Networking Basic
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.
Related Topics
- 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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