Introduction to Ethernet/802.3 LANs
4.1.1 Ethernet/802.3 LAN development
This page will review the devices that are found on a network.
The earliest LAN technologies used either thick Ethernet or thin
Ethernet infrastructures. It is important to understand the limitations of
these infrastructures, as shown in Figure
, in order to understand the advancements in
LAN switching.
The addition of hubs or concentrates into the network offered an
improvement on thick and thin Ethernet technology. A hub is a Layer 1 device
and is sometimes referred to as an Ethernet concentrate or a multi-port
repeater. Hubs allow better access to the network for more users. Hubs
regenerate data signals which allows networks to be extended to greater
distances. Hubs do not make any decisions when data signals are received. Hubs
simply regenerate and amplify the data signals to all connected devices, except
for the device that originally sent the signal.
Ethernet is fundamentally a shared technology where all users on a
given LAN segment compete for the same available bandwidth. This situation is
analogous to a number of cars that try to access a one-lane road at the same
time. Since the road has only one lane, only one car can access it at a time.
As hubs were added to the network, more users competed for the same bandwidth. 
Collisions are a by-product of Ethernet networks. If two or more
devices try to transmit at the same time, a collision occurs. This situation is
analogous to two cars that try to merge into a single lane and cause a
collision. Traffic is backed up until the collision can be cleared. Excessive collisions
in a network result in slow network response times. This indicates that the
network is too congested or has too many users who need to access the network
at the same time.
Layer 2 devices are more intelligent than Layer 1 devices. Layer 2
devices make forwarding decisions based on Media Access Control (MAC) addresses
contained within the headers of transmitted data frames.
A bridge is a Layer 2 device used to divide, or segment, a
network. Bridges collect and selectively pass data frames between two network
segments. In order to do this, bridges learn the MAC address of devices on each
connected segment. With this information, the bridge builds a bridging table
and forwards or blocks traffic based on that table. This results in smaller
collision domains and greater network efficiency. 
Bridges do not restrict broadcast traffic.
However, they do provide greater traffic control within a network.
A switch is also a Layer 2 device and may be referred to as a
multi-port bridge. Switches make forwarding decisions based on MAC addresses
contained within transmitted data frames. Switches learn the MAC addresses of
devices connected to each port and this information is entered into a switching
table.
Switches create a virtual circuit between two connected devices
that want to communicate. When the virtual circuit is created, a dedicated
communication path is established between the two devices. The implementation
of a switch on the network provides micro segmentation. This creates a collision
free environment between the source and destination, which allows maximum
utilization of the available bandwidth. Switches are able to facilitate
multiple, simultaneous virtual circuit connections. This is analogous to a
highway that is divided into multiple lanes and each car has its own dedicated
lane. 
The disadvantage of Layer 2 devices is that they forward broadcast
frames to all connected devices on the network. Excessive broadcasts in a
network result in slow network response times.
A router is a Layer 3 device. Routers make decisions based on
groups of network addresses, or classes, as opposed to individual MAC
addresses. Routers use routing tables to record the Layer 3 addresses of the
networks that are directly connected to the local interfaces and network paths
learned from neighbor routers.
The following are functions of a router:
- Examine inbound
packets of Layer 3 data
- Choose the best
path for the data through the network
- Route the data
to the proper outbound port
Routers do not forward broadcasts unless they are programmed to do
so. Therefore, routers reduce the size of both the collision domains and the
broadcast domains in a network. Routers are the most important devices to
regulate traffic on large networks. Routers enable communication between two
computers regardless of location or operating system.
LANs typically employ a combination of Layer 1, Layer 2, and Layer
3 devices. Implementation of these devices depends on factors that are specific
to the particular needs of the organization. 
The Interactive Media Activity will require students to match
network devices to the layers of the OSI model.
The next page will discuss network congestion.
