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).
HTTP | FTP | SMTP | DNS
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.
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 Fast Ethernet protocol supports transmission up to 100 Mbps. 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 standards include:
- 100BaseT – 100 Mbps over 2-pair category 5 or better UTP cable.
- 100BaseFX – 100 Mbps over fiber cable.
- 100BaseSX -100 Mbps over multimode fiber cable.
- 100BaseBX – 100 Mbps over single mode fiber cable.
Gigabit Ethernet standard is a protocol that has a transmission speed of 1 Gbps (1000 Mbps). It can be used with both fiber optic cabling and copper. The 1000BaseT, the copper cable used for Gigabit Ethernet (see the Cabling section for more information).
- 1000BaseT – 1000 Mbps over 2-pair category 5 or better UTP cable.
- 1000BaseTX – 1000 Mbps over 2-pair category 6 or better UTP cable.
- 1000BaseFX – 1000 Mbps over fiber cable.
- 1000BaseSX -1000 Mbps over multimode fiber cable.
- 1000BaseBX – 1000 Mbps over single mode fiber cable.
The Ethernet standards continue to evolve. with 10 Gigabit Ethernet (10,000 Mbps) and 100 Gigabit Ethernet (100,000 Mbps),
Ethernet Protocol Summary
LocalTalk is a network protocol that was developed by Apple Computer, Inc. for Macintosh computers many years ago. LocalTalk adapters and special twisted pair cable can be used to connect a series of older computers through the serial port (current Macintosh computers have Ethernet cards and/or wireless adapters installed). A primary disadvantage of LocalTalk is speed. Its speed of transmission is only 230 Kbps.
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. 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 dramatically.
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 Interne