ch06.htm

this describes managing multivendor networks

<P>So the promise of OSI is to bring forth standards that provide points of connectivity
among diverse systems. The OSI Reference Model should not, however, be thought of
as the ingredients for multivendor soup. Instead, it is the cookbook from which many
recipes can be selected, altered and tasted. Like most cookbooks, OSI represents
a blend of the old with the new. Many existing products and standards have been included
in the OSI architecture.</P>
<P>When the ISO began its work on the OSI model, it intended to define a layered
architecture to facilitate the development or definition of standards that relate
to the interfacing of open systems. An open system was defined as a system that elected
to participate in the standards. This work was carved into layers to first define
the working boundaries for the model and then enable separate small groups to work
on the issues specific to each layer.</P>
<P>Logistics notwithstanding, the layered approach of the OSI Reference Model also
relates to the historic view of data communications and networking and their relationships
to applications, terminals and users. For example, IBM's Systems Network Architecture
(SNA) is highly structured and layered in many aspects (consider the layered functions
of SNA's physical units, for example). Digital's network involves layering networking
services (DECnet) on top of transport services (Ethernet) to provide information
flow between applications and users. So in the most general of terms, the world of
networking lends itself to a layered dissection.</P>
<P>The OSI Reference Model is divided into seven layers. Each layer contains similar
functions and is as localized as possible. This localization enables layers to change
and evolve as new concepts and technology become available, without forcing changes
in its neighboring layers. In brief, these seven layers are (from bottom to top):

<UL>
	<LI><I>Physical.</I> The physical transmission media.<BR>
	<BR>
	
	<LI><I>Data Link.</I> Low level data packaging and transmission.<BR>
	<BR>
	
	<LI><I>Network.</I> Management of the routes available for data.<BR>
	<BR>
	
	<LI><I>Transport.</I> Delivery and delivery acknowledgment.<BR>
	<BR>
	
	<LI><I>Session.</I> Link management between applications.<BR>
	<BR>
	
	<LI><I>Presentation.</I> Data conversions and transformations.<BR>
	<BR>
	
	<LI><I>Application.</I> End-user and programming services.
</UL>

<P>Information flows from one system down the OSI layers, across the physical media,
and then back up the layers on the other system (see Figure 6.1). As it moves information
from one system to another, each OSI layer communicates with the corresponding OSI
layer on the other system. Please note, however, that although this peer-to-peer
communication between layers is often shown as direct links, the actual path for
the communications flows through the same layers. Each layer of the OSI model depends
on the lower layers to prepare or transport information from one open system to another
(see Figure 6.2).</P>
<P><A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/MuNet/ch06/06fig01.gif  \n\nThis file was not retrieved by Teleport Pro, because it was redirected to an invalid location.  You should report this problem to the site\'s webmaster.  \n\nDo you want to open it from the server?'))window.location='http://docs.rinet.ru:8080/MuNet/ch06/06fig01.gif'" tppabs="http://docs.rinet.ru:8080/MuNet/ch06/06fig01.gif"><B>FIG. 6.1</B></A> <I>Information Flow Through OSI Layers</I></P>
<P><A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/MuNet/ch06/06fig02.gif  \n\nThis file was not retrieved by Teleport Pro, because it was redirected to an invalid location.  You should report this problem to the site\'s webmaster.  \n\nDo you want to open it from the server?'))window.location='http://docs.rinet.ru:8080/MuNet/ch06/06fig02.gif'" tppabs="http://docs.rinet.ru:8080/MuNet/ch06/06fig02.gif"><B>FIG. 6.2</B></A> <I>Actual Communications Path</I></P>
<P>Another way to view the OSI Reference Model is to divide the seven layers into
the following three logical functions:

<UL>
	<LI><I>Physical network.</I> Both the Data Link Layer and the Physical Layer are
	concerned with the movement of data between two points within the known network.
	Data integrity at these layers ensures that no errors are introduced during the transmission
	process--the concern is data content, not context.<BR>
	<BR>
	
	<LI><I>System to system.</I> The Network, Transport, and Session Layers focus on
	moving information from one open system to another. These layers ensure that the
	correct data is delivered to the proper destination.<BR>
	<BR>
	
	<LI><I>End-user services.</I> Both the Presentation and Application Layers provide
	a range of services for the end user. These layers ensure that data is in the proper
	format for the context (for example, making sure that an application screen is correct
	for that user's actual terminal).
</UL>

<P><B>GOSIP</B>&#160;&#160;Not willing or able to be left out of the standards match,
the U.S. government has thrown several hats in the ring. From an organizational perspective,
the government has two significant agencies that define the standards used in networking
and data processing.</P>
<P>The Federal Telecommunications Standard Committee (F&#160;T&#160;SC) develops
or adopts standards for the government's telecommunications needs in advisement to
the National Communications System (NCS). In defining these federal standards, the
F&#160;T&#160;SC works with CCITT, ISO, ANSI and EIA. In some cases existing standards
from these standards organizations are implemented directly, while in other cases
the standards are altered to meet the specific needs of the government.</P>
<P>The U.S. Department of Commerce, in the form of the National Bureau of Standards
(NBS), was given legal responsibility for the development of Federal Information
Processing Standards (FIPS) relating to the government's data processing activities.
Whereas the scope of the FTSC is confined to telecommunications, the scope of FIPS
is much broader. As with the FTSC, the NBS tries to work with existing standards,
especially the federal standards published by the FTSC.</P>
<P>The federal standards put forth by the FTSC are not mandates for all government
agencies. Federal agencies must conform to FIPS by virtue of the same law that brought
FIPS into creation. However, compliance need not be instantaneous; governmental entities
have time to plan and adapt.</P>
<P>Another major offspring from the U.S. Department of Commerce is the Government
Open Systems Interconnection Profile (GOSIP). In 1988, the National Institute of
Science and Technology (NIST) developed GOSIP as a subset of the OSI Reference Model
for the government. Once published, GOSIP then became a FIPS, thereby representing
a major commitment from the U.S. government to embrace parts of the OSI Reference
Model.</P>
<P>GOSIP was planned for implementation in two phases, the first phase taking effect
in 1990, the second scheduled for 1992 (see Table 6.1).
<H4><FONT COLOR="#000077">Table 6.1&#160;&#160;GOSIP Phases</FONT></H4>
<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT"><B>Layer</B></TD>
		<TD ALIGN="LEFT"><B>Name</B></TD>
		<TD ALIGN="LEFT"><B>GOSIP Phase</B></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">1</TD>
		<TD ALIGN="LEFT">Physical</TD>
		<TD ALIGN="LEFT">Phase I: 802.3, 802.4, 802.5</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">2</TD>
		<TD ALIGN="LEFT">Data Link</TD>
		<TD ALIGN="LEFT">Phase I: CSMA/CD, Token Ring, Token Bus</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">3</TD>
		<TD ALIGN="LEFT">Network</TD>
		<TD ALIGN="LEFT">Phase I: X.25, ISO 8473 (connectionless) Phase II: ISDN</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">4</TD>
		<TD ALIGN="LEFT">Transport</TD>
		<TD ALIGN="LEFT">Phase I: ISO 8073 (connection oriented)</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">5</TD>
		<TD ALIGN="LEFT">Session</TD>
		<TD ALIGN="LEFT">Phase I: ISO session (8327 and 9548)</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">6</TD>
		<TD ALIGN="LEFT">Presentation</TD>
		<TD ALIGN="LEFT">Phase I: ISO presentation (8823 and 9576)</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">7</TD>
		<TD ALIGN="LEFT">Application</TD>
		<TD ALIGN="LEFT">Phase I: X.400, FTAM Phase II: X.500, ODA, CMIP/CMIS, VT, RDB Access</TD>
	</TR>
</TABLE>
</P>
<P>Provisions within GOSIP enable the government to continue to use and enhance TCP/IP-based
products that might not be OSI compliant in the strictest sense. Furthermore, exceptions
are granted to small expansions of existing networks as well as to situations in
which total compliance represents an economic hardship (or impossibility).</P>
<P>The U.S. Government is not alone in its GOSIP approach. The United Kingdom has
implemented a similar program and other governments are considering following suit.
These tactics, coupled with the growing maturity of the OSI Reference Model, are
laying the groundwork for practical and functional international OSI networks.</P>
<P>Since the implementation of GOSIP, however, U.S. government agencies have continued
to use other networking protocols besides OSI, most notably TCP/IP. The government
incorrectly expected GOSIP standards to displace proprietary protocols because of
OSI's status as an international standard. Although OSI was expected to be universally
implemented, GOSIP products have been slow to come to market, and have not been widely
accepted or deployed.</P>
<P>The Internet Protocol (IP) suite, on the other hand, has been widely accepted,
and products built on this standard have become widely used commodity products. In
addition, the Internet, which supports IP, has developed substantially since the
imposition of GOSIP; while there has been no such infrastructure developed for GOSIP
itself. Those OSI products that have become available are more expensive, and less
well integrated than comparable IP-based products.</P>
<P>In 1994, the Federal Internetworking Requirements Panel (FIRP) was established
by NIST to reassess the government's requirement for open systems networks and compliance
with the GOSIP standard. At that time, FIRP decided that no one protocol suite should
be imposed to meet all government requirements for internetworking, and that it would
be better to adopt the most effective solution in each of the different areas of
information technology; rather than requiring absolute compliance to one set of standards.
Although the reevaluation applies only to the federal government, it will ultimately
have an impact on American industry as well.


<BLOCKQUOTE>
	<P>
<HR>
<B><font color=#000077>NOTE:</font> </B>The reevaluation of open systems networks applies only to
	the United States; other countries' OSI requirements still stand at this time. n
	
<HR>


</BLOCKQUOTE>

<H3><A NAME="Heading9"></A><FONT COLOR="#000077">Exchange of Standards</FONT></H3>
<P>As you can see, organizations often adopt each other's standards. For example,
the EIA RS-232C standard was adopted by the CCITT as V.24 and V.28, and by ISO as
2110, and the IEEE 802.3 standard was adopted by ECMA as ECMA-80, 81 and 82, and
by ISO as 8802/3.</P>
<P>Good news and bad news intertwine in this exchange of standards. The good news
is that many different organizations with different origins and orientations can
communicate with one another and share information in an open, honest format. The
bad news is that, in many cases, a standard gets subtle changes by each organization
that adopts it; or one organization might subsequently update an adopted standard
while another organization might not. Thus, for example, an EIA standard might not
be 100 percent compatible with what appears to be the equivalent CCITT standard.</P>
<P>Table 6.2 shows which standards are being exchanged among organizations; Table
6.3 lists the names of these and other popular standards.
<H4><FONT COLOR="#000077">Table 6.2&#160;&#160;Examples of Standards Exchange</FONT></H4>
<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT"><B>ISO</B></TD>
		<TD ALIGN="LEFT"><B>CCITT</B></TD>
		<TD ALIGN="LEFT"><B>ECMA</B></TD>
		<TD ALIGN="LEFT"><B>ANSI</B></TD>
		<TD ALIGN="LEFT"><B>EIA</B></TD>
		<TD ALIGN="LEFT"><B>FTSC</B></TD>
		<TD ALIGN="LEFT"><B>FIPS</B></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">646</TD>
		<TD ALIGN="LEFT">V.3</TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT">X3.4</TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT">1-1, 7, 15</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">1155</TD>
		<TD ALIGN="LEFT">V.4, X.4</TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT">X3.15, X3.16</TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT">1010, 1011</TD>
		<TD ALIGN="LEFT">16-1, 17-1</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">1177</TD>
		<TD ALIGN="LEFT">V.4, X.4</TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT">X3.15, X3.16</TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT">1010, 1011</TD>
		<TD ALIGN="LEFT">16-1, 17-1</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">1745</TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT">X3.28</TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT"></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">2022</TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT">X3.41</TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT">35</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">2110</TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT">RS-232C</TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT"></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">2111</TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT">X3.28</TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT"></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">2628</TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT"></TD>
		<TD ALIGN="LEFT">X3.28</TD>