background image
38
Chapter 2
Protocol Attributes
Sequenced Data Transfer
Systems send protocol data units (PDUs) to one another, and each level of
the OSI model has its own type of PDU. Figure 2.4 shows the PDU names for
all seven OSI layers. For example, the Application layer's PDU name is a
Layer 7 PDU. Although this convention can be used for all layers, some lay-
ers use other names as well. For instance, a Layer 3 PDU can be 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 segmented so that it fits the MTU (max-
imum transmission unit). Therefore, several frames may be needed to trans-
fer the original data. Connection-oriented protocols assign a sequence
number to each outgoing and incoming PDU.
F I G U R E 2 . 4
OSI layer PDU names
Figure 2.5 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 the original data is obtained.
Copyright ©2000 SYBEX , Inc., Alameda, CA
www.sybex.com