Packet Size

Packet Size

There is a significant relationship between packet size and transmission time, as
shown in Figure 3.9. In this example, it is assumed that there is a virtual circuit from
station A through nodes 4 and 1 to station B (Figure 3.3). The message to be sent
comprises 40 octets, and each packet contains 3 octets of control information, which
is placed at the beginning of each packet and is referred to as a header. If the entire
message is sent as a single packet of 43 octets (3 octets of header plus 40 octets of
data), then the packet is first transmitted from station A to node 4 .

When the entire packet is received, it can then be transmitted from 4 to 1. When the
entire packet is received at node 1, it is then transferred to station B. Ignoring
switching time, total transmission time is 129 octet-times (43 octets X 3 packet
transmissions).
Suppose now that we break the message up into two packets, each containing
20 octets of the message and, of course, 3 octets each of header, or control information.
In this case, node 4 can begin transmitting the first packet as soon as it has
arrived from A, without waiting for the second packet. Because of this overlap in

transmission, the total transmission time drops to 92 octet-times. By breaking the
message up into five packets, each intermediate node can begin transmission even
sooner and the savings in time is greater, with a total of 77 octet-times for transmission.
However, this process of using more and smaller packets eventually results in
increased, rather than reduced, delay as illustrated in Figure 3.9d. This is because
each packet contains a fixed amount of header, and more packets mean more of
these headers. Furthermore, the example does not show the processing and queuing
delays at each node. These delays are also greater when more packets are handled
for a single message. However, we shall see in the next section that an extremely
small packet size (53 octets) can result in an efficient network design.