Java Reference
In-Depth Information
servers communicate; at the other end of the spectrum, the IEEE 802.3 standard defines
a protocol for how bits are encoded as electrical signals on a particular type of wire.
Open, published protocol standards allow software and equipment from different ven‐
dors to communicate with one another. A web server doesn't care whether the client is
a Unix workstation, an Android phone, or an iPad, because all clients speak the same
HTTP protocol regardless of platform.
The Layers of a Network
Sending data across a network is a complex operation that must be carefully tuned to
the physical characteristics of the network as well as the logical character of the data
being sent. Software that sends data across a network must understand how to avoid
collisions between packets, convert digital data to analog signals, detect and correct
errors, route packets from one host to another, and more. The process is further com‐
plicated when the requirement to support multiple operating systems and heterogene‐
ous network cabling is added.
To hide most of this complexity from the application developer and end user, the dif‐
ferent aspects of network communication are separated into multiple layers. Each layer
represents a different level of abstraction between the physical hardware (i.e., the wires
and electricity) and the information being transmitted. In theory, each layer only talks
to the layers immediately above and immediately below it. Separating the network into
layers lets you modify or even replace the software in one layer without affecting the
others, as long as the interfaces between the layers stay the same.
Figure 1-1
shows a stack of possible protocols that may exist in your network. While
the middle layer protocols are fairly consistent across most of the Internet today, the
top and the bottom vary a lot. Some hosts use Ethernet; some use WiFi; some use PPP;
some use something else. Similarly, what's on the top of the stack will depend completely
on which programs a host is running. The key is that from the top of the stack, it doesn't
really matter what's on the bottom and vice versa. The layer model decouples the ap‐
plication protocols (the main subject of this topic) from the physics of the network
hardware and the topology of the network connections.
