diameter of an Ethernet network can be increased using repeaters as long
as the network diameter does not exceed the collision domain of Ethernet.
All Ethernet nodes must be able to recognize the occurrence of a collision
regardless of the physical location of the nodes since the detection of
collisions is fundamental in the manner Ethernet arbitrates media access.
In this lesson, the concept of switching will be introduced as an
alternative to the deployment of repeaters. Switches can not only increase
the overall network diameter, but will improve the performance of Ethernet
networks as well.
(No part of this article may be reproduced without the written consent of the Industrial Ethernet University.)
Although from the outside a switching hub looks very much like a
repeating hub, they are from different classes of equipment. If you study
the OSI Communications Model, you will notice seven distinct layers
corresponding to different communication services.
At the lowest layer you have
the physical layer which is concerned with the actual signals on the
medium that represent data. These signals are called symbols and repeaters
or repeating hubs receive these symbols and recondition them when
extending networks. References such as 10BASE5 and 10BASE-T are physical
Above the physical layer is the data link layer which handles the
actual transmission and reception of frames sent and received over the
physical layer. Issues such as station addressing (MAC or medium access
control), framing of the data and error detection are handled by the data
link layer. The IEEE 802.3 standard is basically a data link standard
although references to physical layer standards are included as well.
Bridges operate at the data link layer. A bridge and a switch are one in
Above the data link layer is the network layer which addresses the
issues of transferring data, not over just one data link, but over
multiple data links. This is classified as internetworking with the
Internet Protocol (IP) being the most popular internetworking protocol.
Routers are used to direct traffic between multiple data links and the
transmission units are called packets. Switches do not commonly operate at
this layer but there is such a thing as a layer 3 switch. This is actually
a router with some switching functionality.
In terms of hardware, the next
layer of interest is the application layer seven. This is where the
gateways reside when it is necessary to interconnect dissimilar networks
and dissimilar protocols. Gateways are aware of the actual application
being run while all other devices such as repeaters, bridges and routers
are not. We will concentrate only on one class of device called the
BRIDGE (SWITCH) CONSTRUCTION
A switch is a bridge and the
terms will be used interchangeably. The original bridges were two port
devices interconnecting two similar data links to form one larger data
link. If this can be accomplished without disruption, the bridge is
considered a transparent bridge since communication within a data link or
between data links appears the same. You may think that we are describing
a router but we are not. A router would consider each data link as an
actual network with a corresponding network address. A bridge considers
each individual data link as part of one larger data link or one network.
The concept of network addressing is not used and individual station
addresses (MAC addresses) are not duplicated among the various data links.
Unlike the traditional bridge with two ports, the switch has several ports
and is usually referred to as a switching hub or just a switch.
repeating hub, a switch has basically the same Ethernet interface on each
of its ports as found on an Ethernet host adapter. That is because each
port must function just like another Ethernet device. It must be able to
receive and decode Ethernet frames and test for frame integrity as well as
assemble and transmit Ethernet frames. However, each port does not
necessarily require its own MAC address as would be required by an
Ethernet host adapter. Each switch port functions in promiscuous mode by
receiving all frames on its port independent of destination MAC address.
During transmissions, the Ethernet port masquerades as the originating
device by assuming its source address. Therefore, each port on the
switching hub does not require its own MAC address unless bridge
addressing is required (the spanning tree algorithm requires bridge
By having an Ethernet interface on each port, the Ethernet
collision domain terminates at the switch port. With a repeating hub, the
complete hub is part of the collision domain. By having a switch, the
effective network diameter can double with the addition of one switch.
This is because the network can be broken into two distinct data links.
This is one benefit of switches. The effective network diameter can be
increased with the addition of switches. This is especially important at
100 Mbps since the collision domain is only 205 meters wide for
Another difference between a
repeating hub and a switch is that the repeating hub must operate at only
one speed- either 10 Mbps or 100 Mbps. A switching hub can have
multi-speed ports which can adjust to the capabilities of the device
attached to its port. This is called auto-negotiation and different speeds
on different ports are allowed. Some switches have fixed low speed ports
(10 Mbps) and one or more high-speed ports (100 Mbps) for connection to
servers where most of the traffic will be experienced.
By terminating collision
domains at each of its ports, a switch effectively segments the network
into separate collision domains. If only one device is attached to a
switch port (an Ethernet host adapter or another switch port), this is
called microsegmentation. Under these circumstances full-duplex operation
is possible yielding no collisions. However, if a shared Ethernet
collision domain is present on a switch port (multiple host adapters and a
repeating hub), only half-duplex operation is allowed and the switch port
must conform to Ethernet's medium arbitration rules.