Swtiches and Networks
A typical network consists of nodes (computers), a connecting medium
(wired or wireless), and specialized network equipment like routers or
hubs. In the case of the Internet, all these pieces working together allow
your computer to send information to another computer that could be on the
other side of the world!
Switches are a fundamental part of most networks. They make it possible
for several users to send information over a network at the same time
without slowing each other down. Just like routers allow different
networks to communicate with each other, switches allow different nodes (a
network connection point, typically a computer) of a network to
communicate directly with each other in a smooth and efficient manner.
Image courtesy Cisco Systems, Inc. An illustration of a Cisco Catalyst
There are a lot of different types of switches and networks. Switches that
provide a separate connection for each node in a company's internal
network are called LAN switches. Essentially, a LAN switch creates a
series of instant networks that contain only the two devices communicating
with each other at that particular moment. We will focus on Ethernet
networks using LAN switches. You will learn what a LAN switch is and how
transparent bridging works. You will also learn about VLANs, trunking and
In the most basic type of network found today, nodes are simply connected
together using hubs. As a network grows, there are some potential problems
with this configuration:
Scalability: In a hub network, limited shared bandwidth makes it difficult
to accommodate significant growth without sacrificing performance.
Applications today need more bandwidth than ever before. Quite often, the
entire network must be redesigned periodically to accommodate growth.
Latency: The amount of time that it takes a packet to get to its
destination. Since each node in a hub-based network has to wait for an
opportunity to transmit in order to avoid collisions, the latency can
increase significantly as you add more nodes. Or if someone is
transmitting a large file across the network, then all of the other nodes
are waiting for an opportunity to send their own packets. You have
probably seen this before at work. You try to access a server or the
Internet and suddenly everything slows down to a crawl.
Network Failure: In a typical network, one device on a hub can cause
problems for other devices attached to the hub due to wrong speed settings
(100Mbps on a 10Mbps hub) or excessive broadcasts. Switches can be
configured to limit broadcast levels.
Collisions: Ethernet uses a process called Carrier Sense Multiple Access
with Collision Detection (CSMA/CD) to communicate across the network.
Under CSMA/CD, a node will not send out a packet unless the network is
clear of traffic. If two nodes send out packets at the same time, a
collision occurs and the packets are lost. Then both nodes wait a random
amount of time and retransmit the packets. Any part of the network where
there is a possibility that packets from two or more nodes will interfere
with each other is considered to be part of the same collision domain. A
network with a large number of nodes on the same segment will often have a
lot of collisions and therefore a large collision domain.
While hubs provide an easy way to scale up and shorten the distance that
the packets must travel to get from one node to another, they do not break
up the actual network into discrete segments. That is where switches come
Imagine that each vehicle is a packet of data waiting for an opportunity
to continue on its trip.
Think of a hub as a four-way intersection where everyone has to stop. If
more than one car reaches the intersection at the same time, they have to
wait for their turn to proceed. But a switch is like a cloverleaf
intersection. Each car can take an exit ramp to get to their destination
without having to stop and wait for other traffic to go by. Now imagine
what this would be like with a dozen or even a hundred roads intersecting
at a single point. The amount of waiting and the potential for a collision
increases significantly if every car has to check all the other roads
before proceeding. But wouldn't it be amazing if you could take an exit
ramp from any one of those roads to the road of your choosing? That is
exactly what a switch does for network traffic!
A vital difference between a hub and a switch is that all the nodes
connected to a hub share the bandwidth among themselves while a device
connected to a switch port has the full bandwidth all to itself. For
example, if 10 nodes are communicating using a hub on a 10 Mbps network,
then each node may only get a portion of the 10 Mbps if other nodes on the
hub want to communicate as well. But with a switch, each node could
possibly communicate at the full 10 Mbps. Think about our road analogy. If
all of the traffic is coming to a common intersection, then it has to
share that intersection with everyone else. But a cloverleaf allows all of
the traffic to continue at full speed from one road to the next.
In a fully switched network, switches replace all the hubs of an Ethernet
network with a dedicated segment for every node. These segments connect to
a switch, which supports multiple dedicated segments (sometimes in the
hundreds). Since the only devices on each segment are the switch and the
node, the switch picks up every transmission before it reaches another
node. The switch then forwards the frame over the appropriate segment.
Since any segment contains only a single node, the frame only reaches the
intended recipient. This can allow many conversations to occur
simultaneously on a switched network.
Image courtesy Cisco Networks An example of a network using a switch.
Switching allows a network to maintain full-duplex Ethernet. Before
switching, Ethernet was half-duplex, which means that only one device on
the network can transmit at any given time. In a fully switched network,
nodes only communicate with the switch and never directly with each other.
Using our road analogy, half-duplex is similar to the problem of a single
lane, like when road construction closes down the use of one lane of a two
lane road. Traffic is trying to use the same lane in both directions. This
means that traffic coming one way must wait until traffic from the other
direction stops. Otherwise, they will hit head-on!
Fully switched networks employ either twisted pair or fiber optic cabling,
both of which use separate conductors for sending and receiving data. In
this type of environment, Ethernet nodes can forgo the collision detection
process and transmit at will, since they are the only potential devices
that can access the medium. In other words, traffic flowing in each
direction has a lane to itself. This allows nodes to transmit to the
switch at the same time the switch transmits to them, achieving a
collision free environment. Transmitting in both directions also can
effectively double the apparent speed of the network when two nodes are
exchanging information. For example, if the speed of the network is 10
Mbps then each node can transmit at 10Mbps at the same time.
A mixed network with two switches and three hubs.
Most networks are not fully switched because of the costs incurred in
replacing all of the hubs with switches. Instead, a combination of
switches and hubs are used to create an efficient yet cost-effective
network. For example, a company may have hubs connecting the computers in
each department and a switch connecting all of the department-level hubs