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Module 5
Lesson 6
The demand priority access method

Assess the characteristics of the demand priority access method.

What is demand priority?

Demand priority is a newer access method than those previously discussed. Hewlett-Packard developed it specifically for the 100VG-AnyLAN network architecture, which we will discuss in the next module. Demand priority is another contention type of access method, but it is implemented somewhat differently from CSMA/CD and CSMA/CA.

Demand priority is based on the use of hubs that act as multi-port repeaters and also manage network access. Although it offers a number of advantages over other access methods, it is not widely implemented.

TIP:The specifications for the demand priority access method are defined in IEEE 802.12.

How demand priority works

A network using the demand priority method has groups of computers connected to a series of interconnected hubs. Computers communicate only with their hubs; the computers do not access the network directly. Each hub is responsible for monitoring the computers attached to it, listening for requests to transmit. Computers can receive and transmit at the same time, due to the cabling method used.

Administrators can designate that certain types of data will be given higher priority than other types. Because demand priority is a contention method, two computers can request to transmit simultaneously, but the hub will allow the one with the highest priority data to transmit first.

The FlipBook below illustrates how the demand priority access method works.


The table below lists the advantages and disadvantages of the demand priority access method.
Advantages of demand priority Disadvantages of demand priority
  • More efficient; Each hub interacts only with the computers connected directly to it.

  • Flexible; Computers can transmit and receive simultaneously.

  • Selective; Transmissions are not broadcast to the whole network, so there is less congestion of network bandwidth.
  • Relatively costly; Implementation requires expensive equipment.

  • Limited; Few experienced support persons available due to limited usage of demand priority networking.


Next lesson

In the next lesson, you will learn about the relationship of media access methods to networking architecture.
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