The Logical Link Control (LLC) layer is responsible for encapsulating
network layer data and frame synchronization.The network layer provides connectivity between
devices that are connected to separate networks.
Whereas the data link layer ensures communication
between directly connected devices on a local network,
the network layer enables communication between
devices that are in geographically separate locations.
Logical addressing of all the devices on the network and
routing of data packets on the best path between a
sender and a receiver are also specified at the network
layer. Routers are devices that operate at the network
layer. If the data that needs to be transmitted over the
data link layer is too large to be sent at once, the network
layer has the capability to fragment the data into smaller
pieces. As the name indicates, the process of
fragmentation consists of splitting the data into smaller
pieces, sending those pieces on the network, and then
reassembling them when they arrive at another device on
the network. Fragmentation should be avoided as much
as possible as it slows down the transmission of data
considerably. This slowdown occurs because of how
modern routers forward data packets and the
mechanism they implement to accomplish this as fast as
possible. Most modern routers perform data routing in
hardware using specialized integrated circuits called
ASICs. When data becomes fragmented, instead of being
forwarded in hardware using the ASICs, the packets get
process switched, which means they are forwarded based
on an interrupt system using the regular CPU on the
router. Data delivery at the network layer is best effort,
as reliable transmission is not required at this layer. As
we’ll see shortly, reliable transmission is handled at the
next higher layer, the transport layer.
The transport layer, as the name suggests, is
responsible for end-to-end transport of data from the
source to the destination of the data traffic. It segments
data that it receives from the layer above and