What is a Protocol?
A protocol is a set of rules that
governs the communications between
computers on a network. These rules
include guidelines that regulate the
following characteristics of a network:
access method, allowed physical
topologies, types of cabling, and speed of
data transfer.
See the Topology and
Cabling sections of this tutorial for
more information.
The most common protocols are:
Ethernet
The Ethernet protocol is by far the
most widely used. Ethernet uses an access
method called CSMA/CD (Carrier Sense
Multiple Access/Collision Detection). This
is a system where each computer listens to
the cable before sending anything through
the network. If the network is clear, the
computer will transmit. If some other node
is already transmitting on the cable, the
computer will wait and try again when the
line is clear. Sometimes, two computers
attempt to transmit at the same instant.
When this happens a collision occurs. Each
computer then backs off and waits a random
amount of time before attempting to
retransmit. With this access method, it is
normal to have collisions. However, the
delay caused by collisions and
retransmitting is very small and does not
normally effect the speed of transmission
on the network.
The Ethernet protocol allows for linear
bus, star, or tree topologies. Data can be
transmitted over twisted pair, coaxial, or
fiber optic cable at a speed of 10 Mbps up
to 1000 Mbps.
Fast Ethernet
To allow for an increased speed of
transmission, the Ethernet protocol has
developed a new standard that supports 100
Mbps. This is commonly called Fast
Ethernet. Fast Ethernet requires the use
of different, more expensive network
concentrators/hubs and network interface
cards. In addition, category 5 twisted
pair or fiber optic cable is necessary.
Fast Ethernet is becoming common in
schools that have been recently wired.
Gigabit Ethernet
The most recent development in the
Ethernet standard is a protocol that has a
transmission speed of 1 Gbps. Gigabit
Ethernet is primarily used for backbones
on a network at this time. In the future,
it will probably be used for workstation
and server connections also. It can be
used with both fiber optic cabling and
copper. The 1000BaseTX, the copper cable
used for Gigabit Ethernet, is expected to
become the formal standard in 1999.
LocalTalk
LocalTalk is a network protocol that
was developed by Apple Computer, Inc. for
Macintosh computers. The method used by
LocalTalk is called CSMA/CA (Carrier Sense
Multiple Access with Collision Avoidance).
It is similar to CSMA/CD except that a
computer signals its intent to transmit
before it actually does so. LocalTalk
adapters and special twisted pair cable
can be used to connect a series of
computers through the serial port. The
Macintosh operating system allows the
establishment of a peer-to-peer network
without the need for additional software.
With the addition of the server version of
AppleShare software, a client/server
network can be established.
The LocalTalk protocol allows for
linear bus, star, or tree topologies using
twisted pair cable. A primary disadvantage
of LocalTalk is speed. Its speed of
transmission is only 230 Kbps.
Token Ring
The Token Ring protocol was developed
by IBM in the mid-1980s. The access method
used involves token-passing. In Token
Ring, the computers are connected so that
the signal travels around the network from
one computer to another in a logical ring.
A single electronic token moves around the
ring from one computer to the next. If a
computer does not have information to
transmit, it simply passes the token on to
the next workstation. If a computer wishes
to transmit and receives an empty token,
it attaches data to the token. The token
then proceeds around the ring until it
comes to the computer for which the data
is meant. At this point, the data is
captured by the receiving computer. The
Token Ring protocol requires a star-wired
ring using twisted pair or fiber optic
cable. It can operate at transmission
speeds of 4 Mbps or 16 Mbps. Due to the
increasing popularity of Ethernet, the use
of Token Ring in school environments has
decreased.
FDDI
Fiber Distributed Data Interface (FDDI)
is a network protocol that is used
primarily to interconnect two or more
local area networks, often over large
distances. The access method used by FDDI
involves token-passing. FDDI uses a dual
ring physical topology. Transmission
normally occurs on one of the rings;
however, if a break occurs, the system
keeps information moving by automatically
using portions of the second ring to
create a new complete ring. A major
advantage of FDDI is speed. It operates
over fiber optic cable at 100 Mbps.
ATM
Asynchronous Transfer Mode (ATM) is a
network protocol that transmits data at a
speed of 155 Mbps and higher. ATM works by
transmitting all data in small packets of
a fixed size; whereas, other protocols
transfer variable length packets. ATM
supports a variety of media such as video,
CD-quality audio, and imaging. ATM employs
a star topology, which can work with fiber
optic as well as twisted pair cable.
ATM is most often used to interconnect
two or more local area networks. It is
also frequently used by Internet Service
Providers to utilize high-speed access to
the Internet for their clients. As ATM
technology becomes more cost-effective, it
will provide another solution for
constructing faster local area networks.
Protocol Summary
| Protocol |
Cable |
Speed |
Topology |
| Ethernet |
Twisted Pair, Coaxial, Fiber |
10 Mbps |
Linear Bus, Star, Tree |
| Fast Ethernet |
Twisted Pair, Fiber |
100 Mbps |
Star |
| LocalTalk |
Twisted Pair |
.23 Mbps |
Linear Bus or Star |
| Token Ring |
Twisted Pair |
4 Mbps - 16 Mbps |
Star-Wired Ring |
| FDDI |
Fiber |
100 Mbps |
Dual ring |
| ATM |
Twisted Pair, Fiber |
155-2488 Mbps |
Linear Bus, Star, Tree |
|
I.T.
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