The physical layer of the network focuses on hardware elements, 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 (phenomenal at the time) over coaxial cable. The Ethernet protocol allows for bus, star, or tree topologies, depending on the type of cables used and other factors. This heavy coaxial cabling was expensive to purchase, install, and maintain, and very difficult to retrofit into existing facilities.
The current standards are now built around the use of twisted pair wire. Common twisted pair standards are 10BaseT, 100BaseT, and 1000BaseT. The number (10, 100, 1000) ands for the speed of transmission (10/100/1000 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. 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. (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