Last month, I discussed Ethernet routers and some of the concepts
behind routing. This month, we will look at a slightly different piece
of network hardware, the Ethernet switch. I use the term slightly
because, operationally, the current generations of Ethernet switches
have a great deal in common with routers.
LAN hardware and the OSI model
In order to fully understand how switches work, it is useful to know
where the basic network components hubs, bridges and routers fit with
the Open Standards Interconnect (OSI) model. The OSI not only provides
a method for PCs to seamlessly communicate with each other, but it also
helps us recognize the role that various network devices play in
transferring data. You will recall that the OSI model comprises seven
layers; each layer performs specific functions and subsequently passes
data to the next layer up/down the line. It is reasonable to assume
devices that use a higher layer are more intelligent and require more
processing capabilities. Most network hardware is designed to operate
at a specific layer, or, in some cases, more than one. For example,
network interface cards (NIC), or something as simple as the network
cabling, operate at Layer 1, the physical layer, which deals with the
communications hardware. Active hubs also work at Layer 1, since their
main function is to regenerate signals carried over the cabling. This
regeneration can also make it possible to operate different connection
speeds, i.e. 10Mb/s and 100Mb/s within the same network segment.
The OSI model and the layers at which some devices operate.
Layer 2 of the OSI model, the Data Link Layer, assures that data
moves information reliably between two points. You see layer 2 activity
on the flashing lamps typically found on most network devices. Bridges
are an example of a network device operating at Layer 2. The primary
function of the bridge is to segment traffic between two different
networks by filtering data based on the Media Access Control (MAC)
protocol ID number. Bridges can also have more sophisticated features,
such as the ability to talk with other bridges and dynamically route
traffic for the most efficient traffic flow.
Layer 3, the Network Layer, provides the ability to route data based
on Internet Protocol (IP) addressing. Router operations (discussed in
last month's column) are based primarily on Layer 3; however, routers
can also use Layers 1 and 2, depending on the specific features. Next
generation routers will use Layers 4 and higher; however, the power and
complexity required to implement these devices will be sizable.
Hubs and switches
Hubs form the central terminus in a network based on a Star
topology. The earliest hubs were called passive hubs. They were
typically used with coaxial media and were nothing more than a constant
impedance resistive splitter. However, as LANs moved toward using
telephone type wiring and connectors, losses through the cabling were
an issue. Hubs were developed that electronically regenerated received
data, thus enabling longer lengths of cabling. These active hubs became
the standard device used in an Ethernet network.
Switches look and feel like a basic network hub, but that's where
the similarity ends. In terms of functionality, the switch resembles a
high performance bridge. Like the bridge, switches operate at Layers 1
and 2, permit the isolation of LAN traffic and can establish
full-duplex connections with other switches. Perhaps the easiest way to
visualize how a switch differs from a hub is that the hub simply
receives data and regenerates that data to all ports simultaneously,
while a switch sets up each port as an individual path for data. For
example, an eight-port switch will permit up to four separate data
paths to be established. Switches are constructed using a
high-performance backplane designed to support throughputs in excess of
100Mb/s or higher.
In a broadcast environment, where the facility supports a mixed
variety of data traffic such as streaming audio and simple file
transfers, network performance would materially improve simply by
replacing a hub with a switch. Switches, however, do not provide any
meaningful level of security as do devices operating at higher layers,
such as routers.
Varieties of switches
Switches are intelligent devices that read the incoming packets and
make the decision where to send the data. The current generation of
switch is based on either of two technologies store-and-forward or
Switches using the store-and-forward method require that an entire
packet be received before it is sent to the appropriate port. This
permits very efficient translation between connections of different
speeds. Because data is stored before being sent, there is some delay,
or latency, developed. This latency is typically not noticeable in most
10 or 100Mb/s LANs with moderate data traffic.
If latency is an issue, than a switch using the cut-through method
would be advised. Cut-through switches pass packets through immediately
after reading the destination information. In many cases, data has been
passed before the packet has been fully received. Cut-through switches
are pricier than store-and-forward, but are recommended for larger
high-performance LANs or LAN backbones above 100Mb/s.
Most common switches at the local computer emporium are of the
store-and-forward variety. You'll probably find that it is harder to
locate basic hubs anymore, as the price of consumer level switches have
fallen below the $100 price-point.
Because switches operate at layer 2, they permit the passing of
traffic from a variety of network protocols, such as IP, IPX, etc. This
is particularly important if you mix network operating systems, such as
Netware (4.x and lower) and Windows. Some switches permit the ability
to filter data based on criteria such as protocol.
Migrating to switched a LAN environment provides a significant
increase in network performance for a relatively low investment.
Switches can also provide effective load balancing on LANs with mixed
speed connections. Data collisions that would bring Ethernet networks
to a standstill can be minimized or eliminated through the use of
Kevin McNamara, BE Radio's consultant on computer technology, is
president of Applied Wireless Inc., New Market, MD.
All of the Networks articles have been approved by the SBE
Certification Committee as suitable study material that may assist your
preparation for the SBE Certified Broadcast Networking Technologist
exam. Contact the SBE at (317) 846-9000 or go to www.sbe.org for more
information on SBE Certification.