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 what electrical signals should be used to represent a 1 and a 0, how many nanoseconds a
bit lasts, whether transmission may proceed simultaneously in both directions, how the initial connection is
established, how it is torn down when both sides are finished, how many pins the network connector has, and what
each pin is used for.
These design issues largely deal with mechanical, electrical, and timing interfaces, as well as 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
It does so by masking the real errors so the network layer does not see them.
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. I
f 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 may be needed to let the transmitter know when the receiver can accept more data.
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, or more often they can be
updated automatically to avoid failed components.
They can also be determined at the start of each conversation, for example, a terminal session, such as a login to a
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.
Handling congestion is also a responsibility of the network layer, in conjunction with higher layers that adapt the
loadhey place on the network.
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 that used by 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 it, 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 over the course of time.
The transport layer also determines what type of service to provide to the session layer, and, ultimately, to the users of
. 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 exist, such as 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.