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Post by RAVIGARU »


The OSI model has seven layers. The principles that were applied to arrive at the seven layers can be briefly
summarized as follows:

1. A layer should be created where a different abstraction is needed.
2. Each layer should perform a well-defined function.
3. The function of each layer should be chosen with an eye toward defining internationally standardized
4. The layer boundaries should be chosen to minimize the information flow across the interfaces.
5. The number of layers should be large enough that distinct functions need not be thrown together in the
same layer out of necessity and small enough that the architecture does not become unwieldy.

Below we will discuss each layer of the model in turn, starting at the bottom layer. Note that the OSI model itself
is not a network architecture because it does not specify the exact services and protocols to be used in each
layer. It just tells what each layer should do. However, ISO has also produced standards for all the layers,
although these are not part of the reference model itself. Each one has been published as a separate
international standard.

The Physical Layer

The physical layer is concerned with transmitting raw bits over a communication channel. The design issues
have to do with making sure that when one side sends a 1 bit, it is received by the other side as a 1 bit, not as a
0 bit. Typical questions here are how many volts should be used to represent a 1 and how many for a 0, how
many nanoseconds a bit lasts, whether transmission may proceed simultaneously in both directions, how the
initial connection is established and how it is torn down when both sides are finished, and how many pins the
network connector has and what each pin is used for. The design issues here largely deal with mechanical,
electrical, and timing interfaces, and the physical transmission medium, which lies below the physical layer.

The Data Link Layer

The main task of the data link layer is to transform a raw transmission facility into a line that appears free of
undetected transmission errors to the network layer. It accomplishes this task by having the sender break up the
input data into data frames (typically a few hundred or a few thousand bytes) and transmit the frames
sequentially. If the service is reliable, the receiver confirms correct receipt of each frame by sending back an
acknowledgement frame. Another issue that arises in the data link layer (and most of the higher layers as well) is how to keep a fast
transmitter from drowning a slow receiver in data. Some traffic regulation mechanism is often needed to let the
transmitter know how much buffer space the receiver has at the moment. Frequently, this flow regulation and the
error handling are integrated. Broadcast networks have an additional issue in the data link layer: how to control access to the shared channel.
A special sublayer of the data link layer, the medium access control sublayer, deals with this problem.

The Network Layer

The network layer controls the operation of the subnet. A key design issue is determining how packets are
routed from source to destination. Routes can be based on static tables that are ''wired into'' the network and
rarely changed. They can also be determined at the start of each conversation, for example, a terminal session
(e.g., a login to a remote machine). Finally, they can be highly dynamic, being determined anew for each packet,
to reflect the current network load. If too many packets are present in the subnet at the same time, they will get
in one another's way, forming bottlenecks. The control of such congestion also belongs to the network layer.
More generally, the quality of service provided (delay, transit time, jitter, etc.) is also a network layer issue.
When a packet has to travel from one network to another to get to its destination, many problems can arise. The
addressing used by the second network may be different from the first one. The second one may not accept the
packet at all because it is too large. The protocols may differ, and so on. It is up to the network layer to overcome
all these problems to allow heterogeneous networks to be interconnected.
In broadcast networks, the routing problem is simple, so the network layer is often thin or even nonexistent.

The Transport Layer

The basic function of the transport layer is to accept data from above, split it up into smaller units if need be,
pass these to the network layer, and ensure that the pieces all arrive correctly at the other end. Furthermore, all
this must be done efficiently and in a way that isolates the upper layers from the inevitable changes in the
hardware technology. The transport layer also determines what type of service to provide to the session layer,
and, ultimately, to the users of the network. The most popular type of transport connection is an error-free
point-to-point channel that delivers messages or bytes in the order in which they were sent. However, other
possible kinds of transport service are the transporting of isolated messages, with no guarantee about the order of delivery, and the
broadcasting of messages to multiple destinations. The type of service is determined when the connection is
established. (As an aside, an error-free channel is impossible to achieve; what people really mean by this term is
that the error rate is low enough to ignore in practice.)

The transport layer is a true end-to-end layer, all the way from the source to the destination. In other words, a
program on the source machine carries on a conversation with a similar program on the destination machine,
using the message headers and control messages. In the lower layers, the protocols are between each machine
and its immediate neighbors, and not between the ultimate source and destination machines, which may be
separated by many routers. The difference between layers 1 through 3, which are chained, and layers 4 through
7, which are end-to-end, is illustrated

The Session Layer

The session layer allows users on different machines to establish sessions between them. Sessions offer
various services, including dialog control (keeping track of whose turn it is to transmit), token management
(preventing two parties from attempting the same critical operation at the same time), and synchronization
(checkpointing long transmissions to allow them to continue from where they were after a crash).

The Presentation Layer

Unlike lower layers, which are mostly concerned with moving bits around, the presentation layer is concerned
with the syntax and semantics of the information transmitted. In order to make it possible for computers with
different data representations to communicate, the data structures to be exchanged can be defined in an
abstract way, along with a standard encoding to be used ''on the wire.'' The presentation layer manages these
abstract data structures and allows higher-level data structures (e.g., banking records), to be defined and

The Application Layer

The application layer contains a variety of protocols that are commonly needed by users. One widely-used
application protocol is HTTP (HyperText Transfer Protocol), which is the basis for the World Wide Web. When a
browser wants a Web page, it sends the name of the page it wants to the server using HTTP. The server then
sends the page back. Other application protocols are used for file transfer, electronic mail, and network news.
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