Page 29 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Computer Networks/Networks

Notes 2. Each layer is assigned to perform well-defined functions.
3. The function of each layer is based on internationally standardized protocols.
4. The layer boundaries are chosen to minimize the information flow across the interfaces.
5. The number of layers is kept large enough that distinct functions have different layers.
They are also kept small enough that the architecture does not become unwieldy.

Physical Layer (Layer 1)

This layer describes the physical media or communication channel over which the bit stream is
to be transmitted with the objective that when sending side sends a 1 bit, it is received by the
receiving side as a 1 bit, not as a 0 bit. Hence, it defines the electrical and mechanical aspects of
interfacing to a physical medium for transmitting data, as well as setting up, maintaining, and
disconnecting physical links. It is primarily concerned with moving bits from one node to next
over the physical link. The issues concerning with the physical layer involve amplitude of the
pulses to define 1 and 0 level, width of the pulse in microseconds, types and mode of
communications, establishment and breaking of connections at the time of communications,
types of connectors, etc.
It accepts data from the Data Link layer in bit streams for the subsequent transmission over the
physical medium. At this layer, the mechanical (connector type), electrical (voltage levels),
functional (ping assignments), and procedural (handshake) characteristics are defined. RS-232C/
D is an example of a physical layer definition.

Data Link Layer (Layer 2)

It takes the bits that are received by the physical layer and detects error. This establishes an error
free communications path between network nodes over the physical channel, frames messages
for transmission, checks integrity of received messages, manages access to and use of the channel,
ensures proper sequence of transmitted data. Hence, this layer is responsible for the reliable
transfer of data across the Physical link. Its responsibilities include such functions as data flow
control, breaking the input data, frame formatting, transmission of the frames sequentially,
error detection, and link management, etc. In order to provide a reliable service, it also offers
processing of the acknowledgement frames, retransmitting lost or damaged frames, etc.

Did u know? Data link layer is further subdivided into Medium Access (MAC) sub layer to
deal with the access control over the shared channel in broadcast networks.
Network Layer (Layer 3)

The network layer comprises software that addresses the PDUs and transports them to the
ultimate destination, setting up the appropriate paths between the various nodes. Therefore, the
main objective of this layer is to control the operation of the subnet. It is the layer, which
provides Internet Protocol (IP) to use it. It is mainly responsible for providing routing services
from source to destination across the Internet. In doing so, it allows internetworking among
heterogeneous networks using different addressing, length of packet, protocols, etc. The routing
may be static or dynamic. Network layer also plays important role in congestion control.
It also shields the above layers from details about the underlying network (the network topology
and road map) and the routing technology that might have been deployed to connect different
networks together. In addition to routing, this layer is responsible for establishing and

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