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HOME REVIEWS CONTACT ABOUT March 14,
2009
Computer Network Routers, Hubs, and Switches
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When
computer networks are working well, which we hope is most of the time, the inner
workings of the system modules are transparent to the average user. The most common components on a computer
network, not counting cables, are "routers", "hubs", and
"switches". Many of these
modules can be similar cosmetically.
Each assembly typically has Ethernet connectors (RJ45, which looks like
an oversized telephone plug) and LED indicators. However, these modules function differently,
and it is important to understand the differences.
Modern
network hardware operates on the "Open System Interconnection" (OSI)
standard. This standard defines how
communications on a network should be implemented. By conforming to this standard, modules from
different manufacturers can coexist on the same network. Wireless networks (WiFi) and the 802.11X standard
are an additional subset of network systems.
When a
message is sent between computers, it is broken into parts. At the base level, the message is reduced to
"1" and "0" bits.
The next level is a group of bits called a "frame". A frame contains its control information,
including target address and error detection. The next level is a group of
frames called a "packet". The
terms frame and packet are sometimes used interchangeably. If a message is sent on a complex network
like the Internet, some of the packets may take a different path, and be
recombined at the destination.
Error
detection is used with both frames and packets.
The most common routine is called "Cyclic Redundancy Check"
(CRC). CRC sums of all the "1"s in the frame or
packet. This number is stored as a hexadecimal value at the end of the
frame/packet. At the receiving end, the
process is repeated. If the two
hexadecimal values agree, the test passes.
If not, the receiving device requests that the sending device
resend. Most of this functionality is
performed by integrated circuits (chips) inside the network modules.
The Hub
The
network hub operates on the first layer of the OSI standard, called the
"physical layer". The hub is
the most simple of the three modules. A hub
is not aware of the contents of the message that is processed; it handles the
message as bits. It simply records the
signal, and rebroadcast it to all, including back to the port that sent the
message. An "active" hub will
clean the electrical signal of noise and amplify the signal before
rebroadcast. A "passive" hub
does not amplify the received signal; it merely receives a signal, and
rebroadcast the signal as received to each port. Hubs are sometimes used to link multiple
computers with a printer.
The Network Switch
The
network switch operates on the second layer of the OSI standard, called the
"data link layer". The network
switch, as indicated by its name, switches signal paths, so that a message
frame goes to a specific destination. A switch
will improve a networks performance, especially on networks with many
computers. A switch has enough on-board
intelligence to remember the path to each destination. The network switch handles a message in
frames.
When you
connect a computer to a network switch, the switch will record the Media Access
Control or “MAC” address of the computer's network interface card (NIC). This is called address protocol, or
"ARP". When a frame is
received intended for a specific computer, the switch sends the frame only to
that computer. By preventing paths of
the network from being utilized by every frame, network resources are
conserved. Computer A can send a frame
to computer B, while simultaneously, computer C is sending a frame to computer
D.
The Network Router
The
network router operates on the third layer of the OSI standard, called the
"network layer". A router's
name is also indicative of its role.
Routers have some of the same capabilities as switches, but routers are
most often used to connect two or more networks. For example, a router could be used to
connect a wireless network with a conventional local area network (LAN). Another common use is to connect a LAN with
the Internet (a "wide-area network", or "WAN"). In this role, the router uses "Network
Address Translation" (NAT) so that all of the computers attached to the
LAN can share a single IP address. A
network router handles the message in packets.
A router uses the IP addresses in the packets to route them between
multiple networks.
A
personal computer can be configured to handle the function of a router if it is
equipped with router software and two or more network interface cards
(NIC). A separate NIC is needed for each
network.
A router
is capable of advanced functions, including serving as DHCP (domain host
control protocol) server and Firewall. A
Firewall protects computers from potential hazards from the other computers
outside the network. Linking multiple
networks often requires the conversion of protocols.
A router
is an extremely diverse classification.
Routers may provide connectivity inside offices, between different
locations, and between businesses and the Internet. The largest routers connect
Internet service providers, are used in very large business networks, or
connect a business with a satellite link to a distant corporate locale. Advanced routers are powerful computers,
complete with microprocessors. Very sophisticated routers are used by the
Internet to manage the network traffic most efficiently.
A router
maintains a table called “routing information base” (RIB) that tracks
information about the available routes. The RIB can be static (manually
determined by a network administrator) or dynamic (continually updated based on
changing conditions). A RIB is simple on a small LAN, but can be extremely
complex in the very large routers used on the Internet.
Summary
There
are more types of network modules than the three discussed here, and there can
be significant overlap in roles. For
example, an "intelligent hub" can have many of the characteristics of
a network switch. Wireless networks
(WAN) have much in common with their conventional LAN cousins, but additional
protocol is added for the special security and interference concerns specific
to wireless networks. Multiple roles may
be combined into a single assembly.
Network routers sometimes have subassemblies that function as network
switches.