Information Technology Reference
In-Depth Information
issues largely deal with mechanical, electrical, and timing interfaces, as well
as the physical transmission medium, which lies below the physical layer.
2.2.3.1.2 Data Link Layer
The main task of the data link layer is to transform a raw transmission facil-
ity into a line that appears free of undetected transmission errors. 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. 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 lay-
ers 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 trans-
mitter 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.
2.2.3.1.3 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 log-in 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. Handling congestion
is also a responsibility of the network layer, in conjunction with higher layers
that adapt the load they 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 des-
tination, many problems can arise. The addressing used by the second net-
work 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.
2.2.3.1.4 Transport Layer
The basic function of the transport layer is to accept data from above it, split
them up into smaller units if need be, pass these to the network layer, and
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