While repeaters allow LANs
to extend beyond normal distance limitations, they still limit the
number of nodes that can be supported. Bridges and switches, however,
allow LANs to grow significantly larger by virtue of their ability to
support full Ethernet segments on each port. Additionally, bridges and
switches selectively filter network traffic to only those packets needed
on each segment - this significantly increases throughput on each
segment and on the overall network. By providing better performance and
more flexibility for network topologies, bridges and switches will
continue to gain popularity among network managers.
Bridges
The function of a bridge is
to connect separate networks together. Bridges connect different
networks types (such as Ethernet and Fast Ethernet) or networks of the
same type. Bridges map the Ethernet addresses of the nodes residing on
each network segment and allow only necessary traffic to pass through
the bridge. When a packet is received by the bridge, the bridge
determines the destination and source segments. If the segments are the
same, the packet is dropped ("filtered"); if the segments are different,
then the packet is "forwarded" to the correct segment. Additionally,
bridges do not forward bad or misaligned packets.
Bridges are also called
"store-and-forward" devices because they look at the whole Ethernet
packet before making filtering or forwarding decisions. Filtering
packets, and regenerating forwarded packets enables bridging technology
to split a network into separate collision domains. This allows for
greater distances and more repeaters to be used in the total network
design.
Most bridges are
self-learning task bridges; they determine the user Ethernet addresses
on the segment by building a table as packets are passed through the
network. This self-learning capability, however, dramatically raises the
potential of network loops in networks that have many bridges. A loop
presents conflicting information on which segment a specific address is
located and forces the device to forward all traffic. The Spanning Tree
Algorithm is a software standard (found in the IEEE 802.1d
specification) for describing how switches and bridges can communicate
to avoid network loops.
Ethernet Switches
Ethernet switches are an
expansion of the concept in Ethernet bridging. LAN switches can link
four, six, ten or more networks together, and have two basic
architectures: cut-through and store-and-forward. In the past,
cut-through switches were faster because they examined the packet
destination address only before forwarding it on to its destination
segment. A store-and-forward switch, on the other hand, accepts and
analyzes the entire packet before forwarding it to its destination.
It takes more time to
examine the entire packet, but it allows the switch to catch certain
packet errors and keep them from propagating through the network. Today,
the speed of store-and-forward switches has caught up with cut-through
switches so the difference between the two is minimal. Also, there are a
large number of hybrid switches available that mix both cut-through and
store-and-forward architectures.
Both cut-through and
store-and-forward switches separate a network into collision domains,
allowing network design rules to be extended. Each of the segments
attached to an Ethernet switch has a full 10 Mbps of bandwidth shared by
fewer users, which results in better performance (as opposed to hubs
that only allow bandwidth sharing from a single Ethernet). Newer
switches today offer high-speed links, either FDDI, Fast Ethernet or
ATM. These are used to link switches together or give added bandwidth to
high-traffic servers. A network composed of a number of switches linked
together via uplinks is termed a "collapsed backbone" network.
Routers
Routers filter out network
traffic by specific protocol rather than by packet address. Routers also
divide networks logically instead of physically. An IP router can divide
a network into various subnets so that only traffic destined for
particular IP addresses can pass between segments. Network speed often
decreases due to this type of intelligent forwarding. Such filtering
takes more time than that exercised in a switch or bridge, which only
looks at the Ethernet address. However, in more complex networks,
overall efficiency is improved by using routers.
Network Design Criteria
Ethernets and Fast
Ethernets have design rules that must be followed in order to function
correctly. Maximum number of nodes, number of repeaters and maximum
segment distances are defined by the electrical and mechanical design
properties of each type of Ethernet and Fast Ethernet media.
A network using repeaters,
for instance, functions with the timing constraints of Ethernet.
Although electrical signals on the Ethernet media travel near the speed
of light, it still takes a finite time for the signal to travel from one
end of a large Ethernet to another. The Ethernet standard assumes it
will take roughly 50 microseconds for a signal to reach its destination.
Ethernet is subject to the
"5-4-3" rule of repeater placement: the network can only have five
segments connected; it can only use four repeaters; and of the five
segments, only three can have users attached to them; the other two must
be inter-repeater links.
If the design of the
network violates these repeater and placement rules, then timing
guidelines will not be met and the sending station will resend that
packet. This can lead to lost packets and excessive resent packets,
which can slow network performance and create trouble for applications.
Fast Ethernet has modified repeater rules, since the minimum packet size
takes less time to transmit than regular Ethernet. The length of the
network links allows for a fewer number of repeaters. In Fast Ethernet
networks, there are two classes of repeaters. Class I repeaters have a
latency of 0.7 microseconds or less and are limited to one repeater per
network. Class II repeaters have a latency of 0.46 microseconds or less
and are limited to two repeaters per network.
When conditions require
greater distances or an increase in the number of nodes/repeaters, then
a bridge, router or switch can be used to connect multiple networks
together. These devices join two or more separate networks, allowing
network design criteria to be restored. Switches allow network designers
to build large networks that function well. The reduction in costs of
bridges and switches reduces the impact of repeater rules on network
design.
Each network connected via
one of these devices is referred to as a separate collision domain in
the overall network.
When Ethernets Become Too Slow
As more users are added to
a shared network or as applications requiring more data are added,
performance deteriorates. This is because all users on a shared network
are competitors for the Ethernet bus. On a moderately loaded 10Mbps
Ethernet network being shared by 30-50 users, that network will only
sustain throughput in the neighborhood of 2.5Mbps after accounting for
packet overhead, interpacket gaps and collisions.
Increasing the number of
users (and therefore packet transmissions) creates a higher collision
potential. Collisions occur when two or more nodes attempt to send
information at the same time - when they realize that a collision has
occurred, each node shuts off for a random time before attempting
another transmission. With shared Ethernet, the likelihood of collision
increases as more nodes are added to the shared collision domain of the
shared Ethernet. One of the steps to alleviate this problem is to
segment traffic with a bridge or switch. A switch can replace a hub and
improve network performance. For example, an eight-port switch can
support eight Ethernets, each running at a full 10 Mbps. Another option
is to dedicate one or more of these switched ports to a high traffic
device such as a file server.
Multimedia and video
applications demand as much as 1.5 Mbps of continuous bandwidth - as we
have seen above, a single such user can rarely obtain this bandwidth if
they share an average 10Mbps network with 30-50 people. The video will
also look disjointed or "clunky" if the data rate is not sustained.
Greater throughput is required, therefore, to support this application.
When added to the network, Ethernet switches provide a number of
enhancements over shared networks. Foremost is the ability to divide
networks into smaller and faster segments. Ethernet switches examine
each packet, determine where that packet is destined and then forward
that packet to only those ports to which the packet needs to go. Modern
switches are able to do all these tasks at "wire speed," that is, without
delay.
Aside from deciding when to
forward or when to filter the packet, Ethernet switches also completely
regenerate the Ethernet packet. This regeneration and re-timing allows
each port on a switch to be treated as a complete Ethernet segment,
capable of supporting the full length of cable along with all of the
repeater restrictions.
Additionally, bad packets
are identified by Ethernet switches and immediately dropped from any
future transmission. This "cleansing" activity keeps problems isolated
to a single segment and keeps them from disrupting other network
activity. This aspect of switching is extremely important in a network
environment where hardware failures are to be anticipated. Full duplex
doubles the bandwidth on a link, providing 20Mbps for Ethernet and
200Mbps for Fast Ethernet, and is another method used to increase
bandwidth to dedicated workstations or servers. To use full duplex,
special network interface cards are installed in the server or
workstation, and the switch is programmed to support full duplex
operation.
Implementing Fast Ethernet
to increase performance is the next logical step. Higher traffic devices
can be connected to switches or each other via 100 Mbps Fast Ethernet, a
great increase of bandwidth. Many switches are designed with this in
mind, and have Fast Ethernet uplinks available for connection to a file
server or other switches. Eventually, Fast Ethernet can be deployed to
the users' desktops by equipping all computers with Fast Ethernet
network interface cards and using Fast Ethernet switches and repeaters.
With an understanding of
the underlying technologies and products in use in Ethernet networks, we
can now progress to a discussion of some of the most popular real world
applications.
Next: Ethernet
Tutorial Part 3: Sharing Devices
