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 wireless access points, 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 |
|
