The Physical and Link Layers

TCP/IP can be quite interesting, but a knowledge of the first two layers of the OSI model are important too, because without them, even the most basic communication would be impossible.

There are very many standards that define the physical, electrical, and error-control methodologies of data communication. One of the most popular ones in departmental networks is Ethernet, which is available in a variety of cable types and speed capabilities, but the data transmission and error correction strategy is the same in all.

Ethernet used to operate primarily in a mode where every computer on a network section shared the same Ethernet cable. Computers would wait until the line was clear before transmitting. They would then send their data while comparing what they wanted to send with what they actually sent on the cable as a means of error detection. If a mathematical comparison, or cyclic redundancy check (CRC), detected any differences between the two, the server would assume that it transmitted data simultaneously with another server on the cable. It would then wait some random time and retransmit at some later stage when the line was clear again.

Transmitting data only after first sensing whether the cable, which was strung between multiple devices, had the correct signaling levels is a methodology called carrier sense, multiple access or CSMA. The ability to detect garbling due to simultaneous data transmissions, also known as collisions, is called collision detect or CD. You will frequently see references to Ethernet being a CSMA/CD technology for this reason and similar schemes are now being used in wireless networks.

Ethernet devices are now usually connected via a dedicated cable, using more powerful hardware capable of simultaneously transmitting and receiving without interference, thereby making it more reliable and inherently faster than its predecessor versions. The original Ethernet standard has a speed of 10 Mbps; the most recent versions can handle up to 40Gbps!

The 802.11 specifications that define many wireless networking technologies are another example of commonly used layer 1 and 2 components of the OSI model. DSL, cable modem standards and, T1 circuits are all parts of these layers.

The next few sections describe many physical and link layer concepts and the operation of the devices that use them to connect the computers in our offices and homes.