A
protocol is a set of rules that governs the communications between computers on
a network. In order for two computers to talk to each other, they must be
speaking the same language. Many different types of network protocols and
standards are required to ensure that your computer (no matter which operating
system, network card, or application you are using) can communicate with
another computer located on the next desk or half-way around the world. The OSI
(Open Systems Interconnection) Reference Model defines seven layers of
networking protocols. The complexity of these layers is beyond the scope of
this tutorial; however, they can be simplified into four layers to help
identify some of the protocols with which you should be familiar (see fig 1).
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OSI Layer
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Name
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Common Protocols
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7
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Application
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6
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Presentation
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HTTP | FTP | SMTP | DNS
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5
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Session
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4
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Transport
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TCP
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SPX
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3
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Network
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IP
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IPX
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2
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Data Link
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Ethernet
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1
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Physical
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Fig
1. OSI model related to common network protocols
Figure
1 illustrates how some of the major protocols would correlate to the OSI model
in order to communicate via the Internet. In this model, there are four layers,
including:
- Ethernet (Physical/Data Link Layers)
- IP/IPX (Network Layer)
- TCP/SPX (Transport Layer)
- HTTP, FTP, Telnet, SMTP, and DNS (Session/Presentation/Application Layers)
Assuming you want to send an e-mail
message to someone in Italy, we will examine the layers "from the bottom
up" -- beginning with Ethernet (physical/data link kayers).
The physical layer of the network focuses on hardware
issues, such as cables, repeaters, and network interface cards. By far the most
common protocol used at the physical layer is Ethernet. For example, an
Ethernet network (such as 10BaseT or 100BaseTX) specifies the type of cables
that can be used, the optimal topology (star vs. bus, etc.), the maximum length
of cables, etc. (See the Cabling section for more information on Ethernet
standards related to the physical layer).
The data link layer of the network addresses the way that
data packets are sent from one node to another. 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.
Ethernet
The
original Ethernet standard was developed in 1983 and had a maximum speed of 10
Mbps (phenomonal at the time). The Ethernet protocol allows for bus, star, or
tree topologies, depending on the type of cables used and other factors .
The
current standard at the 10 Mbps level is 10BaseT. The "10" stands for
the speed of transmission (10 megabits per second); the "Base" stands
for "baseband" meaning it has full control of the wire on a single
frequency; and the "T" stands for "twisted pair" cable.
Older standards, such as 10Base2 and 10Base5, used coaxial cable, but these
standards are seldom used in new installations. Fiber cable can also be used
at this level in 10BaseFL.
The
network layer is in charge of routing network messages (data) from one computer
to another. The common protocols at this layer are IP (which is paired with TCP
at the transport layer for Internet network) and IPX (which is paired with
SPX at the transport layer for some older Macintosh, Linus, UNIX, Novell and
Windows networks). Because of the growth in Internet-based networks, IP/TCP are
becoming the leading protocols for most networks.
Every
network device (such as network interface cards and printers) have a physical
address called a MAC (Media Access Control) address. When you purchase a
network card, the MAC address is fixed and cannot be changed. Networks using
the IP and IPX protocols assign logical addresses (which are made up of the MAC
address and the network address) to the devices on the network, This can all
become quite complex -- suffice it to say that the network layer takes care of
assigning the correct addresses (via IP or IPX) and then uses routers to send
the data packets to other networks.
The
transport layer is concerned with efficient and reliable trsansportation of the
data packets from one network to another. In most cases, a document,
e-mail message or other piece of information is not sent as one unit. Instead,
it is broken into small data packets, each with header information that
identifies its correct sequence and document.
When
the data packets are sent over a network, they may or may not take the same
route -- it doesn't matter. At the receiving end, the data packets are
re-assembled into the proper order. After all packets are received, a message
goes back to the originating network. If a packet does not arrive, a message to
"re-send" is sent back to the originating network.
TCP,
paired with IP, is by far the most popular protocol at the transport level. If
the IPX protocol is used at the network layer (on networks such as Novell or
Microsoft), then it is paired with SPX at the transport layer.
Several
protocols overlap the session, presentation, and application layers of
networks. There protocols listed below are a few of the more well-known:
- DNS - Domain Name System - translates network address (such as IP addresses) into terms understood by humans (such as URLs)
- DHCP - Dynamic Host Configuration Protocol - can automatically assign Internet addresses to computers and users
- FTP - File Transfer Protocol - a protocol that is used to transfer and manipulate files on the Internet
- HTTP - HyperText Transfer Protocol - An Internet-based protocol for sending and receiving webpages
- IMAP - Internet Message Access Protocol - A protocol for e-mail messages on the Internet
- IRC - Internet Relay Chat - a protocol used for Internet chat and other communications
- POP3 - Post Office protocol Version 3 - a protocol used by e-mail clients to retrieve messages from remote servers
- SMTP - Simple Mail Transfer Protocol - A protocol for e-mail messages on the Internet.
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