Information Technology Reference
In-Depth Information
OSI
TCP/IP
7
Application
Application
6
Presentation
Not present
in the model
5
Session
4
Transport
Transport
3
Network
Internet
2
Data link
Link
1
Physical
FIGURE 2.1
OSI versus TCP/IP network reference model.
Protocol suites that conform to the OSI model must include at least one spe-
cific protocol at each of the seven levels that the model defines (Table 2.1).
2.2.1.4 Datagram
Datagram is essentially another name for data packet. The term datagram
refers to the similarity of this delivery mode to the way in which letters and
telegrams are delivered. The essential feature of datagram networks is that
the delivery of each packet is a one-shot process; no setup is required, and
once the packet is delivered, the network retains no information about it.
2.2.2 Types of Network
There is no generally accepted taxonomy into which all computer networks
fit, but two dimensions stand out as important: transmission technology and
scale. We will now examine each of these in turn. Broadly speaking, there
are two types of transmission technology that are in widespread use: broad-
cast links and point-to-point links.
Point-to-point links connect individual pairs of machines. To go from the
source to the destination on a network made up of point-to-point links, short
messages, called packets in certain contexts, may have to first visit one or
more intermediate machines. Often, multiple routes, of different lengths,
are possible, so finding good ones is important in point-to-point networks.
Point-to-point transmission with exactly one sender and exactly one receiver
is sometimes called unicasting. In contrast, on a broadcast network, the com-
munication channel is shared by all the machines on the network; packets
sent by any machine are received by all the others. An address field within
each packet specifies the intended recipient. Upon receiving a packet, a
machine checks the address field. If the packet is intended for the receiving
 
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