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A Local Area Network (LAN) has been an essential tool for business computing for many years, and great
fun for gaming for not quite so long. And setting up a basic small LAN is now
a very easy task. But networking catalogues are full of bridges and switches and
hubs and routers, making it hard for the beginner to figure out what's going on.
What do you need, and what do you not?
This article will tell you. The first half is the stuff you need to know to put
together a basic Windows network for playing games or ordinary small business
use. It tells you how the common kinds of Ethernet differ, and what to do to make
your network work and keep it working. After that, there's the more technical
information for people who are working with larger networks, or are just curious.
Jump to the technical stuff.
Three Types of Ethernet
This document will deal only with the three most popular kinds of PC networks,
10Base2, 10BaseT and 100BaseT. A number of the terms used here have different meanings when applied to older
and more esoteric networking systems, but only these three flavours are currently
in use for home and small business and, often, large business applications.
10Base2 is also called thin Ethernet or "cheapernet". 10BaseT looks to the computer
like 10Base2, but offers more flexible layout. Both of these are Ethernet; the
term refers both to the kind of cable used in a network, and the kind of signals
sent on the cable. 100BaseT is a ten-times-faster version of 10BaseT, using the
newer "Fast Ethernet" system.
The "10" in the names of the two slower flavours of Ethernet indicates that these
versions have a signalling speed of 10MHz, giving a maximum useful data throughput,
from the user's point of view, of about half a megabyte per second. 100BaseT uses
100MHz signalling and is commensurately faster. The "Base" in the names means
they're "baseband" networks, which means that they have only one channel for data
transmission, so only one device can transmit at a time. This is important; as
baseband networks get more and more machines on them, it's more and more likely
that two machines will try to transmit at once, causing a "collision". After a
collision, the machines that caused it each wait a brief randomly chosen period
of time and try again. This means that even severely congested baseband networks
still work, but they get slower and slower as more and more machines are added.
Special devices like switches, bridges and routers exist to deal with this problem.
The "2" in 10Base2 indicates the maximum segment length in hundreds of meters;
the maximum aggregate length of cables you can use, with up to 30 computers connected,
before you have to start using routers, bridges or switches, of which more later.
In the real world, the maximum reliable 10Base2 segment length is 185 metres.
Just to be awkward, the "T" in 10BaseT and 100BaseT has nothing to do with cable
lengths; it indicates that these systems use unshielded twisted pair (UTP) telephone-type
cable, against the "RG-58" 50 ohm coaxial cable used by 10Base2.
Cables and connectors
Coaxial cable, as used in 10Base2, has a single centre conductor covered with
a layer of insulation, a braided and/or aluminium foil second conductor and then
the outer jacket. It's used in various versions for all sorts of high frequency
applications. The old thick "10Base5" Ethernet cabling has lower loss than RG-58,
and is much thicker, much more expensive and much less flexible. It's technically
incompatible with 10Base2, but will generally work and can be used in a pinch.
The "twisted pair" cabling used for 10BaseT uses pairs of conductors twisted
around each other to reduce susceptibility to induced currents. 10BaseT uses unshielded
twisted pair (UTP) cabling, which is cheap, and comes in different "levels" or
"categories"; the higher the category, the better the data carrying ability. Level 3 cable
is all 10BaseT requires, though many current installations are using Level 4 or
Level 5 in anticipation of faster network standards in the future.
10BaseT does not have a distinct maximum cable length; 100 to 150 meters is the generally accepted limit, but high grade low loss cable can extend
this. This maximum length is the distance each computer can be from its hub, not
the total cable length in the system, so a single 5,000 Naira 17-port hub makes
it easy to cable up a good-sized office.
|"RJ-45" stands for Registered Jack 45. "BNC" variously stands for Bayonet Navy
Connector, British Naval Connector, Bayonet Neill Concelman, or Bayonet Nut Connection,
depending on who you ask.|
10BaseT's dual-twisted-pair cables have RJ-45 modular connectors at the end.
RJ-45s look similar to the RJ-11 modular telephone connectors which are valiantly
attempting to replace the antiquated giant Australian phone plugs, but have eight
pins instead of the RJ-11's six. 10Base2's twist-on BNC connectors violate the
standard rule of computer connectors which states that they should snag as many
other cables as they can when pulled through a mess of wiring. Fortunately, most
RJ-45's redress the balance; the plastic clip on the back of the connector is
not only very good at catching other cables, but impressively fragile as well,
and a broken clip renders the connector very unreliable.
BNCs, however, can be very unreliable when not obviously damaged at all. Frequent
plugging and unplugging can make the centre pin terminal a loose fit, and corrosion
can also cause problems. RJ-45 connectors at least generally LOOK wrong when they're
Setting it up
The step by step procedure for installing a basic small business or game-playing
LAN is now, usually, very simple;¬†install your network cards like any other card,
hook up the cable, and as soon as your operating system knows about it, it works.
In Windows 95 or 98, all you've got to do is make sure you've got your workgroup
name set the same as that of the people you want to connect to, and that all the
computers on the network share at least one network protocol.
About the only problem you're likely to have in setting up a small network is
making the cards work; a standard NE2000-compatible network card needs an IRQ
and a few I/O addresses, and setting one up on a packed machine may require some
resource juggling. This, however, is not a problem peculiar to networks, and since
you can now get dirt cheap Plug and Play network cards, setup is often effortless.
(If it's not, check out my Step By Step column on troubleshooting Plug and Play
There are, however, some basic facts about networks which it helps to know.
Topology is, for network purposes, the layout of computers and cables and other
gadgets in the network. The basic topology for 10Base2 and 10BaseT is simple.
A 10Base2 segment (a segment, in this case, is a network with no bridges or switches
or other fancy devices in it) contains two or more computers, each with a network
card (NIC) fitted with a T-piece which accepts two network cables, or a network cable
and a 50 ohm terminating resistor. Each end of the network must have a terminator
and a cable connected, and everything else has two cables connected. The T-piece
must go right on the network card-¬†you can't use extension cords between the
card and the T-piece.
You can take a computer out of the network by disconnecting its T-piece from
the network card, leaving the cables connected to the T-piece's two arms. Disconnect
in any other way, or remove either terminator, and the network stops working until
you plug it back together. Every machine has to be at least 50 centimetres of
cable away from every other machine, too.
How to wire a 10BaseT crossover cable, for connecting two, but only two, machines
together without a hub. This is the same kind of cable that is used to "cascade"
multiple hubs into one network. The pin numbers are as you look at the contact
side of the plug, with the cable running away from you.
10BaseT, on the other hand, requires a special piece of hardware called a "hub"
if you want to use more than two computers. Two machines can hook together with
a simple crossover cable, but otherwise every machine on the network must have
its own single lead to a port on the hub, which must therefore have enough ports
to support the number of machines you wish to network. Multiple hubs can be connected
together to allow larger networks; see here for more information on hubs.
Both approaches have their pros and cons. Both offer really, really cheap network
cards- about 2000 Naira gets you a vanilla NE2000-clone card with both 10Base2
and 10BaseT connectors. And, for 10Base2, that's pretty much where the spending
stops; the network cards come with T-pieces, so all you need are enough cables
and a couple of terminators, and you're in business.
For 10BaseT, you have to buy a hub as well, and this will set you back about
3,000 Naira for a five port hub, or around twice as much for 17 ports. 10BaseT
hubs are available in various sizes and can be "cascaded" to add more ports to
So why should you bother with 10BaseT? Well, in the 10Base2 configuration, one
dud cable, dodgy T-piece, duff terminator or poor connection makes the whole network
stone dead until the defective component or connection is fixed or, worse yet,
just interrupts the network every now and then. Intermittent problems are the
most annoying. Finding the defective component in 10Base2 is a process of elimination;
you just start somewhere, anywhere, and then "divide and conquer" cut the network
in two and reterminate the two halves, then see which half still has the problem
and divide it again, and so on, until you locate the source of the failure.
In 10BaseT, on the other hand, one bum cable or network adapter will only remove
one machine from the segment. A dead hub will kill the network for every machine
directly connected to that hub, but hubs are much more reliable and less prone
to accidental damage than 10Base2 cables, T-pieces and terminators.
Repeaters, routers and hubs the basics
Each 10Base2 segment can only be 185 metres in length, and can only accommodate
30 computers. For many applications, this is fine, and so you can get away with
a 5,000 Naira-or-less network card in each machine and a few ten buck cables.
But 185 metres can be used up surprisingly quickly in standard into-the-wall-and-up
through-the-ceiling cable installations.
If you need more length, a repeater lets you join 10Base2 segments together.
The Ethernet spec allows for up to four repeaters in a network which, for the
mathematically disinclined, means five segments. But only three of these segments
can be "populated" or have computers connected to them. So your maximum 10Base2
cable length using repeaters is 925 metres, with 555 meters of that being useable
for up to 90 computers.
This rule applies to 10BaseT, as well, because every 10BaseT hub acts as a repeater.
This can result in rather complex layout diagrams, but the basic rule is easy
to remember; the path between any two computers must not include more than four repeaters or hubs, or more than three populated cable segments.
Having 90 computers connected via Ethernet, though, is not a good idea unless
each of them doesn't use the network much. With only 10 megabits per second shared
between 90 machines, all of them trying to move data at once gives each computer
a theoretical maximum bandwidth available to it of about 14 kilobytes per second.
Since there'd be collisions galore from all that simultaneous chatter, the real
bandwidth would be much lower, and the network would grind to a halt.
What you need to do to cut down the chatter is either increase the total shareable
bandwidth by switching to Fast Ethernet (which won't actually help all that much
if you've got 90 computers talking at once; the network will probably still be
painfully slow), or chop the network up into smaller segments, with traffic only
escaping a segment when it's actually addressed to a computer on the outside.
Dividing your LAN up like this is called "internetworking", and allows big networks
to be both faster and physically larger, as it overcomes the maximum cable run
To get around the maximum number of repeaters problem, you have to use bridges.
Bridges are more expensive than repeaters, but they let you extend your network
without breaking the rules, by intelligently filtering and forwarding data based
on the machine it's intended for the bridge has enough brains to know what machine
addresses are on each side of it, and block the passage of traffic addressed to
a section of network which does not contain the intended recipient of the data.
When calculating legal routes, you can reset your repeater count to zero if the
data path goes through a bridge. The Ethernet specification allows no more than
seven bridges on a network. Bridges can have multiple ports, and so connect to
more than two network segments; by using multi-port bridges, you can build huge
networks, because each collision domain can have up to 1024 nodes on it. In practice,
any normal computers will generate enough network traffic that 1000 computers
in one collision domain will hopelessly clog the network, but if they're really,
really quiet, you can do it.
Routers are like bridges, only more so. They do the same data filtering, but
can also connect completely different networks to each other, allowing, for example,
an office network to be connected to the Internet. With the use of routers, there's
no practical limit to how many machines you can network together.
Setting up Windows networking
One of Windows 95's big selling points was that it finally made PC networking
simple enough for anyone to set up. Provided your network card is correctly addressed
by Windows an automatic procedure, for current Plug and Play cards¬†all you have
to do is add the network protocols of your choice in Network Properties.
Windows 98 doesn't change much in this department. If youve set up Windows 95's
networking, you can set up 98's. Once your network card is working with Windows,
add the clients and protocols you need in Network Properties (accessible from
Control Panel, or by right-clicking the Network Neighbourhood icon and selecting
Properties). For most small business networks and Internet access, all you will
need is the Client for Microsoft Networks, the NetBEUI protocol for your network
card and TCP/IP for your Dial-Up Adapter; you only need to install the first yourself,
since the Internet Connection Wizard takes care of the dial-up stuff.
NetBEUI is a fast protocol that works well on networks with fewer than 50 machines,
which covers the majority of situations. If your network uses IPX and/or TCP/IP,
install them for your network card too; if you only want them for multiplayer
gaming, turn off all of the bindings in their Properties windows, and say no to
the dialogue box asking if you would like to change your mind. The bindings let
Windows use these protocols for regular network communication as well as the raw
data transfer the games want, and redundant bindings slow down your network.
If you've got a Windows 95 computer with TCP/IP set up for its network card and
leave it on the default "automatically obtain an IP address" setting, the network
the computer's connected to must have what's called a Dynamic Host Configuration
Protocol (DHCP) server connected to it to dole out an address. Without such a
server, any 95 machines without addresses will fail to communicate over TCP/IP,
and will furthermore slow the network down as they periodically yodel down the
wire, hoping a server's shown up.
Windows 98 does a bit better than this. Machines set to automatically obtain
an address which don't find a server will give themselves an address in the "LINKLOCAL
network" IP address space, which means an address starting with 169.254 and with
two more arbitrary numbers on the end. The LINKLOCAL space is a "class B network",
which means the network ID is the first two numbers. All 169.254.X.X computers
can see each other provided they each have a different combination of the last
two numbers, and since the last two numbers can be from 0 to 255 and from 1 to
254 respectively, up to 64,515 computers can be on this network at once and address
clashes are unlikely.
169.254 addresses are not, however, valid for networks connected to the Internet.
Rest assured that calling your ISP and connecting with TCP/IP for the Dial-Up
Adapter does not constitute "connecting to the Internet" in this sense.
Got a 10Base2 terminator with a little dangly thing hanging off it? It's a ground
chain or strap, or wire. If you screw the lug at the end of the chain, strap or
wire to the chassis of the computer with any convenient mounting screw, you earth
the shield conductor of the network cable.
This may or may not be a good thing.
The 10Base2 spec says the network "may" be grounded in one (and ONLY one) place,
and doing this may reduce network errors. Grounding the network in more than one
place WILL cause errors and may damage equipment thanks to potential differences
between different "grounds". So don't do that.
This is especially bad if you run a 10Base2 cable between buildings, which is
something you're not meant to do. Different buildings often have markedly different
earth potentials, and if the cable's grounded at both ends, a hefty current can
flow through the shield, causing lots of network errors and, possibly, starting
fires. If some hapless person at the other end unplugs the network connector,
or even just touches it if it happens to be grounded at the far end but not the near one, they can
receive an electric shock.
If your network works fine without grounding, there's no need to change. If it's
grounded and you're getting errors, try ungrounding it, or grounding it at the
If your network has a repeater on one end of a segment, it probably automatically
grounds that end. So don't ground the other one.
That's it for the basic stuff...
...now here's the more complex info.
How much is a megabit?
Computer-savvy people know that the kilo-, mega- and giga- prefixes, in computer
usage, don't mean 1,000, 1,000,000 and 1,000,000,000, as they do normally. Computers
use binary arithmetic, so everything is in powers of two, and the three prefixes
indicate 1,024, 1,048,576 and 1,073,741,824. Two to the power of ten, two to the
power of twenty and two to the power of thirty, respectively. Easy, right? Well, sometimes.
RAM (Random Access Memory) is specified in these "proper" megabytes; a 64 megabyte
RAM module has exactly 67,108,864 bytes of storage space, disregarding error-checking
bits, if they're present. But when it comes to hard disk sizes, manufacturers
tend to specify their drives. Raw (unformatted) capacity in nice round millions
and billions of bytes. Unfortunately, they call these measurements "megabytes"
and "gigabytes", in order to make their drives sound bigger.
So a "6.4 gigabyte" hard drive actually has a capacity of about 5.96 real gigabytes,
before you format it and lose another few per cent. You can thank the marketing
people for this.
And now to network speeds. Data communication is specified not in kilobytes and
megabytes, but kilobits and megabits per second; kBps and MBps, respectively, although there's terrible inconsistency
in the use of the upper and lower case "B" to indicate bits and bytes, just to
annoy you further.
A bit is one-eighth of a byte; nobody's ever decimalised the byte, because making it 10 bits would make things
look smaller and slower, which the marketing people find less attractive for some
reason. So, logically, you'd expect one megabyte per second to be eight megabits
per second, right?
Ha! No such luck. Like hard disk capacities, network speeds are expressed in
round numbers, not powers of two. A "64 kilobit" ISDN line moves exactly 64,000
bits per second, or a megabyte (the real kind, not the hard disk kind) every two
minutes and eleven seconds. "10 megabit" Ethernet is good for exactly ten million
bits per second, or 1.192 megabytes per second. And so on.
Bridges, switches and routers in detail
Once upon a time, you could point to a gadget that hooked network segments together
and say, with confidence, what it was. Well, so I'm told, anyway. Today, there
are all sorts of devices designed to move data from one network to another, with
all kinds of fancy features, and their names are a highly unreliable guide to
what sort of device they actually are. The three basic categories of network-joining
device are switches, bridges and routers, but those definitions blur into each
other so much that defining them separately is impossible. The words mean different
things to different companies. You just have to look at the specification sheets
and decide whether a given device is what you need. Here's how to tell.
A bridge, officially, is a stand-alone device or specially configured computer
that connects different LANs, and allows them to act as segments of one LAN.
A bridge can only connect networks which are using an identical network protocol,
like, for example, Ethernet. A bridge with appropriate connections can connect
networks which are using the same protocol on different kinds of connection; bridges
that have a collection of 10BaseT ports and a 10Base2 coaxial connector are common,
for instance. But, canonically, if it can connect networks of different kinds,
like Token Ring to Ethernet, it isn't a bridge.
Since bridges only operate at OSI Layer 2, they cannot connect network segments which couldn't be connected WITHOUT
the bridge there, disregarding media differences. If you're talking TCP/IP, that
means that all segments connected to a bridge must have the same subnet mask;
the second xxx.xxx.xxx.xxx number, subsidiary to the IP address, which determines
what subnet a computer is on. Computers on different subnets can't see each other.
Bridges, unlike simple repeaters, do not retransmit a frame until they've received
the whole thing. This means devices on either side of a bridge can transmit simultaneously
without causing collisions, and so you can use a bridge to segment a network into smaller chunks to reduce
collisions and improve performance. This, indeed, is the major function of the
bridge; chopping up big ungainly networks into smaller "collision domains", so
the overall chatter level doesn't bog the network down. The tiny single frame delay introduced by the bridging system is a small price
All modern bridges are "learning" bridges. This means they have the ability to
figure out, by looking at where data are coming from, what machines are connected
to which of their ports. They can therefore restrict packet retransmission to
only the port they know connects to the network node to which the frame's addressed. If a frame isn't addressed to a machine the
bridge knows about, it retransmits it on all ports except the one it came from,
because everything on the segment the frame came from heard it at the same time
the bridge did. If a given address doesn't transmit for a given period of time,
the bridge removes it from its address table, so as not to fill its table (which
has a limited size) with addresses for machines which may not necessarily even
be on the network any more.
Bridges can even handle being connected up in loops. If this situation were left
uncorrected, it would cause instant and hopeless congestion as every bridge retransmitted
every packet to every other bridge in the loop, and then got it retransmitted
back, ad infinitum. The bridges deal with this by arranging themselves into what's
known as a "spanning tree"; they very quickly shut down connections between bridges
until all of the loops are eliminated. This allows redundant network wiring; if
one cable is cut, the bridges sort out the problem and create a new tree using
a previously ignored cable.
A switch is, essentially, a bridge with knobs on. Or a really smart kind of hub. Or, in its simplest form, maybe just a multi-port bridge. Essentially, switches
are a creation of marketing departments; there may be some under-the-surface differences
between them and previous devices, but from an operational point of view they're
the same as earlier bridges and routers, only faster. Perhaps. A LAN with a switch
joining its segments is referred to as a "switched LAN".
Like a bridge, a switch connects networks and filters packets, only sending on
packets to a given network segment if they're addressed to a device on that segment.
Also like a bridge, your basic switch operates at OSI Layer 2; it cannot change the data it's sending, to route information from one network
flavour to another. Everything connected to a Layer 2 switch has to be configured
as if it were on the one network like a bridge, these switches can only connect
network segments that could be connected anyway, as far as the computer settings
go. Each port on a switch can support a whole LAN or a single station. If only
one station is connected to a switch port, it is said to have a "dedicated LAN".
Classier switches can do Layer 3 routing, and are thus called "Layer 3" or "Multi-layer" switches. They can connect network
segments on different subnets, by routing between them. They can also create "broadcast
firewalls" between ports or groups of ports (and, thus, between any devices you
attach to those ports). These groups are called "bridged groups" or "virtual LANs",
and each behave like a bridged network. This allows devices on one Virtual LAN
to use one IP subnet (or IPX network number, or Appletalk network number range),
and devices on others to use other setups. If the switch is smart enough, it can
give each virtual LAN access to the others as if it were a router.
But, remember, all sorts of things are called "switches" these days. Bear in
mind that something referred to as a switch can also fairly be described as a
bridge or a router.
Switching, the underlying technology that gives switches their name, can be done
in two basic ways; cut-through and store-and-forward. Cut-through is the newer
technology whose introduction, as much as anything, can be said to have spurred
the creation of the "switch" as a distinct gadget. Cut-through switches do nothing
but look at the MAC addresses of the frame headers and forward the frames accordingly.
A cut-through switch doesn't care if the packet inside the frame is valid or not,
and so it will cut network traffic when all is well, but won't prevent malfunctioning
software or hardware from paralysing the network with tons of rubbish packets.
Cut-through forwarding is impossible between media of different speeds, so switches
which support, say, 10BaseT and 100BaseT, do not use cut-through when data moves
between ports of different speeds.
Store-and-forward switching is the older way of doing it. It actually looks at
the packets before sending them on, which means the packets stay in the switch
a little longer, but broken ones are weeded out. Under high loads, cut-through
switches buffer data and hence run no faster than store-and-forward ones, but
they've got a slight performance edge in normal operation.
An overloaded switch can make network congestion even worse than it would be
without the switch, in some cases. If the switch is receiving data for a given
port faster than it can pump it out of that port, it will buffer the data until
it runs out of memory and then start dropping packets throwing them away. The
machines sending the data don't know that it isn't getting through until the network
protocol sorts it out; fragmentary data causes receiving machines to request resends.
This is worse than a plain old collision, because in a collision situation all
of the sending machines know about the problem at once and resend practically
immediately, whereas the resend requests created by a choked switch have to traverse
the network path back from the receiving machine to the sender before anything
A router is a device that connects networks together, like a bridge, but is a
great deal smarter. Routers operate at OSI layer 3, which means they understand both logical and physical addresses when moving
data around, unlike bridges, which work at layer 2 and only understand physical
Routers analyse incoming packets and modify them, if necessary, so they're redirected
to another router or to their initially intended destination. This allows routers
to send packets from one kind of network across another kind of network on their
way to a destination network which can be of yet another kind, via more routers
if necessary. As long as the routers know what computers live where, they can
figure out the necessary route themselves.
Routers maintain a database of addresses which allows them to correctly route
data among the hundreds of millions of in-use addresses accessible via the Internet.
The can do this because their ability to send data to other routers lets them
pay attention only to the "network number", the part of the address that indicates
the network to which the computer they're sending data to is connected, and ignore
the details of the "host number", the exact machine they're aiming at. The router
at the other end can handle getting the data to the exact right machine; the sending
router only has to get it to the right network.
Routers further reduce the number of addresses they need to know about by "summarising"
entries together when a lot of addresses share the same route. Typical Internet
routing tables contain only several tens of thousands of routes, rather than the
hundreds of millions a "dumb" routing strategy would require.
It is this ability that makes routers the life-blood of the Internet, and similarly
important to many other networks, but it also makes them slower than "layer 3
switches", which can generally only do limited routing within the devices directly
connected to them, and do not understand multiple layer 3 protocols.
A truly "Internet-capable" switch has full routing abilities in addition to the quick address lookup capabilities
of a regular switch. If it has to send data to somewhere it hasn't recently received
data from (the arriving data bringing with it routing information for replies),
the switch behaves like a router. This requires much more processing power than
switching and is thus slower. "Slower" in this case is a relative term; router-speed
operation, even from older and cheaper models, can still provide packets much
faster than a T-1 line (1.544 megabits per second) can handle. These cheaper routers
speed of operation roughly matches the available throughput from a T-3 line (43
megabits per second). Routers are getting faster every year, too; top-of-the-line
products can handle a great deal more data.
There is only any point in using a switch/router like this if some large percentage
of the network traffic comes from the same places over and over again local addresses,
in other words. A switch connected to the Internet for public access as well as
to a local network must also be able to preferentially cache routes to local addresses,
or its address table will be rapidly filled with useless once-only Internet access
routes from Net surfers.
Routers can communicate with other routers to provide better traffic management
and avoid slow connections; they can, together, determine the best route through
a complex WAN. Many routers are used for Media Access conversions; linking networks with different
physical connections as well as different Layer 3 protocols, like for example
Token Ring and Ethernet. Many routers also support scads of Layer 3 protocols,
but they don't have to in order to qualify as a router.
Some network protocols are inherently unroutable†SNA (IBM's Systems Network Architecture,
originally a set of mainframe networking protocols), NetBIOS (Network Basic Input
Output System, the basis for Microsoft's popular NetBEUI) and LAT (Digital Equipment
Corporation's Local Area Transport protocol), for instance, none of which have
the innate ability to work with routers. Some of these, like SNA and NetBIOS,
can sort-of-kind-of be routed by being "encapsulated" inside other, routable protocols,
but these solutions are slow and inelegant. Modern routers can work wonders at
integrating disparate kinds of network, such as tend to develop in large companies,
but the elimination of excess protocols is still highly desirable to make management
simpler, even when every protocol on the WAN is routable.
"Static" routers must have their routing tables manually updated. "Dynamic" routers
build and update their own tables.
Bandwidth, and how to get more of it
A network bandwidth is how much data it can move per second. In a plain 10 megabit
Ethernet LAN, that bandwidth is, unsurprisingly, ten megabits per second, or a bit more than a megabyte per second. The actual amount of real
data throughput is considerably lower, because a lot of bandwidth is taken up
by the extra formatting information tacked onto the data to be sent. But if you
just look at the bits being sent, the total number per second, assuming no collisions, is ten million (if you are wondering why this is a round number and not a power
of two, check here).
Things get more complicated when you start playing with bridges and switches on larger networks. Both of these devices, after a brief learning period, forward
traffic only to network segments that actually contain the computer to which the
traffic is addressed.
Depending on the network, you may get a larger performance gain from segmenting
a 10 megabit network than from upgrading it to 100 megabit.
If you have a 10BaseT network with, say, 32 computers on it, you could add an
eight port bridge or switch with a four port hub hanging off each port. This chops
the network into eight segments of four computers each, which means that each
computer can yammer all it likes to its three segment companions without cutting
into the 10 megabit bandwidth of any of the other segments at all. If a given
computer DOES talk to a machine on a different segment, it will only take up bandwidth
on those two segments, leaving half of the network untouched.
Segmented networks can therefore offer impressive "aggregate bandwidth"; the
total amount of data that can be moved around the network by various machines
talking to each other at once. High aggregate bandwidth does not, in this case,
indicate higher bandwidth available to any one network conversation. But this
is usually OK, as for most operations the transfer rate provided by a 10 megabit
network is adequate, provided you can get most or all of it for yourself.
If two machines conduct a 10 megabit conversation between two ports on the bridge
or switch that segments this 32 computer network, and another two conduct a similar
conversation on each of the other three pairs of ports, the network will be saturated
(any extra traffic will produce collisions and slow the LAN down) and an aggregate
bandwidth of only 40 megabits per second will have been achieved. On the other
hand, if computers on the network happen only to talk to other computers on their
own segment, the lack of inter-segment network pollution means the aggregate bandwidth
available will be 80 megabits per second. The worst case scenario arises if three
segments all want to talk to the fourth at once; in this situation they have to
share the fourth segment's bandwidth, and the network's aggregate bandwidth drops
back to 10 megabits per second.
In this situation, an eight-segment 10MBps network clearly offers significantly
less bandwidth, under all circumstances, than an unsegmented 100MBps LAN. But
if you double the number of segments to 16, so each one serves only two computers,
the aggregate bandwidth figures in the above examples jump to 80 and 160 megabits
per second respectively, and the chance of everyone concentrating on one segment
If there's one computer that commonly attracts lots of traffic†a file server,
for instance†that one computer can be given a segment to itself, and can even
be given a 100BaseT network card and be connected to a 10/100MBps dual-mode switch
or bridge. This gives the high-demand computer a dedicated 100 megabit connection,
probably full duplex, to the whole of the rest of the network, even though any given other computer
can only move ten megabits per second. If the 31 other computers all try to access
the file server now, they're sharing 100 megabits per second between them instead
of 10, and will still receive data at a decent rate. Because bridges and switches
prevent collisions between traffic originating on different network segments,
if ten 10MBps computers simultaneously request data from the 100MBps server (and
everything else happens to shut up), they will each get data about as fast as
their network cards can handle it, without a single collision. They will, in fact,
perform just as well in this situation as if they were networked to the server
with 100BaseT all the way.
When there are no switches or bridges or routers to worry about, but only repeaters
(remember, a standard 10BaseT hub is a repeater), network performance is easy
to work out. Everything shares. It's not quite as simple as that,†in a collision
situation NO data gets sent by anyone, so when the network is saturated the total
useful throughput is less than the total bandwidth of the network, but at least
it doesn't matter who's talking to who. A given number of connections will result
in a given aggregate bandwidth.
Another important factor is the internal or "backplane" bandwidth of your bridge
or switch. To avoid causing bottleneck problems at moments of high network use,
you need a backplane bandwidth equal to the aggregate bandwidth of all of a device's
ports. If a bridge, switch or router has this much backplane bandwidth, all of
its ports can be operating at full speed all of the time, and the "data pipe"
inside the device is wide enough to let all of the data through.
ATM: The modern heir to packet switching and circuit switching, Asynchronous Transfer Mode aims to provide
the efficiency and fault-tolerance of the former and the guaranteed delivery of
the latter. Today's network switches are the technological precursors to ATM; switches work with relatively large
data packets of variable length, whereas ATM uses small, equal-sized "cells" of
data and promises far greater speeds than current LANs over short or long distances.
ATM looks, to the connected machines, like a circuit switched system when they
want to transfer data, nothing can impinge upon the 51 or 155 megabit per second
pipe assigned to them. The ATM system installed has to be fast enough, of course,
to handle as many concurrent pipes as are needed.
Broadcast: A "broadcast packet", in Ethernet, is a packet that will be received by every
node on a LAN; it's not addressed to anyone in particular, but to everyone in general.
This leads to the concept of the "broadcast domain", which is every node that
will be reached by a broadcast from any given node. Routers segment broadcast domains; broadcast packets don't get past them.
Bus: A kind of network topology. The bus configuration, as used by 10Base2, has all
of the devices on the network connected in parallel to one cable. This "cable"
is really made up of separate cable segments joined at the T-pieces, but electrically
speaking it can be treated as one wire. Any computer can be disconnected from
this bus without affecting connectivity for everything else, but if the cable
is interrupted anywhere, the whole network goes down.
Category: Twisted pair cable such as is used by 10BaseT and 100BaseT is available in various
specification levels or "categories". 100BaseT requires Category 5 cable, often
referred to as "Cat 5". 10BaseT will work with lower grade, thinner cable, but
a lot of installers use Cat 5 cable anyway because it costs little more and makes
it easy to upgrade. Make sure the cable you use really is Category 5 cable, not
just something labelled "Category 5 quality".
Collision: When two devices on a baseband network like Ethernet try to send data at once,
they talk over each other and cause a collision. When a collision occurs, every
device that's trying to send data pauses for a brief, random period and tries
again. This simple system works less and less well as you get more and more computers
on a network, which is why segmenting big networks with bridges and/or switches is a good idea. Bridges and switches are thus said to segment the "collision
domain"; the group of nodes with whose transmissions it is possible for a given node's transmissions to
Duplex: In computer communications, there are three kinds of connection between two
devices. The first is simplex, in which data can only flow one way. Half duplex
is the system used by regular Ethernet; data can flow either way, but only one
way at a time. Full duplex allows data flow in both directions at once. Ethernet
supports full duplex operation, but only between two devices over twisted pair
cables. Regular 10BaseT or 100BaseT cable has two physical pairs of wires in it,
which in full duplex operation can be used for full bandwidth data transfer in
both directions;†one wire pair per direction. This works because when there are
only two devices involved, so collisions are impossible. The second wire pair is normally needed for collision detection.
Full duplex doubles the aggregate bandwidth of a connection, but doesn't greatly
increase performance unless both devices send a lot of data. Many network transactions
involve a lot of data going one way and only a little going the other, so there's
not much performance difference.
Hub: A hub is a common connection point for network devices. The simplest form of
hub is completely passive¬†it contains no electronics, it's just a collection
of connected ports, and exists only to make wiring more convenient. This sort
of hub doesnt work with 10BaseT or 100BaseT Ethernet, and isn't very practical
for larger 10Base2 networks because of 10Base2's rather limited segment length.
These days, when someone talks about a "passive" hub they probably mean one like
the currently available cheap 10BaseT models. These hubs act as a simple repeater
they copy every packet received at any one of their multiple ports to all of the
other ports, which keeps every hub-to-computer connection as a separate network
The most common kind of hub used in small networks is the "stand-alone" variety.
Despite their name, stand-alone hubs can usually be connected together with regular
twisted pair cable or thick 10Base5 coaxial, so you can add more ports to your
basic hub if your networking needs grow.
"Intelligent" or "manageable" hubs include features so an administrator can monitor
traffic and configure, enable and disable the ports remotely. They still just
copy everything they get to everywhere. The cheap 10BaseT hubs in your local computer
store are probably NOT manageable, which is OK for small networks where no user
is likely to be more than a 30 second walk from the hub anyway. If you've got
hubs, bridges, switches and routers all over the place, though, being able to immediately see what's stopped working
is an obvious advantage.
"Stackable" hubs are designed from the outset to be linked together, and when
you link them they act as one unit for management purposes. Linked stand-alone
hubs don't do this. Stackable hubs provide a cost-effective option for businesses
starting with a medium sized network but with higher aspirations, since many models
allow you to include just one more expensive manageable hub in the stack, and
have it provide management access to all of the others.
The next step up the ladder, only of interest to builders of large networks,
are modular hubs. These use one chassis or "card cage" into which cards, each
of which provides several hub ports, are installed. The cards are cheaper per
port than stackable hubs, because the power supply and casing are provided by
the chassis. Cards can be installed for various different network types as needed,
and a management unit may or may not be installed, according to preference.
"Switching" hubs are smart enough to know what devices are connected to what
ports (figuring it out in the same way as learning bridges), and only copy packets
addressed to those devices, and thus act as a limited kind of bridge† they work like a bridge, but they only have one device connected to each port.
To avoid or at least reduce confusion, the actual word "hub" is generally only
used when you are talking about Ethernet wiring. If you are talking token ring networks instead, you should call call the conceptually similar token ring device
a "multistation access unit", or MAU.
MAC address: Every node on a network has a Media Access Control address, which uniquely identifies
it. On Ethernet networks, every computer's network card has a unique MAC address.
Blocks of MAC addresses (the addresses are 48 bit numbers, so there are more than
281 trillion of them available) are assigned to network card manufacturers and
used sequentially, the result of which should be that no network card has the
same MAC address as any other. In the real world, things like reprogrammable cards
can result in two machines with the same address on one network, which can cause
problems utterly mystifying to the network novice. These problems are, fortunately,
extremely rare. MAC addresses are the identification system used by OSI layer 2.
Network Interface Card: Normally shortened to NIC, this is the technical term for what everyone else
just calls a network card. The NIC is the board you put in your computer so you
can connect the computer to a network. They're almost always made for a particular
kind of network and media, although Ethernet cards commonly have connectors for
10Base2 and 10BaseT, and may also support 100BaseT.
Node: The correct word for a processing location on a network. Things other than computers
can be connected to networks; printers, traffic handling devices and so on. Every
node has a unique hardware address see MAC address.
OSI layers: The seven Open System Interconnection layers are the International Standards
Organisation networking framework definition. Fortunately, users don't need to
know anything about them, except that the lower the layer number, the closer you
are to the hardware. In network communication, control passes from the higher
levels to the lower ones at one end, over the network connection to the next network
station, and back up the levels again. All seven layers put together make up the
entire network system from your application software to the wires, and how each
layer actually works in the real world is defined by a plethora of other protocols.
For example, Ethernet and Token Ring are two different ways of providing the services
defined by OSI layers 1 and 2, the Physical and Data Link layers.
Communication between programs. This is the layer that user programs talk to.
Data representation conversions; this layer translates data, between what the
network requires and what the computers at each end expect.
Establishes and maintains communications channels, so program on different computers
can establish a link. Often combined with the Transport Layer.
Responsible for end-to-end data transmission integrity. Makes sure that the data
actually gets there, with no errors, in the right order, regardless of transmission
Routes data from one network node to another. This layer translates "logical"
device names and addresses into their network hardware equivalents, and does routing,
if necessary, for devices that are more than one network link away.
Data Link Layer
Takes care of moving data from one network node to another, not more than one
Translates the bits generated by all the other layers into signals to send through
the network, and translates them back into bits at the other end.
Packet: A chunk of data transmitted over a packet-switching network. Packet-switching
is any protocol in which data is broken up into these packets and can then follow
various routes to its destination ‚Äď different packets which together comprise
one message may travel via different paths and are assembled when they arrive.
Packets therefore, of necessity, contain a destination address as well as the
data to be transmitted. Packets are often confused with "frames"; frames are the
data structures used by the physical network hardware to move the data. Information
that needs to be sent is parcelled up into a packet by the computer, and the packets
are parcelled up by the network hardware into frames.
Packet switching, as used by the TCP/IP protocol on which the Internet is built,
can be compared with simple "circuit switching", as used by the phone network,
where a dedicated link is established from point to point whenever one device
needs to communicate with another. Circuit switching is faster, works with much
lower-tech equipment and guarantees that data will arrive in the same order it
was sent, important for live audio and video. Packet switching is more efficient
and can tolerate slower and much less reliable connections.
Repeater: A repeater is the dumbest kind of active network-connecting device. It just
takes network traffic in one port and spews it out of one or more others, exactly
as it gets it, but louder. This helps overcome cable losses, and lets cable runs
be longer. Repeaters send while they receive, without waiting for the end of each
packet to see if it's intact or reduce collisions. Repeaters are, hence, useless
as a cure for network congestion. Ordinary 10BaseT and 100BaseT hubs are, in fact, multi-port repeaters.
WAN: Wide Area Network. Any computer network that covers a large geographical area,
and is composed, typically, of more than one Local Area Network. A WAN can be
composed of a multiplicity of network systems. The Internet is the biggest WAN
in the world, both in geographical extent and number of nodes.
Ethernet Standards Quick Reference
||10MBps data rate, star wired bus topology, baseband signalling on unshielded
twisted pair (UTP) cable. 10baseT cables can be up to 100 metres (328 feet) long,
with a minimum cable run between nodes of 2.5 metres (about 8 feet). Maximum of
1,024 nodes per network.|
||100BaseT, also known as "fast Ethernet", is essentially like 10BaseT, but run
at 100MBps instead of 10MBps. It requires category 5 UTP cable.
|100Mbps Ethernet specifications|
||4-pair Category 3, 4 or 5 UTP or STP|
||2-pair Category 5 UTP or STP|
||2-strand fibre-optic cable|
||Thin Ethernet, or "Thinnet". 10Mbps data rate, bus topology, baseband signalling.
The maximum segment length is 185 metres (607 feet). 10Base2 uses RG-58 coaxial
cable, and allows 30 nodes per segment, and 90 nodes per network. The total length
of the network must be less than 925 metres (3033 feet).|
||Thick Ethernet, or "Thicknet". The original "standard Ethernet", now supplanted
in popularity by 10Base2. Typically 10Mbps data rate, baseband signalling, with
a maximum segment length of 500 metres (1650 feet). Uses thick coaxial cable,
RG-8 and RG-11. 100 nodes per segment are permitted, and 300 nodes per network.
A maximum of four repeaters and 100 taps are permitted.|
||Fibre Link Ethernet. 2GBps maximum data transfer, although normally restricted
by hardware capabilities to 100MBps. Uses baseband signalling over fibre optic
cable. Maximum segment length is 2000 metres (6557 feet).|