Lesson #8: Exploring Hubs, Bridges, Routers, & Switches
Management of larger networks and internetworking begins with "structured
wiring". With structured wiring, all of the network stations are physically
star wired to intelligent hubs. Intelligent
hubs are hubs that can be monitored and managed by network operators. This
combination of a star topology and intelligent hubs make troubleshooting and
fault isolation easier and faster because each endstation is attached to the
network on its own individual port, which means it can be monitored
easily and, if necessary, can be easily turned off. In addition,
structured wiring (T569) makes adding users to the network, moving them,
or making other physical changes on the network very simple. Since both
Ethernet and Token Ring networks can use twisted pair cable and can be
configured in a physical star topology, a structured wiring
architecture will support either network technology.
The hub is one of the most important elements of a LAN. It is a
central connection point for wiring the network, and all stations on the LAN
are linked to each other through the hub.
The cornerstone of the network is the intelligent hub, or
concentrator, which serves as the control point
for systems activity, management, and growth. By integrating any combination
of connectivity, internetworking, and management capabilities into
intelligent hubs, network managers can create the perfect
physical network infrastructure for their
environment.
The term concentrator is generally associated with 10BASE-T/100Base-TX
Ethernet networks, while the term Multistation Access Unit
(MAU) is used to refer to the Token Ring wiring hub. Just as these two
LAN technologies use different media access methods, concentrators and MAUs
perform different media access functions internally, but at one level they
perform the same function: They are both network wiring hubs.
A typical hub has multiple user ports to which computers and
peripheral devices such as servers are attached. Each port supports a
single 10BASE-T/100Base-TX twisted pair connection from a network station.
When an Ethernet frame is transmitted to the hub
by one station, it is repeated, or copied, over
onto all of the other ports of the hub. In this way, all of the stations
"see" every packet just as they do on a bus network, so even though each
station is connected to the hub with its own dedicated twisted pair cable, a
hub-based network is still a shared media LAN -- picture it as a LAN in a
box.
Intelligent hubs have been defined as
manageable hubs, meaning that each of the ports on the hub can be
configured, monitored, enabled, or disabled by a network operator from
a hub management console. Hub management can also include gathering
information on a variety of network parameters, such as the numbers of
packets that pass through the hub and each of its ports, what types of
packets they are, whether the packets contain errors, and how many
collisions have occurred. Each hub vendor has some type of management
package it sells with its products. These applications vary in how
much information they can gather, what commands can be issued, and how
the information is presented to the network operator.
Both hubs and MAUs come in three
configurations: standalone hubs, stackable hubs, and modular
hubs. Some products are combinations of the best configurations.
Standalone hubs are -- as the term implies -- single box-level
products with a number of ports. Standalone hubs usually include some
method of linking them to other standalone hubs -- either by
connecting them together with a length of 10BASE5 coaxial cable or
cascading them using twisted pair between individual ports on each hub.
Standalone hubs are usually the least expensive type
of hub and are often not managed. They are best suited for small,
independent workgroups, departments, or offices typically with fewer
than 12 users per LAN.
Network A illustrates four 100BASE-TX hubs connected
together by a single cable. This cable could be a coaxial or an optical
fiber cable. All of the hubs are part of a single LAN.
Network B illustrates two 100BASE-TX hubs cascaded. Here the
cable connecting the two ports is unshielded twisted pair (CAT5) wire. All
of the hubs that are cascaded in this fashion are part of a single LAN.
Stackable hubs look and act like standalone hubs except that several
of them can be stacked or connected together, usually by short lengths
of cable. When they are linked together, they act like a modular hub
in that they can be managed as a single unit. One manageable hub,
used within a stack, can typically provide the management for all
other hubs in the stack. These hubs are ideal when an organization
wants to start with a minimal investment but knows that its LAN will
grow. By utilizing stackable hubs, an organization doesn't need to
invest in a large chassis, which may only have one or two cards in it
for a considerable length of time until more are needed.
Each hub usually represents a single LAN. In many organizations it is
desirable to interconnect all of the LANs, which means linking hubs in some
way. One way to link hubs is to use an interrepeater
link or cascaded segment. This type of connection simply repeats all of
the packets from one hub to the other hub it is linked to, so that in effect the
two hubs are part of the same LAN.
Modular hubs are popular in networks because
they are easily expanded and always have a management option. A
modular hub starts with a chassis, or card cage, with multiple card
slots, each of which accepts a communications card, or module. Each
module acts like a standalone hub; when the communications modules are
placed in the card slots in the chassis, they connect to a
communications backplane that links them together so that a station
connected to a port on one module can easily communicate with a
station on another module.
Modular hubs provide a central point where multiple concentrators located
in different wiring closets can be united into a LAN or WAN. The modular hub
can be equipped with a wide variety of connectivity and network management
modules designed to provide a customized solution for the creation of
enterprise-wide LANs and WANs.
Modular hubs typically range in size from four to 14 slots, so the
network can be easily expanded. Typically, several slots in a modular
hub will be filled with 10BASE-T Ethernet modules. For instance, with
10 modules, each supporting 12 users, a single hub could support 120
users over 10BASE-T. The modules are linked by the high-speed
backplane, which can also be used to connect the communications
modules to a management module that manages all of the cards in the
chassis. In addition to using one management module for a large
number of ports, all of the modules share a common power supply.
Another advantage of some modular hubs is that Ethernet, Token Ring,
and even FDDI communications modules can be placed in the same
chassis, using the same common power supplies.
The term internetworking refers to linking individual LANs
together to form a single internetwork. This internetwork is
sometimes called an enterprise network because it interconnects all of
the computer networks throughout the entire enterprise. Workgroup
LANs on different floors of a building or in separate buildings on a
business campus can be linked together so that all of the computing
systems at that site are interconnected. Geographically distant
company sites can also be tied together in the enterprise-wide
internetwork.
An individual LAN is subject to limits on such things as how far it
can extend, how many stations can be connected to it, how fast data
can be transmitted between stations, and how much traffic it can
support. If a company wants to go beyond those limits -- link more
stations than that LAN can support, for example -- it must install
another LAN and connect the two together in an internetwork.
There are two main reasons for implementing multiple LANs and
internetworking them. One is to extend the geographic coverage of the
network beyond what a single LAN can support -- to multiple floors in
a building, to nearby buildings, and to remote sites. The other key
reason for creating internetworks is to share traffic loads between
more than one LAN. A single LAN can only support so much traffic. If
the load increases beyond its carrying capacity, users will suffer
reduced throughput and much of the productivity achieved by installing
the LAN in the first place will be lost. One way to handle heavy
network traffic is to divide it between multiple internetworked LANs.
There are three major types of devices used for internetworking:
bridges, routers, and switches. Today the most
commonly used internetworking devices are high-speed routers, especially
in wide area internetworks linking geographically remote sites. But
routers are also heavily used in building and campus internetworks.
Bridges have also been popular, even though they offer less
functionality than routers, because they are less expensive to
purchase, implement, and maintain.
LAN switches are a new class of internetworking device, and many
people believe that switched internetworks will become the most common
design for linking building and campus LANs in the future. Today's
LAN switches and switching hubs are the first steps on a migration
path to something called asynchronous transfer mode
(ATM) switching, an emerging technology that will be widely
implemented in both LANs and wide area networks in the coming years.
Bridges and routers are both special kinds of devices used for
internetworking LANs -- that is, linking different LANs or LAN
segments together. Many organizations have LANs located at sites that
are geographically distant from each other. Routers were originally
designed to allow users to connect these remote LANs across a wide
area network, but bridges can also be used for this purpose. By
placing routers or bridges on LANs at two distant sites and connecting
them with a telecommunications link, a user on one of the LANs can
access resources on the other LAN as if those resources were local.
Bridges and routers link adjacent LANs. Local bridges and routers
were first used to extend the area a network could cover by allowing
users to connect two adjacent LANs to maintain performance by reducing
the number of users per segment. Both Ethernet and Token Ring specify
limits on maximum distances between workstations and hubs, hubs and
hubs, and a maximum number of stations that can be connected to a
single LAN. To provide network connectivity for more people, or
extend it to cover a larger area, it is sometimes necessary to link
two different LANs or LAN segments. Bridges and routers can both
provide this function.
Today, however, these internetworking devices are also increasingly
used to segment LANs to maintain performance by
reducing the number of users per segment. When users on a single LAN begin
to experience slower response times, the culprit is often congestion: too
much traffic on the LAN. One method users are employing to deal with
this is to break large LANs with many users into smaller LANs, each
with fewer users. Adding new network users may require the
organization to create new LANs to accommodate them. Implementing new
applications on an existing LAN can create so much incremental traffic
that the organization may need to break the LAN into smaller LANs
segments to maintain acceptable performance levels.
In all of these cases, it is still critical that users on one LAN
be able to reach resources on other LANs within the organization. But
the LANs must be connected in such a way that packets are
filtered, so that only those packets that need to pass from one
LAN to another are forwarded across the link. This keeps the packets
sent between two stations on any one LAN from crossing over onto the
other LANs and thereby congesting them. A general rule of thumb
suggests that 80 percent of the packets transmitted on a typical
workgroup or department LAN are destined for stations on that LAN.
Both bridges and routers can be used to segment LANs.
Bridges are the simpler, and often less expensive,
type of device. Bridges filter packets between LANs by making a
simple forward/don't forward decision on each packet they receive from
any of the networks they are connected to. Filtering is done based on
the destination address of the packet. If a packet's destination is a
station on the same segment where it originated, it is not forwarded.
If it is destined for a station on another LAN, it is connected to a
different bridge port and forwarded to that port. Many bridges today
filter and forward packets with very little delay, making them good
for large traffic volumes.
Routers are more complex internetworking devices and
are also typically more expensive than bridges. They use Network
Layer Protocol Information within each packet to route it from one LAN
to another. This means that a router must be able to recognize all of
the different Network Layer Protocols that may be used on the networks
it is linking together. This is where the term multiprotocol router
comes from -- a device that can route using many different protocols.
Routers communicate with each other and share information that allows
them to determine the best route through a complex internetwork that
links many LANs.
Switches are another type of device used to link several separate
LANs and provide packet filtering between them. A LAN switch is a
device with multiple ports, each of which can support a single
endstation or an entire Ethernet or Token Ring LAN. With a different
LAN connected to each of the switch's ports, it can switch packets
between LANs as needed. In effect, it acts like a very fast multiport
bridge -- packets are filtered by the switch based on the destination
address.
Switches are used to increase performance on an organization's
network by segmenting large networks into many smaller, less congested
LANs, while still providing necessary interconnectivity between them.
Switches increase network performance by providing each port with
dedicated bandwidth, without requiring users to change any existing
equipment, such as NICs, hubs, wiring, or any routers or bridges that
are currently in place. Switches can also support numerous
transmissions simultaneously.
Deploying technology called dedicated LANs
is another advantage of using switches. Each port on an Fast Ethernet switch
supports a dedicated 100 Mbps Ethernet LAN. Usually, these LANs comprise
multiple stations linked to a 100BASE-TX hub, but it is also possible to
connect a single high-performance station, such as a server, to a switch port.
Using LAN switches allows a network designer to create several small
network segments. These smaller segments mean that fewer stations are
competing for bandwidth, thereby diminishing network congestion.
In this case, that one station has an uncontested 100 Mbps Fast Ethernet
LAN all to itself. Packets forwarded over it from other ports on the switch
will never produce any collisions because there are no other stations on the
LAN at that port.
As was noted earlier, LAN switching is a relatively new technology.
Today's switching devices switch relatively large, variable-length LAN
packets between different local area networks. ATM is another type of
switching technology that switches small, fixed-length cells
containing data. ATM networks can be run at much higher data rates
than today's LANs. Eventually, they will be used to carry voice,
video, and multimedia traffic, as well as computer-generated data over
both short and long distances. ATM will be one of the dominant
enterprise networking technologies of the future, and many companies
are beginning to develop strategies to incorporate ATM in their
existing LANs and LAN internetworks.
LAN technology is evolving. In the early 1980s LANs were strictly
local area networks, linking small groups of computers in company
departments. As workgroup LANs proliferated over the past 10 years,
users began connecting them to form internetworks, first with bridges
and later with routers. Today's networks typically comprise a
combination of workgroup and campus hubs, bridges, and routers.
Switches are also beginning to become more prevalent.
The next few years will see networks evolve to include more
sophisticated LAN switches and switching hubs. They will be designed
using several different types of components, both old and new.
Ethernet and Token Ring LANs will be built with stackable workgroup
hubs, which, in turn, will be interconnected by larger modular hubs
that may incorporate LAN switching functionality. Large networks will
include another layer of consolidation with network
center hubs linking workgroup hubs and switches. Routers will
continue to be used as gateways to the wide area network linking other
buildings and remote sites.
For networks to deliver the performance today's users require,
their many components must work together to deliver seamless
connectivity between all of the users and computing systems throughout
the enterprise. Flexibility to grow, power to support applications,
and seamless connectivity are what users expect in the products they
choose to build LANs and enterprise networks.
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Key Terms
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Lesson #9 - Wireless Networking
