ch07.htm

this describes managing multivendor networks

<P>Unlike LANs, which all accomplish the same purpose, WANs offer a unique variety
of technology and approaches. This discussion will focus on some of the better known
approaches--standard phone links, ISDN networking, and X.25 packet-switching--as
well as emerging methods such as ATM, Frame Relay, and SMDS.</P>
<P>Like LANs, WAN links are simply a way of transferring information from point A
to point B. Running on top of both types of links are networking protocols and services
that bring additional functions to the network. For example, IBM's SNA, Digital's
DECnet, TCP/IP, and many other networking protocols all include services that operate
over the physical links. Some protocols are specific to the LAN environment (such
as Digital's LAT or Novell's IPX), while other protocols are better suited for wide
area links (like IBM's SDLC or HP's implementation of HDLC).</P>
<P>The point is, in all cases, no network (wide or local) provides any value without
upper layers of protocols, services and applications.
<H3><A NAME="Heading13"></A><FONT COLOR="#000077">Point-to-Point Links</FONT></H3>
<P>In most cases, long-distance point-to-point links are routed through a telephone
carrier. From a practical point of view, the long-distance telephone carriers have
already done the work of establishing a wide area of physical links, so it makes
sense in some circumstances to use these existing connections.</P>
<P>Before the advent of high-speed digital lines, this world of long distance teleprocessing
was composed of dial lines and leased lines. Dial-up <I>POTS (Plain Old Telephone
Service)</I> lines are one of the few aspects of data communications that has not
changed much over the years, although higher-speed modems have enabled data to be
sent over them much faster. As the name implies, a dial line uses standard voice-grade
lines to create a temporary connection between two computing devices. POTS lines
can operate at speeds of up to 36,600 bps if a noise-free connection can be made.</P>
<P>A leased line is a permanent circuit installed between point A and point B. Because
they are permanent, leased lines can be conditioned to provide less noise and therefore
support high-speed operation (such as 28,800 bps) on a more reliable basis. Leased
lines have been greatly affected by the advent of digital phone circuits.</P>
<P>Before the advent of digital lines, point-to-point links used the same basic approach
to carry data as they did to carry voice. Although, as noted, leased lines could
be purchased with various levels of conditioning, they still used the same analog
approach for transmitting. Modems were developed to bridge the difference between
the analog nature of the phone system and the digital nature of computers. Modems
that translate between the digital and analog formats are described in greater detail
at the end of this chapter.


<BLOCKQUOTE>
	<P>
<HR>
<B><font color=#000077>NOTE:</font> </B>The analog&#160;/digital translation process, MOdulating and
	DEModulating, forms the etymology of the word &quot;modem.&quot; 
<HR>


</BLOCKQUOTE>

<P>As technology and phone systems matured, the nature of the phone network became
much more sophisticated. Satellites were deployed to provide greater coverage without
costly physical connections, and digital circuits were added into many phone systems
to offer high transmission speeds with lower noise (and therefore fewer errors).</P>
<P>In particular, <I>Digital Data Service (DDS)</I> brought increased performance
to leased lines. When compared to digital networking, the analog phone system is
slow and error-prone. Furthermore, because the existing phone system was developed
to address voice transmissions, the way it handles data communications is less than
ideal. Digital service brought increased reliability and performance to leased line
networks.</P>
<P>Higher rates are provided through the use of <I>T1 links. </I>T1 links are multiple,
high-speed links packaged into a single unit. Specifically, a T1 line has an aggregate
throughput of 1.544 Mbps but is, in reality, composed of 24 64-Kbps digital lines.</P>
<P>A T1 user can dedicate these 24 lines to separate functions--for example, some
might carry voice, some video and some data. Or a T1 user can use multiplexing equipment
to run data across all (or a subset) of the separate lines concurrently, to effectively
achieve the full throughput. If a company does not need the full T1 bandwidth, it
might also choose fractional T1 service. In this case, only some of the T1 lines
are connected to the customer's premises. The availability of fractional T1 lines
is dependent on the local phone company's ability to find enough fractional users
to use up an entire T1 link.</P>
<P>Through bridges and routers, geographically distant LANs can be interconnected
over a T1 link. However, since T1 is strictly a North American standard, it cannot
be used to establish an intercontinental WAN.


<BLOCKQUOTE>
	<P>
<HR>
<B><font color=#000077>NOTE:</font> </B>Because of the wide and diverse geography of the U.S., digital
	and T1 services are not available in all areas of the country.&#160; 
<HR>


</BLOCKQUOTE>

<H3><A NAME="Heading14"></A><FONT COLOR="#000077">ISDN</FONT></H3>
<P>ISDN is the planned replacement for the analog circuits used to provide voice
and data communications services worldwide. Development on ISDN was begun in the
1970s by AT&amp;T and formalized in the early 1980s under the mantle of the <I>Consultative
Committee for International Telegraph and Telephony (CCITT).</I> Under the direction
of CCITT, ISDN became worldwide in scope, offering for the first time a fixed set
of interfaces and interface devices that were applicable globally. Thus, the computer
interface used in Germany for ISDN attachment would be the same interface used in
the U.S.</P>
<P>The customer's interface to ISDN is through a <I>service node</I> (see Figure
7.8). The purpose of the service node is to provide an interface from ISDN to the
customer phone system or PBX, a data communications device that interfaces to the
local computer equipment (analogous to a modem or CSU/DSU), or a hybrid device that
performs both functions. This service node interface enables the customer to access
(&#160;given proper security and compatible equipment) any other system also connected
to ISDN. <B></B></P>

<P><A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/MuNet/ch07/07fig08.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/ch07/07fig08.gif'" tppabs="http://docs.rinet.ru:8080/MuNet/ch07/07fig08.gif"><B>FIG. 7.8</B></A><B> </B><I>ISDN Service Node Concept
Interface</I> </P>

<P>For data communications, this approach is more flexible than the traditional point-to-point
leased line or dial-up connections previously discussed. Voice service, on the other
hand, will not be dramatically improved because it also operates at the equivalent
of 64 Kbps over the analog system. The digital nature of the network should, however,
remove some of the static often heard over phones. (Also remember that while static
is annoying to humans, it is devastating to data, and that's why data is not transmitted
across analog lines at these extremely fast speeds.)</P>
<P>From a point of entry perspective, ISDN offers two types of user interfaces:

<UL>
	<LI><I>The Basic Rate Interface (BRI). </I>This interface offers two 64-Kbps data
	and/or voice circuits, known as the B channels, combined with a 16-Kbps management
	and service circuit, or D channel.<BR>
	<BR>
	
	<LI><I>The Primary Rate Interface (PRI).</I> This interface features 23 64-Kbps data
	and/or voice circuits with an additional 64-Kbps circuit for management and ancillary
	services.
</UL>

<P>Because the costs for a PRI far exceed the cost for a BRI, most business needs
are addressed by one or more BRIs. Still, given the higher speeds offered by the
PRI (a total of 1.544 Mbps for the PRI versus a total of 144 Kbps for the BRI), the
PRI is a viable contender to extending LANs using bridges and routers.</P>
<P>Also note that the PRI closely resembles a T1 link. The primary difference lies
in their use and network architecture. T1 is most often used to facilitate high-speed
point-to-point links, whereas ISDN is intended to interface a large number of systems
on a global basis. ISDN also differs from the T1-style link in that it features a
management circuit separate from the data circuits. This additional circuit is present
in both the Basic Rate and Primary Rate Interfaces and delivers some benefits that
are important to ISDN and its marketability. This circuit is separate from the data/voice
channels (see Figure 7.9). In fact, it is termed a <I>D</I> <I>Channel,</I> as opposed
to the <I>B channels</I> that carry the data and voice traffic.</P>
<P><A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/MuNet/ch07/07fig09.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/ch07/07fig09.gif'" tppabs="http://docs.rinet.ru:8080/MuNet/ch07/07fig09.gif"><B>FIG. 7.9</B></A><B> </B><I>ISDN B and D Channels</I></P>
<P>This type D circuit can be used for a number of functions:

<UL>
	<LI><I>Network management.</I> If network monitoring and management functions are
	separated from the network itself (which would be running over the D channels), then
	two benefits accrue. First, the monitoring and management function does not adversely
	affect network performance because it does not occur from within the network. Second,
	if a failure occurs within the network, the monitoring and management structure is
	still available to signal and alert operators to take corrective action.<BR>
	<BR>
	
	<LI><I>Faster call servicing.</I> By using the D Channel to perform call set-up operations,
	ISDN dramatically reduces the amount of time necessary to initially establish a call.
	After the set-up occurs via the D Channel, the B Channel is instantly available for
	the actual voice transmission.<BR>
	<BR>
	
	<LI><I>Automatic Number Identification (ANI).</I> Although the introduction of this
	feature has provoked political controversy in many states, ANI technology has many
	reasonable applications, especially in the customer service area. Specifically, it
	enables a phone call recipient to view (on a special display device) the phone number
	of the person who is calling. In addition, call center applications are available
	that will produce the caller's database record, order information, credit limits
	or other pertinent data instantly on a computer screen, before the call is even answered.<BR>
	<BR>
	
	<LI><I>Advanced information forwarding.</I> As customers learn to use ISDN, they
	will invariably use the D Channel to send advanced information that relates to the
	call going over the B Channel. For example, customers might direct their PBX to send
	account information on the D Channel when they are making a voice call to a vendor.
	Because the account information arrives before the voice call, it can often be processed
	before the two parties converse. Thus, the vendor's representative might have the
	customer's file on his or her workstation when the phone rings.
</UL>

<P>Beyond establishing a digital international data network, ISDN is a key piece
of the standards pie for several reasons:

<UL>
	<LI>The U.S. government has developed federal standards based on ISDN and will soon
	require federal agencies to comply with these standards.<BR>
	<BR>
	
	<LI>The phone companies are committed to ISDN. Like it or not, you will be using
	ISDN at your home and office.<BR>
	<BR>
	
	<LI>ISDN has not overlooked fiber optic technology. Broadband ISDN (BISDN), which
	uses higher speed and more reliable fiber optic communications, is being analyzed
	as a transport within ISDN as well as a service in itself.
</UL>

<P>Laptop PC users now also have the ability to access a network with ISDN services
through new ISDN Basic Rate Interface PCMCIA cards.</P>
<P>The various Regional Bell Operating Companies (RBOCs) are now providing ISDN service
to most major cities, and the number of ISDN lines has increased dramatically over
the past few years. Additionally, all of the major commercial on-line services offer
ISDN access, and many Internet service providers are also offering ISDN Internet
access. The advantages are obvious; file downloads are lightning fast, and there
is less possibility of interruption due to line noise. PC vendors are accommodating
the increasing demand for ISDN by releasing ISDN modems, also known as ISDN terminal
adapters.</P>
<P>Although the cost of an ISDN connection is bound to decrease, costs vary tremendously