$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
$ P/HUN Issue #4 $
$ Volume 2: Phile 9 of 11 $
$ $
$ $
$ USDN VERSUS ISDN $
$ ---------------- $
$ $
$ by $
$ $
$ $
$ LORD MICRO $
$ ********** $
$ $
$ TOLL CENTER BBS - (718)-358-9209 $
$ A 2600 MAGAZINE BBS $
$ $
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
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.
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=