Systems send protocol data units (PDUs) to one another, and each level of the OSI model has
its own type of PDU. Figure 36.4 shows the PDU names for all seven OSI layers. For example,
the Application layer’s PDU name is layer 7 PDU. Although this convention can be used for all
layers, some layers use other names as well. For instance, a layer 3 PDU is called a
packet
and
a layer 2 PDU is called a
frame
. When a system sends data to another system, the data has to
be fragmented so that it fits the MTU (maximum transmission unit). Therefore, several frames
may be needed to transfer the original data. Connection-oriented protocols assign a sequence
number to each outgoing and incoming PDU. This is
sequenced data transfer
.
Figure 36.5 shows you how sequencing works. There is a possibility that the destination system
will receive the PDUs out of order. If this happens, the protocol on the destination system
uses the sequence numbers to put the PDUs back into the correct order so that the original data
is obtained. 1101