Switches
Like hubs,
switches
are the connectivity
points of an Ethernet network. Devices connect to switches via
twisted-pair cabling, one cable for each device. The difference
between hubs and switches is in how the devices deal with the data
that they receive. Whereas a hub forwards the data it receives to
all of the ports on the device, a switch forwards it only to the
port that connects to the destination device. It does this by
learning
the MAC address of the
devices attached to it, and then by matching the destination MAC
address in the data it receives. Figure 3.1 shows how a switch
works.
By forwarding data only to the connection that
should receive it, the switch can improve network performance in
two ways. First, by creating a direct
path
between two devices and
controlling their communication, it can greatly reduce the number
of collisions on the network. As you might recall, collisions occur
on Ethernet networks when two devices attempt to transmit at
exactly the same time. In addition, the lack of collisions enables
switches to communicate with devices in full-duplex mode. In a
full-duplex configuration, devices can send and receive data from
the switch at the same time. Contrast this with half-duplex
communication, in which communication can occur in only one
direction at a time.
Full-duplex
transmission speeds are double
that of a standard, half-duplex, connection. So, a 10Mbps
connection becomes 20Mbps, and a 100Mbps connection becomes
200Mbps.
The net result of these measures is that
switches can offer significant performance improvements over
hub-based networks, particularly when network use is high.
Irrespective of whether a connection is at full
or half duplex, the method of switching dictates how the switch
deals with the data it receives. The following is a brief
explanation of each method:
-
Cut-through
In a cut-through switching environment, the packet begins to be
forwarded as soon as it is received. This method is very fast, but
creates the possibility of errors being propagated through the
network, as there is no error checking.
-
Store-and-forward
Unlike cut-through, in a
store-and-forward switching environment, the entire packet is
received and error checked before being forwarded. The upside of
this method is that errors are not propagated through the network.
The downside is that the error checking process takes a relatively
long time, and store-and-forward switching is considerably slower
as a result.
-
FragmentFree
To take advantage of the error checking of store-and-forward
switching, but still offer performance levels nearing that of
cut-through switching, FragmentFree switching can be used. In a
FragmentFree-switching environment, enough of the packet is read so
that the switch can determine whether the packet has been involved
in a collision. As soon as the collision status has been
determined, the packet is forwarded.
Hub and Switch Cabling
In addition to acting as a connection point for
network devices, hubs and switches can also be connected to create
larger networks. This connection can be achieved through standard
ports with a special cable or by using special ports with a
standard cable.
The ports on a hub to which computer systems are
attached are called
Medium Dependent
Interface-Crossed (
MDI
-X).
The crossed designation is derived from the fact that two of the
wires within the connection are crossed so that the send signal
wire on one device becomes the receive signal of the other. Because
the ports are crossed internally, a standard or
straight-through
cable can be used to connect
devices.
Another type of port, called a
Medium Dependent Interface (
MDI
)
port, is often included on a hub
or switch to facilitate the connection of two switches or hubs.
Because the hubs or switches are designed to see each other as
simply an extension of the network, there is no need for the signal
to be crossed. If a hub or switch does not have an MDI port, hubs
or switches can be connected by using a
crossover
cable
between two MDI-X ports. The crossover cable serves to uncross the
internal
crossing
. You can see diagrams of the cable pinouts for
both a straight-through and crossover cable in Figures 3.2 and 3.3,
respectively.
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In a crossover cable, wires 1 and 3 and wires 2
and 6 are crossed.
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