CompTIA A+ Certification All-in-One For Dummies
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Network access refers to the different methods that computers use to place data on the network. For the compTIA A+ Exam, you need to know about these methods and the advantages and disadvantages of each.


One of the most popular types of access methods is CSMA/CD (Carrier Sense Multiple Access/Collision Detection). Understanding this term is easier if you break it down into its individual parts and examine each part in detail:
  • Carrier sense: All computers on the network are watching, or sensing, the network for network traffic. If the network has data already on the wire, a system will wait until the wire is free of traffic.
  • Multiple access: All computers on the network have equal access to the network at any given time. In other words, anyone can place data on the network whenever he or she chooses. Note, however, that workstations on the network will try not to place data on the wire at the same time the wire is transmitting other data because the two pieces of data will collide, destroying the data. That's why it's so important for workstations to "sense" the wire.

To summarize, carrier sense multiple access suggests that all workstations have access to the network and are watching the network to make sure it is clear of data before they send out their information.

  • Collision detection: When two workstations send information at the same time, the data will collide and be damaged in transit. When two workstations have data that has been involved in a collision, they resend the information out on the network at variable intervals to prevent the data from colliding again.
The nice thing about CSMA/CD is that the workstations decide when to send data, trying to prevent collisions. However, there is always the possibility that multiple workstations will send data out at the exact same moment the network is clear, resulting in data collision.


Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) is similar to CSMA/CD except for one main difference: When a workstation senses that the wire is free, it sends out dummy data first instead of real data. If the dummy data collides with other information on the wire, the workstation has avoided a collision with the real data; if the dummy data does not collide, then the workstation sends the real data. Like CSMA/CD, if the real data collides and does not reach its destination, it will be resent by the sender.

CSMA/CA is used with 802.11 wireless networking.

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