Network Models
A network is a combination of hardware and software that sends data from one location
to another. The hardware consists of the physical equipment that carries signals from
one point of the network to another. The software consists of instruction sets that make
possible the services that we expect from a network.
We can compare the task of networking to the task of solving a mathematics problem
with a computer. The fundamental job of solving the problem with a computer is done
by computer hardware. However, this is a very tedious task if only hardware is involved.
We would need switches for every memory location to store and manipulate data. The
task is much easier if software is available. At the highest level, a program can direct
the problem-solving process; the details of how this is done by the actual hardware can
be left to the layers of software that are called by the higher levels.
Compare this to a service provided by a computer network. For example, the task
of sending an e-mail from one point in the world to another can be broken into several
tasks, each performed by a separate software package. Each software package uses the
services of another software package. At the lowest layer, a signal, or a set of signals, is
sent from the source computer to the destination computer.
In this chapter, we give a general idea of the layers of a network and discuss the
functions of each. Detailed descriptions of these layers follow in later chapters.
2.1 LAYERED TASKS
We use the concept of layers in our daily life. As an example, let us consider two
friends who communicate through postal maiL The process of sending a letter to a
friend would be complex if there were no services available from the post office. Figure
2.1 shows the steps in this task.
Sender, Receiver, and Carrier
In Figure 2.1 we have a sender, a receiver, and a carrier that transports the letter. There
is a hierarchy of tasks.
At the Sellder Site
Let us first describe, in order, the activities that take place at the sender site.
o Higher layer. The sender writes the letter, inserts the letter in an envelope, writes
the sender and receiver addresses, and drops the letter in a mailbox.
o Middle layer. The letter is picked up by a letter carrier and delivered to the post
office.
o Lower layer. The letter is sorted at the post office; a carrier transports the letter.
011 the Way
The letter is then on its way to the recipient. On the way to the recipient's local post
office, the letter may actually go through a central office. In addition, it may be transported
by truck, train, airplane, boat, or a combination of these.
At the Receiver Site
o Lower layer. The carrier transports the letter to the post office.
o Middle layer. The letter is sorted and delivered to the recipient's mailbox.
o Higher layer. The receiver picks up the letter, opens the envelope, and reads it.
According to our analysis, there are three different activities at the sender site and
another three activities at the receiver site. The task of transporting the letter between
the sender and the receiver is done by the carrier. Something that is not obvious
immediately is that the tasks must be done in the order given in the hierarchy. At the
sender site, the letter must be written and dropped in the mailbox before being picked
up by the letter carrier and delivered to the post office. At the receiver site, the letter
must be dropped in the recipient mailbox before being picked up and read by the
recipient.
Services
Each layer at the sending site uses the services of the layer immediately below it. The
sender at the higher layer uses the services of the middle layer. The middle layer uses
the services of the lower layer. The lower layer uses the services of the carrier.
The layered model that dominated data communications and networking literature
before 1990 was the Open Systems Interconnection (OSI) model. Everyone believed
that the OSI model would become the ultimate standard for data communications, but
this did not happen. The TCPIIP protocol suite became the dominant commercial architecture
because it was used and tested extensively in the Internet; the OSI model was
never fully implemented.
In this chapter, first we briefly discuss the OSI model, and then we concentrate on
TCPIIP as a protocol suite.
THE OSI MODEL
Established in 1947, the International Standards Organization (ISO) is a multinational
body dedicated to worldwide agreement on international standards. An ISO standard
that covers all aspects of network communications is the Open Systems Interconnection
model. It was first introduced in the late 1970s. An open system is a set of protocols that
allows any two different systems to communicate regardless of their underlying architecture.
The purpose of the OSI model is to show how to facilitate communication
between different systems without requiring changes to the logic of the underlying hardware
and software. The OSI model is not a protocol; it is a model for understanding and
designing a network architecture that is flexible, robust, and interoperable.
ISO isThe OSI model is a layered framework for the design of network systems that
allows communication between all types of computer systems. It consists of seven separate
but related layers, each of which defines a part of the process of moving information
across a network (see Figure 2.2). An understanding of the fundamentals of the OSI
model provides a solid basis for exploring data communications.
Layered Architecture
The OSI model is composed of seven ordered layers: physical (layer 1), data link (layer 2),
network (layer 3), transport (layer 4), session (layer 5), presentation (layer 6), and
application (layer 7). Figure 2.3 shows the layers involved when a message is sent from
device A to device B. As the message travels from A to B, it may pass through many
intermediate nodes. These intermediate nodes usually involve only the first three layers
of the OSI model.
In developing the model, the designers distilled the process of transmitting data to
its most fundamental elements. They identified which networking functions had related
uses and collected those functions into discrete groups that became the layers. Each
layer defines a family of functions distinct from those of the other layers. By defining
and localizing functionality in this fashion, the designers created an architecture that is
both comprehensive and flexible. Most importantly, the OSI model allows complete
interoperability between otherwise incompatible systems.
Within a single machine, each layer calls upon the services of the layer just below
it. Layer 3, for example, uses the services provided by layer 2 and provides services for
layer 4. Between machines, layer x on one machine communicates with layer x on
another machine. This communication is governed by an agreed-upon series of rules
and conventions called protocols. The processes on each machine that communicate at
a given layer are called peer-to-peer processes. Communication between machines is
therefore a peer-to-peer process using the protocols appropriate to a given layer.
back up through the layers. Each layer in the sending device adds its own information
to the message it receives from the layer just above it and passes the whole package to
the layer just below it.
At layer I the entire package is converted to a form that can be transmitted to the
receiving device. At the receiving machine, the message is unwrapped layer by layer,
with each process receiving and removing the data meant for it. For example, layer 2
removes the data meant for it, then passes the rest to layer 3. Layer 3 then removes the
data meant for it and passes the rest to layer 4, and so on.
Interfaces Between Layers
The passing of the data and network information down through the layers of the sending
device and back up through the layers of the receiving device is made possible by
an interface between each pair of adjacent layers. Each interface defines the information
and services a layer must provide for the layer above it. Well-defined interfaces and
layer functions provide modularity to a network. As long as a layer provides the
expected services to the layer above it, the specific implementation of its functions can
be modified or replaced without requiring changes to the surrounding layers.
Organization ofthe Layers
The seven layers can be thought of as belonging to three subgroups. Layers I, 2, and
3-physical, data link, and network-are the network support layers; they deal withthe physical aspects of moving data from one device to another (such as electrical
specifications, physical connections, physical addressing, and transport timing and
reliability). Layers 5, 6, and 7-session, presentation, and application-can be
thought of as the user support layers; they allow interoperability among unrelated
software systems. Layer 4, the transport layer, links the two subgroups and ensures
that what the lower layers have transmitted is in a form that the upper layers can use.
The upper OSI layers are almost always implemented in software; lower layers are a
combination of hardware and software, except for the physical layer, which is mostly
hardware.
In Figure 2.4, which gives an overall view of the OSI layers, D7 means the data
unit at layer 7, D6 means the data unit at layer 6, and so on. The process starts at layer
7 (the application layer), then moves from layer to layer in descending, sequential
order. At each layer, a header, or possibly a trailer, can be added to the data unit.
Commonly, the trailer is added only at layer 2. When the formatted data unit passes
through the physical layer (layer 1), it is changed into an electromagnetic signal and
transported along a physical link
