ch07.htm

this describes managing multivendor networks

from region to region.
<H3><A NAME="Heading15"></A><FONT COLOR="#000077">Asymmetric Digital Subscriber Line
(ADSL)</FONT></H3>
<P>Although ISDN has been getting most of the press, a similar technology called
<I>asymmetric digital subscriber line (ADSL)</I> promises even more throughput over
an ordinary, narrow copper telephone line. ADSL accomplishes this remarkable feat
through a series of complex compression and digital signal processing algorithms,
and dynamic switching techniques. The ADSL transport technology boosts the capacity
of the existing phone line significantly more than ISDN. Duplex ADSL offers a downstream
data rate of up to 6 Mbps and an upstream channel running at 640 Kbps. ISDN, on the
other hand, ranges from 64 Kbps to 128 Kbps--faster than a standard modem, but still
too slow to handle that interactive TV and other services they keep telling us we'll
all have one of these days.</P>
<P>Most of the regional Bell operating companies are testing ADSL and making plans
to offer it to customers seeking high-bandwidth Internet access. Also, because it
offers two-way communications, some entertainment companies are considering it a
realistic possibility for interactive cable television.</P>
<P>ADSL gives the RBOCs an alternative to costly optical cables because it can transform
their existing copper-wire network into a high-performance system. Here's how it
works: The regular phone wire is configured for ADSL, and then connects to an ADSL
modem on one end and an ADSL circuit switch on the other. The connection then creates:

<UL>
	<LI>A high-speed, unidirectional data channel capable of running between 1.5 Mbps
	to 6.1 Mbps.<BR>
	<BR>
	
	<LI>A medium-speed duplex channel running between 16 Kbps and 640 Kbps.<BR>
	<BR>
	
	<LI>A standard analog connection.
</UL>

<P>Cable companies, set-top box makers, and TV couch potatoes drool over the possibilities.
Look for this technology to be making big news in the near future.
<H3><A NAME="Heading16"></A><FONT COLOR="#000077">X.25</FONT></H3>
<P>The CCITT developed the X.25 standard to define a reliable, relatively low cost
means of routing data through a shared network. An extremely important aspect of
X.25 is that the information being transmitted has been converted into packets.</P>
<P>Packets can be thought of as small fragments of information. Specifically, a block
of information is broken into smaller parts (&#160;packets) before being transmitted
on the physical network. The packet methodology provides faster and more reliable
error detection and correction; it also prevents a system with a huge volume of information
to ship from tying up the network.</P>
<P>In addition to the raw information, each packet also contains information specifying
its origin, its destination, and a number indicating the &quot;piece&quot; of the
information to which it corresponds. This enables each packet to be treated as an
independent entity, so that packets from many different systems can be intermixed
on the network without concern about the order in which they are transmitted or even
the order in which they arrive. Each packet might take the best possible route available
at the time it is ready for transport.</P>
<P>The application end points of the information (that is, the terminal user and
the application program) rarely see the information in its packetized form. As part
of its interface with the network, the computer system converts the information into
packets, and then subsequently reassembles the packets into the original information
(see Figure 7.10).</P>
<P><A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/MuNet/ch07/07fig10.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/ch07/07fig10.gif'" tppabs="http://docs.rinet.ru:8080/MuNet/ch07/07fig10.gif"><B>FIG. 7.10</B></A><B> </B><I>Conceptual Packetizing</I></P>
<P>This packet approach to transmitting data is extremely pervasive in the networking
world. In addition to being used by X.25, this approach is also used by most LANs
and many other data communications protocols (although they are usually referred
to as frames, as discussed in the LAN section of this chapter). Specific to X.25
networks, however, is the concept of a <I>packet switching network (PSN).</I></P>
<P>A PSN is a WAN through which packets are sent. The precise route that packets
take from point A to point B is not fixed and is immaterial to the equipment at point
A or point B, which checks only to see whether the packets arrive intact (again,
order is not a major concern).</P>
<P>Because they don't have prescribed data routes, PSNs are often shown as clouds
in many networking diagrams (see Figure 7.11). When depicted in this manner, information
goes into the cloud at some point and comes out at another. What goes on within the
cloud is not the concern of mere humans.</P>
<P><A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/MuNet/ch07/07fig11.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/ch07/07fig11.gif'" tppabs="http://docs.rinet.ru:8080/MuNet/ch07/07fig11.gif"><B>FIG. 7.11</B></A><B> </B><I>Typical X.25 Representation</I></P>
<P>The inside of the cloud, however, is composed of <I>packet-switching nodes</I>
(also called PSNs, just to make life confusing). The switching nodes can take routes
to other switching nodes, and thus can route or reroute data as necessary. For example,
if a switching node has a packet to forward and the best possible switching node
to receive it is busy, the node holding the packet will reroute it to another node
for subsequent rerouting (see Figure 7.12).</P>
<P><A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/MuNet/ch07/07fig12.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/ch07/07fig12.gif'" tppabs="http://docs.rinet.ru:8080/MuNet/ch07/07fig12.gif"><B>FIG. 7.12</B></A><B> </B><I>Inside of the X.25 &quot;Cloud&quot;</I></P>
<P>Packet-switching networks often are associated with public data networks (PDNs),
but this relationship is certainly casual. A PDN is normally a telephone system (or
telephone company in the U.S.) that offers data services to the public. It does not
have to use packet-switching to move information from point to point. If a PDN does
offer the services of a packet-switching network, it might be referred to as a packet-switching
data network (PSDN) or even a packet-switching public data network (PSPDN). Clearly,
the abbreviations are almost endless.</P>
<P>Furthermore, implementation of packet-switching networks is not limited to telephone
companies. In fact, PSNs can be constructed of telephone links, fiber optic links,
microwave links, satellite links, and other forms of communications. Many large corporations
have used these diverse communication techniques to construct their own private PSN.
Because, in the final analysis, a packet switching network is a cost-effective WAN,
organizations with widely dispersed equipment find this approach most effective in
terms of both cost and function.</P>
<P>The traditional packet-switching cloud is shown in Figure 7.13.</P>
<P><A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/MuNet/ch07/07fig13.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/ch07/07fig13.gif'" tppabs="http://docs.rinet.ru:8080/MuNet/ch07/07fig13.gif"><B>FIG. 7.13</B></A><B> </B><I>X.25 Interfaces</I></P>
<P>Moving outside of this cloud, the interfaces between the computer equipment and
the cloud generally fall into one of two types of devices:

<UL>
	<LI><I>PAD. </I>The packet assembly/disassembly device is a piece of hardware that
	interfaces between the network and computer equipment incapable of sending or receiving
	packets. This function is defined in CCITT standard X.3. The purpose of the PAD,
	then, is to handle the conversion of the raw data into packets for transmission into
	the packet-switching cloud and, conversely, handle the reassembly of information
	from packets received from the cloud. PADs most often are used to interface terminals
	into the packet- switching network, but they are also used to interface computer
	systems that cannot handle packet transformations on their own.<BR>
	<BR>
	
	<LI><I>A communications controller running (normally) the LAP-B protocol.</I> Rather
	than use an external device, such as a PAD, most computers use an internal interface
	to directly connect to the packet-switching network. These interfaces and their corresponding
	software drivers provide much of the same function provided by a PAD. The advantage
	to putting these interfaces into a computer is that computer software can directly
	access the link (whereas in the PAD the link was external and, for the most part,
	invisible to the software). For example, an office automation package can communicate
	with a counterpart package operating on the other &quot;side&quot; of the cloud.
</UL>

<P>For terminal traffic over packet-switching networks, two additional standards
come into play. First, the <I>CCITT X.28</I> standard defines the interface between
an asynchronous terminal and a PAD. Second, the <I>CCITT X.29 </I>standard defines
the control procedures for information exchanges between a PAD and another PAD (or
an integrated controller). Just as X.25 has become synonymous with packet-switching
networks, X.29 has become synonymous with interfacing terminals over packet-switching
networks.
<H3><A NAME="Heading17"></A><FONT COLOR="#000077">LAN Switches</FONT></H3>
<P>Switches are used to extend overcrowded networks by providing each end user with
his own piece of 4 Mbps or 16 Mbps bandwidth. In many cases, this might be more than
each end user needs. In this event, the token ring switch can be used to break one
big token ring into multiple, smaller rings. This approach will also significantly
increase performance.


<BLOCKQUOTE>
	<P>
<HR>
<FONT COLOR="#000077"><B>Switch Technology--Token Ring and FDDI</B></FONT><BR>
	Many vendors are bringing token ring LAN switches to the market. A number of alliances
	illustrate the strength of this market, such as a recent noteworthy alliance between
	Bay Networks (Santa Clara, California) and IBM. Other network vendors, such as Cisco
	Systems and Cabletron Systems, have made similar deals with third parties. As more
	vendors go into this market and volume increases, token ring switch products are
	expected to come down in price and enjoy higher demand.</P>
	<P>FDDI switching is another promising technology for extending network life and
	bandwidth. Digital's GigaSwitch is the leading FDDI switching product, although several
	other vendors are preparing to release FDDI switches as well. 
<HR>


</BLOCKQUOTE>

<P>Token ring networks, like Ethernet networks, have bandwidth limitations, and many
are starting to reach those limitations because of the bigger applications and greater
demands for data that companies are experiencing. The lower pricing structures of
Ethernet and token ring LAN switching devices might encourage individual business
units to make their own purchases. In terms of the overall enterprise, however, this
can be disastrous. It is essential for individual departments to consider the overall
corporate direction when making such purchases, and to make sure that the technology
they are purchasing is compatible with the existing infrastructure and corporate
data needs assessments. If not, they might wind up spending much more money because
they now have to buy additional equipment to connect with the corporate switches
and to address data type and volume transmission requirements decided on by corporate
information communication needs.
<H3><A NAME="Heading18"></A><FONT COLOR="#000077">Tools of the Trade</FONT></H3>
<P>Needless to say, computers and networks do not connect to each other as easy as
phones plug into wall jacks. In networks, the tools of connectivity handle conversion
between analog and digital formats, between one type of physical interface and another,
or between one transmission media and another. In short, these tools are the nuts
and bolts of the erector set called networking.</P>
<P>For LANs, one set of tools is required to make the physical attachment between
the interface in the computer (for example, an Ethernet adapter in a VAX or a token
ring adapter in an AS/400) and the physical network. The tools include:

<UL>
	<LI><I>The Attachment Unit Interface (AUI).</I> This is the cable that attaches the
	interface in the computer to the MAU described below.<BR>
	<BR>
	
	<LI><I>The Medium Attachment Unit (MAU).</I> Also known as a Multistation Access
	Unit when used with a token ring network. This device attaches one or more AUIs to
	the physical LAN. A MAU can provide one-to-one connection with a computer or it can
	be a hub to several systems.
</UL>

<P>When two LANs are joined together, a bridge or router is normally used. When a
bridge links two or more LANs, those LANs form a single, logical LAN. In this case,
all information routed through one LAN goes over the bridge and through the attached
network. Because of this traffic, high speed links are normally required to keep
the bridge from slowing the performance of the network. And finally, because bridges
are implemented at such a low level, all protocols can operate over a bridge.</P>
<P>A router also connects two or more LANs, but routers are much more selective about
the information that they allow to cross over. Specifically, routers are aware (through
self-learning or manual configuration) of which computer addresses apply to which
LANs. Therefore, rather than pass all information across, routers transmit only information
pertinent to the other LAN. Because only selected traffic travels across the link,
lower speed links can be used without affecting overall LAN performance. The router
can also act as a <I>firewall</I> to prevent unwanted access to the network from
outside.</P>
<P>Routers cannot be used in all types of netwo