ch07.htm

this describes managing multivendor networks

	network devices and high-speed hardware. This ANSI standard supports speeds of up
	to 1.06 Gbps.<BR>
	<BR>
	
	<LI><I>Tools of the trade.</I> Implementing wide-area solutions requires some special-purpose
	devices or software that smooth out the differences between the local and wide-area
	connections. These tools include bridges, routers, and gateways. They enable the
	different LAN and WAN strategies to be mixed and matched in a single, unified network.
</UL>

<H2><A NAME="Heading2"></A><FONT COLOR="#000077">Local Area Networks</FONT></H2>
<P>LANs became significant in the world of networking in the late 1980s, following
on the heels of the PC to become the preferred method for connecting multiple PCs
in a self-contained area.</P>
<P>Unfortunately, the networking software and operating systems used with the PC
LANs were quite different from the networking software used on midrange and mainframe
computers and office automation equipment (for example, dedicated word processing
machines and intelligent copiers). This, of course, set up the inevitable conflict
between PCs, office automation equipment, and the larger midrange and mainframe computers.
Although many computer-savvy corporations saw the conflict coming and took steps
to address it head on, other companies first became aware of the conflict when the
requests to run cable hit the maintenance department.</P>
<P>After all, from a simple and fundamental perspective, the laying of the cable
represents a major commitment. Installing the cable requires the unpleasant work
of snaking cable through ceilings and down walls. It requires that the cable be arranged
in such a way that it is manageable and easy to expand (from a networking perspective).
And even worse, the placing of cable is often regulated by local ordinances that
require special casings or materials (Teflon enclosures, for example) if the cable
runs near pipes, electrical work or people. In short, putting the cable in is almost
as much fun as simultaneously remodeling the kitchen and bathroom of your house.
<H3><A NAME="Heading3"></A><FONT COLOR="#000077">LAN Topologies</FONT></H3>
<P>Whether the purpose of the LAN is to interconnect PCs, minicomputers, or both
is almost irrelevant--the first issue is often choosing the topology of the LAN.
This choice dictates the cable, cabling methodology and the networking software that
can operate on the LAN. The three basic topologies are the ring, star, and bus (see
Figure 7.1).</P>
<P><A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/MuNet/ch07/07fig01.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/ch07/07fig01.gif'" tppabs="http://docs.rinet.ru:8080/MuNet/ch07/07fig01.gif"><B>FIG. 7.1</B></A> <I>LAN Topologies</I>

<UL>
	<LI><I>Ring. </I>As its name suggests, a ring LAN joins a set of attachment units
	together via a series of point-to-point connections between each unit. Each attachment
	unit, in turn, interfaces to one or more computers or computing devices. Information
	flows from attachment unit to attachment unit in a single direction, thus forming
	a ring network. Because each PC in a ring network acts as a repeater, performance
	degrades with each additional PC. Consequently, this is typically appropriate only
	in small networks.<BR>
	<BR>
	
	<LI><I>Star. </I>In a star LAN, each computer or computer-related device is connected
	on a point-to-point link to a central device called a hub. The hub acts as the LAN
	traffic manager, setting up communication paths between two devices seeking to exchange
	information. This configuration makes it very easy to isolate problem nodes, and
	is one of the most common LAN models.<BR>
	<BR>
	
	<LI><I>Bus. </I>The simplest form of bus LAN is a set of computers or devices connected
	to a common, linear connection. Under the bus topology, information is transmitted
	over the distance of the network, so each computer can pick up its intended information.
	Links from the main bus line might break off into additional linear links with multiple
	attachments; this type of bus structure is also referred to as a tree because multiple
	branches reach out from the main trunk. This model is used in high-speed PBXs.
</UL>

<P>Star and ring network topologies are sometimes combined into one network to provide
a higher degree of fault tolerance. Because a star network is susceptible to a failure
in the hub, and a ring network is sensitive to a break in the ring, combining both
forms into one offers an alternate route in case one topology fails.
<H3><A NAME="Heading4"></A><FONT COLOR="#000077">LAN Protocols</FONT></H3>
<P>While the LAN topology defines the cabling methodology and the way that information
flows through the network, the LAN discipline determines how the computers interact
with each other on the LAN. The two most used protocols are <I>token passing</I>
and <I>collision sensing.</I></P>
<P>In a token passing network, a special token is passed from computer to computer.
Possession of the token enables a computer to transmit on the network. When the original
transmission returns to the computer that sent it, that transmission is regarded
as complete (whether or not it was actually received) and a new token is generated
to flow to the next station (based on the LAN topology). When a message is successfully
received, the receiving station confirms receipt by changing a flag in the original
transmission. Therefore, by examining the original message when it returns, the sending
station can determine what happened at another end of the ring. Token passing dates
back to 1969 and is one of the earliest multiple-unit, peer-to-peer control procedures.
Token ring networks, although not as widely used as a CSMA/CD network, offer robust
performance because they furnish only a single channel; thereby avoiding any possibility
of collision.</P>
<P>The proper name for collision sensing is <I>Carrier Sense Multiple Access with
Collision Detection (CSMA/CD).</I> With this discipline, each computer listens to
the LAN to sense if another computer is transmitting. If someone else is active on
the LAN, the computer wishing to transmit waits for a preset amount of time before
trying again. When the computer perceives that the LAN is inactive, it transmits.
In the event that two computers transmit at the same time (and their data collides
and is hopelessly corrupted), both sides wait for different lengths of time before
attempting to retransmit. CSMA/CD dates back to the mid-1970s (when Ethernet was
in its infancy) and has grown to be the most common discipline for PC LANs.</P>
<P>The biggest difference between the two disciplines is that token passing is termed
a deterministic discipline while collision sensing is not. A token passing network
is deterministic because each computer is given the opportunity to transmit, but
only at preset time intervals, and only if it is in possession of the token. On a
collision sensing network, however, each computer must, in effect, compete for the
opportunity to transmit.</P>
<P>A third type of discipline, <I>time division,</I> is sometimes used in laboratory
environments for specialized controllers, technical equipment or wireless communications
networks. With this discipline, each unit is given specific amounts of time at specific
intervals to exchange data. Using time division in a conventional data processing
LAN, however, is extremely unusual, not to mention impractical.
<H3><A NAME="Heading5"></A><FONT COLOR="#000077">LAN Implementations</FONT></H3>
<P>Both the discipline and topology define a LAN implementation. Thus, a LAN might
be a token passing ring, a token passing bus, or a collision sensing bus.</P>
<P>The standards for LAN implementations can be roughly broken into two groups: those
that pre-date the work performed by the IEEE in this area, and those that were developed
by the IEEE. Of the LAN implementations that pre-date IEEE's involvement, Ethernet
and token ring implementations have stood the test of time and remain popular.</P>
<P>Ethernet was originally developed by Xerox Corporation in the 1970s as a 3 Mbps
bus LAN using the CSMA/CD discipline. Following the initial release of Ethernet,
both Digital Equipment and Intel joined the development effort and the three companies
released the specification for Ethernet version 1.0 in 1980. The most notable improvement
in Version 1.0 was the increase in the LAN speed from 3 Mbps to 10 Mbps. The Ethernet
specification was then revised again several years later as Ethernet II to provide
a higher degree of compatibility with the IEEE 802.3 standard. The 802.3 standard
has since grown to include a newer specification, known as Fast Ethernet, which boosts
the speed tenfold to 100 Mbps.</P>
<P>In addition, the IEEE 902.9a isochronous Ethernet standard provides a way for
two networks to run over 10Base-T wiring. IsoEthernet permits the integration of
LAN and WAN services, and can extend a company's existing investment in standard
Ethernet. IsoEthernet can deliver voice and video as well. In the past, multimedia
over Ethernet has been limited because of Ethernet's connectionless nature. Traditional
Ethernet generates bursty traffic, which is excellent for sending data, not suitable
for time-sensitive information such as video. This type of time-sensitive traffic
is highly dependent on all packets arriving in the correct order. IsoEthernet is
capable of multiplexing 56 Kbps/64 Kbps ISDN B channels and running both packet and
wideband circuit-switched multimedia services over Category 3 UTP cable. Its encoding
scheme also increases the available bandwidth from 10 Mbps to 16 Mbps. The extra
6 Mbps of bandwidth is used to create a multimedia pipe. IsoEthernet can be integrated
into an existing 10Base-T Ethernet with the addition of an isoEthernet hub, which
permits WAN and LAN services to be synchronized. Workstations must be equipped with
isoEthernet adapter cards, which are connected to the hub. An <I>attachment unit
interface</I> (AUI) then connects the isoEthernet and Ethernet hubs.</P>
<P>Token ring networks have been implemented on a variety of media at a variety of
speeds. Therefore, unlike Ethernet, token ring technology was not successfully introduced
into a generalized data processing network. IBM implemented token ring in its early
PC LANs, as did Apollo for its engineering workstations. But somehow, token ring
did not catch on as Ethernet did. There were several reasons for this, including
the fact that token ring is more expensive to deploy than Ethernet, requires more
planning, and is more difficult to install. More recently, however, token ring networks
have enjoyed a rebirth in popularity for several reasons.</P>
<P>The IEEE organization adopted token ring as a sanctioned network in its IEEE 802.5
spec-ifications.</P>
<P>The market for token ring switches is enjoying tremendous growth as corporate
networks continue to grow at an unprecedented pace. These switches provide users
on overcrowded LANs with their own personal 4 Mbps to 16 Mbps piece of bandwidth.
The switch can also be used to divide a large ring into smaller segments.</P>
<P>Until recently, equipment for switched token ring networks was largely unavailable.
However, token ring networks can suffer from the same geographic limitations as Ethernet,
and vendors are now stepping in to provide the switching equipment users require
to expand their token ring networks. Traditionally, two-port bridges are used in
token ring networks, which impose a significant limitation on its expandability.
Token ring switches can connect the separate rings to each other and to servers,
without the performance limitations of the past. Some products include both token
ring and Ethernet switching facilities in the same box. Most switches also accommodate
high-speed networking, such as ATM or FDDI; many also support RMON management.</P>
<P>Both token ring and Ethernet networks have bandwidth limitations. Many corporate
networks are beginning to reach those limitations, as they bring in more and larger
applications and experience a greater demand for data. Switching technology can help
overcome these limitations by extending an overcrowded network. Whether the switch
is used to divide the ring into smaller segments or to give each user a personal
slice of bandwidth, switches can greatly enhance network performance, thereby extending
the useful life of the existing network.</P>
<P>IBM has made a significant commitment to supporting token ring as the preferred
SNA LAN and has, in fact, provided connections for its broad range of computers and
communications controllers to token ring.</P>
<P>The architecture of the <I>Fiber Distributed Data Interface</I> (FDDI) is modeled
after token ring. FDDI is a high-speed WAN technology that runs at 100 Mbps. Therefore,
if you love FDDI, you must also at least have a passing respect for token ring.</P>
<P>Token ring technology was patented by a European engineer who forced those who
adopted it to pay a royalty. However, this patent has been successfully challenged,
so the economics of token ring networks has taken a turn for the better.
<H2><A NAME="Heading6"></A><FONT COLOR="#000077">LAN Implementation Standards</FONT></H2>
<P>Although both Ethernet and token ring networks function well, they were not recognized
as official standards because they were developed in the private, commercial sector.
To address this need for standardization, the IEEE studied these and other implementations
and developed a series of standards to properly define a series of LAN specifications.</P>
<P>In developing its standards, the IEEE had to walk the line between the OSI Reference
Model and the existing, well-known and widely accepted LAN implementations. In terms
of the OSI Reference Model, for example, IEEE carved the Data Link Layer (layer 2)
into two parts. The upper half of the layer that interfaces with the Network Layer
(layer 3) was termed the<I> Logical Link Control</I> (LLC). The LLC provides a common,
low level point of access, independent from the actual physical media.</P>
<P>The lower half of the layer that interfaces with the Physical Layer (layer 1)
was termed the <I>Medium Access Control</I> (MAC). The MAC addresses the specifics
of the physical network interface; therefore, separate MAC standards are defined
for CSMA/CD, token passing bus and token passing ring. However, note that a single
LLC specification addresses all three MACs.</P>
<P>A message passed from the Network Layer is processed by the LLC protocol, and
an LLC header is added to the data (see Figure 7.2). This new data structure is then
passed on to the MAC where another header and a trailer are added before the data
enters the physical network. The resulting structure that includes the MAC header,
the LLC header, the data and the MAC trailer is termed a <I>frame</I>.
<H3><A NAME="Heading7"></A><FONT COLOR="#000077">IEEE 802.2</FONT></H3>
<P>In IEEE terms, the 802.2 specification defines the LLC (see Figure 7.3). The 802.2
header consists of the following:

<UL>