe, it does provide for the 24 channel- based systems used in the U.S. However, U.S. systems have many unique interface requirements. A new set of proposed services will require forwarding of the calling number for control or processing by either the terminating switching system or the called subcriber. Exchange of such information will likely be accomplished over a local area common channel signaling system or a fulll CCITT standard, signaling system #7 network. Direct data exchange between a network switching unit and a sata bank and/or processor facility will probably evolve from the current trunking scheme to a direct signaling carrier, perhaps CCITT standard #7 with OSI and/or SA protocols. Calls to cellular mobile roamers (i.e., mobile units that have traveled outside their base area) will probably be routed to a central data base for locating routes. A personal locator service for automatically routing calls to the unit's temporary location will require unique system interface and protocols. Privately owned transaction networks may provide this unique interface. While the objective is to eventually use CCITT standard #7 as a vehicle and X.25 as an interface protocol, the USDN must embrace a wide assortment of formats, protocols, and interfaces for the near future. TRANSMISSION TECHNOLOGY TRENDS Ultimately, subscriber loops in the U.S. will be digital, providing two 64K bit-per-second voice or data channels (i.e.,two B channels) and one 16K bit-per-second voice or data channels (i.e., two B channels) and one 16K bit-per-second data only (i.e., one D channel). The 16K bit-per-second channel will probably permit an 8K bit-per-second user data channel or submultiplexed channels of a lower bit rate. Full-duplex (i.e., four wire) operation will be provided by echo-canceling techniques over existing two-wire loops. An alternative approach of time-domain multiplexing may also be used, especially in the neat term. Local Loops: Although modems will not be required at either end of the local loop, network terminating equipment will be required to serve as the multiplexor and, perhaps, as voice CODECs. Users of such circuts can have full, simultaneous, reall-time voice and data channels, as well as seperate control, metering, and low-speed data transmission. With advanced switching centers, each circut can be routed and billed independently. Existing 56K bit-per-second channels on conventional 24-channel digital carrier systems will be replaced by or supplemented with 64K bit-per-second clear channels with extended framing. The local loop plant in many areas is already migrating toward carrier- serving areas, implemented by a subscriber carrier capable of digital transmission. Some local telephone companies are installing glass fiber in their local plants in preparation for the downward migration of direct digital transmission. However, until full, ISDN-type local loops are universally available, near-term adaptations will be offered to satisfy market needs and to prevent users from seeking other communications facilities. Wideband Circuts: Wideband circuits (i.e., multiples of 64K bits per second) over the public switched network may become feasible as newer switching elements are used. Although some transmission links may soon be able to combine DS-O channels for real-time wideband service, limitations in switching centers will restrict their general use. Seperate wideband switching modules, multiplexing on CATV, or extension of wideband LANs may ultimately appear if the need for wideband transmission remains strong. Packet Transmission: Packet transmission is an inherent element of the ISDN. However, the USDN must handle separate packet networks, separate facilities, separate routing, and even separate providers. It is unlikely that American packet networks in the U.S. will ever be combined into an integrated, single-network ISDN. Therefore, the USDN must accommodate such overlay networks and the associated problems of routing, protocol conver- sion, circuit maintenance, billing, and network management. American users will demand and recieve more options for data transport, data processing, and support services than any single network is likely to provide. SWITCHING TECHNOLOGY TRENDS Implementing ISDN standards on the switching systems already in place throughoout the U.S. is a formidable task. Most local end offices have been replaced by SPC analog switches within the past decade. More recently, SPC digital switches have been installed, and this trend will probably accelerate through the 1990s. However, these newer switches are third- generation design; that is, they are designed primarily to handle conventional voice circuit switched traffic within a hierarchical network. These switches are not optimized for data handling, multiple networks, or sophisticated user needs. Although hardware retrofits and software patches are being applied to accommodate LADT, CSDC, and digital loops, such solutions result in limited user services, higher costs, and more complex maintenance requirements. Fourth-Generation Switching: Fourth-generation switching systems, design- ed for USDN requirements, are beginning to appear. A fourth-generation switch is optimized for data; voice switching is simply a special case of data transmission at speeds of 64K bits per second, 32K bits per second, or any other evolutionary compressed voice bit rate. Fourth-generation switches do not have central processors. Each functional unit (e.g., lines or trunks) contains its own processing hardware and software to output packet-format messages (i.e., containing a header message and a data byte) in a uniform deferred format. (The data byte may be a digitized voice sample.) These packets then are routed through a central matrix, which also contains sufficient processing power to route packets to their interim or final destination with only the data contained in the header bits of the call itself. Services such as route translation, tone reception, billing recording, and termination functions are inserted into a call in progress by routing the call packets to specific functional modules on demand. The modules perform the required call functions and return the packetsto the matrix. (or interal network). When the required call-handling functions have been sequentially accomplished, the call is terminated to the desired port and a virtual circuit is established between the calller and called terminals. During the call, the packet header provides control and supervision and performs routine maintenance and alarms. A fourth-generation switch performs required functions-Centrex attendants, toll operators, common-channel signaling, or LAN termination-when the appropriate module is simply plugged in. These functions do not affect existing system service or capacity. Ideally suited to the USDN, the fully distributed control architecture of a true forth-generation switch could also meet the longer-range objectives of the ISDN. Because they do not require a large, costly central processor complex, fourth-generation switches can be economically applied as add-on units or adjuncts to existing SPC switches. Fourth-generation switches thus provide advanced capabilities without the necessity of replacing or retrofitting the existing switches. Some features that can be provided as adjuncts are described in the following paragraphs. Special Services: In the U.S., services that require more sophisticated transmission that provided by standard telephone lines are expanding rapidly. By the end of the 1980s, 50 percent of all lines may require some special treatment. A USDN switch, or a special service adjunct can provide univer- sal line circuits that can be remotely administered for transmission balance, type of transmission, routing, and signaling. The special service adjunct can provide various voice and data arrangements and automatic facilities testing as well as provide and maintain sophisticated data and voice services, often without changing the user's original telephone number. Business Services: Integrated toll, local, Centrex, PBX, and instrument/ terminal systems are not provided in the U.S. because of its regulatory climate. Regulations controlling enhanced services, authority to provide services, equal-access provisions, and franchising of special carriers and servers all affect the USDN but are constraints in the ISDN plans. Because of the uncertainty and ambiguity in such regulatory matters, the business services adjunct can be used with less economic risk than replacing or retrofitting existing switching systems would incur. The business services adjunct permits the existing local office to continue providing the local telephone service for which it was optimized. The business services adjunct can also economically provide such features as: * Citywide, Centrex-like service with universal numbering among user locations. * Centralized attendants and night service * Direct data lines at 64K bits per second * Rerouting of existing PBX trunks with improvement in features * Lan termination for PBX-to-Lan connections and LAN-to-LAN bridging. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=