Lesson
1: Networking Basic
Networking
History |
How a LAN Is Built | LAN
Topologies | LAN/WAN
Devices
LAN/WAN Devices
Let’s now take a look at some of the
devices that move traffic around the network.
The approach taken in this section will be simple. As networking
technology continues to evolve, the actual differences between
networking devices is beginning to blur slightly. Routers
today are switching packets faster and yielding the performance
of switches. Switches, on the other hand, are being designed
with more intelligence and able to act more like routers.
Hubs, while traditionally not intelligent in terms of the
amount of software they run, are now being designed with software
that allows the hub to be “intelligent” acting
more like a switch.
In this section, we’ll keep these different types of
product separate so that you can understand the basics. Let’s
start off with the hub.
Hub
Star topology networks generally have a hub
in the center of the network that connects all of the devices
together using cabling. When bits hit a networking device,
be they hubs, switches, or routers, the devices will strengthen
the signal and then send it on its way.
A hub is simple a multiport repeater. There is usually no
software to load, and no configuration required (i.e. network
administrators don’t have to tell the device what to
do).
Hubs operate very much the same way as a
repeater. They amplify and propagate signals received out
all ports, with the exception of the port from which the data
arrived.
For example in the above image, if system 125 wanted to print
on the printer 128, the message would be sent to all systems
on Segment 1, as well as across the hub to all systems on
Segment 2. System 128 would see that the message is intended
for it and would process it.
Devices on the network are constantly listening for data.
When devices sense a frame of information that is addressed
(and we will talk more about addressing later) for it, then
it will accept that information into memory found on the network
interface card (NIC) and begin processing the data.
In fairly small networks, hubs work very well. However, in
large networks the limitations of hubs creates problems for
network managers. In this example, Ethernet is the standard
being used. The network is also baseband, only one station
can use the network at a time. If the applications and files
being used on this network are large, and there are more nodes
on the network, contention for bandwidth will slow the responsiveness
of the network down.
Bridges
Bridges improve network throughput and operate
at a more intelligent level than do hubs. A bridge is considered
to be a store and forward device that uses unique hardware
addresses to filter traffic that would otherwise travel from
one segment to another. A bridge performs the following functions:
- Reads data frame headers and records source address/port
(segment) pairs
- Reads the destination address of incoming frames
and uses recorded addresses to determine the appropriate
outbound port for the frame.
- Uses memory buffers to store frames during periods
of heavy transmission, and forwards them when
the medium is ready.
Let’s take a look at an example.
The bridge divides this Ethernet LAN into
two segments in the above image, each connecting to a hub
and then to a bridge port. Stations 123-125 are on segment
1 and stations 126-128 are on segment 2.
When station 124 transmits to station 125, the frame goes
into the hub (who repeats it and sends it out all connected
ports) and then on to the bridge. The bridge will not forward
the frame because it recognizes that stations 124 and 125
are on the same segment. Only traffic between segments passes
through the bridge. In this example, a data frame from station
123, 124, or 125 to any station on segment 2 would be forwarded,
and so would a message from any station on segment 2 to stations
on segment 1.
When one station transmits, all other stations must wait until
the line is silent again before transmitting. In Ethernet,
only one station can transmit at a time, or data frames will
collide with each other, corrupting the data in both frames.
Bridges will listen to the network and keep track of who they
are hearing. For instance, the bridge in this example will
know that system 127 is on Segment 2, and that 125 is on segment
1. The bridge may even have a port (perhaps out to the Internet)
where it will send all packets that it cannot identify a destination
for.
Switches
Switches use bridging technology to forward
traffic between ports. They provide full dedicated transmission
rates between two stations that are directly connected to
the switch ports. Switches also build and maintain address
tables just like bridges do. These address tables are known
as “content addressable memory.”
Let’s look at an example.
Replacing the two hubs and the bridge with
an Ethernet switch provides the users with dedicated bandwidth.
Each station has a full 10Mbps “pipe” to the switch.
With a switch at the center of the network, combined with
the 100Mbps links, users have greater access to the network.
Given the size of the files and applications on this network,
additional bandwidth for access to the sever or to the corporate
intranet is possible by using a switch that has both 10Mbps
and 100Mbps Fast Ethernet ports. The 10Mbps links could be
used to support all the desktop devices, including the printer,
while the 100Mbps switch ports would be used for higher bandwidth
needs.
Routers
A router has two basic functions, path determination
using a variety of metrics, and forwarding packets from one
network to another. Routing metrics can include load on the
link between devices, delay, bandwidth, and reliability, or
even hop count (i.e. the number of devices a packet must go
through in order to reach its destination).
In essence, routers will do all that bridges and switches
will do, plus more. Routers have the capability of looking
deeper into the data frame and applying network services based
on the destination IP address. Destination and Source IP addresses
are a part of the network header added to a packet encapsulation
at the network layer.
- SUMMARY -
* LANs
are designed to operate within a limited geographic area
* Key LAN components are
computers, NOS, NICs, hubs, and cables
* Common LAN topologies
include bus, tree, star, and ring
* Common LAN/WAN devices
are hubs, bridges, switches, and routers
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