Understanding LAN Switching
Broadcasts Consume Bandwidth
Now, in terms of broadcast, it's relatively
easy to broadcast in a network, and that's a transmission
mechanism that many different protocols use to communicate
certain information, such as address resolution, for example.Address
resolution is something that all protocols need to do in order
to map Layer 2 MAC addresses up to logical layer, or Layer
3, addresses. For example, in an IP network we do something
known as an ARP, an Address Resolution Protocol.And this allows
us to map Layer 3 IP addresses down to Layer 2 MAC-layer addresses.
Also, in terms of distributing routing protocol information,
we do this by way of broadcasting, and also some key network
services in our networks rely on broadcast mechanisms as well.
And it doesn't really matter what our protocol
is, whether it's AppleTalk or Novell IPX, or TCP IP, for example,
all of these different Layer 3 protocols rely on the broadcast
mechanism. So, in other words, all of these protocols produce
broadcast traffic in a network.
Broadcasts Consume Processor Performance
Now, in addition to consuming bandwidth on
the network, another by-product of broadcast traffic in the
network is that they consume CPU cycles as well.Since broadcast
traffic is sent out and received by all stations on the network,
that means that we must interrupt the CPU of all stations
connected to the network.So here in this diagram you see the
results of a study that was performed with several different
CPUs on a network. And it shows you the relative level of
CPU degradation as the number of broadcasts on a network increases.
So you can see, we did this study based on
a SPARC2 CPU, a SPARC5 CPU and also a Pentium CPU. And as
the number of broadcasts increased, the amount of CPU cycles
consumed, simply by processing and listening to that broadcast
traffic, increased dramatically.So, the other thing we need
to recognize is that a lot of times the broadcast traffic
in our network is not needed by the stations that receive
it.So what we have then in shared LAN technologies is our
broadcast traffic running throughout the network, needlessly
consuming bandwidth, and needlessly consuming CPU cycles.
So hubs are introduced into the network as
a better way to scale our thinand thick Ethernet networks.
It's important to remember, though, that these are still shared
Ethernet networks, even though we're using hubs.
Basically what we have is an individual desktop
connection for each individual workstation or server in the
network, and this allows us to centralize all of our cabling
back to a wiring closet for example. There are still security
issues here, though.It's still relatively easy to tap in and
monitor a network by way of a hub. In fact it's even easier
to do that because all of the resources are generally located
centrally.If we need to scale this type of network we're going
to rely on routers to scale this network beyond the workgroup,
It's makes adds, moves and changes easier
because we can simply go to the wiring closet and move cables
around, but we'll see later on with LAN switching that it's
even easier with LAN switching.Also, in terms of our workgroups,
in a hub or concentrator based network, the workgroups are
determined simply by the physical hub that we plug into. And
once again we'll see later on with LAN switching how we can
improve this as well.