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Using a simplified and conservative approach, the relationship between
the network throughput and the protocol characteristics can be described by
Equation 14.1:
Equation 14.1 Throughput and protocol characteristics
AT is the actual throughput available on the network measured in bytes per
second, NM is the maximum number of packets exchanged per second
between a terminal and the central server, T is the number of terminals, PL
is the size in bytes of the protocol payload per packet, NO is the total
network protocol overhead (e.g. the TCP header size) measured in bytes.
We can assume that the time between the scan of two consecutive
products by the bar code reader is about 1 second. Therefore this is the time
required to exchange all the information about the product and signal
quantity of products acquired. The nominal throughput of an ethernet LAN
is 10 Mbit
s, but in the case of several nodes and heavy traffic we can
consider an actual throughput of 1 Mbit
120 KB
s. The overhead
per packet caused by TCP
IP and the ethernet protocol is about 80 bytes
including the acknowledge packet (Huston 2000). We assume that each
message will require only one TCP packet. Considering the extreme case
where the available throughput is entirely consumed by communication, we
can rewrite Equation 14.1 as follows, expressing the number of supported
terminals as a function of the other factors.
Equation 14.2 Number of terminals
We can estimate the relationship between the payload and the maximum
number of terminals supported by the protocol. NM is a consequence of the
particular application protocol adopted. The graphical representation of this
relationship is shown in Figure 14.3. We consider three types of application
protocol that require the number of messages to buy a product to be 4, 6 and
8. In the case of a supermarket with 60 counters, if the number of messages
required by the application protocol to buy a product is 6, then the payload
limit is 180 bytes per message.
Main features
The supermarket operation support system exhibits four main features:
Counter terminal . This provides a simple interface integrated with
specific devices such as bar code readers.
Central server . This hosts the main computations and interacts with a
database containing all the required information.
Remote communication . The terminals and the server communicate
though a simple and lightweight interaction protocol.
Monitoring . The sales manager should be able to check the current status
of the store and active counters.
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