2.3 Chapter Summary

In this chapter, we started the discussion with the OSI model and what devices work at which layers. It was pointed out that networking for the most part concerns Layers 1 to 4 of the OSI model. Acting as the post office, the job is to get data from point A to point B. The electrical or optical signals on the cable are types of Layer 1 implementations . The Layer 1 physical signals are used by data link Layer 2 as the connection between like Layer 2 switches. Layer 2 switches form the connection between Layer 3 routers. This relationship is most important. An effectively built network starts from the ground ( lowest layer) up. Nothing will get anywhere if you don't plug in the cable.

After the OSI discussion, we discussed the basics of various types of Layer 2 data link technology. Ethernet and the various speeds are examples of LAN broadcast protocols. Frame Relay, PPP, and ATM are examples of WAN nonbroadcast technology. Typically, data moves from a host on a LAN to WAN to LAN through the use of LAN/WAN switches separated by routers. These data link protocols are the local transport between devices for the Layer 3 protocols.

The Layer 3 protocol, we explained, is IP. This is the most widely used Layer 3 protocol by far. It is the protocol that e-mail, the World Wide Web, Internet video, and Internet chat protocols use to get from host to host. It uses an addressing scheme that allows routers to forward IP packets in the appropriate direction until they get to their destination. IP addresses can be grouped together using a subnet mask to represent a range of IP addresses. These ranges can be made smaller to fit smaller networking needs or larger to summarize several networks, all just by moving the bits of the subnet mask right or left respectively.

Lastly, two types of Layer 4 protocol were explained: TCP and UDP. They are completely different in nature and are used according to the upper-layer needs. You may need connectionless, consistent bandwidth in a stream for jitter-free video (UDP) or connection-oriented bursty bandwidth for file-transfer accuracy (TCP).

These layers fit together handsomely. Physical connections transporting Layer 2 protocols carry Layer 3 IP laden with Layer 4 TCP or UDP depending on the needs of the upper-layer application from one place to another. That is networking.